Jonathan Golledge

Osteoprotegerin, vascular calcification and atherosclerosis.

The association of bone pathologies with atherosclerosis has stimulated the search for common mediators linking the skeletal and the vascular system. Since its initial discovery as a key regulator in bone metabolism, osteoprotegerin (OPG) has become the subject of intense interest for its role in vascular disease and calcification. Studies in vitro and in animal models suggest that OPG inhibits vascular calcification. Paradoxically however, clinical studies suggest that serum OPG levels increase in association with vascular calcification, coronary artery disease, stroke and future cardiovascular events. This has led to an extensive debate on the potential of OPG as a biomarker of vascular disease. However the exact significance and mechanisms by which this bone-regulatory protein influences cardiovascular pathophysiology is still unclear. The need for a more complete picture is being addressed in increasing valuable research indicating OPG as not only a marker but also a mediator of vascular pathology modulating osteogenic, inflammatory and apoptotic responses. By integrating the results of recent experimental research, animal models and clinical studies, this review summarises the present understanding of the role of OPG in vascular disease and calcification.

Osteoprotegerin and Osteopontin Are Expressed at High Concentrations Within Symptomatic Carotid Atherosclerosis

Background and Purpose— The aim of this study was to compare the concentration of osteoprotegerin (OPG), receptor activator of nuclear factor κB ligand (RANKL), and osteopontin (OPN) in stable (asymptomatic) and unstable (symptomatic) carotid atherosclerosis. In addition, we were interested in the effect of angiotensin II blockade on the secretion of these proteins by unstable atherosclerosis. Methods— Endarterectomy samples removed from patients with recent (within 6 weeks) or no previous focal neurological symptoms were assessed by immunohistochemistry, Western analysis, and explant culture. Concentrations of OPG, RANKL, and OPN were measured by mean optical density (MOD), densitometry of protein bands, and enzyme-linked immunosorbent assay of supernatants from explant culture, and compared between symptomatic and asymptomatic patients. Results— The concentration of OPG and OPN within the proximal internal carotid (PIC) part of the endarterectomy specimen removed from symptomatic patients was elevated 2- and 4-fold, respectively. Although the concentration of RANKL did not differ according to patients’ symptoms, the quantity of OPG secreted by explants of the PIC was greater in explants from symptomatic patients and could be significantly reduced within 48 hours of incubation with the angiotensin II blocker irbesartan. Conclusion— OPG and OPN are upregulated in symptomatic human carotid atherosclerosis with possible implications for plaque stability. Angiotensin II blockade is able to downregulate OPG secretion in vitro.

Circulating Markers of Abdominal Aortic Aneurysm Presence and Progression

Over the last decade abdominal aortic aneurysm (AAA) has increasingly been recognised as an important cause of mortality in older persons. In 1999 for example AAA was noted to be the 15th leading cause of mortality in the USA [1]. Exact estimates of AAA-related fatalities are hampered by the low rate of post-mortems when sudden death occurs in elderly subjects, however, recent figures suggest that AAA accounts for approximately 15,000 deaths annually in the USA despite the increasing numbers of elective AAA repairs [2,3]. Approximately 25,000 endovascular and open AAA repairs are performed annually in the USA [3]. Ultrasound screening of men >65 years has been demonstrated to reduce AAA-related mortality and selective screening (of men aged ≥65 who have ever smoked) has been introduced in the USA [4]. Most screen-detected AAAs are of small size (<55mm) and surgery for these AAAs has not been demonstrated to improve outcome [5-7]. In an screening study of 12,203 men ≥65 years performed in Australia for example, 814 (6.7%) had a small AAA measuring 30-54mm but only 61 (0.5%) a large AAA (≥55mm) [8]. The increase in identification of small AAAs resulting from screening programs, in association with an ageing population, will highlight the number of deficiencies in current diagnosis and management of this condition. Firstly, there are no accurate non-imaging methods of diagnosing small AAAs, with clinical examination being inaccurate [9]. Secondly, prognostic determinants for AAA are relatively poorly defined [10]. Approximately 70% of 40-55mm AAAs expand within 10 years to a size requiring treatment [6,7]. There are however large intra and inter-patient variations in rates of expansion of small AAAs during follow-up [10]. To date only initial aortic diameter has been consistently shown to predict subsequent increase in aortic diameter [10-13]. Smoking has been associated with increased, and diabetes with decreased, AAA expansion in some, but not all studies [10-13]. More accurate prognostic predictors would offer the possibility of selecting patients for different management pathways rather than relying on aortic diameter alone [10]. Finally the management of small AAAs remains controversial despite randomised controlled trials indicating that open surgical repair of 40-55mm AAAs does not reduce mortality [6,7]. Many centres manage all AAAs ≤55mms conservatively. Estimates based on the UK small aneurysm trial support repeat imaging for 30-40, 41-45, 46-50 and 51-55mm AAAs at 24, 12, 6 and 3 monthly intervals respectively [10]. The increasingly utilisation of endovascular repair of AAA, with its lower peri-operative mortality, has been suggested as more appropriate management for small AAAs, particularly those in the 50-55mm range [14,15]. At present however no randomised controlled trial has been completed examining the outcome of endovascular repair of small AAAs, although one such study is expected to report soon [16]. The lack of any proven medical therapy for prevention of progression and rupture of AAAs represents an important challenge [17]. Only one randomised trial has examined the value of a medication (propranolol) for small AAAs in a cohort of reasonable size (>500 subjects) [18].

