Inactivation of gadd45a sensitizes epithelial cancer cells to ionizing radiation in vivo resulting in prolonged survival

Impact of GADD45A on Radiation Biodosimetry Using Mouse Peripheral Blood

High-dose-radiation exposure in a short period of time leads to radiation syndromes characterized by severe acute and delayed organ-specific injury accompanied by elevated organismal morbidity and mortality. Radiation biodosimetry based on gene expression analysis of peripheral blood is a valuable tool to detect exposure to radiation after a radiological/nuclear incident and obtain useful biological information that could predict tissue and organismal injury. However, confounding factors, including chronic inflammation, can potentially obscure the predictive power of the method. GADD45A (Growth arrest and DNA damage-inducible gene a) plays important roles in cell growth control, differentiation, DNA repair, and apoptosis. GADD45A-deficient mice develop an autoimmune disease, similar to human systemic lupus erythematosus, characterized by severe hematological disorders, kidney disease, and premature death. The goal of this study was to elucidate how pre-existing inflammation in mice, induced by GADD45A ablation, can affect radiation biodosimetry. We exposed wild-type and GADD45A knockout male C57BL/6J mice to 7 Gy of X rays and 24 h later RNA was isolated from whole blood and subjected to whole genome microarray and gene ontology analyses. Dose reconstruction analysis using a gene signature trained on gene expression data from irradiated wild-type male mice showed accurate reconstruction of either a 0 Gy or 7 Gy dose with root mean square error of ± 1.05 Gy (R^2 = 1.00) in GADD45A knockout mice. Gene ontology analysis revealed that irradiation of both wild-type and GADD45A-null mice led to a significant overrepresentation of pathways associated with morbidity and mortality, as well as organismal cell death. However, based on their z-score, these pathways were predicted to be more significantly overrepresented in GADD45A-null mice, implying that GADD45A deletion may exacerbate the deleterious effects of radiation on blood cells. Numerous immune cell functions and quantities were predicted to be underrepresented in both genotypes; however, differentially expressed genes from irradiated GADD45A knockout mice predicted an increased deterioration in the numbers of T lymphocytes, as well as myeloid cells, compared with wild-type mice. Furthermore, an overrepresentation of genes associated with radiation-induced hematological malignancies was associated with GADD45A knockout mice, whereas hematopoietic and progenitor cell functions were predicted to be downregulated in irradiated GADD45A knockout mice. In conclusion, despite the significant differences in gene expression between wild-type and GADD45A knockout mice, it is still feasible to identify a panel of genes that could accurately distinguish between irradiated and control mice, irrespective of pre-existing inflammation status.

Pulsed low dose-rate irradiation response in isogenic HNSCC cell lines with different radiosensitivity

Background

Management of locoregionally recurrent head and neck squamous cell carcinomas (HNSCC) is challenging due to potential radioresistance. Pulsed low-dose rate (PLDR) irradiation exploits phenomena of increased radiosensitivity, low-dose hyperradiosensitivity (LDHRS), and inverse dose-rate effect. The purpose of this study was to evaluate LDHRS and the effect of PLDR irradiation in isogenic HNSCC cells with different radiosensitivity.

Materials and methods

Cell survival after different irradiation regimens in isogenic parental FaDu and radioresistant FaDu-RR cells was determined by clonogenic assay; post irradiation cell cycle distribution was studied by flow cytometry; the expression of DNA damage signalling genes was assesed by reverse transcription-quantitative PCR.

Results

Radioresistant Fadu-RR cells displayed LDHRS and were more sensitive to PLDR irradiation than parental FaDu cells. In both cell lines, cell cycle was arrested in G₂/M phase 5 hours after irradiation. It was restored 24 hours after irradiation in parental, but not in the radioresistant cells, which were arrested in G₁-phase. DNA damage signalling genes were under-expressed in radioresistant compared to parental cells. Irradiation increased DNA damage signalling gene expression in radioresistant cells, while in parental cells only few genes were under-expressed.

Conclusions

We demonstrated LDHRS in isogenic radioresistant cells, but not in the parental cells. Survival of LDHRS-positive radioresistant cells after PLDR was significantly reduced. This reduction in cell survival is associated with variations in DNA damage signalling gene expression observed in response to PLDR most likely through different regulation of cell cycle checkpoints.

