Serum levels of vascular endothelial growth factor and transforming growth factor-beta1 in patients with atrial fibrillation undergoing defibrillation therapy

Vascular Endothelial Barrier Protection Prevents Atrial Fibrillation by Preserving Cardiac Nanostructure

BACKGROUND

Atrial fibrillation (AF), the most common cardiac arrhythmia, is widely associated with inflammation, vascular dysfunction, and elevated levels of the vascular leak-inducing cytokine, vascular endothelial growth factor (VEGF). Mechanisms underlying AF are poorly understood and current treatments only manage this progressive disease, rather than arresting the underlying pathology. The authors previously identified edema-induced disruption of sodium channel (NaV1.5)-rich intercalated disk nanodomains as a novel mechanism for AF initiation secondary to acute inflammation. Therefore, we hypothesized that protecting the vascular barrier can prevent vascular leak-induced atrial arrhythmias.

OBJECTIVES

In this study the authors tested the hypothesis that protecting the vascular barrier can prevent vascular leak-induced atrial arrhythmias. They identified 2 molecular targets for vascular barrier protection, connexin43 (Cx43) hemichannels and pannexin-1 (Panx1) channels, which have been implicated in cytokine-induced vascular leak.

METHODS

The authors undertook in vivo electrocardiography, electron microscopy, and super-resolution light microscopy studies in mice acutely treated with a clinically relevant level of VEGF.

RESULTS

AF incidence was increased in untreated mice exposed to VEGF relative to vehicle control subjects. VEGF also increased the average number of AF episodes. VEGF shifted NaV1.5 signal to longer distances from Cx43 gap junctions, measured by a distance transformation-based spatial analysis of 3-dimensional confocal images of intercalated disks. Similar effects were observed with NaV1.5 localized near mechanical junctions composed of neural cadherin. Blocking connexin43 hemichannels (αCT11 peptide) or Panx1 channels (PxIL2P peptide) significantly reduced the duration of AF episodes compared with VEGF alone with no treatment. Concurrently, both peptide therapies preserved NaV1.5 distance from gap junctions to control levels and reduced mechanical junction-adjacent intermembrane distance in these hearts. Notably, similar antiarrhythmic efficacy was also achieved with clinically-relevant small-molecule inhibitors of Cx43 and Panx1.

CONCLUSIONS

These results highlight vascular barrier protection as an antiarrhythmic strategy following inflammation-induced vascular leak.

Risk of atrial fibrillation is increased in patients with exfoliation syndrome: the Utah project on exfoliation syndrome (UPEXS)

PURPOSE

Exfoliation syndrome, a systemic disorder with ocular manifestations, is associated with lysyl oxidase-like gene variants. Along with transforming growth factor beta-1, lysyl oxidase-like 1 is a key enzyme in stabilizing extracellular matrix and remodelling collagen/elastin. Given the role that transforming growth factor beta-1, lysyl oxidase-like gene variants and fibrosis play in atrial fibrillation, an association with exfoliation syndrome was investigated.

METHODS

An exfoliation syndrome cohort of 2803 patients and an atrial fibrillation cohort of 43 694 patients aged 60-90 years at disease onset were identified using the Utah Population Database (1996-2015). Conditional logistic regression was used to estimate risk of atrial fibrillation in exfoliation syndrome patients and risk of exfoliation syndrome in atrial fibrillation patients compared with respective 5:1 sex- and age-matched control cohorts. Kaplan-Meier curves were examined to assess survival in atrial fibrillation patients by exfoliation syndrome status.

RESULTS

Exfoliation syndrome patients had a 21% greater risk (95% CI 1.06-1.37; p < 0.0001) of atrial fibrillation. This was more pronounced in exfoliation syndrome patients with no hypertension history, who exhibited a 52% increased atrial fibrillation risk (95% CI 1.27-1.82; p < 0.0001). Atrial fibrillation patients exhibited a 20% increased risk of exfoliation syndrome (95% CI 1.07-1.35; p = 0.003), while atrial fibrillation patients with no hypertension had a 72% higher exfoliation risk (95% CI 1.45-2.03; p < 0.0001). Atrial fibrillation patients with exfoliation syndrome had a higher estimated probability of survival (alive at study end or at last follow-up) compared with patients with no exfoliation history (p < 0.0001, log-rank test).

CONCLUSIONS

Exfoliation syndrome patients were at a statistically significant increased risk of atrial fibrillation. Similarly, atrial fibrillation patients were at a statistically significant higher risk of exfoliation, particularly when hypertension history was absent.

Increased fibroblast accumulation in the equine heart following persistent atrial fibrillation

BACKGROUND

Fibroblasts maintain the extracellular matrix homeostasis and may couple to cardiomyocytes through gap junctions and thereby increase the susceptibility to slow conduction and cardiac arrhythmias, such as atrial fibrillation (AF). In this study, we used an equine model of persistent AF to characterize structural changes and the role of fibroblasts in the development of an arrhythmogenic substrate for AF.

MATERIAL AND METHODS

Eleven horses were subjected to atrial tachypacing until self-sustained AF developed and were kept in AF for six weeks. Horses in sinus rhythm (SR) served as control. In terminal open-chest experiments conduction velocity (CV) was measured. Tissue was harvested and stained from selected sites. Automated image analysis was performed to assess fibrosis, fibroblasts, capillaries and various cardiomyocyte characteristics.

