Chao Lily C, Bensinger Steven J, Villanueva Claudio J, Wroblewski Kevin, Tontonoz Peter
Howard Hughes Medical Institute, Department of Pathology and Laboratory Medicine, University of California, Los Angeles, California 90095-1662, USA.
Mol Endocrinol. 2008 Dec;22(12):2596-608. doi: 10.1210/me.2008-0161. Epub 2008 Oct 22.
Members of the nuclear receptor 4A (NR4A) subgroup of nuclear receptors have been implicated in the regulation of glucose and lipid metabolism in insulin-sensitive tissues such as liver and skeletal muscle. However, their function in adipocytes is not well defined. Previous studies have reported that these receptors are rapidly up-regulated after treatment of 3T3-L1 preadipocytes with an adipogenic cocktail. We show here that although Nur77 expression is acutely induced by cAMP agonists in 3T3-L1 cells, it is not induced by other adipogenic stimuli, such as peroxisome proliferator-activated receptor-gamma ligands, nor is it induced during the differentiation of 3T3-F442A preadipocytes, suggesting that Nur77 induction is not an obligatory feature of preadipocyte differentiation. We further demonstrate that inflammatory signals that antagonize differentiation, such as TNFalpha and lipopolysaccharide, acutely induce Nur77 expression both in vitro and in vivo. We also show that NR4A expression in adipose tissue is responsive to fasting/refeeding. Retroviral transduction of each of the NR4A receptors (Nur77, Nurr1, and NOR1) into either 3T3-L1 or 3T3-F442A preadipocytes potently inhibits adipogenesis. Interestingly, NR4A-mediated inhibition of adipogenesis cannot be rescued by peroxisome proliferator-activated receptor-gamma overexpression or activation. Transcriptional profiling of Nur77-expressing preadipocytes led to the identification of gap-junction protein alpha1 (Gja1) and tolloid-like 1 (Tll1) as Nur77-responsive genes. Remarkably, retroviral expression of either Gja1 or Tll1 in 3T3-L1 preadipocytes also inhibited adipocyte differentiation, implicating these genes as potential mediators of Nur77's effects on adipogenesis. Finally, we show that Nur77 expression inhibits mitotic clonal expansion of preadipocytes, providing an additional mechanism by which Nur77 may inhibit adipogenesis.
核受体4A（NR4A）亚组的成员已被证明参与调节胰岛素敏感组织（如肝脏和骨骼肌）中的葡萄糖和脂质代谢。然而，它们在脂肪细胞中的功能尚未明确界定。先前的研究报道，在用成脂混合物处理3T3-L1前脂肪细胞后，这些受体迅速上调。我们在此表明，虽然Nur77的表达在3T3-L1细胞中被cAMP激动剂急性诱导，但它不会被其他成脂刺激（如过氧化物酶体增殖物激活受体γ配体）诱导，在3T3-F442A前脂肪细胞分化过程中也不会被诱导，这表明Nur77的诱导不是前脂肪细胞分化的必然特征。我们进一步证明，拮抗分化的炎症信号，如肿瘤坏死因子α和脂多糖，在体外和体内均可急性诱导Nur77的表达。我们还表明，脂肪组织中的NR4A表达对禁食/再喂食有反应。将每个NR4A受体（Nur77、Nurr1和NOR1）逆转录病毒转导至3T3-L1或3T3-F442A前脂肪细胞中可有效抑制脂肪生成。有趣的是，过氧化物酶体增殖物激活受体γ的过表达或激活无法挽救NR4A介导的脂肪生成抑制作用。对表达Nur77的前脂肪细胞进行转录谱分析，鉴定出缝隙连接蛋白α1（Gja1）和类Tolloid 1（Tll1）为Nur77反应基因。值得注意的是，在3T3-L1前脂肪细胞中逆转录病毒表达Gja1或Tll1也会抑制脂肪细胞分化，表明这些基因是Nur77对脂肪生成影响的潜在介质。最后，我们表明Nur77的表达抑制前脂肪细胞的有丝分裂克隆扩增，这为Nur77抑制脂肪生成提供了另一种机制。
