The central region of Gadd45 is required for its interaction with p21/WAF1

GADD45A: With or without you

The growth arrest and DNA damage inducible (GADD)45 family includes three small and ubiquitously distributed proteins (GADD45A, GADD45B, and GADD45G) that regulate numerous cellular processes associated with stress signaling and injury response. Here, we provide a comprehensive review of the current literature investigating GADD45A, the first discovered member of the family. We first depict how its levels are regulated by a myriad of genotoxic and non-genotoxic stressors, and through the combined action of intricate transcriptional, posttranscriptional, and even, posttranslational mechanisms. GADD45A is a recognized tumor suppressor and, for this reason, we next summarize its role in cancer, as well as the different mechanisms by which it regulates cell cycle, DNA repair, and apoptosis. Beyond these most well-known actions, GADD45A may also influence catabolic and anabolic pathways in the liver, adipose tissue and skeletal muscle, among others. Not surprisingly, GADD45A may trigger AMP-activated protein kinase activity, a master regulator of metabolism, and is known to act as a transcriptional coregulator of numerous nuclear receptors. GADD45A has also been reported to display a cytoprotective role by regulating inflammation, fibrosis and oxidative stress in several organs and tissues, and is regarded an important contributor for the development of heart failure. Overall data point to that GADD45A may play an important role in metabolic, neurodegenerative and cardiovascular diseases, and also autoimmune-related disorders. Thus, the potential mechanisms by which dysregulation of GADD45A activity may contribute to the progression of these diseases are also reviewed below.

Inhibition of p21 activates Akt kinase to trigger ROS-induced autophagy and impacts on tumor growth rate

Owing to its ability to induce cellular senescence, inhibit PCNA, and arrest cell division cycle by negatively regulating CDKs as well as being a primary target of p53, p21 is traditionally considered a tumor suppressor. Nonetheless, several reports in recent years demonstrated its pro-oncogenic activities such as apoptosis inhibition by cytosolic p21, stimulation of cell motility, and promoting assembly of cyclin D-CDK4/6 complex. These opposing effects of p21 on cell proliferation, supported by the observations of its inconsistent expression in human cancers, led to the emergence of the concept of "antagonistic duality" of p21 in cancer progression. Here we demonstrate that p21 negatively regulates basal autophagy at physiological concentration. Akt activation, upon p21 attenuation, driven ROS accumulation appears to be the major underlying mechanism in p21-mediated modulation of autophagy. We also find p21, as a physiological inhibitor of autophagy, to have oncogenic activity during early events of tumor development while its inhibition favors survival and growth of cancer cells in the established tumor. Our data, thereby, reveal the potential role of autophagy in antagonistic functional duality of p21 in cancer.

Regulation of p53 Activity by (+)-Epiloliolide Isolated from Ulva lactuca

Ulva lactuca (U. lactuca) is a green alga distributed worldwide and used as a food and cosmetic material. In our previous study, we determined the effects of U. lactuca methanol extracts on the UVB-induced DNA repair. In the present study, we fractionated U. lactuca methanol extracts to identify the effective compound for the DNA repair. MTT assay demonstrated that (+)-epiloliolide showed no cytotoxicity up to 100 μM in BJ-5ta human dermal fibroblast. Upon no treatment, exposure to UVB 400 J/m² decreased cell viability by 45%, whereas (+)-epiloliolide treatment for 24 h after UVB exposure significantly increased the cell viability. In GO and GESA analysis, a number of differentially expressed genes were uniquely expressed in (+)-epiloliolide treated cells, which were enriched in the p53 signaling pathway and excision repair. Immunofluorescence demonstrated that (+)-epiloliolide increased the nuclear localization of p53. Comet assay demonstrated that (+)-epiloliolide decreased tail moment increased by UVB. Western blot analysis demonstrated that (+)-epiloliolide decreased the levels of p-p53, p21, Bax, and Bim, but increased that of Bcl-2. Reverse transcription PCR (RT-PCR) demonstrated that (+)-epiloliolide decreased the levels of MMP 1, 9, and 13, but increased that of COL1A1. These results suggest that (+)-epiloliolide regulates p53 activity and has protective effects against UVB.

BET bromodomain-containing epigenetic reader proteins regulate vascular smooth muscle cell proliferation and neointima formation

AIMS

Recent studies revealed that the bromodomain and extra-terminal (BET) epigenetic reader proteins resemble key regulators in the underlying pathophysiology of cancer, diabetes, or cardiovascular disease. However, whether they also regulate vascular remodelling processes by direct effects on vascular cells is unknown. In this study, we investigated the effects of the BET proteins on human smooth muscle cell (SMC) function in vitro and neointima formation in response to vascular injury in vivo.

METHODS AND RESULTS

Selective inhibition of BETs by the small molecule (+)-JQ1 dose-dependently reduced proliferation and migration of SMCs without apoptotic or toxic effects. Flow cytometric analysis revealed a cell cycle arrest in the G0/G1 phase in the presence of (+)-JQ1. Microarray- and pathway analyses revealed a substantial transcriptional regulation of gene sets controlled by the Forkhead box O (FOXO1)1-transcription factor. Silencing of the most significantly regulated FOXO1-dependent gene, CDKN1A, abolished the antiproliferative effects. Immunohistochemical colocalization, co-immunoprecipitation, and promoter-binding ELISA assay data confirmed that the BET protein BRD4 directly binds to FOXO1 and regulates FOXO1 transactivational capacity. In vivo, local application of (+)-JQ1 significantly attenuated SMC proliferation and neointimal lesion formation following wire-induced injury of the femoral artery in C57BL/6 mice.

CONCLUSION

Inhibition of the BET-containing protein BRD4 after vascular injury by (+)-JQ1 restores FOXO1 transactivational activity, subsequent CDKN1A expression, cell cycle arrest and thus prevents SMC proliferation in vitro and neointima formation in vivo. Inhibition of BET epigenetic reader proteins might thus represent a promising therapeutic strategy to prevent adverse vascular remodelling.

