Mesenchymal stem cells: A new platform for targeting suicide genes in cancer

1 INTRODUCTION

Based on World Health Organization (WHO), cancer is the second leading cause of death globally which was responsible for 8.8 million mortality in 2015. Nearly 1 in 6 deaths is due to cancer and the number of new cases is expected to rise by about 70% over the next 2 decades worldwide (Siegel, Miller, & Jemal, 2015; Torre et al., 2015). There are many types of cancer treatment including surgery, chemotherapy and radiotherapy (Amer, 2014). Cancerous patients had no survival benefits from current insufficient treatments and in most cases relapse and metastasis occurred (Nowakowski, Drela, Rozycka, Janowski, & Lukomska, 2016; Zhang et al., 2014). Such poor prognosis seems to be linked to detrimental effects on vital noncancerous bodily tissues, deficient drug concentrations in tumors also problems of accessing tumor sites principally in metastatic cancers, and systemic toxicity demonstrates the urgency to explore more effective anti-tumor therapy (Kim & Tannock, 2005; Liu et al., 2015; Pessino & Sobrero, 2006). Targeted therapy is emerging as a supplement or alternative to chemotherapy and/or radiation for various malignant diseases (Mirzaei, Naseri, Rezaee, et al., 2016; Mirzaei, Sahebkar, Salehi, et al., 2016; Mirzaei, Yazdi, Salehi, & Mirzaei, 2016). In the field of targeted therapy, gene therapy appears as a good substitute method for cancer treatment (Wu, Chang, & Huang, 2006). There are different approaches for cancer gene therapy including immunotherapy, oncolytic viruses and gene transfer (McCormick, 2001; Lin & Nemunaitis, 2004; Cross & Burmester, 2006; Mirzaei, Mirzaei, Lee, Hadjati, & Till, 2016; Mirzaei, Naseri, Rezaee, et al., 2016). Immunotherapy employ for immune system stimulation to destroy cancer cells (Blattman & Greenberg, 2004). Oncolytic viruses, that replicates within the cancer cell and cause cell destruction (Chiocca & Rabkin, 2014; Dwyer, Khan, Barry, O'Brien, & Kerin, 2010; Singh, Doley, Kumar, Sahoo, & Tiwari, 2012). Gene transfer serves as a new treatment approach that introduces foreign genes into cancerous cells to promote cell death or slow the progression of the cancer. Gene transfer represents the best way for cancer gene therapy. In this path, we could introduce multiple genes with completely different function to malignant cell such as pro-apoptotic genes, anti-angiogenesis genes, and suicide genes (Cao et al., 1998; Cross & Burmester, 2006; Persano, Crescenzi, & Indraccolo, 2007). Failure to distinguish between normal and tumor cells will probably remain a limiting factor for chemotherapy drugs but Suicide genes form the basis of a strategy for making cancer cells more vulnerable and susceptible to chemotherapy (Karjoo, Chen, & Hatefi, 2016b). Suicide gene therapy system is based on gene transfer into tumor cells, which leads to the exclusive expression of an enzyme able to convert a non-toxic prodrug into a lethal drug (Freeman, 2002; Izmirli, Sonmez, & Gogebakan, 2016; Touati et al., 2014). Achieving the specified purpose requires vehicles that encapsulate the gene and deliver it particularly to cancer cell and cancer local environment of the tumor. In gene therapy and especially cancer gene therapy, at first we need to utilize vehicle so called vector that includes a set of criteria; having tumor tropism (specificity for tumor microenvironments), do not able to elicit an immune response and safety (Hacein-Bey-Abina et al., 2002; Rajab, Nelson, Keung, & Conrad, 2013). From the first reports of gene therapy in 1960–1970 to present, there are three types of gene delivery systems into the target cell (Collins & Thrasher, 2015). Through to their high transduction efficiency, viral vectors are the most frequently used gene delivery strategies. Retrovirus, lentivirus, and adenovirus are common viral vectors. In the field of gene therapy with viral vectors there are two major obstacles in terms of the safety and toxicity of these vectors that limit the clinical potential of this approach (i.e., insertional mutagenesis of retroviral vectors and intensive immune reaction of adenoviral vectors) (Collins et al., 2008; Nayerossadat, Maedeh, & Ali, 2012; Rajab et al., 2013; Waehler, Russell, & Curiel, 2007). Molecular vectors including, naked DNA and application of cationic polymers such as polyethylenimine or poly-l-lysine, cationic peptides, and cationic liposomes. The major drawbacks of this approach are non-specific uptake of the DNA by cancer cells and toxicity of some cationic polymers such as lipoplexes (Dani, 1999; Miller & Vile, 1995; Nayerossadat et al., 2012; Simões et al., 2005; Zhang et al., 2014). Cellular vectors are promising vehicles to deliver various anticancer agents, including small molecule drugs, proteins, suicide genes, nanoparticles, and viruses. Cellular vectors have several advantages such as low immunogenicity, practically unlimited genetic material packaging capacity, simple and low-cost construction which makes them appropriate for large-scale production and most likely safe for clinical gene therapy (Link, Seregina, Traynor, & Burt, 2000; Ohlfest, Freese, & Largaespada, 2005).

