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RATIONALE: Up to 10% of patients with systemic sclerosis (SSc) develop pulmonary arterial hypertension (PAH). This risk persists throughout the disease and is time dependent, suggesting that SSc is a susceptibility factor. Outcome for SSc-PAH is poor compared with heritable or idiopathic forms, despite clinical and pathological similarities. Although susceptibility in heritable PAH and idiopathic PAH is strongly associated with gene mutations leading to reduced expression of bone morphogenetic protein receptor (BMPR) II, these mutations have not been observed in SSc-PAH. OBJECTIVES: To explore BMPRII expression and function in a mouse model of SSc (T[beta]RII[Delta]k-fib) that is susceptible to developing pulmonary hypertension and in SSc lung. METHODS: BMPRII and downstream signaling pathways were profiled in lung tissue and fibroblasts from the T[beta]RII[Delta]k-fib model, which develops pulmonary vasculopathy with pulmonary hypertension that is exacerbated by SU5416. Complementary studies examined SSc or control lung tissue and fibroblasts. MEASUREMENTS AND MAIN RESULTS: Our study shows reduced BMPRII, impaired signaling, and altered receptor turnover activity in a transforming growth factor (TGF)-[beta]-dependent mouse model of SSc-PAH. Similarly, a significant reduction in BMPRII expression is observed in SSc lung tissue and fibroblasts. Increased proteasomal degradation of BMPRII appears to underlie this and may result from heightened TGF-[beta] activity. CONCLUSIONS: We found reduced BMPRII protein in patients with SSc-PAH and a relevant mouse model associated with increased proteasomal degradation of BMPRII. Collectively, these results suggest that impaired BMP signaling, resulting from TGF-[beta]-dependent increased receptor degradation, may promote PAH susceptibility in SSc and provide a unifying mechanism across different forms of PAH. [ABSTRACT FROM AUTHOR]