Abstract
This review provides an overview of the clinical relevance of chemosensitization, giving special reference to the phenolic phytochemicals, curcumin, genistein, epigallocatechin gallate, quercetin, emodin, and resveratrol, which are potential candidates due to their ability to regulate multiple survival pathways without inducing toxicity. We also give a brief summary of all the clinical trials related to the important phytochemicals that emerge as chemosensitizers. The mode of action of these phytochemicals in regulating the key players of the death receptor pathway and multidrug resistance proteins is also abridged. Rigorous efforts in identifying novel chemosensitizers and unraveling their molecular mechanism have resulted in some of the promising candidates such as curcumin, genistein, and polyphenon E, which have gone into clinical trials. Even though considerable research has been conducted in identifying the salient molecular players either contributing to drug efflux or inhibiting DNA repair and apoptosis, both of which ultimately lead to the development of chemoresistance, the interdependence of the molecular pathways leading to chemoresistance is still the impeding factor in the success of chemotherapy. Even though clinical trials are going on to evaluate the chemosensitizing efficacy of phytochemicals such as curcumin, genistein, and polyphenon E, recent results indicate that more intense study is required to confirm their clinical efficacy. Current reports also warrant intense investigation about the use of more phytochemicals such as quercetin, emodin, and resveratrol as chemosensitizers, as all of them have been shown to modulate one or more of the key regulators of chemoresistance. Antioxid. Redox Signal. 18, 1307–1348.
V. Phytochemicals Sensitizing Tumor Cells to Conventional Chemotherapeutic Drugs
VI. The Relevance and Importance of the Phytochemicals Addressed: Clinical Trials
VII. Reduced Bioavailability of Phytochemicals As a Drawback for Their Clinical Use
IX. Some Important Facts To Be Borne in Mind While Using Phytochemicals As Chemosensitizers
I. Introduction
The table provides a brief summary of the number of studies reported in each category. Compounds with maximum number of studies reported were selected for the review.
EGCG, epigallocatechin gallate.
II. Chemotherapeutic Agents and Their Limitations
An ideal chemotherapeutic drug should kill or incapacitate cancer cells without causing damage to normal cells. This ideal situation could be achieved by inducing apoptosis to cancer cells sparing the normal cells, which is not achieved by any of the existing chemotherapeutic drugs. Although there are different types of anti-cancer drugs based on their mechanism of action (alkylating agents, anti-tumor antibiotics, anti-metabolites, mitotic inhibitors, hormones, or others such as tumor necrosis factor-related apoptosis-inducing ligand [TRAIL]), all of them ultimately induce cell death by necrosis, apoptosis, or autophagy by stimulating genotoxic stress (93). Since the chemotherapeutic drugs in current use target rapidly dividing cells, normal dividing cells are not spared, which culminate in side effects. Significant cell death in hematopoietic cells, intestinal epithelial cells, and hair matrix keratinocytes leads to impaired immunity, loss of digestive tract lining, and hair loss (44). The side effects may be short term, which will resolve within a few months after the completion of the therapy, or long term, which will be sustained for many years. Nausea, stomatitis, myelosuppression, alopecia, and thromboembolism are some of the common short-term side effects, while infertility, weight gain, cardiac dysfunction, and secondary leukemia are long-term side effects associated with chemotherapy. Some of the major chemotherapeutic drugs and their side effects are listed in Table 2.
The table just cited provides a brief description of the major chemotherapeutic drugs, their mechanism of action, side effects, and reason for chemoresistance.
5-FU, 5-fluorouracil; Bcl-2, B-cell lymphoma 2; Bcl-xL, B-cell lymphoma-extra large; GRP78, glucose-regulated protein 78; c-FLIP, cellular FLICE-inhibitory protein; cIAP, cellular inhibitor of apoptosis protein; COX-2, cyclooxygenase-2; DTIC, 5-(3,3-dimethyl-1-triazenyl) imidazole-4-carboxamide; HER2/neu, human epidermal growth factor receptor 2/neuregulin; HIF-1α, hypoxia-inducible factor 1-alpha; Hsp, heat shock protein; MAPKs, mitogen-activated protein kinases; Mcl-1, myeloid cell leukemia sequence 1; MMP-9, matrix metallopeptidase 9; MRP, multidrug resistance-associated protein; NF-κB, nuclear factor-kappaB; TRAIL, tumor necrosis factor-related apoptosis-inducing ligand; VEGF, vascular endothelial growth factor; XIAP, X-linked inhibitor of apoptosis protein; NSAID, nonsteroidal anti-inflammatory drug; SERM, selective estrogen receptor modulator.
The development of drug resistance is another important hindrance in the success of cancer chemotherapy. Multidrug resistance (MDR) refers to the ability of tumor cells that are exposed to a single cytotoxic agent to develop resistance to a broad range of structurally and functionally unrelated drugs. Apart from the classic mechanisms of MDR development (over-expression of drug efflux pumps and increased DNA repair mechanism), alterations at the level of apoptosis control serve as a crucial mechanism for the induction of drug resistance. The induction of apoptosis in response to genotoxic damage is opposed in cancer cells because of their attenuated tumor suppressor functions and also by the up-regulation of intrinsic antiapoptotic pathways. Hence, when a drug induces genotoxicity at a level that can circumvent these intrinsic antiapoptotic pathways prevalent in cancer cells, the normal cells are affected, again leading to side effects, which poses a major hurdle in the development of a successful chemotherapeutic drug.
III. Molecular Pathways That Influence Chemosensitivity
Chemotherapy induces cell death by necrosis or programmed cell death, like apoptosis or autophagy. Necrosis is deleterious to the surrounding tissues and in many cases, autophagy protects the cells from oxidative and genotoxic stress. Thus, the efficacy of chemotherapy mainly depends on how well the cancer cell responds to the drug by undergoing apoptosis. The first-level obstacle in the chemotherapeutic efficacy of any drug is the over-expression of intrinsic antiapoptotic molecules in cancer cells. An additional factor that makes the situation more complicated is the development of chemoresistance to a particular drug after its prolonged use. In addition to activating apoptotic cascades, most of the chemotherapeutic agents induce some cell survival pathways that contribute to chemoresistance (145, 230), which accounts for the failure in cancer chemotherapy. Various mechanisms that interfere at different levels of apoptotic signaling which leads to drug resistance have been summarized in Figure 1, and a detailed account of the same is reviewed elsewhere (112). Apart from the over-expression of anti-apoptotic genes, inactivation of pro-apoptotic genes, alteration of the p53 pathway, and altered survival signaling, several other mechanisms also contribute to the acquisition of chemoresistance (Fig. 2). The regulatory molecules involved in chemoresistance induced by major chemotherapeutic drugs have been cited in Table 2.
Different stimuli induce a cell to initiate the cell death program or apoptosis (Fig. 1). Death receptors (DRs) that transmit apoptotic signals initiated by specific ligands play a central role in apoptosis. On ligand binding, these receptors activate death caspases, causing an apoptotic demise of the cell. All DR-mediated signaling pathways, through various intermediates, finally activate caspases that induce DNA fragmentation, leading to apoptosis. More recently, however, evidence has accumulated that noncaspases, including cathepsins, calpains, granzymes, and the proteasome complex, also play roles in mediating and promoting cell death. Apart from this receptor-mediated pathway (extrinsic pathway), apoptosis can be induced through the mitochondrial pathway (intrinsic pathway) by the formation of cytochrome C/apoptosis protease-activating factor-1 (Apaf1)/procaspase 9 complex, which leads to the activation of caspases. This can be achieved by either the cleavage of Bid to tBid by the receptor-mediated pathway or through p53 and B-cell lymphoma 2 (Bcl-2)–associated X protein (Bax), both of which can be antagonized by Bcl-2. All major chemotherapeutic drugs act mainly through the mitochondrial pathway, though the receptor-mediated pathway is also reported as contributing to drug-induced cytotoxicity.
A. Involvement of intrinsic pathway in chemoresistance
p53, the most frequently mutated gene in human cancers, plays a significant role in regulating the apoptotic program induced by chemotherapeutic drugs. As a tumor suppressor, p53 induces cell-cycle arrest, thereby curbing proliferation and activating either DNA repair or apoptosis mechanisms. The efficacy of many chemotherapeutic drugs depends on their ability to induce apoptosis in tumor cells, and the loss of wild-type p53 function in tumors leads to the inhibition of p53-mediated apoptosis, thereby causing increased chemoresistance to various chemotherapeutic drugs, including doxorubicin, cisplatin, 5-Fluorouracil (5-FU), and etoposide. Functional inactivation of p53 may occur by gene mutation or deletion, protein degradation, or viral oncogene binding. Under extreme stress and severe DNA damage, p53 triggers the activation of the genes implicated in the apoptotic cascade. This induction of the apoptotic cascade by p53 is necessary for the cytotoxic effect of chemotherapeutic drugs (75). In most of the cases, mutant p53 may lose this function abrogating the chemotherapeutic efficacy of the drugs, but in some other cases, it may transcriptionally activate a different subset of target genes such as epidermal growth factor receptor (EGFR) and MDR1 (162), which may lead to chemoresistance.
