Protein Phosphorylation Research Group, Section for Diabetes, Metabolism and Endocrinology, Department of Experimental Medical Science, Lund University, Biomedical Centre C11, Klinikgatan 28, 221 84 Lund, Sweden.
Lipid laboratory, Unit of Endocrinology, Department of Medicine Huddinge, Karolinska Institute, Stockholm, Sweden.
Cell Signal. 2020 Dec;76:109786. doi: 10.1016/j.cellsig.2020.109786. Epub 2020 Sep 20.
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.
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.
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 H-2-deoxyglucose.
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.
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.
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表达下调。此外，SIK 同工型对于脂肪细胞中正常的胰岛素信号传导和葡萄糖摄取是必需的，但目前尚不清楚其潜在的分子机制。黏着连接蛋白 JUP，也称为斑联蛋白或γ-连环蛋白，最近被报道可促进肌肉细胞中的胰岛素信号传导。
本研究旨在分析 JUP 是否是脂肪细胞中胰岛素信号传导所必需的，以及这种调节的潜在分子机制。
分析了人类脂肪组织中 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 SIK2、JUP、class IIa HDACs 和 CRTC2 的 siRNA 沉默和/或药理学抑制。通过 Western blot 分析 JUP 蛋白表达，通过 qPCR 分析 mRNA 水平。通过 Western blot 评估胰岛素信号传导，评估磷酸化 PKB/Akt 和 AS160 的水平，以及监测 H-2-脱氧葡萄糖的摄取。
人类脂肪组织中 SIK2 的 mRNA 表达与 JUP 的 mRNA 表达相关。在 3T3-L1-和原代大鼠脂肪细胞中，SIK2 抑制或沉默导致 JUP mRNA 和蛋白表达下调。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 class IIa HDACs 在 SIK2 介导的 JUP 表达调控中发挥作用。
这些发现将 JUP 确定为调节脂肪细胞胰岛素敏感性的新成员，并表明 JUP 表达的变化可能导致 SIK2 对这些细胞中胰岛素信号传导的影响。
