DNA damage-inducible transcripts in mammalian cells

Novel Radiation Response Genes Identified in Gene-Trapped MCF10A Mammary Epithelial Cells

We have used a gene-trapping strategy to screen human mammary epithelial cells for radiation response genes. Relative mRNA expression levels of five candidate genes in MCF10A cells were analyzed, both with and without exposure to radiation. In all five cases, the trapped genes were significantly down-regulated after radiation treatment. Sequence analysis of the fusion transcripts identified the trapped genes: (1) the human androgen receptor, (2) the uncharacterized DREV1 gene, which has known homology to DNA methyltransferases, (3) the human creatine kinase gene, (4) the human eukaryotic translation elongation factor 1 beta 2, and (5) the human ribosomal protein L27. All five genes were down-regulated significantly after treatment with varying doses of ionizing radiation (0.10 to 4.0 Gy) and at varying times (2-30 h after treatment). The genes were also analyzed in human fibroblast and lymphoblastoid cell lines to determine whether the radiation response being observed was cell-type specific. The results verified that the observed radiation response was not a cell-type-specific phenomenon, suggesting that the genes play essential roles in the radiation damage control pathways. This study demonstrates the potential of the gene-trap approach for the identification and functional analysis of novel radiation response genes.

Differences in gene expression profiles in the liver between carcinogenic and non-carcinogenic isomers of compounds given to rats in a 28-day repeat-dose toxicity study

Some compounds have structural isomers of which one is apparently carcinogenic, and the other not. Because of the similarity of their chemical structures, comparisons of their effects can allow gene expression elicited in response to the basic skeletons of the isomers to be disregarded. We compared the gene expression profiles of male Fischer 344 rats administered by daily oral gavage up to 28 days using an in-house oligo microarray. 2-Acetylaminofluorene (2-AAF), 2,4-diaminotoluene (2,4-DAT), 2-nitropropane (2-NP), and 2-nitro-p-phenylenediamine (2-NpP) are hepatocarcinogenic. However, their isomers, 4-acetylaminofluorene (4-AAF), 2,6-diaminotoluene (2,6-DAT), 1-nitropropane (1-NP), and 4-nitro-o-phenylenediamine (4-NoP), are non-hepatocarcinogenic. Because of the limited carcinogenicity of 2-NpP, we attempted to perform two-parametric comparison analyses with (1) a set of 4 isomers: 2-AAF, 2,4-DAT, 2-NP, and 2-NpP as "carcinogenic", and 4-AAF, 2,6-DAT, 1-NP, and 4-NoP as "non-carcinogenic"; and (2) a set of 3 isomers: 2-AAF, 2,4-DAT, and 2-NP, as "carcinogenic", and 4-AAF, 2,6-DAT, and 1-NP as "non-carcinogenic". After ratio filtering and Welch's approximate t-test analysis, 54 and 28 genes were selected from comparisons between the sets of 3 and 4 isomers, respectively, for day 28 data. Using hierarchical clustering analysis with the 54 or 28 genes, 2-AAF, 2,4-DAT, and 2-NP clustered into a "carcinogenic" branch. 2-NpP was in the same cluster as 4-NoP and 4-AAF. This clustering corresponded to the previous finding that 2-NpP is not carcinogenic in male Fischer 344 rats, which indicates that comparing the differences in gene expression elicited by different isomers is an effective method of developing a prediction system for carcinogenicity.

Both the basal transcriptional activity of the GADD45A gene and its enhancement after ionizing irradiation are mediated by AP-1 element

The growth arrest and DNA damage-inducible gene 45A (GADD45A) is involved in the DNA repair, maintenance of genomic stability, cell cycle control and apoptosis, and thus plays an important role in cellular response to DNA damage. The GADD45A gene is responsive to a variety of DNA-damaging agents, including ionizing radiation (IR), methyl methanesulfonate (MMS), and ultraviolet (UV) radiation. It is generally thought that induction of the GADD45A gene after IR exposure is principally p53-dependent, requiring binding of the p53 protein to the p53-recognition sequence in the third intron. However, the involvement of factors other than p53 in transcriptional regulation of the GADD45A gene after IR exposure has not been elucidated. In the present study, we show that the 5'-flanking region containing two OCT sites and a CCAAT box, as well as p53 and AP-1 sites in the third intron, are required for the basal transcriptional activity of the reporter gene. In addition, AP-1 recognition element was shown to be involved in the transcriptional enhancement of the GADD45A gene after X-ray irradiation. Electrophoretic mobility shift analysis (EMSA) and Chromatin immunoprecipitation (ChIP) assay revealed that JunD binds to the third intron of the GADD45A gene. These observations suggest that AP-1 complexes containing JunD, in addition to p53, play an important role not only in transcriptional enhancement by IR but also in basal expression of the GADD45A gene via binding to the AP-1 site in the third intron.

Endoplasmic reticulum stress and apoptosis

Cell death is an essential event in normal life and development, as well as in the pathophysiological processes that lead to disease. It has become clear that each of the main cellular organelles can participate in cell death signalling pathways, and recent advances have highlighted the importance of the endoplasmic reticulum (ER) in cell death processes. In cells, the ER functions as the organelle where proteins mature, and as such, is very responsive to extracellular-intracellular changes of environment. This short overview focuses on the known pathways of programmed cell death triggering from or involving the ER.

Gadd45a Interacts with Aurora-A and Inhibits Its Kinase Activity

Centrosome stability is required for successful mitosis in mammalian cells. Amplification of the centrosome leads to chromosomal missegregation and generation of aneuploidy, which are closely associated with cell transformation and tumorigenesis (Doxsey, S. J. (2001) Nat. Cell Biol. 3, E105-E108; Hinchcliffe, E. H., and Sluder, G. (2001) Genes Dev. 15, 1167-1181; Pihan, G. A., Purohit, A., Wallace, J., Malhotra, R., Liotta, L., and Doxsey, S. J. (2001) Cancer Res. 61, 2212-2219). However, there are currently limited insights into mechanism(s) for this critical biological event. Here we show that Gadd45a, a DNA damage-inducible protein that is regulated by tumor suppressors p53 and BRCA1, participates in the maintenance of centrosome stability. Mouse embryonic fibroblasts derived from gadd45a knock-out mice exhibit centrosome amplification (designated as increased centrosome numbers). Introduction of exogenous Gadd45a into mouse embryonic fibroblasts isolated from gadd45a-null mice substantially restored the normal centrosome profile. In contrast to p21(waf1/cip1), which ensures coordinated initiation of centrosome, Gadd45a had no significant effect on centrosome duplication in S phase. Interestingly Gadd45a was found to physically associate with Aurora-A protein kinase, whose deregulated expression results in centrosome abnormality. Furthermore Gadd45a was demonstrated to strongly inhibit Aurora-A kinase activity and to antagonize Aurora-A-induced centrosome amplification. These findings identify a novel mechanism for Gadd45a in the maintenance of centrosome stability and broaden understandings of p53- and BRCA1-regulated signaling pathways in maintaining genomic fidelity.

High-specificity and high-sensitivity genotoxicity assessment in a human cell line: Validation of the GreenScreen HC GADD45a-GFP genotoxicity assay

The battery of genetic toxicity tests required by most regulatory authorities includes both bacterial and mammalian cell assays and identifies practically all genotoxic carcinogens. However, the relatively high specificity of the Salmonella mutagenicity assay (Ames test) is offset by the low specificity of the established mammalian cell assays, which leads to difficulties in the interpretation of the biological relevance of results. This paper describes a new high-throughput assay that links the regulation of the human GADD45a gene to the production of Green Fluorescent Protein (GFP). A study of 75 well-characterised genotoxic and non-genotoxic compounds with diverse mechanisms of DNA-damage induction (including aneugens) reveals that the assay responds positively to all classes of genotoxic damage with both high specificity and high sensitivity. The current micro-well assay format does not include metabolic activation, but a separate low-throughput protocol demonstrates a successful proof-of-principle for an S9 metabolic activation assay with the model pro-mutagen cyclophosphamide. The test should be of value both as a tool in the selection of candidate compounds for further development, where additional data may be required because of conflicting information from the in vitro test battery, or in product development areas where the use of animals is to be discontinued. As a microplate assay however, it has the qualities of high throughput and low compound use that will facilitate its application in early screening for genotoxic liability.

