Cytosine deaminase expressing human mesenchymal stem cells mediated tumour regression in melanoma bearing mice

Current progress in engineered and nano-engineered mesenchymal stem cells for cancer: From mechanisms to therapy

Mesenchymal stem cells (MSCs), as self-renewing multipotent stromal cells, have been considered promising agents for cancer treatment. A large number of studies have demonstrated the valuable properties of MSC-based treatment, such as low immunogenicity and intrinsic tumor-trophic migratory properties. To enhance the potency of MSCs for therapeutic purposes, equipping MSCs with targeted delivery functions using genetic engineering is highly beneficial. Genetically engineered MSCs can express tumor suppressor agents such as pro-apoptotic, anti-proliferative, anti-angiogenic factors and act as ideal delivery vehicles. MSCs can also be loaded with nanoparticle drugs for increased efficacy and externally moderated targeting. Moreover, exosomes secreted by MSCs have important physiological properties, so they can contribute to intercellular communication and transfer cargo into targeted tumor cells. The precise role of genetically modified MSCs in tumor environments is still up for debate, but the beginning of clinical trials has been confirmed by promising results from preclinical investigations of MSC-based gene therapy for a wide range of malignancies. This review highlights the advanced techniques of engineering/nano-engineering and MSC-derived exosomes in tumor-targeted therapy.

The Dual Role of Mesenchymal Stem Cells in Cancer Pathophysiology: Pro-Tumorigenic Effects versus Therapeutic Potential

Mesenchymal stem/stromal cells (MSCs) are multipotent cells involved in numerous physiological events, including organogenesis, the maintenance of tissue homeostasis, regeneration, or tissue repair. MSCs are increasingly recognized as playing a major, dual, and complex role in cancer pathophysiology through their ability to limit or promote tumor progression. Indeed, these cells are known to interact with the tumor microenvironment, modulate the behavior of tumor cells, influence their functions, and promote distant metastasis formation through the secretion of mediators, the regulation of cell-cell interactions, and the modulation of the immune response. This dynamic network can lead to the establishment of immunoprivileged tissue niches or the formation of new tumors through the proliferation/differentiation of MSCs into cancer-associated fibroblasts as well as cancer stem cells. However, MSCs exhibit also therapeutic effects including anti-tumor, anti-proliferative, anti-inflammatory, or anti-oxidative effects. The therapeutic interest in MSCs is currently growing, mainly due to their ability to selectively migrate and penetrate tumor sites, which would make them relevant as vectors for advanced therapies. Therefore, this review aims to provide an overview of the double-edged sword implications of MSCs in tumor processes. The therapeutic potential of MSCs will be reviewed in melanoma and lung cancers.

A comprehensive review on novel targeted therapy methods and nanotechnology-based gene delivery systems in melanoma

Melanoma, a malignant form of skin cancer, has been swiftly increasing in recent years. Although there have been significant advancements in clinical treatment underlying a well-understanding of melanoma-susceptible genes and the molecular basis of melanoma pathogenesis, the permanency of response to therapy is frequently constrained by the emergence of acquired resistance and systemic toxicity. Conventional therapies, including surgical resection, chemotherapy, radiotherapy, and immunotherapy, have already been used to treat melanoma and are dependent on the cancer stage. Nevertheless, ineffective side effects and the heterogeneity of tumors pose major obstacles to the therapeutic treatment of malignant melanoma through such strategies. In light of this, advanced therapies including nucleic acid therapies (ncRNA, aptamers), suicide gene therapies, and gene therapy using tumor suppressor genes, have lately gained immense attention in the field of cancer treatment. Furthermore, nanomedicine and targeted therapy based on gene editing tools have been applied to the treatment of melanoma as potential cancer treatment approaches nowadays. Indeed, nanovectors enable delivery of the therapeutic agents into the tumor sites by passive or active targeting, improving therapeutic efficiency and minimizing adverse effects. Accordingly, in this review, we summarized the recent findings related to novel targeted therapy methods as well as nanotechnology-based gene systems in melanoma. We also discussed current issues along with potential directions for future research, paving the way for the next-generation of melanoma treatments.

The effects of mesenchymal stem cells on the chemotherapy of colorectal cancer

Colorectal cancer (CRC) has been the third commonest cancer in the world. The prognosis of patients with CRC is related to the molecular subtypes and gene mutations, which is prone to recurrence, metastasis, and drug resistance. Mesenchymal stem cells (MSCs) are a group of progenitor ones with the capabilities of self-renewal， multi-directional differentiation, and tissue re-population, which could be isolated from various kinds of tissues and be differentiated into diverse cell types. In recent years, MSCs are applied for mechanisms study of tissue repairing, graft-versus-host disease (GVHD) and autoimmune-related disease, and tumor development, with the advantages of anti-inflammation, multi-lineage differentiation, and homing capability. Integrating the chemotherapy and MSCs therapy might provide a novel treatment approach for CRC patients. In this review, we summarize the current progress in the integrated treatment of integrating the MSCs and chemotherapy for CRC.

Gene-Directed Enzyme/Prodrug Therapy of Rat Brain Tumor Mediated by Human Mesenchymal Stem Cell Suicide Gene Extracellular Vesicles In Vitro and In Vivo

Simple Summary

Extracellular vesicles— exosomes—secreted by human mesenchymal stem/stromal cells are able to cross the blood–brain barrier and internalize glioblastoma cells. We prepared exosomes possessing a gene message, the product of which is able to convert nontoxic 5-fluorocytosine to cytotoxic drug 5-fluorouracil. Such therapeutic exosomes administered intranasally, intraperitoneally, or subcutaneously to rats bearing intracerebral glioblastoma cells inhibited their growth. The treatment cured a significant number of animals.

Abstract

MSC-driven, gene-directed enzyme prodrug therapy (GDEPT) mediated by extracellular vesicles (EV) represents a new paradigm—cell-free GDEPT tumor therapy. In this study, we tested the efficacy of yeast cytosine deaminase::uracilphosphoribosyl transferase (yCD::UPRT-MSC)-exosomes, in the form of conditioned medium (CM) to inhibit the growth of C6 glioblastoma cells both in vitro and in vivo. MSCs isolated from human adipose tissue, umbilical cord, or dental pulp engineered to express the yCD::UPRT gene secreted yCD::UPRT-MSC-exosomes that in the presence of the prodrug 5-fluorocytosine (5-FC), inhibited the growth of rat C6 glioblastoma cells and human primary glioblastoma cells in vitro in a dose-dependent manner. CM from these cells injected repeatedly either intraperitoneally (i.p.) or subcutaneously (s.c.), applied intranasally (i.n.), or infused continuously by an ALZET osmotic pump, inhibited the growth of cerebral C6 glioblastomas in rats. A significant number of rats were cured when CM containing yCD::UPRT-MSC-exosomes conjugated with 5-FC was repeatedly injected i.p. or applied i.n. Cured rats were subsequently resistant to challenges with higher doses of C6 cells. Our data have shown that cell-free GDEPT tumor therapy mediated by the yCD::UPRT-MSC suicide gene EVs for high-grade glioblastomas represents a safer and more practical approach that is worthy of further investigation.

