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

无机磷是糖酵解、糖异生、能量代谢和骨骼矿化过程中必不可少的营养物质。肾刷状缘膜(BBM)上表达的II型钠依赖磷酸转运体(NPT2)起着生理和病理生理调节磷酸盐稳态的作用。NPT2的表达受转录水平、NPT2的细胞内转位和转录后水平的调控。本研究旨在探讨NPT2在细胞内转位的分子机制。PTH在细胞水平上抑制肾脏磷酸盐转运，通过激活蛋白激酶A和C，选择性地将NPT2从肾近端小管的BBM中清除。甲状旁腺素也能刺激100或77 kDa蛋白质的磷酸化，但不能刺激NPT2。降钙素中也有类似的观察结果。内吞的NPT2被重新分布到细胞质中，但如果PTH治疗延长，它们似乎是针对溶酶体的快速降解，而不是回收到骨髓基质中。在另一种机制中，从大鼠肾脏分离出的独特的NPT2相关蛋白(NPT2α、NPT2β和NPT2γ)参与了研究。NPT2α和NPT2γ糖基化后分别为45 kDa和35 kDa。在分离的BBM囊泡中，N端抗体与45 kDa和40 kDa蛋白反应，C端抗体与37 kDa蛋白反应。这些蛋白质的大小与糖基化形式的大小相对应。功能分析表明，γ对非洲爪哇卵母细胞的NPT2活性有明显的抑制作用。因此，甲状旁腺素和NPT2的短亚型可促进NPT2在刷状缘膜上的快速内化，从而抑制近端小管对磷酸盐的重吸收。

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.