Defective regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase in a somatic cell mutant

Modulation of the bilayer thickness of exocytic pathway membranes by membrane proteins rather than cholesterol

A biological membrane is conceptualized as a system in which membrane proteins are naturally matched to the equilibrium thickness of the lipid bilayer. Cholesterol, in addition to lipid composition, has been suggested to be a major regulator of bilayer thickness in vivo because measurements in vitro have shown that cholesterol can increase the thickness of simple phospholipid/cholesterol bilayers. Using solution x-ray scattering, we have directly measured the average bilayer thickness of exocytic pathway membranes, which contain increasing amounts of cholesterol. The bilayer thickness of membranes of the endoplasmic reticulum, the Golgi, and the basolateral and apical plasma membranes, purified from rat hepatocytes, were determined to be 37.5 +/- 0.4 A, 39.5 +/- 0.4 A, 35.6 +/- 0.6 A, and 42.5 +/- 0.3 A, respectively. After cholesterol depletion using cyclodextrins, Golgi and apical plasma membranes retained their respective bilayer thicknesses whereas the bilayer thickness of the endoplasmic reticulum and the basolateral plasma membrane decreased by 1.0 A. Because cholesterol was shown to have a marginal effect on the thickness of these membranes, we measured whether membrane proteins could modulate thickness. Protein-depleted membranes demonstrated changes in thickness of up to 5 A, suggesting that (i) membrane proteins rather than cholesterol modulate the average bilayer thickness of eukaryotic cell membranes, and (ii) proteins and lipids are not naturally hydrophobically matched in some biological membranes. A marked effect of membrane proteins on the thickness of Escherichia coli cytoplasmic membranes, which do not contain cholesterol, was also observed, emphasizing the generality of our findings.

Isolation of a somatic cell mutant resistant to the induction of apoptosis by oxidized low density lipoprotein

Oxidized low density lipoprotein (oxLDL) induces apoptosis in macrophages, smooth muscle cells, and endothelial cells. To elucidate the molecular mechanism of oxLDL-induced cytotoxicity and determine its tissue specificity, we have used Chinese hamster ovary (CHO)-K1 cells expressing human CD36 (CHO/CD36). Expression of CD36 rendered these cells susceptible to killing by oxLDL. This cytotoxicity was due to the induction of apoptosis. Therefore, CD36 expression is the only requirement for oxLDL-induced apoptosis. Oxysterols apparently mediate the cytotoxicity of oxLDL in macrophage foam cells and endothelial cells. 25-Hydroxycholesterol, at concentrations higher than 1 microg/ml, killed CHO-K1 cells, by apoptosis, in medium supplemented with serum as a source of cholesterol. These effects were not seen in a 25-hydroxycholesterol-resistant CHO/CD36 mutant (OX(R)), which was otherwise capable of undergoing apoptosis in response to staurosporine. This mutant was also resistant to killing by oxLDL, suggesting that oxysterols are at least partially responsible for the toxic effects of oxLDL. Oxysterol-induced apoptosis did not involve regulation of sterol regulatory element-binding protein proteolysis or the cholesterol biosynthetic pathway. 25-Hydroxycholesterol stimulated calcium uptake by CHO-K1 cells within 2 min after addition. Treatment of CHO or THP-1 (macrophage) cells with the calcium channel blocker nifedipine prevented 25-hydroxycholesterol induction of apoptosis. OX(R) showed no enhanced calcium uptake in response to 25-hydroxycholesterol.

Activation of acyl-CoA: cholesterol acyltransferase in rat liver microsomes by 25-hydroxycholesterol

