α-Ketoglutarate abrogates the nuclear localization of HIF-1α in aluminum-exposed hepatocytes

Aluminum (Al), a known environmental pollutant, has been linked to numerous pathologies such as Alzheimer's disease and anaemia. In this study, we show that α-ketoglutarate (KG) mitigates the Al-mediated nuclear accumulation of hypoxia inducible factor-1α (HIF-1α) in cultured human hepatocytes (HepG2). The nuclear localization of HIF-1α appeared to be triggered by the Al-induced perturbation of prolyl hydroxylase 2 (PHD2). This enzyme was markedly diminished in the Al-challenged hepatocytes. The fate of PHD2 and HIF-1α was intricately linked to the mitochondrial dysfunction observed during Al stress. BN-PAGE, immunoblot, and HPLC revealed that the loss of α-ketoglutarate dehydrogenase (KGDH) and succinate dehydrogenase (SDH) activities were coupled to the accumulation of succinate. However, the treatment of the Al-stressed cells with KG recovered the activity and expression of KGDH, SDH, and PHD2 with a concomitant decrease in the levels of HIF-1α in the nucleus. Taken together, these data indicate that the homeostasis of KG plays a pivotal role in aerobic and anaerobic respiration.     

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.     

Issue Information

Complex II [succinate dehydrogenase (succinate‐ubiquinone oxidoreductase); EC 1.3.5.1; SDH] is the only enzyme shared by both the electron transport chain and the tricarboxylic acid (TCA) cycle in mitochondria. Complex II in plants is considered unusual because of its accessory subunits (SDH5–SDH8), in addition to the catalytic subunits of SDH found in all eukaryotes (SDH1–SDH4). Here, we review compositional and phylogenetic analysis and biochemical dissection studies to both clarify the presence and propose a role for these subunits. We also consider the wider functional and phylogenetic evidence for SDH assembly factors and the reports from plants on the control of SDH1 flavination and SDH1–SDH2 interaction. Plant complex II has been shown to influence stomatal opening, the plant defense response and reactive oxygen species‐dependent stress responses. Signaling molecules such as salicyclic acid (SA) and nitric oxide (NO) are also reported to interact with the ubiquinone (UQ) binding site of SDH, influencing signaling transduction in plants. Future directions for SDH research in plants and the specific roles of its different subunits and assembly factors are suggested, including the potential for reverse electron transport to explain the succinate‐dependent production of reactive oxygen species in plants and new avenues to explore the evolution of plant mitochondrial complex II and its utility.     

Mitochondrial reactive oxygen species control the transcription factor CHOP-10/GADD153 and adipocyte differentiation: a mechanism for hypoxia-dependent effect

Recent reports emphasize the importance of mitochondria in white adipose tissue biology. In addition to their crucial role in energy homeostasis, mitochondria are the main site of reactive oxygen species generation. When moderately produced, they function as physiological signaling molecules. Thus, mitochondrial reactive oxygen species trigger hypoxia-dependent gene expression. Therefore the present study tested the implication of mitochondrial reactive oxygen species in adipocyte differentiation and their putative role in the hypoxia-dependent effect on this differentiation. Pharmacological manipulations of mitochondrial reactive oxygen species generation demonstrate a very strong and negative correlation between changes in mitochondrial reactive oxygen species and adipocyte differentiation of 3T3-F442A preadipocytes. Moreover, mitochondrial reactive oxygen species positively and specifically control expression of the adipogenic repressor CHOP-10/GADD153. Hypoxia (1% O2) strongly increased reactive oxygen species generation, hypoxia-inducible factor-1 and CHOP-10/GADD153 expression, and inhibited adipocyte differentiation. All of these hypoxia-dependent effects were partly prevented by antioxidants. By using hypoxia-inducible factor-1alpha (HIF-1alpha)-deficient mouse embryonic fibroblasts, HIF-1alpha was shown not to be required for hypoxia-mediated CHOP-10/GADD153 induction. Moreover, the comparison of hypoxia and CoCl2 effects on adipocyte differentiation of wild type or HIF-1alpha deficient mouse embryonic fibroblasts suggests the existence of at least two pathways dependent or not on the presence of HIF-1alpha. Together, these data demonstrate that mitochondrial reactive oxygen species control CHOP-10/GADD153 expression, are antiadipogenic signaling molecules, and trigger hypoxia-dependent inhibition of adipocyte differentiation.     

Hypoxia-inducible factor 1 activation by aerobic glycolysis implicates the Warburg effect in carcinogenesis

Cancer cells display high rates of aerobic glycolysis, a phenomenon known historically as the Warburg effect. Lactate and pyruvate, the end products of glycolysis, are highly produced by cancer cells even in the presence of oxygen. Hypoxia-induced gene expression in cancer cells has been linked to malignant transformation. Here we provide evidence that lactate and pyruvate regulate hypoxia-inducible gene expression independently of hypoxia by stimulating the accumulation of hypoxia-inducible Factor 1alpha (HIF-1alpha). In human gliomas and other cancer cell lines, the accumulation of HIF-1alpha protein under aerobic conditions requires the metabolism of glucose to pyruvate that prevents the aerobic degradation of HIF-1alpha protein, activates HIF-1 DNA binding activity, and enhances the expression of several HIF-1-activated genes including erythropoietin, vascular endothelial growth factor, glucose transporter 3, and aldolase A. Our findings support a novel role for pyruvate in metabolic signaling and suggest a mechanism by which high rates of aerobic glycolysis can promote the malignant transformation and survival of cancer cells.