CDK Blockade Using AT7519 Suppresses Acute Myeloid Leukemia Cell Survival through the Inhibition of Autophagy and Intensifies the Anti-leukemic Effect of Arsenic Trioxide

The strong storyline behind the critical role of cyclin-dependent kinase (CDK) inhibitor proteins in natural defense against malignant transformation not only represents a heroic perspective for these proteins, but also provides a bright future for the application of small molecule inhibitors of CDKs in the novel cancer treatment strategies. The results of the present study revealed that the inhibition of CDKs using pan-CDK inhibitor AT7519, as revealed by the induction of G1 cell cycle arrest as well as the reduction of cyclins expression, resulted in decreased survival in acute myeloid leukemia (AML)-derived KG-1 cells, either in the context of single agent or in combination with arsenic trioxide (ATO). Apart from alterations in the expression of proliferation and apoptotic genes, the anti-survival property of AT7519 was coupled with the inhibition of autophagy-related genes. Notably, we found that the blockage of autophagy system in KG-1 cells resulted in a superior cytotoxic effect, introducing autophagy as a probable suppressor of cell death. As far as we are aware, to date, no study has reported the contributory mechanisms correlated with the less sensitivity of acute leukemia cells to AT7519 and our study suggested for the first time that the activation of both PI3K and c-Myc signaling pathways could overshadow, at least partly, the efficacy of this agent in KG-1 cells. Overall, due to the pharmacologic safety of AT7519, our study proposed this inhibitor as a promising agent for the treatment of AML either as a single agent or in a combined-modal strategy.

Regulatory Roles of GADD45α in Skeletal Muscle and Adipocyte

GADD45α, a member of the GADD45 family proteins, is involved in various cellular processes including the maintenance of genomic integrity, growth arrest, apoptosis, senescence, and signal transduction. In skeletal muscle, GADD45α plays an important role in regulating mitochondrial biogenesis and muscle atrophy. In adipocytes, GADD45α regulates preadipocyte differentiation, lipid accumulation, and thermogenesis metabolism. Moreover, it has been recently demonstrated that GADD45α promotes gene activation by inducing DNA demethylation. The epigenetic function of GADD45α is important for preadipocyte differentiation and transcriptional regulation during development. This article mainly reviews and discusses the regulatory roles of GADD45α in skeletal muscle development, adipocyte progenitor differentiation, and DNA demethylation.

GADD45A and CDKN1A are involved in apoptosis and cell cycle modulatory effects of viscumTT with further inactivation of the STAT3 pathway

ViscumTT, a whole mistletoe preparation, has shown synergistic induction of apoptosis in several pediatric tumor entities. High therapeutic potential has previously been observed in Ewing's sarcoma, rhabdomyosarcoma, ALL and AML. In this study, we analyzed modulatory effects on the cell cycle by viscumTT in three osteosarcoma cell lines with various TP53 statuses. ViscumTT treatment induced G1 arrest in TP53 wild-type and null-mutant cells, but S arrest in TP53 mutant cells. Blockage of G1/S transition was accompanied by down-regulation of the key regulators CDK4, CCND1, CDK2, CCNE, CCNA. However, investigations on the transcriptional level revealed secondary TP53 participation. Cell cycle arrest was predominantly mediated by transcriptionally increased expression of GADD45A and CDKN1A and decreased SKP2 levels. Enhanced CDKN1A and GADD45A expression further played a role in viscumTT-induced apoptosis with involvement of stress-induced MAPK8 and inactivation of MAPK1/3. Furthermore, viscumTT inhibited the pro-survival pathway STAT3 by dephosphorylation of the two sites, Tyr705 and Ser727, by down-regulation of total STAT3 and its direct downstream targets BIRC5 and C-MYC. Moreover, tests of the efficacy of viscumTT in vivo showing reduction of tumor volume confirmed the high therapeutic potential as an anti-tumoral agent for osteosarcoma.

The β-catenin/CBP-antagonist ICG-001 inhibits pediatric glioma tumorigenicity in a Wnt-independent manner

Pediatric high-grade gliomas (pedHGG) belong to the most aggressive cancers in children with a poor prognosis due to a lack of efficient therapeutic strategies. The β-catenin/Wnt-signaling pathway was shown to hold promising potential as a treatment target in adult high-grade gliomas by abrogating tumor cell invasion and the acquisition of stem cell-like characteristics. Since pedHGG differ from their adult counterparts in genetically and biologically we aimed to investigate the effects of β-catenin/Wnt-signaling pathway-inhibition by the β-catenin/CBP antagonist ICG-001 in pedHGG cell lines. In contrast to adult HGG, pedHGG cells displayed minimal detectable canonical Wnt-signaling activity. Nevertheless, low doses of ICG-001 inhibited cell migration/invasion, tumorsphere- and colony formation, proliferation in vitro as well as tumor growth in vivo/ovo, suggesting that ICG-001 affects pedHGG tumor cell characteristics independent of β-catenin/Wnt-signaling. RNA-sequencing analyses support a Wnt/β-catenin-independent effect of ICG-001 on target gene transcription, revealing strong effects on genes involved in cellular metabolic/biosynthetic processes and cell cycle progression. Among these, high mRNA expression of cell cycle regulator JDP2 was found to confer a better prognosis for pedHGG patients. In conclusion, ICG-001 might offer an effective treatment option for pedHGG patients functioning to regulate cell phenotype and gene expression programs in absence of Wnt/β-catenin signaling-activity.

Correlation between the radiation responses of fibroblasts cultured from individual patients and the risk of late reaction after breast radiotherapy

Late normal tissue toxicity varies widely between patients and limits breast radiotherapy dose. Here we aimed to determine its relationship to DNA damage responses of fibroblast cultures from individual patients. Thirty-five breast cancer patients, with minimal or marked breast changes after breast-conserving therapy consented to receive a 4 Gy test irradiation to a small skin field of the left buttock and have punch biopsies taken from irradiated and unirradiated skin. Early-passage fibroblast cultures were established by outgrowth and irradiated in vitro with 0 or 4 Gy. 53BP1 foci, p53 and p21/CDKN1A were detected by immunofluorescence microscopy. Residual 53BP1 foci counts 24 h after in vitro irradiation were significantly higher in fibroblasts from RT-sensitive versus RT-resistant patients. Furthermore, significantly larger fractions of p53- but not p21/CDKN1A-positive fibroblasts were found in cultures from RT-sensitive patients without in vitro irradiation, and 2 h and 6 d post-irradiation. Exploratory analysis showed a stronger p53 response 2 h after irradiation of fibroblasts established from patients with severe reaction. These results associate the radiation response of fibroblasts with late reaction of the breast after RT and suggest a correlation with severity.

