Abstract 1600: Endogenous Nox4 Is Protective During Ischemia/reperfusion In The Heart
Although excessive production of reactive oxygen species (ROS) causes cellular injuries, ROS also act as signaling molecules and regulate physiological cellular functions. ROS are extensively generated during ischemia/reperfusion (I/R) and cause tissue damages. The NAD(P)H oxidases are major sources of ROS in the cardiovascular system. Among NAD(P)H oxidase proteins, Nox4 is expressed in the heart and cultured cardiac myocytes. To test whether ROS derived from Nox4 induce tissue damage following I/R in the heart, we made mice on FVB background with cardiac-specific overexpression of wild-type Nox4 (Tg-Nox4) and catalytically inactive Nox4 (Tg-DN-Nox4 (P437H)) in which NADPH is unable to bind to Nox4. We confirmed that ROS generation and oxidative stress in the heart are greater in Tg-Nox4 and is lower in Tg-DN-Nox4, compared with non-transgenic mice (NTg), as determined by dihydroethidium fluorescence and 8-hydroxyguanosine staining, respectively. Neither Tg-Nox4 nor Tg-DN-Nox4 exhibited obvious baseline phenotype at 3 months of age. Tg-Nox4, Tg-DN-Nox4, and NTg (n=5, each) at 3 months old were subjected to 45 minutes ischemia, by ligating the left anterior descending artery, followed by 24 hours reperfusion. Unexpectedly, myocardial infarction (MI) size/area at risk (AAR) determined by TTC staining was significantly greater in Tg-DN-Nox4 than in NTg (67% vs 30 %, p<0.05). On the other hand, MI size/AAR in Tg-Nox4 (35 %) was comparable to that in NTg. To elucidate the mechanism by which suppression of Nox4 deteriorates I/R injury, we examined expression of proteins involved in cardiac metabolism. We found that expression levels of peroxisome proliferator-activated receptor α (PPARα), an important transcription factor stimulating mitochondrial fatty acid oxidation, remained high in Tg-DN-Nox4 during I/R (3.0 fold vs NTg), whereas it was suppressed in NTg and Tg-Nox4. Consistently, overexpression of DN-Nox4 enhanced transcriptional activity of PPARα by 2-fold, as determined by a luciferase assay, in cultured myocytes. Taken together, endogenous Nox4 may protect the heart from I/R injury by suppressing fatty acid oxidation through downregulation of PPARα which would otherwise lead to excess ROS generation after I/R.