Abstract
NADPH oxidases (Noxes) represent an enzyme system whose primary function is to produce reactive oxygen species (ROS). Both Nox2 and Nox4 play an important role in regulating oxidative stress and growth/death of cardiomyocytes (CMs). However, the role of Nox2 and Nox4 during ischemia/reperfusion (I/R) is poorly understood. In order to elucidate the function of the Nox isoforms, wild type (WT), Nox2 KO (Nox2-/-), and cardiac specific Nox4 KO (cNox4-/-) mice were subjected to 30 minutes ischemia followed by 24 hours reperfusion. Myocardial infarction size/area at risk (MI/AAR) as evaluated by TTC staining was significantly smaller in Nox2-/- and cNox4 -/- than in WT mice (32 ± 3.1% and 28.0 ± 6.1% vs. 40.3 ± 8.3%, p < 0.05). O 2 - production in Nox2 -/- and cNox4 -/- hearts was significantly lower than in WT hearts, as evaluated with the lucigenin-chemiluminescence assay (433 ± 52, 580 ± 106 vs. 1250 ± 236 RLU, p < 0.05). The I/R experiment was also conducted with mice in which both Nox2 and Nox4 are deleted (double KO mice). O 2 - production in the double KO heart was significantly lower than in the single KO ones (257 ± 42 RLU, p < 0.05). The MI/AAR in double KO mice was, however, significantly greater than that in WT mice (58.0 ± 6.3% vs. 40.3 ± 8.3%, p < 0.05). These results raised the possibility that marked suppression of ROS by combined downregulation of Nox2 and Nox4 exacerbates I/R injury. To elucidate the underlying mechanism, we examined expression of hypoxia inducible factor-1alpha (HIF-1 α ), using DN-Nox transgenic (Tg) mice, which we have shown also exhibited marked suppression of O 2 - and significantly greater MI/AAR after I/R. HIF-1 α is lower in DN-Nox Tg mice than in WT mice in both the ischemic and non-ischemic areas. A genetic cross between DN-Nox Tg and mice lacking prolyl hydroxylase 2, an enzyme mediating hydroxylation and downregulation of HIF-1 α , partially rescued I/R injury in DN-Nox mice (35.8 ± 5.9% vs 65.2 ± 10.3%, p<0.05). Taken together, these data show that both Nox2 and Nox4 mediate increases in oxidative stress and myocardial injury in response to I/R. However, combined downregulation of Nox2 and Nox4 induces marked downregulation of ROS, which in turn exacerbates I/R injury possibly through downregulation of HIF-1α and consequent impairment of hypoxic adaptation.
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