The role of ATM and ATR in the cellular response to hypoxia and re-oxygenation

Known pathogenic gene variants and new candidates detected in sudden unexpected infant death using whole genome sequencing

The purpose of this study is to gain insights into potential genetic factors contributing to the infant's vulnerability to Sudden Unexpected Infant Death (SUID). Whole Genome Sequencing (WGS) was performed on 144 infants that succumbed to SUID, and 573 healthy adults. Variants were filtered by gnomAD allele frequencies and predictions of functional consequences. Variants of interest were identified in 88 genes, in 64.6% of our cohort. Seventy-three of these have been previously associated with SIDS/SUID/SUDP. Forty-three can be characterized as cardiac genes and are related to cardiomyopathies, arrhythmias, and other conditions. Variants in 22 genes were associated with neurologic functions. Variants were also found in 13 genes reported to be pathogenic for various systemic disorders and in two genes associated with immunological function. Variants in eight genes are implicated in the response to hypoxia and the regulation of reactive oxygen species (ROS) and have not been previously described in SIDS/SUID/SUDP. Seventy-two infants met the triple risk hypothesis criteria. Our study confirms and further expands the list of genetic variants associated with SUID. The abundance of genes associated with heart disease and the discovery of variants associated with the redox metabolism have important mechanistic implications for the pathophysiology of SUID.

Known pathogenic gene variants and new candidates detected in Sudden Unexpected Infant Death using Whole Genome Sequencing

Purpose

To gain insights into potential genetic factors contributing to the infant's vulnerability to Sudden Unexpected Infant Death (SUID).

Methods

Whole Genome Sequencing (WGS) was performed on 145 infants that succumbed to SUID, and 576 healthy adults. Variants were filtered by gnomAD allele frequencies and predictions of functional consequences.

Results

Variants of interest were identified in 86 genes, 63.4% of our cohort. Seventy-one of these have been previously associated with SIDS/SUID/SUDP. Forty-three can be characterized as cardiac genes and are related to cardiomyopathies, arrhythmias, and other conditions. Variants in 22 genes were associated with neurologic functions. Variants were also found in 13 genes reported to be pathogenic for various systemic disorders. Variants in eight genes are implicated in the response to hypoxia and the regulation of reactive oxygen species (ROS) and have not been previously described in SIDS/SUID/SUDP. Seventy-two infants met the triple risk hypothesis criteria (Figure 1).

Conclusion

Our study confirms and further expands the list of genetic variants associated with SUID. The abundance of genes associated with heart disease and the discovery of variants associated with the redox metabolism have important mechanistic implications for the pathophysiology of SUID.

Primary biliary cholangitis: molecular pathogenesis perspectives and therapeutic potential of natural products

Primary biliary cirrhosis (PBC) is a chronic cholestatic immune liver disease characterized by persistent cholestasis, interlobular bile duct damage, portal inflammation, liver fibrosis, eventual cirrhosis, and death. Existing clinical and animal studies have made a good progress in bile acid metabolism, intestinal flora disorder inflammatory response, bile duct cell damage, and autoimmune response mechanisms. However, the pathogenesis of PBC has not been clearly elucidated. We focus on the pathological mechanism and new drug research and development of PBC in clinical and laboratory in the recent 20 years, to discuss the latest understanding of the pathological mechanism, treatment options, and drug discovery of PBC. Current clinical treatment mode and symptomatic drug support obviously cannot meet the urgent demand of patients with PBC, especially for the patients who do not respond to the current treatment drugs. New treatment methods are urgently needed. Drug candidates targeting reported targets or signals of PBC are emerging, albeit with some success and some failure. Single-target drugs cannot achieve ideal clinical efficacy. Multitarget drugs are the trend of future research and development of PBC drugs.

Activation of p53 in anoxic freshwater crayfish, Faxonius virilis

Tumor suppressing transcription factor p53 regulates multiple pathways including DNA repair, cell survival, apoptosis, and autophagy. The current work studies stress-induced activation of p53 in anoxic crayfish (Faxonius virilis). Relative levels of target proteins and mRNAs involved in the DNA damage response was measured in normoxic control and anoxic hepatopancreas and tail muscle. Phosphorylation levels of p53 was assessed using immunoblotting at sites known to be phosphorylated (Serine 15 and 37) in response to DNA damage or reduced oxygen signaling. The capacity for DNA binding by phospho-p53 was also measured, followed by transcript analysis of a potentially pro-apoptotic downstream target, the etoposide induced (ei24) gene. Following this, both inhibitor (MDM2) and activator (p19-ARF) protein levels in response to low oxygen stress were studied. The results showed an increase in p53 levels during anoxia in both hepatopancreases and tail muscle. Increased transcript levels of ei24, a downstream target of p53, support the activation of p53 under anoxic stress. Cytoplasmic accumulation of Ser-15 p-p53 was observed during anoxia when proteins from cytoplasmic and nuclear fractions were measured. Increased cytoplasmic concentration is known to initiate an apoptotic response, which can be assumed as a preparatory step to prevent autophagy. The results suggest that p53 might play a protective role in crayfish defense against low oxygen stress. Understanding how anoxia-tolerant organisms are able to protect against DNA damage could provide important clues towards survival under metabolic rate depression and preparation for recovery to minimize damage.

AMPK-mediated phosphorylation on 53BP1 promotes c-NHEJ

AMP-activated protein kinase (AMPK) is an energy sensor that plays roles in multiple biological processes beyond metabolism. Several studies have suggested that AMPK is involved in the DNA damage response (DDR), but the mechanisms remain unclear. Herein, we demonstrate that AMPK promotes classic non-homologous end joining (c-NHEJ) in double-strand break (DSB) repair through recruiting a key chromatin-based mediator named p53-binding protein 1 (53BP1), which facilitates the end joining of distal DNA ends during DDR. We find that the interaction of AMPK and 53BP1 spatially occurs under DSB stress. In the context of DSBs, AMPK directly phosphorylates 53BP1 at Ser1317 and promotes 53BP1 recruitment during DDR for an efficient c-NHEJ, thus maintaining genomic stability and diversity of the immune repertoire. Taken together, our study demonstrates that AMPK is a regulator of 53BP1 and controls c-NHEJ choice by phospho-regulation.

Polycyclic aromatic hydrocarbon and hypoxia exposures result in mitochondrial dysfunction in zebrafish

Organisms are routinely subjected to a variety of environmental and chemical perturbations simultaneously. Often, multi-stressor exposures result in unpredictable toxicity that occurs through unidentified mechanisms. Here, we focus on polycyclic aromatic hydrocarbons (PAHs) and hypoxia, two environmental and physiological stressors that are known to co-occur in the environment. The aim of this study was to assess whether interactive mitochondrial dysfunction resulted from co-exposures of PAHs and hypoxia. Zebrafish embryos were co-exposed to non-teratogenic concentrations of an environmental PAH mixture and hypoxia beginning at 6 hpf for an acute period of 24 h and afterwards were given either no recovery period, 45 min, 5 -hs, or 18 -hs of recovery time in clean conditions. Mitochondrial function and integrity were assessed through the use of both in ovo and in vitro assays. Hypoxia exposures resulted in drastic reductions in parameters relating to mitochondrial respiration, ATP turnover, and mitochondrial DNA integrity. PAH exposures affected ATP production and content, as well as mitochondrial membrane dynamics and lactate content. While PAH and hypoxia exposures caused a broad range of effects, there appeared to be very little interaction between the two stressors in the co-exposure group. However, because hypoxia significantly altered mitochondrial function, the possibility remains that these effects may limit an individual's ability to respond to PAH toxicity and therefore could cause downstream interactive effects.