Association of Osteoprotegerin With Human Abdominal Aortic Aneurysm Progression

Background—Abdominal aortic aneurysm (AAA) is characterized by destruction of the arterial media associated with loss of vascular smooth muscle cells, infiltration of mononuclear cells, and high concentration of metalloproteinases (MMPs) and cytokines. Osteoprotegerin (OPG) has recently been identified in atherosclerosis. The presence and functional importance of OPG in human AAA was investigated. Methods and Results—In 146 men with small AAA followed up by ultrasound for 3 years, serum OPG was weakly correlated with aneurysm growth rate. Western analysis showed 3-, 8-, and 12-fold-greater OPG concentrations in human AAA biopsies compared with biopsies of atherosclerotic narrowed aorta (1.4±0.1 versus 0.5±0.1 ng/mg tissue; P=0.002), postmortem nondiseased abdominal aorta (1.4±0.1 versus 0.2±0.1 ng/mg tissue; P<0.001), and nondiseased thoracic aorta (1.4±0.1 versus 0.1±0.06 ng/mg tissue; P<0.001). Healthy human aortic vascular smooth muscle cells incubated with recombinant human (rh)OPG (0 to 20 ng rhOPG/105 cells per 1 mL per 24 hours) developed an aneurysmal phenotype defined by impaired cell proliferation (P<0.001), increased apoptosis (P<0.01), and increased MMP-9 (92 kDa) expression (P<0.001). Incubation of monocytic THP-1 cells with 1 ng rhOPG/105 cells per 1 mL per 24 hours induced a 2-fold increase in MMP-9 expression (P<0.001), a 1.5-fold increase in MMP-2 activity (P=0.005), and a 2-fold stimulation of IL-6 production in these cells (P=0.02). Finally, secretion of OPG from human AAA explant was abrogated by treatment with the angiotensin II blocker irbesartan, with the reduction in secreted levels averaging 63.0±0.9 ng/mg tissue per 48-hour period. Conclusions—These findings support a role for OPG in the growth of human AAA and suggest a potential benefit for angiotensin II blockade in slowing aneurysm expansion.

Association of statin prescription with small abdominal aortic aneurysm progression

BACKGROUND Statins have been suggested to reduce expansion of abdominal aortic aneurysms (AAAs) independent of lipid-lowering effects. METHODS We assessed the association of statin treatment and serum low-density lipoprotein (LDL) concentrations with small AAA expansion. Six hundred fifty-two patients undergoing surveillance of small AAAs were entered into the study from 5 vascular centers. In a subset, fasting lipids (n = 451) and other biomarkers (n = 216) were measured. The AAA diameter was followed by ultrasound surveillance for a median of 5 years. RESULTS Three hundred forty-nine (54%) of the patients were prescribed statins. Adjusting for other risk factors, statin prescription was not associated with AAA growth (odds ratio [OR] 1.23, 95% CI 0.86-1.76). Above-median AAA growth was positively associated with initial diameter (OR 1.78 per 4.35-mm-larger initial aortic diameter, 95% CI 1.49-2.14) and negatively associated with diabetes (OR 0.37, 95% CI 0.22-0.62). Above-median serum LDL concentration was not associated with AAA growth. Patients receiving statins had lower serum C-reactive protein concentrations but similar matrix metalloproteinase-9 and interleukin-6 concentrations to those not prescribed these medications. CONCLUSIONS We found no association between statin prescription or LDL concentration with AAA expansion. The results do not support the findings of smaller studies and suggest that statins may have no benefit in reducing AAA progression.

Obesity, Adipokines, and Abdominal Aortic Aneurysm Health in Men Study

Background— Obesity is associated with occlusive artery disease but is not considered a risk factor for abdominal aortic aneurysm (AAA). We investigated the association between anthropometric measures of obesity, serum adipokines, and AAA. Methods and Results— As part of a population study, we screened 12 203 men 65 to 83 years of age for AAA using ultrasound; 875 had an AAA (≥30 mm). Cardiovascular risk factors and waist and hip circumference were recorded. Serum adipokines were measured in 952 men, 318 of whom had an AAA. Waist circumference (odds ratio [OR], 1.14; 95% confidence interval [CI], 1.06 to 1.22) and waist-to-hip ratio (OR, 1.22; 95% CI, 1.09 to 1.37) were independently associated with AAA after adjustment for other known risk factors. The association was stronger for AAA ≥40 mm (waist-to-hip ratio: OR, 1.53; 95% CI, 1.26 to 1.85). Serum resistin concentration was strongly independently associated with AAA (OR, 1.53; 95% CI, 1.32 to 1.76) and aortic diameter (&bgr;=0.19, P<0.0001). Serum adiponectin was associated with AAA ≥30 mm (OR, 1.26; 95% CI, 1.07 to 1.50) but not AAA ≥40 mm (OR, 1.03; 95% CI, 0.77 to 1.39). Serum leptin was not associated with AAA. Conclusions— Measures of obesity are independently associated with AAA. Serum resistin concentrations were more strongly associated with aortic diameter than adipokines that are more intimately associated with adiposity. Further studies are required to investigate the mechanisms linking resistin and AAA.

