RIP140, a Janus metabolic switch involved in defense functions

Since its discovery almost 15 years ago, the receptor-interacting protein RIP140 has been determined to be a corepressor of various transcription factors and nuclear receptors. Mice lacking the RIP140 gene are lean, exhibit resistance to high-fat diet-induced obesity and have increased glucose tolerance and insulin sensitivity. In 2008, RIP140 was determined to be a coactivator of nuclear factor-κB, a master transcriptional regulator of inflammation in immune cells, including macrophages. This year, it was reported that RIP140 degradation is involved in the resolution of inflammation and in endotoxin tolerance. The general picture that had emerged indicates that RIP140 is a Janus metabolic switch that convergently regulates metabolic pathways involved in defense functions through its pleotropic interactions with transcription factors.

Signaling pathways controlling metabolic and immune homeostasis are governed at the transcriptional level by the integrated action of nuclear receptors, a type of transcription factor. The nuclear receptors known as peroxisomeproliferator-activated receptors (PPARs) and liver X receptors are lipid-activated transcription factors that have emerged as key regulators of lipid metabolism and inflammation. Coregulators help nuclear receptors to positively or negatively influence the transcription of target genes and thereby integrate the various processes involved in immunometabolic homeostasis. Coregulators, including PGC-1 as a coactivator, Sirt1 as a corepressor and RIP140 as both a coactivator and a corepressor (Figure 1), control the balance between the energy status and the inflammatory response by integrating pathophysiological stimuli in vivo.^1,2

Figure 1.

Coactivator and corepressor functions of RIP140 in macrophages and adipocytes, respectively. (a) Following the exposure of cells to LPS, nuclear translocated RelA interacts with RIP140 to induce the expression of pro-inflammatory cytokines, and Syk phosphorylates RIP140 at Tyr364/418/436. RelA bound to phosphorylated RIP140 further recruits the SOCS1-Rbx1 E3 ligase to the RelA-RIP140 complex at specific chromatin targets. The ligase promotes phospho-RIP140 polyubiquitination and degradation, resulting in a reduction in proinflammatory cytokine production and the promotion of endotoxin tolerance through effects on specific genes. These effects are due to the lack of the specific coactivator RIP140 at those genes. (b) Nuclear RIP140 is recruited by nuclear receptors to repress sets of genes (involved in mitochondrial uncoupling, fatty acid beta-oxidation, oxidative phosphorylation, the TCA cycle and glycolysis) that promote energy consumption. After high fat diet (HFD) feeding, the nuclear export of RIP140 is triggered by its phosphorylation by the DAG-PKCε signaling cascade, followed by arginine methylation by PRMT1. Then, cytoplasmic RIP140 interacts with AS160, impeding the inactivation of AS160 by AKT and leading to the inhibition of GLUT4 translocation to the plasma membrane. In addition, cytoplasmic RIP140 inhibits adiponectin secretion by interacting with AS160 and enhances HSL- and ATGL-induced lipolysis by interacting with perilipin in adipocytes. ↓, activation; ⊥, inhibition. DAG, diacylglycerol; HSL, hormone-sensitive lipase; LPS, lipopolysaccharide; PKC, protein kinase C; PRMT, protein arginine N-methyltransferase; RIP, receptor-interacting protein.

RIP140 was first identified by its ability to interact with estrogen receptors and has been shown to repress their transcriptional activity. Later studies revealed that RIP140 binds and represses a number of other nuclear receptors, including PPARs (PPARα, PPARβ, PPARγ), estrogen-related receptors (ERRα and ERRβ) and thyroid hormone receptors (TRα and TRβ). Researchers have demonstrated that the ability of RIP140 to regulate nuclear receptor function is determined by the relative level of RIP140 expression in comparison with other cofactors, by post-translational modifications, and by interactions with additional transcriptional factors. RIP140 post-translational modifications are chemically diverse. RIP140 primarily functions as a scaffold protein that links nuclear receptors to chromatin remodeling enzymes involved in chromatin condensation and transcriptional repression.³

RIP140 is most highly expressed in white adipose tissue, where it regulates the expression of many genes involved in catabolic pathways (energy consuming), especially those pathways involved in lipid and glucose metabolism. RIP140-null mice express higher levels of genes that regulate fatty acid oxidation and proteins involved in energy consumption, indicating the role of RIP140 as a corepressor that controls the balance between energy conservation and expenditure.^4,5,6

