Abstract
Our group and others have recently demonstrated that peroxisomes contain a number of enzymes involved in cholesterol biosynthesis that previously were considered to be cytosolic or located in the endoplasmic reticulum (ER). Peroxisomes have been shown to contain HMG-CoA reductase, mevalonate kinase, phosphomevalonate kinase, phosphomevalonate decarboxylase, isopentenyl diphosphate isomerase, and FPP synthase. Four of the five enzymes required for the conversion of mevalonate to FPP contain a conserved putative PTS1 or PTS2, supporting the concept of targeted transport into peroxisomes. To date, no information is available regarding the function of the peroxisomal HMG-CoA reductase in cholesterol/isoprenoid metabolism, and the structure of the peroxisomal HMG-CoA reductase has yet to be determined. We have identified a mammalian cell line that expresses only one HMG-CoA reductase protein, and which is localized exclusively to peroxisomes, to facilitate our studies on the function, regulation, and structure of the peroxisomal HMG-CoA reductase. This cell line was obtained by growing UT2 cells (which lack the ER HMG-CoA reductase) in the absence of mevalonate. The surviving cells exhibited a marked increase in a 90-kD HMG-CoA reductase that was localized exclusively to peroxisomes. The wild-type CHO cells contain two HMG-CoA reductase proteins, the well-characterized 97-kD protein localized in the ER, and a 90-kD protein localized in peroxisomes. We have also identified the mutations in the UT2 cells responsible for the lack of the 97-kD protein. In addition, peroxisomal-deficient Pex2 CHO cell mutants display reduced HMG-CoA reductase levels and have reduced rates of sterol and nonsterol biosynthesis. These data further support the proposal that peroxisomes play an essential role in isoprenoid biosynthesis.
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Recent studies by our group and others have demonstrated that a number of the enzymes of the isoprenoid biosynthetic pathway are localized to peroxisomes (Krisans 1996). These include acetoacetyl-CoA thiolase (Thompson and Krisans 1990; Hovik et al. 1991), 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase (Krisans et al. 1988), HMG-CoA reductase (Keller et al. 1985, 1986; Engfelt et al. 1997), mevalonate kinase (Stamellos et al. 1992; Biardi et al. 1994), phosphomevalonate kinase (Biardi and Krisans 1996), phosphomevalonate decarboxylase (Biardi and Krisans 1996), isopentenyl diphosphate (IPP) isomerase (Paton et al. 1997), and farnesyl diphosphate (FPP) synthase (Krisans et al. 1994). Mevalonate kinase and FPP synthase were localized predominantly to peroxisomes, suggesting that the conversion of mevalonate to FPP may occur exclusively in peroxisomes (Krisans et al. 1994).
The peroxisomal enzymes required for conversion of mevalonate to FPP (i.e., mevalonate kinase, phosphomevalonate kinase, mevalonate diphosphate decarboxylase, isopentenyl diphosphate isomerase, and FPP synthase) have now been cloned and sequenced. Four of the five enzymes, mevalonate kinase (Tanaka et al. 1990), phosphomevalonate kinase (Chambliss et al. 1996), and mevalonate diphosphate decarboxylase (Toth and Huwyler 1996) and isomerase (Xuan et al. 1994), contain a conserved putative peroxisomal targeting signal 1 (PTS1) or peroxisomal targeting signal 2 (PTS2) (Subramani 1993). FPP synthase does not contain a currently identifiable PTS1 or PTS2, but it is selectively localized to peroxisomes. It appears likely that targeting to peroxisomes must include alternative mechanisms not yet defined.
Until recently, IPP isomerase was presumed to have a cytosolic localization. However, the following three observations led us to believe that the enzyme may be localized to peroxisomes. (a) In permeabilized cells lacking cytosolic components, mevalonate can be converted to cholesterol in equal amounts to those observed in nonpermeabilized cells, this suggesting that the cytosol does not contain enzymes necessary for the conversion of mevalonate to FPP (Paton et al. 1997). (b) IPP isomerase activity in tissues from patients with peroxisome-deficient diseases (Zellweger and neonatal adrenoleukodystrophy) is 50% of that found in tissues from control patients (Krisans et al. 1994). (c) The deduced amino acid sequence from the human isomerase cDNA, which has been recently cloned (Xuan et al. 1994) and characterized, contains two putative peroxisomal targeting sequences.
