Tumor Necrosis Factor-α Stimulates Lipolysis in Differentiated Human Adipocytes Through Activation of Extracellular Signal-Related Kinase and Elevation of Intracellular cAMP
- 1Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston, MA
- 2Section of Biochemical Physiology, Pulmonary-Critical Care Medicine NHLBI, National Institutes of Health, Bethesda, MD.
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.
Address correspondence and reprint requests to Dr. Andrew S. Greenberg, Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, 711 Washington St., Boston, MA 02111. E-mail:.
Received for publication 31 January 2002 and accepted in revised form 12 July 2002.
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the U.S. Department of Agriculture.
DMEM, Dulbecco’s modified Eagle’s medium; ECL, enhanced chemiluminescence; ERK, extracellular signal-related kinase; FBS, fetal bovine serum; FFA, free fatty acid; HSL, hormone-sensitive lipase; MAPK, mitogen-activated protein kinase; MEK, mitogen-activated protein kinase kinase; PDE3B, phosphodiesterase 3B; PKA, protein kinase A; TNF-α, tumor necrosis factor-α.