The pathogenesis of atherosclerosis: a perspective for the 1990s

Atherosclerosis, the principal cause of heart attack, stroke and gangrene of the extremities, is responsible for 50% of all mortality in the USA, Europe and Japan. The lesions result from an excessive, inflammatory-fibroproliferative response to various forms of insult to the endothelium and smooth muscle of the artery wall. A large number of growth factors, cytokines and vasoregulatory molecules participate in this process. Our ability to control the expression of genes encoding these molecules and to target specific cell types provides opportunities to develop new diagnostic and therapeutic agents to induce the regression of the lesions and, possibly, to prevent their formation.

Skeletal muscle mass and distribution in 468 men and women aged 18-88 yr

We employed a whole body magnetic resonance imaging protocol to examine the influence of age, gender, body weight, and height on skeletal muscle (SM) mass and distribution in a large and heterogeneous sample of 468 men and women. Men had significantly (P < 0.001) more SM in comparison to women in both absolute terms (33.0 vs. 21.0 kg) and relative to body mass (38.4 vs. 30.6%). The gender differences were greater in the upper (40%) than lower (33%) body (P < 0.01). We observed a reduction in relative SM mass starting in the third decade; however, a noticeable decrease in absolute SM mass was not observed until the end of the fifth decade. This decrease was primarily attributed to a decrease in lower body SM. Weight and height explained approximately 50% of the variance in SM mass in men and women. Although a linear relationship existed between SM and height, the relationship between SM and body weight was curvilinear because the contribution of SM to weight gain decreased with increasing body weight. These findings indicate that men have more SM than women and that these gender differences are greater in the upper body. Independent of gender, aging is associated with a decrease in SM mass that is explained, in large measure, by a decrease in lower body SM occurring after the fifth decade.

The smooth muscle cell. II. Growth of smooth muscle in culture and formation of elastic fibers

Smooth muscle derived from the inner media and intima of immature guinea pig aorta were grown for up to 8 wk in cell culture. The cells maintained the morphology of smooth muscle at all phases of their growth in culture. After growing to confluency, they grew in multiple overlapping layers. By 4 wk in culture, microfibrils (110 A) appeared within the spaces between the layers of cells. Basement membrane-like material also appeared adjacent to the cells. Analysis of the microfibrils showed that they have an amino acid composition similar to that of the microfibrillar protein of the intact elastic fiber. These investigations coupled with the radioautographic observations of the ability of aortic smooth muscle to synthesize and secrete extracellular proteins demonstrate that this cell is a connective tissue synthetic cell.

ApoE-deficient mice develop lesions of all phases of atherosclerosis throughout the arterial tree

Initial description of apolipoprotein (apo) E-deficient transgenic mice demonstrated the development of severe hypercholesterolemia due to probable delayed clearance of large atherogenic particles from the circulation. Examination of these mice demonstrated foam cell accumulation in the aortic root and pulmonary arteries by 10 weeks of age. In the present study, the animals were fed either chow or a high-fat, Western-type diet and examined at ages ranging from 6 to 40 weeks. Gross examination by dissection microscopy revealed a predilection for development of lesions in the aortic root, at the lesser curvature of the aortic arch, the principal branches of the aorta, and in the pulmonary and carotid arteries. Monocyte attachment to endothelial cells was observed by light and electron microscopic examination at 6 weeks, the earliest time point examined. Foam cell lesions developed as early as 8 weeks, and after 15 weeks advanced lesions (fibrous plaques) were observed. The latter consisted of a fibrous cap containing smooth muscle cells surrounded by connective tissue matrix that covered a necrotic core with numerous foamy macrophages. Mice fed the Western-type diet generally had more advanced lesions than those fed a chow diet. The apoE-deficient mouse contains the entire spectrum of lesions observed during atherogenesis and is the first mouse model to develop lesions similar to those in humans. This model should provide numerous opportunities to study the pathogenesis and therapy of atherosclerosis in a small, genetically defined animal.

A platelet-dependent serum factor that stimulates the proliferation of arterial smooth muscle cells in vitro

Dialyzed serum from clotted monkey blood ("blood serum") promotes the proliferation of monkey arterial smooth muscle cells in culture, but dialyzed serum prepared from recalcified platelet-poor plasma ("plasma serum") is much less effective. Addition of platelets and calcium to platelet-poor plasma increases the activity of plasma serum to the same level achieved with blood serum. Furthermore, addition to plasma serum of a platelet-free supernatant prepared by exposing purified platelets to thrombin also stimulates the proliferation of smooth muscle cells. Thus, much of the growth-promoting activity of dialyzed serum is directly or indirectly derived from platelets. This finding has important implications for the response of arteries to localized injury and provides a key to further understanding of the role of factors derived from blood serum in promoting cell proliferation in vitro.