Deregulation of the Bcl-2 family of proteins, which control the mitochondrial pathway to apoptosis, has been implicated in chemoresistance. The Bcl-2 family includes pro-survival and pro-apoptotic members that bind to each other and form homo- and heterodimers, thereby regulating the integrity of the mitochondrial membrane in healthy cells, and its permeabilization in response to apoptotic stimuli. The pro-survival proteins of the Bcl-2 family include Bcl-2, Bcl-w, B-cell lymphoma-extra large (Bcl-xL), myeloid cell leukemia sequence 1 (Mcl-1), and A1. The pro-apoptotic BH3-only proteins of this family, such as Bcl-2 interacting mediator (Bim), Bid, Bad, BMF, Noxa, and p53-up-regulated modulator of apoptosis (PUMA), are able to bind to the pro-survival members of the family and inhibit their activity. Once activated, Bax and Bcl-2 homologous antagonist/killer (Bak), the main pro-apoptotic effectors of the Bcl-2-regulated pathway, homo-oligomerize and subsequently form pores in the outer mitochondrial membrane, leading to the release of cytochrome c and SMAC/DIABLO. Several studies have shown that Bcl-2 over-expression confers resistance to various chemotherapeutic drugs, including actinomycin D, cycloheximide, cisplatin, methotrexate, cytarabine, and vincristine (313). Supporting the role of Bcl2 over-expression in chemoresistance, a recent study shows that a potent small-molecule inhibitor of Bcl-2, Bcl-xL, and Bcl-w sensitizes a panel of tumor cell lines to a variety of chemotherapeutic drugs in vitro and in vivo (38). The inactivation of Mcl-1 using antisense oligonucleotides has also been shown to sensitize human melanoma cells to dacarbazine (DTIC) treatment, both in vitro and in vivo (296). Some of the proapoptotic members of the Bcl2 family also have important regulatory roles in chemoresistance. MiR-10b, a micro RNA that is over-expressed in colorectal cancer, is shown to impart chemoresistance through the inhibition of Bim (215). Noxa acts as a mediator between reactive oxygen species (ROS) signaling and apoptosis induction, and its up-regulation helps chronic lymphocytic leukemia cells overcome cisplatin resistance (299). Deficiencies of Bax (31) and Apaf-1 (291) have also been implicated in chemoresistance.
B. Extrinsic pathway and chemoresistance
The extrinsic pathway of apoptosis comprises DRs, their ligands, and the signaling intermediates. DRs are members of the tumor necrosis factor (TNF) receptor gene superfamily, having a broad range of functions, including the regulation of cell death and survival, differentiation, or immune regulation. Cluster of differentiation 95 (CD95) (APO-1/Fas), TNF receptor 1 (TNFRI), TRAIL-R1 (DR4), and TRAIL-R2 (DR5) are the best-characterized DRs, while the exact functions of other receptors such as DR3 or DR6 have not been defined. The ligands of the TNF superfamily comprise CD95L (Fas ligand [FasL]), TNF-α, TRAIL, and TNF-related weak inducer of apoptosis (TWEAK). TRAIL can induce apoptosis exclusively in cancer cells through the activation of TRAIL-R1 and TRAIL-R2, sparing normal cells.
Increased levels of two soluble receptors, namely soluble CD95 (sCD95) and decoy receptor 3 (DcR3), which bind to FasL and antagonize its apoptotic effect, have been shown to contribute to chemoresistance (300). A range of human cancers exhibit increased activity of casein kinase II (CK2), a serine/threonine kinase that has been implicated in cell growth and proliferation. Constitutively activated CK2 in cancer cells shows a high Bcl-xL/tBID ratio (238) and high cellular FLICE-inhibitory protein (c-FLIP) (183), making it resistant to the apoptotic effect of Apo2 ligand (Apo2L)/TRAIL.
c-FLIP, originally identified as a virus-encoded apoptosis-inhibitory protein, contains death effector domains, through which it interacts with caspase-8, thus inhibiting caspase-8 cleavage and blocking apoptosis induced by DRs. It has been shown to have a regulatory role in inducing TRAIL resistance in hepatocellular carcinoma cells (138).
TNF superfamily modulates down-stream signaling pathways such as nuclear factor-kappaB (NF-κB) and PI3 K, and also induces second messengers such as ROS (248). Even though the intracellular level of ROS has been implicated as a mediator of apoptosis, it also plays a major role in the chemosensitivity of cancer cells (227). A high degree of oxidative stress may cause necrosis, while lower levels can lead to apoptosis. ROS are generated by redox-sensitive, pro-survival signaling pathways, which function as intermediates in the transduction of several extracellular signals. Although a complex intracellular redox network exists for protecting cells against oxidative stress, there are pathways involved in ROS-adaptive response that play a critical role in protecting cells against the cytotoxic effects of anticancer agents, leading to chemoresistance. Studies have shown that chemotherapeutic drugs induce apoptosis partly by inducing the formation of ROS in a variety of cell types. Many classes of drugs, including anthracycline antibiotics, alkylating agents, and mitotic inhibitors, induce high levels of oxidative stress in biological systems (55), which lead to drug resistance. The NF-κB pathway, activated by different signaling events, is one of the major down-stream effector pathways regulating ROS signaling, leading to chemoresistance (204).
C. ROS signaling and nuclear factor erythroid 2-related factor 2 in chemoresistance
A cell is said to be in a state of oxidative stress when the amount of ROS generated is higher than the antioxidant system's ability to counteract, in which case a normal cell undergoes apoptosis. Cancer cells, however, overcome this apoptotic signaling through the over-expression of various redox enzymes and the induction of DNA repair mechanisms, which ultimately result in chemoresistance. Increased oxidative stress, thus, plays an important role in the development and progression of cancer (82). Apart from NF-κB, other transcription-like nuclear factor erythroid 2-related factor 2 (Nrf2) and hypoxia-inducible transcription factor 1 alpha (HIF-1α) are also induced by ROS signaling.
Nrf2 is a redox-sensitive transcription factor that binds to antioxidant response element in the promoter region of various phase II detoxifying and antioxidant enzymes and regulates their expression (217). It is ubiquitously expressed in all human organs at low constitutive levels under tight regulation by the anchor protein Kelch-like ECH-associated protein-1 (Keap1). Under oxidative stress, Nrf2 induces the transcription of cellular protective genes to combat carcinogenic reactive intermediates. Therefore, activation of the Nrf2 pathway is important in chemoprevention. However, surprisingly, Nrf2 plays a negative role in chemotherapy response. In the context of cancer, the loss of Keap1 (due to mutation or reduced expression) results in the up-regulation of Nrf2 and its target genes such as HMOX-1and ATP-binding cassette (ABC) transporters, blocking ROS mediated apoptotic pathway, protecting the cancer cells from chemotherapy (163). The over-expression of Nrf2 has been shown to impart chemoresistance in a variety of cancer cells to a broad spectrum of chemotherapeutic drugs (163, 240).
D. Signaling cross-talks in chemoresistance
NF-κB is a ubiquitously expressed transcription factor that regulates diverse functions, including apoptosis, cell proliferation, immune response, and inflammation. Several chemotherapeutic agents, including paclitaxel, vinblastine, vincristine, doxorubicin, daunomycin, 5-FU, cisplatin, tamoxifen, and bortezomib, have been reported to induce NF-κB activation, leading to chemoresistance (172). Cyclooxygenase-2 (COX-2), Cyclin D1, Bcl-2, Bcl-xL, Survivin, and X-linked inhibitor of apoptosis protein (XIAP) have been identified to be responsible for NF-κB-mediated chemoresistance. In addition, there exists crosstalk between NF-κB and other survival pathways such as phosphoinositide 3-OH kinase (PI3K)/Akt and EGFR, further contributing to chemoresistance. Thus, the down-regulation of NF-κB could be considered an effective mechanism for overcoming drug resistance.
Another well-studied factor involved in chemoresistance is hypoxia, where the cells are deprived of adequate oxygen supply. Hypoxic adaptation has been documented as a protective strategy for tumor cells to survive. Hypoxia can up-regulate the expression of hypoxia-inducible genes, particularly HIF-1α, that lead to elevated expression of vascular endothelial growth factor (VEGF), Glut-1, MDR, and Bcl-2, leading to chemoresistance (39). It has been shown that NF-κB can directly modulate the HIF-1α pathway (302) and in certain other contexts, hypoxia is shown to activate NF-κB to bind to κB-binding sites in DNA (146).
PI3K and its downstream target, Akt are important components of the survival signaling pathway that are mediated through the NF-κB pathway or independent of it in response to growth factor stimulation and Ca
Signaling through human epidermal growth factor receptor 2/neuregulin (HER2/neu) is another growth factor receptor-mediated signal transduction pathway that has been implicated in conferring resistance to conventional chemotherapy. HER2/neu is a member of the receptor tyrosine kinase superfamily, and its over-expression has been proved to enhance malignancy, and is associated with the metastasis of breast carcinoma (194). A HER2/PI3K/Akt pathway that mediates MDR in human breast cancer cells has been elucidated (143). HER2/neu has also been shown to recruit mitogenic pathways such as the mitogen-activated protein kinases (MAPKs) (134) apart from PI3K. Moreover, elevated p21Waf1 levels conferred by HER2/neu signaling can also bring about changes in the G2/M transition, and help evade cell death induced by chemotherapeutic drugs (99). Even though p21waf1 was originally identified as a universal cyclin-dependent kinases (CDK) inhibitor with the capacity to induce cell growth arrest, at conditions where Cyclin D is over-expressed, p21 over-expression leads to the inhibition of apoptosis, leading to chemoresistance (99). Studies have shown that over-expression of HER2/neu confers resistance to various chemotherapeutic drugs, including Tamoxifen (62), Taxol (324), doxorubicin, and 5-FU (143).
E. Inhibitor-of-apoptosis proteins as regulators of chemoresistance
Irrespective of the mode of induction of apoptosis, caspases are the key players in regulating the apoptotic machinery. Members of the inhibitors-of-apoptosis protein (IAP) family, survivin, cellular inhibitor of apoptosis protein (cIAP) 1, cIAP2, XIAP, and melanoma-IAP inhibit the activation of the effector caspses 3, 6, and 7, and have been shown to confer resistance to apoptosis induction by chemotherapeutic agents. The induction of survivin has been shown to be the reason for chemoresistance of cancer cells toward various drugs such as doxorubicin, etoposide, paclitaxel, and cisplatin (200). As members of the IAP family, cIAP1, cIAP2, and XIAP have been implicated in the protection of cells from apoptosis induced by etoposide (125).
F. Role of ubiquitin-proteasome pathway in chemoresistance
Apoptosis is regulated by the opposing activities of pro-apoptotic and anti-apoptotic molecules. Cancer cells often have disregulated apoptotic signaling pathways that give malignant cells a survival advantage and can confer resistance to chemotherapeutic agents. The proteasome is involved in the control of apoptosis by modulating the levels of pro- and anti-apoptotic factors. The ubiquitin-proteasome system facilitates the degradation of damaged proteins and the regulators of growth and stress response. Alterations in this proteolytic system are associated with a variety of human pathologies, including cancer. The inhibition of proteasome activity results in an up-regulation of pro-apoptotic factors, such as p53, Bax, and Noxa, while reducing levels of anti-apoptotic proteins such as Bcl-2 and IAP proteins (190). Proteasome inhibitors have been demonstrated to induce apoptosis in numerous malignant cell types when used as a single agent and induce sensitivity to other chemotherapeutic agents in combination.
Bortezomib, a proteasome inhibitor, is shown to play a significant role in combination chemotherapy in multiple myeloma, for the first time, by modulating mechanisms that overcome chemoresistance and support chemosensitization. Proteasome inhibition in multiple myeloma seems to be able to overcome Bcl-2-mediated suppression of apoptosis, P-glycoprotein (P-gp)-mediated MDR, and inducible resistance through NF-κB (220). Proteasome inhibition by bortezomib also enhances the antitumor effects of gemcitabine in experimental pancreatic cancer by overcoming the chemoresistance pathways (12) and abrogates the cisplatin resistance in Fanconi anemia (FA)-proficient ovarian cancer cells (115). Bortezomib and nelfinavir (another proteasome inhibitor) have been reported to sensitize chemoresistant cervical cancer cells to TRAIL receptor antibody treatment (28).
G. Involvement of autophagy in chemoresistance
Autophagy refers to a process in which cytoplasmic components are delivered to lysosomes for bulk degradation in response to intracellular and extracellular stress. It is an evolutionarily conserved mechanism for substance degradation for the maintenance of intracellular homeostasis. A functional autophagy initial protein Beclin1 and the inhibition of mammalian target of rapamycin (mTOR) activity are critical for autophagy. All the molecules that regulate the levels of these two, in turn, regulate autophagy. The important regulators identified are p53/adenosine monophosphate-activated protein kinase (AMPK) pathway, NF-κB, PI3K/Akt, and Bcl-2 family members (335). In a context-specific manner, autophagy can function as either a cytoprotective response or a cell-death-inducing mechanism. The contradictory role of autophagy and its relevance in cancer has been recently reviewed (335).
Accumulating evidence shows that autophagy plays a significant role in chemotherapy and chemoresistance. Many of the chemotherapeutic drugs, including imatinib, paclitaxel, cisplatin, 5-FU, arsenic trioxide (As2O3), TRAIL, and tamoxifen, induce autophagy along with the induction of apoptosis, where autophagy exerts its cytoprotective effect by degrading the drug molecules, helping cancer cells evade apoptosis. In accordance with this, the inhibition of autophagy has been shown to enhance the chemotherapeutic efficacy of cisplatin and 5-FU (97) and various other chemotherapeutic drugs, including imatinib, bortezomib, TRAIL, and tamoxifen (335). Moreover, a switch from apoptosis to autophagy as the principal mechanism of drug-induced cytotoxicity has been shown to be the mechanism of paclitaxel resistance in breast cancer cells (4). At the same time, co-treatment of cisplatin with autophagy inducer, trifluorperazine, is shown to sensitize cisplatin-resistant lung carcinoma cells to cisplatin-induced cell death (275). Different mTOR inhibitors and Bcl-2 inhibitors are used to enhance autophagy, and this could enhance the chemotherapeutic efficacy in cells where there is an inherent defect in apoptosis (335) as observed in the case of Bid-deficient breast carcinoma cells (208). Thus, modulators of autophagy can sensitize chemoresistant cells to chemotherapy.
H. MDR and chemoresistance
One of the major problems that hamper the effective treatment of cancer involving chemotherapy is the acquisition of MDR against anticancer drugs. Increased detoxification of anticancer drugs by glutathione conjugation and the transport of these conjugated forms out of cells by GS-X pump or multidrug resistance-associated protein (MRP) have been proposed to be one of the major reasons for drug resistance (327). ABC transporters are a family of proteins that mediate MDR via ATP-dependent drug efflux pumps. Some of the best-characterized ABC transporters associated with MDR include MDR1/P-gp, MRP1, the lung resistance protein (LRP), and the breast cancer resistance protein (BCRP). MDR is commonly mediated by the over-expression of P-gp, a transmembrane protein that acts as an energy-dependent drug efflux pump. This transporter actively removes a variety of drugs, including anthracyclines, vinca alkaloids, epipodophyllotoxins, actinomycin D, and paclitaxel, which leads to a reduction in their intracellular accumulation and cytotoxicity (279). MRP renders cancer cells resistant to anticancer drugs such as anthracyclines, vinca alkaloids, and epipodophyllotoxins, leading to treatment failures (52). BCRP, another ABC transporter, was identified as a drug efflux pump for anticancer drugs such as anthracyclin antibiotics (70). LRP is frequently over-expressed in multidrug-resistant cells and is reported to cause resistance to drugs such as doxorubicin, vincristine, etoposide, paclitaxel, and gramicidin D (142).
Glucose-regulated protein 78 (GRP78), an endoplasmic reticulum chaperone, is induced in a wide variety of tumors due to glucose starvation and hypoxia in the microenvironment of poorly perfused solid tumors (165). When over-expressed as in carcinoma, GRP78 functions as a co-receptor that binds with various binding partners that are involved in the coordination of multiple signaling pathways, which ultimately results in cell survival or the abrogation of cell death, contributing to chemoresistance. This is achieved through the activation of Akt/PKB and NF-κB with concomitant hampering of transforming growth factor (TGF)-β signaling (250). It has been shown that GRP78 can form an antiapoptotic complex with caspase 7 when it is in a monomeric form that imparts chemoresistance to cancer cells (80). Aberrant GRP78 expression has been correlated with the development of resistance to conventional drugs such as paclitaxel (305), doxorubicin, and (261) etoposide (239).
Since chemoresistance is induced by drug-induced up-regulation of various survival signaling pathways and MDR genes and loss of function of various pro-apoptotic molecules, any compound that can down-regulate those pathways will increase the efficacy of chemotherapy. The major targets to be modulated to overcome chemoresistance are the various members of the DR and p53 pathways, members of the EGFR and Bcl-2 families, and important signaling intermediates such as NF-κB and Akt. It is in this scenario that the concept of chemosensitization emerges. This review provides an overview of polyphenolic phytochemicals derived from edible plants reported to down-regulate the intracellular survival signaling cascades.
IV. Chemosensitizers: Potential Candidates Under Evaluation
Various types of cancers have been observed to exhibit multidrug resistance, which involves cellular and non-cellular mechanisms employed by cancer cells to overcome the effects of structurally and functionally unrelated drugs. The initial research on chemosensitizers was conducted on the reversal of this effect. Classic calcium channel blockers such as verapamil and dexverapamil that block the drug efflux were the candidates, which were selected as chemosensitizers. A phase III randomized study of oral verapamil as a chemosensitizer that reverses drug resistance in patients with refractory myeloma proved that this approach is ineffective due to toxicity (60). The use of dexverapamil as a chemosensitizer for epirubicin therapy in metastatic breast cancer resulted only in a partial response in 4 out of 23 patients studied, suggesting that more potent chemosensitizers are necessary for effective treatment (167).
Many of the chemotherapeutic agents function by inducing apoptosis in cancer cells, as they have the ability to induce DNA damage. However, the damage produced by the therapeutic agents can often be repaired by the base excision repair (BER) proteins, which confer therapeutic resistance. The efficient inhibition of a particular BER protein(s) may increase the efficacy of current chemotherapeutic regimes, and, hence, can act as a chemosensitizer. A small interfering RNA (siRNA) that mediated the silencing of BER proteins, N-Methylpurine DNA glycosylase, or AP-endonuclease is shown to sensitize cancer cells to alkylating chemotherapeutics (1). Other molecules such as checkpoint kinases functioning in the same DNA damage response machinery are other targets for chemosensitization. Checkpoint kinases CHK1 and CHK2 phosphorylate key proteins to elicit cell-cycle blocks. The inhibition of these kinases is believed to sensitize tumor cells to cancer treatments that damage DNA (334). The p53-murine double minute 2 (MDM2) interaction pathway, as it is involved in the DNA damage-response pathway, has been suggested as a novel target for cancer therapy. To this end, several strategies have been explored, including the use of small polypeptides targeted to the MDM2-p53 binding domain and anti-MDM2 antisense oligonucleotides (23).
Since apoptotic machinery is involved in chemoresistance, the modulators that favor apoptosis are potential candidates for functioning as chemosensitizers. The most characterized molecules that are targeted for chemosensitization are anti-apoptotic factors such as NF-κB, Bcl-2, and HIF-1α. Since NF-κB is constitutively activated in almost all tumor types and can be used as a predictor for treatment response (92), its inhibition has been shown to enhance the efficacy of a variety of chemotherapeutic drugs (15, 17, 22, 65, 76). The inhibition of Bcl-2 by DNA anti-sense and RNA interference have been effectively employed to sensitize cancer cells to chemotherapeutic drugs (133). Apart from this, other phytochemicals (18, 22, 202) and chemical inhibitors have been shown to chemosensitize cancer cells through the down-regulation of Bcl-2, and some of them have undergone clinical trials (90, 221). The transcriptional factor HIF-1 is generally considered the major regulator of the hypoxic adaptive response and chemoresistance (203).
