Activation of transforming growth factor β (TGF-β) combined with persistent hypoxia often affects the tumor microenvironment. Disruption of cadherin/catenin complexes induced by these stimulations yields aberrant extracellular matrix (ECM) production, characteristics of epithelial-mesenchymal transition (EMT). Hypoxia-inducible factors (HIF), the hallmark of the response to hypoxia, play differential roles during development of diseases. Recent studies show that localization of cadherin/catenin complexes at the cell membrane might be tightly regulated by protein phosphatase activity. We aimed to investigate the role of stabilized HIF-1α expression by protein phosphatase activity on dissociation of the E-cadherin/β-catenin complex and aberrant ECM expression in lung cancer cells under stimulation by TGF-β. By using lung cancer cells treated with HIF-1α stabilizers or carrying doxycycline-dependent HIF-1α deletion or point mutants, we investigated the role of stabilized HIF-1α expression on TGF-β-induced EMT in lung cancer cells. Furthermore, the underlying mechanisms were determined by inhibition of protein phosphatase activity. Persistent stimulation by TGF-β and hypoxia induced EMT phenotypes in H358 cells in which stabilized HIF-1α expression was inhibited. Stabilized HIF-1α protein expression inhibited the TGF-β-stimulated appearance of EMT phenotypes across cell types and species, independent of de novo vascular endothelial growth factor A (VEGFA) expression. Inhibition of protein phosphatase 2A activity abrogated the HIF-1α-induced repression of the TGF-β-stimulated appearance of EMT phenotypes. This is the first study to show a direct role of stabilized HIF-1α expression on inhibition of TGF-β-induced EMT phenotypes in lung cancer cells, in part, through protein phosphatase activity.
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