DDB2, an essential mediator of premature senescence

The Role of Alternative Splicing Factors, DDB2-Related Ageing and DNA Damage Repair in the Progression and Prognosis of Stomach Adenocarcinoma Patients

DNA damage response is a key signal transduction pathway in triggering ageing and tumor progression. Abnormal alternative splicing (AS) is associated with tumors and ageing. However, the role of AS factors associated with DNA damage repair and ageing in stomach adenocarcinoma (STAD) remains unclear. We downloaded the percentage of splicing (PSI) values for AS in STAD from the TCGA SpliceSeq database. The PSI values of DNA repair gene AS events were integrated with STAD patient survival data for Cox regression analysis. The prediction model for the overall survival (OS) was constructed by the clinical traits. The tumor immune microenvironment was analyzed by CIBERSORT and ESTIMATE. We detected 824 AS events originating from 166 DNA repair genes. Cox regression analysis provided 21 prognostic AS events connected with OS statistically, and a prognostic prediction model was constructed. The expression of these AS factors was higher in STAD tumors. DDB2 high senescence levels were associated with active immune responses and better survival in STAD patients. We built a novel prognostic model founded on DNA repair genes with AS events and identified that DDB2 may be a potential biomarker to apply in clinics.

A protein with broad functions: damage-specific DNA-binding protein 2

Damage-specific DNA-binding protein 2 (DDB2) was initially identified as a component of the damage-specific DNA-binding heterodimeric complex, which cooperates with other proteins to repair UV-induced DNA damage. DDB2 is involved in the occurrence and development of cancer by affecting nucleotide excision repair (NER), cell apoptosis, and premature senescence. DDB2 also affects the sensitivity of cancer cells to radiotherapy and chemotherapy. In addition, a recent study found that DDB2 is a pathogenic gene for hepatitis and encephalitis. In recent years, there have been few relevant literature reports on DDB2, so there is still room for further research about it. In this paper, the molecular mechanisms of different biological processes involving DDB2 are reviewed in detail to provide theoretical support for research on drugs that can target DDB2.

Inhibition of miR-328-3p Impairs Cancer Stem Cell Function and Prevents Metastasis in Ovarian Cancer

Cancer stem cells (CSC) play a central role in cancer metastasis and development of drug resistance. miRNA are important in regulating CSC properties and are considered potential therapeutic targets. Here we report that miR-328-3p (miR-328) is significantly upregulated in ovarian CSC. High expression of miR-328 maintained CSC properties by directly targeting DNA damage binding protein 2, which has been shown previously to inhibit ovarian CSC. Reduced activity of ERK signaling in ovarian CSC, mainly due to a low level of reactive oxygen species, contributed to the enhanced expression of miR-328 and maintenance of CSC. Inhibition of miR-328 in mouse orthotopic ovarian xenografts impeded tumor growth and prevented tumor metastasis. In summary, our findings provide a novel mechanism underlying maintenance of the CSC population in ovarian cancer and suggest that targeted inhibition of miR-328 could be exploited for the eradication of CSC and aversion of tumor metastasis in ovarian cancer. SIGNIFICANCE: These findings present inhibition of miR-328 as a novel strategy for efficient elimination of CSC to prevent tumor metastasis and recurrence in patients with epithelial ovarian cancer.

Targeting LINC00673 expression triggers cellular senescence in lung cancer

Aberrant expression of noncoding RNAs plays a critical role during tumorigenesis. To uncover novel functions of long non-coding RNA (lncRNA) in lung adenocarcinoma, we used a microarray-based screen identifying LINC00673 with elevated expression in matched tumor versus normal tissue. We report that loss of LINC00673 is sufficient to trigger cellular senescence, a tumor suppressive mechanism associated with permanent cell cycle arrest, both in lung cancer and normal cells in a p53-dependent manner. LINC00673-depleted cells fail to efficiently transit from G1- to S-phase. Using a quantitative proteomics approach, we confirm the modulation of senescence-associated genes as a result of LINC00673 knockdown. In addition, we uncover that depletion of p53 in normal and tumor cells is sufficient to overcome LINC00673-mediated cell cycle arrest and cellular senescence. Furthermore, we report that overexpression of LINC00673 reduces p53 translation and contributes to the bypass of Ras-induced senescence. In summary, our findings highlight LINC00673 as a crucial regulator of proliferation and cellular senescence in lung cancer.

