Sterol carrier protein X (SCPx) is a peroxisomal branched-chain beta-ketothiolase specifically reacting with 3-oxo-pristanoyl-CoA: a new, unique role for SCPx in branched-chain fatty acid metabolism in peroxisomes

Fatty Acid Metabolism in Peroxisomes and Related Disorders

One of the functions of peroxisomes is the oxidation of fatty acids (FAs). The importance of this function in our lives is evidenced by the presence of peroxisomal disorders caused by the genetic deletion of proteins involved in these processes. Unlike mitochondrial oxidation, peroxisomal oxidation is not directly linked to ATP production. What is the role of FA oxidation in peroxisomes? Recent studies have revealed that peroxisomes supply the building blocks for lipid synthesis in the endoplasmic reticulum and facilitate intracellular carbon recycling for membrane quality control. Accumulation of very long-chain fatty acids (VLCFAs), which are peroxisomal substrates, is a diagnostic marker in many types of peroxisomal disorders. However, the relationship between VLCFA accumulation and various symptoms of these disorders remains unclear. Recently, we developed a method for solubilizing VLCFAs in aqueous media and found that VLCFA toxicity could be mitigated by oleic acid replenishment. In this chapter, we present the physiological role of peroxisomal FA oxidation and the knowledge obtained from VLCFA-accumulating peroxisome-deficient cells.

Hepatocyte-Specific PEX16 Abrogation in Mice Leads to Hepatocyte Proliferation, Alteration of Hepatic Lipid Metabolism, and Resistance to High-Fat Diet (HFD)-Induced Hepatic Steatosis and Obesity

Obesity results in hepatic fat accumulation, i.e., steatosis. In addition to fat overload, impaired fatty acid β-oxidation also promotes steatosis. Fatty acid β-oxidation takes place in the mitochondria and peroxisomes. Usually, very long-chain and branched-chain fatty acids are the first to be oxidized in peroxisomes, and the resultant short chain fatty acids are further oxidized in the mitochondria. Peroxisome biogenesis is regulated by peroxin 16 (PEX16). In liver-specific PEX16 knockout (Pex16Alb-Cre) mice, hepatocyte peroxisomes were absent, but hepatocytes proliferated, and liver mass was enlarged. These results suggest that normal liver peroxisomes restrain hepatocyte proliferation and liver sizes. After high-fat diet (HFD) feeding, body weights were increased in PEX16 floxed (Pex16fl/fl) mice and adipose-specific PEX16 knockout (Pex16AdipoQ-Cre) mice, but not in the Pex16Alb-Cre mice, suggesting that the development of obesity is regulated by liver PEX16 but not by adipose PEX16. HFD increased liver mass in the Pex16fl/fl mice but somehow reduced the already enlarged liver mass in the Pex16Alb-Cre mice. The basal levels of serum triglyceride, free fatty acids, and cholesterol were decreased, whereas serum bile acids were increased in the Pex16Alb-Cre mice, and HFD-induced steatosis was not observed in the Pex16Alb-Cre mice. These results suggest that normal liver peroxisomes contribute to the development of liver steatosis and obesity.

Role of STAR and SCP2/SCPx in the Transport of Cholesterol and Other Lipids

Cholesterol is a lipid molecule essential for several key cellular processes including steroidogenesis. As such, the trafficking and distribution of cholesterol is tightly regulated by various pathways that include vesicular and non-vesicular mechanisms. One non-vesicular mechanism is the binding of cholesterol to cholesterol transport proteins, which facilitate the movement of cholesterol between cellular membranes. Classic examples of cholesterol transport proteins are the steroidogenic acute regulatory protein (STAR; STARD1), which facilitates cholesterol transport for acute steroidogenesis in mitochondria, and sterol carrier protein 2/sterol carrier protein-x (SCP2/SCPx), which are non-specific lipid transfer proteins involved in the transport and metabolism of many lipids including cholesterol between several cellular compartments. This review discusses the roles of STAR and SCP2/SCPx in cholesterol transport as model cholesterol transport proteins, as well as more recent findings that support the role of these proteins in the transport and/or metabolism of other lipids.

SCP2 variant is associated with alterations in lipid metabolism, brainstem neurodegeneration, and testicular defects

BACKGROUND

The detoxification of very long-chain and branched-chain fatty acids and the metabolism of cholesterol to form bile acids occur largely through a process called peroxisomal β-oxidation. Mutations in several peroxisomal proteins involved in β-oxidation have been reported, resulting in diseases characterized by neurological defects. The final step of the peroxisomal β-oxidation pathway is catalyzed by sterol carrier protein-x (SCPx), which is encoded by the SCP2 gene. Previously, there have been two reports of SCPx deficiency, which resulted from a homozygous or compound heterozygous SCP2 mutation. We report herein the first patient with a heterozygous SCP2 mutation leading to SCPx deficiency.

RESULTS

Clinical presentations of the patient included progressive brainstem neurodegeneration, cardiac dysrhythmia, muscle wasting, and azoospermia. Plasma fatty acid analysis revealed abnormal values of medium-, long-, and very long-chain fatty acids. Protein expression of SCPx and other enzymes involved in β-oxidation were altered between patient and normal fibroblasts. RNA sequencing and lipidomic analyses identified metabolic pathways that were altered between patient and normal fibroblasts including PPAR signaling, serotonergic signaling, steroid biosynthesis, and fatty acid degradation. Treatment with fenofibrate or 4-hydroxytamoxifen increased SCPx levels, and certain fatty acid levels in patient fibroblasts.

CONCLUSIONS

These findings suggest that the patient's SCP2 mutation resulted in decreased protein levels of SCPx, which may be associated with many metabolic pathways. Increasing SCPx levels through pharmacological interventions may reverse some effects of SCPx deficiency. Collectively, this work provides insight into many of the clinical consequences of SCPx deficiency and provides evidence for potential treatment strategies.

A Pex7 Deficient Mouse Series Correlates Biochemical and Neurobehavioral Markers to Genotype Severity—Implications for the Disease Spectrum of Rhizomelic Chondrodysplasia Punctata Type 1

Rhizomelic chondrodysplasia punctata type 1 (RCDP1) is a peroxisome biogenesis disorder caused by defects in PEX7 leading to impairment in plasmalogen (Pls) biosynthesis and phytanic acid (PA) oxidation. Pls deficiency is the main pathogenic factor that determines the severity of RCDP. Severe (classic) RCDP patients have negligible Pls levels, congenital cataracts, skeletal dysplasia, growth and neurodevelopmental deficits, and cerebral hypomyelination and cerebellar atrophy on brain MRI. Individuals with milder or nonclassic RCDP have higher Pls levels, better growth and cognitive outcomes. To better understand the pathophysiology of RCDP disorders, we generated an allelic series of Pex7 mice either homozygous for the hypomorphic allele, compound heterozygous for the hypomorphic and null alleles or homozygous for the null allele. Pex7 transcript and protein were almost undetectable in the hypomorphic model, and negligible in the compound heterozygous and null mice. Pex7 deficient mice showed a graded reduction in Pls and increases in C26:0-LPC and PA in plasma and brain according to genotype. Neuropathological evaluation showed significant loss of cerebellar Purkinje cells over time and a decrease in brain myelin basic protein (MBP) content in Pex7 deficient models, with more severe effects correlating with Pex7 genotype. All Pex7 deficient mice exhibited a hyperactive behavior in the open field environment. Brain neurotransmitters analysis of Pex7 deficient mice showed a significant reduction in levels of dopamine, norepinephrine, serotonin and GABA. Also, a significant correlation was found between brain neurotransmitter levels, the hyperactivity phenotype, Pls level and the severity of Pex7 genotype. In conclusion, our study showed evidence of a genotype-phenotype correlation between the severity of Pex7 deficiency and several clinical and neurobiochemical phenotypes in RCDP1 mouse models. We propose that PA accumulation may underlie the cerebellar atrophy seen in older RCDP1 patients, as even relatively low tissue levels were strongly associated with Purkinje cells loss over time in the murine models. Also, our data demonstrate the interrelation between Pls, brain neurotransmitter deficiencies and the neurobehavioral phenotype, which could be further used as a valuable clinical endpoint for therapeutic interventions. Finally, these models show that incremental increases in Pex7 levels result in dramatic improvements in phenotype.

