Thiazolidinediones (TZD) are PPAR gamma ligands that sensitize tissues to insulin. A cDNA encoding a mitochondrial protein likely to act as uncoupler (uncoupling protein 2, UCP2) has been recently cloned. Since TZD have been reported to increase energy expenditure in animals, we have examined the effects of these drugs on the expression of UCP2 mRNA in cell lines representing white (3T3-L1 and 3T3-F442A) and brown (HIB-1B) adipose tissues and skeletal muscle (L6). Northern blots probed with a mouse UCP2 full-length cDNA showed a mRNA of 1.6 kb both in tissues and the aforementioned cells lines. Within 4 h of exposing these cells to 30 microM darglitazone, there was an increase in UCP2 mRNA which reached a plateau of 5-10 times the basal in about 8 h. In all cells TZDs (darglitazone, troglitazone) were more active than the predominantly PPAR alpha ligands WY-14,613 and clofibrate, or the non-selective ligand linoleic acid. These results indicate that TZDs can stimulate the expression of UCP2 gene probably via PPAR gamma and hence have the potential to increase energy expenditure in adult humans, in whom UCP2 is expressed ubiquitously.