Ability of Hyperoxia to Activate the Unfolded Protein Response (UPR) in the Newborn Lung

Abstract

Hyperoxia generates reactive species which have the potential to interfere with protein folding in pulmonary secretory cells. Given that hyperoxia is known to suppress protein synthesis and increase eIF2α phosphorylation (p‐eIF2α) in cultured lungs cells, we hypothesized that hyperoxia would activate the UPR in the lung. To test our hypothesis, we studied each of the 3 UPR sensors, PERK, ATF6, and IRE1α/XPB1 in the lungs of 4‐day‐old newborn rats exposed to 95%O2 (Ox) for 72 hrs. We found that exposure inhibited pulmonary mRNA translation. Immunoblot analysis of whole lung lysates found that hyperoxia increased p‐eIF2α without activating PERK and failed to alter ATF6 protein expression despite decreasing the expression of BiP protein. We did find that Ox enhanced IRE1α endonuclease activity as evidenced by increased XBP1 splicing. Fractionation indicated that Ox induced the nuclear accumulation of CHOP/GADD153 and Nrf2, downstream effectors of ATF4 and ATF6 activation. Immunofluorescent microscopy localized enhanced p‐eIF2α to airway epithelial and parenchymal cells while electron microscopy denoted engorged and disordered endoplasmic reticulum within alveolar interstial cells. These findings suggest that hyperoxia does induce global UPR activation within the lung, but that specific cell types undergo either partial UPR activation or UPR‐independent activation of select UPR intermediates.