Clinical, biochemical, and molecular characterization of mild (nonclassic) rhizomelic chondrodysplasia punctata

Rhizomelic chondrodysplasia punctata (RCDP) is a heterogenous group of disorders due to defects in genes encoding peroxisomal proteins required for plasmalogen (PL) biosynthesis, specifically PEX7 and PEX5 receptors, or GNPAT, AGPS and FAR1 enzymes. Most patients have congenital cataract and skeletal dysplasia. In the classic form, there is profound growth restriction and psychomotor delays, with most patients not advancing past infantile developmental milestones. Disease severity correlates to erythrocyte PL levels, which are almost undetectable in severe (classic) RCDP. In milder (nonclassic) forms, residual PL levels are associated with improved growth and development. However, the clinical course of this milder group remains largely unknown as only a few cases were reported. Using as inclusion criteria the ability to communicate and walk, we identified 16 individuals from five countries, ages 5-37 years, and describe their clinical, biochemical and molecular profiles. The average age at diagnosis was 2.6 years and most had cataract, growth deficiency, joint contractures, and developmental delays. Other major symptoms were learning disability (87%), behavioral issues (56%), seizures (43%), and cardiac defects (31%). All patients had decreased C16:0 PL levels that were higher than in classic RCDP, and up to 43% of average controls. Plasma phytanic acid levels were elevated in most patients. There were several common, and four novel, PEX7, and GNPAT hypomorphic alleles in this cohort. These results can be used to support earlier diagnosis and improve management in patients with mild RCDP.

Fatty Acid Oxidation in Peroxisomes: Enzymology, Metabolic Crosstalk with Other Organelles and Peroxisomal Disorders

Peroxisomes play a central role in metabolism as exemplified by the fact that many genetic disorders in humans have been identified through the years in which there is an impairment in one or more of these peroxisomal functions, in most cases associated with severe clinical signs and symptoms. One of the key functions of peroxisomes is the β-oxidation of fatty acids which differs from the oxidation of fatty acids in mitochondria in many respects which includes the different substrate specificities of the two organelles. Whereas mitochondria are the main site of oxidation of medium-and long-chain fatty acids, peroxisomes catalyse the β-oxidation of a distinct set of fatty acids, including very-long-chain fatty acids, pristanic acid and the bile acid intermediates di- and trihydroxycholestanoic acid. Peroxisomes require the functional alliance with multiple subcellular organelles to fulfil their role in metabolism. Indeed, peroxisomes require the functional interaction with lysosomes, lipid droplets and the endoplasmic reticulum, since these organelles provide the substrates oxidized in peroxisomes. On the other hand, since peroxisomes lack a citric acid cycle as well as respiratory chain, oxidation of the end-products of peroxisomal fatty acid oxidation notably acetyl-CoA, and different medium-chain acyl-CoAs, to CO₂ and H₂O can only occur in mitochondria. The same is true for the reoxidation of NADH back to NAD⁺. There is increasing evidence that these interactions between organelles are mediated by tethering proteins which bring organelles together in order to allow effective exchange of metabolites. It is the purpose of this review to describe the current state of knowledge about the role of peroxisomes in fatty acid oxidation, the transport of metabolites across the peroxisomal membrane, its functional interaction with other subcellular organelles and the disorders of peroxisomal fatty acid β-oxidation identified so far in humans.

A Mouse Model System to Study Peroxisomal Roles in Neurodegeneration of Peroxisome Biogenesis Disorders

Fourteen PEX genes are currently identified as genes responsible for peroxisome biogenesis disorders (PBDs). Patients with PBDs manifest as neurodegenerative symptoms such as neuronal migration defect and malformation of the cerebellum. To address molecular mechanisms underlying the pathogenesis of PBDs, mouse models for the PBDs have been generated by targeted disruption of Pex genes. Pathological phenotypes and metabolic abnormalities in Pex-knockout mice well resemble those of the patients with PBDs. The mice with tissue- or cell type-specific inactivation of Pex genes have also been established by using a Cre-loxP system. The genetically modified mice reveal that pathological phenotypes of PBDs are mediated by interorgan and intercellular communications. Despite the illustrations of detailed pathological phenotypes in the mutant mice, mechanistic insights into pathogenesis of PBDs are still underway. In this chapter, we overview the phenotypes of Pex-inactivated mice and the current understanding of the pathogenesis underlying PBDs.

Sex-dependent impact of Scp-2/Scp-x gene ablation on hepatic phytol metabolism

While prior studies focusing on male mice suggest a role for sterol carrier protein-2/sterol carrier protein-x (SCP-2/SCP-x; DKO) on hepatic phytol metabolism, its role in females is unresolved. This issue was addressed using female and male wild-type (WT) and DKO mice fed a phytoestrogen-free diet without or with 0.5% phytol. GC/MS showed that hepatic: i) phytol was absent and its branched-chain fatty acid (BCFA) metabolites were barely detectable in WT control-fed mice; ii) accumulation of phytol as well as its peroxisomal metabolite BCFAs (phytanic acid » pristanic and 2,3-pristenic acids) was increased by dietary phytol in WT females, but only slightly in WT males; iii) accumulation of phytol and BCFA was further increased by DKO in phytol-fed females, but much more markedly in males. Livers of phytol-fed WT female mice as well as phytol-fed DKO female and male mice also accumulated increased proportion of saturated straight-chain fatty acids (LCFA) at the expense of unsaturated LCFA. Liver phytol accumulation was not due to increased SCP-2 binding/transport of phytol since SCP-2 bound phytanic acid, but not its precursor phytol. Thus, the loss of Scp-2/Scp-x contributed to a sex-dependent hepatic accumulation of dietary phytol and BCFA.

Effect of Fabp1/Scp-2/Scp-x Ablation on Whole Body and Hepatic Phenotype of Phytol-Fed Male Mice

Liver fatty acid binding protein (Fabp1) and sterol carrier protein-2/sterol carrier protein-x (SCP-2/SCP-x) genes encode proteins that enhance hepatic uptake, cytosolic transport, and peroxisomal oxidation of toxic branched-chain fatty acids derived from dietary phytol. Since male wild-type (WT) mice express markedly higher levels of these proteins than females, the impact of ablating both genes (TKO) was examined in phytol-fed males. In WT males, high phytol diet alone had little impact on whole body weight and did not alter the proportion of lean tissue mass (LTM) versus fat tissue mass (FTM). TKO conferred on dietary phytol the ability to induce weight loss as well as reduce liver weight, FTM, and even more so LTM. Concomitantly TKO induced hepatic lipid accumulation, preferentially threefold increased phospholipid (PL) at the expense of decreased triacylglycerol (TG) and total cholesterol. Increased PL was associated with upregulation of membrane fatty acid transport/translocase proteins (FATP 2,4), cytosolic fatty acid/fatty acyl-CoA binding proteins (FABP2, ACBP), and the rate limiting enzyme in PL synthesis (Gpam). Decreased TG and cholesterol levels were not attributable to altered levels in respective synthetic enzymes or nuclear receptors. These data suggest that the higher level of Fabp1 and Scp2/Scpx gene products in WT males was protective against deleterious effects of dietary phytol, but TKO significantly exacerbated phytol effects in males.

