The removal of cholesterol from Landschütz ascites cells by high-density apolipoprotein

Role of lipoprotein-X in the pathogenesis of cholestatic hypercholesterolemia. Uptake of lipoprotein-X and its effect on 3-hydroxy-3-methylglutaryl coenzyme A reductase and chylomicron remnant removal in human fibroblasts, lymphocytes, and in the rat

Cholestasis is accompanied by the appearance of lipoprotein-X (LP-X) in plasma. This lipoprotein has a high content of unesterified cholesterol and phospholipids and appears to be ineffective in suppressing the enhanced hepatic cholesterogenesis of cholestasis. Its role as a possible causative factor for cholestatic hypercholesterolemia was investigated. When 125I-LP-X was injected into rats, it disappeared rapidly from the circulation. Calculated on the basis of gram wet weight, spleen took up more LP-X than liver. Prior ligation of the bile duct reduced the uptake in spleen. Experiments with isolated perfused rat liver showed that nonparenchymal cells (NPC) took up over eightfold more 125I-LP-X than hepatic parenchymal cells (PC). Incubation of PC, NPC, human lymphocyte suspensions, or fibroblast cultures with LP-X showed that NPC bound more LP-X than PC or fibroblasts. Lymphocytes took up 20-fold more LP-X than PC and the activity of 3-hydroxy-3-methylglutaryl Coenzyme A (HMG-CoA) reductase was depressed by LP-X. Lymphocytes isolated from cholestatic patients showed low activity of this enzyme. The activity was increased by LP-X in isolated perfused livers, but suppressed in isolated microsomes. LP-X competitively inhibited the uptake of chylomicron remnants in isolated perfused livers and hepatocytes. In contrast, degradation of LDL by perfused livers, which were isolated from ethinyl estradiol-treated rats or human fibroblast cultures, remained unchanged in the presence of LP-X. The results indicate that cholesterol transported by LP-X is mainly taken up by the cells of the reticuloendothelial system. It increases the activity of hepatic HMG-CoA reductase and suppresses remnant uptake, thus emphasizing a major role of LP-X in cholestatic hypercholesterolemia.

Secretion of the newly synthesized cholesterol by rat hepatocytes in primary culture

We used monolayer cultured rat hepatocytes as an experimental model to study the secretion of the newly synthesized cholesterol by the liver. Cellular cholesterol was labeled by exposing cultured hepatocytes to [14C]acetate prior to the study of secretion. Secretion of the newly synthesized cholesterol was measured by extracting cholesterol in the culture medium and assaying for the radioactivity of [14C]cholesterol. We found that: (a) cultured hepatocytes could secrete newly synthesized cholesterol in serum-free medium; (b) secreted [14C]cholesterol was bound to macromolecule(s) and the secretion rate was not affected by cycloheximide for up to 5 h; (c) serum added to the culture medium greatly enhanced hepatic cholesterol secretion; (d) serum high-density lipoproteins were most effective, lipoprotein-deficient serum (d greater than 1.21) less effective in stimulating cholesterol secretion, whereas low-density and very-low-density lipoproteins had little effect; (e) when the serum-free culture medium was fractionated by ultracentrifugation, a major portion of the secreted [14C]cholesterol was found in the high-density lipoprotein fraction; (f) part of the medium [14C]cholesterol also turned up in the high-density lipoprotein fraction when lipoprotein-deficient serum was added as the acceptor; (g) secreted [14C]cholesterol was found only in free form, although some of the cellular [14C]cholesterol was found as esters.

Incorporation of cholesterol into serum high density lipoprotein apoprotein and recombinants

The uptake of cholesterol (CHL) by serum high density lipoprotein (HDL) delipidated apoproteins and phospholipid-apoprotein recombinants has been studied with two methods; by incubation with Celite-dispersed cholesterol or with cholesterol crystals. The apoproteins bind very small amounts of cholesterol with a maximum of about 6 micrograms/mg apoprotein. Recombinants with dimyristoyl phosphatidylcholine (DMPC) or egg phosphatidylcholine (EPC) as phospholipid component gave similar values for cholesterol uptake. The initial rate for uptake from Celite-cholesterol by recombinants was high (0.1 mol cholesterol/mol phospholipid/h) and somewhat higher than that for phospholipid vesicles. The maximal uptake was by gel filtration shown to depend on the size of the complexes with values about 0.95 mol cholesterol per phospholipid for vesicular complexes, 0.75 for discoidal complexes and between 0.5 and 0.2 for small 'protein-rich' complexes. During the incubation of recombinants with cholesterol there was considerable decomposition of discoidal complexes and formation of larger ones. The results show that phospholipid-apoprotein complexes are efficient acceptors for cholesterol but also that about 25% of the phospholipid in the discoidal complexes is excluded from interaction with cholesterol by interaction with apoprotein.

