Inducibility, but not stability, of atrial fibrillation is increased by NOX2 overexpression in mice.
Mighiu AS., Recalde A., Ziberna K., Carnicer R., Tomek J., Bub G., Brewer AC., Verheule S., Shah AM., Simon JN., Casadei B.
AIMS: Gp91-containing NADPH oxidases (NOX2) are a significant source of myocardial superoxide production. An increase in NOX2 activity accompanies atrial fibrillation (AF) induction and electrical remodelling in animal models and predicts incident AF in humans; however, a direct causal role for NOX2 in AF has not been demonstrated. Accordingly, we investigated whether myocardial NOX2 overexpression in mice (NOX2-Tg) is sufficient to generate a favourable substrate for AF and further assessed the effects of atorvastatin, an inhibitor of NOX2, on atrial superoxide production and AF susceptibility. METHODS AND RESULTS: NOX2-Tg mice showed a 2-2.5-fold higher atrial protein content of NOX2 compared with wild-type (WT) controls, which was associated with a significant (2-fold) increase in NADPH-stimulated superoxide production (2-hydroxyethidium by HPLC) in left and right atrial tissue homogenates (P = 0.004 and P = 0.019, respectively). AF susceptibility assessed in vivo by transoesophageal atrial burst stimulation was modestly increased in NOX2-Tg compared with WT (probability of AF induction: 88% vs. 69%, respectively; P = 0.037), in the absence of significant alterations in AF duration, surface ECG parameters, and LV mass or function. Mechanistic studies did not support a role for NOX2 in promoting electrical or structural remodelling, as high-resolution optical mapping of atrial tissues showed no differences in action potential duration and conduction velocity between genotypes. In addition, we did not observe any genotype difference in markers of fibrosis and inflammation, including atrial collagen content and Col1a1, Il-1β, Il-6, and Mcp-1 mRNA. Similarly, NOX2 overexpression did not have consistent effects on RyR2 Ca2+ leak nor did it affect PKA or CaMKII-mediated RyR2 phosphorylation. Finally, treatment with atorvastatin significantly inhibited atrial superoxide production in NOX2-Tg but had no effect on AF induction in either genotype. CONCLUSIONS: Together, these data indicate that whilst atrial NOX2 overexpression may contribute to atrial arrhythmogenesis, NOX2-derived superoxide production does not affect the electrical and structural properties of the atrial myocardium. TRANSLATIONAL PERSPECTIVE: Atrial NOX2-derived superoxide is an independent predictor of postoperative AF in humans and contributes to atrial oxidative stress early after AF induction. Whilst experimental models have reported an association of NOX2 with AF-induced remodelling, a causal link between NOX2 activity and AF remains to be established. Here we show that overexpression of the human NOX2 gene in mice resulted in a 2-fold higher atrial superoxide production and a modest increase in AF susceptibility, independent of atrial electrical or structural remodelling. Short-term treatment with atorvastatin normalized atrial superoxide in NOX2-Tg mice without affecting AF susceptibility. Together these findings indicate that atrial NOX2-derived superoxide is more likely a biomarker of AF risk than a primary driver of AF development, and that NOX2 inhibition is unlikely to prevent the new-onset of AF.