At the C-terminal end of human isomerase is a putative PTS1 consisting of YRM (single-letter amino acid notation) and at the N-terminal end is a putative PTS2 sequence consisting of HLX5QL (where X designates any amino acid). The consensus sequence for the PTS1 motif is (S/A/C)(K/H/R)(L/M) (Subramani 1993). However, many tripeptide combinations that do not adhere to this consensus sequence were able to target S. cervisiae malate dehydrogenase to yeast peroxisomes (Elgersma et al. 1996). The PTS2 consensus sequence is usually near the N-terminal end of the protein and is the nonapeptide (R/K)(L/V/I)X5(H/Q)(L/A) (Subramani 1993).
We have cloned the rat and hamster homologues of IPP isomerase and have demonstrated that the protein is targeted to peroxisomes by use of the PTS1 motif (Paton et al. 1997). Because all of the mammalian isomerases have putative PTS1 and PTS2 motifs, we first constructed a eukaryotic expression vector containing the full coding sequence of hamster IPP isomerase with an internal HA epitope tag. An internal HA tag was chosen instead of C- or N-terminal HA tags so as not to disrupt the potential PTS1 or PTS2 function.
To determine the subcellular compartment in which the IPP isomerase is localized, we transfected the full-length construct into CHO cells. The cells were then simultaneously immunolabeled with anti-catalase antibody (Figure 1A) and anti-HA antibody (Figure 1B). The immunofluorescence pattern for catalase was superimposable over the pattern obtained with the HA antibody. Similar results were obtained in human fibroblast cells labeled with anti-catalase antibody and anti-HA antibody (Paton et al. 1997). These results show that hamster IPP isomerase is colocalized with catalase to peroxisomes.
To determine which of the PTS targeting signals are utilized by the hamster isomerase, we utilized two distinct cell lines derived from patients with peroxisomal disorders. One cell line was shown to be deficient in the peroxisomal import of only PTS2 proteins; the other cell line was shown to be deficient in the peroxisomal import of both PTS1 and PTS2 proteins. To first determine if the putative PTS2 is responsible for peroxisomal import, the full-length construct was transfected into the PTS2-deficient cell line. The cells were again double-labeled with anti-catalase antibody and anti-HA antibody. The data again demonstrated a superimposable punctate pattern when labeling with anti-catalase and anti-HA antibodies was performed. These results suggest that IPP isomerase does not use the putative PTS2 for peroxisomal targeting.
Demonstration by double-label immunofluorescence in transiently transfected cells that the C-terminal HRM tripeptide is necessary for peroxisomal targeting. CHO cells transiently transfected with HA-tagged IPP isomerase were labeled for catalase (
To provide direct evidence that the putative PTS1 of isomerase is necessary for peroxisomal targeting, a second expression vector was constructed in which the HRM tripeptide was deleted. This deletion construct was transfected into the PTS2-deficient cell line. The immunofluorescence pattern was consistent with peroxisomal labeling when anti-catalase antibody was used (Figure 1C), whereas a cytosolic labeling pattern was obtained when the anti-HA antibody was used (Figure 1D). These data therefore show that the HRM tripeptide is necessary for the targeting of IPP isomerase to peroxisomes.
The HRM tripeptide is present at the C-terminus of both the rat and the hamster homologue of IPP iso-merase. However, the human homologue has a YRM tripeptide at its C-terminus (Xuan et al. 1994).
The PTS1 consensus sequence of (S/A/C)(K/H/R) (L/M) was derived from extensive mutational analysis where peroxisomal proteins from other organisms or nonperoxisomal proteins were used as reporters. Since the formulation of this consensus sequence, more peroxisomal proteins have been identified whose PTS1-like tripeptide does not fit this exact sequence. However, it has been recently demonstrated that many amino acid substitutions can be made at the first position of a homologous protein without compromising the PTS1 function (Elgersma et al. 1996). This strongly suggests that human isomerase is also targeted to peroxisomes by a PTS1, because the RM dipeptide meets the consensus sequence criteria.
In addition to the above-mentioned enzymes, four of the enzyme activities (dihydrolanosterol oxidase, steroid-14-reductase, steroid-3-ketoreductase, and steroid-8-isomerase) involved in the conversion of lanosterol to cholesterol have also been reported to be present in peroxisomes (Appelkvist et al. 1990).
Figure 2 illustrates our current concept of the compartmentalization of cholesterol biosynthetic enzymes (Krisans 1996). The peroxisome contains enzymes for the conversion of acetyl-CoA to FPP. The conversion of acetyl-CoA to HMG-CoA also occurs in the cytosol, with the further conversion of HMG-CoA to mevalonate taking place in the ER and peroxisomes, whereas the conversion of mevalonate to FPP occurs predominately if not exclusively in the peroxisomes. The incorporation of FPP into squalene occurs only in the ER. Lastly, the metabolism of lanosterol to cholesterol takes place in the ER and may occur also in the peroxisomal compartment.
