Pellino1 regulates reversible ATM activation via NBS1 ubiquitination at DNA double-strand breaks

MDC1 mediates Pellino recruitment to sites of DNA double-strand breaks

Ubiquitylation is critically implicated in the recognition and repair of DNA double-strand breaks. The adaptor protein MDC1 mediates the recruitment of the key DNA damage responsive E3 ubiquitin ligase RNF8 to the break sites. It does so by directly interacting with RNF8 in a phosphorylation-dependent manner that involves the RNF8 FHA domain, thus initiating targeted chromatin ubiquitylation at the break sites. Here, we report that MDC1 also directly binds to two additional E3 ubiquitin ligases, Pellino 1 and 2, which were recently implicated in the DNA damage response. Through a combination of biochemical, biophysical and X-ray crystallographic approaches, we reveal the molecular details of the MDC1-Pellino complexes. Furthermore, we show that in mammalian cells, MDC1 mediates Pellino recruitment to sites of DNA double-strand breaks by a direct phosphorylation-dependent interaction between the two proteins. Taken together, our findings provide new molecular insights into the ubiquitylation pathways that govern genome stability maintenance.

The dilemma of nuclear mechanical forces in DNA damage and repair

Genomic stability, encompassing DNA damage and repair mechanisms, plays a pivotal role in the onset of diseases and the aging process. The stability of DNA is intricately linked to the chemical and mechanical forces exerted on chromatin, particularly within lamina-associated domains (LADs). Mechanical stress can induce DNA damage through the deformation and rupture of the nuclear envelope, leading to DNA bending and cleavage. However, DNA can evade such mechanical stress-induced damage by relocating away from the nuclear membrane, a process facilitated by the depletion of H3K9me3-marked heterochromatin and its cleavage from the lamina. When DNA double-stranded breaks occur, they prompt the rapid recruitment of Lamin B1 and the deposition of H3K9me3. Despite these insights, the precise mechanisms underlying DNA damage and repair under mechanical stress remain unclear. In this review, we explore the interplay between mechanical forces and the nuclear envelope in the context of DNA damage, elucidate the molecular pathways through which DNA escapes force-induced damage, and discuss the corresponding repair strategies involving the nuclear cytoskeleton. By summarizing the mechanisms of force-induced DNA damage and repair, we aim to underscore the potential for developing targeted therapeutic strategies to bolster genomic stability and alleviate the impacts of aging and disease.

Phosphorylated BLM peptide acts as an agonist for DNA damage response

Upon exposure to ionizing irradiation, the MRE11-RAD50-NBS1 complex potentiates the recruitment of ATM (ataxia-telangiectasia mutated) kinase to the double-strand breaks. We show that the lack of BLM causes a decrease in the autophosphorylation of ATM in mice mammary glands, which have lost one or both copies of BLM. In isogenic human cells, the DNA damage response (DDR) pathway was dampened in the absence of BLM, which negatively affected the recruitment of DDR factors onto the chromatin, thereby indicating a direct role of BLM in augmenting DDR. Mechanistically, this was due to the BLM-dependent dissociation of inactive ATM dimers into active monomers. Fragmentation analysis of BLM followed by kinase assays revealed a 20-mer BLM peptide (91-110 aa), sufficient to enhance ATM-dependent p53 phosphorylation. ATM-mediated phosphorylation of BLM at Thr99 within BLM (91-110) peptide enhanced ATM kinase activity due to its interaction with NBS1 and causing ATM monomerization. Delivery of phosphomimetic T99E counterpart of BLM (91-110 aa) peptide led to ATM activation followed by restoration of the DDR even in the absence of ionizing irradiation (both in cells and in BLM knockout mice), indicating its role as a DDR agonist, which can be potentially used to prevent the initiation of neoplastic transformation.