Current status of medical management for abdominal aortic aneurysm

Previous trials indicate that surgical management of small abdominal aortic aneurysms (AAA) does not reduce mortality. The medical management of AAA, however, has to a large degree been ignored until recently. Medical management is not only needed to limit the expansion of small AAAs but also to reduce the high incidence of other cardiovascular events in these patients. In this review current evidence regarding medical therapy for patients with small AAAs is discussed. Four current randomised controlled trials are examining the efficacy of exercise, doxycycline and angiotensin converting enzyme inhibition in limiting AAA progression. A further trial using a mast cell stabilisation agent is expected to start soon. It is anticipated that a range of novel therapies for small AAAs will be identified within the next decade.

Reduced expansion rate of abdominal aortic aneurysms in patients with diabetes may be related to aberrant monocyte–matrix interactions

AIMS Diabetes increases the risk of atherothrombosis, but reduces the risk of abdominal aortic aneurysm (AAA). The reason for this difference is unknown. We examined the role of diabetes and glycation on AAA expansion and extracellular matrix-monocyte interactions. METHODS AND RESULTS We followed 198 patients (20 with diabetes) who had 30-45 mm AAAs with yearly aortic ultrasound for 3 years. Diabetes was independently associated with reduced AAA growth (beta = -0.17, P = 0.01; OR for expansion above median 0.18, 95% confidence interval 0.06-0.57). In vitro incubation of resting human monocytes with glycated bovine serum albumin or monomeric type I collagen increased matrix metalloproteinase (MMP) secretion. In contrast, exposure of activated monocytes to glycated type I collagen lattices induced a marked reduction in MMP and interleukin-6 secretion. This de-activating effect was also demonstrated in cross-linked non-glycated collagen lattices, healthy decellularized aortic media, and decellularized aortic media from diabetes patients with atherosclerosis. In contrast, decellularized aortic media from patients with atherosclerosis, but no diabetes, induced increased MMP secretion. CONCLUSION These findings confirm that the progression of AAA is slower in patients with diabetes and suggest a mechanism by which the aortic media may be protected from degradation in these individuals.

Atherosclerosis and Abdominal Aortic Aneurysm Cause, Response, or Common Risk Factors?

Abdominal aortic aneurysm (AAA) rupture has been recognized as a significant cause of mortality for adults aged >60 years in the developed world for some time.1 AAAs are usually asymptomatic until rupture occurs, and screening programs have been shown to reduce mortality in men aged >65 years.2 Most AAAs detected by ultrasound are <50 mm in diameter, and there is currently no recognized treatment for these AAAs.3,4 Studies aimed at understanding the pathogenesis of AAA are important as they may identify targets for novel therapy. See accompanying article on page 1263 The mechanisms initiating and stimulating progression of AAA are still poorly understood, with most knowledge coming from cross-sectional association studies in humans and increasingly from investigations in animal models.4 Such studies suggest the importance of inflammatory pathways, matrix degradation, thrombosis, hemodynamic forces, and a host of associated signaling molecules in AAA pathogenesis.4,5 On the basis of the new insights from rodent models, a number of novel strategies are being investigated as potential treatments for small AAA.5 To date, there have been very few well-designed randomized controlled trials assessing the efficacy of medication in reducing AAA complications in patients.4 Patients with AAAs frequently have atherosclerosis, and numerous studies show the association of coronary heart disease and peripheral atherosclerosis with AAA.4,6 Whether this association between AAA and atherosclerosis is causal or simply due to common risk factors is unknown. One possibility is that an AAA develops as a pathological response to aortic atherosclerosis, a theory first …

Common variants at 6p21.1 are associated with large artery atherosclerotic stroke

Genome-wide association studies (GWAS) have not consistently detected replicable genetic risk factors for ischemic stroke, potentially due to etiological heterogeneity of this trait. We performed GWAS of ischemic stroke and a major ischemic stroke subtype (large artery atherosclerosis, LAA) using 1,162 ischemic stroke cases (including 421 LAA cases) and 1,244 population controls from Australia. Evidence for a genetic influence on ischemic stroke risk was detected, but this influence was higher and more significant for the LAA subtype. We identified a new LAA susceptibility locus on chromosome 6p21.1 (rs556621: odds ratio (OR) = 1.62, P = 3.9 × 10−8) and replicated this association in 1,715 LAA cases and 52,695 population controls from 10 independent population cohorts (meta-analysis replication OR = 1.15, P = 3.9 × 10−4; discovery and replication combined OR = 1.21, P = 4.7 × 10−8). This study identifies a genetic risk locus for LAA and shows how analyzing etiological subtypes may better identify genetic risk alleles for ischemic stroke.