DDB2 represses ovarian cancer cell dedifferentiation by suppressing ALDH1A1

Cancer stem cells (CSCs), representing the root of many solid tumors including ovarian cancer, have been implicated in disease recurrence, metastasis, and therapeutic resistance. Our previous study has demonstrated that the CSC subpopulation in ovarian cancer can be limited by DNA damage-binding protein 2 (DDB2). Here, we demonstrated that the ovarian CSC subpopulation can be maintained via cancer cell dedifferentiation, and DDB2 is able to suppress this non-CSC-to-CSC conversion by repression of ALDH1A1 transcription. Mechanistically, DDB2 binds to the ALDH1A1 gene promoter, facilitating the enrichment of histone H3K27me3, and competing with the transcription factor C/EBPβ for binding to this region, eventually inhibiting the promoter activity of the ALDH1A1 gene. The de-repression of ALDH1A1 expression contributes to DDB2 silencing-augmented non-CSC-to-CSC conversion and expansion of the CSC subpopulation. We further showed that treatment with a selective ALDH1A1 inhibitor blocked DDB2 silencing-induced expansion of CSCs, and halted orthotopic xenograft tumor growth. Together, our data demonstrate that DDB2, functioning as a transcription repressor, can abrogate ovarian CSC properties by downregulating ALDH1A1 expression.

Gadd45b deficiency promotes premature senescence and skin aging

The GADD45 family of proteins functions as stress sensors in response to various physiological and environmental stressors. Here we show that primary mouse embryo fibroblasts (MEFs) from Gadd45b null mice proliferate slowly, accumulate increased levels of DNA damage, and senesce prematurely. The impaired proliferation and increased senescence in Gadd45b null MEFs is partially reversed by culturing at physiological oxygen levels, indicating that Gadd45b deficiency leads to decreased ability to cope with oxidative stress. Interestingly, Gadd45b null MEFs arrest at the G2/M phase of cell cycle, in contrast to other senescent MEFs, which arrest at G1. FACS analysis of phospho-histone H3 staining showed that Gadd45b null MEFs are arrested in G2 phase rather than M phase. H2O2 and UV irradiation, known to increase oxidative stress, also triggered increased senescence in Gadd45b null MEFs compared to wild type MEFs. In vivo evidence for increased senescence in Gadd45b null mice includes the observation that embryos from Gadd45b null mice exhibit increased senescence staining compared to wild type embryos. Furthermore, it is shown that Gadd45b deficiency promotes senescence and aging phenotypes in mouse skin. Together, these results highlight a novel role for Gadd45b in stress-induced senescence and in tissue aging.

Chromatin association of XRCC5/6 in the absence of DNA damage depends on the XPE gene product DDB2

DDB2 is a multifunctional protein that participates in both nucleotide excision repair and regulation of gene transcription. In colon cancer cells, chromatin association of XRCC5/6, in the absence of DNA damage, depends on DDB2, and the DDB2–XRCC5/6 interaction promotes the transcription of the antiangiogenic gene SEMA3A.

Damaged DNA-binding protein 2 (DDB2), a nuclear protein, participates in both nucleotide excision repair and mRNA transcription. The transcriptional regulatory function of DDB2 is significant in colon cancer, as it regulates metastasis. To characterize the mechanism by which DDB2 participates in transcription, we investigated the protein partners in colon cancer cells. Here we show that DDB2 abundantly associates with XRCC5/6, not involving CUL4 and DNA-PKcs. A DNA-damaging agent that induces DNA double-stranded breaks (DSBs) does not affect the interaction between DDB2 and XRCC5. In addition, DSB-induced nuclear enrichment or chromatin association of XRCC5 does not involve DDB2, suggesting that the DDB2/XRCC5/6 complex represents a distinct pool of XRCC5/6 that is not directly involved in DNA break repair (NHEJ). In the absence of DNA damage, on the other hand, chromatin association of XRCC5 requires DDB2. We show that DDB2 recruits XRCC5 onto the promoter of SEMA3A, a DDB2-stimulated gene. Moreover, depletion of XRCC5 inhibits SEMA3A expression without affecting expression of VEGFA, a repression target of DDB2. Together our results show that DDB2 is critical for chromatin association of XRCC5/6 in the absence of DNA damage and provide evidence that XRCC5/6 are functional partners of DDB2 in its transcriptional stimulatory activity.