Challenges to DNA replication in hypoxic conditions

The term hypoxia refers to any condition where insufficient oxygen is available and therefore encompasses a range of actual oxygen concentrations. The regions of tumours adjacent to necrotic areas are at almost anoxic levels and are known to be extremely therapy resistant (radiobiological hypoxia). The biological response to radiobiological hypoxia includes the rapid accumulation of replication stress and subsequent DNA damage response, including both ATR- and ATM-mediated signalling, despite the absence of detectable DNA damage. The causes and consequences of hypoxia-induced replication stress will be discussed.

Inhibition of ERK1/2 or AKT Activity Equally Enhances Radiation Sensitization in B16F10 Cells

Background

The aim of the study was to evaluate the radiation sensitizing ability of ERK1/2, PI3K-AKT and JNK inhibitors in highly radiation resistant and metastatic B16F10 cells which carry wild-type Ras and Braf.

Methods

Mouse melanoma cell line B16F10 was exposed to 1.0, 2.0 and 3.0 Gy of electron beam radiation. Phosphorylated ERK1/2, AKT and JNK levels were estimated by ELISA. Cells were exposed to 2.0 and 3.0 Gy of radiation with or without prior pharmacological inhibition of ERK1/2, AKT as well as JNK pathways. Cell death induced by radiation as well as upon inhibition of these pathways was measured by TUNEL assay using flow cytometry.

Results

Exposure of B16F10 cells to 1.0, 2.0 and 3.0 Gy of electron beam irradiation triggered an increase in all the three phosphorylated proteins compared to sham-treated and control groups. B16F10 cells pre-treated with either ERK1/2 or AKT inhibitors equally enhanced radiation-induced cell death at 2.0 as well as 3.0 Gy (P < 0.001), while inhibition of JNK pathway increased radiation-induced cell death to a lesser extent. Interestingly combined inhibition of ERK1/2 or AKT pathways did not show additional cell death compared to individual ERK1/2 or AKT inhibition. This indicates that ERK1/2 or AKT mediates radiation resistance through common downstream molecules in B16F10 cells.

Conclusions

Even without activating mutations in Ras or Braf genes, ERK1/2 and AKT play a critical role in B16F10 cell survival upon radiation exposure and possibly act through common downstream effector/s.

Linking hypoxia, DNA damage and proliferation in multicellular tumor spheroids

Background

Multicellular Tumor Spheroids are frequently used to mimic the regionalization of proliferation and the hypoxic environment within avascular tumors. Here we exploit these features to study the activation of DNA damage repair pathways and their correlation to developing hypoxia.

Methods

Activation of DNA damage repair markers, proliferation, cell death, glycogen accumulation and developing hypoxia were investigated using immunofluorescence, immuno-histochemistry, EdU incorporation, Western blots, COMET assays, and pharmacological agents in A673 Ewing sarcoma spheroids and monolayer cultures.

Results

DNA damage marker γ-H2AX is observed in the hypoxic, peri-necrotic region of growing spheroids. While most proliferating cells are seen on the spheroid surface, there are also a few Ki-67 positive cells in the hypoxic zone. The hypoxia-induced phosphorylation of H2AX to form γ-H2AX in spheroids is attenuated by the ATM inhibitor KU55933, but not the ATR inhibitor VE-821.

Conclusion

Tumor spheroids mimic tumor microenvironments such as the anoxic, hypoxic and oxic niches within solid tumors, as well as populations of cells that are viable, proliferating, and undergoing DNA damage repair processes under these different micro-environmental conditions. ATM, but not ATR, is the primary kinase responsible for γ-H2AX formation in the hypoxic core of A673 spheroids. Spheroids could offer unique advantages in testing therapeutics designed to target malignant cells that evade conventional treatment strategies by adapting to the hypoxic tumor microenvironment.

Hypoxic preconditioning decreases nuclear factor κB activity via Disrupted in Schizophrenia-1

Nuclear factor κB is a key mediator of inflammation during conditions of hypoxia. Here, we used models of hypoxic pre-conditioning as mechanism to decrease nuclear factor κB activity induced by hypoxia. Our initial studies suggested that Disrupted in Schizophrenia-1 may be induced by hypoxic pre-conditioning and possibly involved in the regulation of nuclear factor κB. In this study we used Disrupted in Schizophrenia-1 exogenous over-expression and knock-down to determine its effect on ataxia telangiectasia mutated--nuclear factor κB activation cascade. Our results demonstrated that hypoxic pre-conditioning significantly increased the expression of Disrupted in Schizophrenia-1 at mRNA and protein levels both in vitro and in vivo. Over-expression of Disrupted in Schizophrenia-1 significantly attenuated the hypoxia-mediated ataxia telangiectasia mutated phosphorylation and prevented its cytoplasm translocation where it functions to activate nuclear factor κB. We further determined that Disrupted in Schizophrenia-1 activated the protein phosphatase 2A, preventing the phosphorylation of ataxia telangiectasia mutated serine-1981, the main regulatory site of ataxia telangiectasia mutated activity. Cellular levels of Disrupted in Schizophrenia-1 protein significantly decreased nuclear factor κB activation profiles and pro-inflammatory gene expression. Taken together, these results demonstrate that hypoxic pre-conditioning decreases the activation of nuclear factor κB through the transcriptional induction of Disrupted in Schizophrenia-1.

Oxygen regulates molecular mechanisms of cancer progression and metastasis

Oxygen is the basic molecule which supports life and it truly is "god's gift to life." Despite its immense importance, research on "oxygen biology" has never received the light of the day and has been limited to physiological and biochemical studies. It seems that in modern day biology, oxygen research is summarized in one word "hypoxia." Scientists have focused on hypoxia-induced transcriptomics and molecular-cellular alterations exclusively in disease models. Interestingly, the potential of oxygen to control the basic principles of biology like homeostatic maintenance, transcription, replication, and protein folding among many others, at the molecular level, has been completely ignored. Here, we present a perspective on the crucial role played by oxygen in regulation of basic biological phenomena. Our conclusion highlights the importance of establishing novel research areas like oxygen biology, as there is great potential in this field for basic science discoveries and clinical benefits to the society.

