Differential Roles of Rad18 and Chk2 in Genome Maintenance and Skin Carcinogenesis Following UV Exposure

Cell fate regulation governed by p53: Friends or reversible foes in cancer therapy

Cancer is a leading cause of death worldwide. Targeted therapies aimed at key oncogenic driver mutations in combination with chemotherapy and radiotherapy as well as immunotherapy have benefited cancer patients considerably. Tumor protein p53 (TP53), a crucial tumor suppressor gene encoding p53, regulates numerous downstream genes and cellular phenotypes in response to various stressors. The affected genes are involved in diverse processes, including cell cycle arrest, DNA repair, cellular senescence, metabolic homeostasis, apoptosis, and autophagy. However, accumulating recent studies have continued to reveal novel and unexpected functions of p53 in governing the fate of tumors, for example, functions in ferroptosis, immunity, the tumor microenvironment and microbiome metabolism. Among the possibilities, the evolutionary plasticity of p53 is the most controversial, partially due to the dizzying array of biological functions that have been attributed to different regulatory mechanisms of p53 signaling. Nearly 40 years after its discovery, this key tumor suppressor remains somewhat enigmatic. The intricate and diverse functions of p53 in regulating cell fate during cancer treatment are only the tip of the iceberg with respect to its equally complicated structural biology, which has been painstakingly revealed. Additionally, TP53 mutation is one of the most significant genetic alterations in cancer, contributing to rapid cancer cell growth and tumor progression. Here, we summarized recent advances that implicate altered p53 in modulating the response to various cancer therapies, including chemotherapy, radiotherapy, and immunotherapy. Furthermore, we also discussed potential strategies for targeting p53 as a therapeutic option for cancer.

Roles of trans-lesion synthesis (TLS) DNA polymerases in tumorigenesis and cancer therapy

DNA damage tolerance and mutagenesis are hallmarks and enabling characteristics of neoplastic cells that drive tumorigenesis and allow cancer cells to resist therapy. The 'Y-family' trans-lesion synthesis (TLS) DNA polymerases enable cells to replicate damaged genomes, thereby conferring DNA damage tolerance. Moreover, Y-family DNA polymerases are inherently error-prone and cause mutations. Therefore, TLS DNA polymerases are potential mediators of important tumorigenic phenotypes. The skin cancer-propensity syndrome xeroderma pigmentosum-variant (XPV) results from defects in the Y-family DNA Polymerase Pol eta (Polη) and compensatory deployment of alternative inappropriate DNA polymerases. However, the extent to which dysregulated TLS contributes to the underlying etiology of other human cancers is unclear. Here we consider the broad impact of TLS polymerases on tumorigenesis and cancer therapy. We survey the ways in which TLS DNA polymerases are pathologically altered in cancer. We summarize evidence that TLS polymerases shape cancer genomes, and review studies implicating dysregulated TLS as a driver of carcinogenesis. Because many cancer treatment regimens comprise DNA-damaging agents, pharmacological inhibition of TLS is an attractive strategy for sensitizing tumors to genotoxic therapies. Therefore, we discuss the pharmacological tractability of the TLS pathway and summarize recent progress on development of TLS inhibitors for therapeutic purposes.

Impacts of arsenic on Rad18 and translesion synthesis

Arsenite interferes with DNA repair protein function resulting in the retention of UV-induced DNA damage. Accumulated DNA damage promotes replication stress which is bypassed by DNA damage tolerance pathways such as translesion synthesis (TLS). Rad18 is an essential factor in initiating TLS through PCNA monoubiquitination and contains two functionally and structurally distinct zinc fingers that are potential targets for arsenite binding. Arsenite treatment displaced zinc from endogenous Rad18 protein and mass spectrometry analysis revealed arsenite binding to both the Rad18 RING finger and UBZ domains. Consequently, arsenite inhibited Rad18 RING finger dependent PCNA monoubiquitination and polymerase eta recruitment to DNA damage in UV exposed keratinocytes, both of which enhance the bypass of cyclobutane pyrimidine dimers during replication. Further analysis demonstrated multiple effects of arsenite, including the reduction in nuclear localization and UV-induced chromatin recruitment of Rad18 and its binding partner Rad6, which may also negatively impact TLS initiation. Arsenite and Rad18 knockdown in UV exposed keratinocytes significantly increased markers of replication stress and DNA strand breaks to a similar degree, suggesting arsenite mediates its effects through Rad18. Comet assay analysis confirmed an increase in both UV-induced single-stranded DNA and DNA double-strand breaks in arsenite treated keratinocytes compared to UV alone. Altogether, this study supports a mechanism by which arsenite inhibits TLS through the altered activity and regulation of Rad18. Arsenite elevated the levels of UV-induced replication stress and consequently, single-stranded DNA gaps and DNA double-strand breaks. These potentially mutagenic outcomes support a role for TLS in the cocarcinogenicity of arsenite.

