Prolonged treatment (12-24 h) of adipocytes with tumor necrosis factor α (TNFα) stimulates lipolysis. We have investigated the hypothesis that TNFα stimulates lipolysis by blocking the action of endogenous adenosine. Adipocytes were incubated for 48 h with TNFα, 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 TNFα-treated cells, but with adenosine deaminase lipolysis increased in the controls to approximately that in the TNFα-treated cells. This suggests that TNFα blocks adenosine release or prevents its antilipolytic effect. Both N(G)- phenylisopropyl adenosine and nicotinic acid were less potent and efficacious inhibitors of lipolysis in treated cells. A decrease in the concentration of α-subunits of all three G(i) subtypes was detected by Western blotting without a change in G(s) proteins or β-subunits. G(i2)α was about 50% of control, whereas G(i1)α and G(i3)α were about 20 and 40% of control values, respectively. The time course of G(i) down-regulation correlated with the stimulation of lipolysis. Furthermore, down-regulation of G(i) by an alternative approach (prolonged incubation with N(G)-phenylisopropyl adenosine) stimulated lipolysis. These findings indicate that TNFα stimulates lipolysis by blunting endogenous inhibition of lipolysis. The mechanism appears to be a G(i) protein down-regulation.
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