RESULTS

Horses in SR showed a rate-dependent slowing of CV, while in horses with persistent AF this rate-dependency was completely abolished (CV•basic cycle length relation p = 0.0295). Overall and interstitial amounts of fibrosis were unchanged, but an increased fibroblast count was found in left atrial appendage, Bachmann's bundle, intraatrial septum and pulmonary veins (p < 0.05 for all) in horses with persistent AF. The percentage of α-SMA expressing fibroblasts remained the same between the groups.

CONCLUSION

Persistent AF resulted in fibroblast accumulation in several regions, particularly in the left atrial appendage. The increased number of fibroblasts could be a mediator of altered electrophysiology during AF. Targeting the fibroblast proliferation and differentiation could potentially serve as a novel therapeutic target slowing down the structural remodeling associated with AF.

Elevated resting heart rate as a predictor of inflammation and cardiovascular risk in healthy obese individuals

The role of leukocyte inflammatory markers and toll like receptors (TLRs)2/4 in pathologies associated with elevated resting heart rate (RHR) levels in healthy obese (HO) individuals is not well elucidated. Herein, we investigated the relationship of RHR with expression of leukocyte-inflammatory markers and TLRs in HO individuals. 58-obese and 57-lean participants with no history of a major medical condition, were recruited in this study. In HO individuals, the elevated-RHR correlated positively with diastolic blood pressure, cholesterol, pro-inflammatory monocytes CD11b⁺CD11c⁺CD206⁻ phenotype (r = 0.52, P = 0.0003) as well as with activated T cells CD8⁺HLA-DR⁺ phenotype (r = 0.27, P = 0.039). No association was found between RHR and the percentage of CD16⁺CD11b⁺ neutrophils. Interestingly, elevated RHR positively correlated with cells expressing TLR4 and TLR2 (CD14⁺TLR4⁺, r = 0.51, P ≤ 0.0001; and CD14⁺TLR2⁺, r = 0.42, P = 0.001). TLR4⁺ expressing cells also associated positively with the plasma concentrations of proinflammatory or vascular permeability/matrix modulatory markers including TNF-α (r = 0.36, P = 0.005), VEGF (r = 0.47, P = 0.0002), and MMP-9 (r = 0.53, P ≤ 0.0001). Multiple regression revealed that RHR is independently associated with CD14⁺TLR4⁺ monocytes and VEGF. We conclude that in HO individuals, increased CD14⁺TLR4⁺ monocytes and circulatory VEGF levels associated independently with RHR, implying that RHR monitoring could be used as a non-invasive clinical indicator to identify healthy obese individuals at an increased risk of developing inflammation and cardiovascular disease.

Atrial fibrillation: the role of hypoxia-inducible factor-1-regulated cytokines

Atrial fibrillation (AF) is a common arrhythmia that has major morbidity and mortality. Hypoxia plays an important role in AF initiation and maintenance. Hypoxia-inducible factor (HIF), the master regulator of oxygen homeostasis in cells, plays a fundamental role in the regulation of multiple chemokines and cytokines that are involved in different physiological and pathophysiological pathways. HIF is also involved in the pathophysiology of AF induction and propagation mostly through structural remodeling such as fibrosis; however, some of the cytokines discussed have even been implicated in electrical remodeling of the atria. In this article, we highlight the association between HIF and some of its related cytokines with AF. Additionally, we provide an overview of the potential diagnostic benefits of using the mentioned cytokines as AF biomarkers. Research discussed in this review suggests that the expression of these cytokines may correlate with patients who are at an increased risk of developing AF. Furthermore, cytokines that are elevated in patients with AF can assist clinicians in the diagnosis of suspect paroxysmal AF patients. Interestingly, some of the cytokines have been elevated specifically when AF is associated with a hypercoagulable state, suggesting that they could be helpful in the clinician's and patient's decision to begin anticoagulation. Finally, more recent research has demonstrated the promise of targeting these cytokines for the treatment of AF. While still in its early stages, tools such as neutralizing antibodies have proved to be efficacious in targeting the HIF pathway and treating or preventing AF.

Vascular endothelial growth factor promotes atrial arrhythmias by inducing acute intercalated disk remodeling

Atrial fibrillation (AF) is the most common arrhythmia and is associated with inflammation. AF patients have elevated levels of inflammatory cytokines known to promote vascular leak, such as vascular endothelial growth factor A (VEGF). However, the contribution of vascular leak and consequent cardiac edema to the genesis of atrial arrhythmias remains unknown. Previous work suggests that interstitial edema in the heart can acutely promote ventricular arrhythmias by disrupting ventricular myocyte intercalated disk (ID) nanodomains rich in cardiac sodium channels (Na_V1.5) and slowing cardiac conduction. Interestingly, similar disruption of ID nanodomains has been identified in atrial samples from AF patients. Therefore, we tested the hypothesis that VEGF-induced vascular leak can acutely increase atrial arrhythmia susceptibility by disrupting ID nanodomains and slowing atrial conduction. Treatment of murine hearts with VEGF (30–60 min, at clinically relevant levels) prolonged the electrocardiographic P wave and increased susceptibility to burst pacing-induced atrial arrhythmias. Optical voltage mapping revealed slower atrial conduction following VEGF treatment (10 ± 0.4 cm/s vs. 21 ± 1 cm/s at baseline, p < 0.05). Transmission electron microscopy revealed increased intermembrane spacing at ID sites adjacent to gap junctions (GJs; 64 ± 9 nm versus 17 ± 1 nm in controls, p < 0.05), as well as sites next to mechanical junctions (MJs; 63 ± 4 nm versus 27 ± 2 nm in controls, p < 0.05) in VEGF–treated hearts relative to controls. Importantly, super-resolution microscopy and quantitative image analysis revealed reorganization of Na_V1.5 away from dense clusters localized near GJs and MJs to a more diffuse distribution throughout the ID. Taken together, these data suggest that VEGF can acutely predispose otherwise normal hearts to atrial arrhythmias by dynamically disrupting Na_V1.5-rich ID nanodomains and slowing atrial conduction. These data highlight inflammation-induced vascular leak as a potential factor in the development and progression of AF.

