Sirt1 is expressed in lung endothelial cells and Sirt1 deletion increases PM_2.5-induced TM down-regulation in the lung. (A) Immunofluorescence images showing Sirt1 expression (green) in lung tissues of WT C57BL/6 mice. No staining was detected using control normal IgG (data not shown). CD31 (red) was used as a specific marker for endothelial cells. Merged image indicates that Sirt1 is expressed in lung endothelial cells. Images are representative of 6 independent experiments. (B) Sirt1^+/+ and Sirt1^−/− littermates were exposed to PBS or PM_2.5 (100 μg) by intranasal instillation. Twenty-four hours later, lungs were collected. Immunofluorescence microscopy revealed decreased TM expression in the lung after PM_2.5 exposure. Images are representative of 6 independent experiments. (C) Lung homogenates were subjected to immunoblotting to examine TM expression. Blots and normalized bar graphs show TM expression in the lungs of Sirt1^+/+ and Sirt1^−/− littermates. Data are presented as mean + SEM (n ≥ 6 mice/group; *P < .05).