Vitamin C enhances epigenetic modifications induced by 5-azacytidine and cell cycle arrest in the hepatocellular carcinoma cell lines HLE and Huh7

Background

5-Azacytidine (5-AZA), a DNA methyl transferase inhibitor, is a clinically used epigenetic drug for cancer therapy. Recently, we have shown that 5-AZA upregulates ten-eleven translocation (TET) protein expression in hepatocellular carcinoma (HCC) cells, which induce active demethylation. Vitamin C facilitates TET activity and enhances active demethylation. The aim of this study is to investigate whether vitamin C is able to enhance the effect of 5-AZA on active demethylation and to evaluate its consequence in HCC cell lines.

Methods

HCC cell lines (Huh7 and HLE) were treated with 5-AZA and vitamin C. After 48 h of treatment, viability (resazurin conversion), toxicity (lactose dehydrogenase (LDH) release), and proliferation ((proliferating cell nuclear antigen (PCNA)) of single- and combined-treated cells were assessed. The effect of the treatment on 5-hydroxymethylcytosine (5hmC) intensity (immunofluorescence (IF) staining), TET, Snail, GADD45B, and P21 mRNA (real-time PCR) and protein expression (Western blot) were investigated.

Results

Our results indicated that vitamin C enhances the anti-proliferative and apoptotic effect of 5-AZA in HCC cell lines. By further analyzing the events leading to cell cycle arrest, we have shown for the first time in HCC that the combination of 5-AZA and vitamin C leads to an enhanced downregulation of Snail expression, a key transcription factor governing epithelial-mesenchymal transition (EMT) process, and cell cycle arrest.

Conclusions

We conclude that when combined with 5-AZA, vitamin C enhances TET activity in HCC cells, leading to induction of active demethylation. An increase in P21 expression as a consequence of downregulation of Snail accompanied by the induction of GADD45B expression is the main mechanism leading to cell cycle arrest in HCCs.

Inhibitory effect of isoamericanol A from Jatropha curcas seeds on the growth of MCF-7 human breast cancer cell line by G2/M cell cycle arrest

Although various parts of J. curcas (Jatropha curcas L., Euphorbiaceae) have long been used as traditional folk medicines for their antiviral, analgesic, and/or antidotal efficacies, we are the first to investigate the role of anti-carcinogenicity of isoamericanol A (IAA) from the seed extract. Our results showed that IAA is capable of inhibiting cell proliferation in a dose-dependent manner on the human cancer cell lines of MCF-7, MDA-MB231, HuH-7, and HeLa. Flow cytometry analysis showed IAA significantly induces cell cycle arrest at G2/M on MCF-7 cells. At both protein and mRNA levels examined by western blot and real-time PCR, the results revealed increased expression of BTG2 (B-cell translocation gene 2), p21 (p21^WAF1/CIPI), and GADD45A (growth arrest and DNA-damage-inducible, alpha) after IAA treatment, but inversed expression in CDK1 (cyclin-dependent kinase 1) and cyclins B1 and B2. All these effects contribute to G2/M cell cycle arrest. Furthermore, these results coincide with the changes in molecular expressions determined by DNA-microarray analysis. Our findings indicate that IAA has an inhibitory effect on cell proliferation of MCF-7 through cell cycle arrest, giving it great potential as a future therapeutic reagent for cancers.

The Dual Nature of Nek9 in Adenovirus Replication

To successfully replicate in an infected host cell, a virus must overcome sophisticated host defense mechanisms. Viruses, therefore, have evolved a multitude of devices designed to circumvent cellular defenses that would lead to abortive infection. Previous studies have identified Nek9, a cellular kinase, as a binding partner of adenovirus E1A, but the biology behind this association remains a mystery. Here we show that Nek9 is a transcriptional repressor that functions together with E1A to silence the expression of p53-inducible GADD45A gene in the infected cell. Depletion of Nek9 in infected cells reduces virus growth but unexpectedly enhances viral gene expression from the E2 transcription unit, whereas the opposite occurs when Nek9 is overexpressed. Nek9 localizes with viral replication centers, and its depletion reduces viral genome replication, while overexpression enhances viral genome numbers in infected cells. Additionally, Nek9 was found to colocalize with the viral E4 orf3 protein, a repressor of cellular stress response. Significantly, Nek9 was also shown to associate with viral and cellular promoters and appears to function as a transcriptional repressor, representing the first instance of Nek9 playing a role in gene regulation. Overall, these results highlight the complexity of virus-host interactions and identify a new role for the cellular protein Nek9 during infection, suggesting a role for Nek9 in regulating p53 target gene expression.

IMPORTANCE

In the arms race that exists between a pathogen and its host, each has continually evolved mechanisms to either promote or prevent infection. In order to successfully replicate and spread, a virus must overcome every mechanism that a cell can assemble to block infection. On the other hand, to counter viral spread, cells must have multiple mechanisms to stifle viral replication. In the present study, we add to our understanding of how the human adenovirus is able to circumvent cellular roadblocks to replication. We show that the virus uses a cellular protein, Nek9, in order to block activation of p53-regulated gene GADD45A, which is an important player in stress response and p53-mediated cell cycle arrest. Importantly, our study also identifies Nek9 as a transcriptional repressor.

Linking susceptibility genes and pathogenesis mechanisms using mouse models of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) represents a challenging autoimmune disease from a clinical perspective because of its varied forms of presentation. Although broad-spectrum steroids remain the standard treatment for SLE, they have many side effects and only provide temporary relief from the symptoms of the disease. Thus, gaining a deeper understanding of the genetic traits and biological pathways that confer susceptibility to SLE will help in the design of more targeted and effective therapeutics. Both human genome-wide association studies (GWAS) and investigations using a variety of mouse models of SLE have been valuable for the identification of the genes and pathways involved in pathogenesis. In this Review, we link human susceptibility genes for SLE with biological pathways characterized in mouse models of lupus, and discuss how the mechanistic insights gained could advance drug discovery for the disease.

Insights into the regulation of neuronal viability by nucleophosmin/B23

The vastness of the neuronal network that constitutes the human brain proves challenging when trying to understand its complexity. Furthermore, due to the senescent state they enter into upon maturation, neurons lack the ability to regenerate in the face of insult, injury or death. Consequently, their excessive death can be detrimental to the proper functioning of the brain. Therefore, elucidating the mechanisms regulating neuronal survival is, while challenging, of great importance as the incidence of neurological disease is becoming more prevalent in today's society. Nucleophosmin/B23 (NPM) is an abundant and ubiquitously expressed protein that regulates vital cellular processes such as ribosome biogenesis, cell proliferation and genomic stability. As a result, it is necessary for proper embryonic development, but has also been implicated in many cancers. While highly studied in the context of proliferative cells, there is a lack of understanding NPM's role in post-mitotic neurons. By exploring its role in healthy neurons as well as its function in the regulation of cell death and neurodegeneration, there can be a better understanding of how these diseases initiate and progress. Owing to what is thus far known about its function in the cell, NPM could be an attractive therapeutic target in the treatment of neurodegenerative diseases.

