Silencing mitogen-activated protein 4 kinase 4 (MAP4K4) protects beta cells from tumor necrosis factor-α-induced decrease of IRS-2 and inhibition of glucose-stimulated insulin secretion

Abstract

Obesity and type 2 diabetes present partially overlapping phenotypes with systemic inflammation as a common feature, raising the hypothesis that elevated cytokine levels may contribute to peripheral insulin resistance as well as the decreased beta cell functional mass observed in type 2 diabetes. In healthy humans, TNF-α infusion induces skeletal muscle insulin resistance. We now explore the impact of TNF-α on primary beta cell function and the underlying signaling pathways. Human and rat primary beta cells were sorted by FACS and cultured for 24 h±20 ng/ml TNF-α to explore the impact on apoptosis, proliferation, and short-term insulin secretion (1 h, 2.8 mM glucose followed by 1 h, 16.7mMglucose at the end of the 24-h culture period) as well as key signaling protein phosphorylation and expression. Prior exposure to TNF-α for 24 h inhibits glucose-stimulated insulin secretion from primary beta cells. This is associated with a decrease in glucose-stimulated phosphorylation of key proteins in the insulin signaling pathway including Akt, AS160, and other Akt substrates, ERK as well as the insulin receptor. Strikingly, TNF-α treatment decreased IRS-2 protein level by 46 ± 7% versus control, although mRNA expression was unchanged. While TNF-α treatment increased MAP4K4 mRNA expression by 33 ± 5%, knockdown of MAP4K4 by siRNA-protected beta cells against the detrimental effects of TNF-α on both insulin secretion and signaling. We thus identify MAP4K4 as a key upstream mediator of TNF-α action on the beta cell, making it a potential therapeutic target for preservation of beta cell function in type 2 diabetes. © 2009 by The American Society for Biochemistry and Molecular Biology, Inc.