a bonobo humanity?

Recent studies have revealed that children 8-10 years old are being diagnosed with Type II diabetes, high cholesterol, and high blood pressure at an alarming rate.

Lee Haney

As I said in my previous post, biochemistry is almost infinitely complex, so bear with me as I crawl towards an understanding of the role of statins in reducing LDL cholesterol in the blood stream, thus reducing atherosclerosis, a major feature of heart disease.

Remembering, first, that cholesterol is a sterol, which is a modified steroid with a hydroxyl (alcohol) group coming off carbon 3. It’s a mostly hydrophobic lipid with this tiny polar hydroxyl group added. It’s carried around in the bloodstream by lipoproteins.

I’ll turn now to atherosclerosis – though I don’t currently know whether statins can perform roles other than reducing the build-up of plaque in the arteries.

So, generally, our arteries carry oxygenated blood from the heart to other organs and regions of the body. Atherosclerosis is sometimes called ‘hardening of the arteries’, as sklerosis is from Greek, meaning ‘hardening’, but it’s really a narrowing rather than a hardening, or perhaps it’s better described as both, as we’ll see. Arteriosclerosis is a more general name, while atherosclerosis means blockage or narrowing (stenosis) due to an atheroma, an abnormal accumulation of ‘debris’ or plaque consisting of fat (mostly), calcium and sometimes fibrous tissue in the inner arterial wall (endothelium). These atheroma are difficult to detect before they cause heart attacks or disease, because heart arteries are very small and hidden deep within the chest. They’re also quite mobile and elastic with blood flow. Heart attack and stroke sometimes happen when the atheroma ‘bursts’ – the fibrous cap (of smooth muscles cells, cholesterol-rich foam cells, collagen and elastin) which surrounds the atheroma is ruptured, or breaks free from the arterial wall, causing a blood clot (thrombus). These are sudden events, not easily detected beforehand. Alternatively, major problems arise when the atheroma becomes large enough to defeat arterial flexibility.

There can be symptoms, apart from such major dramas as heart attacks and stroke, which may act as warning signs for atherosclerosis. The narrowing of the arteries means that less blood and oxygen is reaching the cardiovascular system (ischemia), and this may result in vomiting, angina (chest pain), and general feelings of faintness and anxiety. Atherosclerosis of the carotid artery, which feeds the brain, may have different symptoms, including headaches, dyspnea and facial numbness. Atherosclerosis can also affect the function of the liver, kidneys and other organs, and the vascular system.

So what causes atheromas? It seems that these accumulations of plaque are the result of monocyte-macrophage activity. Macrophages are types of white blood cells (leucocytes) that perform immune and cleansing functions. However, we don’t really know why the plaque build-up occurs – though it might be initiated by damage to the endothelium. We do know that atherosclerosis can begin early, and that blood LDL cholesterol is a major factor in the activity that leads to this build-up. That’s why researchers have been rather single-minded about ways of reducing LDL cholesterol, and even on increasing HDL cholesterol levels, though there’s little evidence, apparently, that higher HDL levels are beneficial. Nor, interestingly, is there much evidence that lowering triglycerides has a positive effect on heart disease, while study after study has shown that low LDL cholesterol levels are key to avoiding cardiovascular problems.

Okay, now I’m going to take a few steps back to look more deeply at the role of LDL cholesterol in building atheromas and so causing atherosclerosis. Returning to my vague mention of macrophages and monocytes, here’s a clearer picture, drawn mainly from this excellent video.

1. Structure of arterial wall

First, we need to know that the arterial walls are layered. The first layer surrounding the lumen (the tunnel space where the blood flows and where you find red blood cells or RBCs, leukocytes and lipoproteins, etc) is the epithelium, a thin layer of squamous cells. This layer is surrounded by the tunica intima (sometimes the epithelium is described as part of the T intima), an elastic layer quite rich in collagen. It also contains structural cells called fibroblasts, and smooth muscle cells (SMCs). Surrounding the T intima is the tunica media (particularly rich in SMCs), which in turn is surrounded by the thicker, tougher tunica adventitia. In general, the arterial wall becomes stiffer and more fibrous as you move from inner to outer. Atherosclerosis is apparently more of a problem in large and middle-sized arteries which contain more of the protein elastin.