Mesenchymal stem/stromal cells in cancer therapy

The multipotent mesenchymal stem/stromal cells (MSCs), initially discovered from bone marrow in 1976, have been identified in nearly all tissues of human body now. The multipotency of MSCs allows them to give rise to osteocytes, chondrocytes, adipocytes, and other lineages. Moreover, armed with the immunomodulation capacity and tumor-homing property, MSCs are of special relevance for cell-based therapies in the treatment of cancer. However, hampered by lack of knowledge about the controversial roles that MSC plays in the crosstalk with tumors, limited progress has been made with regard to translational medicine. Therefore, in this review, we discuss the prospects of MSC-associated anticancer strategies in light of therapeutic mechanisms and signal transduction pathways. In addition, the clinical trials designed to appraise the efficacy and safety of MSC-based anticancer therapies will be assessed according to published data.

Rat Adipose-Derived Stromal Cells (ADSCs) Increases the Glioblastoma Growth and Decreases the Animal Survival

Many studies have shown that mesenchymal stromal cells (MSCs) and their secreted factors may modulate the biology of tumor cells. However, how these interactions happen in vivo remains unclear. In the present study, we investigated the effects of rat adipose-derived stromal cells (ADSCs) and their conditioned medium (ADSC-CM) in glioma tumor growth and malignancy in vivo. Our results showed that when we co-injected C6 cells plus ADSCs into the rat brains, the tumors generated were larger and the animals exhibited shorter survival, when compared with tumors of the animals that received only C6 cells or C6 cells pre-treated with ADSC-CM. We further showed that the animals that received C6 plus ADSC did not present enhanced expression of CD73 (a gene highly expressed in ADSCs), indicating that the tumor volume observed in these animals was not a mere consequence of the higher density of cells administered in this group. Finally, we showed that the animals that received C6 + ADSC presented tumors with larger necrosis areas and greater infiltration of immune cells. These results indicate that the immunoregulatory properties of ADSCs and its contribution to tumor stroma can support tumor growth leading to larger zones of necrosis, recruitment of immune cells, thus facilitating tumor progression. Our data provide new insights into the way by which ADSCs and tumor cells interact and highlight the importance of understanding the fate and roles of MSCs in tumor sites in vivo, as well as their intricate crosstalk with cancer cells.

Mesenchymal Stem Cells Engineered by Nonviral Vectors: A Powerful Tool in Cancer Gene Therapy

Due to their "tumor homing" and "immune privilege" characteristics, the use of mesenchymal stem cells (MSCs) has been proposed as a novel tool against cancer. MSCs are genetically engineered in vitro and then utilized to deliver tumoricidal agents, including prodrugs and bioactive molecules, to tumors. The genetic modification of MSCs can be achieved by various vectors, and in most cases viral vectors are used; however, viruses may be associated with carcinogenesis and immunogenicity, restricting their clinical translational potential. As such, nonviral vectors have emerged as a potential solution to address these limitations and have gradually attracted increasing attention. In this review, we briefly revisit the current knowledge about MSC-based cancer gene therapy. Then, we summarize the advantages and challenges of nonviral vectors for MSC transfection. Finally, we discuss recent advances in the development of new nonviral vectors, which have provided promising strategies to overcome obstacles in the gene modulation of MSCs.

Intravenously Infused Stem Cells for Cancer Treatment

Despite the recent influx of immunotherapies and small molecule drugs to treat tumors, cancer remains a leading cause of death in the United States, in large part due to the difficulties of treating metastatic cancer. Stem cells, which are inherently tumoritropic, provide a useful drug delivery vehicle to target both primary and metastatic tumors. Intravenous infusions of stem cells carrying or secreting therapeutic payloads show significant promise in the treatment of cancer. Stem cells may be engineered to secrete cytotoxic products, loaded with oncolytic viruses or nanoparticles containing small molecule drugs, or conjugated with immunotherapies. Herein we describe these preclinical and clinical studies, discuss the distribution and migration of stem cells following intravenous infusion, and examine both the limitations of and the methods to improve the migration and therapeutic efficacy of tumoritropic, therapeutic stem cells.

Targeting and Reprograming Cancer-Associated Fibroblasts and the Tumor Microenvironment in Pancreatic Cancer

Pancreatic cancer is the fourth leading cause of cancer deaths in the United States both in female and male, and is projected to become the second deadliest cancer by 2030. The overall five-year survival rate remains at around 10%. Pancreatic cancer exhibits a remarkable resistance to established therapeutic options such as chemotherapy and radiotherapy, due to dense stromal tumor microenvironment. Cancer-associated fibroblasts are the major stromal cell type and source of extracellular matrix proteins shaping a physical and metabolic barrier thereby reducing therapeutic efficacy. Targeting cancer-associated fibroblasts has been considered a promising therapeutic strategy. However, depleting cancer-associated fibroblasts may also have tumor-promoting effects due to their functional heterogeneity. Several subtypes of cancer-associated fibroblasts have been suggested to exhibit tumor-restraining function. This review article summarizes recent preclinical and clinical investigations addressing pancreatic cancer therapy through targeting specific subtypes of cancer-associated fibroblasts, deprogramming activated fibroblasts, administration of mesenchymal stem cells, as well as reprogramming tumor-promoting cancer-associated fibroblasts to tumor-restraining cancer-associated fibroblasts. Further, inter-cellular mediators between cancer-associated fibroblasts and the surrounding tissue microenvironment are discussed. It is important to increase our understanding of cancer-associated fibroblast heterogeneity and the tumor microenvironment for more specific and personalized therapies for pancreatic cancer patients in the future.

Enhancement of the Therapeutic Capacity of Mesenchymal Stem Cells by Genetic Modification: A Systematic Review

Background

The therapeutic capacity of mesenchymal stem cells (also known as mesenchymal stromal cells/MSCs) depends on their ability to respond to the need of the damaged tissue by secreting beneficial paracrine factors. MSCs can be genetically engineered to express certain beneficial factors. The aim of this systematic review is to compile and analyze published scientific literatures that report the use of engineered MSCs for the treatment of various diseases/conditions, to discuss the mechanisms of action, and to assess the efficacy of engineered MSC treatment.

Methods

We retrieved all published studies in PubMed/MEDLINE and Cochrane Library on July 27, 2019, without time restriction using the following keywords: “engineered MSC” and “therapy” or “manipulated MSC” and “therapy.” In addition, relevant articles that were found during full text search were added. We identified 85 articles that were reviewed in this paper.

Results

Of the 85 articles reviewed, 51 studies reported the use of engineered MSCs to treat tumor/cancer/malignancy/metastasis, whereas the other 34 studies tested engineered MSCs in treating non-tumor conditions. Most of the studies reported the use of MSCs in animal models, with only one study reporting a trial in human subjects. Thirty nine studies showed that the expression of beneficial paracrine factors would significantly enhance the therapeutic effects of the MSCs, whereas thirty three studies showed moderate effects, and one study in humans reported no effect. The mechanisms of action for MSC-based cancer treatment include the expression of “suicide genes,” induction of tumor cell apoptosis, and delivery of cytokines to induce an immune response against cancer cells. In the context of the treatment of non-cancerous diseases, the mechanism described in the reviewed papers included the expression of angiogenic, osteogenic, and growth factors.