The indispensable role of the peroxisomal enzymes in isoprenoid biosynthesis is also evident in humans afflicted by the recessive inherited peroxisomal disorders (PDs), e.g., Zellweger syndrome (ZS), in which normal peroxisomes are absent from the cells (Lazarow and Moser 1995). The lack of normal peroxisomes in these cells results in both cytosolic localization and reduced levels of the peroxisomal matrix enzymes. The consequence of this is reflected by the absence or reduced activity of independent peroxisomal enzymatic pathways such as plasmalogen biosynthesis, degradation of very long-chain fatty acids, accumulation of bile acid intermediates, and low plasma cholesterol levels (Lazarow and Moser 1995). Lower rates of cholesterol biosynthesis are also apparent in skin fibroblasts derived from these patients (Hodge et al 1991; Mandel et al. 1995).
Current model of the compartmentalization of the cholesterol biosynthetic pathway. Conversion of acetyl-CoA to HMG-CoA occurs in the cytosol and peroxisomes. The further conversion of HMG-CoA to mevalonate occurs both in the ER and peroxisomes. However, the conversion of mevalonate to FPP occurs predominantly if not exclusively in the peroxisomes. The metabolism of FPP to squalene occurs exclusively in the ER. Lastly, the metabolism of lanosterol to cholesterol occurs in the ER and may also be localized to peroxisomes. FPP is also the precursor for dolichols, ubiquinone, farnesylated proteins, and the isoprenoid moiety of heme A.
Keller et al. (1985) were the first to demonstrate that, in the liver, HMG-CoA reductase is present not only in the ER but also in the peroxisomes. The function of the peroxisomal reductase in cholesterol/isoprenoid metabolism has yet to be defined. However, it is clear that the ER and peroxisomal HMG-CoA reductases can be regulated differently and may therefore play different functional roles (Keller et al. 1986; Rusnak and Krisans 1987). The ER reductase has a diurnal cycle distinct from that of the peroxisomal reductase (Rusnak and Krisans 1987). However, the two reductases can also be regulated coordinately. Both reductase activities are induced by cholestyramine (a bile acid resin) (Keller et al. 1986). Accordingly, to facilitate our studies of the function and regulation of the peroxisomal HMG-CoA reductase, we have identified a mammalian cell line (UT2) that expresses only one HMG-CoA reductase protein of 90 kD and which is localized exclusively to peroxisomes (Engfelt et al. 1997).
The localization of this protein to peroxisomes was demonstrated by four different methods: (a) analytical subcellular fractionation and measurement of enzyme activities; (b) immunoblotting for HMG-CoA reductase in the isolated fractions with a monospecific antibody; (c) immunofluoresence microscopy; and (d) immunoelectron microscopy. All four methods produced consistent results. The conclusion that the 90-kD protein localized in peroxisomes is HMG-CoA reductase is based on the following findings (a) A number of different monospecific HMG-CoA reductase antibodies crossreact with this protein. (b) The proteins were specifically precipitated as they were competed by an excess of the corresponding free peptides. (c) The HMG-CoA reductase antibody specifically immunoprecipitated the HMG-CoA reductase activity. (d) The protein and HMG-CoA reductase activity levels are regulated coordinately. Finally, (e) the HMG-CoA reductase activity is completely abolished in vitro by addition of lovastatin.
We have recently determined that the deficiency of the 97-kD ER HMG-CoA reductase protein in UT2 cells is caused by two mutations within the gene (Engfelt et al. 1998). The UT2 cells contain a mutation in the 5′ splice junction (+1 position) of the intron located between exons 11 and 12. A second similar mutation was found in the 5′ splice junction (+5 position) between exons 13 and 14. Both of these mutations cause aberrant splicing events, which ultimately result in premature stop codons present in the mature mRNA. The presence of the premature stop codons in the cDNA, if translated, would produce nonfunctional, truncated proteins with molecular masses no larger than 66 kD. Furthermore, these proteins would be deficient in the reductase catalytic domain (Engfelt et al. 1998). The identified splice site mutations in the HMG-CoA reductase gene and the isolated HMG-CoA reductase cDNAs are summarized in Figure 3.
