The Anti-Apoptotic Properties of APEX1 in the Endothelium Require the First 20 Amino Acids and Converge on Thioredoxin-1.

The APEX nuclease (multifunctional DNA repair enzyme) 1 (APEX1) has a disordered N-terminus, a redox, and a DNA repair domain. APEX1 has anti-apoptotic properties, which have been linked to both domains depending on cell type and experimental conditions. AIMS: As protection against apoptosis is a hallmark of vessel integrity, we wanted to elucidate whether APEX1 acts anti-apoptotic in primary human endothelial cells and, if so, what the underlying mechanisms are. RESULTS: APEX1 inhibits apoptosis in endothelial cells by reducing Cathepsin D (CatD) cleavage, potentially by binding to the unprocessed form. Diminished CatD activation results in increased Thioredoxin-1 protein levels leading to reduced Caspase 3 activation. Consequently, apoptosis rates are decreased. This depends on the first twenty amino acids in APEX1, because APEX1 (21-318) induces CatD activity, decreases Thioredoxin-1 protein levels, and, thus, increases Caspase 3 activity and apoptosis. Along the same lines, APEX1 (1-20) inhibits Caspase 3 cleavage and apoptosis. Furthermore, re-expression of Thioredoxin-1 via lentiviral transduction rescues endothelial cells from APEX1 (21-318)-induced apoptosis. In an in vivo model of restenosis, which is characterized by oxidative stress, endothelial activation, and smooth muscle cell proliferation, Thioredoxin-1 protein levels are reduced in the endothelium of the carotids. INNOVATION: APEX1 acts anti-apoptotic in endothelial cells. This anti-apoptotic effect depends on the first 20 amino acids of APEX1. CONCLUSION: As proper function of the endothelium during life span is a hallmark for individual health span, a detailed characterization of the functions of the APEX1N-terminus is required to understand all its cellular properties. Antioxid. Redox Signal. 26, 616-629.

Nuclear Factor (Erythroid-Derived 2)-Like 2 and Thioredoxin-1 in Atherosclerosis and Ischemia/Reperfusion Injury in the Heart.

SIGNIFICANCE: Redox signaling is one of the key elements involved in cardiovascular diseases. Two important molecules are the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) and the oxidoreductase thioredoxin-1 (Trx-1). Recent Advances: During the previous years, a lot of studies investigated Nrf2 and Trx-1 as protective proteins in cardiovascular disorders. Moreover, post-translational modifications of those molecules were identified that play an important role in the cardiovascular system. This review will summarize changes in the vasculature in atherosclerosis and ischemia reperfusion injury of the heart and the newest findings achieved with Nrf2 and Trx-1 therein. Interestingly, Nrf2 and Trx-1 can act together as well as independently of each other in protection against atherosclerosis and ischemia and reperfusion injury. CRITICAL ISSUES: In principle, pharmacological activation of a transcription factor-like Nrf2 can be dangerous, since a transcription regulator has multiple targets and the pleiotropic effects of such activation should not be ignored. Moreover, overactivation of Nrf2 as well as long-term treatment with Trx-1 could be deleterious for the cardiovascular system. FUTURE DIRECTIONS: Therefore, the length of treatment with Nrf2 activators and/or Trx-1 has first to be studied in more detail in cardiovascular disorders. Moreover, a combination of Nrf2 activators and Trx-1 should be investigated and taken into consideration. Antioxid. Redox Signal. 26, 630-644.

The aryl hydrocarbon receptor promotes aging phenotypes across species.

The ubiquitously expressed aryl hydrocarbon receptor (AhR) induces drug metabolizing enzymes as well as regulators of cell growth, differentiation and apoptosis. Certain AhR ligands promote atherosclerosis, an age-associated vascular disease. Therefore, we investigated the role of AhR in vascular functionality and aging. We report a lower pulse wave velocity in young and old AhR-deficient mice, indicative of enhanced vessel elasticity. Moreover, endothelial nitric oxide synthase (eNOS) showed increased activity in the aortas of these animals, which was reflected in increased NO production. Ex vivo, AhR activation reduced the migratory capacity of primary human endothelial cells. AhR overexpression as well as treatment with a receptor ligand, impaired eNOS activation and reduced S-NO content. All three are signs of endothelial dysfunction. Furthermore, AhR expression in blood cells of healthy human volunteers positively correlated with vessel stiffness. In the aging model Caenorhabditis elegans, AhR-deficiency resulted in increased mean life span, motility, pharynx pumping and heat shock resistance, suggesting healthier aging. Thus, AhR seems to have a negative impact on vascular and organismal aging. Finally, our data from human subjects suggest that AhR expression levels could serve as an additional, new predictor of vessel aging.

