Hepatic nNOS impaired hepatic insulin sensitivity through the activation of p38 MAPK

Abstract

Neuronal nitric oxide synthase (nNOS) interacts with its adapto r protein NOS1AP through its PZD domain in the neurons. Previously, we had repor ted that NOS1AP enhanced hepatic insulin sensitivity through its PZD-binding do main, which suggested that nNOS might mediate the effect of NOS1AP. This study aimed t o examine the role and underlying mechanisms of nNOS in regulating hepatic insulin sensitivity. nNOS co-localized with NOS1AP in mouse liver. The overexpression of NOS1AP in mouse liver decreased the level of phosphorylated nNOS (p-nNOS (Ser1417)), the active form of nNOS. Conversely, the liver-specific deletion of NOS1AP increase d the level of p-nNOS (Ser1417). The overexpression of nNOS in the liver of high-fat diet-induced obese mice exacerbated glucose intolerance, enhanced intrahepatic lipid accumulation, decreased glycogen storage, and blunted insulin-induced phosphorylation of IRbeta and Akt in the liver. Similarly, nNOS overexpression increased triglyceride production, decreased glucose utilization, and downregulated insulin-induced expression of p-IRbeta, p-Akt, and p-GSK3beta in the HepG2 cells. In contrast, treatment with Nù-propyl-L-arginine (L-NPA), a selective nNOS inhibitor, improved glucose tolerance and upregulated insulininduced phosphorylation of IRbeta and Akt in the liver of ob/ob mice. Furthermore, overexpression of nNOS increased p38MAPK phosphorylation in the HepG2 cells. In contrast, inhibition of p38MAPK with SB203580 significantly reve rsed the nNOS-induced inhibition of insulin-signaling activity (all P < 0.05). This indicated that hepatic nNOS inhibited the insulin-signaling pathway through the activation of p38MAPK. These findings suggest that nNOS is involved in the development of hep atic insulin resistance and that nNOS might be a potential therapeutic target for diabetes.