Src kinases mediate VEGFR2 transactivation by the osteostatin domain of PTHrP to modulate osteoblastic function

Abstract

Parathyroid hormone-related protein (PTHrP) stimulates osteoblastic function through its N- and C-terminal domains. Since the osteogenic action of the latter domain appears to depend at least in part on its interaction with the vascular endothelial growth factor (VEGF) system, we aimed to explore the putative mechanism underlying this interaction in osteoblasts. Using native conditions for protein extraction and immunoblotting, we found that both PTHrP (107-139) and the shorter PTHrP (107-111) peptide (known as osteostatin), at 100 nM, promoted the appearance of a VEGF receptor (VEGFR) 2 protein band of apparent Mr. wt. 230 kDa, which likely represents its activation by dimer formation, in mouse osteoblastic MC3T3-E1 cells. Moreover, osteostatin (100 nM) maximally increased VEGFR2 phosphorylation at Tyr-1059 within 5-10 min in both MC3T3-E1 and rat osteoblastic osteosarcoma UMR-106 cells. This phosphorylation elicited by osteostatin appears to be VEGF-independent, but prevented by the VEGFR2 activation inhibitor SU1498 and also by the Src kinase inhibitors SU6656 and PP1. Furthermore, osteostatin induced phosphorylation of Src, extracellular signal-regulated kinase (ERK) and Akt with a similar time course to that observed for VEGFR2 activation in these osteoblastic cells. This osteostatin-dependent induction of ERK and Akt activation was abrogated by SU6656. Up-regulation of VEGF and osteoprotegerin gene expression as well as the pro-survival effect induced by osteostatin treatment were all prevented by both SU1498 and SU6656 in these osteoblastic cells. Collectively, these findings demonstrate that the osteostatin domain of C-terminal PTHrP phosphorylates VEGFR2 through Src activation, which represents a mechanism for modulating osteoblastic function.