Defective regulation of cholesterol biosynthesis and plasma membrane fluidity in a Chinese hamster ovary cell mutant

Proton NMR visible mobile lipid signals in sensitive and multidrug-resistant K562 cells are modulated by rafts

Background

Most cancer cells are characterized by mobile lipids visible on proton NMR (¹H-NMR), these being comprised mainly of methyl and methylene signals from lipid acyl chains. Erythroleukemia K562 cells show narrow signals at 1.3 and 0.9 ppm, corresponding to mobile lipids (methylene and methyl, respectively), which are reduced when K562 cells are multidrug resistant (MDR). While the significance of the mobile lipids is unknown, their subcellular localization is still a matter of debate and may lie in the membrane or the cytoplasm. In this study, we investigate the role of cholesterol in the generation of mobile lipid signals.

Results

The proportion of esterified cholesterol was found to be higher in K562-sensitive cells than in resistant cells, while the total cholesterol content was identical in both cell lines. Cholesterol extraction in the K562 wild type (K562wt) cell line and its MDR counterpart (K562adr), using methyl-β-cyclodextrin, was accompanied by a rise of mobile lipids in K562wt cells only. The absence of caveolae was checked by searching for the caveolin-1 protein in K562wt and K562adr cells. However, cholesterol was enriched in another membrane microdomain designated as "detergent-insoluble glycosphingomyelin complexes" or rafts. These microdomains were studied after extraction with triton X-100, a mild non-ionic detergent, revealing mobile lipid signals preserved only in the K562wt spectra. Moreover, following perturbation/disruption of these microdomains using sphingomyelinase, mobile lipids increased only in K562wt cells.

Conclusion

These results suggest that cholesterol and sphingomyelin are involved in mobile lipid generation via microdomains of detergent-insoluble glycosphingomyelin complexes such as rafts. Increasing our knowledge of membrane microdomains in sensitive and resistant cell lines may open up new possibilities in resistance reversion.

Transcriptional homeostatic control of membrane lipid composition

Plasma membranes have a structural property, commonly referred to as membrane fluidity, that is compositionally regulated. The two main features of plasma membrane lipid composition that determine membrane fluidity are the ratio of cholesterol to phospholipids and the ratio of saturated to unsaturated fatty acids that are incorporated into the phospholipids. These ratios are determined, at least in part, by regulation of membrane lipid biosynthesis-particularly that of cholesterol and oleate. It now appears that cholesterol and oleate biosynthesis are feedback regulated by a common transcriptional mechanism which is governed by the maturation of the SREBP transcription factors. In this article, we briefly review our current understanding of transcriptional regulation of plasma membrane lipid biosynthesis by sterols and oleate. We also discuss studies related to the mechanism by which the physical state of membrane lipids signals the transcriptional regulatory machinery to control the rates of synthesis of these structural components of the lipid bilayer.

Activation of the silent endogenous cholesterol-7-alpha-hydroxylase gene in rat hepatoma cells: a new complementation group having resistance to 25-hydroxycholesterol

The oxysterol 25-hydroxycholesterol acts both as a regulatory sterol determining the expression of genes governed by sterol regulatory elements and as a substrate for 7-alpha-hydroxylase, the first and rate-limiting enzyme in the bile acid synthetic pathway. Most wild-type nonhepatic cells are killed by the cytotoxic action of 25-hydroxycholesterol. In contrast, liver cells, which express 7-alpha-hydroxylase activity, are resistant to killing by 25-hydroxycholesterol. We examined the possibility that selection for resistance to 25-hydroxycholesterol might lead to the derivation of a cell line expressing 7-alpha-hydroxylase. A rat hepatoma cell line (7-alpha-hydroxylase minus) was transfected with human DNA and screened for resistance to 25-hydroxycholesterol. Although parental hepatoma cells were all killed within a week, a 25-hydroxycholesterol-resistant cell line (L35 cells) which showed stable expression of 7-alpha-hydroxylase activity and mRNA was obtained. These cells exhibited normal inhibition of cholesterol biosynthesis by 25-hydroxycholesterol. Blocking 7-alpha-hydroxylase activity with ketoconazole also blocked the resistance of L35 cells to 25-hydroxycholesterol. Isolation of microsomes from these cells showed levels of 7-alpha-hydroxylase activity (22.9 pmol/min/mg of protein) that were comparable to the activity (33.2 pmol/min/mg) of microsomes isolated from the livers of rats killed during the high point of the diurnal cycle. Parental cells had no detectable activity. These data show a new complementation group for 25-hydroxycholesterol resistance: expression of 7-alpha-hydroxylase. Dexamethasone increased both the activity and the cellular content of mRNA coding for 7-alpha-hydroxylase. Since dactinomycin blocked the ability of dexamethasone to induce mRNA, active transcription is required. Southern analysis of genomic DNA showed that L35 cells contain the rat (endogenous) gene but not the human gene. Furthermore, the RNA expressed by L35 cells is similar in size to rat RNA and is distinct from the human form of 7-alpha-hydroxylase. The combined data indicate that L35 cells are resistant to 25-hydroxycholesterol because they express 7-alpha-hydroxylase. The mechanism responsible involves activation of the endogenous (silent) gene of the parental rat hepatoma cell.

