Differential Sensitivity to β-Cell Secretagogues in Cultured Rat Pancreatic Islets Exposed to Human Interleukin-1β*

Abstract

The early stages of insulin-dependent diabetes mellitus are characterized by a selective inability to secrete insulin in response to glucose, coupled to a better response to nonnutrient secretagogues. The deficient glucose response may be a result of the autoimmune process directed toward the β-cells. Interleukin-1 (IL-1) has been suggested to be one possible mediator of immunological damage of the β-cells. In the present study we characterized the sensitivity of β-cells to different secretagogues after human recombinant IL-1β (rIL-1β) exposure. Furthermore, experiments were performed to clarify the biochemical mechanisms behind the defective insulin response observed in these islets. Rat pancreatic islets were isolated and kept in tissue culture (medium RPMI-1640 plus 10% calf serum) for 5 days. The islets were subsequently exposed to 60 pM human recombinant IL-1β during 48 h in the same culture conditions as above and examined immediately after IL-1 exposure. The rIL-lβ-treated islets showed a marked reduction of glucose-stimulated insulin release. Stimulation with arginine plus different glucose concentrations, and leucine plus glutamine partially counteracted the rIL-1β- induced reduction of insulin release. The activities of the glycolytic enzymes hexokinase, glucokinase, and glyceraldehyde 3-phosphate dehydrogenase, were similar in control and IL-1-exposed islets. Treatment with IL-1 also did not impair the activities of NADH⁺- and NADPH⁺-dependent glutamate dehydrogenase, glutamate-aspartate transaminase, glutamate-alanine transaminase, citrate synthase, and NAD⁺-linked isocitrate dehydrogenase. The oxidation of D-[6-¹⁴C] glucose and L-[U-¹⁴C] leucine were decreased by 50% in IL- 1-treated islets. Furthermore, there was a significant decrease in the ratios of [2-¹⁴C]pyruvate oxidation/[l-¹⁴C]pyruvate decarboxylation and L-[U-¹⁴C]leucine oxidation/L-[l-¹⁴C]leucine decarboxylation, indicating that IL-1 decreases the proportion of generated acetyl-coenzyme-A residues undergoing oxidation. However, in the presence of IL-1 there was a significant increase in L-[U-¹⁴C]glutamate oxidation. These combined observations suggest that exposure to IL-1 induces a preferential decrease in glucose-mediated insulin release and mitochondrial glucose metabolism. This mitochondrial dysfunction seems to reflect an impairment in proximal steps of the Krebs cycle. It is conceivable that the IL-1-induced suppression and shift in islet metabolism can be an explanation for the β-cell insensitivity to glucose observed in the early phases of human and experimental insulin-dependent diabetes mellitus.