Protein phosphatase 4 (PP4) functions as a critical regulator in tumor necrosis factor (TNF)-α-induced hepatic insulin resistance

Protein phosphatase 4 (PP4) was shown to participate in multiple cellular processes, including DNA damage response, cell cycle and embryo development. Recent studies demonstrated a looming role of PP4 in glucose metabolism. However, whether PP4 is involved in hepatic insulin resistance remains poorly understood. The objective of this study was to estimate the role of PP4 in tumor necrosis factor (TNF)-alpha-induced hepatic insulin resistance. db/db mice and TNF-alpha-treated C57BL/6J mice were used as hepatic insulin resistance animal models. In vitro models were established in both HepG2 cells and primary hepatocytes by TNF-alpha treatment. We found that increased expression and activity of PP4 occurred in the livers of db/db mice and TNF-alpha-induced hepatic insulin resistance both in vitro and in vivo. Actually, PP4 silencing and suppression of PP4 activity ameliorated TNF-alpha-induced hepatic insulin resistance, whereas over-expression of PP4 caused insulin resistance. We then further investigated the prodiabetic mechanism of PP4 in TNF-alpha-induced insulin resistance. We found that PP4 formed a complex with IRS-1 to promote phosphorylation of IRS-1 on serine 307 via JNK activation and reduce the expression of IRS-1. Thus, PP4 is an important regulator in inflammatory related insulin resistance.