NF-kB as a key player in regulation of cellular radiation responses and identification of radiation countermeasures

Nuclear factor (NF)-κB is a transcription factor that plays significant role in immunity, cellular survival and inhibition of apoptosis, through the induction of genetic networks. Depending on the stimulus and the cell type, the members of NF-κB related family (RelA, c-Rel, RelB, p50, and p52), forms different combinations of homo and hetero-dimers. The activated complexes (Es) translocate into the nucleus and bind to the 10bp κB site of promoter region of target genes in stimulus specific manner. In response to radiation, NF-κB is known to reduce cell death by promoting the expression of anti-apoptotic proteins and activation of cellular antioxidant defense system. Constitutive activation of NF-κB associated genes in tumour cells are known to enhance radiation resistance, whereas deletion in mice results in hypersensitivity to IR-induced GI damage. NF-κB is also known to regulate the production of a wide variety of cytokines and chemokines, which contribute in enhancing cell proliferation and tissue regeneration in various organs, such as the GI crypts stem cells, bone marrow etc., following exposure to IR. Several other cytokines are also known to exert potent pro-inflammatory effects that may contribute to the increase of tissue damage following exposure to ionizing radiation. Till date there are a series of molecules or group of compounds that have been evaluated for their radio-protective potential, and very few have reached clinical trials. The failure or less success of identified agents in humans could be due to their reduced radiation protection efficacy. In this review we have considered activation of NF-κB as a potential marker in screening of radiation countermeasure agents (RCAs) and cellular radiation responses. Moreover, we have also focused on associated mechanisms of activation of NF-κB signaling and their specified family member activation with respect to stimuli. Furthermore, we have categorized their regulated gene expressions and their function in radiation response or modulation. In addition, we have discussed some recently developed radiation countermeasures in relation to NF-κB activation

GADD45 proteins: roles in cellular senescence and tumor development

The growth arrest and DNA damage 45 (GADD45) family genes regulate DNA repair, cell cycle, cell survival, apoptosis, senescence, and DNA demethylation in the cells under various stress stimuli, such as oxidative stress, UV radiation, and oncogenic stress. Recent studies have provided important insights regarding how different oncogenic stresses activate GADD45 signaling pathway and lead to disparate influences on tumor initiation. In this review, we discuss the deregulation and cellular function of GADD45 proteins in the context of cancer development. We also highlight recent advances in exploring the tumor suppressive function of GADD45 proteins-triggered cellular senescence.

Polyplex exposure inhibits cell cycle, increases inflammatory response, and can cause protein expression without cell division

We sought to evaluate the relationship between cell division and protein expression when using commercial poly(ethylenimine) (PEI)-based polyplexes. The membrane dye PKH26 was used to assess cell division, and cyan fluorescent protein (CFP) was used to monitor protein expression. When analyzed at the whole population level, a greater number of cells divided than expressed protein, regardless of the level of protein expression observed, giving apparent consistency with the hypothesis that protein expression requires cells to pass through mitosis in order for the transgene to overcome the nuclear membrane. However, when the polyplex-exposed population was evaluated for the amount of division in the protein-expressing subpopulation, it was observed that substantial amounts of expression had occurred in the absence of division. Indeed, in HeLa S3 cells, this represented the majority of expressing cells. Of interest, the doubling time for both cell lines was slowed by ~2-fold upon exposure to polyplexes. This change was not altered by the origin of the plasmid DNA (pDNA) transgene promoter (cytomegalovirus (CMV) or elongation factor-1 alpha (EF1α)). Gene expression arrays in polyplex-exposed HeLa S3 cells showed upregulation of cell cycle arrest genes and downregulation of genes related to mitosis. Chemokine, interleukin, and toll-like receptor genes were also upregulated, suggesting activation of proinflammatory pathways. In summary, we find evidence that a cell division-independent expression pathway exists, and that polyplex exposure slows cell division and increases inflammatory response.

GADD45 proteins: central players in tumorigenesis

The Growth Arrest and DNA Damage-inducible 45 (GADD45) proteins have been implicated in regulation of many cellular functions including DNA repair, cell cycle control, senescence and genotoxic stress. However, the pro-apoptotic activities have also positioned GADD45 as an essential player in oncogenesis. Emerging functional evidence implies that GADD45 proteins serve as tumor suppressors in response to diverse stimuli, connecting multiple cell signaling modules. Defects in the GADD45 pathway can be related to the initiation and progression of malignancies. Moreover, induction of GADD45 expression is an essential step for mediating anti-cancer activity of multiple chemotherapeutic drugs and the absence of GADD45 might abrogate their effects in cancer cells. In this review, we present a comprehensive discussion of the functions of GADD45 proteins, linking their regulation to effectors of cell cycle arrest, DNA repair and apoptosis. The ramifications regarding their roles as essential and central players in tumor growth suppression are also examined. We also extensively review recent literature to clarify how different chemotherapeutic drugs induce GADD45 gene expression and how its up-regulation and interaction with different molecular partners may benefit cancer chemotherapy and facilitate novel drug discovery.

Caffeic acid phenethyl ester causes p21 induction, Akt signaling reduction, and growth inhibition in PC-3 human prostate cancer cells

Caffeic acid phenethyl ester (CAPE) treatment suppressed proliferation, colony formation, and cell cycle progression in PC-3 human prostate cancer cells. CAPE decreased protein expression of cyclin D1, cyclin E, SKP2, c-Myc, Akt1, Akt2, Akt3, total Akt, mTOR, Bcl-2, Rb, as well as phosphorylation of Rb, ERK1/2, Akt, mTOR, GSK3α, GSK3β, PDK1; but increased protein expression of KLF6 and p21(Cip1). Microarray analysis indicated that pathways involved in cellular movement, cell death, proliferation, and cell cycle were affected by CAPE. Co-treatment of CAPE with chemotherapeutic drugs vinblastine, paclitaxol, and estramustine indicated synergistic suppression effect. CAPE administration may serve as a potential adjuvant therapy for prostate cancer.