Localization of the structural gene for human apolipoprotein A-I on the long arm of human chromosome 11

Apolipoprotein A-I (apo A-I), the major apolipoprotein in human high density lipoproteins, is involved in the disease atherosclerosis. Cloned apo A-I cDNA (pA1-3) was used as a probe in chromosome mapping studies to detect the human apo A-I structural gene sequence in human-Chinese hamster cell hybrids. Southern blot analysis of 13 hybrids localized the gene to human chromosome 11. Confirmation of the chromosomal assignment was obtained by analysis of a hybrid (J1) containing a single human chromosome, no. 11. Regional mapping was achieved by using deletion subclones of J1 that localized the human apo A-I structural gene to the region 11q13 leads to qter. Since the human apolipoprotein C-III (apo C-III) structural gene is closely linked to apo A-I, it can be assigned to the same region on the long arm of chromosome 11. By extension of methods previously described, it now appears possible to carry out fine-structure analysis of this and related gene regions on chromosome 11 and to study the biochemical concomitants of these genes and of genes on other chromosomes for analysis of their role in atherosclerosis.

Nucleotide sequence of cloned cDNA of human apolipoprotein A-I

ApoA-I is the major human HDL apoprotein. By oligonucleotide hybridization, we have isolated 5 dscDNA clones to human hepatic apo A-I mRNA. One of these clones (pA1-3) was completely sequenced. It has 878 bp plus a poly A tail of 48 and includes all the coding and 3'-untranslated regions of the mRNA and part of the 5'-untranslated region. It predicts a peptide sequence of 267 amino acids (including the 24 amino acid prepropeptides) which is very similar to the sequence reported by Brewer et al., (1978) Biochem. Biophys. Res. Commun. 80:623-630. The predicted signal peptide sequence is highly homologous to the rat apoA-I signal peptide. There is no evidence for any internally repeated segments in apoA-I either at the amino acid or at the DNA level. Using pA1-3 as a probe, we have detected on Northern gels apo A-I mRNA sequences of approximately 1100 nucleotides in human hepatic and baboon hepatic and intestinal RNAs, but not in RNAs from baboon skeletal muscle, kidney or spleen. The demonstration of apo A-I mRNA sequences in specific organs is important to our concept of "reverse cholesterol transport".

Lecithin:cholesterol acyltransferase-mediated modification of discoidal peripheral lymph high density lipoproteins: possible mechanism of formation of cholesterol-induced high density lipoproteins (HDLc) in cholesterol-fed dogs

Peripheral lymph high density lipoproteins (HDL) of the cholesterol-fed dog differ in a number of characteristics from plasma HDL of the same animal. Their high content of free cholesterol, phospholipid, apoprotein E, and apoprotein A-IV, their greater heterogeneity in size, and the presence of many discoidal particles suggest that a portion of lymph HDL is assembled within the interstitial fluid. The present experiments demonstrate that the endogenous lecithin:cholesterol acyltransferase (LCAT) activity of whole peripheral lymph of the cholesterol-fed dog is far less (less than 1%) than that found in the plasma of the same animal (0.3 nmol/hr per ml versus 40.6 nmol/hr per ml). Addition of partially purified LCAT to whole lymph induced many changes in the chemical composition of peripheral lymph lipoproteins. After incubation with LCAT, the free cholesterol and phospholipid contents of lymph HDL decreased, from 17% to 12% and from 46% to 33%, respectively, whereas cholesteryl ester content increased from 7% to 13%. These changes were accompanied by a mass transfer of apoprotein E and cholesterol to the p less than 1.05 g/ml fraction, the complete disappearance of the discoidal particles, and a decrease in size heterogeneity of lymph HDL. These results suggest that, in the cholesterol-fed dog, cholesterol efflux into the interstitial spaces may occur in the absence of significant LCAT activity. Furthermore, our studies suggest that the subsequent reaction between lymph HDL and LCAT within the vascular compartment leads to the generation of apoprotein E and cholesteryl ester-rich cholesterol-induced HDL.

