Endoplasmic Reticulum Stress Is a Danger Signal Boosting Inflammatory Responses in Bronchial Epithelial Cells

Introduction: Endoplasmic reticulum (ER) stress has been associated with proinflammatory signaling and is implicated in several pulmonary pathologies, including cystic fibrosis and other chronic obstructive pulmonary diseases. ER stress activates the unfolded protein response (UPR) in order to restore cell homeostasis. As UPR cascades seem to intersect with immune functions, we hypothesized that UPR signalling might increase inflammatory reactions of Toll-like receptors (TLR) upon microbial stimulation within airway epithelial cells.

Methods: Airway epithelial cells were treated with thapsigargin to induce UPR and subsequently stimulated with TLR agonists. Secreted cytokines and their relative mRNA expression levels were determined by ELISA and qRT-PCR, respectively. Silencing of the UPR branches was done using small interfering RNAs. MAPKs activation was investigated by Western blotting.

Results: ER stress increased reactivity of BEAS-2B and human primary bronchial epithelial cells to microbial stimulation with respect to IL6 and IL8 production. UPR induction resulted in an increased p38 and ERK activity by LPS and polyI:C, yet NFκB activation was not affected. Notably, pharmacological inhibition of p38 and ERK MAPKs could inhibit the observed boosting effect. Knockdown of UPR branches revealed that mainly PERK, and to some extent ATF6, mediated p38 and ERK MAPKs activation leading to synergistic proinflammatory activity with TLR stimulation in airway epithelial cells.

Discussion: We demonstrate that combined activation of ER stress and TLR signaling leads to synergistic proinflammatory activity of bronchial epithelial cells. We suggest that ER stress, present in various chronic pulmonary diseases, acts as a costimulatory danger signal activating MAPKs in airway epithelial cells. MAPKs activation is the crucial trigger for full-blown epithelial cell activation upon TLR stimulation. ER stress might contribute to microbial sensitization and infectious exacerbation of chronic airway diseases.