Review Cholesterol biosynthesis and ER stress in peroxisome deficiency Phyllis L. Faust a , Werner J. Kovacs b, c, * a Department of Pathology and Cell Biology, Columbia University, New York, NY 10032, USA b Institute of Molecular Health Sciences, ETH Zuerich, CH-8093 Zuerich, Switzerland c Competence Center for Systems Physiology and Metabolic Diseases, ETH Zuerich, CH-8093 Zuerich, Switzerland article info Article history: Received 18 June 2013 Accepted 22 October 2013 Available online 7 November 2013 Keywords: Peroxisomes Cholesterol Isoprenoid Pex2 SREBP-2 ER stress Integrated stress response Unfolded protein response Zellweger syndrome abstract Cholesterol biosynthesis is a multi-step process involving more than 20 enzymes in several subcellular compartments. The pre-squalene segment of the cholesterol/isoprenoid biosynthetic pathway is local- ized in peroxisomes. This review intends to highlight recent findings illustrating the important role peroxisomes play in cholesterol biosynthesis and maintenance of cholesterol homeostasis. Disruption of the Pex2 gene leads to peroxisome deficiency and widespread metabolic dysfunction. The Pex2 / mouse model for Zellweger syndrome enabled us to evaluate the role of peroxisomes in cholesterol biosyn- thesis. These studies have shown that Pex2 / mice exhibit low levels of cholesterol in plasma and liver. Pex2 / mice were unable to maintain normal cholesterol homeostasis despite activation of SREBP-2, the master transcriptional regulator of cholesterol biosynthesis, and increased protein levels and activities of cholesterol biosynthetic enzymes. The SREBP-2 pathway remained activated even after normalization of hepatic cholesterol levels in response to bile acid feeding as well as in extrahepatic tissues and the liver of neonatal and longer surviving Pex2 mutants, where cholesterol levels were normal. Several studies have shown that endoplasmic reticulum (ER) stress can dysregulate lipid metabolism via SREBP acti- vation independently of intracellular cholesterol concentration. We demonstrated that peroxisome deficiency activates endoplasmic reticulum stress pathways in Pex2 / mice, especially the integrated stress response mediated by PERK and ATF4 signaling, and thereby leads to dysregulation of the SREBP-2 pathway. Our findings suggest that functional peroxisomes are necessary to prevent chronic ER stress and dysregulation of the endogenous sterol response pathway. The constitutive activation of ER stress pathways might contribute to organ pathology and metabolic dysfunction in peroxisomal disorder patients. Ó 2013 Elsevier Masson SAS. All rights reserved. 1. Introduction Peroxisomes are ubiquitous and highly versatile organelles of eukaryotic cells that have many metabolic functions, including b- oxidation and a-oxidation of fatty acids, ether-phospholipid synthesis, cholesterol and isoprenoid metabolism, bile acid (BA) synthesis, and metabolism of reactive oxygen species (ROS) [1e3]. Peroxisomes can multiply by fission of pre-existing ones [4] or develop de novo from the endoplasmic reticulum (ER) [5]. Proteins involved in peroxisome biogenesis, the peroxins, are encoded by PEX genes. Peroxins are involved in 3 key stages of peroxisomal development: (i) formation of the peroxisomal membrane, (ii) compartmentalization of peroxi- somal matrix proteins, and (iii) peroxisome proliferation (growth and division). Peroxisomal matrix proteins are synthesized on free cyto- solic ribosomes and contain p eroxisomal t argeting s ignals (PTS), either C-terminal tripeptide (PTS1) or N-terminal nonapeptide (PTS2), which are conserved throughout evolution. PTS-containing proteins are recognized by the cytoplasmic import receptors Pex5p and Pex7p, Abbreviations: ACAT, acetoacetyl-CoA thiolase; ATF, activating transcription factor; BA, bile acid; CHOP, C/EBP homologous protein; CoA, coenzyme A; eIF2a, eukaryotic translation initiation factor 2; ER, endoplasmic reticulum; FDPS, farne- syl-diphosphate synthase; Grp, glucose-regulated protein; HDL, high-density lipo- protein; HMGCR, 3-hydroxy-3-methylglutaryl-CoA reductase; HMGCS, 3-hydroxy- 3-methylglutaryl-CoA synthase; IDI, isopentenyl-diphosphate isomerase; Insig, in- sulin-induced gene; IRE1, inositol-requiring enzyme-1; ISA, isotopomer spectral analysis; ISR, integrated stress response; LDL, low-density lipoprotein; MID, mass isotopomer distribution; MVK, mevalonate kinase; MVD, mevalonate-diphosphate decarboxylase; Pex, peroxin; PERK, protein kinase RNA-like endoplasmic reticulum kinase; PMVK, phosphomevalonate kinase; PPARa, peroxisome proliferator- activated receptor alpha; SCAP, SREBP cleavage-activating protein; SREBP, sterol regulatory element-binding protein; UPR, unfolded protein response; VLCFA, very long-chain fatty acid; XBP1, X-box binding protein 1. * Corresponding author. Institute of Molecular Health Sciences, ETH Zuerich, Schafmattstrasse 22, HPL H16, CH-8093 Zuerich, Switzerland. Tel.: þ41 44 633 3084; fax: þ41 44 633 1357. E-mail address: werner.kovacs@biol.ethz.ch (W.J. Kovacs). Contents lists available at ScienceDirect Biochimie journal homepage: www.elsevier.com/locate/biochi 0300-9084/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.biochi.2013.10.019 Biochimie 98 (2014) 75e85