Cholesterol biosynthesis in Zellweger syndrome: normal activity of mevalonate kinase, mevalonate-5'-pyrophosphate decarboxylase and IPP-isomerase in patients' fibroblasts but deficient mevalonate kinase activity in liver

Staying in Healthy Contact: How Peroxisomes Interact with Other Cell Organelles

Peroxisomes share extensive metabolic connections with other cell organelles. Membrane contact sites (MCSs) establish and maintain such interactions, and they are vital for organelle positioning and motility. In the past few years peroxisome interactions and MCSs with other cellular organelles have been explored extensively, resulting in the identification of new MCSs, the tethering molecules involved, and their functional characterization. Defective tethering and compartmental communication can lead to pathological conditions that can be termed 'organelle interaction diseases'. We review peroxisome-organelle interactions in mammals and summarize the most recent knowledge of mammalian peroxisomal organelle contacts in health and disease.

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

We evaluated the major pathways of cholesterol regulation in the peroxisome-deficient PEX2(-/-) mouse, a model for Zellweger syndrome. Zellweger syndrome is a lethal inherited disorder characterized by severe defects in peroxisome biogenesis and peroxisomal protein import. Compared with wild-type mice, PEX2(-/-) mice have decreased total and high-density lipoprotein cholesterol levels in plasma. Hepatic expression of the SREBP-2 gene is increased 2.5-fold in PEX2(-/-) mice and is associated with increased activities and increased protein and expression levels of SREBP-2-regulated cholesterol biosynthetic enzymes. However, the upregulated cholesterogenic enzymes appear to function with altered efficiency, associated with the loss of peroxisomal compartmentalization. The rate of cholesterol biosynthesis in 7- to 9-day-old PEX2(-/-) mice is markedly increased in most tissues, except in the brain and kidneys, where it is reduced. While the cholesterol content of most tissues is normal in PEX2(-/-) mice, in the knockout mouse liver it is decreased by 40% relative to that in control mice. The classic pathway of bile acid biosynthesis is downregulated in PEX2(-/-) mice. However, expression of CYP27A1, the rate-determining enzyme in the alternate pathway of bile acid synthesis, is upregulated threefold in the PEX2(-/-) mouse liver. The expression of hepatic ATP-binding cassette (ABC) transporters (ABCA1 and ABCG1) involved in cholesterol efflux is not affected in PEX2(-/-) mice. These data illustrate the diversity in cholesterol regulatory responses among different organs in postnatal peroxisome-deficient mice and demonstrate that peroxisomes are critical for maintaining cholesterol homeostasis in the neonatal mouse.

An apparent decrease in cholesterol biosynthesis in peroxisomal-defective Chinese hamster ovary cells is related to impaired mitochondrial oxidation

Recent data suggest that impaired mitochondrial activities in Zellweger fibroblasts are related to defective peroxisome biogenesis and vice versa. To investigate the contribution of functional mitochondria to cholesterol biosynthesis, radioactive precursor molecules that form acetyl-CoA via beta-oxidation-independent (pyruvate) or -dependent (palmitate and octanoate) pathways were used. Production of both 14C-labeled cholesterol and 14C-labeled CO(2) from these radioactive tracers was significantly impaired in peroxisomal-defective ZR-82 Chinese hamster ovary cells in comparison to controls. In contrast, cholesterol synthesis from acetate--a tracer directly converted to acetyl-CoA without the involvement of mitochondrial activities--was threefold higher in ZR-82 cells than in controls. Pathways further contributing to cellular cholesterol homeostasis, i.e., receptor-mediated binding of exogenous lipoprotein-associated cholesterol as well as intracellular mobilization of cholesteryl ester deposits were similar in ZR-82 and controls. From these findings, we propose that peroxisomal dysfunction in ZR-82 cells is tightly coupled to impaired mitochondrial activities, e.g., defective mitochondrial beta-oxidation and formation of acetyl-CoA from short chain fatty acids resulting in a decreased rate of CO(2) production, and an apparent decrease in cholesterol biosynthesis. Actually, cholesterol biosynthesis from acetate is increased in the peroxisomal-defective cells. This explains previous conflicting conclusions.

