Regulation Of BRCA1 And BARD1 In Human Ovarian Granulosa Cells

Germ-line mutations in BRCA1 predispose women to early onset, familial breast and ovarian cancers. Genetic analysis clearly establishes that BRCA1 acts as a tumor suppressor in breast and ovarian cancer development. A large amount of evidence has implicated BRCA1 in multiple cellular functions including DNA damage response, transcriptional regulation, and control of centrosome duplication. And recent work from our laboratory in non-epithelial cells also revealed an unexpected role of BRCA1 in modulation of aromatase expression, a key enzyme involved in estrogen biosynthesis. Therefore, the combinatory deficiency of BRCA1 functions in both breast/ovarian epithelial and non-epithelial cells may contribute to the tissue-specificity of BRCA1-associated tumors. In light of the essential role of BRCA1 in tumor suppression, it is conceivable that regulation of its own expression under various conditions constitutes a distinct pathway for the tissue-specific tumor suppression. Indeed, it has been demonstrated that epigenetic modification plays an important role in down-regulation of BRCA1 expression in sporadic breast cancers. In addition, the mRNA and protein abundance of BRCA1 and its partner BARD1 also fluctuate significantly during normal mammary gland development, which presumably reflect the roles of BRCA1 and its partner in cell proliferation and differentiation. In the current study, we focused on the regulation of BRCA1/BARD1 expression during steroidogensis in ovarian granulosa cells. Our work uncovers a reverse correlation between down-regulation of BRCA1/BARD1 and cyclic AMP-dependent expression of aromatase in granulosa cells. Both transcriptional and post-transcriptional mechanisms are involved in reducing BRCA1/BARD1 level upon activation of cAMP-dependent signaling pathway. Interestingly, the basal level of BRCA1/BARD1 in granulosa cells is also tightly controlled in a proteasome-dependent manner. Importantly, we find that the N-terminal regionof BRCAl is intrinsically unstable and may contain a "degron" sequence that confers protein degradation of the native BRCAl protein. Furthermore, our study shows that the same region is heavily ubiquitinated and degraded via a proteasome-mediated pathway. Finally, the relative stability of BRCAl and BARDl is influenced by the protein-protein interaction between the two proteins. Thus, the cAMP-triggered degradation of BRCAl and BARDl in ovarian granulosa cells provides an excellent paradigm for investigating the regulation of this protein complex under a physiological condition.