Absence of peroxisomes in mouse hepatocytes causes mitochondrial and ER abnormalities†

Abstract

Peroxisome deficiency in men causes severe pathology in several organs, particularly in the brain and liver, but it is still unknown how metabolic abnormalities trigger these defects. In the present study, a mouse model with hepatocyte-selective elimination of peroxisomes was generated by inbreeding Pex5-loxP and albumin-Cre mice to investigate the consequences of peroxisome deletion on the functioning of hepatocytes. Besides the absence of catalase-positive peroxisomes, multiple ultrastructural alterations were noticed, including hepatocyte hypertrophy and hyperplasia, smooth endoplasmic reticulum proliferation, and accumulation of lipid droplets and lysosomes. Most prominent was the abnormal structure of the inner mitochondrial membrane, which bore some similarities with changes observed in Zellweger patients. This was accompanied by severely reduced activities of complex I, III, and V and a collapse of the mitochondrial inner membrane potential. Surprisingly, these abnormalities provoked no significant disturbances of adenosine triphosphate (ATP) levels and redox state of the liver. However, a compensatory increase of glycolysis as an alternative source of ATP and mitochondrial proliferation were observed. No evidence of oxidative damage to proteins or lipids nor elevation of oxidative stress defence mechanisms were found. Altered expression of peroxisome proliferator-activated receptor alpha (PPAR-␣) regulated genes indicated that PPAR-␣ is activated in the peroxisomedeficient cells. In conclusion, the absence of peroxisomes from mouse hepatocytes has an impact on several other subcellular compartments and metabolic pathways but is not detrimental to the function of the liver parenchyma. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270-9139/suppmat/index.html). P eroxisomes are indispensable in cellular metabolism, because they harbor enzymes essential for the degradation of very long chain and branched chain fatty acids, for the conversion of cholesterol in bile acids and for the synthesis of ether phospholipids and polyunsaturated fatty acids. They also participate in the catabolism of purines, polyamines, and pipecolic acid. 1 The importance of peroxisomes is stressed by the existence of a group of genetic disorders in which one or more peroxisomal functions are impaired. The most severe is the cerebrohepatorenal syndrome of Zellweger, a peroxisome assembly disorder characterized by the absence of functional peroxisomes due to a disturbance in the peroxisomal protein import machinery. 2 At birth, Zellweger syndrome patients suffer from neurological and eye abnormalities, hypotonia, characteristic craniofacial dysmorphism, and renal cysts. 2 Hepatic pathology develops in the postnatal period, including hepatomegaly, fibrosis, micronodular cirrhosis, cholestasis, hyperbilirubinemia, and elevation of aminotransferases. 2,3 The biochemical hallmarks of this syndrome include the accumulation of very long chain and branched chain fatty aci...

Bile acid treatment alters hepatic disease and bile acid transport in peroxisome‐deficient PEX2 Zellweger mice†

“…Previous studies demonstrated that mitochondria are abnormal in livers of Zellweger patients and mouse Zellweger models [17][18][19][20][21][22][23] and that bile acids may mediate mitochondrial toxicity. 24 Electron microscopy demonstrated mitochondrial abnormalities typically seen in peroxisomal disorders in both early postnatal and P36 untreated PEX2 mutant livers, and these defects persisted in all BA-fed PEX2 Ϫ/Ϫ mice (Fig.…”

Ectopic recombination in the central and peripheral nervous system by aP2/FABP4‐Cre mice: Implications for metabolism research

“…The second Cre line was only crossed with floxed Pex5 mice [15]. Genotyping was performed as previously reported [16]. Upon expression of Cre in the LacZ reporter mouse, a floxed stop codon preceding the LacZ gene is excised resulting in expression of b-galactosidase (b-gal).…”

Carbohydrate Metabolism Is Perturbed in Peroxisome-deficient Hepatocytes Due to Mitochondrial Dysfunction, AMP-activated Protein Kinase (AMPK) Activation, and Peroxisome Proliferator-activated Receptor γ Coactivator 1α (PGC-1α) Suppression

“…During the course of our studies on L-Pex5 knock-out mice, we found initial evidence of disturbed carbohydrate metabolism, including enhanced glycolysis, but we did not characterize these glucose metabolism abnormalities in detail, and neither did we unravel the underlying mechanisms (11). Therefore, an in-depth analysis of glucose homeostasis was performed revealing impaired gluconeogenesis, glycogen synthesis, and insulin signaling in peroxisome-deficient hepatocytes.…”

“…These hepatocytes contained mitochondria with severely distorted inner membranes and loss of the mitochondrial membrane potential, strongly reduced activity of complex I, and more moderate impairment of complex III and V. Several PPAR␣ 2 target genes were induced, indicative of the accumulation of PPAR␣ ligands in peroxisome-deficient hepatocytes. Furthermore, the mice displayed microvesicular steatosis and fibrosis and after 12 months hepatocarcinogenesis (11).…”

“…To better define the role of peroxisomal metabolism in hepatocytes, we recently generated a mouse model with hepatocyterestricted elimination of functional peroxisomes (L-Pex5 knock-out mice) (11). These hepatocytes contained mitochondria with severely distorted inner membranes and loss of the mitochondrial membrane potential, strongly reduced activity of complex I, and more moderate impairment of complex III and V. Several PPAR␣ 2 target genes were induced, indicative of the accumulation of PPAR␣ ligands in peroxisome-deficient hepatocytes.…”