Objective: Our objective was to determine the effects of prolonged exposure to tumor necrosis factor-alpha (TNF-alpha) 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-alpha 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-alpha. Leptin secretion, steady-state levels of leptin mRNA levels, glucose uptake, and lactate production were assessed over 96 h.
Results: TNF-alpha at concentrations of 0.024, 0.24, 2.4 and 24 ng/ml did not affect leptin secretion over 24 h. TNF-alpha at concentrations of 0.24 to 24 ng/ml significantly inhibited leptin secretion over 96 h by 19-60%. TNF-alpha 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-alpha at concentrations of 2.4 and 24 ng/ml significantly increased glucose uptake and lactate production over 96 h by 30-57%. TNF-alpha at a concentration of 0.024 ng/ml did not affect leptin secretion, glucose uptake or lactate production. Overall, for the TNF-alpha concentrations tested, leptin secretion was inversely related to the percent of glucose carbon released as lactate; however, TNF-alpha did not induce a proportional increase of lactate production from glucose.
Conclusion: Short-term (24 h) exposure of isolated adipocytes to TNF-alpha does not affect leptin secretion. Prolonged exposure to TNF-alpha produces a concentration-dependent inhibition of leptin secretion and gene expression. This suggests that the acute effect of TNF-alpha to increase circulating leptin levels in vivo may be indirect. TNF-alpha at higher concentrations increases glucose uptake, but does not increase the conversion of glucose to lactate. Therefore, TNF-alpha appears to induce a dissociation between adipocyte glucose metabolism and leptin production.