Arterial Composition and Metabolism: Esterified Fatty Acids and Cholesterol1 1Publication No. 413 from the Oregon Regional Primate Research Center, supported in part by Grant No. FR 00163 of the National Institutes of Health. The experiments carried out in our laboratory were supported by a grant-in-aid from the National Institutes of Health (HE 09744), Bethesda, Maryland

Distinct regulation of stearoyl-CoA desaturase 1 gene expression by cis and trans C18:1 fatty acids in human aortic smooth muscle cells

Consumption of trans fatty acids is positively correlated with cardiovascular diseases and with atherogenic risk factors. Trans fatty acids might play their atherogenic effects through lipid metabolism alteration of vascular cells. Accumulation of lipids in vascular smooth muscle cells is a feature of atherosclerosis and a consequence of lipid metabolism alteration. Stearoyl-CoA desaturase 1 (scd1) catalyses the production of monounsaturated fatty acids (e.g. oleic acid) and its expression is associated with lipogenesis induction and with atherosclerosis development. We were interested in analysing the regulation of delta-9 desaturation rate and scd1 expression in human aortic smooth muscle cells (HASMC) exposed to cis and trans C18:1 fatty acid isomers (cis-9 oleic acid, trans-11 vaccenic acid or trans-9 elaidic acid) for 48 h at 100 μM. Treatment of HASMC with these C18:1 fatty acid isomers led to differential effects on delta-9 desaturation; oleic acid repressed the desaturation rate more potently than trans-11 vaccenic acid, whereas trans-9 elaidic acid increased the delta-9 desaturation rate. We then correlated the delta-9 desaturation rate with the expression of scd1 protein and mRNA. We showed that C18:1 fatty acids controlled the expression of scd1 at the transcriptional level in HASMC, leading to an increase in scd1 mRNA content by trans-9 elaidic acid treatment, whereas a decrease in scd1 mRNA content was observed with cis-9 oleic acid and trans-11 vaccenic acid treatments. Altogether, this work highlights a differential capability of C18:1 fatty acid isomers to control scd1 gene expression, which presumes of different consequent effects on cell functions.

Lipogenesis in arterial wall and vascular smooth muscle cells of Psammomys obesus: its regulation and abnormalities in diabetes

AIM

Lipogenesis is expressed in vascular smooth muscle cells (VSMCs), and such in situ lipogenesis could be providing the fatty acids for triglyceride synthesis and cholesterol esterification, and contributing to lipid accumulation in the arterial wall. This study investigated both the expression and regulation of lipogenesis in VSMCs to determine if they are modified in Psammomys obesus gerbils fed a high-fat diet as a model of insulin resistance and diabetes.

METHODS

Aortas were collected from diabetic and non-diabetic P. obesus for histological examination, measurement of lipogenic gene expression and VSMC culture.

RESULTS

The aortas of diabetic animals exhibited lipid deposits and foam cells as well as disorganization of elastic fibres. However, lipogenic gene expression was not modified. VSMCs in vitro from the aortas of diabetic animals had, compared with cells from non-diabetic animals, lower mRNA levels of SREBP-1c and ChREBP. An adipogenic medium stimulated moderate FAS and ACC1 expression in cells from both diabetic and non-diabetic animals, but glucose and insulin on their own had no such stimulatory action. Also, triiodothyronine (T3) had a clear stimulatory action, while angiotensin II had a moderate effect, in cells from non-diabetic P. obesus, but not from diabetic animals, whereas LXR agonists stimulated lipogenesis in cells from both animal groups.

CONCLUSION

Lipogenesis is expressed in the arterial walls and VSMCs of P. obesus. However, its expression was not increased in diabetes, and did not respond to either T3 or angiotensin II. Therefore, lipogenesis in situ is unlikely to contribute to the accumulation of lipids in the arterial walls of diabetic P. obesus gerbils.

Lipogenesis in arterial wall and vascular smooth muscular cells: regulation and abnormalities in insulin-resistance

Background

Vascular smooth muscular cells (VSMC) express lipogenic genes. Therefore in situ lipogenesis could provide fatty acids for triglycerides synthesis and cholesterol esterification and contribute to lipid accumulation in arterial wall with aging and during atheroma.

Methods

We investigated expression of lipogenic genes in human and rat arterial walls, its regulation in cultured VSMC and determined if it is modified during insulin-resistance and diabetes, situations with increased risk for atheroma.

Results

Zucker obese (ZO) and diabetic (ZDF) rats accumulated more triglycerides in their aortas than their respective control rats, and this triglycerides content increased with age in ZDF and control rats. However the expression in aortas of lipogenic genes, or of genes involved in fatty acids uptake, was not higher in ZDF and ZO rats and did not increase with age. Expression of lipogenesis-related genes was not increased in human arterial wall (carotid endarterectomy) of diabetic compared to non-diabetic patients. In vitro, glucose and adipogenic medium (ADM) stimulated moderately the expression and activity of lipogenesis in VSMC from control rats. LXR agonists, but not PXR agonist, stimulated also lipogenesis in VSMC but not in arterial wall in vivo. Lipogenic genes expression was lower in VSMC from ZO rats and not stimulated by glucose or ADM.