Some of the other approaches for chemosensitization that have undergone clinical trials are proton pump inhibitors (40), anion channel inhibitors (30), protein tyrosine phosphatase inhibitor (suramin) (303), Vitamin E family members (gamma-tocotrienol) (321), and low-dose radiation (151).
Bortezomib is the first FDA-approved proteasome inhibitor that entered clinical trials, and is very effective for the treatment of dexamethasone-treated multiple myeloma patients (132). Although generally well tolerated, the usage of bortezomib is still hampered by toxicity and resistance, which underscore the need for less toxic proteasome inhibitors (160). Hence, using plant polyphenols possessing proteasome activity would be a better choice for avoiding the potential side effects and chemoresistance. Plant polyphenols that have been identified to possess proteasome-inhibitory activity include epigallocatechin gallate (EGCG), genistein, luteolin, apigenin, chrysin, quercetin, curcumin, and tannic acid. These polyphenols exert a significant effect while overcoming chemoresistance to diverse chemotherapeutic drugs in a broad spectrum of tumors (94, 263). The various chemosensitizers under investigation and their possible mechanism of action are listed in Table 3.
Compounds studied for their chemosensitizing efficacy, the class to which they belong, and their mode of action are summarized in the table just cited.
ABC, ATP-binding cassette; BER, base excision repair; EGFR, epidermal growth factor receptor; MDM2, murine double minute 2; MDR, multidrug resistance; Nrf2, nuclear factor erythroid 2-related factor 2; siRNA, small interfering RNA.
Even though several molecular targets are identified and their modulation by inhibitors or silencers is used to prove their efficacy in vitro, the in vivo translation of most of them is still a controversy. Since chemoresistance is contributed by multiple survival pathways, the inhibition of only one molecule may not be sufficient to circumvent this phenomenon. Hence, compounds that can simultaneously modulate multiple survival signaling pathways might provide a better therapeutic outcome than that of individual inhibitors. Several phytochemicals have been shown to modulate multiple pathways involved in chemoresistance and, hence, are assumed to be of better chemosensitizing efficacy. The phytochemicals reviewed for their chemosensitizing efficacy and their reported mode of action are listed in Table 4.
This table provides a brief summary of the molecular targets regulated by the phytochemicals addressed in the review.
AMPK, adenosine monophosphate-activated protein kinase; AP-1, activator protein 1; Ara-C, cytosine arabinoside; Bak, Bcl-2 homologous antagonist/killer; Bax, Bcl-2–associated X protein; Bim, Bcl-2 interacting mediator; C/EBPβ, CCAAT-enhancer-binding protein β; CHOP, cyclophosphamide, doxorubicin, vincristine, and prednisone; CXCR4, C-X-C chemokine receptor type 4; Cyp1b1, cytochrome p450 1b1; DR, death receptor; ERα, estrogen receptor alpha; ERK1/2, extracellular signal-regulated kinase1/2; FA, Fanconi anemia; FAK, focal adhesion kinase; GADD153, growth arrest and DNA damage induced gene-153; IAPs, inhibitors-of-apoptosis-proteins; IGF-1R, insulin-like growth factor 1 receptor; mTOR, mammalian target of rapamycin; PEA15, phosphoprotein enriched in astrocytes 15; PKC-α, protein kinase C alpha; P-gp, P-glycoprotein; PI3K, phosphoinositide 3-OH kinase; PUMA, p53 up-regulated modulator of apoptosis; RANKL, receptor activator of nuclear factor-kappaB ligand; RhoA, Ras homolog A; ROS, reactive oxygen species; STAT, signal transducer and activator of transcription.
V. Phytochemicals Sensitizing Tumor Cells to Conventional Chemotherapeutic Drugs
Bioprospecting and molecular pharmacology studies have shown that a large number of phytochemicals can modulate the survival pathways induced by cancer cells, carcinogens, and chemotherapeutics. Our detailed investigation shows that a majority of these phytochemicals are phenolic compounds, which can be subdivided into phenolic acids and phenols, polyphenols and polyphenolic flavonoids (Table 1)
The clinical trials evaluating the chemotherapeutic efficacy of the phytochemical addressed in this review are summarized in the table just cited.
HGF, hepatocyte growth factor; HNSCC, head and neck squamous cell carcinoma; QC12, quercetin pro-drug; NSCLC, nonsmall cell lung carcinoma; PSA, prostate-specific antigen.
The chemoprevention trials carried out using the phytochemicals evaluated in this review are listed in the table just cited.
FAP, familial adenomatous polyposis; IGFBP-3, insulin-like growth factor-binding protein 3; TIMP, tissue inhibitor of metalloproteinase.
The table just cited is a brief summary of the in vitro and in vivo chemosensitization studies conducted using the phytochemicals addressed in this review.
BCG, Bacillus Calmette-Guérin.
A. Curcumin
Curcumin (diferuloylmethane), isolated from Curcuma longa is a polyphenol that enhances the chemotherapeutic efficacy of a variety of chemotherapeutic drugs. The key components that regulate the chemosensitizing efficacy of curcumin are recapitulated in Figure 3.
Curcumin potentiates the antitumor effects of antimetabolites such as gemcitabine and 5-FU. It inhibits gemcitabine-induced NF-κB and its downstream targets, leading to the inhibition of proliferation, angiogenesis, and invasion in an orthotopic model of pancreatic cancer (150). Curcumin also augments the cytotoxic effect of gemcitabine in pancreatic adenocarcinoma cells through the down-regulation of COX-2 and phospho-extracellular signal-regulated kinase1/2 (ERK1/2) levels (169). These studies have led to the clinical evaluation of the combination for pancreatic cancer (131) (Table 8). In another study from the same lab, it was shown that curcumin enhanced the antitumor activity of capecitabine through its modulation of cyclin D1, COX-2, matrix metallopeptidase 9 (MMP-9), VEGF, and C-X-C chemokine receptor type 4 (CXCR4), as assessed by an orthotopic mouse model of the human colorectal cancer (149). The chemosensitizing efficacy of curcumin in capecitabine and gemcitabine chemotherapy for colorectal cancer is on clinical evaluation (Table 8). Curcumin has been also shown to sensitize prostate cancer cells to the cytotoxic effect of 5-FU, through a p53-independent cell-cycle arrest and the down-regulation of constitutive NF-κB activation (105). Curcumin enhances the cytotoxic effect of 5-FU and oxaliplatin in colon cancer cells through the down-regulation of COX-2 (71) and the modulation of EGFR and insulin-like growth factor 1 receptor (IGF-1R) (223).
This table depicts the clinical trials using phytochemicals addressed in this review as chemosensitizers in chemotherapy.
The chemosensitizing efficacy of curcumin toward TRAIL is mainly regulated by the modulation of NF-κB and Bcl-2 family members, the key mediators of TRAIL-induced apoptosis. The inhibition of NF-κB activation has been reported to enhance the sensitivity of prostate cancer cells to TRAIL-induced apoptosis in vitro (64) and in vivo (257). A further analysis shows that Akt is the master regulator of the NF-κB and Bcl-2 family members, contributing to this synergism (63). Another mechanism by which curcumin enhances the TRAIL-induced apoptosis is through ROS-mediated up-regulation of the receptor, DR5 (127). In addition to the modulation of Bcl-2 family members, down-regulation of IAPs and up-regulation of TRAIL receptors are reported to be the mechanisms behind curcumin-mediated chemosensitization of androgen unresponsive and responsive TRAIL-resistant cells to TRAIL (256). Curcumin also enhances the antitumor effect of Bacillus Calmette-Guérin (BCG), a widely used drug for bladder cancer, through the down-regulation of NF-κB and the induction of TRAIL receptors (129).
Apart from TRAIL, chemosensization by curcumin enhances the efficacy of various chemotherapeutic agents, including mitotic inhibitors. The in vitro and in vivo studies from our laboratory have shown that the antitumor effects of paclitaxel could be enhanced by curcumin in cervical cancer cells through the down-regulation of paclitaxel-induced activation of NF-κB, Akt, and Bcl-2 (17, 18, 284). This combination was also found to be effective in inhibiting lung metastasis of human breast cancer in nude mice through the down-regulation of paclitaxel-induced NF-κB activation by curcumin (2). One of the mechanisms suggested for this chemosensitizing efficacy of curcumin is by the inhibition of paclitaxel-induced degradation of IκB-α through its proteasomal inhibitory activity, as curcumin is reported to have 20S and 26S proteasomal inhibitory activity in vitro and in vivo (197). Curcumin potentiates cytotoxic effects of paclitaxel in prostate cancer cells through a p53-independent up-regulation of CCAAT-enhancer-binding protein β (C/EBPβ) and p21 and the subsequent inhibition of cell-cycle progression. In addition, curcumin down-regulates the constitutive NF-κB activation in these cells, which may also contribute to the enhanced efficacy of these chemotherapeutic drugs (105). Supporting this observation, the down-regulation of docetaxel-mediated NF-κB activation by curcumin was observed in the in vivo model of ovarian cancer (180). Curcumin also enhances the chemosensitivity of various cancer cells to vincristine (22) or vinblastine (8) through its ability to down-regulate NF-κB or P-gp, respectively. However, curcumin has also been reported to inhibit the therapeutic efficacy of the mitotic inhibitors, paclitaxel (13), and docetaxel (308).