From these data, it is clear that the UT2 cells produce aberrantly spliced HMG-CoA reductase transcripts unable to code for a 97- or 90-kD reductase protein. In addition, we have never obtained alternatively spliced messages capable of coding for a 90-kD protein by use of RT-PCR analysis of CHO or UT2/UT2* cell ER reductase mRNA or by screening of UT2* cell cDNA libraries with the full-length reductase probe. Therefore, these data suggest that the 90-kD HMG-CoA reductase found in CHO and UT2/UT2* cells may be a product of a novel gene.
Summary of ER HMG-CoA reductase genomic mutations and cDNAs identified in UT2/UT2* cells. (
Our hypothesis is that all wild-type cells contain two forms of HMG-CoA reductase. The UT2 cells lack the ER HMG-CoA reductase as a result of chemical mutagenesis (Mosley et al. 1983) and the peroxisomal reductase is suppressed due to growth of the cells in the presence of mevalonate. Therefore, these cells require mevalonate for growth. However, when mevalonate is removed the peroxisomal reductase is upregulated and the cells can grow without mevalonate. When these cells are placed back in UT2 cell medium (containing mevalonate), the peroxisomal reductase activity levels again decrease. Therefore, this is a reversible physiological regulation. These cells provide a model system for study of the function and regulation of the peroxisomal reductase independent of the ER reductase.
In mammals, only one gene has been found to encode HMG-CoA reductase. However yeast, fungi, and plants all contain more than one HMG-CoA reductase gene. Yeast, fungi and Arabidopsis thaliana all contain two genes (Basson et al. 1986; Enjuto et al. 1994; Stermer et al. 1994). A. thaliana HMG-CoA reductase 1 (HMG1) is detected in all tissues, whereas the HMG2 is restricted to young seedlings and roots. A. thaliana HMG1 is believed to function as a housekeeping form of reductase, and HMG2 may have a specialized role in actively dividing cells (Enjuto et al. 1994). Similarly, in yeast the HMG1 and HMG2 genes are differently expressed (Hampton et al. 1996). In addition, in yeast when HMG1 is deleted, the organism remains viable, indicating that HMG2 can replace the function of HMG1. The presence of multiple genes is consistent with the hypothesis that different isoforms of HMG-CoA reductase are involved in separate subcellular pathways for isoprenoid biosynthesis.
We have also performed a detailed analysis of the isoprenoid biosynthetic pathway in the peroxisome-deficient CHO cell lines ZR-78 and ZR-82 (Aboushadi and Krisans 1998). The ZR-78 and the ZR-82 cells are two CHO cell line mutants that show phenotypic resemblance to fibroblasts obtained from patients diagnosed with peroxisomal deficiency diseases. The gene responsible for the mutations in the ZR-78 and ZR-82 cells has been identified as Pex2. The Pex2 gene encodes a peroxisomal integral membrane protein of 35 kD that is involved in peroxisomal assembly and biogenesis and affects both the PTS1 and the PTS2 pathway.
The results showed that total HMG-CoA reductase activity was significantly reduced in the peroxisome-deficient cells compared to the wild-type cells. Analysis of the two reductase proteins in permeabilized cells indicated that, in the ZR-78 and ZR-82 cells, the 90-kD peroxisomal reductase protein was mainly localized to the cytosol. In addition, the rates of both sterol (cholesterol) and nonsterol (dolichols) biosynthesis were significantly lower in the peroxisome-deficient cells when either acetate or mevalonate was used as substrate. In contrast, the rate of dolichol biosynthesis in the peroxisome-deficient cells was similar to that of the wild-type cells when incubated with farnesol. Furthermore, the data also indicated that the peroxisome-deficient cells have increased rates of lanosterol biosynthesis compared to wild-type cells. These data are summarized in Table 1 and support the earlier observation that the enzymatic activities of mevalonate kinase, phosphomevalonate kinase, mevalonate diphosphate decarboxylase, isopentenyl diphosphate isomerase, and FPP synthase were significantly reduced in liver tissue from patients with peroxisome-deficient diseases (Krisans et al. 1994).
Determination of sterol and dolichol biosynthesis in peroxisome-deficient Pex2 CHO cell lines
These data are also in agreement with previous studies in peroxisome-deficient human fibroblast cell lines, which also demonstrated impaired rates of cholesterol biosynthesis (Hodge et al. 1991; Mandel et al. 1995). This decrease is most likely due to the deficiency in the peroxisomal cholesterol biosynthetic enzymes in these cells. This conclusion is supported by the observation that whereas control CHO cells can grow indefinitely in media containing lipoprotein-deficient serum, the peroxisome-deficient CHO cells are able to survive for only 10 days.