HIF-1α signaling is augmented during intermittent hypoxia by induction of the Nrf2 pathway in NOX1-expressing adenocarcinoma A549 cells

Fluctuations in cellular oxygenation causing intermittent hypoxia and oxidative stress affect the regulation of hypoxia-inducible factor (HIF-1) and the nuclear factor erythroid 2-related factor 2 (Nrf2). HIF-1 is primarily induced in hypoxia, whereas Nrf2 is induced in response to oxidative stress. Whereas HIF-1 regulates the expression of genes important for the adaptation of cells to hypoxia, Nrf2 induces antioxidative enzymes such as thioredoxin 1 (Trx1), exerting a cytoprotective role. Here, we investigated the regulation and cross talk of HIF-1α and Nrf2 in intermittent hypoxia in lung adenocarcinoma A549 cells expressing high levels of the NADPH oxidase subunit NOX1. Whereas continuous hypoxia induced only HIF-1α, intermittent hypoxia induced both HIF-1α and Nrf2, including its target Trx1. NOX1 was determined to be crucial for enhanced ROS production in intermittent hypoxia that in turn mediated induction of Nrf2 and Trx1. The regulation of Nrf2 and Trx1 by NOX1 was confirmed by both inhibition of endogenous NOX1 and overexpression of recombinant NOX1 protein. By using a proteasomal inhibitor, NOX1 was demonstrated to activate Nrf2 by protein stabilization. Subsequently, Nrf2-dependent Trx1 induction turned out to enhance HIF-1α signaling in intermittent hypoxia.