Post-transcriptional regulation of osteoblastic platelet-derived growth factor receptor-alpha expression by co-cultured primary endothelial cells.

Platelet-derived growth factor receptor (PDGFR) signaling plays an important role in osteoblast function. Inhibition of PDGFR activity leads to a suppression of osteoblast proliferation, whereas mineralized matrix production is enhanced. In previous experiments, we showed that co-cultivation of human primary endothelial cells and human primary osteoblasts (hOBs) leads to a cell contact-dependent downregulation of PDGFR-alpha expression in the osteoblasts. In this study, we investigated this effect in more detail, revealing that human umbilical vein endothelial cell (HUVEC)-mediated PDGFR-alpha downregulation is dependent on time and cell number. This effect was specific to endothelial cells and was not observed when hOBs were co-cultured with human primary chondrocytes or fibroblasts. Likewise, HUVEC-mediated suppression of PDGFR-alpha expression was only seen in hOBs and mesenchymal stem cells but not in immortalized osteoblastic cell lines. Functional inhibition of gap junctional communication between HUVECs and hOBs by 18alpha-glycyrrhetinic acid had no effect on HUVEC-mediated PDGFR-alpha downregulation, whereas inhibition of p38 mitogen-activated protein kinase (MAPK) prevented the HUVEC-mediated reduction in osteoblastic PDGFR-alpha expression. To delineate the molecular mechanism underlying the PDGFR-alpha downregulation, we examined the effect of HUVEC co-cultivation on osteoblastic PDGFR-alpha promoter activity as well as mRNA stability. Co-cultivation of HUVECs with hOBs significantly shortened the half-life of osteoblastic PDGFR-alpha mRNA, but did not decrease its promoter activity. In summary, our data show that PDGFR-alpha is downregulated in hOBs by co-cultivation with human primary endothelial cells through a p38 MAPK-dependent post-transcriptional mechanism.