Chances are, you know someone with high cholesterol. A parent, a friend, yourself. Too many fatty molecules pulsing through the bloodstream, sticking to the sides of blood vessels, and doubling—or more—the chances of a heart attack. In the United States, one in six adults has cholesterol levels considered too high and doctors write more than 200 million prescriptions a year for cholesterol-lowering drugs.
Statins, the most widely prescribed cholesterol medications, have been successful in curbing cholesterol levels for many people. But in some, statins lead to severe joint and muscle pain or liver inflammation. In others—perhaps due to genetic quirks—statins don’t lower cholesterol levels enough. And when one statin was compared with sugar pills in a clinical trial, the drug lowered the risk of heart attack by only one-third.
Although clinicians have firmly established the link between cholesterol levels and heart disease, there are still more questions than answers when it comes to the nitty-gritty molecular details of this connection. Unraveling the genetics and biochemistry of the body’s natural cholesterol-control mechanisms would do more than satisfy scientists’ curiosity: It could provide targets for better cholesterol drugs and fresh ways to predict earlier in life who is at risk for high cholesterol and related heart disease.
“This is one of the most tightly regulated systems in biology,” says Joe Goldstein of the University of Texas (UT) Southwestern Medical Center at Dallas. Goldstein, an HHMI Trustee, shared the 1985 Nobel Prize in Physiology or Medicine with Michael Brown for their discoveries about cholesterol metabolism. “It’s regulated at so many levels, in so many ways that there’s no shortage of questions about how it works,” he says. That means no shortage of potential drug targets.
Goldstein, Brown, and a handful of other HHMI scientists are still piecing together the full picture of how the human body manages cholesterol. In the process, they’re revealing new ways to stop atherosclerosis and heart attacks: by controlling cholesterol production, absorption, and the immune system’s response.
Despite its bad rap, cholesterol isn’t harmful in moderation. “It’s absolutely required,” says cholesterol researcher Russell DeBose-Boyd, an HHMI early career scientist at UT Southwestern who was a postdoc in the Brown–Goldstein lab. The human body needs cholesterol to function properly—it’s integrated into cellular membranes, in bile it aids digestion, and it plays a key role in the connections between neurons in the brain. But too much cholesterol is toxic for a cell and for the body as a whole. So cells have a complex feedback system to regulate cholesterol levels. The body can make cholesterol, absorb it from food digested in the gut, move it around, and excrete it as bile.
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At any given point, each of these processes can be turned up or down depending on a cell’s needs. “If the cell is deprived of cholesterol, you turn on uptake, and you turn on synthesis,” says DeBose-Boyd. “When the demands are met, synthesis and uptake are both turned off.” But when cholesterol levels from the diet get too high, the body’s system to deal with it becomes overloaded, and molecules idle dangerously in the arteries.
Statins work by halting cholesterol production in cells. They do it by blocking hydroxymethylglutaryl-CoA (HMG-CoA) reductase, an enzyme that carries out an early step of cholesterol synthesis. But the cell reacts to those falling cholesterol levels by making more reductase in an attempt to revive cholesterol synthesis.
“Statins are basically inducing accumulation of the very protein they’re targeting,” says DeBose-Boyd. “We could improve their effectiveness if we can stop that accumulation.” That’s his lab’s goal.
When they are replete with cholesterol, cells not only stop producing HMG-CoA reductase, they also speed up the enzyme’s degradation. In cells deprived of cholesterol and other sterols, reductase molecules stick around, churning out cholesterol, for an average of 10 or 11 hours, says De-Bose Boyd. But with lots of cholesterol around, reductase survives only about an hour.
DeBose-Boyd wants to coax cells to turn on this degradation process even in the low-cholesterol state induced by statins. This would prevent the reductase buildup that limits the drugs’ effectiveness.
