Activation of Gadd34 by diverse apoptotic signals and suppression of its growth inhibitory effects by apoptotic inhibitors

Silencing of the DNA damage repair regulator PPP1R15A sensitizes acute myeloid leukemia cells to chemotherapy

Acute Myeloid Leukemia (AML) is a life-threatening disease whose induction treatment consists of combination chemotherapy with Idarubicin and Cytarabine for fit patients. Treatment failures are frequent, urging the need for novel treatments for this disease. The DNA Damage Response Mechanism (DDR) comprises numerous molecules and pathways intended to arrest the cell cycle until DNA damage is repaired or else drive the cell to apoptosis. AML-derived cell lines after treatment with Idarubicin and Cytarabine were used for studying the expression profile of 84 DDR genes, through PCR arrays. Utilizing de novo AML patient and control samples we studied the expression of PPP1R15A, CDKN1A, GADD45A, GADD45G, and EXO1. Next, we performed PPP1R15A silencing in AML cell lines in two separate experiments using siRNA and CRISPR-cas9, respectively. Our findings highlight that DDR regulators demonstrate increased expression in patients with high cytogenetic risk possibly reflecting increased genotoxic stress. Especially, PPP1R15A is mainly involved in the recovery of the cells from stress and it was the only DDR gene upregulated in AML patients. The PPP1R15A silencing resulted in decreased viability of Idarubicin and Cytarabine-treated cell lines, in contrast to untreated cells. These findings shed light on new strategies to enhance chemotherapy efficacy and demonstrate that PPP1R15A is an important DDR regulator in AML and its downregulation might be a safe and effective way to increase sensitivity to chemotherapy in this disease.

Inhibition of PPP1R15A alleviates osteoporosis via suppressing RANKL-induced osteoclastogenesis

Osteoporosis results from overactivation of osteoclasts. There are currently few drug options for treatment of this disease. Since the successful development of allosteric inhibitors, phosphatases have become attractive therapeutic targets. Protein phosphatase 1, regulatory subunit 15 A (PPP1R15A), is a stress-responsive protein, which promotes the UPR (unfolded protein response) and restores protein homeostasis. In this study we investigated the role of PPP1R15A in osteoporosis and osteoclastogenesis. Ovariectomy (OVX)-induced osteoporosis mouse model was established, osteoporosis was evaluated in the left femurs using micro-CT. RANKL-stimulated osteoclastogenesis was used as in vitro models. We showed that PPP1R15A expression was markedly increased in BMMs derived from OVX mice and during RANKL-induced osteoclastogenesis in vitro. Knockdown of PPP1R15A or application of Sephin1 (a PPP1R15A allosteric inhibitor in a phase II clinical trial) significantly inhibited osteoclastogenesis in vitro. Sephin1 (0.78, 3.125 and 12.5 μM) dose-dependently mitigated the changes in NF-κB, MAPK, and c-FOS and the subsequent nuclear factor of activated T cells 1 (NFATc1) translocation in RANKL-stimulated BMMs. Both Sephin1 and PPP1R15A knockdown increased the phosphorylated form of eukaryotic initiation factor 2α (eIF2α); knockdown of eIF2α reduced the inhibitory effects of Sephin1 on NFATc1-luc transcription and osteoclast formation. Furthermore, Sephin1 or PPP1R15A knockdown suppressed osteoclastogenesis in CD14+ monocytes from osteoporosis patients. In OVX mice, injection of Sephin1 (4, 8 mg/kg, i.p.) every two days for 6 weeks significantly inhibited bone loss, and restored bone destruction and decreased TRAP-positive cells. This study has identified PPP1R15A as a novel target for osteoclast differentiation, and genetic inhibition or allosteric inhibitors of PPP1R15A, such as Sephin1, can be used to treat osteoporosis. This study revealed that PPP1R15A expression was increased in osteoporosis in both human and mice. Inhibition of PPP1R15A by specific knockdown or an allosteric inhibitor Sephin1 mitigated murine osteoclast formation in vitro and attenuated ovariectomy-induced osteoporosis in vivo. PPP1R15A inhibition also suppressed pathogenic osteoclastogenesis in CD14+ monocytes from osteoporosis patients. These results identify PPP1R15A as a novel regulator of osteoclastogenesis and a valuable therapeutic target for osteoporosis.

The PPP1R15 Family of eIF2-alpha Phosphatase Targeting Subunits (GADD34 and CReP)

The vertebrate PPP1R15 family consists of the proteins GADD34 (growth arrest and DNA damage-inducible protein 34, the product of the PPP1R15A gene) and CReP (constitutive repressor of eIF2α phosphorylation, the product of the PPP1R15B gene), both of which function as targeting/regulatory subunits for protein phosphatase 1 (PP1) by regulating subcellular localization, modulating substrate specificity and assembling complexes with target proteins. The primary cellular function of these proteins is to facilitate the dephosphorylation of eukaryotic initiation factor 2-alpha (eIF2α) by PP1 during cell stress. In this review, we will provide a comprehensive overview of the cellular function, biochemistry and pharmacology of GADD34 and CReP, starting with a brief introduction of eIF2α phosphorylation via the integrated protein response (ISR). We discuss the roles GADD34 and CReP play as feedback inhibitors of the unfolded protein response (UPR) and highlight the critical function they serve as inhibitors of the PERK-dependent branch, which is particularly important since it can mediate cell survival or cell death, depending on how long the stressful stimuli lasts, and GADD34 and CReP play key roles in fine-tuning this cellular decision. We briefly discuss the roles of GADD34 and CReP homologs in model systems and then focus on what we have learned about their function from knockout mice and human patients, followed by a brief review of several diseases in which GADD34 and CReP have been implicated, including cancer, diabetes and especially neurodegenerative disease. Because of the potential importance of GADD34 and CReP in aspects of human health and disease, we will discuss several pharmacological inhibitors of GADD34 and/or CReP that show promise as treatments and the controversies as to their mechanism of action. This review will finish with a discussion of the biochemical properties of GADD34 and CReP, their regulation and the additional interacting partners that may provide insight into the roles these proteins may play in other cellular pathways. We will conclude with a brief outline of critical areas for future study.

Whole Transcriptome Sequencing Reveals Cancer-Related, Prognostically Significant Transcripts and Tumor-Infiltrating Immunocytes in Mantle Cell Lymphoma

Mantle cell lymphoma (MCL) is an aggressive B-cell non-Hodgkin lymphoma (NHL) subtype characterized by overexpression of CCND1 and SOX11 genes. It is generally associated with clinically poor outcomes despite recent improvements in therapeutic approaches. The genes associated with the development and prognosis of MCL are still largely unknown. Through whole transcriptome sequencing (WTS), we identified mRNAs, lncRNAs, and alternative transcripts differentially expressed in MCL cases compared with reactive tonsil B-cell subsets. CCND1, VCAM1, and VWF mRNAs, as well as MIR100HG and ROR1-AS1 lncRNAs, were among the top 10 most significantly overexpressed, oncogenesis-related transcripts. Survival analyses with each of the top upregulated transcripts showed that MCL cases with high expression of VWF mRNA and low expression of FTX lncRNA were associated with poor overall survival. Similarly, high expression of MSTRG.153013.3, an overexpressed alternative transcript, was associated with shortened MCL survival. Known tumor suppressor candidates (e.g., PI3KIP1, UBXN) were significantly downregulated in MCL cases. Top differentially expressed protein-coding genes were enriched in signaling pathways related to invasion and metastasis. Survival analyses based on the abundance of tumor-infiltrating immunocytes estimated with CIBERSORTx showed that high ratios of CD8⁺ T-cells or resting NK cells and low ratios of eosinophils are associated with poor overall survival in diagnostic MCL cases. Integrative analysis of tumor-infiltrating CD8⁺ T-cell abundance and overexpressed oncogene candidates showed that MCL cases with high ratio CD8⁺ T-cells and low expression of FTX or PCA3 can potentially predict high-risk MCL patients. WTS results were cross-validated with qRT-PCR of selected transcripts as well as linear correlation analyses. In conclusion, expression levels of oncogenesis-associated transcripts and/or the ratios of microenvironmental immunocytes in MCL tumors may be used to improve prognostication, thereby leading to better patient management and outcomes.

Tumor suppressor BTG1 promotes PRMT1-mediated ATF4 function in response to cellular stress

Cancer cells are frequently exposed to physiological stress conditions such as hypoxia and nutrient limitation. Escape from stress-induced apoptosis is one of the mechanisms used by malignant cells to survive unfavorable conditions. B-cell Translocation Gene 1 (BTG1) is a tumor suppressor that is frequently deleted in acute lymphoblastic leukemia and recurrently mutated in diffuse large B cell lymphoma. Moreover, low BTG1 expression levels have been linked to poor outcome in several solid tumors. How loss of BTG1 function contributes to tumor progression is not well understood. Here, using Btg1 knockout mice, we demonstrate that loss of Btg1 provides a survival advantage to primary mouse embryonic fibroblasts (MEFs) under stress conditions. This pro-survival effect involves regulation of Activating Transcription Factor 4 (ATF4), a key mediator of cellular stress responses. We show that BTG1 interacts with ATF4 and positively modulates its activity by recruiting the protein arginine methyl transferase PRMT1 to methylate ATF4 on arginine residue 239. We further extend these findings to B-cell progenitors, by showing that loss of Btg1 expression enhances stress adaptation of mouse bone marrow-derived B cell progenitors. In conclusion, we have identified the BTG1/PRMT1 complex as a new modifier of ATF4 mediated stress responses.

