Peroxisomal and Mitochondrial Fatty Acid β-Oxidation in Mice Nullizygous for Both Peroxisome Proliferator-activated Receptor α and Peroxisomal Fatty Acyl-CoA Oxidase

Abstract

Fatty acid ␤-oxidation occurs in both mitochondria and peroxisomes. Long chain fatty acids are also metabolized by the cytochrome P450 CYP4A -oxidation enzymes to toxic dicarboxylic acids (DCAs) that serve as substrates for peroxisomal ␤-oxidation. Synthetic peroxisome proliferators interact with peroxisome proliferator activated receptor ␣ (PPAR␣) to transcriptionally activate genes that participate in peroxisomal, microsomal, and mitochondrial fatty acid oxidation. Mice lacking PPAR␣ (PPAR␣ ؊/؊ ) fail to respond to the inductive effects of peroxisome proliferators, whereas those lacking fatty acyl-CoA oxidase (AOX ؊/؊ ), the first enzyme of the peroxisomal ␤-oxidation system, exhibit extensive microvesicular steatohepatitis, leading to hepatocellular regeneration and massive peroxisome proliferation, implying sustained activation of PPAR␣ by natural ligands. We now report that mice nullizygous for both PPAR␣ and AOX (PPAR␣ ؊/؊ AOX ؊/؊ ) failed to exhibit spontaneous peroxisome proliferation and induction of PPAR␣-regulated genes by biological ligands unmetabolized in the absence of AOX. In AOX ؊/؊ mice, the hyperactivity of PPAR␣ enhances the severity of steatosis by inducing CYP4A family proteins that generate DCAs and since they are not metabolized in the absence of peroxisomal ␤-oxidation, they damage mitochondria leading to steatosis. Blunting of microvesicular steatosis, which is restricted to few liver cells in periportal regions in PPAR␣ ؊/؊ AOX ؊/؊ mice, suggests a role for PPAR␣-induced genes, especially members of CYP4A family, in determining the severity of steatosis in livers with defective peroxisomal ␤-oxidation. In agematched PPAR␣ ؊/؊ mice, a decrease in constitutive mitochondrial ␤-oxidation with intact constitutive peroxisomal ␤-oxidation system contributes to large droplet fatty change that is restricted to centrilobular hepatocytes. These data define a critical role for both PPAR␣ 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 ␤-oxidation, with long chain and very long chain fatty acids (LCFAs and VLCFAs) 1 being preferentially oxidized by peroxisomes (1-3). Peroxisomal ␤-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 ␤-oxidation spiral, dehydrogenation of acyl-CoA esters to their corresponding trans-2-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 mo...

Lipid Droplets: Their Role in Nanoparticle-Induced Oxidative Stress

“…40 One possible explanation for LD accumulation is the impairment in mitochondrial fatty acid -oxidation, resulting from mitochondrial membrane damage, and leading to an increase in triglyceride storage. 14, 47,48 Results from our studies suggest that assessment of LD accumulation may serve as a biological marker of oxidative stress and indicate disrupted lipid homeostasis by nanomaterials. Lysosomes are major organelles involved in the degradation of damaged intracellular organelles (e.g., mitochondria) in autophagy, and they are fierce responders to oxidative stress.…”

Regulation of the Liver Fatty Acid Binding Protein Gene (L-FABP) by Hepatocyte NuclearFactor 1aplha: alterations in fatty acid homeostasis in HNF1alpha-deficient mice

“…However, peroxisome assembly can be defective in hepatocytes with an accumulation of fat, thus resulting in the absence of peroxisomes (30). The fact that there was no reduction in the expression of peroxisomal β-oxidation genes and no change in the expression of the genes encoding the mitochondrial β-oxidation enzymes, MCAD and SCAD, in HNF1α-null mice suggests that the accumulation of lipids in their livers is not likely due to a defect in fatty acid oxidation.…”

Functional significance of the two ACOX1 isoforms and their crosstalks with PPARα and RXRα

“…[9][10][11] The generation of PparaÀ/À mice established that PPARa is essential for hepatic peroxisome proliferation and coordinates transcriptional activation of genes coding for ACOX1, L-peroxisomal bifunctional enzyme (L-PBE), peroxisomal thiolase (PTL), cytochrome P 450 4A10 (CYP4A10) and cytochrome P 450 4A14 (CYP4A14), and other lipid metabolism-related key enzymes by structurally diverse synthetic peroxisome proliferators, as well as by several fatty acids and their polyunsaturated derivatives. [12][13][14][15] Although fatty acids are metabolized by the mitochondrial and peroxisomal b-oxidation enzyme systems with partly overlapping substrate specificities, 3,16 the oxidation of the major fraction of medium and long-chain fatty acids (LCFAs, C12-C18) occurs in the mitochondria, whereas oxidation of VLCFAs takes place almost exclusively in peroxisomes. 3,16 Both LCFAs and VLCFAs (4C18) are also metabolized by the microsomal CYP4A10 and CYP4A14 fatty acid o-hydroxylases, resulting in the formation of dicarboxylic acids (DCAs) that are further degraded by the peroxisomal b-oxidation system.…”

“…[12][13][14][15] Although fatty acids are metabolized by the mitochondrial and peroxisomal b-oxidation enzyme systems with partly overlapping substrate specificities, 3,16 the oxidation of the major fraction of medium and long-chain fatty acids (LCFAs, C12-C18) occurs in the mitochondria, whereas oxidation of VLCFAs takes place almost exclusively in peroxisomes. 3,16 Both LCFAs and VLCFAs (4C18) are also metabolized by the microsomal CYP4A10 and CYP4A14 fatty acid o-hydroxylases, resulting in the formation of dicarboxylic acids (DCAs) that are further degraded by the peroxisomal b-oxidation system. 16,17 In several peroxisomal disorders, the peroxisomal fatty acid b-oxidation pathway is defective either with a decreased number or the absence of morphologically distinguishable peroxisomes in the liver.…”

“…2 Mice nullizygous for both Ppara and Acox1 (Ppara(À/À), Acox1(À/À)), have shown a lack of spontaneous hepatic peroxisome proliferation and a marked decrease in hepatic steatosis, which emphasizes the critical role of both PPARa and ACOX1 in hepatic metabolism and in the pathogenesis of specific fatty liver phenotype. 3 Sustained activation of PPARa and transcriptional activation of PPARa target genes in the liver of mice lacking ACOX1 highlight the role of peroxisomal ACOX1 in metabolizing the endogenous natural PPARa ligands. 4 At the transcriptional level, PPARa, which forms a heterodimer with retinoid X receptor (RXR), 5,6 binds to peroxisome proliferator-responsive element (PPRE) present in the 5 0 -flanking region of target genes.…”