Transcriptomal profiling of the cellular response to DNA damage mediated by Slug (Snai2)

Single-cell and bulk transcriptional profiling of mouse ovaries reveals novel genes and pathways associated with DNA damage response in oocytes

Immature oocytes enclosed in primordial follicles stored in female ovaries are under constant threat of DNA damage induced by endogenous and exogenous factors. Checkpoint kinase 2 (CHEK2) is a key mediator of the DNA damage response (DDR) in all cells. Genetic studies have shown that CHEK2 and its downstream targets, p53, and TAp63, regulate primordial follicle elimination in response to DNA damage. However, the mechanism leading to their demise is still poorly characterized. Single-cell and bulk RNA sequencing were used to determine the DDR in wild-type and Chek2-deficient ovaries. A low but oocyte-lethal dose of ionizing radiation induces ovarian DDR that is solely dependent on CHEK2. DNA damage activates multiple response pathways related to apoptosis, p53, interferon signaling, inflammation, cell adhesion, and intercellular communication. These pathways are differentially employed by different ovarian cell types, with oocytes disproportionately affected by radiation. Novel genes and pathways are induced by radiation specifically in oocytes, shedding light on their sensitivity to DNA damage, and implicating a coordinated response between oocytes and pregranulosa cells within the follicle. These findings provide a foundation for future studies on the specific mechanisms regulating oocyte survival in the context of aging, therapeutic and environmental genotoxic exposures.

A Novel Interaction of Slug (SNAI2) and Nuclear Actin

Actin is a protein of central importance to many cellular functions. Its localization and activity are regulated by interactions with a high number of actin-binding proteins. In a yeast two-hybrid (Y2H) screening system, snail family transcriptional repressor 2 (SNAI2 or slug) was identified as a yet unknown potential actin-binding protein. We validated this interaction using immunoprecipitation and analyzed the functional relation between slug and actin. Since both proteins have been reported to be involved in DNA double-strand break (DSB) repair, we focused on their interaction during this process after treatment with doxorubicin or UV irradiation. Confocal microscopy elicits that the overexpression of actin fused to an NLS stabilizes complexes of slug and γH2AX, an early marker of DNA damage repair.

Single-cell and bulk transcriptional profiling of mouse ovaries reveals novel genes and pathways associated with DNA damage response in oocytes

Immature oocytes enclosed in primordial follicles stored in female ovaries are under constant threat of DNA damage induced by endogenous and exogenous factors. Checkpoint kinase 2 (CHEK2) is a key mediator of the DNA damage response in all cells. Genetic studies have shown that CHEK2 and its downstream targets, p53 and TAp63, regulate primordial follicle elimination in response to DNA damage, however the mechanism leading to their demise is still poorly characterized. Single-cell and bulk RNA sequencing were used to determine the DNA damage response in wildtype and Chek2-deficient ovaries. A low but oocyte-lethal dose of ionizing radiation induces a DNA damage response in ovarian cells that is solely dependent on CHEK2. DNA damage activates multiple ovarian response pathways related to apoptosis, p53, interferon signaling, inflammation, cell adhesion, and intercellular communication. These pathways are differentially employed by different ovarian cell types, with oocytes disproportionately affected by radiation. Novel genes and pathways are induced by radiation specifically in oocytes, shedding light on their sensitivity to DNA damage, and implicating a coordinated response between oocytes and pre-granulosa cells within the follicle. These findings provide a foundation for future studies on the specific mechanisms regulating oocyte survival in the context of aging, as well as therapeutic and environmental genotoxic exposures.