Does an imbalance in circulating vascular endothelial growth factors (VEGFs) cause atrial fibrillation in patients with valvular heart disease?

Background

The pathogenesis of atrial fibrillation (AF) remains unclear. Vascular endothelial growth factors (VEGFs) can stimulate fibrosis within the atrium and ventricle. We hypothesized that there is a relationship between the serum VEGFs/soluble vascular endothelial growth factor receptor (sVEGFRs) levels and AF in patients with valvular heart disease (VHD). This provides a new paradigm for studying AF.

Methods

The plasma levels of VEGF-A, VEGF-C, sVEGFR-1 and sVEGFR-2 were detected by enzyme-linked immunosorbent assay (ELISA). A total of 100 people, consisting of AF patients (long-standing, persistent AF; n=49), sinus rhythm (SR) patients (n=31) and healthy controls (n=20), were included in this study.

Results

The plasma levels of VEGF-A were significantly higher in AF patients compared to healthy control (P<0.05). The plasma levels of sVEGFR-1 were significantly higher in AF compared to SR (P<0.05). The plasma levels of sVEGFR-2 were significantly lower in AF patients compared to SR patients and healthy controls (both P<0.05). There was a significant and negative correlation between AF and the sVEGFR-2 levels in the groups (r=-0.432, P=0.000).

Conclusions

An imbalance in VEGFs and sVEGFRs may contribute to AF by breaking the balance of angiogenesis and lymphangiogenesis. Additionally, sVEGFR-2 may be an important biomarker of AF.

Recombinant expression and purification of functional vascular endothelial growth factor-121 in the baculovirus expression system

OBJECTIVE

To express human vascular endothelial growth factor121 (VEGF121) in insect cells.

METHODS

A gene construct containing VEGF was cloned in the pFastBac-HTA vector, followed by transformation in DH10BAC. The recombinant bacmid was then extracted, and transfected into Sf9 insect cells. The transfected cells were harvested, and then VEGF expression was confirmed by western blotting using specific antibodies. The tube formation assay was used for functional assessment of VEGF.

RESULTS

Our results showed that VEGF could be successfully expressed in the baculovirus system. Purified VEGF was able to stimulate in vitro tube formation of human endothelial cells.

CONCLUSIONS

Results from this study demonstrated that the recombinantly-produced VEGF can be considered as a promising candidate for therapeutic purposes.

Angiotensin-converting enzyme-2 overexpression improves atrial remodeling and function in a canine model of atrial fibrillation

BACKGROUND

Atrial fibrosis is an important factor in initiating and maintaining atrial fibrillation. The purpose of this study was to test the hypothesis that atrial angiotensin-converting enzyme-2 (ACE2) overexpression might inhibit atrial collagen accumulation and improve atrial remodeling in a canine atrial pacing model.

METHODS AND RESULTS

Thirty-two mongrel dogs of both genders were divided randomly into 4 groups: sham-operated, control, gene therapy with adenovirus-enhanced green fluorescent protein (Ad-EGFP), and gene therapy with Ad-ACE2. All of the dogs in the control, Ad-EGFP, and Ad-ACE2 groups were paced at 450 bpm for a period of 14 days. The dogs in the sham group were instrumented without pacing. After 2 weeks, all of the dogs underwent a thoracotomy operation and received epicardial gene painting. On post-gene transfer day 21, the animals underwent electrophysiology, histology, and molecular studies. The percentage of fibrosis in the Ad-ACE2 group was markedly lower than the percentage in the control and Ad-EGFP groups. Compared with the other groups, ACE2 expression was increased significantly in the Ad-ACE2 group. Compared with the sham and Ad-ACE2 groups, the expression levels of transforming growth factor-β1 and Smad3 were significantly higher in the Ad-EGFP and control groups; however, the expression levels of Smad7 were lower in the atrial tissue as detected by Western blot and reverse transcription polymerase chain reaction.

CONCLUSIONS

Our results demonstrate that the overexpression of ACE2 inhibits atrial collagen accumulation and improves left atrial remodeling and function in a canine model of atrial fibrillation. Thus, targeted gene ACE2 therapy provides a promising approach for the treatment of atrial fibrillation.

