RESUMEN
BACKGROUND: Injection of parathyroid hormone (PTH) rapidly stimulates renal Pi excretion, in part by downregulating NaPi-IIa (Npt2a/SLC34A1) and NaPi-IIc (Npt2c/SLC34A3) transporters. The mechanisms underlying the effects of PTH on NaPi-IIc are not fully elucidated. METHODS: We analyzed the effect of PTH on inorganic phosphate (Pi) reabsorption in Npt2a-/- mice to eliminate the influence of Npt2a on renal Pi reabsorption. In opossum kidney (OK) cells and Xenopus oocytes, we investigated the effect of NaPi-IIc transporter phosphorylation. Studies of mice with mutations of NaPi-IIc protein in which serine and threonine were replaced with either alanine (A), which prevents phosphorylation, or aspartic acid (D), which mimics the charged state of phosphorylated NaPi-IIc, were also performed to evaluate the involvement of phosphorylation in the regulation of transport function. RESULTS: The Npt2a-/- experiments showed that PTH administration rapidly inactivated NaPi-IIc function in the apical membrane of proximal tubular cells. Analysis of mutant proteins (S71, S138, T151, S174, T583) at putative protein kinase C sites, revealed that S138 markedly suppressed the function and cellular expression of mouse NaPi-IIc in Xenopus oocytes and OK cells. In addition, 138D had a short half-life compared with wild-type protein. CONCLUSIONS: The present study suggests that acute regulation of NaPi-IIc protein by PTH is involved in the inactivation of Na+-dependent Pi cotransporter activity and that phosphorylation of the transporter is involved in the rapid modification.
Asunto(s)
Túbulos Renales Proximales/efectos de los fármacos , Hormona Paratiroidea/farmacología , Fragmentos de Péptidos/farmacología , Fosfatos/metabolismo , Proteína Quinasa C/metabolismo , Procesamiento Proteico-Postraduccional/efectos de los fármacos , Reabsorción Renal/efectos de los fármacos , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIc/metabolismo , Animales , Línea Celular , Femenino , Túbulos Renales Proximales/metabolismo , Masculino , Ratones Noqueados , Zarigüeyas , Fosforilación , Estabilidad Proteica , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIa/deficiencia , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIa/genética , Proteínas Cotransportadoras de Sodio-Fosfato de Tipo IIc/genética , Factores de Tiempo , XenopusRESUMEN
Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) , an autosomal recessive disorder first identified in a large Bedouin tribe, is characterized by hypophosphatemia secondary to renal inorganic phosphate (Pi) wasting, resulting in increased serum1,25-dihydroxyvitamin D3 concentrations with associated intestinal calcium hyperabsorption, hypercalciuria, rickets, and osteomalacia. Recent studies identified several mutations in the NaPi-2c/NPT2c transporter gene (SLC34A3) as the cause of HHRH. The fact that HHRH is caused by NaPi-2c loss-of-function mutations is compatible with the HHRH phenotype and the prevailing view of renal Pi regulation. The NaPi-2c mutants in HHRH show defective processing and stability.