Overexpression of Peroxisome Proliferator-activated Receptor-α (PPARα)-regulated Genes in Liver in the Absence of Peroxisome Proliferation in Mice Deficient in both l- and d-Forms of Enoyl-CoA Hydratase/Dehydrogenase Enzymes of Peroxisomal β-Oxidation System

Yuzhi Jia,Chao Qi,Zhongyi Zhang,Takashi Hashimoto,M Sambasiva Rao,Steven Huyghe,Yasuyuki Suzuki,Paul P Van Veldhoven,Myriam Baes,Janardan K Reddy

doi:10.1074/jbc.m306363200

Abstract

Peroxisomal beta-oxidation system consists of peroxisome proliferator-activated receptor alpha (PPARalpha)-inducible pathway capable of catalyzing straight-chain acyl-CoAs and a second noninducible pathway catalyzing the oxidation of 2-methyl-branched fatty acyl-CoAs. Disruption of the inducible beta-oxidation pathway in mice at the level of fatty acyl-CoA oxidase (AOX), the first and rate-limiting enzyme, results in spontaneous peroxisome proliferation and sustained activation of PPARalpha, leading to the development of liver tumors, whereas disruptions at the level of the second enzyme of this classical pathway or of the noninducible system had no such discernible effects. We now show that mice with complete inactivation of peroxisomal beta-oxidation at the level of the second enzyme, enoyl-CoA hydratase/L-3-hydroxyacyl-CoA dehydrogenase (L-PBE) of the inducible pathway and D-3-hydroxyacyl-CoA dehydratase/D-3-hydroxyacyl-CoA dehydrogenase (D-PBE) of the noninducible pathway (L-PBE-/-D-PBE-/-), exhibit severe growth retardation and postnatal mortality with none surviving beyond weaning. L-PBE-/-D-PBE-/- mice that survived exceptionally beyond the age of 3 weeks exhibited overexpression of PPARalpha-regulated genes in liver, despite the absence of morphological evidence of hepatic peroxisome proliferation. These studies establish that peroxisome proliferation in rodent liver is highly correlatable with the induction mostly of the L- and D-PBE genes. We conclude that disruption of peroxisomal fatty acid beta-oxidation at the level of second enzyme in mice leads to the induction of many of the PPARalpha target genes independently of peroxisome proliferation in hepatocytes, raising the possibility that intermediate metabolites of very long-chain fatty acids and peroxisomal beta-oxidation act as ligands for PPARalpha.

Highlights

In mammals, fatty acid oxidation occurs in mitochondria, peroxisomes, and smooth endoplasmic reticulum [1]
Important is that the induction in liver of enzymes involved in the classical ␤-oxidation system and profound increases in peroxisome population in liver cells caused by structurally diverse peroxisome proliferators has been associated with the development of liver cancer in rats and mice, and these effects are regulated by the nuclear receptor PPAR␣ [1, 6, 7, 10, 14, 34]
The mouse model of disruption of the L-PBE gene further supported the assumption that acyl-CoA oxidase (AOX) substrates function as PPAR␣ ligands because L-PBEϪ/Ϫ mouse livers revealed no such increases in peroxisome number and no upregulation of PPAR␣ target genes in liver [16]

Summary

Introduction

Fatty acid oxidation occurs in mitochondria, peroxisomes, and smooth endoplasmic reticulum [1]. In the noninducible ␤-oxidation pathway, dehydrogenation of branched fatty acyl-CoA esters to their corresponding trans-2-enoyl-CoAs is catalyzed by the branched chain acyl-CoA oxidase or by pristanoylCoA oxidase [4] These enzymes are capable of desaturating straight-chain acyl-CoAs. The second and third reactions of this pathway are performed by D-3-hydroxyacyl-CoA dehydratase/D-3hydroxyacyl-CoA dehydrogenase (D-bi/multifunctional enzyme (DPBE/MFP2)), and the resulting 3-ketoacyl-CoAs are cleaved by the third enzyme of this system, designated sterol carrier protein x (SCPx), which possesses thiolase activity [4, 8]. To investigate the functional implications of the disruption of the metabolism of enoyl-CoAs on lipid metabolism, peroxisome proliferation, and PPAR␣ activation in liver, we generated mice deficient in both L-PBE and D-PBE enzymes so that both peroxisomal ␤-oxidation pathways are effectively disrupted at the second enzyme level Mice deficient in both L-PBE and D-PBE enzymes (L-PBEϪ/ϪD-PBEϪ/Ϫ) exhibited severe growth retardation and postnatal mortality. Oxisome proliferation in rodent liver is highly correlatable with the induction mostly of the L-PBE gene

Methods

Results

Conclusion