The regulatory mechanism of phosphate transport activity mediated by sodium-dependent phosphate transporter

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Inorganic phosphate (phosphate) is an essential nutrient in the processes of glycolysis, gluconeogenesis, energy metabolism and skeletal mineralization. Type II sodium-dependent phosphate transporter (NPT2) expressed on renal brush border membrane (BBM) serves to physiologically and pathophysiologically regulate phosphate homeostasis. The NPT2 expression is regulated by transcriptional level, intracellular translocation of NPT2, and post-transcriptional level. The present study was performed to investigate the molecular mechanism of intracellular translocation of NPT2.PTH inhibits renal phosphate transport at the cellular level involves selective endocytic removal of NPT2 from the BBM of the renal proximal tubule mediated by the activation of both protein kinases A and C activated. PTH also stimulates phosphorylation of 100 or 77 kDa proteins, but not NPT2. Similar observation was found in calcitonin. The endocytosed NPT2 are redistributed to the cytoplasm but they appear to be targeted for rapid lysosomal degradation rather than recycling to the BBM if PTH treatment is prolonged.In another mechanism, the involvement of unique NPT2-related proteins (NPT2α, NPT2β, and NPT2γ) isolated from a rat kidney were investigated. NPT2α and NPT2γ were glycosylated and revealed to be 45- and 35-kDa proteins, respectively. In isolated BBM vesicles, an N-terminal antibody was reacted with the 45- and 40-kDa proteins, and a C-terminal antibody was reacted with the 37-kDa protein. The sizes of these proteins corresponded to those in glycosylated forms. A functional analysis demonstrated that NPT2γ markedly inhibited NPT2 activity in Xenopus oocytes. The findings suggest that this short isoform may function as a dominant negative inhibitor of the full-length transporter.Therefore, it is concluded PTH and short isoform of NPT2 stimulate rapid endocytic internalization of NPT2 at the brush border membrane, leads to an inhibition of phosphate reabsorption in the proximal tubules.