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

Regulation of Mitochondrial and Peroxisomal Metabolism in Female Obesity and Type 2 Diabetes

Obesity and type 2 diabetes (T2D) are widespread metabolic disorders that significantly impact global health today, affecting approximately 17% of adults worldwide with obesity and 9.3% with T2D. Both conditions are closely linked to disruptions in lipid metabolism, where peroxisomes play a pivotal role. Mitochondria and peroxisomes are vital organelles responsible for lipid and energy regulation, including the β-oxidation and oxidation of very long-chain fatty acids (VLCFAs), cholesterol biosynthesis, and bile acid metabolism. These processes are significantly influenced by estrogens, highlighting the interplay between these organelles' function and hormonal regulation in the development and progression of metabolic diseases, such as obesity, metabolic dysfunction-associated fatty liver disease (MAFLD), and T2D. Estrogens modulate lipid metabolism through interactions with nuclear receptors, like peroxisome proliferator-activated receptors (PPARs), which are crucial for maintaining metabolic balance. Estrogen deficiency, such as in postmenopausal women, impairs PPAR regulation, leading to lipid accumulation and increased risk of metabolic disorders. The disruption of peroxisomal-mitochondrial function and estrogen regulation exacerbates lipid imbalances, contributing to insulin resistance and ROS accumulation. This review emphasizes the critical role of these organelles and estrogens in lipid metabolism and their implications for metabolic health, suggesting that therapeutic strategies, including hormone replacement therapy, may offer potential benefits in treating and preventing metabolic diseases.

Peroxisomal cholesterol metabolism regulates yap-signaling, which maintains intestinal epithelial barrier function and is altered in Crohn's disease

Intestinal epithelial cells line the luminal surface to establish the intestinal barrier, where the cells play essential roles in the digestion of food, absorption of nutrients and water, protection from microbial infections, and maintaining symbiotic interactions with the commensal microbial populations. Maintaining and coordinating all these functions requires tight regulatory signaling, which is essential for intestinal homeostasis and organismal health. Dysfunction of intestinal epithelial cells, indeed, is linked to gastrointestinal disorders such as irritable bowel syndrome, inflammatory bowel disease, and gluten-related enteropathies. Emerging evidence suggests that peroxisome metabolic functions are crucial in maintaining intestinal epithelial cell functions and intestinal epithelium regeneration and, therefore, homeostasis. Here, we investigated the molecular mechanisms by which peroxisome metabolism impacts enteric health using the fruit fly Drosophila melanogaster and murine model organisms and clinical samples. We show that peroxisomes control cellular cholesterol, which in turn regulates the conserved yes-associated protein-signaling and contributes to intestinal epithelial structure and epithelial barrier function. Moreover, analysis of intestinal organoid cultures derived from biopsies of patients affected by Crohn's Disease revealed that the dysregulation of peroxisome number, excessive cellular cholesterol, and inhibition of Yap-signaling are markers of disease and could be novel diagnostic and/or therapeutic targets for treating Crohn's Disease. Our studies provided mechanistic insights on peroxisomal signaling in intestinal epithelial cell functions and identified cholesterol as a novel metabolic regulator of yes-associated protein-signaling in tissue homeostasis.

Peroxisomal Localization of a Truncated HMG-CoA Reductase under Low Cholesterol Conditions

3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase, HMGCR) is one of the rate-limiting enzymes in the mevalonate pathway required for cholesterol biosynthesis. It is an integral membrane protein of the endoplasmic reticulum (ER) but has occasionally been described in peroxisomes. By co-immunofluorescence microscopy using different HMGCR antibodies, we present evidence for a dual localization of HMGCR in the ER and peroxisomes in differentiated human monocytic THP-1 cells, primary human monocyte-derived macrophages and human primary skin fibroblasts under conditions of low cholesterol and statin treatment. Using density gradient centrifugation and Western blot analysis, we observed a truncated HMGCR variant of 76 kDa in the peroxisomal fractions, while a full-length HMGCR of 96 kDa was contained in fractions of the ER. In contrast to primary human control fibroblasts, peroxisomal HMGCR was not found in fibroblasts from patients suffering from type-1 rhizomelic chondrodysplasia punctata, who lack functional PEX7 and, thus, cannot import peroxisomal matrix proteins harboring a type-2 peroxisomal targeting signal (PTS2). Moreover, in the N-terminal region of the soluble 76 kDa C-terminal catalytic domain, we identified a PTS2-like motif, which was functional in a reporter context. We propose that under sterol-depleted conditions, part of the soluble HMGCR domain, which is released from the ER by proteolytic processing for further turnover, remains sufficiently long in the cytosol for peroxisomal import via a PTS2/PEX7-dependent mechanism. Altogether, our findings describe a dual localization of HMGCR under combined lipid depletion and statin treatment, adding another puzzle piece to the complex regulation of HMGCR.

Mouse Models to Study Peroxisomal Functions and Disorders: Overview, Caveats, and Recommendations

During the last three decades many mouse lines were created or identified that are deficient in one or more peroxisomal functions. Different methodologies were applied to obtain global, hypomorph, cell type selective, inducible, and knockin mice. Whereas some models closely mimic pathologies in patients, others strongly deviate or no human counterpart has been reported. Often, mice, apparently endowed with a stronger transcriptional adaptation, have to be challenged with dietary additions or restrictions in order to trigger phenotypic changes. Depending on the inactivated peroxisomal protein, several approaches can be taken to validate the loss-of-function. Here, an overview is given of the available mouse models and their most important characteristics.

The Key Role of Peroxisomes in Follicular Growth, Oocyte Maturation, Ovulation, and Steroid Biosynthesis

The absence of peroxisomes can cause disease in the human reproductive system, including the ovaries. The available peroxisomal gene-knockout female mouse models, which exhibit pathological changes in the ovary and reduced fertility, are listed in this review. Our review article provides the first systematic presentation of peroxisomal regulation and its possible functions in the ovary. Our immunofluorescence results reveal that peroxisomes are present in all cell types in the ovary; however, peroxisomes exhibit different numerical abundances and strong heterogeneity in their protein composition among distinct ovarian cell types. The peroxisomal compartment is strongly altered during follicular development and during oocyte maturation, which suggests that peroxisomes play protective roles in oocytes against oxidative stress and lipotoxicity during ovulation and in the survival of oocytes before conception. In addition, the peroxisomal compartment is involved in steroid synthesis, and peroxisomal dysfunction leads to disorder in the sexual hormone production process. However, an understanding of the cellular and molecular mechanisms underlying these physiological and pathological processes is lacking. To date, no effective treatment for peroxisome-related disease has been developed, and only supportive methods are available. Thus, further investigation is needed to resolve peroxisome deficiency in the ovary and eventually promote female fertility.