Potential promising of synthetic lethality in cancer research and treatment

Cancer is a complex disease driven by multiple genetic changes, including mutations in oncogenes, tumor suppressor genes, DNA repair genes, and genes involved in cancer metabolism. Synthetic lethality (SL) is a promising approach in cancer research and treatment, where the simultaneous dysfunction of specific genes or pathways causes cell death. By targeting vulnerabilities created by these dysfunctions, SL therapies selectively kill cancer cells while sparing normal cells. SL therapies, such as PARP inhibitors, WEE1 inhibitors, ATR and ATM inhibitors, and DNA-PK inhibitors, offer a distinct approach to cancer treatment compared to conventional targeted therapies. Instead of directly inhibiting specific molecules or pathways, SL therapies exploit genetic or molecular vulnerabilities in cancer cells to induce selective cell death, offering benefits such as targeted therapy, enhanced treatment efficacy, and minimized harm to healthy tissues. SL therapies can be personalized based on each patient's unique genetic profile and combined with other treatment modalities to potentially achieve synergistic effects. They also broaden the effectiveness of treatment across different cancer types, potentially overcoming drug resistance and improving patient outcomes. This review offers an overview of the current understanding of SL mechanisms, advancements, and challenges, as well as the preclinical and clinical development of SL. It also discusses new directions and opportunities for utilizing SL in targeted therapy for anticancer treatment.

PARP1-TRIM44-MRN loop dictates the response to PARP inhibitors

PARP inhibitors (PARPi) show selective efficacy in tumors with homologous recombination repair (HRR)-defects but the activation mechanism of HRR pathway in PARPi-treated cells remains enigmatic. To unveil it, we searched for the mediator bridging PARP1 to ATM pathways by screening 211 human ubiquitin-related proteins. We discovered TRIM44 as a crucial mediator that recruits the MRN complex to damaged chromatin, independent of PARP1 activity. TRIM44 binds PARP1 and regulates the ubiquitination-PARylation balance of PARP1, which facilitates timely recruitment of the MRN complex for DSB repair. Upon exposure to PARPi, TRIM44 shifts its binding from PARP1 to the MRN complex via its ZnF UBP domain. Knockdown of TRIM44 in cells significantly enhances the sensitivity to olaparib and overcomes the resistance to olaparib induced by 53BP1 deficiency. These observations emphasize the central role of TRIM44 in tethering PARP1 to the ATM-mediated repair pathway. Suppression of TRIM44 may enhance PARPi effectiveness and broaden their use even to HR-proficient tumors.

Ubiquitination and deubiquitination in cancer: from mechanisms to novel therapeutic approaches

Ubiquitination, a pivotal posttranslational modification of proteins, plays a fundamental role in regulating protein stability. The dysregulation of ubiquitinating and deubiquitinating enzymes is a common feature in various cancers, underscoring the imperative to investigate ubiquitin ligases and deubiquitinases (DUBs) for insights into oncogenic processes and the development of therapeutic interventions. In this review, we discuss the contributions of the ubiquitin-proteasome system (UPS) in all hallmarks of cancer and progress in drug discovery. We delve into the multiple functions of the UPS in oncology, including its regulation of multiple cancer-associated pathways, its role in metabolic reprogramming, its engagement with tumor immune responses, its function in phenotypic plasticity and polymorphic microbiomes, and other essential cellular functions. Furthermore, we provide a comprehensive overview of novel anticancer strategies that leverage the UPS, including the development and application of proteolysis targeting chimeras (PROTACs) and molecular glues.

The ubiquitin codes in cellular stress responses

Ubiquitination/ubiquitylation, one of the most fundamental post-translational modifications, regulates almost every critical cellular process in eukaryotes. Emerging evidence has shown that essential components of numerous biological processes undergo ubiquitination in mammalian cells upon exposure to diverse stresses, from exogenous factors to cellular reactions, causing a dazzling variety of functional consequences. Various forms of ubiquitin signals generated by ubiquitylation events in specific milieus, known as ubiquitin codes, constitute an intrinsic part of myriad cellular stress responses. These ubiquitination events, leading to proteolytic turnover of the substrates or just switch in functionality, initiate, regulate, or supervise multiple cellular stress-associated responses, supporting adaptation, homeostasis recovery, and survival of the stressed cells. In this review, we attempted to summarize the crucial roles of ubiquitination in response to different environmental and intracellular stresses, while discussing how stresses modulate the ubiquitin system. This review also updates the most recent advances in understanding ubiquitination machinery as well as different stress responses and discusses some important questions that may warrant future investigation.