Gene expression profile analysis: an emerging approach to investigate mechanisms of genotoxicity

The response to stress triggers transcriptional activation of genes involved in cell survival and/or cell death. Thus, the monitoring of gene expression levels in large gene sets or whole genomes in response to various agents (toxicogenomics) has been proposed as a tool for investigating mechanisms of toxicity. Although standard in vitro genetic toxicity testing provides relatively simple and accurate hazard detection, interpretation of positive findings, i.e., in vitro chromosome aberrations, in terms of relevant risk to humans is difficult, due to the limited insight into the underlying mechanisms. Therefore, the development of experimental approaches capable of differentiating a wide range of genotoxic mechanisms is expected to significantly improve risk assessment. The goal of this review is to summarize current developments in toxicogenomic analysis of genotoxic stress, and to provide a perspective on the application of gene expression profile analysis in genetic toxicology.

GADD45alpha is highly expressed in pancreatic ductal adenocarcinoma cells and required for tumor cell viability

Pancreatic ductal adenocarcinoma is one of the most common causes of cancer death in the western civilization. Recently, NF-kappaB has been shown to be activated in pancreatic ductal adenocarcinoma through constitutive activation of IkappaB kinase (IKK). Inhibition of NF-kappaB by a super-inhibitor of NF-kappaB--delta-N-IkappaBalpha--resulted in impaired proliferation and induction of apoptosis, suggesting an important role of NF-kappaB in pancreatic tumorigenesis. Downstream target genes of IkappaBalpha have not been elucidated in pancreatic ductal adenocarcinoma in detail. Using expression profiling by cDNA array analysis of pancreatic ductal adenocarcinoma cell lines stably transfected with super-IkappaBalpha, we identified GADD45alpha as a significant regulated gene. GADD45alpha is overexpressed in pancreatic ductal adenocarcinoma at the mRNA and protein level. Using RNAi we show that downregulation of GADD45alpha reduces proliferation and induces apoptosis in pancreatic cancer cells. These findings provide evidence that GADD45alpha contributes to pancreatic cancer cell proliferation and viability.

Treatment of cholangiocarcinoma with oncolytic herpes simplex virus combined with external beam radiation therapy

Replication-competent oncolytic herpes simplex viruses (HSV), modified by deletion of certain viral growth genes, can selectively target malignant cells. The viral growth gene gamma(1)34.5 has significant homology to GADD34 (growth arrest and DNA damage protein 34), which promotes cell cycle arrest and DNA repair in response to stressors such as radiation (XRT). By upregulating GADD34, XRT may result in greater oncolytic activity of HSV strains deficient in the gamma(1)34.5 gene. The human cholangiocarcinoma cell lines KMBC, SK-ChA-1 and YoMi were treated with NV1023, an oncolytic HSV lacking one copy of gamma(1)34.5. Viral proliferation assays were performed at a multiplicity of infection (MOI, number of viral particles per tumor cell) equal to 1, either alone or after XRT at 250 or 500 cGy. Viral replication was assessed by plaque assay. In vitro cytotoxicity assays were performed using virus at MOIs of 0.01 and 0.1, with or without XRT at 250 cGy and cell survival determined with lactate dehydrogenase assay. Established flank tumors in athymic mice were treated with a single intratumoral injection of virus (10(3) or 10(4) plaque forming units), either alone or after a single dose of XRT at 500 cGy, and tumor volumes measured. RT-PCR was used to measure GADD34 mRNA levels in all cell lines after a single dose of XRT at 250 or 500 cGy. NV1023 was tumoricidal in all three cell lines, but sensitivity to the virus varied. XRT enhanced viral replication in vitro in all cell lines. Combination treatment with low-dose XRT and virus was highly tumoricidal, both in vitro and in vivo. The greatest tumor volume reduction with combination therapy was seen with YoMi cells, the only cell line with increased GADD34 expression after XRT and the only cell line in which a synergistic treatment effect was suggested. In KMBC and SK-ChA-1 cells, neither of which showed increased GADD34 expression after XRT, tumor volume reduction was less pronounced and there was no suggestion of a synergistic effect in either case. Oncolytic HSV are effective in treating human cholangiocarcinoma cell lines, although sensitivity to virus varies. XRT-enhanced viral replication occurs through a mechanism that is not necessarily dependent on GADD34 upregulation. However, XRT-induced upregulation of GADD34 further promotes tumoricidal activity in viral strains deficient in the gamma(1)34.5 gene, resulting in treatment synergy; this effect is cell type dependent. Combined XRT and oncolytic viral therapy is a potentially important treatment strategy that may enhance the therapeutic ratios of both individual therapies.

Post-transcriptional regulation of thioredoxin by the stress inducible heterogenous ribonucleoprotein A18

Thioredoxin (TRX) is a key protein of the cellular redox metabolism, which expression is increased in several tumors especially gastric tumors. Even though ultraviolet (UV) and hypoxia specifically induce TRX, the mechanisms that lead to increased TRX levels are still ill defined. Here, we show that the heterogenous ribonucleoprotein A18 (hnRNP A18) RNA Binding Domain (RBD) and the arginine, glycine (RGG) rich domain can bind TRX 3′-untranslated region (3′-UTR) independently but both domains are required for maximal binding. Immunoprecipitation (IP) of hnRNP A18-mRNAs complexes and co-localization of hnRNP A18 and TRX transcripts on ribosomal fractions confirm the interaction of hnRNP A18 with TRX transcripts in cells. Moreover, down regulation of hnRNP A18 correlates with a significant reduction of TRX protein levels. In addition, hnRNP A18 increases TRX translation and interacts with the eukaryotic Initiation Factor 4G (eIF4G), a component of the general translational machinery. Furthermore, hnRNP A18 phosphorylation by the hypoxia inducible GSK3β increases hnRNP A18 RNA binding activity in vitro and in RKO cells in response to UV radiation. These data support a regulatory role for hnRNP A18 in TRX post-transcriptional expression possibly through a kissing loop model bridging TRX 3′- and 5′-UTRs through eIF4G.

CHOP plays a pivotal role in the astrocyte death induced by oxygen and glucose deprivation

Ischemia has different consequences on the survival of astrocytes and neurons. Thus, astrocytes show a remarkable resistance to short periods of ischemia that are well known to cause neuronal death. We have used a cell culture model of stroke, oxygen, and glucose deprivation (OGD), to clarify the mechanisms responsible for the exclusive resistance of astrocytes to ischemia. The expression of genes implicated in both ischemia-induced astrocyte death and post-ischemic survival was analysed by the RNA differential display technique. Our study revealed that the expression of the CEBP homologous protein (CHOP)-coding gene is promptly an intensely upregulated following astrocyte oxygen and glucose deprivation. CHOP mRNA induction was accompanied by the activation of other genes (grp78, grp95) that, alike CHOP, are involved in the endoplasmic reticulum (ER) stress response. In addition, drugs that cause ER calcium depletion or protein N-glycosylation inhibition mimicked the effects of OGD on astrocyte survival, further supporting the involvement of ER in the astrocyte responses to OGD. Our experiments also demonstrated that upregulation of CHOP during the ER stress response is required for ischemia to cause astrocyte death. Not only the levels of CHOP mRNA and protein correlate perfectly with the degree of OGD-triggered cell injury, but also astrocyte death induced by OGD is significantly overcome by CHOP antisense oligonucleotide treatment. Nevertheless, we observed that astrocytes undergo apoptosis only when CHOP is permanently upregulated, and not when CHOP increases are transient. Finally, we found that the extent of CHOP induction is determined by the length of the ischemic stimulus. Taken together, our results indicate that permanent upregulation of CHOP is decisive for the induction of astrocyte death by OGD.