Peroxisomal Stress Response and Inter-Organelle Communication in Cellular Homeostasis and Aging

Peroxisomes are key regulators of cellular and metabolic homeostasis. These organelles play important roles in redox metabolism, the oxidation of very-long-chain fatty acids (VLCFAs), and the biosynthesis of ether phospholipids. Given the essential role of peroxisomes in cellular homeostasis, peroxisomal dysfunction has been linked to various pathological conditions, tissue functional decline, and aging. In the past few decades, a variety of cellular signaling and metabolic changes have been reported to be associated with defective peroxisomes, suggesting that many cellular processes and functions depend on peroxisomes. Peroxisomes communicate with other subcellular organelles, such as the nucleus, mitochondria, endoplasmic reticulum (ER), and lysosomes. These inter-organelle communications are highly linked to the key mechanisms by which cells surveil defective peroxisomes and mount adaptive responses to protect them from damages. In this review, we highlight the major cellular changes that accompany peroxisomal dysfunction and peroxisomal inter-organelle communication through membrane contact sites, metabolic signaling, and retrograde signaling. We also discuss the age-related decline of peroxisomal protein import and its role in animal aging and age-related diseases. Unlike other organelle stress response pathways, such as the unfolded protein response (UPR) in the ER and mitochondria, the cellular signaling pathways that mediate stress responses to malfunctioning peroxisomes have not been systematically studied and investigated. Here, we coin these signaling pathways as "peroxisomal stress response pathways". Understanding peroxisomal stress response pathways and how peroxisomes communicate with other organelles are important and emerging areas of peroxisome research.

Knockdown of PEX16 Induces Autophagic Degradation of Peroxisomes

Peroxisome abundance is regulated by homeostasis between the peroxisomal biogenesis and degradation processes. Peroxin 16 (PEX16) is a peroxisomal protein involved in trafficking membrane proteins for de novo peroxisome biogenesis. The present study demonstrates that PEX16 also modulates peroxisome abundance through pexophagic degradation. PEX16 knockdown in human retinal pigment epithelial-1 cells decreased peroxisome abundance and function, represented by reductions in the expression of peroxisome membrane protein ABCD3 and the levels of cholesterol and plasmalogens, respectively. The activation of pexophagy under PEX16 knockdown was shown by (i) abrogated peroxisome loss under PEX16 knockdown in autophagy-deficient ATG5 knockout cell lines, and (ii) increased autophagy flux and co-localization of p62-an autophagy adaptor protein-with ABCD3 in the presence of the autophagy inhibitor chloroquine. However, the levels of cholesterol and plasmalogens did not recover despite the restoration of peroxisome abundance following chloroquine treatment. Thus, PEX16 is indispensable for maintaining peroxisome homeostasis by regulating not only the commonly known biogenesis pathway but also the autophagic degradation of peroxisomes.

Murine deficiency of peroxisomal L-bifunctional protein (EHHADH) causes medium-chain 3-hydroxydicarboxylic aciduria and perturbs hepatic cholesterol homeostasis

Peroxisomes play an essential role in the β-oxidation of dicarboxylic acids (DCAs), which are metabolites formed upon ω-oxidation of fatty acids. Genetic evidence linking transporters and enzymes to specific DCA β-oxidation steps is generally lacking. Moreover, the physiological functions of DCA metabolism remain largely unknown. In this study, we aimed to characterize the DCA β-oxidation pathway in human cells, and to evaluate the biological role of DCA metabolism using mice deficient in the peroxisomal L-bifunctional protein (Ehhadh KO mice). In vitro experiments using HEK-293 KO cell lines demonstrate that ABCD3 and ACOX1 are essential in DCA β-oxidation, whereas both the bifunctional proteins (EHHADH and HSD17B4) and the thiolases (ACAA1 and SCPx) have overlapping functions and their contribution may depend on expression level. We also show that medium-chain 3-hydroxydicarboxylic aciduria is a prominent feature of EHHADH deficiency in mice most notably upon inhibition of mitochondrial fatty acid oxidation. Using stable isotope tracing methodology, we confirmed that products of peroxisomal DCA β-oxidation can be transported to mitochondria for further metabolism. Finally, we show that, in liver, Ehhadh KO mice have increased mRNA and protein expression of cholesterol biosynthesis enzymes with decreased (in females) or similar (in males) rate of cholesterol synthesis. We conclude that EHHADH plays an essential role in the metabolism of medium-chain DCAs and postulate that peroxisomal DCA β-oxidation is a regulator of hepatic cholesterol biosynthesis.

Functional Peroxisomes Are Essential for Efficient Cholesterol Sensing and Synthesis

Cholesterol biosynthesis is a multi-step process involving several subcellular compartments, including peroxisomes. Cells adjust their sterol content by both transcriptional and post-transcriptional feedback regulation, for which sterol regulatory element-binding proteins (SREBPs) are essential; such homeostasis is dysregulated in peroxisome-deficient Pex2 knockout mice. Here, we compared the regulation of cholesterol biosynthesis in Chinese hamster ovary (CHO-K1) cells and in three isogenic peroxisome-deficient CHO cell lines harboring Pex2 gene mutations. Peroxisome deficiency activated expression of cholesterogenic genes, however, cholesterol levels were unchanged. 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) protein levels were increased in mutant cells, whereas HMGCR activity was significantly decreased, resulting in reduced cholesterol synthesis. U18666A, an inhibitor of lysosomal cholesterol export, induced cholesterol biosynthetic enzymes; yet, cholesterol synthesis was still reduced. Interestingly, peroxisome deficiency promoted ER-to-Golgi SREBP cleavage-activating protein (SCAP) trafficking even when cells were cholesterol-loaded. Restoration of functional peroxisomes normalized regulation of cholesterol synthesis and SCAP trafficking. These results highlight the importance of functional peroxisomes for maintaining cholesterol homeostasis and efficient cholesterol synthesis.