Antithrombotics in atrial fibrillation and coronary disease

Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia and approximately 18-45% of AF patients have concomitant coronary artery disease (CAD). Several studies have demonstrated that oral anticoagulation is the mainstay of therapy for stroke prevention in AF. Similarly, antiplatelet therapy including aspirin and P2Y12 inhibitor is recommended in the management of acute coronary syndrome and stable CAD. Despite the high prevalence of CAD with AF, practice guidelines are scarce on the appropriate antithrombotic regimen due to lack of large-scale randomized clinical trials. The use of direct thrombin and factor Xa inhibitors for stroke prevention in AF has also complicated the possible combinations of antithrombotic therapies. This review aims to discuss the available evidence regarding aspirin as an antithrombotic strategy, the role of novel anticoagulants and the specific clinical situations where aspirin may be beneficial in patients with AF and CAD.

Embryology and Anatomy of the Left Atrial Appendage: Why Does Thrombus Form?

The left atrial appendage (LAA) is a long tubular structure that opens into the left atrium. In patients with atrial fibrillation, the LAA develops mechanical dysfunction and fibroelastotic changes on the endocardial surface. The complex anatomy of the LAA makes it a good site for relative blood stasis. In addition, systemic factors exacerbate the hypercoagulable state, eventually resulting in endothelial dysfunction, release of tissue factor, and production of inflammatory cytokines and oxidative free radicals, and eventually initiating the coagulation cascade. Thus, the LAA is susceptible to thrombus formation and is the most common source of systemic thromboembolism.

Associations between cardiac fibrosis and permanent atrial fibrillation in advanced heart failure

Atrial fibrosis is considered as the basis in the development of long-standing atrial fibrillation (AF). However, in advanced heart failure (HF), the independent role of fibrosis for AF development is less clear since HF itself leads to atrial scarring. Our study aimed to differentiate patients with AF from patients without AF in a population consisting of patients with advanced HF. Myocardial samples from the right atrial and the left ventricular wall were obtained during heart transplantation from the explanted hearts of 21 male patients with advanced HF. Long-standing AF was present in 10 of them and the remaining 11 patients served as sinus rhythm controls. Echocardiographic and hemodynamic measurements were recorded prior to heart transplantation. Collagen volume fraction (CVF), transforming growth factor-beta (TGF-beta), and connective tissue growth factor (CTGF) expression in myocardial specimens were assessed histologically and immunohistochemically. The groups were well matched according to age (51.9+/-8.8 vs. 51.3+/-9.3 y) and co-morbidities. The AF group had higher blood pressure in the right atrium (13.6+/-7.7 vs. 6.0+/-5.0 mmHg; p=0.02), larger left atrium diameter (56.1+/-7.7 vs. 50+/-5.1 mm; p=0.043), higher left atrium wall stress (18.1+/-2.1 vs. 16.1+/-1.7 kdynes/m(2); p=0.04), and longer duration of HF (5.0+/-2.9 vs. 2.0+/-1.6 y, p=0.008). There were no significant differences in CVF (p=0.12), in CTGF (p=0.60), and in TGF-beta expression (p=0.66) in the atrial myocardium between the two study groups. In conclusions, in advanced HF, atrial fibrosis expressed by CVF is invariably present regardless of occurrence of AF. In addition to atrial wall fibrosis, increased wall stress might contribute to AF development in long-standing AF.

Oxidative stress and the development of endothelial dysfunction in congenital heart disease with increased pulmonary blood flow: lessons from the neonatal lamb

Congenital heart diseases associated with increased pulmonary blood flow commonly leads to the development of pulmonary hypertension. However, most patients who undergo histological evaluation have advanced pulmonary hypertension, and therefore it has been difficult to investigate aberrations in signaling cascades that precede the development of overt vascular remodeling. This review discusses the role played by both oxidative and nitrosative stress in the lung and their impact on the signaling pathways that regulate vasodilation, vessel growth, and vascular remodeling in the neonatal lung exposed to increased pulmonary blood flow.

Atrial fibrillation induces myocardial fibrosis through angiotensin II type 1 receptor-specific Arkadia-mediated downregulation of Smad7

RATIONALE

Tachycardia-induced atrial fibrosis is a hallmark of structural remodeling of atrial fibrillation (AF). The molecular mechanisms underlying the AF-induced atrial fibrosis remain unclear.

OBJECTIVE

To determine the role of angiotensin II (Ang II)/Ang II type 1 (AT(1)) receptor-coupled transforming growth factor (TGF)-β(1)/Smad signaling pathway in the AF-induced atrial fibrosis.

METHODS AND RESULTS

Rapid atrial pacing (1000 ppm) was applied to the left atrium of rabbit heart to induce atrial fibrillation and fibrosis. Quantitative PCR and Western blot analysis revealed that rapid atrial pacing caused a marked increase in the expression of Ang II, TGF-β(1), phosphorylated Smad2/3 (P-Smad2/3), Arkadia, and hydroxyproline synthesis. However, the expression of Smad7, a key endogenous antagonist of the TGF-β(1)/Smad-mediated fibrosis, was significantly decreased. These changes were dose-dependently reversed by AT(1) receptor antagonist losartan, implicating the involvement of AF-induced release of Ang II and activation of AT(1) receptor-specific pathway. In the adult rabbit cardiac fibroblasts, Ang II increased the expression of TGF-β(1), P-Smad2/3, Smad4, Arkadia, and collagen I synthesis and significantly reduced Smad7 expression. These effects of Ang II were reversed by losartan but not by the AT(2) antagonist (PD123319). In addition, extracellular signal-regulated kinase inhibitor and anti-TGF-β(1) antibody also blocked the Ang II-induced downregulation of Smad7. Silencing of Smad7 gene by small interfering RNA abolished the antagonism of losartan on the fibrogenic effects of Ang II on cardiac fibroblasts, whereas overexpression of Smad7 blocked Ang II-induced increase in collagen I synthesis.

