Ribozyme targeting demonstrates that the nuclear receptor coactivator AIB1 is a rate-limiting factor for estrogen-dependent growth of human MCF-7 breast cancer cells

Human breast tumorigenesis is promoted by the estrogen receptor pathway, and nuclear receptor coactivators are thought to participate in this process. Here we studied whether one of these coactivators, AIB1 (amplified in breast cancer 1), was rate-limiting for hormone-dependent growth of human MCF-7 breast cancer cells. We developed MCF-7 breast cancer cell lines in which the expression of AIB1 can be modulated by regulatable ribozymes directed against AIB1 mRNA. We found that depletion of endogenous AIB1 levels reduced steroid hormone signaling via the estrogen receptor alpha or progesterone receptor beta on transiently transfected reporter templates. Down-regulation of AIB1 levels in MCF-7 cells did not affect estrogen-stimulated cell cycle progression but reduced estrogen-mediated inhibition of apoptosis and cell growth. Finally, upon reduction of endogenous AIB1 expression, estrogen-dependent colony formation in soft agar and tumor growth of MCF-7 cells in nude mice was decreased. From these findings we conclude that, despite the presence of different estrogen receptor coactivators in breast cancer cells, AIB1 exerts a rate-limiting role for hormone-dependent human breast tumor growth.     

Ligand-dependent formation of retinoid receptors, receptor-interacting protein 140 (RIP140), and histone deacetylase complex is mediated by a novel receptor-interacting motif of RIP140

Receptor-interacting protein 140 (RIP140) interacts with retinoic acid receptor and retinoid X receptor in a ligand-dependent manner and suppresses retinoic acid (RA) induction of its target genes. The receptor-interacting motif is mapped to a C-terminal peptide sequence (LTKTNPILYYMLQK) of RIP140. The functional role of this motif in mediating the suppressive effects of RIP140 on RA induction is demonstrated in mutation studies. RA induces coimmunoprecipitation of histone deacetylase 3 with retinoic acid receptor/retinoid X receptor in the presence of wild type RIP140, but not in the presence of the C-terminal motif-deleted RIP140. A decrease in histone acetylation on the promoter region that carries a RA response element is associated with the expression of wild type RIP140, but not with expression of the mutant RIP140, in a dose-dependent manner. These data provide a molecular explanation for RIP140 acting as a novel ligand-dependent, negative modulator of RA-regulated gene expression.