Tumor necrosis factor α stimulates lipolysis in adipocytes by decreasing Gi protein concentrations

Prolonged treatment (12-24 h) of adipocytes with tumor necrosis factor alpha (TNF alpha) stimulates Lipolysis, We have investigated the hypothesis that TNF alpha stimulates lipolysis by blocking the action of endogenous adenosine, Adipocytes were incubated for 48 h with TNF alpha, 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 TNF alpha-treated cells, but with adenosine deaminase lipolysis increased in the controls to approximately that in the TNF alpha-treated cells. This suggests that TNF alpha blocks adenosine release or prevents its antilipolytic effect. Both N-6-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 G(i) subtypes was detected by Western blotting without a change in G(s)proteins or beta-subunits. G(i2)alpha was about 50% of control, whereas G(i1)alpha and G(i3)alpha were about 20 and 40% of control values, respectively. The time course of G(i) down-regulation correlated with the stimulation of Lipolysis, Furthermore, dawn-regulation of Gi by an alternative approach (prolonged incubation with N-6-phenylisopropyladenosine) stimulated lipolysis. These findings indicate that TNF alpha stimulates lipolysis by blunting endogenous inhibition of lipolysis. The mechanism appears to be a G(i) protein down-regulation.