Injury Activates a Dynamic Cytoprotective Network to Confer Stress Resilience and Drive Repair

In healthy individuals, injured tissues rapidly repair themselves following damage. Within a healing skin wound, recruited inflammatory cells release a multitude of bacteriocidal factors, including reactive oxygen species (ROS), to eliminate invading pathogens. Paradoxically, while these highly reactive ROS confer resistance to infection, they are also toxic to host tissues and may ultimately delay repair. Repairing tissues have therefore evolved powerful cytoprotective "resilience" machinery to protect against and tolerate this collateral damage. Here, we use in vivo time-lapse imaging and genetic manipulation in Drosophila to dissect the molecular and cellular mechanisms that drive tissue resilience to wound-induced stress. We identify a dynamic, cross-regulatory network of stress-activated cytoprotective pathways, linking calcium, JNK, Nrf2, and Gadd45, that act to both "shield" tissues from oxidative damage and promote efficient damage repair. Ectopic activation of these pathways confers stress protection to naive tissue, while their inhibition leads to marked delays in wound closure. Strikingly, the induction of cytoprotection is tightly linked to the pathways that initiate the inflammatory response, suggesting evolution of a fail-safe mechanism for tissue protection each time inflammation is triggered. A better understanding of these resilience mechanisms-their identities and precise spatiotemporal regulation-is of major clinical importance for development of therapeutic interventions for all pathologies linked to oxidative stress, including debilitating chronic non-healing wounds.

GADD45α sensitizes cervical cancer cells to radiotherapy via increasing cytoplasmic APE1 level

Radioresistance remains a major clinical challenge in cervical cancer therapy. However, the mechanism for the development of radioresistance in cervical cancer is unclear. Herein, we determined that growth arrest and DNA-damage-inducible protein 45α (GADD45α) is decreased in radioresistant cervical cancer compared to radiosensitive cancer both in vitro and in vivo. In addition, silencing GADD45α prevents cervical cancer cells from undergoing radiation-induced DNA damage, cell cycle arrest, and apoptosis. More importantly, our data show that the overexpression of GADD45α significantly enhances the radiosensitivity of radioresistant cervical cancer cells. These data show that GADD45α decreases the cytoplasmic distribution of APE1, thereby enhancing the radiosensitivity of cervical cancer cells. Furthermore, we show that GADD45α inhibits the production of nitric oxide (NO), a nuclear APE1 export stimulator, by suppressing both endothelial NO synthase (eNOS) and inducible NO synthase (iNOS) in cervical cancer cells. In conclusion, our findings suggest that decreased GADD45α expression significantly contributes to the development of radioresistance and that ectopic expression of GADD45α sensitizes cervical cancer cells to radiotherapy. GADD45α inhibits the NO-regulated cytoplasmic localization of APE1 through inhibiting eNOS and iNOS, thereby enhancing the radiosensitivity of cervical cancer cells.

GADD45A plays a protective role against temozolomide treatment in glioblastoma cells

Glioblastoma multiforme (GBM) is one of the most aggressive cancers. Despite recent advances in multimodal therapies, high-grade glioma remains fatal. Temozolomide (TMZ) is an alkylating agent used worldwide for the clinical treatment of GBM; however, the innate and acquired resistance of GBM limits its application. Here, we found that TMZ inhibited the proliferation and induced the G2/M arrest of GBM cells. Therefore, we performed microarrays to identify the cell cycle- and apoptosis-related genes affected by TMZ. Notably, GADD45A was found to be up-regulated by TMZ in both cell cycle and apoptosis arrays. Furthermore, GADD45A knockdown (GADD45A^kd) enhanced the cell growth arrest and cell death induced by TMZ, even in natural (T98) and adapted (TR-U373) TMZ-resistant cells. Interestingly, GADD45A^kd decreased the expression of O⁶-methylguanine-DNA methyltransferase (MGMT) in TMZ-resistant cells (T98 and TR-U373). In MGMT-deficient/TMZ-sensitive cells (U87 and U373), GADD45A^kd decreased TMZ-induced TP53 expression. Thus, in this study, we investigated the genes influenced by TMZ that were important in GBM therapy, and revealed that GADD45A plays a protective role against TMZ treatment which may through TP53-dependent and MGMT-dependent pathway in TMZ-sensitive and TMZ-resistant GBM, respectively. This protective role of GADD45A against TMZ treatment may provide a new therapeutic strategy for GBM treatment.

