Electronic cigarettes induce mitochondrial DNA damage and trigger toll-like receptor 9-mediated atherosclerosis

Objective: Both electronic cigarette (e-cig) use and toll-like receptor 9 (TLR9) activation have been implicated in promoting atherosclerosis. In this study we aimed to investigate the causative relationship of e-cig exposure on TLR9 activation and atherosclerosis development. Approach and Results: Eight-week-old ApoE-/- mice fed normal chow diet were exposed to e-cig vapor (ECV) for 2 h/day, 5 days/week for 16 weeks. We found that ECV exposure significantly induced atherosclerotic lesions as examined by Oil Red O staining of aortic root and greatly upregulated TLR9 expression in classical monocytes and in the atherosclerotic plaques, which the latter was corroborated by upregulated TLR9 expression in human femoral artery atherosclerotic plaques in e-cig smokers. Intriguingly, we found a significant increase of damaged mitochondria DNA level in the circulating blood of ECV exposed mice. Furthermore, administration of TLR9 antagonist prior to ECV exposure not only alleviated atherosclerotic lesion and the upregulation of TLR9 in plaques, but also attenuated the increase of plasma levels of inflammatory cytokines, reduced the accumulation of lipid and macrophages, and decreased the frequency of blood CCR2+ classical monocytes. Surprisingly, we found that the cytoplasmic mtDNA isolated from ECV extract-treated cells can greatly enhance the expression of TLR9 in reporter cells. Conclusion: E-cig induces mtDNA damage and the mtDNA in circulating blood stimulates the expression of TLR9, which elevate the expression of proinflammatory cytokines in monocyte/macrophage and consequently lead to atherosclerosis. Our results raise the possibility that intervention of TLR9 activation is a potential pharmacologic target of ECV-related inflammation and cardiovascular diseases.