Mitochondrial nicotinamide nucleotide transhydrogenase (NNT) is the main source of NA(D)PH in mitochondria, which is used for detoxification of mitochondrial hydrogen peroxide. Previous studies have shown that the mutation in this enzyme is associated with changes in the redox state of mitochondria, reduction of insulin secretion and increase of diet-induced obesity in C57BL/6J mice from the Jackson Laboratory (B6-J) when compared to strains without this mutation (e.g., C57BL/6JUnib). In this work, we investigate the glucose and lipid metabolism in order to understand the role of NNT in the susceptibility to diabetes and obesity. For this, we studied independent isogenic C57BL6 sub line, with and without mutation of NNT, B6-J and B6-UNI, respectively, as well as congenic lineages (with >99% genetic background homogeneity) with and without the NNT mutation (NNT-/- vs. NNT+/+). These studies were performed with either normal (standard) diet or high fat (HF, 20 weeks) diet. In the first study we evaluated the effect of NNT on glycemic homeostasis. We observed that in relation to the non-mutants (B6-UNI), NNT deficient animals (B6-J) presented glucose intolerance due to both insulin secretion deficiency and peripheral resistance to the hormone. In the pancreatic islets, in addition to the reduction of insulin secretion stimulated by glucose, there was a reduction of autofluorescence of NAD(P)H and increase of H2O2 production. NNT-deficient animals had hepatic insulin resistance as determined by the pyruvate-to-glucose test and reduced hepatic insulin signaling (pAKT). All these findings were observed in both normal and HF diet fed mice. However, in congenic strains, we demonstrated that NNT deficient animals (NNT-/-) were also glucose intolerant, but with secretory insulin function preserved, both in normal and HF diet. Glucose intolerance in NNT-/- animals is explained by increased peripheral insulin resistance, overall (insulin tolerance test) and in liver (conversion of pyruvate to glucose and phosphorylation of AKT). Based on previous studies showing redox abnormalities in hepatic mitochondria with NNT deficiency, our hypothesis is that these abnormalities are responsible for hepatic insulin resistance. Thus, we treated the animals for a month with the antioxidant TEMPOL, which in fact reversed the glucose intolerance caused by NNT deficiency. Thus, we conclude that NNT is important for hepatic insulin signaling, but does not alter insulin secretion. Probably other genetic differences between the B6-J and B6-UNI strains besides NNT mutation are responsible for the impairment of glucose-stimulated insulin secretion in C57BL/6J animals. In the second study we evaluated the role of NNT on lipid metabolism. We observed that NNT-deficient B6-J mice present increased body adiposity and hepatic triglycerides (TG), increased leptinemia and reduced body metabolism (respirometry). These findings were obtained both in the normal diet and in the HF diet. Increased liver fat can be explained by increased dietary lipid retention. When we investigated the congenic lines, we observed that the NNT-/- animals only remain more obese when on a hyperlipidic diet, while hepatic steatosis is present in both normal and HF diet. The mechanisms responsible for hepatic steatosis are associated with reduced hepatic secretion of VLDL-TG. Thus, we propose that mitochondrial dysfunction impairs insulin signaling, and insulin resistance is determinant of hepatic lipid accumulation, mainly because it compromises the secretion of VLDL. In summary, NNT deficiency is determinant of diet-induced adiposity but does not explain differences in fat deposits observed in independent C57BL6 strains under normal dietary. The increase in diet-induced adiposity is explained by reduced body metabolism and hepaticsteatosis by insulin resistance in NNT-deficient animals (AU) |
