Parathyroid hormone-related peptide stimulates oesteogenic cell proliferation through protein kinase C activation of the Ras/mitogen-activated protein kinase signaling pathway

We investigated the mechanisms of parathyroid hormone-related peptide (PTUr P)-mediated effects on osteogenic cells in primary rat bone marrow cell (BM C) cultures. We first demonstrated by reverse transcriptase-polymerase chai n reaction and immunocytochemistry that BMCs express the type I parathyroid hormone/PTHrP receptor. Treatment with. PTHrP increased osteogenic cell pr oliferation as determined by [H-3]thymidine and bromodeoxyuridine incorpora tion and augmented osteogenic colonies. Immunocytochemistry and Western blo tting revealed no direct effect on expression of the osteoblast markers, ty pe I collagen, bone sialoprotein, and osteocalcin, indicating that PTHrP di d not directly stimulate differentiation in this system. PTHrP increased mi togen-activated protein kinase (MAPK) activity in BMC and MAPK activity, an d PTHrP-induced osteogenic cell proliferation could be blocked by the MEK i nhibitor PD-098059. PTHrP also increased Ras activity in BMC. Although wort mannin and HS, inhibitors of phosphoinositol 3-kinase and protein kinase A, respectively, did not block PTHrP-stimulated Ras or MAPK activity, chelery thrin chloride, a known protein kinase C inhibitor, did block these PTHrP a ctions as well as PTHrP-induced osteogenic cell proliferation. These result s demonstrate that PTHrP Stimulates osteogenic cell proliferation in rat ma rrow mesenchymal progenitor cells through protein kinase C-dependent activa tion of the Ras and ALA-PK signaling pathway.