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

Uncoupling protein-2 (UCP-2) is amitochondrial protein expressed in adipocytes and has recently beeninvolved in the control of energy dissipation. Because obesity ischaracterized by an imbalance between energy intake and expenditure andby an enhanced adipocyte-derived secretion of tumor necrosis factor-(TNF-), we asked whether TNF- could directly influence UCP-2expression in adipocytes. Experiments performed in differentiated3T3F442A preadipocytes showed that TNF- (10 ng/ml) induced areduction of UCP-2 trancripts, assessed by Northern blot analysis. Asignificant decrease in UCP-2 expression (40%) was observed after 12 and 24 h of TNF- stimulation of the cells. The characterizationof the mechanisms responsible for the TNF- effect on UCP-2expression demonstrates an involvement of the TNF--induced inducible(i) nitric oxide synthase (NOS) expression. Cell treatment with the NOSinhibitor N^G-nitro-L-arginine methylester (L-NAME; 1 mmol/l) significantly diminished theTNF--mediated sustained downregulation of UCP-2 expression, whereascell treatment with a nitric oxide (NO) donor (10³ mol/lS-nitroso-L-glutathione) mimicked the TNF-effect on UCP-2 expression. Moreover, Western blot analysis clearlyshowed that TNF- alone induces the expression of iNOS after12-24 h treatment of differentiated 3T3F442A cells. Theseexperiments demonstrate that TNF- directly downregulates UCP-2expression via NO-dependent pathways that involve the induction of iNOS expression.