In vitro treatment of 3T3-L1 adipocytes with recombinant Calcium/calmodulin-dependent Protein Kinase IV (CaMKIV) limits ER stress and improves insulin sensitivity through inhibition of autophagy via the mTOR/CREB signaling pathway

Abstract

Background: CaMKIV has been identified as a potential regulator of skeletal muscle glucose metabolism and insulin gene expression in the pancreas. However, the mechanism of CaMKIV involved in adipose insulin resistance is not fully understood. Autophagy has to be shown as a potential therapeutic target for ER (endoplasmic reticulum) stress and insulin resistance. The purpose of this study is to investigate the effects of CaMKIV on the ER stress, autophagic function and insulin signaling in tunicamycin treated adipocytes. Methods: In this study, we treated mature 3T3-L1 adipocytes with tunicamycin to induce ER stress. Then recombinant CaMKIV and/or targeted-siRNA of CREB and mTOR were transfected into tunicamycin treated 3T3-L1 adipocytes. The ER stress markers, autophagy activation, mTOR/CREB signaling and insulin sensitivity were analyzed by western blotting or electron microscopy. Results: CaMKIV not only reversed tunicamycin-induced expression of p-PERK, cleaved-ATF6, Atg7 and LC3II, as well as the reduction of p62 expression, but also improved expression of p-Akt and p-IRS-1. Moreover, CaMKIV inhibited activated ER stress and elevated insulin sensitivity inAtg7 siRNA transfected adipocytes. However, the protective effects of CaMKIV were nullified by suppression of mTOR or CREB in tunicamycin-induced adipocytes. Conclusion: This study proves recombinant CaMKIV inhibits ER stress and improves insulin signaling by regulation of autophagy. The protective effect of CaMKIV in adipocytes at least partly through mTOR/CREB signaling, which could be regarded as novel opportunities for treatment of obesity and type 2 diabetes.