Role of glucocorticoids in the physiopathology of excessive fat deposition and insulin resistance

Glucocorticoids are important hormones in the regulation of metabolic homeostasis. We infused normal rats with dexamethasone given intracerebroventricularly (i.c.v.) for 3 days. This resulted in hyperphagia, hyperinsulinemia, and marked insulin resistance. Similar metabolic defects were observed following i.c.v. infusion of neuropeptide Y (NPY) in normal rats. As central dexamethasone infusion enhanced NPY content in the arcuate nucleus, it suggested that its metabolic effects are mediated by NPY. Moreover, due to the lack of effects observed in vagotomized animals, activation of the parasympathetic nervous system by central dexamethasone infusion is proposed. Glucocorticoid action is known to involve prereceptor metabolism by enzymes such as 11beta-HSD-1 that converts inactive into active glucocorticoids. Mice overexpressing 11beta-HSD-1 in adipose tissue were shown to be obese and insulin resistant. We recently observed that adipose tissue 11beta-HSD-1 mRNA expression is increased at the onset of high-fat diet-induced obesity and positively correlated with the degree of hyperglycemia. In human obesity, increased adipose tissue 11beta-HSD-1 expression and activity were also reported. Resistin is a new adipose tissue-secreted hormone shown to play a role in glucose homeostasis by increasing hepatic glucose production and inhibiting muscle and adipose tissue glucose utilization. We observed increased adipose tissue resistin expression in the early phase of high-fat diet-induced obesity as well as decreased resistin expression in response to leptin. A positive correlation between glycemia and adipose tissue resistin expression further suggested a role of this hormone in the development of insulin resistance. The melanocortin system is another important player in the regulation of energy balance. Peripheral administration of a melanocortin agonist decreased food intake and body weight and favored lipid oxidation, effects that were more marked in obese than in lean rats. It is proposed that both resistin and melanocortin agonists may influence adipose tissue 11beta-HSD-1, thereby decreasing or enhancing glucose metabolism.