Background and purpose Cellular debris causes sterile inflammation after myocardial infarction. Mitochondria constitute about 30 percent of the human heart. Mitochondrial DNA (mtDNA) is a damage‐associated‐molecular‐pattern that induce injurious sterile inflammation. Little is known about mtDNA's inflammatory signalling pathways in cardiomyocytes and how mtDNA is internalized to associate with its putative receptor, toll‐like receptor 9 (TLR9). Experimental Approach We hypothesized that mtDNA can be internalized in cardiomyocytes and induce an inflammatory response. Adult mouse cardiomyocytes were exposed to hypoxia‐reoxygenation and extracellular DNA. Microscale thermophoresis was used to demonstrate binding between nucleolin and DNA. Key results Expression of the pro‐inflammatory cytokines IL‐1β and TNFα were upregulated by mtDNA, but not by nuclear DNA (nDNA), in cardiomyocytes exposed to hypoxia‐reoxygenation. Blocking the RNA/DNA binding protein nucleolin with midkine reduced expression of IL‐1β/TNFα and the nucleolin inhibitor AS1411 reduced interleukin‐6 release in adult mouse cardiomyocytes. mtDNA bound 10‐fold stronger than nDNA to nucleolin. In HEK293‐NF‐κB reporter cells, mtDNA induced NF‐κB activity in normoxia, while CpG‐DNA and hypoxia‐reoxygenation, synergistically induced TLR9‐dependent NF‐κB activity. Protein expression of nucleolin was found in the plasma membrane of cardiomyocytes and inhibition of nucleolin with midkine inhibited cellular uptake of CpG‐DNA. Inhibition of endocytosis did not reduce CpG‐DNA uptake in cardiomyocytes. Conclusion and implications mtDNA, but not nDNA, induce an inflammatory response in mouse cardiomyocytes during hypoxia‐reoxygenation. In cardiomyocytes, nucleolin is expressed on the membrane and blocking nucleolin reduce inflammation. Nucleolin might be a therapeutic target to prevent uptake of immunogenic DNA and reduce inflammation. Linked Articles This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc

Acute myocardial infarction causes lethal cardiomyocyte injury during ischaemia and reperfusion (I/R). Histones have been described as important Danger Associated Molecular Proteins (DAMPs) in sepsis. Aims The objective of this study was to establish whether extracellular histone release contributes to myocardial infarction. Methods and results Isolated, perfused rat hearts were subject to I/R. Nucleosomes and histone H4 release was detected early during reperfusion. Sodium-β-O-Methyl cellobioside sulfate (mCBS), a newly developed histone-neutralising compound, significantly reduced infarct size whilst also reducing the detectable levels of histones. Histones were directly toxic to primary adult rat cardiomyocytes in vitro. This was prevented by mCBS, or HIPe, a recently described, histone-H4 neutralizing peptide, but not by an inhibitor of TLR4, a receptor previously reported to be involved in DAMP-mediated cytotoxicity. Furthermore, TLR4-reporter HEK293 cells revealed that cytotoxicity of histone H4 was independent of TLR4 and NF-κB. In an in vivo rat model of I/R, HIPe significantly reduced infarct size. Conclusion Histones released from the myocardium are cytotoxic to cardiomyocytes, via a TLR4-independent mechanism. The targeting of extracellular histones provides a novel opportunity to limit cardiomyocyte death during I/R injury of the myocardium. Translational perspective Acute myocardial infarction causes lethal cardiomyocyte injury during ischaemia and reperfusion (I/R). New approaches are needed to prevent cardiomyocyte injury and limit final infarct size. We show that histones released from damaged cells, and histone-H4 in particular, causes rapid cardiomyocyte death during I/R. mCBS, a compounds targeting histones non-specifically, was cardioprotective in ex vivo rat hearts, while HIPe, a targeting histone H4 specifically, was cardioprotective in an in vivo rat model. HIPe may have potential as a therapeutic agent in the setting of acute myocardial infarction.

High-doses of anabolic–androgenic steroids (AAS) is efficient for building muscle mass, but pose a risk of cardiovascular side effects. Little is known of the effect of AAS on vasculature, but previous findings suggest unfavorable alterations in vessel walls and vasoreactivity. Here, long-term effect of AAS on vascular function and morphology were examined in male weightlifters, and in a mimicking animal model. Arterial elasticity and morphology were tested with ultrasound, pulse wave velocity (PWV) and carotid intima media thickness (cIMT) in 56 current male AAS users, and 67 non-exposed weightlifting controls (WLC). Female mice were treated with testosterone for 14 days and echocardiography were applied to evaluate vascular function and morphology. Male AAS users had higher PWV (p = 0.044), reduced carotid artery compliance (p = 0.0005), and increased cIMT (p = 0.041) compared to WLC. Similar functional changes were found in the ascending aorta of mice after 7- (p = 0.043) and 14 days (p = 0.001) of testosterone treatment. This animal model can be used to map molecular mechanisms responsible for complications related to AAS misuse. Considering the age-independent stiffening of major arteries and the predictive power of an increase in PWV and cIMT, the long-term users of AAS are at increased risk of severe cardiovascular events.

The innate immune system is rapidly activated during myocardial infarction and blockade of extracellular complement system reduces infarct size. Intracellular complement, however, appears to be closely linked to metabolic pathways and its role in ischemia-reperfusion injury is unknown and may be different from complement activation in the circulation. The purpose of the present study was to investigate the role of intracellular complement in isolated, retrogradely buffer-perfused hearts and cardiac cells from adult male wild type mice (WT) and from adult male mice with knockout of complement component 3 (C3KO). Main findings: (i) Intracellular C3 protein was expressed in isolated cardiomyocytes and in whole hearts, (ii) after ischemia-reperfusion injury, C3KO hearts had larger infarct size (32 ± 9% in C3KO vs. 22 ± 7% in WT; p=0.008) and impaired post-ischemic relaxation compared to WT hearts, (iii) C3KO cardiomyocytes had lower basal oxidative respiration compared to WT cardiomyocytes, (iv) blocking mTOR decreased Akt phosphorylation in WT, but not in C3KO cardiomyocytes, (v) after ischemia, WT hearts had higher levels of ATP, but lower levels of both reduced and oxidized nicotinamide adenine dinucleotide (NADH and NAD+, respectively) compared to C3KO hearts. Conclusion: intracellular C3 protected the heart against ischemia-reperfusion injury, possibly due to its role in metabolic pathways important for energy production and cell survival.

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