Cell therapy is an effective therapeutic approach for treatment of various diseases such as cancer (Goradel et al., 2017; Mirzaei, Mirzaei, Lee, et al., 2016). Among of various cells which could be used for cell therapy, MSC are known as powerful tools for treatment of various diseases such as cancer. MSC can be utilized for targeted-gene delivery (Goradel et al., 2017; Mirzaei, Sahebkar, Jaafari, Goodarzi, & Mirzaei, 2016). MSCs have a low immunogenic potential besides the capacity to home and integrate in to the injured and inflamed sites. MSCs have indicated preferential tropism for tumor, so could transferred suicide genes and their products just in tumor local and reduce side effects about systemic toxicity and could achieve more efficient gene delivery to the target (Dvorak, 1986; Nauta & Fibbe, 2007; Uchibori, Tsukahara, Ohmine, & Ozawa, 2014). The aim of the present review is to discuss about (1) Suicide genes, known as good killers in cancer therapy; (2) Characteristics and application of MSCs in cancer therapy; (3) investigating previous research studies about MSCs in role of a vehicle for suicide genes.

2 SUICIDE GENES AS A GOOD KILLER

The term of gene directed enzyme prodrug therapy (GDEPT) is one of the promising alternatives to conventional chemotherapy because it minimizes the systemic toxicities of conventional chemotherapy drugs and refer to expression of a suicide gene in tumor cells for the in situ conversion of a pro-drug into cytotoxic metabolites (Greco & Dachs, 2001; Springer & Niculescu-Duvaz, 2000). The first attempt to use of suicide genes against cancer was in 1986 by Moolten et al. (Rajab et al., 2013). These points must consider in selection of suicide genes; Intended gene must encodes an enzyme that did not exist in normal cells or there is no enzyme with similar function in cells; Minimal toxicity of prodrug before activation in body and maximum toxicity after converting to active drug in tumor location; high kinetic enzyme activity or high affinity to target prodrug and finally active drug must diffuse in whole tumor mass by Bystander effects (Both, 2009; Rajab et al., 2013; Zarogoulidis et al., 2013). This mentions to the demolition of tumor cells that are not directly expressing the suicide gene. In this effect, gap junctions play substantial role via local diffusion of the active drug. Previous studies revealed that this approach could abolish tumors even if 10% of tumor cells expressed suicide genes, this efficiency is about bystander effects (Freeman et al., 1993; Mesnil & Yamasaki, 2000). There are several mechanisms for Bystander affect; intercellular communication via gap junctions, diffuse through the cell membrane, endocytosis of apoptotic vesicles and stimulation of the immune system against tumor. The gap junctional intercellular communication (GJIC) capacity varied among cancer cell lines and was dependent on the connexin 43 (cx43) expressions (Duarte, Carle, Faneca, De Lima, & Pierrefite-Carle, 2012; Huber, Austin, Richards, Davis, & Good, 1994; Pierrefite-Carle et al., 1999). The overexpression of Cx43 in glioma cells leading to the increase in the number of gap junctions and enhanced the bystander effect of HSV-TK as a suicide gene, reverse situation is in esophageal cancer (Huang et al., 2010; Matono et al., 2003). So evaluation of degree of GJIC has predictive value for determination of response to suicide genes that dependent on GJIC such as HSV-TK. Also studies reported that radiation could induce bystander effect, so suicide cancer therapy juxtaposed with the irradiation could increase tumor eradiation (Azzam & Little, 2004). In the following, we discussed two most applicable suicide genes in cancer gene therapy.

