Koliwad Suneil K, Kuo Taiyi, Shipp Lauren E, Gray Nora E, Backhed Fredrik, So Alex Yick-Lun, Farese Robert V, Wang Jen-Chywan
Gladstone Institute of Cardiovascular Disease, University of California, San Francisco, California 94143, USA.
J Biol Chem. 2009 Sep 18;284(38):25593-601. doi: 10.1074/jbc.M109.025452. Epub 2009 Jul 23.
Glucocorticoids are important regulators of lipid homeostasis, and chronically elevated glucocorticoid levels induce hypertriglyceridemia, hepatic steatosis, and visceral obesity. The occupied glucocorticoid receptor (GR) is a transcription factor. However, those genes regulating lipid metabolism under GR control are not fully known. Angiopoietin-like 4 (ANGPTL4, fasting-induced adipose factor), a protein inhibitor of lipoprotein lipase, is synthesized and secreted during fasting, when circulating glucocorticoid levels are physiologically increased. We therefore tested whether the ANGPTL4 gene (Angptl4) is transcriptionally controlled by GR. We show that treatment with the synthetic glucocorticoid dexamethasone increased Angptl4 mRNA levels in primary hepatocytes and adipocytes (2-3-fold) and in the livers and white adipose tissue of mice (approximately 4-fold). We tested the mechanism of this increase in H4IIE hepatoma cells and found that dexamethasone treatment increased the transcriptional rate of Angptl4. Using bioinformatics and chromatin immunoprecipitation, we identified a GR binding site within the rat Angptl4 sequence. A reporter plasmid containing this site was markedly activated by dexamethasone, indicative of a functional glucocorticoid response element. Dexamethasone treatment also increased histone H4 acetylation and DNase I accessibility in genomic regions near this site, further supporting that it is a glucocorticoid response element. Glucocorticoids promote the flux of triglycerides from white adipose tissue to liver. We found that mice lacking ANGPTL4 (Angptl4(-/-)) had reductions in dexamethasone-induced hypertriglyceridemia and hepatic steatosis, suggesting that ANGPTL4 is required for this flux. Overall, we establish that ANGPTL4 is a direct GR target that participates in glucocorticoid-regulated triglyceride metabolism.
糖皮质激素是脂质稳态的重要调节因子，长期升高的糖皮质激素水平会诱发高甘油三酯血症、肝脂肪变性和内脏肥胖。被占据的糖皮质激素受体（GR）是一种转录因子。然而，在GR调控下调节脂质代谢的那些基因尚未完全明确。血管生成素样4（ANGPTL4，禁食诱导脂肪因子）是脂蛋白脂肪酶的一种蛋白抑制剂，在禁食期间合成并分泌，此时循环中的糖皮质激素水平会生理性升高。因此，我们测试了ANGPTL4基因（Angptl4）是否受GR转录调控。我们发现，用合成糖皮质激素地塞米松处理可使原代肝细胞和脂肪细胞中的Angptl4 mRNA水平升高（2 - 3倍），在小鼠肝脏和白色脂肪组织中升高约4倍。我们在H4IIE肝癌细胞中测试了这种升高的机制，发现地塞米松处理可提高Angptl4的转录速率。通过生物信息学和染色质免疫沉淀，我们在大鼠Angptl4序列中鉴定出一个GR结合位点。含有该位点的报告质粒被地塞米松显著激活，表明存在功能性糖皮质激素反应元件。地塞米松处理还增加了该位点附近基因组区域的组蛋白H4乙酰化和DNase I可及性，进一步支持其为糖皮质激素反应元件。糖皮质激素促进甘油三酯从白色脂肪组织向肝脏的转运。我们发现缺乏ANGPTL4的小鼠（Angptl4(-/-)）在地塞米松诱导的高甘油三酯血症和肝脂肪变性方面有所减轻，这表明ANGPTL4是这种转运所必需的。总体而言，我们确定ANGPTL4是GR的直接靶点，参与糖皮质激素调节的甘油三酯代谢。
