**3. Conclusions**

Although therapy with MSC in regenerative medicine is considered feasible and safe, the literature reported to date reveals dicrepancies respect to the MSCs impact in the tumor microenvironment. This paradoxical effect could be attributed to the differences in the experimental conditions of isolation and expansion, the source and dose of cells used, the route of administration and its timing, and the host characteristics. This chapter highlights the mechanisms of the effects of tumor support or suppression mediated by the MSCs and analyzes the possible mechanisms involved. MSCs demonstrate a tropism for tumors and once they interact with each other and with cancer cells, they promote tumor growth by: inmunosuppression; promotion of angiogenesis; epithelialmesenchymal transition; inhibition of apoptosis; and promotion of metastasis. In contrast, many studies have reported that MSCs can prevent tumor growth by increasing leukocyte infiltration, inhibiting angiogenesis, suppressing Wnt and AKT signaling. Further investigations are necessary to establish the biosecurity of cell therapy in the presence of precancerous lesions.