3 TALENT OF MSCS IN CANCER GENE THERAPY

Adult stem (AS) cells are a potentially valuable source in regenerative medicine. They are dividing to neural stem cells (NSCs) (ectoderm), hematopoietic stem cells (HSCs) (mesoderm), and MSCs (mesoderm) (Robinton & Daley, 2012; Weissman, 2000). In vitro expansion of NSCs is limited to the nervous system; they could not survive outside the nervous system. HSCs have been utilized in allogeneic cell therapy. Although bone marrow (BM) has been the principal source for the isolation of these cells, the harvesting of bone marrow entailed a highly invasive procedure with low cell yields (Prockop, 1997; Sylvester & Longaker, 2004). It is easier to work with MSCs, due to their less invasive and free of ethical issues harvest in compare with other sources (Amara, Touati, Beaune, & de Waziers, 2014; Cavarretta et al., 2010; Compte et al., 2009). MSCs were first obtained from bone marrow and defined as fibroblast-like multipotent stem cells by Friedenstein and coworkers in 1974 (Friedenstein, Chailakhjan, & Lalykina, 1970).

International Society for Cell Therapy (ISCT) proposed minimal criteria to describe human MSCs; adhere to plastic, ability to differentiate into adipocytes, osteoblasts, and chondrocytes, fibroblastoid phenotype and expression of cell surface markers CD105, CD73, CD271, CD90, and ganglioside GD2 in more than 95% of cells and absence of CD45, CD79a, or CD19, CD34, CD14, or CD11b, CD40, CD80, CD86, and the MHC II class cellular receptor HLA-DR (Baird, 2015; Dominici et al., 2006; Goradel et al., 2017; Horwitz et al., 2005; Mirzaei, Sahebkar, Avan, et al., 2016; Mirzaei, Sahebkar, et al., 2017; Mohammadi, Jaafari, Mirzaei, & Mirzaei, 2016). But it should be noted that there is no unique key determinant factor to distinguish MSCs, also the relative expression levels for these markers could be varied according to the species diversity, tissue sample and culture conditions. At first MSCs isolated from bone marrow (BM) but subsequently found in other source such as adipose tissue (AT), umbilical cord blood (UCB), peripheral blood, surrounding blood vessels, synovium, the circulatory system, dental pulp, and amniotic fluid (Morizono et al., 2003; Phinney & Prockop, 2007; Uchibori et al., 2014). In spite of the fact that MSCs only represent a small percentage of the total number of BM populating cells 0.01–0.001%), they extensively used by many researchers (Li, Fan, & Xiong, 2015). Since MSC isolation from BM recognized as an unpleasant experience, adipose tissue another valuable source of MSCs, has drown great attention in recent years (Kucerova et al., 2008).

