Objective: Recent data show that iNOS has an essential role in ER stress in obesity. However, whether iNOS is sufficient to account for obesity-induced ER stress and Unfolded Protein Response (UPR) has not yet been investigated. In the present study, we used iNOS knockout mice to investigate whether high-fat diet (HFD) can still induce residual ER stress-associated insulin resistance.
Methods: For this purpose, we used the intraperitoneal glucose tolerance test (GTT), euglycemic-hyperinsulinemic clamp, western blotting and qPCR in liver, muscle, and adipose tissue of iNOS KO and control mice on HFD.
Results: The results of the present study demonstrated that, in HFD fed mice, iNOS-induced alteration in insulin signaling is an essential mechanism of insulin resistance in muscle, suggesting that iNOS may represent an important target that could be blocked in order to improve insulin sensitivity in this tissue. However, in liver and adipose tissue, the insulin resistance induced by HFD was only partially dependent on iNOS, and, even in the presence of genetic or pharmacological blockade of iNOS, a clear ER stress associated with altered insulin signaling remained evident in these tissues. When this ER stress was blocked pharmacologically, insulin signaling was improved, and a complete recovery of glucose tolerance was achieved.
Conclusions: Taken together, these results reinforce the tissue-specific regulation of insulin signaling in obesity, with iNOS being sufficient to account for insulin resistance in muscle, but in liver and adipose tissue ER stress and insulin resistance can be induced by both iNOS-dependent and iNOS-independent mechanisms.
Keywords: AKT, Protein kinase B; ATF6, activating transcription factor 6; Blocking; ER, endoplasmic reticulum; Endoplasmic reticulum stress; GAPDH, glyceraldehyde 3-phosphate dehydrogenase; GTT, glucose tolerance test; HFD, high-fat diet; IKK, kappa α/β kinase; IRE1, inositol requiring enzyme 1; ITT, insulin tolerance test; Improving; Insulin resistance; JNK, c-JunN-terminal kinase; NO, nitric oxide; PERK, protein kinase RNA-like ER kinase; UPR, unfolded protein response; iNOS; iNOS, inducible nitric oxide synthase; qPCR, real time PCR.