SCOPE:: Fatty acids regulate PPARalpha activity, however, most studies evaluated the binding ability of fatty acids to PPARalpha, which does not necessarily results in PPARalpha transactivation. We therefore examined dose-response relationships between fatty acids and PPARalpha transactivation in HepG2 cells. Secretion of apoA-I protein as-well-as CPT1, ACO, and PPARalpha mRNA expression, all accepted PPARalpha targets, were determined as read-outs. METHODS AND RESULTS:: HepG2 cells transfected with full-length human PPARalpha and a PPRE luciferase reporter were exposed to different fatty acid concentrations. Lauric and lower doses of myristic acid increased PPARalpha transactivation. Palmitic and stearic acid inhibited and their monounsaturated counterparts palmitoleic and oleic acid increased PPARalpha transactivation. Linoleic and gamma-linolenic acid did not influence PPARalpha transactivation, while alpha-linolenic acid strongly increased transactivation. Arachidonic, eicosapentaenoic (EPA), and docosahexaenoic (DHA) acid all activated PPARalpha transactivation at lower doses, but acted at higher concentrations as PPARalpha repressors. In line with these results, alpha-linolenic acid increased and DHA decreased apoA-I protein secretion and PPARalpha mRNA expression. Interestingly, ACO mRNA expression did not change while CPT1 mRNA expression showed the opposite pattern. CONCLUSION:: Fatty acids, reported to bind to PPARalpha, could even repress PPARalpha transactivation though results may vary gene dependently illustrating involvement of multiple regulatory factors. This article is protected by copyright. All rights reserved.