Central role of peroxisomes in isoprenoid biosynthesis

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

Isoprenoid biosynthesis is not compromised in a Zellweger syndrome mouse model

Because several studies indicated that peroxisomes are important for the biosynthesis of isoprenoids, we wanted to investigate whether a reduced availability of isoprenoids could be one of the pathogenic factors contributing to the severe phenotype of the Pex5(-/-) mouse, a model for Zellweger syndrome. Total cholesterol was determined in plasma, brain and liver of newborn mice. In none of these tissues a significant difference was observed between Pex5(-/-) and wild type or heterozygous mice. The hepatic ubiquinone content was found to be even higher in Pex5(-/-) mice as compared to wild type or heterozygous littermates. To investigate whether the Pex5(-/-) fetuses are able to synthesise their own isoprenoids, fibroblasts derived from these mice were incubated with radiolabeled mevalonolactone as a substrate for isoprenoid synthesis. No significant difference was observed between the cholesterol production rates of Pex5(-/-) and normal fibroblasts. Our results show that there is no deficiency of isoprenoids in newborn Pex5(-/-) mice, excluding the possibility that a lack of these compounds is a determinant factor in the development of the disease state before birth.

Synthesis of mevalonate pathway lipids in fibroblasts from Zellweger and X-linked ALD patients

Fibroblasts were cultured to determine the involvement of peroxisomes in cholesterol and dolichol synthesis. For this purpose, the behavior of cells from patients with Zellweger syndrome, with X-linked adrenoleukodystrophy, and from nondiseased control subjects was studied. Cells both after pretreatment with mevinolin and without pretreatment were incubated in a medium containing [3H]-mevalonate. In fibroblasts from patients with peroxisomal defects, the cholesterol content and mevalonate incorporation into cholesterol were decreased by 10-20% in comparison with control cells. Mevinolin pretreatment decreased the incorporation rate of [3H]-mevalonate into cholesterol but increased the labeling of ubiquinone and dolichol both in diseased and control cells. Squalene synthase activity was unchanged, whereas the activity of farnesyl-pyrophosphate synthase was increased in the diseased states. The results show that in patients with peroxisomal deficiency neither the amount nor the rate of synthesis of cholesterol and dolichol is reduced to any greater extent.

Identification and characterization of three novel missense mutations in mevalonate kinase cDNA causing mevalonic aciduria, a disorder of isoprene biosynthesis

Mevalonic aciduria is a rare autosomal recessive metabolic disorder, characterized by psychomotor retardation, failure to thrive, hepatosplenomegaly, anemia and recurrent febrile crises. The disorder is caused by a deficient activity of mevalonate kinase due to mutations in the encoding gene. Thus far, only two disease-causing mutations have been identified. We now report four different missense mutations including three novel ones, which were identified by sequence analysis of mevalonate kinase cDNA from three mevalonic aciduria patients. All mutations affect conserved amino acids. Heterologous expression of the corresponding mutant mevalonate kinases as fusion proteins with glutathione S -transferase in Escherichia coli showed a profound effect of each of the mutations on enzyme activity. In addition, immunoblot analysis of fibroblast lysates from patients using specific antibodies against mevalonate kinase identified virtually no protein. These results demonstrate that the mutations affect not only the activity but also the stability of the mutant proteins.

Differential deficiency of mevalonate kinase and phosphomevalonate kinase in patients with distinct defects in peroxisome biogenesis: evidence for a major role of peroxisomes in cholesterol biosynthesis

Peroxisomes catalyze a number of essential metabolic functions especially related to lipid metabolism. There is increasing evidence suggesting that peroxisomes are also involved in the synthesis of isoprenoids via the mevalonate pathway at least in rat liver. In order to obtain independent evidence for a role of peroxisomes in isoprenoid synthesis in man, we have measured the activity of two key enzymes of the mevalonate pathway in patients suffering from certain defined defects in peroxisome biogenesis. We now report that mevalonate kinase is not only deficient in livers from Zellweger patients in which peroxisome biogenesis is defective, but also in livers from rhizomelic chondrodysplasia punctata (RCDP) Type 1 patients. In the latter group of patients there is a selective defect in peroxisome biogenesis due to a genetic defect in the PTS2-receptor, a mobile receptor-protein guiding peroxisomal proteins with a certain peroxisomal targeting signal (PTS2) to the peroxisome. Phosphomevalonate kinase was found to be strongly deficient in Zellweger patients thus suggesting that this enzyme is also peroxisomal. Taken together, our data indicate that in human liver mevalonate kinase and phosphomevalonate kinase are truly peroxisomal enzymes which strongly suggests that peroxisomes play a major role in cholesterol biosynthesis.

