Kan Chin Fung Kelvin, Singh Amar Bahadur, Stafforini Diana M, Azhar Salman, Liu Jingwen
Department of Veterans Affairs Palo Alto Health Care System, Palo Alto, CA 94304.
Huntsman Cancer Institute, University of Utah, Salt Lake City, UT 84112.
J Lipid Res. 2014 Aug;55(8):1657-67. doi: 10.1194/jlr.M045971. Epub 2014 May 30.
ACSL4 is a member of the long-chain acyl-CoA synthetase (ACSL) family with a marked preference for arachidonic acid (AA) as its substrate. Although an association between elevated levels of ACSL4 and hepatosteatosis has been reported, the function of ACSL4 in hepatic FA metabolism and the regulation of its functional expression in the liver remain poorly defined. Here we provide evidence that AA selectively downregulates ACSL4 protein expression in hepatic cells. AA treatment decreased the half-life of ACSL4 protein in HepG2 cells by approximately 4-fold (from 17.3 ± 1.8 h to 4.2 ± 0.4 h) without causing apoptosis. The inhibitory action of AA on ACSL4 protein stability could not be prevented by rosiglitazone or inhibitors that interfere with the cellular pathways involved in AA metabolism to biologically active compounds. In contrast, treatment of cells with inhibitors specific for the proteasomal degradation pathway largely prevented the AA-induced ACSL4 degradation. We further show that ACSL4 is intrinsically ubiquitinated and that AA treatment can enhance its ubiquitination. Collectively, our studies have identified a novel substrate-induced posttranslational regulatory mechanism by which AA downregulates ACSL4 protein expression in hepatic cells.
ACSL4是长链脂酰辅酶A合成酶（ACSL）家族的成员，对花生四烯酸（AA）作为其底物有明显偏好。尽管已有报道称ACSL4水平升高与肝脂肪变性有关，但ACSL4在肝脏脂肪酸代谢中的功能及其在肝脏中功能表达的调控仍不清楚。在此，我们提供证据表明，AA选择性地下调肝细胞中ACSL4的蛋白表达。AA处理使HepG2细胞中ACSL4蛋白的半衰期缩短约4倍（从17.3±1.8小时降至4.2±0.4小时），且未引起细胞凋亡。罗格列酮或干扰AA代谢为生物活性化合物的细胞途径的抑制剂不能阻止AA对ACSL4蛋白稳定性的抑制作用。相反，用蛋白酶体降解途径特异性抑制剂处理细胞可在很大程度上阻止AA诱导的ACSL4降解。我们进一步表明，ACSL4本质上是泛素化的，并且AA处理可以增强其泛素化。总的来说，我们的研究确定了一种新的底物诱导的翻译后调控机制，通过该机制AA下调肝细胞中ACSL4的蛋白表达。