Even though the NF-κB pathway is the key regulator of curcumin-mediated chemosensitization, other mediators are also involved in it, especially in cisplatin chemotherapy. FA/BRCA pathway, a DNA-damage-responsive pathway, has been reported to be a reason for cisplatin resistance in ovarian tumors (295). Curcumin has been shown to inhibit the FA/BRCA pathway and sensitizes ovarian cancer cells to cisplatin-induced apoptosis (45). In another study, curcumin was shown to be effective in enhancing the cytotoxicity of cisplatin in ovarian cancer cells through the reduction of the autologous production of IL-6 (34). Down-regulation of IAPs by curcumin has also been reported to enhance the effect of cisplatin in hepatic cancer cells (218). Molecular pathways contributing to the chemosensitizing efficacy of curcumin toward various chemotherapeutic drugs have been summarized in Table 4.
B. Genistein
Genistein, (4′, 5, 7-trihydroxyisoflavone), an isoflavone with a heterocyclic diphenolic structure found in soybeans, has been shown to inhibit the growth of various cancer cells in vitro and in vivo without toxicity to normal cells and, hence, has undergone clinical trials (77) (Table 5). The key regulatory molecules involved in genistein-mediated chemosensitization are summarized in Figure 4.
A majority of the research on genistein points out that it is a promising chemosensitizer for chemotherapy with antimetabolites such as gemcitabine and 5-FU. Recent studies show that genistein potentiates the efficacy of gemcitabine through the down-regulation of NF-κB and Akt in osteosarcoma (177, 325) and pancreatic cancer (14). By the same mechanism, it sensitizes pancreatic cancer cells to a combination of erlotinib and gemcitabine (76), an FDA-approved regimen for the treatment of advanced pancreatic cancer. Though the preclinical studies were promising, the clinical trials did not give a positive outcome for this combination, warranting further investigation (77). Down-regulation of COX-2 and enhancement of AMPK activation are other mechanisms suggested for genistein-induced chemosensitization as observed in combination with 5-FU in HT-29 colon cancer cells (111).
Studies from various labs have proved that genistein sensitizes cancers of various origins to TRAIL therapy. Genistein exerts its chemosensitizing efficacy by modulating the receptors of TRAIL, its downstream effectors, or pathways that cross-talk with TRAIL-induced apoptosis. Genistein-mediated up-regulation of DR5 (receptor of TRAIL) sensitizes gastric adenocarcinoma cells to TRAIL-induced apoptosis (123). Proteasomal degradation of the antiapoptotic protein, c-FLIP by genistein has been shown to enhance TRAIL-induced cytotoxicity (273). Down-regulation of Akt is another mechanism by which genistein sensitizes cancer cells to the apoptotic effects of TRAIL (222). TRAIL-resistant human hepatocellular carcinoma cells were sensitized to TRAIL by genistein through down-regulation of a prosurvival signal, p38 MAPK (124).
It has been proposed that genistein could enhance the therapeutic outcome when administered with mitotic inhibitors such as docetaxel to patients with prostate cancer bone metastasis, as in vitro and in vivo studies clearly showed a synergistic cytotoxic effect of these compounds through the modulation of NF-κB pathway, resulting in the inhibition of osteoclastic bone resorption and prostate cancer bone metastasis (176). The same mechanism of genistein-mediated chemosensitization toward docetaxel and cisplatin was observed in BxPC-3 pancreatic cancer cells (175) and was confirmed in an orthotopic model of pancreatic cancer (15).
The combination of cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) is the standard therapy for diffuse large cell lymphoma. In a mouse xenograft model bearing CHOP-resistant lymphoma, genistein enhanced the antitumor activity of CHOP through the down-regulation of NF-κB (202). Perifosine is an orally active alkyl-phosphocholine compound with potential antineoplastic activity. Genistein-combined polysaccharide (GCP) enhanced the perifosine-induced apoptosis in prostate cancer cells. The simultaneous inhibition of Akt and mTOR activity contributes to this enhancement in apoptosis (304). The inhibition of Akt has also been shown to be the mechanism behind the synergistic effect of cytosine arabinoside (Ara-C) and genistein in both in vitro and in vivo models of acute myeloid leukemia (262). Genistein down-regulates HER2/neu along with Akt in breast cancer cells, sensitizing them to doxorubicin (251) and tamoxifen (186). Furthermore, the down-regulation of survivin, EGFR, and estrogen receptor alpha expressions contributed to the synergistic effect of tamoxifen and genistein (186). The signaling events regulating genistein-induced chemosensitization are summarized in Table 4.
Even though genistein has been shown to act as an effective chemosensitizer to various cancers in combination with several chemotherapeutic drugs, its possible use in breast cancer treatment with tamoxifen is not promising. Some of the in vitro studies show that a higher dose of genistein compared with the dietary levels induce apoptosis in combination with tamoxifen in breast cancer cells (186). Later, in vivo studies, however, using low-dose dietary genistein showed that it negates the therapeutic effect of tamoxifen (74, 181). The major studies evaluating the role of genistein as a chemosensitizer are summarized in Table 4.
C. Epigallocatechin gallate
Green tea is a powerful antioxidant with potential chemopreventive properties, which are attributed to the polyphenolic compounds present in it. Among many polyphenolic compounds isolated from green tea, EGCG, an ester of epigallocatechin and gallic acid, is documented as the key active constituent in terms of its chemopreventive potential. Polyphenon E, which is currently being used in clinical trial, is a well-standardized decaffeinated green tea catechin mixture that contains mainly EGCG (65%) along with epicatechin and several polyphenolic compounds. Since this formulation is highly reproducible, Polyphenon E is widely used for clinical trials.
Since the major component of polyphenon E is EGCG, a majority of the research that elucidates the chemosensitizing efficacy of green tea has been done using EGCG. Several studies have proved that EGCG could be potentially used to enhance the therapeutic efficacy of chemotherapeutic drugs and to reduce their toxicity (Table 4) through various mechanisms, as depicted in Figure 5.
The chemosensitizing efficacy of EGCG has been utilized to overcome TRAIL resistance in a variety of cancer cells. It down-regulates the expression of survivin and phosphoprotein enriched in astrocytes 15 (PEA15) that contains a death effector domain whose over-expression has been reported to be involved in the inhibition of both Fas-mediated and TNFRI–mediated apoptosis (54), through an Akt-dependent mechanism (271). It also modulates the extrinsic and intrinsic pathways to sensitize TRAIL-resistant LNCaP cells to TRAIL-mediated apoptosis through the modulation of TRAIL-R1, Fas-activated death domain (FADD), c-FLIP, Bcl-2, and Bcl-xL. Furthermore, the combination also significantly reduced the invasion and migration of LNCaP cells (270). This TRAIL-R1-dependent synergistic effect is also reported in melanoma cells (264). Both in vitro and in vivo studies have shown that EGCG enhances TRAIL-induced apoptosis of hepatocellular carcinoma cells through the down-regulation of NF-κB (216).
Interferon-α (IFN-α) is a pleiotropic cytokine approved by FDA as a therapy for high-risk melanoma patients because of its immunomodulatory effects (243). Many adverse side-effects have been reported in almost every organ system due to IFN-α therapy, which severely hampers its therapeutic efficacy (277). A promising recent report shows that EGCG in combination with suboptimal concentrations of IFN-α increase FasL-mediated apoptosis in melanoma cells, in vitro and in vivo, through an up-regulation of Fas and down-regulation of constitutively active NF-κB (214), suggesting that EGCG can reduce the toxicity and improve the efficacy of IFN-α therapy.
The chemotherapeutic efficacy of 4-hydroxy tamoxifen could be enhanced by EGCG in breast cancer cells (46), glioma cells (265), and lung cancer cells (288). In vivo studies in breast cancer models showed that this enhancement of cytotoxicity is mainly due to EGCG-mediated down-regulation of EGFR, mTOR, Akt, and NF-κB (252). Studies have also shown that EGCG down-regulates P-gp and BCRP in tamoxifen-resistant breast cancer cells (81). The selective expression of telomerase in tumor cells makes telomerase an attractive therapeutic target, and EGCG is known to exhibit telomerase inhibitory effects in a variety of cancers (201, 247). EGCG significantly inhibited telomerase expression and enhanced the cytotoxicity of tamoxifen in human glioma cell lines (265).
The chemosensitizing efficacy of EGCG to paclitaxel chemotherapy depends on the down-regulation of paclitaxel-induced Bcl-2 (228). Another mode of chemosensitization by EGCG is through the modulation of GRP78. Paclitaxel-induced GRP78 activation is shown to be down-regulated by EGCG in breast cancer cells, both in vitro and in vivo (184). The same mechanism has been observed in the synergistic effect of vinblastine and EGCG (305). EGCG also prevents the formation of the antiapoptotic GRP78/caspase7 complex, thereby sensitizing cancer cells to etoposide-induced apoptosis (80).
EGCG is also effective in enhancing the efficacy of alkylating agents. It enhances DTIC-mediated inhibition of melanoma growth and metastasis, through the suppression of MMP-9 secretion and tyrosine phosphorylation of Focal adhesion kinase (FAK) (182). Temozolomide, a DTIC derivative that is widely used for malignant gliomas when given in combination with EGCG, enhances its antitumor activity through the down-regulation of GRP78 in glioma cell lines (232). Even though EGCG enhances the cytotoxicity of cisplatin in glioma cells (265), it protects oral cancer cells against cisplatin-induced cytotoxicity (317).
EGCG pretreatment in human cholangiocarcinoma cell lines enhances the growth inhibitory activities of the antimetabolites, 5-FU, and gemcitabine through the enhancement of mitochondrial membrane depolarization and cytochrome C release (161). Another mechanism suggested for the synergistic enhancement of cytotoxic effect of 5-FU in human head and neck cancer cell lines is the inhibition of EGFR (189). A similar observation of the inhibition of EGF receptor and HER2 has been reported in human colon cancer cells, when treated with polyphenon E (266). Interestingly, Polyphenon E is reported to act synergistically with Atorvastatin, in lung cancer models. Similarly, EGCG is known to enhance the efficacy of 5-FU and etoposide in chemoresistant HT-29 colon cancer cells, though the mechanism is not elucidated (110).
