Background

Salt-inducible kinase 2 (SIK2) is abundant in adipocytes, but downregulated in adipose tissue from individuals with obesity and insulin resistance. Moreover, SIK isoforms are required for normal insulin signalling and glucose uptake in adipocytes, but the underlying molecular mechanisms are currently not known. The adherens junction protein JUP, also termed plakoglobin or γ-catenin, has recently been reported to promote insulin signalling in muscle cells.

Objective

The objective of this study was to analyse if JUP is required for insulin signalling in adipocytes and the underlying molecular mechanisms of this regulation.

Methods

Co-expression of SIK2 and JUP mRNA levels in adipose tissue from a human cohort was analysed. siRNA silencing and/or pharmacological inhibition of SIK2, JUP, class IIa HDACs and CRTC2 was employed in 3T3-L1- and primary rat adipocytes. JUP protein expression was analysed by western blot and mRNA levels by qPCR. Insulin signalling was evaluated by western blot as levels of phosphorylated PKB/Akt and AS160, and by monitoring the uptake of ³H-2-deoxyglucose.

Results

mRNA expression of SIK2 correlated with that of JUP in human adipose tissue. SIK2 inhibition or silencing resulted in downregulation of JUP mRNA and protein expression in 3T3-L1- and in primary rat adipocytes. Moreover, JUP silencing reduced the expression of PKB and the downstream substrate AS160, and consequently attenuated activity in the insulin signalling pathway, including insulin-induced glucose uptake. The known SIK2 substrates CRTC2 and class IIa HDACs were found to play a role in the SIK-mediated regulation of JUP expression.

Conclusions

These findings identify JUP as a novel player in the regulation of insulin sensitivity in adipocytes, and suggest that changes in JUP expression could contribute to the effect of SIK2 on insulin signalling in these cells.

中文翻译：

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JUP/plakoglobin 受盐诱导激酶 2 调节，并且是脂肪细胞中胰岛素诱导的信号传导和葡萄糖摄取所必需的。

背景

盐诱导激酶 2 (SIK2) 在脂肪细胞中含量丰富，但在肥胖和胰岛素抵抗个体的脂肪组织中下调。此外，脂肪细胞中正常的胰岛素信号传导和葡萄糖摄取需要 SIK 同种型，但目前尚不清楚潜在的分子机制。最近有报道称粘附连接蛋白 JUP，也称为斑珠蛋白或 γ-连环蛋白，可促进肌肉细胞中的胰岛素信号传导。

客观的

本研究的目的是分析脂肪细胞中的胰岛素信号传导是否需要 JUP，以及该调节的潜在分子机制。

方法

分析了来自人类队列的脂肪组织中SIK2和JUP mRNA 水平的共表达。在 3T3-L1 和原代大鼠脂肪细胞中采用了 SIK2、JUP、IIa 类 HDAC 和 CRTC2 的 siRNA 沉默和/或药理学抑制。通过蛋白质印迹分析 JUP 蛋白表达，通过 qPCR 分析 mRNA 水平。胰岛素信号通过蛋白质印迹评估为磷酸化 PKB/Akt 和 AS160 的水平，并通过监测³ H-2-脱氧葡萄糖的摄取。

结果

SIK2 的mRNA 表达与人类脂肪组织中JUP 的mRNA 表达相关。SIK2 抑制或沉默导致 3T3-L1 和原代大鼠脂肪细胞中 JUP mRNA 和蛋白质表达的下调。此外，JUP沉默降低了 PKB 和下游底物 AS160 的表达，从而减弱了胰岛素信号通路的活性，包括胰岛素诱导的葡萄糖摄取。发现已知的 SIK2 底物 CRTC2 和 IIa 类 HDAC 在 SIK 介导的 JUP 表达调节中发挥作用。

结论

这些发现表明 JUP 是调节脂肪细胞胰岛素敏感性的新参与者，并表明 JUP 表达的变化可能有助于 SIK2 对这些细胞中胰岛素信号的影响。