Cadaver validation of skeletal muscle measurement by magnetic resonance imaging and computerized tomography

Magnetic resonance imaging (MRI) and computerized tomography (CT) are promising reference methods for quantifying whole body and regional skeletal muscle mass. Earlier MRI and CT validation studies used data-acquisition techniques and data-analysis procedures now outdated, evaluated anatomic rather than adipose tissue-free skeletal muscle (ATFSM), studied only the relatively large thigh, or found unduly large estimation errors. The aim of the present study was to compare arm and leg ATFSM cross-sectional area estimates (cm2) by using standard MRI and CT acquisition and image-analysis methods with corresponding cadaver estimates. A second objective was to validate MRI and CT measurements of adipose tissue embedded within muscle (interstitial adipose tissue) and surrounding muscle (subcutaneous adipose tissue). ATFSM area (n = 119) by MRI [38.9 +/- 22.3 (SD) cm2], CT (39.7 +/- 22.8 cm2), and cadaver (39.5 +/- 23.0 cm2) were not different (P > 0.001), and both MRI and CT estimates of ATFSM were highly correlated with corresponding cadaver values [MRI: r = 0.99, SE of estimate (SEE) 3.9 cm2, P < 0.001; and CT: r = 0.99, SEE = 3.8 cm2, P < 0.001]. Similarly good results were observed between MRI- and CT-measured vs. cadaver-measured interstitial and subcutaneous adipose tissue. For MRI-ATFSM the intraobserver correlation for duplicate measurements in vivo was 0. 99 [SEE = 8.7 cm2 (2.9%), P < 0.001]. These findings strongly support the use of MRI and CT as reference methods for appendicular skeletal muscle, interstitial and subcutaneous adipose tissue measurement in vivo.

Estimation of skeletal muscle mass by bioelectrical impedance analysis

The purpose of this study was to develop and cross-validate predictive equations for estimating skeletal muscle (SM) mass using bioelectrical impedance analysis (BIA). Whole body SM mass, determined by magnetic resonance imaging, was compared with BIA measurements in a multiethnic sample of 388 men and women, aged 18-86 yr, at two different laboratories. Within each laboratory, equations for predicting SM mass from BIA measurements were derived using the data of the Caucasian subjects. These equations were then applied to the Caucasian subjects from the other laboratory to cross-validate the BIA method. Because the equations cross-validated (i.e., were not different), the data from both laboratories were pooled to generate the final regression equation SM mass (kg) = [(Ht<SUP>2</SUP>/ <IT>R</IT> x 0.401) + (gender x 3.825) + (age x -0. 071)] + 5.102 where Ht is height in centimeters; R is BIA resistance in ohms; for gender, men = 1 and women = 0; and age is in years. The r(2) and SE of estimate of the regression equation were 0.86 and 2.7 kg (9%), respectively. The Caucasian-derived equation was applicable to Hispanics and African-Americans, but it underestimated SM mass in Asians. These results suggest that the BIA equation provides valid estimates of SM mass in healthy adults varying in age and adiposity.

Reduction in obesity and related comorbid conditions after diet-induced weight loss or exercise-induced weight loss in men. A randomized, controlled trial

BACKGROUND

The independent effects of diet- or exercise-induced weight loss on the reduction of obesity and related comorbid conditions are not known. The effects of exercise without weight loss on fat distribution and other risk factors are also unclear.

OBJECTIVE

To determine the effects of equivalent diet- or exercise-induced weight loss and exercise without weight loss on subcutaneous fat, visceral fat skeletal muscle mass, and insulin sensitivity in obese men.

DESIGN

Randomized, controlled trial.

SETTING

University research center.

PARTICIPANTS

52 obese men (mean body mass index [+/-SD], 31.3 +/- 2.0 kg/m2) with a mean waist circumference of 110.1 +/- 5.8 cm.

INTERVENTION

Participants were randomly assigned to one of four study groups (diet-induced weight loss, exercise-induced weight loss, exercise without weight loss, and control) and were observed for 3 months.

MEASUREMENTS

Change in total, subcutaneous, and visceral fat; skeletal muscle mass; cardiovascular fitness; glucose tolerance and insulin sensitivity.