Multifaceted control of DNA repair pathways by the hypoxic tumor microenvironment

Hypoxia, as a pervasive feature in the microenvironment of solid tumors, plays a significant role in cancer progression, metastasis, and ultimately clinical outcome. One key cellular consequence of hypoxic stress is the regulation of DNA repair pathways, which contributes to the genomic instability and mutator phenotype observed in human cancers. Tumor hypoxia can vary in severity and duration, ranging from acute fluctuating hypoxia arising from temporary blockages in the immature microvasculature, to chronic moderate hypoxia due to sparse vasculature, to complete anoxia at distances more than 150 μM from the nearest blood vessel. Paralleling the intra-tumor heterogeneity of hypoxia, the effects of hypoxia on DNA repair occur through diverse mechanisms. Acutely, hypoxia activates DNA damage signaling pathways, primarily via post-translational modifications. On a longer timescale, hypoxia leads to transcriptional and/or translational downregulation of most DNA repair pathways including DNA double-strand break repair, mismatch repair, and nucleotide excision repair. Furthermore, extended hypoxia can lead to long-term persistent silencing of certain DNA repair genes, including BRCA1 and MLH1, revealing a mechanism by which tumor suppressor genes can be inactivated. The discoveries of the hypoxic modulation of DNA repair pathways have highlighted many potential ways to target susceptibilities of hypoxic cancer cells. In this review, we will discuss the multifaceted hypoxic control of DNA repair at the transcriptional, post-transcriptional, and epigenetic levels, and we will offer perspective on the future of its clinical implications.

γ-H2AX promotes hepatocellular carcinoma angiogenesis via EGFR/HIF-1α/VEGF pathways under hypoxic condition

Hepatocellular carcinoma (HCC) is one of the most deadly cancers. Using mRNA microarray analysis, we found that H2AX decreased under hypoxic conditions. Hypoxia is an important physiological and pathological stress that induces H2AX phosphorylation (γ-H2AX), but the regulatory mechanism of γ-H2AX remains elusive in the progress of HCC. We report here that increased γ-H2AX expression in HCC is associated with tumor size, vascular invasion, TNM stage and reduced survival rate after liver transplantation (LT). γ-H2AX knockdown was able to effectively inhibit VEGF expression in vitro and tumorigenicity and angiogenesis of HCC in vivo. The mechanism of γ-H2AX on the angiogenic activity of HCC might go through EGFR/HIF-1α/VEGF pathways under hypoxic conditions. Combined γ-H2AX, HIF-1α and EGFR has better prognostic value for HCC after LT. This study suggests that γ-H2AX is associated with angiogenesis of HCC and γ-H2AX or a combination of γ-H2AX/EGFR/HIF-1α is a novel marker in the prognosis of HCC after LT and a potential therapeutic target.

The role of hypoxia inducible factor-1 alpha in bypassing oncogene-induced senescence

Oncogene induced senescence (OIS) is a sustained anti-proliferative response acutely induced in primary cells via activation of mitogenic oncogenes such as Ras/BRAF. This mechanism acts as an initial barrier preventing normal cells transformation into malignant cell. Besides oncogenic activation and DNA damage response (DDR), senescence is modulated by a plethora of other factors, and one of the most important one is oxygen tension of the tissue. The aim of this study was to determine the impact of hypoxia on Ras^V12-induced senescence in human diploid fibroblasts (HDFs). We showed here that hypoxia prevents execution of oncogene induced senescence (OIS), through a strong down-regulation of senescence hallmarks, such as SA- β-galactosidase, H3K9me3, HP1γ, p53, p21^CIP1 and p16^INK4a in association with induction of hypoxia inducible factor-1α (HIF-1α). In addition, hypoxia also decreased marks of H-Ras^V12-induced DDR in both cell lines through down-regulation of ATM/ATR, Chk1 and Chk2 phosphorylation as well as decreased γ-H2AX positivity. Utilizing shRNA system targeting HIF-1α we show that HIF-1α is directly involved in down regulation of p53 and its target p21^CIP1 but not p16^INK4a. In line with this finding we found that knock down of HIF-1α leads to a strong induction of apoptotic response, but not restoration of senescence in Ras expressing HDFs in hypoxia. This indicates that HIF-1α is an important player in early steps of tumorigenesis, leading to suppression of senescence through its negative regulation of p53 and p21^CIP1. In our work we describe a mechanism through which hypoxia and specifically HIF-1α preclude cells from maintaining senescence-driven anti proliferative response. These findings indicate the possible mechanism through which hypoxic environment helps premalignant cells to evade impingement of cellular failsafe pathways.

The versatile functions of ATM kinase

Ataxia-telangiectasia mutated (ATM) kinase, the mutation of which causes the autosomal recessive disease ataxia-telangiectasia, plays an essential role in the maintenance of genome stability. Extensive studies have revealed that activated ATM signals to a massive list of proteins to facilitate cell cycle checkpoints, DNA repair, and many other aspects of physiological responses in the event of DNA double-strand breaks. ATM also plays functional roles beyond the well-characterized DNA damage response (DDR). In this review article, we discuss the recent findings on the molecular mechanisms of ATM in DDR, the mitotic spindle checkpoint, as well as hyperactive ATM signaling in cancer invasion and metastasis.

CRHR1 mediates p53 transcription induced by high altitude hypoxia through ERK 1/2 signaling in rat hepatic cells

We have previously reported that hypoxia activates corticotrophin-releasing hormone (CRH) and the expression of its type-1 receptor (CRHR1) and induces disorders of the brain-endocrine-immune network. p53 is activated by hypoxia and involved in tumorigenesis and apoptosis. Whether CRHR1 regulates p53 transactivation to further influence apoptotic genes remains unclear. Here, we showed that hypoxia at a simulated altitude of 5km or 7km for 8 and 24h increased p53 protein and mRNA, and reduced apoptotic bax and IGFBP3 gene expression while upregulating the cell-arrest gene p21 for 8h in rat liver cells. The upregulation of p53 mRNA and downregulation of bax mRNA induced by hypoxia were blocked by pretreatment with the specific CRHR1 antagonist CP-154,526, but the downregulation of IGFBP3 and upregulation of p21 mRNA were not. Furthermore, CRH stimulated p53 mRNA via the ERK 1/2 pathway in the BRL-3A cell line and this was blocked by the ERK 1/2 antagonist U0126. These data provide novel evidence that the CRHR1-triggered ERK 1/2 pathway is involved in the activation of p53 and suppression of the apoptotic bax gene by hypoxia in rat liver.

