Kai Friedrichs

Myeloperoxidase acts as a profibrotic mediator of atrial fibrillation

Observational clinical and ex vivo studies have established a strong association between atrial fibrillation and inflammation. However, whether inflammation is the cause or the consequence of atrial fibrillation and which specific inflammatory mediators may increase the atrias susceptibility to fibrillation remain elusive. Here we provide experimental and clinical evidence for the mechanistic involvement of myeloperoxidase (MPO), a heme enzyme abundantly expressed by neutrophils, in the pathophysiology of atrial fibrillation. MPO-deficient mice pretreated with angiotensin II (AngII) to provoke leukocyte activation showed lower atrial tissue abundance of the MPO product 3-chlorotyrosine, reduced activity of matrix metalloproteinases and blunted atrial fibrosis as compared to wild-type mice. Upon right atrial electrophysiological stimulation, MPO-deficient mice were protected from atrial fibrillation, which was reversed when MPO was restored. Humans with atrial fibrillation had higher plasma concentrations of MPO and a larger MPO burden in right atrial tissue as compared to individuals devoid of atrial fibrillation. In the atria, MPO colocalized with markedly increased formation of 3-chlorotyrosine. Our data demonstrate that MPO is a crucial prerequisite for structural remodeling of the myocardium, leading to an increased vulnerability to atrial fibrillation.

Myeloperoxidase attracts neutrophils by physical forces

Recruitment of polymorphonuclear neutrophils (PMNs) remains a paramount prerequisite in innate immune defense and a critical cofounder in inflammatory vascular disease. Neutrophil recruitment comprises a cascade of concerted events allowing for capture, adhesion and extravasation of the leukocyte. Whereas PMN rolling, binding, and diapedesis are well characterized, receptor-mediated processes, mechanisms attenuating the electrostatic repulsion between the negatively charged glycocalyx of leukocyte and endothelium remain poorly understood. We provide evidence for myeloperoxidase (MPO), an abundant PMN-derived heme protein, facilitating PMN recruitment by its positive surface charge. In vitro, MPO evoked highly directed PMN motility, which was solely dependent on electrostatic interactions with the leukocytes surface. In vivo, PMN recruitment was shown to be MPO-dependent in a model of hepatic ischemia and reperfusion, upon intraportal delivery of MPO and in the cremaster muscle exposed to local inflammation or to intraarterial MPO application. Given MPOs affinity to both the endothelial and the leukocytes surface, MPO evolves as a mediator of PMN recruitment because of its positive surface charge. This electrostatic MPO effect not only displays a so far unrecognized, catalysis-independent function of the enzyme, but also highlights a principal mechanism of PMN attraction driven by physical forces.

Inflammatory pathways underlying atrial fibrillation

The pathophysiology of atrial fibrillation (AF) remains incompletely understood, despite its prevalence and contributing role in stroke and heart failure. Whereas previous studies have focused on the electrophysiological characteristics of AF, recent reports shed light on structural remodeling of the atria as a prerequisite for AF. Recent evidence suggests that atrial fibrosis is linked not only to stimulation of myocytes and fibroblasts, but also to the activation state of leukocytes. Recruitment of leukocytes with the release of reactive oxygen species, cytokines and growth factors is followed by increased matrix deposition, which leads to adverse atrial remodeling and suggests that inflammatory pathways are a prerequisite for AF. Here we review current evidence demonstrating the interrelation between inflammation and AF.

Transcatheter Treatment of Severe Tricuspid Regurgitation with the Edge-to-Edge: MitraClip Technique.