Impact of dietary phytol on lipid metabolism in SCP2/SCPX/L-FABP null mice

In vitro studies suggest that liver fatty acid binding protein (L-FABP) and sterol carrier protein-2/sterol carrier protein-x (SCP2/SCPx) gene products facilitate uptake and metabolism and detoxification of dietary-derived phytol in mammals. However, concomitant upregulation of L-FABP in SCP2/SCPx null mice complicates interpretation of their physiological phenotype. Therefore, the impact of ablating both the L-FABP gene and SCP2/SCPx gene (L-FABP/SCP2/SCPx null or TKO) was examined in phytol-fed female wild-type (WT) and TKO mice. TKO increased hepatic total lipid accumulation, primarily phospholipid, by mechanisms involving increased hepatic levels of proteins in the phospholipid synthetic pathway. Concomitantly, TKO reduced expression of proteins in targeting fatty acids towards the triacylglycerol synthetic pathway. Increased hepatic lipid accumulation was not associated with any concomitant upregulation of membrane fatty acid transport/translocase proteins involved in fatty acid uptake (FATP2, FATP4, FATP5 or GOT) or cytosolic proteins involved in fatty acid intracellular targeting (ACBP). In addition, TKO exacerbated dietary phytol-induced whole body weight loss, especially lean tissue mass. Since individually ablating SCPx or SCP2/SCPx elicited concomitant upregulation of L-FABP, these findings with TKO mice help to resolve the contributions of SCP2/SCPx gene ablation on dietary phytol-induced whole body and hepatic lipid phenotype independent of concomitant upregulation of L-FABP.

Structural and functional characterization of a novel gene, Hc-daf-22, from the strongylid nematode Haemonchus contortus

BACKGROUND

The strongylid nematode Haemonchus contortus is a parasite of major concern for modern livestock husbandry because hostile environmental conditions may induce diapause in the early fourth-stage larvae.

METHODS

A new gene Hc-daf-22 was identified which is the homologue of Ce-daf-22 and human SCPx. Genome walking and RACE were performed to obtain the whole cDNA and genomic sequence of this gene. Using qRT-PCR with all developmental stages as templates to explore the transcription level and micro-injection was applied to confirm the promoter activity of the 5'-flanking region. Overexpression, rescue and RNA interference experiments were performed in N2, daf-22 mutant (ok 693) strains of C. elegans to study the gene function of Hc-daf-22.

RESULTS

The full length gene of Hc-daf-22 (6,939 bp) contained 16 exons separated by 15 introns, and encoded a cDNA of 1,602 bp (533 amino acids, estimated at about 59.3 kDa) with a peak in L3 and L4 in transcriptional level. The Hc-DAF-22 protein was consisted of a 3-oxoacyl-CoA thiolase domain and a SCP2 domain and evolutionarily conserved. The 1,548 bp fragment upstream of the 5'-flanking region was confirmed to have promoter activity compared with 5'-flanking region of Ce-daf-22. The rescue experiment by micro-injection of daf-22 (ok693) mutant strain showed significant increase in body size and brood size in the rescued worms with significantly reduced or completely absent fat granules confirmed by Oil red O staining, indicating that Hc-daf-22 could partially rescue the function of Ce-daf-22. Furthermore, RNAi with Hc-daf-22 could partially silence the endogenous Ce-daf-22 in N2 worms and mimic the phenotype of daf-22 (ok693) mutants.

CONCLUSION

The gene Hc-daf-22 was isolated and its function identified using C. elegans as a model organism. Our results indicate that Hc-daf-22 shared similar characteristics and function with Ce-daf-22 and may play an important role in peroxisomal β-oxidation and the development in H. contortus.

Metabolic Interplay between Peroxisomes and Other Subcellular Organelles Including Mitochondria and the Endoplasmic Reticulum

Peroxisomes are unique subcellular organelles which play an indispensable role in several key metabolic pathways which include: (1.) etherphospholipid biosynthesis; (2.) fatty acid beta-oxidation; (3.) bile acid synthesis; (4.) docosahexaenoic acid (DHA) synthesis; (5.) fatty acid alpha-oxidation; (6.) glyoxylate metabolism; (7.) amino acid degradation, and (8.) ROS/RNS metabolism. The importance of peroxisomes for human health and development is exemplified by the existence of a large number of inborn errors of peroxisome metabolism in which there is an impairment in one or more of the metabolic functions of peroxisomes. Although the clinical signs and symptoms of affected patients differ depending upon the enzyme which is deficient and the extent of the deficiency, the disorders involved are usually (very) severe diseases with neurological dysfunction and early death in many of them. With respect to the role of peroxisomes in metabolism it is clear that peroxisomes are dependent on the functional interplay with other subcellular organelles to sustain their role in metabolism. Indeed, whereas mitochondria can oxidize fatty acids all the way to CO₂ and H₂O, peroxisomes are only able to chain-shorten fatty acids and the end products of peroxisomal beta-oxidation need to be shuttled to mitochondria for full oxidation to CO₂ and H₂O. Furthermore, NADH is generated during beta-oxidation in peroxisomes and beta-oxidation can only continue if peroxisomes are equipped with a mechanism to reoxidize NADH back to NAD⁺, which is now known to be mediated by specific NAD(H)-redox shuttles. In this paper we describe the current state of knowledge about the functional interplay between peroxisomes and other subcellular compartments notably the mitochondria and endoplasmic reticulum for each of the metabolic pathways in which peroxisomes are involved.

Relative contributions of L-FABP, SCP-2/SCP-x, or both to hepatic biliary phenotype of female mice

Both sterol carrier protein-2/sterol carrier protein-x (SCP-2/SCP-x) and liver fatty acid binding protein (L-FABP) have been proposed to function in hepatobiliary bile acid metabolism/accumulation. To begin to address this issue, the impact of ablating L-FABP (LKO) or SCP-2/SCP-x (DKO) individually or both together (TKO) was examined in female mice. Biliary bile acid levels were decreased in LKO, DKO, and TKO mice; however, hepatic bile acid concentration was decreased in LKO mice only. In contrast, biliary phospholipid level was decreased only in TKO mice, while biliary cholesterol levels were unaltered regardless of phenotype. The loss of either or both genes increased hepatic expression of the major bile acid synthetic enzymes (CYP7A1 and/or CYP27A1). Loss of L-FABP and/or SCP-2/SCP-x genes significantly altered the molecular composition of biliary bile acids, but not the proportion of conjugated/unconjugated bile acids or overall bile acid hydrophobicity index. These data suggested that L-FABP was more important in hepatic retention of bile acids, while SCP-2/SCP-x more broadly affected biliary bile acid and phospholipid levels.

Loss of L-FABP, SCP-2/SCP-x, or both induces hepatic lipid accumulation in female mice

Although roles for both sterol carrier protein-2/sterol carrier protein-x (SCP-2/SCP-x) and liver fatty acid binding protein (L-FABP) have been proposed in hepatic lipid accumulation, individually ablating these genes has been complicated by concomitant alterations in the other gene product(s). For example, ablating SCP2/SCP-x induces upregulation of L-FABP in female mice. Therefore, the impact of ablating SCP-2/SCP-x (DKO) or L-FABP (LKO) individually or both together (TKO) was examined in female mice. Loss of SCP-2/SCP-x (DKO, TKO) more so than loss of L-FABP alone (LKO) increased hepatic total lipid and total cholesterol content, especially cholesteryl ester. Hepatic accumulation of nonesterified long chain fatty acids (LCFA) and phospholipids occurred only in DKO and TKO mice. Loss of SCP-2/SCP-x (DKO, TKO) increased serum total lipid primarily by increasing triglycerides. Altered hepatic level of proteins involved in cholesterol uptake, efflux, and/or secretion was observed, but did not compensate for the loss of L-FABP, SCP-2/SCP-x or both. However, synergistic responses were not seen with the combinatorial knock out animals-suggesting that inhibiting SCP-2/SCP-x is more correlative with hepatic dysfunction than L-FABP. The DKO- and TKO-induced hepatic accumulation of cholesterol and long chain fatty acids shared significant phenotypic similarities with non-alcoholic fatty liver disease (NAFLD).

