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Aug 24

Beyond the Lipid Hypothesis: Plaque Development

Note from Dave–

Rarely have I been so impressed with the aptitude and intelligence of a researcher as I have with Siobhan. Her tenacity and resolve to absorb this subject is nothing short of jaw-dropping and I’m incredibly excited to have her fill in our deep dive on this crucial subject. Enjoy!

 

In the Beginning…

Even before starting the ketogenic diet in August of 2016, I already had a vague notion that the diet heart hypothesis did not have much weight to it: the hypothesis, that is now considered “common knowledge”, is that high cholesterol (LDL-c) causes heart disease. Always an avid reader, and a stubborn critic, I had a decent sense of “something’s not right” surrounding the supposed causes of heart disease. As a result, in 2015, when my doctor told me to take statins at the age of 19, due to my total cholesterol readings of over 300: I refused. I knew enough from my own, curiosity based, research that I did not want them because I was not convinced of the often touted benefits. The topic of heart health and high cholesterol was a sore one, given that my father had had a heart attack in his forties, leaving me with a sense of inevitability that it was in my genes and lurking around every corner.

I did not know much about the mechanisms regarding the development of heart disease and atherosclerosis, beyond the idea that heart disease was likely an inflammation problem. I found some hints in books such as The Case Against Sugar (Gary Taubes), and The Big Fat Surprise (Nina Teicholz), and knew the disease was at least correlated to insulin and sugar consumption. Watching some of Ivor Cummins videos on the topic gave me even more insights towards insulin being a correlative – perhaps even causative – factor, but not how or why this was so. I thought perhaps there was something to the “LDL sticking to the arterial wall and oxidizing” theory, that I had heard in the mainstream, but I began to doubt this as fact when I met Dave at Ketofest, and he asked a very pertinent question.

The Question 

“What if LDL is oxidized before it sticks to the arterial wall?” Dave suggested. He later followed up with the idea that perhaps plaque was a beneficial mechanism of the body that later becomes disordered or overwhelmed and develops into the unhealthy manifestation of plaque buildup. Perhaps too, these two aspects together – the oxidation of LDL and plaque development – work in tandem to form some type of specific, helpful, purpose. With little time at the Ketofest afterparty, and not much time for in-depth discussion, I was left with those two questions simmering at the back of my mind as I talked to him more over the evening.

Further conversations with Dave, which took place over the next few hours, presented me with the idea that the system of cholesterol and fat transportation – with the main characters of LDL, HDL, and chylomicrons – were far more complicated, intuitive, and interesting than I had ever considered before. After Dave’s presentation earlier that day, I became further intrigued by the topic, and found myself excited at the prospect of gaining a better understanding of this oft overlooked and denigrated system. Just like having an enthusiastic teacher in high school genuinely in love with the topic – the excitement for this student was contagious.

The Studying Begins in Earnest

The next day, based on Dave’s recommendation, I began reading Peter Attia’s “The Straight Dope on Cholesterol” series. Admittedly, after reading the first few parts, I knew I was in way over my head. The material was headache-inducing and too complicated for my novice barebones knowledge, but because of my stubborn and determined nature to learn, I set about learning the concepts and terminology to the best of my ability. I took in as much as I could and bided my time until I got home and had better access to information. As soon as I could I was in front of the computer, with a notepad and pen ready, reading through the first part of “The Straight Dope on Cholesterol” again, when I saw the recommendation to read “Therapeutic Lipidology” given by Attia if one wanted to become “an aspiring lipidologist” – I immediately found a copy, and began reading.

The first part of the book focused on basic makeup and functions of lipoproteins, which helped form the building blocks of understanding how they all worked together to form a coherent system. After reading through “Therapeutic Lipidology”, and moving on to “Clinical Lipidology”, I began to investigate the role of plaque formation in cardiovascular disease. I asked myself whether their formation was a mechanism that provides any benefit under normal conditions. I suspected that plaque was part of an inflammatory response, and indeed, not long after plaque was mentioned, so were macrophages and inflammation.

