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Apolipoprotein A-I – a good player with a dark side
Published 27 March 2020
Oktawia Nilsson's doctoral thesis describes Apolipoprotein A-I (shortly named ApoA-I), the main protein of HDL also known as “the good” cholesterol. Based on the experiments performed in the scientific papers included in this thesis, we propose that ApoA-I can be used as a promising treatment for diabetes and cardiovascular diseases. However, some people with genetic predispositions are born with mutated ApoA-I. This not properly functioning ApoA-I, accumulates in the vital organs leading to their failure. Both aspects are in the focus of this thesis and are explained below.
I am sure that most of you have heard of the “good” and the “bad” cholesterol and often were wondering which one it is while looking at the blood test results. I hope that after reading this thesis (or just this chapter), there will be no longer any confusion. Let’s start from the beginning. Our diet is highly enriched in fat, also called lipids. Fat is not soluble in water-based environments, such as blood, and must be therefore transported in the bloodstream with the help of specialised proteins, like ApoA-I. The lipid-protein complexes are called lipoproteins. There is a variety of different lipoproteins which ensure that lipids are transported to cells, where they are used for energy, or to fat tissue, where they can be stored, or to the liver, where the lipids are broken down and removed from the body. When too much fat is consumed, the human organism cannot take care of it properly and this leads to accumulation of lipids in different cells of organs including blood vessels. The build-up of the lipids in the arteries leads to atherosclerosis. The blockade of the blood supply, which provides vital oxygen to the organs, by an atherosclerotic plaque or a blood clots, leads to serious consequences such as heart attack or stroke. The lipoprotein which specialises in the removal of this deadly cholesterol from cells and transport to the liver where it can be eliminated is called HDL, or the “good” cholesterol. On the other hand, the lipoprotein which brings additional cholesterol to the cells, causing its accumulation, is called LDL, or the “bad cholesterol”. People who have high levels of LDL-cholesterol are at a high risk to develop cardiovascular diseases, whereas those with high HDL-cholesterol levels are protected from these diseases. Thus, it is not bad at all to have high cholesterol as long as we are referring to the HDL-cholesterol.
Now, that difference between the “bad” and the “good” cholesterol is clear, I would like to direct your focus on ApoA-I, the main protein of HDL. ApoA-I has a very important function in the removal of the cholesterol from the cells thus preventing the development of atherosclerosis. It interacts with special receptors on the surface of cholesterol-enriched cells and transfers it to the HDL lipoprotein. Another, beneficial function of ApoA-I involves the regulation of blood glucose levels, as described in paper I and paper II included in this thesis. Insulin is a hormone produced by specialised cells, called beta cells located in islets of Langerhans of the pancreas. It is produced after a meal when glucose levels in the blood are high. Glucose is the main energy source for cells and proper regulation of its levels is very important. Insulin interacts with cells of the human body to let them know that they can take up blood glucose and use it for energy. In people with diabetes, these cells do not respond to the signal given by insulin, therefore blood glucose levels stay high. This is called insulin resistance and its development leads to the progression of the disease. To compensate for the high glucose, beta cells produce even more insulin, leading to their exhaustion and dysfunction. Diabetes can be therefore described as a combination of insulin resistance, beta cell dysfunction and consequently high blood glucose levels. Prolonged and untreated diabetes leads to many complications, among which cardiovascular diseases are the most common.
In paper I, we found that ApoA-I can improve the beta cell function to make more insulin in response to high glucose. We used beta cells grown in the lab as well as isolated from mice, to understand how this is happening. We used microscopes to see ApoA-I interaction with the beta cells. We found that ApoA-I is mostly inside the beta cells and that the incubation with ApoA-I leads to more insulin being present close to the cell’s edge. This insulin is ready to be released as soon as there is a need. In the light of our previous experiments, which showed that ApoA-I increases the sensitivity of cells to insulin, we propose that ApoA-I can be used as a future medicine to treat diabetes and cardiovascular diseases, which often go hand-in-hand. One of the most common ways to diagnose diabetes is the measurement of so-called glycated haemoglobin. High blood glucose levels, also referred to as hyperglycemia, existing for a long time leads to the glycation modification, of many proteins including haemoglobin of red blood cells. Haemoglobin is a remarkably stable protein and the levels of its glycation allow to estimate how severe was the hyperglycemia during past up to 12 weeks.
ApoA-I, similarly to haemoglobin, is glycated in hyperglycemia and in paper II we investigated the consequences of this incidence. We created a glycated ApoA-I and researched its functionality. We found that the glycated ApoA-I does not bind to lipids as efficiently as the not modified protein. Additionally, its function in removing cholesterol from cells and lowering glucose levels in mice was not as efficient as compared to not glycated ApoA-I. We concluded that regulation of blood glucose is very important to ensure fully functional ApoA-I.
The second part of this thesis, discussed in paper III and paper IV, is focused on a different aspect of ApoA-I. Some people are born with a genetic mutation located in the gene encoding ApoA-I. These people, often belonging to the same family, have ApoA-I that has different properties. This mutated ApoA-I can associate with one another and form amyloids, which are dysfunctional protein aggregates that can be accumulated in various organs leading to their damage. This condition is called ApoA- I related amyloidosis. Amyloidoses are a group of diseases which are caused by a build- up of different protein amyloids in various organs. Most likely you have heard of Alzheimer’s or Parkinson’s diseases, amyloidoses that cause protein amyloid accumulation in the brain. In case of ApoA-I related amyloidosis, the mutated ApoA-I accumulates in different organs depending on where the mutation occurred in the APOA1 gene. Among the most commonly affected organs are kidney, liver, larynx, and heart. Since this disease is very rare and shows symptoms late in life, it is very difficult to diagnose. As for today, the only available treatment is organ transplant. People with this disease have much lower amounts of HDL in their blood, but surprisingly, they are not in a higher risk to develop cardiovascular diseases. In paper IV, we made and tested four of the mutated ApoA-I and we found that they are more effectively taking cholesterol from cells as compared to normal ApoA-I. This could explain why, despite lower levels of HDL, people with mutated ApoA-I do not have higher risk to develop cardiovascular diseases. In paper III, we used plasma from people who are affected by ApoA-I related amyloidosis for our experiments. We tested how functional are the HDL from their plasma and we came to the same conclusion as before. Their HDL were more effective in removing cholesterol from cells as compared to people who do not have mutated ApoA-I. We wanted to understand what happens to the protein when it has a mutation.
We made mutated ApoA-I and performed special analysis, which allowed us to see if the protein’s structure is changing. We concluded that the mutation makes the protein more flexible. It means that the protein can adapt better to bind more cholesterol from cells. Maybe we could make normal ApoA-I more flexible and therefore more efficient in taking care of the deadly cellular cholesterol? We will see in the future experiments.