Reactive oxygen species generated at mitochondrial complex III stabilize hypoxia-inducible factor-1alpha during hypoxia: a mechanism of O2 sensing

During hypoxia, hypoxia-inducible factor-1alpha (HIF-1alpha) is required for induction of a variety of genes including erythropoietin and vascular endothelial growth factor. Hypoxia increases mitochondrial reactive oxygen species (ROS) generation at Complex III, which causes accumulation of HIF-1alpha protein responsible for initiating expression of a luciferase reporter construct under the control of a hypoxic response element. This response is lost in cells depleted of mitochondrial DNA (rho(0) cells). Overexpression of catalase abolishes hypoxic response element-luciferase expression during hypoxia. Exogenous H(2)O(2) stabilizes HIF-1alpha protein during normoxia and activates luciferase expression in wild-type and rho(0) 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-1alpha stabilization during hypoxia.     

Upregulation of NAD(P)H oxidase 1 in hypoxia activates hypoxia-inducible factor 1 via increase in reactive oxygen species

Hypoxia sensing and related signaling events, including activation of hypoxia-inducible factor 1 (HIF-1), represent key features in cell physiology and lung function. Using cultured A549 cells, we investigated the role of NAD(P)H oxidase 1 (Nox1), suggested to be a subunit of a low-output NAD(P)H oxidase complex, in hypoxia signaling. Nox1 expression was detected on both the mRNA and protein levels. Upregulation of Nox1 mRNA and protein occurred during hypoxia, accompanied by enhanced reactive oxygen species (ROS) generation. A549 cells, which were transfected with a Nox1 expression vector, revealed an increase in ROS generation accompanied by activation of HIF-1-dependent target gene expression (heme oxygenase 1 mRNA, hypoxia-responsive-element reporter gene activity). In A549 cells stably overexpressing Nox1, accumulation of HIF-1alpha in normoxia and an additional increase in hypoxia were noted. Interference with ROS metabolism by the flavoprotein inhibitor diphenylene iodonium (DPI) and catalase inhibited HIF-1 induction. This suggests that H2O2 links Nox1 and HIF-1 activation. We conclude that hypoxic upregulation of Nox1 and subsequently augmented ROS generation may activate HIF-1-dependent pathways.     

NADPH oxidase and cyclooxygenase mediate the ultraviolet B-induced generation of reactive oxygen species and activation of nuclear factor-kappaB in HaCaT human keratinocytes

The detrimental effects of ultraviolet B (UVB) irradiation have been connected with the enhanced generation of reactive oxygen species (ROS) by UVB. However, the exact source of ROS produced by UVB has not been clearly revealed yet. In this study, we determined the source of ROS production and its role in the UVB-induced activation of nuclear factor (NF)-kappaB in HaCaT human keratinocytes. UVB irradiation generated ROS in a dose-dependent manner, and this was significantly inhibited by diphenylene iodonium (DPI), apocynin (Apo) and neopterine (Neo), inhibitors of the NADPH oxidase, and indomethacin (Indo), a cyclooxygenase (COX) inhibitor, but not by the mitochondrial electron transport inhibitors and other cytosolic enzyme inhibitors. In addition, these inhibitors of the NADPH oxidase and COX significantly blocked the UVB irradiation-induced nuclear translocation of NF-kappaB. These results suggest that the NADPH oxidase and COX may be major sources for the UVB-induced ROS generation, and play an essential role in the activation of NF-kappaB which is involved in the expression of a variety of genes induced by UVB in HaCaT cells. These results further suggest that these enzymes may be good targets for the preventive strategy of UVB-induced skin injury.     

HIF-1 prevents the overproduction of mitochondrial ROS after cytokine stimulation through induction of PDK-1

Excessive reactive oxygen species (ROS) are toxic to hematopoietic cells. The majority of cellular ROS are derived from mitochondria and glucose metabolism, and cytokines stimulate this process. During hypoxia, hypoxia inducible factor-1 (HIF-1) attenuates hypoxia-induced mitochondrial ROS production through the induction of pyruvate dehydrogenase kinase-1 (PDK-1). Previously, we found that thrombopoietin (TPO) induces the generation of mitochondrial ROS. Interestingly, the TPO-induced production of mitochondrial ROS promotes the activation of HIF-1. Based on these findings, we speculated that TPO-activated HIF-1 functions as a feedback mechanism to block the overproduction of ROS following TPO stimulation. We found that TPO induces the expression of PDK-1 in a TPO-dependent cell line, UT-7/TPO, in a HIF-1-dependent manner. Inhibition of either HIF-1 or PDK-1 resulted in the increased production of ROS following TPO stimulation. Our observations suggest that HIF-1 functions as a ROS sensor to prevent the overproduction of mitochondrial ROS following cytokine stimulation.     

Induction of HIF-2alpha is dependent on mitochondrial O2 consumption in an O2-sensitive adrenomedullary chromaffin cell line

During low O2 (hypoxia), hypoxia-inducible factor (HIF)-alpha is stabilized and translocates to the nucleus, where it regulates genes critical for survival and/or adaptation in low O2. While it appears that mitochondria play a critical role in HIF induction, controversy surrounds the underlying mechanism(s). To address this, we monitored HIF-2alpha expression and oxygen consumption in an O2-sensitive immortalized rat adrenomedullary chromaffin (MAH) cell line. Hypoxia (2-8% O2) caused a concentration- and time-dependent increase in HIF-2alpha induction, which was blocked in MAH cells with either RNA interference knockdown of the Rieske Fe-S protein, a component of complex III, or knockdown of cytochrome-c oxidase subunit of complex IV, or defective mitochondrial DNA (rho0 cells). Additionally, pharmacological inhibitors of mitochondrial complexes I, III, IV, i.e., rotenone (1 microM), myxothiazol (1 microM), antimycin A (1 microg/ml), and cyanide (1 mM), blocked HIF-2alpha induction in control MAH cells. Interestingly, the inhibitory effects of the mitochondrial inhibitors were dependent on O2 concentration such that at moderate-to-severe hypoxia (6% O2), HIF-2alpha induction was blocked by low inhibitor concentrations that were ineffective at more severe hypoxia (2% O2). Manipulation of the levels of reactive oxygen species (ROS) had no effect on HIF-2alpha induction. These data suggest that in this O2-sensitive cell line, mitochondrial O2 consumption, rather than changes in ROS, regulates HIF-2alpha during hypoxia.