**2.8. Regulation of cell cycle by MSC**

MSCs secrete a variety of cytokines that induce cell cycle arrest of tumor cells, albeit transiently, at the G1 phase through expression of Cyclin A, Cyclin E, Cyclin D2, and p27KIP1 [31, 107, 110]. Human stromal cells of adipose tissue (ADSC) and its conditioned culture medium can suppress tumor growth [107]. In addition, the cell culture medium conditioned by ADSC stimulated the necrosis of the cancer cells after the arrest of the G1 phase in the absence of apoptosis. Finally, when ADSC was introduced in pancreatic adenocarcinoma, the tumor did not grow [107]. Similarly, tumor cells that were cultured with MSC in vitro were also stopped in the G1 phase [111]. However, when the nonobese diabetic-severe combined immunodeficient mice were injected with MSCs and tumor cells, their growth was more increased compared to the injection of tumor cells alone. Although it has been reported that MSCs can induce arrest of the cell cycle of tumor cells in vitro, little is known about the exact mechanisms. In our experiment, the delay or arrest of the cell cycle can be induced in certain types of tumor cells and under certain co-culture conditions (type of medium, cell concentration, or co-culture time). While we cannot explain the exact mechanism (s), several studies performed by different groups, including hours, have shown that the arrest of the tumor cell cycle occurs. It has been shown that MSCs derived from human bone marrow interfere in vitro with small cell lung cancer (A549), esophageal cancer (Eca-109), Kaposi's sarcoma, and proliferative kinetics of the leukemic cell line [112]. The above was not only observed when 0.5 × 105 tumor cells were cultured together with 0.5 × 105 MSCs derived from human bone marrow but also when exposed to medium conditioned by MSC; the cells were stopped during the G1 phase of the cell cycle in both cases by the negative regulation of Cyclin D2 and the induction of apoptosis [111]. MSCs from other sources, including MSCs derived from human fetal skin and MSCs derived from adipose tissue, have also inhibited the growth of human liver cancer cell lines [32], breast cancer (MCF-7) [111], and primary leukemic cells by reducing their proliferation, colony formation, and oncogene expression [30, 32]. Intravenous injection of 4 × 106 MSCs derived from human bone marrow in nude mice carrying Kaposi's sarcoma has inhibited the growth of tumor cells [32]. A similar effect has been observed in an animal model of hepatocellular carcinoma and pancreatic tumors, since the alteration of cell cycle progression has led to the decrease of cell proliferation [30, 31]; the same has happened with melanoma due to increased apoptosis of capillaries [34], and the growth of colon carcinoma in rats has been inhibited when rat EMFs (cell line MPC1cE) were co-mapped with tumor cells in a relationship 1:1 or 1:10 [33]. MSCs derived from human fetal skin (Z3 cell line) also delayed liver tumor growth and decreased tumor size when injected with the same number of cells from the H7402 cell line in SCID mice [36]. Injection of MSCs derived from human adipose tissue (1 × 103 cells/ mm3 ) into established pancreatic cancer xenografts has led to apoptosis and the abrogation of tumor growth in nude (nude) Swiss mice [31]. The role of MSCs in cancer remains paradoxical. Evidence to date has suggested that they are pro as well as antitumorigenic [113–115] and such discrepancy seems to depend on the isolation and expansion conditions, the source and dose of the cell, the route of administration, and the model tumor used.

#### **2.9. MSCs and regulation of cellular signaling**

(BDNF), SDF-1, IGF-1 and IGF-2, transforming growth factor-beta (TGF-b), and IGF-2 binding protein (IGFBP-2) [101, 102]. These factors inhibit the apoptosis of tumor cells and promote tumor proliferation, whereas normal MSCs do not acquire these properties. In addition to the mitogenic properties of growth factors secreted by MSCs, VEGF and FGF-2 can mediate Bcl-2 expression, delaying apoptosis [103], while indirect angiogenic factors can induce VEGF expression and FGF-2 [104]. In addition, SDF-1 was reported to prevent drug-induced apoptosis of chronic lymphocytic leukemia (CLL) cells [105]. In addition, VEGF, FGF-2, HGF, and IGF-1 expressed by MSCs have been reported to stimulate angiogenic and antiapoptotic effects after hypoxic conditioning [101, 106]. Although little is known about how MSCs under hypoxic conditions exert support effects on tumor cells directly, growth factors stimulated by MSCs, stimulated by hypoxia, can provide tumor support effects in the tumor microenviron-