Adipose tissue mostly isolated from subcutaneous tissue, we can get ∼40-fold higher yield of MSC from adipose tissue compared with the bone marrow. The rate of isolation of MSC from human AT is 100%. Great interest has developed in AT-MSC, which is free of ethical concerns and invasiveness (Kern, Eichler, Stoeve, Klüter, & Bieback, 2006). Moreover, it was reported that AT-MSC can be expanded long term without the loss of their phenotype. In contrary, BM-MSCs should be used during early cell passages (below P8) to prevent potential differentiation (Lee et al., 2006). UCB-derived MSC has significantly higher rate of expansion in compared with BM-MSC. Since these cells are obtained from placenta that discarded after birth, so the collection procedure is totally noninvasive and free from ethical considerations (Li et al., 2015). MSCs also have been exploited as tumor specific delivery vehicles for cancer gene therapy by multiple reasons; immunoprivileged status (due to lack of major histocompatibility complex MHC-II and only minimal MHC-I expression) and natural tropism for tumors and their metastases. MSCs do not motivate the host immune response and escape immunological rejection in allogeneic injection (Dvorak, 1986; Kidd et al., 2008; Zhang et al., 2014).

In addition MSCs can be efficiently transduced with viral or non-viral vectors containing a target gene. For example, in separate studies, Bak, Yang, and Wang (2010) and Jiang et al. (2014) demonstrated that hUCB-MSCs and hBM-MSCs can be simply infected by the lentivector and baculovirus respectively at a high efficiency of over 80%. MSCs mostly transduced through viral vectors including lentiviruses, retroviruses, baculovirus, and adenoviruses. All of these viral vehicles produce stable expression, while the later create transient expression of therapeutic genes. Non-viral vectors have been exploited in limited number of studies. The examples are nucleofection, spermine-pullulan (SP), PEIcyclodextrin, and cationic liposomes and cationic polymers (Gao et al., 2010; Uchibori et al., 2009; Zhang et al., 2014). Results revealed that in healthy animal models MSCs mostly migrate to lung, liver and bone but also MSCs have high migration potential to the tissue injury and inflammation (Hu, Fu, Tabata, & Gao, 2010; Kidd et al., 2008). For example, inflammation has long been associated with the development of cancer, MSCs exhibit strong tropism toward tumors and their metastases by expression of receptors such as CCR1, CCR4, CCR7, CCR9, CCR10, CXCR4, CXCR5, CXCR6, RAGE, CX3CR1, VEGFR, c-Kit, and c-Met for Infammatory chemokines and growth factors including SDF-1(CXCL12), MCP-1, HGF, IL-8, NT3, TGF-b, GCSF, VEGF, IL-8, IL-6, CCL2, HMGB1, and SCF (Honczarenko et al., 2006; Reagan & Kaplan, 2011; Ringe et al., 2007). CXCR4 is the most important chemokine receptor implicated in targeted homing of MSCs. MSCs expression profiling demonstrates that overexpression of CXCR4 and down regulation of MMP-2 have greater role in migration capability in to tumor sites, whereas knockdown of SDF-1 plays a negative role in stem cell recruitment to tumors (Menon et al., 2007; Song & Li, 2011). Sato et al demonstrated that epidermal growth factor receptor gene transfection conferred enhanced migratory activity toward GL261 gliomas or B16 melanoma in vivo (Sato et al., 2005). Moreover adhesion molecules, such as b1- and b2-integrins and L-selectin, are involved in MSC migration and homing to tumor site (Hu et al., 2010). A recent report on MSC behavior have highlighted the great impact of irradiation on MSC tropism and engraftment to