Mitochondrial DNA is a key driver in cigarette smoke extract-induced IL-6 expression

Smoking is an independent risk factor for atherosclerosis, the primary pathogenesis of which is inflammation. We recently reported that cigarette smoke extract (CSE) causes cytosolic and extracellular accumulation of both nuclear (n) and mitochondrial (mt) DNA, which leads to inflammation in human umbilical vein endothelial cells (HUVECs). In this study, we examined whether inflammation induction depends more on cytosolic nDNA or mtDNA, and which chemical constituents of CSE are involved. Acrolein (ACR), methyl vinyl ketone (MVK), and 2-cyclopenten-1-one (CPO) were used in the experiments, as these are the major cytotoxic factors in CSE in various cell types. Stimulation with ACR, MVK, or CPO alone resulted in the accumulation of DNA double-strand breaks (DSBs), but not oxidative DNA damage, accumulation of cytosolic DNA, or increased expression of inflammatory cytokines. Simultaneous administration of all three constituents (ALL) resulted in oxidative DNA damage in both the nucleus and mitochondria, accumulation of DSBs, reduced mitochondrial membrane potential, induction of minority mitochondrial outer membrane permeabilization, accumulation of cytosolic free DNA, and increased expression of inflammatory cytokines such as IL-6 and IL-1 alpha. Treatment with N-acetyl-L-cysteine, a reactive oxygen species scavenger, suppressed oxidative DNA damage and the increased expression of IL-6 and IL-1 alpha induced by ALL or CSE. The ALL- or CSE-induced increase in IL-6 expression, but not that of IL-1 alpha, was suppressed by mtDNA depletion. In conclusion, ACR, MVK, and CPO may strongly contribute to CSE-induced inflammation. More importantly, cytosolic free mtDNA is thought to play an important role in IL-6 expression, a central mediator of inflammation.

Automated summary

Smoking is a risk factor for atherosclerosis due to inflammation caused by accumulation of nuclear and mitochondrial DNA in human umbilical vein endothelial cells. The chemical constituents of cigarette smoke extract, including acrolein, methyl vinyl ketone, and 2-cyclopenten-1-one, contribute to DNA damage, reduced mitochondrial membrane potential, and induction of inflammatory cytokines. Cytosolic free mitochondrial DNA plays a crucial role in IL-6 expression, a key mediator of inflammation.