Genetic and chemical characterization of ibuprofen degradation by Sphingomonas Ibu-2

Sphingomonas Ibu-2 has the unusual ability to cleave the acid side chain from the pharmaceutical ibuprofen and related arylacetic acid derivatives to yield corresponding catechols under aerobic conditions via a previously uncharacterized mechanism. Screening a chromosomal library of Ibu-2 DNA in Escherichia coli EPI300 allowed us to identify one fosmid clone (pFOS3G7) that conferred the ability to metabolize ibuprofen to isobutylcatechol. Characterization of pFOS3G7 loss-of-function transposon mutants permitted identification of five ORFs, ipfABDEF, whose predicted amino acid sequences bore similarity to the large and small units of an aromatic dioxygenase (ipfAB), a sterol carrier protein X (SCPx) thiolase (ipfD), a domain of unknown function 35 (DUF35) protein (ipfE) and an aromatic CoA ligase (ipfF). Two additional ORFs, ipfH and ipfI, which encode putative ferredoxin reductase and ferredoxin components of an aromatic dioxygenase system, respectively, were also identified on pFOS3G7. Complementation of a markerless loss-of-function ipfD deletion mutant restored catechol production as did complementation of the ipfF Tn mutant. Expression of subcloned ipfABDEF alone in E. coli did not impart full metabolic activity unless coexpressed with ipfHI. CoA ligation followed by ring oxidation is common to phenylacetic acid pathways. However, the need for a putative SCPx thiolase (IpfD) and DUF35 protein (IpfE) in aerobic arylacetic acid degradation is unprecedented. This work provides preliminary insights into the mechanism behind this novel arylacetic acid-deacylating, catechol-generating activity.

Molecular characterization of ltp3 and ltp4, essential for C24-branched chain sterol-side-chain degradation in Rhodococcus rhodochrous DSM 43269

A previously identified sterol catabolic gene cluster is widely dispersed among actinobacteria, enabling them to degrade and grow on naturally occurring sterols. We investigated the physiological roles of various genes by targeted inactivation in mutant RG32 of Rhodococcus rhodochrous, which selectively degrades sterol side-chains. The ltp3 and ltp4 deletion mutants were each completely blocked in side-chain degradation of β-sitosterol and campesterol, but not of cholesterol. These results indicated a role for ltp3 and ltp4 in the removal of C24 branches specifically. Bioinformatic analysis of the encoded Ltp3 and Ltp4 proteins revealed relatively high similarity to thiolase enzymes, typically involved in β-oxidation, but the catalytic residues characteristic for thiolase enzymes are not conserved in their amino acid sequences. Removal of the C24-branched side-chain carbons of β-sitosterol was previously shown to proceed via aldolytic cleavage rather than by β-oxidation. Our results therefore suggest that ltp3 and ltp4 probably encode aldol-lyases rather than thiolases. This is the first report, to our knowledge, on the molecular characterization of genes with specific and essential roles in carbon-carbon bond cleavage of C24-branched chain sterols in Rhodococcus strains, most likely acting as aldol-lyases. The results are a clear contribution to our understanding of sterol degradation in actinobacteria.

Biochemistry of mammalian peroxisomes revisited

In this review, we describe the current state of knowledge about the biochemistry of mammalian peroxisomes, especially human peroxisomes. The identification and characterization of yeast mutants defective either in the biogenesis of peroxisomes or in one of its metabolic functions, notably fatty acid beta-oxidation, combined with the recognition of a group of genetic diseases in man, wherein these processes are also defective, have provided new insights in all aspects of peroxisomes. As a result of these and other studies, the indispensable role of peroxisomes in multiple metabolic pathways has been clarified, and many of the enzymes involved in these pathways have been characterized, purified, and cloned. One aspect of peroxisomes, which has remained ill defined, is the transport of metabolites across the peroxisomal membrane. Although it is clear that mammalian peroxisomes under in vivo conditions are closed structures, which require the active presence of metabolite transporter proteins, much remains to be learned about the permeability properties of mammalian peroxisomes and the role of the four half ATP-binding cassette (ABC) transporters therein.

Host invasion during rice-blast disease requires carnitine-dependent transport of peroxisomal acetyl-CoA

In lower eukaryotes, beta-oxidation of fatty acids is restricted primarily to the peroxisomes and the resultant acetyl-CoA molecules (and the chain-shortened fatty acids) are transported via the cytosol into the mitochondria for further breakdown and usage. Using a loss-of-function mutation in the Magnaporthe grisea PEROXIN6 orthologue, we define an essential role for peroxisomal acetyl-CoA during the host invasion step of the rice-blast disease. We show that an Mgpex6Delta strain lacks functional peroxisomes and is incapable of beta-oxidation of long-chain fatty acids. The Mgpex6Delta mutant lacked appressorial melanin and host penetration, and was completely non-pathogenic. We further show that a peroxisome-associated carnitine acetyl-transferase (Crat1) activity is essential for such appressorial function in Magnaporthe. CRAT1-minus appressoria showed reduced melanization, but were surprisingly incapable of elaborating penetration pegs or infection hyphae. Exogenous addition of excess glucose during infection stage caused partial remediation of the pathogenicity defects in the crat1Delta strain. Moreover, Mgpex6Delta and crat1Delta mycelia showed increased sensitivity to Calcofluor white, suggesting that weakened cell wall biosynthesis in a glucose-deficient environment leads to appressorial dysfunction in these mutants. Interestingly, CRAT1 was itself essential for growth on acetate and long-chain fatty acids. Thus, carnitine-dependent metabolic activities associated with the peroxisomes, cooperatively facilitate the appressorial function of host invasion during rice-blast infections.

Peroxisomal disorders: genotype, phenotype, major neuropathologic lesions, and pathogenesis

Neurological dysfunction is a prominent feature of most peroxisomal disorders. Enormous progress in defining their gene defects has been achieved. The genes and gene products, peroxins (PEX), in five of the complementation groups have been defined. These studies confirm that Zellweger syndrome (ZS), neonatal adrenoleukodystrophy (NALD), and infantile Refsum disease (IRD) are a disease continuum. The gene defect in adreno-leukodystrophy (ALD) / adrenomyeloneuropathy (AMN) involves an integral peroxisomal membrane protein. Neuropathologic lesions are of three major classes: (i) abnormalities in neuronal migration or differentiation, (ii) defects in the formation or maintenance of central white matter, and (iii) postdevelopmental neuronal degenerations. The central white matter lesions are those of: (i) inflammatory demyelination, (ii) non-inflammatory dysmyelination, and (iii) non-specific reductions in myelin volume or staining with or without reactive astrocytosis. The neuronal degenerations are of two major types: (i) the axonopathy of AMN involving ascending and descending tracts of the spinal cord, and (ii) cerebellar atrophy in rhizomelic chondrodysplasia punctata and probably IRD. We postulate that the abnormal fatty acids in peroxisomal disorders, particularly very long chain fatty acids and phytanic acid, are incorporated into cell membranes and perturb their microenvironments resulting in dysfunction, atrophy and death of vulnerable cells. The advent of mouse models for ZS and ALD is anticipated to provide even greater pathogenetic insights into the peroxisomal disorders.

Sterol homeostasis in the budding yeast, Saccharomyces cerevisiae

Lipid related diseases, such as obesity, type 2 diabetes, and atherosclerosis are epidemics in developed civilizations. A common underlying factor among these syndromes is excessive subcellular accumulation of lipids such as cholesterol and triglyceride. The homeostatic events that govern these metabolites are understood to varying degrees of sophistication. We describe here the utilization of a genetically powerful model organism, budding yeast, to identify and characterize novel aspects of sterol and lipid homeostasis.

