title: Renal expression of the sodium/phosphate cotransporter gene, Npt2, is not required for regulation of renal 1 alpha-hydroxylase by phosphate.
abstract: Several reports have suggested that the regulation of renal 1,25-dihydroxyvitamin D [1,25-(OH)(2)D] synthesis by extracellular phosphate (Pi) is dependent on normal transepithelial Pi transport by the renal tubule. Mice homozygous for the disrupted Na/Pi cotransporter gene Npt2 (Npt2(-/-)) exhibit renal Pi wasting, an approximately 85% decrease in renal brush border membrane Na/Pi cotransport, hypophosphatemia, and an increase in serum 1,25-(OH)(2)D concentration. We undertook 1) to determine the mechanism for the increased circulating levels of 1,25-(OH)(2)D in Npt2(-/-) mice and 2) to establish whether renal 1alpha-hydroxylase was appropriately regulated by dietary Pi in the absence of Npt2 gene expression. On a control diet, the 2.5-fold increase in the serum 1,25-(OH)(2)D concentration in Npt2(-/-) mice, relative to that in Npt2(+/+) littermates, is associated with a corresponding increase in renal mitochondrial 25-hydroxyvitamin D-1 alpha-hydroxylase (1 alpha-hydroxylase) activity and messenger RNA (mRNA) abundance. A low Pi diet elicits an increase in serum 1,25-(OH)(2)D concentration, renal 1alpha-hydroxylase activity, and mRNA abundance in Npt2(+/+) and Npt2(-/-) mice to similar levels in both mouse strains. A high Pi diet has no effect on serum 1,25-(OH)(2)D concentration, renal 1 alpha-hydroxylase activity, or mRNA abundance in Npt2(+/+) mice, but normalizes these parameters in Npt2(-/-) mice. In addition, renal 24-hydroxylase mRNA abundance is significantly reduced in Npt2(-/-) mice compared with that in Npt2(+/+) mice under all dietary conditions. In summary, we demonstrate that 1) increased renal synthesis of 1,25-(OH)(2)D is responsible for the increased serum 1,25-(OH)(2)D concentration in Npt2(-/-) mice; and 2) renal 1alpha-hydroxylase gene expression is appropriately regulated by dietary manipulation of serum Pi in both Npt2(+/+) and Npt2(-/-) mice. Thus, intact renal Na/Pi cotransport is not required for the regulation of renal 1alpha-hydroxylase by Pi.
authors.affiliation: Department of Pediatrics, McGill University-Montreal Children's Hospital Research Institute, Montréal, Québec, Canada H3H 1P3. mdht@www.debelle.mcgill.ca
authors.forename: H S
authors.initials: HS
authors.lastname: Tenenhouse
authors.forename: J
authors.initials: J
authors.lastname: Martel
authors.forename: C
authors.initials: C
authors.lastname: Gauthier
authors.forename: M Y
authors.initials: MY
authors.lastname: Zhang
authors.forename: A A
authors.initials: AA
authors.lastname: Portale
mesh_terms: D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D002352:Carrier Proteins / Q000235:genetics*; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics; D004032:Diet; D005260:Female; D015870:Gene Expression / Q000502:physiology*; D007668:Kidney / Q000201:enzymology / Q000502:physiology*; D008297:Male; D051379:Mice; D018345:Mice, Knockout / Q000235:genetics; D008928:Mitochondria / Q000201:enzymology; D010710:Phosphates / Q000008:administration & dosage / Q000494:pharmacology*; D012333:RNA, Messenger / Q000378:metabolism; D050608:Sodium-Phosphate Cotransporter Proteins; D050609:Sodium-Phosphate Cotransporter Proteins, Type I; D050611:Sodium-Phosphate Cotransporter Proteins, Type III; D013250:Steroid Hydroxylases / Q000235:genetics; D027981:Symporters; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood; D065668:Vitamin D3 24-Hydroxylase
chemical_list: D002352:Carrier Proteins; D010710:Phosphates; D012333:RNA, Messenger; C493549:Slc17a2 protein, mouse; D050608:Sodium-Phosphate Cotransporter Proteins; D050609:Sodium-Phosphate Cotransporter Proteins, Type I; D050611:Sodium-Phosphate Cotransporter Proteins, Type III; D027981:Symporters; D014807:Vitamin D; C097949:1,25-dihydroxyvitamin D; D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; D065668:Vitamin D3 24-Hydroxylase; D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase