Multiple Pathways Coordinating Reprogramming of Endothelial Cells into Osteoblasts by BMP4

Cell type transition occurs during normal development and under pathological conditions.  In prostate cancer bone metastasis, prostate cancer-secreted BMP4 induces endothelial-to-osteoblast (EC-to-OSB) transition, leading to aberrant bone formation.  Such tumor-induced stromal reprogramming supports prostate cancer progression.  Here, we delineate signaling pathways mediating EC-to-OSB transition using endothelial cell lines 2H11 and SVR.  We found that for EC-to-OSB transition to occur, inhibition of angiogenesis, through the Smad1-Notch-Hey1 pathway that inhibits EC migration and tube formation, together with activation of osteogenesis, through the p38MAPK(p44/42ERK,AKT)-GSK3β-βcatenin-Slug pathway that stimulates osterix and osteocalcin expression, are required.  In addition, Smad1-regulated Dlx2 plays a role in converging the Smad1 and β-catenin pathways during EC-to-OSB transition.  Importantly, by just co-expressing the four transcription factors, osterix, Dlx2, Slug and Hey1, we were able to achieve EC-to-OSB transition, leading to bone matrix mineralization even in the absence of BMP4.  Thus, the interplay of cell fate determinants and transcription factors mediates BMP4-induced EC-to-OSB transition during stromal reprogramming.  In human prostate cancer bone metastasis specimens, immunohistochemical analysis showed that β-catenin and pSmad1 are detected in activated osteoblasts rimming the tumor-induced bone. Similarly, in MDA-PCa-118b and C4-2b-BMP4 osteogenic xenografts, β-catenin and pSmad1 are expressed in EC-OSB hybrid cells that rim the tumor-induced bone.  Our results elucidated the pathways and key molecules coordinating prostate cancer-induced stromal programming and provide potential targets for therapeutic intervention.