Conclusion

The therapeutic capacity of MSCs can be enhanced by inducing the expression of certain paracrine factors by genetic modification. Genetically engineered MSCs have been used successfully in various animal models of diseases. However, the results should be interpreted cautiously because animal models might not perfectly represent real human diseases. Therefore, further studies are needed to explore the translational potential of genetically engineered MSCs.

Arming Mesenchymal Stromal/Stem Cells Against Cancer: Has the Time Come?

Since mesenchymal stromal/stem cells (MSCs) were discovered, researchers have been drawn to study their peculiar biological features, including their immune privileged status and their capacity to selectively migrate into inflammatory areas, including tumors. These properties make MSCs promising cellular vehicles for the delivery of therapeutic molecules in the clinical setting. In recent decades, the engineering of MSCs into biological vehicles carrying anticancer compounds has been achieved in different ways, including the loading of MSCs with chemotherapeutics or drug functionalized nanoparticles (NPs), genetic modifications to force the production of anticancer proteins, and the use of oncolytic viruses. Recently, it has been demonstrated that wild-type and engineered MSCs can release extracellular vesicles (EVs) that contain therapeutic agents. Despite the enthusiasm for MSCs as cyto-pharmaceutical agents, many challenges, including controlling the fate of MSCs after administration, must still be considered. Preclinical results demonstrated that MSCs accumulate in lung, liver, and spleen, which could prevent their engraftment into tumor sites. For this reason, physical, physiological, and biological methods have been implemented to increase MSC concentration in the target tumors. Currently, there are more than 900 registered clinical trials using MSCs. Only a small fraction of these are investigating MSC-based therapies for cancer, but the number of these clinical trials is expected to increase as technology and our understanding of MSCs improve. This review will summarize MSC-based antitumor therapies to generate an increasing awareness of their potential and limits to accelerate their clinical translation.

A highly efficient non-viral process for programming mesenchymal stem cells for gene directed enzyme prodrug cancer therapy

Mesenchymal stem cells (MSCs) driven gene-directed enzyme prodrug therapy has emerged as a potential strategy for cancer treatment. The tumour-nesting properties of MSCs enable these vehicles to target tumours and metastases with effective therapies. A crucial step in engineering MSCs is the delivery of genetic material with low toxicity and high efficiency. Due to the low efficiency of current transfection methods, viral vectors are used widely to modify MSCs in preclinical and clinical studies. We show, for the first time, the high transfection efficiency (> 80%) of human adipose tissue derived-MSCs (AT-MSCs) using a cost-effective and off-the-shelf Polyethylenimine, in the presence of histone deacetylase 6 inhibitor and fusogenic lipids. Notably, the phenotypes of MSCs remained unchanged post-modification. AT-MSCs engineered with a fused transgene, yeast cytosine deaminase::uracil phosphoribosyltransferase (CDy::UPRT) displayed potent cytotoxic effects against breast, glioma, gastric cancer cells in vitro. The efficiency of eliminating gastric cell lines were effective even when using 7-day post-transfected AT-MSCs, indicative of the sustained expression and function of the therapeutic gene. In addition, significant inhibition of temozolomide resistant glioma tumour growth in vivo was observed with a single dose of therapeutic MSC. This study demonstrated an efficient non-viral modification process for MSC-based prodrug therapy.

Napabucasin overcomes cisplatin resistance in ovarian germ cell tumor-derived cell line by inhibiting cancer stemness

Background

Cisplatin resistance of ovarian yolk sac tumors (oYST) is a clinical challenge due to dismal patient prognosis, even though the disease is extremely rare. We investigated potential association between cisplatin resistance and cancer stem cell (CSC) markers in chemoresistant oYST cells and targeting strategies to overcome resistance in oYST.

Methods

Chemoresistant cells were derived from chemosensitive human oYST cells by cultivation in cisplatin in vitro. Derivative cells were characterized by chemoresistance, functional assays, flow cytometry, gene expression and protein arrays focused on CSC markers. RNAseq, methylation and microRNA profiling were performed. Quail chorioallantoic membranes (CAM) with implanted oYST cells were used to analyze the micro-tumor extent and interconnection with the CAM. Tumorigenicity in vivo was determined on immunodeficient mouse model. Chemoresistant cells were treated by inhibitors intefering with the CSC properties to examine the chemosensitization to cisplatin.

Results

Long-term cisplatin exposure resulted in seven-fold higher IC₅₀ value in resistant cells, cross-resistance to oxaliplatin and carboplatin, and increased migratory capacity, invasiveness and tumorigenicity, associated with hypomethylation of differentially methylated genes/promotors. Resistant cells exhibited increased expression of prominin-1 (CD133), ATP binding cassette subfamily G member 2 (ABCG2), aldehyde dehydrogenase 3 isoform A1 (ALDH3A1), correlating with reduced gene and promoter methylation, as well as increased expression of ALDH1A3 and higher overall ALDH enzymatic activity, rendering them cross-resistant to DEAB, disulfiram and napabucasin. Salinomycin and tunicamycin were significantly more toxic to resistant cells. Pretreatment with napabucasin resensitized the cells to cisplatin and reduced their tumorigenicity in vivo.

Conclusions

The novel chemoresistant cells represent unique model of refractory oYST. CSC markers are associated with cisplatin resistance being possible targets in chemorefractory oYST.

Mesoporous Silica Nanoparticles for Co-Delivery of Drugs and Nucleic Acids in Oncology: A Review

Mesoporous silica nanoparticles have attracted much attention in recent years as drug and gene delivery systems for biomedical applications. Among their most beneficial features for biomedicine, we can highlight their biocompatibility and their outstanding textural properties, which provide a great loading capacity for many types of cargos. In the context of cancer nanomedicine, combination therapy and gene transfection/silencing have recently been highlighted as two of its most promising fields. In this review, we aim to provide an overview of the different small molecule drug-nucleic acid co-delivery combinations that have been developed using mesoporous silica nanoparticles as carriers. By carefully selecting the chemotherapeutic drug and nucleic acid cargos to be co-delivered by mesoporous silica nanoparticles, different therapeutic goals can be achieved by overcoming resistance mechanisms, combining different cytotoxic mechanisms, or providing an additional antiangiogenic effect. The examples here presented highlight the great promise of this type of strategies for the development of future therapeutics.

Suicide Gene Therapy Mediated with Exosomes Produced by Mesenchymal Stem/Stromal Cells Stably Transduced with HSV Thymidine Kinase

Mesenchymal stem/stromal cells (MSCs) prepared from various human tissues were stably transduced with the suicide gene herpes simplex virus thymidine kinase (HSVTK) by means of retrovirus infection. HSVTK-transduced MSCs express the suicide gene and in prodrug ganciclovir (GCV) presence induced cell death by intracellular conversion of GCV to GCV-triphosphate. The homogenous population of HSVTK-MSCs were found to release exosomes having mRNA of the suicide gene in their cargo. The exosomes were easily internalized by the tumor cells and the presence of ganciclovir caused their death in a dose-dependent manner. Efficient tumor cell killing of glioma cell lines and primary human glioblastoma cells mediated by HSVTK-MSC exosomes is reported. Exosomes produced by suicide gene transduced MSCs represent a new class of highly selective tumor cell targeted drug acting intracellular with curative potential.