CONCLUSIONS

Ang II/AT(1) receptor-specific activation of Arkadia-mediated poly-ubiquitination and degradation of Smad7 may decrease the inhibitory feedback regulation of TGF-β(1)/Smad signaling and serves as a key mechanism for AF-induced atrial fibrosis.

Atrial fibrosis and atrial fibrillation: the role of the TGF-β1 signaling pathway

BACKGROUND

Atrial fibrosis concurs with chronic atrial fibrillation (AF), a phenomenon that contributes to the resistance to restore and maintain sinus rhythm (SR). Fibrogenesis represents a complex process in which the transforming growth factor-β1 (TGF-β1) pathway may play a major role, e.g. in the setting of myocardial infarction. The present study addresses the potential contribution of the TGF-β1 signaling pathway to atrial fibrosis in patients with AF.

METHODS AND RESULTS

Right atrial appendages of 163 patients were excised during heart surgery and grouped according to rhythm (SR vs. AF) and AF duration. Five groups were defined: SR, paroxysmal/chronic persistent AF (<6 months), chronic permanent AF (CAF) of 7-24 months, 25-60 months, and >60 months duration. Collagen content of atria, determined morphometrically, revealed a steady and significant increase in patients with SR (14.6±8.9%) up to patients with CAF of >60 months (28.1±7.1%). Likewise, expression of TGF-β1 mRNA and protein, TGF-β-receptor-II protein, profibrotic phospho-Smad-2 and -4 proteins increased. However, the TGF-β(1) effect appeared to decline with increasing AF duration, characterized by a decrease in TGF-β-receptor-I protein, increases of TGF-β inhibiting Smad-7 protein and a reduction of ph-Smad-2.

CONCLUSIONS

Human atrial fibrogenesis in patients with atrial fibrillation is accompanied by a biphasic response, an early increase and later loss of responsiveness to TGF-β(1). It appears that fibrosis progresses despite compensatory changes in the TGF-β-signaling pathway. The sequential changes in the contribution of different profibrotic processes during the establishment of AF may offer the opportunity to selectively interfere with the atrial remodeling process at different stages.

Atrial fibrillation is associated with cardiac hypoxia

BACKGROUND

Atrial fibrillation (AF), the most common human arrhythmia, is responsible for substantial morbidity and mortality and may be promoted by selective atrial ischemia and atrial fibrosis. Consequently, we investigated markers for hypoxia and angiogenesis in AF.

METHODS

Right atrial appendages (n=158) were grouped according to heart rhythm [sinus rhythm (SR) or AF]. The degree of fibrosis and microvessel density of all patients were determined morphometrically using Sirius-Red- and CD34/CD105-stained sections, respectively. Next, sections (n=77) underwent immunostaining to detect hypoxia- and angiogenesis-related proteins [hypoxia-inducible factor (HIF)1 alpha, HIF2 alpha, vascular endothelial growth factor (VEGF), VEGF receptor 2 (KDR), phosphorylated KDR (pKDR), carboanhydrase IX, platelet-derived growth factor] and the apoptosis-related B-cell lymphoma 2 protein.

RESULTS

Fibrosis progressed significantly from 14.7+/-0.8% (SR) to 22.3+/-1.4% (AF). While the positive cytoplasmic staining of HIF1 alpha, HIF2 alpha, VEGF, KDR, and pKDR rose significantly from SR to AF, their nuclear fractions fell (only pKDR significantly). The median CD34/CD105-positive microvessel size increased significantly from SR to AF.

CONCLUSIONS

AF is closely associated with an atrial up-regulation of hypoxic and angiogenic markers. Whether this is cause, effect, or co-phenomenon of fibrosis remains to be investigated. It is conceivable that fibrosis might lead to an increased O(2) diffusion distance and thus induce ischemic signaling, which, in turn, leads to angiogenesis.

Mechanisms of thrombogenesis in atrial fibrillation: Virchow's triad revisited

Atrial fibrillation is the most common sustained cardiac arrhythmia, which is associated with a high risk of stroke and thromboembolism. Increasing evidence suggests that the thrombogenic tendency in atrial fibrillation is related to several underlying pathophysiological mechanisms. Abnormal changes in flow are evident by stasis in the left atrium, and seen as spontaneous echocontrast. Abnormal changes in vessel walls-essentially, anatomical and structural defects-include progressive atrial dilatation, endocardial denudation, and oedematous or fibroelastic infiltration of the extracellular matrix. Additionally, abnormal changes in blood constituents are well described, and include haemostatic and platelet activation, as well as inflammation and growth factor changes. These changes result in the fulfilment of Virchow's triad for thrombogenesis, and accord with a prothrombotic or hypercoagulable state in this arrhythmia. In this Review, we present an overview of the established and purported mechanisms for thrombogenesis in atrial fibrillation.

