Calcium/calmodulin-dependent kinase IV controls glucose-induced Irs2 expression in mouse beta cells via activation of cAMP response element-binding protein

Irs2, which is upregulated by glucose, is important for beta cell plasticity. Cyclic AMP response element-binding protein (CREB) stimulates beta cell Irs2 expression and is a major calcium/calmodulin-dependent kinase (CaMK)(IV) target in neurons. We therefore hypothesised that CaMKIV mediates glucose-induced Irs2 expression in beta cells via CREB activation. The functions of CaMKIV and CREB were investigated in MIN6 beta cells and mouse islets using the CaMK inhibitor KN62, the calcium chelator bapta-(AM) and the voltage-dependent calcium channel inhibitor nifedipine. Small interfering RNAs were used to silence endogenous CaMKIV production and expression vectors to overproduce constitutively active and dominant negative forms of CaMKIV and CREB. Irs1 and Irs2 expression were determined by quantitative PCR and Western blotting, and the role of CREB was also investigated by assessing its phosphorylation on serine 133. Increasing the glucose concentration from 2.5 to 25 mmol/l stimulated CREB phosphorylation on serine 133 and specifically stimulated Irs2 but not Irs1 expression. Similarly, overproduction of a constitutively active form of CaMKIV promoted sustained CREB phosphorylation and a significant increase in Irs2 but not Irs1 expression. In contrast, these stimulatory effects of glucose were all suppressed by overproducing an inactive CaMKIV mutant. Inhibition of glucose-induced calcium influx with nifedipine or chelation of intracellular calcium with bapta-(AM), as well as silencing of CaMKIV or inhibition of its activity with KN62 resulted in similar observations. Finally, overproduction of a dominant negative form of CREB completely suppressed glucose and CaMKIV stimulation of Irs2 expression. Our results suggest that the Ca2+/CaMKIV/CREB cascade plays a critical role in the regulation of Irs2 expression in beta cells.