Inclusion of endophenotypes in a standard GWAS facilitate a detailed mechanistic understanding of genetic elements that control blood lipid levels

Dyslipidemia is the primary cause of cardiovascular disease, which is a serious human health problem in large parts of the world. Therefore, it is important to understand the genetic and molecular mechanisms that regulate blood levels of cholesterol and other lipids. Discovery of genetic elements in the regulatory machinery is often based on genome wide associations studies (GWAS) focused on end-point phenotypes such as total cholesterol level or a disease diagnosis. In the present study, we add endophenotypes, such as serum levels of intermediate metabolites in the cholesterol synthesis pathways, to a GWAS analysis and use the pig as an animal model. We do this to increase statistical power and to facilitate biological interpretation of results. Although the study population was limited to ~ 300 individuals, we identify two genome-wide significant associations and ten suggestive associations. Furthermore, we identify 28 tentative associations to loci previously associated with blood lipids or dyslipidemia associated diseases. The associations with endophenotypes may inspire future studies that can dissect the biological mechanisms underlying these previously identified associations and add a new level of understanding to previously identified associations.

Longitudinal study of Pex1-G844D NMRI mouse model: A robust pre-clinical model for mild Zellweger spectrum disorder

Zellweger spectrum disorders (ZSD) are inborn errors of metabolism caused by mutations in PEX genes that lead to peroxisomal biogenesis disorder (PBD). No validated treatment is able to modify the dismal progression of the disease. ZSD mouse models used to develop therapeutic approaches are limited by poor survival and breeding restrictions. To overcome these limitations, we backcrossed the hypomorphic Pex1 p.G844D allele to NMRI background. NMRI mouse breeding restored an autosomal recessive Mendelian inheritance pattern and delivered twice larger litters. Mice were longitudinally phenotyped up to 6 months of age to make this model suitable for therapeutic interventions. ZSD mice exhibited growth retardation and relative hepatomegaly associated to progressive hepatocyte hypertrophy. Biochemical studies associated with RNA sequencing deciphered ZSD liver glycogen metabolism alterations. Affected fibroblasts displayed classical immunofluorescence pattern and biochemical alterations associated with PBD. Plasma and liver showed very long-chain fatty acids, specific oxysterols and C₂₇ bile acids intermediates elevation in ZSD mice along with a specific urine organic acid profile. With ageing, C₂₆ fatty acid and phytanic acid levels tended to normalize in ZSD mice, as described in patients reaching adulthood. In conclusion, our mouse model recapitulates a mild ZSD phenotype and is suitable for liver-targeted therapies evaluation.

Peroxisome-Deficiency and HIF-2α Signaling Are Negative Regulators of Ketohexokinase Expression

Ketohexokinase (KHK) is the first and rate-limiting enzyme of fructose metabolism. Expression of the two alternatively spliced KHK isoforms, KHK-A and KHK-C, is tissue-specific and KHK-C is predominantly expressed in liver, kidney and intestine and responsible for the fructose-catabolizing function. While KHK isoform choice has been linked to the development of disorders such as obesity, diabetes, cardiovascular disease and cancer, little is known about the regulation of total KHK expression. In the present study, we investigated how hypoxic signaling influences fructose metabolism in the liver. Hypoxia or von Hippel-Lindau (VHL) tumor suppressor loss leads to the stabilization of hypoxia-inducible factors alpha (HIF-1α and HIF-2α) and the activation of their signaling to mediate adaptive responses. By studying liver-specific Vhl, Vhl/Hif1a, and Vhl/Epas1 knockout mice, we found that KHK expression is suppressed by HIF-2α (encoded by Epas1) but not by HIF-1α signaling on mRNA and protein levels. Reduced KHK levels were accompanied by downregulation of aldolase B (ALDOB) in the livers of Vhl and Vhl/Hif1a knockout mice, further indicating inhibited fructose metabolism. HIF-1α and HIF-2α have both overlapping and distinct target genes but are differentially regulated depending on the cell type and physiologic or pathologic conditions. HIF-2α activation augments peroxisome degradation in mammalian cells by pexophagy and thereby changes lipid composition reminiscent of peroxisomal disorders. We further demonstrated that fructose metabolism is negatively regulated by peroxisome-deficiency in a Pex2 knockout Zellweger mouse model, which lacks functional peroxisomes and is characterized by widespread metabolic dysfunction. Repression of fructolytic genes in Pex2 knockout mice appeared to be independent of PPARα signaling and nutritional status. Interestingly, our results demonstrate that both HIF-2α and peroxisome-deficiency result in downregulation of Khk independent of splicing as both isoforms, Khka as well as Khkc, are significantly downregulated. Hence, our study offers new and unexpected insights into the general regulation of KHK, and therefore fructolysis. We revealed a novel regulatory function of HIF-2α, suggesting that HIF-1α and HIF-2α have tissue-specific opposing roles in the regulation of Khk expression, isoform choice and fructolysis. In addition, we discovered a previously unknown function of peroxisomes in the regulation of fructose metabolism.

TMEM135 regulates primary ciliogenesis through modulation of intracellular cholesterol distribution

Recent evidence has linked the lysosomal cholesterol accumulation in Niemann–Pick type C1 with anomalies associated with primary ciliogenesis. Here, we report that perturbed intracellular cholesterol distribution imposed by lysosomal cholesterol accumulation during TMEM135 depletion is closely associated with impaired ciliogenesis. TMEM135 depletion does not affect the formation of the basal body and the ciliary transition zone. TMEM135 depletion severely blunts Rab8 trafficking to the centrioles without affecting the centriolar localization of Rab11 and Rabin8, the upstream regulators of Rab8 activation. Although TMEM135 depletion prevents enhanced IFT20 localization at the centrioles, ciliary vesicle formation is not affected. Furthermore, enhanced IFT20 localization at the centrioles is dependent on Rab8 activation. Supplementation of cholesterol in complex with cyclodextrin rescues Rab8 trafficking to the centrioles and Rab8 activation, thereby recovering primary ciliogenesis in TMEM135‐depleted cells. Taken together, our data suggest that TMEM135 depletion prevents ciliary vesicle elongation, a characteristic of impaired Rab8 function. Our study thus reveals a previously uncharacterized effect of erroneous intracellular cholesterol distribution on impairing Rab8 function and primary ciliogenesis.

