Atherosclerosis and Dyslipidemia

Atherosclerosis

Atherosclerosis is a cardiovascular disease in which plaques form in the tunica intima of blood vessels.  Atherosclerosis is an inflammatory disease, and tends to develop in sites where the endothelium becomes damaged.  Useful reading in the Silverthorn textbook is found on pages 501-504, particularly Figure 15.21 (p. 503), which describes the development of atherosclerotic plaque.

Heart disease occurs because the coronary arteries supplying the heart are a major site where atherosclerotic plaques form.  The growth of atherosclerotic plaque can cause blood vessels to narrow, reducing blood flow, and causing chest pain (angina). Even more damaging is when a plaque ruptures and triggers the formation of a clot, which blocks blood flow and oxygen delivery causing tissue damage.  When a clot blocks blood flow in a coronary artery, this is known as a myocardial infarction (heart attack). 

The major factors that increase the risk for atherosclerotic cardiovascular disease are male sex, increased age, smoking, high blood pressure, and dyslipidemia.  Dyslipemia means abnormal levels of circulating lipids.  Increased risk for atherosclerotic cardiovascular disease is associated with a high level of LDL cholesterol and a low level of HDL cholesterol.  But what do we mean when we say "LDL cholesterol"?

Lipoproteins

Cholesterol is a lipid that is an important component of cell membranes and is the precursor used in the synthesis of steroid hormones, bile salts (necessary for fat digestion and absorption), and vitamin D.  Cholesterol, like other lipids, is nonpolar and poorly soluble in the aqueous extracellular fluid.  Cholesterol and other lipids travel through the circulation packaged in particles called lipoproteins.  

Lipoproteins are particles that contain several thousand molecules.  The core of the lipoprotein is filled with nonpolar molecules:  cholesterol esters and triacylglycerol (TAG; fat).  The outer layer of the lipoprotein is coated with amphipathic phospholipids in a single layer, oriented so that their nonpolar tails face the core of the lipoprotein and their polar heads face outward, enabling the lipoprotein to be soluble in the fluids of the body.  The outside of the lipoprotein contains amphipathic proteins called apolipoproteins that stabilize the lipoprotein and provide an identity tag.

Lipoproteins can be differentiated on the basis of their density, but also by the types of apolipoproteins they contain. The degree of lipid in a lipoprotein affects its density—the lower the density of a lipoprotein, the more lipid it contains relative to protein. The four major types of lipoproteins are chylomicrons, very low-density lipoprotein (VLDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL).

Although all cells can make cholesterol, cholesterol is mainly synthesized in the liver by hepatocytes.  Hepatocytes also synthesize triacylglycerol (TAG) from excess carbohydrates.  Hepatocytes package cholesterol and TAG in lipoproteins for export to other cells in the body. 

The figure below summarizes the fates of the various lipoproteins. Refer to it as you read about the different lipoproteins.

• Chylomicrons and VLDL deliver TAG to cells in the body. Two types of lipoproteins are triglyceride-rich: the chylomicrons and VLDL. Chylomicrons are synthesized by enterocytes from lipids absorbed in the small intestine. VLDL is synthesized in the liver. The function of these lipoproteins is to deliver energy-rich triacylglycerol (TAG) to cells in the body (pink pathway). TAG is stripped from chylomicrons and VLDL through the action of lipoprotein lipase, an enzyme that is found on the surface of endothelial cells. This enzyme digests the TAG to fatty acids and monoglycerides, which can then diffuse into the cell to be oxidized, or in the case of an adipose cell, to be re-synthesized into TAG and stored in the cell.


• LDL delivers cholesterol to cells in the body. As VLDL particles are stripped of triacylglycerol, they become more dense. These particles are remodeled at the liver and transformed into LDL. The function of LDL is to deliver cholesterol to cells, where it is used in membranes, or for the synthesis of steroid hormones (blue pathway). Cells take up cholesterol by receptor-mediated endocytosis. The apolipoprotein found in LDL, apolipoprotein B, specifically binds to the LDL receptor and is internalized in an endocytic vesicle. Receptors are recycled to the cell surface, while hydrolysis in an endolysosome releases cholesterol for use in the cell.