Although RIP140 is located predominantly in the nucleus, its fine localization seems to be regulated by alternate signaling pathways. RIP140 is subject to sumoylation, which is accompanied by the relocalization of RIP140 from nuclear foci to a more diffuse nuclear distribution. This change in localization correlates with an increase in the transcriptional repressive activity of RIP140. In contrast, RIP140 phosphorylation by diacylglycerol-induced protein kinase Cε followed by methylation by protein arginine N-methyltransferase-1 leads to RIP140 nuclear export and the inhibition of its trans-repressive properties. Several roles for RIP140 in the cytoplasm have been identified, distinct from this protein's roles in transcriptional regulation. Cytoplasmic RIP140 inhibits glucose metabolism by reducing insulin-stimulated GLUT4 trafficking, glucose uptake and adiponectin secretion, but increases lipolysis via an interaction with perilipin. This interaction results in the more efficient recruitment of hormone-sensitive lipase to lipid droplets and adipose triglyceride lipase (ATGL), which forms a complex with CGI-58, an activator of ATGL. Consequentially, hormone-sensitive lipase can more readily access its substrate, and ATGL is activated, ultimately enhancing lipolysis (Figure 1b).^7,8,9,10

In contrast to the corepressor activities of nuclear RIP140 in adipocytes, RIP140 can also function as a coactivator in macrophages. It is proposed that RIP140 is recruited to nuclear factor-κB-dependent cytokine promoters and stimulates transcription by acting as a bridging factor that stabilizes the formation of a trimeric complex with the RelA subunit and the CRE-binding protein coactivator. The absence of RIP140 does not appear to immunocompromise mice, indicating that the regulatory mechanisms of RIP140 remain poorly understood.¹¹

In the recent issue of Nature Immunology, Ho et al. used the endotoxin tolerance (ET) model to elucidate the mechanism by which RIP140 controls the inflammatory response (Figure 1a).¹² They elucidated the coactivator function of RIP140 in macrophages step by step. First, they showed that ET induction is inversely correlated with the abundance of RIP140, suggesting that the degradation of RIP140 may resolve inflammation and promote the establishment of ET in a gene-specific manner. They confirmed this result by showing that both the transfection of macrophages with Y3F-mutant RIP140 (substitution of phenylalanine for tyrosine at positions 364, 418 and 436 of the Syk target sites) and the reconstitution of mice with Y3F-mutant RIP140 transfected macrophages resulted in resistance to ET at the cellular and animal levels, respectively. Y3F-mutant RIP140 expressing cells can produce TNF and IL-1β, even after a second stimulation with lipopolysaccharide (LPS). Second, the authors identified tyrosine phosphorylation as a new post-translational modification of RIP140 that occurs after LPS stimulation. Several experiments indicate that Syk is responsible for the LPS-induced tyrosine phosphorylation of RIP140. Syk exhibits similar activation kinetics after LPS challenge and also has an important role in resolving inflammation. In addition, the treatment of LPS-exposed RAW cells with a Syk inhibitor helps maintain their level of RIP140 protein. An in vitro kinase assay showed that Syk phosphorylates RIP140 at its tyrosine residues and that Y3F effectively blocks the Syk-mediated tyrosine phosphorylation of RIP140. Third, Ho et al. showed that the proteasome-mediated degradation of ubiquitinated RIP140 is responsible for the LPS-triggered decrease in RIP140 protein in LPS-tolerant macrophages using MG132. They identified Rbx1 as an RIP140-interacting protein using bacterial two-hybrid screening and determined that Rbx1 contributes to the LPS-triggered ubiquitination of RIP140. Ho et al. also found that RIP140 is a direct target of the SOCS1-Rbx1 E3 ligase. These results show that LPS triggers RIP140 degradation by promoting the ubiquitination of RIP140 mediated by the SOCS1-Rbx1 E3 ligase. Fourth, they found that RIP140 associates with SOCS1 in a RelA-dependent manner in co-immunoprecipitation assays, showing that RelA acts as an adapter by which the SOCS1-Rbx1 E3 ligase can control the degradation of RIP140. Finally, these authors demonstrated that RIP140 degradation contributes to the establishment of endotoxin tolerance in a gene-specific manner because the ectopic expression of Y3F-mutant RIP140 resulted in the higher expression of genes that are targets of RIP140 without affecting the expression of genes that are not targets of RIP140 in both primary macrophages and RAW cells.¹²

Briefly, the LPS-stimulated degradation of RIP140 is initiated by Syk-mediated tyrosine phosphorylation, followed by the RelA-dependent recruitment of the SOCS1-Rbx1 E3 ligase, and this degradation of RIP140 contributes to the loss of RelA binding and to active histone modification at the promoters of genes encoding proinflammatory cytokines in ET macrophages.

Generally, the roles of nuclear RIP140 are divided into two groups: corepressor functions in metabolic tissues that inhibit energy expenditure and coactivator functions in monocytes/macrophages that enhance innate inflammation. In addition, the cytoplasmic functions of RIP140 foster immune/responses by increasing lipolysis and decreasing adiponectin secretion. RIP140-induced metabolic changes might prime the RIP140-induced immune/inflammatory response functions to allow the host to survive attack by environmental invaders.

The author declares no competing financial interests.