The healing process
Most people, when they think about atherosclerotic plaques, think of them as starting very small – as fatty streaks and suchlike. Then they inevitably get bigger and bigger over many years. However, this is not correct:
‘Atherosclerosis was originally considered to be an ongoing process that was inevitably associated with age. However, plaques are highly dynamic, and are able to progress, stabilize or regress depending on their surrounding milieu. A great deal of research attention has been focused on understanding the involvement of high-density lipoprotein in atherosclerotic plaque regression. However, atherosclerotic plaque regression encompasses a variety of processes that can be grouped into three main areas: removal of lipids and necrotic material; restoration of endothelial function and repair of denuded areas; and cessation of vascular smooth muscle cell proliferation and phenotype reversal.’ 1
In short, progression is not inevitable. Plaques can shrink down in size, the smooth muscle proliferation (often considered and irreversible components of plaques) reversed, and endothelial function restored. In truth, you will most likely not end up with a perfectly healed plaque with no signs it was ever there. You will be left with a bit of a ‘scar’ or some sort. However, the important point is that we are not looking at a one-way street. The body can heal plaques. (Probably not once calcified, but that is another issue).
This leads me onto the third part of the process of CVD. As I have been discussing in this series, the process of CVD has four basic components:
- Endothelial damage
- Clot formation/dysfunctional clot formation
- Clot repair/dysfunctional clot repair
- The final, fatal, blood clot
Up to now I have mainly talked about endothelial damage, and clot formation, which are plaque ‘growth’ factors. However, repair is also very important. Anything that can interfere with the repair process is going to make plaques grow, rather than regress.
The key players in repair are: monocytes, macrophages and Endothelial Progenitor Cells (EPCs). As mentioned several times before, once the endothelium is damaged, and a clot formed, EPCs are attracted to the area to form a new layer of endothelium. So, clearly EPCS are critical players. Just to quote one paper:
‘BACKGROUND: Cardiovascular risk factors contribute to atherogenesis by inducing endothelial-cell injury and dysfunction. We hypothesized that endothelial progenitor cells derived from bone marrow have a role in ongoing endothelial repair and that impaired mobilization or depletion of these cells contributes to endothelial dysfunction and cardiovascular disease progression.
CONCLUSIONS: In healthy men, levels of endothelial progenitor cells may be a surrogate biologic marker for vascular function and cumulative cardiovascular risk. These findings suggest that endothelial injury in the absence of sufficient circulating progenitor cells may affect the progression of cardiovascular disease.’2 Which means that with fewer EPCS, plaques grow faster.
The critical part that EPCs have to play is also seen in patients who have angioplasty, or stents. Immediately following the procedure, the bone marrow starts making more EPCs.
‘In conclusion, endothelial injury from angioplasty can lead to time-dependent mobilization or homing of EPCs; mature EPC subpopulations are actively mobilized, and may contribute more to endothelial reparation; and the mobilization amplitude of the main EPC subpopulations is significantly influenced by the degree of endothelial injury and certain clinical factors.’3
It follows that, if you have fewer EPCs the risk of CVD will be much higher. This is clearly seen in Systemic Lupus Erythematosus (mentioned a few times before). The paper quoted from below looked at SLE, and the number of EPCs, and also haematopoietic stem Cells (HSCs) – which are the precursor to EPCs – found in the bone marrow:
‘SLE patients have lower levels of circulating HSC and EPC, even during clinical remission. Our data suggest that increased HSC apoptosis (cell death) is the underlying cause for this depletion. These observations indicate that progenitor cell mediated endogenous vascular repair is impaired in SLE, which may contribute to the accelerated development of atherosclerosis.’4
Other conditions, or factors, that reduce EPC numbers include:
- Type II diabetes
- Rheumatoid arthritis
To name but four.
Of course there tends to be a tight association between factors that damage the endothelial cells, and factors that reduce EPC number. This appears to be primarily modulated by nitric oxide levels. Anything that increases NO levels in endothelial cells and also helps to protect them from damage appears to increase EPC production in the bone marrow.
A non-exhaustive list of things can do this this are:
- ACE-inhibitors (used for BP reduction)
Yes, the dreaded statins… Boooo! In truth, for many years I have accepted (albeit with great reluctance), that statins do have some benefits in CVD. Not enough, in my opinion, to overcome the damage that they can do. However, the benefit is there, it is real.
I knew it could be nothing to do with the impact of statins on lowering LDL, as LDL has nothing to do with CVD (well, almost nothing). So there had to be another effect. And that effect is, in my opinion, almost entirely to do with the ability of statins to increase nitric oxide (NO) production:
‘Endothelial nitric oxide (eNO) bioavailability is severely reduced after myocardial infarction (MI) and in heart failure. Statins enhance eNO availability by both increasing eNO production and reducing NO inactivation. We therefore studied the effect of statin treatment on eNO availability after MI and tested its role for endothelial progenitor cell mobilization, myocardial neovascularization, left ventricular (LV) dysfunction, remodeling, and survival after MI….. These findings suggest that increased eNO availability is required for statin-induced improvement of endothelial progenitor cell mobilization, myocardial neovascularization, LV dysfunction, interstitial fibrosis, and survival after MI. eNO bioavailability after MI likely represents an important therapeutic target in heart failure after MI and mediates beneficial effects of statin treatment after MI.5’
Yes, when you decide to look through a different prism, you can find that things you thought were one thing, turn out to be another thing entirely. Professor Michael Oliver – a trenchant critic of the cholesterol hypothesis before statins came along – changed his mind, once he saw that statins lowered LDL and lowered CVD risk. Case proven – he said.
No, case not proven. Instead, if you look at EPCs and nitric oxide (NO) and take the view of CVD that it is all due to endothelial dysfunction, blood clotting and impaired repair, you can see exactly where statins may fit into the picture.
Next: The final event.