TRAF6 Restricts p53 Mitochondrial Translocation, Apoptosis, and Tumor Suppression

Mitochondrial p53 is involved in apoptosis and tumor suppression. However, its regulation is not well studied. Here, we show that TRAF6 E3 ligase is a crucial factor to restrict mitochondrial translocation of p53 and spontaneous apoptosis by promoting K63-linked ubiquitination of p53 at K24 in cytosol, and such ubiquitination limits the interaction between p53 and MCL-1/BAK. Genotoxic stress reduces this ubiquitination in cytosol by S13/T330 phosphorylation-dependent translocation of TRAF6 from cytosol to nucleus, where TRAF6 also facilitates the K63-linked ubiquitination of nuclear p53 and its transactivation by recruiting p300 for p53 acetylation. Functionally, K63-linked ubiquitination of p53 compromised p53-mediated apoptosis and tumor suppression. Colorectal cancer samples with WT p53 reveal that TRAF6 overexpression negatively correlates with apoptosis and predicts poor response to chemotherapy and radiotherapy. Together, our study identifies TRAF6 as a critical gatekeeper to restrict p53 mitochondrial translocation, and such mechanism may contribute to tumor development and drug resistance.

Transcriptome analysis of glioma cells for the dynamic response to γ-irradiation and dual regulation of apoptosis genes: a new insight into radiotherapy for glioblastomas

Ionizing radiation (IR) is of clinical importance for glioblastoma therapy; however, the recurrence of glioma characterized by radiation resistance remains a therapeutic challenge. Research on irradiation-induced transcription in glioblastomas can contribute to the understanding of radioresistance mechanisms. In this study, by using the total mRNA sequencing (RNA-seq) analysis, we assayed the global gene expression in a human glioma cell line U251 MG at various time points after exposure to a growth arrest dose of γ-rays. We identified 1656 genes with obvious changes at the transcriptional level in response to irradiation, and these genes were dynamically enriched in various biological processes or pathways, including cell cycle arrest, DNA replication, DNA repair and apoptosis. Interestingly, the results showed that cell death was not induced even many proapoptotic molecules, including death receptor 5 (DR5) and caspases were activated after radiation. The RNA-seq data analysis further revealed that both proapoptosis and antiapoptosis genes were affected by irradiation. Namely, most proapoptosis genes were early continually responsive, whereas antiapoptosis genes were responsive at later stages. Moreover, HMGB1, HMGB2 and TOP2A involved in the positive regulation of DNA fragmentation during apoptosis showed early continual downregulation due to irradiation. Furthermore, targeting of the TRAIL/DR5 pathway after irradiation led to significant apoptotic cell death, accompanied by the recovered gene expression of HMGB1, HMGB2 and TOP2A. Taken together, these results revealed that inactivation of proapoptotic signaling molecules in the nucleus and late activation of antiapoptotic genes may contribute to the radioresistance of gliomas. Overall, this study provided novel insights into not only the underlying mechanisms of radioresistance in glioblastomas but also the screening of multiple targets for radiotherapy.

LGR5 is a proneural factor and is regulated by OLIG2 in glioma stem-like cells

The biological functional roles of LGR5 (leucine-rich repeat containing G protein-coupled receptor 5, also known as GPR49), a novel potential marker for stem-like cells in glioblastoma (GSCs), is poorly acknowledged. Here, we demonstrated that LGR5 was detected in glioblastoma tissues and GSCs. Bioinformatics analysis revealed that LGR5 is closely related to neurogenesis and neuronal functions, and preferentially expressed in Proneural subtype of GBMs. Furthermore, LGR5 is regulated by Proneural factor OLIG2, which is important for both neurogenesis and GSC maintenance. Biological experiments in GSC cells validated the bioinformatics analysis results and revealed that LGR5 regulated the tumor sphere formation capacity, an important stem cell property for GSCs. Therefore, LGR5 expression may be functionally correlated with the neurogenic competence, and be regulated by OLIG2 in GSCs.

