Increased glucocorticoid sensitivity in pancreatic beta-cells : effects on glucose metabolism and insulin release

Abstract

Type 2 diabetes mellitus (T2DM) is characterized by three pathological alterations: (1) insulin resistance in peripheral tissues, (2) increased hepatic glucose production and (3) impaired insulin secretion from the pancreatic beta-cells. Glucocorticoids (GCs) exert profound effects on glucose homeostasis. They decrease glucose uptake and increase hepatic glucose production. In addition, they may directly inhibit insulin release. The main aim of this thesis was to investigate the effect of GCs on beta-cell function, insulin release and glucose homeostasis. Furthermore, we studied the mechanisms behind this possible effect. For this purpose two different animal models were used. First, transgenic mice (TG mice) with an increased GC sensitivity restricted to their beta-cells were generated by overexpressing the glucocorticoid receptor (GR) under the control of insulin promoter 1. The second animal model, was ob/ob mice, in which hyperglycemia co-exists with obesity.

At the age of 3-4 months, TG mice had normal fasting blood glucose but decreased glucose tolerance (paper 1). In an intravenous glucose tolerance test, decreased glucose tolerance was observed in TG mice compared to controls. TG mice exhibited significantly higher blood glucose concentrations than control mice at 60 min after intravenous injection of glucose. Measurement of plasma insulin levels 5 min after glucose load demonstrated a decrease in acute insulin response in the TG mice.

Following the natural history of glucose impairment in TG mice, we found that the transgenic mice developed hyperglycemia both in the fed and overnight-fasted states at the age of 12-15 months (paper III). The basal and stimulated insulin secretion was significantly decreased in the old TG mice both under in vivo and in vitro conditions. Importantly, glucose elimination after i.p. insulin administration did not differ between the groups, suggesting that insulin sensitivity was normal in TG mice. Therefore, it seems that the development of manifest diabetes in these animals is due to a direct inhibitory effect of GCs on insulin release, and is not related to reduction in insulin sensitivity.

The mechanisms behind the progressive deterioration of glucose tolerance in TG mice are not known. However, data presented in paper Il and III shed some light on the mechanisms behind the impaired insulin release in TG mice. First, the activity of islet Glucose-6-phosphatase (G6Pase) was increased in transgenic mice. Secondly, we found a significantly higher density Of alpha2-adrenergic receptor (alpha2-AR) in the islets of TG mice compared to controls. Furthermore, expression Of alpha2-AR mRNA was increased in islets from TG mice.

G6Pase and alpha2-AR are two of the genes that have been reported to be regulated by GC and may be related to the regulation of insulin secretion. Previous reports from some animal models of T2 DM have shown an increased G6Pase activity in the islets which can be attenuated by GC treatment. GCs upregulate also alpha2-AR expression and signaling in beta-cell lines. Thus, reduced glucose-stimulated insulin release and development of hyperglycemia in TG mice, can be due to enhanced expression of G6Pase and alpha2-AR.

The inhibitory effect of GC on islets is part partly mediated by 11-betahydroxysteroid dehydrogenase type 1 (11 beta-HSD-1) through local production of active GC. In the last study (IV), we found evidence for activity and expression of 11beta-HSD-1 in pancreatic islets from ob/ob mice. Glucose-stimulated insulin release was inhibited with 11-dehydrocorticosterone in a dose dependent manner. Carbenoxolone restored this effect. Thus, 11beta-HSD-1 is an amplifier of GC action in the beta-cells of ob/ob mice.

Taken together, our results demonstrate that GCs directly inhibit insulin release and identify the pancreatic beta-cells as an important target for the diabetogenic action of GC. Antagonism of GR and their signaling pathways are thus promising therapeutic targets for treatment of T2DM.