Iron overload, mitochondrial dysfunction and oxidative stress play pathogenic roles in the diabetes of hereditary hemochromatosis (HH) and type 2 diabetes. We have studied a mouse model of HH (Hfe-/-) that exhibits decreased insulin secretion associated with oxidative stress, desensitization of glucose-stimulated insulin secretion, and beta cell apoptosis. Glucose tolerance, however, is supranormal because of increased insulin sensitivity, and the mice are resistant to diet-induced obesity. Our studies of humans with HH revealed a high prevalence of impaired glucose tolerance (IGT, 30%) and diabetes (22%). HH with IGT, as in the mice, is characterized by low insulin secretion but increased insulin sensitivity. HH subjects with diabetes were all overweight, suggesting that resultant insulin resistance could not be compensated because of impaired insulin secretion. Mitochondria from Hfe-/- mice exhibit increased oxidant stress, in part from a novel mechanism of altered Mn transport. Metabolic and genomic analysis suggests that a shift in preference from glucose to fatty acid oxidation results from a second novel mechanism of increased adiponectin action. We hypothesize that fat metabolism is highly regulated by iron in normal physiology and disease. Depending on its tissue distribution, excess iron has both pro-diabetic (e.g. beta cell damage) and anti-diabetic (e..g enhanced fat oxidation) effects. We will continue the strategy of parallel mouse/human studies and address these aims: 1. Determine if humans with HH exhibit a preference for fat oxidation over glucose oxidation by characterizing them by indirect calorimetry, FSIVGTT, serum adiponectin and measurment of the metabolic fate of [1,2-13C]glucose. Candidate genetic polymorphisms that may contribute to the diabetes morbidity of HH will also be assessed. 2. Determine whether phlebotomy will reverse the full phenotype of HH, i.e. decreased insulin secretion, increased insulin sensitivity, and altered fat oxidation. 3. In the mouse model investigate the mechanism for mitochondrial dysfunction by studying ATP production, oxygen consumption, substrate preference, activities of key mitochondrial proteins, and the role of decreased Cu and Mn levels. 4. Study the mechanism of altered hepatic metabolism by assessing fat and glucose oxidation and synthesis, and the actions of adiponectin, insulin, and glucagon in isolated hepatocytes. These results will guide future studies to determine the role of these phenomena in type 2 diabetes.
|Gabrielsen, J Scott; Gao, Yan; Simcox, Judith A et al. (2012) Adipocyte iron regulates adiponectin and insulin sensitivity. J Clin Invest 122:3529-40|