Acetylation dependent translocation of EWSR1 regulates CHK2 alternative splicing in response to DNA damage

Ewing sarcoma breakpoint region 1 (EWSR1) is a member of FET (FUS/EWSR1/TAF15) RNA-binding family of proteins. The Ewing sarcoma oncoprotein EWS-FLI1 has been extensively studied, while much less is known about EWSR1 itself, especially the potential role of EWSR1 in response to DNA damage. Here, we found that UV irradiation induces acetylation of EWSR1, which is required for its nucleoli translocation. We identified K423, K432, K438, K640, and K643 as the major acetylation sites, p300/CBP and HDAC3/HDAC10 as the major acetyltransferases and deacetylases, respectively. Mechanically, UV-induced EWSR1 acetylation repressed its interaction with spliceosomal component U1C, which caused abnormal splicing of CHK2, suppressing the activity of CHK2 in response to UV irradiation. Taken together, our findings uncover acetylation as a novel regulatory modification of EWSR1, and is essential for its function in DNA damage response.

Roles of Chk2/CHEK2 in guarding against environmentally induced DNA damage and replication-stress

Checkpoint kinase 2 (human CHEK2; murine Chk2) is a critical mediator of the DNA damage response and has established roles in DNA double strand break (DSB)-induced apoptosis and cell cycle arrest. DSBs may be invoked directly by ionizing radiation but may also arise indirectly from environmental exposures such as solar ultraviolet (UV) radiation. The primary forms of DNA damage induced by UV are DNA photolesions (such as cyclobutane pyrimidine dimers CPD and 6-4 photoproducts) which interfere with DNA synthesis and lead to DNA replication fork stalling. Persistently stalled and unresolved DNA replication forks can "collapse" to generate DSBs that induce signaling via Chk2 and its upstream activator the ataxia telangiectasia-mutated (ATM) protein kinase. This review focuses on recently defined roles of Chk2 in protecting against DNA replication-associated genotoxicity. Several DNA damage response factors such as Rad18, Nbs1 and Chk1 suppress stalling and collapse of DNA replication forks. Defects in the primary responders to DNA replication fork stalling lead to generation of DSB and reveal "back-up" roles for Chk2 in S-phase progression and genomic stability. In humans, there are numerous variants of the CHEK2 gene, including CHEK2*1100delC. Individuals with the CHEK2*1100delC germline alteration have an increased risk of developing breast cancer and malignant melanoma. DNA replication fork-stalling at estrogen-DNA adducts and UV-induced photolesions are implicated in the etiology of breast cancer and melanoma, respectively. It is likely therefore that the Chk2/CHEK2-deficiency is associated with elevated risk for tumorigenesis caused by replication-associated genotoxicities that are exacerbated by environmental genotoxins and intrinsic DNA-damaging agents.

De novo transcriptome analysis and gene expression profiling of fish scales isolated from Carassius auratus during space flight: Impact of melatonin on gene expression in response to space radiation

Astronauts are inevitably exposed to two major risks during space flight, microgravity and radiation. Exposure to microgravity has been discovered to lead to rapid and vigorous bone loss due to elevated osteoclastic activity. In addition, long-term exposure to low-dose-rate space radiation was identified to promote DNA damage accumulation that triggered chronic inflammation, resulting in an increased risk for bone marrow suppression and carcinogenesis. In our previous study, melatonin, a hormone known to regulate the sleep-wake cycle, upregulated calcitonin expression levels and downregulated receptor activator of nuclear factor-κB ligand expression levels, leading to improved osteoclastic activity in a fish scale model. These results indicated that melatonin may represent a potential drug or lead compound for the prevention of bone loss under microgravity conditions. However, it is unclear whether melatonin affects the biological response induced by space radiation. The aim of the present study was to evaluate the effect of melatonin on the expression levels of genes responsive to space radiation. In the present study, to support the previous data regarding de novo transcriptome analysis of goldfish scales, a detailed and improved experimental method (e.g., PCR duplicate removal followed by de novo assembly, global normalization and calculation of statistical significance) was applied for the analysis. In addition, the transcriptome data were analyzed via global normalization, functional categorization and gene network construction to determine the impact of melatonin on gene expression levels in irradiated fish scales cultured in space. The results of the present study demonstrated that melatonin treatment counteracted microgravity- and radiation-induced alterations in the expression levels of genes associated with DNA replication, DNA repair, proliferation, cell death and survival. Thus, it was concluded that melatonin may promote cell survival and ensure normal cell proliferation in cells exposed to space radiation.

