Peroxisomal and Mitochondrial Fatty Acid b-Oxidation in Mice Nullizygous for Both Peroxisome Proliferator-activated Receptor a and Peroxisomal Fatty Acyl-CoA Oxidase GENOTYPE CORRELATION WITH FATTY LIVER PHENOTYPE*

Fatty acid b-oxidation occurs in both mitochondria and peroxisomes. Long chain fatty acids are also metabolized by the cytochrome P450 CYP4A v-oxidation enzymes to toxic dicarboxylic acids (DCAs) that serve as substrates for peroxisomal b-oxidation. Synthetic peroxisome proliferators interact with peroxisome proliferator activated receptor a (PPARa) to transcriptionally activate genes that participate in peroxisomal, microsomal, and mitochondrial fatty acid oxidation. Mice lacking PPARa (PPARa) fail to respond to the inductive effects of peroxisome proliferators, whereas those lacking fatty acyl-CoA oxidase (AOX), the first enzyme of the peroxisomal b-oxidation system, exhibit extensive microvesicular steatohepatitis, leading to hepatocellular regeneration and massive peroxisome proliferation, implying sustained activation of PPARa by natural ligands. We now report that mice nullizygous for both PPARa and AOX (PPARa AOX) failed to exhibit spontaneous peroxisome proliferation and induction of PPARa-regulated genes by biological ligands unmetabolized in the absence of AOX. In AOX mice, the hyperactivity of PPARa enhances the severity of steatosis by inducing CYP4A family proteins that generate DCAs and since they are not metabolized in the absence of peroxisomal b-oxidation, they damage mitochondria leading to steatosis. Blunting of microvesicular steatosis, which is restricted to few liver cells in periportal regions in PPARa AOX mice, suggests a role for PPARa-induced genes, especially members of CYP4A family, in determining the severity of steatosis in livers with defective peroxisomal b-oxidation. In agematched PPARa mice, a decrease in constitutive mitochondrial b-oxidation with intact constitutive peroxisomal b-oxidation system contributes to large droplet fatty change that is restricted to centrilobular hepatocytes. These data define a critical role for both PPARa and AOX in hepatic lipid metabolism and in the pathogenesis of specific fatty liver phenotype. In animal cells, mitochondria as well as peroxisomes oxidize fatty acids via b-oxidation, with long chain and very long chain fatty acids (LCFAs and VLCFAs) being preferentially oxidized by peroxisomes (1–3). Peroxisomal b-oxidation is carried out by two distinct groups of enzymes. The classical first group utilizes straight chain saturated fatty acyl-CoAs as substrates, whereas the second group acts on branched chain acyl-CoAs (3, 4). In the classical L-3-hydroxy-specific b-oxidation spiral, dehydrogenation of acyl-CoA esters to their corresponding trans2-enoyl-CoAs is catalyzed by fatty acyl-CoA oxidase (AOX), whereas the second and third reactions, hydration and dehydrogenation of enoyl-CoA esters to 3-ketoacyl-CoA, are catalyzed by a single enzyme, enoyl-CoA hydratase/L-3-hydroxyacyl-CoA dehydrogenase (L-bifunctional enzyme (L-PBE)) (3). The third enzyme of this classical system, 3-ketoacyl-CoA thiolase (PTL), cleaves 3-ketoacyl-CoAs to acetyl-CoA and an acylCoA that is two carbon atoms shorter than the original molecule, and it can re-enter the b-oxidation spiral (1, 2). In the second D-3-hydroxy-specific b-oxidation pathway, dehydrogenation of acyl-CoA esters to their corresponding trans-2-enoylCoAs is catalyzed by the branched chain acyl-CoA oxidase (2). The recently identified D-3-hydroxyacyl-CoA dehydratase/D-3hydroxyacl-CoA dehydrogenase (D-bifunctional enzyme, (DPBE)) then converts enoyl-CoAs to 3-ketoacyl-CoAs via D-3hydroxyacyl-CoAs (3). The third enzyme of this second system is designated as sterol carrier protein x (SCPx), which possesses 3-ketoacyl-CoA thiolase activity (5). The first step in mitochondrial b-oxidation is the a-b dehydrogenation of the acyl-CoA ester by a family of four chain-length-specific acylCoA dehydrogenases, which include very long chain, long