Inhibition of proteasome activity blocks the ability of TNFα to down-regulate Gi proteins and stimulate lipolysis

Prolonged treatment of rat adipocytes with TNF alpha increases lipolysis through a mechanism mediated, in part, by down-regulation of inhibitory G proteins (G(i)). Separately, downregulation of G(i) by prolonged treatment with an A(1)-adenosine receptor agonist, N-6-phenylisopropyl adenosine (PIA) increases lipolysis. To investigate the role of proteolysis in TNF alpha and PIA-mediated G(i) down-regulation and stimulation of lipolysis, we used the protease inhibitors lactacystin (proteasome inhibitor) and calpeptin (calpain inhibitor). Rat adipocytes were preincubated for IL h with lactacystin (10 muM) or calpeptin (50 muM), before 30-h treatment with either TNF alpha (50 ng/ml) or PIA (300 nM). We then measured lipolysis (glycerol release), abundance of alpha -subunits of G(i)1 and G(i)2 in plasma membranes (Western blotting) and protease activities (in specific fluorogenic assays). TNF alpha and PIA stimulated lipolysis approximately 2-fold and caused G(i) down-regulation. Although neither lactacystin nor calpeptin affected basal lipolysis, lactacystin completely inhibited both TNF alpha and PIA-stimulated lipolysis (the 50% inhibitory concentration was similar to2 muM), whereas calpeptin had no effect. Similarly, lactacystin but not calpeptin blocked both PIA and TNF alpha -induced G(i) down-regulation. These findings provide further evidence that the chronic lipolytic effect of TNFa and PIA is secondary to G(i) down-regulation and suggest that the mechanism involves proteolytic degradation mediated through the proteasome pathway.