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The Wilms' tumor gene (wt1) encodes a transcription factor involved in urogenital development, in particular in renal differentiation, and in hematopoietic differentiation. Differentiation of a number of solid tumor and leukemic cells lines can be mediated by 1,25-dihydroxyvitamin D3. This is predominantly mediated by the nuclear receptor for 1,25-dihydroxyvitamin D3, the vitamin D receptor (VDR). In initial experiments addressing a possible link between WT1 and VDR, we observed a correlated expression of WT1 and VDR mRNA in samples from renal tissues. HT29 colon carcinoma cells, stably transfected to express WT1, exhibited elevated endogenous VDR levels compared with control cells transfected with a control construct. Elevated VDR expression was found in wt1-transfected human embryonic kidney 293 cells, as well. In transient cotransfection experiments, we observed an activation of a vdrpromoter reporter by WT1 through a WT1 recognition element, indicating transcriptional regulation of the vdrgene expression by WT1. The responsive sequence element was specifically bound by wild-type, but not by mutated WT1, in electrophoretic mobility shift assays. HT29 colon carcinoma cells, which respond to 1,25-dihydroxyvitamin D3with slow induction of growth arrest, were investigated for the influence of WT1 on 1,25-dihydroxyvitamin D3-mediated growth suppression. Although HT29 cells transfected with a control construct responded moderately to 1,25-dihydroxyvitamin D3, the response of HT29 cells expressing WT1 was strikingly enhanced. Stimulation with dihydroxyvitamin D3caused an up to 3-fold reduction in the growth rate of different HT29 clones expressing WT1 as compared with control cells lacking WT1 expression. Thus, induction of VDR by WT1 leads to an enhanced response to 1,25-dihydroxyvitamin D3. We conclude that the vitamin D receptor gene is a target for transcriptional activation by WT1, suggesting a possible physiological role of this regulatory pathway.

The availability of the mouse vitamin D receptor (mVDR) gene has allowed a characterization of a TATA-less promoter containing a cluster of four Sp1 sites named Sp1-1, Sp1-2, Sp1-3, and Sp1-4 (F. Jehan and H. F. DeLuca, 1997, Proc. Natl. Acad. Sci. USA 94, 10138–10143). By means of primer extension analysis, S1 nuclease mapping and ribonuclease protection assay, the start site has been deduced, as has the existence of other minor transcription start sites. Initiation of transcription at the major site is located 4 bp upstream of the 5′ end of the mVDR cDNA sequence and very close to the putative Sp1 sites. A second minor promoter might exist between exon 1 and exon 2 of the mVDR gene. The nucleotide sequence of the Sp1 region is well conserved between the mouse, the human, and the chicken VDR genes, suggesting an important role for these Sp1 sites. Gel shift analysis of the four Sp1 sites of the mVDR promoter has confirmed specific binding complexes to Sp1-1, Sp1-2, and Sp1-4 (Sp1-3 rather binds an unknown complex that is unable to bind the canonical Sp1 GGGGCGGGGC). Deletion or mutation of all the Sp1 sites eliminates promoter activity. However, mutation or deletion of individual Sp1 sites did not dramatically change the promoter activity, except for mutation of Sp1-3 that increases promoter activity. We, therefore, conclude that the mVDR promoter is controlled by the Sp1 sites and is the main VDR promoter in intestine and kidney.