Targeting ATR in vivo using the novel inhibitor VE-822 results in selective sensitization of pancreatic tumors to radiation

Combined radiochemotherapy is the currently used therapy for locally advanced pancreatic ductal adenocarcinoma (PDAC), but normal tissue toxicity limits its application. Here we test the hypothesis that inhibition of ATR (ATM-Rad3-related) could increase the sensitivity of the cancer cells to radiation or chemotherapy without affecting normal cells. We tested VE-822, an ATR inhibitor, for in vitro and in vivo radiosensitization. Chk1 phosphorylation was used to indicate ATR activity, γH2AX and 53BP1 foci as evidence of DNA damage and Rad51 foci for homologous recombination activity. Sensitivity to radiation (XRT) and gemcitabine was measured with clonogenic assays in vitro and tumor growth delay in vivo. Murine intestinal damage was evaluated after abdominal XRT. VE-822 inhibited ATR in vitro and in vivo. VE-822 decreased maintenance of cell-cycle checkpoints, increased persistent DNA damage and decreased homologous recombination in irradiated cancer cells. VE-822 decreased survival of pancreatic cancer cells but not normal cells in response to XRT or gemcitabine. VE-822 markedly prolonged growth delay of pancreatic cancer xenografts after XRT and gemcitabine-based chemoradiation without augmenting normal cell or tissue toxicity. These findings support ATR inhibition as a promising new approach to improve the therapeutic ration of radiochemotherapy for patients with PDAC.

Intrauterine growth restriction modifies the normal gene expression in kidney from rabbit fetuses

The aim of this work was to study the effect of intrauterine growth restriction (IUGR) on fetal kidneys. The IUGR was induced by uteroplacental vessels ligature in a model of pregnant rabbit. We centralized the study in the gene expression of essential proteins for fetal kidney development and kidney protection against hypoxia, osmotic stress, and kidney injury. The gene expression of HIF-1α, NFAT5, IL-1β, NGAL, and ATM were studied by qRT-PCR and Western blot in kidneys from control and IUGR fetuses. Experimental IUGR fetuses were significantly smaller than the control animals (39 vs. 48 g, p<0.05). The number of glomeruli was decreased in IUGR kidneys, without morphological alterations. IUGR increased the gene expression of HIF-1α, NFAT5, IL-1β, NGAL, and ATM (p<0.05) in kidneys of fetuses undergoing IUGR, suggesting that fetal blood flow restriction produce alterations in gene expression in fetal kidneys.

Human embryonic stem cells fail to activate CHK1 and commit to apoptosis in response to DNA replication stress

Pluripotent cells of the early embryo, to which embryonic stem cells (ESCs) correspond, give rise to all the somatic cells of the developing fetus. Any defects that occur in their genome or epigenome would have devastating consequences. Genetic and epigenetic change in human ESCs appear to be an inevitable consequence of long-term culture, driven by selection of variant cells that have a higher propensity for self-renewal rather than either differentiation or death. Mechanisms underlying the potentially separate events of mutation and subsequent selection of variants are poorly understood. Here, we show that human ESCs and their malignant counterpart, embryonal carcinoma (EC) cells, both fail to activate critical S-phase checkpoints when exposed to DNA replication inhibitors and commit to apoptosis instead. Human ESCs and EC cells also fail to form replication protein A, γH2AX, or RAD51 foci or load topoisomerase (DNA) II binding protein 1 onto chromatin in response to replication inhibitors. Furthermore, direct measurements of single-stranded DNA (ssDNA) show that these cells fail to generate the ssDNA regions in response to replication stress that are necessary for the activation of checkpoints and the initiation of homologous recombination repair to protect replication fork integrity and restart DNA replication. Taken together, our data suggest that pluripotent cells control genome integrity by the elimination of damaged cells through apoptosis rather than DNA repair, and therefore, mutations or epigenetic modifications resulting in an imbalance in cell death control could lead to genetic instability.

Activation of the ATM-Snail pathway promotes breast cancer metastasis

The DNA damage response (DDR) is critical for the maintenance of genetic stability and serves as an anti-cancer barrier during early tumorigenesis. However, the role of the DDR in tumor progression and metastasis is less known. Here, we demonstrate that the ATM kinase, one of the critical DDR elements, is hyperactive in late stage breast tumor tissues with lymph-node metastasis and this hyperactivity correlates with elevated expression of the epithelial-mesenchymal transition marker, Snail. At the molecular level, we demonstrate that ATM regulates Snail stabilization by phosphorylation on Serine-100. Using mass spectrometry, we identified HSP90 as a critical binding protein of Snail in response to DNA damage. HSP90 binds to and stabilizes phosphorylated Snail. We further provide in vitro and in vivo evidence that activation of ATM-mediated Snail phosphorylation promotes tumor invasion and metastasis. Finally, we demonstrate that Snail Serine-100 phosphorylation is elevated in breast cancer tissues with lymph-node metastasis, indicating clinical significance of the ATM-Snail pathway. Together, our findings provide strong evidence that the ATM-Snail pathway promotes tumor metastasis, highlighting a previously undescribed role of the DDR in tumor invasion and metastasis.

NFAT5 is activated by hypoxia: role in ischemia and reperfusion in the rat kidney

The current hypothesis postulates that NFAT5 activation in the kidney's inner medulla is due to hypertonicity, resulting in cell protection. Additionally, the renal medulla is hypoxic (10–18 mmHg); however there is no information about the effect of hypoxia on NFAT5. Using in vivo and in vitro models, we evaluated the effect of reducing the partial pressure of oxygen (PO₂) on NFAT5 activity. We found that 1) Anoxia increased NFAT5 expression and nuclear translocation in primary cultures of IMCD cells from rat kidney. 2) Anoxia increased transcriptional activity and nuclear translocation of NFAT5 in HEK293 cells. 3) The dose-response curve demonstrated that HIF-1α peaked at 2.5% and NFAT5 at 1% of O₂. 4) At 2.5% of O₂, the time-course curve of hypoxia demonstrated earlier induction of HIF-1α gene expression than NFAT5. 5) siRNA knockdown of NFAT5 increased the hypoxia-induced cell death. 6) siRNA knockdown of HIF-1α did not affect the NFAT5 induction by hypoxia. Additionally, HIF-1α was still induced by hypoxia even when NFAT5 was knocked down. 7) NFAT5 and HIF-1α expression were increased in kidney (cortex and medulla) from rats subjected to an experimental model of ischemia and reperfusion (I/R). 7) Experimental I/R increased the NFAT5-target gene aldose reductase (AR). 8) NFAT5 activators (ATM and PI3K) were induced in vitro (HEK293 cells) and in vivo (I/R kidneys) with the same timing of NFAT5. 8) Wortmannin, which inhibits ATM and PI3K, reduces hypoxia-induced NFAT5 transcriptional activation in HEK293 cells. These results demonstrate for the first time that NFAT5 is induced by hypoxia and could be a protective factor against ischemic damage.

