Tumor necrosis factor alpha stimulates lipolysis in adipocytes by decreasing Gi protein concentrations.
Prolonged treatment (12-24 h) of adipocytes with tumor necrosis factor alpha (TNFalpha) stimulates lipolysis. We have investigated the hypothesis that TNFalpha stimulates lipolysis by blocking the action of endogenous adenosine. Adipocytes were incubated for 48 h with TNFalpha, and lipolysis was measured in the absence or presence of adenosine deaminase. Without adenosine deaminase, the rate of glycerol release was 2-3-fold higher in the TNFalpha-treated cells, but with adenosine deaminase lipolysis increased in the controls to approximately that in the TNFalpha-treated cells. This suggests that TNFalpha blocks adenosine release or prevents its antilipolytic effect. Both N6-phenylisopropyl adenosine and nicotinic acid were less potent and efficacious inhibitors of lipolysis in treated cells. A decrease in the concentration of alpha-subunits of all three Gi subtypes was detected by Western blotting without a change in Gs proteins or beta-subunits. Gi2alpha was about 50% of control, whereas Gi1alpha and Gi3alpha were about 20 and 40% of control values, respectively. The time course of Gi down-regulation correlated with the stimulation of lipolysis. Furthermore, down-regulation of Gi by an alternative approach (prolonged incubation with N6-phenylisopropyl adenosine) stimulated lipolysis. These findings indicate that TNFalpha stimulates lipolysis by blunting endogenous inhibition of lipolysis. The mechanism appears to be a Gi protein down-regulation.
Gasic
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Bassett Research Institute, The Mary Imogene Bassett Hospital, Cooperstown, New York 13326, USA.
Tian
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B
Green
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A
D017667:Adipocytes / Q000378:metabolism*; D000818:Animals; D019204:GTP-Binding Proteins / Q000378:metabolism*; D008066:Lipolysis / Q000187:drug effects*; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology*
D014409:Tumor Necrosis Factor-alpha; D019204:GTP-Binding Proteins