Statins do more than just lower cholesterol

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Statins do more than just lower cholesterol Carl J Vaughan, Michael B Murphy, Brendan M Buckley Clinical trials have demonstrated that inhibitors of 3hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (statins) greatly reduce cardiovascular-related morbidity and mortality in patients with and without coronary artery disease.1,2 These drugs were designed to inhibit the rate-limiting enzyme of cholesterol synthesis in the liver, thereby decreasing hepatic production of lowdensity lipoprotein (LDL), and upregulating expression of hepatic LDL receptors, thus lowering concentrations of circulating LDL. With lower plasma LDL concentrations, plaque development should be retarded or should regress because of lipid loss, which would render the plaque less occlusive and less likely to disrupt or cause thrombosis. The use of simvastatin in the Scandinavian (4S) secondary intervention study1 and the use of pravastatin in the West of Scotland Coronary Prevention Study (WOSCOPS) primary intervention trial2 supported the hypothesis that drugs that lower plasma cholesterol concentration are of benefit to patients with coronary artery disease. However, the clinical benefit of the drugs used in these studies is manifest early in the course of lipidlowering therapy before plaque regression could occur. Quantitative angiographic assessments of the impact of statin therapy on coronary atherosclerosis have demonstrated that improvement in arterial topographical morphology occurs slowly and only to a small extent.3 In the Multicentre Anti-Atheroma Study (MAAS),3 statistically significant improvement in arterial morphology occurred with simvastatin after 4 years and was not evident after 2 years.3 However, it is difficult to attribute the time scale of less than 2 years, in which clinical benefit appeared in the 4S and WOSCOPS trials, solely to a decrease in LDL-cholesterol concentration. About 9 years elapsed before any real clinical benefit was found in the Program on the Surgical Control of the Hyperlipidaemias (POSCH) trial.4 In the POSCH trial, plasma cholesterol was decreased by partial ilial bypass to a degree similar to that achieved in the statin trials and it may be that its clinical effects are attributable purely to changes in circulating lipoproteins. The activity of HMG-CoA reductase limits the rate of synthesis not only of cholesterol but also of a range of other molecules involved in functions such as cellular respiration and cell-cell recognition. Therefore it should not be surprising that the statins, as inhibitors of this enzyme, might modify constituents of the vascular milieu other than LDL cholesterol. Rudolph Virchow (1821–1902) originally proposed that vascular thrombosis was caused by a triad of changes: in Lancet 1996; 348: 1079–82 Department of Pharmacology and Therapeutics, Clinical Sciences Unit, University College Cork, Ireland (C J Vaughan MB, M B Murphy MD, B M Buckley DPhil) Correspondence to: Dr Brendan M Buckley, The Cork Clinic, Bon Secours Hospital, College Road, Cork, Ireland

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the blood vessel wall, in blood flow, and in the constituents of the blood. We suggest that the clinical benefits of simvastatin and pravastatin therapy are best explained by their direct effects, in each component of the triad, on atherosclerotic and thrombotic mechanisms within arteries, as well as through the more conventionally accepted means of decreasing plasma LDL concentrations.