tumor location, mediated at least in part by apoptosis and subsequent enhanced local release of inflammatory signals such as CCL2 and CCR8. For example, UCB-MSCs-based TRAIL gene delivery to irradiated glioma tumors enhanced apoptosis in glioma cells. Besides radiotherapy could increase MSC localization in LoVo, HT-29 (colon), and MDA-231 breast cancer cells (Kim et al., 2010; Klopp et al., 2007; Zielske, Livant, & Lawrence, 2009). Every safe cell-based therapy will require the capability to harness the unwanted growth of cells. Local injection of MSC developed some side effects, that is, ectopic ossification and calcification foci in mouse and rat models of myocardial infarction. Moreover bilateral diffuse pulmonary arises after bone marrow transplant in a dog. In a distinct study, spontaneous osteosarcoma formation in culture has been reported in murine-derived MSCs (Breitbach et al., 2007; Sale & Storb, 1983; Yoon, Park, Tkebuchava, Luedeman, & Losordo, 2004). Albeit MSCs have been utilized for treatment of many patients without major undesirable effects but some results, both in vitro and in vivo models suggest that must be cautions in this regard. MSCs exhibit pro-tumorigenic or tumor-supporting roles through inhibition of immune system and apoptosis, stimulation of EMT (epithelial–mesenchymal transition) and enhancement of angiogenesis, proliferation, migration, and metastasis (Djouad et al., 2003; Martin et al., 2010; Ramasamy et al., 2007). Unmodified MSC have antitumor properties in leukemias, glioma, and melanoma, and breast, hepatoma, and lung cancer cells (Ramasamy et al., 2007; Sun et al., 2009). In this regard, Ma et al. (2012) observed the growth inhibition of breast CSCs by hUCMSC both in vitro and in vivo. Ohlsson, Varas, Kjellman, Edvardsen, and Lindvall (2003) showed that mesenchymal progenitor (immortalized MSCs) are effective at inhibiting the in vivo proliferation of rat colon carcinoma cells. The underlying mechanism about antitumor effect of MSCs is likely related to down-regulation of Wnt, Akt, PI3K, and NF-kB signaling pathways by factors released from MSC in tumor position (Dai et al., 2011; Ma, Hao, Zhang, & Zhang, 2012). Some studies in order to increase the innate antitumor effects of MSCs carried out gene modification with oncogenes or other genes promoting cell proliferation but these modifications are associated with higher risk of tumorigenicity (Miletic et al., 2007). Engineered MSCs was first use for direct delivery of interferon ß (IFN-ß) gene to melanoma xenografts mice (Studeny et al., 2002). Considerable reduced in tumor growth rate and significant prolongation of survival of tumor-bearing mice motivated more researches to study therapeutic efficacy of genetically modified MSCs as cellular vehicles for several cancer treatments. MSCs explored for local secretion of therapeutic cytokine proteins such as; IFN-ß, IL-12, IL-24, and IL-2, delivery of prodrug activator genes (i.e., actually suicide genes) that will be explained in the following, amplifying and deliver of oncolytic viruses such as conditionally replicating oncolytic adenoviruses (CRAd), ICOVIR-5 and measles virus (MV), secretion of pro-apoptotic proteins such as TRAIL, TNF-related apoptosis ligand, anti-angiogenic agents such as TSP1 and endostatin and growth factor antagonists, that is, NK4 (antagonist of hepatocyte growth factor [HGF]) (Dwyer et al., 2010; Li, Ou, Wang, & Tang, 2016; Shah, 2012). MSCs loaded with nanoparticles containing chemotherapy drug, paclitaxel have been shown high efficient tumoricidal effect (Sadhukha, O'Brien, & Prabha, 2014).