Metabolic interactions between peroxisomes and mitochondria with a special focus on acylcarnitine metabolism

Carnitine plays an essential role in mitochondrial fatty acid β-oxidation as a part of a cycle that transfers long-chain fatty acids across the mitochondrial membrane and involves two carnitine palmitoyltransferases (CPT1 and CPT2). Two distinct carnitine acyltransferases, carnitine octanoyltransferase (COT) and carnitine acetyltransferase (CAT), are peroxisomal enzymes, which indicates that carnitine is not only important for mitochondrial, but also for peroxisomal metabolism. It has been demonstrated that after peroxisomal metabolism, specific intermediates can be exported as acylcarnitines for subsequent and final mitochondrial metabolism. There is also evidence that peroxisomes are able to degrade fatty acids that are typically handled by mitochondria possibly after transport as acylcarnitines. Here we review the biochemistry and physiological functions of metabolite exchange between peroxisomes and mitochondria with a special focus on acylcarnitines.

Gene-based meta-analysis of genome-wide association studies implicates new loci involved in obesity

To date, genome-wide association studies (GWASs) have identified >100 loci with single variants associated with body mass index (BMI). This approach may miss loci with high allelic heterogeneity; therefore, the aim of the present study was to use gene-based meta-analysis to identify regions with high allelic heterogeneity to discover additional obesity susceptibility loci. We included GWAS data from 123 865 individuals of European descent from 46 cohorts in Stage 1 and Metabochip data from additional 103 046 individuals from 43 cohorts in Stage 2, all within the Genetic Investigation of ANthropometric Traits (GIANT) consortium. Each cohort was tested for association between ∼2.4 million (Stage 1) or ∼200 000 (Stage 2) imputed or genotyped single variants and BMI, and summary statistics were subsequently meta-analyzed in 17 941 genes. We used the 'VErsatile Gene-based Association Study' (VEGAS) approach to assign variants to genes and to calculate gene-based P-values based on simulations. The VEGAS method was applied to each cohort separately before a gene-based meta-analysis was performed. In Stage 1, two known (FTO and TMEM18) and six novel (PEX2, MTFR2, SSFA2, IARS2, CEP295 and TXNDC12) loci were associated with BMI (P < 2.8 × 10(-6) for 17 941 gene tests). We confirmed all loci, and six of them were gene-wide significant in Stage 2 alone. We provide biological support for the loci by pathway, expression and methylation analyses. Our results indicate that gene-based meta-analysis of GWAS provides a useful strategy to find loci of interest that were not identified in standard single-marker analyses due to high allelic heterogeneity.

Dysregulation of Plasmalogen Homeostasis Impairs Cholesterol Biosynthesis

Plasmalogen biosynthesis is regulated by modulating fatty acyl-CoA reductase 1 stability in a manner dependent on cellular plasmalogen level. However, physiological significance of the regulation of plasmalogen biosynthesis remains unknown. Here we show that elevation of the cellular plasmalogen level reduces cholesterol biosynthesis without affecting the isoprenylation of proteins such as Rab and Pex19p. Analysis of intermediate metabolites in cholesterol biosynthesis suggests that the first oxidative step in cholesterol biosynthesis catalyzed by squalene monooxygenase (SQLE), an important regulator downstream HMG-CoA reductase in cholesterol synthesis, is reduced by degradation of SQLE upon elevation of cellular plasmalogen level. By contrast, the defect of plasmalogen synthesis causes elevation of SQLE expression, resulting in the reduction of 2,3-epoxysqualene required for cholesterol synthesis, hence implying a novel physiological consequence of the regulation of plasmalogen biosynthesis.

Sox17 regulates liver lipid metabolism and adaptation to fasting

Liver is a major regulator of lipid metabolism and adaptation to fasting, a process involving PPARalpha activation. We recently showed that the Vnn1 gene is a PPARalpha target gene in liver and that release of the Vanin-1 pantetheinase in serum is a biomarker of PPARalpha activation. Here we set up a screen to identify new regulators of adaptation to fasting using the serum Vanin-1 as a marker of PPARalpha activation. Mutagenized mice were screened for low serum Vanin-1 expression. Functional interactions with PPARalpha were investigated by combining transcriptomic, biochemical and metabolic approaches. We characterized a new mutant mouse in which hepatic and serum expression of Vanin-1 is depressed. This mouse carries a mutation in the HMG domain of the Sox17 transcription factor. Mutant mice display a metabolic phenotype featuring lipid abnormalities and inefficient adaptation to fasting. Upon fasting, a fraction of the PPARα-driven transcriptional program is no longer induced and associated with impaired fatty acid oxidation. The transcriptional phenotype is partially observed in heterozygous Sox17+/− mice. In mutant mice, the fasting phenotype but not all transcriptomic signature is rescued by the administration of the PPARalpha agonist fenofibrate. These results identify a novel role for Sox17 in adult liver as a modulator of the metabolic adaptation to fasting.

Peroxisomes are required for lipid metabolism and muscle function in Drosophila melanogaster

Peroxisomes are ubiquitous organelles that perform lipid and reactive oxygen species metabolism. Defects in peroxisome biogenesis cause peroxisome biogenesis disorders (PBDs). The most severe PBD, Zellweger syndrome, is characterized in part by neuronal dysfunction, craniofacial malformations, and low muscle tone (hypotonia). These devastating diseases lack effective therapies and the development of animal models may reveal new drug targets. We have generated Drosophila mutants with impaired peroxisome biogenesis by disrupting the early peroxin gene pex3, which participates in budding of pre-peroxisomes from the ER and peroxisomal membrane protein localization. pex3 deletion mutants lack detectible peroxisomes and die before or during pupariation. At earlier stages of development, larvae lacking Pex3 display reduced size and impaired lipid metabolism. Selective loss of peroxisomes in muscles impairs muscle function and results in flightless animals. Although, hypotonia in PBD patients is thought to be a secondary effect of neuronal dysfunction, our results suggest that peroxisome loss directly affects muscle physiology, possibly by disrupting energy metabolism. Understanding the role of peroxisomes in Drosophila physiology, specifically in muscle cells may reveal novel aspects of PBD etiology.

