Monoamine Oxidase B (MAO-B) as an Inducer of Mitochondrial Reactive Oxygen Species (ROS) Production and Myofibroblast Differentiation in Cardiac Fibroblasts of Mice.

MAO-B-specific inhibition, either in knockout (KO) mice or pharmacologically, preserves left ventricular function and reduces cardiac fibrosis after myocardial infarction or pressure overload. We investigated whether stimulation of MAO-B in cardiac fibroblasts provokes ROS production and myofibroblast development. Fibroblast-specific MAO-B knockdown (KD) mice were created by crossing Col1a2CreERT mice with MAO-Bfl/fl mice. The KD was induced by tamoxifen injection. Fibroblasts of KD mice and wild types (WTs) were isolated and reduced MAO-B expression in KD fibroblasts was confirmed. In isolated mitochondria from the left ventricle of these mice, ROS production was reduced under stimulation with the specific MAO-B substrate β-phenylethylamine (PEA). Mitochondrial ROS production in fibroblasts, detected by MitoSox Red staining, increased under PEA (1000 µM) stimulation only in WT fibroblasts. mRNA of the marker genes for myofibroblast differentiation, Col1a1 and periostin, increased 2- or 3-fold, respectively, in WT but not in MAO-B KD fibroblasts. The enhanced migration potential under PEA was reduced in MAO-B KD fibroblasts. In conclusion, stimulation of MAO-B in cardiac fibroblasts leads to the formation of mitochondrial ROS, enhancement of myofibroblast marker gene expression and migration of the cells. Excessive fibrosis caused by elevated MAO-B activity in myocardial infarction can therefore contribute to cardiac dysfunction.