Institute of Anatomy, University of Zurich, Zurich, Switzerland.
Pflugers Arch. 2010 Aug;460(3):677-87. doi: 10.1007/s00424-010-0841-1. Epub 2010 Jun 5.
Renal phosphate reabsorption across the brush border membrane (BBM) in the proximal tubule is mediated by at least three transporters, NaPi-IIa (SLC34A1), NaPi-IIc (SLC34A3), and Pit-2 (SLC20A2). Parathyroid hormone (PTH) is a potent phosphaturic factor exerting an acute and chronic reduction in proximal tubule phosphate reabsorption. PTH acutely induces NaPi-IIa internalization from the BBM and lysosomal degradation, but its effects on NaPi-IIc and Pit-2 are unknown. In rats adapted to low phosphate diet, acute (30 and 60 min) application of PTH decreased BBM phosphate transport rates both in the absence and the presence of phosphonoformic acid, an inhibitor of SLC34 but not SLC20 transporters. Immunohistochemistry showed NaPi-IIa expression in the S1 to the S3 segment of superficial and juxtamedullary nephrons; NaPi-IIc was only detectable in S1 segments and Pit-2 in S1 and weakly in S2 segments of superficial and juxtamedullary nephrons. PTH reduced NaPi-IIa staining in the BBM with increased intracellular and lysosomal appearance. NaPi-IIa internalization was most prominent in S1 segments of superficial nephrons. We did not detect changes in NaPi-IIc and Pit-2 staining over this time period. Blockade of lysosomal protein degradation with leupeptin revealed NaPi-IIa accumulation in lysosomes, but no lysosomal staining for NaPi-IIc or Pit-2 could be detected. Immunoblotting of BBM confirmed the reduction in NaPi-IIa abundance and the absence of any effect on NaPi-IIc expression. Pit-2 protein abundance was also significantly reduced by PTH. Thus, function and expression of BBM phosphate cotransporters are differentially regulated allowing for fine-tuning of renal phosphate reabsorption.
近端肾小管刷状缘膜（BBM）的肾磷酸盐重吸收至少由三种转运体介导，即 NaPi-IIa（SLC34A1）、NaPi-IIc（SLC34A3）和 Pit-2（SLC20A2）。甲状旁腺激素（PTH）是一种有效的降磷因子，可在急性和慢性阶段降低近端肾小管的磷酸盐重吸收。PTH 可急性诱导 NaPi-IIa 从 BBM 内化和溶酶体降解，但对 NaPi-IIc 和 Pit-2 的影响尚不清楚。在适应低磷酸盐饮食的大鼠中，急性（30 和 60 分钟）应用 PTH 降低了 BBM 磷酸盐转运速率，无论是在没有还是存在膦甲酸（SLC34 但不是 SLC20 转运体的抑制剂）的情况下。免疫组织化学显示 NaPi-IIa 在浅层和近髓肾单位 S1 至 S3 段的表达；NaPi-IIc 仅在 S1 段检测到，Pit-2 在浅层和近髓肾单位的 S1 和 S2 段弱表达。PTH 减少了 BBM 中 NaPi-IIa 的染色，同时增加了细胞内和溶酶体的出现。NaPi-IIa 的内化在浅层肾单位的 S1 段最为明显。在这段时间内，我们没有检测到 NaPi-IIc 和 Pit-2 染色的变化。用亮肽素阻断溶酶体蛋白降解显示 NaPi-IIa 在溶酶体中的积累，但没有检测到 NaPi-IIc 或 Pit-2 的溶酶体染色。BBM 的免疫印迹证实了 NaPi-IIa 丰度的减少，并且对 NaPi-IIc 表达没有任何影响。PTH 还显著降低了 Pit-2 蛋白的丰度。因此，BBM 磷酸盐共转运体的功能和表达受到差异调节，从而实现了肾脏磷酸盐重吸收的精细调节。