25-Hydroxycholesterol stimulated acyl-CoA:cholesterol acyltransferase (ACAT) activity in rat liver microsomes in vitro with half-maximal stimulation at 16.8 microM oxysterol and a maximal activity that was three times that in its absence. The current study was conducted to determine the effect of 25-hydroxycholesterol on rates and extent of intervesicular cholesterol transfers within microsomes and to determine whether this activation of ACAT could be accounted for on the basis of increased cholesterol availability for the enzyme. Cholesterol transfer kinetics were assessed in systems that either enriched or depleted microsomal cholesterol. Incubation of microsomes at 37 degrees C with phosphatidylcholine:cholesterol liposomes or purified plasma membranes resulted in enrichment of microsomal cholesterol. Incubation of microsomes with just phosphatidylcholine liposomes resulted in depletion of cholesterol. The extent of cholesterol enrichment or depletion depended on incubation time and the initial concentration of cholesterol in donor and acceptor vesicles. The rate and extent of cholesterol transfer from liposomes to microsomes were slightly increased when 25-hydroxycholesterol was present during the transfer process. Irrespective of the treatment, 25-hydroxycholesterol continued to stimulate the ACAT activity of the treated microsomes. Microsomes that were enriched or depleted of cholesterol in the absence of 25-hydroxycholesterol yielded as much enzyme activities when assayed in the presence of 25-hydroxycholesterol as with the systems that contained 25-hydroxycholesterol during both the transfer process and enzyme assays. The results suggest that a major part of the activation of microsomal ACAT by 25-hydroxycholesterol is not ascribable to increased substrate availability for the enzyme.

Biological effects of oxysterols: current status

A review of relevant literature on biological activities of oxysterols (OS) and cholesterol is presented. The data clearly demonstrate manifold biological activities, often detrimental, for OS compared with little or no such activity of a deleterious nature for cholesterol itself. Cholesterol is perhaps the single most important compound in animal tissue and, as such, it is difficult to imagine it as a toxin or hazard. In contrast, OS exhibit cytotoxicity to a wide variety of cells leading to angiotoxic and atherogenic effects; alter vascular permeability to albumin; alter prostaglandin synthesis and stimulate platelet aggregation, an important process facilitating atherosclerosis and thrombosis; alter the functionality of low density lipoprotein (LDL) receptors, possibly stimulating hypercholesterolaemia; modify cholesteryl ester accumulation in various cells, inducing foam cell formation; and enrich the LDL particle in cholesteryl esters, possibly increasing its atherogenicity. Furthermore, OS are mutagenic and carcinogenic, although some have been studied as antitumour agents based on their cytotoxic properties. Moreover, numerous studies have implicated OS in membrane and enzyme alterations that are interrelated with many of the foregoing effects. The authors find that OS deserve much more attention than cholesterol itself in terms of research activity but that unfortunately the reverse is true with regard to funding.

Somatic cell genetic analysis of two classes of CHO cell mutants expressing opposite phenotypes in sterol-dependent regulation of cholesterol metabolism

Two different classes of hamster cell mutants (25RA cells and M1 cells) express opposite phenotypes in sterol dependent regulation. In 25RA cells, sterols added in growth medium fail to cause down-regulation of sterol synthesis rate and low density lipoprotein (LDL) receptor activity, while in M1 cells, removal of lipids from growth medium fail to cause up-regulation of sterol synthesis rate and LDL receptor activity. Cell hybridization analysis showed that the 25RA phenotype is semidominant, while the M1 phenotype is recessive. Using 25RA as the parental cells, we isolated eight independent mutant cells (DM cells) and showed that all of them belong to the same genetic complementation group as the M1 mutant, indicating that the normal (unmutated) M1 gene product(s) is required to express the 25RA phenotype. We next performed gene transfer experiments using hamster cell genomic DNAs containing the functional human M1 gene as the donor, and the double mutant cell DM7 as the recipient. The resultant transfectant cells express the 25RA cell phenotype (instead of the wild-type cell phenotype). This result, along with the results obtained from cell hybridization analysis, shows that the 25RA and M1 cell phenotypes are caused by mutations at two different genes.