Feasibility of novel PPP1R15A and proposed ANXA11 single nucleotide polymorphisms as predictive markers for bevacizumab regimen in metastatic colorectal cancer

PURPOSE

Bevacizumab improves survival in patients with metastatic colorectal cancer (mCRC) under chemotherapy, but few predictive markers have been identified.

METHODS

To investigate chemosensitive single nucleotide polymorphisms (SNPs) of mCRC, we performed exome sequencing and RNA sequencing in 19 patients. A clinical association analysis was performed with the other 116 patients who had received chemotherapy to bevacizumab regimens. In vivo biodistribution studies and [(18)F]FDG-PET imaging were performed on mice bearing human colorectal cancer (HCT116 and SW480) xenografts after injection of bevacizumab with 5-FU, leucovorin, and irinotecan (FOLFIRI).

RESULTS

PPP1R15A rs557806 showed the most significant association with FRB-driven tumor IR in exome sequencing and the highest correlation (r = 0.74) with drug responses in RNA sequencing. Patients homozygous for the reference alleles (GG) of PPP1R15A rs557806 exhibited greater disease control rate and a tendency toward greater objective response rate (ORR) than those with homozygous or heterozygous substitution alleles (GC and CC; P = 0.027 and 0.073, respectively). In xenografted mice, HCT116 clones transfected with the G allele at PPP1R15A rs557806 were more sensitive to bevacizumab regimens than those with the C allele. Tumor volume of xenografts with the G allele was significantly lower than that of xenografts with the C allele (P = 0.004, day 13). [(18)F]FDG uptake decreased to 75 % in HCT116 xenograft-bearing mice with the G allele, whereas [(18)F]FDG uptake was 42 % in mice xenografts with the C allele (P = 0.032). ANXA11 rs1049550, a predictive biomarker of SNP described in our previous study, was validated using the xenograft model. Tumor volume and [(18)F]FDG uptake analyses showed that tumors in the SW480 xenografts expressing the substitution allele (T) at ANXA11 rs1049550 were more susceptible to FOLFIRI plus bevacizumab-induced suppression than those expressing the reference allele (C) (P = 0.001 and 0.026, respectively).

CONCLUSION

ANXA11 rs1049550 and PPP1R15A rs557806 may improve the identification of mCRC patients sensitive to bevacizumab regimens, and further validation is required in large cohorts.

Riproximin: A type II ribosome inactivating protein with anti-neoplastic potential induces IL24/MDA-7 and GADD genes in colorectal cancer cell lines

Riproximin (Rpx) is a type II ribosome inactivating protein, which was extracted and purified from the seeds of Ximenia americana. Previous studies demonstrated cytotoxicity of Rpx against a variety of cell lines originating from solid and non-solid cancers. In this study, we investigated the mechanistic aspects of Rpx in selected human and rat colorectal cancer (CRC) cell lines. Cytotoxic levels of Rpx were determined by MTT assay, while cytostatic and apoptotic effects were investigated by flow cytometry and nuclear staining procedures. Effects of Rpx exposure on colony formation/migration of CRC cells and expressional modulations in anticancer/stress-related genes were also studied. Rpx showed significant and comparable levels of cytotoxicity in CRC cells as determined by inhibitory concentration (IC) values. Similar inhibitory effects were found for clonogenicity, while more pronounced inhibition of migration was observed in response to Rpx exposure. Profound arrest in S phases of the cell cycle was noted especially in primary CRC cells. Apoptotic effects were more prominent in rat CRC cells as indicated by Annexin V-FITC assay and Hoechst 33342 nuclear staining. Rpx exposure induced significantly increased levels of the IL24/MDA-7, a well characterized anticancer gene, in all CRC cells. In addition, following Rpx treatment, high expression levels of growth arrest and DNA damage (GADD family) genes were also observed. Increased expression of two additional GADD genes (34 and 153) only in rat CRC cells (CC531) conferred higher sensitivity towards Rpx and subsequent anti-proliferative/apoptotic effects as compared to human CRC cells (SW480 and SW620). The present investigation indicates the anticancer potential of Rpx in CRC and favor further evaluation of this natural compound as therapeutic agent.

Alteration of DNA damage signaling pathway profile in radiation-treated glioblastoma stem-like cells

The present study aimed to investigate the alteration of the DNA damage signaling pathway profile in radiation-treated glioblastoma stem-like cells (GSLCs), and also aimed to explore potential targets for overcoming glioblastoma radioresistance. Serum-free medium was used to isolate and culture GSLCs. Cell growth was detected using a cell counting kit-8 assay and cell sorting analysis was performed by flow cytometry. X-ray irradiation was produced by a Siemens-Primus linear accelerator. Reverse transcription-quantitative polymerase chain reaction (qPCR)was performed to investigate target genes. SPSS 15.0 was used for all statistical analyses. Human glioblastoma U251 and U87 cells were cultured in serum-free medium supplemented with epidermal growth factor and fibroblast growth factor 2, which constitutes tumor sphere medium, and demonstrated sphere formation, with significantly increased the proportion of CD133⁺ and Nestin⁺ cells, which are referred to as GSLCs. The present data revealed that treatment with 10 Gy X-ray radiation alters the expression profile of DNA damage-associated genes in GSLCs. The expression levels of 12 genes demonstrated a ≥2-fold increase in the irradiated U87 GSLCs compared with the untreated U87 GSLCs. Three genes, consisting of XPA, RAD50 and PPP1R15A, were selected from the 12 genes by gene functional searching and qPCR confirmatory studies, as these genes were considered to be potential targets for overcoming radioresistance. The expression of XPA, RAD50 and PPP1R15A is significantly increased in U87 and U251 radiation resistant GSLCs, indicating three potential targets for overcoming the radioresistance of GSLCs.

Negative regulation of GADD34 on myofibroblasts during cutaneous wound healing

The growth arrest and DNA damage-inducible protein, GADD34, has been proved to be involved in TGF-β signaling pathway and correlates with cell death, which are two important mechanisms in regulating myofibroblast differentiation and apoptosis during tissue repair. But roles of GADD34 in myofibroblasts differentiation and apoptosis remain unknown. To investigate the function of GADD34 in these processes, we subjected WT and GADD34^−/− mice to dermal wound healing. Here we show that GADD34^−/− mice exhibited accelerated wound closure compared with WT mice. In addition, GADD34^−/− mice showed increased number of myofibroblasts, elevated collagen production, and decreased cell apoptosis during wound healing. Moreover, we found that GADD34^−/− mice showed increased phosphorylation of Smad3 and lower level of cleaved caspase-3. Thus these results indicate that GADD34 appears to suppress myofibroblast differentiation through inhibiting Smad3-dependent TGFβ signal pathway and promote its apoptosis by activating caspase-3 pathway.

Novel Target Genes Responsive to Apoptotic Activity by Ocimum gratissimum in Human Osteosarcoma Cells

Osteosarcoma (OS) is a type of bone cancer. Eighty percent of this tumor will metastasize to the lungs or liver, and as a result, patients generally need chemotherapy to improve survival possibility. Recently, antitumor activity has been reported in Ocimum gratissimum aqueous extract (OGE), which has been the focus of recent extensive studies on therapeutic strategies due to its antioxidant properties. We performed pharmacogenomics analyses for the effect of OGE on human osteosarcoma U2-OS and HOS cell growth. Cell viability, Western blot and flow cytometry analysis were performed before performing pharmacogenomics analyses for the effect of OGE on human osteosarcoma U2-OS and HOS cell growth, including cDNA microarray and RT-PCR assays. Cell viability assays revealed that OGE significantly and dose-dependently decreased the viability of U2-OS and HOS cells. Increases in cell shrinkage, Sub-G1 fragments and the activation of caspase 3 indicated that OGE induced cell apoptosis in U2-OS and HOS cells. There was no change in human osteoblast hFOS cells. cDNA microarray assay demonstrated that the expression of cell cycle regulators, apoptosis-related factors and cell proliferation markers were all modified by OGE treatment. RT-PCR analysis also confirmed the down-regulation of SKA2 and BUB1B, and the up-regulation of PPP1R15A, SQSTM1, HSPA1B, and DDIT4 by OGE treatment. The finding of anticancer activity in OGE and the identification of some potential target genes raise the expectation that OGE may become a useful therapeutic drug for human OS.