Mutual regulation of TGFβ-induced oncogenic EMT, cell cycle progression and the DDR

TGFβ signaling and the DNA damage response (DDR) are two cellular toolboxes with a strong impact on cancer biology. While TGFβ as a pleiotropic cytokine affects essentially all hallmarks of cancer, the multifunctional DDR mostly orchestrates cell cycle progression, DNA repair, chromatin remodeling and cell death. One oncogenic effect of TGFβ is the partial activation of epithelial-to-mesenchymal transition (EMT), conferring invasiveness, cellular plasticity and resistance to various noxae. Several reports show that both individual networks as well as their interface affect chemo-/radiotherapies. However, the underlying mechanisms remain poorly resolved. EMT often correlates with TGFβ-induced slowing of proliferation, yet numerous studies demonstrate that particularly the co-activated EMT transcription factors counteract anti-proliferative signaling in a partially non-redundant manner. Collectively, evidence piled up over decades underscore a multifaceted, reciprocal inter-connection of TGFβ signaling / EMT with the DDR / cell cycle progression, which we will discuss here. Altogether, we conclude that full cell cycle arrest is barely compatible with the propagation of oncogenic EMT traits and further propose that 'EMT-linked DDR plasticity' is a crucial, yet intricate facet of malignancy, decisively affecting metastasis formation and therapy resistance.

Translational Implications for Radiosensitizing Strategies in Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood and adolescence that includes FP-RMS, harboring the fusion oncoprotein PAX3/7-FOXO1 and FN-RMS, often mutant in the RAS pathway. Risk stratifications of RMS patients determine different prognostic groups and related therapeutic treatment. Current multimodal therapeutic strategies involve surgery, chemotherapy (CHT) and radiotherapy (RT), but despite the deeper knowledge of response mechanisms underpinning CHT treatment and the technological improvements that characterize RT, local failures and recurrence frequently occur. This review sums up the RMS classification and the management of RMS patients, with special attention to RT treatment and possible radiosensitizing strategies for RMS tumors. Indeed, RMS radioresistance is a clinical problem and further studies aimed at dissecting radioresistant molecular mechanisms are needed to identify specific targets to hit, thus improving RT-induced cytotoxicity.

Slug inhibition increases radiosensitivity of nasopharyngeal carcinoma cell line C666-1

Slug is associated with the radioresistance of nasopharyngeal carcinoma (NPC) and the main current approach of treatment for NPC is radiotherapy. Hence, the aim of the current study was to determine the effect of Slug silencing on the radiosensitivity of NPC cells. Lentiviral-mediated transfection of Slug RNA interference (RNAi) in NPC cell line C666-1 was performed in vitro. Following Slug inhibition, its expression was detected using western blotting. A clonogenic survival assay and flow cytometry were then performed to evaluate the clonogenic cell survival, cell cycle distribution and apoptosis of C666-1 cells following irradiation. The results indicated that Slug RNAi decreased cell proliferation, and increased cell apoptosis and G₀/G₁ arrest. Thus, lentiviral-mediated transfection of Slug RNAi enhanced the radiosensitivity of the NPC cell line C666-1, and Slug may therefore be a potential target to improve radiotherapy in treatment of NPC and reduce the radioresistance of NPC.

Slug inhibition increases radiosensitivity of oral squamous cell carcinoma cells by upregulating PUMA

As a new strategy, radio-gene therapy was widely used for the treatment of cancer patients in recent few years. Slug was involved in the radioresistance of various cancers and has been found to have an anti-apoptotic effect. This study aims to investigate whether the modulation of Slug expression by siRNA affects oral squamous cell carcinoma sensitivity to X-ray irradiation through upregulating PUMA. Two oral squamous cell carcinoma cell lines (HSC3 and HSC6) were transfected with small interfering RNA (siRNA) targeting Slug and subjected to radiotherapy in vitro. After transfection with Slug siRNA, both HSC3 and HSC6 cells showed relatively lower expression of Slug and higher expression of PUMA. The Slug siRNA transfected cells showed decreased survival and proliferation rates, an increased apoptosis rate and enhanced radiosensitivity to X-ray irradiation. Our results revealed that Slug siRNA transfection in combination with radiation increased the expression of PUMA, which contributed to radiosensitivity of oral squamous cell carcinoma cells. Thus, controlling the expression of Slug might contribute to enhance sensitivity of HSC3 and HSC6 cells toward X-ray irradiation in vitro by upregulating PUMA.

