Modulation of estrogen receptor transactivation and estrogen-induced gene expression by ormeloxifene - A triphenylethylene derivative

The study was aimed to investigate the interaction of D,L-ormeloxifene (Orm), a triphenylethylene and its hydroxy derivative with estrogen receptor subtypes alpha and beta, its influence on ERE-driven transcriptional activation and progesterone receptor expression. In competitive binding experiments using human recombinant ER alpha and ER beta, Orm showed interaction with both ER subtypes, with more selectivity and higher affinity towards ER alpha (8.8%) as compared to ER beta (3%). In case of 7-hydroxy derivative, the relative binding affinity for both ERs was enhanced several folds. Orm showed lower Ki, i.e. higher affinity for ER alpha (250nM) than for ER beta (750nM). It was observed that Orm promoted the formation of ER-ERE complexes in uterine tissue extract whereas its hydroxy derivative showed inhibitory effects. Transient co-transfection assay in COS-1 cells using ERE-luciferase reporter construct, revealed that Orm showed estrogenic response whereas its hydroxy- derivative was potent antiestrogen at ER alpha at transcription level. In immature rats, Orm (2 mg/kg) was associated with less increase in uterine weight and in luminal epithelial cell height than E-2 or Tam. Orm also induced the expression of PR mRNA but the expression level was significantly less than estradiol treated group. These results suggest that ER-ERE complexes formed under the influence of 7-hydroxy Orm appear to be transcriptionally less effective hence antagonizing the E-2-regulated gene expression in this target tissue. (C) 2006 Elsevier Inc. All rights reserved.