he fate of cholesterol from a lunchtime cheeseburger partly depends on two opposing players in the bloodstream: the bad low density lipoproteins (LDL and VLDL) and the good high density lipoprotein (HDL). Good HDL can pick up the soft, waxy cholesterol deposited on arteries by bad LDL and deliver it to the liver for disposal in bile. More cholesterol can be disposed when HDL levels are high, partly explaining why high HDL levels are associated with a low risk for atherosclerosis and heart disease.
But since most HDL acts like a taxi, dropping cholesterol off at the liver and returning to the blood for more, the speed of HDL processing in the liver is also important in determining atherosclerosis risk. But little is known about how the processing works. Discovering how the liver takes up HDL may reveal drug targets that can increase HDL flow through the liver and decrease atherosclerosis. Dr. David Silver, an associate research scientist with Dr. Alan Tall, director of the division of molecular medicine and Tilden Weger Bieler Professor of Medicine, has identified the surprising way the liver takes in HDL and, possibly, a way to regulate the responsible receptor.
The receptor in question, SRB1 (scavenger receptor class B type 1) was discovered six years ago by another laboratory. Though SRB1 is one of three ways HDL delivers cholesterol to the liver for disposal, its absence is devastating to arteries. Without SRB1, knockout mice still have high levels of HDL in their blood, but they also have a 45 percent reduction in the amount of cholesterol in their bile and show high rates of atherosclerosis. SRB1 has a significant role for HDL transport into the liver, Dr. Silver says.
At the time of SRB1s identification, most researchers thought HDL was stripped of its cholesterol at the surface of the liver cell and never entered the cell. Based on this thinking, the original model of SRB1s mechanism claimed the receptor formed a channel for cholesterol to enter the liver cell, leaving the protein component of HDL behind in the blood to pick up more cholesterol.
But Dr. Silver and Dr. Tall thought it more likely the receptor brought the whole HDL complex into the cell since SRB1 is structurally similar to a receptor for oxidized LDL, which works by endocytosis. To test whether SRB1 entered liver cells, Dr. Silver fluorescently labeled HDL and SRB1 in cultured liver cells (though not in the same cell) and looked for their locations under a confocal microscope. Contrary to the original model of SRB1, which maintained that SRB1 and HDL remain outside the cell, both proteins were found inside the cell. And both were found in the same locations, early endosomes (sorting compartments for proteins and lipids that enter cells) and in regions near the bile duct, indicating SRB1 not only binds HDL at the cell surface, but that it also moves HDL to intracellular compartments.
The new model, in overthrowing a 20-year-old assumption, generated a fair amount of controversy, admits Dr. Silver. The work was published in the July 6, 2001, issue of the Journal of Biological Chemistry, but since then, he adds, most people are beginning to accept the mechanism, even if they only believe it works in the liver. SRB1 is present in other cells.
Dr. Silver says knowing how SRB1 works opens up entirely new areas of research that may lead to drugs that increase the flow of HDL through the liver and decrease the risk of atherosclerosis. Drugs are now available that lower LDL, but no drugs have been developed that primarily increase HDL levels or activity.
With this in mind, Dr. Silver is looking for other proteins that control the level of the SRB1 in the membrane. If the amount of SRB1 were increased on the surface of the liver cell by a drug, more cholesterol could potentially be removed from the body.
One such protein is PDZK1, named for its sandwich-like pdz-binding domain, already known to bind SRB1. Dr. Silver found he could increase the amount of SRB1 on the cell surface by overexpressing PDZK1. Overexpression of both increases HDL uptake. Dr. Silver presented the PDZK1 work in late February at the Knowledge Foundations third annual international conference on HDL cholesterol.
Dr. Silver is now making transgenic mice that overexpress PDZK1 to see if the genetic change will affect HDL levels in vivo. Other work must still be done to understand how the other two HDL pathways work in conjunction with the SRB1 pathway. Putting all these proteins together will help us understand how HDL uptake works, Dr. Silver says, and give us new ideas for therapeutic intervention.
Pfizer is very interested in HDL, but developing drugs that increase SRB1 has proven hard to do, says Dr. Tall, who is also director of the Specialized Center of Research in Atherosclerosis. Pfizer Inc. supported part of the work with a grant to Dr. Silver, and the company has first rights of refusal to any technology that comes from the research. I think Davids work is interesting because it gives a new angle on SRB1 and may reveal other points of attack, Dr. Tall says.