A Crucial Piece to the Cholesterol Puzzle


When you go for a checkup and get a blood test chances are, your doctor will take a look at your cholesterol levels to make sure they aren’t too high. Cholesterol is an essential lipid to human metabolism; however, having high levels of cholesterol is also linked to atherosclerosis. Atherosclerosis is the formation of plaques on the inside of your arteries, which subsequently causes them to narrow and harden.

by Michaela Go | staff writer | SQ Vol. 10 (2012-2013)

Background

When you go for a checkup and get a blood test chances are, your doctor will take a look at your cholesterol levels to make sure they aren’t too high. Cholesterol is an essential lipid to human metabolism; however, having high levels of cholesterol is also linked to atherosclerosis. Atherosclerosis is the formation of plaques on the inside of your arteries, which subsequently causes them to narrow and harden.

Comparison between a normal artery and an artherosclerotic artery.

Comparison between a normal artery and an artherosclerotic artery.

These plaques may even grow large enough to restrict blood flow or completely block arteries that provide blood to crucial organs causing tissue damage to extremities or organs, stroke, or heart attack. According to the National Heart Lung and Blood Institute, “Coronary heart disease (atherosclerosis of the coronary arteries) is the #1 killer of both men and women in the United States.” Fortunately, breakthrough research led by UCSD researcher Christopher Glass, M.D., Ph.D., is paving the way towards new and much needed treatments for atherosclerosis.
Dr. Glass’s involvement in atherosclerotic research really started with his fascination with the macrophage cell and its diverse functions in immunity and homeostasis. Macrophages are cells that consume foreign or dangerous identified cells and matter. As macrophages consume other cells, they also consume the cell’s store of cholesterol. Most of the time, these cells have effective means of metabolizing and getting rid of excess cholesterol. Macrophages that do not properly metabolize the extra cholesterol accumulate cholesterol in the form of “foamy lipid droplets,” giving these cholesterol-filled cells the name “macrophage foam cells.” These macrophage foam cells are one of the most abundant cell types within atherosclerotic lesions.
He began studying the molecular biology of the macrophage, but since these cells play such an important role in atherosclerosis, putting his work in the context of “one of the most common diseases of our society…was a very attractive angle.”

Linking Inflammation and atherosclerosis

There is a definite correlation between inflammation of the arteries and accumulation of macrophages with high cholesterol and atherosclerotic lesions. This correlation lead many scientists to believe that the accumulation of cholesterol within macrophages caused macrophages to express more pro-inflammatory genes; in other words, to be more “activated” and cause inflammation. However Glass’ research shows that, unexpectedly, the opposite is actually true. When induced to fill up with cholesterol, these macrophages were actually deactivated—their pro-inflammatory genes were turned off. Thus, Glass believes that something else, not cholesterol, is driving the inflammatory response and thwarting these macrophages’ ability to inhibit inflammation. “Exactly how the increase of cholesterol is causing inflammation in my mind is still an unanswered question.”

The reason inflammation in the context of atherosclerosis is such an important topic of research is because while inflammation is usually self-limiting, it may become continuous and cause chronic inflammatory diseases. In this case, the body recognizes plaques deposited in the arteries as abnormal and launches an inflammatory response. The inflamed arteries then further promote atherosclerotic lesions launching a positive-feedback cycle of inflammation and plaque formation.

Glass set out to identify the molecule accumulating inside the macrophage foam cells that was responsible for inhibiting inflammation. Further research identified this molecule as desmosterol, a precursor in the cholesterol biosynthesis pathway that is eventually converted into cholesterol. Because desmosterol is able to inhibit the inflammatory response, Glass hopes it can be used as a basis for a novel treatment aimed toward the prevention of inflammation and the propagation of atherosclerotic lesions.

Unexpected Findings

Macrophages were generated in mice by feeding them a diet high in cholesterol and fat and additionally knocking out the mice’s LDL receptors, which bind free floating low-density-lipoproteins (LDL). Knocking out the LDL receptors helped to facilitate even higher cholesterol levels and accumulation of macrophage foam cells within the mice. Analysis of the macrophages’ genes, transcription factors and lipids revealed significant changes in almost all of the lipid classes and gene expression.[1]

Genes linked to inflammation and cholesterol synthesis were unexpectedly downregulated. This downregulation of pro-inflammatory genes was very shocking because the accumulation of macrophage foam cells were supposed to activate inflammation. This discovery was so surprising that after presenting the data to a group of colleagues, someone suggested that the samples may have been mixed up, “because the suppression of inflammation was so counter to what everyone was expecting.”

Desmosterol turned out to not only be important in inhibiting inflammation, but is also very important in cholesterol level regulation. During their analysis, Glass and his team followed two important proteins that regulate cholesterol biosynthesis, Sterol Regulatory Element-Binding Proteins (SREBPs) and Liver X Receptors (LXRs). The SREBP pathway functions to turn on the cholesterol biosynthetic pathway when cholesterol levels are low. The LXR pathway, on the other hand, helps rid the cell of too much cholesterol. It also turns off cholesterol biosynthesis to prevent the synthesis of new cholesterol when cholesterol levels are already high.

Glass and his colleagues found that desmosterol, the last intermediate in the biosynthesis pathway, was the molecule responsible for both down regulating cholesterol synthesis and upregulating the LXR pathway. This was surprising because they didn’t expect the responsible molecule to be an intermediate of cholesterol biosynthesis—a pathway that should be turning off. Glass’ explanation for this unexpected behavior is that during the accumulation of cholesterol within the cell, only the last step of cholesterol biosynthesis is turned off instead of the whole pathway. This still hinders cholesterol biosynthesis, but because desmosterol is prevented from turning into cholesterol, it allows desmosterol to accumulate.

What’s Next?

Now at the position of finding out that desmosterol is responsible for stopping the synthesis of new cholesterol, getting rid of excess cholesterol, and inhibiting inflammation, Glass and his team hypothesize that somehow the LXR pathway must be becoming inactivated in the artery wall. If this is indeed the case, they want to find out how this is happening and are setting up a number of experiments to look at the inactivation of the LXR system.
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They are also looking into making desmosterol mimetics (small desmosterol-like molecules) that will have the same function as normal functioning desmosterol. They hope that this can be beneficial in the context of hypercholesterol and atherosclerosis. Current cholesterol medications aim to prevent atherosclerosis by merely lowering cholesterol levels. However, Glass’s discovery has the potential to give rise to a new medication that additionally hinders the inflammatory response, prevents excess cholesterol biosynthesis, and promotes cholesterol excretion.

At this point in time, Glass says although he thinks he knows how the desmosterol mimetic will affect the macrophage, he cannot accurately predict what will happen in the liver, which is really the organ that controls total cholesterol levels. “And that’s really one of the questions that we want to look at either using desmosterol mimetics or other approaches to tweak this pathway in vivo.” Although their research is still at the basic science level, Glass believes that these findings have the potential to be translated into new therapies.

“These findings really changed the way I thought, and that’s what a discovery is…a finding that changes the way you think about things.”

WRITTEN BY MICHAELA GO. Michaela Go is a Biochemistry and Cell Biology major from Muir College. She will graduate in 2014.

References

1.www.ncbi.nlm.nih.gov/pmc/articles/PMC1810472/

2.http://www.heart.org/HEARTORG/Conditions/Inflammation-and-Heart-Disease_UCM_432150_Article.jsp


[NOTE: It is important to note that Glass’ studies were conducted with macrophage cells generated in the peritoneal cavity of mice. The shortage of macrophages in the artery wall makes analysis very difficult.]

 



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