Tumor necrosis factor-α stimulates lipolysis in differentiated human adipocytes through activation of extracellular signal-related kinase and elevation of intracellular cAMP

Tumor necrosis factor-α (TNF-α) stimulates lipolysis in human adipocytes. However, the mechanisms regulating this process are largely unknown. We demonstrate that TNF-α increases lipolysis in differentiated human adipocytes by activation of mitogen-activated protein kinase kinase (MEK), extracellular signal-related kinase (ERK), and elevation of intracellular cAMP. TNF-α activated ERK and increased lipolysis; these effects were inhibited by two specific MEK inhibitors, PD98059 and U0126. TNF-α treatment caused an electrophoretic shift of perilipin from 65 to 67 kDa, consistent with perilipin hyperphosphorylation by activated cAMP-dependent protein kinase A (PKA). Coincubation with TNF-α and MEK inhibitors caused perilipin to migrate as a single 65-kDa band. Consistent with the hypothesis that TNF-α induces perilipin hyperphosphorylation by activating PKA, TNF-α increased intracellular cAMP ~1.7-fold, and the increase was abrogated by PD98059. Furthermore, H89, a specific PKA inhibitor, blocked TNF-α--induced lipolysis and the electrophoretic shift of perilipin, suggesting a role for PKA in TNF-α--induced lipolysis. Finally, TNF-α decreased the expression of cyclic-nucleotide phosphodiesterase 3B (PDE3B) by ~50%, delineating a mechanism by which TNF-α could increase intracellular cAMP. Cotreatment with PD98059 restored PDE3B expression. These studies suggest that in human adipocytes, TNF-α stimulates lipolysis through activation of MEK-ERK and subsequent increase in intracellular cAMP.
Obesity and type 2 diabetes are associated with increased concentrations of circulating free fatty acids (FFAs) that are thought to elicit systemic insulin resistance (1). The flux of FFA is [...]