The
retinoid X receptor beta (RXR beta; H-2RIIBP) forms heterodimers with various
nuclear hormone receptors and binds multiple
hormone response elements, including the
estrogen response element (ERE). In this report, we show that endogenous RXR beta contributes to ERE binding activity in nuclear extracts of the human
breast cancer cell line MCF-7. To define a possible regulatory role of RXR beta regarding
estrogen-responsive transcription in
breast cancer cells, RXR beta and a reporter gene driven by the
vitellogenin A2 ERE were transfected into
estrogen-treated MCF-7 cells. RXR beta inhibited ERE-driven reporter activity in a dose-dependent and
element-specific fashion. This inhibition occurred in the absence of the RXR
ligand 9-cis
retinoic acid. The RXR beta-induced inhibition was specific for
estrogen receptor (ER)-mediated ERE activation because inhibition was observed in ER-negative MDA-MB-231 cells only following transfection of the
estrogen-activated ER. No inhibition of the basal reporter activity was observed. The inhibition was not caused by simple competition of RXR beta with the ER for ERE binding, since deletion mutants retaining
DNA binding activity but lacking the N-terminal or C-terminal domain failed to inhibit reporter activity. In addition, cross-linking studies indicated the presence of an auxiliary nuclear factor present in MCF-7 cells that contributed to RXR beta binding of the ERE. Studies using known heterodimerization partners of RXR beta confirmed that RXR beta/
triiodothyronine receptor alpha heterodimers avidly bind the ERE but revealed the existence of another
triiodothyronine-independent pathway of ERE inhibition. These results indicate that
estrogen-responsive genes may be negatively regulated by RXR beta through two distinct pathways.