MicroRNA-203 inhibits cellular proliferation and invasion by targeting Bmi1 in non-small cell lung cancer

MicroRNAs are proposed to serve vital functions in the regulation of tumor progression and invasion. However, the expression levels of miR-203 in non-small cell lung cancer (NSCLC) and its clinical significance remain unknown. In the present study, the association between B-cell-specific moloney murine leukemia virus insertion site 1 (Bmi1) and miR-203 was investigated. miR-203 was demonstrated to act as a tumor suppressor by regulating the expression of Bmi1. miR-203 expression levels were downregulated in NSCLC tissues while Bmi1 expression was upregulated in NSCLC tissues and cell lines. Furthermore, downregulated Bmi1 or enhanced miR-203 expression inhibited NSCLC cell proliferation and invasion in vitro. In addition, a dual-luciferase reporter assay was performed, which identified Bmi1 as a novel target of miR-203. In conclusion, the present study demonstrated that miR-203 functions as a tumor suppressor and is important in inhibiting the proliferation of NSCLC cells through targeting Bmi1. These findings indicate that miR-203 may be useful as a novel potential therapeutic target for NSCLC.

Aberrant expression of osteopontin and E-cadherin indicates radiation resistance and poor prognosis for patients with cervical carcinoma

Radiotherapy is the first-line treatment for all stages of cervical cancer, whether it is used for radical or palliative therapy. However, radioresistance of cervical cancer remains a major therapeutic problem. Consequently, we explored if E-cadherin (a marker of epithelial-mesenchymal transition) and osteopontin could predict radioresistance in patients with locally advanced cervical squamous cell carcinoma (LACSCC). Patients were retrospectively reviewed and 111 patients divided into two groups (radiation-resistant and radiation-sensitive groups) according to progression-free survival (PFS). In pretreated paraffin-embedded tissues, we evaluated E-cadherin and osteopontin expression using immunohistochemical staining. The percentage of patients with high osteopontin but low E-cadherin expression in the radiation-resistant group was significantly higher than those in the radiation-sensitive group (p<0.001). These patients also had a lower 5-year PFS rate (p<0.001). Our research suggests that high osteopontin but low E-cadherin expression can be considered as a negative, independent prognostic factor in patients with LACSCC ([Hazard ratios (95% CI) 6.766 (2.940, 15.572)], p<0.001).

Epigenetic reprogramming modulates malignant properties of human liver cancer

Reversal of DNA hypermethylation and associated gene silencing is an emerging cancer therapy approach. Here we addressed the impact of epigenetic alterations and cellular context on functional and transcriptional reprogramming of hepatocellular carcinoma (HCC) cells. Our strategy employed a 3-day treatment of established and primary human HCC-derived cell lines grown as a monolayer at various cell densities with the DNMT1 inhibitor zebularine (ZEB) followed by a 3D culture to identify cells endowed with self-renewal potential. Differences in self-renewal, gene expression, tumorigenicity, and metastatic potential of spheres at generations G1-G5 were examined. Transient ZEB exposure produced differential cell density-dependent responses. In cells grown at low density, ZEB caused a remarkable increase in self-renewal and tumorigenicity associated with long-lasting gene expression changes characterized by a stable overexpression of cancer stem cell-related and key epithelial-mesenchymal transition genes. These effects persisted after restoration of DNMT1 expression. In contrast, at high cell density, ZEB caused a gradual decrease in self-renewal and tumorigenicty, and up-regulation of apoptosis- and differentiation-related genes. A permanent reduction of DNMT1 protein using short hairpin RNA (shRNA)-mediated DNMT1 silencing rendered HCC cells insensitive both to cell density and ZEB effects. Similarly, WRL68 and HepG2 hepatoblastoma cells expressing low DNMT1 basal levels also possessed a high self-renewal, irrespective of cell density or ZEB exposure. Spheres formed by low-density cells treated with ZEB or shDNMT1 displayed a high molecular similarity which was sustained through consecutive generations, confirming the essential role of DNMT1 depletion in the enhancement of cancer stem cell properties.