Conclusion

Lipogenic genes are expressed in arterial wall and VSMC; this expression is stimulated (VSMC) by glucose, ADM and LXR agonists. During insulin-resistance and diabetes, this expression is not increased and resists to the actions of glucose and ADM. It is unlikely that this metabolic pathway contribute to lipid accumulation of arterial wall during insulin-resistance and diabetes and thus to the increased risk of atheroma observed in these situations.

Skin cholesterol and DNA in young patients with myocardial infarction

Disturbances in cholesterol metabolism are connected with an increased risk of clinical complications to atherosclerosis. Serum cholesterol has been used as an index of such disturbances. However, recently the significance of local tissue, and cellular factors in cholesterol metabolism and atherogenesis have been better appreciated. As easily accessible sources of cells and tissues, skin biopsies have been suggested to increase the possibilities to assess the extent of atherosclerosis in an individual. In order to test this hypothesis, skin biopsies were taken from 24 male patients, who had sustained a myocardial infarction before the age of 40, and from 42 healthy, randomly selected male volunteers, matched for age and serum cholesterol. Cholesterol and DNA contents were measured in epidermis and dermis separately; no significant differences were found between the groups. A significant, positive correlation between serum and dermis cholesterol was found in both groups. Our data do not support the hypothesis that skin biopsies discriminate individuals with atherosclerosis better than serum cholesterol. It is possible that previous data, suggesting a correlation between skin cholesterol and atherosclerosis, might reflect the well known correlation between serum cholesterol and atherosclerosis.

Adipocytic differentiation and liver x receptor pathways regulate the accumulation of triacylglycerols in human vascular smooth muscle cells

Lipid accumulation by vascular smooth muscle cells (VSMC) is a feature of atherosclerotic plaques. In this study we describe two mechanisms whereby human VSMC foam cell formation is driven by de novo synthesis of fatty acids leading to triacylglycerol accumulation in intracellular vacuoles, a process distinct from serum lipoprotein uptake. VSMC cultured in adipogenic differentiation medium accumulated lipids and were induced to express the adipocyte marker genes adipsin, adipocyte fatty acid-binding protein, C/EBPalpha, PPARgamma, and leptin. However, complete adipocyte differentiation was not observed as numerous genes present in mature adipocytes were not detected, and the phenotype was reversible. The rate of lipid accumulation was not affected by PPARgamma agonists, but screening for the effects of other nuclear receptor agonists showed that activation of the liver X receptors (LXR) dramatically promoted lipid accumulation in VSMC. Both LXRalpha and LXRbeta were present in VSMC, and their activation with TO901317 resulted in induction of the lipogenic genes fatty acid synthetase, sterol regulatory element binding protein (SREBP1c), and stearoyl-CoA desaturase. 27-Hydroxycholesterol, an abundant oxysterol synthesized by VSMC acted as an LXR antagonist and, therefore, may have a protective role in preventing foam cell formation. Immunohistochemistry showed that VSMC within atherosclerotic plaques express adipogenic and lipogenic markers, suggesting these pathways are present in vivo. Moreover, the development of an adipogenic phenotype in VSMC is consistent with their known phenotypic plasticity and may contribute to their dysfunction in atherosclerotic plaques and, thus, impinge on plaque growth and stability.

Cholesterol vehicle in experimental atherosclerosis 24: avocado oil

OBJECTIVE

To determine atherogenicity of avocado oil relative to saturated (coconut oil), monounsaturated (olive oil) and polyunsaturated (corn oil) fats.

METHODS

New Zealand White rabbits were fed a semipurified diet containing 0.2% cholesterol and 14% fat for 90 days. They were then necropsied and severity of atherosclerosis was determined visually.

RESULTS

Coconut oil was the most atherogenic fat. Corn oil was only slightly less atherogenic than either olive or avocado oils. Percentage of serum HDL cholesterol was highest in the rabbits fed the two monounsaturated fats.

CONCLUSION

Avocado oil is of the same order of atherogenicity as corn oil and olive oil.

Atherosclerosis induced by infection with Marek's disease herpesvirus in chickens

BACKGROUND

This research was suggested after crystals that we observed in herpesvirus-infected cell cultures were identified as cholesterol. Other reports and the development of defined reagents led us to select the use of Marek's disease herpesvirus (MDV) infection of chickens to demonstrate a potential role of herpesviruses in the pathogenesis of atherosclerosis. Available for our use were a clone-purified strain of MDV of known virulence, genetically selected, specific pathogen-free chickens, and appropriate isolation facilities to design controlled experiments to fulfill Koch's postulates.

METHODS AND RESULTS

Experiments were performed to test the roles of both MDV and dietary cholesterol in atherosclerosis. The birds were examined 7 months after MDV infection with and without cholesterol feeding for gross and microscopic arterial lesions. Atherosclerotic lesions were found only in infected normocholesterolemic or hypercholesterolemic birds. The character and distribution of these lesions closely resembled those found in the chronic human arterial disease. Atherosclerotic lesions were not found in uninfected birds even if the birds were hypercholesterolemic.

CONCLUSIONS

Evidence was obtained from other experiments that after MDV infection, cholesterol and cholesteryl esters accumulated in cell cultures and in atherosclerotic lesions. These changes were associated with altered enzymatic activity of the cholesterol synthesis cycle. Immunization with turkey herpesvirus vaccine or SB-1 vaccine prevented atherosclerotic lesions.

Mature human atherosclerotic plaque contains peroxidized phosphatidylcholine as a major lipid peroxide

The initial stage of atherosclerotic plaque formation involves oxidation of the phosphatidyl-choline moiety of low density lipoprotein (LDL) and subsequent uptake by macrophages. Ongoing uptake in developing plaque also may involve oxidized LDL and would require an oxidizing environment in plaque lipids. Atherosclerotic plaque lipids from 12 patients undergoing peripheral vascular procedures were extracted in chloroform: methanol (2:1). This extract was applied to a 25 cm 5 micron silica HPLC column and eluted with a ternary gradient mobile phase utilizing a laser light scattering (ELSD) mass detector. Individual lipid fractions were then analyzed. Cholesterol, both free and esterified, was the most prominent lipid in plaque (104 +/- 74 mg/gm tissue. However, lipid peroxides were present in much higher concentrations (3.52 +/- 2.84 FU X 10(4)/mg phospholipid) and overall level (21.27 +/- 10.10 FU X 10(4)/gm plaque) in the phospholipid component (*p< 0.05). Phosphatidyl-choline (PC) accounted for 63% of the total phospholipid peroxides recovered (6.31 +/- 5.09 mg/gm plaque; *p<0.05). PC and phosphatidylinositol (PI) content were linearly related to lipid peroxide fluorescence (PC; r=0.696; p=0.01) (PI; r=0.809; p=0.001). Lipid peroxides in human atherosclerotic plaque are present primarily in the phospholipid component and phosphatidyl-choline forms the bulk of these peroxides. PC may play an important role in ongoing plaque lipid accumulation.

Atherosclerosis in young white males: arterial collagen and cholesterol

As part of a multicenter study on atherosclerosis, we examined defined segments of thoracic and abdominal aortas from 118 white males, age 15-34 years, who died from external causes. One half of each aorta specimen was graded for lesions. Intima-media preparations were assayed for collagen and cholesterol in two standardized regions (dorsal and ventral) derived from the alternate half of each segment. Even though the mean extent of intimal surface involvement with raised lesions remained minimal (0-6%), the data revealed a remarkable transition in vessel wall chemistry over this time span. For example, the amount of collagen per unit surface area increases with age in all vessel segments except the ventral domain of the thoracic aorta. The amount of collagen as a percent of total vessel protein rises with age only in the ventral and dorsal regions of the abdominal aorta. Free and esterified cholesterol levels per unit surface area increase with age in all vessel segments. There is a significant correlation between collagen and esterified cholesterol per unit surface area in all vessel regions with the exception of the abdominal ventral segment. In the latter segment increases in collagen per unit surface area occur without a corresponding increase in cholesterol level suggesting that connective tissue proliferation may actually precede lipid deposition in the genesis of atherosclerosis. Esterified cholesterol is present at higher levels in the dorsal domains of the thoracic and abdominal aortas than in the ventral domains. These findings provide chemical data confirming that the dorsal domains is the most lesion-prone region of these vessel segments.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of lysophospholipase by cholesterol in rabbit aorta

Lysophospholipase activity was measured in rabbit aorta using 1-[1-14C]palmitoyl-sn-glycero-3-phosphocholine as a substrate. The enzyme did not require Ca2+ for its activation and the maximal activation was attained in the presence of EGTA. Cholesterol dose-dependently inhibited the lysophospholipase activity in the soluble fraction and IC50 value was approximately 15 microM. Lineweaver-Burk plot revealed that cholesterol competitively inhibited lysophospholipase and Km values in the presence and absence of cholesterol (15.5 microM) were 12.3 and 2.8 microM, respectively. Vmax values were approximately 475 pmol/min.mg. The results suggest that cholesterol can interact with the enzyme per se, resulting in the inhibition of the lysophospholipase activity in rabbit aorta.

Lipid composition of aorta of Watanabe heritable hyperlipemic and comparably hypercholesterolemic fat-fed rabbits. Plasma lipid composition determines aortic lipid composition of hypercholesterolemic rabbits

Aortic and plasma lipid compositions were compared during a 12-month period in Watanabe heritable hyperlipemic (WHHL), comparably hypercholesterolemic fat-fed, and age-matched control normolipidemic rabbits to determine whether exposure to equivalent degrees of endogenous or exogenous hypercholesterolemia led to differences in the composition and concentration of lipids deposited in the artery wall. Although there were marked differences in the distribution of cholesterol among the lipoproteins in the WHHL versus the fat-fed rabbits, the contents of both free and esterified cholesterol were elevated to an equivalent degree in the aorta and plasma. In contrast, the triglyceride content of both the plasma and aorta were elevated only in the WHHL rabbits. However, aortic total phospholipids were increased in both the WHHL and fat-fed animals as compared to control rabbits. In the control rabbits, there was an age-dependent enrichment in aortic relative to plasma cholesteryl-oleate consistent with low density lipoprotein (LDL) receptor-directed intracellular cholesteryl ester processing. In contrast, enrichment in cholesteryl-oleate in aortae relative to plasma was not detected in either WHHL or fat-fed groups, suggesting that the plasma cholesteryl ester composition was the primary determinant of the aortic cholesterol composition. Thus, during chronic hypercholesterolemia, deposition of lipids in the artery wall appears to be determined by plasma lipoprotein levels and composition if the LDL receptor is either absent on a genetic basis or suppressed due to a high-fat, high-cholesterol diet.

Association of low-density lipoprotein with particulate connective tissue matrix components enhances cholesterol accumulation in cultured subendothelial cells of human aorta

Low-density lipoproteins (LDL) were incubated with elastin particles, collagenase-resistant debris isolated from human aorta, and latex beads of 1.13 microns in diameter. As a result of incubation, insoluble LDL-associates were formed. These associates, as well as LDL-heparin-fibronectin-gelatin complexes described by other workers, were added to a 7-day primary culture of enzyme-isolated cells of human aortic subendothelial intima. The culture contained a mixed cell population made up mostly of typical and modified smooth muscle cells. 24 h later, total cholesterol, phospholipid, triacylglycerol, free cholesterol and cholesteryl ester levels were measured. Addition of insoluble LDL-complexes as well as LDL-associates to culture brought about a substantial accumulation of intracellular lipids; primarily, cholesteryl esters. The total cholesterol level in cultured cells was raised 3- to 8-fold. Addition of free LDL or LDL-free particles had no effect on the content of intracellular lipids. The results obtained allow the assumption that the occurrence of the LDL-mediated accumulation of intracellular lipids is due mainly to the LDL penetration inside the cell via 'nonspecific' phagocytosis and not through a regulated receptor-dependent pathway.

Studies on fine structure and location of lipids in quick-freeze replicas of atherosclerotic aorta of WHHL rabbits

The fine structure of intracellular and extracellular lipids in the atherosclerotic aorta of Watanabe-heritable hyperlipidemic (WHHL) rabbits was demonstrated by a quick-freeze etching technique. Many lipid droplets, with and without a membrane, were observed in the foam cells. Membrane-free droplets were observed as onion-like structure with a concentric lamellar structure surrounded by 10 nm filaments. Droplets surrounded by a limited membrane probably correspond to lipid-laden lysosomes. In the extracellular connective tissue space, marked accumulation of lipids with a vesicular structure was seen among collagen fibers. The appearance of these lipids was similar to that of lipids in lysosomes of foam cells.

Low-density lipoprotein apheresis and regression of atherosclerotic plaque in vitro

Short-term organ culture and primary cell culture of human aorta were used to study the effect of selective removal of low-density lipoproteins (LDLs) from the surrounding medium (LDL apheresis) on the lipid content of cultured tissue and cells, respectively. LDL apheresis was performed by passing the culture medium through immunosorbent containing agarose-bound goat antibodies against human LDL. LDL apheresis promoted the decrease of lipids of all classes in the cultured atherosclerotic plaque. However, only the cholesteryl ester level, but not free cholesterol, triglyceride, or phospholipid levels, was lowered in the cells cultured from the plaque. One may thus assume that LDL apheresis facilitates regression of lipoidosis by reducing the content of lipids in atherosclerotic lesions.

High acyl-CoA cholesterol acyl transferase activity in fetal rabbit aorta: evidence for the presence of stimulating factor(s) in amniotic fluid

The activity of Acyl CoA-cholesterol acyl transferase was markedly high in fetal aortas when compared to maternal and adult male rabbits. This activity dropped by 50% at 1 week of age. This high activity in fetal aorta a) did not appear to be due to changes in plasma cholesterol levels or to the later development of endogenous inhibitor in the aorta, but rather b) due to stimulatory factor(s) present in amniotic fluid.

Evidence that lysolecithin is an important causal agent of atherosclerosis

We examine the hypothesis of Portman et al. (1970) that lysolecithin is a causal agent of atherosclerosis. Four lines of argument support this hypothesis. (1) Lysolecithin is present, taken up and can act. Large amounts of lysolecithin are formed in plasma concomitant with triglyceride transport and it is readily taken up by arteries and retained for some time. Lysolecithin in aortic intima increases several-fold early in the induction of atherosclerosis in animals. From model system studies it is plausible that physiological doses of lysolecithin have physiologically significant effects. (2) At least one plausible mechanism of action can be formulated: stimulation of smooth muscle cell division due to lysolecithin-increased Ca2+ uptake. (3) The hypothesis is consistent with, and rationalizes, many literature observations in that inferred lysolecithin levels or production rates are appropriately correlated with a variety of positive and negative risk factors to a degree highly unlikely by chance. We found no data contradicting the hypothesis and only one piece weakening it. (4) A mechanism is outlined showing that low density lipoprotein receptor deficiency, the severest known risk factor, should cause the delivery of very high lysolecithin doses to artery walls. We conclude that the evidence, although indirect, is strong enough to give direct tests a high priority.

The prostacyclin analogue iloprost and prostaglandin E1 suppress sterol synthesis in freshly isolated human mononuclear leukocytes

The effects of the stable prostacyclin analogue iloprost, prostaglandin E1 and prostaglandin F2 alpha on sterol synthesis were investigated in freshly isolated human mononuclear leukocytes. Incubation of cells for 6 h in a medium containing lipid-depleted serum led to a 3-fold rise in the rate of sterol synthesis from [14C]acetate or tritiated water. Iloprost and prostaglandin E1 added in increasing concentrations at zero time resulted in an inhibition of the synthesis of sterols, the suppression being 50 and 55% at a concentration of 1 mumol/1, respectively. Both prostaglandins yielded a sigmoidal log dose-effect curve. In contrast, prostaglandin F2 alpha had no influence on sterol synthesis up to a concentration of 1 mumol/1. The action of the prostacyclin analogue and prostaglandin E1 on the relative rate of sterol synthesis was not immediate, since the prostaglandins had no effect when given at 6 h to the incubation medium, and the incorporation of [14C]acetate into sterols was measured thereafter. The results suggest that prostacyclin and prostaglandin E1 affect cholesterol synthesis and therefore may play a role in the regulation of cellular cholesterol homeostasis and in the development of atherosclerosis.

Lipids in cells of atherosclerotic and uninvolved human aorta. I. Lipid composition of aortic tissue and enzyme-isolated and cultured cells

Phospholipid, triglyceride, cholesterol, and cholesteryl ester contents were measured in unaffected and atherosclerotic areas of human aorta and in a suspension of enzyme-isolated cells from these segments. Aortic tissue and the cells isolated from it, as well as intimal and medial cells, significantly differ in lipid content. As lipoidosis develops in an atherosclerotic lesion, lipids accumulate unevenly in the tissue and cells. In zones of fatty infiltration, lipids accumulate, apparently, mainly inside cells while in the fatty streak and atherosclerotic plaque they predominate in the extracellular space. In a suspension of cells derived from both an atherosclerotic lesion and the underlying media, cholesteryl esters are the main component of excessive fat. In the primary culture of cells enzyme-isolated from unaffected intima, fatty streak, and plaque, the lipid content and composition are retained until Days 12 to 14 and are similar to those of freshly isolated cells.

Lipid composition of rabbit aortic wall following removal of endothelium by balloon catheter

To determine the lipid content of the neointima formed following balloon catheter deendothelialization, we measured the chemical composition of intimal medial samples of the aortic wall of rabbits at 6, 12, and 24 months after the operation. The areas of endothelial regrowth and the denuded areas were outlined by intravenous injection of Evan's blue dye. Lipids were extracted by chloroform/methanol (2:1, vol/vol) separated by thin-layer chromatography, eluted, and estimated by gas-liquid chromatography. No tendency to regression was observed. By contrast we observed a progressive increase over time in the concentration of most lipid classes in the areas of endothelial regrowth. There is evidence indicating that the increase in the lipid concentration parallels the increase in the glycosaminoglycan content in the areas of endothelial regrowth. It is possible that endothelial injury causes some irreversible changes in the composition of the neointima in areas of endothelial regeneration.

Free cholesterol not carried by lipoproteins in human serum

Considerable amounts of nonesterified cholesterol were found in human serum freed from lipoproteins. This cholesterol, when incubated with synaptosomal plasma membranes of dog brain, evokes changes of the ouabain-sensitive ATPase activity, as the exogenously added cholesterol does.