Depending on the microenvironment, MSCs can exert an antiproliferative effect. Lu et al. demonstrated that MSCs had an inhibitory effect on mouse tumor hepatoma cells in a cell-dependent manner through the activation of intrinsic caspase 3 pathway [107]. Lu et al. reported that MSCs increased p21 gene expression, involved in the arrest of the cell cycle. These data demonstrate that MSCs exerted tumor inhibitory effects in the absence of host immunosuppression, inducing arrest of the G0/G1 phase and apoptosis of cancer cells [107]. The same tumor suppressor activity by MSCs was observed in xenografted SCID mice with disseminated non-Hodgkin lymphoma [108]. A single injection of MSCs which increased the survival of the animals included those who presented more aggressive lymphomas. In turn, significant induction of endothelial cell apoptosis was observed when co-cultured with MSCs, suggesting that MSCs exert anti-angiogenic activity through endothelial cell apoptosis [108]. These findings were consistent with the results reported by Karnoub et al. where they demonstrated that MSCs exhibited potent anti-angiogenic activity in Kaposi's sarcomas with high vascularity and endothelial cell cultures in vitro by inducing apoptosis of tumor epithelial and endothelial cells through the Dkk-1 protein [32, 34]. Additionally, Dasari et al. reported that downregulation of the antiapoptotic inhibitor, inhibitor of the apoptosis protein linked to X (XIAP), in the presence of human umbilical cord bloodderived mesenchymal stem cell (hUCBSC) induced apoptosis of glioma cells and xenograft by the activation of caspase-3 and caspase-9 [109]. Recently, MSCs cultured at high density express IFN type I, which leads to cell death of breast cancer cells, MCF-7 and MDR-MB-231 cells [98]. In addition, MSCs prepared with IFN-gamma or cultured with three-dimensional systems can

ment through angiogenic and antiapoptotic effects (**Figure 4**).

72 Stromal Cells - Structure, Function, and Therapeutic Implications

**2.7. MSCs can induce apoptosis of cancer cells and endothelial cells**

express TRAIL, which induces specific apoptosis of tumor cells. [97, 98].

MSCs secrete a variety of cytokines that induce cell cycle arrest of tumor cells, albeit transiently, at the G1 phase through expression of Cyclin A, Cyclin E, Cyclin D2, and p27KIP1 [31, 107, 110]. Human stromal cells of adipose tissue (ADSC) and its conditioned culture medium can suppress tumor growth [107]. In addition, the cell culture medium conditioned by ADSC stimulated the necrosis of the cancer cells after the arrest of the G1 phase in the absence of

**2.8. Regulation of cell cycle by MSC**

The main signaling pathway involved in the control of cell survival is the pathway of phosphoinositide 3-kinase (PI3K)/AKT and WNT/beta-catenin. The activation of this pathway induces proliferation, growth, and migration, and increases cellular metabolism [116–118]. In the biology of a tumor cell, numerous studies have reported the requirement for the activation of the AKT-signaling cascade for the migration, invasion, and survival of tumor cells. Additionally, the WNT pathway has also been associated with the development of various types of carcinomas, including breast, liver, colon, skin, stomach, and ovary [119]. In a murine model of Kaposi's sarcoma, Kakhoo et al. reported that MSCs injected intravenously were able to migrate to the tumor and inhibit tumor cell proliferation by inhibiting AKT [32]. On the other hand, they observed in glioma cells that PTEN was upregulated in the presence of HUCBSCs, with the consequent downregulation of AKT [109]. In addition to inhibiting the PI3K/AKT pathway, MSCs can also suppress the WNT/beta-catenin pathway through the induced expression of the pro-apoptotic protein DKK-1 [31, 36, 37]. These recent findings demonstrated that beta-catenin can be negatively regulated in different human carcinoma cell lines (hepatocellular, H7402 and HepG2, breast, MCF-7, hematopoietic, K562 and HL60) by the secretion of DKK-1 by the MSCs. On the other hand, when the activity of DKK-1 was inhibited by the use of anti-DKK-1 neutralizing antibodies or interfering RNA, the inhibitory effects of MSCs on tumor progression disappeared [31, 36, 37].

IDO indoleamine 2,3-dioxygenase

PD-1/2 programmed death-1/2

SDF-1 stromal derived factor-1

EGF epithelial growth factor

NGF neurotrophic growth factor

ECM stromal extracellular matrix

AKT Serine-threonine kinase

PI3K Phosphoinositide 3-kinase

HGF Hepatocyte Growth Factor

IGF-1 insulin dependent growth factor-1

TGF-α transforming growth factor-alpha VEGF vascular endothelial growth factor

MSCs multipotent stromal mesenchymal stem cells

XIAP X-linked inhibitor of apoptosis protein

ZEB1/2 zinc finger E-box binding homeobox 1/2

SCID severe combined immunodeficiency

CAFs carcinoma-associated fibroblasts

EMT epithelial–mesenchymal transition

PDGF platelet derived growth factor

TWIST twist related protein-1

MMP-2 metalloproteinase-2

SERPINE1 serpin family E member 1

TGF-β transforming growth factor-beta

Wnt Wingless-Type MMTV Integration Site Family, Member 1

PD-L1/2 programmed death-1/2 ligand

HLA-G histocompatibility antigen, class I, G

Multipotent Stromal Cells in a Tumor Microenvironment http://dx.doi.org/10.5772/intechopen.77345 75

CD80 T-lymphocyte activation antigen CD80 CD86 T-lymphocyte activation antigen CD86

IL-6 interleukin-6 IL-10 interleukin-6