Epigallocatechin-3-gallate (EGCG) protects against chromate-induced toxicity in vitro

Hexavalent chromium [Cr(VI)] is a human carcinogen that results in the generation of reactive oxygen species (ROS) and a variety of DNA lesions leading to cell death. Epigallocatechin-3-gallate (EGCG), the major polyphenol present in green tea, possesses potent antioxidative activity capable of protecting normal cells from various stimuli-induced oxidative stress and cell death. Here we demonstrated that co-treatment with EGCG protected human normal bronchial epithelial BEAS-2B cells from Cr(VI)-induced cell death in a dose-dependent manner. Cr(VI) induces apoptosis as the primary mode of cell death. Co-treatment of BEAS-2B cells with EGCG dose-dependently suppressed Cr(VI)-induced apoptosis. Fluorescence microscopic analyses and quantitative measurement revealed that EGCG significantly decreased intracellular levels of ROS induced by Cr(VI) exposure. Using a well-established K(+)/SDS precipitation assay, we further showed that EGCG was able to dose-dependently reduce DNA-protein cross-links (DPC), lesions that could be partially attributed to Cr(VI)-induced oxidative stress. Finally, analyses of Affymetrix microarray containing 28,869 well-annotated genes revealed that, among the 3412 genes changed more than 1.5-fold by Cr(VI) treatment, changes of 2404 genes (70%) were inhibited by pretreatment of EGCG. Real-time PCR confirmed the induction of 3 genes involved in cell death and apoptosis by Cr(VI), which was eliminated by EGCG. In contrast, Cr(VI) reduced the expression of 3 genes related to cellular defense, and this reduction was inhibited by EGCG. Our results indicate that EGCG protects BEAS-2B cells from Cr(VI)-induced cytotoxicity presumably by scavenging ROS and modulating a subset of genes. EGCG, therefore, might serve as a potential chemopreventive agent against Cr(VI) carcinogenesis.

Cellular iron depletion and the mechanisms involved in the iron-dependent regulation of the growth arrest and DNA damage family of genes

Iron plays a crucial part in proliferation while iron deficiency results in G(1)/S arrest, DNA damage, and apoptosis. However, the precise role of iron in cell cycle control remains unclear. We showed that iron depletion using the iron chelators, desferrioxamine (DFO), or 2-hydroxy-1-napthylaldehyde isonicotinoyl hydrazone (311), increased the mRNA levels of the growth arrest and DNA damage 45α gene, GADD45α (Darnell, G. and Richardson, D. R. (1999) Blood 94, 781-792). In this study, we examined the effect of iron depletion on up-regulating GADD family members involved in growth control, including cell cycle arrest, apoptosis, and DNA repair, making them therapeutic targets for tumor suppression. We showed the GADD family members were up-regulated by cellular iron depletion. Further, up-regulation of GADD45α after iron deprivation was independent of hypoxia-inducible factor-1α (HIF-1α), octamer-1 (Oct-1), p53 and early growth response 1 (Egr1). We then analyzed the regulatory elements responsible for iron depletion-mediated regulation of GADD45α and identified the specific transcription factor/s involved. This region was within -117 bp and -81 bp relative to the start codon where the consensus sequences of three transcription factors are located: the CCAAT-binding factor/nuclear factor-Y (NF-Y), the stabilizing molecule v-MYB and the enhancer, CCAAT enhancer-binding protein (CEBPα). Mutation analysis, shRNA studies, Western blotting, and electrophoretic mobility shift assays led to the identification of NF-Y in the transcriptional up-regulation of GADD45α after iron depletion. Furthermore, like GADD45α, NF-YA was up-regulated after iron chelation and down-regulated by iron supplementation. These results are important for understanding the mechanisms of iron depletion-mediated cell cycle arrest, DNA damage repair, and apoptosis.

Association with endoplasmic reticulum promotes proteasomal degradation of GADD34 protein

Stress-induced endogenous and ectopically expressed GADD34 proteins were present both in the cytoplasm and in membranes, with their membrane association showing similar biochemical properties. Deletion of N-terminal sequences in GADD34-GFP proteins highlighted an amphipathic helix, whose hydrophobic surface, specifically valine 25 and leucine 29, mediated endoplasmic reticulum (ER) localization. Substitution of leucines for three arginines on the polar surface indicated that the same helix also mediated the association of GADD34 with mitochondria. Fluorescence protease protection and chemical modification of cysteines substituted in the membrane-binding domain pointed to a monotopic insertion of GADD34 into the outer layer of the ER membrane. Fluorescence recovery after photobleaching showed that ER association retards the mobility of GADD34 in living cells. Both WT GADD34 and the mutant, V25R, effectively scaffolded the α-isoform of protein phosphatase-1 (PP1α) and enabled eIF2α dephosphorylation. However, the largely cytosolic V25R protein displayed a reduced rate of proteasomal degradation, and unlike WT GADD34, whose ectopic expression resulted in a dilated or distended ER, V25R did not modify ER morphology. These studies suggested that the association of with ER modulates intracellular trafficking and proteasomal degradation of GADD34, and in turn, its ability to modify ER morphology.

Gene expression analysis of cell death induction by taurolidine in different malignant cell lines

Background

The anti-infective agent Taurolidine (TRD) has been shown to have cell death inducing properties, but the mechanism of its action is largely unknown. The aim of this study was to identify potential common target genes modulated at the transcriptional level following TRD treatment in tumour cell lines originating from different cancer types.

Methods

Five different malignant cell lines (HT29, Chang Liver, HT1080, AsPC-1 and BxPC-3) were incubated with TRD (100 μM, 250 μM and 1000 μM). Proliferation after 8 h and cell viability after 24 h were analyzed by BrdU assay and FACS analysis, respectively. Gene expression analyses were carried out using the Agilent -microarray platform to indentify genes which displayed conjoint regulation following the addition of TRD in all cell lines. Candidate genes were subjected to Ingenuity Pathways Analysis and selected genes were validated by qRT-PCR and Western Blot.

Results

TRD 250 μM caused a significant inhibition of proliferation as well as apoptotic cell death in all cell lines. Among cell death associated genes with the strongest regulation in gene expression, we identified pro-apoptotic transcription factors (EGR1, ATF3) as well as genes involved in the ER stress response (PPP1R15A), in ubiquitination (TRAF6) and mitochondrial apoptotic pathways (PMAIP1).

Conclusions

This is the first conjoint analysis of potential target genes of TRD which was performed simultaneously in different malignant cell lines. The results indicate that TRD might be involved in different signal transduction pathways leading to apoptosis.

Desipramine induces apoptosis in rat glioma cells via endoplasmic reticulum stress-dependent CHOP pathway

Various antidepressants, mainly tricyclic antidepressants (TCAs) and selective serotonin reuptake inhibitors (SSRIs), have exhibited potent anticancer properties in different cancer cell types. In the present study, desipramine (DMI), a representative of TCAs, was examined with respect to its apoptosis-inducing activity in rat C6 glioma cells and the underlying mechanism of action. DMI induced typical apoptotic morphology of chromatin condensation in rat glioma C6 cells and activated intracellular caspase 9 and caspase 3 with no change in mitochondrial membrane potential. Simultaneously, DMI significantly elevated expression of endoplasmic reticulum stress regulator CHOP/GADD153 and its targeting molecule GADD34. However, knockdown of CHOP by CHOP-specific short interfering RNA (siRNA) could decrease the activity of intracellular caspase 3 and the cytotoxicity of DMI to C6 cells. These results revealed that the CHOP-dependent endoplasmic reticulum (ER) stress pathway is responsible for DMI-induced apoptosis in C6 cells.

Brevetoxin 2 alters expression of apoptotic, DNA damage, and cytokine genes in Jurkat cells

Brevetoxins are potent neurotoxins that exert their toxicity through activation of voltage-gated sodium channels. Exposure to brevetoxins cause severe respiratory inflammation in marine mammals and humans. Brevetoxin activation of voltage-gated sodium channels on immune cells can lead to several biological responses including cell proliferation, gene transcription, cytokine production and even apoptosis. Jurkat E6-1 T cells were treated with brevetoxin 2 for 4 hours at a dose previously shown to induce apoptosis and DNA damage. Changes in gene expression were then assessed via PCR arrays. Gene expression analysis revealed significant change in expression of 17 genes related to apoptosis, 21 genes related to DNA damage signaling, and 19 genes encoding common cytokines. The gene expression data supports the idea that brevetoxins trigger complex reactions involving both inflammation and cell death.

Cardiac expression of microsomal triglyceride transfer protein is increased in obesity and serves to attenuate cardiac triglyceride accumulation

Obesity causes lipid accumulation in the heart and may lead to lipotoxic heart disease. Traditionally, the size of the cardiac triglyceride pool is thought to reflect the balance between uptake and β-oxidation of fatty acids. However, triglycerides can also be exported from cardiomyocytes via secretion of apolipoproteinB-containing (apoB) lipoproteins. Lipoprotein formation depends on expression of microsomal triglyceride transfer protein (MTP); the mouse expresses two isoforms of MTP, A and B. Since many aspects of the link between obesity-induced cardiac disease and cardiac lipid metabolism remain unknown, we investigated how cardiac lipoprotein synthesis affects cardiac expression of triglyceride metabolism-controlling genes, insulin sensitivity, and function in obese mice. Heart-specific ablation of MTP-A in mice using Cre-loxP technology impaired upregulation of MTP expression in response to increased fatty acid availability during fasting and fat feeding. This resulted in cardiac triglyceride accumulation but unaffected cardiac insulin-stimulated glucose uptake. Long-term fat-feeding of male C57Bl/6 mice increased cardiac triglycerides, induced cardiac expression of triglyceride metabolism-controlling genes and attenuated heart function. Abolishing cardiac triglyceride accumulation in fat-fed mice by overexpression of an apoB transgene in the heart prevented the induction of triglyceride metabolism-controlling genes and improved heart function. The results suggest that in obesity, the physiological increase of cardiac MTP expression serves to attenuate cardiac triglyceride accumulation albeit without major effects on cardiac insulin sensitivity. Nevertheless, the data suggest that genetically increased lipoprotein secretion prevents development of obesity-induced lipotoxic heart disease.

