Regression of atherosclerosis in primates

Apoptosis does not mediate macrophage depletion in rabbit atherosclerotic plaques after dietary lipid lowering

Unstable atherosclerotic plaques are characterized by a thin fibrous cap that contains few smooth muscle cells (SMCs) and numerous foam cells of macrophage origin. Previously we and others demonstrated that macrophages disappear from atherosclerotic plaques after dietary lipid lowering. However, it remains unclear whether loss of macrophages after lipid lowering occurs via increased apoptosis, decreased macrophage replication and/or recruitment, or via a combination of both. Rabbits were fed a diet supplemented with cholesterol (0.3%) for 24 weeks followed by a normal diet for 4, 12, or 24 weeks. After 24 weeks of cholesterol supplement, plaques showed apoptosis in both macrophages and SMCs, as determined by terminal deoxynucleotidyl transferase dUTP nick-end labeling. Cell replication (Ki-67 immunolabeling) was predominantly present in macrophages. After 24 weeks of cholesterol withdrawal, the thickness and areas of the plaques were unchanged. Nevertheless, plaques showed a considerable loss of macrophages. This event was associated with a reduced immunoreactivity for vascular cell adhesion molecule-1 (VCAM-1) in the endothelial cells starting 4 weeks after cholesterol withdrawal. Apoptosis did not increase after lipid lowering but showed a steady decline. Apart from decreased VCAM-1 expression, a strong decrease in Ki-67 immunolabeling was observed after 12 weeks of cholesterol withdrawal. Our findings suggest that loss of macrophages in atherosclerotic plaques after dietary lipid lowering is not related to induction of macrophage apoptosis but mainly a consequence of impaired monocyte recruitment followed by decreased macrophage replication. This information is essential for understanding the effects of aggressive lipid lowering on plaque stability.

Lipid and macrophage accumulations in arteries of children and the development of atherosclerosis

About one-half of infants in the first 6 mo of life have small collections of macrophages and macrophages filled with lipid droplets (foam cells) in susceptible segments of the coronary arteries. In subsequent years, fewer children have foam cells but around puberty (12-15 y) foam cell accumulations mostly larger than those in infants occur in 69% of adolescents. Lesions that represent the previously missing link between foam cell accumulations and atheromas have now been identified in a subgroup of highly susceptible locations. Such "preatheroma" lesions contain small pools of lipid droplets and dead cell remnants (extracellular lipid) in addition to macrophage foam cells. Atheromas, which emerge in some adolescents and young adults in the same locations, have a lipid core in which increased extracellular lipid displaces structural smooth muscle cells and the normal extracellular matrix. As soon as lipid cores form, calcium granules appear in some smooth muscle cells and among the extracellular lipid of the core. The degree of calcification is variable and, in youth, generally small. In the age group of 16-19 y, 15% of persons have either preatheromas or atheromas in coronary arteries; foam cell accumulations only are present in an additional 53% of 16-19-y-olds. Because the lipid cores of atheromas may be an underlying cause of lesion rupture, hematomas, and thrombosis, and because their development begins soon after puberty, it would be prudent to attempt to lower the influx of excessive atherogenic lipoproteins into the arterial wall by that age.

Cell composition, replication, and apoptosis in atherosclerotic plaques after 6 months of cholesterol withdrawal

Unstable human atherosclerotic plaques are characterized by a thin fibrous cap that contains few smooth muscle cells (SMCs) and numerous foam cells of macrophagic origin. Apoptosis of SMCs in the fibrous cap could destabilize the plaque and promote plaque rupture. In an experimental approach, we have studied apoptotic cell death and related proteins in atherosclerotic plaques of cholesterol-fed rabbits and examined the effects of cholesterol withdrawal. The induced atherosclerotic plaques at the thoracic aorta were composed of both fibromuscular tissue and foam cells. The presence of SMCs overlying macrophage accumulation was reminiscent of the structure of human atherosclerotic plaques. The plaques showed signs of cell replication and apoptotic cell death (1.8+/-0.5% terminal deoxynucleotidyl transferase end-labeling [TUNEL]-positive nuclei). Cell replication was confined mostly to the macrophages, whereas 34% of the TUNEL-labeled cells were SMCs. Both the macrophages and SMCs in the plaques expressed BAX, a proapoptotic protein of the BCL-2 family. After 6 months of cholesterol withdrawal, the thickness of the plaques in all localizations of the aorta was unchanged, but apoptosis was nearly absent (<0.1% of nuclei). Moreover, macrophages disappeared from the plaques, whereas the SMCs that remained present lost their lipid accumulation and strongly reduced their BAX expression. These changes were associated with a reduction of cell replication and increased deposition of fibrillar collagen fibers in the plaques, which pointed to plaque stabilization. In conclusion, the cell composition but not the thickness of atherosclerotic plaques was profoundly altered after a 6-month cholesterol withdrawal period. These changes were associated with a strong reduction of cell replication and apoptotic cell death. Moreover, the expression of the proapoptotic factor, BAX, was reduced in the remaining cells, which were mainly SMCs. These findings could help to explain the benefit of lipid-lowering therapy on plaque stabilization.

Conjugated equine estrogens alone, but not in combination with medroxyprogesterone acetate, inhibit aortic connective tissue remodeling after plasma lipid lowering in female monkeys

The objective of this study was to determine the arterial responses to plasma lipid lowering alone or in combination with (1) estrogen replacement therapy or (2) hormone replacement therapy in surgically postmenopausal female monkeys with preexisting atherosclerosis. Eighty-eight female cynomolgus macaques were ovariectomized, fed an atherogenic diet for 24 months, and then assigned by randomized stratification into 4 groups. One group (baseline, n=20) was necropsied at the end of the atherogenic diet period; the remaining 3 groups were fed a plasma lipid-lowering diet (regression) for 30 months. These regression groups were control (diet only), CEE (receiving conjugated equine estrogens alone), and CEE+MPA (receiving CEE and continuous medroxyprogesterone acetate). A previous report described coronary artery functional and histological results; the present report describes biochemical and histological results from the abdominal aorta. Aortic plaque size was not different between groups, similar to previous findings in the coronary arteries. Aortic cholesterol content (milligrams per gram lipid-free dry weight) was lower in the regression groups compared with baseline, both for free cholesterol (mean, control=19.1, CEE=15.7, CEE+MPA=14.4, and baseline=32.7; P<0.001) and for esterified cholesterol (mean, control=18.9, CEE=15.4, CEE+MPA=14.2, and baseline=58.7; P<0.001). This cholesterol efflux could lead to increased plaque stability without changing the physical size of the lesion. Alterations in aortic connective tissue composition were observed in the regression groups. When expressed as a percentage of the lipid-free tissue weight, the aortic elastin content of the control (mean=14.9) and the CEE+MPA (mean=14.0) groups was lower than that of the baseline group (mean=19.0), which was not different from that of the CEE group (mean=15.8). Aortic collagen content, as estimated by hydroxyproline content per milligram of lipid-free tissue, was higher in the control group (mean=67.4) and the CEE+MPA group (mean=66.1) than in the baseline group (mean=56.2; P<0.05). Collagen content of the CEE group (mean=58.9) was not different from that of the baseline group. When the regression groups were considered separately, the aortic collagen content of the CEE group was lower than that of the control group (P<0.05) and tended to be lower than that of the CEE+MPA group (P=0.10), suggesting that CEE therapy (but not CEE+MPA) inhibits potentially detrimental connective tissue alterations that accompany lesion regression. These results have implications for combinations of lipid-lowering and hormone replacement therapies in relation to vascular remodeling and abdominal aortic aneurysm development.

Long-term induction and regression of diet-induced atherosclerotic lesions in rhesus monkeys. II. Morphometric evaluation of lesions by light microscopy in coronary and carotid arteries

Atherosclerotic lesions were induced in rhesus monkeys (Macaca mulatta) by feeding them a high-saturated fatty acid and high-cholesterol diet. After 5.4 years the extent of lesions in three major coronary arteries and the right carotid artery was evaluated morphometrically by light microscopy in one group of animals (group P). The remaining animals were switched to a low-cholesterol diet that remained high in saturated fatty acids and provided the same percentage of total calories as did the atherogenic diet. Lesion regression was then evaluated in one group of monkeys 1.9 years (group R4) and in another group of monkeys 3.7 years (group R5) after withdrawal of cholesterol alone from the diet. In group P, the mean intimal thickness varied between 26 and 47 microns, maximum intimal thickness between 70 and 92 microns, and luminal reduction between 9% and 12% in the three major coronary arteries. Luminal reduction varied between 1% and 11% in right carotid artery segments. After 1.9 years of consuming the basal diet, group R4 animals were no different from group P animals with respect to morphometric measures. Total intimal and medial areas of the left anterior descending (LAD) coronary artery in groups P and R4 were also similar. In contrast, after 3.7 years of consuming the basal diet, group R5 animals showed consistently although not statistically significantly lower values than those in group P for the morphometric measures in coronary arteries and total intimal area in the LAD. Similar results were obtained for the common carotid and external carotid arteries. Thus, our study shows that long-term diet-induced lesions in coronary arteries and in common and external segments of the right carotid artery regressed only when the animals were fed the basal diet for 3.7 years. We conclude that atherosclerotic lesions induced in coronary and carotid arteries can regress toward normal to a certain extent, but they require a longer time for regression than do other arterial segments. These findings support the results of clinical trials in human subjects.

Plaque changes and arterial enlargement in atherosclerotic monkeys after manipulation of diet and social environment

To study the effects of dietary and social manipulations on lesion progression in male monkeys with established atherosclerosis, 83 animals fed a diet containing 1 mg cholesterol per kcal for 14 months were either necropsied (baseline group, n = 21) or assigned to one of three experimental conditions: 1) a diet containing a high amount of fat and cholesterol and a stressful social situation (HiFC-stress, n = 18); 2) a diet lower in fat and cholesterol and a stressful social situation (LoFC-stress, n = 21); or 3) the low-fat, low-cholesterol diet and a nonstressful social situation (LoFC-no stress, n = 23). After 28 months, all animals were necropsied. Coronary atherogenesis was arrested among monkeys in the LoFC-stress and LoFC-no stress conditions compared with that of animals in the baseline condition (plaque areas of 0.35 mm2, 0.30 mm2, and 0.38 mm2, respectively). Lesions in animals fed the LoFC diet (both stress and no-stress groups) were significantly smaller than those in monkeys in the HiFC-stress condition (0.96 mm2). Furthermore, aortic cholesterol content was significantly decreased and luminal areas were relatively larger among monkeys in both LoFC conditions compared with animals in the baseline and HiFC-stress conditions (p < 0.05 for all). The results demonstrate that a low-fat, low-cholesterol diet can halt plaque development, reduce arterial cholesterol content, and permit compensatory arterial enlargement, processes that were unaffected by social stress in this investigation.

Hypertension sustains plaque progression despite reduction of hypercholesterolemia

To assess the effect of hypertension on diet-induced coronary artery plaques after a return to a nonatherogenic diet, 10 cynomolgus monkeys were fed an induction regimen containing 2% cholesterol and 25% peanut oil for 6 months and then were subjected to midthoracic aortic coarctation to induce hypertension. The animals were then fed a nonatherogenic "prudent" ration for 6 additional months (hypertension-regression group). Twelve additional monkeys were fed the atherogenic diet for 6 months; six were killed (lesion-induction control group) and six were changed to the prudent diet for 6 additional months without coarctation (normotension-regression control group). At the end of the induction period, cholesterol levels averaged 744 +/- 178 mg/dl for the 22 animals and were similar for the three groups throughout the induction period. For the animals restored to the nonatherogenic diet (hypertension-regression and normotension-regression groups), serum cholesterol levels fell to 486 +/- 252 mg/dl at 1 month, to 341 +/- 162 mg/dl at 2 months, and to 234 +/- 78 mg/dl at 6 months. There was no significant difference between the hypertensive and normotensive animals. Six months after coarctation, blood pressure proximal to the coarctations for the hypertension-regression group ranged from 100/60 to 220/145 mm Hg with a mean of 166/103 +/- 36/28 mm Hg. Cross-sectional area of coronary plaques was somewhat lower for the normotension-regression control group compared with the lesion-induction control group, but the difference was not significant. Plaque area was, however, markedly greater in the hypertension-regression group than in either the lesion-induction or the normotension-regression groups (p less than 0.05 for each) despite progressive reduction in hyperlipidemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of serum lipid and apolipoprotein concentrations to predict extent of diet-induced atherosclerotic lesions in aortas and coronary arteries and to demonstrate regression of lesions in individual rhesus monkeys

In most studies, the assessment of lesion "regression" is based on comparisons of group means between the "progression" and regression groups. This comparison depends on the assumption that the extent and distribution of lesions produced by the end of the lesion-induction period in the regression animals are equal to those observed in the progression group. To determine whether significant regression of lesions occurs in an individual regression animal, it is necessary to obtain a measure of the lesions produced in these animals at the end of the lesion-induction period. We achieved this goal by developing models using multiple stepwise regression analysis that related steady-state serum cholesterol and apolipoprotein B and A-I concentrations measured during a lesion-induction period in 27 rhesus monkeys fed an atherogenic high-saturated-fat/high-cholesterol diet for 2 years. The models were developed to estimate the percent of intimal surface with lesions, the esterified cholesterol content (micrograms/cm2) for the artery segments, and three histomorphometric measures (mean intimal thickness, mean maximal intimal thickness, and mean percent stenosis) for the coronary arteries. In these models, multiple R2 ranged from 0.42 to 0.74 for the aortas and peripheral arteries, indicating that approximately one half to three fourths of the variance in lesions was accounted for. For the three histomorphometric measures in coronary arteries, however, the multiple R2 was 0.27 or 0.28, indicating that only approximately one fourth of the variance in lesions was accounted for.(ABSTRACT TRUNCATED AT 250 WORDS)

Persistence of foam cells in rabbit xanthoma after normalization of serum cholesterol level

Xanthoma was produced in diet-induced hypercholesterolemic rabbits by intradermal dextran sulfate injections. The serum cholesterol level returned to the normal range at about 10 weeks after ending the cholesterol diet. Gross observations after cessation of the cholesterol diet revealed a decrease in xanthomatous infiltrations. However, the dense foam cell infiltrations and cholesterol accumulations showed no signs of regression at even 9 months after ending the cholesterol diet. Signs of foam cell migration into the blood stream were not observed. The persistence of the xanthoma may be due to a lack of acceptors, such as high-density lipoproteins, that remove the cholesterol from the foam cells. During our 9-month observation period, some foam cells were degenerated and a few were fused with each other to transform into Touton-type giant cells. Nonfoamy histiocytes were infiltrated around these degenerating foam cells. The histiocytes may have transformed into foam cells by incorporating the lipids of the degenerated foam cells.

Macrophages, macrophage foam cells, and eccentric intimal thickening in the coronary arteries of young children

We surveyed the incidence and location of macrophages and macrophage foam cells in the coronary artery intima of 63 children that died in the first 5 years of life. We related the data on macrophages and macrophage foam cells to intimal smooth muscle cells and to measurements of intima:media area and thickness. All morphometric data were obtained from coronary arteries that were fixed by perfusion with glutaraldehyde under pressure, embedded in Maraglas, and cut into 1-micron cross-sections, and 65-nm fine sections. Coronary artery intima was always thicker (eccentric thickening) at bifurcations in the half of the circumference opposite to the flow divider. This was true for both male and female children. The remaining part of the coronary artery intima was less thick (diffuse thickening). Both types of intimal thickening were composed of an inner layer in which glycosaminoglycan ground substance predominated and a deeper musculoelastic layer. Fifty-nine children (94%) had intimal macrophages. Twenty children also had macrophage foam cells. Of 33 children aged to 8 months, 15 (45%) had macrophage foam cells. Of the 30 children older than 8 months, 5 (17%) had macrophage foam cells. Macrophages and macrophage foam cells occurred in the GAG-rich layer of the intima as isolated cells. In 5 infants macrophage foam cells occurred also as clusters of many cells. Macrophages were more numerous in cases that also had macrophage foam cells. Macrophages were 6 times, and macrophage foam cells 5 times more numerous in eccentric intimal thickening than in diffuse intimal thickening.

