SMAD3 and atrial fibrillation: The results showed that (1) the left atrial tissues of older AF patients, aging mouse, and senescence human atrial fibroblasts had more severe atrial fibrosis and higher protein expression levels of p300, p53/acetylated p53 (ac‐p53)/p21, Smad3/p‐Smads, and fibrosis‐related factors.