It has been showed that MSCs could have dual effects on cancerous condition. Anti-cancer and tumorgensis effects of MSCs led to some concerns about utilization of them as therapeutic agents. It has been showed that express of different types of MSCs surface markers play critical roles in the effect of MSCs in various conditions.

Finally, three main points regarding employing of MSCs as vectors remain to be defined: (1) MSCs are cells with active physiological process which using of them are not simple delivery platform. They could release a wide array of different molecules (i.e., growth factors, cytokines, and chemokines). Hence, they may provide various signals which could lead to enhancing of tumor burden and metastases. (2) Cancerous microenvironments may lead to inducing of malignant transformation in the injected MSCs. (3) How many MSCs need to be given and when and where they should be administrated. Hence, better understanding of MSCs physiology within the tumor sites, and more robust studies characterizing their homing mechanisms could enhance suggested therapies.

4 MSCS-EXOSOMES AS NEW THERAPEUTIC APPROACH FOR CANCER THERAPY

Among of various signaling and cellular effects of MSCs, exosomes released from MSCs have critical roles for transferring specific signals to host cells. Exosomes are known as nano vesicles which could carry a variety of molecules and markers such as DNAs, mRNAs, microRNAs, and proteins (Banikazemi et al., 2017; Borujeni et al., 2017; Mirzaei, Sahebkar, Jaafari, et al., 2016). The cargos are able to change behavior of host cells via targeting various cellular and molecular targets (Mirzaei, Sahebkar, Jaafari, et al., 2016; Saadatpour et al., 2016). For example, microRNAs (miRNAs) are one of important cargos which could be carried by exosomes. MiRNAs are small non-coding RNAs which have critical roles in regulating of a variety of vital biological processes such as growth, angiogenesis, and differentiation (Golabchi et al., 2017; Hashemi Goradel et al., 2017; Keshavarzi, Sorayayi, et al., 2017; Mashreghi et al., 2017; Mirzaei, Masoudifar, et al., 2017; Mirzaei, Yazdi, Salehi, et al., 2016; Rashidi, Malekzadeh, Goodarzi, Masoudifar, & Mirzaei, 2017; Salarinia et al., 2016). It has been showed that deregulation of them could be associated with imitation and progression of a wide range of diseases such as cardiovascular diseases, stroke, diabetes, and cancer (Fathullahzadeh, Mirzaei, Honardoost, Sahebkar, & Salehi, 2016; Gholamin et al., 2017; Hoseini et al., 2017; Keshavarzi, Darijani, et al., 2017; Mirzaei, 2017; Rabieian et al., 2017; Rashidi, Hoseini, Sahebkar, & Mirzaei, 2016; Simonian, Mosallayi, & Mirzaei, 2017). These molecules exert their effects via targeting various cellular and molecular pathways involved in various physiological processes (Gholamin et al., 2016; Mirzaei, Fathullahzadeh, et al., 2017; Mirzaei, Khataminfar, Mohammadparast, et al., 2016; Mirzaei, Momeni, et al., 2017; Mirzaei, Sahebkar, Mohammadi, et al., 2016; Mohammadi, Goodarzi, Jaafari, Mirzaei, & Mirzaei, 2016; Moridikia, Mirzaei, Sahebkar, & Salimian, 2017). Hence, targeting of miRNAs by exosomes could lead to change behavior of host cells and could be associated with emerging of diseases condition or therapeutic effects (Mirzaei, Gholamin, Shahidsales, et al., 2016; Mirzaei, Naseri, Rezaee, et al., 2016). Exosomes could be released from various types of cells such as cancer cells, and MSCs. These nano-particles could exert their inflammatory or anti-inflammatory effects on host cells (Rani, Ryan, Griffin, & Ritter, 2015). It has been showed that MSCs employed exosomes for paracrine functions. Paracrine functions of MSCs are known as one of important mechanisms involved in anti-inflammatory effects of MSCs (Rani et al., 2015). Hence, it seems that utilization of exosomes released from MSCs could be used as effective candidate for treatment of various diseases.