CONCLUSION

These results identify DNA methylation as a key epigenetic regulatory mechanism determining the pool of cancer stem cells in liver cancer and possibly other solid tumors.

Slug/β-catenin-dependent proinflammatory phenotype in hypoxic breast cancer stem cells

Cancer stem cell survival relies on the activation of inflammatory pathways, which is speculatively triggered by cell autonomous mechanisms or by microenvironmental stimuli. Here, we observed that hypoxic bone marrow stroma-derived transforming growth factor-β 1 promotes the growth of human breast cancer stem cells as mammospheres. The ensuing Slug-dependent serine 139 phosphorylation of the DNA damage sensor H2AX in breast cancer stem cells induces tumor necrosis factor-α and IL-8 mRNAs, whose stability is enhanced by cytoplasmic β-catenin. β-Catenin also up-regulates and binds miR-221, reducing the stability of the miR-221 targets Rad51 and ERα mRNAs. Our data show that the Slug/β-catenin-dependent activation of DNA damage signaling triggered by the hypoxic microenvironment sustains the proinflammatory phenotype of breast cancer stem cells.

Signaling between transforming growth factor β (TGF-β) and transcription factor SNAI2 represses expression of microRNA miR-203 to promote epithelial-mesenchymal transition and tumor metastasis

TGF-β promotes tumor invasion and metastasis by inducing an epithelial-mesenchymal transition (EMT). Understanding the molecular and epigenetic mechanisms by which TGF-β induces EMT may facilitate the development of new therapeutic strategies for metastasis. Here, we report that TGF-β induced SNAI2 to promote EMT by repressing miR-203. Although miR-203 targeted SNAI2, SNAI2 induced by TGF-β could directly bind to the miR-203 promoter to inhibit its transcription. SNAI2 and miR-203 formed a double negative feedback loop to inhibit each other's expression, thereby controlling EMT. Moreover, we found that miR-203 was significantly down-regulated in highly metastatic breast cancer cells. The restoration of miR-203 in highly metastatic breast cancer cells inhibited tumor cell invasion in vitro and lung metastatic colonization in vivo by repressing SNAI2. Taken together, our results suggest that the SNAI2 and miR-203 regulatory loop plays important roles in EMT and tumor metastasis.

Convergence of oncogenic and hormone receptor pathways promotes metastatic phenotypes

Cyclin D1b is a splice variant of the cell cycle regulator cyclin D1 and is known to harbor divergent and highly oncogenic functions in human cancer. While cyclin D1b is induced during disease progression in many cancer types, the mechanisms underlying cyclin D1b function remain poorly understood. Herein, cell and human tumor xenograft models of prostate cancer were utilized to resolve the downstream pathways that are required for the protumorigenic functions of cyclin D1b. Specifically, cyclin D1b was found to modulate the expression of a large transcriptional network that cooperates with androgen receptor (AR) signaling to enhance tumor cell growth and invasive potential. Notably, cyclin D1b promoted AR-dependent activation of genes associated with metastatic phenotypes. Further exploration determined that transcriptional induction of SNAI2 (Slug) was essential for cyclin D1b-mediated proliferative and invasive properties, implicating Slug as a critical driver of disease progression. Importantly, cyclin D1b expression highly correlated with that of Slug in clinical samples of advanced disease. In vivo analyses provided strong evidence that Slug enhances both tumor growth and metastatic phenotypes. Collectively, these findings reveal the underpinning mechanisms behind the protumorigenic functions of cyclin D1b and demonstrate that the convergence of the cyclin D1b/AR and Slug pathways results in the activation of processes critical for the promotion of lethal tumor phenotypes.

SLUG silencing increases radiosensitivity of melanoma cells in vitro

BACKGROUND

Melanoma radioresistance has been attributed to the presence of tumor cells with highly efficient DNA damage repair mechanisms. We examined the expression of genes involved in DNA damage repair and DNA damage sensing, and assessed their modulation by SLUG silencing, which is potentially capable of increasing radiosensitivity.