The chemosensitizing efficacy of EGCG enhances the therapeutic outcome of COX-2 inhibitors, such as Sulindac, which is restricted in chemotherapy due to its side effects (78). Sulindac-induced apoptosis of lung cancer cells is enhanced when given in combination with EGCG, although the mechanism is not clear (288). EGCG in combination with celecoxib, another COX-2-inhibitor, significantly enhanced GADD153 gene expression and synergistically induced the apoptosis of nonsmall cell lung carcinoma (NSCLC) cells, possibly through the suppression of Bcl-2 and the up-regulation of BAX (287). Thus, the dose of COX-2 inhibitors could be reduced with a better therapeutic outcome and lesser toxicity if used in combination with EGCG (287).
Several studies have shown that EGCG could reverse MDR phenotypes in vitro (336) and could sensitize resistant cells to doxorubicin cytotoxicity by enhancing their intracellular accumulation (233). In vivo studies support this observation and showed that the mechanism works through the inhibition of P-gp activity (331). The regulatory molecules contributing to EGCG-mediated chemosensitization have been summarized in Table 4.
D. Quercetin
Quercetin (3, 3′, 4′, 5, 7-pentahydroxyflavone) is a flavonoid that is ubiquitously present in fruits and vegetables, and has been described as a potential anticancer agent. Numerous studies have shown that quercetin augments the efficacy of anticancer drugs and sensitizes cancer cells to chemotherapy. It chemosensitizes the therapeutic effect of antitumor antibiotics (doxorubicin), TRAIL, and alkylating agents (DTIC, cisplatin) through mechanisms summarized in Figure 6.
Recent reports reveal that the chemosensitizing efficacy of quercetin to doxorubicin chemotherapy depends on its ability to down-regulate HIF1-α, HER2/neu, or by inducing persistent T-cell tumor-specific responses. Both in vitro and in vivo studies have shown that quercetin-mediated down-regulation of HIF1-α in cancer cells makes them sensitive to doxorubicin, sparing normal cells (72). Another mechanism of this synergistic action is through the down-regulation of HER2/neu (121), which also results in the enhanced sensitivity to the drug. One of the major problems encountered in patients receiving chemotherapy is the considerable immune dysfunction that is associated with it. It is shown that quercetin induced persistent T-cell tumor-specific responses, promoted lymphocyte proliferation, and regulated T-helper type 1/2 (Th1/Th2) cytokine imbalance, leading to the rejection of the tumor when used with doxorubicin (73). Hence, this could be considered an effective approach for inducing immune responses against tumors. The induction of apoptosis and the inhibition of invasion by quercetin is shown to be mediated through the activation of protein kinase C alpha (PKC-α) pathway, and its combination with doxorubicin, when used as a PKC inhibitor, has been shown to antagonize the effects of quercetin (333). Quercetin-mediated up-regulation of P-gp is another mechanism that is reported to abolish the synergistic effect of doxorubicin and quercetin (56).
The ability of quercetin to redistribute TRAIL receptors and other components of death-inducing signaling complex (DISC) into lipid rafts sensitizes colon cancer cells to TRAIL-mediated apoptosis and is one of the major mechanisms of its chemosensitizing efficacy (231). Quercetin has also been shown to significantly up-regulate DR5 expression, leading to enhanced TRAIL-induced apoptosis (128). The dephosphorylation of Akt (141) or its down-regulation (272), down-regulation of survivin (140), up-regulation of DR5, and down-regulation of c-FLIP (138) have also been suggested as mechanisms for the synergistic effect of quercetin and TRAIL. Apart from this, quercetin has also been shown to mediate the chemosensitization of the apoptosis-resistant human thymoma-derived cell line to CD95 monoclonal antibody through the enhancement of PKC-α activity (246). When TRAIL is used as a chemotherapeutic drug, quercetin enhances apoptosis in transformed cell lines without affecting normal cells (245). However, in HeLa cells, it did not exert any synergism with TRAIL (290).
DTIC, the FDA-approved alkylating drug used in the treatment of melanoma (36), has only moderate response rates (11). Quercetin treatment promotes an ataxia-telangiectasia mutated (ATM)-dependent phosphorylation of p53 (297) or induces its transcriptional activity (298), thereby sensitizing melanoma cells to DTIC or its analogue, Temozolomide. Several studies have shown the effectiveness of quercetin as a potent and nontoxic compound that could cause the reversal of MDR in several tumor models (37). It can sensitize HeLa cells to cisplatin-induced apoptosis through the down-regulation of heat shock protein (Hsp72) and MRP (117). The synergistic effect of quercetin and cisplatin could also be mediated through the down-regulation of Bcl-2 and Bcl-xL with the concomitant up-regulation of Bax and the induction of mitochondrial membrane permeabilization, as observed in Hep-2cells (148). The major chemotherapeutic drugs influenced by quercetin-mediated chemosensitization are listed in Table 4.
E. Emodin
Emodin (1, 3, 8-trihydroxy-6-methylanthraquinone) is a naturally occurring anthraquinone obtained from Cassia obtusifolia, Fallopia japonica, Polygonum cuspidatum, and Rheum palmatum. The central players in emodin-mediated chemosensitization are illustrated in Figure 7. Emodin is a tyrosine kinase inhibitor and is shown to inhibit HER2/neu tyrosine kinase activity that leads to a preferential suppression of growth, represses the transformation of HER2/neu over-expressing breast cancer cells (328), and sensitizes these cells to paclitaxel in nude mice models (330). It also sensitizes chemoresistant ovarian cancer cells to paclitaxel-induced apoptosis by increasing the cellular concentration of paclitaxel as well as down-regulating the expression of antiapoptotic molecules such as XIAP and survivin (173). Several studies have shown that the ROS-producing effect of emodin significantly contributes to the chemosensitizing efficacy of this compound. Huang et al. reported that emodin, having little effect on normal cells, can synergize cytotoxicity of paclitaxel in prostate carcinoma cells in vitro and in vivo through ROS generation and suppression of P-gp and HIF-1α (109).
Emodin-mediated generation of ROS and suppression of P-gp and HIF-1α has also been reported to be a mechanism behind the chemosensitizing efficacy of emodin in cisplatin chemotherapy (109). It enhances the cytotoxic activity of cisplatin, carboplatin, and oxaliplatin through ROS-mediated reduction of cellular glutathione level and reduced expression of MRP1 (311). Emodin-mediated reduction in tyrosine phosphorylation of HER2/neu suppresses the proliferation of HER2/neu-overexpressing nonsmall cell lung cancer cells and sensitizes these cells to the cytotoxic activity of cisplatin in a synergistic manner (329). Another mechanism reported for the synergistic effect of emodin and cisplatin is through the reduction in the expression of ERCC1 DNA repair gene and ERK1/2 activation (144). Interestingly, the synergistic combination of emodin and cisplatin was observed in ovarian cancer cells when emodin was pretreated while, the co-treatment abolished the synergism, possibly due to the up-regulation of the human copper transporter 1 (153).
Low concentrations of emodin have been reported to sensitize HeLa cells to As2O3 through ROS-mediated inhibition of NF-κB, activator protein 1 (AP-1) and survivin (320, 322). Emodin has been shown to induce ROS-mediated suppression of P-gp and HIF-1α sensitizing prostate carcinoma cells to As2O3, in vitro and in vivo (109). Emodin-induced oxidative stress in conjunction with As2O3 can also inhibit Ras homolog A activation, thereby sensitizing human gastric carcinoma cells to anoikis (apoptosis resulting from loss of cell–matrix interactions) (29).
Apart from paclitaxel, platinum drugs, and As2O3, emodin chemosensitizes a variety of other chemotherapeutic drugs. It enhances the cytotoxic effect of celecoxib to suppress anchorage-dependent and independent growth of rat cholangiocarcinoma cells through the inhibition of Akt (158). In both in vitro and in vivo models of pancreatic cancer, emodin has been shown to enhance the efficacy of gemcitabine through down-regulation of survivin, possibly by blocking the translocation of β-catenin to the nucleus (95). The inhibition of CK2 activity through reduction of the Bcl-xL/tBID ratio by emodin is another mechanism by which it sensitizes Apo2L/TRAIL-resistant colon cancer cells to apoptosis (238). A similar mechanism has also been reported for emodin-mediated chemosensitization in primary endometrial carcinoma explants (183). Emodin-mediated enhancement of cytotoxicity has also been reported in combination with gefitinib (41) and mitomycin C (285) through down-regulation of Rad51 expression and suppression of ERK1/2 activation in NSCLC cell lines. The HER2/neu tyrosine kinase inhibitory function of emodin also contributes to the chemosensitization of these cells to doxorubicin and etoposide (329). However, emodin is reported to rescue melanoma cells from the cytotoxic effects of doxorubicin and paclitaxel (236). The molecular targets of emodin-mediated chemosensitization is summed up in Table 4.
