Phuong Oanh T. Tran

Prostaglandin E2 Mediates Inhibition of Insulin Secretion by Interleukin-1β

Interleukin-1β (IL-1β) and prostaglandin E2 (PGE2), frequently co-participants in inflammatory states, are two well recognized inhibitors of glucose-induced insulin secretion. Previous reports have concluded that the inhibitory effects of these two autacoids on pancreatic β cell function are not related because indomethacin, a potent prostaglandin synthesis inhibitor, does not prevent IL-1β effects. However, indomethacin is not a specific cyclooxygenase inhibitor, and its other pharmacologic effects are likely to inhibit insulin secretion independently. Since we recently observed that IL-1β induces cyclooxygenase-2 (COX-2) gene expression and PGE2 synthesis in islet β cells, we have reassessed the possibility that PGE2 mediates IL-1β effects on β function. By using two cell lines (HIT-T15 and βHC13) as well as Wistar rat isolated pancreatic islets, we examined the ability of two COX-2-specific antagonists, NS-398 and SC-236, to prevent IL-1β inhibition of insulin secretion. Both drugs prevented IL-1β from inducing PGE2 synthesis and inhibiting insulin secretion; adding back exogenous PGE2 re-established inhibition of insulin secretion in the presence of IL-1β. We also found that EP3, the PGE2 receptor subtype whose post-receptor effect is to decrease adenylyl cyclase activity and, thereby, insulin secretion, is the dominant mRNA subtype expressed. We conclude that endogenous PGE2 mediates the inhibitory effects of exogenous IL-1β on β cell function.

d-Glyceraldehyde Causes Production of Intracellular Peroxide in Pancreatic Islets, Oxidative Stress, and Defective Beta Cell Function via Non-mitochondrial Pathways

d-Glyceraldehyde (d-GLYC) is usually considered to be a stimulator of insulin secretion but theoretically can also form reactive oxygen species (ROS), which can inhibit beta cell function. We examined the time- and concentration-dependent effects of d-GLYC on insulin secretion, insulin content, and formation of ROS. We observed that a 2-h exposure to 0.05–2 mm d-GLYC potentiated glucose-stimulated insulin secretion (GSIS) in isolated Wistar rat islets but that higher concentrations inhibited GSIS. A 24-h exposure to 2 mm d-GLYC inhibited GSIS, decreased insulin content, and increased intracellular peroxide levels (2.14 ± 0.31-fold increase, n = 4, p < 0.05). N-Acetylcysteine (10 mm) prevented the increase in intracellular peroxides and the adverse effects of d-GLYC on GSIS. In the presence of 11.1 but not 3.0 mm glucose, koningic acid (10 μm), a specific glyceraldehyde-3-phosphate dehydrogenase inhibitor, increased intracellular peroxide levels (1.88 ± 0.30-fold increase, n = 9, p < 0.01) and inhibited GSIS (control GSIS = p < 0.001; koningic acid GSIS, not significant). To determine whether oxidative phosphorylation was the source of ROS formation, we cultured rat islets with mitochondrial inhibitors. Neither rotenone or myxothiazol prevented d-GLYC-induced increases in islet ROS. Adenoviral overexpression of manganese superoxide dismutase also failed to prevent the effect of d-GLYC to increase ROS levels. These observations indicate that exposure to excess d-GLYC increases reactive oxygen species in the islet via non-mitochondrial pathways and suggest the hypothesis that the oxidative stress associated with elevated d-GLYC levels could be a mechanism for glucose toxicity in beta cells exposed chronically to high glucose concentrations.

Prevention of oxidative stress by adenoviral overexpression of glutathione-related enzymes in pancreatic islets.

Abstract: Chronic exposure to supraphysiologic glucose concentrations causes functional damage to cells and tissues, a process known as glucose toxicity. Recent research indicates that one important mechanism for glucose toxicity is oxidative stress. Glucose has been shown to form reactive oxygen species through several metabolic pathways. The pancreatic islet is distinguished by its relatively low antioxidant enzyme content and activity, which render it especially susceptible to oxidative stress. Adenoviral overexpression of glutathione peroxidase as well as gamma‐glutamylcysteine ligase have been shown to protect the islet against oxidative stress. Antioxidants have been shown to brake the worsening of diabetes by improving beta cell function in animal models. These observations suggest that enhancing antioxidant defense mechanisms in pancreatic islets may be a valuable pharmacologic approach to managing diabetes.

Glucose Toxicity of the Pancreatic ß-Cell

For purposes of this chapter, glucose toxicity is defined as pathophysiologic and largely irreversible damage to the pancreatic islet f3-cell caused by chronic exposure to supraphysiologic glucose concentrations. This leads to profound disturbances in insulin synthesis, which in turn leads to decreased insulin content and secretion. Using various experimental models, many authors have described negative effects of glucose on 13-cell function and termed these effects variously as glucose toxicity, 13- cell exhaustion, or glucose desensitization (1-50). While the distinction between 13- cell exhaustion and glucose toxicity is not always clear, we favor the concept that the two lie in a pathophysiologic continuum. f3-cell exhaustion is the earlier form and likely to be reversible, whereas glucose toxicity is the later form and less reversible. In marked contrast, we view “glucose desensitization” as a temporary, physiological state of p-cell refractoriness induced by repeated or prolonged exposure to high glucose concentrations, which is reversed in a time-dependent manner after restoration of normal glucose concentrations. This term implies an intrinsic and reversible alteration in stimulus-secretion coupling rather than damage to the 13-cell.