METHODS

Two melanoma cell lines (M14 and M79) were used to evaluate in vitro radiation-induced cytotoxicity before and after SLUG silencing. mRNA expression levels of BRCA1, ERCC1, DNA-PK, PARP, MGMT, ATM and TGM2 were determined by real-time RT-PCR, and protein expression levels of SLUG, caspase 3, p21, PUMA and pMAPK by Western blotting.

RESULTS

The cytotoxic effect of radiation was high in M14 and low in M79 cells. SLUG silencing increased the interference of radiation on cell cycle distribution and cell killing by 60 % and 80 % in M79 cells after a 2.4 Gy and 5 Gy radiation dose, respectively. It also led to a significant inhibition of expression of genes involved in DNA damage repair and DNA damage sensing in all cell lines maintained after radiation. An almost total inhibition was observed for TGM2, which is expressed at a high basal level in the most radioresistant cell line (M79). Protein expression of PUMA was induced by radiation and was enhanced after SLUG silencing.

CONCLUSIONS

Our results reveal a pivotal role of SLUG in regulating a cellular network involved in the response to DNA damage, and highlight the importance of TGM2 in radiosensitivity modulation. SLUG silencing appears to increase radiation sensitivity of the melanoma cells tested.

EMT in breast cancer stem cell generation

The concept of cancer stem cells (CSCs) has been proposed to explain the ability of single disseminated cancer cells to reconstitute tumours with heterogeneity similar to that of the primary tumour they arise from. Although this concept is now commonly accepted, the origin of these CSCs remains a source of debate. First proposed to arise through stem/progenitor cell transformation, CSCs might also or alternatively arise from differentiated cancer cells through epithelial to mesenchymal transition (EMT), an embryonic transdifferentiation process. Using breast carcinomas as a study model, I propose revisiting the role of EMT in generating CSCs and the debate on potential underlying mechanisms and biological significance.

Failsafe program escape and EMT: a deleterious partnership

The epithelial to mesenchymal transition (EMT) is a latent embryonic process which can be aberrantly reactivated during tumor progression. It is generally viewed as one of the main forces driving metastatic dissemination, by providing cells with invasive and motility capabilities. The aberrant reactivation of embryonic EMT inducers has now been additionally linked to escape from senescence and apoptosis, which suggests a role in tumor initiation. This oncogenic potential relies on the ability of EMT inducers to neutralize both the RB and p53 oncosuppressive pathways. RB and p53 have recently been described as key factors in the maintenance of epithelial morphology, which suggests an unexpected and intimate crosstalk between EMT and the corresponding safety programs. In this review, we attempt to understand how these two cell processes are interlinked and might facilitate cell transformation and tumor initiation.

Sulfur mustard toxicity: history, chemistry, pharmacokinetics, and pharmacodynamics

Sulfur mustard (SM) and similar bifunctional agents have been used as chemical weapons for almost 100 years. Victims of high-dose exposure, both combatants and civilians, may die within hours or weeks, but low-dose exposure causes both acute injury to the eyes, skin, respiratory tract and other parts of the body, and chronic sequelae in these organs are often debilitating and have a serious impact on quality of life. Ever since they were first used in warfare in 1917, SM and other mustard agents have been the subjects of intensive research, and their chemistry, pharmacokinetics and mechanisms of toxic action are now fairly well understood. In the present article we review this knowledge and relate the molecular-biological basis of SM toxicity, as far as it has been elucidated, to the pathological effects on exposure victims.

Integrated proteomics and genomics analysis reveals a novel mesenchymal to epithelial reverting transition in leiomyosarcoma through regulation of slug

