[Effects of hypoxic exposure on coordinative expression of cytochrome oxidase subunits I and IV in rat cerebral cortex]

This study was intended to evaluate the effects of hypoxic exposure on gene expression and coordination of cytochrome oxidase (COX) subunits I (COX I) and IV (COX IV) encoded by mtDNA and nDNA respectively in rat cerebral cortex. Male Wistar rats were exposed to hypoxia in a hypobaric chamber simulating high altitude at 5000 m for 2, 5, 15 and 30 d. Control rats were fed outside the hypobaric chamber (the height was 300 m above sea level). Rats were sacrificed and mitochondria from cerebral cortex were isolated by differential centrifugation at each time point. COX I and COX IV proteins in isolated rat cerebral cortex mitochondria were detected by Western blot analysis and mRNA in the cerebral cortex by RT-PCR. The ratios of protein and mRNA were used to estimate the coordinative expression of two subunits. The results showed that COX I mRNA increased significantly at 2 and 5 d, and decreased to the control level at 15 and 30 d; COX IV mRNA remarkably increased at 2, 5 and 15 d, and dropped below the control level at 30 d. The mRNA ratio of COX IV to COX I reached a peak at 15 d, but showed no differences between other time points. The Western blot analysis of COX I and COX IV in isolated rat cerebral cortex mitochondria showed no obvious changes during hypoxic exposure. Our findings demonstrate that hypoxia can affect mRNA expression of COX I and COX IV and their coordination, while protein expression of both subunits are stable and coordinative. This study suggests that the expression of COX I and COX IV proteins during hypoxic exposure is coordinately regulated by post-transcriptional mechanisms.