Identification of fatty acid oxidation disorder patients with lowered acyl-CoA thioesterase activity in human skin fibroblasts

BACKGROUND

Acyl-CoA thioesterases are enzymes that hydrolyze acyl-CoAs to the free fatty acid and coenzyme A (CoASH). These enzymes have been identified in several cellular compartments and are thought to regulate intracellular levels of acyl-CoAs, free fatty acids and CoASH. However, to date no patients deficient in acyl-CoA thioesterases have been identified.

DESIGN

Acyl-CoA thioesterase activity was measured in human skin fibroblasts. Western-blot analysis was used to determine Type-II acyl-CoA thioesterase protein levels in patients.

RESULTS

Acyl-CoA thioesterase activity was found in human fibroblasts with all saturated acyl-CoAs from C4-CoA to C18-CoA, with highest activity detected with lauroyl-CoA and myristoyl-CoA (C12-CoA and C14-CoA). An antibody that recognizes the major isoforms of Type-II acyl-CoA thioesterases precipitated the majority of acyl-CoA thioesterase activity in fibroblasts, showing that the main thioesterase activity detected in fibroblasts is catalyzed by Type-II thioesterases. Measurement of acyl-CoA thioesterase activity from fibroblasts of 34 patients with putative fatty acid oxidation disorders resulted in the identification of three patients with lowered Type-II acyl-CoA thioesterase activity in fibroblasts. These patients also had lowered expression of Type-II acyl-CoA thioesterase protein in fibroblasts as judged by Western-blot analysis. However, mutation analysis failed to identify any mutation in the coding sequences for the mitochondrial acyl-CoA thioesterase II (MTE-II) or the cytosolic acyl-CoA thioesterase II (CTE-II).

CONCLUSIONS

We have described three patients with lowered Type-II acyl-CoA thioesterase protein and activity in human skin fibroblasts, which is the first description of patients with a putative defect in acyl-CoA thioesterases.

Sexually dimorphic metabolism of branched-chain lipids in C57BL/6J mice

Despite the importance of branched chain lipid oxidation in detoxification, almost nothing is known regarding factors regulating peroxisomal uptake, targeting, and metabolism. One peroxisomal protein, sterol carrier protein-x (SCP-x), is thought to catalyze a key thiolytic step in branched chain lipid oxidation. When mice with substantially lower hepatic levels of SCP-x were tested for susceptibility to dietary stress with phytol (a phytanic acid precursor and peroxisome proliferator), livers of phytol-fed female but not male mice i). accumulated phytol metabolites (phytanic acid, pristanic acid, and Delta-2,3-pristanic acid); ii). exhibited decreased fat tissue mass and increased liver mass/body mass; iii). displayed signs of histopathological lesions in the liver; and iv). demonstrated significant alterations in hepatic lipid distributions. Moreover, both male and female mice exhibited phytol-induced peroxisomal proliferation, as demonstrated by liver morphology and upregulation of the peroxisomal protein catalase. In addition, levels of liver fatty acid binding protein, along with SCP-2 and SCP-x, increased, suggesting upregulation mediated by phytanic acid, a known ligand agonist of the peroxisomal proliferator-activated receptor alpha. In summary, the present work establishes a role for SCP-x in branched chain lipid catabolism and demonstrates a sexual dimorphic response to phytol, a precursor of phytanic acid, in lipid parameters and hepatotoxicity.

Regulation of sterol carrier protein gene expression by the forkhead transcription factor FOXO3a

The SCP gene encodes two proteins, sterol carrier protein X (SCPx) and SCP2, that are independently regulated by separate promoters. SCPx has been shown to be the thiolase involved in the breakdown of branched-chain fatty acids and in the biosynthesis of bile acids. The in vivo function of SCP2 however remains to be established. The transcriptional regulation of SCPx and SCP2 is unclear, but their promoter regions contain several putative regulatory domains. We show here that both SCPx and SCP2 are upregulated by the daf-16-like Forkhead transcription factor FOXO3a (also known as FKHRL1) on the level of promoter activity. It was recently described that Forkheads regulate protection against (oxidative) stress in both Caenorhabditis elegans and mammalian cells. We looked into a role for SCP2 in the cellular defense against oxidative damage and found that a fluorescent fatty acid analog bound to SCP2 is protected against H2O2/Cu2+-induced oxidative damage. We propose a model for the way in which SCP2 could protect fatty acids from peroxidation.

Sterol carrier protein-2/sterol carrier protein-x expression differentially alters fatty acid metabolism in L cell fibroblasts

Sterol carrier protein-2 (SCP-2) and SCP-x are ubiquitous proteins found in all mammalian tissues. Although both proteins interact with fatty acids, their relative contributions to the uptake, oxidation, and esterification of straight-chain (palmitic) and branched-chain (phytanic) fatty acids in living cells has not been resolved. Therefore, the effects of each gene product on fatty acid metabolism was individually examined. Based on the following, SCP-2 and SCP-x did not enhance the uptake/translocation of fatty acids across the plasma membrane into the cell: i) a 2-fold increase in phytanic and palmitic acid uptake was observed at long incubation times in SCP-2- and SCP-x-expressing cells, but no differences were observed at initial time points; ii) uptake of 2-bromo-palmitate, a nonoxidizable, poorly metabolizable fatty acid analog, was unaffected by SCP-2 or SCP-x overexpression; and iii) SCP-2 and SCP-x expression did not increase targeting of radiolabeled phytanic and palmitic acid to the unesterified fatty acid pool. Moreover, SCP-2 and SCP-x expression enhanced fatty acid uptake by stimulating the intracellular metabolism via fatty acid oxidation and esterification. In summary, these data showed for the first time that SCP-2 and SCP-x stimulate oxidation and esterification of branched-chain as well as straight-chain fatty acids in intact cells.

Expression of fatty acid binding proteins inhibits lipid accumulation and alters toxicity in L cell fibroblasts

High levels of saturated, branched-chain fatty acids are deleterious to cells and animals, resulting in lipid accumulation and cytotoxicity. Although fatty acid binding proteins (FABPs) are thought to be protective, this hypothesis has not previously been examined. Phytanic acid (branched chain, 16-carbon backbone) induced lipid accumulation in L cell fibroblasts similar to that observed with palmitic acid (unbranched, C(16)): triacylglycerol >> free fatty acid > cholesterol > cholesteryl ester >> phospholipid. Although expression of sterol carrier protein (SCP)-2, SCP-x, or liver FABP (L-FABP) in transfected L cells reduced [(3)H]phytanic acid uptake (57-87%) and lipid accumulation (21-27%), nevertheless [(3)H]phytanic acid oxidation was inhibited (74-100%) and phytanic acid toxicity was enhanced in the order L-FABP >> SCP-x > SCP-2. These effects differed markedly from those of [(3)H]palmitic acid, whose uptake, oxidation, and induction of lipid accumulation were not reduced by L-FABP, SCP-2, or SCP-x expression. Furthermore, these proteins did not enhance the cytotoxicity of palmitic acid. In summary, intracellular FABPs reduce lipid accumulation induced by high levels of branched-chain but not straight-chain saturated fatty acids. These beneficial effects were offset by inhibition of branched-chain fatty acid oxidation that correlated with the enhanced toxicity of high levels of branched-chain fatty acid.