Disturbances in cholesterol, bile acid and glucose metabolism in peroxisomal 3-ketoacylCoA thiolase B deficient mice fed diets containing high or low fat contents

The peroxisomal 3-ketoacyl-CoA thiolase B (ThB) catalyzes the thiolytic cleavage of straight chain 3-ketoacyl-CoAs. Up to now, the ability of ThB to interfere with lipid metabolism was studied in mice fed a laboratory chow enriched or not with the synthetic agonist Wy14,643, a pharmacological activator of the nuclear hormone receptor PPARα. The aim of the present study was therefore to determine whether ThB could play a role in obesity and lipid metabolism when mice are chronically fed a synthetic High Fat Diet (HFD) or a Low Fat Diet (LFD) as a control diet. To investigate this possibility, wild-type (WT) mice and mice deficient for Thb (Thb(-/-)) were subjected to either a synthetic LFD or a HFD for 25 weeks, and their responses were compared. First, when fed a normal regulatory laboratory chow, Thb(-/-) mice displayed growth retardation as well as a severe reduction in the plasma level of Growth Hormone (GH) and Insulin Growth Factor-I (IGF-I), suggesting alterations in the GH/IGF-1 pathway. When fed the synthetic diets, the corrected energy intake to body mass was significantly higher in Thb(-/-) mice, yet those mice were protected from HFD-induced adiposity. Importantly, Thb(-/-) mice also suffered from hypoglycemia, exhibited reduction in liver glycogen stores and circulating insulin levels under the LFD and the HFD. Thb deficiency was also associated with higher levels of plasma HDL (High Density Lipoproteins) cholesterol and increased liver content of cholesterol under both the LFD and the HFD. As shown by the plasma lathosterol to cholesterol ratio, a surrogate marker for cholesterol biosynthesis, whole body cholesterol de novo synthesis was increased in Thb(-/-) mice. By comparing liver RNA from WT mice and Thb(-/-) mice using oligonucleotide microarray and RT-qPCR, a coordinated decrease in the expression of critical cholesterol synthesizing genes and an increased expression of genes involved in bile acid synthesis (Cyp7a1, Cyp17a1, Akr1d1) were observed in Thb(-/-) mice. In parallel, the elevation of the lathosterol to cholesterol ratio as well as the increased expression of cholesterol synthesizing genes were observed in the kidney of Thb(-/-) mice fed the LFD and the HFD. Overall, the data indicate that ThB is not fully interchangeable with the thiolase A isoform. The present study also reveals that modulating the expression of the peroxisomal ThB enzyme can largely reverberate not only throughout fatty acid metabolism but also cholesterol, bile acid and glucose metabolism.

Liver kinase b1 is required for white adipose tissue growth and differentiation

White adipose tissue (WAT) is not only a lipogenic and fat storage tissue but also an important endocrine organ that regulates energy homeostasis, lipid metabolism, appetite, fertility, and immune and stress responses. Liver kinase B1 (LKB1), a tumor suppressor, is a key regulator in energy metabolism. However, the role of LKB1 in adipogenesis is unknown. The current study aimed to determine the contributions of LKB1 to adipogenesis in vivo. Using the Fabp4-Cre/loxP system, we generated adipose tissue-specific LKB1 knockout (LKB1(ad-/-)) mice. LKB1(ad-/-) mice exhibited a reduced amount of WAT, postnatal growth retardation, and early death before weaning. Further, LKB1 deletion markedly reduced the levels of insulin receptor substrate 1 (IRS1), peroxisome proliferator-activated receptor γ, CCAAT/enhancer-binding protein α, and phosphorylated AMP-activated protein kinase (AMPK). Consistent with these results, overexpression of constitutively active AMPK partially ablated IRS1 degradation in LKB1-deficient cells. LKB1 deletion increased the levels of F-box/WD repeat-containing protein (Fbw) 8, the IRS1 ubiquitination E3 ligase. Silencing of Fbw8 increased IRS1 levels. Finally, promoter analysis and DNA chromatin immunoprecipitation analysis identified three sterol regulatory element (SRE) sites in the Fbw8 promoter, where SRE-binding protein 1c binds and induces the expression of Fbw8. Taken together, these data indicate that LKB1 controls IRS1-dependent adipogenesis via AMPK in WAT.

Peroxisome interactions and cross-talk with other subcellular compartments in animal cells

Peroxisomes are remarkably plastic and dynamic organelles, which fulfil important functions in hydrogen peroxide and lipid metabolism rendering them essential for human health and development. Despite great advances in the identification and characterization of essential components and molecular mechanisms associated with the biogenesis and function of peroxisomes, our understanding of how peroxisomes are incorporated into metabolic pathways and cellular communication networks is just beginning to emerge. Here we address the interaction of peroxisomes with other subcellular compartments including the relationship with the endoplasmic reticulum, the peroxisome-mitochondria connection and the association with lipid droplets. We highlight metabolic cooperations and potential cross-talk and summarize recent findings on peroxisome-peroxisome interactions and the interaction of peroxisomes with microtubules in mammalian cells.

Farnesyl diphosphate synthase, the target for nitrogen-containing bisphosphonate drugs, is a peroxisomal enzyme in the model system Dictyostelium discoideum

NBP (nitrogen-containing bisphosphonate) drugs protect against excessive osteoclast-mediated bone resorption. After binding to bone mineral, they are taken up selectively by the osteoclasts and inhibit the essential enzyme FDPS (farnesyl diphosphate synthase). NBPs inhibit also growth of amoebae of Dictyostelium discoideum in which their target is again FDPS. A fusion protein between FDPS and GFP (green fluorescent protein) was found, in D. discoideum, to localize to peroxisomes and to confer resistance to the NBP alendronate. GFP was also directed to peroxisomes by a fragment of FDPS comprising amino acids 1–22. This contains a sequence of nine amino acids that closely resembles the nonapeptide PTS2 (peroxisomal targeting signal type 2): there is only a single amino acid mismatch between the two sequences. Mutation analysis confirmed that the atypical PTS2 directs FDPS into peroxisomes. Furthermore, expression of the D. discoideum FDPS–GFP fusion protein in strains of Saccharomyces cerevisiae defective in peroxisomal protein import demonstrated that import of FDPS into peroxisomes was blocked in a strain lacking the PTS2-dependent import pathway. The peroxisomal location of FDPS in D. discoideum indicates that NBPs have to cross the peroxisomal membrane before they can bind to their target.

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. Herein, we demonstrate a more profound dysregulation of cholesterol homeostasis in newborn Pex2(-/-) mice congenic on a 129S6/SvEv (129) genetic background, and substantial differences between newborn versus postnatal Pex2(-/-) mice in factors that activate ER stress. These differences extend to relationships between activation of genes regulated by SREBP-2 versus PPARα. The SREBP-2 pathway is induced in neonatal Pex2(-/-) livers from 129 and SW/129 strains, despite normal hepatic cholesterol levels. ER stress markers are increased in newborn 129 Pex2(-/-) livers, which occurs in the absence of hepatic steatosis or accumulation of peroxins in the ER. Moreover, the induction of SREBP-2 and ER stress pathways is independent of PPARα activation in livers of newborn 129 and SW/129 Pex2(-/-) mice. Two-week-old wild-type mice treated with the peroxisome proliferator WY-14,643 show strong induction of PPARα-regulated genes and decreased expression of SREBP-2 and its target genes, further demonstrating that SREBP-2 pathway induction is not dependent on PPARα activation. Lastly, there is no activation of either SREBP-2 or ER stress pathways in kidney and lung of newborn Pex2(-/-) mice, suggesting a parallel induction of these pathways in peroxisome-deficient mice. These findings establish novel associations between SREBP-2, ER stress and PPARα pathway inductions.

