Multihormonal Control of<i>ob</i>Gene Expression and Leptin Secretion from Cultured Human Visceral Adipose Tissue: Increased Responsiveness to Glucocorticoids in Obesity¹

The direct role of hormones on leptin synthesis has not yet been studied in cultured adipose cells or tissue from lean and obese subjects. Moreover, this hormonal regulation has never been addressed in human visceral fat, although this site plays a determinant role in obesity-linked disorders. In this study, we investigated the hormonal control of ob expression and leptin production in cultured visceral adipose tissue from lean and obese subjects. We more particularly focused on the interactions between glucocorticoids and insulin. We also briefly tackled the role of cAMP, which is still unknown in man. Visceral (and subcutaneous) adipose tissues from eight obese (body mass index, 41 +/- 2 kg/m2) and nine nonobese (24 +/- 1 kg/m2) subjects were sampled during elective abdominal surgery, and explants were cultured for up to 48 h in MEM. The addition of dexamethasone to the medium increased ob gene expression and leptin secretion in a time-dependent manner. Forty-eight hours after dexamethasone (50 nmol/L) addition, the cumulative integrated ob messenger ribonucleic acid (mRNA) and leptin responses were, respectively, approximately 5- and 4-fold higher in obese than in lean subjects. These responses closely correlated with the body mass index. The stimulatory effect of the glucocorticoid was also concentration dependent (EC50 = approximately 10 nmol/L). Although the maximal response was higher in obese than in lean subjects, the EC50 values were roughly similar in both groups. Unlike dexamethasone, insulin had no direct stimulatory effect on ob gene expression and leptin secretion. Singularly, insulin even inhibited the dexamethasone-induced rise in ob mRNA and leptin release. This inhibition was observed in both lean and obese subjects, whereas the expected stimulation of insulin on glucose metabolism and the accumulation of mRNA species for the insulin-sensitive transporter GLUT4 and glyceraldehyde-3-phosphate dehydrogenase occurred in lean patients only. This inhibitory effect was already detectable at 10 nmol/L insulin and was also observed in subcutaneous fat. Although a lowering of intracellular cAMP concentrations is involved in some of the effects of insulin on adipose tissue, this cannot account for the present finding, because the addition of cAMP to the medium also decreased ob mRNA and leptin secretion (regardless of whether dexamethasone was present). In conclusion, glucocorticoids, at physiological concentrations, stimulated leptin secretion by enhancing the pretranslational machinery in human visceral fat. This effect was more pronounced in obese subjects due to a greater responsiveness of the ob gene and could contribute to the metabolic abnormalities associated with central obesity by para/endocrine actions of hyperleptinemia on adipocytes and liver. Unlike dexamethasone, insulin had no direct stimulatory effect on ob gene expression and leptin secretion, and even prevented the positive response to dexamethasone by a cAMP-independent mechanism that remained functional despite insulin resistance.