Adaptive changes in the expression of nuclear and mitochondrial encoded subunits of cytochrome c oxidase and the catalytic activity during hypoxia

Abstract

The effects of physiologically relevant hypoxia on the catalytic activity of cytochrome c oxidase (CytOX), mitochondrial gene expression, and both nuclear and mitochondrial encoded CytOX mRNA levels were investigated in murine monocyte macrophages, mouse C2C12 skeletal myocytes and rat adrenal pheochromocytoma PC12 cells. Our results suggest a coordinated down regulation of mitochondrial genome-coded CytOX I and II and nuclear genome-coded CytOX IV and Vb mRNAs during hypoxia. Hypoxia also caused a severe decrease in mitochondrial transcription rates, and associated decrease in mitochondrial transcription factor A. The enzyme from hypoxia exposed cells exhibited altered subunit content as revealed by blue native gel electrophoresis. There was a generalized decline in mitochondrial function that led to a decrease in total cellular heme and ATP pools. We also observed a decrease in mitochondrial heme aa 3 content and decreased levels of CytOX subunit I, IV and Vb, though the catalytic efficiency of the enzyme (TN for cytochrome c oxidase) remained nearly the same. Increased glycolytic flux and alterations in the kinetic characteristics of the CytOX might be the two mechanisms by which hypoxic cells maintain adequate ATP levels to sustain life processes. Reoxygenation nearly completely reversed hypoxia-mediated changes in CytOX mRNA contents, rate of mitochondrial transcription, and the catalytic activity of CytOX enzyme. Our results show adaptive changes in CytOX structure and activity during physiological hypoxia.

Oxygen Initiation of Respiration and Mitochondrial Biogenesis in Rice

“…1), this indicates that some form of post-transcriptional regulation may explain the observed changes in cox2 transcript levels. This appears to differ from the situation in yeast, where levels of cox2 are regulated at the translational level under low oxygen conditions (55), but is similar to observations in mammalian cells showing that cox2 transcript levels respond to changes in oxygen availability (56). Furthermore, cox2 protein levels were found to change significantly compared with other mitochondrial encoded proteins in response to sucrose starvation of Arabidopsis cell culture (49), indicating that regulation of cox2 independent of other mitochondrial encoded components exists in other systems.…”

The role of protein kinase A in hypoxia-induced PC-12 cell death.

“…Hypoxia has been shown in PC-12 cells to decrease the activity of enzymes involved in oxidative phosphorylation, including Complexes I, II, and IV of the electron transport chain (8). Furthermore, hypoxic upregulation of glycolytic enzymes, such as hexokinase and phosphofructokinase, has been shown in PC-12 cells (8).…”

“…Furthermore, hypoxic upregulation of glycolytic enzymes, such as hexokinase and phosphofructokinase, has been shown in PC-12 cells (8). This decreased oxidative phosphorylation can present a significant problem in energy homeostasis, since glycolysis produces a net gain of only 2 ATP relative to the approximately 38 ATP that can be produced by oxidative phosphorylation and glycolytic processes combined.…”

“…COX IV is the terminal electron acceptor in the mitochondrial electron transport chain which converts molecular oxygen (0 2 ) to water. COX IV consists of 13 subunits, 3 of which (subunits I, II, and II) are encoded by mitochondrial DNA and synthesized directly in the mitochondria (8).…”

“…The other 10 subunits of cytochrome oxidase (IV, Vb, Via, Vlb, Vic, VilA, Vllb, and VIII) are synthesized in the cytosol by the nuclear genome, and then transported into the mitochondria (8).…”