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September 2003

   
Cholesterol Up Close    

A Century of Research
Cholesterol is essential for the functioning of all human organs, but it is nevertheless the cause of coronary heart disease—a condition that is responsible for more than one-third of all deaths in the Western world. It is in fact the number one killer in the United States and in other industrialized nations.

Over the course of nearly a century of investigation, researchers have developed four lines of evidence—experimental, genetic, epidemiologic, and therapeutic—that irrefutably established the causal connection between cholesterol-carrying low-density lipoprotein (LDL) and atherosclerosis. Building on that knowledge, scientists have been successful in developing an effective course of therapy—the statin drugs.

Few other major diseases have been subject to such intensive and ultimately fruitful research. Here, briefly, is how the history of cholesterol research unfolded.

THE EXPERIMENTAL EVIDENCE
The first hint that cholesterol was related to atherosclerosis goes back to 1910, when the German chemist Adolph Windaus reported that atherosclerotic plaques from aortas of human subjects contained 20- to 26-fold higher concentrations of cholesterol than did normal aortas. Three years later, the Russian pathologist Nikolai Anitschov fed pure cholesterol to rabbits, which produced marked hypercholesterolemia and severe atherosclerosis of the aorta. This was the first experimental production of atherosclerosis, and Anitschov's experiment has been repeated many thousands of times ever since in virtually every animal species from pigeons to humans.

Windaus and Anitschov studied aortic plaques rather than the coronary artery plaques that are responsible for heart attacks. Aortic plaques in humans had been noted by 19th-century pathologists, who believed that coronary artery plaques were rare; they also believed that when a thrombotic occlusion of an atherosclerotic plaque did occur in a coronary artery, it was always a fatal event. This view persisted until 1918, when the syndrome of nonfatal myocardial infarction was recognized by James Herrick, a Chicago clinician, who made the first use of the electrocardiograph to diagnose heart attacks in patients who presented with crushing chest pain. Herrick provided the first clear demonstration that thrombosis of a coronary artery was not always fatal and that coronary heart disease was responsible for the acute chest pain that had been previously ascribed to all kinds of causes, from indigestion to apoplexy.

THE GENETIC EVIDENCE
The genetic connection between cholesterol and heart attacks was first made in 1938 by Norwegian clinician Carl Müller, who described several large families in which high blood-cholesterol levels and premature heart attacks together were an inherited trait. The genetic understanding of this syndrome, which came to be known as familial hypercholesterolemia (FH), was greatly advanced 25 years later by the astute observations of Lebanese clinician Avedis K. Khachadurian, who delineated two clinically distinct forms of FH in inbred families—the homozygous form, in which affected individuals manifest severe hypercholesterolemia at birth (with plasma cholesterol levels of about 800 mg/dl) and heart attacks that occur as early as 5 years of age, and the heterozygous form, characterized by levels in the 300- to 400-mg/dl range and premature heart attacks that occur typically between 35 and 60 years of age. The incidence of heart attacks in children with homozygous FH provided strong genetic evidence that hypercholesterolemia alone can produce atherosclerosis.

The mounting clinical interest in cholesterol led to an intense effort in the 1950s to determine the process by which cholesterol was synthesized in the body. Most of the crucial steps in this complex pathway, involving 30 enzymatic reactions, were worked out by four biochemists—Konrad E. Bloch, Feodor Lynen, John Cornforth, and George Popják—in a triumph of technical virtuosity that combined organic chemistry, enzymology, and one of the earliest uses of radioisotopes. The major outlines of this pathway were completed by 1960.

THE EPIDEMIOLOGIC EVIDENCE
The epidemiologic side of the cholesterol-coronary connection unfolded in 1955 when John Gofman, a biophysicist at the University of California at Berkeley, used the newly developed ultracentrifuge to separate plasma lipoproteins by flotation. Gofman found not only that heart attacks correlated with elevated levels of cholesterol but also that the cholesterol was contained in one lipoprotein particle, LDL. Gofman also observed that heart attacks were less frequent when the blood contained elevated levels of another cholesterol-carrying lipoprotein, high-density lipoprotein (HDL).

