Catecholamine-induced lipolysis in obesity.

Catecholamines are the only hormones with pronounced lipolytic action in man. A number of in vivo and in vitro studies suggest that there is lipolytic resistance to catecholamines in subcutaneous adipose tissue, which is the major fat depot in obese subjects. This is due to multiple alterations in catecholamine signal transduction, involving decreased expression and function of beta2-adrenoceptors, increased function of alpha2-adrenoceptors and decreased ability of cyclic monophosphate (AMP) to stimulate hormone sensitive lipase. A sedentary life-style, which usually characterizes obesity, may contribute to the catecholamine resistance. However, hereditary/genetic factors may also be involved. Recently, decreased expression and function of hormone sensitive lipase has been found in subcutaneous adipocytes of non-obese subjects with heredity for obesity. In addition, polymorphisms in the genes for beta2-adrenoceptors, beta3-adrenoceptors and hormone sensitive lipase, associate with obesity. On the other hand, catecholamine-induced lipolysis in visceral adipose tissue is increased in obesity due to increased function of beta3-adrenoceptors (major finding), decreased function of alpha2-adrenoceptors and increased ability of cyclic AMP to stimulate lipolysis. When the findings in different adipose regions are considered together, it appears that there is a redistribution of lipolysis and thereby fatty acid mobilization in obesity, favouring the visceral fat depot. This leads to an increase in the circulating fatty acid levels in the portal vein, which connects visceral fat with the liver. As a consequence, the liver function may be altered leading to hyperinsulinemia, hyperglycemia and dyslipidemia, which usually accompany the obese state.