MMP9 and Myocardial fibrosis: HIF-1α can increase the expression of matrix metalloproteinases (MMPs) and transforming growth factor (TGF)-β in AF through promoting atrial fibrosis (Ogi et al., 2010), and inhibiting the expression of HIF-1α can decrease the levels of TGF-β and MMP-9 accompanied by the less myocardial fibrosis in rabbit models (Su et al., 2014).