Introduction Although the presence of inflammation is well recognised in pulmonary arterial hypertension (PAH), the underlying mechanisms remain poorly understood. Mutations in the bone morphogenetic protein type II receptor gene, BMPR2, underlie the majority of heritable and a significant fraction of idiopathic PAH Here, we sought to determine whether loss of BMPR2 was directly associated with inflammation, the underlying mechanisms, and possible therapeutic interventions.
Methods The initial approach employed human and mouse pulmonary arterial smooth muscle cells (PASMCs) from controls and subjects with a mutation in BMPR2. Cells were studied at baseline and in response to lipopolysaccharide (LPS), in the presence or absence of pharmacological inhibitors of known pro-inflammatory pathways. Cytokines were measured by ELISA Reactive oxygen species were measured using dihydroethidium staining. For in vivo studies, we employed the heterozygous null bmpr2 mouse (bmpr2+/–) and wild-type littermates.
Results Chronic low dose LPS resulted in a marked elevation of right ventricular systolic pressure to ∼40 mmHg in bmpr2+/– mice, but not in wild-type mice (Fig.1). In vitro studies showed that mouse bmpr2+/– and human BMPR2mut PASMCs produced more IL-6 and IL-8 at baseline and after stimulation with LPS For example, bmpr2+/– PASMCs produced 1533±280 pg/ml of IL-6 (versus 341±10 pg/ml in controls, P<0.01) after LPS stimulation. Reduced BMPR2 was also associated with increased phospho-STAT3 levels and an abnormal pro-proliferative response to IL-6. Acute administration of LPS to the bmpr2+/– mouse reproduced these results in vivo, with increased lung and serum IL-6 and IL-8. The link between reduced BMPR2 and increased cytokine expression was found to be associated with increased superoxide generation and loss of extracellular superoxide dismutase (SOD3). Treatment of BMPR2 deficient PASMCs with the superoxide dismutase mimetic, Tempol, normalised the exaggerated secretion of IL-6 and IL-8. Co-administration of Tempol prevented the PAH phenotype in bmpr2+/– mice exposed to low dose LPS (Fig. 1).
Conclusions We have determined a novel link between BMPR2 loss and increased expression of pro-inflammatory cytokines, which is dependent on mishandling of superoxide. Our results suggest that pro-inflammatory stimuli may represent disease-provoking triggers for patients with BMPR2 deficiency. Promotion of SOD3 activity may represent a novel therapeutic approach in PAH.