Orientational order in the choline headgroup of sphingomyelin: A 14N-NMR study

An aqueous dispersion of fully hydrated bovine sphingomyelin was studied using 14N-NMR spectroscopy. Spectra were obtained as a function of temperature over the range 15-80 degrees C, in both the liquid crystal and gel phases. In the liquid crystal phase, powder pattern lineshapes were obtained, whose quadrupolar splitting slowly decreases with increasing temperature. The spectra are increasingly broadened as the temperature is lowered through the phase transition into the gel phase. The linewidths and the second moments of these spectra indicate that the onset of a broad phase transition occurs at approx. 35 degrees C, in agreement with previous calorimetric and 31 P-NMR measurements. There is no evidence from the lineshapes for an hexagonal phase in this system, and this conclusion is supported by X-ray diffraction measurements carried out on aqueous dispersions of sphingomyelin in both phases. Assuming that the static nitrogen quadrupole coupling constant is the same for both sphingomyelin and dipalmitoyl-L-alpha-phosphatidylcholine (DPPC), the decrease observed in the quadrupolar splitting of sphingomyelin compared to that of DPPC indicates that the orientational order of the choline headgroup in liquid crystalline sphingomyelin is not the same as that of its counterpart in DPPC. Preliminary relaxation time measurements of T1 and T2 are presented which suggest that there are also dynamic differences between sphingomyelin and DPPC in the choline headgroup.

Flux of plasma lipoproteins into human arterial intima. Comparison between grossly normal and atheromatous intima

Arterial specimens were obtained from 12 patients during arterial repair surgery and 3 patients during nephrectomy following a bolus injection of autologous radio-iodinated very low density lipoprotein (VLDL) up to 30 h before surgery. Radioactivity in apoprotein B (apo B) was determined in lipoprotein fractions isolated from saline extracts of washed intima in the density ranges less than 1.006 g/ml, 1.006-1.019 g/ml and 1.019-1.063 g/ml [intimal VLDL, intermediate-(IDL) and low density lipoprotein (LDL), respectively]. Specific radioactivity of apo B was measured in corresponding plasma protein fractions. In atheromatous intima, minimum influx rates of VLDL-apo B [12.1 +/- 4.3 (SEM) ng . cm-1 . h-1] and IDL-apo B (13.6 +/- 5.1) over 24 h were significantly higher than fluxes of VLDL-apo B (1.2 +/- 0.34) and IDL-apo B (1.4 +/- 1.1) into normal intima (P less than 0.05). Minimum influx rate of LDL-apo B was also significantly higher in atheromatous intima (196 +/- 39) compared with normal intima (25.8 +/- 13.5, P less than 0.01) and was 16-18 fold higher than corresponding VLDL- and IDL-apo B fluxes in both types of intima. Collagenase digestion of intimal samples after saline extraction yielded further significant mass of VLDL but this fraction had little radioactivity compared with saline extractable VLDL.

Properties of acyl-CoA:cholesterol O-acyltransferase in aortic microsomes from atherosclerotic rabbits

Acyl-CoA:cholesterol O-acyltransferase (EC 2.3.1.26) was in microsomal fractions from atherosclerotic rabbit aortic tissue. Activity was increased over 70-fold following 8-11 weeks of cholesterol feeding. Comparison of the lipid composition of aortic microsomal fractions from control and cholesterol-fed animals showed a 2-fold increase in the molar ratio of unesterified cholesterol to phospholipid in the cholesterol-fed group, although no change in phospholipid content or composition was found. Aortic microsomes were fractionated by sucrose density gradient centrifugation. Acyl-CoA:cholesterol O-acyltransferase was localized in fractions containing neutral alpha-glucoside activity but was clearly separated from 5'-nucleotidase activity. The cholesteryl ester formed during in vitro incubation of incubations of microsomes with either [1-14C]-oleoyl CoA or [7-3H]cholesterol was localized in the same region of the density gradient as acyl-CoA:cholesterol O-acyltransferase. The studies indicate that the increased acyl-CoA:cholesterol O-acyltransferase activity found in cells from aortic atherosclerotic lesions is due to both an increased amount of enzyme and to an expanded pool of microsomal cholesterol which is available for esterification.

Correlation in the human aorta of APO B fractions with tissue cholesterol and collagen content

The amounts of buffer- and Triton-extracted apo B (LDL-protein), as well as the sum of these two fractions, were correlated with the total tissue cholesterol and hydroxyproline content (as a measure of collagen) in grossly normal intima, fatty streaks, and fibrous plaques of human aortas obtained at autopsy. Quantitative values of buffer- and Triton-extracted apo B were obtained by sequentially extracting homogenates of aortic intima with an aqueous buffer and one containing Triton X-100, and measuring the apo B content in each extract by an electroimmunoassay relative to plasma LDL or Triton-treated LDL. Significant positive correlations were obtained between the following: tissue cholesterol and both buffer-extracted and total-extracted apo B in grossly normal intima; tissue cholesterol and Triton-extracted apo B in microdissected fibrotic caps and cores of fibrous plaques, as well as in whole plaques. A positive correlation was also obtained between tissue cholesterol and total-extracted apo B in the necrotic core. A significant negative correlation was found between Triton-extracted apo B and collagen in whole plaques. The calculated mean percent of total tissue cholesterol in the different aortic regions that could be present as part of an intact LDL particle were: 100% in grossly normal intima, 16% in fatty streaks, and 11% in fibrous plaques. The positive correlation between Triton-extracted apo B and cholesterol in plaques suggests one or both of the following: the extracellular pool of cholesterol or some material increasing concurrently with cholesterol interacts with apo B or another part of the LDL particle; or the apo B containing lipoprotein is trapped in the hydrophobic environment of extracellular lipid. Both possibilities would render the particle less soluble in aqueous buffers. The negative correlation between Triton-extracted apo B and tissue collagen and the lack of a significant correlation between buffer-extracted apo B and collagen content suggests that collagen is probably not responsible for apo B retention in the aortic intima.

