Regulation mechanism of adipocyte differentiation through prostaglandin D2 receptor

脂质运载蛋白型前列腺素D合成酶（L-PGDS）在脂肪细胞中表达，由于L-PGDS基因敲除小鼠表现出这些功能的异常，因此被认为参与了2型糖尿病葡萄糖耐量和动脉粥样硬化的调节。然而，L-PGDS在脂肪细胞中的作用及其基因表达调控机制尚不清楚。我将L-PGDS的小干扰RNA应用于小鼠3 T3-L1细胞，发现它抑制了这些细胞向脂肪细胞的分化。小鼠L-PGDS启动子的报告基因分析表明，在-233处的肝受体同源物-1（LRH-1）的应答元件在前脂肪细胞3 T3-L1细胞中起关键作用。此外，我们确定了两个甾醇调控元件（SRE）在-194为顺式元件激活L-PGDS基因表达的脂肪细胞3 T3-L1细胞。合成的肝X受体激动剂T0901317通过激活脂肪细胞3 T3-L1细胞中SRE结合蛋白-1c（SREBP-1c）的表达诱导L-PGDS mRNA的表达。电泳迁移率变动分析和染色质免疫沉淀分析结果表明，LRH-1和SREBP-1c分别与L-PGDS基因启动子中的结合元件结合。小干扰RNA介导的LRH-1或SREBP-1c抑制分别降低前脂肪细胞或脂肪细胞3 T3-L1细胞中L-PGDS基因的表达。这些结果表明，L-PGDS基因表达在前脂肪细胞中被LRH-1激活，在脂肪细胞中被SREBP-1c激活。肝脏X受体通过激活SREBP-1c介导的L-PGDS上调是促进脂肪细胞分化的新途径。

Lipocalin-type prostaglandin (PG) D synthase (L-PGDS) is expressed in adipocytes and is proposed to be involved in the regulation of glucose tolerance and atherosclerosis in type 2 diabetes, because L-PGDS gene knock-out mice show abnormalities in these functions. However, the role of L-PGDS and the regulation mechanism governing its gene expression in adipocytes remain unclear. I applied small interference RNA of L-PGDS to mouse 3T3-L1 cells and found that it suppressed differentiation of these cells into adipocytes. Reporter analysis of the mouse L-PGDS promoter demonstrated that a responsive element for liver receptor homolog-1 (LRH-1) at -233 plays a critical role in preadipocytic 3T3-L1 cells. Moreover, we identified two sterol regulatory elements (SREs) at -194 to be ciselements for activation of L-PGDS gene expression in adipocytic 3T3-L1 cells. L-PGDS mRNA was induced in response to synthetic liver X receptor agonist, T0901317, through activation of the expression of SRE-binding protein-1c (SREBP-1c) in the adipocytic 3T3-L1 cells. The results of electrophoretic mobility shift assay and chromatin immunoprecipitation assay revealed that LRH-1 and SREBP-1c bound to their respective binding elements in the promoter of L-PGDS gene. Small interference RNA-mediated suppression of LRH-1 or SREBP-1c decreased L-PGDS gene expression in preadipocytic or adipocytic 3T3-L1 cells, respectively. These results indicate that L-PGDS gene expression is activated by LRH-1 in preadipocytes and by SREBP-1c in adipocytes. Liver X receptor-mediated up-regulation of L-PGDS through activation of SREBP-1c is a novel pathway to enhance adipocyte differentiation.