Role of the RIP140 corepressor in ovulation and adipose biology

RIP140 is a ligand-dependent corepressor for most, if not all, nuclear receptors. It is expressed widely in many different tissues, but the phenotype of mice devoid of RIP140 indicates that it plays a crucial role in the ovary and in adipose biology. Ovarian expression of RIP140 is cell-type-specific during follicular development and it is essential for oocyte release during ovulation, but not for luteinization of mature ovarian follicles. In adipose tissue, RIP140 is essential for normal fat accumulation and RIP140-null mice show decreased lipid storage even on a high-fat diet, with upregulation of mitochondrial uncoupling protein (UCP1) in some fat depots. Thus RIP140 plays a crucial role in female fertility and in energy homeostasis, and could be a target for infertility treatment, new contraceptive strategies or prevention of obesity.     

Toso regulates the balance between apoptotic and nonapoptotic death receptor signaling by facilitating RIP1 ubiquitination

The regulation of cellular survival and apoptosis is of critical importance for the immune system to maintain immune homeostasis and to establish tolerance. Here, we demonstrate that the immune specific cell surface molecule Toso exhibits antiapoptotic effects on death receptor signaling by a novel regulatory mechanism involving the adaptor kinase RIP1. The antiapoptotic function of Toso depends on RIP1 ubiquitination and involves the recruitment of the death adaptor FADD to a Toso/RIP1 protein complex. In response to CD95L and TNFα, Toso promotes the activation of MAPK and NF-κB signaling pathways. Because of this relative augmentation of survival versus apoptotic signals, Toso raises the threshold for death receptor-mediated apoptosis. Our analysis of Toso-deficient mice revealed that Toso is essential for TNFα-mediated liver damage. Furthermore, the antiapoptotic function of Toso could be blocked by a Toso-specific monoclonal antibody, opening up new therapeutic prospects for the treatment of immune disorders and hematologic malignancies.     

Molecular interaction of retinoic acid receptors with coregulators PCAF and RIP140

p300/CBP-associating factor (PCAF) is a ligand-dependent coactivator, whereas receptor-interacting protein 140 (RIP140) is a ligand-dependent negative coregulator for retinoic acid (RA) receptor (RAR) and retinoid X receptor (RXR). To compare these molecular interactions and to determine the effect of RXR ligands, we focus on PCAF/RAR/RXR complex formation in this study for a comparison to RIP140/RAR/RXR complex formation. The LBD of RXR is identified as its primary PCAF-interacting motif. BIAcore studies determine the Kd of RAR/RXR association with PCAF as 9.35 nM in the presence of RXR ligand AGN194204, and 47.2 nM in the absence of ligand. Cross-linking study demonstrates tri-molecular complex consisting of one RAR/RXR pair and one PCAF. In competition experiments, RIP140 strongly competes with PCAF for interaction with RAR/RXR both in vitro and in vivo. Chromatin immunoprecipitation demonstrates recruitment of RIP140 and PCAF to the endogenous RA-regulated gene, the RARbeta2 promoter. This study presents kinetic evidence for competition of RIP140 with PCAF for ligand-dependent interactions with RAR/RXR, and provides kinetic explanation for the suppressive activity of RIP140 in RA-activated gene expression.     

RIP140 contributes to foam cell formation and atherosclerosis by regulating cholesterol homeostasis in macrophages