Activation of the silent endogenous cholesterol-7-alpha-hydroxylase gene in rat hepatoma cells: a new complementation group having resistance to 25-hydroxycholesterol

The oxysterol 25-hydroxycholesterol acts both as a regulatory sterol determining the expression of genes governed by sterol regulatory elements and as a substrate for 7-alpha-hydroxylase, the first and rate-limiting enzyme in the bile acid synthetic pathway. Most wild-type nonhepatic cells are killed by the cytotoxic action of 25-hydroxycholesterol. In contrast, liver cells, which express 7-alpha-hydroxylase activity, are resistant to killing by 25-hydroxycholesterol. We examined the possibility that selection for resistance to 25-hydroxycholesterol might lead to the derivation of a cell line expressing 7-alpha-hydroxylase. A rat hepatoma cell line (7-alpha-hydroxylase minus) was transfected with human DNA and screened for resistance to 25-hydroxycholesterol. Although parental hepatoma cells were all killed within a week, a 25-hydroxycholesterol-resistant cell line (L35 cells) which showed stable expression of 7-alpha-hydroxylase activity and mRNA was obtained. These cells exhibited normal inhibition of cholesterol biosynthesis by 25-hydroxycholesterol. Blocking 7-alpha-hydroxylase activity with ketoconazole also blocked the resistance of L35 cells to 25-hydroxycholesterol. Isolation of microsomes from these cells showed levels of 7-alpha-hydroxylase activity (22.9 pmol/min/mg of protein) that were comparable to the activity (33.2 pmol/min/mg) of microsomes isolated from the livers of rats killed during the high point of the diurnal cycle. Parental cells had no detectable activity. These data show a new complementation group for 25-hydroxycholesterol resistance: expression of 7-alpha-hydroxylase. Dexamethasone increased both the activity and the cellular content of mRNA coding for 7-alpha-hydroxylase. Since dactinomycin blocked the ability of dexamethasone to induce mRNA, active transcription is required. Southern analysis of genomic DNA showed that L35 cells contain the rat (endogenous) gene but not the human gene. Furthermore, the RNA expressed by L35 cells is similar in size to rat RNA and is distinct from the human form of 7-alpha-hydroxylase. The combined data indicate that L35 cells are resistant to 25-hydroxycholesterol because they express 7-alpha-hydroxylase. The mechanism responsible involves activation of the endogenous (silent) gene of the parental rat hepatoma cell.

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.

Heat-induced alterations in cell membrane permeability and cell inactivation of transformed mouse fibroblasts

Hyperthermia, has recently been extended in many permutations as a modality of anticancer treatment, but the mechanisms underlying heat-induced cell inactivation are poorly understood. In this study, the role of the cell permeability barrier in the process of heat cytotoxicity are examined. Changes in cell membrane permeability were determined by following the efflux of normally impermeant metabolites, e.g. nucleotides, in cultures of Swiss mouse 3T3 cells, and their transformed derivatives, 3T6 cells. The increase in cell membrane permeability as a function of temperature and exposure duration was found to be characterized by a sigmoid curve, with a threshold value, above which the permeability markedly increased. A correlation was found between cell membrane permeabilization and cell inactivation. Both heat-induced permeabilization and heat cytotoxicity were more pronounced in the transformed cells, as compared to their untransformed counterparts. The temperature-dependent permeabilization was more effective in the presence of the ionophore amphotericin B. The data suggest that heat-induced lesion in the cell membrane has a major role in hyperthermia cytotoxicity.

Surface morphological study of Ehrlich ascites tumor cells exposed to microwave irradiation and heat

Microwave irradiation of EAT cells caused an increase in length and number of surface microvilli. The tumor cells tend to form large aggregates by means of extensive interdigitation of surface microvilli. On the other hand, heat hyperthermia caused a decrease of surface microvilli but an increase of surface blebs. Hence the surface morphology of EAT cells after in vitro exposure to microwave irradiation differs markedly from that after heat hyperthermia.

Secondary mutation resistant to 7-ketocholesterol rescues a sterol metabolic defect in amphotericin B-resistant Chinese hamster cell line

Amphotericin B-resistant mutants isolated from Chinese hamster V79 cells (1) are defective in cholesterol synthesis and more sensitive to an oxygenated sterol analog, 7-ketocholesterol, than their parental cell line. We isolated 7-ketocholesterol-resistant mutants from an amphotericin B-resistant mutant, AMBR-1. The 7-ketocholesterol-resistant mutants had regained increased level of free cholesterol, and they showed somewhat similar dose-response curves to amphotericin B as that of V79. Sterol synthesis from acetate, but not from mevalonate, in 7-ketocholesterol-resistant clones was threefold higher than that of AMBR-1. 7-Ketocholesterol-resistant clone, unlike AMBR-1, could form colonies in the presence of lipoprotein-depleted serum. The results are discussed in terms of probable change in the sterol biosynthetic pathway by the different lesions.

