Molecular and cellular mechanisms of adipose secretion: Comparison of leptin and angiotensinogen

Besides their function of lipid storage, the adipose cells secrete a number of proteins of physiopathological importance. To get further insights into this function, which remains poorly characterized, we sought to compare th e mechanisms and regulation of secretion of two individual proteins in the same cells. Leptin and angiotensinogen were chosen and assessed by radioimm unoassay and quantitative immunoblotting, respectively, in primary culture of epididymal adipose cells from young obese Zucker rats. Leptin was secret ed at a steady rate of 4 ng/10(6) cells/h over 2-6 h. Despite secretion, le ptin cellular content remained stable at 3 ng/10(6) cells. In contrast, the rate of angiotensinogen secretion decreased regularly from 45 arbitrary un its/10(6) cells/h at 2 h, to half this value at 6 h, although cell content remained constant at 100 arbitrary units/10(6) cells. Inhibition of protein synthesis by cycloheximide depleted the cells from leptin, but not from an giotensinogen for up to 6 h. Insulin increased leptin secretion (+75%) and cell content (+70 %), without affecting angiotensinogen. Secretion of both proteins was inhibited by Golgi-disturbing agents, brefeldin A and monensin . The presence of brefeldin A led to a specific rise in leptin cell content , an effect inhibited by cycloheximide and enhanced by insulin (+80%). Thes e data show that leptin and angiotensinogen are both secreted through Golgi -dependent pathways and that their respective intracellular pool exhibit di stinct turn-over rate and insulin sensitivity. These characteristics might account for the differential response of these adipose proteins to variatio ns in the systemic environment. (C) 2001 Wiley-Liss, Inc.