Transforming Growth Factor-β 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)-β family are important regulators of skeletal development. In this study, we investigated the effect of TGF-β1 on inorganic phosphate (Pi) transport and on expression of the type III Pi carriers Glvr-1 and Ram-1 in murine ATDC5 chondrocytes. TGF-β1 induced a selective, dose- and time-dependent increase in sodium-dependent Pi transport in ATDC5 cells. This response was dependent on RNA and protein synthesis and reflected a change in the maximal rate of the transport system, suggesting that TGF-β1 induces the synthesis of new Pi carriers and their insertion into the plasma membrane. Consistently, Northern blotting analysis showed a dose-dependent increase in Glvr-1 messenger RNA expression in response to TGF-β1, which preceded the maximal stimulation of Pi transport by several hours. Glvr-1 thus likely mediates at least part of the increase in Pi uptake induced by TGF-β1. Ram-1 messenger RNA expression was not affected by TGF-β1. TGF-β1 activated the Smad signaling pathway and the mitogen-activated protein kinases ERK and p38 in ATDC5 cells. Unlike the regulation of Pi transport by receptor tyrosine kinase agonists in osteoblasts, the effect of TGF-β1 on Pi uptake in ATDC5 cells did not involve protein kinase C or mitogen-activated protein kinases, suggesting that a specific, possibly Smad-dependent, signal mediates this response. In conclusion, TGF-β1 stimulates Pi transport and Glvr-1 expression in chondrocytes, suggesting that, like proliferation, differentiation, and matrix synthesis, Pi handling is subject to regulation by TGF-β family members in bone-forming cells.