Regression of experimental atherosclerotic lesions in rhesus monkeys consuming a high saturated fat diet

Atherosclerotic lesions were induced in rhesus monkeys by feeding them a high-fat, high-cholesterol diet for 2 years. Arteries were examined after autopsy of a subgroup of animals (group P) and cholesterol was removed from the diet of the remaining animals. Lesions were examined in other subgroups after 30 weeks (group R1) and after 52 weeks (group R2). A control group (group C) was fed the diet without cholesterol throughout the study. The mean total serum cholesterol concentration before, during, and after lesion induction was 151, 390, and 157 mg/dl, respectively. The mean percent of surface area with fatty streak or fibrous plaque and the free and esterified cholesterol content of the artery increased in all six arterial segments examined in group P. The means for percent of surface with fatty streak and for arterial cholesterol content or concentration (but not for extent of surface with fibrous plaque) were consistently less in groups R1 and R2 than in group P, although they remained greater in groups R1 and R2 than in group C. The mean intimal thickness for coronary arteries was 10-fold greater in group P than in group C and 60 percent less in groups R1 and R2 than in group P; there was, however, much variability among animals and these differences among groups were not statistically significant. By using several measures in several arterial systems, we have shown that there was regression of diet-induced atherosclerotic lesions in rhesus monkeys while they were fed a diet high in saturated fat but without cholesterol for 30 or 52 weeks.

Role of macrophages in regression of atherosclerosis

The exact role of macrophages in regression is still not clear. It appears that some of their functions are beneficial, while others are detrimental. Among their beneficial functions are: (1) their ability to phagocytize cellular and extracellular debris and remove them outside the arterial wall. This function may be enhanced by the macrophage's own secretion of fibronectin; (2) their ability to solubilize necrotic debris by their complement of hydrolytic enzymes, thus, rendering them diffusible through the arterial wall; and, (3) their secretion of SMC mitogen and components of the arterial wall. Our work supports the role of macrophages in the removal of necrotic debris by the mechanisms cited in (1) and (2) above. On the other hand, macrophages may be detrimental to regression if they secrete an excess of the same hydrolytic enzymes, mentioned above as being beneficial, and if directed towards normal arterial wall components. This can result in disorganization and degradation of these components, and in more necrosis, as was seen at the six-week regression period in our sequential study. Cell debris resulting from necrosis of SMC and from death of macrophages themselves may form nidi for calcific bodies to occur. Our work suggests this may be the case during regression. Finally, excess stimulation of SMC, mitogen, and the secretion of the arterial wall components may contribute to the lesion growth and could explain the lack of regression in some species and under certain conditions. In conclusion, our hypothesis that the macrophage is a "friend" during regression appears to be only partially true, and their presence at this phase of the disease may be a "two-edged sword." On one hand, they may help in the removal of necrosis, while on the other hand, they may accelerate calcification.

Reduction in tissue LDL accumulation during coronary artery regression in cynomolgus macaques