Atrial extracellular matrix remodelling in patients with atrial fibrillation

Atrial fibrillation (AF) is the most frequent clinical arrhythmia. Atrial fibrosis is an important factor in initiating and maintaining AF. However, the collagen turnover and its regulation in AF has not been completely elucidated. We tested the hypothesis that the extracellular matrix changes are more severe in patients with permanent AF in comparison with those in patients in sinus rhythm (SR). Intraoperative biopsies from the right atrial appendages (RAA) and free walls (RFW) from 24 patients with AF undergoing a mini-Maze procedure and 24 patients in SR were investigated with qualitative and quantitative immunofluorescent and Western blot analyses. As compared with SR, all patients with AF exhibited dysregulations in collagen type I and type III synthesis/degradation. Tissue inhibitors of metalloproteinases (TIMP2) was significantly enhanced only in RAA-AF. As compared with SR, collagen VI, matrix metalloproteinases MMP2, MMP9 and TIMP1 were significantly increased while TIMP3 and TIMP4 remained unchanged in all AF groups. Reversion-inducing cysteine-rich protein with Kazal motifs (RECK), a newly discovered MMPs inhibitor, was elevated in RFW as compared to RAA-AF (P<0.05) and RFW-SR (P<0.05). The level of transforming growth factor (TGF)-β1 was higher in AF than SR. Smad2 and phosphorylated Smad2 showed an elevation in RFW-AF as compared to RFW-SR, RAA-AF, and RAA-SR groups (P<0.05). Conclusions: Atrial fibrosis in AF is characterized by severe alterations in collagen I and III synthesis/degradation associated with disturbed MMP/TIMP systems and increased levels of RECK. TGF-β1 contributes to atrial fibrosis via TGF-β1-Smad pathway by phospho-rylating Smad2. These processes culminate in accumulations of fibrillar and non-fibrillar collagens leading to excessive atrial fibrosis and maintainance of AF.

Transforming growth factor-β1 decreases cardiac muscle L-type Ca2+ current and charge movement by acting on the Cav1.2 mRNA

Transforming growth factors-β (TGF-βs) are essential to the structural remodeling seen in cardiac disease and development; however, little is known about potential electrophysiological effects. We hypothesized that chronic exposure (6–48 h) of primary cultured neonatal rat cardiomyocytes to the type 1 TGF-β (TGF-β1, 5 ng/ml) may affect voltage-dependent Ca2+ channels. Thus we investigated T- ( ICaT) and L-type ( ICaL) Ca2+ currents, as well as dihydropyridine-sensitive charge movement using the whole cell patch-clamp technique and quantified CaV1.2 mRNA levels by real-time PCR assay. In ventricular myocytes, TGF-β1 did not exert significant electrophysiological effects. However, in atrial myocytes, TGF-β1 reduced both ICaL and charge movement (55% at 24–48 h) without significantly altering ICaT, cell membrane capacitance, or channel kinetics (voltage dependence of activation and inactivation, as well as the activation and inactivation rates). Reductions of ICaL and charge movement were explained by concomitant effects on the maximal values of L-channels conductance ( Gmax) and charge movement (Qmax). Thus TGF-β1 selectively reduces the number of functional L-channels on the surface of the plasma membrane in atrial but not ventricular myocytes. The TGF-β1-induced ICaL reduction was unaffected by supplementing intracellular recording solutions with okadaic acid (2 μM) or cAMP (100 μM), two compounds that promote L-channel phosphorylation. This suggests that the decreased number of functional L-channels cannot be explained by a possible regulation in the L-channels phosphorylation state. Instead, we found that TGF-β1 decreases the expression levels of atrial CaV1.2 mRNA (70%). Thus TGF-β1 downregulates atrial L-channel expression and may be therefore contributing to the in vivo cardiac electrical remodeling.

Angiogenic factors in atrial fibrillation: a possible role in thrombogenesis?

BACKGROUND

The precise pathophysiological processes underlying the prothrombotic or hypercoagulable state in atrial fibrillation (AF) remain uncertain. We hypothesized a relationship between abnormal endothelial damage/dysfunction, coagulation, and angiogenic factors, thereby contributing to increased thrombogenicity.

METHODS

Plasma levels of von Willebrand factor (vWF, an index of endothelial damage/dysfunction) and tissue factor (TF, an index of coagulation), as well as the angiogenic factors, vascular endothelial growth factor (VEGF), angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2), were measured by enzyme-linked immunosorbant assay (ELISA) in 59 chronic AF patients. Data were compared to 40 age- and sex-matched healthy controls in sinus rhythm.

RESULTS

Plasma vWF, VEGF and Ang-2 were significantly higher in AF patients compared to healthy controls (P=0.005, P=0.0055 and P<0.0001 respectively) but there were no significant differences in plasma Ang-1 or TF levels between the two groups (P=0.925 and P=0.121 respectively). Significant correlations were found between VEGF and vWF levels (Spearman, r=0.262, P=0.011) and between VEGF and Ang-2 (r=0.333, P=0.001).

CONCLUSIONS

Raised VEGF in association with Ang-2 and vWF may reflect a link between abnormal endothelial damage/dysfunction and angiogenic factors. These may act together to alter TF expression and endothelial integrity, thereby contributing to the prothrombotic state in AF.

Effect of Atrial Fibrillation on Hematopoietic Progenitor Cells

Background— Injury to the heart causes hematopoietic progenitor cells (HPCs) to migrate to the site of damage and to undergo cell differentiation. Studies suggest that myocardial progenitor cells invade atrial tissue. So far it is unclear, however, whether an atrial disease per se affects circulating HPCs.