2. Plaque formation

Plaque formation begins, it’s believed, when there’s damage to the thin endothelial layer (only one cell thick) as well as an abundance of circulating low density lipoproteins (LDLs). LDLs (mostly lipid with a small amount of protein) can then move through the damaged layer into the T intima where they become oxidised by ‘reactive oxygen species’ (free radicals) and other enzymes such as metallo-proteases, released by the endothelial cells. These oxidised LDLs, which are now ‘trapped’ in the T intima, will activate endothelial cells to express receptors for white blood cells (leukocytes), particularly the largest types of leucocyte, known as monocytes. So we have this accumulation of oxidised LDLs activating endothelial cells to express adhesion molecules for leucocytes, which brings monocytes and T helper cells into the T intima layer. This movement into tissue transforms monocytes into macrophages (not sure how that happens), and these macrophages then ‘take up’ or engulf the oxidised LDLs and form foam cells. By this time the lipid material dumped into the T intima has created something like a lake of fat, known as a ‘fatty streak’. Foam cells are central to the process of plaque formation and atherosclerosis, as they induce more SMCs into the T intima from the T media by means of a released growth factor, IGF-1 (insulin-like growth factor), and this leads to increased synthesis of collagen in the region, which hardens the plaque build-up, a build-up further fostered by foam cell death which releases more lipid material. Foam cells also release pro-inflammatory cytokines and reactive oxygen species as well as chemokines which attract more macrophages to the site. Upon death they also release DNA material that attracts neutrophils, a very common type of white blood cell. All of this will increase inflammation or plaque build-up in the region.

3. Effects

As mentioned, SMCs contribute to the containment of this inflamed lipid area by releasing proteins such as collagen and elastin, which is used to build a fibrous cap around it. They also stiffen the formation, the atheroma as it’s called, by adding calcium. All of this has the effect of enlarging the atheroma and so reducing the diameter of the arterial lumen in the area, which raises blood pressure as the blood tries to maintain an adequate flow. The calcification of the area also considerably reduces the flexibility of the arterial wall, again resulting in increased blood pressure. Rupture of the fibrous cap may result, which may lead to thrombosis.

So where do statins come in here? Let me quote from an abstract of one academic paper: Statins in atherosclerosis: lipid-lowering agents with antioxidant capabilities, published in 2004:

Statins (3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors) are the first-line choice for lowering total and LDL cholesterol levels and they have been proven to reduce the risk of CHD [chronic heart disease]. Recent data suggest that these compounds, in addition to their lipid-lowering ability, can also reduce the production of reactive oxygen species and increase the resistance of LDL to oxidation. It may be that the ability of statins to limit the oxidation of LDL contributes to their effectiveness at preventing atherosclerotic disease.

Note that oxidation of LDL has the effect of fixing it in the T intima, as mentioned above, so if it’s true, as I presume it is, that statins inhibit LDL oxidation, as well as having other benefits, then they can’t be a bad thing, as long as there aren’t serious side-effects. I’ll continue to explore this topic, as it’s teaching me a lot about the blood, the liver and the circulatory system, inter alia – and it’s great fun. Dr Ben Goldacre has written a book Do statins work? the battle for perfect evidence-based medicine, which hasn’t been released yet, but I intend to get my hands on it and devour it, along with more videos and articles. In the meantime I hope it’s not too controversial to go on saying that the best way to reduce that nasty (but not too nasty) LDL cholesterol is to eat a healthy diet and engage in effective exercise.

PS: haha I know this’ll be unreadable to most, but if anybody finds any egregious error in this, let me know.

References

Atherosclerosis video – Nucleus Medical Media (2009)

Atherosclerosis – pathophysiology, video by Armando Hasudungen (2014)

Atherosclerosis – part 1, Khan Academy video

https://www.ncbi.nlm.nih.gov/pubmed/15177118

https://training.seer.cancer.gov/anatomy/cardiovascular/blood/classification.html

Cholesterol metabolism part 1, video by Ben1994 (2015)

Cholesterol metabolism part 2, video by Ben1994 (2015)

Cholesterol metabolism part 3, video by Ben1994 (2015)