Identification of a cDNA clone for mouse apoprotein A-1 (apo A-1) and its use in characterization of apo A-1 mRNA expression in liver and small intestine

A cDNA clone for mouse apoprotein A-1 (apo A-1), the major apoprotein of plasma high density lipoproteins, has been identified. In addition to structural and physiological evidence, a genetic polymorphism for mouse plasma apo A-1 has been used to confirm that this DNA sequence corresponds to the apo A-1 gene. Use of this clone in molecular hybridization studies has shown that the concentration of apo A-1 mRNA is similar in liver and small intestine and is constant along the entire length of the small intestine. We provide evidence that the same apo A-1 gene is expressed in both liver and small intestine. Apo A-1 mRNA is also present in the stomach and esophagus at 10-15% the concentration found in small intestine but is undetectable in other tissues (such as large intestine, pancreas, heart, kidney, spleen, and brain). Finally, we show that there is a differential effect of a diet high in saturated fat and cholesterol on apo A-1 mRNA levels in liver and small intestine.

Uptake of [3H]cholesterol from low density lipoprotein by cultured human fibroblasts

The uptake of [3H]cholesterol from low density lipoprotein (LDL) was studied in LDL receptor-positive and receptor-negative human fibroblasts. In both cell lines the uptake depended upon temperature, time of incubation and the concentration of LDL in the medium. Although the incorporation of 125I-labeled LDL was minimal after 2 h of incubation in the receptor-negative (homozygous familial hypercholesterolemia, FH) cells, the uptake of [3H]cholesterol was only slightly less than that of the receptor-positive (WI-38) cells. With longer periods of incubation, a larger difference in labeled cholesterol incorporation was observed; this appeared to be due to a continued accumulation of the steroid in the WI-38 cells. After 8 and 24 h of incubation, some of the [3H]cholesterol was present as the ester in the WI-38 cells, but not the FH cells. Modified (reduced and methylated) LDL did not enter WI-38 cells by the receptor-mediated pathway during 2 h of incubation, as indicated by 125I uptake. [3H]Cholesterol uptake, however, was not significantly different from modified and unmodified LDL. While experiments indicated that significant amounts of cholesterol moved rapidly from LDL to cultured cells with a dependence on time and LDL concentration, no increase in total cell cholesterol was detected in either cell line. FH cells contained less total cholesterol and had a higher 3H specific activity than the WI-38 cells. These data suggest that there may be important mechanisms in addition to the LDL pathway for the movement of lipids into cells.

Hypertriglyceridemia: mechanisms, clinical significance, and treatment

The association between hypertriglyceridemia and coronary heart disease is explored followed by a discussion of the mechanisms of the disorder and guidelines on patient evaluation and treatment.

Cholesterol movement between human skin fibroblasts and phosphatidylcholine vesicles

Cholesterol readily exchanges between human skin fibroblasts and unilamellar phospholipid vesicles. Only a fraction of the exchangeable cholesterol and only 10-15% of the total cellular free cholesterol is available for net movement or depletion to cholesterol-free phosphatidylcholine vesicles. [14C]Cholesterol introduced into the fibroblast plasma membrane by exchange from lipid vesicles does not readily equilibrate with fibroblast cholesterol labelled endogenously from [3H]mevalonic acid. While endogenously-synthesized [3H]cholesterol readily becomes incorporated into a pool of esterified cholesterol, little, if any, of the [14C]cholesterol introduced into the fibroblast membrane by exchange from lipid vesicles becomes available for esterification. We interpret these findings as suggesting that: (1) net cholesterol movement from fibroblasts to an acceptor membrane is limited to a small percentage of the plasma membrane cholesterol, and (2) separate pools of cholesterol exist in human skin fibroblasts, one associated with the plasma membrane and the second associated with intracellular membranes, and equilibration of cholesterol between the two pools is a very limited process.

The effect of bezofibrate and clofibrate on cholesterol accumulation, esterification and removal in cultured 3T3 fibroblasts

3T3 mouse fibroblasts were used to determine the effect of bezafibrate and clofibrate on the cellular metabolism of cholesterol. In cells incubated in normal medium these agents decreased the incorporation of 3H-labelled oleic acid relative to 14C-labelled linoleic acid into the cholesterol ester fraction. When the 3T3 fibroblasts were incubated with cationised low density lipoprotein (LDL) the amount of esterified cholesterol which accumulated in the cells was greatly increased. This accumulation of cholesterol ester was reduced by bezafibrate and clofibrate. These agents decreased the incorporation of both 3H-labelled oleic acid and 14C-labelled linoleic acid into the cholesterol ester fraction of the cells, with a preferential effect on oleic acid as indicated by a reduction in the 3H/14C ratio. When cells which had been preincubated with cationised LDL were reincubated in normal medium, the removal of esterified cholesterol from the cells was increased by both bezafibrate and clofibrate. The mechanism of the effects of these agents on the metabolism of cellular cholesterol is discussed.

