Hypothalamic substrate partitioning in energy balance and glucose homeostasis

The incidence of obesity in the U.S. has increased dramatically in recent years to epidemic proportions whereas mechanisms underlying obesity and feeding behavior remain to be determined. Fuel partitioning involving a shift away from glycolysis and toward alternative metabolic pathways such as beta-oxidation occurs during nutritional deprivation associated with Creb mediated responses to fasting, estradiol-induced anorexia, and glucocorticoid-induced obesity (Louet, Hayhurst et al. 2002; Wortman, Clegg et al. 2003; Attia, Connaughton et al. 2010). During fasting Creb is induced (Altarejos, Goebel et al. 2008) and binds to liver carnitine palmitoyltransferase I (L-CPT I), the rate-limiting enzyme for lipid metabolism (Louet, Hayhurst et al. 2002). To assess the role of hypothalamic Creb in energy balance, the binding of Creb to specific hypothalamic genes was examined in mouse strains that are resistant or sensitive to diet-induced obesity, the latter of which express higher levels of hypothalamic Creb than the former. We observed that Creb binding to the pro-obesity gene Agrp in mice prone to obesity, and that hypoglycemia induces Creb binding to Agrp. However, neither genetic ablation or a dominant negative Creb construct enhanced sensitivity to diet-induced obesity. Substrate partitioning during nutrition deprivation in the context of estradiol-induced anorexia was also assessed. Previous studies have shown estradiol inhibits Cpt1a within the hypothalamus (Cheng, Isoda et al. 2009) and pharmalogical inhibition of fatty acid synthase leads to a build up of malonyl-coA in the ventral medial hypothalamus which inhibits beta-oxidation (Lopez, Lelliott et al. 2006). We therefore assessed which hypothalamic metabolites correlate with feeding under a variety of conditions. We also assessed if AAV-mediated overexpression of hypothalamic Cpt1a would promote obesity. Metabolomic profiles indicated that a high-fat diet produces a switch from glucose utilization to beta-oxidation in the liver associated with hyperphagia. Several hypothalamic metabolite concentrations also correlated with feeding. Specifically, food intake positively correlated with the intermediates of the pentose phosphate pathway (xylulose- and ribulose-5' phosphate) and with fatty acid degradation (glycerol 3-phosphate) indicating that fuel partitioning away from glycolysis and toward alternative beta-oxidation is correlated with increased feeding. This correlational observation was strikingly supported by the result that experimental enhancement of hypothalamic expression of Cpt1a increased weight gain and food intake on a high-fat diet. Glucocorticoids also promote a metabolic switch away from glycolysis and towards beta-oxidation, in part by inducing pyruvate dehydrogenase kinase 4 (PDK4) gene expression (Attia, Connaughton et al. 2010). Glucocorticoids also promote obesity in human diseases, for example in Cushing's disease in which excess cortisol production causes increase and redistribution of body fat (Orth 1995). During a screen for hypothalamic genes induced by fasting, we identified Fkbp51, an antagonist to the glucocorticoid receptor, to be induced by fasting. To assess if blocking hypothalamic glucocorticoid action would block the glucocorticoid negative feedback loop and thus predispose to diet-induced obesity, we used AAV to increase hypothalamic expression of Fkbp51. This manipulation resulted in increased weight gain on a high-fat diet and impaired glucose tolerance associated with chronic elevation of plasma corticosterone, without change in food intake. Taken together these studies support that hypothalamic fuel partitioning away from glucose utilization toward beta-oxidation promotes obese phenotypes.