Inhibition of proteasome activity blocks the ability of TNF alpha to down-regulate G(i) proteins and stimulate lipolysis

Endocrinology. 2001 Dec;142(12):5069-75. doi: 10.1210/endo.142.12.8518.


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, down-regulation 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 1 h with lactacystin (10 microM) or calpeptin (50 microM), 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 approximately 2 microM), 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 TNF alpha and PIA is secondary to G(i) down-regulation and suggest that the mechanism involves proteolytic degradation mediated through the proteasome pathway.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acetylcysteine / analogs & derivatives*
  • Acetylcysteine / pharmacology
  • Adipocytes / metabolism*
  • Animals
  • Calpain / antagonists & inhibitors
  • Calpain / physiology
  • Cells, Cultured
  • Cysteine Endopeptidases / physiology*
  • Cysteine Proteinase Inhibitors / pharmacology
  • Dipeptides / pharmacology
  • Down-Regulation / physiology
  • GTP-Binding Protein alpha Subunits, Gi-Go / metabolism*
  • Lipolysis / drug effects
  • Lipolysis / physiology*
  • Male
  • Multienzyme Complexes / antagonists & inhibitors
  • Multienzyme Complexes / physiology*
  • Phenylisopropyladenosine / pharmacology
  • Proteasome Endopeptidase Complex
  • Rats
  • Rats, Sprague-Dawley
  • Tumor Necrosis Factor-alpha / physiology*


  • Cysteine Proteinase Inhibitors
  • Dipeptides
  • Multienzyme Complexes
  • Tumor Necrosis Factor-alpha
  • lactacystin
  • calpeptin
  • Phenylisopropyladenosine
  • Calpain
  • Cysteine Endopeptidases
  • Proteasome Endopeptidase Complex
  • GTP-Binding Protein alpha Subunits, Gi-Go
  • Acetylcysteine