Transforming growth factor-beta stimulates inorganic phosphate transport and expression of the type III phosphate transporter Glvr-1 in chondrogenic ATDC5 cells

Members of the transforming growth factor (TGF)-beta family are important r egulators of skeletal development. In this study, we investigated the effec t of TGF-beta 1 on inorganic phosphate (Pi) transport and on expression of the type III Pi carriers Glvr-1 and Ram-1 in murine ATDC5 chondrocytes. TGF -beta 1 induced a selective, dose- and time-dependent increase in sodium-de pendent Pi transport in ATDC5 cells. This response was dependent on RNA and protein synthesis and reflected a change in the maximal rate of the transp ort system, suggesting that TGF-beta 1 induces the synthesis of new Pi carr iers and their insertion into the plasma membrane. Consistently, Northern b lotting analysis showed a dose-dependent increase in Glvr-1 messenger RNA e xpression in response to TGF-beta 1, which preceded the maximal stimulation of Pi transport by several hours. Glvr-1 thus likely mediates at least par t of the increase in Pi uptake induced by TGF-beta 1. Ram-1 messenger RNA e xpression was not affected by TGF-beta 1. TGF-beta 1 activated the Smad sig naling pathway and the mitogen-activated protein kinases ERK and p38 in ATD C5 cells. Unlike the regulation of Pi transport by receptor tyrosine kinase agonists in osteoblasts, the effect of TGF-beta 1 on Pi uptake in ATDC5 ce lls did not involve protein kinase C or mitogen-activated protein kinases, suggesting that a specific, possibly Smad-dependent, signal mediates this r esponse. In conclusion, TGF-beta 1 stimulates Pi transport and Glvr-1 expre ssion in chondrocytes, suggesting that, like proliferation, differentiation , and matrix synthesis, Pi handling is subject to regulation by TGF-beta fa mily members in bone-forming cells.