We have isolated a highly expressed splice variant mRNA of murine estrogen receptor-beta (ERbeta), mERbeta2, containing an in-frame 54 nucleotide insertion between exons 5 and 6 of wild-type mERbeta1. The predicted ERbeta2 protein contains 18 amino acids inserted in the ligand binding domain of mERbeta1. Recombinant protein generated by in vitro transcription/translation showed that mERbeta2 had markedly reduced ligand binding (K(D)=17.7+/-4.7 nM, mean+/-s.e.m., n=3) compared with mERbeta1-bound (3)H-estradiol (K(D)=0.56+/- 0.19 nM, mean+/-s.e.m., n=3). Both receptors bound similarly to palindromic estrogen responsive elements (EREs) in vitro and in vivo, and similarly bent DNA. Transcriptional activity was assessed using transient transfection analysis into a homologous murine cell line, NIH 3T3 cells. mERbeta1 transactivated ERE-tk-CAT reporter genes similarly to mERalpha, whereas mERbeta2 had little activity except at high ligand concentrations. However, under conditions in which mERbeta2 is unlikely to be ligand saturated, co-transfected mERbeta2 inhibited activity of mERalpha and possibly mERbeta1 on ERE-tk-CAT genes. Using a 'novel raloxifene responsive' gene reporter system (TGF-beta3-CAT), we found the ability of estradiol and LY117018 to activate both mERalpha and mERbeta1 on this promoter was identical, and mERbeta2 activity in the presence of either estradiol or LY117018 was only slightly less than that observed with either mERbeta1 or mERalpha. Both mERbeta1 and mERbeta2 when liganded with LY117018 inhibited transcription at a classical ERE-regulated promoter under these transfection conditions, which was in marked contrast to their stimulatory effect at the transforming growth factor-beta3 promoter. These data suggest that responsiveness of gene expression to a relatively highly expressed variant murine ERbeta isoform, mERbeta2, is both ligand and promoter specific. Determination of the relative level of expression of mERbeta1 mRNA and mERbeta2 mRNA in mouse tissues indicated predominance of mERbeta2 mRNA in some but not all tissues. These data suggest that the mERbeta2 may have some tissue-specific and promoter-specific modulatory effects.

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