Recent Progress of Stem Cell Therapy in Cancer Treatment: Molecular Mechanisms and Potential Applications

The insufficient and unspecific target of traditional therapeutic approaches in cancer treatment often leads to therapy resistance and cancer recurrence. Over the past decades, accumulating discoveries about stem cell biology have provided new potential approaches to cure cancer patients. Stem cells possess unique biological actions, including self-renewal, directional migration, differentiation, and modulatory effects on other cells, which can be utilized as regenerative medicine, therapeutic carriers, drug targeting, and generation of immune cells. In this review, we emphasize the mechanisms underlying the use of various types of stem cells in cancer treatment. In addition, we summarize recent progress in the clinical applications of stem cells, as well as common risks of this therapy. We finally give general directions for future studies, aiming to improve overall outcomes in the fight against cancer.

Therapeutic Mesenchymal Stromal Cells for Immunotherapy and for Gene and Drug Delivery

Mesenchymal stromal cells (MSCs) possess several fairly unique properties that, when combined, make them ideally suited for cellular-based immunotherapy and as vehicles for gene and drug delivery for a wide range of diseases and disorders. Key among these are: (1) their relative ease of isolation from a variety of tissues; (2) the ability to be expanded in culture without a loss of functionality, a property that varies to some degree with tissue source; (3) they are relatively immune-inert, perhaps obviating the need for precise donor/recipient matching; (4) they possess potent immunomodulatory functions that can be tailored by so-called licensing in vitro and in vivo; (5) the efficiency with which they can be modified with viral-based vectors; and (6) their almost uncanny ability to selectively home to damaged tissues, tumors, and metastases following systemic administration. In this review, we summarize the latest research in the immunological properties of MSCs, their use as immunomodulatory/anti-inflammatory agents, methods for licensing MSCs to customize their immunological profile, and their use as vehicles for transferring both therapeutic genes in genetic disease and drugs and genes designed to destroy tumor cells.

Disulfiram Overcomes Cisplatin Resistance in Human Embryonal Carcinoma Cells

Cisplatin resistance in testicular germ cell tumors (TGCTs) is a clinical challenge. We investigated the underlying mechanisms associated with cancer stem cell (CSC) markers and modalities circumventing the chemoresistance. Chemoresistant models (designated as CisR) of human embryonal carcinoma cell lines NTERA-2 and NCCIT were derived and characterized using flow cytometry, gene expression, functional and protein arrays. Tumorigenicity was determined on immunodeficient mouse model. Disulfiram was used to examine chemosensitization of resistant cells. ALDH1A3 isoform expression was evaluated by immunohistochemistry in 216 patients' tissue samples. Chemoresistant cells were significantly more resistant to cisplatin, carboplatin and oxaliplatin compared to parental cells. NTERA-2 CisR cells exhibited altered morphology and increased tumorigenicity. High ALDH1A3 expression and increased ALDH activity were detected in both refractory cell lines. Disulfiram in combination with cisplatin showed synergy for NTERA-2 CisR and NCCIT CisR cells and inhibited growth of NTERA-2 CisR xenografts. Significantly higher ALDH1A3 expression was detected in TGCTs patients' tissue samples compared to normal testicular tissue. We characterized novel clinically relevant model of chemoresistant TGCTs, for the first time identified the ALDH1A3 as a therapeutic target in TGCTs and more importantly, showed that disulfiram represents a viable treatment option for refractory TGCTs.

Engineered Mesenchymal Stem Cells for Targeting Solid Tumors: Therapeutic Potential beyond Regenerative Therapy

Mesenchymal stem cells (MSCs) have previously demonstrated considerable promise in regenerative medicine based on their ability to proliferate and differentiate into cells of different lineages. More recently, there has been a significant interest in using MSCs as cellular vehicles for targeted cancer therapy by exploiting their tumor homing properties. Initial studies focused on using genetically modified MSCs for targeted delivery of various proapoptotic, antiangiogenic, and therapeutic proteins to a wide variety of tumors. However, their use as drug delivery vehicles has been limited by poor drug load capacity. This review discusses various strategies for the nongenetic modification of MSCs that allows their use in tumor-targeted delivery of small molecule chemotherapeutic agents. SIGNIFICANCE STATEMENT: There has been considerable interest in exploiting the tumor homing potential of MSCs to develop them as a vehicle for the targeted delivery of cytotoxic agents to tumor tissue. The inherent tumor-tropic and drug-resistant properties make MSCs ideal carriers for toxic payload. While significant progress has been made in the area of the genetic modification of MSCs, studies focused on identification of molecular mechanisms that contribute to the tumor tropism along with optimization of the engineering conditions can further improve their effectiveness as drug delivery vehicles.

Mesenchymal stromal cells for bone sarcoma treatment: Roadmap to clinical practice

Over the past few decades, there has been growing interest in understanding the molecular mechanisms of cancer pathogenesis and progression, as it is still associated with high morbidity and mortality. Current management of large bone sarcomas typically includes the complex therapeutic approach of limb salvage or sacrifice combined with pre- and postoperative multidrug chemotherapy and/or radiotherapy, and is still associated with high recurrence rates. The development of cellular strategies against specific characteristics of tumour cells appears to be promising, as they can target cancer cells selectively. Recently, Mesenchymal Stromal Cells (MSCs) have been the subject of significant research in orthopaedic clinical practice through their use in regenerative medicine. Further research has been directed at the use of MSCs for more personalized bone sarcoma treatments, taking advantage of their wide range of potential biological functions, which can be augmented by using tissue engineering approaches to promote healing of large defects. In this review, we explore the use of MSCs in bone sarcoma treatment, by analyzing MSCs and tumour cell interactions, transduction of MSCs to target sarcoma, and their clinical applications on humans concerning bone regeneration after bone sarcoma extraction.

Isocytosine deaminase Vcz as a novel tool for the prodrug cancer therapy

Background

The cytosine deaminase (CD)/5-fluorocytosine (5-FC) system is among the best explored enzyme/prodrug systems in the field of the suicide gene therapy. Recently, by the screening of the environmental metagenomic libraries we identified a novel isocytosine deaminase (ICD), termed Vcz, which is able of specifically converting a prodrug 5-fluoroisocytosine (5-FIC) into toxic drug 5-fluorouracil (5-FU). The aim of this study is to test the applicability of the ICD Vcz / 5-FIC pair as a potential suicide gene therapy tool.

Methods

Vcz-expressing human glioblastoma U87 and epithelial colorectal adenocarcinoma Caco-2 cells were treated with 5-FIC, and the Vcz-mediated cytotoxicity was evaluated by performing an MTT assay. In order to examine anti-tumor effects of the Vcz/5-FIC system in vivo, murine bone marrow-derived mesenchymal stem cells (MSC) were transduced with the Vcz-coding lentivirus and co-injected with 5-FIC or control reagents into subcutaneous GL261 tumors evoked in C57/BL6 mice.

