Cell Reprogramming Techniques: Contributions to Cancer Therapy

Abstract

The reprogramming of terminally differentiated cells over the past few years has become important for induced pluripotent stem cells (iPSCs) in the field of regenerative medicine and disease drug modeling. At the same time, iPSCs have also played an important role in human cancer research. iPSCs derived from cancer patients can be used to simulate the early progression of cancer, for drug testing, and to study the molecular mechanism of cancer occurrence. In recent years, with the application of cellular immunotherapy in cancer therapy, patient-derived iPSC-induced immune cells (T, natural killer, and macrophage cells) solve the problem of immune rejection and have higher immunogenicity, which greatly improves the therapeutic efficiency of immune cell therapy. With the continuous progress of cancer differentiation therapy, iPSC technology can reprogram cancer cells to a more primitive pluripotent undifferentiated state, and successfully reverse cancer cells to a benign phenotype by changing the epigenetic inheritance of cancer cells. This article reviews the recent progress of cell reprogramming technology in human cancer research, focuses on the application of reprogramming technology in cancer immunotherapy and the problems solved, and summarizes the malignant phenotype changes of cancer cells in the process of reprogramming and subsequent differentiation.

Introduction

With the development of cancer treatment research, more and more advanced technologies have been applied to cancer research, including induced pluripotent stem cell (iPSC) technology (Oshima and Aoi, 2015). As early as 2006, Yamanaka screened four genes (Oct4, Sox2, c-Myc, and Klf4), and introduced them into mouse somatic cells, which can be reprogrammed to have similar characteristics to embryonic cells and have a tendency to differentiate toward the endoderm, mesoderm, and ectoderm (Takahashi and Yamanaka, 2006). In 2007, Yamanaka introduced four transcription factors into adult fibroblasts and obtained the human iPSC (hiPSC) (Takahashi et al., 2007). iPSC research has ushered in a new era. With the continuous development of iPSC research, this technology has been increasingly applied to cancer research.

Past studies have often used animal models to simulate human cancer. However, the limitations of animal models in reproducing human cancers should not be ignored (Talbert et al., 2019), and the emergence of iPSC technology can solve the shortcomings of animal models well. One stereotype associated with iPSCs is that their best medical use is in organ regenerative medicine and transplantation therapy (Afanasyeva et al., 2021; Saha et al., 2021). However, in our view, cancer modeling, cancer drug screening, and cellular immunotherapy are equally important as cancer cell reprogramming. This includes modeling cancer stem cells (CSCs), because CSCs are difficult to isolate from cancer cells, effective reprogramming of tumor cells using iPSC technology has emerged as a potential strategy for identifying tumor stem cell-related oncogenes and suppressor genes (Pan et al., 2017).

Moreover, the iPSC technique of reprogramming terminally differentiated tumor cells into an embryonic-like state reveals the possibility of reprogramming malignant cells into a more controllable stem cell-like state, and the phenomenon of changing the malignant degree of tumor cells occurs (Gong et al., 2019; Yilmazer et al., 2015).

In addition, as iPSC technology continues to evolve, it is also a reliable source of cellular immunotherapy research. While cell immunotherapy is successfully applied in clinics, the availability of immune cells and immune cell therapy is facing great difficulties. The emergence of cell reprogramming technology provides an effective solution to solve these difficulties. While studying the specificity of immune cells, iPSC technology can also be used as a reliable source of T cells, natural killer (NK) cells, macrophages, and other immune cells (Ackermann et al., 2022; Goldenson et al., 2022; Kawamoto et al., 2021). With the widespread application of iPSCs in the field of cancer immunotherapy, immune cells can also be genetically modified before differentiation, so that the obtained immune cells have a stronger antitumor activity. Thus, cell reprogramming techniques have the potential to tailor specific immune cells to patients (Zimmermannova et al., 2021).

Moreover, as reprogramming techniques improve, so does their safety in vivo. iPSC technology is also used to reprogram tumor cells in cancer patients. More importantly, the malignant function of tumor cells is changed, and with the change of the cell cycle, the ability of proliferation and migration is weakened, and the tumor cells are transformed into benign cells, undoubtedly providing a new idea for cancer treatment (Gong et al., 2019).

In this review, we aim to discuss recent advances in iPSC cancer models for cancer research, the effects of cancer cell reprogramming on cancer cells, and its potential use in immune cell therapy (Fig. 1).

FIG. 1.