F. Resveratrol
Resveratrol (3, 4′, 5-trihydroxystilbene), a polyphenol that is present mainly in grapes and red wine, has also been shown to sensitize cancer cells to conventional chemotherapeutic drugs by modulating the molecular players of chemoresistance (Fig. 8). The possible use of resveratrol as a chemosensitizer and its limitations have been recently reviewed (98). The key regulators of resveratrol-mediated chemosensitization appear to be the members of the Bcl-2 family, NF-κB pathway and IAPs, and TRAIL is the most reported drug that has been potentiated by resveratrol. Resveratrol induces redistribution of DRs into lipid rafts in colon cancer cells, thereby sensitizing these cells to DR ligands such as TNF, anti-CD95 antibodies, and TRAIL, among which TRAIL appears to be the most potent ligand (66). It has been shown that resveratrol sensitizes tumor cells to TRAIL-induced apoptosis through survivin depletion alone (86, 87). However, later studies indicate that apart from survivin, the up-regulation of pro-apoptotic molecules such as Bax, Bak, PUMA, Noxa, Bim, TRAIL-R1, and TRAIL-R2 with concomitant down-regulation of anti-apoptotic molecules such as Bcl-2, Bcl-xL, and XIAP contribute to the chemosensitizing efficacy of resveratrol (256). Down-regulation of clusterin by inhibition of Src/JAK-signal transducer and activator of transcription (STAT) 1 pathway is a novel mechanism adopted by resveratrol to overcome TRAIL resistance in prostate cancer (249). Resveratrol also increases the sensitivity of DR5-positive melanomas to exogenous TRAIL through the inhibition of STAT3, NF-κB, c-FLIP, and Bcl-xL (114).
Apart from TRAIL, other chemotherapeutic drugs are also potentiated by resveratrol, possibly through the down-regulation of intrinsically over-expressed antiapoptotic molecules. The ability of resveratrol to inhibit ribonucleotide reductase, a key enzyme of de novo dNTP synthesis, is shown to sensitize HL-60 human promyelocytic leukemia cells to Ara-C and tiazofurin, two anti metabolites used in the treatment of leukemia (104). Cytochrome p450 1b1 (Cyp1b1) has been shown to be up-regulated in a large number of cancers and plays an important role in the development of resistance to chemotherapy (191). Resveratrol effectively down-regulates Cyp1b1, thus enhancing apoptosis induced by antimetabolites such as 5-FU and gemcitabine in chemoresistant cholangiocarcinoma tumor models, even though the precise mechanism for the enhanced chemoresistance associated with Cyp1b1 over-expression is unknown (84). The constitutive activation of NF-κB and the over-expression of its possible downstream gene products such as Bcl-2, Bcl-xL, COX-2, cyclin D1, MMP-9, and VEGF are reported in pancreatic cancer. The down-regulation of these molecules by resveratrol enhanced the antitumor activity of gemcitabine in vitro and in an orthotopic mouse model of human pancreatic cancer (101). It has been shown that some analogs of resveratrol mediate the nuclear redistribution of cyclin A and form a cyclin A/cdk2 complex in human colon cancer cell lines. Although the exact mechanism is unclear, this redistribution of cyclin A sensitizes these cells to 5-FU-mediated cytotoxicity (53).
The main mechanism behind the chemosensitization of resveratrol to paclitaxel chemotherapy is through the down-regulation of Bcl-2 family members and MDR1/P-gp. Resveratrol-mediated inhibition of ERK1/2 and AP-1 pathways leading to reduced Bcl-xL expression has been suggested as the mechanism behind the chemosensitization of non-Hodgkin's lymphoma and multiple myeloma cell lines to paclitaxel (120). In addition to Bcl-2, MDR1/P-gp is also down-regulated by resveratrol in KBv200, a classic multidrug-resistant cell line, which leads to the sensitization of this cell line to paclitaxel (235). Down-regulation of survivin is another mechanism by which resveratrol enhances the growth inhibitory effects of paclitaxel (87).
Chemosenstization of resveratrol to doxorubicin or vincristine is shown to be mediated through the down-regulation of MDR1/P-gp and Bcl-2 in human uterine cancer cells (241), doxorubicin-resistant acute myeloid leukemia cells (156), and multidrug-resistant oral cancer cells (235). Down-regulation of survivin by resveratrol is also found to be a mechanism of chemosensitization to doxorubicin in addition to VP16, actinomycin D, cytarabine, and methotrexate (87). Bortezomib and thalidomide, which are approved drugs for the treatment of multiple myeloma when used in combination with resveratrol, potentiated their cytotoxicity in multiple myeloma cell line U266 through the down-regulation of constitutively activated NF-κB and STAT3 expression (21). Reports of chemosensitizing efficacy of resveratrol toward various chemotherapeutic drugs have been summarized in Table 4.
Though resveratrol can be effectively used in combination with several chemotherapeutic agents, it shows adverse effects when used along with paclitaxel, vincristine, and daunorubicin in some tumor cells (3, 98). Among the studies reported on the synergistic action of resveratrol and paclitaxel, half of them support its use as a chemosensitizer, while the rest questions the possibility. Three studies from the same group have reported that this combination is not effective, as resveratrol induces S phase arrest in the cells, preventing them from entering into a G2/M phase, where paclitaxel exerts its effect (244). Studies on bladder cancer cells (188) and breast cancer cells (85) support this notion, and they further show that the ROS scavenging effect of resveratrol also contribute to this adverse effect (85). None of the studies, however, explain how the cells proliferated after S-phase arrest.
The reason behind this contradictory results appears to be the difference in time at which resveratrol is added to the cells. In all the studies in which resveratrol attenuated the cytotoxicity, both the compounds were given simultaneously, while in all studies in which resveratrol acts synergistically with the drug, the cells were pretreated with resveratrol. The study by Mao (188) supports this explanation, in which they observed synergism when they pretreated the bladder cancer cells with resveratrol, while they observed an adverse effect of resveratrol when it was given simultaneously with Taxol.
We have summarized the in vitro and in vivo studies evaluating the chemosensitizing efficacy of the phytochemicals reviewed, in Table 7.
VI. The Relevance and Importance of the Phytochemicals Addressed: Clinical Trials
The toxicity of most of the phytochemicals addressed in this review has been evaluated in clinical trials, and the limiting dose has been determined. Most of the studies showed that curcumin at a dose of 8 g/day is well tolerated without toxicity (19, 43). Clinical trial in prostate cancer patients showed that Genistein is well tolerated upto 600 mg/day (198). A daily dose of Polyphenon E, which contained 800 mg of EGCG, did not induce any toxicity in patients with prostate cancer (192). A bolus dose of 1400 mg/m2 of quercetin is recommended as nontoxic and safe in a clinical trial (83). A repeat dose study of resveratrol in healthy volunteers showed that a dose of 5 g/day can be effectively used as a cancer chemopreventive (27).
The major chemotherapeutic clinical trials using the phytochemicals described in the review are summarized in Table 5. The therapeutic efficacy of curcumin has been evaluated in many clinical trials. A phase II trial of oral administration of curcumin in patients with advanced pancreatic cancer did not show promising results, as only 2 of the 21 patients responded to the therapy (68), may be due to its poor bioavailability (260). In another study on colorectal cancer, patients showed that curcumin treatment improves the general health of patients with increased apoptotic tumor cells and enhanced expression of p53 molecule in tumor tissue (102). Genistein is shown to reduce the serum prostate-specific antigen (PSA) in patients with localized prostate cancer without any effects on hormones in a randaomized placebo-controlled double blind phase II clinical trial. Even though it was well tolerated and had a beneficial effect on blood cholesterol, the therapeutic efficacy is yet to be confirmed (164). In another study, it showed a clinical outcome in only 2 out of 14 patients (225). Even though polyphenon E is well tolerated in prostate cancer patients (192) and chronic lymphoid leukemia patients (254), its therapeutic outcome was observed only in prostate cancer patients as assessed by a reduction in PSA, hepatocyte growth factor, and VEGF levels. SRT501, a micronized resveratrol, significantly increased apoptosis in malignant hepatic tissues in patients with hepatic metastasis without toxicity (106). In a phase I pilot clinical trial in colon cancer patients, resveratrol blocked the Wnt pathway by inhibiting Wnt target genes (213).
Many of the phytochemicals addressed in the review have undergone clinical trials as chemopreventives as listed in Table 6. The chemopreventive efficacy of curcumin has been studied in colorectal cancer (91) and in a variety of other cancers (43). The daily intake of curcumin reduced lesion size in oral leukoplakia and submucous fibrosis (237). The combination of quercetin and curcumin was effective in reducing the number and size of ileal and rectal adenomas in familial adenomatous polyposis patients without toxicity (57). The chemopreventive efficacy, of quercetin, however, was not proved in colon cancer (24). Although genistein showed a bimodal effect and was effective in low doses in a phase II chemoprevention study in bladder cancer patients (195), its use as a chemopreventive in breast cancer was not promising (136). In another study enrolling 1459 women, risk reduction was observed in ER, PR positive women after genistein uptake (59). In a study including 60 men with elevated PSA and high-grade preneoplastic lesions, only one person developed prostate cancer after EGCG intake. It was also effective against benign hyperplasia (20). However, the chemopreventive efficacy of EGCG was not proved in a clinical trial in prostate cancer patients (307) and cancer among women (306). In healthy volunteers, the chemopreventive efficacy of resveratrol was promising as assessed by decreased levels of IGF1 and insulin-like growth factor-binding protein 3 (IGFBP-3) (26). In healthy volunteers, it modulated the enzyme systems involved in carcinogen activation and detoxification (49).
The chemosensitization trials by the phytochemicals of interest are listed in Table 8. Out of the six studies evaluating the chemosensitization efficacy of curcumin, results of three are published. Curcumin uptake did not significantly alter the prognosis of pancreatic cancer in combination with gemcitabine (79, 131). However, a phase I trial with docetaxel and curcumin showed significant results in advanced breast cancer (19). Recently, a phase II clinical trial of a micronized resveratrol formulation (SRT501), in combination with the proteasome inhibitor, bortezomib in patients with multiple myeloma was suspended due to safety concerns (NCT00920556). The trial was halted when 5 out of 24 patients developed a kidney condition called cast nephropathy, though it is uncertain whether the kidney failures were actually related to the resveratrol treatment, as cast nephropathy is commonly associated with multiple myeloma (130).