Peroxisomal beta-oxidation and peroxisome proliferator-activated receptor alpha: an adaptive metabolic system

beta-Oxidation occurs in both mitochondria and peroxisomes. Mitochondria catalyze the beta-oxidation of the bulk of short-, medium-, and long-chain fatty acids derived from diet, and this pathway constitutes the major process by which fatty acids are oxidized to generate energy. Peroxisomes are involved in the beta-oxidation chain shortening of long-chain and very-long-chain fatty acyl-coenzyme (CoAs), long-chain dicarboxylyl-CoAs, the CoA esters of eicosanoids, 2-methyl-branched fatty acyl-CoAs, and the CoA esters of the bile acid intermediates di- and trihydroxycoprostanoic acids, and in the process they generate H2O2. Long-chain and very-long-chain fatty acids (VLCFAs) are also metabolized by the cytochrome P450 CYP4A omega-oxidation system to dicarboxylic acids that serve as substrates for peroxisomal beta-oxidation. The peroxisomal beta-oxidation system consists of (a) a classical peroxisome proliferator-inducible pathway capable of catalyzing straight-chain acyl-CoAs by fatty acyl-CoA oxidase, L-bifunctional protein, and thiolase, and (b) a second noninducible pathway catalyzing the oxidation of 2-methyl-branched fatty acyl-CoAs by branched-chain acyl-CoA oxidase (pristanoyl-CoA oxidase/trihydroxycoprostanoyl-CoA oxidase), D-bifunctional protein, and sterol carrier protein (SCP)x. The genes encoding the classical beta-oxidation pathway in liver are transcriptionally regulated by peroxisome proliferator-activated receptor alpha (PPAR alpha). Evidence derived from mice deficient in PPAR alpha, peroxisomal fatty acyl-CoA oxidase, and some of the other enzymes of the two peroxisomal beta-oxidation pathways points to the critical importance of PPAR alpha and of the classical peroxisomal fatty acyl-CoA oxidase in energy metabolism, and in the development of hepatic steatosis, steatohepatitis, and liver cancer.

Human metabolism of phytanic acid and pristanic acid

Phytanic acid is a methyl-branched fatty acid present in the human diet. Due to its structure, degradation by beta-oxidation is impossible. Instead, phytanic acid is oxidized by alpha-oxidation, yielding pristanic acid. Despite many efforts to elucidate the alpha-oxidation pathway, it remained unknown for more than 30 years. In recent years, the mechanism of alpha-oxidation as well as the enzymes involved in the process have been elucidated. The process was found to involve activation, followed by hydroxylase, lyase and dehydrogenase reactions. Part, if not all of the reactions were found to take place in peroxisomes. The final product of phytanic acid alpha-oxidation is pristanic acid. This fatty acid is degraded by peroxisomal beta-oxidation. After 3 steps of beta-oxidation in the peroxisome, the product is esterified to carnitine and shuttled to the mitochondrion for further oxidation. Several inborn errors with one or more deficiencies in the phytanic acid and pristanic degradation have been described. The clinical expressions of these disorders are heterogeneous, and vary between severe neonatal and often fatal symptoms and milder syndromes with late onset. Biochemically, these disorders are characterized by accumulation of phytanic and/or pristanic acid in tissues and body fluids. Several of the inborn errors involving phytanic acid and/or pristanic acid metabolism have been characterized on the molecular level.

Gene structure, intracellular localization, and functional roles of sterol carrier protein-2

Since its discovery three decades ago, sterol carrier protein-2 (SCP-2) has remained a fascinating protein whose physiological function in lipid metabolism remains an enigma. Its multiple proposed functions arise from its complex gene structure, post-translational processing, intracellular localization, and ligand specificity. The SCP-2 gene has two initiation sites coding for proteins that share a common 13 kDa SCP-2 C-terminus: (1) One site codes for 58 kDa SCP-x which is partially post-translationally cleaved to 13 kDa SCP-2 and a 45 kDa protein. (2) A second site codes for 15 kDa pro-SCP-2 which is completely post-translationally cleaved to 13 kDa SCP-2. Very little is yet known regarding how the relative proportions of the two transcripts are regulated. Although all three proteins contain a C-terminal SKL peroxisomal targeting sequence, it is unclear why all three proteins are not exclusively localized in peroxisomes. However, the recent demonstration that the SCP-2 N-terminal presequence in pro-SCP-2 dramatically modulated the intracellular targeting coded by the C-terminal peroxisomal targeting sequence may account for the observation that as much as half of total SCP-2 is localized outside the peroxisome. The tertiary and secondary structure of the 13 kDa SCP-2, but not that of 15 kDa pro-SCP-2 and 58 kDa SCP-x, are now resolved. Increasing evidence suggests that the 58 kDa SCP-x and 45 kDa proteins are peroxisomal 3-ketoacyl-CoA-thiolases involved in the oxidation of branched chain fatty acids. Since 15 kDa pro-SCP-2 is post-translationally completely cleaved to 13 kDa SCP-2, relatively little attention has been focused on this protein. Finally, although the 13 kDa SCP-2 is the most studied of these proteins, because it exhibits diversity of its ligand partners (fatty acids, fatty acyl CoAs, cholesterol, phospholipids), new potential physiological function(s) are still being proposed and questions regarding potential compensation by other proteins with overlapping specificity are only beginning to be resolved.

Prenatal and postnatal development of peroxisomal lipid-metabolizing pathways in the mouse

The ontogeny of the following peroxisomal metabolic pathways was evaluated in mouse liver and brain: alpha-oxidation, beta-oxidation and ether phospholipid synthesis. In mouse embryos lacking functional peroxisomes (PEX5(-/-) knock-out), a deficiency of plasmalogens and an accumulation of the very-long-chain fatty acid C(26:0) was observed in comparison with control littermates, indicating that ether phospholipid synthesis and beta-oxidation are already active at mid-gestation in the mouse. Northern analysis revealed that the enzymes required for the beta-oxidation of straight-chain substrates are present in liver and brain during embryonic development but that those responsible for the degradation of branched-chain substrates are present only in liver from late gestation onwards. The expression pattern of transcripts encoding enzymes of the alpha-oxidation pathway suggested that alpha-oxidation is initiated in the liver around birth and is not active in brain throughout development. Remarkably, a strong induction of the mRNA levels of enzymes involved in alpha-oxidation and beta-oxidation was observed around birth in the liver. In contrast, enzyme transcripts that were expressed in brain were present at rather constant levels throughout prenatal and postnatal development. These results suggest that the defective ether phospholipid synthesis and/or peroxisomal beta-oxidation of straight-chain fatty acids might be involved in the pathogenesis of the prenatal organ defects in peroxisome-deficient mice and men.

Biogenesis of nonspecific lipid transfer protein and sterol carrier protein x: studies using peroxisome assembly-defective pex cell mutants

Nonspecific lipid transfer protein (nsLTP; also called sterol carrier protein 2) with a molecular mass of 13 kDa is synthesized as a larger 15-kDa precursor (pre-nsLTP) with an N-terminal 20-amino acid extension presequence, as well as with the peroxisome targeting signal type 1 (PTS1), Ala-Lys-Leu, at the C terminus. The precursor pre-nsLTP is processed to mature nsLTP by proteolytic removal of the presequence, most likely after being imported into peroxisomes. Sterol carrier protein x (SCPx), a 59-kDa branched-chain fatty acid thiolase of peroxisomes, contains the entire pre-nsLTP moiety at the C-terminal part and is converted to the 46-kDa form and nsLTP after the transport to peroxisomes. We investigated which of these two potential topogenic sequences functions in biogenesis of nsLTP and SCPx. Morphological and biochemical analyses, making use of Chinese hamster ovary cell pex mutants such as the PTS1 receptor-impaired pex5 and PTS2 import-defective pex7, as well as green fluorescent protein chimeras, revealed that both pre-nsLTP and SCPx are imported into peroxisomes by the Pex5p-mediated PTS1 pathway. Nearly half of the pre-nsLTP remains in the cytosol, as assessed by subcellular fractionation of the wild-type Chinese hamster ovary cells. In an in vitro binding assay, only mature nsLTP, but not pre-nsLTP, from the cell lysates interacted with the Pex5p. It is likely, therefore, that modulation of the C-terminal PTS1 by the presequence gives rise to cytoplasmic localization of pre-nsLTP.