Effects of delipidated serum and lipoprotein-deficient serum on sterol biosynthesis and efflux in cultured skin fibroblasts - a comparison of the behaviour of cells from a control with those from a heterozygote and homozygote for familial hypercholesterolaemia

The biosynthesis and efflux of sterols from cells into the medium were investigated in skin fibroblasts from a control, a patient with obligate heterozygous familial hypercholesterolaemia and a patient with the homozygous condition. The behaviour of the cells was studied in two lipid free media (lipoprotein deficient and delipidated serum), with and without the addition of low density lipoproteins (LDL) in order to find experimental conditions which showed maximum differences between the three cell lines. Incorporation of [14C]acetate into sterols in the presence (repression) and absence (induction) of LDL was similar in the normal and heterozygous cells, whereas the homozygous cells showed reduced repression and increased induction. In all three cell lines induction of sterol synthesis was greater with delipidated than lipoprotein deficient serum. The efflux of sterols in both the presence and absence of LDL did not differ between the three cell lines, but it was greater when LDL was added to the medium and when delipidated serum was used. Sterol biosynthesis and efflux from the cells of the heterozygote did not differ significantly from those of the control.

Elastin-lipid interaction in the arterial wall. Part 2. In vitro binding of lipoprotein-lipids to arterial elastin and the inhibitory effect of high density lipoproteins on the process

The mechanism of lipoprotein binding to arterial elastin, and the inhibitory effect of high density lipoprotein (HDL) on the in vitro complex formation between plasma low density lipoprotein (LDL) and elastin were studied. The binding of LDL-cholesterol, phospholipids and triacylglycerols to delipidated elastin increased progressively with time over 24 h of incubation. The results of a kinetic study on lipoprotein-cholesterol concentration in the incubation medium, suggesting that the ability to bind cholesterol to elastin decreases in the following order: very low density lipoprotein (VLDL), LDL, intermediate density lipoprotein (IDL) and HDL, and that the capacities to bind to a fixed amount of elastin decrease in the order: LDL, IDL, VLDL and HDL. When a definite amount of LDL was incubated with elastin in the presence of increasing concentrations of HDL, the binding of lipids to elastin progressively decreased. On the other hand, no release of cholesterol, bound to elastin during preincubation with LDL, could be detected in additional incubations with HDL, apoHDL or apoHDL-phospholipid complex.

Effect of exogenous lipids and lipoproteins on the primary immune response in vitro

Cholesterol and certain lipoproteins have regulatory effects on the primary immune responses of murine spleen cells in vitro. The plaque-forming cell (PFC) responses to sheep red blood cells of trinitrophenylated Brucella abortus were studied in complete, lipid-depleted or lipoprotein-reconstituted media. The requirement for exogenous low density lipoprotein (LDL) and its cholesterol moiety was established by comparison of the yield of PFC in cell cultures deprived of lipoproteins with that in cultures to which specific classes of lipoproteins were added. The spleen cells in complete medium yielded about 10-fold greater PFC responses than cells in lipoprotein-deficient medium. In lipoprotein-deficient media, human LDL completely reversed the decreased immune response, LDL lipids and free cholesterol partially reversed the deficit, the human high density lipoproteins and an apo B phospholipid complex were ineffective. In complete media, cholesterol at higher concentrations (100--200 microgram/ml) and LDL lipids partially inhibited the primary immune response. Exogenous cholesterol was required for the in vitro response to both thymus-dependent and thymus-independent antigens.

Increased plasma HDL-cholesterol and apo A-1 in sedentary middle-aged men after physical conditioning

