CtIP, a BRCA1 binding protein, is a transcriptional corepressor of estrogen receptor α.

Abstract

Abstract Abstract #606 The role of estrogen and estrogen receptor (ER) transcriptional regulation in breast carcinogenesis is indisputable. In the last few years there has been dramatic progress in understanding how receptor transcriptional coactivator and corepressor complexes operate.
 We demonstrated that CtIP (CtBP interacting protein) is downregulated in tamoxifen resistant breast cancer lines. Furthermore, CtIP protein expression correlates with response to neo-adjuvant endocrine therapy and patients with progressive disease express lower CtIP protein in their primary breast carcinomas than those who respond. CtIP expression directly correlates with ER alpha expression, and inversely correlates with disease free survival and breast cancer metastasis. Importantly, silencing endogenous CtIP confers tamoxifen resistance in tamoxifen sensitive breast cancer cells and re-expression restored tamoxifen sensitivity in resistant breast cancer cells (Wu et al. Molec. Cancer Res. 2007; 5: 1285-1295). Our findings indicate that CtIP silencing is a novel mechanism for the development of tamoxifen resistance in breast cancer.
 CtIP interacts with the BRCT domains of BRCA1 where most mutations occur in familial BRCA1 breast cancers. CtIP also interacts with the transcriptional corepressor CtBP. Emerging evidence suggests that CtIP is involved in transcriptional repression and may be a tumor suppressor gene. We hypothesize that CtIP plays a key role in repression of ER transcription and such repressive function is linked to the inhibitory growth effects exerted by tamoxifen. We further speculate that CtIP inhibition of ER transcription is mediated by its ability to bridge BRCA1 with CtBP to form a transcriptional corepressor complex able to modulate ER activity via the interaction between BRCA1 and ER and that CtIP is in fact required for BRCA1 to exert its transcriptional repression activity on ER. We will present our work showing that CtIP, BRCA1 and CtBP form a multiprotein complex, that requires CtIP, and associates with ER in breast cancer cells. More importantly, we will show that BRCA1, CtIP and CtBP associate with ER on the ER-dependent promoter of the pS2 gene in the absence of ligand. Stimulation by ligand leads to dissociation of the complex and ER-dependent gene expression. Furthermore, silencing CtIP expression using RNAi in E2-starved MCF-7 cells led to an increase of basal transcription of the pS2 gene. CtIP was also observed to repress E2 stimulation of ERE-luciferase in transfected MCF-7 cells. We are currently characterizing the role of the CtIP corepressor complex in ER-transcriptional activity. We will present the results of our efforts to identify specific ER-dependent promoters repressed by the CtIP corepressor. In addition, we will present results of studies aimed at characterizing CtIP-specific changes in chromatin remodeling and ER-cofactor recruitment at ER-dependent promoters. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 606.