Growth arrest-inducing genes are activated in Dbl-transformed mouse fibroblasts

The Dbl oncogene is a guanine nucleotide exchange factor for Rho GTPases and its activity has been linked to the regulation of gene transcription. Dbl oncogene expression in NIH3T3 cells leads to changes in morphological and proliferative properties of these cells, inducing a highly transformed phenotype. To gain insights into Dbl oncogene-induced transformation we compared gene expression profiles between Dbl oncogene-transformed and parental NIH3T3 cells by cDNA microarray. We found that Dbl oncogene expression is associated with gene expression modulation involving upregulation of 51 genes and downregulation of 49 genes. Five of the overexpressed genes identified are known to exert antiproliferative functions. Our observations suggest that the expression of Dbl oncogene in NIH3T3 may lead to the induction of genes associated with cell cycle arrest, possibly through the activation of stress-induced kinases.

Doxorubicin prevents endoplasmic reticulum stress-induced apoptosis

Several cellular stress signaling pathways initiate apoptosis in eukaryotic cells, but the interactions and coordination between the pathways have not been elucidated. In this study, apoptosis was triggered in MCF7 human breast carcinoma cells using doxorubicin, a topoisomerase inhibitor, and an endoplasmic reticulum (ER) stress inducer, thapsigargin, the latter causing the unfolded protein response (UPR). Interestingly, compared to treatment with doxorubicin or thapsigargin alone, cell death was reduced by treatment with both stress inducers. In contrast to another topoisomerase inhibitor, etoposide, doxorubicin markedly decreased apoptosis induced by thapsigargin; this doxorubicin effect was accompanied by reduced expression of the UPR-specific proapoptotic protein, C/EBP-homologous protein, and its upstream transcription factor, ATF4. We further found that doxorubicin downregulates the expression of ATF4 mRNA, indicating that doxorubicin interferes with the UPR at the level of ATF4 transcription. Taken together, the data suggest that ER stress-initiated cell death might be regulated by doxorubicin.

Cloning and Characterization of a p53 and DNA Damage Down-regulated Gene PIQ that Codes for a Novel Calmodulin-Binding IQ Motif Protein and Is Up-regulated in Gastrointestinal Cancers

We have identified a p53 and DNA damage-regulated gene that encodes a novel IQ motif protein, which we have named p53 and DNA damage-regulated IQ motif protein (PIQ). PIQ has two isoforms, long (PIQ-L) and short (PIQ-S), and both bind to calmodulin in the presence and absence of calcium. PIQ expression is down-regulated by p53 and DNA damage-inducing agents, whereas PIQ itself represses the expression of p53 up-regulated modulator of apoptosis that is a key mediator of p53-induced apoptosis. Thus, PIQ is a novel protein that may function to bridge a crosstalk between p53 and calmodulin-regulated cellular processes. We further show that PIQ expression is up-regulated in a number of primary colorectal and gastric tumors when compared with matching normal tissues, suggesting that PIQ may be involved in tumorigenesis and could serve as a valuable diagnostic/prognostic marker for gastrointestinal tumors.

Gadd45 beta and Gadd45 gamma are critical for regulating autoimmunity

The number of effector T cells is controlled by proliferation and programmed cell death. Loss of these controls on self-destructive effector T cells may precipitate autoimmunity. Here, we show that two members of the growth arrest and DNA damage-inducible (Gadd45) family, β and γ, are critical in the development of pathogenic effector T cells. CD4⁺ T cells lacking Gadd45β can rapidly expand and invade the central nervous system in response to myelin immunization, provoking an exacerbated and prolonged autoimmune encephalomyelitis in mice. Importantly, mice with compound deficiency in Gadd45β and Gadd45γ spontaneously developed signs of autoimmune lymphoproliferative syndrome and systemic lupus erythematosus. Our findings therefore identify the Gadd45β/Gadd45γ-mediated control of effector autoimmune lymphocytes as an attractive novel target for autoimmune disease therapy.

Analysis of gene-expression profiles after gamma irradiation of normal human fibroblasts

PURPOSE

To understand comprehensive transcriptional profile of normal human fibroblast in response to irradiation.

METHODS AND MATERIALS

To identify genes whose expression is influenced by gamma radiation, we used a cDNA microarray to analyze expression of 23,000 genes in normal human fibroblasts at 7 timepoints (1, 3, 6, 12, 24, 48, and 72 hours) after 5 different doses (0.5, 2, 5, 15, and 50 Gy) of exposure.

RESULTS

Among the genes that showed altered expression patterns, some were already known to be regulated by irradiation, for instance ODC, EGR1, FGF2, PCNA, PKC, and several p53-target genes, including p53DINP1, BTG2, GADD45, and MDM2. The time course of each dose showed that from 350 to 600 genes were affected as to their expression; induction profiles characteristic to each dose were demonstrated. Of the total identified, only 89 genes were up-regulated; the vast majority was down-regulated over the 72-hour time course. We identified 21 genes that were distinctly induced by irradiation; 11 of them were functionally known, and 6 of those were p53-target genes.

CONCLUSIONS

The results underscored the complexity of the transcriptional responses to irradiation, and the data should serve as a basis for global characterization of radiation-regulated genes and pathways.

ATR and GADD45alpha mediate HIV-1 Vpr-induced apoptosis

The human immunodeficiency virus type-1 (HIV-1) accessory gene vpr encodes a conserved 96-amino-acid protein that is necessary and sufficient for the HIV-1-induced block of cellular proliferation. Expression of vpr in CD4+ lymphocytes results in G2 arrest, followed by apoptosis. In a previous study, we identified the ataxia telangiectasia-mutated (ATM) and Rad3-related protein (ATR) as a cellular factor that mediates Vpr-induced cell cycle arrest. In the present study, we report that the breast cancer-associated protein-1 (BRCA1), a known target of ATR, is activated in the presence of Vpr. In addition, the gene encoding the growth arrest and DNA damage-45 protein alpha (GADD45alpha), a known transcriptional target of BRCA1, is upregulated by Vpr in an ATR-dependent manner. We demonstrate that RNAi-mediated silencing of either ATR or GADD45alpha leads to nearly complete suppression of the proapoptotic effect of Vpr. Our results support a model in which Vpr-induced apoptosis is mediated via ATR phosphorylation of BRCA1, and consequent upregulation of GADD45alpha.

Ultraviolet Light (UVB and UVA) Induces the Damage-Responsive Transcription Factor CHOP/gadd153 in Murine and Human Epidermis: Evidence for a Mechanism Specific to Intact Skin

C/EBP-homologous protein (CHOP)/gadd153 (or CHOP) is a transcription factor induced by endoplasmic reticulum (ER) stress. Forcible overexpression of CHOP causes apoptosis in keratinocytes in culture. Here, we asked whether CHOP might be increased in the skin after UVB (280-320 nm) exposure, thus implicating CHOP in sunburn cell (SBC) formation. SKH-1 hairless mice were exposed to a ultraviolet (UV) source (80 mJ per cm2; approximately 74% UVB, approximately 16% UVA), and skin biopsies examined by immunohistology and immunoprecipitation. Compared with non-irradiated epidermis, CHOP expression was significantly increased at 30 min, and reached maximal levels by 24 h. Similar increases in CHOP following UVB exposure were observed in human buttock skin. The time course of CHOP expression preceded SBC formation and another marker of apoptosis, caspase-3 cleavage. Intracellular CHOP accumulated mainly in cytoplasmic and perinuclear locations, with little remaining in the nucleus. To examine mechanisms, cultured keratinocytes were irradiated in vitro and examined by western blotting. Under conditions that eliminated ER stress because of cell handling, CHOP did not accumulate (and was in fact decreased) in the cells. Thus, induction of CHOP in keratinocytes requires factors present only in the native skin. Overall, the data suggest that CHOP participates in adaptive responses of the epidermis following UVB/UVA exposure in vivo.

