Mesenchymal stem cells reduce hypoxia-induced apoptosis in alveolar epithelial cells by modulating hypoxic signaling

Abstract

Acute or chronic alveolar injuries provoke massive apoptosis of alveolar epithelial cells (AEC) that compromises an efficient repair of the alveolar epithelium and leads to lung diseases such as ARDS or IPF. These disorders are commonly associated with local alveolar hypoxia aggravating their progression through the stimulation of AEC apoptosis. Administration of allogenic mesenchymal stem cells (MSC) has been shown to limit lung inflammation and fibrosis in murine models of alveolar injury, through a still poorly understood paracrine mechanism. We studied whether MSC could protect AEC from hypoxia-induced apoptosis and the mechanisms involved. Rat primary AEC were exposed for 4-48 hrs to normoxia or hypoxia (1.5% O 2 ) in the presence or absence of human MSC conditioned medium (hMSC-CM). AEC apoptosis and reactive oxygen species (ROS) accumulation were assessed by flow cytometry. Expression of hypoxia-inducible transcription factors (HIF) and their targets were studied by immunoblotting and qPCR. hMSC-CM significantly reduced hypoxia-induced apoptosis of AEC. Such a anti-apoptotic effect was also obtained with ROS scavenger N-acetylcystein or HIF1a inhibitor YC-1. hMSC-CM decreased the protein expression of HIF1α and HIF2α and of their pro-apoptotic target Bnip3 in hypoxic AEC. hMSC-CM also reduced ROS accumulation in hypoxic AEC by enhancing the activity of anti-oxidant enzymes and prevented the induction of CHOP, a pro-apoptotic factor induced by ROS signaling. The paracrine effect of hMSC was partly dependent on KGF and HGF secretion. hMSC prevent via a paracrine effect hypoxia-induced apoptosis of AEC by modulating hypoxic and ROS signaling.