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

Abstract

Tumor necrosis factor-alpha (TNF-alpha) stimulates lipolysis in human adipocytes. However, the mechanisms regulating this process are largely unknown. We demonstrate that TNF-alpha 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-alpha activated ERK and increased lipolysis; these effects were inhibited by two specific MEK inhibitors, PD98059 and U0126. TNF-alpha 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-alpha and MEK inhibitors caused perilipin to migrate as a single 65-kDa band. Consistent with the hypothesis that TNF-alpha induces perilipin hyperphosphorylation by activating PKA, TNF-alpha increased intracellular cAMP approximately 1.7-fold, and the increase was abrogated by PD98059. Furthermore, H89, a specific PKA inhibitor, blocked TNF-alpha-induced lipolysis and the electrophoretic shift of perilipin, suggesting a role for PKA in TNF-alpha-induced lipolysis. Finally, TNF-alpha decreased the expression of cyclic-nucleotide phosphodiesterase 3B (PDE3B) by approximately 50%, delineating a mechanism by which TNF-alpha could increase intracellular cAMP. Cotreatment with PD98059 restored PDE3B expression. These studies suggest that in human adipocytes, TNF-alpha stimulates lipolysis through activation of MEK-ERK and subsequent increase in intracellular cAMP.