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112 | 2013
Annuaire du Collège de France 2011-2012
Résumé des cours et travaux 112e année
Conférenciers invités

Why Hearts Attack

Résumé des conférences du Collège de France (2011-2012)
Michael Brown
p. 832-836

Notes de la rédaction

Les enregistrements des conférences de Michael Brown, professeur invité, sont disponibles sur le site Internet du Collège de France : http://www.college-de-france.fr/site/pierre-corvol/guestlecturer-2011-2012__1.htm.

Texte intégral

Why Hearts Attack

1Heart attacks are the most frequent cause of death in France as in every western industrialized nation. The disease begins in the teenage years when cholesterol-carrying lipoproteins begin to penetrate into the walls of the coronary arteries that carry blood to the heart muscle. Lipoprotein penetration causes an inflammatory reaction that narrows the channel. Eventually, a blood clot forms, blocking the flow of blood and depriving the heart muscle of oxygen. The result is the crushing chest pain of a heart attack.

2The cholesterol-carrying protein that deposits in arteries is called low-density lipoprotein or LDL. Incontrovertible evidence has established that LDL causes atherosclerosis. Indeed, the number of heart attacks rises in direct proportion to plasma LDL. Moreover, drugs that lower LDL prevent heart attacks.

3In the 1970s, Dr. Joseph Goldstein and I discovered that the concentration of LDL in blood is determined by a receptor on the surface of cells in the liver. The receptor binds to LDL and carries it into the liver cell where it is rapidly degraded. When LDL receptor levels are high, LDL is rapidly removed from plasma and the circulating level is low. When LDL receptors are reduced, LDL remains in plasma and the level increases. LDL receptors are regulated by genetic and environmental factors. About 1 in 500 people inherits a mutant LDL receptor gene, leading to elevated LDL levels and premature heart attacks. Even in people with normal receptor genes, diets that are high in cholesterol and animal fats suppress the production of LDL receptors resulting in heart attacks.

4LDL receptors can be increased by the consumption of vegetarian diets that are low in cholesterol and saturated fats. They can also be increased by drugs called statins that block cholesterol synthesis in the liver and induce the liver to produce more LDL receptors. Statin treatment has been demonstrated to reduce heart attacks and extend life in people at high risk for heart attacks. Progress in understanding the relation between plasma LDL and heart attacks has already reduced the frequency of heart attacks.

The Story of Statins

5In the 1970s, Joseph Goldstein and I discovered that the level of low density lipoprotein (LDL) in plasma is controlled by the number of LDL receptors on liver cells. We also found that the production of LDL receptors is regulated. When liver cells are deficient in cholesterol, the gene for the LDL receptor is activated, and large numbers of LDL receptors are produced. As a result, plasma LDL levels fall. One way to reduce liver cholesterol levels is to block cholesterol synthesis. The possibility of blocking synthesis was first realized in 1976 with the publication of a major discovery from Japan.

6Akira Endo, a biochemist at the Sankyo drug company in Tokyo, screened many fungal extracts seeking for a molecule that would block cholesterol synthesis. He found a potent molecule which he called ML-236B. He showed that ML-236B is a competitive inhibitor of 3-hydroxy 3-methylglutaryl coenzyme A reductase (HMG CoA reductase), the rate determining enzyme in the cholesterol biosynthetic pathway.

7After Endo published his discovery, Dr. Goldstein and I collaborated with him to determine whether the block in cholesterol synthesis would lead to an increase in LDL receptors and a fall in plasma LDL. We showed this to be the case initially in cultured human fibroblasts and subsequently in dogs. ML-236B never reached the marketplace because the Sankyo company feared that it might cause cancer. This fear was not correct.

8At this point, an American drug company, Merck, took up the challenge. They discovered a molecule called lovastatin that was closely related to ML-236B. They conducted clinical trials showing that lovastatin lowered plasma LDL levels. On the basis of this lowering, lovastatin was approved by the US FDA in 1987 and it reached the market as Mevacor, the first statin drug. This was shortly followed with several other statins isolated from other molds. Many clinical trials have shown that statins reduce plasma LDL-cholesterol levels. As a result, heart attacks are prevented, and life is extended. Currently, statin drugs are taken daily by more than 30 million people.

9In recent years Dr. Goldstein and I discovered the mechanism by which statins raise LDL receptors. The key is a membrane-bound transcription factor called Sterol Regulatory Element Binding Protein (SREBP) that senses the level of cholesterol in cell membranes. Statins block cholesterol synthesis, which causes the level of cholesterol in cell membranes to fall. This releases SREBP from its position in endoplasmic reticulum membranes, allowing it to reach the nucleus. There it activates the LDL receptor gene, increasing LDL receptors and increasing the uptake of LDL-cholesterol. This restores cellular cholesterol levels and lowers the level of LDL in blood. Statin-mediated activation of SREBP has the potential to reduce dramatically deaths from heart attacks in Western countries.

