Effects of zinc ions on PM_2.5-induced lung inflammation in mice

Background Fine particulate matter (PM_2.5) is one of the main components of air pollutants which contribute to haze pollution in China in recent years. PM_2.5 exposure can cause potential health hazards to human beings. Zinc is one of the important heavy metal components of PM_2.5, but whether it is involved in PM_2.5-induced lung toxicity and its pathogenic mechanism remain unclear.

Objective The study investigates the effects of PM_2.5 and exogenous zinc ions on the lung inflammation in mice.

Methods PM_2.5 samples were collected from Xinxiang City from November 2015 to March 2016. Twenty-four BALB/c male mice aged 6-8 weeks were randomly divided into a PBS control group, a PM_2.5 group, a low ZnSO₄ group (30 μg·kg^-1, same with the zinc ion concentration of the PM_2.5 samples), and a high ZnSO₄ group (60μg·kg^-1, two times the zinc ion concentration of the PM_2.5 samples), with six mice in each group. Intratracheal instillation was administered once the other day, and for three times in total. The mice were sacrificed on the second day after the last exposure. Pathological changes of lung tissues and infiltration of inflammatory cells were observed by HE staining. The number of inflammatory cells in bronchoalveolar lavage fluid (BALF) was detected by flow cytometry. The total protein and lactic acid dehydrogenase (LDH) levels in BALF were determined with BCA protein assay kit and LDH assay kit respectively. The expression level of interleukin (IL)-1β protein was examined by ELISA.

Results Compared with the control group, the PM_2.5 group, low ZnSO₄ group, and high ZnSO₄ group showed obvious inflammatory cell infiltration, alveolar wall fracture, and lung structure destruction. Compared with the control group(14.78±2.82)×10⁴mL^-1, the total number of cells in BALF increased significantly in the PM_2.5 group(24.11±5.30)×10⁴ mL^-1 and two ZnSO₄ groupsboth (24.00±4.61)×10⁴ mL^-1 (P < 0.05), but there was no difference among the exposure groups. Compared with the control group(1.40±0.14) g·L^-1, the total protein levels of BALF in the PM_2.5 group, low ZnSO₄ group, and high ZnSO₄ group were significantly increased(2.58±0.50), (2.69±0.41), (3.62±0.98)g·L^-1 (P < 0.05), and the increase in the high ZnSO₄ group was more significant than that in the PM_2.5 group. Compared with the control group(44.18±11.54)U·L^-1, the PM_2.5 group, low ZnSO₄ group, and high ZnSO₄ group(443.02±123.33), (185.16±64.00), (428.37±70.71)U·L^-1 showed increased LDH levels in BALF (P < 0.05), and the PM_2.5 group and high ZnSO₄ group were obvious. Compared with the control group(9.54±2.38)×10³ng·L^-1, the PM_2.5 group, low ZnSO₄ group, and high ZnSO₄ group(20.51±3.04)×10³, (15.98±0.53)×10³, (26.36±4.87)×10³ ng·L^-1 were significantly increased in IL-1β levels (P < 0.05), and the increase was significant in the high ZnSO₄ group compared with the other two groups (P < 0.05).

Conclusion Zinc ions in PM_2.5 could induce lung inflammation in mice.