Interplay between the chalcone cardamonin and selenium in the biosynthesis of Nrf2-regulated antioxidant enzymes in intestinal Caco-2 cells.

Selenoenzymes and nuclear factor erythroid 2-related factor 2 (Nrf2)-regulated phase II enzymes comprise key components of the cellular redox and antioxidant systems, which show multiple interrelations. Deficiency of the micronutrient selenium (Se) and impaired biosynthesis of selenoproteins have been reported to result in induction of Nrf2 target genes. Conversely, transcription of the selenoenzymes glutathione peroxidase 2 (GPx2) and thioredoxin reductase 1 (TrxR1) is up-regulated upon Nrf2 activation. Here, we have studied the interplay between Se and the secondary plant metabolite cardamonin, an Nrf2-activating chalcone, in the regulation of Nrf2-controlled antioxidant enzymes. Se-deficient and Se-repleted (sodium selenite-supplemented) human intestinal Caco-2 cells were exposed to cardamonin. Uptake of cardamonin by the Caco-2 cells was independent of their Se status. Cardamonin strongly induced gene expression of GPx2 and TrxR1. However, cardamonin treatment did not result in elevated GPx or TrxR activity and protein levels, possibly relating to a concomitant down-regulation of O-phosphoseryl-tRNA(Sec) kinase (PSTK), an enzyme involved in translation of selenoprotein mRNAs. On the other hand, induction of the Nrf2-regulated enzyme heme oxygenase 1 (HO-1) by cardamonin was diminished in Se-replete compared to Se-deficient cells. Our findings suggest that cardamonin interferes with the biosynthesis of Nrf2-regulated selenoenzymes, in contrast to the Nrf2-activating isothiocyanate compound sulforaphane, which has been shown earlier to synergize with Se-mediated cytoprotection. Conversely, the cellular Se status apparently affects the cardamonin-mediated induction of non-selenoprotein antioxidant enzymes such as HO-1.

Endothelial cells in health and disease.

According to the World Health Organization, from 2014, cardiovascular diseases (CVD) are the number one cause of death worldwide. One of the key players in maintaining proper cardiovascular function is the endothelium, the inner layer of all blood vessels. This monolayer of cells on one hand serves as a barrier between blood and the surrounding tissue and on the other hand regulates many aspects of vessel function. Therefore, it is not surprising that interventions reducing the risk for CVD improve endothelial function. There is a clear correlation between endothelial dysfunction, in which the endothelial homeostasis is disturbed, and the development and progression of many CVD. Thus, the development of diagnostic tools for early detection of disturbances in endothelial homeostasis or interventions aimed at improving endothelial function after insults require a comprehensive knowledge not only of the cellular reactions to the positive or negative stimuli but also of the molecular mechanisms relaying these responses. Thus, this Forum on "endothelial cells in health and disease" focuses on key molecules and processes intimately involved in endothelial cell function and covers areas from endothelial nitric oxide synthase-dependent processes, over the group of Phox-Bem1 domain proteins, cytochrome P450 epoxygenase-derived metabolites, and pre-mRNA splicing to microRNAs. Finally, one has to conclude that keeping endothelial homeostasis is the central key for a healthy long life of the human individual.

Detection of a functional xenobiotic response element in a widely employed FoxO-responsive reporter construct.

FHRE-Luc is a promoter reporter construct that is widely used to assess the activity of FoxO (forkhead box, class O) transcription factors. We here demonstrate that this promoter construct responds to exposure of HepG2 human hepatoma cells to known agonists of the aryl hydrocarbon receptor (AhR), 3-methylcholanthrene, benzo(a)pyrene, and 6-formylindolo[3,2-b]carbazole. However, FHRE-Luc activation did not coincide with FoxO DNA binding or changes in Akt-induced FoxO phosphorylation after treatment with AhR agonists. Testing FHRE-Luc deletion constructs and using AhR-deficient cells, we found that FHRE-Luc activation by AhR agonists is due to a functional xenobiotic-response element (XRE) spanning the backbone/insert border of the reporter plasmid. In conclusion, care must be taken when using FHRE-Luc to assess FoxO activity in response to stimuli that potentially interfere with xenobiotic signaling.