Changes in the vessel wall The process of plaque fissuring, which results in thrombosis, triggers most acute coronary events. Most lesions prone to fissuring and rupture have a large core of lipid-laden macrophages and a thin fibrous cap underlying the endothelium. Although these vulnerable lesions account for 10–20% of all lesions, they are responsible for 80–90% of acute clinical events.5 The reduction in clinical events secondary to lipid lowering has been conventionally attributed to the selective depletion of both the lipid and foam-cell content of this vulnerable subset of plaques by altering the balance between LDL accumulation and efflux in the plaque. This alteration of composition makes the plaque less likely to fissure, disrupt, or cause acute thrombosis. Whereas changes in plaque composition and development are likely to contribute significantly to the reduction in clinical endpoints in the long term, concurrent changes in other constituents or actions of the vessel wall, or both, may contribute to, and explain, the early benefit seen with statin treatment. Atherosclerosis is considered to be a chronic inflammatory disorder characterised by the presence of monocytes or macrophages and T lymphocytes in atherosclerotic lesions as well as the proliferation of smooth muscle cells, elaboration of extracellular matrix, and neovascularisation. Macrophages participate in the uptake and metabolism of lipids in the early stages of atherogenesis6 and may accelerate atherogenesis by other mechanisms (figure 1) including secretion of mitogenic factors similar to platelet-derived growth factor (PDGF), which stimulate smooth-muscle proliferation and plaque neovascularisation.7 Macrophages have been implicated in the pathophysiology of acute coronary syndromes as they produce enzymes which include members of the metalloproteinase family (interstitial collagenase, gelatinase, and stromelysin) that digest and weaken the plaque cap, making disruption more likely.8 The site of plaque disruption, in addition to containing a large population of inflammatory cells, also expresses, in abundance, HLA-DR antigens, which indicates an active inflammatory response. Pravastatin has been shown to influence cholesterol metabolism in macrophages directly in vivo and in vitro, in a manner analogous to its effect in hepatocytes.9 Single dose administration to normocholesterolaemic and hypercholesterolaemic individuals decreases cholesterol 1079

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smooth-muscle-cell proliferation. A modulation of smooth-muscle-cell proliferation by non-sterol products synthesised from mevalonate has also been demonstrated in other studies.14,15 These findings indicate that the beneficial adjunctive effects of statin therapy may also be mediated by a decrease in cholesterol precursors and are not solely because of the effects on LDL levels.

Metalloproteinases

Neovascularisation Foam cell

PDGF

Macrophage

Cytokines T-lymphocyte Smooth-muscle cell

Figure 1: Role of macrophages in plaque formation Activated macrophages produce various substances that contribute to atherosclerosis and acute coronary syndromes: uptake and endogenous synthesis of cholesterol by macrophages leads to foam-cell formation; release of metalloproteinases weakens the plaque cap; secretion of platelet-derived growth factor (PDGF) induces mitogenesis and promotes plaque neovascularisation; macrophages also elaborate cytokines which stimulate smooth-muscle cell and lymphocyte proliferation.

synthesis in macrophages by 62% and 47%, respectively. Pravastatin also causes a dose-dependent inhibition of macrophage cholesterol synthesis in vitro and increases the cellular degradation of LDL. Inhibition of endogenous cholesterol synthesis by macrophages has the potential to reduce macrophage activation, foam-cell formation, and the thrombogenicity of plaques, and alters the lipid to cell ratio of the atherosclerotic lesion, which may make the plaque less prone to rupture. In usual dose, statins also modulate immune function in vitro. Statins have been shown to alter regulation of DNA transcription, regulate natural-killer-cell cytotoxicity, and inhibit antibody-dependent cellular cytotoxicity.10 These observations in vitro are substantiated in clinical practice by the observation that cardiac transplant recipients have a reduced incidence of cardiac rejection, better survival, and reduced cardiac transplant vasculopathy when treated with pravastatin.11 In that study a correlation between cholesterol levels and the development of vasculopathy 1 year after transplantation was apparent. Evidence that statins alter other biological processes in the vessel wall is also accumulating. A study examining the effects of simvastatin on PDGF-induced DNA synthesis and PDGF-␤ chain gene expression in human glomerular mesangial cells showed that exposure of cells to simvastatin completely inhibited PDGF-induced DNA synthesis.12 PDGF is known to contribute to macrophage, platelet, smooth muscle cell, and fibroblast migration and proliferation within blood vessels as well as in atherosclerotic lesions.7 Potentially, statins may attenuate PDGF-induced mitogenesis in plaques and limit deleterious mitogenic responses that lead to migration and activation of smooth-muscle cells and macrophages. This inhibition of smooth-muscle-cell proliferation by statins can be overcome by mevalonate.13 This finding suggests a pro-proliferative role for other cholesterol precursors in 1080