• HDL is involved in reverse cholesterol transport. Excess cholesterol is eliminated from the body via the liver, which secretes cholesterol in bile or converts it to bile salts. The liver removes LDL and other lipoproteins from the circulation by receptor-mediated endocytosis. Additionally, excess cholesterol from cells is brought back to the liver by HDL in a process known as reverse cholesterol transport (green pathway). HDL (or really, the HDL precursor) is synthesized and secreted by the liver and small intestine. The HDL precursor is like an empty bag that travels through the circulation, gathering cholesterol to form mature HDL. HDL returns the cholesterol to the liver via various pathways.

Cholesterol homeostasis

The liver is central to the regulation of cholesterol levels in the body. Not only does it synthesize cholesterol for export to other cells, but it also removes cholesterol from the body by converting it to bile salts and putting it into the bile where it can be eliminated in the feces. Furthermore, the liver synthesizes the various lipoproteins involved in transporting cholesterol and other lipids throughout the body.

Cholesterol synthesis in the liver is under negative feedback regulation.  The rate-limiting step in cholesterol synthesis involves the enzyme HMG-CoA reductase.  Increased cholesterol inside the hepatocyte causes decreased HMG-CoA reductase activity by causing degradation of the enzyme.  Cholesterol also inhibits a factor that travels to the nucleus to stimulate trasncription of LDL receptors and HMG-CoA reductase.  As shown in the figure, when cholesterol is high inside the hepatocyte, it has two responses that can lower cholesterol:

1. It decreases synthesis by decreasing HMG-CoA reductase activity.
2. It decreases cholesterol uptake by decreasing LDL receptors.
   

Conversely, when cholesterol is low inside the hepatocyte, it upregulates cholesterol synthesis through increased transcription of HMG-CoA reductase.  And it upregulates cholesterol clearance from the circulation by increasing expression of LDL receptors.  The LDL receptor is key to the regulation of cholesterol levels in the body, because it allows the hepatocyte to "see" how much LDL is in the circulation.

Disorders and Drug Treatments

Familial Hypercholesterolemia

The link between cholesterol and heart disease was recognized through the study of individuals with familial hypercholesterolemia (FH). There are two forms of FH:  a severe homozygous form which is very rare (affecting 1 in 1,000,000 individuals) and a more common heterozygous form (affecting 1 in 500 individuals).  Individuals with heterozygous FH have levels of LDL that are roughly twice as high as normal, and they may start to have heart attacks and strokes in their 30s and 40s.  In the more severe homozygous form of FH, LDL is 8 times higher than normal and heart attacks and strokes occur in childhood.  FH clearly illustrates that there is a link between the level of LDL cholesterol and the risk for atherosclerotic cardiovascular disease. 

The genetic defect in FH was determined through the work of Michael Brown and Joseph Goldstein (for which they were awarded the 1985 Nobel Prize in Physiology or Medicine).  Most FH is caused by a mutation in the LDL receptor. In FH homozygotes without functioning LDL receptors, LDL is not cleared from the circulation. As well, because cholesterol cannot get into cells efficiently, there is no negative feedback suppression of cholesterol synthesis in the liver.

Another mutation that can cause FH is a mutation in apolipoprotein B, the apolipoprotein found in LDL.  Apolipoprotein B is the component of LDL that specifically binds to the LDL receptor.  

