Cigarette smoke induces endoplasmic reticulum stress response and proteasomal dysfunction in human alveolar epithelial cells

Abstract

New findings r What is the central question of this study?The endoplasmic reticulum stress response caused by cigarette smoke may lead to excessive apoptosis with disruption of the epithelial barrier, thus contributing to chronic obstructive pulmonary disease. One way of promoting cell survival is to facilitate degradation of cigarette smoke-induced protein damage through the ubiquitin-proteasome pathway. Direct effects of gas-phase cigarette smoke on proteasomal activities have not been demonstrated previously. r What is the main finding and what is its importance?We show that cigarette smoke induces protein damage and triggers the endoplasmic reticulum stress response in human alveolar epithelial cells. A significant reduction of all three proteasomal activities was found. Ineffective degradation of damaged proteins could lead to a sustained epithelial stress response and development of chronic obstructive pulmonary disease.Cigarette smoking is the major risk factor for chronic obstructive pulmonary disease. Cigarette smoke (CS) causes oxidative stress and severe damage to proteins in the lungs. One of the main systems to protect cells from the accumulation of damaged proteins is the ubiquitin-proteasome pathway. In the present study, we aimed to find out whether exposure of alveolar epithelial cells to CS induces an endoplasmic reticulum (ER) stress response by accumulation of damaged proteins that are inefficiently degraded by the proteasomes. The hypothesis was tested in a human alveolar epithelial cell line (A549) exposed to gas-phase CS. Exposure to gas-phase CS for 5 min caused an increase in the amount of ubiquitin-protein conjugates within 4 h. Cigarette smoke exposure also induced the ER stress response marker eIF2α, followed by a significant reduction of nascent protein synthesis and increase in the level of free intracellular amino acids. Moreover, CS exposure significantly reduced all three proteasomal activities (caspase-, trypsinand chymotrypsin-like activity) within 4 h, which was still present after 24 h. It can be concluded that gas-phase CS induces ER stress in A549 alveolar epithelial cells, leading to inadequate protein turnover caused by an accumulation of damaged proteins, reduction in nascent protein synthesis and inhibition of the proteasome. We suggest that prolonged ER stress may lead to excessive A. Somborac-Bačura and M. van der Toorn contributed equally to this work.

Histone deacetylase 6–mediated selective autophagy regulates COPD-associated cilia dysfunction

“…The mechanisms underlying CS-induced epithelial cell injury and dysfunction remain unclear, but may include a protease/antiprotease imbalance, inflammation, oxidative stress, and programmed cell death (2)(3)(4). Recent studies have suggested additional pathway mechanisms involving altered protein homeostasis (proteostasis) (5,6), including ER stress, inhibition of the ubiquitin-proteasome system (7)(8)(9), and autophagy (10)(11)(12)(13), all of which potentially contribute to the pathogenesis of chronic lung diseases and emphysema (5).…”

Endoplasmic reticulum stress in lung disease

“…One plausible explanation is that recovery of protein synthesis is pro-survival during responses to transient ER stress, but becomes toxic during chronic ER stress, when ongoing protein synthesis overwhelms the adaptive potential of the system. Evidence has implicated these pathways in the responses to oxidative stress imposed by cigarette smoke, but this requires further exploration [25][26][27][28]. Although ATF4 signalling is activated in the airways of many patients with chronic obstructive pulmonary disease [44], a direct mechanistic link with disease pathogenesis remains elusive.…”

Autophagy: A Crucial Moderator of Redox Balance, Inflammation, and Apoptosis in Lung Disease

“…CS exposure may impair the delivery of bacteria to lysosomes (115), suggesting that defective xenophagy in alveolar macrophages of smokers may contribute to recurrent infections in COPD patients. CS affects oxidative protein folding in the ER (74), and it can induce the UPR in human alveolar epithelial cells (165), which may lead to ER stress and impaired protein homeostasis in the lungs of COPD patients (13,105,111).…”