Hypoxia-induced alveolar epithelial-mesenchymal transition requires mitochondrial ROS and hypoxia-inducible factor 1

Zhou G, Dada LA, Wu M, Kelly A, Trejo H, Zhou Q, Varga J, Sznajder JI. Hypoxia-induced alveolar epithelial-mesenchymal transition requires mitochondrial ROS and hypoxia-inducible factor 1. Am J Physiol Lung Cell Mol Physiol 297: L1120-L1130, 2009. First published October 2, 2009; doi:10.1152/ajplung.00007.2009.-Patients with acute lung injury develop hypoxia, which may lead to lung dysfunction and aberrant tissue repair. Recent studies have suggested that epithelial-mesenchymal transition (EMT) contributes to pulmonary fibrosis. We sought to determine whether hypoxia induces EMT in alveolar epithelial cells (AEC). We found that hypoxia induced the expression of alpha-smooth muscle actin (alpha-SMA) and vimentin and decreased the expression of E-cadherin in transformed and primary human, rat, and mouse AEC, suggesting that hypoxia induces EMT in AEC. Both severe hypoxia and moderate hypoxia induced EMT. The reactive oxygen species ( ROS) scavenger Euk-134 prevented hypoxia-induced EMT. Moreover, hypoxia-induced expression of alpha-SMA and vimentin was prevented in mitochondria-deficient rho(0) cells, which are incapable of ROS production during hypoxia. CoCl2 and dimethyloxaloylglycine, two compounds that stabilize hypoxia-inducible factor (HIF)-alpha under normoxia, failed to induce alpha-SMA expression in AEC. Furthermore, overexpression of constitutively active HIF-1 alpha did not induce alpha-SMA. However, loss of HIF-1 alpha or HIF-2 alpha abolished induction of alpha-SMA mRNA during hypoxia. Hypoxia increased the levels of transforming growth factor (TGF)-beta 1, and preincubation of AEC with SB431542, an inhibitor of the TGF-beta 1 type I receptor kinase, prevented the hypoxia-induced EMT, suggesting that the process was TGF-beta 1 dependent. Furthermore, both ROS and HIF-alpha were necessary for hypoxia-induced TGF-beta 1 upregulation. Accordingly, we have provided evidence that hypoxia induces EMT of AEC through mitochondrial ROS, HIF, and endogenous TGF-beta 1 signaling.