Background: Current surgical and medical treatment options for severe tricuspid regurgitation (TR) are limited, and additional interventional approaches are required. In the present observational study, the safety and feasibility of transcatheter repair of chronic severe TR with the MitraClip system were evaluated. In addition, the effects on clinical symptoms were assessed. Methods: Patients with heart failure symptoms and severe TR on optimal medical treatment were treated with the MitraClip system. Safety, defined as periprocedural adverse events such as death, myocardial infarction, stroke, or cardiac tamponade, and feasibility, defined as successful implantation of 1 or more MitraClip devices and reduction of TR by at least 1 grade, were evaluated before discharge and after 30 days. In addition, functional outcome, defined as changes in New York Heart Assocation class and 6-minute walking distance, were assessed. Results: We included 64 consecutive patients (mean age 76.6±10 years) deemed unsuitable for surgery who underwent MitraClip treatment for chronic, severe TR for compassionate use. Functional TR was present in 88%; in addition, 22 patients were also treated with the MitraClip system for mitral regurgitation as a combined procedure. The degree of TR was severe or massive in 88% of patients before the procedure. The MitraClip device was successfully implanted in the tricuspid valve in 97% of the cases. After the procedure, TR was reduced by at least 1 grade in 91% of the patients, thereof 4% that were reduced from massive to severe. In 13% of patients, TR remained severe after the procedure. Significant reductions in effective regurgitant orifice area (0.9±0.3cm2 versus 0.4±0.2cm2; P<0.001), vena contracta width (1.1±0.5 cm versus 0.6±0.3 cm; P=0.001), and regurgitant volume (57.2±12.8 mL/beat versus 30.8±6.9 mL/beat; P<0.001) were observed. No intraprocedural deaths, cardiac tamponade, emergency surgery, stroke, myocardial infarction, or major vascular complications occurred. Three (5%) in-hospital deaths occurred. New York Heart Association class was significantly improved (P<0.001), and 6-minute walking distance increased significantly (165.9±102.5 m versus 193.5±115.9 m; P=0.007). Conclusions: Transcatheter treatment of TR with the MitraClip system seems to be safe and feasible in this cohort of preselected patients. Initial efficacy analysis showed encouraging reduction of TR, which may potentially result in improved clinical outcomes.

Myeloperoxidase deficiency preserves vasomotor function in humans

Aims Observational studies have suggested a mechanistic link between the leucocyte-derived enzyme myeloperoxidase (MPO) and vasomotor function. Here, we tested whether MPO is systemically affecting vascular tone in humans. Methods and results A total of 12 135 patients were screened for leucocyte peroxidase activity. We identified 15 individuals with low MPO expression and activity (MPOlow), who were matched with 30 participants exhibiting normal MPO protein content and activity (control). Nicotine-dependent activation of leucocytes caused attenuation of endothelial nitric oxide (NO) bioavailability in the control group (P < 0.01), but not in MPOlow individuals (P = 0.12); here the MPO burden of leucocytes correlated with the degree of vasomotor dysfunction (P = 0.008). To directly test the vasoactive properties of free circulating MPO, the enzyme was injected into the left atrium of anaesthetized, open-chest pigs. Myeloperoxidase plasma levels peaked within minutes and rapidly declined thereafter, reflecting vascular binding of MPO. Blood flow in the left anterior descending artery and the internal mammary artery (IMA) as well as myocardial perfusion decreased following MPO injection when compared with albumin-treated animals (P < 0.001). Isolated IMA-rings from animals subjected to MPO revealed markedly diminished relaxation in response to acetylcholine (P < 0.01) and nitroglycerine as opposed to controls (P < 0.001). Conclusion Myeloperoxidase elicits profound effects on vascular tone of conductance and resistance vessels in vivo. These findings not only call for revisiting the biological functions of leucocytes as systemic and mobile effectors of vascular tone, but also identify MPO as a critical systemic regulator of vasomotion in humans and thus a potential therapeutic target.

