Suppression of gene expression by tethering KRAB domain to promoter of ER target genesProceedings of Xth International Congress on Hormonal Steroids, Quebec, Canada, 17–21 June 1998.

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Estrogens play an important role in the development and progression of breast cancer. Although estrogen antagonist treatment often results in the arrest or remission of breast cancer growth, most breast cancers recur and become resistant to estrogen ablative therapy. The molecular mechanisms underlying these actions remain largely undefined. It is hypothesized that tumor cells of an advanced stage may develop compensatory pathways to stimulate the expression of estrogen receptor (ER) target genes or downstream events, independent of estrogen action. In this study, we developed a chimeric repressor to turn off ER target genes with the aim of directly investigating the role of ER target genes in tumor progression. The chimeric repressor contains the ER DNA-binding domain that recognizes estrogen response elements (EREs), a Krupple-associated box (KRAB) repressor domain which silences target genes when tethered to their promoter regions and a truncated progesterone ligand-binding domain which responds only to the exogenous synthetic ligand, RU486. The ability of the chimeric repressor to block ER mediated transcription was assessed in transient transfection assays. ER-induced reporter activity was inhibited by the repressor in a dose-dependent manner, with the maximum effect of more than 80% reduction. The inhibitory activity of the chimeric repressor was tightly under the control of RU486. Effective suppression by the repressor on the natural promoter of ER target gene, complement factor 3 (C3), was also observed. The inhibitory activity was specific to ER, since the repressor has no effect on other nuclear receptor systems tested. Furthermore, the repressor could inhibit the 4-hydroxy-tamoxifen (4OH-T)-induced ER activity. Taken together, our results demonstrate that the inducible repressor we have designed could specifically inhibit ER target gene expression in response to an exogenous synthetic ligand. This repressor will provide a useful tool to study the role of ER target genes in breast cancer progression and it may be potentially useful for gene therapy of breast cancer.