Extracellular Superoxide Dismutase Protects Cardiomyocytes During Hyperoxia

Abstract

Reactive oxygen species are implicated in tissue damage in many cardiovascular diseases. The current study was designed to test the hypothesis that exposure to high inspired oxygen concentrations (100%) damages cardiac mitochondria and that a biogenic response is needed for cell survival. The study was done in mice with a transgene (TG) for extracellular superoxide dismutase (EC-SOD) or with the EC-SOD gene knockout (KO). Adult EC-SOD mice and their wild type littermates and EC-SOD KO mice and their wild type littermates were exposed to 100% oxygen for 6 hours and for 72 hours, and the hearts were subsequently removed under deep anesthesia. The mortality rate in KO mice after 72 hours (25%) was significantly higher than wild type littermates (10%) and TG mice (4.2%). In the EC-SOD TG mice, 6 hours of exposure to hyperoxia activated the transcription of nuclear genes important for mitochondrial biogenesis – nuclear respiratory factors (NRF-1 and NRF-2) and their co-regulator PGC-1α – leading to expression of mitochondrial transcription factor A (Tfam), which regulates mitochondrial transcription and replication. Hyperoxia also induced activation of NRF-1 and NRF-2 in EC-SOD KO mice after 6 hours but down-regulated Tfam and PGC1α transcripts. But after 72 hours, mRNA expression for NRF-2, PGC-1α and Tfam increased in these mice. The protein expression of the pro-survival Protein Kinase B (Akt) was significantly increased by hyperoxia in the TG hearts after 6 hours (ppp In conclusion, systemic hyperoxia consistently increases mitochondrial biogenesis markers in the mouse heart, most prominently in EC-SOD TG mice, and co-incidentally with increases in Akt, p-Bad and p-GSK-3 R. This suggests that extracellular reactive oxygen products activate mitochondrial biogenesis and the pro-survival pathway in TG mice. Also, hyperoxia decreased the activation of Tfam and PGC-1α transcription after 6 hours and increased it again after 72 hours in KO mice. This was associated with a decrease in expression of Akt, p-Bad and p-GSK-3β and an increase in p38, indicating that either hyperoxia activated mitochondrial biogenesis and pro-survival pathways through p38 or that hyperoxia produced more oxidative damage to the mitochondria in the KO. In the latter case, the increase in p38 may be a signal for activation of apoptotic pathways.