A DDB2 mutant protein unable to interact with PCNA promotes cell cycle progression of human transformed embryonic kidney cells

DNA damage binding protein 2 (DDB2) is a protein involved in the early step of DNA damage recognition of the nucleotide excision repair (NER) process. Recently, it has been suggested that DDB2 may play a role in DNA replication, based on its ability to promote cell proliferation. We have previously shown that DDB2 binds PCNA during NER, but also in the absence of DNA damage; however, whether and how this interaction influences cell proliferation is not known. In this study, we have addressed this question by using HEK293 cell clones stably expressing DDB2(Wt) protein, or a mutant form (DDB2(Mut)) unable to interact with PCNA. We report that overexpression of the DDB2(Mut) protein provides a proliferative advantage over the wild type form, by influencing cell cycle progression. In particular, an increase in the number of S-phase cells, together with a reduction in p21(CDKN1A) protein level, and a shorter cell cycle length, has been observed in the DDB2(Mut) cells. These results suggest that DDB2 influences cell cycle progression thanks to its interaction with PCNA.

DDB2 increases radioresistance of NSCLC cells by enhancing DNA damage responses

Radiotherapy resistance is one of the major factors limiting the efficacy of radiotherapy in lung cancer patients. The extensive investigations indicate the diversity in the mechanisms underlying radioresistance. Here, we revealed that DNA damage binding protein 2 (DDB2) is a potential regulator in the radiosensitivity of non-small cell lung cancer (NSCLC) cells. DDB2, originally identified as a DNA damage recognition factor in the nucleotide excision repair, promotes the survival and inhibits the apoptosis of NSCLC cell lines upon ionizing radiation (IR). Mechanistic investigations demonstrated that DDB2 is able to facilitate IR-induced phosphorylation of Chk1, which plays a critical role in the cell cycle arrest and DNA repair in response to IR-induced DNA double-strand breaks (DSBs). Indeed, knockdown of DDB2 compromised the G2 arrest in the p53-proficient A549 cell line and reduced the efficiency of homologous recombination (HR) repair. Taken together, our data indicate that the expression of DDB2 in NSCLC could be used as a biomarker to predict radiosensitivity of the patients. Targeting Chk1 can be used to increase the efficacy of radiotherapy in patients of NSCLC possessing high levels of DDB2.

The emerging role of deubiquitination in nucleotide excision repair

Nucleotide excision repair (NER) protects genome stability by eliminating DNA helix distorting lesions, such as those induced by UV radiation. The addition and removal of ubiquitin, namely, ubiquitination and deubiquitination, have recently been demonstrated as general mechanisms to regulate protein functions. Accumulating evidence shows that several NER factors are subjected to extensive regulation by ubiquitination and deubiquitination. Thus, the balance between E3 ligases and deubiquitinating enzyme activities can dynamically alter the ubiquitin landscape at DNA damage sites, thereby regulating NER efficiency. Current knowledge about XPC ubiquitination by different ubiquitin E3 ligases highlights the importance of ubiquitin linkage types in regulating XPC binding and release from damaged DNA. Here, we discuss the emerging roles of deubiquitinating enzymes and their ubiquitin linkage specificities in NER.

Oxidative stress and age-related changes in T cells: is thalassemia a model of accelerated immune system aging?

Iron overload in β-thalassemia major occurs mainly due to blood transfusion, an essential treatment for β-thalassemia major patients, which results in oxidative stress. It has been thought that oxidative stress causes elevation of immune system senescent cells. Under this condition, cells normally enhance in aging, which is referred to as premature immunosenescence. Because there is no animal model for immunosenescence, most knowledge on the immunosenescence pattern is based on induction of immunosenescence. In this review, we describe iron overload and oxidative stress in β-thalassemia major patients and how they make these patients a suitable human model for immunosenescence. We also consider oxidative stress in some kinds of chronic virus infections, which induce changes in the immune system similar to β-thalassemia major. In conclusion, a therapeutic approach used to improve the immune system in such chronic virus diseases, may change the immunosenescence state and make life conditions better for β-thalassemia major patients.