Methods and Results— Seventeen patients with persistent atrial fibrillation (persistAF), 12 with paroxysmal AF (paroxAF), and 17 matched patients with sinus rhythm (SR) were studied. HPCs (CD34 + and CD34 + /CD117 + ) were quantified with the use of a fluorescence-activated cell sorter; stromal cell–derived factor-1α (SDF-1α), vascular endothelium growth factor (VEGF), and atrial natriuretic peptide (ANP) were determined by immunoassays. In patients with persistAF, blood samples were obtained before as well as 10 minutes, 24 hours, and 48 hours after electrical cardioversion. CD34 + HPCs (AF, 7.0±2.3×103/mL versus SR, 5.0±1.6×10 3 /mL; P <0.01) were increased during persistAF only. Highest SDF-1α levels were also observed during persistAF. Successful and unsuccessful cardioversion decreased CD34 + HPCs temporarily (7.0±2.3×10 3 /mL versus 24 hours: 5.0±1.5×10 3 /mL; P <0.05). Forty-eight hours after successful cardioversion, SDF-1α and CD34 + HPC levels started to decline, reaching control levels after 59±19 days. CD34 + /CD117 + and VEGF levels, however, were increased by DC energy but not by AF. ANP levels correlated with CD34 + HPC ( r =0.76; P <0.01) and SDF-1α ( r =0.56; P <0.01). HPCs from patients with AF had a greater tendency to differentiate into cells expressing (cardio)myocyte markers ANP and myocyte enhancer factor-2.

Conclusions— PersistAF appears to increase the potential of HPCs for (cardio)myogenesis. Restitution of CD34 + HPC levels, mediated by SDF-1α and possibly ANP, occurs within several weeks after successful cardioversion.

Alterations in TGF-beta1 expression in lambs with increased pulmonary blood flow and pulmonary hypertension

The mechanisms responsible for pulmonary vascular remodeling in congenital heart disease with increased pulmonary blood flow remain unclear. We developed a lamb model of congenital heart disease and increased pulmonary blood flow utilizing an in utero placed aortopulmonary vascular graft (shunted lambs). Morphometric analysis of barium-injected pulmonary arteries indicated that by 4 wk of age, shunts had twice the pulmonary arterial density of controls (P < 0.05), and their pulmonary vessels showed increased muscularization and medial thickness at both 4 and 8 wk of age (P < 0.05). To determine the potential role of TGF-beta1 in this vascular remodeling, we investigated vascular changes in expression and localization of TGF-beta1 and its receptors TbetaRI, ALK-1, and TbetaRII in lungs of shunted and control lambs at 1 day and 1, 4, and 8 wk of life. Western blots demonstrated that TGF-beta1 and ALK-1 expression was elevated in shunts compared with control at 1 and 4 wk of age (P < 0.05). In contrast, the antiangiogenic signaling receptor TbetaRI was decreased at 4 wk of age (P < 0.05). Immunohistochemistry demonstrated shunts had increased TGF-beta1 and TbetaRI expression in smooth muscle layer and increased TGF-beta1 and ALK-1 in endothelium of small pulmonary arteries at 1 and 4 wk of age. Moreover, TbetaRI expression was significantly reduced in endothelium of pulmonary arteries in the shunt at 1 and 4 wk. Our data suggest that increased pulmonary blood flow dysregulates TGF-beta1 signaling, producing imbalance between pro- and antiangiogenic signaling that may be important in vascular remodeling in shunted lambs.

Expression of VEGF and its receptors Flt-1 and Flk-1/KDR is altered in lambs with increased pulmonary blood flow and pulmonary hypertension

Utilizing in utero aortopulmonary vascular graft placement, we developed a lamb model of congenital heart disease and increased pulmonary blood flow. We showed previously that these lambs have increased pulmonary vessel number at 4 wk of age. To determine whether this was associated with alterations in VEGF signaling, we investigated vascular changes in expression of VEGF and its receptors, Flt-1 and KDR/Flk-1, in the lungs of shunted and age-matched control lambs during the first 8 wk of life. Western blot analysis demonstrated that VEGF, Flt-1, and KDR/Flk-1 expression was higher in shunted lambs. VEGF and Flt-1 expression was increased at 4 and 8 wk of age (P <0.05). However, KDR/Flk-1 expression was higher in shunted lambs only at 1 and 4 wk of age (P <0.05). Immunohistochemical analysis demonstrated that, in control and shunted lambs, VEGF localized to the smooth muscle layer of vessels and airways and to the pulmonary epithelium while increased VEGF expression was localized to the smooth muscle layer of thickened media in remodeled vessels in shunted lambs. VEGF receptors were localized exclusively in the endothelium of pulmonary vessels. Flt-1 was increased in the endothelium of small pulmonary arteries in shunted animals at 4 and 8 wk of age, whereas KDR/Flk-1 was increased in small pulmonary arteries at 1 and 4 wk of age. Our data suggest that increased pulmonary blood flow upregulates expression of VEGF and its receptors, and this may be important in development of the vascular remodeling in shunted lambs.

