Peroxisome proliferator-activated receptor alpha (PPARalpha ) turnover by the ubiquitin-proteasome system controls the ligand-induced expression level of its target genes.
Peroxisome proliferator-activated receptor alpha (PPARalpha ) turnover by the ubiquitin-proteasome system controls the ligand-induced expression level of its target genes.
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发表时间：
2002
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通讯作者：
C. Blanquart;O. Barbier;J. Fruchart;B. Staels;C. Glineur
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作者：
C. Blanquart;O. Barbier;J. Fruchart;B. Staels;C. Glineur
过氧化物酶体增殖物激活受体α(PPARpha)是核受体家族中的一种配体激活的转录因子。PPARα参与调节脂糖代谢和控制炎症反应。最近的研究表明，许多核受体通过泛素-蛋白酶体途径降解。由于PPARpha呈现昼夜节律表达，而且PPARpha在某些病理生理条件下(如急性期炎症反应)受到快速调节，我们假设PPARpha蛋白水平必须受到严格控制。在这里，我们研究了控制PPARpha蛋白水平的机制及其对PPARpha靶基因转录控制的影响。脉冲追逐实验表明，PPARpha是一种短暂的蛋白质，它的配体的加入稳定了这个核受体。通过使用PPARpha和血凝素标记的泛素的表达载体进行瞬时共转染实验，证明了PPARpha蛋白是泛素化的，其配体减少了这种核受体的泛素化，从而为脉冲追逐实验中观察到的配体依赖的稳定提供了机制。此外，用选择性蛋白酶体抑制剂MG132处理，可增加泛素化的PPARpha水平，并抑制其在转基因细胞中的降解。此外，MG132处理可提高HepG2细胞内源性PPARα的水平。瞬时转染法和定量逆转录-聚合酶链式反应表明，抑制PPARα的降解增加了其转录活性和靶基因如apoA-II和脂肪酸转运蛋白(FATP)的表达。综上所述，这些数据表明PPARpha被泛素-蛋白酶体系统以配体依赖的方式降解。对其降解的调节为该核受体的转录活性提供了一种新的调节机制。
Peroxisome proliferator activated-receptor alpha (PPARalpha) is a ligand-activated transcription factor belonging to the nuclear receptor family. PPARalpha 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 PPARalpha exhibits a circadian expression rhythm and since PPARalpha is rapidly regulated under certain pathophysiological conditions such as the acute phase inflammatory response, we hypothesized that PPARalpha protein levels must be under tight control. Here, we studied the mechanisms controlling PPARalpha protein levels and their consequences on the transcriptional control of PPARalpha target genes. Using pulse-chase experiments, it is shown that PPARalpha is a short-lived protein and that addition of its ligands stabilizes this nuclear receptor. By transient cotransfection experiments using expression vectors for PPARalpha and hemagglutinin-tagged ubiquitin, it is demonstrated that PPARalpha 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 PPARalpha and inhibits its degradation in transfected cells. Furthermore, MG132 treatment enhances the level of endogenous PPARalpha in HepG2 cells. Finally, transient transfection and quantitative reverse transcription-PCR show that inhibition of PPARalpha 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 PPARalpha 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.