Macrophages – Traveling Doctors of the Immune System

Macrophages are the traveling doctors of the immune system, I soon discovered, roaming through the bloodstream, and tissues, until they find a site of infection or damage – ranging from a physical cut on your finger to a virus sequestered in the artery wall1. The macrophages then land on the site, and depending on the situation, have multiple tools at their disposal. They can increase production of various growth hormones to use for repair of damaged cells2, can induce phagocytosis to envelop and digest viruses and harmful bacteria3, or can initiate inflammation. This inflammation is actually a useful tool that can be used to kill bacteria, just like a fever but in a localized area. I found this relationship, between macrophages and inflammation, interesting as it appeared that the relationship was more complicated than it first appeared, even in the case of atherosclerosis.

For example, macrophages don’t only cause inflammation in the artery wall, contrary to my original belief, but come in different ‘types’ used in various circumstances throughout the course of the disease. M1 type macrophages, for example, are responsible for initiating and sustaining an inflammatory response, and are found in developing atherosclerosis. M2 macrophages, on the other hand, have properties that are primarily anti-inflammatory and associated with either resolution of an infection / injury4 or a chronic infection5. In receding atherosclerosis, M1 as well as M2 macrophages can be found inside the plaque, and an increase in both types of macrophage are present in vulnerable plaque as well. Thus, the presence of M2 type macrophages is not entirely straightforward, as it is not only associated with resolution of atherosclerosis but also worsening development6. Perhaps, then, what causes macrophage sites to appear in the first place will provide a hint as to how atherosclerosis develops.

In other words, if macrophages are causing an inflammatory response in the arteries, as is found in atherosclerosis, then what are macrophages creating inflammation in response to? Surely, such a specialized mechanism would not arbitrarily be called upon to invoke an inflammatory response unless needed. The response must inherently be responding to something disrupting the system. Indeed, upon studying further I found that monocytes, the precursor to macrophages, come upon modified LDL lodged in the arterial wall7, 8, and initiate inflammation. So perhaps there is some unintentional lodging of modified LDL in the arterial wall which kick-starts the whole process – however, I did not find any evidence to support the theory of “unintentional” lodging of modified LDL. In fact, it was quite the opposite.

Not So Unintentional

Consider, for example, Lectin-like oxidized low-density lipoprotein (LDL) receptor-1 (LOX-1) and Scavenger Receptor class A I (SR-AI), two of at least ten so-called ‘scavenger receptors’, some of which appear at the endothelial layer in the increased presence of modified LDL9, and are able to recognize harmful particles, including modified LDL10, and pathogens11. The role of scavenger receptors is to halt the progress of dangerous material through the bloodstream, and some of them are uniquely able to recognize modified LDL, but do not recognize/signal to unmodified, healthy LDL12, and as such only ‘see’ low density lipoproteins that are damaged or otherwise modified13. This receptor signals to this changed LDL to pull over out of the active bloodstream, park at the artery wall and stay there until a roaming monocyte comes across it. In other words, just like a police officer who pulls over a driver for having a flagged license plate, this system appears to be a form of ensuring that certain types of LDL are taken out of the system to be dealt with – a healthy immune response.

This caused me to believe that the hypothesis of LDL “crashing” into the artery wall, as is speculated in common discourse, was an inaccurate one, and in fact, LDL was intentionally taken out of circulation so that it could be passed off to roaming macrophages for clearance. Further, endothelial cells are not the only ones to utilize scavenger receptors in this modified LDL clearance pathway, either14, 15. Macrophages, likewise, once at the site where modified LDL has been ‘parked’ produce their own scavenger receptors which, again, specifically take in modified LDL and other debris such as pathogens16. Once these particles are taken in, macrophages continue encouraging inflammation17, 18, 19 – perhaps to neutralize damaging particles that have been ‘pulled over’ and contained.