Results

5-FIC alone showed no significant toxic effects on U87 and Caco-2 cells at 100 μM concentration, whereas the number of cells of both cell lines that express Vcz cytosine deaminase gene decreased by approximately 60% in the presence of 5-FIC. The cytotoxic effects on cells were also induced by media collected from Vcz-expressing cells pre-treated with 5-FIC. The co-injection of the Vcz-transduced mesenchymal stem cells and 5-FIC have been shown to augment tumor necrosis and increase longevity of tumorized mice by 50% in comparison with control group animals.

Conclusions

We have confirmed that the novel ICD Vcz together with the non-toxic prodrug 5-FIC has a potential of being a new enzyme/prodrug system for suicide gene therapy.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-5409-7) contains supplementary material, which is available to authorized users.

Mesenchymal Stem Cell Exosome-Mediated Prodrug Gene Therapy for Cancer

Exosomes derived from human mesenchymal stem cells (MSCs) engineered to express the suicide gene yeast cytosine deaminase::uracil phosphoribosyl transferase (yCD::UPRT) represent a new therapeutic approach for tumor-targeted innovative therapy. The yCD::UPRT-MSC-exosomes carry mRNA of the suicide gene in their cargo. Upon internalization by tumor cells, the exosomes inhibit the growth of broad types of cancer cells in vitro, in the presence of a prodrug. Here we describe the method leading to the production and testing of these therapeutic exosomes. The described steps include the preparation of replication-deficient retrovirus possessing the yCD::UPRT suicide gene, and the preparation and selection of MSCs transduced with yCD::UPRT suicide gene. We present procedures to obtain exosomes possessing the ability to induce the death of tumor cells. In addition, we highlight methods for the evaluation of the suicide gene activity of yCD::UPRT-MSC-exosomes.

Suicide-gene transfection of tumor-tropic placental stem cells employing ultrasound-responsive nanoparticles

A Trojan-horse strategy for cancer therapy employing tumor-tropic mesenchymal stem cells transfected with a non-viral nanovector is here presented. In this sense, ultrasound-responsive mesoporous silica nanoparticles were coated with a polycation (using two different molecular weights), providing them with gene transfection capabilities that were evaluated using two different plasmids. First, the expression of Green Fluorescent Protein was analyzed in Decidua-derived Mesenchymal Stem Cells after incubation with the silica nanoparticles. The most successful nanoparticle was then employed to induce the expression of two suicide genes: cytosine deaminase and uracil phosphoribosyl transferase, which allow the cells to convert a non-toxic pro-drug (5-fluorocytosine) into a toxic drug (5-Fluorouridine monophosphate). The effect of the production of the toxic final product was also evaluated in a cancer cell line (NMU cells) co-cultured with the transfected vehicle cells, Decidua-derived Mesenchymal Stem Cells. STATEMENT OF SIGNIFICANCE: Cell-mediated cancer therapy has recently attracted great interest. Tumor-homing cells can exert anticancer effects through innate capacities, via transfection with a therapeutic gene or acting as vehicles of therapeutic nanoparticles. In this work, an ultrasound-responsive mesoporous silica nanoparticle (capable of carrying an anticancer drug) is engineered to act as a non-viral transfection agent for tumor-tropic human placental mesenchymal stem cells. The successful transfection of the vehicle cells is evaluated employing different expression plasmids. After transfection with two suicide genes, the vehicle cells are capable of converting a non-toxic pro-drug into a highly toxic molecule, which can also kill surrounding cancer cells in an in vitro co-culture model. This work opens the gate for a plethora of strategies in which both genes and drug-loaded nanoparticles can be transported towards tumor tissues by easily available human mesenchymal stem cells.

Mesenchymal stem cells in preclinical cancer cytotherapy: a systematic review

Mesenchymal stem cells (MSC) comprise a heterogeneous population of rapidly proliferating cells that can be isolated from adult (e.g., bone marrow, adipose tissue) as well as fetal (e.g., umbilical cord) tissues (termed bone marrow (BM)-, adipose tissue (AT)-, and umbilical cord (UC)-MSC, respectively) and are capable of differentiation into a wide range of non-hematopoietic cell types. An additional, unique attribute of MSC is their ability to home to tumor sites and to interact with the local supportive microenvironment which rapidly conceptualized into MSC-based experimental cancer cytotherapy at the turn of the century. Towards this purpose, both naïve (unmodified) and genetically modified MSC (GM-MSC; used as delivery vehicles for the controlled expression and release of antitumorigenic molecules) have been employed using well-established in vitro and in vivo cancer models, albeit with variable success. The first approach is hampered by contradictory findings regarding the effects of naïve MSC of different origins on tumor growth and metastasis, largely attributed to inherent biological heterogeneity of MSC as well as experimental discrepancies. In the second case, although the anti-cancer effect of GM-MSC is markedly improved over that of naïve cells, it is yet apparent that some protocols are more efficient against some types of cancer than others. Regardless, in order to maximize therapeutic consistency and efficacy, a deeper understanding of the complex interaction between MSC and the tumor microenvironment is required, as well as examination of the role of key experimental parameters in shaping the final cytotherapy outcome. This systematic review represents, to the best of our knowledge, the first thorough evaluation of the impact of experimental anti-cancer therapies based on MSC of human origin (with special focus on human BM-/AT-/UC-MSC). Importantly, we dissect the commonalities and differences as well as address the shortcomings of work accumulated over the last two decades and discuss how this information can serve as a guide map for optimal experimental design implementation ultimately aiding the effective transition into clinical trials.

Tumor-Targeted Immunotherapy by Using Primary Adipose-Derived Stem Cells and an Antigen-Specific Protein Vaccine

Cancer is a leading cause of mortality and a major public health problem worldwide. For biological therapy against cancer, we previously developed a unique immunotherapeutic platform by combining mesenchymal stem cells with an antigen-specific protein vaccine. However, this system possesses a few limitations, such as improperly immortalized mesenchymal stem cells (MSCs) along with transfected oncogenic antigens in them. To overcome the limitations of this platform for future clinical application, we freshly prepared primary adipose-derived stem cells (ADSCs) and modified the E7' antigen (E7') as a non-oncogenic protein. Either subcutaneously co-inoculated with cancer cells or systemically administered after tumor growth, ADSC labeled with enhanced green fluorescent protein (eGFP) and combined with modified E7' (ADSC-E7'-eGFP) cells showed significant antitumor activity when combined with the protein vaccine in both colon and lung cancer in mice. Specifically, this combined therapy inhibited tumor through inducing cell apoptosis. The significantly reduced endothelial cell markers, CD31 and vascular endothelial growth factor (VEGF), indicated strongly inhibited tumor angiogenesis. The activated immune system was demonstrated through the response of CD4+ T and natural killer (NK) cells, and a notable antitumor activity might be contributed by CD8+ T cells. Conclusively, these evidences imply that this promising immunotherapeutic platform might be a potential candidate for the future clinical application against cancer.

