Insights into the involvement of noncoding RNAs in 5-fluorouracil drug resistance

Abstract

5-Fluorouracil is a classic chemotherapeutic drug that is widely used to treat various cancers. However, patients often exhibit primary or acquired drug resistance during treatment with 5-fluorouracil chemotherapy. 5-Fluorouracil resistance is a multifactorial event that involves abnormal enzyme metabolism, transport deregulation, cell cycle disorders, apoptosis resistance, and mismatch repair deficiency. Despite advancements in bioresearch technologies in the past several decades, the molecular mechanisms of 5-fluorouracil resistance have not been completely clarified. Recently, microarray analyses have shown that noncoding RNAs (i.e. microRNAs and long noncoding RNAs) play a vital role in 5-fluorouracil resistance in multiple cancer cell lines. These noncoding RNAs can function as oncogenes or tumor suppressors, contributing to 5-fluorouracil drug resistance. In this review, we discuss the effects of microRNAs on 5-fluorouracil sensitivity via targeting of metabolic enzymes, the cell cycle, apoptosis, autophagy, the epithelial–mesenchymal transition, and cancer stem cells. In particular, we focus on summarizing current knowledge on the molecular mechanisms through which long noncoding RNAs mediate 5-fluorouracil drug resistance. Moreover, we describe the specific microRNAs that may function as markers for prediction of chemotherapeutic response to 5-fluorouracil. This review will help to improve the current understanding of how to reverse 5-fluorouracil resistance and may facilitate the establishment of new strategies for alleviating drug resistance in the future.

Keywords

microRNA long noncoding RNA 5-fluorouracil drug resistance

Introduction

5-Fluorouracil (5-FU) is a basic chemotherapeutic drug that has been widely used since 1957 to treat various cancers, including gastrointestinal cancer, breast cancer, and head and neck cancers.¹ 5-FU can decrease the recurrence rate of tumors and improve progression-free survival (PFS) and overall survival in patients, particularly in patients with colorectal cancer, when used alone or when combined with other newer chemotherapies. However, the clinical applications of 5-FU are limited because of the occurrence of 5-FU resistance. Some patients are initially insensitive to 5-FU at the beginning of treatment, and this is defined as primary drug resistance. However, many patients are initially sensitive to 5-FU and then become insensitive after treatment, which is defined as acquired drug resistance. Despite advancements in bioresearch technologies in the past several decades, the molecular mechanisms of 5-FU resistance remain largely unknown.

5-FU is one of the first classic antimetabolites generated by suppression of biosynthetic processes. 5-FU generates several types of active metabolites and contributes to disruption of DNA and RNA syntheses. Importantly, the active metabolites of 5-FU can inhibit the activity of thymidylate synthase (TS) enzyme, which is essential for converting intracellular deoxyuridine monophosphate (dUMP) into deoxythymidine monophosphate (dTMP).^2,3 Dihydropyrimidine dehydrogenase (DPD) is the initial and rate-limiting enzyme in the catabolism of 5-FU, functioning to convert 5-FU to dihydrofluorouracil (DHFU).⁴ Many studies have shown that high expression of these metabolic enzymes leads to 5-FU resistance.

In addition to abnormal expression of metabolic enzymes, noncoding RNAs (e.g. microRNAs (miRNAs) and lncRNAs) have also been shown to play vital roles in 5-FU resistance in multiple cancer cell lines. miRNAs are small noncoding RNAs that regulate gene expression post-transcriptionally by binding to the 3′-untranslated region (UTR) of messenger RNAs (mRNAs).^5–8 Many studies have shown that abnormal miRNA expression is involved in 5-FU resistance in cancer cell lines and that miRNA expression may be significantly altered following treatment with 5-FU. For example, Kurokawa et al.⁹ demonstrated that miR-19b and miR-21 are highly overexpressed in 5-FU-resistant colon cancer cells. Additionally, He et al.¹⁰ showed that miR-21 expression is suppressed following treatment with 5-FU and that low miR-21 levels in patients with hepatocellular carcinoma (HCC) are associated with longer time to relapse. Another study found that miRNA expression is involved in 5-FU-induced autophagy. Notably, the use of miRNA mimics or inhibitors can partly restore the sensitivity of cells to 5-FU both in vitro and in vivo.¹¹ Thus, miRNAs may be effective molecular targets for mediating 5-FU resistance. Additionally, some lncRNAs have been shown to contribute to 5-FU resistance in human cancer cells.

