Action of growth hormone in adipose tissue.

Growth hormone (GH) has diverse and still not fully elucidated effects on adipose tissue. Both preadipocytes and mature adipocytes possess specific GH receptors. GH may mediate its actions via these receptors, but some effects are indirectly mediated through the GH-mediated secretion of insulin-like growth factor-I (IGF-I). IGF-I may then act back on the adipose tissue in an autocrine/paracrine manner. In primary cultures of adipose precursor cells obtained from human or rat adipose tissue, GH is found to stimulate the proliferation of these immature cells and reduce their differentiation to mature adipocytes. During long-term incubation and in in vivo studies, GH has a pronounced lipolytic effect. Whether this lipolytic effect is a direct effect of GH or more indirectly mediated, for instance, via inhibition of the action of antilipolytic compounds (e.g. adenosine, prostaglandins and insulin) is presently unknown. Finally, GH produces a pronounced inhibition of adipose tissue lipoprotein lipase activity. This enzyme plays a main role for hydrolysing triglycerides in the blood circulation in the adipose tissue and then for triglyceride accumulation in adipose cells. Thus, GH inhibits adipocyte differentiation, reduces triglyceride accumulation and increases lipolyses--all mechanisms which reduce adipose tissue mass.