Prodrug suicide gene therapy for cancer targeted intracellular by mesenchymal stem cell exosomes

The natural behavior of mesenchymal stem cells (MSCs) and their exosomes in targeting tumors is a promising approach for curative therapy. Human tumor tropic mesenchymal stem cells (MSCs) isolated from various tissues and MSCs engineered to express the yeast cytosine deaminase::uracil phosphoribosyl transferase suicide fusion gene (yCD::UPRT-MSCs) released exosomes in conditional medium (CM). Exosomes from all tissue specific yCD::UPRT-MSCs contained mRNA of the suicide gene in the exosome's cargo. When the CM was applied to tumor cells, the exosomes were internalized by recipient tumor cells and in the presence of the prodrug 5-fluorocytosine (5-FC) effectively triggered dose-dependent tumor cell death by endocytosed exosomes via an intracellular conversion of the prodrug 5-FC to 5-fluorouracil. Exosomes were found to be responsible for the tumor inhibitory activity. The presence of microRNAs in exosomes produced from naive MSCs and from suicide gene transduced MSCs did not differ significantly. MicroRNAs from yCD::UPRT-MSCs were not associated with therapeutic effect. MSC suicide gene exosomes represent a new class of tumor cell targeting drug acting intracellular with curative potential.

Mechanoresponsive stem cells to target cancer metastases through biophysical cues

Despite decades of effort, little progress has been made to improve the treatment of cancer metastases. To leverage the central role of the mechanoenvironment in cancer metastasis, we present a mechanoresponsive cell system (MRCS) to selectively identify and treat cancer metastases by targeting the specific biophysical cues in the tumor niche in vivo. Our MRCS uses mechanosensitive promoter-driven mesenchymal stem cell (MSC)-based vectors, which selectively home to and target cancer metastases in response to specific mechanical cues to deliver therapeutics to effectively kill cancer cells, as demonstrated in a metastatic breast cancer mouse model. Our data suggest a strong correlation between collagen cross-linking and increased tissue stiffness at the metastatic sites, where our MRCS is specifically activated by the specific cancer-associated mechano-cues. MRCS has markedly reduced deleterious effects compared to MSCs constitutively expressing therapeutics. MRCS indicates that biophysical cues, specifically matrix stiffness, are appealing targets for cancer treatment due to their long persistence in the body (measured in years), making them refractory to the development of resistance to treatment. Our MRCS can serve as a platform for future diagnostics and therapies targeting aberrant tissue stiffness in conditions such as cancer and fibrotic diseases, and it should help to elucidate mechanobiology and reveal what cells "feel" in the microenvironment in vivo.

Nano-Engineered Mesenchymal Stem Cells Increase Therapeutic Efficacy of Anticancer Drug Through True Active Tumor Targeting

Tumor-targeted drug delivery has the potential to improve therapeutic efficacy and mitigate non-specific toxicity of anticancer drugs. However, current drug delivery approaches rely on inefficient passive accumulation of the drug carrier in the tumor. We have developed a unique, truly active tumor-targeting strategy that relies on engineering mesenchymal stem cells (MSC) with drug-loaded nanoparticles. Our studies using the A549 orthotopic lung tumor model show that nano-engineered MSCs carrying the anticancer drug paclitaxel (PTX) home to tumors and create cellular drug depots that release the drug payload over several days. Despite significantly lower doses of PTX, nano-engineered MSCs resulted in significant inhibition of tumor growth and superior survival. Anticancer efficacy of nano-engineered MSCs was confirmed in immunocompetent C57BL/6 albino female mice bearing orthotopic Lewis Lung Carcinoma (LL/2-luc) tumors. Furthermore, at doses that resulted in equivalent therapeutic efficacy, nano-engineered MSCs had no effect on white blood cell count, whereas PTX solution and PTX nanoparticle treatments caused leukopenia. Biodistribution studies showed that nano-engineered MSCs resulted in greater than 9-fold higher AUC_lung of PTX (1.5 μg.day/g) than PTX solution and nanoparticles (0.2 and 0.1 μg.day/g tissue, respectively) in the target lung tumors. Furthermore, the lung-to-liver and the lung-to-spleen ratios of PTX were several folds higher for nano-engineered MSCs relative to those for PTX solution and nanoparticle groups, suggesting that nano-engineered MSCs demonstrate significantly less off-target deposition. In summary, our results demonstrate that nano-engineered MSCs can serve as an efficient carrier for tumor-specific drug delivery and significantly improved anti-cancer efficacy of conventional chemotherapeutic drugs. Mol Cancer Ther; 17(6); 1196-206. ©2018 AACR.

Application of Mesenchymal Stem Cells for Therapeutic Agent Delivery in Anti-tumor Treatment

Mesenchymal stem cells (MSCs) are non-hematopoietic progenitor cells, which can be isolated from different types of tissues including bone marrow, adipose tissue, tooth pulp, and placenta/umbilical cord blood. There isolation from adult tissues circumvents the ethical concerns of working with embryonic or fetal stem cells, whilst still providing cells capable of differentiating into various cell lineages, such as adipocytes, osteocytes and chondrocytes. An important feature of MSCs is the low immunogenicity due to the lack of co-stimulatory molecules expression, meaning there is no need for immunosuppression during allogenic transplantation. The tropism of MSCs to damaged tissues and tumor sites makes them a promising vector for therapeutic agent delivery to tumors and metastatic niches. MSCs can be genetically modified by virus vectors to encode tumor suppressor genes, immunomodulating cytokines and their combinations, other therapeutic approaches include MSCs priming/loading with chemotherapeutic drugs or nanoparticles. MSCs derived membrane microvesicles (MVs), which play an important role in intercellular communication, are also considered as a new therapeutic agent and drug delivery vector. Recruited by the tumor, MSCs can exhibit both pro- and anti-oncogenic properties. In this regard, for the development of new methods for cancer therapy using MSCs, a deeper understanding of the molecular and cellular interactions between MSCs and the tumor microenvironment is necessary. In this review, we discuss MSC and tumor interaction mechanisms and review the new therapeutic strategies using MSCs and MSCs derived MVs for cancer treatment.

Engineering Hematopoietic Cells for Cancer Immunotherapy: Strategies to Address Safety and Toxicity Concerns

Advances in cancer immunotherapies utilizing engineered hematopoietic cells have recently generated significant clinical successes. Of great promise are immunotherapies based on chimeric antigen receptor-engineered T (CAR-T) cells that are targeted toward malignant cells expressing defined tumor-associated antigens. CAR-T cells harness the effector function of the adaptive arm of the immune system and redirect it against cancer cells, overcoming the major challenges of immunotherapy, such as breaking tolerance to self-antigens and beating cancer immune system-evasion mechanisms. In early clinical trials, CAR-T cell-based therapies achieved complete and durable responses in a significant proportion of patients. Despite clinical successes and given the side effect profiles of immunotherapies based on engineered cells, potential concerns with the safety and toxicity of various therapeutic modalities remain. We discuss the concerns associated with the safety and stability of the gene delivery vehicles for cell engineering and with toxicities due to off-target and on-target, off-tumor effector functions of the engineered cells. We then overview the various strategies aimed at improving the safety of and resolving toxicities associated with cell-based immunotherapies. Integrating failsafe switches based on different suicide gene therapy systems into engineered cells engenders promising strategies toward ensuring the safety of cancer immunotherapies in the clinic.