In this review, we discuss the effects of miRNAs on 5-FU sensitivity. Additionally, we summarize current knowledge on the molecular mechanisms through which lncRNAs mediate 5-FU drug resistance. Moreover, we describe the specific miRNAs that may function as markers for prediction of chemotherapeutic response to 5-FU. This review will help to improve the current understanding of how to reverse 5-FU resistance and may facilitate the establishment of new strategies for alleviating drug resistance in the future.

Involvement of miRNAs in chemotherapy resistance

Within the past decade, many studies have reported an association between miRNAs and 5-FU drug resistance. miRNAs have been shown to be involved in chemotherapy resistance through various mechanisms, including modulation of metabolic enzymes, adenosine triphosphate–binding cassette (ABC) transporter proteins, the cell cycle, apoptosis signaling pathways, and the epithelial–mesenchymal transition (EMT).^12–17 Importantly, miRNAs can target enzymes involved in drug metabolism, thereby reducing drug sensitivity. For example, dihydrofolate reductase (DHFR), which converts dihydrofolate to tetrahydrofolate, is critical for methotrexate drug resistance. Mishra et al. found a single-nucleotide polymorphism (SNP) present in the DHFR 3′-UTR near a miR-24 binding site that results in DHFR overexpression and methotrexate drug resistance. Knockdown of miR-24 can increase the sensitivity of DHFR wild-type cells but not mutant cells to methotrexate.¹⁸ Additionally, ABC transporter proteins and P-glycoprotein (P-gp), which play vital roles in drug resistance,^19,20 are targeted by miR-508-5p, and the interaction of miR-508-5p with P-gp and ZNRD1 is involved in resistance to chemotherapy.²¹ Thus, these findings suggest that miRNAs may be good targets for cancer therapy.

Apoptosis and autophagy are important mechanisms involved in chemotherapy resistance. Xia et al.²² showed that miR-15b and miR-16 modulate multidrug resistance in gastric cancer cells through targeting of BCL2, demonstrating that miRNAs can sensitize gastric cancer cells to chemotherapy drugs via stimulation of the apoptosis signaling pathway. Interestingly, Zhang et al.²³ showed that miR-22 functions as a vital switch between apoptosis and autophagy by targeting antiproliferative B-cell translocation gene 1 (BTG1) to regulate 5-FU sensitivity.

Additional studies have also shown that cell cycle dysregulation can induce resistance to chemotherapeutics in cancer cells. p21 (WAF1/CIP1), a potent cyclin-dependent kinase inhibitor, is a direct and functional target of miR-224. Overexpression of miR-224 can promote chemoresistance in human lung cancer cells via regulation of the G₁/S transition by targeting p21.²⁴

In contrast to the molecular mechanisms described above, modulation of the EMT can have unique effects on cancer cell resistance. During the EMT, cancer cells acquire a mesenchymal phenotype and cancer stem cell (CSC) characteristics, becoming resistant to chemoradiotherapy.^25–27 Liao et al.²⁸ showed that miR-203 contributes to drug resistance by promoting the EMT via targeting of the transcription factor SNAI2, suggesting that this miRNA may serve as a potential target for overcoming chemotherapy resistance in human glioblastoma.

Taken together, these studies have shown that miRNAs play diverse roles in mediating drug resistance through multiple pathways.

Involvement of miRNAs in 5-FU drug resistance

Recently, microarray analyses have shown that miRNAs play vital roles in 5-FU resistance in various cancer cell lines. These results have improved the current understanding of the gene networks and pathways through which miRNAs regulate 5-FU drug resistance. Here, we summarize the major mechanisms of miRNA-mediated 5-FU drug resistance described in recent studies (Figure 1).

Figure 1.

Schematic representation of the miRNA pathways related to resistance to 5-FU.

miRNAs regulate 5-FU metabolic enzymes

The enzymes responsible for 5-FU metabolism include TS, DPD, and thymidine phosphorylase (TP).²⁹ Recently, many studies have demonstrated that miRNAs can directly or indirectly regulate enzymes involved in 5-FU metabolism (Table 1). Gotanda et al. found that miR-433 regulates TS expression post-transcriptionally and can alter 5-FU sensitivity in HeLa cells. Additionally, overexpression of this miRNA and synergy with 5-FU significantly suppress cell proliferation.³⁰ Similarly, Li et al.³¹ found that miR-203 modulates 5-FU chemosensitivity by targeting TS in colon cancer cells. These studies suggested that miRNAs may be therapeutic targets in 5-FU-resistant cells.