Hepatosteatosis in peroxisome deficient liver despite increased β-oxidation capacity and impaired lipogenesis

Peroxisome deficiency in liver causes hepatosteatosis both in patients and in mice. Here, we studied the mechanisms that contribute to this lipid accumulation and to activation of peroxisome proliferator activated receptor α (PPARα) by using liver-specific Pex5(-/-) mice (L-Pex5(-/-) mice). Surprisingly, steatosis was accompanied both by increased mitochondrial β-oxidation capacity, confirming previous observations, and by impaired de novo lipid synthesis mediated by reduced expression of sterol regulatory element binding protein 1c and its targets. As a consequence, when challenged with a high fat diet, L-Pex5(-/-) mice were protected from adiposity. Hepatic fatty acid uptake was strongly increased whereas the expression of apolipoproteins and the lipoprotein assembly factor microsomal triglyceride transfer protein were markedly reduced resulting in reduced secretion of very low density lipoproteins. Most of these changes seemed to be orchestrated by the endogenous activation of PPARα, challenging the assumption that PPARα activation in hepatocytes requires fatty acid synthase dependent de novo fatty acid synthesis. Expression of cholesterol synthesizing enzymes and cholesterol levels were not affected in peroxisome deficient liver. In conclusion, increased fatty acid uptake driven by endogenous PPARα activation and reduced fatty acid secretion cause hepatosteatosis in peroxisome deficient livers.

A Drosophila model for the Zellweger spectrum of peroxisome biogenesis disorders

Human peroxisome biogenesis disorders are lethal genetic diseases in which abnormal peroxisome assembly compromises overall peroxisome and cellular function. Peroxisomes are ubiquitous membrane-bound organelles involved in several important biochemical processes, notably lipid metabolism and the use of reactive oxygen species for detoxification. Using cultured cells, we systematically characterized the peroxisome assembly phenotypes associated with dsRNA-mediated knockdown of 14 predicted Drosophila homologs of PEX genes (encoding peroxins; required for peroxisome assembly and linked to peroxisome biogenesis disorders), and confirmed that at least 13 of them are required for normal peroxisome assembly. We also demonstrate the relevance of Drosophila as a genetic model for the early developmental defects associated with the human peroxisome biogenesis disorders. Mutation of the PEX1 gene is the most common cause of peroxisome biogenesis disorders and is one of the causes of the most severe form of the disease, Zellweger syndrome. Inherited mutations in Drosophila Pex1 correlate with reproducible defects during early development. Notably, Pex1 mutant larvae exhibit abnormalities that are analogous to those exhibited by Zellweger syndrome patients, including developmental delay, poor feeding, severe structural abnormalities in the peripheral and central nervous systems, and early death. Finally, microarray analysis defined several clusters of genes whose expression varied significantly between wild-type and mutant larvae, implicating peroxisomal function in neuronal development, innate immunity, lipid and protein metabolism, gamete formation, and meiosis.

Peroxisome biogenesis: something old, something new, something borrowed

Eukaryotic cells are characterized by their varied complement of organelles. One set of membrane-bound, usually spherical compartments are commonly grouped together under the term peroxisomes. Peroxisomes function in regulating the synthesis and availability of many diverse lipids by harnessing the power of oxidative reactions and contribute to a number of metabolic processes essential for cellular differentiation and organismal development.

A role for the peroxisomal 3-ketoacyl-CoA thiolase B enzyme in the control of PPARα-mediated upregulation of SREBP-2 target genes in the liver

Peroxisomal 3-ketoacyl-CoA thiolase B (Thb) catalyzes the final step in the peroxisomal β-oxidation of straight-chain acyl-CoAs and is under the transcription control of the nuclear hormone receptor PPARα. PPARα binds to and is activated by the synthetic compound Wy14,643 (Wy). Here, we show that the magnitude of Wy-mediated induction of peroxisomal β-oxidation of radiolabeled (1-(14)C) palmitate was significantly reduced in mice deficient for Thb. In contrast, mitochondrial β-oxidation was unaltered in Thb(-/-) mice. Given that Wy-treatment induced Acox1 and MFP-1/-2 activity at a similar level in both genotypes, we concluded that the thiolase step alone was responsible for the reduced peroxisomal β-oxidation of fatty acids. Electron microscopic analysis and cytochemical localization of catalase indicated that peroxisome proliferation in the liver after Wy-treatment was normal in Thb(-/-) mice. Intriguingly, micro-array analysis revealed that mRNA levels of genes encoding cholesterol biosynthesis enzymes were upregulated by Wy in Wild-Type (WT) mice but not in Thb(-/-) mice, which was confirmed at the protein level for the selected genes. The non-induction of genes encoding cholesterol biosynthesis enzymes by Wy in Thb(-/-) mice appeared to be unrelated to defective SREBP-2 or PPARα signaling. No difference was observed in the plasma lathosterol/cholesterol ratio (a marker for de novo cholesterol biosynthesis) between Wy-treated WT and Thb(-/-) mice, suggesting functional compensation. Overall, we conclude that ThA and SCPx/SCP2 thiolases cannot fully compensate for the absence of ThB. In addition, our data indicate that ThB is involved in the regulation of genes encoding cholesterol biosynthesis enzymes in the liver, suggesting that the peroxisome could be a promising candidate for the correction of cholesterol imbalance in dyslipidemia.