Sterol-resistant transcription in CHO cells caused by gene rearrangement that truncates SREBP-2

Sterol-resistant CHO cells (SRD-1 cells) fail to repress sterol synthesis and LDL receptor gene transcription when incubated with 25-hydroxycholesterol. Here we trace the defect to a rearrangement in the gene encoding SREBP-2, a membrane-bound transcription factor that regulates cholesterol homeostasis. SREBP-2 is an 1139-amino acid protein that is bound to extranuclear membranes via a carboxy-terminal attachment domain. In sterol-depleted cells a protease liberates the amino-terminal fragment (approximately 480 amino acids). This fragment, which contains the transcriptional activation and bHLH-Zip domains, translocates to the nucleus. 25-Hydroxycholesterol abolishes protease activity and halts transcription. SRD-1 cells produce a soluble, truncated form of SREBP-2 (amino acids 1-460) that lacks the membrane attachment domain and activates transcription directly, bypassing the sterol-regulated proteolytic step. Although SRD-1 cells produce full-length SREBP-2 from the wild-type allele and a related transcription factor, SREBP-1, they fail to cleave both of these precursors, indicating that the truncated form of SREBP-2 down-regulates the protease through a form of end-product feedback inhibition. The current data provide genetic evidence for the previously proposed model in which cholesterol homeostasis is controlled by sterol-regulated proteolysis of a membrane-bound bHLH-Zip transcription factor.

Treatment of CHO-K1 cells with 25-hydroxycholesterol produces a more rapid loss of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity than can be accounted for by enzyme turnover

A key enzyme in the regulation of mammalian cellular cholesterol biosynthesis is 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase). It is well established that treatment with the compound 25-hydroxycholesterol lowers HMG-CoA reductase activity in cultured Chinese hamster ovary (CHO-K1) cells. After brief incubation (0-4 h) with 25-hydroxycholesterol (0.5 microgram/ml), cellular HMG-CoA reductase activity is decreased to 40% of its original level. This also occurs in the presence of exogenous mevinolin, a competitive inhibitor of HMG-CoA reductase which has previously been shown to inhibit its degradation. The inhibition of HMG-CoA reductase activity by 25-hydroxycholesterol is complete after 2 h. Radio-immune precipitation analysis of the native enzyme under these conditions shows a degradation half-life which is considerably longer than that of the observed inhibition. Studies with sodium fluoride, phosphatase 2A, bacterial alkaline phosphatase and calf alkaline phosphatase indicate that the observed loss of activity is not due to phosphorylation. These data are not consistent with described mechanisms of HMG-CoA reductase activity regulation by phosphorylation or degradation but are consistent with a novel mechanism that regulates the catalytic efficiency of this enzyme.

Further characterization of a somatic cell mutant defective in regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase

Two enzymes of mammalian cellular mevalonate biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase and HMG-CoA reductase, have been shown to be regulated by exogenous sterols. It has been demonstrated that these enzymes are regulated, at least in part, by transcriptional control of their synthesis. We have previously described a somatic cell mutant (CR1) of the CHO-K1 cell line that is defective in regulation of the activity of these enzymes in response to exogenous sterols. In this report, we demonstrate that this mutant is defective in regulation of the mRNA levels for HMG-CoA reductase and HMG-CoA synthase by 25-hydroxycholesterol and mevinolin. In the case of HMG-CoA reductase, this loss of apparent transcriptional control is not accompanied by a comparable loss in regulation of synthesis of this enzyme. This observation is consistent with prior studies suggesting that HMG-CoA reductase can be regulated translationally. We also show that CR1 cells exhibit a constitutively rapid rate of degradation of HMG-CoA reductase.