Normal prostate-derived stromal cells stimulate prostate cancer development

Stromal cells play a decisive role in regulating tumor progression. In this study, we assessed the significance of normal prostate-derived stromal cells (PSCs) in prostate cancer development. An in vivo s.c. tumor model was established as follows: Group 1, DU145 cells alone; Group 2, DU145 + PSCs; Group 3, DU145 cells alone injected into pre-castrated mice; and Group 4, DU145 + PSCs injected into pre-castrated mice. Following injection, tumors were only detectable in the first two groups, with more aggressive growth in Group 2 than in Group 1 (P < 0.05). Immunohistochemical analysis revealed significantly higher proliferation (P < 0.05), but not apoptosis or altered expression of androgen receptor in Group 2, as compared with Group 1. In vitro, DU145 cells isolated from Group 1 tumors showed lower viability and migratory capability than those from Group 2. cDNA microarray on isolated DU145 cells from Groups 1 and 2 revealed the differential expression of genes regulating cell cycle progression and cell mobility, including GADD45A, RHOV, KLK11, and PCK1. Our results suggest that stromal cells derived from normal prostate potentiate the development of tumor growth in vivo, which is achieved at least in part through the regulation of cell-cycle- and migration-related gene expression within the tumor cells.

Apoptosis is the essential target of selective pressure against p53, whereas loss of additional p53 functions facilitates carcinoma progression

The high frequency of p53 mutation in human cancers indicates the important role of p53 in suppressing tumorigenesis. It is well established that the p53 regulates multiple, distinct cellular functions such as cell-cycle arrest and apoptosis. Despite intensive studies, little is known about which function is essential, or if multiple pathways are required, for p53-dependent tumor suppression in vivo. Using a mouse brain carcinoma model that shows high selective pressure for p53 inactivation, we found that even partially abolishing p53-dependent apoptosis by Bax inactivation was sufficient to significantly reduce the selective pressure for p53 loss. This finding is consistent with previous reports that apoptosis is the primary p53 function selected against during Eμ-myc-induced mouse lymphoma progression. However, unlike observed in the Eμ-myc-induced lymphoma model, attenuation of apoptosis is not sufficient to phenocopy the aggressive tumor progression associated with complete loss of p53 activity. We conclude that apoptosis is the primary tumor suppressive p53 function and the ablation of additional p53 pleiotropic effects further exacerbates tumor progression.

Over-expression of Gadd45a enhances radiotherapy efficacy in human Tca8113 cell line

AIM

To investigate the effect of the growth arrest- and DNA damage-inducible Gadd45a gene on the radiosensitivity of human tongue squamous cell carcinoma cell line to ionizing radiation (IR).

METHODS

Short interfering ribonucleic acid (si-RNA) targeting Gadd45a or an irrelevant mRNA (nonsense si-RNA) was chemically synthesized. The constructed si-RNAs were transfected into Tca8113 cells and Gadd45a expression was determined using quantitative real-time PCR and Western-blot. After 24-h exposure to IR at a dose rate of 4 Gy/min, apoptosis of Tca8113 cells was detected using flow cytometry, and radiosensitivity was measured using MTT assays.

RESULTS

IR apparently increased the expression of Gadd45a at mRNA and protein levels in Tca8113 cells. The effect was efficiently inhibited by transfection with Gadd45a si-RNA (P<0.01). Furthermore, silencing Gadd45a gene significantly increased cell viability and decreased the percentage of apoptotic cells during irradiation, which indicated that IR-induced Gadd45a over-expression could increase the radiosensitivity of Tca8113 cells.

CONCLUSION

These results indicated that targeting Gadd45a may have important therapeutic implications in sensitizing Tca8113 cells to IR.

Identification of radiation-induced expression changes in nonimmortalized human T cells

In the event of a radiation accident or attack, it will be imperative to quickly assess the amount of radiation exposure to accurately triage victims for appropriate care. RNA-based radiation dosimetry assays offer the potential to rapidly screen thousands of individuals in an efficient and cost-effective manner. However, prior to the development of these assays, it will be critical to identify those genes that will be most useful to delineate different radiation doses. Using global expression profiling, we examined expression changes in nonimmortalized T cells across a wide range of doses (0.15-12 Gy). Because many radiation responses are highly dependent on time, expression changes were examined at three different times (3, 8, and 24 h). Analyses identified 61, 512 and 1310 genes with significant linear dose-dependent expression changes at 3, 8 and 24 h, respectively. Using a stepwise regression procedure, a model was developed to estimate in vitro radiation exposures using the expression of three genes (CDKN1A, PSRC1 and TNFSF4) and validated in an independent test set with 86% accuracy. These findings suggest that RNA-based expression assays for a small subset of genes can be employed to develop clinical biodosimetry assays to be used in assessments of radiation exposure and toxicity.