A Germline CHEK2 Mutation in a Family with Papillary Thyroid Cancer

Approximately 5% of all cases of papillary thyroid cancer (PTC) are inherited. However, the susceptibility gene(s) for nonsyndromic familial PTC (FPTC) remain unclear. We performed whole genome sequencing of peripheral blood DNA samples from two affected family members with PTC. CHEK2 transcript expression and the protein levels of CHK2 and p53 were evaluated in the thyroid tissues from two affected members of the kindred and sporadic PTC cases. The entire CHEK2 coding sequence was examined by Sanger sequencing in blood DNA samples from 242 sporadic PTC patients. We identified a novel heterozygous germline mutation in CHEK2 (c.417C→A) that was detected in all available affected members of a kindred with FPTC. This variant was found in only 1 out of 264,200 persons in the Genome Aggregation Database and the NHLBI Trans-Omics for Precision Medicine program. The CHEK2 c.417C→A variant introduces a premature termination codon (Y139X). We found reduced CHK2 protein expression in tumor samples from the two patients who carried the variant as compared with sporadic cases without the variant. The Y139X loss-of-function variant led to reduced p53 phosphorylation and decreased p53 protein level. In addition, two rare missense variants (R180C and H371Y) in CHEK2 were identified in 5 (2%) of 242 patients with sporadic PTC. Our findings suggest that the CHEK2 Y139X variant may be associated with FPTC.

circRAD18 sponges miR-208a/3164 to promote triple-negative breast cancer progression through regulating IGF1 and FGF2 expression

As a new rising star of non-coding RNA, circular RNAs (circRNAs) emerged as vital regulators with biological functions in diverse of cancers. However, the function and precise mechanism of the vast majority of circRNAs in triple-negative breast cancer (TNBC) occurrence and progression have not been clearly elucidated. In the current study, we identified and further investigated hsa_circ_0002453 (circRAD18) by analyzing our previous microarray profiling. Expression of circRAD18 was found significantly upregulated in TNBC compared with normal mammary tissues and cell lines. circRAD18 was positively correlated with T stage, clinical stage and pathological grade and was an independent risk factor for TNBC patients. We performed proliferation, colony formation, cell migration, apoptosis and mouse xenograft assays to verify the functions of circRAD18. Knockdown of circRAD18 significantly suppressed cell proliferation and migration, promoted cell apoptosis and inhibited tumor growth in functional and xenograft experiments. Through luciferase reporter assays, we confirmed that circRAD18 acts as a sponge of miR-208a and miR-3164 and promotes TNBC progression through upregulating IGF1 and FGF2 expression. Altogether, our research revealed the pivotal role of circRAD18-miR-208a/3164-IGF1/FGF2 axis in TNBC tumorigenesis and metastasis though the mechanism of competing endogenous RNAs. Thus, circRAD18 may serve as a novel prognostic biomarker and potential target for TNBC treatment in the future.

Knockdown of circRAD18 Mitigates Breast Cancer Progression through the Regulation of miR-613/HK2 Axis

Background

Breast cancer (BC) remains the most prevalent malignancy and the leading cause of cancer death. Circular RNAs (circRNAs) have been discovered to serve as crucial regulators in BC. In the current work, we aimed to study the impact of circRAD18 (hsa_circ_0002453) on BC progression and mechanism governing it.