Previous studies have demonstrated an inverse relationship between plasma high density lipoproteins (HDL) cholesterol and coronary heart disease risk. In the present study we investigated prospectively the effect of a moderate physical conditioning programme on plasma lipids and lipoproteins, especially HDL-cholesterol and apolipoprotein A-I (apo A-I), the major apoprotein of HDL. Healthy, sedentary, middle-aged men were randomly selected and assigned either to a training group (n = 24, age 40 +/- 3.4, mean +/- SD) or to a control group (n = 13, age 39 +/- 5.0). Training consisted of various indoor and outdoor sports activities 45 min/day, 3 times/week for 12 weeks at an intensity of approximately 80% of measured maximal oxygen uptake (VO2 max). The trained subjects were studied at 4, 8 and 12 weeks. The training increased VO2 max by 12% (P less than 0.01). Increases were observed in both apo A-I (10%, P less than 0.02) and HDL-cholesterol (8%, P less than 0.02) after training, with significant increases already after 4 and 8 weeks, respectively. Furthermore, decreases in total plasma cholesterol (5%, P less than 0.004) and plasma triglycerides (26%, P less than 0.003) were found without changes in body weight, body composition, cigarette smoking, alcohol consumption or the percentage composition of dietary intake. Fasting serum-insulin concentrations decreased significantly during training. No changes were noted in the control group. The present study demonstrates prospectively that moderate physical training can increase HDL.

The effects of serum albumin and phospholipid on sterol excretion in tissue culture cells

Serum albumin was as effective as whole serum or alpha-globulins in facilitating sterol release from strain L mouse fibroblasts. Commercial bovine serum albumin preparations, however, had markedly different absolute effects in this regard. These differences were attributable to the variation in phospholipid content of these products. All but one of these albumins enhanced sterol release when supplemented with phospholipid. The exception was fatty acid-poor albumin which contained an adequate amount of phospholipid. Among the phospholipids examined, lecithin proved to be most effective, while phosphatidylethanolamine had little potentiating influence. As the unsaturation of the test lecithins increased, enhancement of sterol release decreased. The potentiating effect of the phospholipid was in turn dependent on the protein used, since the phenomenon was not observed with non-serum proteins like ovalbumin or with non-transport serum proteins such as gamma-globulins. The results of these studies raise the possibility that serum albumin together with phospholipid can play an important role in sterol release in tissue culture cells.

Deposition and mobilization of cholesterol ester in cultured human skin fibroblasts

Human skin fibroblasts in culture served as a model system to study intracellular cholesterol ester deposition in mesenchymal cells. Confluent cultures were exposed to homologous low density lipoprotein alone and together with chloroquine. In the presence of low density lipoprotein alone, even at half circulating serum concentrations, cellular free cholesterol increased no more than 12%, while the increase in cholesterol ester ranged from 13--100% during 48 h of incubation. Addition of chloroquine to the culture medium containing low density lipoprotein resulted in a very marked increase in cholesterol ester and the ratio of cellular esterified cholesterol to free cholesterol rose up to 2.2. In the presence of chloroquine the sum of uptake and degradation of 125I-labeled low density lipoprotein was enhanced and at higher chloroquine concentrations the more pronounced inhibition of degradation resulted in the intracellular retention of undegraded protein. Upon removal of the chloroquine-containing medium, there was a slight fall in the cellular cholesterol after 24 h incubation in a medium containing 10% fetal calf serum. Replacement of the fetal calf serum by lipoprotein-deficient serum and a mixture of high density apolipoprotein and sphingomyelin increased very significantly the loss of total cholesterol from the cells. At the same time the rate of loss of the retained labeled low density lipoprotein was also increased. The present results provide an adequate and reproducible model system for the study of cholesterol accumulation in human mesenchymal cells, which is one of the basic changes in atheromatosis. The availability of cholesterol ester laden cells also provides a good system for the study of agents active in cholesterol removal.

Lipid metabolism in cultured aortic smooth muscle cells and comparison with other cell types. Part I. Composition of cells grown in hyperlipemic serum

The lipid compositions of cultured rabbit aortic smooth muscle cells, adventitial fibroblasts and skin fibroblasts were determined for cells grown in media containing either normolipemic or hyperlipemic serum. No significant changes were found in cell phospholipid composition. Each of the threee cell types responded similarly to hyperlipemic serum, accumulating esterified cholesterol and triglyerides.

Cholesterol content and sterol synthesis in human skin fibroblasts and rat aortic smooth muscle cells exposed to lipoprotein-depleted serum and high density apolipoprotein/phospholipid mixtures