Multiple applications of iPSC technology in cancer treatment research. iPSC, induced pluripotent stem cell.

iPSCs Were Used for Cancer Modeling

iPSCs have been used for many years to simulate neurodegeneration, mental retardation, heart disease, diabetes, and metabolic disorders (Cai et al., 2020; Pozo et al., 2022; Tian et al., 2019), but they have only recently been used in cancer research. Due to the complexity and heterogeneity of human cancer, the mouse model cannot fully summarize the occurrence and development of human cancer, and therefore, it is very necessary to build a more similar human model to study the mechanism and clinical application of cancer. Due to their multidifferentiation potential and easy availability from human skin cells, iPSCs can be used to model cancer, screen drug compounds, validate targets, and test drug efficacy and safety (Table 1).

Table 1.

Cancer type	iPSC model	Cell origin	Modeling type	Reference
Osteosarcoma	LFS-iPSC	Fibroblasts of patients	OB derived from LFS patient-specific iPSCs were used to model OS and to profile the disease characteristics of osteosarcoma.	Zhou et al. (2018)
Osteosarcoma	LFS-iPSC	Fibroblasts of patients	To investigate the carcinogenic effect of SFRP2 on the development of p53 mutation-associated OS.	Kim et al. (2018)
Leukemia	MDS and MDS/AML-iPSC	Patient's BM or PB mononuclear cells (BMMC or PBMC)	Delineate the stages of myeloid malignancy, including familial predisposition, low-risk MDS, high-risk MDS, and MDS/AML.	Kotini et al. (2017)
Leukemia	MDS and MDS/AML-iPSC	Clonal iPSCs generated by single-cell reprogramming in patients with MDS and AML at different disease stages	iPSCs were combined with CRISPR-Cas9 technology to establish clonal evolution model of AML. During differentiation, iPSCs introduce different mutations to produce different precancerous stages, including CH and MDS, eventually leading to transplantable leukemia, and reproduce the transcriptional and chromatin accessibility characteristics of primary human MDS and AML.	Wang et al. (2021)
Prostate cancer	Prostate cancer iPSC	Human prostate cancer cell line 22RV1	Modeling prostate CSCs.	Xu et al. (2018), Zhang et al. (2020)
Liver cancer	miPS-Huh7cm	Human HCC cell line Huh7	From miPSC to liver CSC to simulate liver CSC.	Afify et al. (2020)
Pancreatic cancer	BRCA1 iPSC	PBMC from patients carrying exon 17 lines of BRCA1 gene deletion	iPSC-derived BRCA1 gene +/− iMSCs, demonstrating that POSTN may play an important role in the occurrence, progression and spread of breast cancer.	Portier et al. (2021)
Breast cancer	iPS-like cell	Human pancreatic ductal adenocarcinoma primary cells from a patient	iPSC technology provides a cellular model of early pancreatic cancer progression.	Kim et al. (2013)
Ovarian cancer	BRCA1^mut-iPSC	iPSCs were produced in three patients diagnosed with epithelial ovarian cancer	Human FTE-like organs from BRCA1^mut-iPSC provide a reliable in vitro physiological model of FTE injury generation and early cancer development.	Yucer et al. (2021)
NF1	hiPSC	Commercially available BJFF.6 healthy human fibroblasts reprogrammed into iPSCs (nerve fibers 1^+/+)	hiPSC-derived neurofibroma and cNF mouse models to simulate the pathogenesis of human neurofibroma.	Mo et al. (2021)
Angiomyolipoma	hiPSC	hiPSC line 77-patient, 77-TSC2-NULL, and control 77-TSC2wt	Transplantation of hiPSC-derived renal AML organoids was used in vivo to simulate TSC-associated human kidney disease.	Hernandez et al. (2021)

AML, acute myeloid leukemia; BM, bone marrow; BMMC, bone marrow mononuclear cell; CH, clonal hematopoietic; cNF; CSC, cancer stem cell; HCC, hepatocellular carcinoma; hiPSC, human induced pluripotent stem cell; LFS, Li-Fraumeni syndrome; MDS, myelodysplastic syndrome; miPSC, mouse induced pluripotent stem cell; NF1, neurofibromatosis type 1; OB, osteoblasts; OS, osteosarcoma; PB, peripheral blood; PBMC, peripheral blood mononuclear cell; SFRP2, secreted fried-associated protein 2; TSC, tuberous sclerosis complex.

iPSC for hematologic cancer modeling

Common types of leukemia are acute myeloid leukemia (AML), CML, ALL, and CLL. AML, a highly common type of leukemia, has the highest incidence in adult patients (Brunning, 2003). In addition, AML can be subdivided into a variety of types; there are different subtypes of genetic abnormalities that need to be matched with different treatments (Newell and Cook, 2021). Evolutionary models of leukemia can help illuminate this process and inform therapeutic interventions early in the disease, but their creation faces huge challenges (Vetrie et al., 2020). iPSC technology can be applied to study the evolution process of human leukemia and establish the evolution model of leukemia to solve this problem.

