Lipopolysaccharide Induces Autophagic Cell Death through the PERK-Dependent Branch of the Unfolded Protein Response in Human Alveolar Epithelial A549 Cells

Abstract

Background: Alveolar epithelial cell death plays a critical role in the pathogenesis of lipopolysaccharide (LPS)-induced acute lung injury. Increased autophagy has a dual effect on cell survival. However, it is not known whether autophagy promotes death or survival in human alveolar epithelial cells exposed to LPS. Methods: Genetic and pharmacological approaches were used to evaluate the effect of autophagy on A549 cell viability upon LPS exposure. The endoplasmic reticulum (ER) stress and unfolded protein response (UPR) pathways were examined with immunoblotting studies to further explore underlying mechanisms. Results: Treatment with LPS (50 µg/ml) led to autophagy activation and decreased cell viability in A549 cells. Blocking autophagy via short interfering RNA or inhibitor significantly decreased, whereas rapamycin increased, the LPS-induced effect on viability. ER stress was activated in LPS-stimulated A549 cells, and ER stress inhibitor reduced LPS-induced autophagy. LPS activated only the PERK pathway and had rarely effect on the ATF6 and IRE1 branches of the UPR in A549 cells. Moreover, the knockdown of PERK and ATF4 attenuated LPS-induced autophagy and promoted cell survival. Conclusion: In human alveolar epithelial A549 cells, LPS induces autophagic cell death that depends on the activation of the PERK branch of the UPR upon ER stress.

Cytoprotective Effect of Heat Shock Protein 27 Against Lipopolysaccharide-Induced Apoptosis of Renal Epithelial HK-2 Cells

“…In response to LPS, tubular epithelial apoptosis is induced and caspase-3 is cleaved [28,29], whereas Hsp27 exerts an anti-apoptotic role by inhibition of caspase-dependent apoptosis [10]. In accordance with previous reports, we demonstrated that LPS decreased HK-2 cells viability and induced apoptosis.…”

Up-Regulated ATF4 Expression Increases Cell Sensitivity to Apoptosis in Response to Radiation

“…ATF4 induces the transcription of genes involved in redox protection (e.g., HMOX-1 and GCLC), amino acid synthesis (AsnS) and ER stress (e.g., REDD1). ATF4 also up-regulates GADD34, which is involved in apoptosis [9], and genes involved in cell fate decisions (e.g., GADD153/CHOP, ATF3, and CHAC1), which induce cell-cycle arrest and trigger apoptosis in cells exposed to intense chronic stress [10, 26-28]. ATF4 results in the initiation of the transcription of the apoptosis-associated CHOP gene and the autophagy-associated Map1lc3b, which selenite links ER stress to apoptosis and autophagy during treatment for leukemia [29].…”

“…ATF4 over-expressing cell lines were used to show that ATF4 increased drug resistance to cisplatin, doxorubicin, etoposide, SN-38 and vincristine [8]. Li et al reported that the knockdown of PERK and ATF4 attenuated LPS-induced autophagy and promoted cell survival [9]. ATF4 is the master coordinator of the integrated stress response (ISR), which is an adaptive pathway that is triggered by multiple stressors.…”

Endoplasmic Reticulum Stress Induces HRD1 to Protect Alveolar Type II Epithelial Cells from Apoptosis Induced by Cigarette Smoke Extract

“…UPR could activate downstream signaling pathways PERK, eIF2α and IRE1 to temperately repress protein synthesis, enhance protein folding capacity in ER and facilitate the degradation of unfolded protein to recover ER function. While ER stress may promote cells to adapt to extrinsic stimuli, persistent or strong ER stress often leads to cell apoptosis [8][9][10][11][12]. As a common air contaminant, cigarette smoke induces ER stress in lung cells.…”