Mechanisms of peroxisome proliferation by perfluorooctanoic acid and endogenous fatty acids

1. The effects of endogenous fatty acids and perfluorooctanoic acid (PFOA) and its analogs on peroxisomal acyl CoA oxidase (ACO) and microsomal laurate hydroxylase (LH) activities mere evaluated in primary cultures of rat hepatocytes and activation of peroxisome proliferator-activated receptor cu (PPAR alpha) in CV-1 cells. The rank order for the stimulation of ACO activity in hepatocytes for selected compounds was PFOA>>octanoic acid>octanedioic acid, perfluorooctanol (inactive). Increases in ACO activity by PFOA, like those of ciprofibrate, were associated with a marked increase in peroxisome number and cytosolic occupancy volume. Maximal effects of ciprofibrate and PFOA on the stimulation of ACO activity were not additive, suggesting that these two compounds share a common pathway of peroxisome proliferation. 2. Saturated monocarboxylic acids of C-4 to C-18 chain length were inactive, and, among dicarboxylic acids, only small elevations (40-45%) in ACO activity were observed with the long-chain C-12 and C-16 dioic acids. Of the C-18 fatty acids tested, only oleic and linoleic acids, at 1 mM, produced a two-to threefold elevation in ACO and LH activities. In comparison with endogenous fatty acids, PFOA was more potent and exhibited a different time course and greater magnitude of stimulation of ACO and LH activities in cultured hepatocytes. 3. Addition of mitochondrial beta-oxidation inhibitors (3-mercaptopropionic and 2-bromooctanoic acids) did not alter ACO activity in the presence of octanoic acid or octanedioic acid; nor did they modify the stimulation of ACO activity by PFOA. The carnitine palmitoyltransferase I inhibitor 2-bromo palmitic acid produced a 2.5-fold increase in ACO stimulatory activity and reduced bath ciprofibrate-and PFOA-mediated stimulations of ACO activity. 4. Cycloheximide treatment reduced PFOA- and ciprofibrate induced ACO activities; however, the response to oleic acid was not blocked and increased slightly. 5. In rat and human PPAR alpha transactivation assays, the rank order of activation was ciprofibrate> PFOA>oleic acia>>octanoic acid>octanedioic acid,or perfluorooctanol (inactive). PFOA, ciprofibrate and oleic acid were activators of rPPAR alpha at concentrations that correlated favorably with the changes in ACO activity in cell culture. Octanoic acid did not increase ACO activity and was a weak activator of PPAR alpha. 6. Our findings suggest that fatty acids such as oleic acid (endogenous fatty acids) and PFOA (a stable fatty acid) act through more than one pathway to increase ACO activity in rat hepatocytes. We conclude that the potent effects of PFOA are primarily mediated by a mechanism that includes the activation of liver PPAR alpha. (C) 1998 Elsevier Science Inc.