Estrogen receptor-α, RBCK1, and protein kinase C β 1 cooperate to regulate estrogen receptor-α gene expression

Estrogen receptor alpha (ER alpha) is initially overexpressed in two-thirds of all breast cancers and is involved in its development and proliferation. We previously reported that the RanBP-type and C3HC4-type zinc finger containing 1 (RBCK1) interacts with the ER alpha promoter and that RBCK1 expression positively correlates with ER alpha levels, expression of ER alpha downstream target genes, and proliferation of breast cancer cells. Based on this, and that RBCK1 positively correlates with ER alpha expression in breast cancer samples, we propose RBCK1 as a potential therapeutic target in breast cancer acting as a modulator of ER alpha expression. To further explore this, the molecular mechanism by which RBCK1 regulates ER alpha expression has to be defined. Here, we show that ER alpha, RBCK1, and the RBCK1-interacting protein protein kinase C beta 1 (PKC beta(1)) co-occupy a previously identified ER alpha binding region in the proximal ER alpha promoter. We describe a number of mechanistic details of this complex including that RBCK1 recruitment to the ER alpha promoter B is facilitated by ER alpha, which in turn facilitates PKC beta(1) recruitment and PKC beta(1)-dependent histone modifications. Furthermore, ER alpha regulation of its own mRNA expression is facilitated by RBCK1 recruitment, suggesting an ER alpha coactivator function of RBCK1. The interaction between RBCK1 and ER alpha was dependent on the E3 ubiquitin ligase domain of RBCK1 and the activating function-1 domain of ER alpha. The ligand-binding function of ER alpha does not influence the interaction with RBCK1. In summary, our data provide insight into the molecular mechanism by which ER alpha expression is modulated in breast cancer cells. Journal of Molecular Endocrinology (2012) 49, 277-287