Essential roles of FoxM1 in Ras-induced liver cancer progression and in cancer cells with stem cell features

BACKGROUND & AIMS

Overexpression of FoxM1 correlates with poor prognosis in hepatocellular carcinoma (HCC). Moreover, the Ras-signaling pathway is found to be ubiquitously activated in HCC through epigenetic silencing of the Ras-regulators. We investigated the roles of FoxM1 in Ras-driven HCC, and on HCC cells with stem-like features.

METHODS

We employed a transgenic mouse model that expresses the oncogenic Ras in the liver. That strain was crossed with a strain that harbor floxed alleles of FoxM1 and the MxCre gene that allows conditional deletion of FoxM1. FoxM1 alleles were deleted after development of HCC, and the effects on the tumors were analyzed. Also, FoxM1 siRNA was used in human HCC cell lines to determine its role in the survival of the HCC cells with stem cell features.

RESULTS

Ras-driven tumors overexpress FoxM1. Deletion of FoxM1 inhibits HCC progression. There was increased accumulation of reactive oxygen species (ROS) in the FoxM1 deleted HCC cells. Moreover, FoxM1 deletion caused a disproportionate loss of the CD44+ and EpCAM+ HCC cells in the tumors. We show that FoxM1 directly activates expression of CD44 in human HCC cells. Moreover, the human HCC cells with stem cell features are addicted to FoxM1 for ROS-regulation and survival.

CONCLUSION

Our results provide genetic evidence for an essential role of FoxM1 in the progression of Ras-driven HCC. In addition, FoxM1 is required for the expression of CD44 in HCC cells. Moreover, FoxM1 plays a critical role in the survival of the HCC cells with stem cell features by regulating ROS.

DDB2 modulates TGF-β signal transduction in human ovarian cancer cells by downregulating NEDD4L

The expression of DNA damage-binding protein 2 (DDB2) has been linked to the prognosis of ovarian cancer and its underlying transcription regulatory function was proposed to contribute to the favorable treatment outcome. By applying gene microarray analysis, we discovered neural precursor cell expressed, developmentally downregulated 4-Like (NEDD4L) as a previously unidentified downstream gene regulated by DDB2. Mechanistic investigation demonstrated that DDB2 can bind to the promoter region of NEDD4L and recruit enhancer of zeste homolog 2 histone methyltransferase to repress NEDD4L transcription by enhancing histone H3 lysine 27 trimethylation (H3K27me3) at the NEDD4L promoter. Given that NEDD4L plays an important role in constraining transforming growth factor β signaling by targeting activated Smad2/Smad3 for degradation, we investigated the role of DDB2 in the regulation of TGF-β signaling in ovarian cancer cells. Our data indicate that DDB2 enhances TGF-β signal transduction and increases the responsiveness of ovarian cancer cells to TGF-β-induced growth inhibition. The study has uncovered an unappreciated regulatory mode that hinges on the interaction between DDB2 and NEDD4L in human ovarian cancer cells. The novel mechanism proposes the DDB2-mediated fine-tuning of TGF-β signaling and its downstream effects that impinge upon tumor growth in ovarian cancers.

Differential Molecular Stress Responses to Low Compared to High Doses of Ionizing Radiation in Normal Human Fibroblasts

Understanding the mechanisms producing low dose ionizing radiation specific biological effects represents one of the major challenges of radiation biology. Although experimental evidence does suggest that various molecular stress response pathways may be involved in the production of low dose effects, much of the detail of those mechanisms remains elusive. We hypothesized that the regulation of various stress response pathways upon irradiation may differ from one another in complex dose-response manners, causing the specific and subtle low dose radiation effects. In the present study, the transcription level of 22 genes involved in stress responses were analyzed using RT-qPCR in normal human fibroblasts exposed to a range of gamma-doses from 1 to 200 cGy. Using the alkali comet assay, we also measured the level of DNA damages in dose-response and time-course experiments. We found non-linear dose responses for the repair of DNA damage after exposure to gamma-radiation. Alterations in gene expression were also not linear with dose for several of the genes examined and did not follow a single pattern. Rather, several patterns could be seen. Our results suggest a complex interplay of various stress response pathways triggered by low radiation doses, with various low dose thresholds for different genes.