Hypoxia activates tumor suppressor p53 by inducing ATR-Chk1 kinase cascade-mediated phosphorylation and consequent 14-3-3γ inactivation of MDMX protein

It has been known that p53 can be induced and activated by hypoxia, an abnormal condition that often occurs in rapidly growing solid tumors or when normal tissues undergo ischemia. Although the ATR-Chk1 kinase cascade was associated with hypoxia-induced p53 activation, molecules that directly link this hypoxia-ATR-Chk1 pathway to p53 activation have been elusive. Here, we showed that hypoxia could induce phosphorylation of MDMX at Ser-367 and enhance the binding of this phosphorylated MDMX to 14-3-3γ, consequently leading to p53 activation. A Chk1 inhibitor or knockdown of ATR and Chk1 inhibited the phosphorylation of MDMX at Ser-367 and impaired the binding of MDMX to 14-3-3γ in addition to p53 activation in response to hypoxia. In primary mouse embryonic fibroblast cells that harbor a mutant MDMX, including the S367A mutation, hypoxia also failed to induce the binding of this mutant MDMX to 14-3-3γ and to activate p53 and its direct targets. These results demonstrate that hypoxia can activate p53 through inactivation of MDMX by the ATR-Chk1-MDMX-14-3-3γ pathway.

Hypoxic activation of ATR and the suppression of the initiation of DNA replication through cdc6 degradation

Many severely hypoxic cells fail to initiate DNA replication, but the mechanism underlying this observation is unknown. Specifically, while the ataxia-telangiectasia-rad3 related (ATR) kinase has been shown to be activated in hypoxic cells, several studies have not been able to document down-stream consequences of ATR activation in these cells. By clearly defining the DNA replication initiation checkpoint in hypoxic cells, we now demonstrate that ATR is responsible for activating this checkpoint. We show that the hypoxic activation of ATR leads to the phosphorylation dependent degradation of the cdc25a phosphatase. Down regulation of cdc25a protein by ATR in hypoxic cells decreases CDK2 phosphorylation and activity, which results in the degradation of cdc6 by APC/C^Cdh1. These events do not occur in hypoxic cells when ATR is depleted, and the initiation of DNA replication is maintained. We therefore present a novel mechanism of cdc6 regulation in which ATR can play a central role in inhibiting the initiation of DNA replication via the regulation of cdc6 by APC/C^Cdh1. This model provides insight into the biology and therapy of hypoxic tumors.

New insights into the roles of ATM and DNA-PKcs in the cellular response to oxidative stress

Reactive oxygen species (ROS) are induced by a variety of endogenous and exogenous sources. At pathologically high levels, ROS cause damage to biological molecules, including DNA. The damage sustained by DNA likely plays a key role in the pathogenesis of aging and carcinogenesis. Extensive research has established in detail the mechanism of cellular response to oxidative stress. Attention is now focused on identifying the molecular contributions of the key DNA damage response kinases ataxia telangiectasia mutated (ATM), DNA-dependent protein kinase catalytic subunit (DNA-PKcs), and ATM- and Rad3-related (ATR) in the oxidative stress response. In this review, we will provide an update of the current evidence regarding the involvement of these related DNA damage response kinases in oxidative DNA lesion repair and signaling responses. The growing understanding of the involvement of ATM, DNA-PKcs, and ATR in the oxidative stress response will offer new possibilities for the treatment of ROS-related diseases.

Exposure to hypoxia following irradiation increases radioresistance in prostate cancer cells

BACKGROUND

Tumor hypoxia is a common feature of prostate tumors associated with the stabilization of hypoxia-inducible-factor 1alpha (HIF-1α) and poor prognosis following radiation therapy. Lack of oxygen at the time of irradiation is associated with radioresistance, but recent reports suggest radioresponse is also modulated by the dynamic nature of tumor hypoxia.

OBJECTIVE

We proposed to evaluate the effect of post-irradiation hypoxic exposure on the radioresponse of 2 prostate cancer (CaP) cell lines (22Rv1, DU145) and to examine whether it correlates with modified cellular responses induced by hypoxia.

METHODS AND RESULTS

Aerobic and hypoxic CaP cells exposed to hypoxia (24 h) after irradiation (4 Gy) gained a survival advantage compared with cells fully oxygenated after irradiation. This survival advantage was associated with induction of a G2/M cell cycle arrest, reduced induction of apoptosis and decreased amount of senescent cells. These modified cellular responses appeared mediated by HIF-1α.

CONCLUSION

Our data suggest that targeting hypoxia after irradiation may benefit patients with aggressive hypoxic prostate tumors.

Regulation of cell cycle components during exposure to anoxia or dehydration stress in the wood frog, Rana sylvatica

The wood frog (Rana sylvatica) exhibits a well-developed natural anoxia and dehydration tolerance. The degree of stress tolerance depends on numerous biochemical adaptations, including stress-induced hypometabolism that helps to preserve long-term viability by reducing ATP demand. We hypothesized that the mechanisms involved in cell cycle control could act to aid in the establishment of the hypometabolic state required for stress survival. Selected proteins involved in the proliferation of cells were evaluated using immunoblotting in liver and skeletal muscle of wood frogs comparing controls with animals subjected to either 24-hr anoxia exposure under a nitrogen gas atmosphere or dehydration to 40% of total body water lost (all at 5°C). Levels of cyclins (type A, B, D, and E) decreased significantly under both stresses in liver and skeletal muscle. Similar reductions were seen for Cyclin-dependant kinases (Cdk) types 2, 4, and 6 in both liver and skeletal muscle; however, an increase in the relative amount of phosphorylated inactive p-Cdk (Thr14/Tyr15) was observed in liver under both stresses. Levels of positive regulators of Cdk activity (Cdc25 type A and C) were significantly reduced in both tissues under both stresses, whereas negative regulators of Cdk activity (p16(INK4a) and p27(KIP1) ) increased significantly in liver under both anoxia and dehydration stress (but not in muscle). This study provides the first report of differential regulation of cell cycle components in an anoxia and dehydration tolerant vertebrate, the wood frog, suggesting that cell cycle suppression is an active part of stress resistance and life extension in hypometabolic states.

Perspectives in cell cycle regulation: lessons from an anoxic vertebrate

The ability of an animal, normally dependent on aerobic respiration, to suspend breathing and enter an anoxic state for long term survival is clearly a fascinating feat, and has been the focus of numerous biochemical studies. When anoxia tolerant turtles are faced with periods of oxygen deprivation, numerous physiological and biochemical alterations take place in order to facilitate vital reductions in ATP consumption. Such strategies include reversible post-translational modifications as well as the implementation of translation and transcription controls facilitating metabolic depression. Although it is clear that anoxic survival relies on the suppression of ATP consuming processes, the state of the cell cycle in anoxia tolerant vertebrates remain elusive. Several anoxia tolerant invertebrate and embryonic vertebrate models display cell cycle arrest when presented with anoxic stress. Despite this, the cell cycle has not yet been characterized for anoxia tolerant turtles. Understanding how vertebrates respond to anoxia can have important clinical implications. Uncontrollable cellular proliferation and hypoxic tumor progression are inescapably linked in vertebrate tissues. Consequentially, the molecular mechanisms controlling these processes have profound clinical consequences. This review article will discuss the theory of cell cycle arrest in anoxic vertebrates and more specifically, the control of the retinoblastoma pathway, the molecular markers of cell cycle arrest, the activation of checkpoint kinases, and the possibility of translational controls implemented by microRNAs.