Lipid metabolism in peroxisomes in relation to human disease

Peroxisomes were long believed to play only a minor role in cellular metabolism but it is now clear that they catalyze a number of important functions. The importance of peroxisomes in humans is stressed by the existence of a group of genetic diseases in man in which one or more peroxisomal functions are impaired. Most of the functions known to take place in peroxisomes have to do with lipids. Indeed, peroxisomes are capable of 1. fatty acid beta-oxidation 2. fatty acid alpha-oxidation 3. synthesis of cholesterol and other isoprenoids 4. ether-phospholipid synthesis and 5. biosynthesis of polyunsaturated fatty acids. In Chapters 2-6 we will discuss the functional organization and enzymology of these pathways in detail. Furthermore, attention is paid to the permeability properties of peroxisomes with special emphasis on recent studies which suggest that peroxisomes are closed structures containing specific membrane proteins for transport of metabolites. Finally, the disorders of peroxisomal lipid metabolism will be discussed.

Identification of catalytic residues in human mevalonate kinase

cDNA encoding human mevalonate kinase has been overexpressed and the recombinant enzyme isolated. This stable enzyme is a dimer of 42-kDa subunits and exhibits a Vm = 37 units/mg, Km(ATP) = 74 microM, and Km(DL-MVA) = 24 microM. The sensitivity of enzyme to water-soluble carbodiimide modification of carboxyl groups prompted evaluation of four invariant acidic amino acids (Glu-19, Glu-193, Asp-204, and Glu-296) by site-directed mutagenesis. Elimination of Glu-19's carboxyl group (E19A, E19Q) destabilizes the enzyme, whereas E19D is stable but exhibits only approximately 2-fold changes in Vm and Km values. E296Q is a stable enzyme, which exhibits kinetic parameters comparable to those measured for wild-type enzyme. E193A is a labile protein, whereas E193Q is stable, exhibiting >50-fold diminution in Vm and elevated Km values for ATP (approximately 20-fold) and mevalonate (approximately 40-fold). Such effects would be compatible with a role for Glu-193 in interacting with the cation of the MgATP substrate. D204A and D204N are stable enzymes lacking substantial mevalonate kinase activity. The active sites of these Asp-204 mutants are intact, based on their ability to bind a spin-labeled ATP analog with stoichiometries and equilibrium binding constants that are comparable to those determined for wild-type enzyme. Competitive displacement experiments demonstrate that the Asp-204 mutants can bind ATP with Kd values that are comparable to estimates for wild-type enzyme. The >40,000-fold diminution in kcat for the Asp-204 mutants and the demonstration that they contain an otherwise intact active site support assignment of a crucial catalytic role to Asp-204. The assignment of Asp-204 as the catalytic base that facilitates deprotonation of the C-5 hydroxyl of mevalonic acid would be compatible with the experimental observations.

Identification and functional characterization of an active-site lysine in mevalonate kinase

We report the construction of an expression plasmid for rat mevalonate kinase and the overexpression of recombinant enzyme in Escherichia coli. The homogeneous enzyme had a specific activity of 30 units/mg and an observed subunit molecular mass of 42 kDa. The Michaelis constants (Km) for DL-potassium mevalonate (288 microM) and for ATP (1.24 mM) were in agreement with values reported for enzymes isolated from rat liver (Tanaka, R. D., Schafer, B. L., Lee, L. Y., Freudenberger, J. S., and Mosley, S. T. (1990) J. Biol. Chem. 265, 2391-2398). Recombinant rat mevalonate kinase was inactivated by the lysine-specific reagent, pyridoxal phosphate (PLP). ATP (5 mM) afforded protection against inactivation, suggesting reaction of PLP with an active-site lysine. Mapping, isolation, and Edman degradation of the ATP-protectable peptide from [3H]PLP-inactivated borohydride-reduced mevalonate kinase allow assignment of lysine 13, a residue invariant in known mevalonate kinase sequences, as the modification site. These results represent the first identification of an active-site residue in mevalonate kinase. The function of lysine 13 was evaluated by replacing this residue with methionine. Vm of the mutant protein is diminished by 56-fold, suggesting that lysine 13 facilitates catalysis. Kd values of wild-type and mutant proteins for ATP were determined in electron spin resonance competition experiments. The observed 56-fold diminution in affinity for the mutant enzyme supports an additional role for lysine 13 in stabilization of ATP binding.