Expression of 25(OH)D₃24-hydroxylase in distal nephron: coordinate regulation by 1,25(OH)₂D₃and cAMP or PTH

Abstract

Previous studies using microdissected nephron segments reported that the exclusive site of renal 25-hydroxyvitamin D3-24-hydroxylase (24OHase) activity is the renal proximal convoluted tubule (PCT). We now report the presence of 24OHase mRNA, protein, and activity in cells that are devoid of markers of proximal tubules but express characteristics highly specific for the distal tubule. 24OHase mRNA was undetectable in vehicle-treated mouse distal convoluted tubule (DCT) cells but was markedly induced when DCT cells were treated with 1,25 dihydroxyvitamin D3[1,25(OH)2D3]. 24OHase protein and activity were also identified in DCT cells by Western blot analysis and HPLC, respectively. 8-Bromo-cAMP (1 mM) or parathyroid hormone [PTH-(1—34); 10 nM] was found to potentiate the effect of 1,25(OH)2D3on 24OHase mRNA. The stimulatory effect of cAMP or PTH on 24OHase expression in DCT cells suggests differential regulation of 24OHase expression in the PCT and DCT. In the presence of cAMP and 1,25(OH)2D3, a four- to sixfold induction in vitamin D receptor (VDR) mRNA was observed. VDR protein, as determined by Western blot analysis, was also enhanced in the presence of cAMP. Transient transfection analysis in DCT cells with rat 24OHase promoter deletion constructs demonstrated that cAMP enhanced 1,25(OH)2D3-induced 24OHase transcription but this enhancement was not mediated by cAMP response elements (CREs) in the 24OHase promoter. We conclude that 1) although the PCT is the major site of localization of 24OHase, 24OHase mRNA and activity can also be localized in the distal nephron; 2) both PTH and cAMP modulate the induction of 24OHase expression by 1,25(OH)2D3in DCT cells in a manner different from that reported in the PCT; and 3) in DCT cells, upregulation of VDR levels by cAMP, and not an effect on CREs in the 24OHase promoter, is one mechanism involved in the cAMP-mediated modulation of 24OHase transcription.

Regulation of Vitamin D hydroxylases gene expression by 1,25-dihydroxyvitamin D3 and cyclic AMP in cultured human syncytiotrophoblasts

“…cAMP effects on CYP24A1 are probably mediated by phosphorylation of CREB and binding to putative CRE sites at the promoter level [29,30]. In addition, the observation that calcitriol induced the strongest transcriptional response on CYP24A1 gene, suggested, as previously shown in kidney [31], that cAMP upregulation of VDR is probably one mechanism involved in cAMP-mediated modulation of CYP24A1 transcription [31,32]. Furthermore, Dhawan et.…”

Vitamin D receptor is required for dietary calcium-induced repression of calbindin-D9k expression in mice

“…Since changes in the VDR content can alter expression of many target genes [9][10][11], we then investigated whether changes in VDR expression were involved in the calciuminduced repression of CaBP-D9k in wild-type mice. As shown in Fig.…”

Extracellular calcium is a direct effecter of VDR levels in proximal tubule epithelial cells that counter-balances effects of PTH on renal Vitamin D metabolism

“…However, PTH also enhances the efficiency of proximal tubule 1␣-hydroxyation by repressing 1,25(OH) 2 D 3 -receptor (VDR) transcription and mediating 1,25(OH) 2 D 3 -24-hydroxylase (CYP24) down-regulation [7][8][9][10][11][12]. In vitro studies that have examined a role of [Ca 2+ ] e on either CYP27B1 or CYP24 systems have failed to demonstrate any direct effects at physiologically meaningful concentrations, while in vivo studies depict a general correlation between the rise in serum [Ca 2+ ] e with the reciprocal suppression and induction of CYP27B1 and CYP24, respectively.…”