Atherosclerosis:
A Lesson in Lesions


Vadim Ivanov, Ph.D.
LPI Cardiovascular Research Group

Photo of Vadim Ivanov, Ph.D.

Cardiovascular disease, the leading cause of death in industrialized countries, is due mainly to atherosclerosis: a thickening of the artery wall formed by the overgrowth of smooth muscle cells and an accumulation of leucocytes and extracellular matrix components. The pathogenesis of atherosclerosis was discussed in the Spring/Summer 1996 LPI Newsletter.

What initiates and propagates the atherosclerotic lesion? Thousands of scientists have being trying to answer this question for decades. Obviously, the right answer would put the end to the misery caused by atherosclerosis.

Epidemiologic studies have revealed risk factors for atherosclerosis, such as age, male sex, smoking, lack of physical activity, excess weight, high blood pressure, diabetes, and elevated blood plasma levels of cholesterol, triglycerides, Lp(a), and homocysteine. These studies have resulted in advice to prevent the disease by changing behavior and dietary preferences.

Biochemical studies, including investigations of experimental models of atherosclerosis in laboratory animals and cell cultures, morphological and biochemical analysis of atherosclerotic lesions, and other work, showed that many different factors and biochemical processes in different organs are involved in the complicated development of atherosclerotic lesions.

Many hypotheses to explain atherosclerosis have been developed, including those involving cholesterol, viral infection, local arterial inflammation, response-to-injury, and autoimmune response. The chronic character of human atherosclerosis (arterial depositions of fat can be observed even at the first decade of an individual's life, and the total affected area constantly increases with age) and numerous sites of impaired metabolism cause some to propose that the disease is part of the general process of aging. Every hypothesis concentrates on a particular aspect of the atherosclerotic process. Although these hypotheses are not mutually exclusive, no "universal" theory explains every observation and predicts how we can defeat atherosclerosis.

One hypothesis proposed by Linus Pauling and a colleague suggests that an ascorbic acid deficiency underlies atherosclerosis. According to their hypothesis, lipoprotein(a), or Lp(a), acts as a surrogate for vitamin C to repair damage in the arterial wall, and Lp(a) deposition leads to plaque formation. Vitamin C may act to strengthen the arterial wall, thereby precluding the development of plaque. Lp(a) blood levels are primarily genetically determined, and it has not been conclusively demonstrated that they can be modulated by vitamin C. Dr. Pauling published three case reports in which he described the therapeutic daily use of 3 to 6 grams of vitamin C and lysine to ameliorate angina pectoris, or chest pain, possibly by decreasing arterial plaque. Lysine, a positively-charged essential amino acid, may act by binding to Lp(a) and assisting in its removal from plaque. More recent evidence indicates that vitamin C improves the relaxation of blood vessels, which may explain the ameliorating effect of vitamin C on angina pectoris.

Protection from oxidative stress
Two lines of evidence connect the pathogenesis of atherosclerosis to oxidative processes. The first was the discovery of the capacity of plasma low-density lipoprotein (LDL, the main carrier of lipids and cholesterol in the body) to undergo free radical-induced oxidative modification, which changes the biological properties of LDL. Oxidized LDL, but not normal LDL, through the interaction with specific quot;scavenger" receptors on the surface of macrophages and smooth muscle cells, induces the formation of "foam" cells overloaded with lipids, a major factor in arterial narrowing. Oxidized LDL also exhibits cytotoxic, chemoattractant and cell growth stimulating activities.

Other evidence for the involvement of oxidative stress in atherosclerosis evolved from clinical epidemiologic studies showing a strong inverse correlation between the dietary intake of antioxidants, i.e., vitamins C, E and beta-carotene, and the risk of developing cardiovascular disease. An important report published in 1993 in The New England Journal of Medicine linked the supernutritional intake of vitamin E (>100 IU per day) to significantly decreased incidence of coronary heart disease. Intriguing data have come from studies on the Mediterranean and French diets. The French diet contains much fat but doesn't result in a high incidence of heart disease (the so called "French paradox"). Components of this diet exhibit a capacity either to contribute to the body's antioxidant system or to change the composition of LDL, making it more resistant to oxidation. What are these protective factors?