Table 1.

miRNAs regulating 5-FU metabolic enzymes.

Metabolic enzymes	MiRNAs	References
TS	MiR-192, 215, 433, 203	Gotanda et al.,³⁰ Li et al.,³¹ and Boni³²
DPD	MiR-27a/b, 134, 528-5p, 494, 21^a	Hirota et al.,³³ Offer et al.,³⁴ Chai et al.,³⁵ and Deng et al.³⁶
TP	MiR-21^a	Deng et al.³⁶

TS: thymidylate synthase; DPD: dihydropyrimidine dehydrogenase; TP: thymidine phosphorylase.

Indirect.

In contrast, although miR-192 and miR-215 directly target TS expression at the post-transcriptional level, overexpression of these miRNAs does not sensitize cancer cells to 5-FU treatment, but rather increases resistance to 5-FU. Further analysis revealed that overexpression of miR-192 and miR-215 leads to G₁/S arrest, subsequently preventing 5-FU cytotoxicity.³²

DPD also can be regulated by some miRNAs. For example, Hirota et al. showed that DPD is a target of multiple miRNAs, including miR-27a, miR-27b, miR-582-5p, and miR-134. Moreover, they further verified these results in lung cancer tissues and found that miR-27b could cause overexpression of DPD.³³ In later studies, Offer et al.³⁴ confirmed that miR-27a/b represses DPD expression via binding to two conserved recognition sites in DPD in the liver. Thus, miR-27 may represent a therapeutic target for the modulation of DPD enzyme function, thereby promoting 5-FU sensitivity. Similarly, Chai et al.³⁵ showed that DPD is a direct target of miR-494 in colon cancer. We also showed that miR-21 is highly expressed in HT-29/5-FU cells and it can indirectly regulate TP and DPD to influence 5-FU chemotherapy sensitivity.³⁶ Thus, our findings imply that miR-21 may represent a novel therapeutic target for overcoming 5-FU resistance in colon cancer cells.

miRNAs regulate cell cycle components involved in 5-FU resistance

Many experimental and clinical studies have shown that disorders of cell cycle regulation are hallmarks of tumor cells.^37–39 5-FU is a cell cycle–specific agent that affects the S phase of proliferation and disrupts DNA synthesis. Nie et al.⁴⁰ showed that miR-365 can inhibit cell cycle progression and increase 5-FU sensitivity by targeting cyclin D1, which is required for progression through the G₁ phase of the cell cycle. Chen et al.⁴¹ demonstrated that miR-381 plus 5-FU activates Cdc2, sensitizing renal cancer cells to 5-FU. Additionally, we showed that forkhead box M1 (FOXM1), which is crucial for cell mitosis, is an important factor mediating miR-320-induced 5-FU resistance. Moreover, this miR-320-FOXM1 axis was shown to be clinically significant in 50 matched colon cancer tissues.⁴² In contrast, some studies have shown that miRNAs promote 5-FU chemoresistance mediated by G₁-phase arrest. For example, miR-140 reduces osteosarcoma and colon cancer cell proliferation through G₁- and G₂-phase arrest by targeting histone deacetylase 4 (HDAC4).⁴³ However, overexpression of miR-140 induces resistance to 5-FU in these cell lines, similar to the functions of miR-192 and miR-215 described above. Thus, we speculate that miRNAs may modify cells to prevent 5-FU cytotoxicity if the miRNAs only suppress S-phase progression in the cells. Because individual miRNAs have many target genes, they may function synergistically with other molecular mechanisms, such as apoptosis, to regulate the sensitivity of 5-FU, depending on which mechanism yields the dominant effect (Table 2).

Table 2.

Effects of miRNAs on 5-FU sensitivity via targeting of the cell cycle, apoptosis, autophagy, EMT, CSCs, MMR, and other pathways.