Somatic cell genetics and the study of cholesterol metabolism

The regulation of cholesterol biosynthesis by extracellular cholesterol occurs both in whole animal tissue and in permanent somatic cell lines in culture. Permanent mammalian cells lines, under optimized growth conditions, are easily manipulated both biochemically and genetically. The Chinese hamster ovary cell line (CHO-K1) is the most widely used cell line for genetic studies. CHO-K1 is a pseudo-diploid mammalian cell exhibiting a short doubling time and a relatively high plating efficiency. Somatic cell mutants can be generated through mutagenesis and also by drug adaptation. Following mutagenesis, auxotrophs may be isolated either by selection or by screening. Most selection procedures for mutants of cholesterol metabolism must be done in serum depleted of cholesterol which requires the endogenous biosynthetic pathway to be intact. Mutants failing to produce cholesterol do not replicate their DNA and exhibit reduced concentrations of cholesterol in their membranes. BUdR and polyene antibiotics have both been used to select against the wild-type cells which incorporate these compounds and are killed, allowing the survival of the mutant cells. Both mevalonate and cholesterol auxotrophs have been isolated with the BUdR technique and have proven useful for elucidation of the early steps in cholesterol biosynthesis, particularly for the ratelimiting enzyme HMG-CoA reductase. Somatic cell fusion of a mutant and wild-type cell followed by chromosomal segregation, routinely used to map human genes, has also been used to map the human gene for HMG-CoA synthase. Such hybrids also provide valuable information on the dominance or recessivity of a specific lesion. DNA-mediated gene transfer into somatic cell mutants allows the selection of DNA sequences which complement the mutation, and is also useful for analysis of regions of regulatory significance. Mutants, resistant to the regulatory effects of oxygenated sterols, can be isolated following mutagenesis. Mutants of this type vary the lipid content of their membranes in response to cholesterol concentration in the medium. All such mutants tested exhibit a pleiotropic regulatory effect on more than one enzyme in the cholesterol biosynthetic pathway. Adaptation to drugs such as compactin and mevinolin, which inhibit HMG-CoA reductase, have been used to produce mutants which overexpress enzymes in the pathway. These amplified cells are useful sources of specific mRNAs for construction of cDNA libraries and gene isolation. Structure-function relationships of membrane sterols can be studied in cholesterol auxotrophs where changes in acyl-chain ordering can be manipulated by exogenous sterols in the medium.

Requirement for 24(S),25-epoxycholesterol for the viability of cultured fibroblasts

Previous studies on a somatic cell mutant auxotrophic for mevalonate (Mev-1) have shown that these cells rapidly lose viability when deprived of mevalonic acid in culture medium supplemented with serum cholesterol. Testing of all known end products of mevalonate metabolism in cultured mammalian cells has been conducted to determine the basis for this mevalonate requirement. It has been found that the recently discovered mevalonate metabolite 24(S),25-epoxycholesterol produces a partial restoration of viability of Mev-1 cells starved for mevalonate, whereas other structurally similar oxysterols do not. It appears that 24(S),25-epoxycholesterol has a specific, vital cellular function in CHO-K1 cells.

Comparison of glycine metabolism in mouse lymphoma cells either sensitive or resistant to L-asparaginase

Previous work suggested a relationship between glycine metabolism and the effect of L-asparaginase upon tumor cells. Therefore, L5178Y (sensitive) or L5178Y/L-ASE (resistant) ascites lymphoma cells were incubated with 14C-labeled glyoxylate, glycine, serine, or asparagine, and the metabolism to other amino acids was measured by high performance liquid chromatography. Metabolic differences between the two cells lines were found. Under control conditions, the interconversion rate of glycine and serine via serine hydroxymethyltransferase (SHMT) was higher in sensitive than in resistant cells. The transformation rate of glyoxylate to serine was also higher in sensitive cells. These results may indicate a difference in the activity of SHMT. An alternate explanation would be that transport or diffusion of serine and glycine into sensitive cells is greater than into resistant cells. Several crucial metabolic differences were observed between the two cell types when L-asparaginase was added. A key difference is the decrease of glycine synthesis from glyoxylate observed in the sensitive cells compared to resistant cells which show no change. This suggests that asparagine is used for transamination of glyoxylate. Also, only sensitive cells appear to compensate for L-asparaginase-induced loss of glycine formation from glyoxylate by increasing glycine synthesis from serine. Alterations in sensitive tumor glycine metabolism may be an important function of L-asparaginase anticancer activity.