Quantitative aspects of free and esterified sterols in Saccharomyces cerevisiae under various conditions

For extraction of free and esterified sterols from yeast cells, a method was devised in which both forms of sterols were extracted with light petroleum after the treatment of the cells with acetone, and then with dimethylsulfoxide. The content of sterol esters in the cells under aerobic conditions markedly increased with time, amounting to 95% of the total sterols under some conditions. However, the formed sterol esters were decreased, accompanied with an increase of free sterols, when the cells were put under anaerobic conditions. Variations of radioactivities of both sterols which had been labeled in the side chain by incubation of the cells with [Me[-14C]methionine were examined on the cells grown under various conditions. No variation was observed on the cells under aerobic conditions. On the other hand, the labeled esters were hydrolyzed to yield free sterols in the cells under anaerobic conditions. In the cells under aerobic conditions, the free sterols were found to consist mainly of ergosterol, whereas the esterified sterols contained considerable amounts of zymosterol, lanosterol, and other intermediate sterols besides ergosterol.

Apolipoprotein B (apoB) retention in atherosclerotic intracranial arteries

Low (LDL) and very low (VLDL) density lipoproteins retained in grossly normal and atherosclerotic human intracranial arteries have been quantitated using an electro-immunoassay directed against apolipoprotein B (apoB), the major protein of these two lipoprotein fractions. Buffer-homogenates of grossly normal arteries contained apoB amounts ranging from less than 0.04 to 1.58 microng/mg tissue dry weight, while those of atherosclerotic plaques gave values ranging from 0.80 to 3.9 microng per mg tissue dry weight. These results were consistent with immunofluorescence studies localizing apoB in these arteries. Plaques also contained a remaining fraction of tightly-bound apoB as evidenced by positive immunofluorescence in sections of pellets from buffer homogenates. This was in contrast to the negative results from grossly normal arteries. These results would suggest that retention of apoB by intracranial arteries correlates positively with vessel lesions. Arterial apoB is present in both grossly normal regions and plaques in a loosely-bound form, possibly representing intact lipoprotein. ApoB is also present in a tightly-bound form in plaques.

Destruction of endogenous low density lipoprotein in incubated intima

The effect of incubation on the content of endogenous intact plasma lipoprotein (LP) has been examined in minced samples of normal intima and lesions from 38 patients. Both the electrophoretically mobile and the immobilized LP fractions decreased on incubation, and the rate of destruction was proportional to LP concentration (r=0.832, p less than 0.001). Mincing the intima with EDTA before incubation increased the rate of destruction about 4-fold in fibrous lesions but not in lesions containing numerous fat-filled cells. The destruction of LP was highly dependent on pH; the rate was highest below pH 5.5 and destruction was almost completely inhibited above pH 6.4. In standard cathepsin assays haemoglobin substrate was hydrolysed at a rate comparable to the rate of destruction of LP. The results suggest that LP may be degraded by a lysosomal cathepsin in intima.

The synthesis of lipids from [1-14C]acetate by human venous endothelium in tissue culture

Endothelial cells from human umbilical cords were harvested, using a trypsin technique, grown in tissue culture and lipid synthesis studied using [1-14C]acetate as a precursor. Radiosubstrate was incorporated into fatty acids, mono-, di- and triglycerides, cholesterol esters and phospholipids. Radioactivity was also present in the culture medium in the mono-and diglyceride fractions and in the phospholipids running with the solvent front.

The release of an immobilized lipoprotein fraction from atherosclerotic lesions by incubation with plasmin