Gadd45 proteins: relevance to aging, longevity and age-related pathologies

The Gadd45 proteins have been intensively studied, in view of their important role in key cellular processes. Indeed, the Gadd45 proteins stand at the crossroad of the cell fates by controlling the balance between cell (DNA) repair, eliminating (apoptosis) or preventing the expansion of potentially dangerous cells (cell cycle arrest, cellular senescence), and maintaining the stem cell pool. However, the biogerontological aspects have not thus far received sufficient attention. Here we analyzed the pathways and modes of action by which Gadd45 members are involved in aging, longevity and age-related diseases. Because of their pleiotropic action, a decreased inducibility of Gadd45 members may have far-reaching consequences including genome instability, accumulation of DNA damage, and disorders in cellular homeostasis - all of which may eventually contribute to the aging process and age-related disorders (promotion of tumorigenesis, immune disorders, insulin resistance and reduced responsiveness to stress). Most recently, the dGadd45 gene has been identified as a longevity regulator in Drosophila. Although further wide-scale research is warranted, it is becoming increasingly clear that Gadd45s are highly relevant to aging, age-related diseases (ARDs) and to the control of life span, suggesting them as potential therapeutic targets in ARDs and pro-longevity interventions.

Crystal structure of human Gadd45γ [corrected] reveals an active dimer

The human Gadd45 protein family plays critical roles in DNA repair, negative growth control, genomic stability, cell cycle checkpoints and apoptosis. Here we report the crystal structure of human Gadd45γ [corrected], revealing a unique dimer formed via a bundle of four parallel helices, involving the most conserved residues among the Gadd45 isoforms. Mutational analysis of human Gadd45γ [corrected] identified a conserved, highly acidic patch in the central region of the dimer for interaction with the proliferating cell nuclear antigen (PCNA), p21 and cdc2, suggesting that the parallel dimer is the active form for the interaction. Cellular assays indicate that: (1) dimerization of Gadd45γ [corrected] is necessary for apoptosis as well as growth inhibition, and that cell growth inhibition is caused by both cell cycle arrest and apoptosis; (2) a conserved and highly acidic patch on the dimer surface, including the important residues Glu87 and Asp89, is a putative interface for binding proteins related to the cell cycle, DNA repair and apoptosis. These results reveal the mechanism of self-association by Gadd45 proteins and the importance of this self-association for their biological function.

Enhanced Gadd45 expression and delayed G2/M progression are p53-dependent in zinc-supplemented human bronchial epithelial cells

Zinc is an essential nutrient for humans; however, this study demonstrated for the first time that an elevated zinc status, created by culturing cells at optimal plasma zinc concentration attainable by oral zinc supplementation, is cytotoxic for normal human bronchial epithelial (NHBE) cells. p53 plays a central role in the modulation of cell signal transduction in response to the stress from DNA damage, hypoxia and oncogene activation. The present study was designed to determine whether the previously reported increased Gadd45 expression and delayed G2/M cell cycle progression in zinc-supplemented NHBE cells is p53-dependent, and to decipher the mechanisms responsible for the regulation of Gadd45 expressions by p53, and elucidate the Gadd45 functions in impaired cell growth and cell cycle progression in NHBE cells. Cells were cultured for one passage in different concentrations of zinc: <0.4 micromol/L (ZD) as severe zinc-deficient; 4 micromol/L (ZN) as normal zinc level in culture medium; 16 micromol/L (ZA) as normal human plasma zinc level; and 32 micromol/L (ZS) as the high end of plasma zinc attainable by oral supplementation. Inhibition of cell growth and upregulation of p53 mRNA and protein expression were observed in ZS cells. Most importantly, ZS treatment also enhanced Gadd45 nuclear protein level and promoter activity, decreased CDK1-Cyclin B1 level and delayed G2/M cell cycle progression. These changes were normalized to those observed in ZN by treating ZS cells with Pifitherin, an inhibitor of p53 transactivation activity. Thus, our findings support the p53 dependency of the Gadd45-CDK1/Cyclin B1-G2/M cell cycle progression pathway in ZS NHBE cells.

Gadd45 proteins as critical signal transducers linking NF-kappaB to MAPK cascades

The growth arrest and DNA damage-inducible 45 (Gadd45) proteins are a group of critical signal transducers that are involved in regulations of many cellular functions. Accumulated data indicate that all three Gadd45 proteins (i.e., Gadd45alpha, Gadd45beta, and Gadd45gamma) play essential roles in connecting an upstream sensor module, the transcription Nuclear Factor-kappaB (NF-kappaB), to a transcriptional regulating module, mitogen-activated protein kinase (MAPK). This NF-kappaB-Gadd45(s)-MAPK pathway responds to various kinds of extracellular stimuli and regulates such cell activities as growth arrest, differentiation, cell survival, and apoptosis. Defects in this pathway can also be related to oncogenesis. In the first part of this review, the functions of Gadd45 proteins, and briefly NF-kappaB and MAPK, are summarized. In the second part, the mechanisms by which Gadd45 proteins are regulated by NF-kappaB, and how they affect MAPK activation, are reviewed.

Twilight effects of low doses of ionizing radiation on cellular systems: a bird's eye view on current concepts and research

The debate about the health risks from low doses of radiation is vigorous and often acrimonious since many years and does not appear to weaken. Being far from completeness, this review presents only a bird's eye view on current concepts and research in the field. It is organized and divided in two parts. The first is dedicated to molecular responses determined by radiation-induced DNA ruptures. It focuses its attention on molecular pathways that are activated by ATM and tries to describe the variegated functions and specific roles of Chk2 and p53 and other proteins in sensing, promoting and executing DNA repair. The second part is more concerned with the risk associated with exposure to low dose radiation and possible effects that the radiation-affected cell may undergo. These effects include induction of apoptosis and mitotic catastrophe, bystander effect and genomic instability, senescence and hormetic response. Current hypotheses and research on these issues are briefly discussed.