Effects of WY-14643 on peroxisomal enzyme activity and hormone secretion in immortalized human trophoblast cells

In our previous report, clofibric acid increased both the enzyme activities of peroxisomes (catalase and fatty acyl-CoA oxidase) and the secretion of progesterone in immortalized human extravillous trophoblast cells (TCL-1) (F. Hashimoto et al., Biochem. Pharm., 68, 313 (2004)). WY-14643 is reported to be stronger inducer of peroxisomes in rodents than clofibric acid. Therefore, the effects of WY-14643 on the activities of peroxisomal enzymes and hormone secretion in TCL-1 were studied. After incubation for 3 d with WY-14643, WY-14643 (>/=0.15 mM) suppressed the rate of increase in DNA and protein. The specific activities of catalase were increased by 0.1 mM WY-14643. The specific activities of fatty acyl-CoA oxidase were hardly changed by WY-14643. The concentration of progesterone in the medium was increased by 0.1 mM WY-14643, but human chorionic gonadotropin was decreased by 0.2 mM WY-14643. After a discontinuous Nycodenz-density gradient centrifugation of the light mitochondrial fraction of the cells, catalase activity was distributed in lower density fractions than cytochrome-c oxidase (a mitochondria marker enzyme) activity, but the distribution was not changed by WY-14643. These results suggest that WY-14643 inhibits the proliferation of trophoblast cells. The density of peroxisomes in human trophoblast cells is lower than that of mitochondria, and it is not affected by WY-14643. WY-14643 may increase the progesterone secretion. Effects of WY-14643 on metabolism of human trophoblast cells are different from those of clofibric acid.

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. SREBP-2 pathway induction was also observed in neonatal and longer surviving PEX2 mutants, where hepatic cholesterol levels were normal. Abnormal expression patterns for SREBP-1c and Insig-2a, and novel regulation of Insig-2b, further demonstrate that peroxisome deficiency widely affects the regulation of related metabolic pathways. We have provided the first demonstration that peroxisome deficiency activates hepatic endoplasmic reticulum (ER) stress pathways, especially the integrated stress response mediated by PERK and ATF4 signaling. Our studies suggest a mechanism whereby ER stress leads to dysregulation of the endogenous sterol response mechanism and concordantly activates oxidative stress pathways. Several metabolic derangements in peroxisome-deficient PEX2-/- liver are likely to trigger ER stress, including perturbed flux of mevalonate metabolites, altered bile acid homeostasis, changes in fatty acid levels and composition, and oxidative stress.

Coordinate induction of PPAR alpha and SREBP2 in multifunctional protein 2 deficient mice

Mice with inactivation of the D-specific multifunctional protein 2 (MFP2), a crucial enzyme of peroxisomal beta-oxidation, develop multiple pathologies in diverse tissues already starting in the postnatal period. Gene expression profiling performed on liver of 2-day-old pups revealed up-regulation of PPAR alpha responsive genes in knockout mice. Surprisingly, also genes involved in cholesterol biosynthesis were markedly induced. Real-time PCR confirmed the induction of PPAR alpha target genes and of HMGCR and SREBP2, both involved in cholesterol synthesis, in lactating and in adult MFP2 knockout mice. In accordance, the rate of cholesterol biosynthesis was significantly increased in liver of knockout mice but the hepatic cholesterol concentration was unaltered. In MFP2/PPAR alpha double knockout mice, up-regulations of SREBP2 and HMGCR were markedly attenuated. These data demonstrate a tight interrelationship between induction of PPAR alpha by endogenous ligands and up-regulation of genes of cholesterol biosynthesis through increased expression of SREBP2.

Quantifying raft proteins in neonatal mouse brain by 'tube-gel' protein digestion label-free shotgun proteomics

Background

The low concentration and highly hydrophobic nature of proteins in lipid raft samples present significant challenges for the sensitive and accurate proteomic analyses of lipid raft proteins. Elimination of highly enriched lipids and interfering substances from raft samples is generally required before mass spectrometric analyses can be performed, but these procedures often lead to excessive protein loss and increased sample variability. For accurate analyses of the raft proteome, simplified protocols are needed to avoid excessive sample handling and purification steps.

Results

We have devised a simple protocol using a 'tube-gel' protein digestion that, when combined with mass spectrometry, can be used to obtain comprehensive and reproducible identification and quantitation of the lipid raft proteome prepared from neonatal mouse brain. Lipid rafts (detergent-resistant membranes using Triton X-100 extraction) prepared from neonatal mouse brain were directly incorporated into a polyacrylamide tube-gel matrix without prior protein separation. After in-gel digestion of proteins, nanospray LC-MS/MS was used to analyze the extracted peptides, and the resulting spectra were searched to identify the proteins present in the sample. Using the standard 'label-free' proteomics approach, the total number of MS/MS spectra for the identified proteins was used to provide a measure of relative protein abundances. This approach was successfully applied to lipid rafts prepared from neonatal mouse brain. A total of 216 proteins were identified: 127 proteins (58.8%) were predicted to be membrane proteins, or membrane-associated proteins and 175 proteins (~80%) showed less than a 2-fold variation in the relative abundance in replicate samples.

Conclusion

The tube-gel protein digestion protocol coupled with nanospray LC-MS/MS (TubeGeLC-MS/MS) offers a simple and reproducible method for identifying and quantifying the changes of relative abundances in lipid raft proteins from neonatal mouse brain and could become a useful approach for studying lipid raft proteins from various tissues.

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

The marked deficiency of peroxisomal organelle assembly in the PEX2(-/-) mouse model for Zellweger syndrome provides a unique opportunity to developmentally and biochemically characterize hepatic disease progression and bile acid products. The postnatal survival of homozygous mutants enabled us to evaluate the response to bile acid replenishment in this disease state. PEX2 mutant liver has severe but transient intrahepatic cholestasis that abates in the early postnatal period and progresses to steatohepatitis by postnatal day 36. We confirmed the expected reduction of mature C24 bile acids, accumulation of C27-bile acid intermediates, and low total bile acid level in liver and bile from these mutant mice. Treating the PEX2(-/-) mice with bile acids prolonged postnatal survival, alleviated intrahepatic cholestasis and intestinal malabsorption, reduced C27-bile acid intermediate production, and prevented older mutants from developing severe steatohepatitis. However, this therapy exacerbated the degree of hepatic steatosis and worsened the already severe mitochondrial and cellular damage in peroxisome-deficient liver. Both untreated and bile acid-fed PEX2(-/-) mice accumulated high levels of predominantly unconjugated bile acids in plasma because of altered expression of hepatocyte bile acid transporters. Significant amounts of unconjugated bile acids were also found in the liver and bile of PEX2 mutants, indicating a generalized defect in bile acid conjugation.

CONCLUSION

Peroxisome deficiency widely disturbs bile acid homeostasis and hepatic functioning in mice, and the high sensitivity of the peroxisome-deficient liver to bile acid toxicity limits the effectiveness of bile acid therapy for preventing hepatic disease.

