Directed by Michael S. Brown, M.D. and Joseph L. Goldstein, M.D.
Heart attacks and ischemic strokes are caused by cholesterol-carrying proteins called low density lipoproteins (LDL) that circulate in blood and infiltrate the walls of blood vessels, thereby initiating an inflammatory reaction that leads to obstruction of the vessel by a blood clot. More than two-thirds of all people in industrialized societies have circulating levels of LDL that are high enough to inflame their blood vessels, particularly when the vessels have been damaged by ancillary factors such as high blood pressure, diabetes and cigarette smoke. Why do so many people have dangerously high levels of LDL? Can we devise a treatment that lowers the level of LDL and prevents heart attacks and strokes?
Inflammatory cholesterol-rich atherosclerotic plaques in coronary arteries nourishing heart muscle.
These were the questions that our laboratory set out to answer more than 30 years ago. We began with the discovery of the LDL receptor, a cell surface protein that binds LDL particles and removes them from blood. Studies of the LDL receptor exposed receptor-mediated endocytosis, a general process by which cells use coated pits to internalize and degrade macromolecules that have bound to receptors. We called this uptake process the LDL Receptor Pathway. We found that one in 500 people throughout the world inherits a mutant gene for the LDL receptor. These people cannot remove LDL from blood at a normal rate. LDL builds up in blood and heart attacks occur by middle age.
But what about the rest of us - the two-thirds of people who maintain dangerously high levels of LDL despite two normal LDL receptor genes? Their problem arises from high fat diets acting together with subtle mutations in regulatory genes that predispose to diet-induced elevations in LDL.
How do high fat diets elevate blood LDL? The answer emerged from our studies of the regulation of the LDL receptor gene in liver. Liver is the primary organ that uses LDL receptors to remove LDL from blood. In liver, the activity of the LDL receptor gene, and hence the production of LDL receptors, is controlled by a feedback mechanism. When humans or animals ingest high fat diets, liver cells accumulate too much cholesterol. In defense the cells repress the transcription of the LDL receptor gene. With fewer receptors, the liver takes up LDL less efficiently from blood, and blood LDL levels rise.
In recent years we have uncovered the mechanism for this feedback regulation. We call it the SREBP Pathway. The key is a novel membrane-bound transcription factor called Sterol Regulatory Element Binding Protein (SREBP). Unlike nearly all other transcription factors, SREBP is produced as a membrane- bound protein attached to the endoplasmic reticulum of the cell. In cholesterol-starved cells SREBP is transported to the Golgi apparatus where it is cleaved sequentially by two proteases (designated S1P and S2P) that release the active fragment of SREBP into the cytosol. This fragment, called the bHLH fragment, then enters the nucleus where it activates the gene for the LDL receptor, and also the genes encoding all known enzymes of de novo cholesterol biosynthesis. The cellular cholesterol deficiency is then repaired by a combination of increased uptake of LDL and increased de novo synthesis of cholesterol. When high fat diets are consumed, the ingested cholesterol builds up in liver membranes and this blocks the processing of the SREBP. Instead of moving to the Golgi for cleavage the SREBP remains in the endoplasmic reticulum and it has no access to its target genes in the nucleus. As a result, the liver cells produce fewer LDL receptors, and this causes LDL to accumulate in blood.
Our laboratory identified and cloned the two Golgi proteases that process SREBP, and a sterol-regulated escort protein called SCAP that carries SREBP to the Golgi. We also identified INSIG as the protein that anchors SCAP and SREBP in the endoplasmic reticulum when intracellular cholesterol levels are high.
The work on LDL receptors and SREBP explains the action of a popular class of drugs called statins that are the most potent agents in lowering blood LDL levels. These drugs inhibit 3-hydroxy-3-methylglutaryl coenzyme A reductase, thereby blocking cholesterol synthesis in liver. This causes liver cholesterol levels to fall. As a result, SREBP is transported to the Golgi where it is processed to yield the active fragment that activates the LDL receptor gene. As a result of the increased LDL receptors, blood LDL falls.
Statins are currently taken by more than 20 million people every day. Studies of more than 50,000 treated subjects demonstrate conclusively that the LDL-lowering effect of statins leads to a reduction of 30-40% in heart attacks. These studies were all conducted in people who already had arterial damage. We believe that this reduction will be much greater when people begin to take statins at an earlier age, preventing, rather than reversing, the blood vessel damage.
Currently, the thrust of our laboratory is to understand at a molecular level how SCAP senses the level of cholesterol in cell membranes and how this sensing mechanism controls the movement of SCAP to the Golgi. This is a fundamental problem in membrane biochemistry and cell biology. We are also studying the actions of SREBP at the whole-animal level using gene-manipulated mice and other animal models. Our experimental systems range from whole animals to cultured cells to isolated proteins to isolated genes. We employ all modern techniques of cell biology, molecular biology and biochemistry.
Over the years, 22 graduate students have completed their thesis work in our laboratory, and we have been assisted by more than 100 postdoctoral fellows from two dozen countries. Our students and fellows work in a fast-paced, exciting, interactive and mutually supportive atmosphere that maximizes the ability of each person to live up to her or his potential.
