Oxidative stress and the endothelium

Oxidative stress plays a major role in cardiovascular disease. Despite the widespread acceptance of this view, it remains a challenge to establish the importance of this link in specific disease states because of the numerous pro-oxidant substances and also the many endogenous antioxidant systems that contribute to the imbalance that we term ‘oxidative stress’. It seems that the vascular endothelium is particularly sensitive to oxidative stress, and this is one of the mechanisms that results in widespread endothelial dysfunction in most cardiovascular diseases. Moreover, when the endothelium becomes diseased it is itself a source of oxidative stress; however, other components of the vasculature, such as the adventitia, are also important sources of oxidative stress. The symposium Oxidative stress and the endothelium at Life Sciences Meeting, Glasgow, Thursday 12th July 2007 aimed to bring together important new data and concepts that are contributing to the rapid advances in our understanding of this area. Mark Kearney spoke on Insulin resistance and endothelial cell dysfunction, Patrick Pagano spoke on The adventitial fibroblast NAD(P)H oxidase as a paracrine mediator of vascular hypertrophy: signal for vascular disease?, James Lieper spoke on Disruption of methylarginine metabolism impairs vascular homeostasis, and David Newby spoke on Endothelial dysfunction and atherothrombosis. The multimeric enzyme complex NAD(P)H oxidase, or Nox, is the major enzyme system that generates superoxide in the vasculature (Pagano et al. 1995; Cifuentes & Pagano, 2006). A major physiological activator is angiotensin II, acting on AT1 receptors and then protein kinase C to stimulate Nox (Pagano et al. 1998; Cifuentes et al. 2000; Touyz et al. 2005). Activation of this pathway helps to explain many of the acute and chronic cardiovascular actions of angiotensin II and the role of the renin–angiotensin system in cardiovascular disease and endothelial dysfunction (Cifuentes et al. 2000; Touyz et al. 2005). Much (or in some studies almost all) of the Nox in blood vessels is located in the adventitia (Pagano et al. 1997; Di Wang et al. 1998; Rey & Pagano, 2002; Fig. 1), and it has been shown that reactive oxygen species generated by the adventitia can reach the endothelium and limit the activity of NO (Rey et al. 2002). Photomicrograph of a cross-section of a rabbit aorta stained with Nitro Blue Tetrazolium to reveal location of reactive oxygen species formation in the adventitia Reproduced from Pagano et al. (1997), with permission. Copyright ©1997 National Academy of Sciences, USA. Since NO reacts with superoxide, NO acts primarily as an antioxidant; nevertheless, this reaction generates the highly reactive if short-lived oxidant, peroxynitrite. Thus, impaired NO output from endothelium is one of the causes of oxidative stress. The endogenous inhibitor of NO synthase, asymmetric dimethylarginine (ADMA), is generated and destroyed by specific enzyme pathways (Leiper & Vallance, 2006). Patients who have atherosclerosis have been found to have high blood levels of ADMA, and the ADMA level has been found to predict overall cardiovascular risk (Furuki et al. 2007; Leiper et al. 2007). A small molecule inhibitor of the enzyme that degrades ADMA (dimethylarginine dimethylaminohydrolase, DDAH) was found to inhibit NO-dependent arterial relaxation (Rossiter et al. 2005), demonstrating that DDAH is functionally active in the intact artery and limits NO production. Experimental manipulation of DDAH activity has been found to alter cell phenotype, showing the importance of ADMA to modulate NO and oxidative stress and thus to regulate cell physiology (Smith et al. 2003; Fig. 2). Photographs of human umbilical vein endothelial cells plated onto Matrigel Left panel shows that untreated cells develop the typical endothelial morphology and form into tube-like structures. Right panel shows that cells treated with the DDAH inhibitor 4124W (1 mm) for 24 h form fewer tube-like structures. Reproduced from Smith et al. (2003), with permission The endothelium maintains an antithrombotic surface by synthesis of tissue plasminogen activator (t-PA), an endogenous mechanism to remove any thrombi should they form on the vessel lining. In human subjects, t-PA is released from the vascular endothelium when stimulated by substance P (Oliver et al. 2005). The amount of t-PA released is reduced in subjects who are cigarette smokers compared with non-smokers (Robinson et al. 2006b), which explains why cigarette smoking leads to endothelial dysfunction and cardiovascular disease. Moreover, inhalation of vehicle exhaust impairs the production of endothelial t-PA. in volunteers (Mills et al. 2005; Fig. 3), thus demonstrating a link between exposure to air pollution and increased cardiovascular risk. Insulin resistance increases cardiovascular risk and has been shown to be linked to excess reactive oxygen species formation and reduced NO formation (Kearney et al. 2007). Release of t-PA antigen was reduced in subjects 6 h after exposure to diesel exhaust (•) compared with air (○) during intrabrachial infusion of bradykinin Reproduced from Mills et al. (2005), with permission. Many aspects of endothelial function can be disrupted by oxidative stress, and thus prevention of oxidative stress can protect against or reverse cardiovascular pathologies. Cell-permeable agents have been developed that specifically inhibit Nox, and these tools reduce vascular oxidative stress and ameliorate the neointimal hyperplasia that develops following arterial injury (Weaver et al. 2006). Patients with atherosclerosis have endothelial dysfunction and have also been found to have excess platelet activation (Robinson et al. 2006a), thus demonstrating the importance of a healthy endothelium to prevent the formation of thrombi. The research discussed is revealing mechanisms underlying the production and effects of oxidative stress by cardiovascular risk factors and cardiovascular disease. Future research will aim to develop ways to prevent damage to the endothelium, to protect NO synthase, to prevent formation of reactive oxygen species and to protect vulnerable pathways or cells against damage when exposed to oxidative stress.