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. Our present findings clearly show and confirm previous studies that the pre-squalene segment of the cholesterol biosynthetic pathway is localized to peroxisomes. In addition, our data are consistent with the hypothesis that acetyl-CoA derived from peroxisomal beta-oxidation of very long-chain fatty acids and medium-chain dicarboxylic acids is preferentially channeled to cholesterol synthesis inside the peroxisomes without mixing with the cytosolic acetyl-CoA pool.

Peroxisomes and bile acid biosynthesis

Peroxisomes play an important role in the biosynthesis of bile acids because a peroxisomal beta-oxidation step is required for the formation of the mature C24-bile acids from C27-bile acid intermediates. In addition, de novo synthesized bile acids are conjugated within the peroxisome. In this review, we describe the current state of knowledge about all aspects of peroxisomal function in bile acid biosynthesis in health and disease. The peroxisomal enzymes involved in the synthesis of bile acids have been identified, and the metabolic and pathologic consequences of a deficiency of one of these enzymes are discussed, including the potential role of nuclear receptors therein.

Generalised and conditional inactivation of Pex genes in mice

During the past 10 years, several Pex genes have been knocked out in the mouse with the purpose to generate models to study the pathogenesis of peroxisome biogenesis disorders and/or to investigate the physiological importance of the Pex proteins. More recently, mice with selective inactivation of a Pex gene in particular cell types were created. The metabolic abnormalities in peroxisome deficient mice paralleled to a large extent those of Zellweger patients. Several but not all of the clinical and histological features reported in patients also occurred in peroxisome deficient mice as for example hypotonia, cortical and cerebellar malformations, endochondral ossification defects, hepatomegaly, liver fibrosis and ultrastructural abnormalities of mitochondria in hepatocytes. Although the molecular origins of the observed pathologies have not yet been resolved, several new insights on the importance of peroxisomes in different tissues have emerged.

Peroxisomal cholesterol biosynthesis and Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS), caused by 7-dehydrocholesterol-reductase (DHCR7) deficiency, shows variable severity independent of DHCR7 genotype. To test whether peroxisomes are involved in alternative cholesterol synthesis, we used [1-(14)C]C24:0 for peroxisomal beta-oxidation to generate [1-(14)C]acetyl-CoA as cholesterol precursor inside peroxisomes. The HMG-CoA reductase inhibitor lovastatin suppressed cholesterol synthesis from [2-(14)C]acetate and [1-(14)C]C8:0 but not from [1-(14)C]C24:0, implicating a peroxisomal, lovastatin-resistant HMG-CoA reductase. In SLOS fibroblasts lacking DHCR7 activity, no cholesterol was formed from [1-(14)C]C24:0-derived [1-(14)C]acetyl-CoA, indicating that the alternative peroxisomal pathway also requires this enzyme. Our results implicate peroxisomes in cholesterol biosynthesis but provide no link to phenotypic variation in SLOS.

"New" hepatic fat activates PPARalpha to maintain glucose, lipid, and cholesterol homeostasis

De novo lipogenesis is an energy-expensive process whose role in adult mammals is poorly understood. We generated mice with liver-specific inactivation of fatty-acid synthase (FAS), a key lipogenic enzyme. On a zero-fat diet, FASKOL (FAS knockout in liver) mice developed hypoglycemia and fatty liver, which were reversed with dietary fat. These phenotypes were also observed after prolonged fasting, similarly to fasted PPARalpha-deficiency mice. Hypoglycemia, fatty liver, and defects in expression of PPARalpha target genes in FASKOL mice were corrected with a PPARalpha agonist. On either zero-fat or chow diet, FASKOL mice had low serum and hepatic cholesterol levels with elevated SREBP-2, decreased HMG-CoA reductase expression, and decreased cholesterol biosynthesis; these were also corrected with a PPARalpha agonist. These results suggest that products of the FAS reaction regulate glucose, lipid, and cholesterol metabolism by serving as endogenous activators of distinct physiological pools of PPARalpha in adult liver.

p90Rsk is not involved in cytostatic factor arrest in mouse oocytes

Vertebrate oocytes arrest in metaphase of the second meiotic division (MII), where they maintain a high cdc2/cyclin B activity and a stable, bipolar spindle because of cytostatic factor (CSF) activity. The Mos-MAPK pathway is essential for establishing CSF. Indeed, oocytes from the mos-/- strain do not arrest in MII and activate without fertilization, as do Xenopus laevis oocytes injected with morpholino oligonucleotides directed against Mos. In Xenopus oocytes, p90Rsk (ribosomal S6 kinase), a MAPK substrate, is the main mediator of CSF activity. We show here that this is not the case in mouse oocytes. The injection of constitutively active mutant forms of Rsk1 and Rsk2 does not induce a cell cycle arrest in two-cell mouse embryos. Moreover, these two mutant forms do not restore MII arrest after their injection into mos-/- oocytes. Eventually, oocytes from the triple Rsk (1, 2, 3) knockout present a normal CSF arrest. We demonstrate that p90Rsk is not involved in the MII arrest of mouse oocytes.

Absence of peroxisomes in mouse hepatocytes causes mitochondrial and ER abnormalities

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-alpha) regulated genes indicated that PPAR-alpha is activated in the peroxisome-deficient 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).

Intracellular cholesterol transport

Intracellular cholesterol transport is essential for the maintenance of cholesterol homeostasis. Many aspects of cholesterol metabolism are well-known, including its synthesis in the endoplasmic reticulum, its extracellular transport in plasma lipoproteins, its uptake by the low-density lipoprotein receptor, and its regulation of SREBP and LXR transcription factors. These fundamental pathways in cholesterol metabolism all rely on its proper intracellular distribution among subcellular organelles and the plasma membrane. Transport involving the ER and endosomes is essential for cholesterol synthesis, uptake, and esterification, whereas cholesterol catabolism by enzymes in mitochondria and ER generates steroids, bile acids, and oxysterols. Cholesterol is a highly hydrophobic lipid that requires specialized transport in the aqueous cytosol, involving either vesicles or nonvesicular mechanisms. The latter includes hydrophobic cavity transporters such as StAR-related lipid transfer (START) proteins. Molecular understanding of intracellular cholesterol trafficking has lagged somewhat behind other aspects of cholesterol metabolism, but recent advances have defined some transport pathways and candidate proteins. In this review, we discuss cholesterol transport among specific intracellular compartments, emphasizing the relevance of these pathways to cholesterol homeostasis.