RESULTS

Body weight decreased by 7.5 kg (8%) in both weight loss groups and did not change in the exercise without weight loss and control groups. Compared with controls, cardiovascular fitness (peak oxygen uptake) in the exercise groups improved by approximately 16% (P < 0.01). Although total fat decreased in both weight loss groups (P < 0.001), the average reduction was 1.3 kg (95% CI, 0.3 to 2.3 kg) greater in the exercise-induced weight loss group than in the diet-induced weight loss group (P = 0.03). Similar reductions in abdominal subcutaneous, visceral, and visceral fat-to-subcutaneous fat ratios were observed in the weight loss groups (P < 0.001). Abdominal and visceral fat also decreased in the exercise without weight loss group (P = 0.001). Plasma glucose and insulin values (fasting and oral glucose challenge) did not change in the treatment groups compared with controls (P = 0.10 for all comparisons). Average improvement in glucose disposal was similar in the diet-induced weight loss group (5.6 mg/kg skeletal muscle per minute) and in the exercise-induced weight loss group (7.2 mg/kg skeletal muscle per minute) (P > 0.2). However, these values were significantly greater than those in the control and exercise without weight loss groups (P < 0.001).

CONCLUSIONS

Weight loss induced by increased daily physical activity without caloric restriction substantially reduces obesity (particularly abdominal obesity) and insulin resistance in men. Exercise without weight loss reduces abdominal fat and prevents further weight gain.

Inhibition of neointimal smooth muscle accumulation after angioplasty by an antibody to PDGF

Approximately 30 to 40 percent of atherosclerotic coronary arteries treated by angioplasty or by bypass surgery occlude as a result of restenosis. This restenosis is due principally to the accumulation of neointimal smooth muscle cells, which is also a prominent feature of the advanced lesions of atherosclerosis. The factors responsible for the accumulation of intimal smooth muscle cells have not been identified. Platelet-derived growth factor (PDGF) is a potent smooth muscle chemoattractant and mitogen. It is present in platelets and can be formed by endothelium, smooth muscle, and monocyte-derived macrophages. The development of an intimal lesion in the carotid artery of athymic nude rats induced by intraarterial balloon catheter deendothelialization was inhibited by a polyclonal antibody to PDGF. These data demonstrate that endogenous PDGF is involved in the accumulation of neointimal smooth muscle cells associated with balloon injury and may be involved in restenosis after angioplasty, and perhaps in atherogenesis as well.

Studies of hypercholesterolemia in the nonhuman primate. I. Changes that lead to fatty streak formation

Morphologic studies resulting from events that occur during the development of the lesions of atherosclerosis were studied in chronic, diet-induced hypercholesterolemia in a series of nonhuman primates. Within 12 days of hypercholesterolemia in Macaca nemestrina, monocytes became adherent to the surface of the endothelium. These monocytes appeared to migrate subendothelially, accumulate lipid, and become lipid-laden macrophages (foam cells). Within a month, a "serofibrinous insudate" formed together with variable numbers of subendothelial lipid-laden macrophages. By the second month, foam cells increased in number, often in multilayers, to form a fatty streak. Concomitantly, the luminal surface of the arteries became increasingly irregular due to the subendothelial accumulation of foam cells. Numerous monocytes continued to attach to the endothelial surface over the fatty streaks, and many of them appeared to enter the intima and participate in the growth of the fatty streaks. Lipid-laden smooth muscle cells appeared in small numbers and formed two to four layers between the macrophages and the internal elastic lamella at 2 to 3 months. During the third month of hypercholesterolemia, endothelial cell continuity over the lipid-laden macrophages became interrupted, exposing the underlying foam cells to circulating blood. Foam cells were then readily observed in whole blood smears, suggesting that many of the lipid-laden macrophages leave the intima and enter the circulation. After 4 months, significant endothelial denudation was found in the iliac artery and many exposed macrophages were covered by adherent platelets in the form of a mural thrombus. Thus, the early components of atherosclerosis induced by chronic hypercholesterolemia centered around the monocyte-macrophage and its interaction with endothelium in the induction of the fatty streak. Subsequent changes that lead to macrophage-smooth muscle interactions, platelet-macrophage interactions, and platelet-endothelial interactions appeared to set the stage for the development of more advanced proliferative lesions.

Cell biology of atherosclerosis

The process of atherosclerosis is a life-threatening disease that affects critical organs including the heart and brain. It results from the influence of noxious agents associated with hyperlipidemia, hypertension, diabetes, cigarette smoking, homocysteinemia, and other agents that may, in altering the homeostatic condition of the artery wall, injure the endothelium and smooth muscle. The result is a protective, inflammatory, fibroproliferative response that becomes excessive and in its excess results in the disease process we call atherosclerosis.