The crystal structure and dimerization interface of GADD45gamma

Gadd45 proteins are recognized as tumor and autoimmune suppressors whose expression can be induced by genotoxic stresses. These proteins are involved in cell cycle control, growth arrest, and apoptosis through interactions with a wide variety of binding partners. We report here the crystal structure of Gadd45gamma, which reveals a fold comprising an alphabetaalpha sandwich with a central five-stranded mixed beta-sheet with alpha-helices packed on either side. Based on crystallographic symmetry we identified the dimer interface of Gadd45gamma dimers by generating point mutants that compromised dimerization while leaving the tertiary structure of the monomer intact. The dimer interface comprises a four-helix bundle involving residues that are the most highly conserved among Gadd45 isoforms. Cell-based assays using these point mutants demonstrate that dimerization is essential for growth inhibition. This structural information provides a new context for evaluation of the plethora of protein-protein interactions that govern the many functions of the Gadd45 family of proteins.

Suppression of Gadd45 alleviates the G2/M blockage and the enhanced phosphorylation of p53 and p38 in zinc supplemented normal human bronchial epithelial cells

An adequate zinc status is essential for optimal cellular functions and growth. Yet, excessive zinc supplementation can be cytotoxic and can impair cell growth. Gadd45 plays a vital role as cellular stress sensor in the modulation of cell signal transduction in response to stress. The present study was designed to determine the influence of zinc status on Gadd45 expression and cell cycle progression in zinc deficient and supplemented normal human bronchial epithelial (NHBE) cells, and to decipher the molecular mechanism(s) exerted by the suppression of Gadd45 expression on cell growth and cell cycle progression in this cell type. Cells were cultured for one passage in different concentration of zinc: <0.4 muM (ZD) as severe zinc deficient; 4 muM as normal zinc level in culture medium; 16 microM (ZA) as normal human plasma zinc level; and 32 muM (ZS) as the high end of plasma zinc attainable by oral supplementation. Inhibition of cell growth, upregulation of Gadd45 mRNA and protein expression, and blockage of G2/M cell cycle progression were observed in ZS cells. In contrast, little or no changes in these parameters were seen in ZD cells. The siRNA-mediated knocking down of Gadd45 was found to relieve G2/M blockage in ZS cells, which indicated that the blockage was Gadd45 dependent. Moreover, the enhanced phosphorylation of p38 and p53 (ser15) in ZS cells was normalized after suppression of Gadd45 by siRNA, implicating that the enhanced phosphorylation of these proteins was Gadd45 dependent. Thus, we demonstrated for the first time that an elevated zinc status modulated signal transduction to produce a delay at G2/M during cell cycle progression in NHBE cells.

Nitric oxide down-regulates polo-like kinase 1 through a proximal promoter cell cycle gene homology region

Polo-like kinase 1 (PLK1) is an evolutionarily conserved serine/threonine kinase essential for cell mitosis. As a master cell cycle regulator, p21/Waf1 plays a critical role in cell cycle progression. Nitric oxide (NO.) has been shown to down-regulate PLK1 and up-regulate p21/Waf1 independent of cGMP. Here, the respective roles of p38 MAPK and p21/Waf1 in NO.-mediated PLK1 repression were investigated using differentiated U937 cells that lack soluble guanylate cyclase. NO. was shown to down-regulate both PLK1 mRNA and protein. Nuclear run-on assays and mRNA stability studies demonstrated that the effect of NO. on PLK1 expression was associated with decreased transcription without changes in transcript stability. SB202190, a p38 MAPK inhibitor, prevented transcriptional repression of PLK1 by NO.. Transfection with dominant-negative p38 MAPK mutant eliminated the NO. effect on both p21/Waf1 and PLK1 gene expression. Knockdown of p21/Waf1 with siRNA also substantially reduced the regulatory effect of NO. on PLK1. Reporter gene experiments showed that NO. decreased activity of the PLK1 proximal promoter, an effect that was blocked by p38 MAPK inhibitor. Deletion or mutation of the CDE/CHR promoter site, an element regulated by p21/Waf1, increased base-line promoter activity and abolished NO. repression of the PLK1 promoter. Likewise, electrophoretic mobility shift assays with CDE/CHR probe revealed a NO.-mediated change in protein-probe complex formation. Competition with various unlabeled CDE/CHR mutant sequences showed that NO. increased nuclear protein binding to intact CHR. These results demonstrate that a NO.-p38 MAPK-p21/Waf1 signal transduction pathway represses PLK1 through a canonical CDE/CHR promoter element.

Coordination of JNK1 and JNK2 is critical for GADD45alpha induction and its mediated cell apoptosis in arsenite responses

Arsenite is a well documented environmental pathogen, whereas it has also been applied as medication to treat various neoplasmas. The pathogenic and therapeutic effects of arsenite are associated with cellular apoptotic responses. However, the molecular mechanisms of arsenite-induced apoptosis are not very well understood. Our previous study has shown that arsenite exposure is able to activate JNKs, which subsequently mediate the apoptotic outcome. The present study further revealed that the coordination of JNK1 and JNK2 was critical for the arsenite-induced expression of GADD45alpha (growth arrest and DNA damage 45alpha), which in turn mediated the cellular apoptosis. The arsenite-induced apoptosis and GADD45alpha expression were significantly impaired in mouse embryonic fibroblasts deficient in either jnk1 (JNK1-/-) or jnk2 (JNK2-/-). Knockdown of GADD45alpha by its specific small interfering RNA also dramatically reduced the apoptotic responses, and overexpression of GADD45alpha in either JNK1-/- or JNK2-/- mouse embryonic fibroblasts partially resensitized the cell death. Furthermore, it was found that the regulation of GADD45alpha by JNK1 and JNK2 was achieved through mediating the activation of c-Jun, since in the JNK1-/- and JNK2-/- cells the c-Jun activation was impaired, and overexpression of the dominant negative mutant of c-Jun (TAM67) in wild type cells could also block GADD45alpha induction as well as cellular apoptosis. Our results demonstrate that the coordination of JNK1 and JNK2 is critical for c-Jun/GADD45alpha-mediated cellular apoptosis induced by arsenite.

