Introduction: Atrial fibrillation (AFib) and heart failure (HF) often co-exist in patients and are associated with worsened cardiovascular outcomes. The overarching aim of this study is to understand the molecular mechanisms leading to AFib development in human LV.

Methods and results: Ventricular myocardium from patients with aortic stenosis and preserved LV function with sinus rhythm or rate-controlled AFib was studied. LV myocardium from patients with sinus rhythm and patients with AFib showed no differences in fibrosis. We measured the in vivo redox state of cardiac proteins in AFib patients and compared to SR patients. We used the OxICAT method coupled with mass spectrometry (MS). OxICAT allows the precise quantification of oxidative thiol modifications in hundreds of different proteins in a single experiment. It uses cysteine-specific isotope-coded affinity tag (ICAT) reagent to differentially label oxidized and reduced cysteines, which can then be detected with MS. By systematically labeling all reduced cysteines in the tissue sample first and then labeling the remaining cysteines that were in an oxidized state in the tissue. Using this method, we were able to detect and quantify many peptides that were oxidized in AFib patients compared to SR patients. We detected many peptides from different cardiac proteins to be highly oxidized. Additionally, we found significantly high level of many markers of inflammation (e.g. ICAM, VCAM, IL6, TNFα) and oxidative stress (H2O2, LPO, and 3-Nitrotyrosin2) in AFib patients using Western blot and ELISA. Fractionations from mitochondria and cytosol showed a significant high H2O2 and LPO with reduced GSH levels, this was accompanied by many mitochondrial peptides that were significantly oxidized. In addition to a significant elevated expression and activity of NADPH oxidase 2 (Nox2) and Nox4 in AFib patients compared to SR patients. Enhanced inflammasome and oxidized cardiomyocytes was associated with upregulation of Neuropilin-1 (NRP-1) by IL-6 in AFib patients compared to SR patients. Neutrophils are among the cells recruited to release proteolytic enzymes, including neutrophil elastase. The activity of neutrophil elastase was increased in in AFib patients compared to SR patients, this was accompanied by increased proteolytic activity and apoptosis of cardiomyocyte in Afib patients compared to SR patients.

Conclusion: Our study suggests a heavily inflamed and oxidized microenvironment in the AFib patients compared to SR patients that has a significant impact on cardiac protein oxidation and contributes thereby to the cardiomyocyte function modulation.