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|Research by Russell DeBose-Boyd, Helen Hobbs, and Peter Tontonoz (l-r) on how the body manages cholesterol is revealing new targets for cholesterol-lowering drugs. Photos: DeBose-Boyd: UT Southwestern Medical Center at Dallas, Hobbs: UT Southwestern Medical Center at Dallas, Tontonoz: Paul Fetters|
“It looks like there is a switch for this whole cholesterol system where it’s either on or it’s off,” says Goldstein, just down the hall from DeBose-Boyd’s lab. “Understanding this switch is really fundamental to understanding the system.” HMG-CoA reductase is normally located on the outer membrane of a cell’s endoplasmic reticulum (ER), a packaging center that directs newly made proteins to their destinations in the rest of the cell.
DeBose-Boyd discovered that in times of high cholesterol, a protein called Insig binds to reductase and removes it from the ER, according to work published June 2010 in the Journal of Biological Chemistry. From there, the reductase ends up in lipid droplets. “They’re basically little balls of fat in the cell,” he says. Exactly how this happens, he’s not sure, but somewhere in the lipid droplet or the cell’s watery cytosol, the reductase is broken into pieces, no longer functional.
DeBose-Boyd’s lab has also revealed that it’s not cholesterol that triggers Insig to bind to reductase and ship it out of the ER. It’s a molecule related to cholesterol, called dihydrolanosterol (DHL). Because it’s not identical to cholesterol, DHL can potentially turn on HMG-CoA reductase degradation without the risk associated with increasing cholesterol levels.
“If we were to get a drug from this work, it’d have to be designed after DHL,” says DeBose-Boyd. But such a drug is still hypothetical. No pharmaceutical company will pursue it until DeBose-Boyd or others reveal the full picture of how reductase degradation works—the role of the lipid droplets, how DHL mediates Insig, and how the final degradation happens.
Other researchers are aiming new drugs at the part of the system that imports cholesterol. When it travels in blood, cholesterol is packaged inside lipoproteins—either low-density lipoproteins (LDL), considered the bad guys for their accumulation in arteries, or high-density lipoproteins (HDL), the “good” lipoproteins that carry cholesterol to the liver for excretion. People with low levels of LDL and high levels of HDL have the lowest chance of atherosclerosis and heart disease. These days, cholesterol reduction is measured by tracking LDL; the role of HDL is not as clear-cut.
For more than three decades, HHMI investigator Helen Hobbs has been tracking down individuals with extreme LDL and HDL levels—either low or high—and analyzing their genetics. Hobbs, a physician and researcher at UT Southwestern, got her start in research with a postdoctoral fellowship in the Brown–Goldstein lab. She hopes to uncover genetic mutations that hint at new drug targets for managing cholesterol. She’s already revealed one promising candidate—a protein that sweeps the bloodstream clear of LDL—and it’s in the pharmaceutical pipeline.
In 2003, a research group in France identified a gene called PCSK9 that helps control LDL levels. Hobbs had already been following the genetics and cholesterol levels of almost 3,500 people as part of the Dallas Heart Study, her large-scale attempt to find genetic causes of heart disease. So her team tested a handful of participants for mutations in the PCSK9 gene. They found them—in 2 percent of their African-American participants. They repeated the work in a larger population and showed that PCSK9 mutations were associated with a 28 percent reduction in LDL and an 88 percent decrease in coronary heart disease.
“Studies like the Dallas Heart Study are absolutely key to this field,” says Joe Goldstein. “The way to find interesting mutations is to find a population and look at those extremes.”
The research team led by Hobbs then relied on basic biochemistry to piece together PCSK9’s function. They discovered that the protein encoded by PCSK9 is required to degrade the LDL receptor—the protein that pulls LDL from the bloodstream into a cell’s interior. Removal of functioning PCSK9 protein is an ideal recipe for treating high cholesterol: LDL receptors increase, LDL in the bloodstream decreases, and atherosclerosis risk drops.
“This is the single biggest story in the translational medicine side of cholesterol research right now,” says Goldstein. “Hobbs has taken PCSK9 all the way from a finding in a population to learn the real importance of the protein to medicine. And now we have a really good drug target.”
