Plasma von Willebrand factor levels and surrogates of atherosclerosis

Authors


Gregory Y. H. Lip, Haemostasis Thrombosis and Vascular Biology Unit, University Department of Medicine, City Hospital, Birmingham B18 7QH, UK.
Tel.: +44 121 554 3801; fax: +44 121 554 4083; e-mail: g.y.h.lip@bham.ac.uk

Endothelial dysfunction is a frequentlyused term to refer to abnormalities in the various properties of the endothelium, where the latter literally fails to perform its physiological functions, such as blood pressure control or hemostasis [1]. Indeed, a continuum is likely to exist between normality, endothelial activation (for example, by inflammatory cytokines), endothelial dysfunction (e.g. impairment of nitric oxide, resulting in loss of regulation of vascular tone) and endothelial damage (as may be caused by smoking, oxidized LDL, cytopathic viruses, etc.) [1].

The terms endothelial perturbation, disturbance, dysfunction and damage tend to be used interchangeably, although there are distinct differences in the pathophysiology behind each one. Also, measurement of different markers/methods of endothelial activation, dysfunction and damage will probe different aspects of the physiology and pathology of the endothelial cell. Unfortunately, none of the measurement techniques have been described as a ‘gold standard’. Indeed, there are several methods currently employed to assess the endothelium. For example, the most widely studied methods are the estimation of plasma markers [such as von Willebrand factor (VWF), E-selectin and thrombomodulin], circulating endothelial cells (CECs) and studies of forearm circulation in response to hypoxia-induced stress (‘flow-mediated dilatation’, FMD) or intra-arterially administered drugs such as acetyl choline [1–3].

Some of the plasma markers of endothelial damage/dysfunction have been related to overt vascular disease and the risk factors for vascular disease, such as hypertension, and appropriate treatment of risk factors [1,2]. Also, indices of endothelial (patho)physiology also appear to be related to each other – for example, plasma VWF has been correlated with FMD in hypertensives [4], and CECs (indicating endothelial damage) have been highly correlated with plasma VWF levels [5]. Furthermore, baseline plasma VWF levels have been predictive of an adverse prognosis in cardiovascular disease [1,2,5,6]. Less definitive, however, are the inter-relationships of indices of endothelial abnormalities in totally asymptomatic individuals, and the causal (and more importantly, sequential) relationships to the subsequent development of overt vascular disease.

In this edition of the Journal, Paramo et al. [7] describe an inter-relationship between VWF levels and two markers of vascular damage, carotid intima-media thickness (IMT, also an index of atherosclerosis) and urinary microalbumin excretion. They investigated 857 middle-aged, predominantly (80%) male subjects who had cardiovascular risk factors (dyslipidemics, hypertensives, diabetics and smokers) but who were not known to have atheromatous disease. They showed that VWF levels correlated with age, blood pressure, body mass index, cholesterol level, glucose level and high-sensitivity C-reactive protein (hsCRP), broadly in agreement with many previous studies [8]. One novel aspect of this work, however, is to show the correlation between two markers of vascular damage (IMT, microalbuminuria) and an easily measured plasma marker of endothelial damage/dysfunction in middle-aged asymptomatic subjects, independent of potential confounders.

Of note, previous authors have looked at carotid IMT in the context of Framingham risk assessment, and other studies have actually measured VWF and carotid IMT in small numbers of subjects [9]. The difficulty of cross-sectional studies of this nature still remains the potential confounding effects of comorbidity and concomitant therapies, which can be major factors. For example, in the study by Paramo et al. [7], 51% were dyslipidemic, 32.2% were hypertensive, 11.4% were diabetic and 35% were smokers – and all these factors can influence the endothelium. Limited information is provided on concomitant drug therapies, as well as the ‘additive’ effect of risk factors. Indeed, a patient with diabetes plus hypertension is likely to have more endothelial abnormalities than diabetes alone or hypertension alone. The severity of disease, as well as duration of insult, also play a major part and it is unlikely that statistical adjustments can fully account for all biological diversity and variability, and all pathophysiological processes.

Why the quest for a measurable index of endothelial abnormalities? VWF is now a well-established marker of endothelial damage/dysfunction and has the advantage of being a relatively simple assay to perform [10,11]. In contrast, carotid IMT is a measure of the degree of atheroma present in an individual, at least in one local site (the carotid artery) and it is assumed that IMT abnormalities reflect generalized disease [12,13] – which may not be the case at the individual level – although increased carotid IMT predicts both strokes and myocardial infarction [13]. The IMT is also an anatomical index of disease burden rather than a functional measure of a secreted molecule, in contrast with measuring VWF. However, the measurement of IMT is somewhat operator and equipment dependent, which may be less convenient for large-scale epidemiological studies. Thus, the measurement of plasma indices of endothelial abnormalities, such as VWF, has clear advantages over carotid IMT in terms of inter- (and intra-) observer variability, availability of resources and time constraints; in any case, B-mode ultrasound of adequate quality is not available widely outside dedicated research centers.

Why measure urinary microalbuminuria? Normal kidneys prevent protein loss in the urine, and the presence of microalbuminuria is therefore abnormal. Microalbumnuria as a marker of cardiovascular disease has been best studied in diabetic patients, where microalbuminuria has been identified as an indication for angiotensin-converting enzyme inhibition in order to prevent progression to end-stage renal failure [14]. More recently, microalbuminuria has been correlated with carotid IMT, in both diabetics and non-diabetics [15], suggesting that it may be an marker of generalized atherosclerosis – although an indirect one – as well as a risk for further renal damage. In hypertension, there is also a relationship between urinary microalbuminuria and plasma VWF levels [16]. Urinary microalbumin excretion is thought to reflect atherosclerosis, presumably via abnormalities in structure of the glomerular endothelium being markers of more widespread endothelial abnormalities.

The work by Paramo et al. [7] provides a link between two very different measurements of vascular biology. Despite the link of atherosclerosis to VWF, this study has not rendered measurement of carotid IMT obsolete. Despite VWF being relatively specific to endothelial cells, it can sometimes be a relatively non-specific marker of endothelial perturbation, with many different endothelial insults resulting in altered plasma VWF levels. Urinary microalbumin measurement is also a pretty non-specific test, with virtually any renal insult leading to some degree of proteinuria, and is therefore not widely used as a test of atheromatous burden.

However, Paramo et al. [7] do improve our understanding of the initiation and progression of arterial atherothrombotic disease, from the initial insult to clinically overt atheroma. The correlations between markers of clinically important atherosclerosis and markers of endothelial damage/dysfunction is certainly of interest, as it reinforces the evidence for the progression from endothelial activation to dysfunction and then damage, with atherogenesis. These findings are consistent with the hypothesis that VWF provides an important pathophysiological link between endothelial cell abnormalities and surrogates of diffuse atherosclerosis, but this may be a little premature, and requires a longitudinal study that addressed vascular disease progression, for example, with serial measurements of carotid IMT, or some serial quantification of vascular disease progression, whether in the heart, brain or peripheral arteries.

Instead of performing single-point assessments, it may be much more valuable to calculate various biological parameters across time. Indeed, single-point assessment for a cross-sectional analysis has many limitations, whether related to individual, seasonal, temporal diurnal variation, etc. Thus, the finding of a positive correlation in a cross-sectional study such as that by Paramo et al. [7] implies an association and is not causality, being very different from a definitive pathophysiological link being proven; however, this still remains a possibility, given the prognostic significance of raised VWF levels in apparently healthy people [17]. What needs to be more clearly characterized is the progression from normality to endothelial damage/dysfunction (in asymptomatic subjects) to overt (symptomatic) vascular disease.

Ancillary