Mechanisms of suppression of alveolar epithelial cell GM-CSF expression in the setting of hyperoxic stress

Abstract

Pulmonary expression of granulocyte/macrophage colony-stimulating factor (GM-CSF) is critically important for normal functional maturation of alveolar macrophages. We found previously that lung GM-CSF is dramatically suppressed in mice exposed to hyperoxia. Alveolar epithelial cells (AEC) are a major source of GM-CSF in the peripheral lung, and in vivo hyperoxia resulted in greatly reduced expression of GM-CSF protein by AEC ex vivo. We now explore the mechanisms responsible for this effect, using primary cultures of murine AEC exposed to hyperoxia in vitro. Exposure of AEC to 80% oxygen/5% CO2 for 48 h did not induce overt toxicity, but resulted in significantly decreased GM-CSF protein and mRNA expression compared with cells in normoxia. Similar effects were seen when AEC were stressed with serum deprivation, an alternative inducer of oxidative stress. The effects in AEC were opposite those in a murine lung epithelial cell line (MLE-12 cells), in which hyperoxia induced GM-CSF expression. Both hyperoxia and serum deprivation resulted in increased intracellular reactive oxygen species (ROS) in AEC. Hyperoxia and serum deprivation induced significantly accelerated turnover of GM-CSF mRNA. Treatment of AEC with catalase during oxidative stress preserved GM-CSF protein and mRNA and was associated with stabilization of GM-CSF mRNA. We conclude that hyperoxia-induced suppression of AEC GM-CSF expression is a function of ROS-induced destabilization of GM-CSF mRNA. We speculate that AEC oxidative stress results in significantly impaired pulmonary innate immune defense due to effects on local GM-CSF expression in the lung. lung; innate immunity; growth factors; oxidative stress

Key Role of MicroRNA in the Regulation of Granulocyte Macrophage Colony-stimulating Factor Expression in Murine Alveolar Epithelial Cells during Oxidative Stress

“…Similarly, in primary AEC GM-CSF expression is greatly reduced during oxidative stress. Our recent studies have shown that this decreased expression is due to accelerated turnover of GM-CSF mRNA (9). In the present study we have provided new information concerning the mechanisms by which expression is suppressed in response to oxidative stress.…”

“…Regulation of AEC GM-CSF mRNA Turnover-Mechanistic studies have indicated that an increase in mRNA turnover is a key step in the suppression of GM-CSF expression during oxidative stress (9). We have found that that addition of recombinant GM-CSF protein during hyperoxia reversed this suppression ( Fig.…”

“…GM-CSF mRNA is constitutively expressed in the normal alveolar wall (8). GM-CSF expression is sensitive to oxidative stress and is suppressed by hyperoxia in AEC in vitro (9). Expression of GM-CSF mRNA in whole lung and in the alveolar wall is also suppressed by exposure of mice to hyperoxia (8).…”

“…We have shown previously that AEC expression of GM-CSF is subject to suppression by exposure to hyperoxia, with important in vivo consequences, including increased susceptibility to lethal pneumonia and AEC apoptosis (8). AEC expression of GM-CSF during oxidative stress in vitro is regulated by changes in turnover of GM-CSF mRNA (9). MicroRNAs (miRNA) are a recently recognized class of non-coding short RNAs, ϳ22 nucleotides in length, which regulate gene expression through effects on mRNA stability or translation.…”

Alveolar epithelial cells orchestrate DC function in murine viral pneumonia

“…We and others have previously demonstrated that AECs are a substantial source of GM-CSF in the inflamed distal lung (39,40), and GM-CSF was shown to be induced in a TLR-dependent way in alveolar epithelium following bacterial challenge in vitro (41). Nonetheless, leukocytes of lymphoid and myeloid lineages, especially resident macrophages, were also found to be sources of GM-CSF in inflamed tissues (42).…”