Surviving Starvation: The Essential Role of the Ghrelin/Growth Hormone Axis

10Starvation is a major cause of death in the world, killing more than 6 million children each year. Throughout evolution famine has been a constant threat to all animal species. As a result, evolution has selected for powerful mechanisms to maintain life even under conditions in which nutrients are severely limited. When body fat disappears, it is essential that the body maintain levels of plasma glucose that are sufficient to nourish the brain. The famous French scientist Claude Bernard was the first to call attention to this compensation with his famous quotation: « La fixité du mileu intérieur est la condition d’une vie libre et indépendante. »

11Recently, our laboratory found that a peptide hormone, ghrelin, is essential to the survival of chronically starved mice. Ghrelin was discovered in 1999 by Kojima and Kangawa in Japan. It is a peptide of 28 amino acids that has a unique modification, namely, an 8 carbon fatty acid, octanoate, attached in thioester linkage to a serine at position three. The octanoate modification is essential for ghrelin activity, and it is conserved in all vertebrate species.

12Most ghrelin is produced primarily in the stomach. Its concentration in plasma rises steeply before meals, and it declines dramatically following eating. Administration of excess ghrelin to rodents and humans increases food intake. It was therefore postulated that ghrelin provides the signal that makes animals hungry prior to meals. This hypothesis was disproven when the gene for ghrelin and its receptor were eliminated in mice through homologous recombination. When deprived of food, the ghrelin deficient mice became just as hungry as wild-type mice. Moreover, when fed a high-calorie diet the ghrelin-deficient mice became just as obese as wild-type mice.

13Ghrelin was discovered because of its ability to release growth hormone from pituitary cells. Nevertheless, ghrelin-deficient mice grew normally and had no signs of growth hormone deficiency. Therefore, the true function of ghrelin was an enigma.

14Our laboratory became interested in ghrelin because of the octanoate modification. Ghrelin is the only animal protein that is known to be modified by an 8 carbon fatty acid. We identified the unique enzyme that is responsible for the attachment of octanoate to ghrelin. We called the enzyme Ghrelin O-Acyltransferase (GOAT).

15We eliminated the gene for GOAT in mice through homologous recombination. The GOAT-deficient mice produced no ghrelin. However, just like the ghrelin-deficient mice they grew normally, ate normally and gained weight normally on a high fat diet.

16To search for another function for ghrelin, we subjected our GOAT-knockout mice to severe calorie restriction in a protocol that reproduces the effects of famine. The mice were fed only 40% of the calories that they would normally consume each day. After three days on this diet, the wild-type and GOAT-knockout mice both lost 80% of their body fat and 30% of their body weight. In both strains the fasting blood glucose level declined to the range of 60 mg/dL. Over the succeeding four days, the wild type mice maintained blood glucose above 40 mg/dL, and they showed normal physical activity. On the other hand, the GOAT-knockout mice exhibited a progressive fall in their fasting glucose levels. By day seven the blood glucose levels had fallen to the range of 20 mg/dL, and the animals began to die.

17In wild-type mice, the calorie restriction led to an increase in plasma ghrelin levels that was progressive over seven days. Plasma growth hormone levels also rose markedly. These increases did not occur in the GOAT-knockout mice. Infusion of ghrelin or growth hormone prevented the fall in blood sugar in the GOAT-knockout mice.

18Profound hypoglycemia in the GOAT-knockout mice was associated with reductions in plasma lactate levels and a reduced synthesis of glucose by the liver. Infusion of lactate restored glucose production and plasma glucose rose. We conclude that ghrelin is essential during famine to maintain viable levels of blood sugar. Ghrelin acts by causing major increases in growth hormone. The growth hormone maintains production of lactate that is sufficient to support normal glucose production. Ghrelin is therefore an essential hormone in the resistance of mice to chronic starvation.

Partnerships, Puzzles and Paradigms: The Joys of a 40 Year Scientific Collaboration

19Joseph Goldstein and I met in June 1966 when we both joined the Massachusetts General Hospital in Boston as medical interns. Joe had graduated from Southwestern Medical School in Dallas, Texas, one of the youngest medical schools in the United States. I graduated from the University of Pennsylvania School of Medicine, the oldest medical school in the United States. We came from different backgrounds. Joe was born and raised in a tiny rural community in the Deep South. I was born in Brooklyn, New York, and I grew up big-city Philadelphia, Pennsylvania. Despite these differences, we were drawn together by our common interests in medical science. We each had a burning desire to solve medical puzzles, and hopefully to help develop more powerful treatments.

