Vascular calcification and inorganic phosphate

Vascular calcification is highly correlated with elevated serum phosphate l evels in uremic patients. To shed light an this process, we examined the ab ility of extracellular inorganic phosphate (Pi) levels to regulate human ao rtic smooth muscle cell (HSMC) culture mineralization in vitro. When cultur ed in media containing normal physiological levels of Pi (1.4 mmol/L Pi), H SMC grew in monolayers and did not mineralize. In contrast, HSMC cultured i n media containing Pi levels comparable to those seen in hyperphosphatemic individuals (>1.4 mmol/L), showed dose-dependent increases in cell culture calcium deposition. Mechanistic studies showed that elevated Pi treatment o f HSMC also enhanced the expression of the osteogenic markers, osteocalcin and Cbfa-1. The effects of elevated Pi on HSMC were mediated by a sodium-de pendent phosphate cotransporter (NPC), as Indicated by the ability of the s pecific NPC inhibitor, phosphonoformic acid (PFA), to dose-dependently inhi bit Pi-Induced calcium deposition as well as osteocalcin and Cbfa-1 gene ex pression. Using polymerase chain reaction and Northern blot analyses, the N PC in HSMC was Identified as Pit-1 (Glvr-1), a member of the type III NPCs. Interestingly, platelet-derived growth factor-BB (PDGF-BB), a potent ather ogenic stimulus, Increased the maximum velocity (Vmax) but not the affinity (Km) of phosphate uptake, enhanced the expression of Pit-1 mRNA, and induc ed HSMC culture calcification in a time- and dose-dependent manner. Importa ntly, in the presence of PDGF, HSMC culture calcification occurred under no rmophosphatemic conditions. These data suggest that elevated Pi may directl y stimulate HSMC to undergo phenotypic changes that predispose to calcifica tion and may help explain both the phenomena of human metastatic calcificat ion under hyperphosphatemic conditions as well as increased calcification i n PDGF-rich atherosclerotic lesions. (C) 2001 by the National Kidney Founda tion, Inc.