NOX1 and pulmonary arterial hypertension: Basal MMP9 transcript was elevated in PAH-hPASMCs, and serotonin further stimulated MMP9 in both control and PAH-hPASMCs, effects attenuated in the presence of 5-HT1BR antagonist, SB224289, and Nox1 inhibitor, ML171 (Figure 4D).