Regulation of ob gene expression and leptin secretion by insulin and dexamethasone in rat adipocytes

Leptin, the ob gene product, is produced by adipocytes, and it acts to decr ease caloric intake and increase energy expenditure. To better understand t he molecular mechanisms of hormone-regulated leptin synthesis and secretion , we assessed the ability of insulin and dexamethasone to acutely modulate ob gene expression and leptin secretion in rat adipocytes. Incubation of ra t adipocytes with 100 nmol/l insulin for 2 h had no effect on ob mRNA level s, but it stimulated a twofold increase in leptin secretion. Dexamethasone (100 nmol/l) stimulated both a two- to fourfold increase in ob mRNA and a t wofold increase in leptin secretion, Consonant with a posttranscriptional a nd transcriptional regulatory mechanism for insulin- and dexamethasone-stim ulated leptin secretion, respectively, actinomycin D blocked dexamethasone- stimulated leptin secretion but did not affect; insulin-stimulated leptin s ecretion, Cycloheximide treatment did not, significantly affect ob mRNA acc umulation, but it reduced total secreted leptin. Interestingly, however, in sulin was still able to stimulate a twofold increase in leptin secretion, T hese data suggest that insulin, but not dexamethasone, is able to stimulate leptin secretion from a preexisting intracellular pool, although de novo p rotein synthesis is required for the fun insulin-stimulated effect. Signali ng pathways involved in leptin synthesis/secretion were also evaluated. The phosphalidylinositol 3-kinase inhibitor LY294002, the Map/Erk kinase inhib itor PD98059, and the immunosuppressant rapamycin had no effect on basal le vels of leptin secretion. However, all three inhibitors markedly decreased both insulin- and dexamethasone-stimulated leptin secretion. These findings suggest a complex set of signaling pathways involved in mediating insulin- and dexamethasone-stimulated leptin synthesis and secretion.