Exposure timing is a determinant of fine particulate matter (PM2.5) pulmonary, vascular, and metabolic toxicity in male mice.

Exposure to fine particulate matter (PM2.5) air pollution is associated with an increased cardiometabolic disease risk. However, although exposure timing is recognized as an important toxicity determinant, the chronotoxicity of PM2.5 exposure is less explored. To evaluate whether PM2.5 sensitivity depends on exposure timing, adult male C57/BL6 mice were exposed to concentrated PM2.5 (CAP, 6 h/day, 30 days) either during the inactive (light) phase (Zeitgeber time, ZT1 to 7) or the active (dark) phase (ZT17 to 23). Metabolic health was assessed by measuring body weight, fasting blood glucose, and plasma insulin levels. Oxidative stress and inflammatory responses in aortas and lungs were examined by quantitative real-time (qRT)-PCR, and pulmonary and circulating redox changes were measured calorimetrically. Although CAP exposure during the active phase increased blood glucose levels, inactive-phase exposure more profoundly increased antioxidant enzyme and inflammatory mRNA abundance in lungs and aortas. Inactive-phase exposure also intensified pulmonary and systemic lipid peroxidation and increased the depletion of circulating nitric oxide and lung glutathione. Examining the ability to protect against CAP-induced pulmonary oxidative stress, we found that the pulmonary antioxidant enzyme mRNA expression showed circadian rhythmicity that peaked during the active phase. These data suggest that a superior pulmonary antioxidant defense potential during the active phase could contribute to the protection against the PM2.5-induced vascular and pulmonary effects but not its impact on glucose homeostasis. Our study identified that exposure sensitivity depends on the exposure timing, which is of significance as it informs on timely susceptibility windows that could help to mitigate PM2.5 toxicity.