Atypical protein kinase C regulates dual pathways for degradation of the oncogenic coactivator SRC-3/AIB1

SRC-3/AIB1 is a steroid receptor coactivator with potent growth-promoting activity, and its overexpression is sufficient to induce tumorigenesis. Previous studies indicate that the cellular level of SRC-3 is tightly regulated by both ubiquitin-dependent and ubiquitin-independent proteasomal degradation pathways. Atypical protein kinase C (aPKC) is frequently overexpressed in cancers. In the present study, we show that aPKC phosphorylates and specifically stabilizes SRC-3 in a selective ER-dependent manner. We further demonstrate that an acidic residue-rich region in SRC-3 is an important determinant for aPKC-mediated phosphorylation and stabilization. The mechanism of the aPKC-mediated stabilization appears due to a decreased interaction between SRC-3 and the C8 subunit of the 20S core proteasome, thus preventing SRC-3 degradation. Our results demonstrate a potent signaling mechanism for regulating SRC-3 levels in cells by coordinate enzymatic inhibition of both ubiquitin-dependent and ubiquitin-independent proteolytic pathways.     

The nuclear receptor coactivator PGC-1alpha exhibits modes of interaction with the estrogen receptor distinct from those of SRC-1

Estrogen receptor (ER) function is mediated by multi-domain co-regulator proteins. A fluorescently labelled fragment of the human PGC-1alpha co-regulator (residues 91-408) bearing the two motifs most strongly implicated in interactions with nuclear receptors (NR box2 and NR box3), was used to characterize in vitro binding of PGC-1alpha to ER. Anisotropy measurements revealed that the affinity of this PGC-1alpha fragment for human ERalpha and beta was fairly strong in the presence of estradiol (approximately 5 nM), and that unlike a similar fragment of SRC-1 (570-780), PGC-191-408 exhibited ligand-independent interactions with ER, particularly with ERbeta (Kd approximately 30 nM). Competition experiments of the complex between ERalpha and fluorescently labelled PGC-1 91-408 with unlabelled SRC-1 570-780 showed that PGC-1 91-408 was an efficient competitor of SRC-1 570-780, while the inverse was not true, underscoring their distinct modes of binding. The anisotropy data provide strong evidence for a ternary complex between ERalpha, SRC-1 570-780 and PGC-1 91-408. GST-pull-down experiments with deletion mutants of ERalpha revealed that the constitutive binding of PGC-1 91-408 requires the presence of the linker domain between the DNA binding and ligand binding domains (DBD and LBD). Homology modeling studies of the different regions of full length PGC-1alpha confirmed the lack of compact tertiary structure of the N-terminal region bearing the NR box motifs, and suggested a slightly different mode of interaction compared to the NR box motifs of SRC-1. They also provided reasonable structural models for the coiled-coil dimerization motif at residues 633-675, as well as the C-terminal putative RNA binding domain, raising important questions concerning the stoichiometry of its complex with the nuclear receptors.     

The SRC-3/AIB1 coactivator is degraded in a ubiquitin- and ATP-independent manner by the REGgamma proteasome

Steroid receptor coactivator-3 (SRC-3/AIB1) is an oncogene frequently amplified and overexpressed in breast cancers. Here we report that SRC-3 interacts with REGgamma, a proteasome activator known to stimulate the trypsin-like activity of the 20S proteasome. RNAi knockdown and gain-of-function experiments suggest that REGgamma promotes SRC-3 protein degradation. Cellular levels of REGgamma expression affect estrogen-receptor target-gene expression and cell growth as a result of its ability to promote degradation of the SRC-3 protein. In vitro proteasome proteolysis assays using purified REGgamma, SRC-3, and the 20S proteasome reinforce these conclusions and demonstrate that REGgamma promotes the degradation of SRC-3 in a ubiquitin- and ATP-independent manner. This work demonstrates the first example of a physiologically relevant endogenous cellular target for the REGgamma-proteasome complex. It also highlights the fact that an alternative mode of proteasome-mediated protein degradation, independent of the 19S proteasome regulatory cap, targets the SRC-3 protein for degradation.     

Tissue- and nuclear receptor-specific function of the C-terminal LXXLL motif of coactivator NCoA6/AIB3 in mice

Although the LXXLL motif of nuclear receptor (NR) coactivators is essential for interaction with NRs, its role has not been assessed in unbiased animal models. The nuclear receptor coactivator 6 (NCoA6; also AIB3, PRIP, ASC-2, TRBP, RAP250, or NRC) is a coactivator containing an N-terminal LXXLL-1 (L1) and a C-terminal L2. L1 interacts with many NRs, while L2 interacts with the liver X receptor α (LXRα) and the estrogen receptor α (ERα). We generated mice in which L2 was mutated into AXXAL (L2m) to disrupt its interaction with LXRα and ERα. NCoA6^L2m/L2m mice exhibited normal reproduction, mammary gland morphogenesis, and ERα target gene expression. In contrast, when treated with an LXRα agonist, lipogenesis and the LXRα target gene expression were significantly reduced in NCoA6^L2m/L2m mice. The induction of Cyp7A1 expression by a high-cholesterol diet was impaired in NCoA6^L2m/L2m mice, which reduced bile acid synthesis in the liver and excretion in the feces and resulted in cholesterol accumulation in the liver and blood. These results demonstrate that L2 plays a tissue- and NR-specific role: it is required for NCoA6 to mediate LXRα-regulated lipogenesis and cholesterol/bile acid homeostasis in the liver but not required for ERα function in the mammary gland.