Confluent cultures of human skin fibroblasts and rat aortic smooth muscle cells were shown to lose 15-27% of their cellular cholesterol upon replacement of the fetal calf serum with human high density lipoprotein (50 mug cholesterol/ml) or lipoprotein-depleted serum at a concentration equivalent to 40% whole serum. Addition to the latter medium of high density apoliproprotein/phospholipid mixtures resulted in further enhancement of cellular cholesterol loss which was evident by 12 h of incubation. Human skin fibroblasts that had been enriched in cholesterol by previous incubation with low density lipoprotein lost their cholesterol in the presence of a high density apolipoprotein/sphingomyelin mixture as readily as non-enriched cells. Concomitant with the marked cholesterol depletion there was a stimulation of sterol synthesis from acetate. The more pronounced loss of cellular cholesterol induced by the presence of phosphatidylcholine or sphingomyelin resulted in a greater incorporation of acetate into sterol in both smooth muscle cells and skin fibroblasts. The present findings indicate that peripheral cells, in spite of their capacity to synthesize cholesterol, depend on exogenous cholesterol for the maintenance of normal levels. It is suggested that the native cholesterol "acceptor" in the lipoprotein-depleted serum is an apolipoprotein which under the experimental conditions can form a complex with phospholipids and might also represent the physiological cholesterol "acceptor" in peripheral lymph.

Relationships between plasma lipoprotein cholesterol concentrations and the pool size and metabolism of cholesterol in man

The plasma concentration of unesterified and esterified cholesterol within very low density (VLDL), low density (LDL) and high density (HDL) lipoproteins have been examined in relation to the metabolism and pool size of cholesterol in normal and hyperlipidaemic subjects. Cholesterol metabolism was assessed as faecal endogenous neutral and acidic steroid excretion, a 2-pool model of cholesterol turnover, and in vitro plasma cholesterol esterifying activity. VLDL total cholesterol (TC) concentration was positively correlated with cholesterol turnover, endogenous neutral steroid excretion, bile acid excretion and the absolute rate of plasma cholestrol esterification. The correlations with cholesterol turnover and neutral steroid excretion, but not that with bile acid excretion, remained significant when these were corrected for their relationships to body weight. LDL-TC was negatively correlated with the fractional rate of plasma cholesterol esterification and, in subjects with primary type IIa hyperlipoproteinaemia, also with the rate constant for cholesterol elimination from the rapidly exchanging cholesterol pool. No correlation was found between LDL-TC concentration and bile acid excretion. HDL-TC concentration was negatively correlated with both the rapidly and slowly exchanging pools of tissue cholesterol, after correction for their relationships to body weight and adiposity. In contrast, cholesterol pool sizes were not correlated with the concentration of VLDL or LDL-TC; nor was there any relationship to plasma cholesterol esterifying activity. No correlation was found between the relative proportions of unesterified cholesterol within any lipoprotein fraction and either the pool size or metabolism of cholesterol. These findings accord with previous reports of enhanced cholesterol metabolism in subjects with elevated VLDL concentrations and of impaired plasma LDL and cholesterol clearance in patients with primary type IIa hyperlipoproteinaemia. The demonstration that HDL-TC concentration is negatively correlated with body cholesterol pool size supports in vitro evidence for a role of HDL IN TISSUE CHOLESTEROL CLEARENCE.

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.

Decreased serum cholesterol-binding reserve in premature myocardial infarction

Incubation of human serum with crystalline cholesterol which had been pulverised by sonication resulted in a measurable uptake of cholesterol by the serum. This uptake was designated "serum cholesterol-binding reserve" (S.C.B.R.). Among more than 200 men and women examined, S.C.B.R. values varied from less than 10 to over 200 mg/dl, while the values from repeated determinations on two individuals over several months varied within 24 mg/dl. S.C.B.R. could be attributed to two serum-lipoprotein subfractions--S.F.V. separated from very-low-density lipoprotein and S.F.H. from high-density lipoprotein, by gel filtration. Without further purification, S.F.V. solubilised 4-5 mg and S.F.H. solubilised 0-36 mg of additional cholesterol/mg of protein, while the remaining bulk of the lipoproteins lacked this property. It is proposed that S.F.V. and S.F.H. have physiological roles in retarding atherogenesis by removing cholesterol from the arterial intima and carrying it back to the circulating serum. Accordingly, individuals who have low S.C.B.R. values, being deficient in S.F.V. and S.F.H., are at higher risk for the development of atherosclerosis and coronary heart-disease. This hypothesis was tested by comparing S.C.B.R. values of patients with premature myocardial infarction with values of controls. The results indicated a trend of increasing S.C.B.R. values with increasing levels of serum cholesterol and triglycerides among the controls, but this trend was virtually lost among the patients. The S.C.B.R. values also were lower among patients than controls, and the difference was statistically significant between patients and controls with serum-cholesterol above 250 mg/dl or fasting serum-triglycerides above 160 mg/dl. These results are consistent with the proposed hypothesis.