Cellular senescence and protein degradation: breaking down cancer

Autophagy and the ubiquitin-proteasome pathway (UPP) are the major protein degradation systems in eukaryotic cells. Whereas the former mediate a bulk nonspecific degradation, the UPP allows a rapid degradation of specific proteins. Both systems have been shown to play a role in tumorigenesis, and the interest in developing therapeutic agents inhibiting protein degradation is steadily growing. However, emerging data point to a critical role for autophagy in cellular senescence, an established tumor suppressor mechanism. Recently, a selective protein degradation process mediated by the UPP was also shown to contribute to the senescence phenotype. This process is tightly regulated by E3 ubiquitin ligases, deubiquitinases, and several post-translational modifications of target proteins. Illustrating the complexity of UPP, more than 600 human genes have been shown to encode E3 ubiquitin ligases, a number which exceeds that of the protein kinases. Nevertheless, our knowledge of proteasome-dependent protein degradation as a regulated process in cellular contexts such as cancer and senescence remains very limited. Here we discuss the implications of protein degradation in senescence and attempt to relate this function to the protein degradation pattern observed in cancer cells.

Protein kinase D1 is essential for Ras-induced senescence and tumor suppression by regulating senescence-associated inflammation

Oncogene-induced senescence (OIS) is an initial barrier to tumor development. Reactive oxygen species (ROS) is critical for oncogenic Ras OIS, but the downstream effectors to mediate ROS signaling are still relatively elusive. Senescent cells develop a senescence-associated secretory phenotype (SASP). However, the mechanisms underlying the regulation of the SASP are largely unknown. Here, we identify protein kinase D1 (PKD1) as a downstream effector of ROS signaling to mediate Ras OIS and SASP. PKD1 is activated by oncogenic Ras expression and PKD1 promotes Ras OIS by mediating inflammatory cytokines interleukin-6 (IL-6) and interleukin-8 (IL-8) via modulation of NF-κB activity. We demonstrate that ROS-protein kinase Cδ (PKCδ)-PKD1 axis is essential for the establishment and maintenance of IL-6/IL8 induction. In addition, ablation of PKD1 causes the bypass of Ras OIS, and promotes cell transformation and tumorigenesis. Together, these findings uncover a previously unidentified role of ROS-PKCδ-PKD1 pathway in Ras OIS and SASP regulation.

Chromatin retention of DNA damage sensors DDB2 and XPC through loss of p97 segregase causes genotoxicity

DNA damage recognition subunits like DDB2 and XPC protect the human skin from ultraviolet (UV) light-induced genome instability and cancer, as demonstrated by the devastating inherited syndrome xeroderma pigmentosum. Here, we show that the beneficial DNA repair response triggered by these two genome caretakers critically depends on a dynamic spatiotemporal regulation of their homeostasis. The prolonged retention of DDB2 and XPC in chromatin, due to a failure to readily remove both recognition subunits by the ubiquitin-dependent p97/VCP/Cdc48 segregase complex, leads to impaired DNA excision repair of UV lesions. Surprisingly, the ensuing chromosomal aberrations in p97-deficient cells are alleviated by a concomitant down regulation of DDB2 or XPC. Also, genome instability resulting from an excess of DDB2 persisting in UV-irradiated cells is prevented by concurrent p97 over-expression. Our findings demonstrate that DNA damage sensors and repair initiators acquire unexpected genotoxic properties if not controlled by timely extraction from chromatin.

Interferon-γ induces senescence in normal human melanocytes

BACKGROUND

Interferon-γ (IFN-γ) plays an important role in the proceedings of vitiligo through recruiting lymphocytes to the lesional skin. However, the potential effects of IFN-γ on skin melanocytes and the subsequent contribution to the vitiligo pathogenesis are still unclear.

OBJECTIVE

To investigate the effects of IFN-γ on viability and cellular functions of melanocytes.