Flavonoids and other plant derived polyphenols, present in considerable amounts in fresh fruits, vegetables, and, especially, in grape juice and red wines, have been shown to be powerful water- and/or lipid-soluble antioxidants capable of preventing LDL oxidation induced by free radicals. As much as 1 gram of flavonoids may be consumed each day in an ordinary diet, thus comprising a significant contribution to the antioxidant defense system. Experiments conducted by the LPI cardiovascular group have demonstrated that some flavonoids possess an antioxidant activity comparable to or even exceeding that of ascorbic acid. Interestingly, some flavonoids synergistically increased the antioxidant activity of ascorbic acid to prevent LDL oxidation. The beneficial properties of certain plants may be explained by the presence of some especially effective flavonoids like resveratrol, which has also been found in red wines.

Regular consumption of another common component of the Mediterranean diet, olive oil, has been shown to significantly protect LDL particles from free-radical induced oxidation, possibly because of the substitution of the oleic acid in olive oil for other fatty acids in LDL. The double bond in fatty acids is the primary target for free-radical attack. Olive oil is monounsaturated, i.e. it has a single double bond, and provides little opportunity for attack by free radicals, whereas polyunsaturated fatty acids, such as sunflower oil, provide more targets for free-radical attack because of their double bonds in long hydrophobic chains. Oxidation of fatty acids by free radicals is the first essential step in the oxidative modification of LDL.

While the oxidative hypothesis of the pathogenesis of atherosclerosis can account for much, it has not yet given us the complete answer. It does not explain, for instance, why consumption of fish oil, containing polyunsaturated n-3 fatty acids, inhibits the development and progression of atherosclerosis, despite the vulnerability of these fatty acids to oxidation. The fish oil fatty acids most likely inhibit blood clot formation, an important aspect of atherosclerotic lesion formation and the clinical manifestations of atherosclerosis, such as heart attack or stroke.

Inhibition of smooth muscle cells
Another important factor in the genesis of atherosclerosis is the high proliferation rate of arterial smooth muscle cells (SMC) that have migrated into the intima layer of the artery. We have studied the effect of ascorbic acid on the proliferation rate of cultured SMC isolated from the aorta of guinea pigs. Our results demonstrate that ascorbic acid, in a dose-dependent manner, induces synthesis and secretion of a previously unknown specific inhibitor of SMC proliferation. We believe that this inhibitor is a protein that accumulates in the extracellular matrix by binding to one of its major constituents, hyaluronic acid. Currently, we are working on the isolation of this protein, to be followed by characterization of its structure and biochemical properties.

Genetic influences
Genes can be responsible for atherosclerosis, as illustrated by familial hypercholesterolemia. In this disease, a defect in the gene coding for a cell surface receptor that specifically binds low-density lipoprotein causes a dramatic increase in plasma LDL cholesterol concentration, which in turn leads to extensive atherosclerotic lesions and an early death (usually before the age of 20 years for homozygotes, people who have inherited the defective gene from both parents). Currently, treatment of this genetic disease targets the control of blood plasma cholesterol by lowering both the level of its uptake from food and its internal biosynthesis and by increasing the catabolic, or breakdown, rate of LDL. These treatments retard the development of atherosclerotic lesions and increase the patient's life expectancy.

On the other hand, some individuals with high levels of LDL cholesterol in blood from high dietary fat consumption do not have any clinical manifestation of atherosclerosis. Studies with mice have uncovered nine discrete genes that have alleles of "resistance" to atherosclerosis, and scientists are trying to elucidate the molecular mechanisms of their action. It is likely that some fortunate individuals have similar genes or combination of genes that protect them from heart disease.

How can we best protect ourselves from heart disease before all the definitive data is at hand? We can work at modifying the known risk factors: stop smoking (smoking generates enormous quantities of free radicals), avoid stress, keep an appropriate weight, maintain good physical activity, and reduce the intake of fats. These behavior changes have already resulted in a significant decrease in the incidence of heart disease. Additionally, one can avoid fried foods (frying causes the oxidation of fatty acids, and pre-oxidized fatty acids become incorporated into LDL, increasing its susceptibility to oxidation) and enjoy a good intake of antioxidants and bioflavonoids in fruits and vegetables.

Last updated May, 1997

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