A noncanonical IRAK4-IRAK1 pathway counters DNA damage-induced apoptosis independently of TLR/IL-1R signaling

Interleukin-1 receptor (IL-1R)-associated kinases (IRAKs) are core effectors of Toll-like receptors (TLRs) and IL-1R in innate immunity. Here, we found that IRAK4 and IRAK1 together inhibited DNA damage-induced cell death independently of TLR or IL-1R signaling. In human cancer cells, IRAK4 was activated downstream of ATR kinase in response to double-strand breaks (DSBs) induced by ionizing radiation (IR). Activated IRAK4 then formed a complex with and activated IRAK1. The formation of this complex required the E3 ubiquitin ligase Pellino1, acting structurally but not catalytically, and the activation of IRAK1 occurred independently of extracellular signaling, intracellular TLRs, and the TLR/IL-1R signaling adaptor MyD88. Activated IRAK1 translocated to the nucleus in a Pellino2-dependent manner. In the nucleus, IRAK1 bound to the PIDD1 subunit of the proapoptotic PIDDosome and interfered with platform assembly, thus supporting cell survival. This noncanonical IRAK signaling pathway was also activated in response to other DSB-inducing agents. The loss of IRAK4, of IRAK4 kinase activity, of either Pellino protein, or of the nuclear localization sequence in IRAK1 sensitized p53-mutant zebrafish to radiation. Thus, the findings may lead to strategies for overcoming tumor resistance to conventional cancer treatments.

Biology of Pellino1: a potential therapeutic target for inflammation in diseases and cancers

Pellino1 (Peli1) is a highly conserved E3 Ub ligase that exerts its biological functions by mediating target protein ubiquitination. Extensive evidence has demonstrated the crucial role of Peli1 in regulating inflammation by modulating various receptor signaling pathways, including interleukin-1 receptors, Toll-like receptors, nuclear factor-κB, mitogen-activated protein kinase, and phosphoinositide 3-kinase/AKT pathways. Peli1 has been implicated in the development of several diseases by influencing inflammation, apoptosis, necrosis, pyroptosis, autophagy, DNA damage repair, and glycolysis. Peli1 is a risk factor for most cancers, including breast cancer, lung cancer, and lymphoma. Conversely, Peli1 protects against herpes simplex virus infection, systemic lupus erythematosus, esophageal cancer, and toxic epidermolysis bullosa. Therefore, Peli1 is a potential therapeutic target that warrants further investigation. This comprehensive review summarizes the target proteins of Peli1, delineates their involvement in major signaling pathways and biological processes, explores their role in diseases, and discusses the potential clinical applications of Peli1-targeted therapy, highlighting the therapeutic prospects of Peli1 in various diseases.

Ubiquitination Links DNA Damage and Repair Signaling to Cancer Metabolism

Changes in the DNA damage response (DDR) and cellular metabolism are two important factors that allow cancer cells to proliferate. DDR is a set of events in which DNA damage is recognized, DNA repair factors are recruited to the site of damage, the lesion is repaired, and cellular responses associated with the damage are processed. In cancer, DDR is commonly dysregulated, and the enzymes associated with DDR are prone to changes in ubiquitination. Additionally, cellular metabolism, especially glycolysis, is upregulated in cancer cells, and enzymes in this metabolic pathway are modulated by ubiquitination. The ubiquitin-proteasome system (UPS), particularly E3 ligases, act as a bridge between cellular metabolism and DDR since they regulate the enzymes associated with the two processes. Hence, the E3 ligases with high substrate specificity are considered potential therapeutic targets for treating cancer. A number of small molecule inhibitors designed to target different components of the UPS have been developed, and several have been tested in clinical trials for human use. In this review, we discuss the role of ubiquitination on overall cellular metabolism and DDR and confirm the link between them through the E3 ligases NEDD4, APC/C^CDH1, FBXW7, and Pellino1. In addition, we present an overview of the clinically important small molecule inhibitors and implications for their practical use.

Prevalence of germline mutations in cancer susceptibility genes in Chinese patients with renal cell carcinoma

Background

Germline pathogenic variants are estimated to affect 3-5% of patients with renal cell carcinoma (RCC). The identification of patients with hereditary RCC is important for cancer screening and treatment guidance.

Methods

Whole-exome sequencing (WES) (n=69) or gene panel sequencing containing 139 genes (n=54) related to germline cancer predisposition was used to analyze germline mutations in 123 patients with RCC admitted to Department of Urology, The Third Medical Center of Chinese PLA General Hospital. Chi-square test (χ²) was used to analyze relationship between clinicopathologic parameters and germline mutations.

