Postoperative atrial fibrillation is a frequent complication in cardiac surgery. The aberrant activation of signal transducer and activator of transcription 3 (STAT3) contributes to the pathogenesis of atrial fibrillation. MicroRNA-21 (miR-21) promotes atrial fibrosis. Recent studies support the existence of reciprocal regulation between STAT3 and miR-21. Here, we test the hypothesis that these 2 molecules might form a feedback loop that contributes to postoperative atrial fibrillation by promoting atrial fibrosis.

A sterile pericarditis model was created using atrial surfaces dusted with sterile talcum powder in rats. The inflammatory cytokines interleukin (IL)-1β, IL-6, transforming growth factor-β, and tumor necrosis factor-α, along with STAT3 and miR-21, were highly upregulated in sterile pericarditis rats. The inhibition of STAT3 by S3I-201 resulted in miR-21 downregulation, which ameliorated atrial fibrosis and decreased the expression of the fibrosis-related genes, α-smooth muscle actin, collagen-1, and collagen-3; reduced the inhomogeneity of atrial conduction; and attenuated atrial fibrillation vulnerability. Meanwhile, treatment with antagomir-21 decreased STAT3 phosphorylation, alleviated atrial remodeling, abrogated sterile pericarditis–induced inhomogeneous conduction, and prevented atrial fibrillation promotion. The culturing of cardiac fibroblasts with IL-6 resulted in progressively augmented STAT3 phosphorylation and miR-21 levels. S3I-201 blocked IL-6 induced the expression of miR-21 and fibrosis-related genes in addition to cardiac fibroblast proliferation. Transfected antagomir-21 decreased the IL-6–induced cardiac fibroblast activation and STAT3 phosphorylation. The overexpression of miR-21 in cardiac fibroblasts caused the upregulation of STAT3 phosphorylation, enhanced fibrosis-related genes, and increased cell numbers.

Our results have uncovered a novel reciprocal loop between STAT3 and miR-21 that is activated after heart surgery and can contribute to atrial fibrillation.