Dietary copper supplementation restores β-cell function of Cohen diabetic rats: a link between mitochondrial function and glucose-stimulated insulin secretion

Abstract

β-Cell mitochondrial dysfunction as well as proinflammatory cytokines have been suggested to contribute to reduced glucose-stimulated insulin secretion (GSIS) in type 2 diabetes. We recently demonstrated that Cohen diabetic sensitive (CDs) rats fed a high-sucrose, low-copper diet (HSD) developed hyperglycemia and reduced GSIS in association with peri-islet infiltration of fat and interleukin (IL)-1β-expressing macrophages, whereas CD resistant (CDr) rats remained normoglycemic on HSD. We examined: 1) the correlation between copper concentration in the HSD and progression, prevention, and reversion of hyperglycemia in CDs rats, 2) the relationship between activity of the copper-dependent, respiratory-chain enzyme cytochrome c oxidase (COX), infiltration of fat, IL-1β-expressing macrophages, and defective GSIS in hyperglycemic CDs rats. CDs and CDr rats were fed HSD or copper-supplemented HSD before and during hyperglycemia development. Blood glucose and insulin concentrations were measured during glucose tolerance tests. Macrophage infiltration and IL-1β expression were evaluated in pancreatic sections by electron-microscopy and immunostaining. COX activity was measured in pancreatic sections and isolated islets. In CDs rats fed HSD, GSIS and islet COX activity decreased, while blood glucose and infiltration of fat and IL-1β-expressing macrophages increased with time on HSD (P < 0.01 vs. CDr-HSD rats, all parameters, respectively). CDs rats maintained on copper-supplemented HSD did not develop hyperglycemia, and in hyperglycemic CDs rats, copper supplementation restored GSIS and COX activity, reversed hyperglycemia and infiltration of fat and IL-1β-expressing macrophages (P < 0.01 vs. hyperglycemic CDs-HSD rats, all parameters, respectively). We provide novel evidence for a critical role of low dietary copper in diminished GSIS of susceptible CDs rats involving the combined consequence of reduced islet COX activity and pancreatic low-grade inflammation.

IL-1β hampers glucose-stimulated insulin secretion in Cohen diabetic rat islets through mitochondrial cytochromecoxidase inhibition by nitric oxide

“…Moreover, treatment with the iNOS inhibitor N -nitro-L-arginine prevented the IL-1␤-induced COX inhibition and GSIS reduction in isolated islets, whereas copper supplementation in vivo prevented the increase in iNOS expression and the development of hyperglycemia. These findings are reinforced by our previous studies showing that IL-1␤-expressing macrophages infiltrating the exocrine pancreas of hyperglycemic CDs rats reduce COX activity and ␤-cell function, both of which were reversed by copper supplementation (48,49). Thus, taken together, our previous and current data confirm the role of NO in IL-1␤-mediated GSIS reduction by reducing COX activity below a certain critical level.…”

“…In our previous in vivo study, low-copper HSD induced hyperglycemia in CDs rats, whereas coppersupplemented HSD reversed these phenomena (48,49). Our current ex vivo experiments on isolated islets of rats fed copper-supplemented HSD as well as in vitro coincubation of islets with the complex Cu-GHL (44) and IL-1␤ revealed that COX activity and GSIS similarly recovered upon copper supplementation.…”

“…We have observed previously that the development of hyperglycemia in CDs rats fed HSD is associated with peri-islet infiltration of fat and macrophages ex-pressing IL-1␤ (49). We also showed that copper supplementation to HSD improves GSIS, suggesting the involvement of COX (48). In the current study, we examine the mechanism by which interleukin-1␤ modulates GSIS to clarify the role of COX in GSIS of isolated islets from CDs and CDr rats.…”

“…All studies were performed on 3-mo-old male rats fed one of three dietary protocols: 1) a regular diet (purchased from Teklad, 2018; Harlan Laboratories) containing adequate (16 ppm) copper (49); 2) HSD containing a low (1.2 ppm) copper concentration, 72% sucrose, 18% vitamin-free casein, 5% salt mixture USP XIII no. 2 (MP Biomedicals), butter (4.5%), corn oil (0.5%), and vitamins (7,49,50); and 3) HSD supplemented with adequate amounts (16 ppm) of copper (48).…”

“…CDs and CDr male rats fed for 8 wk on a regular diet were either 1) maintained on a regular diet for 30 additional days, 2) switched to the diabetogenic HSD for 30 days (49), or 3) fed HSD for 30 days and then switched to HSD supplemented with copper (HSD ϩ Cu) for an additional period of 20 days (48). Studies were performed on islets isolated from rats with one of these three diets.…”