Growth-rate-related and hydroxysterol-induced changes in membrane fluidity of cultured hepatoma cells: correlation with 3-hydroxy-3-methyl glutaryl CoA reductase activity

3-hydroxy-3-methylglutaryl-coenzyme A reductase (EC 1.1.1.3.4.) activity and cell membrane fluidity measured by fluorescence polarization using 1,6 diphenyl, 1,3,5-hexatriene as fluorescent probe have been concomitantly examined in HTC hepatoma cells, both in relation to growth rate and in response to treatment with hydroxylated sterols. A high level of HMG-CoA reductase activity was observed in cells at log phase of growth which progressively decreased to reach a sustained low level at stationary phase. Similarly, membrane fluidity markedly decreased in relation to growth rate. Hydroxylated sterols such as 7 beta-hydroxycholesterol or 25-hydroxycholesterol strongly inhibited HMG-CoA reductase activity whereas a water-soluble derivative of 7 beta-hydroxycholesterol sodium 3,7-bishemisuccinate had no effect. Within the same range of concentrations 7 beta-hydroxycholesterol and 25-hydroxycholesterol strongly decreased membrane fluidity when the water-soluble derivative was ineffective. Thus, the present results provide evidence for a correlation between the two tested parameters and suggest a dependency of HMG-CoA reductase activity on cell membrane fluidity.

Analysis of the coordinate expression of 3-hydroxy-3-methylglutaryl coenzyme A synthase and reductase activities in Chinese hamster ovary fibroblasts

Decreased activities of both 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) synthase and HMG CoA reductase are observed in the presence of sterol in the Chinese hamster ovary (CHO) fibroblast. In three different genotypes of CHO cell mutants resistant to 25-hydroxycholesterol both enzyme activities exhibit a decreased response to 25-hydroxycholesterol compared to wild-type cells. Permanently repressed levels of both HMG CoA synthase and HMG CoA reductase activities are observed in another CHO mutant, phenotypically a mevalonate auxotroph. Mevinolin, a competitive inhibitor of HMG CoA reductase, has no effect on HMG CoA synthase activity measured in vitro. Incubation of CHO cells with sublethal concentrations of mevinolin produces an inhibition of the conversion of [14C]acetate to cholesterol and results in elevated levels of both HMG CoA synthase and HMG CoA reductase activities. Studies of CHO cells in sterol-free medium supplemented with cycloheximide indicate that continuous protein synthesis is not required for the maximal expression of HMG CoA synthase activity and provide an explanation for the lack of temporal similarity between HMG CoA synthase and reductase activities after derepression. These results support the hypothesis of a common mode of regulation for HMG CoA synthase and HMG CoA reductase activities in CHO fibroblasts.

Complementary recessive 25-hydroxycholesterol-resistant somatic cell mutants--assay of 25-hydroxycholesterol binding activity

Complementation analysis of recessive 25-hydroxycholesterol-resistant mutants of the CHO-Kl cell shows the existence of at least two complementation groups, one of which is missing a binding activity for 25-hydroxycholesterol. Both complementation groups are shown to be refractory to inhibition of cellular HMG-CoA reductase activity and in the inhibition of biosynthesis of this enzyme by 25-hydroxycholesterol.

Characterization of dominant hamster cell mutants resistant to oxygenated sterols

Stable mutants of Dede and CHO cells, resistant to suppression of cholesterogenesis by oxygenated sterols, have been isolated in a single step. Luria-Delbrück fluctuation analysis indicated a random occurrence of resistant at a rate of 1 x 10(-7) mutations/cell/generation. Cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A reductase activity, and growth of the mutant cells were coordinately resistant to oxygenated sterols in the culture medium, and this resistance was expressed as a dominant trait in somatic cell hybrids of the wild-type and mutant cells. The dominant resistance was employed in the selection of various cells hybrids. There was complete additivity of reductase activities in mixed lysates of inhibited wild-type and uninhibited mutant cells, indicating that cytosolic (in)activation factors were not causative of this resistance. We suggest that oxygenated sterols are (co)repressors in suppression of the synthesis of the reductase and that the resistance mutant phenotypes result from altered regulatory loci.