Wang et al. (2021) combined iPSCs and CRISPR-Cas9 technology to develop clonal evolution models of AML. After hematopoietic differentiation, iPSCs can differentiate into different stages of malignancies, including clonal hematopoietic and myelodysplastic syndromes, and finally constructed cell models that tracked the evolution of human leukemia (Jonas, 2017). In addition, AML-iPSCs have been used to differentiate into different mature myeloid cell types, such as monocytes and granulocytes in vitro. To detect the effect of the AML1-ETO fusion protein on differentiation (Gerritsen et al., 2019; Mandoli et al., 2016), stage-specific iPSCs provide a powerful new tool for functional studies of bone marrow diseases (Kotini et al., 2017). Following the modeling of iPSC systems using CRISPR-Cas9 gene correction or mutation introduction, the combination of these two technologies can be used to model drug-specific responses to cancer and provide a new platform for testing genetic and pharmacological interventions.

iPSC for solid tumor modeling

Renal tumor

With the discovery of cell reprogramming and advances in the utility of the CRISPR-Cas9 system in human cells, the combination of these two technologies has enabled the scientific community to obtain human cells that were previously unavailable and have facilitated the creation of new tools that have an impact on basic science and drug discovery, including the kidney field.

Renal angiomyolipoma (AML), although a rare tumor, occurs in more than 80% of tuberous sclerosis complex patients (Bausch et al., 2021). Renal AML is primarily treated with rapalogs and often relapses after treatment interruption due to limited drug therapy. The lack of animal models has been a barrier to developing and improving rapalog-based therapies (Trnka and Kennedy, 2021). Renal AML organoids derived from hiPSC can be used to model AML to address the drug dependence of rapamycin. After the transplantation of iPSC-AML-like organs into the kidneys of immunodeficient mice, the related pathogenesis of human AML can be simulated in animals and tested in vivo. Due to the lack of a renal rare tumor disease model, iPSC technology is applied to the development of this model, which has played a huge role in the treatment of this disease (Hernandez et al., 2021).

Osteosarcoma

As one of the most common types of bone cancer, osteosarcoma is associated with TP53 inactivation in 95% of patients (Kansara et al., 2014). The autosomal dominant cancer caused by its genetic mutation is often referred to as the Li-Fraumeni syndrome (LFS) (Simpson and Brown, 2018). As a type of cancer with high incidence in patients with LFS, osteosarcoma is in urgent need of tumor models to study the pathogenesis, and the establishment of an LFS-iPSC model is precisely used to study osteosarcoma. Mouse models of LFS have been developed to study this mutation, but they do not fully generalize the tumor profile found in patients with LFS. In 2015, the osteosarcoma model derived from LFS-iPSCs was successfully established for the first time, and the iPSCs derived from LFC patients were induced to differentiate into mesenchymal stem cells and further differentiate into osteosarcoma primitive cells—osteoblasts.

The obtained LFS-osteoblasts have carcinogenic characteristics associated with osteosarcoma, which well simulates the pathogenesis of bone and meat (Lee et al., 2015). Therefore, the establishment of the LFS-iPSC model is of great significance for the study of the pathological mechanism of cancer caused by the TP53 mutation and can be used to simulate the occurrence and development of tumors (Lin et al., 2017; Pang et al., 2020).

Neuropathic tumor

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder caused by a mutation in the NF1 tumor suppressor gene, resulting from the inheritance of a mutant copy of the NF1 tumor suppressor gene (Rosenbaum and Wimmer, 2014). Cluster NFs (pNFs) are one of the signs of this disease, but there is a lack of an effective clinical model. iPSCs were used to simulate NF1 disease, and the heterogeneous sphere system consisting of iPSC-derived differentiated SCs and pNF-derived Fbs (nerve fiber spheres) was the most powerful model of NF formation (Mazuelas et al., 2020).

NF1 is caused by a genetic mutation that predisposes patients to develop peripheral Schwann cell line schwannoma (neurofibroma) (Guo et al., 2020). The transformation of hiPSCs into Schwann lineage cells will help elucidate the progression and pathogenesis of NF1 (Mo et al., 2021). In conclusion, these studies demonstrate that iPSCs play an important role in NF1 studies. Because nerve cells are difficult to obtain, iPSCs will contribute to the development of neurotumor research models and provide an unprecedented opportunity for the treatment of neurofibroma.

Ovarian and breast cancer

Ovarian cancer is a malignant tumor that is the leading cause of death in women, and various treatment methods have not significantly improved the survival rate of ovarian cancer. Among them, epithelial ovarian cancer has a high incidence and is highly heterogeneous. Its different cell sources, individual differences, and biological behaviors lead to drug resistance, and specific models are urgently needed for drug screening and clinical studies (Matulonis, 2018). High-grade serous carcinoma (HGSCS), as one of the most common subtypes of ovarian cancer, has a very high fatality rate. About 70% of patients with ovarian cancer have been diagnosed with HGSCS (Koshiyama et al., 2017). Yucer et al. (2021) used iPSC technology to establish an iPSC-based model of BRCA1^mut.

