Novel regulatory function for NHERF-1 in Npt2a transcription

Several lines of evidence show that sodium/hydrogen exchanger regulatory factor 1 (NHERF-1) regulates the expression and activity of the type IIa sodium-dependent phosphate transporter (Npt2a) in renal proximal tubules. We have previously demonstrated that expression of a COOH-terminal ezrin binding domain-deficient NHERF-1 in opossum kidney (OK) cells decreased expression of Npt2a in apical membranes but did not affect responses to parathyroid hormone. We hypothesized that NHERF-1 regulates apical membrane expression of Npt2a in renal proximal tubule cells. To address this hypothesis, we compared regulation of Npt2a expression and function in NHERF-deficient OK cells (OK-H) and wild-type cells (OK-WT). In OK-H cells, phosphate uptake and expression of Npt2a protein in apical membranes were significantly lower than in OK-WT cells. Transient transfection of green fluorescent protein-tagged Npt2a cDNA into OK-H cells resulted in aberrant localization of an Npt2a fragment to the cytosol but not to the apical membrane. OK-H cells also exhibited a marked decrease in Npt2a mRNA expression. As demonstrated by luciferase assay, Npt2a promoter activity was significantly decreased in OK-H cells compared with that shown in OK-WT cells. Transfection of OK-H cells with human NHERF-1 restored Npt2a expression at both the protein and mRNA levels and regulation by parathyroid hormone. Expression of NHERF-1 constructs with mutations in the PDZ domains or the ezrin binding domain in OK-H cells suggested that the PDZ2 domain is critical for apical translocation of Npt2a and for expression at the mRNA level. Our data demonstrate for the first time that NHERF-1 regulates Npt2a transcription and membrane insertion.