TGFbeta regulates the expression of G alpha(i2) via an effect on the localization of ras

The negative chronotropic response of the heart to parasympathetic stimulation is mediated via the interaction of M(2) muscarinic receptors, Galpha(i2) and the G-protein coupled inward rectifying K(+) channel, GIRK1. Here TGFbeta(1) is shown to decrease the expression of Galpha(i2) in cultured chick atrial cells in parallel with attenuation of the negative chronotropic response to parasympathetic stimulation. The response to the acetylcholine analogue, carbamylcholine, decreased from a 95+/-2% (+/-SEM, n=8) inhibition of beat rate in control cells to 18+/-2% (+/-SEM,n =8) in TGFbeta(1) treated cells. Data support the conclusion that TGFbeta regulation of Galpha(i2) expression was mediated via an effect on Ras. TGFbeta(1) inhibited Galpha(i2) promoter activity by 56+/-6% (+/-SEM, n=4) compared to control. A dominant activating Ras mutant reversed the effect of TGFbeta on Galpha(i2) expression and stimulated Galpha(i2) promoter activity 1.7 fold above control. A dominant negative Ras mutant mimicked the effect of TGFbeta(1) on Galpha(i2) promoter activity. TGFbeta had no effect on the ratio of GDP/GTP bound Ras, but markedly decreased the level of membrane associated Ras and increased the level of cytoplasmic Ras compared to control. Furthermore, farnesol, a precursor to farnesylpyrophosphate, the substrate for the farnesylation of Ras, not only reversed TGFbeta(1) inhibition of Ras localization to the membrane, but also reversed TGFbeta(1) inhibition of Galpha(i2)promoter activity. FTI-277, a specific inhibitor of the farnesylation of Ras, mimicked the effect of TGFbeta(1) on Ras localization and Galpha(i2) promoter activity. These data suggest a novel relationship between TGFbeta signaling, regulation of Ras function and the autonomic response of the heart.

Is the hypercoagulable state in atrial fibrillation mediated by vascular endothelial growth factor?

BACKGROUND AND PURPOSE

Tissue factor (TF; an initiator of coagulation) and vascular endothelial growth factor (VEGF; a marker of angiogenesis) are involved in the hypercoagulable state associated with malignancy. We investigated their roles in chronic atrial fibrillation (AF), a condition also associated with increased risk of stroke and thromboembolism, as well as a prothrombotic or hypercoagulable state.

METHODS

We studied 25 patients with AF (20 men; mean+/-SD age, 62+/-13 years) who were compared with 2 control groups in sinus rhythm: 30 healthy control subjects (17 men; mean age, 60+/-9 years) and 35 patient control subjects with coronary artery disease (CAD; 27 men; mean age, 60+/-12 years). Plasma levels of TF, VEGF, and the VEGF receptor sFlt-1 were measured by enzyme-linked immunosorbent assay.

RESULTS

VEGF, sFlt-1, and TF were significantly different between the 3 groups, with abnormal levels in AF and CAD patients compared with control subjects (P<0.001, P=0.022, and P=0.008, respectively). Among the AF patients, TF levels were significantly correlated with VEGF (Spearman's r=0.65, P<0.001) and sFlt (r=0.54, P=0.006) levels. Only TF and VEGF levels were significantly correlated in CAD patients (r=0.39, P=0.02). There were no significant correlations among the healthy control subjects.

CONCLUSIONS

Patients with chronic AF have high TF levels, in keeping with the prothrombotic state associated with this arrhythmia. The relationships between TF and VEGF and its receptor sFlt-1 in AF suggest a possible role for VEGF in the hypercoagulable state found in AF, as seen in malignancy and atherosclerosis.

Autocrine regulation of myocyte Cx43 expression by VEGF

Cardiac myocytes can rapidly adjust their expression of gap junction channel proteins in response to changes in load. Previously, we showed that after only 1 hour of linear pulsatile stretch (110% of resting cell length; 3 Hz), expression of connexin43 (Cx43) by cultured neonatal rat ventricular myocytes is increased by approximately 2-fold and impulse propagation is significantly more rapid. In the present study, we tested the hypothesis that vascular endothelial growth factor (VEGF), acting downstream of transforming growth factor-beta (TGF-beta), mediates stretch-induced upregulation of Cx43 expression by cardiac myocytes. Incubation of nonstretched cells with exogenous VEGF (100 ng/mL) or TGF-beta (10 ng/mL) for 1 hour increased Cx43 expression by approximately 1.8-fold, comparable to that observed in cells subjected to pulsatile stretch for 1 hour. Stretch-induced upregulation of Cx43 expression was blocked by either anti-VEGF antibody or anti-TGF-beta antibody. Stretch-induced enhancement of conduction was also blocked by anti-VEGF antibody. ELISA assay showed that VEGF was secreted into the culture medium during stretch. Furthermore, stretch-conditioned medium stimulated Cx43 expression in nonstretched cells. This effect was also blocked by anti-VEGF antibody. Upregulation of Cx43 expression stimulated by exogenous TGF-beta was blocked by anti-VEGF antibody, but VEGF-stimulation of Cx43 expression was not blocked by anti-TGF-beta antibody. Thus, stretch-induced upregulation of Cx43 expression is mediated, at least in part, by VEGF, which acts downstream of TGF-beta. Because the cultures contained only approximately 5% nonmyocytic cells, these results indicate that myocyte-derived VEGF, secreted in response to stretch, acts in an autocrine fashion to enhance intercellular coupling.