By generating 3D organoid models from a variety of BRCA1^mut patients, it can clarify the treatment-dependent response of BRCA1^mut, and this information can help predict the drug sensitivity of individual patients based on their unique genetics. The model covers not only mutations in cancer but also the entire genetic spectrum of cancer patients, which provides a good research platform for the study of known and unknown mutations of HGSCS.

For patients at risk for breast and/or ovarian cancer, there is a lack of effective models for how the deletion or mutation of the BRCA1 gene affects this process. To simulate the matrix of BRCA1 deletion, specific iPSC models were developed from patients with BRCA1 deletion. The establishment of this model is of great significance for the study of the effect of BRCA1 haploid deletion on BRCA1-related cancer tumors. At the same time, this model also provides a new approach to the treatment of BRCA1-related hereditary breast cancer (Portier et al., 2021).

Pancreatic cancer

Different studies have shown that in some cases, reprogrammed cancer cells can mimic the early formation of cancer by inducing differentiation and exhibiting the corresponding early phenotype. In past studies, human pancreatic cancer cells undergoing reprogramming can be used to completely summarize each stage of pancreatic cancer development, from early- to late-stage cancer development, and establish iPSCs from clinical samples of human pancreatic cancer (Kim and Zaret, 2019; Kim et al., 2013).

Pancreatic ductal adenocarcinoma (PDAC) is one of the most representative types of pancreatic cancer, and CSCs are an important part of the heterogeneous cell population of PDAC solid tumors. The research shows that the model transformed from iPSCs to PDAC-CSCs is a new model for the research of PDAC. The model of the CSC line (iPSC-CSCs) established by iPSC showed different histopathological characteristics of ADM, PanIN, and PDAC lesions in xenotransplantation. The establishment of this model will hopefully put forward new views on the occurrence and development of pancreatic cancer and input forward constructive suggestions for the elimination of CSCs in the process of its occurrence and development (Calle et al., 2016).

Hepatocellular carcinoma

Hepatocellular carcinoma (HCC) ranks fifth in the global cancer mortality rate (Chidambaranathan-Reghupaty et al., 2021). All liver cancer cells contain CSCs. So far, the molecular mechanism regulating the development of HCC stem cells remains unknown. It is of great significance to study the molecular mechanism of liver CSC formation by reprogramming HCC to obtain liver CSCs. Afify et al. (2020) used liver cancer cell lines to transform mouse-iPSCs to establish a model of liver CSC transformation and showed different stages of differentiation during the transformation process. The establishment of this model is helpful to understand the different stages of liver cancer (Afify et al., 2020).

In addition, HCC contains stem-like cell subpopulations expressing various stem cell markers. Reprogramming techniques not only provide a way to induce CSCs from cancer cells but also provide insight into the signaling pathways that uncover their disturbances since many reprogramming factors are oncogenes (Liang et al., 2022; Liu et al., 2020). To study the role of mbd3/NuRD in liver iCSC generation, Li et al. (2017) reprogrammed liver cancer cells into iCSCs, providing a good model for the study of this molecule.

Cell Therapy: iPSCs Were Used in Cancer Immunotherapy

iPSC and T cell

Chimeric antigen receptor (CAR)-T cell immunotherapy is a new cellular immunotherapy technology that has developed very rapidly in recent years, and it has shown remarkable therapeutic effects in the treatment of tumors, especially in some hematologic tumors (June and Sadelain, 2018; Sterner and Sterner, 2021). However, this therapy also has many challenges, including T cells' difficulty in infiltrating solid tumors, inhibition and depletion in complex tumor microenvironments, limited efficiency of gene modification of primary T cells, high cell heterogeneity, and high cost due to personalized manufacturing processes (Sterner and Sterner, 2021). To overcome these obstacles, various strategies have been used, including optimizing CARs, improving T cell function, optimizing T cell subpopulation proportions, or using other types of immune cells (Klichinsky et al., 2020; Noy and Pollard, 2014), and manufacturing “off-the-shelf” general-purpose T cells.

