Baicalein protects human keratinocytes against PM2.5-induced apoptosis by suppressing oxidative stress, endoplasmic reticulum stress, and ERK/JNK signaling

Excessive exposure to fine particulate matter (PM_2.5; ≤2.5 μm) induces oxidative stress through the overproduction of Reactive Oxygen Species (ROS), ultimately leading to keratinocyte injury and Apoptosis. In this study, we investigated whether baicalein, a flavonoid with potent antioxidant properties, protects human HaCaT keratinocytes against PM_2.5-induced endoplasmic reticulum (ER) stress-mediated Apoptosis. Cells were pretreated with baicalein prior to PM_2.5 exposure, and confocal microscopy, flow cytometry, and immunoblotting were used to assess cell viability, ROS generation, oxidative macromolecular damage, mitochondrial dysfunction, ER stress signaling, and apoptotic cell death. PM_2.5 exposure markedly elevated intracellular ROS levels, leading to lipid peroxidation, protein carbonylation, and oxidative DNA damage, whereas baicalein pretreatment attenuated these effects and restored cell viability. Moreover, baicalein prevented PM_2.5-induced mitochondrial membrane depolarization and caspase-dependent Apoptosis, as evidenced by decreased Bax, cleaved caspase-9, cleaved Caspase-3, and cleaved poly (ADP-ribose) polymerase levels, together with increased Bcl-2 expression. Additionally, baicalein effectively suppressed PM_2.5-induced intracellular CA²⁺ overload and ER stress-related signaling, including activation of protein kinase R-like ER kinase, eukaryotic initiation factor 2α, inositol-requiring enzyme 1α, activating transcription factor 6, X-box binding protein 1, C/EBP homologous protein, and 78 kDa glucose-regulated protein. Pharmacological inhibition of ER stress with tauroursodeoxycholic acid partially recapitulated the cytoprotective effects of baicalein, underscoring the importance of ER stress modulation in its protective mechanism. Furthermore, baicalein significantly inhibited PM_2.5-induced activation of extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK), two key components of the mitogen-activated protein kinase signaling cascade involved in oxidative stress-induced Apoptosis. Collectively, these findings demonstrate that baicalein protects keratinocytes against PM_2.5-induced Apoptosis by suppressing ROS accumulation, alleviating ER stress-associated CA²⁺ dysregulation, and inhibiting ERK/JNK-dependent apoptotic signaling.