Hepatocyte growth factor (HGF) and 1,25-dihydroxyvitamin D together stimulate human bone marrow-derived stem cells toward the osteogenic phenotype by HGF-induced up-regulation of VDR

Bone formation and remodeling require generation of osteoprogenitors from bone marrow stem cells (MSC), which are regulated by growth factors and hormones, with putative roles in mesenchymal cell differentiation. Hepatocyte growth factor (HGF) is a pleiotropic growth factor, and together with its high affinity receptor cMet are widely expressed in normal tissues. 1,25-dihydroxyvitamin D (1,25OHD) is the most active metabolite of vitamin D; produced mainly in the kidney, but also by osteoblasts. We previously reported that HGF and 1,25OHD act together to increase osteogenic differentiation of human MSC (hMSC) potentially through increasing p53. Although p53 does not induce the vitamin D receptor (VDR), p63, a member of the p53 family of transcription factors has been reported to up-regulate VDR expression in some tumor cell lines, and thus might play a part in HGF-regulated VDR expression. Our hypothesis is that the combination of HGF and 1,25OHD can induce hMSC differentiation by up-regulation of 1,25OHD and/or VDR expression to increase cell response(s) to 1,25OHD. Using real-time RT-qPCR, Western blots, luciferase reporter assays, and siRNAs, as well as antibodies to specific signaling molecules we showed that HGF induced VDR gene expression, as well as up-regulated p63 gene expression. p63 gene knockdown by siRNA eliminated the effects of HGF on VDR gene expression as measured by RT-qPCR, Western blots and luciferase reporter assay, and downstream on osteogenic differentiation markers, including alkaline phosphatase staining. Differentiation is a coordinated process of cell cycle exit and tissue-specific gene expression. These results suggest HGF might be a good candidate to coordinate the regulation of these two processes during hMSC osteogenic differentiation. p63 could be a key connecting molecule on the pathway of HGF-induced VDR expression. Understanding the role of these factors and their actions could have important clinical implications for the use of hMSC in the development of novel stem cell therapies.