Transforming growth factor-β1 protects against pulmonary artery endothelial cell apoptosis via ALK5

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Abstract
Transforming growth factor (TGF)-β1 has been reported to cause endothelial cell apoptosis. However, conflicting data have also demonstrated that TGF-β1 promotes endothelial cell survival. In this study, the effect of TGF-β1 on apoptosis of cultured bovine pulmonary artery endothelial cells (PAEC) induced by multiple stimuli was investigated. TGF-β1 protected against apoptosis of bovine PAEC induced by serum deprivation or the VEGF receptor inhibitor SU-5416, but not by UV light exposure or TNFα. Neither caspase-8 nor caspase-12 was activated by serum deprivation or the VEGF receptor blocker. However, blockade of VEGF receptors activated caspase-9, an effect that was abolished by TGF-β1. Furthermore, serum deprivation and inhibition of VEGF receptors significantly decreased the protein level of Bcl-2, an effect that was also abrogated by TGF-β1. In addition, the baseline level of Bcl-2 was enhanced by TGF-β1 and reduced by inhibition of activin receptor-like kinase 5 (ALK5), a TGF-β1 type I receptor. Furthermore, inhibition of ALK5 caused apoptosis of bovine PAEC. These results suggest that TGF-β1 signaling is critical for maintenance of bovine PAEC survival. Finally, the protective effects of TGF-β1 on bovine PAEC apoptosis and Bcl-2 reduction were abolished by ALK5 inhibition, but not by inhibition of non-SMAD signaling pathways. Also, TGF-β1 activated SMAD2 and SMAD1/5, an effect that was abolished by ALK5 inhibition. The results of this study suggest that TGF-β1 protects against bovine PAEC apoptosis, possibly through ALK5-mediated Bcl-2 induction and subsequent inhibition of the mitochondria-mediated intrinsic pathway of apoptosis. Understanding the mechanism by which TGF-β1 promotes endothelial cell survival may provide a better treatment for apoptosis-dependent vascular diseases, such as emphysema.

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