The effect of reagents that increase membrane fluidity on the activity of 3-hydroxyl-3-methyl glutaryl coenzyme A reductase in the CHO-K1 cell

The compounds cetyl trimethyl ammonium bromide (CTAB) and ethanol both decrease the order parameter of a spin probe embedded in cholesterol-lecithin liposomes, but CTAB produces lowering of the order parameter comparable to that produced by ethanol at a 10,000-fold lower concentration. Treatment of CHO-K1 cells with CTAB or ethanol at concentrations that produce comparable increases of membrane fluidity produce to 2- to 3-fold increase of microsomal membrane cholesterol to phospholipid ratio and a 2- to 3-fold increase of the activity of the rate-limiting enzyme of cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Cells treated with CTAB or ethanol show a progressively decreasing capacity to accumulate alpha-aminoisobutyric acid with increasing drug treatment, but cells pre-treated with CTAB are relatively resistant to the effects of CTAB on alpha-aminoisobutyrate transport. The increase in HMG-CoA reductase by CTAB or ethanol is not observed when these compounds are added directly to cell extracts but, rather, is only observed after 8 hours or exposure of intact cells to these drugs. Actinomycin D and cycloheximide treatment prevent the increase in enzyme activity, and the increase is also blocked in a regulatory mutant of the CHO-K1 cell with permanently repressed HMG-CoA reductase activity. These data are consistent with a homeoviscous adaptation mechanism in the CHO-K1 cell, in which increased activity of HMG-CoA reductase, through a process requiring RNA and protein synthesis, compensates for conditions that increase membrane fluidity by increased cellular cholesterol biosynthesis and cholesterol to phospholipid ratio.

Plasma lipoproteins of leukemic guinea pigs (L2C) can regulate cholesterol biosynthesis by lymphocytes of normal guinea pigs. A comparative study of plasma lipoproteins of normal and neoplastic animals

The defect of regulation of cholesterol biosynthesis by leukemic (L2C) guinea pig lymphocytes is not a consequence of serum lipoprotein modifications which would make them unable to participate in the regulatory process. Low density lipoprotein of leukemic animals, in parallel to normal low density lipoprotein, can inhibit the cholesterol biosynthesis by normal cells. Surprisingly, very low density lipoprotein of leukemic animals have the same inhibitory property. Analyses of serum of leukemic animals showed a larger amount of the different lipoprotein fractions (+323% very low density, +27% low density lipoproteins, the high density lipoprotein staying undetectable in control and leukemic sera) than in normal serum. L2C leukemia produces low density lipoprotein slightly richer in unesterified cholesterol and very low density lipoprotein markedly modified by an increased proportion of unesterified cholesterol, phospholipids and apoprotein B. The inhibitory power of leukemic very low density lipoprotein is discussed by analogy with corresponding power of normal low density lipoprotein which can operate either by the way of binding to the low density lipoprotein receptor or by exchange of unesterified cholesterol between the lipoprotein and the cell.

N-carbamoyloxyurea-resistant Chinese hamster ovary cells with elevated levels of ribonucleotide reductase activity

We describe the isolation and characterization of a Chinese hamster ovary cell line selected for resistance to N-carbamoyloxyurea. Using the mammalian cell permeabilization assay developed in our laboratory, a detailed analysis of the target enzyme, ribonucleotide reductase (EC 1.17.4.1), was carried out. Both drug-resistant and parental wild-type cells required the same optimum conditions for enzyme activity. The Ki values for N-carbamoyloxyurea inhibition of CDP reduction were 2.0 mM for NCR-30A cells and 2.3 mM for wild-type cells, while the Ki value for ADP reduction was 2.3 mM for both cell lines. Although the Ki values remained essentially unchanged, the Vmax values for NCR-30A cells were 1.01 nmoles dCDP formed/5 X 10(6) cells/hour and 1.83 nmoles dADP/5 X 10(6) cells/hour, while those for the wild-type cells were 0.49 nmoles dCDP produced/5 X 10(6) cells/hour and 1.00 nmoles dADP/5 X 10(6) cells/hour. This approximate twofold increase in reductase activity as least partially accounts for a 2.6-fold increase in D10 value for cellular resistance to N-carbamoyloxyurea exhibited by NCR-30A cells. The NCR-30A cell line was also cross-resistant to the antitumor agents, hydroxyurea and guanazole. No differences in Ki values for inhibition of CDP and ADP reduction by these two drugs were detected and cellular resistance could be entirely accounted for by the elevation in activity of the reductase in the NCR-30A cell line. The properties of N-carbamoyloxyurea-resistance cells indicate they should be useful for further investigations into the regulation of mammalian enzyme activity.