METHODS

Primary human melanocytes were treated with IFN-γ. Cell viability, apoptosis, cell cycle melanin content and intracellular reactive oxygen species (ROS) level were measured. mRNA expression was examined by real-time PCR. The release of interleukin 6 (IL-6) and heat shock protein 70 (HSP-70) was monitored by ELISA. β-galactosidase staining was utilized to evaluate melanocyte senescence.

RESULTS

Persistent IFN-γ treatment induced viability loss, apoptosis, cell cycle arrest and senescence in melanocytes. Melanocyte senescence was characterized as the changes in pigmentation and morphology, as well as the increase of β-galactosidase activity. Increase of p21Cip1/Waf1 protein was evident in melanocytes after IFN-γ treatment. IFN-γ induction of senescence was attenuated by siRNAs against p21, Janus kinase 2 (JAK2) or signal transducer and activator of transcription 1 (STAT1), but not by JAK1 siRNA nor by p53 inhibitor pifithrin-α. IFN-γ treatment increased the accumulation of intracellular ROS in melanocytes, while ROS scavenger N-acetyl cysteine (NAC) effectively inhibited IFN-γ induced p21 expression and melanocyte senescence. IL-6 and HSP-70 release was significantly induced by IFN-γ treatment, which was largely inhibited by NAC. The increase of IL-6 and HSP-70 release could also be observed in senescent melanocytes.

CONCLUSION

IFN-γ can induce senescence in melanocytes and consequently enhance their immuno-competency, leading to a vitiligo-prone milieu.

DDB2 suppresses tumorigenicity by limiting the cancer stem cell population in ovarian cancer

Ovarian cancer is an extremely aggressive disease associated with a high percentage of tumor recurrence and chemotherapy resistance. Understanding the underlying mechanism of tumor relapse is crucial for effective therapy of ovarian cancer. DNA damage-binding protein 2 (DDB2) is a DNA repair factor mainly involved in nucleotide excision repair. Here, a novel role was identified for DDB2 in the tumorigenesis of ovarian cancer cells and the prognosis of patients with ovarian cancer. Overexpressing DDB2 in human ovarian cancer cells suppressed its capability to recapitulate tumors in athymic nude mice. Mechanistic investigation demonstrated that DDB2 is able to reduce the cancer stem cell (CSC) population characterized with high aldehyde dehydrogenase activity in ovarian cancer cells, probably through disrupting the self-renewal capacity of CSCs. Low DDB2 expression correlates with poor outcomes among patients with ovarian cancer, as revealed from the analysis of publicly available gene expression array datasets. Given the finding that DDB2 protein expression is low in ovarian tumor cells, enhancement of DDB2 expression is a promising strategy to eradicate CSCs and would help to halt ovarian cancer relapse.

IMPLICATIONS

DDB2 status has prognostic potential, and elevating its expression eradicates CSCs and could reduce ovarian cancer relapse.

DNA damage-binding complex recruits HDAC1 to repress Bcl-2 transcription in human ovarian cancer cells

Elevated expression of the antiapoptotic factor Bcl-2 is believed to be one of the contributing factors to an increased relapse rate associated with multiple cisplatin-resistant cancers. DNA damage-binding protein complex subunit 2 (DDB2) has recently been revealed to play an important role in sensitizing human ovarian cancer cells to cisplatin-induced apoptosis through the downregulation of Bcl-2, but the underlying molecular mechanism remains poorly defined. Here, it is report that DDB2 functions as a transcriptional repressor for Bcl-2 in combination with DDB1. Quantitative ChIP and EMSA analysis revealed that DDB2 binds to a specific cis-acting element at the 5'-end of Bcl-2 P1 promoter. Overexpression of DDB2 resulted in marked losses of histone H3K9,14 acetylation along the Bcl-2 promoter and enhancer regions, concomitant with a local enrichment of HDAC1 to the Bcl-2 P1 core promoter in ovarian cancer cells. Coimmunoprecipitation and in vitro binding analyses identified a physical interaction between DDB1 and HDAC1, whereas downregulation of HDAC1 significantly enhanced Bcl-2 promoter activity. Finally, in comparison with wild-type DDB2, mutated DDB2, which is unable to repress Bcl-2 transcription, mediates a compromised apoptosis upon cisplatin treatment. Taken together, these data support a model wherein DDB1 and DDB2 cooperate to repress Bcl-2 transcription. DDB2 recognizes and binds to the Bcl-2 P1 promoter, and HDAC1 is recruited through the DDB1 subunit associated with DDB2 to deacetylate histone H3K9,14 across Bcl-2 regulatory regions, resulting in suppressed Bcl-2 transcription.

