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Oxidative Stress, ß-Cell Apoptosis, and Decreased Insulin Secretory Capacity in Mouse Models of Hemochromatosis

Research Service of the Veterans Affairs Medical Center (R.C.C., D.A.M.), and Departments of Medicine and Biochemistry, University of Utah School of Medicine (R.C.C., H.A.J., R.S.A., M.W.H., D.L.J., J.P.K., D.A.M.), Salt Lake City, Utah 84132

Address all correspondence and requests for reprints to: Dr. Donald A. McClain, Division of Endocrinology, University of Utah School of Medicine, 30 North 2030 East, Salt Lake City, Utah 84132. E-mail: donald.mcclain{at}hsc.utah.edu.

The pathogenesis of diabetes associated with hemochromatosis is not known. We therefore examined glucose homeostasis and ß-cell function in mouse models of hemochromatosis. Mice with targeted deletion of the hemochromatosis gene (Hfe^–/–) on the 129/Sv genetic background exhibited a 72% increase in iron content in the islets of Langerhans compared with wild-type controls. Insulin content was decreased in Hfe^–/– mice by 35%/pancreas and 25%/islet. Comparable decreases were seen in the mRNA levels of ß-cell-specific markers, ins1, ins2, and glucose transporter 2. By 6–8 months, islets from Hfe^–/– mice were 45% smaller, associated with increased staining for activated caspase 3 and terminal deoxynucleotidyl transferase-mediated deoxy-UTP nick end labeling. Islets from Hfe^–/– mice were also desensitized to glucose, with half-maximal stimulation of insulin secretion seen at 16.7 ± 0.9 mM glucose in perifused islets from Hfe^–/– mice compared with 13.1 ± 0.6 mM glucose in wild-type animals. Carbonyl protein modification, a marker for oxidative stress, was increased by 58% in Hfe^–/– islets. Despite decreased islet size, Hfe^–/– mice exhibited enhanced glucose tolerance. Fasting serum insulin levels were comparable between Hfe^–/– and Hfe^+/+ mice, but were 48% lower in the Hfe^–/– mice 30 min after challenge. Similar results were seen in mice carrying an Hfe mutation analogous to the common human mutation (C282Y) and in mice fed excess dietary iron. Hfe^–/–mice on the C57BL6 background exhibited decreased glucose tolerance at 10–12 months due to an inability to increase insulin levels as they aged. We conclude that iron excess results in ß-cell oxidant stress and decreased insulin secretory capacity secondary to ß-cell apoptosis and desensitization of glucose-induced insulin secretion. This abnormality alone, however, is insufficient to cause diabetes.

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