PM2.5 Exposure Elicits Oxidative Stress Responses and Mitochondrial Apoptosis Pathway Activation in HaCaT Keratinocytes

Background: PM2.5 (aerodynamic diameter <= 2.5 mu m) is a dominant and ubiquitous air pollutant that has become a global concern as PM2.5 exposure has been linked to many adverse health effects including cardiovascular and pulmonary diseases. Emerging evidence supports a correlation between increased air PM2.5 levels and skin disorders although reports on the underlying pathophysiological mechanisms are limited. Oxidative stress is the most common mechanism of PM2.5-induced adverse health effects. This study aimed to investigate PM2.5-induced oxidative damage and apoptosis in immortalized human keratinocyte (HaCaT) cells. Methods: HaCaT cells were exposed to 0, 25, 50, 100, or 200 mu g/ml PM2.5 for 24 h. Reactive oxygen species (ROS) generation, lipid peroxidation products, antioxidant activity, DNA damage, apoptotic protein expression, and cell apoptosis were measured. Results: PM2.5 exposure (0-200 mu g/ml) for 24 h resulted in increased ROS levels (arbitrary unit: 201.00 +/- 19.28, 264.50 +/- 17.91, 305.05 +/- 19.57, 427.95 +/- 18.32, and 436.70 +/- 17.77) and malondialdehyde production (0.54 +/- 0.05 nmol/mg prot, 0.61 +/- 0.06 nmol/mg prot, 0.68 +/- 0.05 nmol/mg prot, 0.70 +/- 0.05 nmol/mg prot, and 0.76 +/- 0.05 nmol/mg prot), diminished superoxide dismutase activity (6.47 +/- 0.28 NU/mg prot, 5.97 +/- 0.30 NU/mg prot, 5.15 +/- 0.42 NU/mg prot, 4.08 +/- 0.20 NU/mg prot, and 3.76 +/- 0.37 NU/mg prot), and increased DNA damage and apoptosis in a dose-dependent manner in HaCaT cells. Moreover, cytochrome-c, caspase-3, and caspase-9 expression also increased proportionately with PM2.5 dosing. Conclusion: PM2.5 might elicit oxidative stress and mitochondria-dependent apoptosis that likely manifests as skin irritation and damage.