Effects of tumor necrosis factor alpha on leptin secretion and gene expression: Relationship to changes of glucose metabolism in isolated rat adipocytes

OBJECTIVE: Our objective was to determine the effects of prolonged exposure to tumor necrosis factor-alpha (TNF-α) on leptin secretion from and leptin (OB) gene expression in isolated adipocytes. Because glucose uptake and the metabolism of glucose beyond lactate are important determinants of leptin production in adipocytes, we examined the effects of TNF-α on glucose uptake and lactate production and their relationship to leptin secretion. DESIGN AND METHODS: Isolated rat adipocytes were anchored in a defined matrix of basement membrane components and cultured with media containing 5 mM glucose, 0.16 nM insulin and several concentrations of TNF-α. Leptin secretion, steady-state levels of leptin mRNA levels, glucose uptake, and lactate production were assessed over 96 h. RESULTS: TNF-α at concentrations of 0.024, 0.24, 2.4 and 24 ng/ml did not affect leptin secretion over 24 h. TNF-α at concentrations of 0.24 to 24 ng/ml significantly inhibited leptin secretion over 96 h by 19-60%. TNF-α at concentrations of 0.024 to 24 ng/ml significantly decreased steady-state levels of leptin mRNA after 96 h by 32-95%. In addition, TNF-α at concentrations of 2.4 and 24 ng/ml significantly increased glucose uptake and lactate production over 96 h by 30-57%. TNF-α at a concentration of 0.024 ng/ml did not affect leptin secretion, glucose uptake or lactate production. Overall, for the TNF-α concentrations tested, leptin secretion was inversely related to the percent of glucose carbon released as lactate; however, TNF-α did not induce a proportional increase of lactate production from glucose. CONCLUSION: Short-term (24 h) exposure of isolated adipocytes to TNF-α does not affect leptin secretion. Prolonged exposure to TNF-α produces a concentration-dependent inhibition of leptin secretion and gene expression. This suggests that the acute effect of TNF-α to increase circulating leptin levels in vivo may be indirect, TNF-α at higher concentrations increases glucose uptake, but does not increase the conversion of glucose to lactate. Therefore, TNF-α appears to induce a dissociation between adipocyte glucose metabolism and leptin production.