Regulation of pathways of extramitochondrial fatty acid oxidation and liver fatty acid-binding protein by long-chain monocarboxylic fatty acids in hepatocytes. Effect of inhibition of carnitine palmitoyltransferase I.

The regulation of the extramitochondrial fatty acid oxidation pathways located in the peroxisomes and the endoplasmic reticulum is not fully understood. Although both long-chain dicarboxylic fatty acids, which are poorly metabolized in hepatocytes, and non-beta-oxidizable fatty acid analogs induce peroxisomal beta-oxidation and liver fatty acid-binding protein (L-FABP) by a pretranslational mechanism, monocarboxylic long-chain fatty acids, which are rapidly esterified and oxidized, do not. To establish whether impaired utilization and, hence, sustained intracellular levels of monocarboxylic long-chain fatty acids increase their efficacy as inducers, the effect of oleic acid on cytochrome P-450 4A1, peroxisomal beta-oxidation, and L-FABP during inhibition of mitochondrial beta-oxidation was determined. In primary hepatocyte cultures, oleic acid had no inducing effect, but in the presence of 2-tetradecylglycidic acid (TDGA), an inhibitor of carnitine palmitoyltransferase I, it induced P-450 4A1, peroxisomal beta-oxidation, and L-FABP pretranslationally. An increase in peroxisomal beta-oxidation was also noted in the presence of etomoxir, another inhibitor of carnitine palmitoyltransferase I. Exposure of hepatocytes to TDGA for 1 h led to an expected decrease in incorporation of radiolabel from [1-14C]oleate into CO2 and water-soluble products. In contrast, long-term exposure to TDGA increased incorporation of [1-14C]oleate into oxidation products, most likely due to an adaptive induction of peroxisomal beta-oxidation. Both acute and long-term exposure of hepatocytes to TDGA decreased incorporation of oleic acid into triglycerides, an effect that may have contributed to the intracellular accumulation of fatty acids. These results provide support for a mechanism by which long-chain fatty acids or specific metabolites, including long-chain acyl-CoA esters and long-chain dicarboxylic acids, act as signals in the induction of P-450 4A1, peroxisomal beta-oxidation, and L-FABP under conditions in which long-chain fatty acids accumulate due to impaired entry into the mitochondrial beta-oxidation pathway.