Background/Purpose:

Pulmonary arterial hypertension associated is an important complication of scleroderma (PAH-SSc) and has a poor prognosis compared to idiopathic (iPAH) or heritable (hPAH) forms of the disease. BMPRII mutations with concomitant effects on BMP signalling are an established cause of hPAH and some iPAH but these are not present in PAH-SSc. Furthermore, alterations in signalling through other TGFβ superfamily pathways have reciprocal effects on the BMP pathway.

Methods:

We investigated BMP signalling in the lung in a mouse model of SSc in which TGFβ signalling is upregulated. Experiments were performed on whole lung isolates and explant cultured fibroblasts (n=6) from the TβRIIΔk-fib mouse and compared with wild type (WT) controls. Structural and biochemical analysis of components of the TGFβ superfamily and downstream signalling pathway was investigated by Western blot, immunohistochemistry and confirmed using qPCR measurement. Migration assays investigated the effects of PDGF-BB on lung fibroblasts from TβRIIΔk-fib and WT controls (n=3). Confirmatory biochemical and functional studies on scleroderma fibroblasts were also performed.

Results:

The TβRIIΔk-fib model has increased levels of pSmad 2/3, indicative of enhanced TGFβ signalling. Consistent with an imbalance in the TGFβ/BMP axis we observed a significant reduction in BMPRII protein expression in the TβRIIΔk-fib model, both in whole lung isolates (1.43, 0.38) (p<0.05), and explant cultured fibroblasts (0.2985, 0.09) (p<0.05) (figure1). Explant cultured fibroblasts exhibited a blunted induction of phospho-Smad1 in response to BMP ligands (p<0.05) (figure1). TβRIIΔk-fib lung fibroblasts also exhibited enhanced migratory response compared to wild type controls (p<0.05). These findings were confirmed in scleroderma fibroblast studies, highlighting a reduction in BMPRII levels and an increased migratory response compared to healthy controls (p<0.05).

Conclusion:

Association of altered BMPRII expression with PAH is well established in hPAH. Here we demonstrate the TβRIIΔk-fib model of SSc that develops a constitutive pulmonary vasculopathy, exhibits reduced expression of BMPRII as a reciprocal response to increased TGFβ signalling, with associated downstream signalling alterations independent of mutations in the BMPRII. Collectively our data suggests loss of BMPRII expression by non-genetic means may contribute to the development of PH in the TβRIIΔk-fib model of SSc.