Although statin drugs, otherwise known as HMG-coenzyme A reductase inhibitors, are very effective in reducing the serum levels of LDL, the so-called “bad” cholesterol, it is not clear that this is the reason why they reduce the incidence of heart attacks. Over the last decade or so, there has developed among the research community a strong opinion that statins prevent heart attacks via a so-called pleiotrophic effect unrelated to their ability to reduce serum LDL [7]. Recently, the idea that this pleiotrophic effect is tied to statins’ observed ability to increase the production of nitric oxide by endothelial cells lining the artery walls has gained considerable traction [1, 12, 14]. It’s been argued that nitric oxide has a lot of benefits in terms of relaxing the artery wall, as well as anti-proliferative and anti-migratory properties, which are considered to be protective features in cardiovascular disease in vessels [4].
Armed with this knowledge, I became interested in finding other examples from the literature of mechanisms that increase the production of nitric oxide. I stumbled upon an article on a rare disease called paroxysmal nocturnal hemoglobinurea (PNH) [13], first described in the literature in 1986 [8], only a few years after statins began to be widely prescribed.
This condition immediately caught my attention, because the health issues associated with it fit well with a set of side effects I had learned were associated with statins, such as difficulty swallowing, abdominal pain, and erectile dysfunction. Furthermore, the condition is caused by a genetic defect in a protein called PIG-A [2], which is essential to orchestrate glycosylphosphatidylinositol (GPI)-anchoring of proteins to cell membranes. Statins also interfere with GPI-anchoring, directly through their obstruction of the mevalonate pathway, by inhibiting the synthesis of isoprenoids, which, in turn, are essential for the GPI-mediated membrane attachment of signaling molecules like Rho GTPases [12].
PNH results in two major direct effects, one related to red blood cells (RBCs) and the other to platelets. The RBCs depend on the impaired protein to protect them from hemolysis – a breaking apart of their membrane and a spilling of their contents into the blood stream. Platelets depend on the same protein to protect them from forming blood clots.
RBCs have an important role and a difficult job in transporting a highly toxic element – oxygen – to all the tissues. Once they’ve picked up oxygen, the resulting oxyhemoglobin is a strong oxidizing agent. It is therefore very important that RBCs sequester hemoglobin within their walls to protect blood proteins and fats from oxidative damage. RBCs ordinarily produce an abundance of coenzyme Q10 (ubiquinone) to protect their own membrane from oxidation damage, and statins interfere with the production of coenzyme Q10 [11]. The genetic disease, sickle cell anemia, is associated with an excess of oxidizing agents in the blood and therefore the sickled RBCs maintain far more coenzyme Q10 than normal RBCs [9]. Statins’ known ability to deplete the supply of Coenzyme Q10 would result in excess oxidation damage to RBC membranes, which would further increase their vulnerability to hemolysis. This may mean that blacks, who have a much higher incidence of sickle cell anemia (due to the protection it affords against malaria), may also have a greater susceptibility to statin side effects.
What does all of this have to do with nitric oxide? It turns out that hemoglobin is an avid scavenger of nitric oxide – it binds strongly to the gas and disables its ability to signal arterial relaxation. Furthermore, RBCs contain a protein, L-arginase, that actively degrades the substrate for NO synthesis, L-arginine. As a consequence of hemoglobin and L-arginine now roaming freely in the blood stream, spilled out from the destroyed RBCs, there will be severe arterial constriction unless the endothelial cells greatly increase their production of nitric oxide. The mechanism is mediated by the protein erythropoietin (epo) which both stimulates the production of replacement RBCs from stem cells in the bone marrow and acts as a signaling agent to induce NO production in the artery wall [5]. So I would like to suggest that statins increase the production of nitric oxide as a compensatory mechanism to its active destruction by hemoglobin and its reduced substrate supply due to L-arginase.
I also found a series of articles on a fascinating breed of genetically engineered mice, which produce excessive human epo and therefore have too many RBCs in their blood serum [15, 10, 5]. These mice also dramatically overproduce NO. They don’t fare well, however; they develop paralysis in their hind legs and die young.
