Expression of estrogen receptor co-regulators SRC-1, RIP140 and NCoR and their interaction with estrogen receptor in rat uterus, under the influence of ormeloxifene

Ormeloxifene binds competitively to ERs and antagonizes estrogen-induced gene expression in the uterus. However its detailed molecular mechanisms are not well understood. Present study was aimed to examine the changes in expression pattern of co-regulatory proteins SRC-1 (co-activator), RIP140 and NCoR (co-repressors) and their interaction with ER alpha in rat uterus under the influence of ormeloxifene (Orm) and tamoxifen (Tam). Adult ovariectomized rats were treated with estradiol (E-2) (5 mu g/100g), or Orm or Tam (200 mu g/100 g, s.c.) alone or along with E-2, for 3 days. RT-PCR analysis of uterine RNA and immunoblotting of uterine extracts revealed that expression of SRC-1, RIP140 and NCoR was insensitive to E-2 or Orm or Tam treatment. Direct protein-protein interaction experiments using co-immunoprecipitation revealed that E-2-induced the interaction of ER alpha with co-activator SRC-1. In rats given Orm, alone or along with E-2, there was a significant reduction in E-2-induced effect on ER alpha-SRC-1 interaction. In case of ER beta and SRC-1, Orm reduced interaction only in the absence of E-2. Interaction of RIP140 or NCoR with ER alpha was found to be more in rats treated with Orm along with E-2 as compared to that in E-2-treated rats whereas no such recruitment was found in Tam treated rats. Interaction of RIP140 with ER beta was insensitive to Orm or Tam treatment whereas the interaction of NCoR with ER alpha and ER beta was increased in Orm treated rats. Ormeloxifene also showed inhibitory effects on uterine ER-ERE binding and estrogen-induced expression of progesterone receptor. Taken together, these findings demonstrate that ormeloxifene antagonizes ER alpha-mediated transcription by inhibiting the recruitment of SRC-I and inducing the recruitment of RIP140 and NCoR. (C) 2009 Elsevier Ltd. All rights reserved.