Gadd45a expression induces Bim dissociation from the cytoskeleton and translocation to mitochondria

Gadd45a, a p53- and BRCA1-regulated stress protein, has been implicated in the maintenance of genomic fidelity, probably through its roles in the control of cell cycle checkpoint and apoptosis. However, the mechanism(s) by which Gadd45a is involved in the induction of apoptosis remains unclear. We show here that inducible expression of Gadd45a protein causes dissociation of Bim, a Bcl2 family member, from microtubule-associated components and translocation to mitochondria. The Bim accumulation in mitochondria enhances interaction of Bim with Bcl-2, relieves Bax from Bcl-2-bound complexes, and subsequently results in release of cytochrome c into the cytoplasm. Suppression of endogenous Bim greatly inhibits Gadd45a induction of apoptosis. Interestingly, Gadd45a interacts with elongation factor 1alpha (EF-1alpha), a microtubule-severing protein that plays an important role in maintaining cytoskeletal stability, and inhibits EF-1alpha-mediated microtubule bundling, indicating that the interaction of Gadd45a with EF-1alpha disrupts cytoskeletal stability. A mutant form of Gadd45a harboring a deletion of EF-1alpha-binding domain fails to inhibit microtubule stability and to induce Bim translocation to mitochondria. Furthermore, coexpression of EF-1alpha antagonizes Gadd45a's property of suppressing cell growth and inducing apoptosis. These findings identify a novel link that connects stress protein Gadd45a to the apoptotic machinery and address the importance of cytoskeletal stability in apoptotic response to DNA damage.

Involvement of GADD153 and Cardiac Ankyrin Repeat Protein in Hypoxia-induced Apoptosis of H9c2 Cells

Oxidative stress is the main cause of cardiac injury during ischemia/reperfusion but the molecular mechanism for this process is unclear. In this study, it was found that hypoxia induces apoptosis in rat embryonic heart-derived H9c2 cells leading to the induction of GADD153, which is an apoptosis-related gene. Therefore, this study addressed the molecular role of GADD153 in hypoxia-induced apoptosis. The stable or inducible overexpression of GADD153 sensitized the H9c2 cells to apoptotic cell death. The results suggest that the transactivation domain of the GADD153 might be responsible for this cell execution and play a role in the nucleoplasmic localization of GADD153. The cells transiently transfected with the antisense GADD153 were more resistant to hypoxia-induced apoptosis than the vector control cells. Furthermore, GADD153 transcriptionally down-regulated the expression of the cardiac ankyrin repeat protein gene (CARP), which is a nuclear transcriptional co-factor that negatively regulates the expression of the cardiac gene. The ectopic expression of CARP in H9c2 cells increased the resistance to hypoxia-induced apoptosis. These results suggest that GADD153 overexpression and the concomitant down-regulation of CARP might have a causative role in the apoptotic cell injury of hypoxic H9c2 cells.

The protective effect of dantrolene on ischemic neuronal cell death is associated with reduced expression of endoplasmic reticulum stress markers

The endoplasmic reticulum (ER) plays an important role in ischemic neuronal cell death. In order to determine the effect of dantrolene, a ryanodine receptor antagonist, on ER stress response and ischemic brain injury, we investigated changes in ER stress-related molecules, that is phosphorylated form of double-stranded RNA-activated protein kinase (PKR)-like ER kinase (p-PERK), phosphorylated form of eukaryotic initiation factor 2alpha (p-eIF2alpha), activating transcription factor-4 (ATF-4), and C/EBP-homologous protein (CHOP), as well as terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) in the peri-ischemic area and ischemic core region of rat brain after transient middle cerebral artery occlusion (MCAO). In contrast to the cases treated with vehicle, the infarct volume and TUNEL-positive cells were significantly reduced at 24 h of reperfusion by treatment with dantrolene. The immunoreactivities for p-PERK, p-eIF2alpha, ATF-4, and CHOP were increased at the ischemic peripheral region after MCAO, which were partially inhibited by dantrolene treatment. The present results suggest that dantrolene significantly decreased infarct volume and provided neuroprotective effect on rats after transient MCAO by reducing ER stress-mediated apoptotic signal pathway activation in the ischemic area.

Gadd45a, a p53- and BRCA1-regulated stress protein, in cellular response to DNA damage

Mammalian cells exhibit complex, but intricate cellular responses to genotoxic stress, including cell cycle checkpoints, DNA repair and apoptosis. Inactivation of these important biological events may result in genomic instability and cell transformation, as well as alterations of therapeutic sensitivity. Gadd45a, a p53- and BRCA1-regulated stress-inducible gene, has been characterized as one of the important players that participate in cellular response to a variety of DNA damage agents. Interestingly, the signaling machinery that regulates Gadd45a induction by genotoxic stress involves both p53-dependent and -independent pathways; the later may employ BRCA1-related or MAP kinase-mediated signals. Gadd45a protein has been reported to interact with multiple important cellular proteins, including Cdc2 protein kinase, proliferating cell nuclear antigen (PCNA), p21Waf1/Cip1 protein, core histone protein and MTK/MEKK4, an up-stream activator of the JNK/SAPK pathway, indicating that Gadd45a may play important roles in the control of cell cycle checkpoint, DNA repair process, and signaling transduction. The importance of Gadd45a in maintaining genomic integrity is well manifested by the demonstration that disruption of endogenous Gadd45a in mice results in genomic instability and increased carcinogenesis. Therefore, Gadd45a appears to be an important component in the cellular defense network that is required for maintenance of genomic stability.

B23 Regulates GADD45a Nuclear Translocation and Contributes to GADD45a-induced Cell Cycle G2-M Arrest

Gadd45a is an important player in cell cycle G2-M arrest in response to genotoxic stress. However, the underlying mechanism(s) by which Gadd45a exerts its role in the control of cell cycle progression remains to be further defined. Gadd45a interacts with Cdc2, dissociates the Cdc2-cyclin B1 complex, alters cyclin B1 nuclear localization, and thus inhibits the activity of Cdc2/cyclin B1 kinase. These observations indicate that Gadd45a nuclear translocation is closely associated with its role in cell cycle G2-M arrest. Gadd45a has been characterized as a nuclear protein, but it does not contain a classical nuclear localization signal, suggesting that Gadd45a nuclear translocation might be mediated through different nuclear import machinery. Here we show that Gadd45a associates directly with B23 (nucleophosmin), and the B23-interacting domain is mapped at the central region (61-100 amino acids) of the Gadd45a protein using a series of Myc tag-Gadd45a deletion mutants. Deletion of this central region disrupts Gadd45a association with B23 and abolishes Gadd45a nuclear translocation. Suppression of endogenous B23 through a short interfering RNA approach disrupts Gadd45a nuclear translocation and results in impaired Gadd45a-induced cell cycle G2-M arrest. These findings demonstrate a novel association of B23 and Gadd45a and implicate B23 as an important regulator in Gadd45a nuclear import.

The physiological unfolded protein response in the thyroid epithelial cells

The perturbation of the endoplasmic reticulum (ER) homeostasis by the pharmacological agents or mutant secretory proteins invokes the adaptive cellular reaction, unfolded protein response (UPR). Here we show the activation of UPR under the physiological conditions of the hormone-stimulated expression of thyroglobulin (Tg), the major secretory product of thyroid cells. The early increase in the Tg synthesis within 1h after stimulation apparently triggers UPR as it is evidenced by the activation of ATF6- and PERK-dependent pathways of UPR and subsequent expression of the UPR target genes, ER molecular chaperones. However, little or no activation of the other UPR sensor, IRE1, was observed, as judged by the absence of the spliced mRNA of its downstream effector, transcription factor XBP1. We conclude that the physiological UPR in differentiating thyrocytes differs substantially from the drug-induced stress by its weaker manifestation and distinct pattern of the activation of UPR sensors.

Roles of CHOP/GADD153 in endoplasmic reticulum stress

Endoplasmic reticulum (ER) is the site of synthesis and folding of secretory proteins. Perturbations of ER homeostasis affect protein folding and cause ER stress. ER can sense the stress and respond to it through translational attenuation, upregulation of the genes for ER chaperones and related proteins, and degradation of unfolded proteins by a quality-control system. However, when the ER function is severely impaired, the organelle elicits apoptotic signals. ER stress has been implicated in a variety of common diseases such as diabetes, ischemia and neurodegenerative disorders. One of the components of the ER stress-mediated apoptosis pathway is C/EBP homologous protein (CHOP), also known as growth arrest- and DNA damage-inducible gene 153 (GADD153). Here, we summarize the current understanding of the roles of CHOP/GADD153 in ER stress-mediated apoptosis and in diseases including diabetes, brain ischemia and neurodegenerative disease.