MiRNAs	Up or down	Targets	Synergy with 5-FU	References
MiR-365	Down	Cyclin D1	Increase sensitivity	Nie et al.⁴⁰
MiR-381	Down	Cdc2^a	Increase sensitivity	Chen et al.⁴¹
MiR-320	Down	FOXM1	Increase sensitivity	Wan et al.⁴²
MiR-140	Down	HDAC4	Decrease sensitivity	Song et al.⁴³
MiR-192	Down	TS	Decrease sensitivity	Boni et al.³²
MiR-215	Down	TS	Decrease sensitivity	Boni et al.³²
MiR-96	Down	XIAP and UBE2N	Increase sensitivity	Kim et al.⁴⁴
MiR-365	Down	BCL2	Increase sensitivity	Nie et al.⁴⁰
MiR-204	Down	BCL2	Increase sensitivity	Sacconi et al.⁴⁵
MiR-129	Down	BCL2	Increase sensitivity	Karaayvaz et al.⁴⁶
MiR-34a	Down	BCL2	Increase sensitivity	Wang et al.⁴⁷
MiR-122	Down	BCL2	Increase sensitivity	Yin et al.⁴⁸
MiR-15a/16b	Down	BCL2 and BCL-xL	Increase sensitivity	Xia et al.²²
MiR-10b	Down	BIM	Increase sensitivity	Nishida et al.⁴⁹
MiR-195	Down	BCL-W	Increase sensitivity	Yang et al.⁵⁰
MiR-23a	Up	APAF-1	Decrease sensitivity	Shang et al.⁵¹
MiR-200c	Down	Bmi1	Increase sensitivity	Yin et al.⁵²
MiR-203	Down	Bmi1	Increase sensitivity	Yin et al.⁵²
MiR-101	Down	EZH2	Increase sensitivity	Xu et al.⁵³
MiR-22	Down	BTG1	Increase sensitivity	Zhang et al.²³
MiR-488	Down	SATB1	Increase sensitivity	Li et al.⁵⁴
MiR-224	Up	–	Decrease sensitivity	Amankwatia et al.⁵⁵
MiR-612	Down	AKT2	Increase sensitivity	Tang et al.⁵⁶
MiR-21	Up	PDCD4 and PTEN	Decrease sensitivity	Yu et al.⁵⁷
MiR-142-5p	Down	CD133/Lgr5ABCG2	Increase sensitivity	Shen et al.⁵⁸
MiR-21	Up	hMSH2/6	Decrease sensitivity	Valeri et al.⁵⁹
MiR-155	Up	MMR	Decrease sensitivity	Valeri et al.⁶⁰
MiR-125b	Down	HK II	Increase sensitivity	Jiang et al.⁶¹
MiR-122	Down	PKM2	Increase sensitivity	He et al.⁶²
MiR-497	Down	IGF1-R	Increase sensitivity	Guo et al.⁶³
MiR-221	Up	–	Decrease sensitivity	Zhao et al.⁶⁴
MiR-520g	Down	P21	Increase sensitivity	Zhang et al.⁶⁵
MiR-148b	Down	AMPKα1	Increase sensitivity	Zhao et al.⁶⁶
MiR-141	Down	Keap1	Increase sensitivity	Shi et al.⁶⁷
MiR-137	Down	PTN	Increase sensitivity	Xiao et al.⁶⁸
Let-7g	Down	Cyclin A	Increase sensitivity	Tang et al.⁶⁹

Indirect.

miRNAs regulate apoptotic and autophagic pathways involved in 5-FU resistance

There are two basic apoptotic pathways activated during development: the intrinsic pathway and the extrinsic pathway. The intrinsic pathway, also called the mitochondrial signaling pathway, includes many apoptosis-related proteins, such as Bcl-2 and caspase family proteins.^70,71 Apoptosis resistance is a major hallmark of cancer, and many miRNAs have been shown to be involved in regulating apoptotic signaling pathways to affect 5-FU sensitivity⁷² (Table 2). However, some studies have provided direct evidence for the miRNA-dependent modulation of apoptotic proteins. For example, Kim et al.⁴⁴ found that miR-96 can target XIAP and UBE2N to modify 5-FU sensitivity. Additionally, BCL2 can be regulated by miR-365,⁴⁰ miR-204,⁴⁵ miR-129,⁴⁶ miR-34a,⁴⁷ and miR-122,⁴⁸ and upregulation of these miRNAs can improve 5-FU sensitivity through apoptotic signaling pathways. However, although miR-15a and miR-16b can target the 3′-UTR of BCL2, overexpression of these miRNAs does not improve 5-FU sensitivity in resistant gastric cancer cells.²² Thus, other molecular mechanisms may function to neutralize the apoptotic effects of these miRNAs. miRNAs also can target other core components of the BCL2 family. For example, miR-10b targets Bcl-2-like protein 11 (BIM),⁴⁹ miR-195 targets BCL-W,⁵⁰ and miR-15b silences Bcl-xL;⁷³ these events lead to increased apoptosis and modification of 5-FU sensitivity.

