Muraoka Masaaki, Fukushima Aiko, Viengchareun Say, Lombès Marc, Kishi Fukuko, Miyauchi Akira, Kanematsu Mariko, Doi Junko, Kajimura Junko, Nakai Ryo, Uebi Tatsuya, Okamoto Mitsuhiro, Takemori Hiroshi
Laboratory of Cell Signaling and Metabolism, National Institute of Biomedical Innovation, 7-6-8, Asagi, Saito, Ibaraki, Osaka 567-0085, Japan.
Am J Physiol Endocrinol Metab. 2009 Jun;296(6):E1430-9. doi: 10.1152/ajpendo.00024.2009. Epub 2009 Apr 7.
Salt-inducible kinase 2 (SIK2) is expressed abundantly in adipose tissues and represses cAMP-response element-binding protein (CREB)-mediated gene expression by phosphorylating the coactivator transducer of regulated CREB activity (TORC2). Phosphorylation at Ser(587) of SIK2 diminishes its TORC2 phosphorylation activity. In 3T3-L1 white adipocytes, SIK2 downregulates lipogenic gene in response to nutritional stresses. To investigate the impact of SIK2 on the function of brown adipose tissue (BAT), we used T37i brown adipocytes, mice with diet-induced obesity, and SIK2 mutant (S587A) transgenic mice. When T37i adipocytes were treated with insulin, the levels of peroxisome proliferator-activated receptor-coactivator-1alpha (PGC-1alpha) and uncoupling protein-1 (UCP-1) mRNA were increased, and the induction was inhibited by overexpression of SIK2 (S587A) mutant or dominant-negative CREB. Insulin enhanced SIK2 phosphorylation at Ser(587), which was accompanied by decrease in phospho-TORC2. Similarly, the decrease in the level of SIK2 phosphorylation at Ser(587) was observed in the BAT of mice with diet-induced obesity, which was negatively correlated with TORC2 phosphorylation. To confirm the negative correlation between SIK2 phosphorylation at Ser(587) and TORC2 phosphorylation in BAT, SIK2 mutant (S587A) was overexpressed in adipose tissues by using the adipocyte fatty acid-binding protein 2 promoter. The expression of recombinant SIK2 (S587A) was restricted to BAT, and the levels of phospho-TORC2 were elevated in BAT of transgenic mice. Male transgenic mice developed high-fat diet-induced obesity, and their BAT expressed low levels of PGC-1alpha and UCP-1 mRNA, suggesting that SIK2-TORC2 cascade may be important for the regulation of PGC-1alpha and UCP-1 gene expression in insulin signaling in BAT.
盐诱导激酶2（SIK2）在脂肪组织中大量表达，并通过磷酸化环磷酸腺苷反应元件结合蛋白（CREB）调节活性的共激活因子转导子（TORC2）来抑制CREB介导的基因表达。SIK2第587位丝氨酸（Ser(587)）的磷酸化会降低其TORC2磷酸化活性。在3T3-L1白色脂肪细胞中，SIK2会响应营养应激而下调脂肪生成基因。为了研究SIK2对棕色脂肪组织（BAT）功能的影响，我们使用了T37i棕色脂肪细胞、饮食诱导肥胖的小鼠以及SIK2突变体（S587A）转基因小鼠。当用胰岛素处理T37i脂肪细胞时，过氧化物酶体增殖物激活受体共激活因子-1α（PGC-1α）和解偶联蛋白-1（UCP-1）的mRNA水平升高，而SIK2（S587A）突变体或显性负性CREB的过表达会抑制这种诱导。胰岛素增强了SIK2在Ser(587)处的磷酸化，同时伴随着磷酸化TORC2的减少。同样，在饮食诱导肥胖的小鼠的BAT中也观察到SIK2在Ser(587)处的磷酸化水平降低，这与TORC2磷酸化呈负相关。为了证实BAT中SIK2在Ser(587)处的磷酸化与TORC2磷酸化之间的负相关，通过使用脂肪细胞脂肪酸结合蛋白2启动子在脂肪组织中过表达SIK2突变体（S587A）。重组SIK2（S587A）的表达局限于BAT，转基因小鼠BAT中的磷酸化TORC2水平升高。雄性转基因小鼠出现高脂饮食诱导的肥胖，其BAT中PGC-1α和UCP-1的mRNA表达水平较低，这表明SIK2-TORC2级联反应可能对BAT中胰岛素信号通路中PGC-1α和UCP-1基因表达的调节很重要。
