Leptin's Potential Interactions with Counter-Regulatory Hormones and Glucocorticoid Receptor Signaling in Type 1 Diabetic Rats
Type of DegreePhD Dissertation
Nutrition, Dietetics and Hospitality Management
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Diabetes is a global epidemic and the seventh leading cause of death in the United States. One of the characteristics of diabetes is hyperglycemia. Untreated diabetes can lead to ketoacidosis, which may lead to coma or even death. The main types of diabetes are type 1 diabetes, type 2 diabetes, and gestational diabetes. type 1 diabetes is an autoimmune disease of beta-cells in the pancreas that leads to insulin deficiency. Patients with type 2 diabetes, the most common form, can still make insulin, but they are insulin resistant. For type 1 diabetes, insulin supplementation is the most common treatment so far. However, for insulin-resistant type 2 diabetes, extra insulin treatment is not effective. It has been found that leptin can also decrease blood glucose concentrations in diabetic animals. The effect of leptin administration into the brain of previously uncontrolled diabetic animals on normalizing blood glucose concentrations is independent of insulin. However, the mechanism by which central leptin administration normalizes blood glucose concentrations in diabetic animals is not fully understood. Previous studies suggest that leptin blocks the responsiveness of glucagon by inhibiting cyclic adenosine monophosphate (cAMP) signaling and reducing hepatic glucose output from gluconeogenesis in type 1 diabetic animals. Preliminary studies showed that some leptin-treated diabetic rats had hypoglycemia after a short-term fast. During fasting, cAMP-CREB (cAMP response element binding protein) pathway is activated by counter-regulatory hormones such as epinephrine to increase hepatic glucose output. It has been suggested that the transition from the fed to fasted state lowers circulating leptin concentrations, leading to the activation of the hypothalamic-pituitary-adrenal (HPA) axis, an increase in corticosterone, and an increase in hepatic glucose production. We hypothesize that a similar effect occurs during diabetes. Therefore, central leptin treatment would be expected to attenuate the activation of the HPA axis, prevent the increase in corticosterone, and inhibit hepatic glucose production in diabetic animals. If this is true, dexamethasone treatment would be expected to increase blood glucose concentrations in leptin-treated diabetic rats. To determine the glucose responsiveness of leptin-treated nondiabetic and diabetic rats to counter-regulatory hormone during fasted and fed states and whether alterations in blood glucose concentrations are associated with biochemical changes in hepatic cAMP-CREB pathway, we monitored the blood glucose concentration change during fasted and fed state followed by an intraperitoneal (IP) injection of epinephrine to each rat and analyzed the level of hepatic gluconeogenic enzymes and plasma hormone contents. To examine the hypothesis that dexamethasone treatment increases blood glucose concentrations in chronic leptin-treated diabetic rats, we injected subcutaneous dexamethasone to half of the rats after the blood glucose levels were normalized by central leptin treatment and determined their blood glucose concentrations during glucose tolerance tests. As has been seen previously, chronic central leptin normalized blood glucose concentrations in diabetic rats. Leptin attenuated the increase of blood glucose in response to epinephrine in both diabetic and non-diabetic groups during fasting (p=0.044) and attenuated the effect of epinephrine in fed non-diabetic rats (p=0.0001). Diabetes increased hepatic phosphoenolpyruvate carboxykinase (PEPCK), glucose-6-phosphatase, CREB, and glycogen synthase levels (p<0.05), while hepatic glycogen phosphorylase content was decreased by diabetes (p<0.05). Leptin treatment of diabetic rats negated all these changes, except for glucose 6-phosphatase. Leptin did not alter the content of these proteins in nondiabetic rats. Plasma glucagon levels tended to increase in diabetic groups (p=0.0503). Plasma insulin concentrations were under the detectable limits in all groups. No differences were found in plasma corticosterone and hepatic cAMP concentrations between groups. Dexamethasone injections caused a large increase in the daily blood glucose concentrations, despite animals continuing to receive daily injections of leptin. During the 6-hour fast, blood glucose decreased in both groups; however, the blood glucose concentrations remained greater in the dexamethasone-treated rats. After receiving glucose injections during the glucose tolerance test, blood glucose concentrations increased for the first 30 minutes then started to decrease for both groups. However, the concentrations of blood glucose in the dexamethasone group (~200mg/dL) were higher than the vehicle group (~100mg/dL) throughout the whole test. When the blood glucose concentrations were expressed as the difference from time 0, there were no differences between the two groups. From these studies, we conclude that chronic central leptin treatment blocks the cAMP-CREB pathway and attenuates the effects of epinephrine to increase hepatic glucose production. These results support the hypothesis that dexamethasone administration would negate the ability of leptin to normalize blood glucose concentrations in diabetic rats. This suggests that alterations in glucocorticoid signaling are downstream of the site at which leptin normalizes blood glucose concentrations.