5 MSCS AS A VEHICLE FOR TARGETING SUICIDE GENES

Identification of new drug delivery platform is one of the major landscapes in cancer gene therapy (Mohammadi, Jaafari, et al., 2016). Despite a numerous of novel therapeutic approaches have emerged, none has yet confirmed the ability to cure various cancers. In this regard, MSCs emerge to be promising new therapeutic option. The natural tropism of MSCs for tumor sites makes them excellent vehicles for tumor-targeted therapies (Mohammadi, Jaafari, et al., 2016). Also, it has been showed that use of genetically modified MSCs may overcome limitations related with systemic administration of some cytokines and anti-neoplastic agents with a short half-life and high toxicity (Mohammadi, Jaafari, et al., 2016). Viral vectors for directed enzyme prodrug therapy (GDEPT) or suicide gene therapy have been utilized by multiple studies. This approach has some limitations due to its low specificity for tumor cells and systemic toxicity. Utilization of viral vectors for targeting various genes such as suicide gene are associated with various limitations such as high immunogenicity (Mirzaei, Sahebkar, Jaafari, et al., 2016). Hence, several studies applied improved viral vectors for overcoming to various limitations. Moreover, it has been showed that other vectors such as Piggybac could be used for targeting various genes and be associated with same or better results than viral vectors (Mirzaei, Sahebkar, Jaafari, et al., 2016). Hence, utilization of Piggybac or improved viral vectors could be employed as new delivery system with high potential for targeting various genes such as suicide genes. Genetically engineered MSCs using a GDEPT strategy could be a good resolve for these obstructions. MSC-targeted gene therapy is a two-step procedure. In the first step, the gene for the foreign enzyme (bacterial, yeast, or viral) is targeted to the tumor by transduction of MSCs. In the second step, transcription of the gene encoding the prodrug-drug converting enzyme could generate a lethal substance at the tumor site. With looking to previous studies we found that the retroviral suicide gene construct is often used for MSC transduction and most frequent transduced suicide genes are CD and HSV-TK. A large number of approaches have been applied using AT-MSCs and BM-MSCs as a delivery system for anti-neoplastic agents (Amara et al., 2014). A key characteristic of manipulated MSCs is that they have no difference with naïve MSCs in terms of proliferation, differentiation, and tumor homing potentials as well as surface antigenicity (Park et al., 2013). Until now many in vitro and in vivo studies carried out using MSCs in cancer therapy. A brief report of studies within this research area is embedded in Table 1. Due to this fact that MSCs express very low levels of receptors, like most healthy tissues, are unaffected by TRAIL-induced apoptosis. As a result, recruitment of MSCs secreting TRAIL brings brilliant results in cancer treatment (Grisendi et al., 2010; Kim et al., 2008; Loebinger, Eddaoudi, Davies, & Janes, 2009; Mohr et al., 2008). Moreover co-transfection of the TRAIL gene with some cases of double suicide gene approaches (i.e., HSV-TK) that confront with some difficulties including antagonistic antitumor activity, could improve the efficiency of tumoricidal effect (Kim et al., 2013; Martinez-Quintanilla et al., 2013). In order to achieve efficient homing of MSCs to the tumor site, the median amount of administered therapeutic cells recommended to be less than 10% of the tumor mass (Hung et al., 2005). Therefore, it is logical to use multiple injections of the therapeutic cells at a low dose rather than single injection at high dose. For example, Kim et al. (2013) reported that mouse with metastatic renal cell carcinoma (RCC) after two and three injections of small-divided doses of MSC/TRAIL-TK represents 50% and 100% survival, respectively. The potency of consecutive suicide gene therapy was also evaluated in another study in which repeated intracerebral injection of CDy:UPRT-AT-MSC lead to 88% increase in the survival time of rat with glioblastoma (Altanerova et al., 2012). Some studies employed combined therapeutic modalities for malignant disease treatment. Combination of suicide gene- MSCs with chemotherapy drug shows satisfying results in some cases. Ando et al. (2014) revealed that NSCLC progression prevents following proteasome inhibitor bortezomib administration, together with MSC- Ad.iC9. While, this drug is ineffective for the treatment of NSCLC, alone. In another study MSCs-TK and Valproic acid (VPA) has been used for treatment of glioma-bearing mice (Ryu et al., 2012). The combined treatment had dramatic inhibitory effects on tumor growth and prolonged the survival rate in mice.

6 FUTURE PERSPECTIVE

Current data indicate that cancer incidence is steadily increasing in the World. There is high mortality statistic available in current decades. All of these highlighted the urgent need for more safer and more effective therapies. One of the principal challenges along cancer treatment is how to destroy malignant tumors without damaging healthy cells. A new approach that shows great promise in this area is employment of a suicide gene. In this way we need an appropriate carrier for therapeutic gene delivery specific to cancer site. The application of anti-cancer gene-expressing MSCs for targeted cancer therapy is a novel and promising strategy. MSCs with important characteristic such as strong tumor tropism, unlimited packaging capacity and unique immunologic tolerance, could overcome current obstacles and successfully deliver these suicide genes. Although MSCs have anticancer capacity but based on some reports could have positive role in tumor progression. Eventually more researches required to find novel insights into MSCs biology, potential clinical application, and molecular mechanism for homing to tumor site.