Control of cellular GADD34 levels by the 26S proteasome

GADD34, the product of a growth arrest and DNA damage-inducible gene, is expressed at low levels in unstressed cells. In response to stress, the cellular content of GADD34 protein increases and, on termination of stress, rapidly declines. We investigated the mechanisms that control GADD34 levels in human cells. GADD34 proteins containing either an internal FLAG or a C-terminal green fluorescent protein epitope were degraded at rates similar to endogenous GADD34. However, the addition of epitopes at the N terminus or deletion of N-terminal sequences stabilized GADD34. N-terminal peptides of GADD34, either alone or fused to heterologous proteins, exhibited rapid degradation similar to wild-type GADD34, thereby identifying an N-terminal degron. Deletion of internal PEST repeats had no impact on GADD34 stability but modulated the binding and activity of protein phosphatase 1. Proteasomal but not lysosomal inhibitors enhanced GADD34 stability and eukaryotic initiation factor 2alpha (eIF-2alpha) dephosphorylation, a finding consistent with GADD34's role in assembling an eIF-2alpha phosphatase. GADD34 was polyubiquitinated, and this modification enhanced its turnover in cells. A stabilized form of GADD34 promoted the accumulation and aggregation of the mutant cystic fibrosis transmembrane conductance regulator (CFTRDeltaF508), highlighting the physiological importance of GADD34 turnover in protein processing in the endoplasmic reticulum and the potential impact of prolonged GADD34 expression in human disease.

A microarray study of gene and protein regulation in human and rat brain following middle cerebral artery occlusion

BACKGROUND

Altered gene expression is an important feature of ischemic cerebral injury and affects proteins of many functional classes. We have used microarrays to investigate the changes in gene expression at various times after middle cerebral artery occlusion in human and rat brain.

RESULTS

Our results demonstrated a significant difference in the number of genes affected and the time-course of expression between the two cases. The total number of deregulated genes in the rat was 335 versus 126 in the human, while, of 393 overlapping genes between the two array sets, 184 were changed only in the rat and 36 in the human with a total of 41 genes deregulated in both cases. Interestingly, the mean fold changes were much higher in the human. The expression of novel genes, including p21-activated kinase 1 (PAK1), matrix metalloproteinase 11 (MMP11) and integrase interactor 1, was further analyzed by RT-PCR, Western blotting and immunohistochemistry. Strong neuronal staining was seen for PAK1 and MMP11.

CONCLUSION

Our findings confirmed previous studies reporting that gene expression screening can detect known and unknown transcriptional features of stroke and highlight the importance of research using human brain tissue in the search for novel therapeutic agents.

TRB3 protects cells against the growth inhibitory and cytotoxic effect of ATF4

Tribbles homolog 3 (TRB3) is a pseudokinase the level of which is increased in response to various stresses. We and other researchers have previously shown that TRB3 interacts with activating transcription factor 4 (ATF4) and may function as a negative feedback regulator of ATF4. In the present study, we investigate the effect of ATF4 and TRB3 on cell growth and viability, using both the enforced expression and silencing of the genes. HEK293 cells overexpressing ATF4 show retarded growth in the complete medium and decreased viability in the glucose-free medium. The enforced expression of ATF4 increases the level of reactive oxygen species (ROS) and the supplementation of the medium with ROS scavenging and reducing compounds supports the growth and survival of cells overexpressing ATF4. The deleterious effects of elevated ATF4 are suppressed by the coexpression of TRB3, which downregulates ATF4 transcriptional activity and results in the decrease of intracellular ROS. Also, the coexpression of TRB3 rescues postmitotic neuronally differentiated PC12 cells from the apoptosis evoked by ATF4 overexpression. The silencing of ATF4 and TRB3 genes by RNA interference reveals that endogenous ATF4 promotes and TRB3 suppresses the death of glucose-deprived SaOS2 cells. Together, the results indicate that TRB3 protects cells against the growth inhibitory and cytotoxic effect of ATF4.

Radiation-induced cellular DNA damage repair response enhances viral gene therapy efficacy in the treatment of malignant pleural mesothelioma

BACKGROUND

Malignant pleural mesothelioma (MPM) treated with radiotherapy (RT) has incomplete responses as a result of radiation-induced tumoral stress response that repairs DNA damage. Such stress response is beneficial for oncolytic viral therapy. We hypothesized that a combination of RT and NV1066, an oncolytic herpes virus, might exert an additive or synergistic effect in the treatment of MPM.

METHODS

JMN, a MPM cell line, was infected with NV1066 at multiplicities of infection of .05 to .25 in vitro with and without radiation (1 to 5 Gy). Virus replication was determined by plaque assay, cell kill by lactate dehydrogenase assay, and GADD34 (growth arrest and DNA damage repair 34, a DNA damage-repair protein) by real-time reverse transcriptase-polymerase chain reaction and Western blot test. Synergistic cytotoxicity dependence on GADD34 upregulation was confirmed by GADD34 small inhibitory RNA (siRNA).

RESULTS

Synergism was demonstrated between RT and NV1066 across a wide range of doses. As a result of such synergism, a dose-reduction for each agent (up to 5500-fold) can be accomplished over a wide range of therapeutic-effect levels without sacrificing tumor cell kill. This effect is correlated with increased GADD34 expression and inhibited by transfection of siRNA directed against GADD34.

CONCLUSIONS

RT can be combined with oncolytic herpes simplex virus therapy in the treatment of malignant pleural mesothelioma to achieve synergistic efficacy while minimizing dosage and toxicity.

Hemodialysis modulates gene expression profile in skeletal muscle

BACKGROUND

Uremia alters diverse metabolic pathways involving multiple organ systems, including skeletal muscle. Skeletal muscle has an important role in nutrition, metabolism, oxidative stress, and inflammation. We hypothesized that hemodialysis (HD) will change the genomic fingerprinting associated with uremia and facilitate expression of a distinct set of genes.

METHODS

Five patients with end-stage renal disease (ESRD) were studied. Skeletal muscle biopsy specimens from the vastus lateralis were obtained before (pre-HD) and during the last 10 minutes of HD (post-HD). Oligonucleotide microarray (version 2, GeneChip arrays; Affymetrix U95A, Santa Clara, CA) was used to analyze global transcriptional modification in skeletal muscle by HD. Pre-HD data were compared with data from 3 subjects without renal failure.

RESULTS

In skeletal muscle of patients with ESRD, 83 genes were upregulated and 8 genes were downregulated pre-HD compared with controls. Pathway analysis linked 55 genes to 5 gene networks involved in the regulation of cell cycle, cell proliferation, cellular organization, apoptosis, and inflammation. During HD, expression of 22 genes increased and 1 (TOB1) decreased. Pathway analysis mapped 20 genes to 2 genetic networks involved in: (1) inflammation, cell proliferation, and cell signaling; and (2) apoptosis, cell function, protein synthesis, and tissue morphology. Reverse-transcription polymerase chain reaction confirmed increased expression of GADD45A, BTG2, PDE4B, and CEBPD and downregulation of TOB1 in skeletal muscle intradialysis.

CONCLUSION

In response to the uremic milieu, skeletal muscle goes through very active transcriptional and translational changes. HD activates a diverse, yet biologically linked, network of genes related to inflammation and apoptosis in skeletal muscle.

Double-stranded RNA binding of influenza B virus nonstructural NS1 protein inhibits protein kinase R but is not essential to antagonize production of alpha/beta interferon

Expression of alpha/beta interferon (IFN-alpha/beta) in virus-infected vertebrate cells is a key event in the establishment of a sustained antiviral response, which is triggered by double-stranded RNA (dsRNA) produced during viral replication. These antiviral cytokines initiate the expression of cellular proteins with activities that limit the replication and spread of the invading viruses. Within this response, the dsRNA-dependent protein kinase R (PKR) that is expressed at constitutive levels and upregulated by IFN-alpha/beta acts as an important antiviral effector that can block the cellular translational machinery. We previously demonstrated that efficient replication of influenza B virus depends on the viral dsRNA-binding NS1 protein that inhibits the transcriptional activation of IFN-alpha/beta genes. Here we tested the postulate that the viral NS1 protein counteracts antiviral responses through sequestering intracellular dsRNA by analyzing a collection of recombinant influenza B viruses. As expected, viruses expressing dsRNA-binding-defective NS1 proteins were strongly attenuated for replication in IFN-competent hosts. Interestingly, these virus mutants failed to prevent activation of PKR but could effectively limit IFN induction. Conversely, a mutant virus expressing the N-terminal dsRNA-binding domain of NS1 prevented PKR activation, but not IFN induction, suggesting an important role for the NS1 C-terminal part in silencing the activation route of IFN-alpha/beta genes. Thus, our findings indicate an unexpected mechanistic dichotomy of the influenza B virus NS1 protein in the suppression of antiviral responses, which involves at least one activity that is largely separable from dsRNA binding.