Lipid vesicle-mediated alterations of membrane cholesterol levels: effects on Na+ and K+ currents in squid axon

We demonstrate that cholesterol can exchange from sonicated lipid vesicles to a perfused squid axon membrane and that vesicles with varying cholesterol/phospholipid (C/P) mole ratios can be used to achieve either net loading or net depletion of axon membrane cholesterol. Two types of evidence were obtained which show that net loading or depletion of cholesterol was achieved: (i) changes in the cholesterol/phospholipid (C/P) mole ratios of axons, and (ii) visualization of cholesterol depleted from the preparation by cholesterol-free vesicles by thin-layer chromatography. The C/P mole ratios indicate that cholesterol levels in the preparation were increased or decreased by 30-40%. Increasing or decreasing membrane cholesterol levels were ineffective in altering the Na+ or K+ occurrents in voltage-clamped axons. In addition, we determined that cholesterol "flip-flop" across the axonal membrane occurred with a t 1/2 of 7.3 to 15.3 min.

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.

Effect of sterol side chains on growth and membrane fatty acid composition of Saccharomyces cerevisiae

Saccharomyces cerevisiae GL7 cells require exogenous sterol and unsaturated fatty acid for growth. When grown in the presence of cholesterol or 7-dehydrocholesterol, the cells incorporated less saturated fatty acid into phospholipids than cells grown with ergosterol, stigmasterol, or beta-sitosterol as the sterol source. This lower saturated fatty acid content was most pronounced in phosphatidylethanolamine, slightly less so in phosphatidylcholine, and least evident in phosphatidylserine and phosphatidylinositol. Growing the cells with the various sterols did not affect the ratios of individual phospholipids. The ability of strain GL7 to use 7-dehydrocholesterol as the only sterol supplement for growth was dependent upon the nature of the unsaturated fatty acids added to the growth medium. In the presence of linoleic, linolenic, or a mixture of palmitoleic and oleic acids, excellent growth was observed with either ergosterol, cholesterol, or 7-dehydrocholesterol. However, when the medium was supplemented with either oleic or petroselenic acid, the cells grew more slowly (oleic) or much more poorly (petroselenic) with 7-dehydrocholesterol than with ergosterol. A specific relationship between sterol structure and membrane fatty acid composition in yeast cells is implied.

Adaptive alteration in phospholipid composition of plasma membranes from a somatic cell mutant defective in the regulation of cholesterol biosynthesis

A somatic cell mutant (CR1) of a Chinese hamster ovary cell (CHO-K1) which has previously been shown to be defective in the regulation of cholesterol biosynthesis accumulates more cholesterol than the parental cell line in plasma membranes. Although such an increase in membrane cholesterol should lead to an increase in the order parameter of these membranes, as measured with an electron spin resonance spin probe, the order parameters of mutant and wild-type plasma membranes are identical--apparently because of an adaptive alteration in membrane phospholipid composition. The phospholipid compositions of mutant and wild-type cell plasma membranes are compared and the mutant is shown to have a threefold higher level of oleic acid and a twofold lower level of phosphatidylethanolamine than the wild type. These results are consistent with model studies which show that these compositional changes lead to lower-order parameters for phospholipid dispersions.

Effect of alkyl-substituted precursors of cholesterol on artificial and natural membranes and on the viability of Mycoplasma capricolum

Various alkyl-substituted sterols and stanols representative of the intermediates in cholesterol biosynthesis from lanosterol have been compared with respect to (a) their effect on the physical state of lecithin vesicles, (b) their efficacy as growth factors for the sterol auxotroph Mycoplasma capricolum, and (c) their effect on the physical state of the respective mycoplasma membranes. By all three criteria, sterol effectiveness progresses in the order lanosterol less than 4,4-dimethylcholestanol less than or equal to 4 beta-methylcholestanol less than 4 alpha-methylcholestanol less than cholestanol less than cholesterol. Since the corresponding steps in cholesterol biosynthesis occur in the same order, we conclude that the nuclear modifications of the lanosterol structure by oxidative demethylation serve to improve the membrane function of the sterol molecule.

Rate limitation of (Na+ + K+)-stimulated adenosinetriphosphatase by membrane acyl chain ordering

A somatic cell mutant (CR1) of the Chinese hamster ovary cell line (CHO-K1) that is defective in the regulation of cholesterol biosynthesis can be grown under conditions in which plasma membranes from these cells display various cholesterol contents and acyl chain order parameters. The (Na+ + K+)-stimulated adenosinetriphosphatase (ATP phosphohydrolase, EC 3.6.1.3) from these cells was shown to vary in activity by a factor of 10 as the order parameter was varied, and the activity exhibited an exponential dependence on this parameter. Under these conditions the number of Na+,K+-ATPase molecules was shown to remain constant by affinity labeling with [gamma-32P]ATP in the absence of Na+. Control experiments showed that alteration in cholesterol content without change in order parameter did not result in altered enzyme activity. It is concluded that, under our conditions, the rate of catalysis by the Na+,K+-ATPase is determined by the order parameter. These studies suggest a physical mechanism by which variation of membrane lipid composition or other factors that determine membrane lipid acyl chain order parameter can result in variation in membrane enzyme activity.