MOLECULAR-CLONING AND CHARACTERIZATION OF HUMAN ESTROGEN-RECEPTOR BETA-CX - A POTENTIAL INHIBITOR OF ESTROGEN ACTION IN HUMAN

We have identified and characterized a novel human estrogen receptor ( ER) beta isoform, ER beta cx, which is truncated at the C-terminal reg ion but has an extra 26 amino acids due to alternative splicing. The E R beta cx transcript is expressed in testis, ovary, thymus and prostat e as well as in human cultured cell lines such as HEC-1, HOS-TE85 and Saos-2 cells. ER beta cx protein is also immunodetectable in these hum an cells. Biochemical analysis reveals that the average dissociation c onstants (K-d) of ER alpha and ER beta for 17 beta-estradiol (E-2) are 0.2 and 0.6 nM respectively, but ER beta cx has no ligand binding abi lity. ER alpha and ER beta proteins bind to the estrogen response elem ent, whereas ER beta cx does not form any shifted complex in gel shift assays. In a transient expression assay, ER beta cx shows no ligand-d ependent transactivation ability of a basal promoter and also cannot i nteract with a cofactor, TIFl alpha, in the presence or absence of E-2 . ER beta cx preferentially forms a heterodimer with ER alpha rather t han that with ER beta, inhibiting DNA binding by ER alpha. Interesting ly however, it shows a significant dominant negative activity only aga inst ER alpha transactivation. Thus, this study indicates that ER beta cx potentially inhibits ER alpha-mediated estrogen action and that al ternative splicing of the C-terminal region and its inhibitory propert ies are characteristic of several members of nuclear receptor isoforms .