VII. Reduced Bioavailability of Phytochemicals As a Drawback for Their Clinical Use
One of the major drawbacks of the clinical usage of phytochemicals is that their biological effects are frequently diminished or even lost due to incomplete absorption and first-pass metabolism. Extremely low serum levels, limited tissue distribution, apparent rapid metabolism, and short half life are the major reasons attributed to the poor bioavailability of most of these phytochemicals.
Intense research is carried out to increase the bioavailability of curcumin either by using an adjuvant that blocks the metabolic pathways or by modifying it as water soluble albumin bound nanoparticle (139), gold nanoparticle (187), poly(lactic-co-glycolic acid) (PLGA), and nonoparticle (210), and all these studies have been shown to increase the bioavailability in vitro. Recent reports confirm the superior bioavailability of curcumin-loaded PLGA nanoparticles in vivo (7, 282). Another approach is to use curcumin analogues such as BCM-95 (biocurcumax) or dimethoxycurcumin, which was shown to have enhanced biological activity. Curcumin formulations in liposomes, micelles, and phospholipid complexes as well as a combination of curcumin with low doses of piperine (the black pepper alkaloid) have also provided promising results (6). The various modifications that could increase the bioavailability which may lead to better drug delivery have been summarized in Table 9.
The table just cited is a brief summary of studies using various modifications of the phytochemicals reviewed to improve their bioavailability and solubility in aqueous medium.
HACS, high-amylose corn starch; PLGA, poly(lactic-co-glycolic acid).
The oral bioavailability of genistein has been shown to improve when genistein nanoparticles were prepared by the nano precipitation technique using Eudragit® E100 as carrier (293). Another study shows that Fe3O4-CMCH-genistein nano-conjugate exhibits a significantly enhanced inhibition effect on gastric cancer cells than the free genistein (268). Experiments using high-amylose corn starch-genistein complexes in rats showed that it can improve the bioavailability of genistein (51).
Several approaches are used for optimizing the bioavailability of EGCG. Piperine could be used as a potential dietary modulator that enhances the bioavailability of EGCG by inhibiting its glucuronidation in the small intestines, as well as by inhibiting gastric emptying and gastrointestinal transit, which may result in increased absorption (159). Various nano formulations have also been reported to increase the bioavailability of EGCG (107, 269).
The in vivo efficacy of quercetin is limited by poor bioavailability, primarily due to its low solubility and consequent low absorption in the gut. Co-crystallization of quercetin with caffeine and methanol has been shown to improve the physicochemical characteristics and bioavailability of quercetin (278). A polymer (Guar Gum)-based matrix tablet using quercetin has been developed as a colon-targeted drug delivery system that improves the absorption of quercetin in the colon and hence enhances the bioavailability (274). A polymeric nanocapsuled quercetin has been shown to have 20-fold better therapeutic efficacy compared with free quercetin in preventing gastric ulcers (33).
Literature survey on methods to improve the bioavailability of emodin shows that not much work has been done on this aspect. A recent study, however, has shown that emodin-loaded solid lipid nanoparticles is a promising oral drug delivery system for treating breast cancer and is nontoxic to normal human mammary epithelial cells (309). The significance of this formulation in increasing the bioavailability is yet to be evaluated, in vivo.
It is shown that combining resveratrol with piperine significantly improves its in vivo bioavailability (126). A recent study reports that SRT501, a micronized resveratrol formulation, enhances the bioavailability of resveratrol by 3.6-fold through increased surface area and improved suspension properties in colorectal cancer patients, compared with the nonmicronized resveratrol and was found to be well tolerated (106). However, a phase II clinical study to assess the safety and activity of SRT501 alone or in combination with bortezomib in patients with multiple myeloma was halted, because some of the patients developed cast nephropathy, a common condition caused by myeloma that can lead to kidney failure (NCT00920556).
VIII. Phytochemicals As Radiosensitizers
In addition to sensitizing cancer cells to chemotherapeutic drugs, phytochemicals are also known to make cancer cells more susceptible to radiation, thereby acting as radiosensitizers. Several phytochemicals such as curcumin, genistein, EGCG, quercetin, resveratrol, plumbagin, flavopiridol, and parthenolide have been reported to possess radiosensitizing properties. They could enhance the activity of radiation through various processes as in chemosensitization, such as inhibiting survival signaling pathways, enhancing pro-apoptotic pathways, decreasing the antioxidant potential, and hence raising the level of ROS, which has been extensively reviewed elsewhere (212). In addition to this, certain other mechanisms are also reported for radiosensitization. Enhancing the radiation potential through inhibition of telomerase activity (9) and inhibition of the antioxidant enzyme thioredoxin reductase-1 (119) are some of the recent mechanisms identified for curcumin-mediated radiosensitization. In vivo protection from radiation-induced acute myelotoxicity is achieved by pretreatment with genistein, which imparts quiescence to hematopoietic stem cells (61). Inhibition of one of the DNA damage response proteins, ATM kinase, thereby enhancing the effect of radiation, is another novel mechanism reported for quercetin-mediated radiosensitization (178). Resveratrol is known to induce radiosensitization by potentiating radiation-induced ceramide accumulation (253), a key trigger in radiation-induced apoptosis.
IX. Some Important Facts To Be Borne in Mind While Using Phytochemicals As Chemosensitizers
Except emodin, all the phytochemicals addressed here exert antioxidant activity by scavenging ROS and by the synthesis of antioxidant or phase 2 detoxification enzymes through the Nrf2 pathway, which enhances the cellular defense capacity against oxidative stress. Emodin, however, depends on free radical generation for its cytotoxic effects, and the regulatory role of Nrf2 in its mode of action is unexplored. Although the antioxidant activity of the phytochemicals contributes to the chemopreventive efficacy, it may counteract the efficacy of drugs that depend on ROS generation for their chemotherapeutic effect, when used as a chemosensitizer. The reported inhibitory role of curcumin in the therapeutic efficacy of paclitaxel (13), docetaxel (308), doxorubicin (280), and camptothecin (280), all of which exert their therapeutic efficacy at least in part through ROS generation, may be due to the counteracting ROS scavenging and Nrf2 activating property of curcumin. Several in vitro and in vivo studies, however, strongly support the synergistic effect of curcumin with these drugs (Table 4). Similarly, a counteracting effect of resveratrol is reported in paclitaxel, vincristine, and daunorubicin-mediated chemotherapy, which again is due to the ROS scavenging effect (244) and possible Nrf2 activation (163) by resveratrol. May be in the scenario where the chemosensitizer is an ROS scavenger and the chemotherapeutic drug acts through ROS generation, a pretreatment of chemosensitizer is preferred to get the chemosensitizing effect as suggested by a majority of the positive studies. The possible opposing role of Nrf2 activation by these phytochemicals in the chemosensitizing efficacy has not yet been explored. If that emerges as a major impediment in the success of the clinical use of these compounds, effective Nrf2 blockers in combination with the existing therapeutic regimen may improve the clinical outcome. Several phytochemicals such as brusatol (240), luteolin (294), procyanidin (219), and Ginsenoside Rg3 (166) have been shown to down-regulate Nrf2 to enhance the efficacy of a variety of chemotherapeutic drugs.
Another important aspect of phytochemicals that has to be addressed before their clinical use as chemosensitizers is their ability to modulate autophagy. Curcumin is known to induce autophagy in a variety of cell types, and in a majority of the cases where it was treated alone, it induced cell death (207, 318). In hepatocellular carcinoma cells, curcumin is shown to enhance the effect of doxorubicin through the induction of apoptosis and autophagy simultaneously (234). Genistein (211), quercetin (116), and resveratrol (179) have been shown to synergistically induce apoptosis and autophagy in combination with chemotherapeutics such as temozolomide. In some other instances, resveratrol is shown to attenuate the effect of doxorubicin by the induction of autophagy (316). Emodin is also shown to abrogate the cytotoxic effects of TNF through up-regulation of autophagy (100). Thus, in this scenario, the role of photochemical-mediated autophagy in modulating its chemosensitizing efficacy in combination with major chemotherapeutic agents in vitro and in vivo remains to be explored.
X. Conclusions
Increased failure rate in cancer chemotherapy is mainly attributed to inherent and acquired resistance pathways. Moreover, the inability of these drugs to specifically target cancer cells may contribute to unwanted side effects to the patients receiving chemotherapy. Hence, the use of nontoxic chemosensitizers, which can enhance the efficacy of the chemotherapeutic drugs, will bring down the optimal dose of the drug so that the cost and side effects of chemotherapy can be minimized. Hence, the present situation demands research on targeting the molecules involved in chemoresistance.
Several molecules involved in DNA damage response have been considered as candidates to be targeted to surmount chemoresistance. The major targets include the BER pathway (1) checkpoint kinases (334) and MDM2/p53 (23). The known anti-apoptotic factors such as NF-κB (135), Bcl-2 (133), and HIF1-α (203) are also important in this aspect. Recently, proteasome inhibitors and Nrf2 inhibitors have also been identified as chemosensitizers. Current research is focused on targeting these molecules individually using siRNA or other specific inhibitors. The advantage of using natural compounds is that they can circumvent the effects of different antiapoptotic molecules simultaneously with minimum side effects. The pharmacological safety and chemotherapeutic efficacy of most of the compounds addressed in this review have been evaluated in clinical trials (48, 57, 68, 79, 83, 164, 224, 254). Several clinical trials intended to evaluate the chemosensitizing efficacy of these compounds are completed and/or ongoing, as tabulated in Table 8. So far, the successes in the preclinical trials are not translated to clinical trials demanding extensive research in improving the strategies. If these studies give a promising outcome, the approach of chemosensitization using nontoxic natural products will emerge as a boon to cancer chemotherapy.
Footnotes
Acknowledgment
The authors acknowledge Arunkumar T. Thulasidasan for technical assistance.