Peroxisome Proliferator-activated Receptor α (PPARα) Turnover by the Ubiquitin-Proteasome System Controls the Ligand-induced Expression Level of Its Target Genes

Abstract

Peroxisome proliferator activated-receptor ␣ (PPAR␣) is a ligand-activated transcription factor belonging to the nuclear receptor family. PPAR␣ is implicated in the regulation of lipid and glucose metabolism and in the control of inflammatory response. Recently, it has been demonstrated that a number of nuclear receptors are degraded by the ubiquitin-proteasome pathway. Since PPAR␣ exhibits a circadian expression rhythm and since PPAR␣ is rapidly regulated under certain pathophysiological conditions such as the acute phase inflammatory response, we hypothesized that PPAR␣ protein levels must be under tight control. Here, we studied the mechanisms controlling PPAR␣ protein levels and their consequences on the transcriptional control of PPAR␣ target genes. Using pulse-chase experiments, it is shown that PPAR␣ is a short-lived protein and that addition of its ligands stabilizes this nuclear receptor. By transient cotransfection experiments using expression vectors for PPAR␣ and hemagglutinin-tagged ubiquitin, it is demonstrated that PPAR␣ protein is ubiquitinated and that its ligands decrease the ubiquitination of this nuclear receptor, thus providing a mechanism for the ligand-dependent stabilization observed in pulse-chase experiments. In addition, treatment with MG132, a selective proteasome inhibitor, increases the level of ubiquitinated PPAR␣ and inhibits its degradation in transfected cells. Furthermore, MG132 treatment enhances the level of endogenous PPAR␣ in HepG2 cells. Finally, transient transfection and quantitative reverse transcription-PCR show that inhibition of PPAR␣ degradation increases its transcriptional activation and expression of target genes such as apoA-II and fatty acid transport protein (FATP). Taken together, these data demonstrate that PPAR␣ is degraded by the ubiquitin-proteasome system in a ligand-dependent manner. Regulation of its degradation provides a novel regulatory mechanism of transcriptional activity of this nuclear receptor.The peroxisome proliferator-activated receptors (PPARs) 1 are members of the nuclear receptor superfamily that act as ligand-dependent transcription factors. PPAR␣ is highly expressed in liver, skeletal and cardiac muscle, and proximal tubular epithelium of kidney. A significant expression of PPAR␣ has also been shown in endothelial cells, smooth muscle cells, and cells involved in the inflammatory process (1). The ligands of PPAR␣ are natural fatty acids and derivatives such as leukotriene B4 and 8-S-hydroxyeicosatetraenoic acid or oxidized phospholipids from oxidized low density lipoprotein. The fibrates hypolipidemic drugs are synthetic PPAR␣ ligands (1). PPAR␣ plays a role in intracellular fatty acid metabolism and in triglyceride metabolism by regulating genes involved in the transport and degradation of fatty acids in mitochondria and peroxisomes (2). PPAR␣ is also implicated in the metabolism of lipids and lipoproteins. As a result, PPAR␣ activation decreases the hepatic very low density lipoprotein secretion and plasma triglyceride levels (3). Fur...

The 26S proteasome system degrades the ERM transcription factor and regulates its transcription-enhancing activity

“…It has recently been demonstrated that inhibition of proteosomal degradation enhances the transcriptional response mediated by some nuclear receptors (Blanquart et al, 2002;Deroo et al, 2002;Pollenz, 2002) and the Ets protein E1AF (Takahashi et al, 2005). We show here that blocking protein turnover with the proteasome inhibitor ALLN or MG132 also induces a change in ERM transcriptional activity.…”

Sequestration of Thermogenic Transcription Factors in the Cytoplasm during Development of Brown Adipose Tissue

“…Several nuclear hormone receptors, such as RXR␣/␥, RAR␣, thyroid hormone receptor, and PPAR␥, are degraded by the ubiquitin-proteasome system (43)(44)(45). Recently, Blanquart et al (46) have shown that the protein stability of PPAR␣ is increased in the presence of its ligand WY 14,643 by decreasing its ubiquitination. Consistent with these reports, we show with a pulse-chase experiment that the stability of PPAR␥ increases in differentiated brown adipocyte cultures after treatment with norepinephrine in a manner similar to that caused by proteasome inhibitors.…”

“…5B). It has been shown that nuclear hormone receptors, including PPAR␣ and PPAR␥, are targets for the ubiquitin-proteasome system of protein degradation (43)(44)(45)(46). Because ligand activation has been shown to influence the stability of nuclear receptor proteins, the effect of norepinephrine on PPAR␥ was examined.…”

Thioredoxin-mediated Negative Autoregulation of Peroxisome Proliferator-activated Receptor α Transcriptional Activity

“…It may be reasonable to expect some mechanisms that rapidly sense and tightly control the activity of PPAR␣ according to metabolic needs under normal physiological condition. Degradation of PPAR␣ by the ubiquitin-proteasome system and decrease of the ubiquitination by the ligands of PPAR␣ may represent one of such mechanisms (Blanquart et al, 2002). Because feedback regulation, by which a transcription factor is negatively regulated by its target gene product, has been found for several nuclear receptors such as thyroid hormone receptor and retinoid receptor (Thompson and Bottcher, 1997;Kerley et al, 2001), a question may be raised as if any PPAR␣-target gene product can also negatively regulate PPAR␣ activity.…”

“…The significance of PPAR␣ in physiology and disease is evidenced by the fact that it and PPAR␥, another subtype of PPARs, are molecular targets for the lipid-lowering fibrate drugs and insulin-sensitizing thiazolidinedione (TZD), respectively (Evans et al, 2004). Besides activation of PPAR␣ by fatty acid and various exogenous ligands such as fibrates, the regulation of the PPAR␣ transcription activity by coactivators (Dowell et al, 1997), corepressors (Dowell et al, 1999), other transcription factors (TFs; Zhou et al, 1999), and posttranslational modifications including phosphorylation (Juge-Aubry et al, 1999) and ubiquination (Blanquart et al, 2002) have been extensively studied. However, redox regulation of PPAR␣ activity has not been reported so far.…”