Nicotine alters alveolar-barrier integrity by blocking autophagy and occludin trafficking

Tobacco smoke inhalation disrupts the integrity of the alveolar-capillary barrier (ACB) and contributes to the pathogenesis of multiple chronic pulmonary diseases as chronic obstructive pulmonary disease (COPD). Nicotine, a major component of both cigarette smoke and electronic cigarette (e-cigarette) aerosol, is predominantly deposited on the alveolar surface, where it is rapidly absorbed, yet its specific contribution to ACB dysfunction remains insufficiently characterized. Our objective was to decipher the effects of nicotine across a wide concentration range (2.5 µM to 250 mM) on the modulation of the ACB in vitro. To this end, the following endpoints were investigated: cell viability (MTS and BrdU assays), cytotoxicity (LDH assay), mitochondrial oxidative stress (MitoSOX assay) and barrier integrity using Trans-Epithelial/Endothelial Electric Resistance (TEER) following exposure to increasing nicotine concentrations. Autophagic flux (LC3B-II, p62 and LAMP2 protein expression and localization) and occludin modulation and trafficking were also examined and compared with the effects of bafilomycin A1. Nicotine exposure slowed cell proliferation and enhanced mitochondrial reactive oxygen species (ROS) production, effects that were partially reversed by N-acetyl-cysteine (NAC). Nicotine inhibited autophagic flux through a mechanism distinct from that of bafilomycin A1, and this inhibition was partially reversed by NAC. In parallel, nicotine compromised barrier integrity, inducing a TEER decrease associated with occludin internalization and defective lysosomal degradation and recycling. These findings identify nicotine as an independent disruptor of ACB integrity, acting through oxidative stress, impaired autophagy and junctional remodeling, mechanisms relevant to smoking-related pulmonary diseases beyond COPD, including those associated with e-cigarette use.