Radiation therapy potentiates effective oncolytic viral therapy in the treatment of lung cancer

BACKGROUND

Replication-competent oncolytic herpes simplex viruses with deletion of the gamma(1)34.5 gene preferentially replicate in and kill malignant cells. The gamma(1)34.5 gene codes for ICP 34.5, a protein that enhances viral replication, and is homologous to growth arrest and DNA damage protein 34 (GADD34), a radiation-inducible DNA repair gene. We hypothesized that radiation therapy may potentiate efficacy of oncolytic viral therapy by upregulating GADD34 and promoting viral replication.

METHODS

The A549 and H1299 lung cancer cell lines were infected with NV1066, an oncolytic herpes simplex virus, at multiplicities of infection (number of viral particles per tumor cell) of 0.1 to 0.5 in vitro with radiation (2 to 10 Gy) or without radiation. Viral replication was determined by plaque assay, cell-to-cell spread was determined by flow cytometry, cell kill was determined by lactate dehydrogenase assay, and GADD34 induction was determined by real-time reverse transcription-polymerase chain reaction and Western blot method. Evidence of synergistic cytotoxicity dependence with GADD34 induction is further confirmed by small inhibitory RNA inhibition of GADD34 expression.

RESULTS

Using both the isobologram method and combination index method of Chou and Talalay, significant synergism was demonstrated between radiation therapy and NV1066 both in vitro and in vivo. As a result of such synergism, a dose reduction for each agent (2- to 6,000-fold) can be accomplished for a wide range of therapeutic effect levels without sacrificing tumor cell kill. This effect is correlated with increased GADD34 expression and inhibited by transfection of small inhibitory RNA directed against GADD34.

CONCLUSIONS

These data provide the cellular basis for the clinical investigation of combined use of radiation therapy with oncolytic herpes simplex virus therapy in the treatment of lung cancer to achieve synergistic efficacy while minimizing dosage and toxicity.

mda-7/IL-24: multifunctional cancer-specific apoptosis-inducing cytokine

"Differentiation therapy" provides a unique and potentially effective, less toxic treatment paradigm for cancer. Moreover, combining "differentiation therapy" with molecular approaches presents an unparalleled opportunity to identify and clone genes mediating cancer growth control, differentiation, senescence, and programmed cell death (apoptosis). Subtraction hybridization applied to human melanoma cells induced to terminally differentiate by treatment with fibroblast interferon (IFN-beta) plus mezerein (MEZ) permitted cloning of melanoma differentiation associated (mda) genes. Founded on its novel properties, one particular mda gene, mda-7, now classified as a member of the interleukin (IL)-10 gene family (IL-24) because of conserved structure, chromosomal location, and cytokine-like properties has become the focus of attention of multiple laboratories. When administered by transfection or adenovirus-transduction into a spectrum of tumor cell types, melanoma differentiation associated gene-7/interleukin-24 (mda-7/IL-24) induces apoptosis, whereas no toxicity is apparent in normal cells. mda-7/IL-24 displays potent "bystander antitumor" activity and also has the capacity to enhance radiation lethality, to induce immune-regulatory activities, and to inhibit tumor angiogenesis. Based on these remarkable attributes and effective antitumor therapy in animal models, this cytokine has taken the important step of entering the clinic. In a Phase I clinical trial, intratumoral injections of adenovirus-administered mda-7/IL-24 (Ad.mda-7) was safe, elicited tumor-regulatory and immune-activating processes, and provided clinically significant activity. This review highlights our current understanding of the diverse activities and properties of this novel cytokine, with potential to become a prominent gene therapy for cancer.

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.

Escherichia coliverotoxin 1 mediates apoptosis in human HCT116 colon cancer cells by inducing overexpression of the GADD family of genes and S phase arrest

The Escherichia coli verotoxin 1 (VT1) inhibits protein synthesis, cell proliferation, and damages endothelial cell in the hemolytic uremic syndrome. VT1 can specifically bind and act on endothelial cells as well as on many tumor cells because these cells express its high affinity receptor, globotriaosylceramide. This indicates that VT1 may have both antiangiogenic and antineoplastic activities. We investigated this potential of VT1 by incubating several colon cancer cell lines with VT1 for different time periods and found that HCT116 cells were especially sensitive to VT1. A combination of morphological studies, flow cytometry, DNA laddering and annexin V staining confirmed that VT1 irreversibly arrests these cells in S phase within 24 h and prolonged incubation triggers DNA fragmentation. Concomitant to the activation of the S phase checkpoint, increased levels of mRNA and proteins of growth arrest and DNA damage-inducible gene family that include GADD34, GADD45alpha, and GADD45beta was observed. Interestingly, no significant changes in expression of key cell cycle related proteins such as cdk2, cdk4, p21, p27, and p53 was found during the S phase arrest and apoptosis. We therefore suggest that GADD proteins might play an important role in VT1 induced S phase arrest and programmed cell death in HCT116 cells.

Alterations in Gene Expression Induced by the Lipid Peroxidation Product, 4-Hydroxy-2-nonenal

Reactive intermediates produced during oxidative stress are believed to contribute to the development and progression of a variety of age-related diseases. 4-hydroxy-2-nonenal (HNE), an abundant product of polyunsaturated fatty acid oxidation and decomposition, reacts extensively with DNA and proteins, depletes intracellular glutathione, and alters many cell signaling cascades, including those regulating apoptosis. Using microarray technology, global changes in gene expression were monitored temporally at subcytotoxic and cytotoxic HNE doses in RKO human colorectal carcinoma cells, with the highest level of changes being observed at cytotoxic doses. Gene expression alterations were confirmed using real-time reverse transcription polymerase chain reaction, Western blotting, and transcription factor reporter profiling. Significant alterations were observed in genes regulated by the antioxidant, heat shock, ER stress, and nutrient deprivation responses. Our results demonstrate that HNE simultaneously affects multiple stress signaling pathways, many of which represent potential mechanisms through which HNE alters cellular viability and response to damage.

Inhibition of Single Minded 2 gene expression mediates tumor-selective apoptosis and differentiation in human colon cancer cells

A Down's syndrome associated gene, Single Minded 2 gene short form (SIM2-s), is specifically expressed in colon tumors but not in the normal colon. Antisense inhibition of SIM2-s in a RKO-derived colon carcinoma cell line causes growth inhibition, apoptosis, and inhibition of tumor growth in a nude mouse tumoriginicity model. The mechanism of cell death in tumor cells is unclear. In the present study, we investigated the pathways underlying apoptosis. Apoptosis was seen in a tumor cell-specific manner in RKO cells but not in normal renal epithelial cells, despite inhibition of SIM2-s expression in both of these cells by the antisense. Apoptosis was depended on WT p53 status and was caspase-dependent; it was inhibited by a pharmacological inhibitor of mitogen-activated protein kinase activity. Expression of a key stress response gene, growth arrest and DNA damage gene (GADD)45alpha, was up-regulated in antisense-treated tumor cells but not in normal cells. In an isogenic RKO cell line expressing stable antisense RNA to GADD45alpha, a significant protection of the antisense-induced apoptosis was seen. Whereas antisense-treated RKO cells did not undergo cell cycle arrest, several markers of differentiation were deregulated, including alkaline phosphatase activity, a marker of terminal differentiation. Protection of apoptosis and block of differentiation showed a correlation in the RKO model. Our results support the tumor cell-selective nature of SIM2-s gene function, provide a direct link between SIM2-s and differentiation, and may provide a model to identify SIM2-s targets.

Potential molecular mechanism for rodent tumorigenesis: mutational generation of Progression Elevated Gene-3 (PEG-3)

Progression Elevated Gene-3 (PEG-3) was cloned using subtraction hybridization as an upregulated transcript associated with transformation and tumor progression of rat embryo fibroblast cells. PEG-3 is a unique gene facilitating tumor progression by modulating multiple pathways in transformed cells, including genomic stability, angiogenesis and invasion. PEG-3 originates from mutation in the growth arrest and DNA damage inducible gene GADD34. A one base deletion in rat GADD34 results in a frame-shift and premature appearance of a stop-codon resulting in a C-terminally truncated molecule that is PEG-3. We now document that mutation in the GADD34 gene is a frequent event during transformation and/or immortalization of rodent cells. Sequencing of the GADD34 gene in a number of independent rat tumor cell lines revealed that in a majority of these the GADD34 gene is mutated to either PEG-3 or a PEG-3-like gene with similar C-terminal truncations. An important function of GADD34 is to inhibit cell growth, predominantly by apoptosis, and we demonstrate that PEG-3 or C-terminal truncations of human GADD34 resembling PEG-3 prevent growth inhibition by both human and rat GADD34. Phosphorylation of p53 by GADD34 is one mechanism by which it inhibits growth and PEG-3 could prevent GADD34-induced p53 phosphorylation. In contrast, PEG-3 was unable to block other GADD34-induced changes, including eIF2 alpha dephosphorylation, indicating that its effects on GADD34 may be related more to its effect on cell growth rather than a global inhibitor of all GADD34 functions. We hypothesize that mutational generation of PEG-3 or a similar molecule is a critical event during rodent carcinogenesis. The inherent property of PEG-3 to function as a dominant negative of the growth inhibitory property of GADD34 might rescue cells from DNA damage-induced apoptosis leading to growth independence and tumorigenesis.

