Fatty acid chain length and saturation influences PPARα transcriptional activation and repression in HepG2 cells

Scope: Fatty acids regulate peroxisome proliferator activated receptor alpha (PPAR alpha) activity, however, most studies evaluated the binding ability of fatty acids to PPAR alpha, which does not necessarily result in PPAR alpha transactivation. We therefore examined dose-response relationships between fatty acids and PPAR alpha transactivation in HepG2 cells. Secretion of apoA-I protein as well as CPT1, ACO, and PPAR alpha mRNA expression, all accepted PPAR alpha targets, were determined as read-outs. Methods and results: HepG2 cells transfected with full- length human PPAR alpha and a PPAR response element luciferase reporter were exposed to different fatty acid concentrations. Lauric and lower doses of myristic acid increased PPAR alpha transactivation. Palmitic and stearic acid inhibited and their monounsaturated counterparts, palmitoleic and oleic acid, increased PPAR alpha transactivation. Linoleic and gamma-linolenic acid did not influence PPAR alpha transactivation, while alpha-linolenic acid strongly increased transactivation. Arachidonic, eicosapentaenoic acid, and docosahexaenoic acid all activated PPAR alpha transactivation at lower doses, but acted at higher concentrations as PPAR alpha repressors. In line with these results, alpha-linolenic acid increased and docosahexaenoic acid decreased apoA-I protein secretion and PPAR alpha mRNA expression. Interestingly, ACO mRNA expression did not change while CPT1 mRNA expression showed the opposite pattern. Conclusion: We found that fatty acids, reported to bind strongly to PPAR alpha, could even repress PPAR alpha transactivation illustrating that these binding assays should be interpreted with caution.