N‑acetyl‑L‑cysteine protects rat lungs and RLE‑6TN cells from cigarette smoke‑induced oxidative stress

Cigarette smoke (CS) is a key contributor of chronic obstructive pulmonary disease (COPD); however, its role in the pathogenesis of COPD has not been fully elucidated. N‑acetyl‑L‑cysteine (NAC), as an antioxidant, has been used in the treatment of COPD; however, the mechanisms of action of NAC are not fully understood. Alveolar epithelial type 2 (ATII) cells serve an essential role in the maintenance of alveolar integrity. The aim of the present study was to identify the effect of CS on rat lungs and ATII cells. A subacute lung injury model of Wistar rats was established using CS exposure for 4 weeks. Interalveolar septa widening, infiltration of inflammatory cells, edema fluid in airspaces and abnormal enlargement of airspaces were observed through H&E staining. ELISA revealed that NAC could protect against CS‑induced increases in serum levels of malondialdehyde and decreases in serum levels of superoxide dismutase. Additionally, 8‑hydroxy‑deoxyguanosine was detected using immunohistochemical staining, and this was also expressed at increased levels in the lung tissue of the CS‑exposed group. In addition, the expression levels of Bcl‑2, BAX and caspase‑3 p12 in lung tissue were detected by western blotting or immunohistochemical staining. The expression levels of Bcl‑2 decreased and those of caspase3 p12 were increased in response to CS exposure when compared with those in the control group. These effects were prevented by treatment with NAC. <i>In vitro</i>, the effect of CS extract (CSE) on rat lung epithelial‑6‑T‑antigen negative (RLE‑6TN) cells was observed, flow cytometry was used to detect intracellular reactive oxygen species (ROS) levels and the occurrence of apoptosis, and the content of glutathione (GSH) was detected using a colorimetric assay. Additionally, the expression levels of heme oxygenase‑1 (HO‑1), p53 and Bcl‑2 were examined by western blotting, and HO‑1 mRNA expression was also examined using reverse transcription‑quantitative PCR. The results of the present study revealed that CSE induced apoptosis of RLE‑6TN cells, accompanied by increased levels of intracellular ROS and exhaustion of GSH. Significantly increased protein levels of HO‑1 and p53, as well as decreased protein levels of Bcl‑2 were also observed. These effects were prevented by administration of NAC. Overall, these findings suggested that CS could promote apoptosis in rat lung tissues and alveolar epithelial cells by inducing intracellular oxidative injury, and NAC may serve an antioxidant role by replenishing the intracellular GSH content.