Protein regulation: Tag wrestling with relatives of ubiquitin

126

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...

Section: Discussionmentioning

confidence: 77%

“…Finally, ubiquitinprotein isopeptide ligase (E3) catalyzes the covalent bond of ubiquitin to the target protein. Following this process, multiubiquitinated proteins are rapidly degraded by the 26 S proteasome (13).…”

mentioning

confidence: 99%

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

Blanquart¹,

Barbier²,

Fruchart

et al. 2002

126

“…Many proteins with a structural similarity to ubiquitin present in cells, Ubls (ubiquitin-like proteins), are divided into two subclasses: small, type-1 Ubls, such as SUMO-1 and NEDD8, that are ligated to target proteins in a similar manner to ubiquitin, and type-2 Ubls containing ubiquitin-like structures within a variety of large proteins having distinct functions, such as Elongin B, Rad23, and Parkin. Although ubiquitin and type-1 Ubls are central players in post-translational protein modification, the significance of type-2 Ubls remains obscure (42,43). The lack of a diglycine motif at the C-terminal end of the ubiquitin domain suggests that Herp is a type-2 Ubl.…”

Section: Discussionmentioning

confidence: 99%

Herp, a New Ubiquitin-like Membrane Protein Induced by Endoplasmic Reticulum Stress

Kokame

Agarwala

Kato

et al. 2000

297

286

Hyperhomocysteinemia, a risk factor for vascular disease, injures endothelial cells through undefined mechanisms. We previously identified several homocysteineresponsive genes in cultured human vascular endothelial cells, including the endoplasmic reticulum (ER)-resident molecular chaperone GRP78/BiP. Here, we demonstrate that homocysteine induces the ER stress response and leads to the expression of a novel protein, Herp, containing a ubiquitin-like domain at the N terminus. mRNA expression of Herp was strongly upregulated by inducers of ER stress, including mercaptoethanol, tunicamycin, A23187, and thapsigargin. The ER stress-dependent induction of Herp was also observed at the protein level. Immunochemical analyses using Herpspecific antibodies indicated that Herp is a 54-kDa, membrane-associated ER protein. Herp is the first integral membrane protein regulated by the ER stress response pathway. Both the N and C termini face the cytoplasmic side of the ER; this membrane topology makes it unlikely that Herp acts as a molecular chaperone for proteins in the ER, in contrast to GRP78 and other ER stress-responsive proteins. Herp may, therefore, play an unknown role in the cellular survival response to stress.

“…Ubls are conjugated to target proteins by an enzymatic cascade involving an activating enzyme (E1), a conjugating enzyme (E2), and a ligase (E3) (15)(16)(17). Ubl modification of signaling molecules and transcription factors has a large impact on gene expression (13,14).…”

mentioning

confidence: 99%

Virus Infection Triggers SUMOylation of IRF3 and IRF7, Leading to the Negative Regulation of Type I Interferon Gene Expression

Kubota¹,

Matsuoka

Chang

et al. 2008

142

132