Nitric oxide-dependent downregulation of adipocyte UCP-2 expression by tumor necrosis factor-alpha

Uncoupling protein-2 (UCP-2) is a mitochondrial protein expressed in adipoc ytes and has recently been involved in the control of energy dissipation. B ecause obesity is characterized by an imbalance between energy intake and e xpenditure and by an enhanced adipocyte-derived secretion of tumor necrosis factor-alpha (TNF-alpha), we asked whether TNF-alpha could directly influe nce UCP-2 expression in adipocytes. Experiments performed in differentiated 3T3F442A preadipocytes showed that TNF-alpha (10 ng/ml) induced a reductio n of UCP-2 trancripts, assessed by Northern blot analysis. A significant de crease in UCP-2 expression (40%) was observed after 12 and 24 h of TNF-alph a stimulation of the cells. The characterization of the mechanisms responsi ble for the TNF-alpha effect on UCP-2 expression demonstrates an involvemen t of the TNF-alpha-induced inducible (i) nitric oxide synthase (NOS) expres sion. Cell treatment with the NOS inhibitor N-G-nitro-L-arginine methyl est er (L-NAME; 1 mmol/l) significantly diminished the TNF-alpha-mediated susta ined downregulation of UCP-2 expression, whereas cell treatment with a nitr ic oxide (NO) donor (10(-3) mol/l S-nitroso-L-glutathione) mimicked the TNF -alpha effect on UCP-2 expression. Moreover, Western blot analysis clearly showed that TNF-alpha alone induces the expression of iNOS after 12-24 h tr eatment of differentiated 3T3F442A cells. These experiments demonstrate tha t TNF-alpha directly downregulates UCP-2 expression via NO-dependent pathwa ys that involve the induction of iNOS expression.