Fenofibrate, a peroxisome proliferator-activated receptor alpha (PPARalpha) activator, increases the expression of the cytosolic aspartate aminotransferase (cAspAT) gene in human liver cells, which may partially explain the increase of this enzyme in the serum of individuals undergoing fenofibrate treatment. Conversely, in rodents, fenofibrate represses the expression of the cAspAT gene. We compared the mechanisms of fenofibrate action in human and rat hepatoma cells. Transfection assays of the wild-type and mutated rat promoters in Fao and H4IIEC3 cells established a critical role for sequences similar to nuclear receptor responsive elements in the -404/-366 bp region. Nuclear proteins bound to these sequences and the amounts of protein bound were decreased by fenofibrate treatment, probably accounting for the decreased gene expression. Pharmacological studies confirmed the involvement of PPARalpha. However, this receptor did not bind directly to these sequences. The human promoter was cloned and the regulatory region localized between -2663/-706 bp. Co-transfection assays suggested that, in humans, the PPARalpha was also involved in the increase in expression of the cAspAT gene due to fibrates, without the presence of a canonical PPAR responsive element.