To address these issues, iPSC technology could provide further benefits to T cell therapy development by improving the speed, cell count, and quality control of allogeneic products, which could help improve therapeutic efficacy.

iPSC technology solves the problem of T cell depletion

In the process of immunotherapy, the depletion of antigen-specific T cells causes many difficulties in treatment. Due to the phenomenon of T cell exhaustion, especially for solid tumors with harsh microenvironments, the injected cells will not persist (Rodriguez-Garcia et al., 2021). Therefore, to solve this problem, some researchers reprogrammed antigen-specific CD8⁺T cells cloned and amplified from HIV-1 infected patients to return them to their original undifferentiated state and then induced them to differentiate into CD8⁺T cells with enhanced proliferation ability and telomere extension. These newly induced cells had the same antigen-specific killing activity as the previous cells and showed the same T cell receptor (TCR) gene rearrangement pattern as the patient's original T cell clone. The treatment difficulties caused by antigen-specific T cell depletion were effectively solved (Nishimura et al., 2013).

It can be seen that iPSC-induced T cells have the ability to resist exhaustion, and iPSC-T cells have persistence in vivo. Moreover, because hiPSCs are sustainably available, it opens the possibility of engineered T cell harvesting for human use, which requires more clinical validation and safety testing (Sadeqi Nezhad et al., 2021). Moreover, iPSC-induced T cells play a significant role in the treatment of various cancers, such as Epstein–Barr virus-associated lymphomas, melanoma, and liver cancer (Harada et al., 2022; Itoh et al., 2023; Lu et al., 2021).

The effectiveness of T cells obtained by the iPSC technique was higher

Currently, effective T cell immunotherapies include CAR-T therapy and TCR-T therapy, both of which require modification of a patient's T cells. However, the restriction of T cell proliferation and multiple gene editing modifications may have an impact on the elimination of tumors (Jiang et al., 2019). iPSCs provide a renewable source of cells for immune cell therapy because they can be grown indefinitely in vitro. The study demonstrated that the application of gene editing to iPSCs followed by T cell differentiation was more effective in killing tumor cells than before. Moreover, gene editing of iPSCs did not affect the antitumor activity of subsequently differentiated T cells (Yano and Kaneko, 2018).

Kaneko's (2022) team has already combined CAR with iPS-T cells they built, and showed that gene editing does not affect the effectiveness of CAR. In addition, CAR was added to iPS-T cells and injected into mice models of leukemia. Those CAR iPS-T cells that received gene editing to evade the immune system showed stronger anticancer effects and longer survival than cells that did not receive editing (Kaneko, 2022).

The combination of CRISPR technology and reprogramming technology can effectively improve the low antigen specificity of TCR. CRISPR technology can knock out the recombinant enzyme gene in T-iPSCs to prevent additional TCR rearrangement, thus improving their antitumor activity (Minagawa et al., 2018). In addition, T-iPSCs can produce CD8 αβ T cells during differentiation, and the T cells expressing CD8 αβ have higher antigen-specific cytotoxicity than CD8 αα (Maeda et al., 2016).

iPSC and NK cells

Following the success of CAR-T therapy in cancer treatment, researchers are also looking at CAR-NK cell therapy. Compared with CAR-T cells, CAR-NK cells have the unique advantage that they are not limited by GVHD and have a higher antiangiogenic immune activity in terms of persistence (Marofi et al., 2021). At the same time, NK cells face some challenges. The clinical application of NK cells requires enough cells, but the NK cells, both in vivo and in vitro, show a limited expansion trend (Karagiannis and Kim, 2021). Second, NK cells have been controversial in the treatment of solid tumors, and they lack available targets, which is in urgent need of methods to improve (Maddineni et al., 2022; Siegler et al., 2018). The application of iPSC in immunotherapy provides a way to solve the above obstacles.

Increased action time of iPSC-NK cells

Although many studies have shown that compared with T cells the modification of NK cells before acquisition and clinical application is of great simplicity, NK cells are still wastage when applied in treatment before treatment, resulting in a short time of action in vivo, which also leads to defects in the antitumor effect of NK cells (Bachanova et al., 2014; Klingemann et al., 2016). Therefore, modifying NK cells with iPSCs to maintain their persistence in vivo may help enhance their antitumor activity. Some studies combined specially modified CAR constructs that could enhance the antitumor activity of NK cells with iPSCs to create CAR-iPSCs, and then differentiated NK cells with persistent antitumor activity in vivo. It is capable of massive expansion and can treat solid malignancies (Karagiannis and Kim, 2021).

Recent studies have shown that NK cells can provide effective immunotherapy for ovarian cancer. To verify that iPSC-NK cells can be used as a powerful source of ovarian cancer immune cell therapy, the killing ability of NK cells from different sources was evaluated in vivo. The results showed that the killing ability of NK cells from peripheral blood (PB) and iPSC-NK cells on ovarian cancer cells was comparable with that of PB-NK cells. However, due to the powerful amplification ability of iPSC-NK cells, it can be used to treat more patients than PB-NK cells (Hermanson et al., 2016).

