Parathyroid hormone-dependent endocytosis of renal type IIc Na-P_icotransporter

Abstract

Hereditary hypophosphatemic rickets with hypercalciuria results from mutations of the renal type IIc Na-Pi cotransporter gene, suggesting that the type IIc transporter plays a prominent role in renal phosphate handling. The goal of the present study was to investigate the regulation of the type IIc Na-Pi cotransporter by parathyroid hormone (PTH). Type IIc Na-Pi cotransporter levels were markedly increased in thyroparathyroidectomized (TPTX) rats. Four hours after administration of PTH, type IIc transporter protein levels were markedly decreased in the apical membrane fraction but recovered to baseline levels at 24 h. Immunohistochemical analyses demonstrated the presence of the type IIc transporter in the apical membrane and subapical compartments in the proximal tubular cells in TPTX animals. After administration of PTH, the intensity of immunoreactive signals in apical and subapical type IIc transporter decreased in the renal proximal tubular cells in TPTX rats. Colchicine completely blocked the internalization of the type IIc transporter. In addition, leupeptin prevented the PTH-mediated degradation of the type IIa transporter in lysosomes but had no effect on PTH-mediated degradation of the lysosomal type IIc transporter. In PTH-treated TPTX rats, the internalization of the type IIc transporter occurred after administration of PTH(1-34) (PKA and PKC activator) or PTH(3-34) (PKC activator). Thus the present study demonstrated that PTH is a major hormonal regulator of the type IIc Na-Pi cotransporter in renal proximal tubules. phosphate transporter; proximal tubule; regulation INORGANIC PHOSPHATE (P i ) reabsorption in the renal proximal tubule is required for body P i homeostasis, and Na ϩ -dependent P i (Na-P i ) transporters in the brush-border membrane (BBM) of proximal tubular cells mediate the rate-limiting step in the overall P i -reabsorptive process (17, 19 -21). The type IIa and type IIc Na-P i cotransporters are expressed in the apical membrane of proximal tubular cells and mediate Na-P i cotransport, and the extent of P i reabsorption in the proximal tubules is determined largely by the abundance of the type IIa Na-P i cotransporter (17, 19 -21, 28). Indeed, Na-P i cotransporter type IIa-deficient mice (Npt2a Ϫ/Ϫ mice) exhibit increased urinary P i excretion, hypophosphatemia, and an appropriate adaptive increase in the circulating concentration of 1,25(OH) 2 D 3 (2, 28). Additional biochemical findings in Npt2aϪ/Ϫ mice include hypercalcemia, hypercalciuria, decreased serum parathyroid hormone (PTH) levels, and elevated serum alkaline phosphatase activity (2, 28). These biochemical features are typical of patients with hereditary hypophosphatemic rickets with hypercalciuria (HHRH), a Mendelian disorder of renal P i reabsorption (6, 30). However, in contrast to patients with HHRH, Npt2a Ϫ/Ϫ mice do not exhibit rickets and osteomalacia (2, 7, 28), and linkage analyses indicated that the Npt2a gene is not a candidate for HHRH patients (9, 33).More recently, two groups demonstrated that HHRH results f...

Type IIc Sodium–Dependent Phosphate Transporter Regulates Calcium Metabolism

“…19,20 The precipitation of cortical membrane was also performed as previous described. 21 The homogenate of the tissue in 0.3 M sucrose containing protease inhibitor cocktail tablets (Complete Mini, EDTA-free; Roche Diagnostics, Indianapolis, IN) was centrifuged at 600 ϫ g for 10 min. The supernatant was then centrifuged at 105,000 ϫ g for 45 min, and the membrane pellet was used for Western blotting.…”

“…19,21 For immunostaining, serial sections (5 m) were incubated with affinity-purified Npt2a antibodies (1:1000) and Npt2c antibodies (1: 1000). Sections were then treated with Envision (ϩ) rabbit peroxidase (Dako, Carpinteria, CA) for 30 min at room temperature.…”

Post-transcriptional regulation of the Type IIA sodium-phosphate cotransporter by parathyroid hormone.

“…It is interesting to note that P i -depleted animals fail to produce a hypophosphatemic response to PTH administration, despite normal activation of PKA 156 .…”

The SLC34 family of sodium-dependent phosphate transporters

“…NaPiIIc removal from the brush border membrane in response to PTH or intake of phosphate-rich diets occurs with much slower kinetics than for NaPi-IIa and requires hours [112,127,126]. In contrast to NaPi-IIa, NaPi-IIc may undergo partial recycling [126].…”