Background: Heme oxygenase-1 (HO-1), which is suggested to play a role in defending the organism against oxidative stress-mediated injuries, can be induced by diverse factors including viruses and iron. As coxsackievirus B3 (CVB3)-infected SWR/J mice susceptible for chronic myocarditis were found to have a significant iron incorporation and HO-1 upregulation in the myocardium, we aimed to investigate the molecular interplay between HO-1 expression and iron homeostasis in the outcome of viral myocarditis.
Methods and results: In susceptible SWR/J mice, but not in resistant C57BL/6 mice, we observed at later stages of CVB3 myocarditis significant iron deposits in macrophages and also in cardiomyocytes, which were spatially associated with oxidative stress, upregulation of HO-1 and caspase-3 activation. HO-1, which is also expressed in cultivated RAW 264.7 macrophages upon incubation with iron and/or CVB3, could be downregulated by inhibition of NO/iNOS using L-NAME. Moreover, specific inhibition of HO-1 by tin mesoporphyrin revealed a suppression of superoxide production in iron and/or CVB3-treated macrophages. The molecular relationship of HO-1 and caspase-3 activation was proven by downregulation with HO-1 siRNA in iron- and/or CVB3-treated cultivated cells. Importantly, iron was found to increase viral replication in vitro.
Conclusion: These results indicate that HO-1 induces a paracrine signalling in macrophages via reactive oxygen species production, mediating apoptosis of heart muscle cells at later stages of myocarditis. Notably, in genetically susceptible mice iron potentiates the detrimental effects of CVB3 by the NO/HO-1 pathway, thus increasing cardiac pathogenicity.