In addition, we report that CAR-modified NK cells derived from hiPSCs exhibit enhanced antitumor activity in a mouse ovarian cancer model, similar to the anticancer effects of CAR-T immunotherapy (Woan et al., 2021).

iPSC-NK cells solve the limitation of cell

Obtaining large numbers of NK cells is relatively tedious and time-consuming. Currently, only a small number of NK cells can be obtained from a single related donor for immunotherapy, and its high cost and low availability have become one of the biggest obstacles to the use of NK cells for immunotherapy. More and more research is being done to find better ways to solve this problem, to get enough cells for treatment, and to get them easily. These include in vitro amplification of PB-NK cells (Siegler et al., 2018), differentiation and expansion of NK cells from umbilical cord progenitor cells (Dolstra et al., 2017; Heipertz et al., 2021), and treatment of the NK-92 cell line (Heipertz et al., 2021; Michel et al., 2022).

iPSCs are an interesting source of NK cells. iPSCs are capable of efficient cloning, growth, and expansion, as well as in vitro differentiation, and in the preclinical research environment, there are multiple differentiation schemes to obtain immune effector cells from iPSCs (Arias et al., 2021). This feature of iPSCs enables them to generate a large number of unified NK cells by inducing differentiation. Not only can iPSC-NK cells expand NK cells in large quantities, but UCB-NK cells have the same growth rate, express surface markers, and have a higher antitumor activity than PB-NK cells (Ni et al., 2013; Zhou et al., 2022).

iPSC-NK cells showed high activity

Although large quantities of NK cells can be obtained from the expansion of the NK-92 cell line for cancer therapy, the cell line often lacks many markers of NK cells, which limits the activity of NK cells in vivo. hiPSC is now widely used as a new tool to produce NK cells with high antitumor activity (Wang et al., 2019). In contrast, NK (iNK) cells derived from iPSCs can have a higher ability to expand for clinical applications, can perform gene editing at the iPSC stage, and can enhance iNK cell proliferation in vivo. Therefore, iNK cells (differentiation induced by iPSCs) can be used as an important source of a large number of NK cells for the treatment of malignant tumors, which can effectively play an active role in vivo and make a great contribution to the killing of malignant tumors (Zhu et al., 2020b).

Interleukin-15 (IL-15) signaling regulates the proliferation and growth of NK cells, cytokine-inducible SH2-containing protein (CIS, encoded by the gene CISH) is an important negative regulator of the IL-15 signaling pathway, and inhibition of CISH promotes the activity of NK cells (Christodoulou et al., 2021; Zhang et al., 2018). The CISH gene in NK cells was knocked out using the iPSC platform, and then, the NK cells were amplified, which not only ensured the knockdown efficiency of NK cells but also had a higher antitumor activity (Zhu et al., 2020a).

iPSC and macrophage

Following the success of CAR-T cell therapy, CAR macrophages have also attracted the interest of developers (Arango Duque and Descoteaux, 2014). At the same time, studies have shown that macrophages are more likely to infiltrate tumors (the proportion of macrophages in tumor tissues can reach 40%–50%). Specific CAR modification of macrophages can improve the tumor antigen presentation and phagocytosis activity of macrophages, opening new possibilities for the treatment of solid tumors (Cassetta and Pollard, 2018; Klichinsky et al., 2020). Similarly, iPSC technology is applied to macrophages.

Researchers obtained CAR-iPSCs by combining CAR with iPSCs and obtained macrophages after induced differentiation with phagocytic phenotype. These results proved that iPSCs can be successfully applied to obtain macrophages and can be used as a productive source of macrophages (Zhang et al., 2020). hiPSCs can effectively generate large, homogeneous populations of human macrophages using a fully defined regimen of farm-free and serum-free differentiation. Macrophages differentiated from iPSCs expressed classical surface cell markers and had phagocytic activity, which showed no difference compared with macrophages derived from PB (Cao et al., 2020; Su et al., 2022; van Wilgenburg et al., 2013).

With the deepening of the research on macrophages, the research on tumor-associated macrophages (TAMs) is vital to the study of human cancer macrophages. As TAM is difficult to be isolated in cancer cells, it is also an obstacle to the study. iPSCs can differentiate into TAM in vitro and become an effective source of TAMs (Heideveld et al., 2020), which can effectively solve the source problem.

iPSC Technology Promotes the Transformation of Cancer Cells into Benign States

Many studies have shown that epigenetic modification and cell reprogramming processes are important for tumor cell transformation and the development of malignant tumor phenotypes. iPSC technology has been successfully applied to a variety of human cancer cells in the last decade of research. Nuclear reprogramming of cancer cells using iPSC technology can weaken or eliminate malignancy by altering the epigenetic state of cancer cells. In this study, we discuss whether reprogramming cancer cells can be applied to developing new therapies and the challenges this approach faces.

