Gut microbiota contributes to protection against high-fat diet-induced insulin resistance in iNOS knockout mice

Inducible nitric oxide synthase (iNOS) is activated in obesity and is involved in the regulation of cellular pathways, including the unfolded protein response and insulin signaling. We previously showed that iNOS deficiency is associated with reduced obesity-induced insulin resistance (IR), particularly in skeletal muscle, in iNOS knockout (iNOS KO) mice fed a high-fat diet (HFD). However, the mechanisms underlying protection against HFD-induced IR in iNOS KO mice remain incompletely understood. Several studies have reported associations between gut microbiota composition and metabolic outcomes, including body weight regulation and insulin resistance, with differences observed between lean and obese subjects. However, the potential contribution of the gut microbiota to the metabolic phenotype observed in iNOS KO mice has not been fully investigated. To address this question, we assessed glucose homeostasis, adipose tissue inflammation, gut microbiota composition, and intestinal barrier integrity. Fecal microbiota transfer (FMT) experiments were also performed between HFD-fed iNOS KO and C57BL/6J mice in both directions. HFD-fed iNOS KO mice exhibited reduced adipose tissue inflammation, characterized by decreased numbers of mast cells and pro-inflammatory M1 macrophages, together with increased numbers of anti-inflammatory M2 macrophages, compared with HFD-fed C57BL/6J mice. The gut microbiota profile of HFD-fed iNOS KO mice was associated with increased expression of genes related to intestinal tight junction integrity. Fecal microbiota transfer from HFD-fed iNOS KO mice was associated with improved glucose tolerance, enhanced insulin sensitivity, and increased expression of genes related to intestinal tight junction integrity in recipient mice. These findings suggest that gut microbiota contributes, at least in part, to the protection against HFD-induced IR in iNOS KO mice and support the potential of targeting gut microbiota as a therapeutic strategy to improve metabolic disturbances.