Regulation of renal parathyroid hormone receptor expression by 1, 25-dihydroxyvitamin D3 and retinoic acid

The renal distal convoluted tubule (DCT) is the major site of parathyroid hormone (PTH) and 1alpha,25-dihydroxyvitamin D3 [1, 25(OH)2D3]-regulated calcium absorption. 1,25(OH)2D3 augments PTH-stimulated calcium transport by DCT cells, while having no effect of its own. 1,25(OH)2D3 mediates its effects on gene expression by binding to a nuclear vitamin-D receptor (VDR), which then associates with the retinoid-X receptor (RXR) as a heterodimer. We studied the effects of 1,25(OH)2D3, 9-cis- and all-trans-retinoic acid on PTH/PTHrP receptor expression. mRNAs for the PTH/PTHrP, VDR, and RXR receptors were detected in immortalized DCT cells by reverse transcriptase-polymerase chain reaction. Changes in PTH/PTHrP receptor mRNA expression were quantified by slot blot hybridization. 1,25(OH)2D3 maximally increased PTH/PTHrP receptor mRNA levels by 70%. The stimulation was specific since 1,25(OH)2D3 treatment had no effect on the expression of adrenergic receptor or Na+/H+ exchanger mRNA levels. Likewise, the inactive form, 25(OH)2D3 had no effect on PTH/PTHrP receptor mRNA expression. In combination with the putative RXR ligand, 9-cis-retinoic acid, 1,25(OH)2D3 increased PTH/PTHrP receptor mRNA levels 4-fold. 9-cis-Retinoic acid had no effect of its own on steady-state PTH/PTHrP receptor mRNA expression. The putative ligand for the retinoic acid receptor, all-trans-retinoic acid, increased PTH/PTHrP receptor mRNA expression alone and in combination with 1,25(OH)2D3. 9-cis-Retinoic acid alone, and in combination with 1,25(OH)2D3, also increased specific PTH/PTHrP receptor binding to plasma membranes isolated from DCT cells. These results indicate that 1,25(OH)2D3 upregulated PTH/PTHrP receptor expression at both mRNA and protein levels in a manner consistent with VDR/RXR heterodimers transactivating the PTH/PTHrP receptor gene by binding a vitamin D response element in the PTH/PTHrP gene.