In the lung of PAH patients, accelerated turnover of the extracellular matrix because of increased MMP activity occurs.23 Loss of balance between MMPs and TIMPs initiates extracellular matrix and vascular remodeling and is involved in a variety of pathologies.43 Serotonin increased expression of the profibrotic signaling proteins MMP2 and MMP9 in hPASMCs. This evidence concerns the gene MMP9 and pulmonary arterial hypertension.