Hobbs identified a person in the Dallas Heart Study who has no PCSK9 and appears to be completely healthy, which has reassured pharmaceutical companies about the safety of the protein as a drug target. A PCSK9 inhibitor is now in early phase human studies with the pharmaceutical company Regeneron. It blocks PCSK9 from binding to and degrading the LDL receptor and results in a dramatic reduction in LDL levels.
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A Role for Inflammation
Cholesterol build-up causes inflammation too, which is a risk factor for atherosclerosis. That inflammation pathway offers another target for drug developers.
When cholesterol accumulates along artery walls, macrophages—immune cells that recognize foreign material—are the first cells to encounter the clumps. The reaction of the macrophage to the cholesterol can either help clear the artery or make problems worse.
“A macrophage is a scavenger for extracellular garbage,” says HHMI investigator Peter Tontonoz of the University of California, Los Angeles. “And when there are cholesterol deposits, they’re recognized by the macrophage as junk that it wants to clear.” Normally, this is a good thing—macrophages help remove LDL from the artery wall. But when a macrophage is overwhelmed with too much cholesterol to process, it turns into a foam cell—so named because the LDL in its interior looks like foamy bubbles.
Foam cells are the first sign of an atherosclerotic plaque. The foamy macrophage produces inflammatory molecules and recruits other immune cells to the site, setting up an inflammatory response, a hallmark of coronary artery disease. “The reason the plaque eventually gets so big and complicated is that the macrophage talks to and recruits other cell types,” says Tontonoz.
But what scientists have struggled to understand is why the macrophage recruits inflammatory molecules when it fills with cholesterol. When the macrophage eats other foreign material, it clears them with no inflammation.
Tontonoz has an answer: a protein called LXR. Originally identified by HHMI investigator David Mangelsdorf, of UT Southwestern, LXR switches between an inactive form, in the presence of low cholesterol, and an active form, in the presence of high cholesterol. In its active form, LXR causes the cell to pump cholesterol out and stop taking cholesterol in.
There are different versions of LXR in different cell types, including macrophages. Mangelsdorf and Tontonoz published a 2003 paper showing that LXR also has anti-inflammatory effects. Tontonoz has since discovered that mice without LXR are more susceptible to a host of diseases, including listeria and tuberculosis. Other studies have shown that drugs increasing the activity of LXR in macrophages have the potential to stop the formation of a foam cell—by pumping cholesterol out—and to decrease arterial inflammation. The combination could stop atherosclerosis.
As Tontonoz has explored the pathway of LXR, he’s also discovered how it arrests cholesterol input, and it’s a familiar mechanism: degradation. In a July 2009 paper in Science, Tontonoz reported that one of the proteins that LXR turns on is a protein called Idol. Idol in macrophages has the same job as Hobbs’s PCSK9 in the liver—degradation of LDL receptors. So Idol, like PCSK9, could be a target for new pharmaceuticals. Already, compounds activating LXR are in the pharmaceutical pipeline.
Pieces of the Puzzle
For every 10 milligrams per deciliter of blood that you decrease your LDL, you have a 10 percent decrease in coronary heart disease risk, says Hobbs. Statins have been an effective way to achieve this LDL reduction, but for some patients, they’re not effective enough to stop heart disease. The network of proteins and genes that regulate cholesterol in the body is complex and far-reaching. Statins affect only one part of this system.
The next cholesterol drug—be it a compound that blocks PCSK9, degrades HMG-CoA reductase, or turns on LXR—will likely be used in concert with statins to come at the problem from two angles.
You can’t predict which aspect of the field will lead to the next breakthrough, says Goldstein. “You have to wait and see. But the important thing is to keep looking at this from new angles.”
As scientists forge ahead in probing those new angles and revealing each part of the cholesterol puzzle, they get closer to that next breakthrough, and the promises of the next drug come into focus.