Heavy metal ion-induced insulin-mimetic signaling.

The serine/threonine kinase Akt is a mediator of insulin effects on target cells and, as such, is a major regulator of fuel metabolism. Akt was demonstrated to be activated in a phosphoinositide 3'-kinase-dependent fashion by stressful stimuli, including reactive oxygen species (ROS) and certain heavy metal ions. This minireview focuses on activation of the PI3K/Akt signaling cascade by exposure of cells to transition metal ions, such as Cu(II), Zn(II) or Ni(II), and discusses potential mechanisms of Akt activation and the role of ROS therein and consequences for signaling processes downstream of Akt, including modulation of FoxO-family transcription factors. In addition, we speculate on the significance of these findings with respect to processes with which FoxO proteins are known to be involved, i.e. stress-induced senescence and selenium homeostasis.

Nickel and copper ion-induced stress signaling in human hepatoma cells: analysis of phosphoinositide 3'-kinase/Akt signaling.

Nickel compounds may act as carcinogens, affecting both initiation and promotion stages of carcinogenesis due, in large parts, to their capability of inducing DNA damage and of modulating cellular signaling cascades known to affect cellular proliferation, respectively. We have previously demonstrated that the phosphoinositide 3-kinase (PI3K)/Akt signaling cascade is stimulated in cells exposed to copper ions, resulting in phosphorylation and nuclear exclusion of FoxO transcription factors. Here, human hepatoma cells were exposed to nickel or copper ions, followed by comparative analysis of PI3K/Akt-dependent signaling. Exposure of hepatoma cells to copper ions resulted in extensive oxidation of cellular glutathione, while no such effect was detected with nickel ions. Similarly, copper ions were more than 100-fold more toxic to cells than nickel, as deduced from analyses of colony forming abilities. Despite this lack of oxidative and cytotoxic action, exposure of hepatoma cells to Ni(2+) resulted in a significant activation of Akt that was abrogated by inhibitors of PI3K. Interestingly, activation of Akt--although coincident with a phosphorylation of Akt substrates, such as glycogen synthase kinase-3--did not result in significant nuclear exclusion of FoxO1a. In line with this finding, no significant modulation of the activity of a FoxO-responsive promoter construct was observed in cells exposed to nickel ions. In summary, exposure of HepG2 human hepatoma cells to nickel ions results in stimulation of the Ser/Thr kinase Akt in a PI3K-dependent fashion, activation most likely being independent of oxidative processes. In sharp contrast to copper ions, nickel-induced Akt activation is not propagated further downstream to FoxO-dependent signaling beyond the phosphorylation of FoxO1a and 3a.

Modulation of FoxO signaling in human hepatoma cells by exposure to copper or zinc ions.

Cells respond to heavy metal stress by activating signaling cascades regulating cellular proliferation and survival. We here demonstrate that the anti-apoptotic kinase Akt is activated in HepG2 human hepatoma cells exposed to copper or zinc ions. Cu2+- and Zn2+-induced phosphorylation of Akt was blocked by phosphoinositide 3-kinase (PI3K) inhibitors, wortmannin and LY294002. Moreover, several endogenous Akt substrates were phosphorylated, including glycogen synthase kinase-3 and transcription factors of the FoxO family, FoxO1a and FoxO4. Exposure to Cu2+ or Zn2+ elicited the subcellular redistribution of an overexpressed FoxO1a-EGFP fusion protein from nucleus to cytoplasm, which was not seen with a mutant FoxO1a form devoid of Akt phosphorylation sites. Both FoxO phosphorylation and nuclear exclusion were blocked by wortmannin. Likewise, the subcellular translocation from nucleus to cytoplasm of the Caenorhabditis elegans FoxO ortholog, DAF-16, was caused in starved worms exposed to copper ions. Activity of the promoter of the human glucose 6-phosphatase gene, known to be regulated by insulin and FoxO1a, was demonstrated in reporter gene assays to be attenuated in hepatoma cells exposed to Cu2+. However, this suppression of glucose 6-phosphatase promoter activity was independent of modulation of the PI3K/Akt pathway. In summary, the PI3K/Akt pathway is activated in human hepatoma cells exposed to Cu2+ or Zn2+, resulting in the phosphorylation and subcellular relocalisation of transcription factor FoxO1a. Furthermore, copper is demonstrated to exert an insulin-mimetic effect also independently of the PI3K/Akt/FoxO pathway.