Aims/hypothesis

Salt-inducible kinase 2 (SIK2) is downregulated in adipose tissue from obese or insulin-resistant individuals and inhibition of SIK isoforms results in reduced glucose uptake and insulin signalling in adipocytes. However, the regulation of SIK2 itself in response to insulin in adipocytes has not been studied in detail. The aim of our work was to investigate effects of insulin on various aspects of SIK2 function in adipocytes.

Methods

Primary adipocytes were isolated from human subcutaneous and rat epididymal adipose tissue. Insulin-induced phosphorylation of SIK2 and HDAC4 was analyzed using phosphospecific antibodies and changes in the catalytic activity of SIK2 with in vitro kinase assay. SIK2 protein levels were analyzed in primary adipocytes treated with the proteasome inhibitor MG132.

Results

We have identified a novel regulatory pathway of SIK2 in adipocytes, which involves insulin-induced phosphorylation at Thr484. This phosphorylation is impaired in individuals with a reduced insulin action. Insulin stimulation does not affect SIK2 catalytic activity or cellular activity towards HDAC4, but is associated with increased SIK2 protein levels in adipocytes.

Conclusion/interpretation

Our data suggest that downregulation of SIK2 in the adipose tissue of insulin-resistant individuals can partially be caused by impaired insulin signalling, which might result in defects in SIK2 expression and function.

中文翻译：

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胰岛素诱导脂肪细胞中的Thr484磷酸化和SIK2的稳定化

目的/假设

盐诱导的激酶2（SIK2）在肥胖或胰岛素抵抗者的脂肪组织中被下调，SIK亚型的抑制导致脂肪细胞中葡萄糖的摄取和胰岛素信号的减少。然而，尚未详细研究SIK2自身对脂肪细胞中胰岛素的响应的调节。我们的工作目的是研究胰岛素对脂肪细胞中SIK2功能各个方面的影响。

方法

从人皮下和大鼠附睾脂肪组织中分离出原代脂肪细胞。使用磷酸特异性抗体分析胰岛素诱导的SIK2和HDAC4的磷酸化，并通过体外激酶测定分析SIK2的催化活性的变化。在用蛋白酶体抑制剂MG132处理的原代脂肪细胞中分析了SIK2蛋白水平。

结果

我们已经确定了脂肪细胞中SIK2的新型调控途径，该途径涉及胰岛素诱导的Thr484磷酸化。这种磷酸化在胰岛素作用降低的个体中受损。胰岛素刺激不影响SIK2催化活性或对HDAC4的细胞活性，但与脂肪细胞中SIK2蛋白水平升高有关。

结论/解释

我们的数据表明，胰岛素抵抗患者的脂肪组织中SIK2的下调可能部分由胰岛素信号传导受损引起，这可能导致SIK2表达和功能缺陷。