Angiotensin II early regulated genes in H295R human adrenocortical cells

Evidence for the dysregulation of aldosterone synthesis in cardiovascular pathophysiology has renewed interest in the control of its production. Cellular mechanisms by which angiotensin II (ANG II) stimulates aldosterone synthesis in the adrenal zona glomerulosa are incompletely understood. To elucidate the mechanism of intracellular signaling by ANG II stimulation in the adrenal, we have studied immediate-early regulated genes in human adrenal H295R cells using cDNA microarrays. H295R cells were stimulated with ANG II for 3 h. Gene expression was analyzed by microarray technology and validated by real-time RT-PCR. Eleven genes were found to be upregulated by ANG II. These encode the proteins for ferredoxin, Nor1, Nurr1, c6orf37, CAT-1, A20, MBLL, M-Ras, RhoB, GADD45α, and a novel protein designated FLJ45273 . Maximum expression levels for all genes occurred 3–6 h after ANG II stimulation. This increase was dose dependent and preceded maximal aldosterone production. Other aldosterone secretagogues, K+and endothelin-1 (ET-1), also induced the expression of these genes with variable efficiency depending on the gene and with lower potency than ANG II. ACTH had negligible effect on gene expression except for the CAT-1 and Nurr1 genes. These ANG II-stimulated genes are involved in several cellular functions and are good candidate effectors and regulators of ANG II-mediated effects in adrenal zona glomerulosa.

Molecular mechanisms of arsenic carcinogenesis

Arsenic is a metalloid compound that is widely distributed in the environment. Human exposure of this compound has been associated with increased cancer incidence. Although the exact mechanisms remain to be investigated, numerous carcinogenic pathways have been proposed. Potential carcinogenic actions for arsenic include oxidative stress, genotoxic damage, DNA repair inhibition, epigenetic events, and activation of certain signal transduction pathways leading to abberrant gene expression. In this article, we summarize current knowledge on the molecular mechanisms of arsenic carcinogenesis with an emphasis on ROS and signal transduction pathways.

Gadd45 Proteins Induce G2/M Arrest and Modulate Apoptosis in Kidney Cells Exposed to Hyperosmotic Stress

Gadd45 proteins are induced by hyperosmolality in renal inner medullary (IM) cells, but their role for cell adaptation to osmotic stress is not known. We show that a cell line derived from murine renal IM cells responds to moderate hyperosmotic stress (540 mosmol/kg) by activation of G(2)/M arrest without significant apoptosis. If the severity of hyperosmotic stress exceeds the tolerance limit of this cell line (620 mosmol/kg) apoptosis is strongly induced. Using transient overexpression of ectopic Gadd45 proteins and simultaneous analysis of transfected versus non-transfected cells by laser-scanning cytometry, we were able to measure the effects of Gadd45 super-induction during hyperosmolality on G(2)/M arrest and apoptosis. Our results demonstrate that induction of all three Gadd45 isoforms inhibits mitosis and promotes G(2)/M arrest during moderate hyperosmotic stress but not in isosmotic controls. Furthermore, all three Gadd45 proteins are also involved in control of apoptosis during severe hyperosmotic stress. Under these conditions Gadd45gamma induction strongly potentiates apoptosis. In contrast, Gadd45alpha/beta induction transiently increases caspase 3/7 and annexin V binding before 12 h but inhibits later stages of apoptosis during severe hyperosmolality. These results show that Gadd45 isoforms function in common but also in distinct pathways during hyperosmolality and that their increased abundance contributes to the low mitotic index and protection of genomic integrity in cells of the mammalian renal inner medulla.

Retinoic acid regulates C/EBP homologous protein expression (CHOP), which negatively regulates myeloid target genes

Retinoic acid (RA) promotes granulocytic differentiation of normal hematopoietic cells and acute promyelocytic leukemia (APL) blasts by transcriptional modulation of myeloid regulatory genes. In this study, we have identified the C/EBP homologous protein (CHOP) as a novel retinoid-responsive gene using a polymerase chain reaction (PCR)-based cDNA subtraction method. All-trans retinoic acid (ATRA) induced a biphasic expression of CHOP mRNA in the NB4 and HL60 AML cell lines. Levels of CHOP expression increased within 1 hour of exposure to ATRA. ATRA expression became nearly absent between 6 and 24 hours, and a second phase of induction occurred after 48 hours. Retinoid-dependent regulation of CHOP expression was also observed in normal human neutrophils but not in peripheral blood mononuclear cells. In addition, retinoid-dependent regulation of CHOP expression was not observed in retinoid-nonresponsive cell lines HL60R and NB4-R2. CHOP expression was regulated at the transcriptional level and was independent of new protein synthesis. CHOP heterodimerized with C/EBPepsilon and negatively regulated the myeloid-specific gene lactoferrin. Furthermore, CHOP transcriptionally inhibited C/EBPalpha- and C/EBPepsilon-dependent induction of secondary granule gene expression. RA signaling in granulocytic differentiation involves regulated expression of CHOP and C/EBPepsilon in a coordinated fashion.

Medaka as a model system for the characterisation of cell cycle regulators: a functional analysis of Ol-Gadd45gamma during early embryogenesis

Numerous studies, mostly performed on mammalian cell cultures, have implicated the Gadd45 family of small acidic proteins in cell cycle control (arrest and/or engagement in the apoptotic pathway). We report here the cloning, detailled expression pattern and functional characterisation in embryonic development of Ol-Gadd45gamma, the Oryzias latipes ortholog of mammalian Gadd45gamma. Its expression pattern, notably in the developing brain (optic tectum) strongly suggests that it is involved in cell cycle exit. Gain-of-function experiments (through mRNA injection) slowed down early development, and produced embryos clearly reduced in size, while morpholino knockdowns resulted in small embryos over-sensitive to DNA damage (UV irradiation). We further demonstrated that, following Ol-Gadd45gamma overexpression, cells are proliferation-arrested before both G1/S and G2/M cell cycle checkpoints, while in the MO-Ol-Gadd45 loss-of-function experiments cells are engaged in apoptosis rather than prevented from proliferating. These results show that Ol-Gadd45gamma is likely to play an important role in coordinating cell fate decisions during neurogenesis; they also demonstrate that the medakafish is a promising model to analyse in vivo the developmental control of the cell cycle.

Nucleophosmin sets a threshold for p53 response to UV radiation

Because activation of p53 can trigger cell cycle arrest and apoptosis, it is necessary for a cell to suppress this activation until it is absolutely required for survival. The mechanisms underlying this important regulatory event are poorly understood. Here we show that nucleophosmin (NPM) acts as a natural repressor of p53 by setting a threshold for p53 activation in response to UV radiation. NPM binds to the p53 N terminus and inhibits p53 transcriptional activity by more than 70%. Our data indicate that the levels of NPM in a cell determine the UV dose at which the tumor suppressor p53 can be phosphorylated on Ser15. Moreover, we show that NPM is a substrate for the UV-activated protein kinase ATR and inhibits the UV-induced p53 phosphorylation at Ser15. In addition, NPM forms a complex with p53 and ATR in vivo. These data suggest that NPM is an early responder to DNA damage that prevents premature activation of p53. In normal cells, NPM could contribute to suppressing p53 activation until its functions are absolutely required while in cancer cells overexpression of NPM could contribute to p53 inactivation and tumor progression.

Effect of two nitrogenous diphenyl ether pesticides on mast cell activation

We examined the effect of two nitrogenous diphenyl ether pesticides, nitrofen (NIP) and chlornitrofen (CNP), on mast cell activation. RBL-2H3 (rat basophilic leukemia) cells were exposed to NIP or CNP for 30 min to investigate their effect on degranulation, and for 3 h to investigate their effect on cytokine production and gene expression. NIP and CNP increased IgE receptor-mediated beta-hexosaminidase release, MCP-1 release, and TNF-alpha release in a dose-dependent manner. The increasing effect of CNP on their release was greater than that of NIP. In the gene expression experiment, 30 microg/ml CNP significantly upregulated Egr-1, MCP-1 and GADD45a gene expression. These results suggest that at higher concentrations (more than 30 microg/ml) the nitrogenous diphenyl ether pesticides had both a degranulation-enhancing effect and proinflammatory cytokine-production enhancing effect through the expression of some transcription factors in RBL-2H3 cells.

