Problem Studying cancer tumors microenvironment may reveal a novel role in driving cancer progression and metastasis. The biological interaction between stromal (mesenchymal) stem cells (MSCs) and cancer cells remains incompletely understood. Herein, we investigated the effects of tumor cells’ secreted factors as represented by a panel of human cancer cell lines (breast (MCF7 and MDA-MB-231); prostate (PC-3); lung (NCI-H522); colon (HT-29) and head & neck (FaDu)) on the biological characteristics of MSCs. Background Over the past several years, significant amount of research has emerged documenting a role for MSCs in promoting epithelial-to-mesenchymal transition (ETM), and accelerating tumor growth and metastasis. In addition, MSCs are being introduced into therapy for a number of clinical indications and there is a concern of possible promoting effects on tumour growth by MSCs. On the other hand, several other studies reported that MSCs exert tumor suppressive effects. Therefore, understanding the crosstalk between MSCs and tumor growth is very crucial for the safe utilization of MSCs in regenerative medicine Hypothesis Given this complex interplay between MSCs and tumor cells, the goal of this study was to assess the cellular and molecular changes in MSCs in response to secreted factors present in conditioned media (CM) from a panel of human tumor cell lines covering a spectrum of human cancers (Breast, Prostate, Lung, colon, and head and neck). Research Morphological changes were assessed using fluorescence microscopy. Changes in gene expression were assessed using Agilent microarray and qRT-PCR. GeneSpring X and DAVID tools were used for bioinformatic and signaling pathway analyses. Cell migration was assessed using transwell migration system. SB-431542, PF-573228, and PD98059 were used to inhibit TGFb, FAK, and MAPKK pathways, respectively. IL1b was measured using ELISA. Observations MSCs exposed to secreted factors present in conditioned media (CM) from FaDu, MDA-MB-231, PC-3, and NCI-H522, but not from MCF7 and HT-29, developed an elongated, spindle-shaped morphology with bipolar processes. In association with phenotypic changes, genome-wide gene expression and bioinformatics analysis revealed an enhanced pro-inflammatory response of those MSCs. Pharmacological inhibitions of FAK and MAPKK severely impaired the pro-inflammatory response of MSCs to tumor CM (~80-99%, and 55-88% inhibition, respectively), while inhibition of the TGFb pathway was found to promote the pro-inflammatory response (~3-fold increase). In addition, bioinformatics and pathway analysis of gene expression data from tumor cell lines combined with experimental validation revealed tumor-derived IL1b as one mediator of the pro-inflammatory phenotype observed in MSCs exposed to tumor CM. MSCs exhibited significant tropism toward secreted factors from the aforementioned tumor cell lines, while both normal and MSCs exposed to tumor conditioned media were capable of attracting human peripheral blood mononuclear cells (PBMCs). Conclusions: Our data revealed tumor-derived IL1b as one mediator of the pro-inflammatory response in MSCs exposed to tumor CM, which was found to be positively regulated by FAK and MAPK signaling, and negatively regulated by TGFb signaling. Thus, our data support a model where MSCs could promote cancer progression through becoming pro- inflammatory cells within the cancer stroma.