Exposure to acute hypoxia induces a transient DNA damage response which includes Chk1 and TLK1

Severe hypoxia has been demonstrated to induce a replication arrest which is associated with decreased levels of nucleotides. Chk1 is rapidly phosphorylated in response to severe hypoxia and in turn deactivates TLK1 through phosphorylation. Loss of Chk1 has been shown to sensitize cells to hypoxia/reoxygenation. After short (acute) exposure to hypoxia this is due to an increased rate of reoxygenation-induced replication restart and subsequent p53-dependent apoptosis. After longer (chronic) exposure to hypoxia S phase cells do not undergo reoxygenation-induced replication restart. Cells exposed to these levels of hypoxia however are sensitive to loss of Chk1. This suggests a new role for Chk1 in the cell cycle response to reoxygenation.

ATR (ataxia telangiectasia mutated- and Rad3-related kinase) is activated by mild hypothermia in mammalian cells and subsequently activates p53

In vitro cultured mammalian cells respond to mild hypothermia (27-33 °C) by attenuating cellular processes and slowing and arresting the cell cycle. The slowing of the cell cycle at the upper range (31-33 °C) and its complete arrest at the lower range (27-28 °C) of mild hypothermia is effected by the activation of p53 and subsequent expression of p21. However, the mechanism by which cold is perceived in mammalian cells with the subsequent activation of p53 has remained undetermined. In the present paper, we report that the exposure of Chinese-hamster ovary-K1 cells to mildly hypothermic conditions activates the ATR (ataxia telangiectasia mutated- and Rad3-related kinase)-p53-p21 signalling pathway and is thus a key pathway involved in p53 activation upon mild hypothermia. In addition, we show that although p38MAPK (p38 mitogen-activated protein kinase) is also involved in activation of p53 upon mild hypothermia, this is probably the result of activation of p38MAPK by ATR. Furthermore, we show that cold-induced changes in cell membrane lipid composition are correlated with the activation of the ATR-p53-p21 pathway. Therefore we provide the first mechanistic detail of cell sensing and signalling upon mild hypothermia in mammalian cells leading to p53 and p21 activation, which is known to lead to cell cycle arrest.

mTORC1 signaling under hypoxic conditions is controlled by ATM-dependent phosphorylation of HIF-1α

The mTOR complex-1 (mTORC1) coordinates cell growth and metabolism, acting as a restriction point under stress conditions such as low oxygen tension (hypoxia). Hypoxia suppresses mTORC1 signaling. However, the signals by which hypoxia suppresses mTORC1 are only partially understood, and a direct link between hypoxia-driven physiological stress and the regulation of mTORC1 signaling is unknown. Here we show that hypoxia results in ataxia telangiectasia mutated (ATM)-dependent phosphorylation of hypoxia-inducible factor 1-alpha (HIF-1α) on serine(696) and mediates downregulation of mTORC1 signaling. Deregulation of these pathways in pediatric solid tumor xenografts suggests a link between mTORC1 dysregulation and solid tumor development and points to an important role for hypoxic regulation of mTORC1 activity in tumor development.

Novel Ser/Thr protein phosphatase 5 (PP5) regulated targets during DNA damage identified by proteomics analysis

The DNA damage response likely includes a global phosphorylation signaling cascade process for sensing the damaged DNA condition and coordinating responses to cope with and repair the perturbed cellular state. We utilized a label-free liquid chromatography-mass spectrometry approach to evaluate changes in protein phosphorylation associated with PP5 activity during the DNA damage response. Biological replicate analyses of bleomycin-treated HeLa cells expressing either WT-PP5 or mutant inactive PP5 lead to the identification of six potential target proteins of PP5 action. Four of these putative targets have been previously reported to be involved in DNA damage responses. Using phospho-site specific antibodies, we confirmed that phosphorylation of one target, ribosomal protein S6, was selectively decreased in cells overexpressing catalytically inactive PP5. Our findings also suggest that PP5 may play a role in controlling translation and in regulating substrates for proline-directed kinases, such as MAP kinases and cyclin-dependent protein kinases that are involved in response to DNA damage.

Functional significance for a heterogenous ribonucleoprotein A18 signature RNA motif in the 3'-untranslated region of ataxia telangiectasia mutated and Rad3-related (ATR) transcript

The predominantly nuclear heterogenous ribonucleoprotein A18 (hnRNP A18) translocates to the cytosol in response to cellular stress and increases translation by specifically binding to the 3'-untranslated region (UTR) of several mRNA transcripts and the eukaryotic initiation factor 4G. Here, we identified a 51-nucleotide motif that is present 11.49 times more often in the 3'-UTR of hnRNP A18 mRNA targets than in the UniGene data base. This motif was identified by computational analysis of primary sequences and secondary structures of hnRNP A18 mRNA targets against the unaligned sequences. Band shift analyses indicate that the motif is sufficient to confer binding to hnRNP A18. A search of the entire UniGene data base indicates that the hnRNP A18 motif is also present in the 3'-UTR of the ataxia telangiectasia mutated and Rad3-related (ATR) mRNA. Validation of the predicted hnRNP A18 motif is provided by amplification of endogenous ATR transcript on polysomal fractions immunoprecipitated with hnRNP A18. Moreover, overexpression of hnRNP A18 results in increased ATR protein levels and increased phosphorylation of Chk1, a preferred ATR substrate, in response to UV radiation. In addition, our data indicate that inhibition of casein kinase II or GSK3beta significantly reduced hnRNP A18 cytosolic translocation in response to UV radiation. To our knowledge, this constitutes the first demonstration of a post-transcriptional regulatory mechanism for ATR activity. hnRNP A18 could thus become a new target to trigger ATR activity as back-up stress response mechanisms to functionally compensate for absent or defective responders.

Crosstalk between the DNA damage response, histone modifications and neovascularisation

Neovascularisation is critical in several malignant and inflammatory conditions, as well as in the course of eye disorders. During new vessel formation, endothelial cell functions, such as proliferation and sprouting are very important and are regulated by a variety of growth factors. The DNA damage response machinery as well as factors regulating histone modifications, such as histone deacetylases, regulate cell fate as well as gene expression. Recent evidence has pointed to potential interactions among BRCA1, H2AX and SIRT1 in these intracellular pathways and neovascularisation, which will be reviewed here.

