cells. Isolated mitochondria increase ROS generation during hypoxia, as does the bacterium Paracoccus denitrificans. These findings reveal that mitochondria-derived ROS are both required and sufficient to initiate HIF-1␣ stabilization during hypoxia.Hypoxia initiates transcription of a number of gene products that help to sustain the supply of O 2 to tissues and to enhance cell survival during severe O 2 deprivation. Gene products that augment O 2 supply at the tissue level include erythropoietin (Epo) 1 which increases the proliferation of erythrocytes, tyrosine hydroxylase which is necessary for the synthesis of the neurotransmitter dopamine in the carotid bodies, and the angiogenic factor VEGF which stimulates growth of new capillaries (1-3). At the cellular level, gene products that enhance survival during hypoxia include the glycolytic enzymes and the glucose transporters Glut1 and Glut3 (4). The induction of these genes is mediated by hypoxia-inducible factor-1 (HIF-1) (5-7), a heterodimeric transcription factor consisting of HIF-1␣ and the aryl hydrocarbon nuclear translocator (ARNT or HIF-1␤) subunits (7-9). The significance of HIF-1 in transcriptional regulation was recently demonstrated by the marked decrease in mRNA expression of VEGF and glycolytic enzymes seen during hypoxia in HIF-1␣-or ARNT-deficient murine embryonic stem cells (10 -12).The mechanism by which HIF-1 activation is initiated during hypoxia remains unclear. Both HIF-1␣ and ARNT mRNAs are constitutively expressed, indicating that functional activity of the HIF-1␣⅐ARNT complex is regulated by post-transcriptional events. ARNT levels are not significantly affected by [O 2 ], whereas HIF-1␣ protein is rapidly degraded under normoxic conditions by the ubiquitin-proteasome system (13,14). Hypoxia enhances HIF-1␣ protein levels by inhibiting its degradation, thereby allowing it to accumulate, to dimerize with ARNT, and to bind to the hypoxia-responsive element (HRE) in the promoter or enhancer regions of various genes. Thus, the functional HIF-1␣⅐ARNT complex is primarily regulated by the abundance of the HIF-1␣ subunit.Although much has been learned about the role of HIF-1 in controlling the expression of hypoxia-responsive genes, the underlying mechanism by which cells detect the decrease in [O 2 ] and initiate the stabilization of HIF-1␣ is not known. Presently, four diverse O 2 -sensing mechanisms have been proposed to mediate the transcriptional response to hypoxia (15). Two of these models postulate the involvement of an iron-containing unit in the form of either a heme group or an iron/sulfur cluster, which undergoes a change in activity during hypoxia that triggers the transcriptional response. These models are supported by the observation that cobaltous ions, or alternatively the iron chelator desferrioxamine (DFO), stabilize HIF-1␣ under normoxic conditions (16). However, no specific proteins with this role have been identified in mammalian systems. Two other models involve the generation of reactive oxygen species (ROS) by a f...

Inhibition of mitochondrial respiratory chain complex I by rotenone had been found to induce cell death in a variety of cells. However, the mechanism is still elusive. Because reactive oxygen species (ROS) play an important role in apoptosis and inhibition of mitochondrial respiratory chain complex I by rotenone was thought to be able to elevate mitochondrial ROS production, we investigated the relationship between rotenone-induced apoptosis and mitochondrial reactive oxygen species. Rotenone was able to induce mitochondrial complex I substrate-supported mitochondrial ROS production both in isolated mitochondria from HL-60 cells as well as in cultured cells. Rotenone-induced apoptosis was confirmed by DNA fragmentation, cytochrome c release, and caspase 3 activity. A quantitative correlation between rotenone-induced apoptosis and rotenone-induced mitochondrial ROS production was identified. Rotenone-induced apoptosis was inhibited by treatment with antioxidants (glutathione, N-acetylcysteine, and vitamin C). The role of rotenone-induced mitochondrial ROS in apoptosis was also confirmed by the finding that HT1080 cells overexpressing magnesium superoxide dismutase were more resistant to rotenone-induced apoptosis than control cells. These results suggest that rotenone is able to induce apoptosis via enhancing the amount of mitochondrial reactive oxygen species production.

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The signaling networks that drive the aging process, associated functional deterioration, and pathologies has captured the scientific community's attention for decades. While many theories exist to explain the aging process, the production of reactive oxygen species (ROS) provides a signaling link between engagement of cellular senescence and several age-associated pathologies. Cellular senescence has evolved to restrict tumor progression but the accompanying senescence-associated secretory phenotype (SASP) promotes pathogenic pathways. Here, we review known biological theories of aging and how ROS mechanistically control senescence and the aging process. We also describe the redox-regulated signaling networks controlling the SASP and its important role in driving age-related diseases. Finally, we discuss progress in designing therapeutic strategies that manipulate the cellular redox environment to restrict age-associated pathology.

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