Atherosclerosis, a syndrome with abnormal arterial walls, is one of the major causes that lead to the development of various cardiovascular diseases. The key initiator of atherosclerosis is cholesterol accumulation. The uncontrolled cholesterol deposition, mainly involving low-density lipoprotein (LDL), causes atheroma plaque formation, which initiates chronic inflammation due to the recruitment of inflammatory cells such as macrophages. Macrophages scavenge excess peripheral cholesterol and transport intracellular cholesterol to high-density lipoprotein (HDL) for excretion or storage. Cholesterol-laden macrophage-derived foam cell formation is the main cause of atherogenesis. It is critical to understand the regulatory mechanism of cholesterol homeostasis in the macrophage in order to prevent foam cells formation and further develop novel therapeutic strategies against atherosclerosis. Here we identified a protein, RIP140 (receptor interacting protein 140), which enhances macrophage-derived foam cell formation by reducing expression of reverse cholesterol transport genes, A TP-binding membrane cassette transporter A-1 (ABCA1) and ATP-binding membrane cassette transporter G-1 (ABCG1). In animal models, we found that reducing RIP140 levels by crossing macrophage-specific RIP140 knockdown (MϕRIP140KD) mice with ApoE null mice effectively ameliorates high-cholesterol diet-induced atherosclerosis. Our data suggest that reducing RIP140 levels in macrophages significantly inhibits atherosclerosis, along with markers of inflammation and the number of macrophages in a western diet fed ApoE null mouse. This study provides a proof-of-concept for RIP140 as a risk biomarker of, and a therapeutic target for, atherosclerosis.     

The nuclear corepressor, NCoR, regulates thyroid hormone action in vivo

The thyroid hormone receptor (TR) has been proposed to regulate expression of target genes in the absence of triiodothyronine (T₃) through the recruitment of the corepressors, NCoR and SMRT. Thus, NCoR and SMRT may play an essential role in thyroid hormone action, although this has never been tested in vivo. To accomplish this, we developed mice that express in the liver a mutant NCoR protein (L-NCoRΔID) that cannot interact with the TR. L-NCoRΔID mice appear grossly normal, however, when made hypothyroid the repression of many positively regulated T₃-target genes is abrogated, demonstrating that NCoR plays a specific and sufficient role in repression by TR in the absence of T₃. Remarkably, in the euthyroid state, expression of many T₃-targets is also up-regulated in L-NCoRΔID mice, demonstrating that NCoR also determines the magnitude of the response to T₃ in euthyroid animals. Although positive T₃ targets were up-regulated in L-NCoRΔID mice in the hypo- and euthyroid state, there was little effect seen on negatively regulated T₃ target genes. Thus, NCoR is a specific regulator of T₃-action in vivo and mediates repression by the unliganded TR in hypothyroidism. Furthermore, NCoR appears to play a key role in determining the tissue-specific responses to similar levels of circulating T₃. Interestingly, NCoR recruitment to LXR is also impaired in this model, leading to activation of LXR-target genes, further demonstrating that NCoR recruitment regulates multiple nuclear receptor signaling pathways.     

Multiple signaling defects in the absence of RIP140 impair both cumulus expansion and follicle rupture

The nuclear receptor corepressor RIP140 is essential in the ovary for ovulation, but is not required for follicle growth and luteinization. To identify genes that may be subject to regulation by RIP140 or play a role in ovulation, we compared ovarian gene expression profiles in untreated immature wild-type and RIP140 null mice and after treatment with pregnant mare serum gonadotropin and human chorionic gonadotropin. Many genes involved in signaling, extracellular matrix formation, cell-cell attachment, and adhesion were aberrantly regulated in the absence of RIP140, varying according to the hormone status of the mice. Notable among these was the reduced expression of a number of genes that encode components of signaling pathways and matrix proteins required for cumulus expansion, a key remodeling process necessary for ovulation. Histological analysis confirmed that cumulus expansion in RIP140 null mice is reduced, oocyte detachment from the mural cell wall is impaired, and follicles fail to rupture in response to LH. Although the expression of many genes involved in cumulus cell expansion was reduced, there was a subset of genes involved in extracellular matrix formation and cell-cell interactions that was up-regulated and may interfere with ovarian tissue remodeling. We propose that widespread gene dysregulation in ovarian tissues in the absence of RIP140 leads to the anovulatory phenotype. This helps to define an important role for RIP140 in the regulation of multiple processes leading to ovulation.