Melanoma

As early as 2009, Utikal et al. (2009) reprogrammed the R545 melanoma cell line, and the iPSCs derived from this cell line showed endogenous expression of OCT4, KLF4, and c-MYC, and demethylation of the OCT4 and NANOG promoters in vivo. Interestingly, mouse chimeras derived from reprogrammed melanoma cells remained tumor free at 5 months after termination of Yamanaka factor expression, indicating tumorigenic loss of melanoma cells after reprogramming. In addition. Huang et al. (2014) demonstrated that melanoma cells carrying the BRAF mutation were reprogrammed to achieve a metastable pluripotent state and that the reprogrammed cells differentiated in vivo and in vitro. Notably, cells that differentiated in vivo lost their typical melanoma markers and did not develop new melanomas, and lost their oncogene dependence (Huang et al., 2014).

The above results indicate that the tumorigenicity of melanoma is weakened or lost after the reprogramming process, suggesting that cancer cell reprogramming can be used as a new cancer therapy.

Gastrointestinal tumors

Miyoshi et al. (2010) successfully reprogrammed eight cancer cells from colorectal cancer, esophageal cancer, gastric cancer, pancreatic cancer, liver cancer, and bile duct cancer, and the reprogrammed iPC inhibited the tumor suppressor gene P16 (INK4A). In addition, reprogrammed cancer cells are more sensitive to chemotherapy agents 5-fluorouracil and have shown significant regression of cell proliferation and invasiveness in the treatment of induction (Miyoshi et al., 2010). According to the results of the study, the inhibition of tumor suppressor genes and inducing changes in cancer cells' sensitivity to drugs are caused by epigenetic modifications after reprogramming.

With the introduction of identified transcription factors, the prevalence of malignant phenotypes of digestive cancer cells is reduced, but the induction efficiency is low. Reprogramming of colorectal cancer cells (HCT116) and mutated TP53-deficient HCT116 cells under hypoxia has been reported, and the experimental results show that the efficiency of specific programming is improved compared with classical reprogramming schemes. In addition, both types of cells undergoing reprogramming showed decreased malignancy of tumors, mainly manifested by decreased proliferation, invasion, and tumorigenic ability (Hoshino et al., 2012).

Moreover, in primary human PDAC studies, the reprogramming of PDAC cells using a virus-free system has been most effective. Reprogrammed PDAC cells functionally differ from parent cells, significantly reducing tumorigenicity in vitro and in vivo (Khoshchehreh et al., 2019). These results support the possibility of new treatments for gastrointestinal cancers through reprogramming.

Skin cancer

Researchers demonstrated that skin cancer cells can be effectively reprogrammed into iPSC-like cells (called mirPS) using microRNA-302 and differentiate into different tissue types in vitro. Interestingly, mirPS significantly suppressed the cell cycle, resulting in a reduced rate of cell division, suggesting that epigenetic changes in reprogramming skin cancer cells may lead to benign transformation (Lin et al., 2008).

Genetic mutations in epidermal cells are prone to cause highly invasive cutaneous squamous cell carcinoma (cSCC), whose early onset and rapid progression are important reasons for the high mortality of patients. After parental cells of cSCC patients were reprogrammed into iPSCs and induced to differentiate into keratinocytes, both in vitro and in vivo experiments showed significantly reduced cell proliferation after differentiation, suggesting that reprogramming and redifferentiation alter tumor cell function (Rami et al., 2020).

Leukemia

Many studies have shown that the tumorigenicity of cancer cells is closely related to their differentiation status (Khoshchehreh et al., 2019). In the 2017 study, the iPSC model of AML was successfully established. The model retained the original genetic characteristics, and after differentiation into hematopoietic cells, recovered the ability to cause leukemia in vivo, and reconstructed DNA methylation and gene expression patterns of leukemia. However, when differentiated into nonhematopoietic lineages (neurons and cardiomyocytes), they are no longer tumorigenic (Green et al., 2013). Thus, reprogramming cancer cells while maintaining safety can effectively transform tumor cells into benign cells, providing a new direction for cancer treatment.

Osteosarcoma

The ability to achieve a terminal differentiation state or its effect on the elimination of tumorigenicity has not been demonstrated in human cancer cells undergoing nuclear reprogramming in the past. Zhang et al. (2013) used patient-derived complex karyotype solid tumors as research objects and found that sarcomas can be terminally differentiated into mature connective tissues and erythrocytes after reprogramming, and terminal differentiation is accompanied by the loss of proliferation and tumorigenicity, and this loss of malignancy is irreversible.