IMPLICATIONS

Increasing the expression of DDB complex may provide a molecular strategy for cancer therapy.

DDB2 suppresses epithelial-to-mesenchymal transition in colon cancer

Colon cancer is one of the deadliest cancers worldwide because of its metastasis to other essential organs. Metastasis of colon cancer involves a complex set of events, including epithelial-to-mesenchymal transition (EMT) that increases invasiveness of the tumor cells. Here, we show that the xeroderma pigmentosum group E (XPE) gene product, damaged DNA-binding protein (DDB)-2, is downregulated in high-grade colon cancers, and it plays a dominant role in the suppression of EMT of the colon cancer cells. Depletion of DDB2 promotes mesenchymal phenotype, whereas expression of DDB2 promotes epithelial phenotype. DDB2 constitutively represses genes that are the key activators of EMT, indicating that DDB2 is a master regulator of EMT of the colon cancer cells. Moreover, we observed evidence that DDB2 functions as a barrier for EMT induced by hypoxia and TGF-β. Also, we provide evidence that DDB2 inhibits metastasis of colon cancer. The results presented here identify a transcriptional regulatory pathway of DDB2 that is directly linked to the mechanisms that suppress metastasis of colon cancer.

A novel cis-acting element from the 3'UTR of DNA damage-binding protein 2 mRNA links transcriptional and post-transcriptional regulation of gene expression

The DNA damage-binding protein 2 (DDB2) is an adapter protein that can direct a modular Cul4-DDB1-RING E3 Ligase complex to sites of ultraviolet light-induced DNA damage to ubiquitinate substrates during nucleotide excision repair. The DDB2 transcript is ultraviolet-inducible; therefore, its regulation is likely important for its function. Curiously, the DDB2 mRNA is reportedly short-lived, but the transcript does not contain any previously characterized cis-acting determinants of mRNA stability in its 3' untranslated region (3'UTR). Here, we used a tetracycline regulated d2EGFP reporter construct containing specific 3'UTR sequences from DDB2 to identify novel cis-acting elements that regulate mRNA stability. Synthetic 3'UTRs corresponding to sequences as short as 25 nucleotides from the central region of the 3'UTR of DDB2 were sufficient to accelerate decay of the heterologous reporter mRNA. Conversely, these same 3'UTRs led to more rapid induction of the reporter mRNA, export of the message to the cytoplasm and the subsequent accumulation of the encoded reporter protein, indicating that this newly identified cis-acting element affects transcriptional and post-transciptional processes. These results provide clear evidence that nuclear and cytoplasmic processing of the DDB2 mRNA is inextricably linked.

The yin-yang of DNA damage response: roles in tumorigenesis and cellular senescence

Senescent cells are relatively stable, lacking proliferation capacity yet retaining metabolic activity. In contrast, cancer cells are rather invasive and devastating, with uncontrolled proliferative capacity and resistance to cell death signals. Although tumorigenesis and cellular senescence are seemingly opposite pathological events, they are actually driven by a unified mechanism: DNA damage. Integrity of the DNA damage response (DDR) network can impose a tumorigenesis barrier by navigating abnormal cells to cellular senescence. Compromise of DDR, possibly due to the inactivation of DDR components, may prevent cellular senescence but at the expense of tumor formation. Here we provide an overview of the fundamental role of DDR in tumorigenesis and cellular senescence, under the light of the Yin-Yang concept of Chinese philosophy. Emphasis is placed on discussing DDR outcome in the light of in vivo models. This information is critical as it can help make better decisions for clinical treatments of cancer patients.