Some studies have provided indirect evidence of miRNA-dependent modulation of apoptotic proteins (Table 2). For example, Shang et al. found that miR-23a regulates 5-FU chemosensitivity by modulating the apoptotic protease activating factor 1 (APAF-1)/caspase-9 apoptotic signaling pathway. Additionally, antisense miR-23a plus 5-FU activates caspase-3 and -7 expression in colorectal cancer cells.⁵¹ Moreover, Yin et al. showed that miR-200c and miR-203 directly repress Bmi1 expression and reverse Bmi1-mediated resistance to 5-FU.⁵² Interestingly, the miRNA-Bmi1 interaction modulates the response to 5-FU through the p53 apoptotic signaling pathway. Indeed, high expression of miRNAs has also been shown to contribute to cancer cell resistance to 5-FU by targeting core transcription factors, leading to increased resistance to 5-FU-induced apoptosis. For example, Zhang et al. showed that miR-191 decreases the sensitivity of colon cancer cells to 5-FU by targeting CCAAT/enhancer-binding protein beta (C/EBPβ), inhibition of which can increase apoptosis rates in 5-FU-treated cancer cells.Moreover, overexpression of miR-191 significantly inhibits the caspase-3 apoptotic pathway, whereas inhibition of this miRNA clearly increases the apoptotic pathway induced by 5-FU.⁷⁴

Autophagy is another characteristic of cancer cells and is a critical mechanism mediating 5-FU resistance during therapeutic and metabolic stress in cancer cells⁷⁵ (Table 2). Hou et al. identified four downregulated miRNAs and 27 upregulated miRNAs in nutrient-starved HT-29 cells. Pathway and gene ontology (GO) network analyses showed that most of these miRNAs were involved in autophagy signaling pathways, highlighting the potential functions of miRNAs in the regulation of 5-FU-related autophagy.¹¹ Intriguingly, autophagic and apoptotic pathways often crosstalk during 5-FU treatment. Xu et al.⁵³ showed that miR-101 acts in concert with 5-FU to induce apoptosis and inhibit autophagy by targeting enhancer of zeste homolog 2 (EZH2). As described above, miR-22 also functions to silence BTG1 in colorectal cancer cells.²³

Taken together, these findings demonstrate that miRNAs are involved in various biological processes, including cell cycle regulation, apoptosis, and autophagy, to mediate 5-FU sensitivity. Additional studies are still needed to fully elucidate the miRNA-mediated mechanisms regulating this complex network.

miRNAs regulate the EMT and CSCs to affect 5-FU resistance

The EMT converts epithelial cells into mesenchymal cells, and cells acquire stem cell–like features during this process. Importantly, once cancer cells acquire the capacity to undergo the EMT, chemoresistance is increased and metastasis can occur.⁷⁶ Additionally, chemotherapy can induce the EMT and modulate the expression of various miRNAs to promote cancer cell progression (Table 2). For example, Li et al. showed that many chemotherapeutic drugs, including 5-FU, suppress the expression of miR-488, leading to activation of the epidermal growth factor receptor (EGFR)/nuclear factor kappaB (NF-κB) signaling pathway by targeting SATB1 mRNA. They also found that NF-κB and miR-448 form a feedback loop during chemotherapy to induce the EMT.⁵⁴ Amankwatia et al. showed that miR-224 is differentially expressed in colon cancer tissues and in KRAS wild-type and mutant cells. Moreover, inhibition of this miRNA significantly increases 5-FU chemosensitivity by blocking the phosphorylation of extracellular signal–regulated kinase (ERK) and AKT.⁵⁵ Importantly, bioinformatics analysis has shown that miR-224 may play an important role in the EMT through its regulation of EGFR/AKT/ERK/NF-κB signaling in 5-FU resistance, suggesting that miRNAs may have clinical utility in the regulation of this pathway. Tang et al.⁵⁶ showed that miR-612 suppresses both the EMT and stemness of liver cancer cells via the Wnt/β-catenin pathway and targeting of AKT2.