Elevated gadd153/chop expression and enhanced c-Jun N-terminal protein kinase activation sensitizes aged cells to ER stress

The endoplasmic reticulum (ER), as a processing plant for the folding and posttranslational modification of proteins, is exquisitely sensitive to changes in its internal environment. Various conditions, collectively termed 'ER stress', can perturb ER functions, leading to the activation of a complex response known as the unfolded protein response. Here, we investigated the response of hepatocytes derived from young (4-5 months) and aged (24-26 months) rats to two agents, thapsigargin (TG) and tunicamycin (TM), which act via different mechanisms to induce ER stress. Old hepatocytes displayed greater cell death than young cells following treatment with TG or TM, associated with higher expression of the pro-apoptotic gene gadd153 (also known as chop) and enhanced c-Jun N-terminal protein kinase (JNK) activation. Pharmacologic inhibition of JNK decreased the expression of TG-stimulated gadd153 in old cells and reduced their sensitivity to TG-induced cell death. Inhibition of p38, on the other hand, enhanced TG-induced gadd153 expression and JNK activation, and augmented TG-induced cell death. Additional experiments implicated the PERK/eIF-2 alpha signaling pathway as a contributor to the higher Gadd153 expression and JNK activation, and greater sensitivity of old cells to ER stress.

c-Myc represses the proximal promoters of GADD45a and GADD153 by a post-RNA polymerase II recruitment mechanism

The c-Myc cellular oncogene has diverse activities, including transformation, proliferation, and apoptosis. These activities are dependent on the ability of c-Myc to regulate gene transcription. c-Myc downregulates the GADD45a and GADD153 (DDTI3) genes that are induced in response to genotoxic stresses and that encode protein products with antiproliferative activities. We show that c-Myc represses the expression of GADD45a and GADD153 in response to thapsigargin, a nongenotoxic stress, as well as other endoplasmic reticulum (ER) stress agents. c-Myc represses both the basal expression and the magnitude of ER stress induction of GADD gene transcription. This repression requires the minimal promoter region of GADD45a and GADD153 and is not dependent on the ER stress element or p53-binding sites in the regulatory regions of these genes. Further analysis by chromatin immunoprecipitation shows that c-Myc binds to the minimal promoter region of GADD45a and GADD153 in vivo. c-Myc-associated protein X (Max) is also bound to both GADD gene promoters, whereas c-Myc interacting zinc-finger protein 1 (Miz-1) is bound to the GADD153, but not GADD45a, promoter. RNA polymerase II (RNAPII) is recruited to the GADD gene promoters in the presence and absence of c-Myc, which suggests that c-Myc represses these genes through a post-RNAPII recruitment mechanism.

Loss of expression of the growth inhibitory gene GADD45gamma, in human pituitary adenomas, is associated with CpG island methylation

Inappropriate expression of cell-cycle regulatory genes and/or their protein products are a frequent finding in pituitary tumours; however, genetic changes associated with or responsible for their dysregulation are in general uncommon. In a search for novel genes, and employing cDNA-representational difference analysis, the gene encoding GADD45gamma was recently isolated and identified as being under-represented in pituitary adenomas. GADD45gamma is a member of a family of genes that are induced by DNA damage and function in the negative regulation of cell growth. In this study, we further confirm this initial report that the majority of pituitary adenomas (22 of 33; 67%) do not express GADD45gamma as determined by RT-PCR analysis. Loss of expression was not associated with either loss of heterozygosity or mutations within the coding region of this gene. In marked contrast, epigenetic change, namely methylation of the GADD45gamma genes CpG island, was a frequent finding (19 of 33 adenoma; 58%) and was significantly associated with tumours in which GADD45gamma transcript was not expressed (18 of 22; 82%; P=0.002). In common with the primary tumours, methylation-associated gene silencing of the GADD45gamma gene was also found in the pituitary tumour cell line AtT20. The treatment of AtT20 cells with the demethylating agent, 5-Aza-2'-deoxycytidine, induced the re-expression of this gene. These findings show that silencing of the GADD45gamma gene in pituitary tumours is primarily associated with methylation of the genes CpG island. Methylation has functional importance since reversal of this epigenetic change in a pituitary-derived cell line is associated with re-expression. Silencing of GADD45gamma, a negative regulator of cell growth, is most likely responsible for conferring a selective growth advantage during tumour evolution and outgrowth.

CHOP activation by photodynamic therapy increases treatment induced photosensitization

BACKGROUND AND OBJECTIVES

C/EBP homologous protein (CHOP) is an endoplasmic reticulum (ER) stress inducible transcription factor involved in the development of apoptosis, growth arrest, and differentiation. CHOP deficient (chop - / - ) mouse embryonic fibroblasts (MEFs) exposed to ER stresses such as tunicamycin exhibit decreased apoptosis and reduced toxicity when compared to chop + / + control cells. Overexpression of the 70 kDa heat shock stress protein (HSP-70) can inhibit apoptotic pathways. The biological significance of photodynamic therapy (PDT) protocols that induce cellular damage resulting in differential CHOP and stress protein expression patterns was examined.

STUDY DESIGN/MATERIALS AND METHODS

Wild type mouse radiation induced fibrosarcoma (RIF) cells as well as MEFs with chop + / + and chop - / - genotypes were used with either a mitochondrial and ER localizing porphyrin (PH) photosensitizer or a lysosomal localizing chlorin (NPe6) photosensitizer. PDT induced cytotoxicity, apoptosis, and stress protein expression patterns were determined as a function of cell type and photosensitizer.

RESULTS

PH mediated PDT induced expression of CHOP and 78 kDa glucose regulated protein (GRP-78), but not HSP-70 while NPe6 mediated PDT induced protein expression of HSP-70 but did not activate CHOP or GRP-78 expression. Enhanced apoptosis and toxicity were observed in chop + / + cells following exposure to tunicamycin or PH mediated PDT when compared to identical treatments in chop - / - cells. NPe6 mediated PDT induced minimally detectable apoptosis in both chop + / + and chop - / - cells and only a modest increase in survival for chop - / - cells.

CONCLUSIONS

These results demonstrate that PDT activation of CHOP, GRP-78, and HSP-70 varied as a function of photosensitizer subcellular localization and that a single oxidative stress response was not observed following PDT. We also show that CHOP expression increased apoptosis following PH mediated PDT and that increased CHOP expression is associated with enhanced PDT photosensitization.

Comprehensive search for X-ray-responsive elements and binding factors in the regulatory region of the GADD45a gene

The growth arrest and DNA damage-inducible protein 45alpha (GADD45a) gene is responsive to a variety of DNA-damaging agents. It is known that induction of the GADD45a gene is regulated in a p53-dependent manner after ionizing irradiation. Our previous study showed that X-ray irradiation increased the transcription rate of the GADD45a gene much earlier than the maximum accumulation of stabilized p53 protein in human myeloblastic leukemia ML-1 cells. We hypothesized that some transcription factor(s) may cooperate with p53 in regulating the GADD45a gene early after the irradiation of ML-1 cells. This idea is supported by recent studies showing that the p53-dependent activation of several genes in human and mouse cells requires some additional transcription factors, such as Sp1, GKLF, Ets1, and IRF-1. To examine the possible involvement of cooperating factors in transcriptional regulation of the GADD45a gene by ionizing radiation, we comprehensively searched for the X-ray-inducible binding locus of the nuclear factor throughout the upstream region (-2244 bp/+89 bp) and the third intron (+1389 bp/+2488 bp) of the GADD45a gene by EMSA using 136 probes. The X-ray-responsive binding of nuclear factors was detected at eight loci. Oct, NF-kappaB, HNF, NF-AT, and KLF family transcription factors were identified by a competition assay. It is possible that some of these factors cooperate with p53 to mediate transcriptional regulation of the GADD45a gene after ionizing irradiation.

