Oxygen Tension Regulates Mitochondrial DNA-encoded Complex I Gene Expression

Abstract

Oxygen is a major regulator of nuclear gene expression. However, although mitochondria consume almost all of the O 2 available to the cells, little is known about how O 2 tension influences the expression of the mitochondrial genome. We show in O 2 -sensitive excitable rat PC12 cells that, among the mtDNA-encoded genes, hypoxia produced a specific down-regulation of the transcripts encoding mitochondrial complex I NADH dehydrogenase (ND) subunits, particularly ND4 and ND5 mRNAs and a stable mRNA precursor containing the ND5 and cytochrome b genes. This unprecedented effect of hypoxia was fast (developed in <30 min) and fairly reversible and occurred at moderate levels of hypoxia (O 2 tensions in the range of 20 -70 mm Hg). Hypoxic down-regulation of the mitochondrial complex I genes was paralleled by the reduction of complex I activity and was retarded by iron chelation, suggesting that an iron-dependent post-transcriptional mechanism could regulate mitochondrial mRNA stability. It is known that cell respiration is under tight control by the amount of proteins in mitochondrial complexes of the electron transport chain. Therefore, regulation of the expression of the mitochondrial (mtDNA)-encoded complex I subunits could be part of an adaptive mechanism to adjust respiration rate to the availability of O 2 and to induce fast adaptive changes in hypoxic cells.Oxygen (O 2 ) is absolutely required for cell survival due to its fundamental role in mitochondrial oxidative phosphorylation. Decreases in O 2 tension (PO 2 ) trigger acute and chronic adaptive cellular responses to increase O 2 delivery to the tissues and to minimize the deleterious effects of hypoxia. In mammals, acute hypoxia (occurring over a time scale of seconds to minutes) regulates the activity of ion channels existing in specific O 2 -sensitive cells, which mediate fast respiratory and cardiovascular counterregulatory adjustments (for review, see Refs. 1 and 2 and references therein). Protracted (chronic) hypoxia (lasting hours or days) activates hypoxia-inducible transcription factors (hypoxia-inducible factor-1␣ (HIF-1␣) 2 and isoforms) that regulate the expression of a broad and growing set of nuclear genes involved in anaerobic glucose metabolism, angiogenesis, or oxygen transport as well as in development, tissue remodeling, and tumor transformation (3-5).In recent years, it has been shown that HIF activity depends on prolyl and asparaginyl hydroxylases, which utilize ambient O 2 as substrate. Under normoxic conditions, hydroxylation of proline and asparagine accelerates HIF-1␣ degradation by the proteasome (6, 7) and prevents its transcriptional activity (8, 9). Inhibition of HIF-1␣ hydroxylation during hypoxia leads to its stabilization and nuclear translocation and to the subsequent activation of HIF-dependent genes (10 -12). Stabilization of HIF-1␣ by hypoxia seems to require the release of reactive oxygen species (ROS) from mitochondria to the cytosol (13-16).Despite the progress in the understanding of the signaling pathway responsible for...

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Fluctuation of Brain Tissue Oxygen Partial Pressure: A Biochemical Landmark in the Arctic Ground Squirrel's Spontaneous Arousal

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“…14 C]Glucose uptake assay Glucose transport activity was measured in adipose cells by [U- 14 C]glucose uptake as described [9]. Adipose cells were incubated with or without insulin for 30 min before measurement of [U- 14 C]glucose uptake.…”

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