Tachycardia of atrial myocytes induces collagen expression in atrial fibroblasts through transforming growth factor β1

Aims We investigated the molecular mechanism of rapid-depolarization-induced atrial fibrosis. Methods and results We used a direct atrial myocyte fibroblast contact co-culture and a fibroblast-specific transforming growth factor beta 1 (TGF-beta 1), connective tissue growth factor (CTGF) and procollagen type I alpha-1 (COL1A1) luciferase reporter system to investigate the possible molecular mechanism of rapid-depolarization-induced atrial fibrosis. Mouse atrial fibroblasts were first transfected with promoter-luciferase reporters, and then co-cultured with HL-1 atrial myocytes. Rapid depolarization of atrial myocytes by rapid electrical field stimulation induced increased TGF-beta 1, CTGF and COL1A1 promoter activities in the co-cultured atrial fibroblasts (2.4 +/- 0.3-fold increase, P = 0.008 for TGF-beta 1; 2.9 +/- 0.4-fold increase, P < 0.001 for CTGF; and 2.1 +/- 0.2-fold increase, P 0.008 for COL1A1). Rapid depolarization of atrial myocytes increased paracrine secretion of angiotensin II (Ang II) and reactive oxygen species in the co-culture medium. Rapid electrical field stimulation-induced ROS generation in atrial myocytes was attenuated by the membrane NADPH oxidase inhibitor, apocynin. Atrial myocyte-induced expression of TGF-beta 1, CTGF and COL1A1 in atrial fibroblasts was attenuated by co-treatment with the Ang II receptor blocker, losartan, and apocynin. Atrial myocyte- induced COL1A1 expression in atrial fibroblasts was attenuated by anti-TGF-beta 1 antibody and RNA interference knockdown of the TGF-beta 1 receptor. Conclusion We first demonstrated that tachycardia of atrial myocytes induced paracrine secretion of Ang II and reactive oxygen species, which in turn induced expression of CTGF and procollagen in co-cultured atrial fibroblasts through increasing TGF-beta 1 expression. The results may imply that use of an Ang II receptor blocker, in combination with an antioxidant, blocks rapid-depolarization-induced atrial fibrosis.