Gadd45a contributes to p53 stabilization in response to DNA damage

p53 is an important molecule in cellular response to DNA damage. After genotoxic stress, p53 protein stabilizes transiently and accumulates in the nucleus, where it functions as a transcription factor and upregulates multiple downstream-targeted genes, including p21(Waf1/Cip1), Gadd45a and Bax. However, regulation of p53 stabilization is complex and may mainly involve post-translational modification of p53, such as phosphorylation and acetylation. Using mouse embryonic fibroblasts (MEFs) derived from Gadd45a knockouts, we found that disruption of Gadd45a greatly abolished p53 protein stabilization following UVB treatment. Phosphorylation of p53 at Ser-15 was substantially reduced in Gadd45a-/- MEFs. In addition, p53 induction by UVB was shown to be greatly abrogated in the presence of p38 kinase inhibitor, but not c-Jun N-terminal kinase (JNK) and extracellular-signal regulated kinase (ERK), suggesting that p38 protein kinase is involved in the regulation of p53 induction. Along with the findings presented above, inducible expression of Gadd45a enhanced p53 accumulation after cell exposure to UVB. Taken together, the current study demonstrates that Gadd45a, a conventional downstream gene of p53, may play a role as an upstream effector in p53 stabilization following DNA damage, and thus has defined a positive feedback signal in the activation of the p53 pathway.

p53-independent induction of Gadd45 by histone deacetylase inhibitor: coordinate regulation by transcription factors Oct-1 and NF-Y

Histone deacetylase (HDAC) inhibitors cause growth arrest at the G1 and/or G2/M phases, and induce differentiation and/or apoptosis in a wide variety of tumour cells. The growth arrest at G1 phase by HDAC inhibitors is thought to be highly dependent on the upregulation of p21/WAF1, but the precise mechanism by which HDAC inhibitors cause G2/M arrest or apoptosis in tumour cells is unknown. Gadd45 causes cell cycle arrest at the G2/M phase transition and participates in genotoxic stress-induced apoptosis. We show here that it is also induced by a typical HDAC inhibitor, trichostatin A (TSA), through its promoter, in a p53-independent manner. To identify the mechanism of activation of the gadd45 promoter, we performed luciferase reporter analyses and electrophoretic mobility shift assays. These revealed that both the Oct-1 and CCAAT sites are needed for the full activation by TSA. We also found that the transcription factors Oct-1 and NF-Y specifically bind to each site. Thus, HDAC inhibitors can induce Gadd45 through its promoter without the need for functional p53, and both the Oct-1 and NF-Y concertedly participate in TSA-induced activation of the gadd45 promoter.

SPRR1B overexpression enhances entry of cells into the G0 phase of the cell cycle

Many studies have established the role of SPRR1B during squamous differentiation of skin and respiratory epithelial cells. However, its role in nonsquamous cells is largely unknown. We reported that expression of SPRR1B in Chinese hamster ovary (CHO) cells is increased as they enter the G0 phase of the cell cycle. The purpose of this study was to further investigate the SPRR1B expression pattern in nonsquamous tumors and to study its role in these cells. Expression of SPRR1B was detected by Northern blotting in a higher percentage of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone-induced compared with beryllium metal-induced rat lung adenocarcinomas. In situ hybridizations confirmed that SPRR1B is expressed in individual or clusters of cells of nonsquamous cells from mouse, rat, and human adenocarcinomas. The same pattern of expression was observed in adenocarcinomas formed in nude mice from cell lines established from adenocarcinomas. SPRR1B expression was downregulated in the cell lines derived from adenocarcinoma when cells were enriched in G0 at low confluence. Tetraploidy was induced in CHO, mouse, and human tumor cell lines by stably overexpressing SPRR1B, whereas control cells showed no change in ploidy. Inducible expression of this protein for shorter periods using the ecdyson system did not affect growth rate or the ploidy of CHO cells but accelerated entry into G0/G1 compared with controls. These findings indicate that SPRR1B is likely coupled primarily to signals responsible for withdrawal from the proliferative state rather than the final stages of cellular quiescence and that its overexpression for prolonged periods may disrupt the normal progression of mitosis.

Delineation of a negative feedback regulatory loop that controls protein translation during endoplasmic reticulum stress

Transient protein synthesis inhibition is an important protective mechanism used by cells during various stress conditions including endoplasmic reticulum (ER) stress. This response centers on the phosphorylation state of eukaryotic initiation factor (eIF)-2 alpha, which is induced by kinases like protein kinase R-like ER kinase (PERK) and GCN2 to suppress translation and is later reversed so translation resumes. GADD34 was recently identified as the factor that activates the type 1 protein serine/threonine phosphatase (PP1), which dephosphorylates eIF-2 alpha during cellular stresses. Our study delineates a negative feedback regulatory loop in which the eIF-2 alpha-controlled inhibition of protein translation leads to GADD34 induction, which promotes translational recovery. We show that activating transcription factor-4 (ATF4), which is paradoxically translated during the eIF-2 alpha-mediated translational block, is required for the transactivation of the GADD34 promoter in response to ER stress and amino acid deprivation. ATF4 directly binds to and trans-activates a conserved ATF site in the GADD34 promoter during ER stress. Examination of ATF4-/- MEFs revealed an absence of GADD34 induction, prolonged eIF-2 alpha phosphorylation, delayed protein synthesis recovery, and diminished translational up-regulation of BiP during ER stress. These studies demonstrate the essential role of GADD34 in the resumption of protein synthesis, define the pathway for its induction, and reveal that cytoprotective unfolded protein response targets like BiP are sensitive to the eIF-2 alpha-mediated block in translation.

Selective up-regulation of the growth arrest DNA damage-inducible gene Gadd45 alpha in sensory and motor neurons after peripheral nerve injury

The growth arrest and DNA damage-inducible gene 45 alpha (Gadd45a) was one of 240 genes found previously by high density oligonucleotide microarray analysis to be regulated in the rat L4 and L5 dorsal root ganglia 3 days after transection of the sciatic nerve (>four-fold up-regulation). The Gadd45a mRNA expression profile investigated by northern blot, RNase protection assay and in situ hybridization in the rat shows negligible constitutive mRNA levels in embryonic, neonatal or adult intact dorsal root ganglia. Within 24 h of a sciatic nerve injury, a very large induction is found that persists for as long as regeneration of injured fibres is prevented by peripheral nerve ligation. When axons are allowed to regrow following sciatic nerve crush injury, Gadd45a expression is terminated at later time points, when levels of other markers of injury return towards normal. Colocalization with activating transcription factor 3-LI and c-jun mRNA implies that all peripherally injured primary sensory and motor neurons express Gadd45a mRNA. Injury to the central axons of dorsal root ganglion neurons produces only a minimal induction of Gadd45a while peripheral inflammation is without effect. Gadd45a is a specific marker of the presence of peripheral axonal injury in adult primary sensory and motor neurons.

Down-regulation of proliferating cell nuclear antigen gene expression occurs during cell cycle arrest induced by human fecal water in colonic HT-29 cells

Cancer is a disease in which the cell cycle is altered, and the elucidation of the mechanisms by which constituents of human fecal water influence the cell cycle can lead to noninvasive measurement of colon cancer risk. The purpose of the present study was to investigate the effect of human fecal water on HT-29 cell-cycle progression with sodium selenite as a reference for comparison. Both human fecal water (2.5-5.0%) and selenite (3-4 micro mol/L) significantly inhibited cell growth. Cell-cycle analysis revealed that human fecal water decreased the proportion of S + G2 phase cells and increased that of G1 phase cells. In contrast, selenite decreased G1 phase cells and increased proportions of S and G2 phase cells. Both 5% human fecal water and 4 micro mol/L selenite greatly increased the mRNA level of the cyclin-dependent kinase inhibitor gene p21(waf1). Interestingly, the mRNA levels of cyclin A and proliferating cell nuclear antigen (PCNA) were dramatically decreased by 69 and 62%, respectively, in HT-29 cells treated with fecal water but not selenite. In contrast, the mRNA level of DNA damage-inducible transcript 1, gadd45, was significantly increased by 2.28-fold in HT-29 cells treated with selenite but not fecal water. Furthermore, a PCNA gene promoter was cloned into a luciferase reporter construct and its activity was significantly reduced in a dose-dependent manner in cells treated with fecal water but not selenite. Collectively, these results suggest that human fecal water and selenite can differentially induce growth arrest genes, and that PCNA gene expression is uniquely and highly sensitive to human fecal water.

