The vitamin D receptor functions as a transcription regulator in the absence of 1,25-dihydroxyvitamin D3

Abstract

The vitamin D receptor (VDR) is a critical mediator of the biological actions of 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃). As a nuclear receptor, ligand activation of the VDR leads to the protein's binding to specific sites on the genome that results in the modulation of target gene expression. The VDR is also known to play a role in the hair cycle, an action that appears to be 1,25(OH)₂D₃-independent. Indeed, in the absence of the VDR as in hereditary 1,25-dihydroxyvitamin D resistant rickets (HVDRR) both skin defects and alopecia emerge. Recently, we generated a mouse model of HVDRR without alopecia wherein a mutant human VDR lacking 1,25(OH)₂D₃-binding activity was expressed in the absence of endogenous mouse VDR. While 1,25(OH)₂D₃ failed to induce gene expression in these mice, resulting in an extensive skeletal phenotype, the receptor was capable of restoring normal hair cycling. We also noted a level of secondary hyperparathyroidism that was much higher than that seen in the VDR null mouse and was associated with an exaggerated bone phenotype as well. This suggested that the VDR might play a role in parathyroid hormone (PTH) regulation independent of 1,25(OH)₂D₃. To evaluate this hypothesis further, we contrasted PTH levels in the HVDRR mouse model with those seen in Cyp27b1 null mice where the VDR was present but the hormone was absent. The data revealed that PTH was indeed higher in Cyp27b1 null mice compared to VDR null mice. To evaluate the mechanism of action underlying such a hypothesis, we measured the expression levels of a number of VDR target genes in the duodena of wildtype mice and in transgenic mice expressing either normal or hormone-binding deficient mutant VDRs. We also compared expression levels of these genes between VDR null mice and Cyp27b1 null mice. In a subset of cases, the expression of VDR target genes was lower in mice containing the VDR as opposed to mice that did not. We suggest that the VDR may function as a selective suppressor/de-repressor of gene expression in the absence of 1,25(OH)₂D₃.

Keywords: 1,25-Dihydroxyvitamin D(3)-independent action; Cyp27b1 null mice; Transcription repressor; Transcriptional de-repressor; VDR null mice.

Fig. 1

Analysis of serum PTH levels. PTH levels were assessed in EDTA-plasma obtained from wildtype (WT), VDR null mice (VDR^−/−; white bar), wildtype humanized VDR mice (hVDR^WT/VDR^−/−), 3 mutant humanized VDR mice (T805/VDR^−/−, T806/VDR^−/− and T807/VDR^−/−; gray bars) and Cyp27b1 null mice (Cyp27b1^−/−; black bar) as indicated. Data are presented as the mean ± SEM (n=5–7 per strain). *p < 0.05 compared with wildtype mice, ^#p < 0.05 compared with VDR null mice.

Fig. 2

Basal expression of VDR target genes in duodenum of humanized VDR null mice. Expression of the indicated VDR target genes was measured in duodenum of wildtype (WT; white bars), wildtype humanized VDR mice (hVDR^WT/VDR^−/−; gray bars) and 3 mutant humanized VDR mice (T805/VDR^−/−, T806/VDR^−/− and T807/VDR^−/−; black bars) by qPCR. Expression levels of the gene transcripts were normalized to Gapdh and presented as the mean ± SEM (5–7 mice per strain). *p < 0.05 compared with wildtype mice, ^#p < 0.05 compared with wildtype humanized VDR mice, ^@p < 0.05 compared with T805/VDR^−/− and T806/VDR^−/− mice.

Fig. 3

Basal expression of VDR target genes in duodenum of Cyp27b1 null mice. Expression of the indicated VDR target genes was measured in duodenum of VDR null mice (VDR^−/−; gray bars) and Cyp27b1 null mice (Cyp27b1^−/−; black bars) by qPCR. Expression levels of the gene transcripts were normalized to Gapdh and presented as the mean ± SEM (5–7 mice per strain). *p < 0.05 compared with VDR null mice.