RORα Binds to E2F1 To Inhibit Cell Proliferation and Regulate Mammary Gland Branching Morphogenesis

Retinoic acid receptor-related orphan nuclear receptor alpha (ROR alpha) is a potent tumor suppressor that reduces cell proliferation and inhibits tumor growth. However, the molecular mechanism by which it inhibits cell proliferation remains unknown. We demonstrate a noncanonical nuclear receptor pathway in which ROR alpha binds to E2F1 to inhibit cell cycle progression. We showed that ROR alpha bound to the heptad repeat and marked box region of E2F1 and suppressed E2F1-regulated transcription in epithelial cells. Binding of ROR alpha inhibited E2F1 acetylation and its DNA-binding activity by recruiting histone deacetylase 1 (HDAC1) to the protein complexes. Knockdown of HDAC1 or inhibition of HDAC activity at least partially rescued transcription factor activity of E2F1 that was repressed by ROR alpha. Importantly, ROR alpha levels were increased in mammary ducts compared to terminal end buds and inversely correlated with expression of E2F1 target genes and cell proliferation. Silencing ROR alpha in mammary epithelial cells significantly enhanced cell proliferation in the ductal epithelial cells and promoted side branching of the mammary ducts. These results reveal a novel link between ROR alpha and E2F1 in regulating cell cycle progression and mammary tissue morphogenesis.