CSCs, which are thought to give rise to tumors and to be involved in the development and progression of cancer, have also been shown to be involved in 5-FU resistance. For example, targeting of miR-21 reduces the number of colon cancer CSCs during 5-FU treatment.⁵⁷ In support of the ability of miRNAs to target multiple mRNAs, Shen et al.⁵⁸ showed that miR-142-5p binds to and inhibits CD133, Lgr5, and ABCG2, thereby increasing the sensitivity of the cells to 5-FU.

Taken together, these studies have provided important insights into the novel mechanisms through which miRNA is involved in 5-FU resistance (Table 2), further supporting the potential applications of miRNAs as therapeutic targets in cancer therapy.

Other mechanisms through which miRNAs regulate 5-FU resistance

The DNA mismatch repair (MMR) system plays a critical role in the maintenance of genomic stability, and deficiencies in MMR proteins cause cancer cells to become resistant to 5-FU.⁷⁷ Valeri et al.⁵⁹ provided strong evidence that miR-21 induces 5-FU resistance in colon cancer cells by downregulation of human mutS homolog 2 (hMSH2). Moreover, they demonstrated that miR-21 dramatically reduces the efficacy of 5-FU in vivo. We have also confirmed these findings in HT-29 cells.³⁶ Additional studies have also shown that miR-155 significantly downregulates core MMR proteins, including hMSH2, hMSH6, and hMLH1, further supporting the role of miR-155 in drug sensitivity⁶⁰ (Table 2).

Recently, many studies have shown that promising approaches to cancer treatment may be obtained based on the Warburg effect.⁷⁸ For example, Jiang et al.⁶¹ found that hexokinase II, a critical enzyme in glucose metabolism, is a direct target of miR-125b and that overexpression of this miRNA sensitizes liver cancer cells to 5-FU. Additionally, He et al.⁶² showed that glucose metabolism is significantly upregulated in 5-FU-resistant cells. Importantly, they found that increased miR-122 expression in 5-FU-resistant cells resensitizes the cells to 5-FU treatment by targeting pyruvate kinase (PKM2). Thus, the application of miRNAs in the inhibition of glycolysis may represent an effective therapeutic strategy to overcome 5-FU resistance (Table 2).

Tissue miRNAs as predictors of 5-FU response

Many studies have shown that miRNAs could be prognostic biomarkers for selection of patients who will benefit from 5-FU treatment, thereby avoiding toxicity in patients who will not respond to 5-FU.^79–81 Among these miRNAs, miR-21 has emerged as a potential biomarker for 5-FU sensitivity. In addition to in vitro and in vivo data showing that miR-21 is involved in 5-FU resistance, Tomimaru et al.⁸² also showed that miR-21 can predict clinical response to interferon (IFN)-α/5-FU combination therapy in patients with liver cancer. Subsequent studies have confirmed that miR-21 is a good molecular marker in patients with colon cancer and pancreatic cancer who have undergone 5-FU treatment.⁸³ However, more clinical studies are needed to definitively establish the ability of miR-21 to predict the clinical effects of 5-FU treatment.

Interestingly, Perez-Carbonell et al.⁸⁴ found that miR-320e is associated with adverse clinical outcomes in patients with stage III colorectal cancer treated with 5-FU-based adjuvant chemotherapy in an independent cohort of 237 patients with stages II–IV colorectal cancer. Zhang et al.⁸⁵ found that overexpression of miR-363 predicted poor response to 5-FU-based chemotherapy in patients with gastric cancer. Other miRNAs, such as miR-200 family members⁸⁶ and miR-215,⁸⁷ may also be indicators of 5-FU response. These data suggest that tissue miRNA may be a potential biomarker for predicting the response to 5-FU treatment.