Blood flow, vasomotor tone, and endothelial function The endothelium is involved in the regulation of vasomotor tone, inhibition of platelet activity, inhibition of thrombosis, and promotion of fibrinolysis. Endotheliumderived relaxing factor (EDRF) is a prominent modulator of normal endothelial function (figure 2). EDRF production is stimulated by various factors including platelet activation, shear stress, and concentrations of thrombin, serotonin, and catecholamines. The importance of EDRF and changes in EDRF production in atherosclerosis have been clearly established. Normal atheroma-free coronary arteries dilate when exposed to acetylcholine, an EDRF agonist, whereas atheromatous arteries constrict paradoxically when so exposed. This constriction may contribute to episodic myocardial ischaemia in patients with coronary artery disease.16,17 Improvements in endothelial function and vasomotion have been demonstrated in hypercholesterolaemic patients treated with statins. In one study, therapy with pravastatin led to an 80% reduction in the epicardial coronary artery constrictor response to acetylcholine in conjunction with a 60% increase in coronary blood flow after 6 months of therapy.18 In another study lovastatin produced similar improvements in endothelial vasomotor function.19 Improvements in myocardial perfusion as demonstrated by thallium-201 single-photon emission computed Platelet

Fibrinogen

PDGF

Tx A 2 Tx B 2

FPA

TAT-III

PAI-1

TM

Macrophage

Smoothmuscle cell Thrombosis

Fibrinolysis EDRF

PGI2

PAI

Figure 2: Thrombosis, fibrinolysis, and the endothelium Platelet activation leads to the production of thromboxanes which promote platelet aggregation and vasoconstriction. Platelets also secrete PDGF which is a potent mitogen. The endothelium releases a number of factors that inhibit platelet aggregation and vasodilation. EDRF=endothelium derived relaxing factor; FPA=fibrinopeptide A; PDGF=platelet derived growth factor; PGI2=prostacyclin; PAI-1= plasminogen activator inhibitor-1; TAT-III=thrombin anti-thrombin-III complex; TM=thrombomodulin; TxA 2=thromboxane A 2; Tx B 2=thromboxane B 2.

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tomography have been documented in patients with coronary artery disease after short-term statin therapy.'O In this study the time scale during which improvement occurred was 12 weeks, an interval too short for anatomic regression to occur. In addition to abnormal coronary vasomotion, forearm blood flow abnormalities have also been demonstrated in hypercholesterolaemic patients and normalisation has been achieved by statin therapy.21 T h e endothelial dysfunction that accompanies hypercholesterolaemia may be because of an abnormality in the nimc oxide Larginine pathway which is corrected when LDL concentrations are decreased by statins. Changes in cardiovascular reactivity and blood pressure can provoke plaque rupture and conmbute to coronary vasospasm and ischaemia. In a randomised, double-blind, crossover study Straznicky and colleaguesZzevaluated the effects of short-term cholesterol reduction on cardiovascular reactivity in mildly hypertensive patients given pravastatin or placebo. Cardiovascularreactivity was assessed by measurement of blood-pressure responses to incremental infusions of angiotensin I1 and noradrenaline, by cold pressor testing and isomeaic exercise. Compared with placebo, pravastatin caused a significant fall in diastolic blood pressure responses (expressed as the infusion rate required to raise blood pressure by 20 mm Hg) to both angiotensin I1 and noradrenaline. Improvements in endothelial function are probably at the basis of these improvements in cardiovascular reactivity. Attenuation of cardiovascular reactivity would be expected to lower the haemodynamic stress on plaques and lessen the probability of acute disruption and thrombosis.