Leiomyosarcoma is one of the most common mesenchymal tumors. Proteomics profiling analysis by reverse-phase protein lysate array surprisingly revealed that expression of the epithelial marker E-cadherin (encoded by CDH1) was significantly elevated in a subset of leiomyosarcomas. In contrast, E-cadherin was rarely expressed in the gastrointestinal stromal tumors, another major mesenchymal tumor type. We further sought to 1) validate this finding, 2) determine whether there is a mesenchymal to epithelial reverting transition (MErT) in leiomyosarcoma, and if so 3) elucidate the regulatory mechanism responsible for this MErT. Our data showed that the epithelial cell markers E-cadherin, epithelial membrane antigen, cytokeratin AE1/AE3, and pan-cytokeratin were often detected immunohistochemically in leiomyosarcoma tumor cells on tissue microarray. Interestingly, the E-cadherin protein expression was correlated with better survival in leiomyosarcoma patients. Whole genome microarray was used for transcriptomics analysis, and the epithelial gene expression signature was also associated with better survival. Bioinformatics analysis of transcriptome data showed an inverse correlation between E-cadherin and E-cadherin repressor Slug (SNAI2) expression in leiomyosarcoma, and this inverse correlation was validated on tissue microarray by immunohistochemical staining of E-cadherin and Slug. Knockdown of Slug expression in SK-LMS-1 leiomyosarcoma cells by siRNA significantly increased E-cadherin; decreased the mesenchymal markers vimentin and N-cadherin (encoded by CDH2); and significantly decreased cell proliferation, invasion, and migration. An increase in Slug expression by pCMV6-XL5-Slug transfection decreased E-cadherin and increased vimentin and N-cadherin. Thus, MErT, which is mediated through regulation of Slug, is a clinically significant phenotype in leiomyosarcoma.

Slug/SNAI2 regulates cell proliferation and invasiveness of metastatic prostate cancer cell lines

Many metastatic cancers recapitulate the epithelial-to-mesenchymal transition (EMT) resulting in enhanced cell motility and invasiveness. The EMT is regulated by several transcription factors, including the zinc finger protein SNAI2, also named Slug, which appears to exert additional functions during development and cancer progression. We have studied the function of SNAI2 in prostate cancer cells. Quantitative RT-PCR analysis showed strong SNAI2 expression particularly in the PC-3 and PC3-16 prostate carcinoma cell lines. Knockdown of SNAI2 by specific siRNA induced changes in EMT markers and inhibited invasion of both cell lines into a matrigel matrix. SNAI2 siRNA-treated cells did not tolerate detachment from the culture plates, likely at least in part due to downregulation of integrin alpha6beta4. SNAI2 knockdown disturbed the microtubular and actin cytoskeletons, especially severely in PC-3 cells, resulting in grossly enlarged, flattened, and sometimes multinuclear cells. Knockdown also decreased cell proliferation, with a prominent G0/G1 arrest in PC3-16. Together, our data imply that SNAI2 exerts strong effects on the cytoskeleton and adhesion of those prostate cancer cells that express it and is necessary for their proliferation and invasiveness.

Snail and slug mediate radioresistance and chemoresistance by antagonizing p53-mediated apoptosis and acquiring a stem-like phenotype in ovarian cancer cells

The transcriptional repressors Snail and Slug contribute to cancer progression by mediating epithelial-mesenchymal transition (EMT), which results in tumor cell invasion and metastases. We extend this current understanding to demonstrate their involvement in the development of resistance to radiation and paclitaxel. The process is orchestrated through the acquisition of a novel subset of gene targets that is repressed under conditions of stress, effectively inactivating p53-mediated apoptosis, while another subset of targets continues to mediate EMT. Repressive activities are complemented by a concurrent derepression of specific genes resulting in the acquisition of stem cell-like characteristics. Such cells are bestowed with three critical capabilities, namely EMT, resistance to p53-mediated apoptosis, and a self-renewal program, that together define the functionality and survival of metastatic cancer stem cells. EMT provides a mechanism of escape to a new, less adverse niche; resistance to apoptosis ensures cell survival in conditions of stress in the primary tumor; whereas acquisition of "stemness" ensures generation of the critical tumor mass required for progression of micrometastases to macrometastases. Our findings, besides achieving considerable expansion of the inventory of direct genes targets, more importantly demonstrate that such elegant cooperative modulation of gene regulation mediated by Snail and Slug is critical for a cancer cell to acquire stem cell characteristics toward resisting radiotherapy- or chemotherapy-mediated cellular stress, and this may be a determinative aspect of aggressive cancer metastases.