Tumor regression by phenethyl isothiocyanate involves DDB2

Phenethyl isothiocyanate (PEITC) is a promising cancer chemopreventive agent commonly found in edible cruciferous vegetables. It has been implicated also for therapy, and is in clinical trial for lung cancer. Here, we provide evidence that the tumor suppressive effect of PEITC is related to its ability to induce expression of damaged DNA binding protein 2 (DDB2), a DNA repair protein involved also in apoptosis and premature senescence. DDB2 expression is attenuated in a wide variety of cancers including the aggressive colon cancers. We show that, in colon cancer cells, reactive oxygen species, which are induced by PEITC, augment expression of DDB2 through the p38MAPK/JNK pathway, independently of p53. PEITC-induced expression of DDB2 is critical for inhibition of tumor progression by PEITC. Tumors derived from DDB2-deficient colon cancer cells are refractory to PEITC-treatments, resulting from deficiencies in apoptosis and senescence. The DDB2-proficient tumors, on the other hand, respond effectively to PEITC. The results show that PEITC can be used to induce expression of DDB2, and that expression of DDB2 is critical for effective response of tumors to PEITC.

Damaged DNA binding protein 2 in reactive oxygen species (ROS) regulation and premature senescence

Premature senescence induced by DNA damage or oncogene is a critical mechanism of tumor suppression. Reactive oxygen species (ROS) have been implicated in the induction of premature senescence response. Several pathological disorders such as cancer, aging and age related neurological abnormalities have been linked to ROS deregulation. Here, we discuss how Damaged DNA binding Protein-2 (DDB2), a nucleotide excision repair protein, plays an important role in ROS regulation by epigenetically repressing the antioxidant genes MnSOD and Catalase. We further revisit a model in which DDB2 plays an instrumental role in DNA damage induced ROS accumulation, ROS induced premature senescence and inhibition of skin tumorigenesis.

Identification of blood biomarkers of aging by transcript profiling of whole blood

Immunological changes that inevitably occur with aging are related to the onset of various diseases including autoimmune diseases, immunodeficiency, as well as other age-reflecting (AR) diseases. They are becoming serious problems in the global trend of longevity. To understand the AR changes, we searched for genes whose expression profiles in the whole peripheral blood change dramatically as a function of age using the Agilent whole human genome 44K microarray. After examining two cohorts consisting of 154 healthy people between age 23 and 77, we discovered 16 transcripts strongly and reproducibly correlated with age. Analysis using a publicly available gene expression dataset for a variety of human immune cells revealed that some of these transcripts were highly expressed in specific cell types whose number and function are known to change with age. This analysis shed light on the molecular mechanism of AR immunological system changes. Because of its simplicity, the assay system is expected to be useful for understanding individual health conditions.

p21 cooperates with DDB2 protein in suppression of ultraviolet ray-induced skin malignancies

Exposure to ultraviolet rays (UV) in sunlight is the main cause of skin cancer. Here, we show that the p53-induced genes DDB2 and p21 are down-regulated in skin cancer, and in the mouse model they functionally cooperate to prevent UV-induced skin cancer. Our previous studies demonstrated an antagonistic role of DDB2 and p21 in nucleotide excision repair and apoptosis. Surprisingly, we find that the loss of p21 restores nucleotide excision repair and apoptosis in Ddb2(-/-) mice, but it does not protect from UV-mediated skin carcinogenesis. In contrast, Ddb2(-/-)p21(-/-) mice are significantly more susceptible to UV-induced skin cancer than the Ddb2(-/-) or the p21(-/-) mice. We provide evidence that p21 deletion in the Ddb2(-/-) background causes a strong increase in cell proliferation. The increased proliferation in the Ddb2(-/-)p21(-/-) background is related to a severe deficiency in UV-induced premature senescence. Also, the oncogenic pro-proliferation transcription factor FOXM1 is overexpressed in the p21(-/-) background. Our results show that the anti-proliferative and the pro-senescence pathways of DDB2 and p21 are critical protection mechanisms against skin malignancies.

Never-ageing cellular senescence

Cellular senescence was historically discovered as a form of cellular ageing of in vitro cultured cells. It has been under the spotlight following the evidence of oncogene-induced senescence in vivo and its role as a potent tumour suppressor mechanism. Presently, a PubMed search using keywords ‘cellular senescence and cancer’ reveals 8398 number of references (by April 2011) showing that while our knowledge of senescence keeps expanding, the complexity of the phenomenon keeps us – researchers in the field of cancer biology – fascinated and busy. In this short review, we summarise the many cellular pathways leading to cellular senescence and we discuss the latest experimental evidence and the questions emerging in the field.