Pattern of Development, and Potential Outcomes

The Macrophage

Contrary to my original inclinations, which was that macrophages drew in LDL at the beginning stages of atherosclerosis for repairing damaged cells, it appeared that once taken in via scavenger receptors, the materials of modified LDL et al. are contained in lipid pools20, 21, 22, thus forming foam cells. These lipid pools are later cleared through Reverse Cholesterol Transport (RCT) via HDL23, 24, which appears to not only be beneficial in regards to atherosclerotic development25, 26, but also may help prevent the development of a necrotic core inside the foam cell which contributes to vulnerable plaque formation27, 28 (I will expand more on this in upcoming parts of the series). The development of foam cells and plaque appeared to me to follow a pattern, and marks the beginning of the true derangement in the atherosclerotic process, in my view. The pattern, as I saw it, was as follows:

  1. A monocyte comes across a site of damage (such as modified LDL in the arterial wall).
  2. The monocyte lands on the site, forms into a macrophage and begins the inflammatory process.
  3. At the same time scavenger receptors form on the macrophage, pulling in more modified LDL and storing them in lipid pools, which create foam cells.
  4. Plaque forms in the arterial intima (the lining beneath the endothelial layer) with foam cells surrounding the artery, housing the modified LDL. It is important to note here that the foam cells and plaque do not encroach on the caliber of the artery at this stage. Everything is housed underneath the endothelial layer and the lipid pool underneath the surrounding plaque.29, 30
  5. HDL is called by the macrophage, which initiates Reverse Cholesterol Transport (RCT) for the clearance of cholesterol from the foam cells.
  6. Phagocytes (e.g. macrophages, etc) are used to contain apoptotic cells, or other debris, from the affected area and initiate a ‘controlled breakdown’ throughout the entire process.31

After this process there are two basic outcomes:

  1. The lipid pools housed in the foam cells eventually, over time, deplete. This leaves non-breaching stable plaque around the artery, characterized by a smaller lipid pool, and thick cap (plaque)32;
  2. The lipid pool becomes engorged, and develops a necrotic core composed of OxLDL, etc. If sufficiently exposed, macrophages may undergo programmed or accidental cell death33, which may result in weakening and destabilization of the plaque covering the foam cell site34. Finally, if the plaque is sufficiently weakened this chain of events can culminate in either a partial burst of the cap or a complete rupture (thrombosis) resulting in a cardiac event35. If the burst is not fatal, or in some cases ‘silent’ or asymptomatic, then the plaque can be repaired from the interior of the artery resulting in a patch. However, if the lipid pool fails to be cleared sufficiently, these silent ruptures will continue, over the course of decades, and the patches of the plaque will accumulate, one on top of the other, resulting in partial or complete blockage of the artery.36, 37

An Important Realization Leads to More Questions

I found that one of the things that stood out to me most after learning about these initial stages of atherosclerosis, and something that should be emphasized, is that non-modified LDL being drawn into macrophages via LDLr (the receptor for native/non-modified LDL) does not encourage foam cell formation38, nor do scavenger receptors recognize unmodified LDL as previously mentioned. Which means that normal, healthy LDL presence in macrophages does not contribute to foam cell formation. I repeat: normal, healthy, LDL presence in macrophage sites does not promote foam cell formation. I found this especially striking considering the current hypothesis that high LDL-p encourages mechanical crashes and build-up of lipids in the arteries, which then promotes plaque formation. However, it is only modified LDL, not healthy LDL, that encourages foam cell formation and plaque accumulation.

After realizing this and learning the mechanisms of how the plaque actually forms, I began to ask myself: if I am right about everything up to this point being normal, healthy mechanisms to clear out pathogens, endogenous or exogenous, from the blood – e.g. an immune response – what exactly is drawn in by scavenger receptors? What exactly is stored in foam cell lipid pools? Perhaps most importantly, what causes this derangement of a seemingly normal immune response in the first place? This is what I focused on the next phase of my journey to find out the mechanisms of atherosclerosis and the complex involvement of lipoproteins, which I will address in detail in the next post of Beyond the Lipid Hypothesis in Part 2: LDL Modification.

 

Sources

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