Changes In the conrtltuentr of Mood Platelets contribute to both atherosclerosis and thrombosis and play a central role in acute coronary syndromes (figure 2). Hypercholesterolaemia is associated with hypercoagulability as well as enhanced platelet reactivity at sites of acute vascular damage." Patients with hypercholesterolaemia have higher resting levels of malondialdehyde, thromboxane B,, and P-thromboglobulin, which suggests that high blood cholesterol induces lipid peroxidation and platelet activation.z' The following studies indicate that statins may modulate this prothrombotic state. Therapy with lovastatin in patients with type-IIa hypercholesterolaemia caused a significant reduction in serum fibrinogen levels and in adenosine-diphosphate-induced aggregation." Baseline platelet thrombus formation in an ex-vivo system was significantly higher in hypercholesterolaemic patients than in normocholesterolaemic patients and platelet aggregation decreased at both low and high shear rates in ' hypercholesterolaemicpatients with documented coronary artery disease after 2-3 months treatment with pravastatin." The precise mechanism responsible for this effect on platelets is unclear but may be through decreased platelet thromboxane production. High LDL levels appear to cause platelet hyper-reactivity in association with enhanced thromboxane A, biosynthesis. Enhanced thromboxane A, production has also been demonstrated in the majority of patients with type IIa hypercholesterolaemia, and simvastatin has been shown to reduce ex-vivo platelet aggregation and the production of thromboxane B, and thromboxane A, in patients with this dy~lipidaemia.~' Vol348 -October 19,1996

The calcium content of plasma membrane and cytosol of platelets is altered by hypercholesterolaemia, and platelet reactivity is increased. The finding that the cholesterol content of the membranes of both erythrocytes and platelets is reduced in patients treated with pravastatin2' indicates that the properties of these membranes may be altered in a manner that renders them less prone to participation in thrombosis. Additionally, since increased fibrinogen binding to platelets has also been documented in familial hypercholesterolaemia,Uand statin therapy has been shown to reduce serum fibrinogen levels, it is plausible that statins also decrease thrombus formation by decreasing platelet interactions with fibrinogen. In conjunction with alterations in platelet reactivity, humoral thrombogenic and hypofibrinolytic factors have also been found in hypercholesterolaemia. Significantly elevated mean plasma levels of thrombin-antithrombin 111 complex, fibrinopepetide A, thrombomodulin, and plasminogen activator inhibitor 1 (PAI-1) have been described in patients with hypercholesterolaemia,ZPand all of these are significantly reduced after pravastatin treatment. The effects of statins in tending to normalise this state point to the possibility that statins may in part act as anti-thrombotic agents.

conclurlon Although the components of Vichow's t i a d are far less discrete than originally envisaged, the mad is still a conceptually useful way to explore the vascular milieu with respect to both thrombosis and atherosclerosis. The complex interactions between endothelium, platelets, and macrophages cannot be viewed separately. The production of a myriad of factors by platelets and macrophages on one hand, and by the endothelium on the other, makes any evaluation of the in-vivo effects of statin therapy difficult. Attention has been almost entirely focused on the extent to which therapy with these drugs lowers plasma LDL concentrations and the established clinical benefits of statin therapy have been directly, and virtually exclusively, attributed to this effect. However, it is increasingly likely that we cannot attribute the relatively early reduction in mortality seen in c l i c a l uials of statin therapy solely to LDL-dependent reduction in plaque volume, plaque lipid, and regression of coronary atherosclerosis. There is experimental evidence that statins have effects on immune function, macrophage metabolism, and cell proliferation independent of changes in plasma LDL concentrations and that their modulation of the pathophysiological determinants of acute coronary syndromes accounts for the early c l i c a l benefit observed. We suggest that statins improve haemorheological characteristics involved in coagulation and vasomotion early in the course of therapy (figure 3). The time c o m e c

0

2 U

e!

actions

.B -

!

5

Plaque geometry

Time Figure 3 Time courses of putative beneffcialeffecta of M t i ~

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of improvement in endothelial function as well as amelioration of aberrant components of the coagulation cascade is very likely to yield clinical benefit relatively quickly. However, alterations in plaque geometry, lipid content, and thrombogenicity are likely to require therapy for longer before clinical benefit becomes evident.

References 1

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