Statin drugs also interfere with mobility – many of the web side effect reports written by people who have taken statin drugs talk about muscle weakness, difficulty walking, and decreased mobility. These phrases all came up as highly significantly over-represented in statin reviews compared to age-matched reviews in our studies on statin side effects. Comparing the collective count of a number of phrases associated with “difficulty walking” in reviews on statin drugs vs other drugs yielded a skewed distribution with a p-value less than 0.0005.
Another indicator that I am on the right track with this idea comes from the observation that statins induce an increased synthesis of heme oyygenase in macrophages [3]. Macrophages typically produce heme oxygenase in order to break down hemoglobin into bilirubin and carbon monoxide, thus detoxifying the hemoglobin. So this is a strong indication that statins induce excess free hemoglobin in the blood, with the most plausible source being wrecked RBCs. The macrophages in atherosclerotic plaque have been shown to avidly take up hemoglobin and break it down with heme oxygenase, with the resulting accumulation of iron deposits in the plaque. It is believed that this iron is a significant contributor to the inflammatory processes in the plaque [6].
The really disturbing part of this story for me is the link between PNH and deep vein thrombosis – half of the deaths associated with PNH are due to venous thrombosis [13]. We have been hearing a lot more lately about deep vein thrombosis, with a warning issued regarding long airplane rides and massive prescriptions of blood thinners like Coumadin (i.e., rat poison!) to attempt to avert it. In PNS, deep vein thromobosis is a consequence of the platelets’ increased potential to form blood clots, due to the defect in GPI-anchoring of proteins, and I suspect that the leg paralysis in the mice is a consequence of suppressed circulation in the legs due to the excess risk of thrombosis. Might statin therapy be a direct contributor to the increased incidence of this highly critical condition?
PNH also leads to pulmonary hypertension, which in turn leads to increased risk to heart failure. This is hypothesized to be a direct consequence of the scavenging of nitric oxide by hemoglobin, which then induces increased pressure in the blood vessels supplying the lungs, putting excess strain on the heart [13]. It’s another one of those biological cascades that makes sense in that oxygen supply needs to be suppressed when there is so much free hemoglobin, because the hemoglobin is far more destructive to cell membranes and proteins when it is oxidized. Our studies showed a significant (p<0.05) increased risk to heart failure associated with statin therapy. Whether this occurs directly due to the depletion of coenzyme Q10 and cholesterol in the heart muscle, or indirectly due to the cascade from erupted RBCs to artery constriction in the lungs to inadequate oxygen supply to the heart is anybody’s guess. I suspect both paths contribute.
So now we must go back to the question of the pleiotrophic effect of statins that results in a reduced incidence of heart attacks. Experts are in agreement that statins don’t actually reduce the plaque, despite the fact that they interfere with the supply of cholesterol and fat. I don’t personally believe that nitric oxide synthesis is the right answer, as it would be more than cancelled out by the scavenging of NO by free hemoglobin. My own best guess at the moment is that statins interfere with the cells’ communication lines directly through their disruption of G-protein signaling mechanisms. I believe the consequence is that the little heart attacks don’t happen, because the cells can’t orchestrate a coordinated plan. As a result, the big heart attacks are more deadly. This idea is analogus to the consequences of preventing the small forest fires and watching the large ones rage out of control. Another analogy is with earthquakes – when the little ones don’t happen, the pressure builds up and the big one is highly destructive. This would explain why statins don’t consistently show improvement in mortality rates due to cardiovascular disease, despite their significant reduction in the frequency of heart attacks.
References
[1] P. Balakumar, S. Kathuria, G. Taneja, Sanjeev Kalra, and Nanjaian Mahadevan, “Is targeting eNOS a key mechanistic insight of cardiovascular defensive potentials of statins?” Journal of Molecular and Cellular Cardiology, To Appear, 2011.
[2] M. Bessler, P.J. Mason, P. Hillmen, T. Miyata, N. Yamada, J. Takeda, L. Luzzatto and T. Kinoshita, “Paroxysmal nocturnal haemoglobinuria (PNH) is caused by somatic mutations in the PIG-A gene,” The EMBO Journal 13(1):110-117, 1994.