The function of GADD34 is a recovery from a shutoff of protein synthesis induced by ER stress: elucidation by GADD34-deficient mice

GADD34 is a protein that is induced by stresses such as DNA damage. The function of mammalian GADD34 has been proposed by in vitro transfection, but its function in vivo has not yet been elucidated. Here we generated and analyzed GADD34 knockout mice. Despite their embryonic stage- and tissue-specific expressions, GADD34 knockout mice showed no abnormalities at fetal development and in early adult life. However, in GADD34-/- mouse embryonic fibroblasts (MEFs), recovery from a shutoff of protein synthesis was delayed when MEFs were exposed to endoplasmic reticulum (ER) stress. The phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2alpha) at Ser51 induced by thapsigargin or DTT was prolonged in GADD34-/- MEF, although following treatment with tunicamycin, the eIF2alpha phosphorylation level did not change in either GADD34+/+ or GADD34-/- cells. ER stress stimuli induced expressions of Bip (binding Ig protein) and CHOP (C/EBP homologous protein) in MEF of wild-type mice. These expressions were strongly reduced in GADD34-/- MEF, which suggests that GADD34 up-regulates Bip and CHOP. These results indicate that GADD34 works as a sensor of ER stress stimuli and recovers cells from shutoff of protein synthesis.

CR6-interacting factor 1 interacts with Gadd45 family proteins and modulates the cell cycle

The Gadd45 family of proteins includes Gadd45alpha, MyD118/Gadd45beta, and CR6/OIG37/Gadd45gamma. These proteins play important roles in maintaining genomic stability and in regulating the cell cycle. This study reports the cloning of a novel protein called CR6-interacting factor 1 (CRIF1) which interacts with Gadd45alpha, MyD118/Gadd45beta, and CR6/OIG37/Gadd45gamma. CRIF1 binds specifically to the Gadd45 family proteins, as determined by an in vitro glutathione S-transferase pull-down assay and an in vivo mammalian cell two-hybrid assay along with coimmunoprecipitation assays. CRIF1 mRNA is highly expressed in the thyroid gland, heart, lymph nodes, trachea, and adrenal tissues. CRIF1 localizes exclusively to the nucleus and colocalizes with Gadd45gamma. Recombinant CRIF1 inhibits the histone H1 kinase activity of immunoprecipitated Cdc2-cyclin B1 and Cdk2-cyclin E, and the inhibitory effects were additive with Gadd45 proteins. Overexpression of CRIF1 increases the percentage of cells in G1, decreases the percentage of cells in S phase, and suppresses growth in NIH3T3 cells. The down-regulation of endogenous CRIF1 by the transfection of the small interfering RNA duplexes resulted in the inactivation of Rb by phosphorylation and decreased the G1 phase cell populations. Expression of CRIF1 is barely detectable in adrenal adenoma and papillary thyroid cancer and much lower than in adjacent normal tissue. The results presented here suggest that CRIF1 is a novel nuclear protein that interacts with Gadd45 and may play a role in negative regulation of cell cycle progression and cell growth.

Role of Gadd45alpha in the density-dependent G1 arrest induced by p27(Kip1)

p27(Kip1), an inhibitor of cyclin-dependent kinases, is an important regulator of cell cycle progression. We have previously shown that p27(Kip1) inhibits the G0 to S transition when ectopically expressed in p27-47 mouse fibroblasts arrested at high but not low densities. In the study described here, we identify Gadd45alpha, a member of the growth arrest- and DNA damage-inducible family of proteins, as a potential mediator of the density-dependent effects of p27(Kip1) on cell proliferation. Gadd45alpha mRNA and protein were more abundant in p27-47 cells arrested at high densities than at low densities. Amounts of both decreased and remained low when cells arrested at high densities were exposed to mitogens in the absence, but not in the presence, of ectopically expressed p27(Kip1). Importantly, enforced expression of Gadd45alpha prevented density-arrested mouse fibroblasts from initiating DNA synthesis in response to mitogens. We suggest that amounts of Gadd45alpha above a certain threshold are growth inhibitory and that such amounts are achieved in cells arrested at high but not low densities. For cultures arrested at high densities, the resumption of cell cycle traverse requires a sustained reduction in Gadd45alpha abundance, a process that is induced by mitogens and inhibited by p27(Kip1).

Comparative analysis of the genetic structure and chromosomal mapping of the murine Gadd45g/CR6 gene

Gadd45g/CR6, Gadd45b/MyD118, and Gadd45a/Gadd45 are members of a gene family that displays distinct patterns of gene expression in response to stimuli that induce differentiation, growth arrest, and/or apoptosis. All three of these highly conserved proteins interact with a number of critical cell cycle and cell survival regulatory proteins such as PCNA, p21(WAF1/CIP1), CDK1 (cdc2-p34), and MTK1/MEKK4, and have been reported to influence the activity of the p38 and JNK kinases. Species-blot analysis showed that Gadd45g is an evolutionarily conserved gene and sequence analysis showed that Gadd45g has a gene structure conserved with that of other members of its gene family. A comparison of the putative transcription factor binding sites found in the sequences of the gene family members suggests, that like Gadd45b, NF-kappaB and STATs may be responsible for the differences in regulation of expression observed between Gadd45g and Gadd45a. Analysis of the Gadd45b/MyD118 promoter shows that there are three different enhanceosome-like regions that may allow cell-type specific responses to TGF-beta1 by the Gadd45b/MyD118 promoter. Fluorescent in situ hybridization (FISH) confirmed the localization of the Gadd45g gene to mouse chromosome band 13A5-B, which has been reported to contain a quantitative trait locus that regulates body weight in mice. This suggests that alleles of the Gadd45g gene may function in the regulation of body weight, in addition to its currently recognized roles in differentiation and stress responses.

cDNA microarray analysis reveals chop-10 plays a key role in Sertoli cell injury induced by bisphenol A

We examined the time course of changes in gene expression in detail using cDNA microarray analysis of mouse Sertoli TTE3 cells treated with bisphenol A (BPA). A subtoxic dose of BPA (200 microM) transiently increased intracellular Ca(2+) concentration and time-dependently induced an increase in mRNA level of 78-kDa glucose-regulated protein, indicating that BPA induces endoplasmic reticulum stress. Of the 865 genes analyzed, 31 genes showed increased levels of expression. TaqMan analysis confirmed that the mRNA levels of chop-10, fra-2, c-myc, and ornithine decarboxylase were increased, and showed that chop-10 is the most sensitive gene. The expression level of chop-10 protein and cell injury induced by BPA were significantly reduced in stable TTE3 cells overexpressing full-length chop-10 antisense RNA. We conclude that chop-10 plays a key role in Sertoli cell injury induced by BPA.

Interferon-gamma induces reactive oxygen species and endoplasmic reticulum stress at the hepatic apoptosis

Interferon-gamma (IFN-gamma) induces cell-cycle arrest and p53-independent apoptosis in primary cultured hepatocytes. However, the detailed mechanism, including regulating molecules, is still unclear. In this study, we found that IFN-gamma induced generation of reactive oxygen species (ROS) in primary hepatocytes and that pyrrolidinedithiocarbamate (PDTC), an anti-oxidant reagent, completely suppressed IFN-gamma-induced hepatic apoptosis. PDTC blocked apoptosis downstream from IRF-1 and upstream from caspase activation, suggesting that the generation of ROS occurred between these stages. However, IFN-gamma also induced the generation of ROS in IRF-1-deficient hepatocytes, cells insensitive to IFN-gamma-induced apoptosis. Moreover, a general cyclooxygenase (COX) inhibitor, indomethacin (but not the cyclooxygenase 2-specific inhibitor, NS-398) also inhibited the apoptosis without blocking the generation of ROS. Both PDTC and indomethacin also blocked IFN-gamma-induced release of cytochrome c from mitochondria. These results suggest that ROS are not the only or sufficient mediators of IFN-gamma-induced hepatic apoptosis. In contrast, we also found that IFN-gamma induced endoplasmic reticulum (ER) stress proteins, CHOP/GADD153 and caspase 12, in wild-type primary hepatocytes, but induced only caspase 12 and not CHOP/GADD153 protein in IRF-1-deficient hepatocytes. These results suggest that IFN-gamma induces ER stress in primary hepatocytes. Both the ROS and ER stress induced by IFN-gamma may be complementary mediators that induce apoptosis in primary hepatocytes.