Increased GADD gene expression in human colon epithelial cells exposed to deoxycholate

The colonic epithelium is often exposed to high concentrations of secondary bile acids, which stresses the epithelial cells, leading potentially to activation of stress-response genes. To examine this possibility in vitro, the purpose of this study was to determine if expression of certain growth arrest and DNA damage-inducible genes (GADD) is upregulated in human colonic epithelial cells exposed to deoxycholate (DOC). DNA macroarray screening of a small cluster of stress/apoptosis-related genes in DOC-treated HCT-116 colonocytes revealed clearly higher expression of only GADD45, which was confirmed by gene-specific relative RT-PCR analysis. Subsequently, it was found that DOC also increased GADD34 mRNA expression. However, mRNA expression of GADD153 was increased most markedly in DOC-treated HCT-116 colonocytes, which express wild-type p53. However, the upregulation of GADD34, GADD45, and GADD153 mRNA expression apparently did not require p53, based on the finding that DOC increased expression of all three GADD genes in HCT-15 colonocytes, which express mutant p53. In further studying GADD153 in particular, the effect of DOC on GADD153 mRNA was prevented by actinomycin-D (Act-D), but not by antioxidants or MAPK inhibitors. DOC also caused GADD153 protein to be expressed in close parallel with increased GADD153 mRNA expression. Induction of GADD153 protein by DOC was prevented by either anisomycin or cycloheximide. These findings suggest that DOC-induced upregulation of GADD153 mRNA expression occurred at the level of transcription without involving reactive oxygen species and MAPK signaling, and that the expression of GADD153 protein was due also to translation of pre-existing, and not just newly synthesized, mRNA.

Microarray analysis of Aeromonas hydrophila cytotoxic enterotoxin-treated murine primary macrophages

We performed microarray analyses of murine peritoneal macrophages to examine cellular transcriptional responses to a cytotoxic enterotoxin of Aeromonas hydrophila. While 66% of altered genes were common to both primary macrophages and the murine macrophage cell line RAW 264.7, Act caused expression changes of 28 genes specifically in murine peritoneal macrophages.

GADD34 induces p53 phosphorylation and p21/WAF1 transcription

Recently, others and we have shown that one of the functions of GADD34 is a recovery from a shutoff of protein synthesis induced by endoplasmic reticulum stress. GADD34 has been shown to induce growth arrest and apoptosis. Main protein of apoptosis is p53, especially phosphorylation of p53. And one of the main proteins of growth arrest is p21/WAF1. Here we analyzed the effects of GADD34 on p53 phosphorylation and p21/WAF1 transcription. Transfected Myc-tagged p53 was dose-dependently phosphorylated at Ser15 by increasing the amount of GADD34. Transfection of GADD34 also induced the endogenous phosphorylation of p53 and enhanced p21 protein expression. Transfection of GADD34 induced p21/WAF1 promoter activity. This activity was dependent on p53, because GADD34 transfection to p53-deficient cells produced only a slight increase of p21/WAF1 promoter activity. The p21/WAF1 promoter activity was greatly enhanced by the transfection of p53. Both GADD34 and p53 transfection induced much higher p21/WAF1 promoter activity. The promoter activity of p21/WAF1 was very low in GADD34 deficient MEF. The transfection of GADD34 increased the p21/WAF1 promoter activity in GADD34 deficient MEF.

Up-regulation of a growth arrest and DNA damage protein (GADD34) in the ischaemic human brain: implications for protein synthesis regulation and DNA repair

GADD34 is a growth arrest and DNA damage inducible gene up-regulated in response to DNA damage, cell cycle arrest and apoptosis. It is thought that GADD34 may play a crucial role in cell survival in ischaemia. GADD34 expression was assessed immunohistochemically in post-mortem human hippocampal tissue obtained from patients surviving for defined periods (0-24 h; 24 h-7 days) after a cardiac arrest and in age-matched control subjects. In control brain, cytoplasm staining in GADD34 immunopositive cells was faint but present throughout the hippocampus and cortex. There was minimal change in GADD34 expression in the group surviving 0-24 h after cardiac arrest. However GADD34 immunostaining was markedly increased in selectively vulnerable regions in the 24 h-7 day survival group. Increased GADD34 staining was present in ischaemic neurones and in some morphologically normal neurones after cardiac arrest. Extensive ischaemic damage was found to correlate with elevated GADD34 immunostaining in the CA1 layer of the hippocampus (**P < 0.0016). In addition, GADD34 was found to colocalize with proliferating cell nuclear antigen in some neurones. The up-regulation of GADD34 in response to global ischaemia in the human brain plus its influence on protein synthesis and DNA repair suggests that this protein may have the potential to influence cell survival.

Gene expression profiling reveals novel targets of estramustine phosphate in prostate cancer cells

Estramustine phosphate (EMP) is a compound widely used for the treatment of hormone-refractory prostate cancer. In order to better understand the precise molecular mechanism(s) by which EMP exerts its effects on hormone-resistant PC3 prostate cancer cells, we have utilized microarray to interrogate 22,215 known genes to determine the gene expression profiles altered by EMP treatment. The purpose of this investigation was to identify gene expression profile first and then in future studies determine the specific role of these genes in EMP-induced apoptosis in prostate cancer cells. We found a total of 726 genes which showed >2 fold change after EMP treatment. Clustering analysis showed 12 different types of expression alteration. These genes were also subjected to cluster analysis according to their biological functions. We found that EMP regulated the expression of genes, which are critically involved in the regulation of cell growth, cell cycle, apoptosis, iron homeostasis, cytoskeleton and cell signaling transduction. Real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis was used to confirm the results of microarray, and the results of real-time quantitative RT-PCR were consistent with the microarray data. From these results, we conclude that EMP caused changes in the expression of a large number of genes that are related to the control of cell survival and physiological behaviors. The gene expression profiles may provide comprehensive molecular mechanism(s) by which EMP exerts its pleiotropic effects on prostate cancer cells. EMP-induced regulation of these genes may be further exploited for devising therapeutic strategies for prostate cancer.

A Constitutively Active Arylhydrocarbon Receptor Induces Growth Inhibition of Jurkat T Cells through Changes in the Expression of Genes Related to Apoptosis and Cell Cycle Arrest

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to suppress T cell-dependent immune reactions through the activation of the arylhydrocarbon receptor (AhR). Our previous findings suggest that TCDD inhibits the activation and subsequent expansion of T cells following antigen stimulation in mice, leading to a decreased level of T cell-derived cytokines involved in antibody production. In the present study, we investigated the effects of activated AhR on T cells by transiently expressing a constitutively active AhR (CA-AhR) mutant in AhR-null Jurkat T cells. In agreement with our previous findings, CA-AhR markedly inhibited the growth of Jurkat T cells. The inhibited cell growth was found to be concomitant with both an increase in the annexin V-positive apoptotic cells and the accumulation of cells in the G(1) phase. The growth inhibition was also shown to be mediated by both xenobiotic response element (XRE)-dependent and -independent mechanisms, because an A78D mutant of the CA-AhR, which lacks the ability of XRE-dependent transcription, partially inhibited the growth of Jurkat T cells. Furthermore, we demonstrated that CA-AhR induces expression changes in genes related to apoptosis and cell cycle arrest. These expression changes were shown to be solely mediated in an XRE-dependent manner, because the A78D mutant of the CA-AhR did not induce them. To summarize, these results suggest that AhR activation causes apoptosis and cell cycle arrest, especially through expression changes in genes related to apoptosis and cell cycle arrest by the XRE-dependent mechanism, leading to the inhibition of T cell growth.

The transcriptosomal response of human A549 lung cells to a hydrogen peroxide-generating system: relationship to DNA damage, cell cycle arrest, and caspase activation

Intracellular oxidative stress is a dynamic situation characterized by the accumulation of reactive oxygen metabolites, such as hydrogen peroxide. This is traditionally associated with both macromolecular damage and adaptive changes in gene expression, aimed at preventing cellular demise. However, the overall extent of such genetic changes is not well characterized. Here we present a comprehensive analysis of altered mRNA profiles in human A549 type II lung epithelial cells in response to hydrogen peroxide, at concentrations failing to induce necrotic toxicity. The results of an Affymetrix-based screen of the steady-state levels of mRNAs for several thousand genes revealed a complex pattern of transcriptional and/or posttranscriptional response to oxidative stress, which can be functionally related to both the oxidation and repair of damaged DNA, the induction and permanency of cell cycle arrest, and caspase-3 activation. Many of the genetic events can be related to activation of the p53/p21 pathway, but many other novel inductions and suppressions were detected, revealing the intricacy of the response. The data also disclosed a potential interaction between hydrogen peroxide treatment and increased sensitivity to cell killing by TRAIL, which could be functionally confirmed at the level of induction of caspase-3 activity.