Serum miRNAs as a predictive factor of the curative effects of 5-FU

Many recent studies have investigated the levels of miRNAs in serum and plasma, demonstrating the presence of multiple miRNAs in blood samples; because of the ease of collection of blood samples, such miRNAs may have potential value for early diagnosis of cancer and prediction of chemotherapeutic effects.^88–90 Chen et al. identified five miRNAs (miR-221, miR-222, miR-122, miR-144, and miR-19a) that showed differential expression in the serum of patients treated with FOLFOX as a first-line chemotherapy regimen. Moreover, they found that serum miR-19a could be a potential biomarker for predicting outcomes of 5-FU-based chemotherapy and that high miR-19a expression could predict both intrinsic and acquired drug resistance.⁹¹ Additionally, Kjersem et al.⁹² showed that serum miR-326 may serve as a marker for predicting outcomes in patients with colon cancer treated with 5-FU and oxaliplatin-based chemotherapy. Thus, these studies have provided additional evidence for the clinical importance of miRNAs in the prediction of sensitivity to 5-FU-based chemotherapy. Additional studies are expected to identify more serum miRNAs and lncRNAs that can be used as noninvasive markers for predicting the curative effects of 5-FU.

Involvement of lncRNAs in 5-FU drug resistance

The mechanisms described above may provide a molecular basis for 5-FU resistance and treatment failure. However, protein-coding genes and miRNAs cannot completely explain the complex mechanisms of 5-FU resistance, and it remains challenging to improve outcomes in patients exhibiting resistance to 5-FU chemotherapy.

LncRNAs are a class of noncoding RNA (ncRNA) with a length greater than 200 nt, with limited or no protein-coding capacity. Some lncRNAs have been functionally characterized in patients’ tissues and serum and function similarly to genes to mediate tumor-suppressing or oncogenic effects.⁹³ Recent studies have shown that lncRNAs also play a vital role in epigenetic, transcriptional, and post-transcriptional gene expression, thereby mediating 5-FU drug resistance.^94–98 Xiong et al.⁹⁹ showed that the expression levels of many lncRNAs are altered following treatment with 5-FU in colon cancer cells; a microarray analysis showed that these aberrantly expressed lncRNAs are involved in the phosphoinositol 3-kinase (PI3K)/Akt and NF-κB signaling pathways. Moreover, Lee et al.¹⁰⁰ used two 5-FU-resistant cell lines to analyze the expression of lncRNAs. They found that snaR is downregulated in 5-FU-resistant cells and that loss of snaR increases cell viability after 5-FU treatment, suggesting that snaR may be a negative regulator of colon cancer cell growth in response to 5-FU. Han et al.¹⁰¹ showed that LEIGC functions as a tumor-suppressive lncRNA in gastric cancer and that overexpression of this lncRNA can enhance the sensitivity of cells to 5-FU by inhibiting the EMT. Additionally, upregulation of CCAT2 decreases chemosensitivity to 5-FU in breast cancer.¹⁰² However, although lncRNAs are associated with 5-FU therapeutic resistance, the specific molecular mechanisms through which lncRNAs regulate 5-FU sensitivity in cancer cells are not yet clear. Additional studies are needed to improve the current understanding of the molecular mechanisms through which lncRNAs regulate 5-FU resistance.

Conclusion and perspectives

In addition to the above-mentioned miRNAs involved in 5-FU resistance, many other miRNAs have also been reported to be involved in 5-FU resistance. For example, miR-497,⁶³ miR-221,⁶⁴ miR-520g,⁶⁵ miR-148b,⁶⁶ miR-141,⁶⁷ miR-137,⁶⁸ and let-7g⁶⁹ have been shown to regulate the sensitivity of cells to 5-FU. Clearly, miRNAs are critical factors mediating 5-FU resistance and can function through different molecular mechanisms. Future studies are required to identify more miRNAs and lncRNAs involved in 5-FU resistance to develop effective strategies for preventing or managing resistance to this widely used chemotherapeutic agent. In particular, circulating miRNAs may be used as biomarkers for 5-FU-based cancer therapy. Hence, understanding how noncoding RNAs are involved in 5-FU resistance will improve the current understanding of how to reverse 5-FU resistance and may provide insights into new pathways of drug resistance in the future.

Footnotes

Acknowledgements

J.D., Y.W., and J.L. contributed equally to this work.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

This work was supported by the National Natural Science Foundation of China (grant nos 81160281 and 81441083), the Jiangxi Province Talent 555 Project, and the National Natural Science Foundation of Jiangxi Province (grant nos 20152ACB20024 and 20151BBG70228).

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