MMP1 and pulmonary arterial hypertension: There was no difference in MMP-1 expression in PASMCs between controls and PAH patients 9; however, cells co-cultured with monocyte-derived macrophages exhibited an increase in MMP-1 expression, suggesting that infiltrated macrophages may play a role in the regulation of MMPs and that the additional effects of MMPs on PASMCs may be important in the early stages of vascular remodeling.