The role of Cyclin D1 in regulating cell proliferation through binding of Cdk4/6 and subsequently phosporylating and inactivating retinoblastoma protein (Rb) has been well characterized. Overexpression in 30 to 40% of human breast cancers, associates cyclin D1 with a poor prognosis in ERα positive cancers. Cyclin D1 is well known to be required for ERα, ErbB2, or Ras-induced cellular proliferation and hyperplasia. It enhances ERα transactivation through a stable coactivator complex formation in the either absence or presence of estrogen. Given that ERα function is repressed by BRCA1 and activated by cyclin D1 in human breast cancer, we examined the possibility that cyclin D1 may contribute to the activation of ERα through a formation of a specific regulatory protein complex and disassociation of co-repressor complex.

We have previously shown that BRCA1 breast tumor susceptibility gene product inhibited ligand-bound ERα activity and that cyclin D1 selectively antagonized BRCA1-induced repression of liganded ERα transactivation independently of its G1 phase cdk-holoenzyme function in human breast and prostate cancer cells. A novel helix-loop-helix domain of cyclin D1 was required for both ERα binding and rescue of BRCA1-mediated ERα transcriptional repression. At present study, we demonstrated that ERα co-eluted with BRCA1 at > 4 mDa and at 670 kDa with coactivators P/CAF, AIB1 and SRC1. Estradiol recruited cyclin D1 into the 670 kDa ERα-coactivator complex which excluded BRCA1. Cyclin D1-/- cells showed dramatic reduction ERα coactivator complex, and proportional increase in the abundance of ERα in the >4 mDa complex, implicating cyclin D1 in the regulation of the ERα distribution between the BRCA1 and ERα-coactivator complexes. We further identified a WW motif (residues W360 and W383 spaced by 22 amino acids), which is required for BRCA1 and co-activator association. Mutation of either residue enhanced ERα transactivation in response to ERα agonists and antagonists. However, simultaneous mutation of both residues completely abolished ERα activity due to its deficiency in cyclin D1 and coactivator binding. Studies described propose cyclin D1 function in antagonizing BRCA1 repression of ERα by recruitment of the ERα to its coactivators. As cyclin D1 abundance is regulated by oncogenic and mitogenic signals, the antagonism of the BRCA1-mediated ERα repression by cyclin D1 may contribute to the selective induction of BRCA1-regulated target genes.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA