Solubilization, purification, and characterization of a truncated form of rat hepatic squalene synthetase

Rat hepatic microsomal squalene synthetase (EC 2.5.1.21) was induced 25-fold by feeding rats with diet containing the hydroxymethylglutaryl-coenzyme A reductase inhibitor, fluvastatin, and cholestyramine, a bile acid sequestrant. A soluble squalene synthetase protein with an estimated mass of 32-35 kDa, as determined by gel filtration chromatography on Sephacryl S-200 column, was solubilized out of the microsomes by controlled proteolysis with trypsin. Approximately 25% of the activity was recovered in a soluble form. The enzyme was purified to homogeneity utilizing a series of column chromatography purification steps on DEAE-cellulose, hydroxylapatite, and phenyl-Sepharose sequentially. The purified enzyme showed a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Initial kinetic analysis indicated an S0.5 values for trans-farnesyl diphosphate of 1.0 microM and for NADPH of 40 microM. The Vmax with respect to trans-farnesyl diphosphate was calculated at 1.2 mumol/min/mg. NADH also serves as substrate for the reaction with S0.5 value of 800 microM. Western blot analysis utilizing rabbit antisera raised against the purified, trypsin-truncated enzyme showed a single band for the isolated solubilized enzyme at 32-33 kDa and a band for the intact microsomal enzyme at about 45-47 kDa.

Antiatherogenic properties of calcium antagonists. State of the art

Atherosclerosis is an arterial disease characterized by localized accumulation of collagen, elastin, lipids, and calcium at sites associated with macrophage infiltration and altered smooth muscle metabolism. Studies in several types of animal models, especially cholesterol-fed rabbits, have shown that calcium competitors, calcium chelators, anticalcifying agents, and calcium antagonists can reduce the accumulation of atherogenic lesion components and decrease the progression of lesions. Although there are some conflicting data in the animal model studies, it is now apparent that several classes of calcium antagonists inhibit the progression of early arterial lesions induced by cholesterol-feeding in animals. The dihydropyridine class of calcium antagonists may be more potent as anti-atherosclerotic agents than the other classes. Mechanisms involving regulation of endothelial cell, smooth muscle cell, and macrophage metabolism may be responsible for the effects of calcium antagonists on early lesion progression. Recent studies in cell culture-model systems suggest that calcium antagonists may significantly alter activities that regulate lipoprotein-derived cholesterol accumulation by arterial wall cells. Some of these activities are independent of calcium flux across voltage-operated calcium channels. Thus, calcium antagonists may reduce the progression of atherogenic lesions by a combination of decreasing calcium accumulation within arterial wall cells and by altering calcium channel-independent metabolic activities, which affect lesion development.

Protective action of calcium channel antagonists in atherogenesis and experimental vascular injury

Experimental research using in vitro and in vivo models of vascular injury have delineated several common mechanisms that characterize the arterial damage in diseases such as atherosclerosis and hypertension. Changes in endothelial permeability, smooth muscle cell proliferation, and accumulation of connective tissue matrix are major common mechanisms. Chronic hyperlipidemia is a major determinant of the proliferative arterial lesions in atherogenic models. Calcium antagonists of very diverse structure and function have been shown to have antiatherogenic potential in several animal model systems of arterial injury. Calcium channel-blockers of several chemical classes have been demonstrated to alter endothelial function, intimal smooth muscle proliferation, and lipid accumulation in the arterial wall. Cell culture model systems have elucidated several potential mechanisms that may contribute to the antiatherogenic potential of the calcium channel-blockers. These activities may in part involve protection of arterial cells from calcium overload via inhibition of calcium flux across voltage-regulated ion channels. However, other activities of these drugs, such as inhibition of cholesterol esterification and matrix protein formation, appear to function independently of calcium flux. A hypothesis is presented that lipophilic calcium channel-blockers are accumulated in cell membranes and perturb metabolic function as a result of altering local membrane structure.

The antiatherogenic potential of calcium antagonists

Atherosclerosis is an arterial disease characterized by focal accumulation of collagen, elastin, lipids, and calcium at sites associated with macrophage infiltration and altered smooth muscle metabolic function. Studies in several types of animal models, especially cholesterol-fed rabbits, have shown that calcium competitors, calcium chelators, anticalcifying agents, and calcium channel blockers can reduce the accumulation of atherogenic lesion components and thus apparently decrease the progression of lesions. Although there are some conflicting data in the animal model studies using calcium channel antagonists, as a result of differences in experimental designs, it is now apparent that several classes of calcium channel blockers inhibit the progression of early arterial lesions induced by cholesterol feeding. The dihydropyridine calcium channel blockers appear to be more potent antiatherosclerotic agents than other classes of calcium channel antagonists. Several mechanisms involving regulation of endothelial cell, smooth muscle cell, and macrophage metabolic functions may be responsible for the calcium channel blocker effects on early lesion progression. For example, recent studies in cell culture model systems suggest that calcium channel blockers may significantly alter activities that regulate lipoprotein-derived cholesterol accumulation by cells. Some of these activities are independent of calcium flux across voltage-operated calcium channels. Thus, calcium channel blockers may reduce the progression of atherogenic lesions by a combination of decreasing calcium accumulation within arterial wall cells and by altering calcium-independent metabolic activities.

Antiatherogenic properties of calcium antagonists

A generalized accumulation of cholesterol, calcium and matrix materials (collagen, elastin and proteoglycans) occurs in an age-dependent manner in major arteries. Human atherogenesis is a disease of arteries characterized by a focal accumulation of fibrous matrix elements, lipids and calcium at lesion sites. Studies in cholesterol-fed animal models have indicated that calcium competitors and chelating agents can reduce calcium, lipid and matrix accumulation in arterial lesions and reduce the extent of lesion formation. These agents generally alter soft and hard tissue calcium pools or have deleterious side-effect profiles. Antiatherogenic studies with calcium antagonists (which have been shown to be safe in human clinical studies) have created confusion because of conflicting results. It is apparent, however, that high doses of calcium antagonists can significantly decrease atherogenic lesion development in cholesterol-fed rabbits. The antiatherogenic effects of calcium antagonists may be the result of changes in intracellular calcium pools within smooth muscle cells, which may lead to alterations in cellular metabolic activity or may be due to activities not related to calcium channel effects. Several mechanisms involving regulation of lipoprotein receptor synthesis, lipoprotein uptake or degradation, cholesterol ester hydrolytic activity and arterial matrix synthesis are discussed as potential sites of activity for calcium antagonists. A dihydropyridine channel antagonist, PN 200-110 (isradipine), has been shown to be a very potent antiatherogenic agent in the rabbit and also to be a potent inhibitor of smooth muscle cell matrix synthesis.

Lipoprotein synthesis and secretion by cultured rat hepatocytes. Parallel inhibition of secretion of VLDL, HDL and albumin by monensin

The biosynthesis and secretion of very-low-density lipoproteins (VLDL) and high-density lipoproteins (HDL) by cultured normal rat hepatocytes was investigated with particular emphasis on its modification by monensin. This acidic ionophore coordinately inhibited the rates of secretion of the several VLDL apolipoproteins and the VLDL lipids, suggesting an effect late in the process of biosynthesis and secretion, probably at the stage of exiting from the Golgi apparatus. The secretion of immunoreactive albumin into the medium was comparably inhibited, implying that the pathway and mechanisms involved in albumin secretion may be closely similar to those for VLDL synthesis and secretion. Secretion of phospholipids and of apolipoproteins E and A-I in the HDL fraction increased progressively with time over 18 h in control incubations but was strongly inhibited by monensin. During extended incubation with monensin at high concentrations (10 microM), there was a net release to the medium of a number of hepatocyte proteins, including some that comigrated with apolipoprotein A-I and apolipoprotein C, making it appear that monensin increased the secretion of these apolipoproteins. However, using labeled amino acids, it was shown by autoradiography and by immunoprecipitation that secretion of newly-synthesized, radioactive apolipoprotein A-I and apolipoprotein C was actually inhibited by monensin. These results are compatible with the conclusion that HDL synthesis and secretion may occur by mechanisms closely related to those for synthesis and secretion of albumin and VLDL.

Isolation and analysis of lipoproteins secreted by rat liver hepatocytes

A procedure has been developed for the small-scale isolation and characterization of lipoproteins secreted by cultured rat liver hepatocytes. The lipoproteins in the culture medium were separated into VLDL, LDL, HDL and a fraction with d greater than 1.21 on single-spin density-gradients. The lipoproteins were removed from the gradients by adsorption onto Cab-O-Sil, a hydrated colloidal silica. The lipid components were extracted from the silica with CHCl3/CH3OH and the apoproteins solubilized in a buffer that contained 2% sodium dodecyl sulfate and 6 M urea. The proteins were analyzed on 3-20% acrylamide electrophoresis gels that contained 1% sodium dodecyl sulfate. The two major rat-plasma lipoproteins, VLDL and HDL, were well separated by the gradients. The Cab-O-Sil was shown to bind 90-95% of the HDL and VLDL in the fractions from the gradient. The recovery of the lipid components was essentially quantitative. The recovery of the apolipoproteins was only about 60% but with very good precision. Over a 20 h period, the lipid phosphorus associated with secreted lipoproteins increased linearly. The secretion of apolipoprotein A1 and apolipoprotein E associated with HDL and apolipoprotein B associated with VLDL also increased as a nearly linear function with time. The secretion of apolipoprotein E associated with VLDL was linear only up to approx. 6 h. The availability of this procedure should greatly facilitate further studies on the characterization of lipoproteins secreted by hepatocytes and mechanisms that regulate lipoprotein synthesis and secretion.

Androgen and FSH synergistically stimulate lipoprotein degradation and utilization by ovary granulosa cells

Androgen can directly modulate the induction of steroidogenic enzymes by FSH (follicle stimulating hormone) in ovary granulosa cells. In studies of its mechanism of action, we examined the androgen effect on granulosa cell interaction with lipoproteins, the physiologic source of cholesterol. After granulosa cells were cultured for 48 hours with and without androgen and/or FSH, the cells were incubated for 24 hours with 125I-lipoproteins [human high density lipoprotein (HDL), rat HDL, or human low density lipoprotein (LDL)]. The media were then analyzed for lipoprotein protein coat degradation products (mainly 125I-monoiodotyrosine) and progestin [mainly 20 alpha-dihydroprogesterone (20 alpha-DHP)]. In the absence of FSH and androgen, 2 X 10(5) granulosa cells degraded basal levels of all three lipoproteins, but produced no measurable 20 alpha-DHP. The addition of 10(-7) M androstenedione (A), testosterone (T), or 5 alpha-dihydrotestosterone (DHT) had no effect on lipoprotein protein degradation or 20 alpha-DHP production. FSH alone stimulated lipoprotein protein degradation by 50 to 300% while the addition of androgen synergistically augmented the FSH-stimulated 20 alpha-DHP production as well as protein coat degradation of all three lipoproteins. DHT and T were both effective, indicating that androgens themselves, and not estrogen products, were responsible for the effect on lipoprotein protein degradation and 20 alpha-DHP production. The addition of a 10-fold excess cyproterone acetate (an anti-androgen) inhibited the effect of T, suggesting that the action of T was mediated by the granulosa cell androgen receptor. Androgen and FSH also synergistically stimulated the production of 3H-progestin when the granulosa cells were incubated with either 3H-cholesterol ester core labeled human HDL or similarly labeled human LDL. This report demonstrates that androgen, in combination with FSH, augments the steroidogenic pathway of the granulosa cell from the degradation of lipoprotein and utilization of the cholesterol ester core, to the production of progestin product.

Activity of acyl-CoA: cholesterol acyltransferase and 3-hydroxy-3-methylglutaryl-CoA reductase in subfractions of hepatic microsomes enriched with cholesterol

The influence of membrane cholesterol on the activities of acyl-CoA: cholesterol acyltransferase and 3-hydroxy-3-methylglutaryl-CoA reductase was examined in three microsomal subfractions (RNA-rich, RNA-poor, and smooth) that had been enriched with cholesterol by incubation with mixed lipoproteins from hypercholesterolemic rabbit serum. Acyl-CoA: cholesterol acyltransferase activity was significantly stimulated in the three subfractions, particularly in the RNA-rich microsomal component. 3-Hydroxy-3-methylglutaryl-CoA reductase, on the other hand, was suppressed (30%) in only one (RNA-poor) of the three microsomal subfractions, despite a 1.4-fold increase in the concentration of membrane cholesterol. An attempt was made to distinguish between an effect based exclusively on an increase in available cholesterol substrate and an activation of acyl-CoA: cholesterol acyltransferase in RNA-rich microsomes enriched with cholesterol. An experimental design was devised so that substrate cholesterol was provided in the form of heated smooth microsomes and acyl-CoA: cholesterol acyltransferase was provided as a separate preparation in the form of RNA-rich microsomes. Appropriate controls were carried out to test for transfer of cholesteryl ester between the two sets of particles. The results suggested that cholesterol enhanced acyl-CoA: cholesterol acyltransferase activity by serving both as a substrate and as a non-substrate modulator.

Dissociation of tissue uptake of cholesterol ester from that of apoprotein A-I of rat plasma high density lipoprotein: selective delivery of cholesterol ester to liver, adrenal, and gonad

The metabolic fate of homologous high density lipoprotein (HDL) was studied in the rat, tracing the apoprotein A-I (apo A-I) and cholesterol ester moieties simultaneously. The apo A-I was labeled with covalently linked 125I-labeled tyramine cellobiose, which accumulates in the cells degrading the apoprotein; [3H]cholesterol ethers, which cannot be hydrolyzed or mobilized after uptake, were incorporated into the lipid core of reconstituted HDL to reflect the fate of the cholesterol esters. Several lines of evidence, including direct comparison with biologically labeled HDL, are presented to support the validity of this approach. The liver was the major organ of cholesterol ether uptake, accounting for 65% of the total; the adrenal gland and ovary were the most active organs per gram (wet) of weight. Uptake of cholesterol ether was 7-fold greater than that of apo A-I in adrenal, 4-fold greater in the ovary, and greater than 2-fold greater in the liver. The remaining tissues took up apo A-I and cholesterol ethers at more nearly equal rates. Transfer of HDL-associated cholesterol ethers and 125I-labeled apo A-I to other lipoprotein fractions was not observed; thus, the results reflect direct uptake from HDL itself. Whereas uptake of low density lipoprotein appears to involve endocytosis of intact particles, uptake of HDL in at least some rat tissues involves additional, more complex, transfer mechanisms.

Effects of insulin and glucose on very low density lipoprotein triglyceride secretion by cultured rat hepatocytes

The effect of insulin on hepatic triglyceride synthesis and secretion is controversial. Previously, we have described a cell culture system of adult rat hepatocytes that synthesize and secrete very low density lipoprotein (VLDL) triglycerides with small and irreproducible effects of insulin on triglyceride metabolism. To study the primary effects of insulin on hepatic triglyceride metabolism a method was developed utilizing fibronectin-coated culture dishes that allowed adhesion, spreading, and maintenance of hepatocytes for 2-3 d in the absence of serum and insulin. This culture system allowed mass measurements of both cellular and secreted VLDL triglycerides for long time periods after the addition of physiological concentrations of insulin to hormone-free culture medium. In the absence of insulin and after an initial 4 h in culture, the medium was replenished and triglyceride mass was measured at the end of 18-h incubations. VLDL triglyceride accumulated in the culture medium at a linear rate over this time-course with increasing accumulation as the medium glucose concentration was raised from 2.5 to 25 mM glucose (1.77+/-0.24 to 3.09+/-0.76 mug triglyceride/mg cell protein per h). There was no apparent significant lipolysis or hepatocellular reuptake of secreted VLDL triglycerides. In the absence of insulin cellular triglyceride levels were unchanged between 3 and 24 h in culture while insulin (50-500 muU/ml) significantly increased cellular triglyceride content at all glucose concentrations tested (0-25 mM). The addition of insulin to the culture medium progressively reduced the rate of VLDL triglyceride secretion accompanied by an increase in cellular triglyceride at insulin concentrations > 50 muU/ml. Most or all of the observed increase in cell triglyceride content could in all experiments be accounted for by the insulin-induced inhibition of VLDL secretion. Incorporation of [2-(3)H]glycerol into cellular and VLDL triglycerides as a function of insulin concentration was also measured. Glycerol incorporation data at 20-22 h after plating of the cells closely paralleled the insulin-induced changes in cellular and VLDL triglyceride as determined by mass analysis. The observed effects of insulin occurred at concentrations close to the physiological range and suggest that the direct hepatic effect is to suppress VLDL secretion although the net effect in vivo will clearly reflect many additional accompanying changes.

Viscosity of culture medium as a regulator of synthesis and secretion of very low density lipoproteins by cultured hepatocytes

It has been postulated that hyperlipidemia in the nephrotic syndrome is due to overproduction of lipoproteins and that low colloid osmotic pressure (due to hypoalbuminemia) triggers this. Secretion of very low density lipoproteins (VLDL) by cultured rat hepatocytes has been shown to be inhibited by albumin, globulins, and dextrans, but the effect did not correlate with osmolarity. In the present studies we tested the hypothesis that viscosity rather than osmolarity might be the parameter determining the effectiveness of macromolecules in inhibiting VLDL synthesis and secretion by cultured rat hepatocytes. Synthesis and secretion of VLDL was measured in terms of incorporation of [3H] glycerol into medium triglycerides and also from changes in the mass of secreted VLDL triglycerides and apoproteins. The viscosity of the culture medium was increased by addition of dextran-500, gelatin or methylcellulose MX 880. Synthesis and secretion of VLDL was inhibited in direct proportion to increasing viscosity. At a viscosity of 2, which is about that of normal plasma, VLDL secretion was reduced by 20%. An inhibition of 60-70% in secretion and 30-40% in synthesis of VLDL lipid and protein components was observed at a relative viscosity of approximately 3.7. This viscosity was obtained by addition of any of the following: 3% dextran, 3% gelatin, 0.2% methylcellulose, or a combination of 0.1% methylcellulose plus 2% gelatin. Thus, similar viscosities resulted in similar degrees of inhibition despite differences of up to 16-fold in mass concentration and up to 20-fold in osmolarity.

Defective catabolism of low-density lipoprotein by fibroblasts from patients with I-cell disease

Skin fibroblast cultures from patients with I-cell disease (mucolipidosis II) are characterized by multiple lysosomal enzyme deficiencies The present studies deal with the consequences of these deficiencies with respect to the metabolism of plasma low-density lipoproteins. Degradation of the protein moiety was defective in I-cells compared with control cells, but the binding and internalization of low density lipoprotein were much less affected. Measurements of low-density lipoprotein degradation in homogenates demonstrated directly for the first time a deficiency of acid proteinase activity in I-cell fibroblasts. Comparison of results in 6-h incubations with those in 24-h incubations showed accumulation of intracellular low-density lipoprotein in I-cell fibroblasts and an accelerating rate of degradation, possibly attributable to intracellular accumulation of low-density lipoprotein substrate. The significance of these findings with respect to low-density lipoprotein metabolism in vivo is discussed.

Lipoproteins stimulate androgen production by cultured rat testis cells

The effects of human high density lipoprotein (hHDL), human low density lipoprotein (hLDL), and rat high density lipoprotein (rHDL) on androgen production by cultured rat testis cells were investigated. Enzymatically dissociated testis cells from hypophysectomized adult rats were cultured in a serum-free medium. During the first 2 days of culture, the addition of human or rat lipoprotein alone stimulated testis cell testosterone (T) production by 100 to 250% in a dose-dependent manner. Likewise, treatment with hCG caused a 2.5-fold increase in T production. Furthermore, the effect of lipoproteins plus hCG was synergistic; the stimulation of T production by concomitant treatment with hCG and lipoproteins was greater than the sum of each added individually. Rat HDL augmented hCG stimulated T production in a concentration-dependent manner and maximum synthesis was achieved at 100 micrograms protein/ml (240% increase) with an ED50 value of 25 micrograms/ml rHDL. At low concentrations (10-30 micrograms protein/ml), all lipoproteins tested had similar stimulatory effects on T production but at the highest dose tested (300 micrograms protein/ml), hHDL was more effective than rHDL. The stimulatory effect of lipoprotein was shown to be time-dependent. Maximum stimulation of T production by lipoprotein was seen during the initial 48 h of culture, whereas lipoproteins were ineffective during the first 5 h and during days 8 to 10 of culture. The present data are consistent with the concept that lipoproteins provide cholesterol substrate while gonadotropins stimulate tha rate limiting enzyme(s) which convert cholesterol to T. This primary culture model of testis cells in serum-free medium is responsive to serum lipoproteins and offers a unique opportunity to study the direct effect of lipoproteins and gonadotropins on testicular steroidogenesis.

Degradation of rat and human lipoproteins by cultured rat ovary granulosa cells

We have investigated the degradation of 125I-labeled rat and human lipoproteins by rat ovary granulosa cells cultured in serum-free medium. The granulosa cells degrade rat [125I]iodo high density lipoprotein (HDL) to acid-soluble products, mainly monoiodotyrosine. The degradation of 125I-labeled rat HDL is a specific, saturable, high affinity (Km = 21 micrograms protein/ml) process. In studies of rat [125I]iodo-HDL degradation and progestin (progesterone plus 20 alpha-dihydroprogesterone) production by the same granulosa cell cultures, the cholesterol potentially made available to the cells by degradation can account for the majority of the substrate necessary for the increased progestin production. Granulosa cells degrade human [125I]iodo-HDL by a specific, saturable, high affinity (Km = 20 micrograms protein/ml) process. The degradation of human [125I]iodo-HDL can account for only 20% of the cholesterol substrate necessary for increased progestin production. The degradation of human [125I]iodo-low density lipoprotein (LDL) is saturable and a high affinity (Km = 8 micrograms protein/ml) process, but can be inhibited significantly by a 10-fold excess of unlabeled human HDL. In contrast to both rat [125I]iodo-HDL and human [125I]iodo-HDL, the degradation of human [125I]iodo-LDL can potentially provide twice the cholesterol necessary for increased progestin production. Pronase treatment of the granulosa cells inhibits human [125I]iodo-LDL degradation but stimulates rat [125I]iodo-HDL degradation, indicating that the mechanisms of degradation are separate. The data demonstrate that cultured rat ovary granulosa cells degrade rat HDL, human HDL, and human LDL, and this process has the potential for providing cholesterol for cellular steroid hormone synthesis.

New function for high density lipoproteins. Isolation and characterization of a bacterial lipopolysaccharide-high density lipoprotein complex formed in rabbit plasma

The addition of bacterial lipopolysaccharide (LPS) from Salmonella minnesota R595 to rabbit plasma results in a marked reduction of the hydrated buoyant density of the parent R595 LPS, from 1.38 to less than 1.2 g/cm3. Using immunopurified anti-R595 LPS antibody covalently linked to Sepharose 4B, we were able to separate the altered R595 LPS (d less than 1.2 g/cm3) from the remainder of the plasma proteins by elution of the bound material with 2.5 M KSCN. The KSCN eluate was shown to have a d less than 1.2 g/cm3 and to contain both R595 LPS as well as protein and lipid characteristic of high density lipoprotein (HDL). The major protein in the KSCN eluate is a single polypeptide chain with an apparent molecular weight of 26,000 in sodium dodecyl sulfate and an amino acid composition essentially identical to that of apoprotein AI, the major protein of rabbit HDL. The lipid composition of the KSCN eluate is similar to that of HDL, although marked differences in the cholesterol ester/cholesterol ration and the phosphatidyl choline/phosphatidyl ethanolamine ratio were observed when the KSCN eluate and rabbit HDL were compared. The formation of this R595 LPS-protein-lipid complex in plasma accounts for the marked reduction in buoyant density found when LPS is added to plasma.

Regulation of cholesterol esterification and biosynthesis in monolayer cultures of normal adult rat hepatocytes

Adult rat parenchymal liver cells were isolated and cultured in monolayers. Cholesterol esterification in the intact cultured cells was determined by measuring incorporation of tritiated oleic acid into cell cholesterol ester. Addition of 10 microgram/ml of 25-hydroxycholesterol to the medium gave a 3- to 6-fold increase in cholesterol esterification, while the incorporation of oleic acid into phospholipids and triglycerides remained unaltered. Similar stimulation of cholesterol esterification by 25-hydroxycholesterol was also found if [14C]mevalonolactone or [3H]cholesterol (the latter presented to the cells in high density lipoproteins) were used as precursors. The stimulatory effect of 25-hydroxycholesterol was maximal after only 15-min incubation and was independent of protein synthesis. After 4 to 6 h of incubation with 25-hydroxycholesterol, its stimulatory effect was reduced significantly, and after 18 h of incubation no stimulation was observed. Thus, the liver cells in some fashion adapt to the continuing presence of 25-hydroxycholesterol. Isolated microsomes prepared from cells previously incubated with 25-hydroxycholesterol showed acyl-CoA:cholesterol acyltransferase activity 3 times that of microsomes from control cells. Incubation of isolated microsomes with 25-hydroxycholesterol increased acyl-CoA:cholesterol acyltransferase activity 2-fold. The net cellular content of ester cholesterol increased after 2 to 6 h incubation of hepatocytes with 25-hydroxycholesterol; there was a net decrease in cellular free cholesterol. Mevalonolactone (10 mM) also stimulated cholesterol esterification and inreased the cellular content of ester cholesterol (3- to 4-fold). The effectiveness of mevalonolactone did not diminish with longer periods of preincubation. Furthermore, the stimulatory effects of 25-hydroxycholesterol and mevalonolactone added together were at least 50 to 100% greater than the effects of either agent alone, suggesting that the mechanisms by which they increase cellular cholesterol esterification are different. Both 25-hydroxycholesterol and mevalonolactone rapidly inhibited microsomal 3-hydroxy-3-methylglutaryl-CoA reductase activity. Pure cholesterol had no effect on cellular cholesterol esterification or on 3-hydroxy-3-methylglutaryl-CoA reductase activity at concentrations up to 10 microgram/ml.

New function for high density lipoproteins. Their participation in intravascular reactions of bacterial lipopolysaccharides

The addition of bacterial lipopolysaccharide (LPS) from Escherichia coli 0111:B4 or Salmonella minnesota R595 to plasma (or serum) resulted in a marked reduction of the hydrated buoyant density of the parent LPS (0111:B4 [d = 1.44 g/cm3] and R595 [d = 1.38 g/cm3]), to d less than 1.2 g/cm3. This reduction in buoyant density to less than 1.2 g/cm3 of the LPS required plasma (or serum) lipid. Delipidation of plasma (or serum) by extraction with n-butanol/diisopropyl ether (40/60, vol:vol) prevented the conversion of the parent LPS to a form with d less than 1.2 g/cm3. Reversal of the effect of delipidation was accomplished by the addition of physiologic concentrations of high density lipoprotein (HDL). In contrast, as much as two times normal serum concentration of low density or very low density lipoprotein were ineffective. The ability of normal plasma (or serum) to inhibit the pyrogenic activity of LPS, lost after delipidation, was also restored after the addition of HDL. Preliminary results suggested that prior modifications of the LPS, probably disaggregation, may be required before interaction with HDL.

Metabolic studies in an unusual case of asymptomatic familial hypobetalipoproteinemia with hypolphalipoproteinemia and fasting chylomicronemia

A new kindred with asymptomatic hypobetalipoproteinemia is reported. The proband, age 67, differs from previously described cases in several respects: (a) unusually low levels of low density lipoprotein (LDL) cholesterol (4-8 mg/dl); (b) normal triglyceride levels; (c) low levels of high density lipoprotein; (d) mild fat malabsorption; and (e) a defect in chylomicron clearance. On a high-carbohydrate diet his plasma triglyceride levels, instead of rising, actually fell. Turnover of triglycerides in very low density lipoproteins (VLDL) was low (2.8 mg/kg per h). Fractional catabolic rate of LDL protein was just above the normal range (0.655/d) but net turnover was <10% of normal (0.65 mg/kg per d). The half-life of his chylomicrons was 29 min, five times the normal value. Postheparin lipoprotein lipase activity was normal and apolipoprotein C-II, the activator protein for lipoprotein lipase, was present and functional. Apolipoprotein C-III(1), however, was not detected in the VLDL fraction, a finding previously reported in patients with abetalipoproteinemia. Fecal excretion of cholesterol was almost twice normal; total sterol balance was increased by congruent with40%. The unusual features in the proband that distinguish him from previously described cases and from his affected first-degree relatives suggested that, in addition to the basic gene defect affecting LDL metabolism, he might have a second abnormality affecting clearance of chylomicrons and VLDL. The ratio of apolipoprotein E(3) to E(2) in his VLDL fraction was 0.93, just below the lower limit of normal, suggesting heterozygosity for E(3) deficiency. Whether or not this contributes to his hypertriglyceridemia remains to be established.

Unaltered catabolism of desialylated low-density lipoprotein in the pig and in cultured rat hepatocytes

Removal of the terminal sialic acid residues from many serum glycoproteins results in exposure of their penultimate galactose residues and rapid clearance from circulation by the liver. Low-density lipoprotein is a glycoprotein containing 21 galactose and 9 sialic acid residues per particle. Studies in this laboratory and others have shown that both the liver and extrahepatic tissues contribute to the degradation of low-density lipoprotein. This study was undertaken to determine whether desialylation of pig low-density lipoprotein alters its removal from circulation. Low-density lipoprotein was incubated at 37 degrees C with an agarose-bound neuraminidase, proteinase-free, from Clostridium perfringens. After 18 h at pH 5.0, 70% of the sialic acid residues were removed. The desialylated 131I-labelled and native 125I-labelled low-density lipoproteins were simultaneously injected into a pig, and their disappearance from plasma was followed for 96 h. The turnovers of the two were identical. In contrast, neuraminidase-treated fetuin was cleared about 200-fold faster than native fetuin. Studies were also performed in cultured rat hepatocytes. Rates of degradation of native and neuraminidase-treated low-density lipoprotein were similar, whereas asialo-fetuin was degraded at six to ten times the rate of native fetuin. Thus desialylation does not appear to alter low-density-lipoprotein catabolism by hepatic or extrahepatic cells.

Uptake and degradation of high density lipoprotein: comparison of fibroblasts from normal subjects and from homozygous familial hypercholesterolemic subjects

Fibroblasts cultured from the skin of subjects with homozygous familial hyperlipoproteinemia (HFH) internalize and degrade low density lipoproteins at a much lower rate than do fibroblasts from normal subjects. Evidence has been presented that this reflects the absence from such mutant cells of specialized binding sites with high affinity for low density lipoproteins. The specificity of this membrane defect in familial hypercholesterolemia is further supported by the present studies comparing the metabolism of low density lipoproteins (LDL) and high density lipoproteins (HDL) in normal fibroblasts and in fibroblasts from HFH patients. The surface binding (trypsin-releasable (125)I) of (125)I-labeled LDL by HFH cells was approximately 30% of that by normal cells at a concentration of 5 micro g LDL protein per ml. At the same concentration the internalization (cell-associated (125)I after trypsinization) and degradation (trichloroacetic acid-soluble non-iodide (125)I) of (125)I-labeled LDL were less than 10% of the values obtained with normal cells. In contrast, the binding of (125)I-labeled HDL to HFH cells was actually somewhat greater than that to normal cells. Despite this, the internalization and degradation of (125)I-labeled HDL by HFH cells averaged only 70% of that by normal cells. [(3)H]- or [(14)C]Sucrose uptake, a measure of fluid uptake by pinocytosis, was similar in normal and HFH fibroblasts. These findings are consistent with the proposal that fibroblasts from subjects with HFH lack high-affinity receptors for LDL. These receptors do not play a significant role in HDL binding and uptake. Instead, as previously proposed, HDL appears to bind randomly on the cell surface and its internalization is not facilitated by the specific mechanism that internalizes LDL. The small but significant abnormalities in HDL binding and internalization, however, suggest that there may be additional primary or secondary abnormalities of membrane structure and function in HFH cells. Finally, the observed overall rate of uptake of LDL (that internalized plus that degraded) by HFH fibroblasts was considerably greater than that expected from fluid endocytosis alone. This implies that adsorptive endocytosis, associated with binding to low-affinity sites on the cell surface, may play a significant role in LDL degradation by HFH cells, even though it does not regulate endogenous cholesterol synthesis in these cells.

Lipoprotein and cholesterol metabolism in rabbit arterial endothelial cells in culture

Like all other peripheral cells types thus far studied in culture, endothelial cells derived from the rabbit aorta bind, internalize and degrade low density lipoprotein (LDL) at a significant rate. At any given LDL concentration, the metabolism by rabbit endothelial cells was slower than that by fibroblasts or smooth muscle cells. Thus, longer incubations were required to achieve a net increment in cell cholesterol content or to suppress endogenous sterol synthesis; after 18-24 h incubation in the presence of LDL at 100 microgram LDL protein/ml inhibition was greater than 80% relative to the rate in cells incubated in the absence of lipoproteins. High density lipoproteins (HDL) were also taken up and degraded but did not inhibit sterol synthesis. Studies of LDL binding to the cell surface suggested the presence of at least two classes of binding sites; the high-affinity binding sites were fully saturated at very low LDL concentrations (about 5 microgram LDL protein/ml). However, the degree of inhibition of endogenous sterol synthesis increased progressively with increasing LDL concentrations from 5 to 100 microgram LDL/ml, suggesting that uptake from the low affinity sites in this cell line contributes to the suppression of endogenous sterol synthesis. The internalization and degradation of LDL also increased with concentrations as high as 700 microgram/ml. Thus, in vivo, where the cells are exposed to LDL concentrations far above that needed to saturate the high affinity sites, most of the LDL degradation would be attributable to LDL taken up from low affinity sites. As noted previously in swine arterial smooth muscle cells and in human skin fibroblasts, unlabeled HDL reduced the binding, internalization and degradation of labeled LDL. Cells incubated for 24 h in the presence of high concentrations of LDL alone showed a net increment in cell cholesterol content; the simultaneous presence of HDL in the medium significantly reduced this LDL-induced increment in cell cholesterol content. The possible relationship between LDL uptake and degradation by these cells in vitro is discussed in relationship to their transport function in vivo.

Interactions of native and modified human low density lipoproteins with human skin fibroblasts

125I-labeled low density lipoprotein (LDL) covalently bonded to Sepharose beads was not degraded by normal human fibroblasts nor did it trigger inhibition of sterol synthesis. The Sepharose beads loaded with LDL bound very tightly to the surface both of normal fibroblasts and fibroblasts from a subject with homozygous familial hypercholesterolemia; control Sepharose beads (activated sites covered with glycine) did not adhere to either cell type. LDL was extracted by a modification of the method of Gustafson (Gustafson, A. (1965) J. Lipid Res. 6, 512-517), so as to remove essentially all cholesterol, cholesterol ester and triglyceride. This modified LDL was bound, internalized and degraded as well as or better than native LDL. However, it failed to suppress sterol synthesis. These results provide additional evidence that the sterol moiety of the LDL is the key component affecting sterol synthesis. They also imply that the neutral lipids of LDL play a minor role in the binding of LDL to cell membranes and that the apoprotein rather than molecular size and shape is the critical factor.

Binding, internalization, and degradation of high density lipoprotein by cultured normal human fibroblasts

Comparative studies were made of the metabolism of plasma high density lipoprotein (HDL) and low density lipoprotein (LDL) by cultured normal human fibroblasts. On a molar basis, the surface binding of (125)I-HDL was only slightly less than that of (125)I-LDL, whereas the rates of internalization and degradation of (125)I-HDL were very low relative to those of (125)I-LDL. The relationships of internalization and degradation to binding suggested the presence of a saturable uptake mechanism for LDL functionally related to high-affinity binding. This was confirmed by the finding that the total uptake of (125)I-LDL (internalized plus degraded) at 5 micro g LDL protein/ml was 100-fold greater than that attributable to fluid or bulk pinocytosis, quantified with [(14)C]sucrose, and 10-fold greater than that attributable to the sum of fluid endocytosis and adsorptive endocytosis. In contrast, (125)I-HDL uptake could be almost completely accounted for by the uptake of medium during pinocytosis and by invagination of surface membrane (bearing bound lipoprotein) during pinocytosis. These findings imply that, at most, only a small fraction of bound HDL binds to the high-affinity LDL receptor and/or that HDL binding there is internalized very slowly. The rate of (125)I-HDL degradation by cultured fibroblasts (per unit cell mass) exceeded an estimate of the turnover rate of HDL in vivo, suggesting that peripheral tissues may contribute to HDL catabolism. In accordance with their differing rates of uptake and cholesterol content, LDL increased the cholesterol content of fibroblasts and selectively inhibited sterol biosynthesis, whereas HDL had neither effect.

Interaction between high density and low density lipoproteins uptake and degradation by cultured human fibroblasts

High density lipoprotein (HDL) inhibited the binding (trypsin-releasable radioactivity), internalization (cell-associated radioactivity after trypsinization), and degradation (TCA-soluble non-iodide radioactivity) of (125)I-low density lipoprotein ((125)I-LDL) by cultured normal human fibroblasts. At HDL:LDL molar ratios of 25:1 (protein ratios about 5:1), these parameters were reduced by about 25%. Unlabeled LDL was about 25 times more effective in reducing (125)I-LDL binding, implying that if HDL and LDL bind at common sites the affinity of HDL for these sites is very low or that the interaction is on some other basis. The fractional reduction in (125)I-LDL binding at a given HDL: (125)I-LDL ratio was independent of (125)I-LDL concentration and occurred equally with fibroblasts from a subject with homozygous familial hypercholesterolemia. Reciprocally, the binding, internalization, and degradation of (125)I-HDL were reduced by LDL. Preincubation of fibroblasts with HDL (or LDL) reduced the subsequent binding of (125)I-LDL (or (125)I-HDL) during a second incubation. In other studies HDL reduced the net increase in cell cholesterol content induced by incubation with LDL. HDL alone had no net effect on cell cholesterol content. These findings suggest that HDL reduces both the high affinity and the low affinity binding of LDL to human fibroblasts and that this in turn reduces the internalization and degradation of LDL. The effect of HDL on the LDL-induced changes in cell cholesterol content could be in part on this basis and in part on the basis of an HDL-stimulated release of cholesterol from the cells. These effects of HDL in vitro may be relevant to the negative correlations reported from in vivo studies between plasma HDL concentration and both body cholesterol pool size and the prevalence of clinically manifest atherosclerosis but further studies will be needed to establish this.

Growth control of differentiated fetal rat hepatocytes in primary monolayer culture. IX. Specific inhibition of DNA synthesis initiation by very low density lipoprotein and possible significance to the problem of liver regeneration

Rat serum very low density lipoprotein (VLDL) inhibits initiation of DNA synthesis in fetal rat hepatocyte cultures; cells engaged in synthesizing DNA resist inhibition. VLDL action is specific and apparently blocks prereplicative protein synthesis. These and other results, from studies of altered blood VLDL levels and [3H] thymidine incorporation into isolated liver nuclei in 70% hepatectomized normal and mutant hyperlipoproteinemic rats, as well as from infusion studies with a "mitogenic" hormone solution, suggest that hepatic VLDL metabolism is linked to the suppression of hepatocyte proliferation.

Uptake and degradation of low density lipoprotein by swine arterial smoot muscle cells with inhibition of cholesterol biosynthesis

We have previously proposed on the basis of studies in hepatectomized animals that low density lipoproteins are degraded at a significant rate by peripheral tissues. To test the capacity of one peripheral cell type to catabolize low density lipoprotein, cultures of swine aortic smooth muscle cells were incubated with homologous 125I-labeled low density lipoprotein and uptake and degradation measured. Degradation of 125I-labeled low density lipoprotein to products soluble in trichloroacetic acid showed an initial lag period of 1--2 h after which the rate increased and remained linear for the following 15 h. Rates of degradation increased sharply with low density lipoprotein concentration over the lower range (from 0--25 mug protein/ml) and then more slowly up to the highest concentration tested, 300 mug protein/ml. Even at very low concentrations, 1 mug low density lipoprotein protein/ml (less than 10% of the plasma low density lipoprotein concentration), the in vitro degradation rate (per kg of smooth muscle cells) exceeded the in vivo degradation rate (per kg of total body weight). To the extent that smooth muscle cells are representative of other peripheral cells, the results support the proposal that peripheral degradation of low density lipoprotein apoprotein may be quantitatively important. The rate of incorporation of labeled acetate into sterols was suppressed in cells incubated with whole serum, low density and very low density lipoproteins, or suspensions of free cholesterol. In this respect, the results were similar to those observed in human skin fibroblasts studied concurrently. However, high density lipoprotein inhibited sterol synthesis by about 25% in swine smooth muscle cells while it had no effect in human skin fibroblasts.

Binding, internalization, and degradation of low density lipoprotein by normal human fibroblasts and by fibroblasts from a case of homozygous familial hypercholesterolemia

Skin fibroblasts from a patient with homozygous familial hypercholesterolemia (HFH) were compared with normal skin fibroblasts with regard to binding, internalization, and degradation of iodinated human low density lipoprotein (LDL). Like other cell lines from HFH patients, the mutant cells showed no suppression of sterol synthesis by LDL. Surface binding, measured at 0 degrees to eliminate the appreciable internalization that was shown to occur at 37 degrees, was on the average slightly less for HFH cells than normal cells at low LDL concentrations but comparable or even greater at high LDL concentrations (greater than 60 mug of LDL protein per ml). A major defect observed was in the rate of internalization of LDL at 37 degrees, which was only 1-10% of that in normal cells. LDL degradation was also markedly reduced but not to the same extent. Thus, a larger fraction of the LDL taken up appeared to be degraded by the mutant cells. The most striking defect observed, then, was not in surface binding of LDL but in rate of LDL internalization. While this might be secondary to a defect in specific binding sites of LDL, the magnitude of the observed differences in binding at low temperature seems too small to account for the huge differences in internalization (13- to 115-fold).

Simple charring method for determination of lipids

A rapid method is described for charring 5-300 micrograms of lipids (with concentrated sulfuric acid in a test tube) and estimating them with a reproducibility of +/-1%.