Abstract 3624: Nox4 Promotes Myocardial Ischemia/Reperfusion Injury
Reactive oxygen species (ROS) play an important role in mediating tissue damage due to ischemia/reperfusion (I/R). NAD(P)H oxidases (Noxs) are major sources of ROS in the cardiovascular system. In order to elucidate the in vivo function of Nox4, a major isoform in the heart, we generated cardiac-specific Nox4 −/− (c-Nox4 −/−) mice, using αMHC-Cre. Three month old wild-type (WT) and c-Nox4 −/− mice were subjected to 30 minutes ischemia followed by 24 hours reperfusion. Although the area at risk (AAR) and the capillary density in c-Nox4 −/− mice did not differ from those in WT mice, myocardial infarction size (MI)/AAR determined by TTC staining was significantly smaller in c-Nox4 −/− than in WT mice (12.0±3.1% vs. 26.3±5.3%, p<0.05). Similarly, mice systemically deficient in Nox2 also exhibited a smaller MI/AAR than in WT (9.9±3.9 vs. 16.8±4.0%, p<0.05) in response to I/R. These results suggest that both Nox4 and Nox2 mediate myocardial injury in response to I/R. Interestingly, these results differed from our previous result that I/R induced a greater MI/AAR in transgenic mice with cardiac-specific expression of dominant negative Nox4 (Tg-DN-Nox4) (Tg, 67.2±9.0% vs. NTg, 18.0±8.2%, p<0.05). To reconcile the discrepancy, we measured superoxide-producing activity in these mice, using a chemiluminescence method. Superoxide production was more severely attenuated in Tg-DN-Nox4 mouse hearts (WT, 1250±236; c-Nox4 −/−, 580±106; Tg-DN-Nox4, 216±106 RLU; p<0.05), suggesting that although partial inhibition of Nox may be protective, more severe reduction of Noxs may be detrimental during I/R. To elucidate the mechanism by which knockout of Nox4 reduces I/R injury, we examined expression of proteins involved in cardiac metabolism during I/R. Among them, expression of peroxisome proliferator-activated receptor α (PPARα), a transcription factor promoting mitochondrial fatty acid oxidation, was markedly suppressed in c-Nox4 −/−, but was upregulated in Tg-DN-Nox4 mice during I/R, suggesting that suppression of fatty acid oxidation may contribute to reduced oxidative stress in c-Nox4−/− mice. In summary, cardiac specific deletion of Nox4 significantly attenuated I/R injury, and was accompanied by downregulation of PPARα.