Inducible activation of CEBPB, a gene negatively regulated by BCR/ABL, inhibits proliferation and promotes differentiation of BCR/ABL-expressing cells

Translational regulation by oncogenic proteins may be a rapid and efficient mechanism to modulate gene expression. We report here the identification of the CEBPB gene as a target of translational regulation in myeloid precursor cells transformed by the BCR/ABL oncogene. Expression of CEBPB was repressed in 32D-BCR/ABL cells and reinduced by imatinib (STI571) via a mechanism that appears to depend on expression of the CUG-repeat RNA-binding protein CUGBP1 and the integrity of the CUG-rich intercistronic region of c/ebpbeta mRNA. Constitutive expression or conditional activation of wild-type CEBPB induced differentiation and inhibited proliferation of 32D-BCR/ABL cells in vitro and in mice, but a DNA binding-deficient CEBPB mutant had no effect. The proliferation-inhibitory effect of CEBPB was, in part, mediated by the CEBPB-induced GADD45A gene. Because expression of CEBPB (and CEBPA) is low in the blast crisis (BC) stage of chronic myelogenous leukemia (CML) and is inversely correlated with BCR/ABL tyrosine kinase levels, these findings point to the therapeutic potential of restoring C/EBP activity in CML-BC and, perhaps, other types of acute leukemia.

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.

Linking JNK signaling to NF-kappaB: a key to survival

In addition to marshalling immune and inflammatory responses, transcription factors of the NF-kappaB family control cell survival. This control is crucial to a wide range of biological processes, including B and T lymphopoiesis, adaptive immunity, oncogenesis and cancer chemoresistance. During an inflammatory response, NF-kappaB activation antagonizes apoptosis induced by tumor necrosis factor (TNF)-alpha, a protective activity that involves suppression of the Jun N-terminal kinase (JNK) cascade. This suppression can involve upregulation of the Gadd45-family member Gadd45beta/Myd118, which associates with the JNK kinase MKK7/JNKK2 and blocks its catalytic activity. Upregulation of XIAP, A20 and blockers of reactive oxygen species (ROS) appear to be important additional means by which NF-kappaB blunts JNK signaling. These recent findings might open up entirely new avenues for therapeutic intervention in chronic inflammatory diseases and certain cancers; indeed, the Gadd45beta-MKK7 interaction might be a key target for such intervention.

Mechanism of liver regeneration after partial hepatectomy using mouse cDNA microarray

BACKGROUND/AIMS

The liver has the capacity to regenerate after partial hepatectomy. In order to clarify the mechanism of liver regeneration, we observed the initial stage, especially the mechanism of gene expression during progress from G0 to S phase (0-24 h), and attempted to identify new genes controlling progress to the S phase.

METHODS

We applied large-scale gene expression analysis with complementary DNA microarrays in mouse hepatectomy models to clarify the mechanism of liver regeneration after partial hepatectomy.

RESULTS

As a result, 23 new immediate-early gene candidates such as interleukin-1 receptor associated kinase-1 and karyopherin alpha-1, which are involved in transportation within the nucleus, were discovered. Candidates for new genes concerned with the progress to the S phase were discovered: inhibitor of DNA binding 2 (ID2) and inhibitor of DNA binding 3 (ID3), both new liver regeneration factors that promoted progress to the S phase, and GADD45 gamma (growth arrest and DNA-damage-inducible protein) as a factor inhibiting that process.

CONCLUSIONS

The above results not only suggest the importance of NFkappaB in the initial stage of liver regeneration but also points to the orderly maintenance of the proliferation of the cells in liver regeneration.

Gadd45beta is important for perpetuating cognate and inflammatory signals in T cells

Gadd45beta (growth arrest and DNA damage-inducible, beta) is involved in cell cycle arrest, apoptosis, signal transduction and cell survival. In T cells, Gadd45b was rapidly induced by T cell receptor (TCR) and inflammatory signals. Deficiency of Gadd45beta in CD4+ T cells impaired their responses to TCR stimulation or inflammatory cytokines. ERK, p38 and JNK activation were all substantially suppressed in Gadd45beta-deficient CD4+ T cells. Cytokine production by Gadd45beta-deficient CD4+ T cells was also impaired. Furthermore, Gadd45beta mediated inflammatory cytokine production by dendritic cells, and Gadd45beta-deficient mice showed an impaired T helper type 1 response during Listeria monocytogenes infection. Gadd45beta is therefore a critical feedback regulator that perpetuates both cognate and inflammatory signals.

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.

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.

p53-related pathways and the molecular pathogenesis of melanoma

Cutaneous malignant melanoma (CMM) is a life-threatening cancer that can have a poor prognosis with high metastatic potential. Its incidence is rapidly increasing worldwide. Its molecular alterations involve multiple pathways, including those related to p53. Since 1981, more than 380 papers containing the terms 'p53 and melanoma' as key words in the Abstract have been published in the literature. However, in spite of these extensive investigations, a review of p53 and associated genes in CMM is still lacking. To remedy this issue, this review seeks to provide a brief overview of p53 and discuss the genes targeted along its related pathways.

Transcriptomic classification of antitumor agents: application to the analysis of the antitumoral effect of SR31747A

SR31747A is a sigma ligand that exhibits a potent antitumoral activity on various human tumor cell lines both in vitro and in vivo. To understand its mode of action, we used DNA microarray technology combined with a new bioinformatic approach to identify genes that are modulated by SR31747A in different human breast or prostate cancer cell lines. The SR31747A transcriptional signature was also compared with that of seven different representative anticancer drugs commonly used in the clinic. To this aim, we performed a two-dimensional hierarchical clustering analysis of drugs and genes which showed that 1) standard molecules with similar mechanism of action clustered together and 2) SR31747A does not belong to any previously characterized class of standard anticancer drugs. Moreover, we showed that 3) SR31747A mainly exerted its antiproliferative effect by inhibiting the expression of genes playing a key role in DNA replication and cell cycle progression. Finally, contrasting with other drugs, we obtained evidence that 4) SR31747A strongly inhibited the expression of three key enzymes of the nucleotide synthesis pathway (i.e., dihydrofolate reductase, thymidylate synthase, and thymidine kinase) with the latter shown both at the mRNA and protein levels. These results, obtained through a novel molecular approach to characterize and compare anticancer agents, showed that SR31747A exhibits an original mechanism of action, very likely through unexpected targets whose modulations may account for its antitumoral effect.