Materials and Methods

The expression levels of circRAD18, miR-613 and hexokinase 2 (HK2) mRNA were determined by quantitative real-time polymerase chain reaction (qRT-PCR). CircRAD18 identification was performed using RNase R digestion and actinomycin D assay. Cell viability, colony formation, apoptosis, migration, invasion and glycolysis were measured by Cell Counting Kit-8 assay, colony formation assay, flow cytometry, transwell analysis and extracellular acidification rate detection assay, respectively. Western blot was used to assess the levels of E-Cadherin, Vimentin, N-Cadherin and HK2 protein. The targeted interplay between miR-613 and circRAD18 or HK2 was detected by dual-luciferase reporter assay. Xenograft model assay was performed to observe the role of circRAD18 in vivo.

Results

CircRAD18 was highly expressed in BC tissues and cells. CircRAD18 depletion hindered BC cell malignant behaviors, as evidenced by the inhibition in cell viability, colony formation, migration, invasion, epithelial to mesenchymal transition and glycolysis, as well as the promotion in cell apoptosis. CircRAD18 directly interacted with miR-613, and miR-613 mediated the repressive effect of circRAD18 knockdown on BC cell malignant behaviors. Moreover, HK2 was a direct target of miR-613, and circRAD18 positively regulated HK2 expression via sponging miR-613. Additionally, circRAD18 knockdown repressed tumor growth in vivo by miR-613.

Conclusion

Our current work suggested that circRAD18 silencing suppressed BC cell malignant behaviors in vitro and tumor growth in vivo at least partly via the regulation of the miR-613/HK2 axis, highlighting that circRAD18 might be a promising therapeutic target for BC treatment.

DNA damage response signaling pathways and targets for radiotherapy sensitization in cancer

Radiotherapy is one of the most common countermeasures for treating a wide range of tumors. However, the radioresistance of cancer cells is still a major limitation for radiotherapy applications. Efforts are continuously ongoing to explore sensitizing targets and develop radiosensitizers for improving the outcomes of radiotherapy. DNA double-strand breaks are the most lethal lesions induced by ionizing radiation and can trigger a series of cellular DNA damage responses (DDRs), including those helping cells recover from radiation injuries, such as the activation of DNA damage sensing and early transduction pathways, cell cycle arrest, and DNA repair. Obviously, these protective DDRs confer tumor radioresistance. Targeting DDR signaling pathways has become an attractive strategy for overcoming tumor radioresistance, and some important advances and breakthroughs have already been achieved in recent years. On the basis of comprehensively reviewing the DDR signal pathways, we provide an update on the novel and promising druggable targets emerging from DDR pathways that can be exploited for radiosensitization. We further discuss recent advances identified from preclinical studies, current clinical trials, and clinical application of chemical inhibitors targeting key DDR proteins, including DNA-PKcs (DNA-dependent protein kinase, catalytic subunit), ATM/ATR (ataxia–telangiectasia mutated and Rad3-related), the MRN (MRE11-RAD50-NBS1) complex, the PARP (poly[ADP-ribose] polymerase) family, MDC1, Wee1, LIG4 (ligase IV), CDK1, BRCA1 (BRCA1 C terminal), CHK1, and HIF-1 (hypoxia-inducible factor-1). Challenges for ionizing radiation-induced signal transduction and targeted therapy are also discussed based on recent achievements in the biological field of radiotherapy.

In vitro proliferation of human epidermal melanocytes biopsied from multiple anatomical sites

BACKGROUND

Vitiligo is an acquired disease that involves low pigment variation in the skin. The use of cultured melanocytes for treatment of recalcitrant vitiligo has become a well-established treatment modality. In vitro cultured melanocytes present an effective autologous transplantation treatment modality for vitiligo. The present study investigated the in vitro culture of epidermal melanocytes sampled from multiple body parts, as well as the differences in total propagation time.

METHODS

Skin specimens were collected from 30 participants (14 males and 16 females) who supplied normal colored skin sections from various regions of their body, including the face, chest, abdomen, buttock, and extremities. Subsequently, all of these specimens were treated with an identical melanocyte purification and culturing process.

RESULTS

Melanocytes from the face demonstrated the most rapid growth and longest total propagation time. Melanocytes from the buttock, abdomen, and extremities displayed similar results to one another, and melanocytes from the chest and back had the slowest growth and shortest total propagation time.

CONCLUSION

Selecting the most favorable site to obtain epidermal melanocytes will reduce the required quantity of skin and culturing time, and maximize the growth and total propagation time of melanocytes. Therefore, care should be exercised when selecting the region of skin when culturing epidermal melanocytes.