Mesenchymal Stromal/Stem Cells: A New Era in the Cell-Based Targeted Gene Therapy of Cancer

In recent years, in light of the promising potentials of mesenchymal stromal/stem cells (MSCs) for carrying therapeutic anticancer genes, a complete revisitation on old chemotherapy-based paradigms has been established. This review attempted to bring forward and introduce the novel therapeutic opportunities of using genetically engineered MSCs. The simplicities and advantages of MSCs for medical applications make them a unique and promising option in the case of cancer therapy. Some of the superiorities of using MSCs as therapeutic gene micro-carriers are the easy cell-extraction procedures and their abundant proliferation capacity in vitro without losing their main biological properties. Targeted therapy by using MSCs as the delivery vehicles of therapeutic genes is a new approach in the treatment of various types of cancers. Some of the distinct properties of MSCs, such as tumor-tropism, non-immunogenicity, stimulatory effect on the anti-inflammatory molecules, inhibitory effect on inflammatory responses, non-toxicity against the normal tissues, and easy processes for the clinical use, have formed the basis of attention to MSCs. They can be easily used for the treatment of damaged or injured tissues, regenerative medicine, and immune disorders. This review focused on the drugability of MSCs and their potential for the delivery of candidate anticancer genes. It also briefly reviewed the vectors and methods used for MSC-mediated gene therapy of malignancies. Also, the challenges, limitations, and considerations in using MSCs for gene therapy of cancer and the new methods developed for resolution of these problems are reviewed.

Mesenchymal stem cells as therapeutic agents and in gene delivery for the treatment of glioma*

Mesenchymal stem cells (MSCs) are plastic-adherent cells with a characteristic surface phenotype and properties of self-renewal, differentiation, and high proliferative potential. The characteristics of MSCs and their tumor-tropic capability make them an ideal tool for use in cell-based therapies for cancer, including glioma. These cells can function either through a bystander effect or as a delivery system for genes and drugs. MSCs have been demonstrated to inhibit the growth of glioma and to improve survival following transplantation into the brain. We briefly review the current data regarding the use of MSCs in the treatment of glioma and discuss the potential strategies for development of a more specific and effective therapy.

Cell-mediated enzyme prodrug cancer therapies

Cell-directed gene therapy is a promising new frontier for the field of targeted cancer therapies. Here we discuss the current pre-clinical and clinical use of cell-mediated enzyme prodrug therapy (EPT) directed against solid tumors and avenues for further development. We also discuss some of the challenges encountered upon translating these therapies to clinical trials. Upon sufficient development, cell-mediated enzyme prodrug therapy has the potential to maximize the distribution of therapeutic enzymes within the tumor environment, localizing conversion of prodrug to active drug at the tumor sites thereby decreasing off-target toxicities. New combinatorial possibilities are also promising. For example, when combined with viral gene-delivery vehicles, this may result in new hybrid vehicles that attain heretofore unmatched levels of therapeutic gene expression within the tumor.

Targeted antitumor therapy mediated by prodrug-activating mesenchymal stromal cells

Mesenchymal stromal cells (MSCs) were introduced as tumor-targeted vehicles suitable for delivery of the gene-directed enzyme/prodrug therapy more than 10 years ago. Over these years key properties of tumor cells and MSCs, which are crucial for the treatment efficiency, were examined; and there are some critical issues to be considered for the maximum antitumor effect. Moreover, engineered MSCs expressing enzymes capable of activating non-toxic prodrugs achieved long-term curative effect even in metastatic and hard-to-treat tumor types in pre-clinical scenario(s). These gene-modified MSCs are termed prodrug-activating MSCs throughout the text and represent promising approach for further clinical application. This review summarizes major determinants to be considered for the application of the prodrug-activating MSCs in antitumor therapy in order to maximize therapeutic efficiency.

Genetically engineered suicide gene in mesenchymal stem cells using a Tet-On system for anaplastic thyroid cancer

Anaplastic thyroid cancer (ATC) is the most aggressive malignancy of the thyroid, during which undifferentiated tumors arise from the thyroid follicular epithelium. ATC has a very poor prognosis due to its aggressive behavior and poor response to conventional therapies. Gene-directed enzyme/prodrug therapy using genetically engineered mesenchymal stromal cells (MSC) is a promising therapeutic strategy. The doxycycline (DOX)-controlled Tet inducible system is the most widely utilized regulatory system and could be a useful tool for therapeutic gene-based therapies. For example, use a synthetic "tetracycline-on" switch system to control the expression of the therapeutic gene thymidine kinase, which converts prodrugs to active drugs. The aim of this study was to develop therapeutic MSCs, harboring an inducible suicide gene, and to validate therapeutic gene expression using optical molecular imaging of ATC. We designed the Tet-On system using a retroviral vector expressing herpes simplex virus thymidine kinase (HSV1-sr39TK) with dual reporters (eGFP-Fluc2). Mouse bone marrow-derived mesenchymal stromal cells (BM-MSC) were transduced using this system with (MSC-Tet-TK/Fluc2) or without (MSC-TK/Fluc) the Tet-On system. Transduced cells were screened and characterized. Engineered MSCs were co-cultured with ATC (CAL62/Rluc) cells in the presence of the prodrug ganciclovir (GCV) and stimulated with DOX. The efficiency of cell killing monitored by assessing Rluc (CAL62/Rluc) and Fluc (MSC-Tet-TK/Fluc and MSC-TK/Fluc) activities using IVIS imaging. Fluc activity increased in MSC-Tet-TK/Fluc cells in a dose dependent manner following DOX treatment (R2 = 0.95), whereas no signal was observed in untreated cells. eGFP could also be visualized after induction with DOX, and the HSV1-TK protein could be detected by western blotting. In MSC-TK/Fluc cells, the Fluc activity increased with increasing cell number (R2 = 0.98), and eGFP could be visualized by fluorescence microscopy. The Fluc activity and cell viability of MSC-Tet-TK/Fluc and MSC-TK/Fluc cells decreased significantly following GCV treatment. A bystander effect of the therapeutic cells confirmed in co-cultures of CAL62 cells, an anaplastic thyroid cancer cell line, with either MSC-Tet-TK/Fluc cells or MSC-TK/Fluc cells. The Rluc activity in MSC-Tet-TK/Fluc co-cultures, derived from the CAL62/Rluc cells, decreased significantly with GCV treatment of DOX treated cultures, whereas no significant changes were observed in untreated cultures. In addition, the Fluc activity of MSC-Tet-TK/Fluc cells also decreased significantly with DOX treatment whereas no signal was present in untreated cultures. A bystander effect also be demonstrated in co-cultures with MSC-TK/Fluc cells and CAL62/Rluc; both the Rluc activity and the Fluc activity were significantly decreased following GCV treatment. We have successfully developed a Tet-On system of gene-directed enzyme/prodrug delivery using MSCs. We confirmed the therapeutic bystander effect in CAL62/Rluc cells with respect to MSC-Tet-TK/Fluc and MSC-TK/Fluc cells after GCV treatment with and without DOX. Our results confirm the therapeutic efficiency of a suicide gene, with or without the Tet-On system, for ATC therapy. In addition, our findings provide an innovative therapeutic approach for using the Tet-On system to eradicate tumors by simple, repeated administration of MSC-Tet-TK/Fluc cells with DOX and GCV.

iPS-derived MSCs from an expandable bank to deliver a prodrug-converting enzyme that limits growth and metastases of human breast cancers

One attractive strategy to treat cancers is to deliver an exogenous enzyme that will convert a non-toxic compound to a highly toxic derivative. The strategy was tested with viral vectors but was disappointing because the efficiency of transduction into tumor cells was too low. Recent reports demonstrated that the limitation can be addressed by using tissue-derived mesenchymal stromal cells (MSCs) to deliver enzyme/prodrug systems that kill adjacent cancer cells through bystander effects. Here we addressed the limitation that tissue-derived MSCs vary in their properties and are difficult to generate in the large numbers needed for clinical applications. We prepared a Feeder Stock of MSCs from induced pluripotent stem cells (iPSs) that provided an extensively expandable source of standardized cells. We then transduced the iPS-derived MSCs to express cytosine deaminase and injected them locally into a mouse xenogeneic model of human breast cancer. After administration of the prodrug (5-fluorocytosine), the transduced iPS-MSCs both limited growth of preformed tumors and decreased lung metastases.