[3] F. Gueler, J-K Park, S. Rong, T. Kirsch, C. Lindschau, W. Zheng, M. Elger, A. Fiebeler, D. Fliser, F.C. Luft, and H. Haller, “Statins Attenuate Ischemia-Reperfusion Injury by Inducing Heme Oxygenase-1 in Infiltrating Macrophages,” The American Journal of Pathology, 170(4):1192-1199, Apr. 2007.
[4] D.G. Harrison, “Endothelial control of vasomotion and nitric oxide production a potential target for risk factor management,” Cardiol Clin 14:115, 1996.
[5] K. Heinicke, O. Baum, O.O. Ogunshola, J. Vogel, T. Stallmach, D.P. Wolfer, S. Keller, K. Weber, P.D. Wagner, M. Gassmann and V. Djonov, “Excessive erythrocytosis in adult mice overexpressing erythropoietin leads to hepatic, renal, neuronal, and muscular degeneration,” Am J Physiol Regul Integr Comp Physiol 291:R947-R956, 2006.
[6] W. Li, L.H. Xu, and X.M. Yuan, “Macrophage hemoglobin scavenger receptor and ferritin accumulation in human atherosclerotic lesions,” Ann N Y Acad Sci. 1030, 196-201, Dec 2004.
[7] James K. Liao, and Ulrich Laufs “Pleiotropic Effects of Statins,” Annual Review of Pharmacology and Toxicology 45: 89-118, 2005.
[8] J.T. McCarthy, B.A. Staats, “Pulmonary hypertension, hemolytic anemia, and renal failure: a mitomycin-associated syndrome,” Chest, 89:608-611, 1986.
[9] P. Niklowitz, T. Menke, T. Wiesel, E. Mayatepek, J. Zschocke, J.G. Okun , and W. Andler, “Coenzyme Q10 in plasma and erythrocytes: comparison of antioxidant levels in healthy probands after oral supplementation and in patients suffering from sickle cell anemia,” Clin Chim Acta. 326(1-2):155-61, Dec. 2002.
[10] O.O. Ogunshola, V. Djonov, R. Staudt, J. Vogel, and M. Grassmann, “Chronic excess erythrocytosis induces endothelial activation and damage in mouse brain,” Am J Physiol Regul Integr Comp Physiol 290: R678-R684, 2006.
[11] G. de Pinieux, P. Chariot, M. Ammi-Said, F. Louarn, J.L. LeJonc, A. Astier, B. Jacotot, and R. Gherardi, “Lipid-lowering drugs and mitochondrial function: effects of HMG-CoA reducase inhibitors on serum ubiquinone and blood lactate/pyruvate ratios.” Br. J. Clin. Pharmacol. 42: 333-337, 1996.
[12] Yoshiyuki Rikitake, James K. Liao, “Rho GTPases, Statins, and Nitric Oxide,” Circulation Research 97:1232-1235, 2005.
[13] R.P. Rother, L. Bell, P. Hillmen, and M.T. Gladwin, “The Clinical Sequelae of Intravascular Hemolysis and Extracellular Plasma Hemoglobin: A Novel Mechanism of Human Disease,” JAMA, 293(13): 1653-1662, Apr 6, 2005.
[14] M. Sata, H. Nishimatsu, E. Suzuki, S. Sugiura, M. Yoshizumi, Y. Ouchi, Y. Hirata, and R. Nagai, “Endothelial nitric oxide synthase is essential for the HMG-CoA reductase inhibitor cerivastatin to promote collateral growth in response to ischemia.” The FASEB Journal 15(13):2530-2532, Nov 1, 2001.
[15] C. Wiessner, P.R. Allegrini, D. Ekatodramis, U.R. Jewell, T. Stallmach, and M. Gassmann, “Increased Cerebral Infarct Volumes in Polyglobulic Mice Overexpressing Erythropoietin,” J Cereb Blood Flow Metab, 21(7):857-864, 2001.