Histone H2AX is integral to hypoxia-driven neovascularization

H2A histone family member X (H2AX, encoded by H2AFX) and its C-terminal phosphorylation (gamma-H2AX) participates in the DNA damage response and mediates DNA repair. Hypoxia is a physiological stress that induces a replication-associated DNA damage response. Moreover, hypoxia is the major driving force for neovascularization, as the hypoxia-mediated induction of vascular growth factors triggers endothelial cell proliferation. Here we studied the role of the hypoxia-induced DNA damage response in endothelial cell function and in hypoxia-driven neovascularization in vivo. Hypoxia induced replication-associated generation of gamma-H2AX in endothelial cells in vitro and in mice. Both in cultured cells and in mice, endothelial cell proliferation under hypoxic conditions was reduced by H2AX deficiency. Whereas developmental angiogenesis was not affected in H2afx(-/-) mice, hypoxia-induced neovascularization during pathologic proliferative retinopathy, in response to hind limb ischemia or during tumor angiogenesis was substantially lower in H2afx(-/-) mice. Moreover, endothelial-specific H2afx deletion resulted in reduced hypoxia-driven retina neovascularization and tumor neovascularization. Our findings establish that H2AX, and hence activation of the DNA repair response, is needed for endothelial cells to maintain their proliferation under hypoxic conditions and is crucial for hypoxia-driven neovascularization.

Reactive oxygen species-independent oxidation of thioredoxin in hypoxia: inactivation of ribonucleotide reductase and redox-mediated checkpoint control

We have investigated the role of cellular redox state on the regulation of cell cycle in hypoxia and shown that whereas cells expressing mutant thioredoxin (Trx) or a normal level of Trx undergo increased apoptosis, cells overexpressing Trx are protected against apoptosis. We show that hypoxia activates p53 and Chk1/Chk2 proteins in cells expressing normal or mutant Trx but not in cells overexpressing Trx. We also show that the activity of ribonucleotide reductase decreases in hypoxia in cells expressing redox-inactive Trx. Although hypoxia has been shown to induce reactive oxygen species (ROS) generation in the mitochondria resulting in enhanced p53 expression, our data demonstrate that hypoxia-induced p53 expression and phosphorylation are independent of ROS. Furthermore, hypoxia induces oxidation of Trx, and this oxidation is potentiated in the presence of 6-aminonicotinamide, an inhibitor of glucose-6-phosphate dehydrogenase. Taken together our study shows that Trx redox state is modulated in hypoxia independent of ROS and is a critical determinant of cell cycle regulation.

The Warburg effect suppresses oxidative stress induced apoptosis in a yeast model for cancer

Background

Otto Warburg observed that cancer cells are often characterized by intense glycolysis in the presence of oxygen and a concomitant decrease in mitochondrial respiration. Research has mainly focused on a possible connection between increased glycolysis and tumor development whereas decreased respiration has largely been left unattended. Therefore, a causal relation between decreased respiration and tumorigenesis has not been demonstrated.

Methodology/Principal Findings

For this purpose, colonies of Saccharomyces cerevisiae, which is suitable for manipulation of mitochondrial respiration and shows mitochondria-mediated cell death, were used as a model. Repression of respiration as well as ROS-scavenging via glutathione inhibited apoptosis and conferred a survival advantage during seeding and early development of this fast proliferating solid cell population. In contrast, enhancement of respiration triggered cell death.

Conclusion/Significance

Thus, the Warburg effect might directly contribute to the initiation of cancer formation - not only by enhanced glycolysis - but also via decreased respiration in the presence of oxygen, which suppresses apoptosis.

Proinflammatory cytokine-induced cellular senescence of biliary epithelial cells is mediated via oxidative stress and activation of ATM pathway: a culture study

Cellular senescence is reportedly involved in cholangiopathy in primary biliary cirrhosis and oxidative stress is proposed as a pathogenetic factor in biliary epithelial cells (BECs). This study investigated the involvement of proinflammatory cytokines (IFN-beta, IFN-gamma and TNF-alpha) and ataxia telangiectasia-mutated (ATM)/p53/ p21(WAF1/Cip1) pathway with respect to oxidative stress in cellular senescence of BECs. H(2)O(2) treatment (oxidative stress) induced phosphorylation (activation) of ATM and p53 and also p21(WAF1/Cip1) expression in BECs. Treatment with inflammatory cytokines generated reactive oxygen species (ROS) in cultured BECs followed by activation of the ATM/p53/p21(WAF1/Cip1) pathway and the induction of cellular senescence. Pre-treatment with ATM inhibitor (2-aminopurine) and antioxidant (N-acetylcysteine) significantly blocked the cellular senescence of BECs induced by oxidative stress or inflammatory cytokines. In conclusion, proinflammatory cytokines induce ROS generation and activate the ATM/p53/p21(WAF1/Cip1) pathway, followed by biliary epithelial senescence. This senescent process may be involved in the development of destructive cholangiopathy in humans.

The increase of cell-membranous phosphatidylcholines containing polyunsaturated fatty acid residues induces phosphorylation of p53 through activation of ATR

The G1 phase of the cell cycle is marked by the rapid turnover of phospholipids. This turnover is regulated by CTP:phosphocholine-cytidylyltransferase (CCT) and group VIA Ca(2+)-independent-phospholipase A(2) (iPLA(2)). We previously reported that inhibition of iPLA(2) arrests cells in G1 phase of the cell cycle by activating the p53-p21 checkpoint. Here we further characterize the mechanism of p53 activation. We show that specific inhibition of iPLA(2) induces a time dependent phosphorylation of Ser15 in p53 in the absence of DNA damage. This phosphorylation requires the kinase ataxia-telangiectasia and Rad-3-related (ATR) but not the ataxia-telangiectasia-mutated (ATM) kinase. Moreover, we identify in cell membranes a significant increase of phosphatidylcholines (PCs) containing chains of polyunsaturated fatty acids and a decrease of PCs containing saturated fatty acids in response to inhibition of iPLA(2). The time course of phosphorylation of Ser15 in p53 correlates with increasing levels of PCs containing polyunsaturated fatty acids. We further demonstrate that the PCs with linoleic acid in their sn-2 position (18:2n6) induce phosphorylation of Ser15 in p53 in an ATR-dependent manner. Our findings establish that cells can regulate the levels of polyunsaturated fatty acids in phospholipids through iPLA(2)-mediated deacylation of PCs. Disruption of this regulation increases the proportions of PCs containing polyunsaturated fatty acids and activates the ATR-p53 signalling pathway.

Gene regulation under low oxygen: holding your breath for transcription

Oxygen is both an environmental and developmental signal that governs important cellular pathways. Therefore, hypoxia (or low oxygen tensions) is part of both physiological and pathological processes. To deal with hypoxic conditions, cells and organisms have evolved exquisite mechanisms for adaptation and survival. The cellular responses are reliant on controlled transcriptional and post-transcriptional events, where certain genes are positively regulated and others either remain inactive or are actively repressed. It has been known for some time that, during hypoxia, transcription is mainly regulated by the hypoxia inducible factor (HIF). However, recently it has been demonstrated that additional transcription factors are also activated and that non-HIF-dependent processes are involved in the hypoxic stress response. Therefore, gene expression following hypoxia is the result of combined effects on transcription, translation and adjustment mechanisms such as the induction of microRNAs and changes in chromatin.