Sterol carrier protein-2 localization in endoplasmic reticulum and role in phospholipid formation

Although sterol carrier protein-2 (SCP-2; also called nonspecific lipid transfer protein) binds fatty acids and fatty acyl-CoAs, its role in fatty acid metabolism is not fully understood. L-cell fibroblasts stably expressing SCP-2 were used to resolve the relationship between SCP-2 intracellular location and fatty acid transacylation in the endoplasmic reticulum. Indirect immunofluorescence double labeling and laser scanning confocal microscopy detected SCP-2 in peroxisomes > endoplasmic reticulum > mitochondria > lysosomes. SCP-2 enhanced incorporation of exogenous [(3)H]oleic acid into phospholipids and triacylglycerols of overexpressing cells 1.6- and 2.5-fold, respectively, stimulated microsomal incorporation of [1-(14)C]oleoyl-CoA into phosphatidic acid in vitro 13-fold, and exhibited higher specificity for unsaturated versus saturated fatty acyl-CoA. SCP-2 enhanced the rate-limiting step in microsomal phosphatidic acid biosynthesis mediated by glycerol-3-phosphate acyltransferase. SCP-2 also enhanced microsomal acyl-chain remodeling of phosphatidylethanolamine up to fivefold and phosphatidylserine twofold, depending on the specific fatty acyl-CoA, but had no effect on other phospholipid classes. In summary, these results were consistent with a role for SCP-2 in phospholipid synthesis in the endoplasmic reticulum.

Sterol carrier protein-2

The compartmentalization of cholesterol metabolism implies target-specific cholesterol trafficking between the endoplasmic reticulum, plasma membrane, lysosomes, mitochondria and peroxisomes. One hypothesis has been that sterol carrier protein-2 (SCP2, also known as the non-specific lipid transfer protein) acts in cholesterol transport through the cytoplasm. Recent studies employing gene targeting in mice showed, however, that mice lacking SCP2 and the related putative sterol carrier known as SCPx, develop a defect in peroxisomal beta-oxidation. In addition, diminished peroxisomal alpha-oxidation of phytanic acid (3,7,11, 15-tetramethylhexadecanoic acid) in these null mice was attributed to the absence of SCP2 which has a number of properties supporting a function as carrier for fatty acyl-CoAs rather than for sterols.

Inactivation of the peroxisomal multifunctional protein-2 in mice impedes the degradation of not only 2-methyl-branched fatty acids and bile acid intermediates but also of very long chain fatty acids

According to current views, peroxisomal beta-oxidation is organized as two parallel pathways: the classical pathway that is responsible for the degradation of straight chain fatty acids and a more recently identified pathway that degrades branched chain fatty acids and bile acid intermediates. Multifunctional protein-2 (MFP-2), also called d-bifunctional protein, catalyzes the second (hydration) and third (dehydrogenation) reactions of the latter pathway. In order to further clarify the physiological role of this enzyme in the degradation of fatty carboxylates, MFP-2 knockout mice were generated. MFP-2 deficiency caused a severe growth retardation during the first weeks of life, resulting in the premature death of one-third of the MFP-2(-/-) mice. Furthermore, MFP-2-deficient mice accumulated VLCFA in brain and liver phospholipids, immature C(27) bile acids in bile, and, after supplementation with phytol, pristanic and phytanic acid in liver triacylglycerols. These changes correlated with a severe impairment of peroxisomal beta-oxidation of very long straight chain fatty acids (C(24)), 2-methyl-branched chain fatty acids, and the bile acid intermediate trihydroxycoprostanic acid in fibroblast cultures or liver homogenates derived from the MFP-2 knockout mice. In contrast, peroxisomal beta-oxidation of long straight chain fatty acids (C(16)) was enhanced in liver tissue from MFP-2(-/-) mice, due to the up-regulation of the enzymes of the classical peroxisomal beta-oxidation pathway. The present data indicate that MFP-2 is not only essential for the degradation of 2-methyl-branched fatty acids and the bile acid intermediates di- and trihydroxycoprostanic acid but also for the breakdown of very long chain fatty acids.

Role and organization of peroxisomal beta-oxidation

In mammals, peroxisomes are involved in breakdown of very long chain fatty acids, prostanoids, pristanic acid, dicarboxylic fatty acids, certain xenobiotics and bile acid intermediates. Substrate spectrum and specificity studies of the four different beta-oxidation steps in rat and/or in man demonstrate that these substrates are degraded by separate beta-oxidation systems composed of different enzymes. In both species, the enzymes acting on straight chain fatty acids are palmitoyl-CoA oxidase, an L-specific multifunctional protein (MFP-1) and a dimeric thiolase. In liver, bile acid intermediates undergo one cycle of beta-oxidation catalyzed by trihydroxycoprostanoyl-CoA oxidase (in rat), or branched chain acyl-CoA oxidase (in man), a D-specific multifunctional protein (MFP-2) and SCPX-thiolase. Finally, pristanic acid is degraded in rat tissues by pristanoyl-CoA oxidase, the D-specific multifunctional protein-2 and SCPX-thiolase. Although in man a pristanoyl-CoA oxidase gene is present, so far its product has not been found. Hence, pristanoyl-CoA is believed to be desaturated in human tissues by the branched chain acyl-CoA oxidase. Due to the stereospecificity of the oxidases acting on 2-methyl-branched substrates, an additional enzyme, 2-methylacyl-CoA racemase, is required for the degradation of pristanic acid and the formation of bile acids.

Isolation and subunit composition of native sterol carrier protein 2/3-oxoacyl-coenzyme A thiolase from normal rat liver peroxisomes

In the present report we describe a method for the complete purification of native sterol carrier protein 2/3-oxoacyl-CoA thiolase (SCP-2/thiolase) from normal rat liver peroxisomes. The isolation procedure is based on the alteration in chromatographic properties of the enzyme in the presence of low concentrations of CoA. The purified preparation of SCP-2/thiolase consisted of 58- and 46-kDa polypeptides. Peroxisomes prepared freshly from normal rat liver contained three SCP-2/thiolase isoforms, separable by conventional chromatography. Immunochemical, molecular sieving, and chemical cross-linking experiments indicated that these isoforms represent thiolytically active homo- and heterodimeric combinations of the 46- and 58-kDa subunits (2 x 58, 58-46, and 2 x 46-kDa proteins).

Immunological detection of alkaline-diaminobenzidine-negativeperoxisomes of the nematode Caenorhabditis elegans purification and unique pH optima of peroxisomal catalase

We purified catalase-2 of the nematode Caenorhabditis elegans and identified peroxisomes in this organism. The peroxisomes of C. elegans were not detectable by cytochemical staining using 3, 3'-diaminobenzidine, a commonly used method depending on the peroxidase activity of peroxisomal catalase at pH 9 in which genuine peroxidases are inactive. The cDNA sequences of C. elegans predict two catalases very similar to each other throughout the molecule, except for the short C-terminal sequence; catalase-2 (500 residues long) carries a peroxisomal targeting signal 1-like sequence (Ser-His-Ile), whereas catalase-1 does not. The catalase purified to near homogeneity from the homogenate of C. elegans cells consisted of a subunit of 57 kDa and was specifically recognized by anti-(catalase-2) serum but not by anti-(catalase-1) serum. Subcellular fractionation and indirect immunoelectron microscopy of the nematode detected catalase-2 inside vesicles judged to be peroxisomes using morphological criteria. The purified enzyme (220 kDa) was tetrameric, similar to many catalases from various sources, but exhibited unique pH optima for catalase (pH 6) and peroxidase (pH 4) activities; the latter value is unusually low and explains why the peroxidase activity was undetectable using the standard alkaline diaminobenzidine-staining method. These results indicate that catalase-2 is peroxisomal and verify that it can be used as a marker enzyme for C. elegans peroxisomes.

