MODULATION OF VITAMIN-D-RECEPTOR AND ESTROGEN-RECEPTOR BY 1,25(OH)(2)-VITAMIN D-3 IN T-47D HUMAN BREAST-CANCER CELLS

1,25(OH)(2)-vitamin D-3 inhibits breast cancer cell proliferation thro ugh interaction with the vitamin D receptor (VDR). Regulation of VDR i s under the influence of several factors which include the functional ligand for this receptor (1,25(OH)(2)-vitamin D-3) as well as heterolo gous steroid hormones. We evaluated the nature of homologous regulatio n in T-47D human breast cancer cells with a radiolabelled ligand bindi ng assay and a ribonuclease protection assay for VDR. Significant VDR up-regulation, as measured by hormone binding assays, occurred with pr e-incubations with 10(-9) M through 10(-6) M 1,25(OH)(2)-vitamin D-3 ( P < 0.05). A 7-fold VDR up-regulation with 10(-8) M 1,25(OH)(2) vitami n D-3 occurred at 4 h treatment and was not associated with an increas e in VDR mRNA expression on ribonuclease protection assay. This suppor ts the hypothesis that up-regulation of VDR is probably the result of ligand-induced stabilization of pre-existing receptor. All-trans-retin oic acid, the progesterone analog R-5020, and prednisone were found to induce heterologous up-regulation of the VDR. We then determined with ligand binding assays whether 1,25(OH)(2)-vitamin D-3 could influence receptor levels for another hormone in a manner analogous to the hete rologous regulation of VDR. Regulation of estrogen receptor (ER) by 1, 25(OH)(2)-vitamin D-3 was studied in T-47D and MDA-MB-231 breast cance r cells. Incubation of T-47D cells, which are ER (+), with 10(-8) M 1, 25(OH)(2)-vitamin D-3 did not result in up-regulation of ER. Yet estro gen binding was significantly up-regulated in a cell line that is ER ( -), MDA-MB-231. The increased estrogen binding was associated with a s hift in binding affinity and ribonuclease protection assay showed abse nce of ER mRNA in these cells, suggesting an up-regulation of estrogen binding proteins and not of the ER itself.