GADD45-induced cell cycle G2-M arrest associates with altered subcellular distribution of cyclin B1 and is independent of p38 kinase activity

In response to DNA damage, the cell cycle checkpoint is an important biological event in maintaining genomic fidelity. Gadd45, a p53-regulated and DNA damage inducible protein, has recently been demonstrated to play a role in the G2-M checkpoint in response to DNA damage. In the current study, we further investigated the biochemical mechanism(s) involved in the GADD45-activated cell cycle G2-M arrest. Using the tetracycline-controlled system (tet-off), we established GADD45-inducible lines in HCT116 (wild-type p53) and Hela (inactivated p53 status) cells. Following inducible expression of the Gadd45 protein, cell growth was strongly suppressed in both HCT116 and Hela cells. Interestingly, HCT116 cells revealed a significant G2-M arrest but Hela cells failed to arrest at the G2-M phases, indicating that the GADD45-activated G2-M arrest requires normal p53 function. The GADD45-induced G2-M arrest was observed independent of p38 kinase activity. Importantly, induction of Gadd45 protein resulted in a reduction of nuclear cyclin B1 protein, whose nuclear localization is critical for the completion of G2-M transition. The reduced nuclear cyclin B1 levels correlated with inhibition of Cdc2/cyclin B1 kinase activity. Additionally, overexpression of cyclin B1 substantially abrogated the GADD45-induced cell growth suppression. Therefore, GADD45 inhibition of Cdc2 kinase activity through alteration of cyclin B1 subcellular localization may be an essential step in the GADD45-induced cell cycle G2-M arrest and growth suppression.

Xiro homeoproteins coordinate cell cycle exit and primary neuron formation by upregulating neuronal-fate repressors and downregulating the cell-cycle inhibitor XGadd45-gamma

The iroquois (iro) homeobox genes participate in many developmental processes both in vertebrates and invertebrates, among them are neural plate formation and neural patterning. In this work, we study in detail Xenopus Iro (Xiro) function in primary neurogenesis. We show that misexpression of Xiro genes promotes the activation of the proneural gene Xngnr1 but suppresses neuronal differentiation. This is probably due to upregulation of at least two neuronal-fate repressors: XHairy2A and XZic2. Accordingly, primary neurons arise at the border of the Xiro expression domains. In addition, we identify XGadd45-gamma as a new gene repressed by Xiro. XGadd45-gamma encodes a cell-cycle inhibitor and is expressed in territories where cells will exit mitosis, such as those where primary neurons arise. Indeed, XGadd45-gamma misexpression causes cell cycle arrest. We conclude that, during Xenopus primary neuron formation, in Xiro expressing territories neuronal differentiation is impaired, while in adjacent cells, XGadd45-gamma may help cells stop dividing and differentiate as neurons.

GADD45b and GADD45g are cdc2/cyclinB1 kinase inhibitors with a role in S and G2/M cell cycle checkpoints induced by genotoxic stress

Gadd45a (Gadd45), Gadd45b (MyD118), and Gadd45g (CR6) constitute a family of evolutionarily conserved, small, acidic, nuclear proteins, which have been implicated in terminal differentiation, growth suppression, and apoptosis. How Gadd45 proteins function in negative growth control is not fully understood. Recent evidence has implicated Gadd45a in inhibition of cdc2/cyclinB1 kinase and in G2/M cell cycle arrest. Yet, whether Gadd45b and/or Gadd45g function as inhibitors of cdc2/cyclinB1 kinase and/or play a role in G2/M cell cycle arrest has not been fully established. In this work, we show that Gadd45b and Gadd45g specifically interact with the Cdk1/CyclinB1 complex, but not with other Cdk/Cyclin complexes, in vitro and in vivo. Data also has been obtained that Gadd45b and Gadd45g, as well as GADD45a, interact with both Cdk1 and cyclinB1, resulting in inhibition of the kinase activity of the Cdk1/cyclinB1 complex. Inhibition of Cdk1/cyclinB1 kinase activity by Gadd45b and Gadd45a was found to involve disruption of the complex, whereas Gadd45g did not disrupt the complex. Moreover, using RKO lung carcinoma cell lines, which express antisense Gadd45 RNA, data has been obtained, which indicates that all three Gadd45 proteins are likely to cooperate in activation of S and G2/M checkpoints following exposure of cells to UV irradiation.

cDNA array reveals mechanosensitive genes in chondrocytic cells under hydrostatic pressure

Hydrostatic pressure (HP) has a profound effect on cartilage metabolism in normal and pathological conditions, especially in weight-bearing areas of the skeletal system. As an important component of overall load, HP has been shown to affect the synthetic capacity and well-being of chondrocytes, depending on the mode, duration and magnitude of pressure. In this study we examined the effect of continuous HP on the gene expression profile of a chondrocytic cell line (HCS-2/8) using a cDNA array containing 588 well-characterized human genes under tight transcriptional control. A total of 51 affected genes were identified, many of them not previously associated with mechanical stimuli. Among the significantly up-regulated genes were immediate-early genes, and genes involved in heat-shock response (hsp70, hsp40, hsp27), and in growth arrest (GADD45, GADD153, p21(Cip1/Waf1), tob). Markedly down-regulated genes included members of the Id family genes (dominant negative regulators of basic helix-loop-helix transcription factors), and cytoplasmic dynein light chain and apoptosis-related gene NIP3. These alterations in the expression profile induce a transient heat-shock gene response and activation of genes involved in growth arrest and cellular adaptation and/or differentiation.

Insulin-like growth factor I induces MDM2-dependent degradation of p53 via the p38 MAPK pathway in response to DNA damage

In many tissues, the insulin-like growth factor I (IGF-I) receptor (IGF-IR) is known to functionally oppose apoptosis. Recently, we demonstrated a direct role for the IGF-IR in the rescue of DNA-damaged fibroblasts by activating a DNA repair pathway (Héron-Milhavet, L., Karas, M., Goldsmith, C. M., Baum, B. J., and LeRoith, D. (2001) J. Biol. Chem. 276, 18185-18192). p53 is a nuclear transcription factor that can block progression of the cell cycle, modulate DNA repair, and trigger apoptosis. In this work, we tested the effect of IGF-I on the regulation of the p53 signaling cascade. The DNA-damaging agent 4-nitroquinoline 1-oxide was applied to NIH-3T3 cells overexpressing normal IGF-IRs (NWTb3 cells). We showed that after 4-nitroquinoline 1-oxide-induced DNA damage, IGF-I induced exclusion of the p53 protein from the nucleus and led to its degradation in the cytoplasm, whereas p53 mRNA was unaffected. Degradation of the p53 protein was associated with an increase in MDM2, an upstream modulator of the half-life and activity of the p53 protein. p53 degradation was also associated with down-regulation of p21. We further showed that the effects of IGF-I on mdm2 transcription and on MDM2/p19 ARF association were mediated by the p38 MAPK pathway. In conclusion, we describe a novel role for IGF-I in the regulation of the MDM2/p53/p21 signaling pathway during DNA damage.