HDAC1 and pulmonary arterial hypertension: In monocrotaline (MCT)‐induced PAH rats, the upregulation of HDAC1 caused an increase in matrix metalloproteinase‐9 (MMP‐9) and tissue inhibitor of metalloproteinase‐1 (TIMP‐1), and MMP‐2 and TIMP‐2 ratios by inhibiting miR‐34a levels [115].