A large amount of plasma low density lipoprotein is present in human aortic intima, and this can be removed and measured by electrophoresis directly from the minced tissue into an antibody-containing gel. We now find that, in addition to this electrophoretically mobile lipoprotein, there is an immobilized lipoprotein fraction than can be released from lesions by incubation of the tissue sample with plasmin or other proteolytic enzymes after the mobile lipoprotein has been removed. The concentration of immobilized lipoprotein is highly correlated with the concentration of the residual cholesterol (not mobile on electrophoresis) that has accumulated in the tissue (r = 0.702; P less than 0.001). Thus, in normal intima and early gelatinous lesions it is about 15% of the concentration of mobile lipoprotein, whereas in the atheroma lipid layers of fibrous or gelatinous plaques it may be 2 or 3 times greater than the concentration of mobile lipoprotein. This suggests that immobilization of plasma lipoprotein is an intermediate step in the irreversible deposition of extracellular cholesterol in atherosclerotic lesions. Incubation with plasmin allowed maximum release of lipoprotein: plasmin = crude collagenase greater than trypsin greater than "pure" collagenase greater than chondroitinase ABC in order of their relative effectiveness. The concentration of immobilized lipoprotein was significantly correlated (r = 0.793; P less than 0.001) with the concentration in the tissue of fibrin or other insoluble derivatives of fibrinogen ("fibrin"). In aliquots of lesions incubated with varying amounts of plasmin for varying times there was a constant relation between release of lipoprotein and release of fibrin-degradation products. Together, these findings suggest that the lipoprotein is associated with insoluble "fibrin". This appears to be of considerable clinical interest, suggesting a synergism between lipoprotein and fibrinogen in the accumulation of lipid in lesions.

Effect of hyperlipidaemic diet and clofibrate treatment on rat erythrocyte lipids

Lipids of rat plasma and erythrocytes were analysed during hyperlipidaemic diet and clofibrate treatment. The diet increased the plasma cholesterol and phospholipid concentrations after 3 weeks' lipid feeding. Clofibrate in a dose of 200 mg/kg body weight did not inhibit the increase in plasma cholesterol and phospholipid contents, but significantly decreased the concentration of plasma triglycerides. The lipid diet slightly increased the erythrocyte cholesterol and phospholipid concentrations without affecting the red cell morphology or reticulocytosis. After 1 week's treatment, clofibrate in a dose of 200 mg/kg body weight caused a 2-fold increase in the cholesterol content and a rise in the cholesterol-phospholipid ratio of red blood cells of rats fed the hyperlipidaemic diet. These results suggest that the erythrocyte cell membrane can increase its cholesterol content and possibly play a role in cholesterol transport.

Turnover of phospholipids in rat aortic smooth muscle cells in culture

Smooth muscle cells derived from rat aortic media were explanted and grown in culture for 14 to 60 days. During that time they formed a confluent multilayer and depostied extracellular material resembling newly formed elastin. The lipid composition of the cells in culture differed slightly from the parent cells in the intact aorta with respect to a higher phospholipid/DNA ratio and a higher lecithin content. The cholesterol content resembled that of parent cells. After incubation with labeled precursor the cultured cells show an active lipid synthesis; choline is incorporated mainly into lecithin, whereas glycerol and palmitate appear in phospholipids and to a lesser extent in neutral lipids. After a 2 hour pulse and up to 96 hour chase there is a linear fall in the specific acitivty of lecithin with a half-time of 28 to 30 hours. The rate of fall in specific activity of glycerol- or choline-labeled lecithin was found to be similar, indicating that choline does not turn over by an exchange reaction and is a suitable marker for studying phospholipid turnover in cultured cells. The results provide a basis for investigation of the effect of increasing cellular cholesterol content on phospholipid turnover.

Lipoprotein uptake and metabolism by rat aortic smooth muscle cells in tissue culture

Aortic smooth muscle cells from the rat were successfully grown in tissue culture and shown to have characteristic morphology. 125 I-labeled homologous very low density lipoproteins and high density lipoproteins were taken up by these smooth muscle cells during incubation for 48 hours at the stationary phase. Despite multiple washings, a large proportion of the lipoprotein radioactivity associated with the cells was apparently surface bound and trypsin releasable. With both lipoprotein fractions, lipid and protein uptake by the cells measured after trypsinization was related to time and to the amount of lipoprotein protein added to the medium. Compared with protein, there was a disproportionately greater entry of lipid radioactivity into the cells. Light and electron microscope autoradiography localized the label intracellularly over the cell cytoplasm, cell boundaries, and, in some cells, over lysosomes. On the basis of either protein uptake or whole particle uptake, approximately four times as much high density lipoprotein as very low density lipoprotein was taken up by the smooth muscle cells. To assess metabolism and degradation of high density lipoproteins, aortic smooth muscle cells were incubated in fresh unlabeled medium for 48 hours after exposure to 125 I-labeled high density lipoproteins. A large proportion of radioactivity released was trichloroacetic acid precipitable, suggesting some release of whole lipoprotein protein; however, these lipoproteins appeared to be modified when they were tested with anti-high density lipoprotein antiserum. Also, water-soluble radioactivity (presumably protein breakdown products) was released in amounts that averaged 3% of the protein label in the cells. These results indicate that although aortic smooth muscle cells growing in tissue culture can rapidly take up lipids and lipoproteins, catabolism of lipoprotein protein is slow. Correlative biochemical and ultrastructural analysis suggests the possibility of regurgitation of noncatabolized lipoprotein protein by reverse endocytosis.