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Smurf1 ubiquitin ligase causes downregulation of BMP receptors and is induced in monocrotaline and hypoxia models of pulmonary arterial hypertension

Reduced bone morphogenetic protein (BMP) receptor (BMPR) expression and BMP signaling have been implicated in vascular cell proliferation and remodeling associated with pulmonary arterial hypertension (PAH). The low penetrance of the BMPR II disease gene in familial PAH suggests that additional genetic or environmental factors are involved in clinical manifestation of PAH. Smurf1 ubiquitin ligase, together with inhibitory SMAD 6/7, forms a negative feedback loop for the attenuation of BMP signals by downregulating BMPR and signaling molecules and, in addition, functions in the integration of MAPK/Ras mitogenic pathways. The present study found that Smurf1 was significantly elevated in pulmonary arteries of monocrotaline and hypoxia-induced PAH rats. In the pulmonary artery of hypoxia-exposed mice, elevation of Smurf1 and SMAD7 was correlated with reduced expression of BMPR II protein. Over-expression of Smurf1 in cultured cells induced ubiquitination and degradation of BMPR I and II whereas ligase-inactive Smurf1 reduced ubiquitination and elevated their protein levels, thus serving a dominant-negative function. Smurf1-induced receptor degradation was inhibited by both proteasomal and lysosomal inhibitors. Thus, Smurf1 reduces steady-state levels of BMPRs by ubiquitination and subsequent degradation involving proteasomes and lysosomes. Therefore, these results show that Smurf1 induction could be a key event for triggering downregulation of BMP signaling and causing vascular cell proliferation and remodeling in PAH and that abrogating Smurf1 function could be a strategy for PAH therapeutics.