Mechanism of Cigarette Smoke-induced Injury to Alveolar Epithelial Cells

Smoking is the leading preventable risk factor for disease and death worldwide. Tobacco and its smoke contain a complex mix of over 9 500 chemical substances, including oxidative gases, heavy metals, and 83 known carcinogens. Long-term smoking is a significant risk factor for respiratory diseases such as acute lung injury, emphysema, and pulmonary fibrosis. Damage to alveolar epithelial cells (AECs) is a common pathological feature in these smoking-related lung diseases. AECs, which line the surface of the alveoli, play a crucial role in preventing overexpansion or collapse, secreting cell factors and surfactants, containing abundant mitochondria, and being essential for lung tissue maturation, gas exchange, metabolism, and repair after damage. Damage to these cells can lead to pulmonary edema and alveolar collapse. Cigarette smoke (CS) can disrupt alveolar epithelial cell function through various pathways, resulting in cell death, tissue damage, and the development of lung diseases.This review summarizes recent research on the damage caused by CS to AECs, showing that CS can promote cell death and damage through induction of oxidative stress, autophagy, endoplasmic reticulum stress, mitochondrial dysfunction, inflammation, and epithelial-mesenchymal transition. It also affects the proliferative function of alveolar type II epithelial cells. The review highlights that CS-induced oxidative stress is a key factor in causing various types of damage, with TRP ion channels serving as important triggers. Inhibiting CS-induced oxidative damage can significantly prevent cell death and subsequent diseases such as pulmonary emphysema. The activation of the same pathway induced by CS can lead to different types of cell damage, potentially encouraging the development of different diseases. CS can either directly induce or indirectly promote cell inflammation through endoplasmic reticulum stress, mitochondrial dysfunction, and senescence. There are interconnected relationships between these mechanisms, and SIRT1 is an important protein in preventing CS- induced AECs damage. Increasing SIRT1 activity can alleviate CS-induced autophagy, endoplasmic reticulum stress, and senescence in various cell damages; its substrate NAD+ is already used clinically, and its effectiveness in COPD treatment deserves further exploration. The impact of CS on cells varies based on concentration: lower concentrations stimulate stress responses or apoptosis, while higher concentrations lead to apoptosis or necrosis through various mechanisms, ultimately impairing lung epithelial function. When external stimuli exceed the cells' self-healing capacity, they can cause damage to cells, lung epithelial barriers, and alveoli, promoting the development of related lung diseases. Key proteins that play a protective role may serve as potential targets to mitigate cell damage. This review provides insights into the various mechanisms through which CS induces damage to AECs, covering important transcription factors, DNA repair proteins, and membrane channel proteins, paving the way for the study of new mechanisms and pathways. However, there are still unanswered questions, such as the need for further exploration of the upstream pathways of CS-induced autophagy in AECs and the intrinsic mechanisms of CS in enhancing the stem cell properties of AECs and its relationship to the occurrence of lung cancer. It is expected that this article will provide a theoretical basis for future research on the mechanisms of lung epithelial cell damage caused by CS or its individual components and inspire clinical strategies for the prevention and treatment of smoking-related lung diseases.