Mutations in the bone morphogenetic protein type-II receptor (BMPR-II) underlie ∼70% of heritable pulmonary arterial hypertension (PAH) cases. However, the low penetrance in mutation carriers implies a ‘second hit’ is required for disease onset. A possible trigger is inflammation. Circulating tumour necrosis factor alpha (TNFα) is raised in PAH patients and TNFα can alter smooth muscle cell proliferation. Thus, we examined the effect of TNFα on BMP signalling.
Human pulmonary arterial smooth muscle cells (PASMCs) were derived from patients with BMPR-II mutations and from disease-free controls. Quantitative polymerase chain reaction (qPCR) and western blotting revealed that 24 hours of TNFα stimulation reduced the expression of BMP2 and BMPR-II but increased BMP6 expression. TNFa reduced BMP2-mediated signalling, as expected. In contrast, BMP6 treatment in the presence of TNFα increased smad1/5 activation and upregulated ID1 expression, implying that BMP6 can signal independently of BMPR-II Furthermore, BMPR-II mutant PASMCs proliferated in response to BMP6 and TNFα. Quantitative PCR revealed increased ActR-IIa expression following TNFα stimulation and siRNA knockdown of ActR-IIa abrogated this enhanced proliferative response.
In summary, TNFα increases BMP6 expression, which in the absence of functional BMPR-II, can bind to receptor complexes involving ActR-IIa to accelerate PASMC proliferation. The potential contribution of this novel mechanism to vascular remodelling in the systemic and pulmonary circulation requires further study.