The epidemiologic connection between blood cholesterol and coronary atherosclerosis was firmly established by a physiologist at the University of Minnesota, Ancel Keys, whose classic Seven Countries Study showed that the incidence of heart attacks in 15,000 middle-aged men followed for 10 years was linearly proportional to the blood level of cholesterol. Keys also found that the cholesterol level rose in proportion to the saturated-fat content of the diet. Men living in eastern Finland, where the mean cholesterol level was 260 mg/dl, had eight times more coronary deaths in a 10-year period than men living in a Japanese fishing village where the mean cholesterol level was 165 mg/dl. Men living in Italy, where the mean cholesterol level (200 mg/dl) was intermediate between that of Japan and eastern Finland, had three times fewer coronary deaths than in Finland and three times more than in Japan. Subsequent studies showed that this wide range of cholesterol levels resulted from a correspondingly wide variation of LDL levels in the blood.

In 1974, the authors of this article discovered that the level of LDL in blood is controlled by the activity of a cell-surface protein we called the LDL receptor, which binds LDL and delivers it to cells where the lipoprotein is degraded; the cholesterol is then used there for metabolic and structural purposes. We also found that FH is caused by genetic defects in this receptor that ultimately block removal of LDL from the blood. These studies provided the first molecular link between LDL cholesterol and atherosclerosis.

THE THERAPEUTIC EVIDENCE
In 1976 Akira Endo, a Japanese scientist at the Sankyo Co. Ltd., discovered a fungal metabolite that could block cholesterol synthesis by inhibiting the enzyme HMG-CoA reductase. This discovery led to the first statin. In collaboration with Endo, we showed that the inhibition of cholesterol synthesis led to an up-regulation of LDL receptors, which explained how these drugs could selectively lower LDL, the bad cholesterol, without lowering HDL, the good cholesterol. We encouraged Merck & Co. Inc. to develop these drugs for therapeutic use, and in 1986 the FDA approved the first statin for human consumption. In 2003, more than 25 million people worldwide will take statins.

 
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Nobel Prize winners Joseph Goldstein (left) and Michael Brown provided the first molecular link between LDL cholesterol and atherosclerosis.

In 1994 the landmark "4S" (Scandinavian Simvastatin Survival Study) was completed. Sponsored by Merck and conducted by physicians in four Scandinavian countries, it showed for the first time that statins, by lowering LDL levels, could not only prevent myocardial infarctions but could generally prolong life. In several large multicenter trials, involving nearly 50,000 people followed for three to five years, treatment with statins lowered LDL levels by 25-35 percent and reduced the frequency of heart attacks by 25-30 percent—even in high-risk people who had "normal" LDL levels at entry. In these individuals, the high risk came from other predisposing conditions such as chronic smoking, hypertension, or diabetes. They benefited from statin therapy presumably because the predisposing conditions render the coronary arteries prone to inflammation at LDL levels considered "normal" in Western societies.

After nearly 100 years of exploration, we now have four lines of persuasive evidence—experimental, genetic, epidemiologic, and therapeutic—that implicate the cholesterol-carrying LDL particle as the primary cause of atherosclerosis. Very few, if any, chronic diseases of adults have ever been subjected to such intensive research, and in very few, if any, chronic diseases of adults has the cause been so convincingly demonstrated in so many ways.

Joseph L. Goldstein and Michael S. Brown shared the 1985 Nobel Prize in Physiology or Medicine for their research on the mechanism underlying cholesterol metabolism.

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Photos: Reid Horn, Courtesy of Brown-Goldstein Laboratory

Reprinted from the HHMI Bulletin,
September 2003, pages 10-19.
©2003 Howard Hughes Medical Institute

 
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CLOGGED ARTERIES
Coronary arteries of a 50-year-old man who died of a myocardial infarction. They have been opened lengthwise...