BAG-1 p50 isoform interacts with the vitamin D receptor and its cellular overexpression inhibits the vitamin D pathway

Human BAG-1 is an anti-apoptotic protein with four protein isoforms (BAG-1 p50, p46, p33, and p29), BAG-1 p46 was originally isolated in a screen for proteins binding to the glucocorticoid receptor; it binds and modulates the action of several members of the nuclear steroid hormone receptor superfam ily, The vitamin D receptor (VDR) is another member of this superfamily, an d the vitamin D pathway is important for prevention and therapy of osteopor osis, renal failure, cancer, and psoriasis, Therefore, we investigated the effect of the recently isolated BAG-1 p50 on the vitamin D pathway. By use of Far Western blot analysis and glutathione S-transferase BAG-1 p50 bindin g assays, BAG-1 p50 was demonstrated to interact with the VDR, and the BAG- 1 p50 N-terminus was required. In U87 cells that were stably transfected wi th BAG-1 p50, binding of the VDR to its response element in electrophoretic mobility shift assays was blocked, enhancement of transcriptional activati on was inhibited, cell growth rate was enhanced, cell growth inhibition ind uced by 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3] was blocked, and 1,25(OH )(2)D-3-mediated VDR induction was inhibited. These results suggest that BA G-1 p50 is a novel regulator of the vitamin D signaling pathway, and its ov erexpression may lead to cellular resistance to 1,25(OH)(2)D-3 therapy. (C) 2001 Academic Press.