PLATELET-DERIVED GROWTH-FACTOR STIMULATES SODIUM-DEPENDENT P-I TRANSPORT IN OSTEOBLASTIC CELLS VIA PHOSPHOLIPASE C-GAMMA AND PHOSPHATIDYLINOSITOL 3'-KINASE

Inorganic phosphate (P-i) is a major regulator of cell metabolism. The P-i transport activity in the plasma membrane is a main determinant o f the intracellular level of this ion. In bone-forming cells, P-i tran sport is important for the calcification of the bone matrix In this st udy, the effect of platelet-derived growth factor (PDGF) on P-i transp ort activity and the signaling mechanism involved in this cellular res ponse were analyzed, The results indicate that PDGF is a potent and se lective stimulator of sodium-dependent P-i transport in the mouse calv ariaderived MC3T3-E1 osteoblast-like cells. The change in P-i transpor t induced by PDGF-BB was dependent on translational processes and affe cted the V-max of the P-i transport system, These observations suggest ed that enhanced P-i transport activity in response to PDGF resulted f rom insertion of newly synthesized P-i transporters in the plasma memb rane. The role of activation of mitogen activated protein (MAP) kinase , phospholipase C (PLC)gamma or phosphatidylinositol 3-kinase (PI-3-ki nase), in mediating this effect of PDGF, was investigated. A selective inhibitor of the PDGF receptor tyrosine kinase activity (CGP 53716) c ompletely blocked PDGF-induced protein tyrosine phosphorylation of sev eral proteins including the PDGF receptor, PLC gamma, MAP kinase, and association of the p85 subunit of PI-3'-kinase. Associated with this e ffect, the increase in P-i transport induced by PDGF was completely bl unted by 5 mu M CGP 53716. Inhibition of MAP kinase activity by cAMP a gonists did not influence P-i transport stimulation induced by PDGF. H owever, inhibitors of protein kinase C completely blocked this respons e. A selective inhibitor of PI-3-kinase, LY294002, also significantly reduced this effect of PDGF. In summary, these results indicate that P DGF is a potent and selective stimulator of P-i transport in osteoblas tic cells. The mechanism responsible for this effect is not mediated b y MAP kinase but involves tyrosine phosphorylation-dependent activatio n of PLC gamma and PI-3-kinase.