A pathogenic role of visceral fat beta 3-adrenoceptors in obesity

Abstract

Increased release of free fatty acids (FFA) from visceral fat cells to the portal venous system may cause several metabolic disturbances in obesity. However, this hypothesis and the underlying mechanism remain to be demonstrated. In this study catecholamine-induced lipid mobilization through lipolysis in omental adipose tissue was investigated in vitro in 25 markedly obese subjects (body mass index range 35-56 kg/m2) undergoing weight reduction surgery and in 19 nonobese subjects (body mass index range 20-28 kg/m2) undergoing cholecystectomy. Release of FFA and glycerol, induced by norepinephrine or adrenergic receptor subtype-specific agonists, were determined in isolated omental fat cells. The obese subjects had higher fat cell volume, blood pressure, plasma insulin levels, blood glucose, plasma triglycerides, and plasma cholesterol than the controls. There was evidence of upper-body fat distribution in the obese group. The rate of FFA and glycerol response to norepinephrine was increased twofold in the cells of obese subjects; no significant reutilization of FFA during catecholamine-induced lipolysis was observed in any of the groups (glycerol/FFA ratio near 1:3). There were no differences in the lipolytic sensitivity to beta 3- or beta 2-adrenoceptor specific agonists between the two groups. However, beta 3-adrenoceptor sensitivity was approximately 50 times enhanced (P = 0.0001), and the coupling efficiency of these receptors was increased from 37 to 56% (P = 0.01) in obesity. Furthermore, the obese subjects demonstrated a sixfold lower alpha 2-adrenoceptor sensitivity (P = 0.04). beta 3-Adrenoceptor sensitivity, but not alpha 2-, beta 1-, or beta 2-adrenoceptor sensitivity, correlated with norepinephrine-induced lipolysis (r = -0.67, P = 0.0001) and fat cell volume (r = -0.71, P = 0.0001). In conclusion, catecholamine-induced rate of FFA mobilization from omental fat cells is accelerated due to elevated rate of lipolysis in obesity, mainly because of an increased beta 3-adrenoceptor function, but partly also because of a decreased alpha 2-adrenoceptor function. This promotes an increased release of FFA to the portal system, which may contribute to the parallel metabolic disturbances observed in upper-body obesity.