Induction of GADD gene expression by phenethylisothiocyanate in human colon adenocarcinoma cells

Phenethylisothiocyanate (PEITC), a potential cancer chemopreventive agent, induces colon cancer cell death, but the mechanism is not entirely clear. Therefore, the aim of this study was to further clarify the molecular effects of PEITC in causing death of human colon adenocarcinoma cells. When incubated with PEITC, HCT-116 colonocytes showed morphological features characteristic of apoptosis, such as irregular cell shape, translocation of plasma membrane phosphatidylserine, and also chromatin condensation and fragmentation. These changes occurred after single-strand breaks in DNA were detected, suggesting that PEITC induced irreparable DNA damage, which in turn triggered the process of apoptosis. DNA macroarray analysis of a selected small cluster of apoptosis-related genes revealed noticeably higher expression of only GADD45, which was confirmed by gene-specific relative RT-PCR analysis. This led to investigation of other GADD gene members possibly affected by PEITC. Whereas GADD34 mRNA expression increased just slightly, there was an appreciable elevation of the mRNA for GADD153, which is recognized as a pro-apoptotic gene. The effect of PEITC on GADD153 was attenuated by either actinomycin D or N-acetylcysteine, suggesting that PEITC-induced upregulation of GADD153 mRNA expression was partly at the level of transcriptional activation involving reactive oxygen species. Additionally, PEITC-induced upregulation of GADD153 mRNA expression did not appear to require p53, based on the observation that PEITC also increased GADD153 mRNA expression in HCT-15 colonocytes, which are known to express mutant p53. These findings suggest that PEITC creates an oxidative cellular environment that induces DNA damage and GADD153 gene activation, which in turn helps trigger apoptosis.

Gadd34 functional domains involved in growth suppression and apoptosis

Gadd34 (also known as MyD116) was originally described as a growth arrest and DNA damage-inducible gene. Increased expression of Gadd34 was subsequently found to correlate with apoptosis, and forced overexpression of the protein leads to apoptosis. Gadd34 protein modulates protein phosphatase type 1 activity through both direct binding to the protein, as well as through binding to other proteins that also modulate phosphatase activity. In addition, Gadd34 has a region of homology with the herpes simplex virus type 1 ICP34.5 protein that is involved in the prevention of apoptosis in infected cells. Recently it was reported that a novel rat Gadd34-related gene, PEG-3, was upregulated in transformed cells, and that forced expression of this gene led to increased tumorigenic potential of cells implanted into nude mice and increased angiogenesis of these tumors. We have found, however, that PEG-3 does not exist in normal rat cells, which have a single diploid complement of Gadd34. Sequence analysis of the rat Gadd34 gene and comparison with PEG-3 indicates that PEG-3 is most likely a mutant of Gadd34 that perhaps arose as a result of transformation. This finding suggests that truncated Gadd34 may interfere with normal Gadd34 function in transfected cells. However, human Gadd34 lacking the viral homology domain does not interfere with normal Gadd34-induced apoptosis in cultured cells. This suggests that viral similarity sequences may be required for Gadd34-mediated functions other than apoptosis.

Human BAG-1 proteins bind to the cellular stress response protein GADD34 and interfere with GADD34 functions

The cellular stress response protein GADD34 mediates growth arrest and apoptosis in response to DNA damage, negative growth signals, and protein malfolding. GADD34 binds to protein phosphatase PP1 and can attenuate the translational elongation of key transcriptional factors through dephosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha). Recently, we reported the involvement of human SNF5/INI1 (hSNF5/INI1) protein in the functions of GADD34 and showed that hSNF5/INI1 binds GADD34 and stimulates the bound PP1 phosphatase activity. To better understand the regulatory and functional mechanisms of GADD34, we undertook a yeast two-hybrid screen with full-length GADD34 as bait in order to identify additional protein partners of GADD34. We report here that human cochaperone protein BAG-1 interacts with GADD34 in vitro and in SW480 cells treated with the proteasome inhibitor z-LLL-B to induce apoptosis. Two other proteins, Hsp70/Hsc70 and PP1, associate reversibly with the GADD34-BAG-1 complex, and their dissociation is promoted by ATP. BAG-1 negatively modulates GADD34-bound PP1 activity, and the expression of BAG-1 isoforms can also mask GADD34-mediated inhibition of colony formation and suppression of transcription. Our findings suggest that BAG-1 may function to suppress the GADD34-mediated cellular stress response and support a role for BAG-1 in the survival of cells undergoing stress.

Growth arrest and DNA damage-inducible protein GADD34 targets protein phosphatase 1 alpha to the endoplasmic reticulum and promotes dephosphorylation of the alpha subunit of eukaryotic translation initiation factor 2

The growth arrest and DNA damage-inducible protein, GADD34, associates with protein phosphatase 1 (PP1) and promotes in vitro dephosphorylation of the alpha subunit of eukaryotic translation initiation factor 2, (eIF-2 alpha). In this report, we show that the expression of human GADD34 in cultured cells reversed eIF-2 alpha phosphorylation induced by thapsigargin and tunicamycin, agents that promote protein unfolding in the endoplasmic reticulum (ER). GADD34 expression also reversed eIF-2 alpha phosphorylation induced by okadaic acid but not that induced by another phosphatase inhibitor, calyculin A (CA), which is a result consistent with PP1 being a component of the GADD34-assembled eIF-2 alpha phosphatase. Structure-function studies identified a bipartite C-terminal domain in GADD34 that encompassed a canonical PP1-binding motif, KVRF, and a novel RARA sequence, both of which were required for PP1 binding. N-terminal deletions of GADD34 established that while PP1 binding was necessary, it was not sufficient to promote eIF-2 alpha dephosphorylation in cells. Imaging of green fluorescent protein (GFP)-GADD34 proteins showed that the N-terminal 180 residues directed the localization of GADD34 at the ER and that GADD34 targeted the alpha isoform of PP1 to the ER. These data provide new insights into the mode of action of GADD34 in assembling an ER-associated eIF-2 alpha phosphatase that regulates protein translation in mammalian cells.

Molecular sequelae of proteasome inhibition in human multiple myeloma cells

The proteasome inhibitor PS-341 inhibits IkappaB degradation, prevents NF-kappaB activation, and induces apoptosis in several types of cancer cells, including chemoresistant multiple myeloma (MM) cells. PS-341 has marked clinical activity even in the setting of relapsed refractory MM. However, PS-341-induced apoptotic cascade(s) are not yet fully defined. By using gene expression profiling, we characterized the molecular sequelae of PS-341 treatment in MM cells and further focused on molecular pathways responsible for the anticancer actions of this promising agent. The transcriptional profile of PS-341-treated cells involved down-regulation of growth/survival signaling pathways, and up-regulation of molecules implicated in proapoptotic cascades (which are both consistent with the proapoptotic effect of proteasome inhibition), as well as up-regulation of heat-shock proteins and ubiquitin/proteasome pathway members (which can correspond to stress responses against proteasome inhibition). Further studies on these pathways showed that PS-341 decreases the levels of several antiapoptotic proteins and triggers a dual apoptotic pathway of mitochondrial cytochrome c release and caspase-9 activation, as well as activation of Jun kinase and a Fas/caspase-8-dependent apoptotic pathway [which is inhibited by a dominant negative (decoy) Fas construct]. Stimulation with IGF-1, as well as overexpression of Bcl-2 or constitutively active Akt in MM cells also modestly attenuates PS-341-induced cell death, whereas inhibitors of the BH3 domain of Bcl-2 family members or the heat-shock protein 90 enhance tumor cell sensitivity to proteasome inhibition. These data provide both insight into the molecular mechanisms of antitumor activity of PS-341 and the rationale for future clinical trials of PS-341, in combination with conventional and novel therapies, to improve patient outcome in MM.

The human SNF5/INI1 protein facilitates the function of the growth arrest and DNA damage-inducible protein (GADD34) and modulates GADD34-bound protein phosphatase-1 activity

The growth arrest and DNA damage-inducible protein (GADD34) mediates growth arrest and apoptosis in response to DNA damage, negative growth signals, and protein malfolding. GADD34 binds to protein phosphatase-1 (PP1) and can attenuate translational elongation of key transcriptional factors through dephosphorylation of eukaryotic initiation factor-2alpha. We reported previously that the human trithorax leukemia fusion protein (HRX) can bind to GADD34 and abrogate GADD34-mediated apoptosis in response to UV irradiation. We found that hSNF5/INI1, a component of the hSWI/SNF chromatin remodeling complex, also binds to GADD34 and can coexist with GADD34 and HRX fusion proteins as a trimolecular complexes in vivo. In the present report, we demonstrate that hSNF5/INI1 binds to GADD34 in part through the PP1 docking site within a domain homologous to herpes simplex virus-1 ICP34.5. We found that hSNF5/INI1 can bind PP1 independently and weakly stimulate its phosphatase activity in solution and in complex with GADD34. hSNF5/INI1 and PP1 do not compete for binding to GADD34 but rather form a stable heterotrimeric complex with GADD34. We also show that Epstein-Barr nuclear protein 2, which binds hSNF5/INI1, can disrupt hSNF5/INI1 binding to GADD34 and partially reverse the GADD34-mediated growth suppression function in Ha-ras expressing HIH-3T3 (3T3-ras) cells. These results implicate hSNF5/INI1 in the function of GADD34 and suggest that hSNF5/INI1 may regulate PP1 activity in vivo.

