Stimulation of sodium-dependent phosphate transport and signaling mechanisms induced by basic fibroblast growth factor in MC3T3-E1 osteoblast-like cells

Physiological and pathological observations indicate that basic fibroblast growth factor (bFGF) is an important regulator of osteoblastic cell differe ntiation and in particular of cranial ossification. Experimental evidence s uggests that inorganic phosphate (P-i) transport could be an important func tion of bone matrix calcification. In the present study, we address the inf luence of bFGF on P-i transport activity in MC3T3-E1 osteoblast-like cells derived from mouse calvaria. The results indicate that bFGF is a potent and selective stimulator of sodium-dependent P-i transport in these cells. The change in P-i transport activity induced by bFGF depends on transcription and translation and corresponds to a change in the maximum velocity of the P-i transport system (V-max). These observations suggest that enhanced P-i transport activity in response to bFGF may result from insertion of newly s ynthesized P-i transporters into the plasma membrane. A selective inhibitor of fibroblast growth factor receptor (FGFR) tyrosine kinase, SU5402, blunt ed the stimulation of P-i transport induced by bFGF. It also prevented the increase in protein tyrosine phosphorylation induced by bFGF, including pho sphorylation of FGFR-1, FGFR-2, phospholipase C-gamma (PLC-gamma), and Shc as well as the recruitment of the Grb2/Sos signaling complex. In addition, bFGF-induced the activation of the mitogen-activated protein (MAP) kinases extracellular signal-regulated kinase (ERK) and p38, effects that were prev ented by SU5402. Both the protein kinase C (PKC) inhibitor calphostin C and PKC down-regulation suppressed the stimulatory effect of bFGF on P-i trans port. Selective inhibitors of ERK and p38 MAP kinases slightly reduced this cellular response with a significant effect observed with the highest conc entration of the p38 MAP kinase inhibitor. In conclusion, the results of th is study indicate that bFGF selectively stimulates P-i transport in calvari a-derived osteoblastic cells. The main signaling mechanism responsible for this effect involves tyrosine phosphorylation of PLC-gamma and activation o f PKC, with a possible contribution of the p38 MAP kinase pathway.