UVC inhibits HIF-1α protein translation by a DNA damage- and topoisomerase I-independent pathway

Hypoxia inducible factor 1 (HIF-1) is a key player in cancer progression and an attractive target for cancer therapy. Several small molecule inhibitors of HIF-1alpha also induce a DNA damage response. However, whether or not DNA damage is required for or associated with the inhibition of HIF-1alpha protein accumulation is poorly understood. In this report we investigated the effects of distinct DNA damaging conditions on the hypoxic induction of HIF-1alpha protein in cancer cell lines. We demonstrate that in addition to topotecan (TPT), a known inhibitor of HIF-1alpha, UVC, but not other DNA damaging agents (cisplatin, ionizing radiation and doxorubicin), inhibited HIF-1alpha protein accumulation in a dose-dependent, p53-independent fashion. Low doses UVC decreased HIF-1alpha translation without affecting global protein synthesis. Inhibition of HIF-1alpha by UVC required ongoing RNA transcription, but not DNA replication. Moreover, a functional ATR was required for the activation of DNA damage-dependent responses by both UVC and TPT, but was dispensable for the inhibition of HIF-1alpha protein. Notably, unlike TPT, inhibition of HIF-1alpha protein by UVC did not require topoisomerase I, suggesting a similar yet distinct mode of action. Our data reveal that UVC is a novel signal associated with inhibition of HIF-1alpha protein accumulation, and they uncouple the DNA damage-dependent signaling pathway exerted by UVC and TPT from HIF-1alpha inhibition.

Radiation-induced cell signaling: inside-out and outside-in

Exposure of tumor cells to clinically relevant doses of ionizing radiation causes DNA damage as well as mitochondria-dependent generation of reactive oxygen species. DNA damage causes activation of ataxia telangiectasia mutated and ataxia telangiectasia mutated and Rad3-related protein, which induce cell cycle checkpoints and also modulate the activation of prosurvival and proapoptotic signaling pathways, such as extracellular signal-regulated kinase 1/2 (ERK1/2) and c-Jun NH(2)-terminal kinase 1/2, respectively. Radiation causes a rapid reactive oxygen species-dependent activation of ERBB family and other tyrosine kinases, leading to activation of RAS proteins and multiple protective downstream signaling pathways (e.g., AKT and ERK1/2), which alter transcription factor function and the apoptotic threshold of cells. The initial radiation-induced activation of ERK1/2 can promote the cleavage and release of paracrine ligands, which cause a temporally delayed reactivation of receptors and intracellular signaling pathways in irradiated and unirradiated bystander cells. Hence, signals from within the cell can promote activation of membrane-associated receptors, which signal back into the cytosol: signaling from inside the cell outward to receptors and then inward again via kinase pathways. However, cytosolic signaling can also cause release of membrane-associated paracrine factors, and thus, paracrine signals from outside of the cell can promote activation of growth factor receptors: signaling from the outside inward. The ultimate consequence of these signaling events after multiple exposures may be to reprogram the irradiated and affected bystander cells in terms of their expression levels of growth-regulatory and cell survival proteins, resulting in altered mitogenic rates and thresholds at which genotoxic stresses cause cell death. Inhibition of signaling in one and/or multiple survival pathways enhances radiosensitivity. Prolonged inhibition of any one of these pathways, however, gives rise to lineages of cells, which have become resistant to the inhibitor drug, by evolutionary selection for the clonal outgrowth of cells with point mutations in the specific targeted protein that make the target protein drug resistant or by the reprogramming of multiple signaling processes within all cells, to maintain viability. Thus, tumor cells are dynamic with respect to their reliance on specific cell signaling pathways to exist and rapidly adapt to repeated toxic challenges in an attempt to maintain tumor cell survival.

Spermidine/spermine N1-acetyltransferase overexpression in kidney epithelial cells disrupts polyamine homeostasis, leads to DNA damage, and causes G2 arrest

Expression of spermidine/spermine N1-acetyltransferase (SSAT) increases in kidneys subjected to ischemia-reperfusion injury (IRI). Increased expression of SSAT in vitro leads to alterations in cellular polyamine content, depletion of cofactors and precursors of polyamine synthesis, and reduced cell proliferation. In our model system, a >28-fold increase in SSAT levels in HEK-293 cells leads to depletion of polyamines and elevation in the enzymatic activities of ornithine decarboxylase and S-adenosylmethionine decarboxylase, suggestive of a compensatory reaction to increased polyamine catabolism. Increased expression of SSAT also led to DNA damage and G2 arrest. The increased DNA damage was primarily due to the depletion of polyamines. Other factors such as increased production of H2O2 due to polyamine oxidase activity may play a secondary role in the induction of DNA lesions. In response to DNA damage the ATM/ATR → Chk1/2 DNA repair and cell cycle checkpoint pathways were activated, mediating the G2 arrest in SSAT-expressing cells. In addition, the activation of ERK1 and ERK2, which play integral roles in the G2/M transition, is impaired in cells expressing SSAT. These results indicate that the disruption of polyamine homeostasis due to enhanced SSAT activity leads to DNA damage and reduced cell proliferation via activation of DNA repair and cell cycle checkpoint and disruption of Raf → MEK → ERK pathways. We propose that in kidneys subjected to IRI, one mechanism through which increased expression of SSAT may cause cellular injury and organ damage is through induction of DNA damage and the disruption of cell cycle.

SUMOylation attenuates sensitivity toward hypoxia- or desferroxamine-induced injury by modulating adaptive responses in salivary epithelial cells

Hypoxic stress activates various signal transduction pathways including posttranslational modification with the ubiquitin-like SUMO protein (SUMOylation). However, the molecular mechanisms by which SUMOylation regulates hypoxic responses remain unclear. Here, we investigated the ability of rat salivary Pa-4 epithelial cells to resist cell injury elicited by 1% O(2)- or hypoxia-mimetic desferroxamine (DFO)-stimulated SUMOylation processes. By using Pa-4 cells stably transduced with lenti-SUMO-1 and a cell-permeant peptide harboring SUMO-binding motif to interfere with SUMO-dependent protein-protein interactions, we demonstrate that SUMOylation augments cell survival against DFO treatment. This appeared to be partly mediated through attenuation of Protein Kinase C (PKC)-delta activation and caspase-3 cleavage, hallmarks of pro-apoptotic signaling. Intriguingly, DFO-induced phosphorylation of DNA damage marker ataxia-telangiectasia-mutated protein S1981 preceded activation of PKCdelta and caspase-3. Constitutive SUMOylation facilitated 1% O(2)- or DFO-induced nuclear factor kappaB transactivation, possibly via activation of genotoxic signaling cascade. In addition, we observed transient preservation of transepithelial electrical resistance during the early stage of hypoxia (1% O(2)) as well as enhanced transepithelial electrical resistance recovery after prolonged hypoxia in SUMO-1-expressing cell monolayers. In conclusion, our results unveil a previously unrecognized mechanism by which SUMOylation and activation of ataxia-telangiectasia-mutated protein, PKCdelta, caspase-3, and nuclear factor kappaB signaling pathways modulate salivary adaptive responses to stress in cells exposed to either 1% O(2) or DFO.