Tumour-associated mesenchymal stem/stromal cells: emerging therapeutic targets

Mesenchymal stem cells, also known as mesenchymal stromal cells (MSCs), exist in many tissues and are known to actively migrate to sites of tissue injury, where they participate in wound repair. Tumours can be considered "wounds that never heal" and, in response to cues from a tumour, MSCs are continuously recruited to and become integral components of the tumour microenvironment. Recently, it has become apparent that such tumour-associated MSCs (TA-MSCs) have an active role in tumour initiation, promotion, progression and metastasis. In this Review, we discuss recent advances in our understanding of the pathogenic role of TA-MSCs in regulating the survival, proliferation, migration and drug resistance of tumour cells, as well as the influence of MSCs on the immune status of the tumour microenvironment. Moreover, we discuss therapeutic approaches that target TA-MSC upstream or downstream modulators or use MSCs as vehicles for the delivery of tumoricidal agents. It is anticipated that new insights into the functions of TA-MSCs will lead to the development of novel therapeutic strategies against tumours.

Engineered Mesenchymal Stem Cells as an Anti-Cancer Trojan Horse

Cell-based gene therapy holds a great promise for the treatment of human malignancy. Among different cells, mesenchymal stem cells (MSCs) are emerging as valuable anti-cancer agents that have the potential to be used to treat a number of different cancer types. They have inherent migratory properties, which allow them to serve as vehicles for delivering effective therapy to isolated tumors and metastases. MSCs have been engineered to express anti-proliferative, pro-apoptotic, and anti-angiogenic agents that specifically target different cancers. Another field of interest is to modify MSCs with the cytokines that activate pro-tumorigenic immunity or to use them as carriers for the traditional chemical compounds that possess the properties of anti-cancer drugs. Although there is still controversy about the exact function of MSCs in the tumor settings, the encouraging results from the preclinical studies of MSC-based gene therapy for a large number of tumors support the initiation of clinical trials.

Metastatic Ovarian Cancer Can Be Efficiently Treated by Genetically Modified Mesenchymal Stromal Cells

Due to late diagnosis, often recurrence, formation of metastases and resistance to commonly used chemotherapeutics human ovarian carcinoma represents a serious disease with high mortality. Adipose tissue-derived mesenchymal stromal cells (AT-MSC) can serve as vehicles for therapeutic genes and we engineered AT-MSC to express either Herpes simplex virus thymidine kinase (HSVtk-MSC), which phosphorylates ganciclovir (GCV) to its toxic metabolites or yeast fused cytosine deaminase::uracil phosphoribosyltransferase (CD::UPRT-MSC), which converts 5-fluorocytosine (5-FC) to highly toxic 5-fluorouracil (5-FU). Here, we reported different responses of cytotoxicity mediated by CD::UPRT-MSC/5-FC treatment on human ovarian carcinoma cell lines-SKOV-3 and A2780 used in adherent or three-dimensional (3D) cell culture and we proved high potential of 3D model to predict results in our in vivo experiments. Both tumor cell lines showed similarly high chemosensitivity to the used treatment in adherent culture, but 3D model revealed severe discrepancy-only 36% of SKOV-3 cells but even 90% of A2780 cells were eliminated. This result served as a prognostic marker-we were able to achieve significantly decreased tumor volumes of subcutaneous xenografts of A2780 cells in nude mice and we prolonged tumor-free survival in 33% of animals bearing highly metastatic ovarian carcinoma after CD::UPRT-MSC/5-FC treatment.

Practical Issues with the Use of Stem Cells for Cancer Gene Therapy

Stem cell-based drug delivery for cancer therapy has steadily gained momentum in the past decade as several studies have reported stem cells' inherent tropism towards tumors. Since this science is still in its early stages and there are many factors that could significantly impact tumor tropism of stem cells, some contradictory results have been observed. This review starts by examining a number of proof-of-concept studies that demonstrate the potential application of stem cells in cancer therapy. Studies that illustrate stem cells' tumor tropism and discuss the technical difficulties that could impact the therapeutic outcome are also highlighted. The discussion also emphasizes stem cell imaging/tracking, as it plays a crucial role in performing reliable dose-response studies and evaluating the therapeutic outcome of treatment protocols. In each section, the pros and cons associated with each method are highlighted, limitations are underlined, and potential solutions are discussed. The overall intention is to familiarize the reader with important practical issues related to stem cell cancer tropism and in vivo tracking, underline the shortcomings, and emphasize critical factors that need to be considered for effective translation of this science into the clinic.

Tyrosine kinase inhibitor SU11274 increased tumorigenicity and enriched for melanoma-initiating cells by bioenergetic modulation

Background

Small molecule inhibitor of tyrosine kinase activity, compound SU11274, was reported to have antitumorigenic and antimetastatic effect in melanoma. In this study, we evaluated, whether similar effect could be achieved also in other melanoma cells including highly tumorigenic and hypermetastatic variant.

Methods

The effect of SU11274 was evaluated in adherent and non-adherent melanosphere cultures of human melanoma cells M14, M4Beu, A375 and EGFP-A375/Rel3. Tumorigenicity of SU11274-treated cells was tested by limiting dilution assay in xenograft model in vivo.

Results

Here we show that SU11274 enriched for melanoma-initiating cells in vivo. SU11274 substantially decreased number of cells in adherent and spheroid cultures, but increased their tumorigenic potential as determined by higher frequency of tumor-initiating cells in vivo. SU11274 treatment was not associated with any significant alteration in the expression of stem cell markers, but the inhibitor stimulated higher level of pluripotent markers. SU11274-treated melanoma cells exhibited higher ATP content and lactate release indicative of increased glycolysis. Our data suggest that the SU11274 altered bioenergetic state of the cells. Indeed, pharmacological intervention with a glycolytic inhibitor dichloroacetate significantly reduced SU11274-promoted increase in melanoma-initiating cells and decreased their tumorigenicity.

Conclusions

Our data suggest critical role of glycolysis regulation in melanoma-initiating cells. Moreover, these data unravel substantial plasticity of melanoma cells and their adoptive mechanisms, which result in ambivalent response to therapeutic targeting.