Homocystine-induced arteriosclerosis. The role of endothelial cell injury and platelet response in its genesis

The atherogenic mechanism of homocystinemia has been defined by measuring endothelial cell loss and regeneration, platelet consumption, and intimal lesion formation in a primate model. Three groups of baboons were studied: (a) 8 control animals; (b) 15 animals after 3 mo of continuous homocystinemia; and (c) 11 animals after 3 mo of combined homocystinemia and oral treatment with dipyridamole. Experimental homocystinemia caused patchy endothelial desquamation comprising about 10% of the aortic surface despite a 25-fold increase in endothelial cell regeneration. Neither endothelial cell loss nor regeneration was changed significantly by dipyridamole. Homocystine-induced vascular deendothelialization produced a threefold increase in platelet consumption that was interrupted by dipyridamole inhibition of platelet function. All homocystinemic animals developed typical arteriosclerotic or preatherosclerotic intimal lesions composed of proliferating smooth muscle cells averaging 10-15 cell layers surrounded by large amounts of collagen, elastic fibers, glycosaminoglycans, and sometimes lipid. Intimal lesion formation was prevented by dipyridamole therapy. We conclude that homocystine-induced endothelial cell injury resulted in arteriosclerosis through platelet-mediated intimal proliferation of smooth muscle cells that can be prevented by drug-induced platelet dysfunction.

Skeletal muscle attenuation determined by computed tomography is associated with skeletal muscle lipid content

The purpose of this investigation was to validate that in vivo measurement of skeletal muscle attenuation (MA) with computed tomography (CT) is associated with muscle lipid content. Single-slice CT scans performed on phantoms of varying lipid concentrations revealed good concordance between attenuation and lipid concentration (r(2) = 0.995); increasing the phantom's lipid concentration by 1 g/100 ml decreased its attenuation by approximately 1 Hounsfield unit (HU). The test-retest coefficient of variation for two CT scans performed in six volunteers was 0.51% for the midthigh and 0.85% for the midcalf, indicating that the methodological variability is low. Lean subjects had significantly higher (P < 0.01) MA values (49.2 +/- 2.8 HU) than did obese nondiabetic (39.3 +/- 7.5 HU) and obese Type 2 diabetic (33.9 +/- 4. 1 HU) subjects, whereas obese Type 2 diabetic subjects had lower MA values that were not different from obese nondiabetic subjects. There was also good concordance between MA in midthigh and midcalf (r = 0.60, P < 0.01), psoas (r = 0.65, P < 0.01), and erector spinae (r = 0.77, P < 0.01) in subsets of volunteers. In 45 men and women who ranged from lean to obese (body mass index = 18.5 to 35.9 kg/m(2)), including 10 patients with Type 2 diabetes mellitus, reduced MA was associated with increased muscle fiber lipid content determined with histological oil red O staining (P = -0.43, P < 0. 01). In a subset of these volunteers (n = 19), triglyceride content in percutaneous biopsy specimens from vastus lateralis was also associated with MA (r = -0.58, P = 0.019). We conclude that the attenuation of skeletal muscle in vivo determined by CT is related to its lipid content and that this noninvasive method may provide additional information regarding the association between muscle composition and muscle function.

Expression and secretion of type beta transforming growth factor by activated human macrophages

Alveolar macrophages activated with concanavalin A and peripheral blood monocytes activated with lipopolysaccharide secrete type beta transforming growth factor (TGF-beta). There is minimal TGF-beta secretion in unactivated monocytes, even though TGF-beta mRNA is expressed in these cells at a level similar to that in activated, lipopolysaccharide-treated cultures. U937 lymphoma cells, which have monocytic characteristics, also express mRNA for TGF-beta. Freshly isolated monocytes, both control and lipopolysaccharide-treated, secrete an acid-labile binding protein that inhibits TGF-beta action. We conclude the following: (i) that expression of TGF-beta mRNA is unrelated to monocyte activation, (ii) that secretion of TGF-beta is induced by monocyte activation, and (iii) that cosecretion of TGF-beta and its monocyte/macrophage-derived binding protein may modulate growth factor action. In contrast, monocytic expression of other growth factor genes, such as the B chain of platelet-derived growth factor, is not constitutive and requires activation.

