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We undertook a study of molecular interference of nuclear orphan receptors. Nuclear receptor response element-1 (NRRE-1) from the human medium-chain acyl coenzyme A dehydrogenase (MCAD) gene promoter was shown to contain three hexamer elements (site 1 through 3) that are known to interact with a number of nuclear receptors including chicken ovalbumin upstream promoter transcription factor (COUP-TF) and estrogen-related receptor alpha (ERRalpha). We demonstrated that the peroxisome proliferator-activated receptor alpha/9-cis-retinoic acid receptor alpha (PPARalpha/RXRalpha) heterodimer complex can also bind to the two hexamer repeat sequences (between site 1 and site 3) arranged as an everted imperfect repeat separated by 14 bp (ER14). Mutations of the putative core elements have shown that these three sites are differentially involved in ERRalpha and PPARalpha/RXRalpha binding. Homodimer of ERRalpha was shown to interact between site 1 and site 3 (ER14). To date, no nuclear receptor is known to bind to response elements over such long intervals. Interestingly, site 1 was shown to be essential for ERRalpha binding while site 3 supports its binding only in the presence of site 1. Furthermore, it was shown that the binding profile of ERRalpha and PPARalpha/RXRalpha are competitive rather than making a high order complex within NRRE-1. At the cellular level, transcriptional activation driven by the PPARalpha/RXRalpha complex was counteracted by the expression of ERRalpha in HeLa cells. These results suggest that ERRalpha and PPARalpha/RXRalpha could interfere with each other's function through binding to similar DNA elements, thereby finetuning the transcriptional outcome of the target gene. Our findings suggest a mechanism whereby multiple nuclear receptors can activate or repress DNA binding or transcription via a single pleiotropic regulatory element.

The effects of two vitamin D analogues, EB1089 and CB1093, on insulin-like growth factor binding protein (IGFBP) expression have been examined in MCF-7 and Hs578T human breast cancer cell lines. Both vitamin D analogues inhibited IGF-1 stimulated growth of MCF-7 cells and enhanced the production of IGFBP-3 as determined by Western-ligand blotting. Recombinant human IGFBP-3 inhibited the growth of MCF-7 cells over the concentration range 1-235 ng/ml. Hs578T cells were unresponsive to the mitogenic effects of IGF-1 but growth was inhibited by the two vitamin D analogues. Treatment of Hs578T cells with EB1089 and CB1093 (10 nM) as well as 100 nM 9-cis retinoic acid (9-cis RA) or all-trans retinoic acid (ATRA) was associated with increased accumulation of IGFBP-3 in conditioned medium. Furthermore, cotreatment of Hs578T cells with EB1089 and 9-cis RA led to augmented effects on both inhibition of cell growth and IGFBP-3 accumulation in conditioned medium as assessed by Western ligand blotting and radioimmunoassay. These findings suggest a role for IGFBP-3 in the growth inhibitory effects of vitamin D analogues.

The vitamin D receptor (VDR) stimulates transcription as a 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3))-activated heterodimer with retinoid X receptor (RXR). RXR also forms homodimers to mediate 9-cis retinoic acid (9-cis RA)-induced gene expression. Both receptors possess a C-terminal hormone-dependent activation function-2 (AF-2), a highly conserved region that binds coactivators to transduce the transcriptional signal. By replacing single amino acids within the AF-2 of human RXR alpha (hRXR alpha) or mouse RXR beta (mRXR beta), the contribution of these residues to transactivation by the RXR-VDR heterodimer and the RXR-RXR homodimer was evaluated. In 9-cis RA-responsive homodimers, the second and fourth positions of the AF-2 (leucine and glutamate respectively) are essential. However, in the context of an RXR-VDR heterodimer activated by 1,25(OH)(2)D(3), alteration of these two RXR residues has little effect. Instead, AF-2 residues located towards the C-terminus, such as the penultimate position (L455 in hRXR alpha or L441 in mRXR beta), are crucial for RXR-VDR heterodimers. Indeed, L455A mutant RXR exerts a dominant negative effect on RXR-VDR transcriptional responsiveness to 1,25(OH)(2)D(3). Further experiments with a mutant hRXR alpha (F313A) which elicits 9-cis RA-independent transactivation as a homodimer demonstrate that, when heterodimerized with VDR, this RXR mutant is incapable of activating the RXR-VDR heterocomplex in the absence of the VDR ligand. Taken together, these results indicate that RXR is a subordinate, yet essential transcriptional partner in RXR-VDR-mediated activation of gene expression. Furthermore, a functional switch in RXR AF-2 signaling occurs between RXR residues in the homodimeric versus the heterodimeric states, likely reflecting different interactions between subregions of the AF-2 and coactivator(s).