Asparaginyl endopeptidase (AEP) regulates myocardial apoptosis in response to radiation exposure via alterations in NRF2 activation.

Radiation-induced heart disease (RIHD) leads to myocardial dysfunction and metabolic abnormalities in patients treated with thoracic irradiation which restricts the long-term survival benefits of radiotherapy. There is no specific or effective manner of intervention currently available. Asparaginyl endopeptidase (AEP) plays a pivotal role in the maintenance of cellular functions through regulating proteolytic cleavage as peptidase enzyme. We aimed to investigate the role of unique cardiac AEP in cardiac function by modulating key signaling elements in the myocardium. The murine heart was exposed to a single dose of 14 Gy radiation. Cellular signaling and apoptosis was analyzed in human and rat cardiomyocytes treated with various doses of radiation, we observed expression of AEP was increased by immunohistochemical staining in murine heart exposed to radiation. The AEP production along with its increased level of mRNA expression was associated with increased doses of radiation (0, 2, 5, 10 Gy) in cardiomyocytes. The myocardial cells transfected with AEP overexpression showed overall cellular viability enhancement, DNA damage inhibition, the foci formation of γ-H2AX suppressed and DNA repair enhancement significantly after radiation exposure. Small interfering RNA-mediated AEP knockdown was with reduced cardiomyocyte viability, elevated apoptotic rate, increased γ-H2AX foci formation and inhibited DNA repair as well after irradiation. After radiation exposure of 10 Gy, the expression of AEP increased in P53 overexpressing cardiomyocytes and decreased in the P53 knockdown cells, indicates that radiation-induced expression of AEP might be regulated by P53. Moreover, treatments with either AEP overexpression or knockdown showed enhanced NRF2 activity in the nuclear or suppressed NRF2 expression in the cytoplasm of myocardial cells after irradiation, respectively, defined a possible regulatory effect of AEP associated with diminished NRF2 translocation and activation by radiation exposure, including impair myocardium and myocardial apoptosis. These findings suggest that increased levels of AEP in failing myocardium after irradiation is mediated by P53 and regulate a novel pathway that involves NRF2 activation. AEP is essential for maintaining cellular redox homeostasis of cardiac function.