Neurologic neoplasms

Epigenetic changes are often observed in cancer, and extensive epigenetic reset of cancer cells can be achieved using reprogramming techniques. It has been reported that exogenous expression of reprogramming factor OCT4 and KLF4 can program glioblastoma (GBM) into GBM iPSCs (GiPSCs), which remain highly malignant after xenotransplantation of GiPSC-derived neural progenitor cells, but when GiPSC is directed to non-neural cell types, persistent expression of reactivated tumor suppressors and decreased invasive behavior were observed (Stricker et al., 2013). These data suggest that exogenous expression of reprogramming-related genes can inhibit malignant behavior.

The Challenges of Tumor Cell Reprogramming

Tumor cell line limitation

In previous studies, various tumor cell lines have been reprogrammed for disease modeling and other purposes. However, studies have shown that not all tumor cell lines can be reprogrammed through epigenetic modification. Miyoshi et al. (2010) tried to reprogram 20 gastrointestinal cancer cell lines to produce iPSCs, and only 8 were able to successfully reprogram, and these include colorectal cancer (DLD1, HT29), esophageal cancer (TE10), gastric cancer (MKN45), pancreatic cancer (MIAPaCa-2, PAN-1), HCC (PLC), and bile duct cell carcinoma (HuCCT). Noguchi et al. (2015) demonstrated that PANC1 cells were able to reprogram successfully, whereas three other cell lines (MIAPaCa-2, PSN-1, and AsPC-1) failed to express pluripotent markers after the introduction of reprogramming factors. Iskender et al. (2016) demonstrated that the bladder cancer cell line T24 produces iPSC-like cells. However, another bladder cancer cell line, HTB-9, did not respond to pluripotent induction and failed to produce cells with pluripotent characteristics.

These studies indicate that, despite the consistency of induction methods, the generation of iPSCs from cancer cells does not apply to all cancer cell lines, which also hinders the research related to tumor cell reprogramming.

Selection of reprogramming methods

In the selection of reprogramming methods, the use of viruses as vectors will have a negative impact on the differentiation ability of iPSCs and may even lead to malignant tumors (Bouma et al., 2017). In addition, with the increase of iPSC passage algebra, the occurrence of abnormal karyotype will also lead to the tumorigenicity of iPSCs (Schlaeger et al., 2015). Due to the low stability of exogenous mRNA, the reprogramming process requires continuous transfection for several days, and so, it is laborious, error-prone, and expensive for the alternative unintegrated carrier mRNA (Gaignerie et al., 2018). Moreover, the selection of reprogramming methods faces many challenges and technical difficulties in translating iPSC technology into clinical practice.

Many approaches that mediate reprogramming of human and cancer cells, such as transcription factors (Deng et al., 2021), small molecules (Guan et al., 2022), microRNA (Miyazaki et al., 2015), CRISPR-Cas9 (Yoo et al., 2022), and liposome nanoparticles (Liu et al., 2022), have made great achievements, making them increasingly useful and convenient for preclinical application. We believe that as technology advances, this bottleneck will be solved.

Conclusion and Prospect

At present, the application of iPSC technology in cancer research is more and more extensive. Countless research results have reported that iPSC, as a tool for cancer research, brings great convenience to cancer research. We reviewed the application of iPSC technology in cancer research in the past 10 years. As a new technology, iPSC technology has played a huge role in cancer research, which is mainly manifested in the establishment of cancer models. Providing a source of personalized immunotherapy, epigenetic reprogramming and terminal differentiation reduce cancer malignancy.

When animal models are not readily available for cancer research, cell reprogramming techniques can be used to reprogram patient-origin cells to simulate their cancer progression, providing a platform for drug testing and genetic target research. At the same time, with the development of iPSC technology, people directly reprogram cancer cells, bringing new hope to cancer treatment. Cancer cells undergoing reprogramming or subsequent differentiation show reduced tumorigenicity and malignancy. Although the road to clinical application of cancer cell reprogramming is still far away, more and more studies are solving this problem, such as using small chemical molecules to induce cell reprogramming to solve the safety problems posed by the four transcription factors.

Then, with the promotion of immunotherapy in cancer treatment, it has become a new hope in cancer treatment, but there also emerged more and more problems, such as the limited source of immune cells, low antitumor activity, short duration in vivo, immune rejection, and other problems. The limitation of immunotherapy can be well solved with the help of the multidirectional differentiation potential of iPSCs. Immune cells induced by iPSCs not only overcome the above obstacles, but also obtained higher amplification ability and immunogenicity, and had stronger antitumor activity.

iPSC technology provides many conveniences for the treatment of cancer. The effectiveness of iPSCs in cancer treatment is a double-edged sword, and the use of iPSCs in therapeutic applications must be made more cautious to make them become beneficial tools for cancer treatment research.

Footnotes

Authors' Contributions

T.G.: Visualization, writing—original draft, and writing—review and editing. Q.W.: Writing—review and editing.

Author Disclosure Statement

The authors declare they have no conflicting financial interests.

Funding Information

No funding was received for this article.

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