Aberrant oxidation of the cholesterol side chain in bile acid synthesis of sterol carrier protein-2/sterol carrier protein-x knockout mice

Peroxisomal beta-oxidation plays an important role in the metabolism of a wide range of substrates, including various fatty acids and the steroid side chain in bile acid synthesis. Two distinct thiolases have been implicated to function in peroxisomal beta-oxidation: the long known 41-kDa beta-ketothiolase identified by Hashimoto and co-workers (Hijikata, M., Ishii, N., Kagamiyama, H., Osumi, T., and Hashimoto, T. (1987) J. Biol. Chem. 262, 8151-8158) and the recently discovered 60-kDa SCPx thiolase, that consists of an N-terminal domain with beta-ketothiolase activity and a C-terminal moiety of sterol carrier protein-2 (SCP2, a lipid carrier or transfer protein). Recently, gene targeting of the SCP2/SCPx gene has shown in mice that the SCPx beta-ketothiolase is involved in peroxisomal beta-oxidation of 2-methyl-branched chain fatty acids like pristanic acid. In our present work we have investigated bile acid synthesis in the SCP2/SCPx knockout mice. Specific inhibition of beta-oxidation at the thiolytic cleavage step in bile acid synthesis is supported by our finding of pronounced accumulation in bile and serum from the knockout mice of 3alpha,7alpha, 12alpha-trihydroxy-27-nor-5beta-cholestane-24-one (which is a known bile alcohol derivative of the cholic acid synthetic intermediate 3alpha,7alpha,12alpha-trihydroxy-24-keto-cholestano yl-coenzyme A). Moreover, these mice have elevated concentrations of bile acids with shortened side chains (i.e. 23-norcholic acid and 23-norchenodeoxycholic acid), which may be produced via alpha- rather than beta-oxidation. Our results demonstrate that the SCPx thiolase is critical for beta-oxidation of the steroid side chain in conversion of cholesterol into bile acids.

Type-II 3-oxoacyl-CoA thiolase of the nematode Caenorhabditis elegans is located in peroxisomes, highly expressed during larval stages and induced by clofibrate

We examined the expression and localization of type-II 3-oxoacyl-CoA thiolase in the nematode Caenorhabditis elegans. Type-II thiolase acts on 3-oxoacyl-CoA esters with a methyl group at the alpha carbon, whereas conventional thiolases do not. Mammalian type-II thiolase, which is also termed sterol carrier protein x (SCPx) or SCP2/3-oxoacyl-CoA thiolase, is located in the peroxisomes and involved in phytanic acid degradation and most probably in bile acid synthesis. The nematode enzyme lacks the SCP2 domain, which carries the peroxisomal-targeting signal, but produces bile acids in a cell-free system. Northern and Western blot analyses demonstrated that C. elegans expressed type-II thiolase throughout its life cycle, especially during the larval stages, and that the expression was significantly enhanced by the addition of clofibrate at 5 mM or more to the culture medium. Whole-mount in situ hybridization and immunostaining of L4 larvae revealed that the enzyme was mainly expressed in intestinal cells, which are multifunctional like many of the cell types in C. elegans. Subcellular fractionation and indirect immunoelectron microscopy of the nematode detected the enzyme in the matrix of peroxisomes. These results indicate the fundamental homology between mammalian SCPx and the nematode enzyme regardless of whether the SCP2 part is fused, suggesting their common physiological roles.

Enoyl-CoA hydratase deficiency: identification of a new type of D-bifunctional protein deficiency

D-bifunctional protein is involved in the peroxisomal beta-oxidation of very long chain fatty acids, branched chain fatty acids and bile acid intermediates. In line with the central role of D-bifunctional protein in the beta-oxidation of these three types of fatty acids, all patients with D-bifunctional protein deficiency so far reported in the literature show elevated levels of very long chain fatty acids, branched chain fatty acids and bile acid inter-mediates. In contrast, we now report two novel patients with D-bifunctional protein deficiency who both have normal levels of bile acid intermediates. Complementation analysis and D-bifunctional protein activity measurements revealed that both patients had an isolated defect in the enoyl-CoA hydratase domain of D-bifunctional protein. Subsequent mutation analysis showed that both patients are homozygous for a missense mutation (N457Y), which is located in the enoyl-CoA hydratase coding part of the D-bifunctional protein gene. Expression of the mutant protein in the yeast Saccharomyces cerevisiae confirmed that the N457Y mutation is the disease-causing mutation. Immunoblot analysis of patient fibroblast homogenates showed that the protein levels of full-length D-bifunctional protein were strongly reduced while the enoyl-CoA hydratase component produced after processing within the peroxisome was undetectable, which indicates that the mutation leads to an unstable protein.

Rhizomelic chondrodysplasia punctata, a peroxisomal biogenesis disorder caused by defects in Pex7p, a peroxisomal protein import receptor: a minireview

Rhizomelic chondrodysplasia punctata (RCDP) is a lethal autosomal recessive disease corresponding to complementation group 11 (CG11), the second most common of the thirteen CGs of peroxisomal biogenesis disorders (PBDs). RCDP is characterized by proximal limb shortening, severely disturbed endochondrial bone formation, and mental retardation, but there is an absence of the neuronal migration defect found in the other PBDs. Plasmalogen biosynthesis and phytanic acid oxidation are deficient, but very long chain fatty acid (VLCFA) oxidation is normal. At the cellular level, RCDP is unique in that the biogenesis of most peroxisomal proteins is normal, but a specific subset of at least four, and maybe more, peroxisomal matrix proteins fail to be imported from the cytosol. In this review, we discuss recent advances in understanding RCDP, most prominently the cloning of the affected gene, PEX7, and identification of PEX7 mutations in RCDP patients. Human PEX7 was identified by virtue of its sequence similarity to its Saccharomyces cerevisiae ortholog, which had previously been shown to encode Pex7p, an import receptor for type 2 peroxisomal targeting sequences (PTS2). Normal human PEX7 expression rescues the cellular defects in cultured RCDP cells, and cDNA sequence analysis has identified a variety of PEX7 mutations in RCDP patients, including a deletion of 100 nucleotides, probably due to a splice site mutation, and a prevalent nonsense mutation which results in loss of the carboxyterminal 32 amino acids. Identification of RCDP as a PTS2 import disorder explains the observation that several, but not all, peroxisomal matrix proteins are mistargeted in this disease; three of the four proteins deficient in RCDP have now been shown to be PTS2-targeted.

Peroxisomal disorders: clinical, biochemical, and molecular aspects

Peroxisomes are subcellular organelles catalyzing a number of indispensable functions in cellular metabolism. The importance of peroxisomes in man is stressed by the existence of an expanding group of genetic diseases in which there is an impairment in one or more peroxisomal functions. Much has been learned in recent years about these functions and many of the enzymes involved have been characterized, purified and their cDNAs cloned. This has allowed resolution of the enzymatic and molecular basis of many of the single peroxisomal enzyme deficiencies. Similarly, the molecular basis of the peroxisome biogenesis disorders is also being resolved rapidly thanks to the successful use of CHO as well as yeast mutants. In this paper we will provide an overview of the peroxisomal disorders with particular emphasis on their clinical, biochemical and molecular characteristics.

Peroxisomal beta-oxidation enzymes

The enzymes involved in beta-oxidation spiral are schematically classified into two groups. The first group consists of palmitoyl-CoA oxidase, the L-bifunctional protein, which has been called as the bifunctional protein, and 3-ketoacyl-CoA thiolase. The second group consists of the newly confirmed enzymes, branched chain oxidase, the D-bifunctional protein, and sterol carrier protein x. The enzymes of the first group are inducible and act on the straight chain acyl-CoA substrates. But the enzymes of the second group are non-inducible and act on branched chain acyl-CoAs. Accordingly, bile acid formation and oxidation of pristanic acid derived from phytol are catalyzed by the enzymes of the second group but not by those of the first group. The functions of the peroxisomal system and methods of analysis of the enzymes are briefly summarized.