Activation of Signal Transducers and Activators of Transcription 3 and Focal Adhesion Kinase by Stromal Cell-Derived Factor 1 Is Required for Migration of Human Mesenchymal Stem Cells in Response to Tumor Cell-Conditioned Medium

Mesenchymal stem cells (MSCs) migrate to tumors both in vitro and in vivo. Gene expression profiling analysis reveals that stromal cell-derived factor 1 (SDF-1) is significantly upregulated in MSCs exposed to tumor cell-conditioned medium, when compared with cells treated with control medium, suggesting that SDF-1 signaling is important in mediating MSC migration. This study investigates downstream signaling during MSC migration in response to tumor cell-conditioned medium and recombinant SDF-1 protein treatments. We observed that both recombinant SDF-1 and tumor cell-conditioned medium were able to activate downstream signaling via signal transducer and activator of transcription 3 (STAT3) and extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) as revealed by increased phosphorylation of STAT3 and ERK1/2 in human MSCs (hMSCs). Significant impairment of in vitro migration was observed in the presence of MAPK/ERK kinase (MEK) inhibitor PD98059, whereas two Janus kinase 2 (Jak2) inhibitors completely abolished migration induced by tumor cell-conditioned medium. Impaired MSC migration correlated with decreased levels of phosphorylated STAT3 and ERK1/2, suggesting that SDF-1 stimulation activates Jak2/STAT3 as well as MEK/ERK1/2 signaling, which in turn promotes migration of MSCs toward tumor cells. Furthermore, stimulation of hMSCs with recombinant SDF-1 and tumor cell-conditioned medium also significantly activated the focal adhesion kinases (FAKs) and paxillin, which correlated with reorganization of F-actin filaments in hMSCs. Decreased phosphorylation of FAK and paxillin as well as disruption of cytoskeleton organization was observed following Jak2 and MEK inhibitor treatment. Taken together, our results provide insight into the molecular pathways responsible for MSC migration toward the tumor microenvironment and may provide the molecular basis for modifying MSCs for therapeutic purposes.