Role of the sodium-dependent phosphate cotransporter, Pit-1, in vascular smooth muscle cell calcification

Vascular calcification is associated with cardiovascular morbidity and mortality. Hyperphosphatemia is an important contributor to vascular calcification. Our previous studies demonstrated that elevated phosphate induces calcification of smooth muscle cells (SMC) in vitro. Inhibition of phosphate transport by phosphonoformic acid blocked phosphate-induced calcification, implicating sodium-dependent phosphate cotransporters in this process. In the present study, we have investigated the role of the type III sodium-dependent phosphate cotransporter, Pit-1, in SMC calcification in vitro. Human SMC stably expressing Pit-1 small interfering double-stranded RNA (SMC-iRNA) were established using a retroviral system. SMC-iRNA had decreased Pit-1 mRNA and protein levels and sodium-dependent phosphate transport activity compared with the control transduced cells (SMC-CT) (2.9 versus 9.78 nmol/mg protein per 30 minutes, respectively). Furthermore, phosphate-induced SMC calcification was significantly inhibited in SMC-iRNA compared with SMC-CT at all time points examined. Overexpression of Pit-1 restored phosphate uptake and phosphate-induced calcification in Pit-1 deficient cells. Mechanistically, although Pit-1-mediated SMC calcification was not associated with apoptosis or cell-derived vesicles, inhibition of phosphate uptake in Pit-1 knockdown cells blocked the induction of the osteogenic markers Cbfa-1 and osteopontin. Our results indicate that phosphate uptake through Pit-1 is essential for SMC calcification and phenotypic modulation in response to elevated phosphate.