20After finishing our residencies, both of us moved to the National Institutes of Health, just outside of Washington D.C. There we worked with profound basic scientists. Joe worked in the laboratory of Marshall Nirenburg, who would later receive the Nobel Prize for solving the genetic code. I trained with Earl Stadtman, one of the world’s leading enzymologists. There I developed a fascination with metabolic regulation that was to last for my whole life.

21As a medical student, Joe had made a commitment to return to the Department of Medicine at Southwestern Medical School after his training. I was initially very skeptical about Dallas. Texas was a very conservative state where political views were substantially different from my own. My wife, Alice, shared this concern.

22At this point, Joe introduced me to Donald Seldin, the chairman of medicine at Southwestern. I was deeply impressed. Seldin had the broadest, and also the deepest, knowledge of medical science that I had ever seen in one individual. He was building a department filled with physician-scientists, all of whom took care of patients, and all of whom supervised laboratories designed to solve the puzzles of human disease.

23After much consideration, I decided to join Southwestern Medical College. While at the NIH, Dr. Goldstein and I helped to care for two children who were suffering from multiple heart attacks caused by extremely high levels of plasma Low Density Lipoprotein (LDL) cholesterol. In Dallas we obtained skin biopsies from similar patients and studied their cholesterol homeostasis in cell culture. This work led to the discovery of the LDL receptor and the process of receptor-mediated endocytosis. Later many other proteins were shown to enter cells by this process. Receptor-mediated endocytosis was our first paradigm.

24We early found that the production of LDL receptors is regulated. Cells produce abundant LDL receptors when they need cholesterol. When cholesterol accumulates, the gene for the LDL receptor is silenced and the number of receptors declines. We found the secret to this control mechanism when we isolated a protein called Sterol Regulatory Element Binding Protein (SREBP) that activates transcription, not only of the LDL receptor, but also of all the genes that are necessary to produce cholesterol and fatty acids in cells. We discovered that SREBPs are synthesized as membrane-bound proteins. The active fragment must be liberated proteolytically from the membrane in order to activate transcription. When cholesterol accumulates in membranes, SREBP proteolysis is blocked and the transcription of all target genes declines. We called this process regulated intramembrane proteolysis (RIP), and it turned out to be our second paradigm. Several other transcription factors are now known to be synthesized as membrane-bound proteins and to be liberated proteolytically just like SREBP.

25The third paradigm emerged from our studies of a human disease called Niemann-Pick C Disease. In this disease, LDL cholesterol enters the cell through the LDL receptor. The cholesterol is delivered to lysosomes, but there is a block in the removal of cholesterol from lysosomes. As a result, cholesterol accumulates in lysosomes of cells throughout the body. Affected children die from progressive malfunction of the brain, liver and lung.

26Two genes had been identified as the cause of NPC disease. One encodes NPC-2, a soluble protein that floats in the lumen of lysosomes. The other encodes NPC-1, a complex polytopic membrane protein with 13 membrane-spanning helices. Homozygous mutations in either gene cause the identical syndrome. Using purified NPC-2 and NPC-1 proteins, we showed that NPC-2 binds cholesterol immediately after it is released from LDL. The NPC-2 transports the cholesterol to the membrane where NPC-1 resides. There, NPC-2 transfers cholesterol directly to the amino-terminal domain of NPC-1. The transfer is carried out such that the hydrophobic cholesterol is never exposed to the water phase. The transfer to NPC-1 occurs by a direct sliding mechanism. We call it the hydrophobic handoff and this is the third paradigm. We expect that hydrophobic handoffs maybe a general mechanism by which water-insoluble lipids are transferred from one protein or one organelle to another.

27Throughout the 40 years of our scientific collaboration, Joe Goldstein and I have thoroughly enjoyed sharing our discoveries. Neither of us takes separate credit for the discoveries that have emerged from this partnership. All of our experiments are conducted only after the two of us agree upon the approach. Although other scientists have collaborated as intensely as we do, our collaboration, spanning over four decades may be the lengthiest, at least among biologists who have been recognized with the Nobel Prize.

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Cours et travaux du Collège de France. Annuaire 112e année, Collège de France, Paris, avril 2013, p. 832-836. ISBN 978-2-7226-0198-7

Référence électronique

Michael Brown, « Why Hearts Attack », L’annuaire du Collège de France [En ligne], 112 | 2013, mis en ligne le 28 août 2013, consulté le 24 février 2018. URL : http://journals.openedition.org/annuaire-cdf/1109

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Auteur

Michael Brown

Professeur University of Texas Southwestern Medical School (États-Unis)

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Droits d’auteur

Collège de France

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