The aim of this study was to determine whether the amount of LDL that had accumulated in the coronary arteries of cynomolgus monkeys during an 18-month period on a hypercholesterolemic (H) diet was reduced during subsequent periods of 6 and 12 months on a normolipemic (N) diet. This was performed by assessing the accumulation of LDL in the left anterior descending (LAD) branch of the left coronary artery within an area 4 mm from its origin, since this region contained the largest lesions in the LAD. LDL accumulation was estimated by measuring the percent cross-sectional area of artery occupied by reaction product depicting apo B by an immunoperoxidase procedure. The following reduction in mean (+/- SD) percent cross-sectional area occupied by reaction product was found in 8 animals on the progression diet (group I), 7 animals on the 6-month regression diet (group II), and 9 animals on the 12-month regression diet (group III), respectively: 21.7 +/- 4.7, 6.9 +/- 5.5, and 2.3 +/- 1.3. Differences between group I and either group II or III were statistically significant (using the Wilcoxon signed-ranks test). In group I, LDL was localized primarily in the necrotic core and around pools of foam cells. In groups II and III fewer foam cells and smaller pools of extracellular debris were qualitatively evident, and LDL was localized closer to the lumen and along collagen fibers. These results suggest that lowering of the plasma LDL level following termination of a hypercholesterolemic diet also induces a decrease in the LDL content in coronary artery lesions, even without significant reductions in lesion size, and that this decrease might be responsible for the decrease in foam cells.

Prevention of progression of coronary atherosclerosis by treatment of hyperlipidaemia: a seven year prospective angiographic study

The progression of coronary atherosclerosis was assessed by repeat angiography in 28 patients and 20 controls with hyperlipidaemia (serum cholesterol concentration greater than 7.2 mmol/l (278 mg/100 ml) or serum triglyceride concentration greater than 2.0 mmol/l (177 mg/100 ml), or both) and symptomatic coronary artery disease of two or three vessels. Twenty eight patients (26 men and two women) were treated with diet and drugs (clofibrate or nicotinic acid, or both) to lower lipid concentrations. Twenty men taking part in a simultaneous study served as non-randomised controls. They received medical treatment for coronary artery disease but no treatment to reduce lipid concentrations. The initial levels of coronary risk factors and the angiographic state were comparable in the two groups. In the 28 patients total cholesterol, total triglyceride, and low density lipoprotein cholesterol concentrations were reduced by an average 18%, 38%, and 19% respectively by treatment for hyperlipidaemia and high density lipoprotein cholesterol concentration was increased on average by 10%. The treatment maintained these concentrations during a follow up of seven years. By all criteria coronary lesions progressed significantly less in the patients than the controls: the angiographic state remained completely unchanged in nine (32%) of the patients compared with only one (8%) of the surviving controls; of the arterial segments at risk, 46 (16.5%) progressed in the patients compared with 50 (38.2%) in the controls (p less than 0.001); and the coronary obstruction increased less in patients than in controls (p less than 0.05). Cardiac survival was 89% in seven years in the patients compared with 65% in five years in the controls (p less than 0.01). The anginal symptoms diminished or remained stable in 16 of the 24 patients who survived until the end of the study. The progression of coronary atheromatosis was significantly greater in those patients who during the seven years of treatment had an average total cholesterol concentration, VLDL plus LDL cholesterol concentration, or ratio of LDL to HDL cholesterol concentration above the respective median value than in those with the corresponding values below median. On the other hand, the patients with HDL cholesterol concentrations above the median during treatment showed less progression than those with lower HDL cholesterol concentrations. The increase in coronary obstruction was inversely related to the average HDL cholesterol concentration during treatment. The progression was not, however, related to LDL cholesterol concentration during treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Experimental models of atherosclerosis regression

Regression of atherosclerosis has been demonstrated in several species of animals, including rabbits, chickens, rats, dogs, pigeons, pigs, and nonhuman primates. Regression has been associated with withdrawal of cholesterol from the diet or with the ingestion of cholestyramine, alfalfa meal, or alfalfa saponins in monkeys fed high-cholesterol diets. Although regression of atherosclerosis has been documented by sequential contrast arteriography in humans, the correspondence of regression in animals and in humans needs to be established.

Autoradiographic and ultrastructural studies on the human fibro-atheromatous plaque

Foam cells, either myogenic or macrophagic, are commonly detected in experimental and human fibro-atheromatous plaques. Their role in human atherosclerosis is not yet understood. This paper reports on a preliminary autoradiographic study combined with ultrastructural observations in the human fibro-atheromatous plaque. Most of the autoradiographic silver grains appeared on foam cells and monocyte-like cells, thus suggesting a local proliferation of these cells.