Induction of pro-inflammatory cytokines (IL-1 and IL-6) and lung inflammation by Coronavirus-19 (COVI-19 or SARS-CoV-2): anti-inflammatory strategies

Coronavirus-19 (COVI-19) involves humans as well as animals and may cause serious damage to the respiratory tract, including the lung: coronavirus disease (COVID-19). This pathogenic virus has been identified in swabs performed on the throat and nose of patients who suffer from or are suspected of the disease. When COVI-19 infect the upper and lower respiratory tract it can cause mild or highly acute respiratory syndrome with consequent release of pro-inflammatory cytokines, including interleukin (IL)-1β and IL-6. The binding of COVI-19 to the Toll Like Receptor (TLR) causes the release of pro-IL-1β which is cleaved by caspase-1, followed by inflammasome activation and production of active mature IL-1β which is a mediator of lung inflammation, fever and fibrosis. Suppression of pro-inflammatory IL-1 family members and IL-6 have been shown to have a therapeutic effect in many inflammatory diseases, including viral infections. Cytokine IL-37 has the ability to suppress innate and acquired immune response and also has the capacity to inhibit inflammation by acting on IL-18Rα receptor. IL-37 performs its immunosuppressive activity by acting on mTOR and increasing the adenosine monophosphate (AMP) kinase. This cytokine inhibits class II histocompatibility complex (MHC) molecules and inflammation in inflammatory diseases by suppressing MyD88 and subsequently IL-1β, IL-6, TNF and CCL2. The suppression of IL-1β by IL-37 in inflammatory state induced by coronavirus-19 can have a new therapeutic effect previously unknown. Another inhibitory cytokine is IL-38, the newest cytokine of the IL-1 family members, produced by several immune cells including B cells and macrophages. IL-38 is also a suppressor cytokine which inhibits IL-1β and other pro-inflammatory IL-family members. IL-38 is a potential therapeutic cytokine which inhibits inflammation in viral infections including that caused by coronavirus-19, providing a new relevant strategy.

The elastic fiber. I. The separation and partial characterization of its macromolecular components

The two morphologically different constituents of the mature elastic fiber, the central amorphous and the peripheral microfibrillar components, have been separated and partially characterized. A pure preparation of elastic fibers was obtained from fetal bovine ligamentum nuchae by extraction of the homogenized ligament with 5 M guanidine followed by digestion with collagenase. The resultant preparation consisted of elastic fibers which were morphologically identical with those seen in vivo. The microfibrillar components of these elastic fibers were removed either by proteolytic enzymes or by reduction of disulfide bonds with dithioerythritol in 5 M guanidine. The microfibrils solubilized by both methods were rich in polar, hydroxy, and sulfur-containing amino acids and contained less glycine, valine, and proline than the amorphous component of the elastic fiber. In contrast, the amino acid composition of the amorphous component was identical with that previously described for elastin. This component demonstrated selective susceptibility to elastase digestion, but was relatively resistant to the action of other proteolytic enzymes and to reduction. These observations establish that the microfibrils consist of a different connective tissue protein (or proteins) that is neither collagen nor elastin. During embryologic development the microfibrils form an aggregate structure before the amorphous component is secreted. These microfibrils may therefore play a primary role in the morphogenesis of the elastic fiber.

Upregulation of VCAM-1 and ICAM-1 at atherosclerosis-prone sites on the endothelium in the ApoE-deficient mouse

Focal recruitment of monocytes and lymphocytes is one of the earliest detectable cellular responses in the formation of lesions of atherosclerosis. This localized accumulation of leukocytes is a multistep process in which the endothelium remains intact and may regulate leukocyte recruitment by expressing specific adhesion molecules. To examine the relationship of adhesion molecule expression to initiation factors and the sites of lesion formation, we analyzed the expression of vascular cell adhesion molecule-1 (VCAM-1), intercellular adhesion molecule-1 (ICAM-1), and platelet-endothelial cell adhesion molecule-1 (PECAM-1) en face on the aortic endothelium of control mice and homozygous apolipoprotein E-deficient (ApoE -/-) mice that develop complex lesions of atherosclerosis similar to those in humans. In control mice, VCAM-1 staining was weak and limited to sites of altered blood flow. In contrast, in the ApoE -/- mice, VCAM-1 appeared to be localized over the surface of groups of endothelial cells in lesion-prone sites. Expression of VCAM-1 preceded lesion formation, and increased expression above control levels appeared to be correlated with the extent of exposure to plasma cholesterol. Although ICAM-1 was the most prominent adhesion molecule in lesion-prone sites, its expression appeared to be independent of plasma cholesterol levels and was upregulated in both ApoE -/- and control mice. At lesion-prone sites associated with altered blood flow, ICAM-1 was located over the surface of each endothelial cell and on microvilli, whereas VCAM-1 was confined to the cell periphery in non-lesion-prone sites. PECAM-1 was localized at the cell periphery throughout the aorta, and its expression did not appear to be regulated. Thus, the levels, localization, and characteristics of expression of VCAM-1, ICAM-1, and PECAM-1 appear to be differentially regulated. Upregulation of VCAM-1 and ICAM-1 is associated with sites of lesion formation.