MMP9 and pulmonary arterial hypertension: Such that, expression of MMP2 is mainly determined by the ubiquitous Sp1 family of transcription factors, where expression of MMP2 in the main part is constitutive, with only modest sensitivity to induction by growth factors or cytokines.42 This is in line with our data showing that MMP2 transcript is induced by serotonin solely in PAH-hPASMCs, whereas MMP9 mRNA expression is sensitive to serotonin stimulation in both control and PAH-hPASMCs.