BMP9 Modulates IL-33 Signaling to Mitigate EndMT in Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a progressive disorder involving disrupted BMP (bone morphogenetic protein) signaling, pulmonary inflammation, and endothelial-to-mesenchymal transition (EndMT). We hypothesized that IL (interleukin)-33 signaling contributes to PAH progression by inducing EndMT and interacting with BMP9, a key modulator of inflammation and vascular remodeling.

IL-33 expression was assessed in lung tissues from Sugen/hypoxia and control mice, as well as in pulmonary arterial endothelial cells (PAECs) and lung tissues from patients with PAH and healthy donors. EndMT and signaling pathways were analyzed in PAECs and microvascular endothelial cells (MVECs) exposed to IL-33, BMP9, and sST2 (soluble supression of tumorigenicity 2) using quantitative polymerase chain reaction, Western blotting, ELISA, and immunostaining. Plasma BMP9 and sST2 levels were quantified in patients with PAH.

Immunofluorescent analysis revealed elevated IL-33 expression in pulmonary endothelial cells of Sugen/hypoxia mice compared with controls, consistent with findings in PAECs from patients with PAH. BMP9 significantly upregulated sST2 expression in human PAEC and microvascular endothelial cells, inhibited IL-33 target gene expression, and effectively suppressed IL-33-induced EndMT. Notably, BMP9 demonstrated greater efficacy in preventing EndMT compared with rsST2 (recombinant soluble ST2) or ST2L-neutralizing antibodies. Circulating BMP9 and sST2 levels in the plasma of patients with PAH were positively correlated in specific patient groups stratified by sex, age, and New York Heart Association functional class, suggesting a protective role of BMP9 in modulating IL-33-induced EndMT.

BMP9 plays a protective role against IL-33-induced EndMT in PAECs by upregulating sST2 expression and neutralizing IL-33, suggesting that targeting the IL-33 signaling pathway may represent a promising therapeutic strategy to mitigate EndMT in PAH.