Dyslipidemia

More common in the general population is dyslipidemia, which is the term that is used if lipid levels are outside the normal range.  In a typical lipid profile, the fasting levels of total cholesterol, LDL cholesterol, HDL cholesterol, and triglycerides are determined. High levels of LDL cholesterol (the so-called “bad cholesterol”) greatly increase the risk for atherosclerosis because LDL particles contribute to the formation of atherosclerotic plaques. Low levels of HDL cholesterol (the so-called "good cholesterol") are an independent risk factor, because reverse cholesterol transport works to prevent plaque formation, or may even cause regression of plaques once they have formed. HDL may also have anti-inflammatory properties that help reduce the risk of atherosclerosis.  Fasting triglyceride levels are used to estimate the level of VLDL. High levels of triglycerides are also associated with an increased risk for atherosclerosis, although the mechanism is not entirely clear.

Drug treatments

The most important drugs for the treatment of dyslipidemia are by far, the statins. Statins have been shown in multiple clinical trials to reduce cardiovascular events and mortality.

• Statins
  These drugs lower LDL cholesterol by inhibiting HMG-CoA reductase, the rate-limiting enzyme in cholesterol synthesis. They are designed to mainly inhibit HMG-CoA reductase in the liver. Inhibition of cholesterol synthesis further decreases circulating LDL because reduced levels of cholesterol in the hepatocyte remove the inhibition on transcription of LDL receptors, causing the hepatocyte to increase LDL receptors and LDL clearance from the circulation (see figure).

In the past, several different drugs have been used to treat dyslipidemia, however the most recent treatment guidelines recommend mainly statin therapy at different intensities according to the patient's risk for cardiovascular disease. However, statins may cause adverse effects in some patients, or in others, statins by themselves may not provide sufficient lowering of LDL cholesterol.  These patients may benefit from the use of the other two drugs listed below.

• PCSK9 inhibitors
  PCSK9* is a secreted protein that binds to LDL receptors and targets them for breakdown. PCSK9 inhibitors are monoclonal antibody drugs that bind to PCSK9 and block its ability to cause LDL receptor degradation. They lower LDL cholesterol because with increased numbers of LDL receptors, more LDL can be removed from the circulation.
  

  Two PCSK9 inhibitor drugs were approved in 2015: alirocumab (Praluent™) and evolocumab (Repatha™). Because they are monoclonal antibody drugs, they must be administered by injection.  In December 2021, the FDA approved inclisiran (Leqvio™) a small interfering RNA drug that is also a PCSK9 inhibitor.

  PCSK9 inhibitors have been approved as a second-line treatment for patients who can’t tolerate statins, or who are unable to get effective lowering of LDL cholesterol by using a statin alone. In clinical trials, these drugs were able to substantially lower LDL cholesterol. Large randomized, controlled clinical trials have shown that treatment with PCSK9 inhibitors can lower cardiovascular events such as heart attack and stroke.  Because of their high cost and the requirement that they be administered by injections, PCSK9 inhibitors are mainly recommended for use in high risk patients, such as patients with familial hypercholesterolemia.
  

  *PCSK9 stands for "proprotein convertase subtilisin/kexin type 9".

• Ezetimibe
  Ezetimibe inhibits cholesterol absorption in the small intestine. This reduces absorption of dietary cholesterol, but also promotes cholesterol excretion, since biliary cholesterol accounts for some of the cholesterol that passes through the small intestine.  Ezetimibe effectively lowers LDL cholesterol, and, when paired with a statin, can provide the same degree of cholesterol lowering with a lower dose of statin. 
  




Quick Quiz
	Fill in Answer	Correct	False	Correct Answer
1. Name the triglyceride-rich lipoprotein that is synthesized by the liver.
2. Name the rate-limiting enzyme in cholesterol synthesis.
3. HDL returns cholesterol to the liver in a process known as ___________.
4. What is the mechanism used by cells to take up LDL cholesterol from the circulation?
5. What protein is required for the above process?
6. What is the name for the disorder in which the above protein is non-functional?
7. Name the secreted protein that binds to LDL receptors and leads to their degradation.
	(Spelling must be correct)