Peroxisome deficiency-induced ER stress and SREBP-2 pathway activation in the liver of newborn PEX2 knock-out mice.

Disruption of the Pex2 gene leads to peroxisome deficiency and widespread metabolic dysfunction. We previously demonstrated that peroxisomes are critical for maintaining cholesterol homeostasis, using peroxisome-deficient Pex2(-/-) mice on a hybrid Swiss Webster×129S6/SvEv (SW/129) genetic background. Peroxisome deficiency activates hepatic endoplasmic reticulum (ER) stress pathways, leading to dysregulation of the endogenous sterol response mechanism.

Peroxisome deficiency causes a complex phenotype because of hepatic SREBP/Insig dysregulation associated with endoplasmic reticulum stress.

Regulation of hepatic cholesterol biosynthesis, lipogenesis, and insulin signaling intersect at the transcriptional level by control of SREBP and Insig genes. We previously demonstrated that peroxisome-deficient PEX2-/- mice activate SREBP-2 pathways but are unable to maintain normal cholesterol homeostasis. In this study, we demonstrate that oral bile acid treatment normalized hepatic and plasma cholesterol levels and hepatic cholesterol synthesis in early postnatal PEX2 mutants, but SREBP-2 and its target gene expressions remained increased.

IDI2, a second isopentenyl diphosphate isomerase in mammals.

We recently described the identification of a novel isopentenyl diphosphate isomerase, IDI2 in humans and mice. Our current data indicate that, in humans, IDI2 is expressed only in skeletal muscle. Expression constructs of human IDI2 in Saccharomyces cerevisiae can complement isomerase function in an idi1-deficient yeast strain.

Localization of the pre-squalene segment of the isoprenoid biosynthetic pathway in mammalian peroxisomes.

Previous studies have indicated that the early steps in the isoprenoid/cholesterol biosynthetic pathway occur in peroxisomes. However, the role of peroxisomes in cholesterol biosynthesis has recently been questioned in several reports concluding that three of the peroxisomal cholesterol biosynthetic enzymes, namely mevalonate kinase, phosphomevalonate kinase, and mevalonate diphosphate decarboxylase, do not localize to peroxisomes in human cells even though they contain consensus peroxisomal targeting signals. We re-investigated the subcellular localization of the cholesterol biosynthetic enzymes of the pre-squalene segment in human cells by using new stable isotopic techniques and data computations with isotopomer spectral analysis, in combination with immunofluorescence and cell permeabilization techniques.

Disturbed cholesterol homeostasis in a peroxisome-deficient PEX2 knockout mouse model.

We evaluated the major pathways of cholesterol regulation in the peroxisome-deficient PEX2(-/-) mouse, a model for Zellweger syndrome. Zellweger syndrome is a lethal inherited disorder characterized by severe defects in peroxisome biogenesis and peroxisomal protein import. Compared with wild-type mice, PEX2(-/-) mice have decreased total and high-density lipoprotein cholesterol levels in plasma.

Isopentenyl-diphosphate isomerases in human and mouse: evolutionary analysis of a mammalian gene duplication.

Isopentenyl diphosphate isomerase (IDI) activates isopentenyl diphosphate (IPP) for polymerization by converting it to its highly nucleophilic isomer dimethylallyl diphosphate (DMAPP). In plants, this central reaction of isoprenoid biosynthesis is catalyzed by various highly conserved isozymes that differ in expression pattern and subcellular localization. Here we report the identification of an IDI duplication in mammals.

Loss of compartmentalization causes misregulation of lysine biosynthesis in peroxisome-deficient yeast cells.

To characterize the metabolic role of peroxisomes in yeast cells under physiological conditions, we performed a comprehensive meta-analysis of published microarray data. Previous studies of yeast peroxisomes have mainly been focused on the function of peroxisomes under extreme conditions, such as growth on oleate or methanol as the sole carbon source, and may therefore not be representative of the normal physiological role of yeast peroxisomes. Surprisingly, our analysis of the microarray data reveals that the only pathway responding to peroxisome deficiency in mid-log phase is lysine biosynthesis, whereas classical peroxisomal pathways such as beta-oxidation are unaffected.

A second gene for peroxisomal HMG-CoA reductase? A genomic reassessment.

HMG-CoA reductase (HMGCR) catalyzes the conversion of HMG-CoA to mevalonate, the rate-limiting step of eukaryotic isoprenoid biosynthesis, and is the main target of cholesterol-lowering drugs. The classical form of the enzyme is a transmembrane-protein anchored to the endoplasmic reticulum. However, during the last years several lines of evidence pointed to the existence of a second isoform of HMGCR localized in peroxisomes, where mevalonate is converted further to farnesyl diphosphate.

Rat liver acyl-CoA synthetase 4 is a peripheral-membrane protein located in two distinct subcellular organelles, peroxisomes, and mitochondrial-associated membrane.

Obesity and non-insulin-dependent diabetes favor storage of fatty acids in triacylglycerol over oxidation. Recently, individual acyl-CoA synthetase (ACS) isoforms have been implicated in the channeling of fatty acids either toward lipid synthesis or toward oxidation. Although ACS1 had been localized to three different subcellular regions in rat liver, endoplasmic reticulum, mitochondria, and peroxisomes, the study had used an antibody raised against the full-length ACS1 protein which cross-reacts with other isoforms, probably because all ACS family members contain highly conserved amino acid sequences.

Central role of peroxisomes in isoprenoid biosynthesis.

Peroxisomes contain enzymes catalyzing a number of indispensable metabolic functions mainly related to lipid metabolism. The importance of peroxisomes in man is stressed by the existence of genetic disorders in which the biogenesis of the organelle is defective, leading to complex developmental and metabolic phenotypes. The purpose of this review is to emphasize some of the recent findings related to the localization of cholesterol biosynthetic enzymes in peroxisomes and to discuss the impairment of cholesterol biosynthesis in peroxisomal deficiency diseases.