Extracellular mtDNA activates NF-κB via toll-like receptor 9 and induces cell death in cardiomyocytes
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Acute myocardial infarction (AMI) causes sterile inflammation, which exacerbates tissue injury. Elevated levels of circulating mitochondrial DNA (mtDNA) have been associated with AMI. We hypothesized that mtDNA triggers an innate immune response via TLR9 and NF-κB activation, causing cardiomyocyte injury. Murine cardiomyocytes express TLR9 mRNA and protein and were able to internalize fluorescently labeled mouse mtDNA. Incubation of human embryonic kidney cells with serum from AMI patients containing naturally elevated levels of mtDNA induced TLR9-dependent NF-κB activity. This effect was mimicked by isolated mtDNA. mtDNA activated NF-κB in reporter mice both in vivo and in isolated cardiomyocytes. Moreover, incubation of isolated cardiomyocytes with mtDNA induced cell death after 4 and 24 h. Laser confocal microscopy showed that incubation of cardiomyocytes with mtDNA accelerated mitochondrial depolarization induced by reactive oxygen species. In contrast to mtDNA, isolated total DNA did not activate NF-κB nor induce cell death. In conclusion, mtDNA can induce TLR9-dependent NF-κB activation in reporter cells and activate NF-κB in cardiomyocytes. In cardiomyocytes, mtDNA causes mitochondrial dysfunction and death. Endogenous mtDNA in the extracellular space is a danger signal with direct detrimental effects on cardiomyocytes.
KeywordsMitochondrial DNA mtDNA Inflammation TLR9 NF-κB Myocardial infarction
Harald Carlsen and Jan Øivind Moskaug, Department of Nutrition, Institute of Basic Medical Sciences, University of Oslo, kindly provided NF-κB luciferase reporter mice. The authors acknowledge the expertise of Gerbrand Koster for technical advice (NorMIC imaging cluster, Department of Biosciences, University of Oslo) and technical assistance was expertly performed by Torun Flatebø and Sowmya Sanjeevini.
This work was supported by the Norwegian Health Association, UNIFOR, the Norwegian Research Council, the University of Oslo, and the Novo Nordisk Foundation. Marte Bliksøen was supported by a grant from South-Eastern Regional Health Trust.
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