Ionizing radiation potentiates the antitumor efficacy of oncolytic herpes simplex virus G207 by upregulating ribonucleotide reductase

BACKGROUND

Replication-competent herpes simplex virus-1 (HSV-1) mutants have an oncolytic effect on human and animal cancers. The aim of this study was to determine whether G207, an HSV-1 mutant, can be combined with ionizing radiation (IR) to increase antitumor activity while decreasing treatment-associated toxicity.

METHODS

This study was performed by using G207, a replication-competent HSV-1 mutant deficient in viral ribonucleotide reductase (RR) and the gamma(1)34.5 neurovirulence protein. The antitumor activity of G207 or IR was tested against HCT-8 human colorectal cancer cells in vitro and in an in vivo mouse subcutaneous tumor model.

RESULTS

We demonstrated that G207 has significant oncolytic effect on HCT-8 cells in vitro in a cytotoxicity assay and in vivo in a mouse flank tumor model and that these effects are improved with low-dose IR. We further illustrated that the increased tumoricidal effect is dependent on the up-regulation of cellular RR by IR measured by a functional bioassay for RR activity. Chemical inhibition of RR by hydroxyurea abrogates the enhanced effect. In contrast to G207, R3616, the parent virus of G207 that expresses functional RR, does not exhibit enhanced oncolysis when combined with IR.

CONCLUSIONS

These data encourage clinical investigation of combination radiation therapy and HSV oncolytic therapy.

mda-7 (IL-24) Mediates selective apoptosis in human melanoma cells by inducing the coordinated overexpression of the GADD family of genes by means of p38 MAPK

Subtraction hybridization identified melanoma differentiation-associated gene-7 (mda-7) as a gene induced during terminal differentiation in human melanoma cells. On the basis of structure, chromosomal localization and cytokine-like properties, mda-7 is classified as IL-24. Administration of mda-7/IL-24 by means of a replication-incompetent adenovirus (Ad.mda-7) induces apoptosis selectively in diverse human cancer cells without inducing harmful effects in normal fibroblast or epithelial cells. The present studies investigated the mechanism underlying this differential apoptotic effect. Infection of melanoma cells, but not normal immortal melanocytes, with Ad.mda-7 induced a time- and dose-dependent increase in expression, mRNA and protein, of a family of growth arrest and DNA damage (GADD)-inducible genes, which correlated with induction of apoptosis. Among the members of the GADD family of genes, GADD153, GADD45 alpha, and GADD34 displayed marked, and GADD45 gamma showed minimal induction. Treatment of melanoma cells with SB203580, a selective inhibitor of the p38 mitogen-activated protein kinase (MAPK) pathway, effectively inhibited Ad.mda-7-induced apoptosis. Additional support for an involvement of the p38 MAPK pathway in Ad.mda-7-mediated apoptosis was documented by using an adenovirus expressing a dominant negative mutant of p38 MAPK. Infection with Ad.mda-7 increased the phosphorylation of p38 MAPK and heat shock protein 27 in melanoma cells but not in normal immortal melanocytes. In addition, SB203580 effectively inhibited Ad.mda-7-mediated induction of the GADD family of genes in a time- and dose-dependent manner, and it effectively blocked Ad.mda-7-mediated down-regulation of the antiapoptotic protein BCL-2. Inhibition of GADD genes by an antisense approach either alone or in combination also effectively blocked Ad.mda-7-induced apoptosis in melanoma cells. These results support the hypothesis that Ad.mda-7 mediates induction of the GADD family of genes by means of the p38 MAPK pathway, thereby resulting in the selective induction of apoptosis in human melanoma cells.

Myeloid differentiation (MyD) primary response genes in hematopoiesis

Myeloid Differentiation (MyD) primary response and Growth Arrest DNA-Damage (Gadd) genes comprise a set of overlapping genes, including known (IRF-1, EGR-1, Jun) and novel (MyD88, Gadd45alpha MyD118/Gadd45beta, GADD45gamma, MyD116/Gadd34) genes, that have been cloned by virtue of there being co-ordinately induced upon the onset of terminal myeloid differentiation. This review delineates the role MyD genes play in blood cell development, where they function as positive regulators of terminal differentiation, lineage specific blood cell development and control of blood cell homeostasis, including growth inhibition and apoptosis.

Myeloid differentiation (MyD)/growth arrest DNA damage (GADD) genes in tumor suppression, immunity and inflammation

Myeloid differentiation (MyD) primary response and growth arrest DNA damage (Gadd) genes comprise a set of overlapping genes, including known (IRF-1, EGR-1, Jun) and novel (MyD88, Gadd45alpha, MyD118/Gadd45beta, GADD45gamma, MyD116/ Gadd34) genes, that have been cloned by virtue of being co-ordinately induced upon the onset of terminal myeloid differentiation and following exposure of cells to stress stimuli. In recent years it has become evident that MyD/Gadd play a role in blood cell development, where they function as positive regulators of terminal differentiation, lineage-specific blood cell development and control of blood cell homeostasis, including growth inhibition and apoptosis. MyD/Gadd are also involved in inflammatory responses to invading micro-organisms, and response to environmental stress and physiological stress, such as hypoxia, which results in ischemic tissue damage. An intricate network of interactions among MyD/GADD genes and gene products appears to control their diverse functions. Deregulated growth, increased cell survival, compromised differentiation and deficiencies in DNA repair are hallmarks of malignancy and its progression. Thus, the role MyD/Gadd play in negative growth control, including cell cycle arrest and apoptosis, and in DNA repair, make them attractive molecular targets for tumor suppression. The role MyD/Gadd play in innate immunity and host response to hypoxia also make these genes and gene products attractive molecular targets to treat immunity and inflammation disorders, such as septic shock and ischemic tissue damage.

Regulator-driven functional diversification of protein phosphatase-1 in eukaryotic evolution

We have used the (nearly) completed eukaryotic genome sequences to trace the evolution of thirteen families of established vertebrate regulators of type-1 protein phosphatases (PP1). Two of these families are present in all lineages of the eukaryotic crown and therefore qualify as candidate primordial regulators that determined the surface of PP1. The set of regulators of PP1 has continued to expand ever since, often in response to functional innovations in different eukaryotic lineages. In particular, the development of metazoan multicellularity was accompanied by an explosive increase in the number of regulators of PP1. The further increase in the functional diversity of PP1 in the vertebrate lineage was mainly achieved by the duplication of genes for regulatory subunits and by the conversion of already existing proteins into regulators of PP1. Unexpectedly, our analysis has also enabled us to classify nine poorly characterized proteins as likely regulators of PP1.

Involvement of multiple kinase pathways in stimulation of gene transcription by hypertonicity

Osmolality of the mammalian renal medulla is high because of the operation of the urinary concentrating mechanism. To understand molecular events during the early phase of cellular adaptation to hypertonicity, we performed comprehensive searches for genes induced in response to hypertonicity using a cell line (mIMCD3) derived from the inner medullary collecting duct of mouse kidney. PCR-based subtractive hybridization of cDNA pools and cDNA microarray analysis were used. We report 12 genes whose mRNA expression is significantly increased within 4 h after exposure to hypertonicity. The increase in mRNA expression was the result of increased transcription. Many are either stress response genes or growth regulatory genes, supporting the notion that hypertonicity evokes the stress response and growth regulation in cells. Experiments using inhibitors revealed that mitogen-activated protein kinases were commonly involved in signaling for the induction of genes by hypertonicity. Tyrosine kinases and phosphatidylinositol 3-kinase also play a significant role. Signaling pathways for stimulation of transcription appeared quite diverse in that each gene was sensitive to different combinations of inhibitors.

Interaction between growth arrest-DNA damage protein 34 and Src kinase Lyn negatively regulates genotoxic apoptosis

Genotoxic stresses activate intracellular signaling molecules, which lead to growth arrest, DNA repair, and/or apoptosis. Among these molecules are the growth arrest and DNA damage protein 34 (GADD34) and the Src-related protein tyrosine kinase Lyn. Here, we report that these two proteins physically and functionally interact to regulate DNA damage-induced apoptosis. Multiple isolates of GADD34 and the related murine protein MyD116 were identified as binding partners of Lyn in a yeast two-hybrid screen. The specific interaction was confirmed by in vitro association of GADD34 with glutathione S-transferase fusion proteins containing the Src Homology 3 (SH3) domain of Lyn, as well as coimmunoprecipitation of GADD34 and Lyn from mammalian cells. GADD34 was tyrosine-phosphorylated in vivo in a Lyn-dependent manner. Lyn efficiently phosphorylated affinity-purified GADD34 in vitro. Lyn negatively regulated the proapoptotic function of GADD34 in a kinase-dependent manner. Expression of wild-type, but not kinase-inactive, Lyn weakened promotion of apoptosis by GADD34 following treatment with methyl-methanesulfonate or ionizing radiation in HEK293 and HeLa cells. In contrast, pretreatment of cells with the Src-specific tyrosine kinase inhibitor PP1 strengthened promotion of apoptosis by GADD34. We propose that Lyn regulates the proapoptotic function of GADD34 by binding and phosphorylating it.