Diagnostic value of MPO plasma levels in patients admitted for suspected myocardial infarction

BACKGROUND Besides its well-established role in atherosclerosis, myeloperoxidase (MPO) has gained attention as a prognostic indicator in cardiovascular disease. Previous studies assessed MPO retrospectively and at a single time point. The current study aimed to evaluate the prognostic information of MPO prospectively and in consecutive measurements in patients presenting with chest pain. METHODS MPO plasma levels were determined in 274 consecutive chest pain patients admitted to the emergency room. RESULTS A total of 100 patients (36.5%) were finally diagnosed for acute myocardial infarction (AMI). Patients with AMI had significantly higher MPO levels than patients without AMI. Importantly, MPO levels were elevated in patients finally diagnosed for AMI even when troponin I (TNI) was negative (cutoff: 0.032 ng/ml). Overall, MPO yielded a negative predictive value (NPV) of 85.5% (95% confidence interval (CI): 82.6-88.4) and a sensitivity for diagnosing AMI of 80.0% (95% CI: 75.8-84.2) compared to a NPV of 91.7% (95% CI: 89.5-94.0) and a sensitivity of 85.9% (95% CI: 82.3-89.5) for TNI. For patients with a symptom onset of ≤ 2 h the sensitivity of MPO increased to 95.8% (95% CI: 93.7-97.9) whereas the sensitivity of TNI dropped to 50.0% (95% CI: 44.8-55.2). The negative predictive value of MPO for this group of patients was 95.6% (95% CI: 94.0-97.3) compared to 73.3% (95% CI: 69.8-76.9) for TNI. DISCUSSION The current data underscore the role of MPO as diagnostic marker in acute coronary disease; however the additive information derived from MPO is restricted to patients presenting in the early phase of symptom onset.

Protective Effects of 10-nitro-oleic Acid in a Hypoxia-Induced Murine Model of Pulmonary Hypertension

Pulmonary arterial hypertension (PAH) is characterized by adverse remodeling of pulmonary arteries. Although the origin of the disease and its underlying pathophysiology remain incompletely understood, inflammation has been identified as a central mediator of disease progression. Oxidative inflammatory conditions support the formation of electrophilic fatty acid nitroalkene derivatives, which exert potent anti-inflammatory effects. The current study investigated the role of 10-nitro-oleic acid (OA-NO2) in modulating the pathophysiology of PAH in mice. Mice were kept for 28 days under normoxic or hypoxic conditions, and OA-NO2 was infused subcutaneously. Right ventricular systolic pressure (RVPsys) was determined, and right ventricular and lung tissue was analyzed. The effect of OA-NO2 on cultured pulmonary artery smooth muscle cells (PASMCs) and macrophages was also investigated. Changes in RVPsys revealed increased pulmonary hypertension in mice on hypoxia, which was significantly decreased by OA-NO2 administration. Right ventricular hypertrophy and fibrosis were also attenuated by OA-NO2 treatment. The infiltration of macrophages and the generation of reactive oxygen species were elevated in lung tissue of mice on hypoxia and were diminished by OA-NO2 treatment. Moreover, OA-NO2 decreased superoxide production of activated macrophages and PASMCs in vitro. Vascular structural remodeling was also limited by OA-NO2. In support of these findings, proliferation and activation of extracellular signal-regulated kinases 1/2 in cultured PASMCs was less pronounced on application of OA-NO2.Our results show that the oleic acid nitroalkene derivative OA-NO2 attenuates hypoxia-induced pulmonary hypertension in mice. Thus, OA-NO2 represents a potential therapeutic agent for the treatment of PAH.

Fibrosis in Atrial Fibrillation - Role of Reactive Species and MPO.

Atrial fibrosis with enhanced turnover and deposition of matrix proteins leads to inhomogeneous atrial electrical conduction and gives rise to electrical reentry circuits resulting in atrial fibrillation. The multifactorial pathogenesis of atrial fibrosis involves resident cardiac cells as well as infiltrating leukocytes, both generating and sequestering matrix metalloproteinases (MMPs), a key enzyme family involved in fibrosis. A growing body of evidence points toward an important role of reactive oxygen species (ROS) in the release and activation of pro-MMPs and the stimulation of pro-fibrotic cascades. Myeloperoxidase (MPO), a bactericidal enzyme released from activated polymorphonuclear neutrophils (PMN) is not only associated with a variety of cardiovascular diseases, but has also been shown to be mechanistically linked to atrial fibrosis and fibrillation. MPO catalyzes the generation of reactive species like hypochlorous acid, which affect intracellular signaling cascades in various cells and advance activation of pro-MMPs and deposition of atrial collagen resulting in atrial arrhythmias. Thus, inflammatory mechanisms effectively promote atrial structural remodeling and importantly contribute to the initiation and perpetuation of atrial fibrillation.

