Stimulation of Na-dependent phosphate transport by platelet-derived growth factor in rat aortic smooth muscle cells

We investigated the effect of platelet-derived growth factor B homodimer (PDGF-BB) on inorganic phosphate (Pi) transport activity, which has been reported to be involved in the mechanism of atherosclerosis, in A-10 rat aortic vascular smooth muscle cells (VSMCs). PDGF-BB time- and dose-dependently stimulated Pi transport in A-10 cells. Using northern blot analysis, the PDGF-BB-enhanced Pi transporter (PiT) in A-10 cells was identified as Pit-1 (Glvr-1), a member of the type III Na-dependent PiT. An inhibitor of PDGF β-receptor tyrosine kinase suppressed PDGF-BB-induced Pi transport. Both a protein kinase C (PKC) inhibitor calphostin C and PKC down regulation suppressed the stimulatory effect of PDGF-BB on Pi transport. On the other hand, inhibition of mitogen-activated protein (MAP) kinases by selective inhibitors did not affect Pi transport. Ly294002, a phosphatidylinositol (PI) 3-kinase inhibitor, partially attenuated PDGF-BB-induced Pi transport. A selective inhibitor of S6 kinase, rapamycin, reduced this effect of PDGF-BB, while Akt kinase inhibitor did not. In summary, these results indicated that PDGF-BB is a potent and selective stimulator of Pi transport in VSMCs. The mechanism responsible for this effect is not mediated by MAP kinase, but involves activation of PKC, PI 3-kinase and S6 kinase.