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Atrial fibrosis, the hallmark of structural remodeling in atrial fibrillation (AF), is characterized by abnormal proliferation of fibroblasts and excessive deposition of extracellular matrix. Transforming growth factor-β1 (TGF-β1)/activin receptor-like kinase 5 (ALK5)/Smad2/3/4 pathway has been reported to be involved in the process. Recent studies implicated activin A and its specific receptor activin receptor-like kinase 4 (ALK4) in stimulating fibrosis in non-cardiac organs. We recently reported that ALK4 is upregulated in pressure-overloaded heart and ALK4 haplodeficiency attenuated the pressure overload-induced cardiac fibrosis. However, the role of activin A/ALK4/Smad2/3 in the pathogenesis of atrial fibrosis and AF remains unknown. In this study, we aim to investigate the role of activin A/ALK4 signaling pathway in atrial fibrosis and AF.
Wild type (WT) littermates and ALK4+/- mice were subcutaneously infused with angiotensin-II (Ang-II) (750ng/kg/min) to mimick atrial fibrosis progression. At the 28th day post Ang-II infusion, echocardiography was performed with ultrasound instrument and inducibility of AF was tested. The atrium was either fixed in formalin for histological staining, or weighted and snap-frozen for quantitative real-time PCR analysis and Western Blotting analysis. Atrial fibroblasts isolated from WT littermates and ALK4+/- mice were stimulated by Ang-II to test relative gene expressions.
Our study provided experimental and clinical evidence for the involvement of activin A and ALK4 in the pathophysiology of atrial fibrosis and AF. Patients with AF had higher activin A and ALK4 expression in atrium as compared to individuals devoid of AF. After Ang-II infusion, ALK4-deficient mice showed lower expression of ALK4 in atrium, reduced activation of atrial fibroblasts, blunted atrial enlargement and atrial fibrosis, and further reduced the AF vulnerability upon right atrial electrophysiological studies as compared to wild type littermates. Moreover, we found activin A was induced by Ang-IIand then activated the ALK4/Smad2/3 pathway. Ang-II-mediated activation of activin A/ALK4 pathway could be inhibited by losartan, follistatin, ALK4 haplodeficiency or ALK4 inhibitor (SB431542).
Our results demonstrate that apart from the well-known TGF-b1/ALK5 pathway, the activation of activin A/ALK4/smad2/3 pathway played an important role in the pathogenesis of Ang II-mediated atrial fibrosis and inducibility of AF. Downregulation of ALK4 retards above pathological process by preventing the activation of atrial fibroblasts, and might be a potential therapeutic target for AF.