Induction of Atrial Fibrillation by Neutrophils Critically Depends on CD11b/CD18 Integrins

Background Recent observational clinical and ex-vivo studies suggest that inflammation and in particular leukocyte activation predisposes to atrial fibrillation (AF). However, whether local binding and extravasation of leukocytes into atrial myocardium is an essential prerequisite for the initiation and propagation of AF remains elusive. Here we investigated the role of atrial CD11b/CD18 mediated infiltration of polymorphonuclear neutrophils (PMN) for the susceptibility to AF. Methods and Results C57bl/6J wildtype (WT) and CD11b/CD18 knock-out (CD11b−/−) mice were treated for 14 days with subcutaneous infusion of angiotensin II (Ang II), a known stimulus for PMN activation. Atria of Ang II-treated WT mice were characterized by increased PMN infiltration assessed in immunohistochemically stained sections. In contrast, atrial sections of CD11b−/− mice lacked a significant increase in PMN infiltration upon Ang II infusion. PMN infiltration was accompanied by profoundly enhanced atrial fibrosis in Ang II treated WT as compared to CD11b−/− mice. Upon in-vivo electrophysiological investigation, Ang II treatment significantly elevated the susceptibility for AF in WT mice if compared to vehicle treated animals given an increased number and increased duration of AF episodes. In contrast, animals deficient of CD11b/CD18 were entirely protected from AF induction. Likewise, epicardial activation mapping revealed decreased electrical conduction velocity in atria of Ang II treated WT mice, which was preserved in CD11b−/− mice. In addition, atrial PMN infiltration was enhanced in atrial appendage sections of patients with persistent AF as compared to patients without AF. Conclusions The current data critically link CD11b-integrin mediated atrial PMN infiltration to the formation of fibrosis, which promotes the initiation and propagation of AF. These findings not only reveal a mechanistic role of leukocytes in AF but also point towards a potential novel avenue of treatment in AF.

Nitrated fatty acids suppress angiotensin II-mediated fibrotic remodelling and atrial fibrillation

AIM Atrial fibrosis, one of the most striking features in the pathology of atrial fibrillation (AF), is promoted by local and systemic inflammation. Electrophilic fatty acid nitroalkenes, endogenously generated by both metabolic and inflammatory reactions, are anti-inflammatory mediators that in synthetic form may be useful as drug candidates. Herein we investigate whether an exemplary nitro-fatty acid can limit atrial fibrosis and AF. METHODS AND RESULTS Wild-type C57BL6/J mice were treated for 2 weeks with angiotensin II (AngII) and vehicle or nitro-oleic acid (10-nitro-octadec-9-enoic acid, OA-NO2, 6 mg/kg body weight) via subcutaneous osmotic minipumps. OA-NO2 significantly inhibited atrial fibrosis and depressed vulnerability for AF during right atrial electrophysiological stimulation to levels observed for AngII-naive animals. Left atrial epicardial mapping studies demonstrated preservation of conduction homogeneity by OA-NO2. The protection from fibrotic remodelling was mediated by suppression of Smad2-dependent myofibroblast transdifferentiation and inhibition of Nox2-dependent atrial superoxide formation. CONCLUSION OA-NO2 potently inhibits atrial fibrosis and subsequent AF. Nitro-fatty acids and possibly other lipid electrophiles thus emerge as potential therapeutic agents for AF, either by increasing endogenous levels through dietary modulation or by administration as synthetic drugs.