Results

A total of 13 (10.57%) patients carried pathogenic or likely pathogenic germline mutations in 10 cancer predisposition genes, including VHL, FH, FLCN, SDHB, MUTYH, RAD51C, NBN, RAD50, FANCI, and FANCM. A total of 6 of these 10 cancer predisposition genes were associated with maintenance of genomic stability and DNA repair. Patients harboring pathogenic germline mutations tended to have an earlier RCC onset. The prevalence of deleterious mutations was higher in patients with bilateral or multifocal RCC compared to patients without bilateral or multifocal RCC. Patients with non-clear cell RCC (nccRCC) were significantly more likely to have RCC-associated gene mutations.

Conclusions

To our knowledge, this is the first report of pathogenic germline mutations in the FANCI and FANCM genes and heterozygous germline missense mutation in exon 5 of the FH gene c.563A>T:p.N188I in RCC. Young RCC patients, patients with bilateral or multifocal RCC, or patients with nccRCC are more likely to have pathogenic/potentially pathogenic germline mutations.

A Non-Canonical IRAK Signaling Pathway Triggered by DNA Damage

Interleukin-1 receptor (IL-1R)-associated kinases (IRAKs) are core effectors of Toll-like receptor (TLR) and IL-1R signaling, with no reported roles outside of innate immunity. We find that vertebrate cells exposed to ionizing radiation (IR) sequentially activate IRAK4 and IRAK1 through a phosphorylation cascade mirroring that induced by TLR/IL-1R, resulting in a potent anti-apoptotic response. However, IR-induced IRAK1 activation does not require the receptors or the IRAK4/1 adaptor protein MyD88, and instead of remaining in the cytoplasm, the activated kinase is immediately transported to the nucleus via a conserved nuclear localization signal. We identify: double-strand DNA breaks (DSBs) as the biologic trigger for this pathway; the E3 ubiquitin ligase Pellino1 as the scaffold enabling IRAK4/1 activation in place of TLR/IL-1R-MyD88; and the pro-apoptotic PIDDosome (PIDD1-RAIDD-caspase-2) as a critical downstream target in the nucleus. The data delineate a non-canonical IRAK signaling pathway derived from, or ancestral to, TLR signaling. This DSB detection pathway, which is also activated by genotoxic chemotherapies, provides multiple actionable targets for overcoming tumor resistance to mainstay cancer treatments.

SIRT1 in the cardiomyocyte counteracts doxorubicin-induced cardiotoxicity via regulating histone H2AX

AIMS

Cardiotoxicity by doxorubicin predicts worse prognosis of patients. Accumulation of damaged DNA has been implicated in doxorubicin-induced cardiotoxicity. SIRT1, an NAD+-dependent histone/protein deacetylase, protects cells by deacetylating target proteins. We investigated whether SIRT1 counteracts doxorubicin-induced cardiotoxicity by mediating Ser139 phosphorylation of histone H2AX, a critical signal of the DNA damage response.

METHODS AND RESULTS

Doxorubicin (5 mg/kg per week, x4) was administered to mice with intact SIRT1 (Sirt1f/f) and mice that lack SIRT1 activity in cardiomyocytes (Sirt1f/f;MHCcre/+). Reductions in left ventricular fractional shortening and ejection fraction by doxorubicin treatment were more severe in Sirt1f/f;MHCcre/+ than in Sirt1f/f. Myocardial expression level of type-B natriuretic peptide was 2.5-fold higher in Sirt1f/f;MHCcre/+ than in Sirt1f/f after doxorubicin treatment. Sirt1f/f;MHCcre/+ showed larger fibrotic areas and higher nitrotyrosine levels in the heart after doxorubicin treatment. Although doxorubicin-induced DNA damage evaluated by TUNEL staining was enhanced in Sirt1f/f;MHCcre/+, the myocardium from Sirt1f/f;MHCcre/+ showed blunted Ser139 phosphorylation of H2AX by doxorubicin treatment. In H9c2 cardiomyocytes, SIRT1 knockdown attenuated Ser139 phosphorylation of H2AX, increased DNA damage, and enhanced caspase-3 activation under doxorubicin treatment. Immunostaining revealed that acetylation level of H2AX at Lys5 was higher in hearts from Sirt1f/f;MHCcre/+. In H9c2 cells, acetyl-Lys5-H2AX level was increased by SIRT1 knockdown and reduced by SIRT1 overexpression. Ser139 phosphorylation in response to doxorubicin treatment was blunted in a mutant H2AX with substitution of Lys5 to Gln (K5Q) that mimics acetylated lysine compared with that in wild-type H2AX. Expression of K5Q-H2AX as well as S139A-H2AX, which cannot be phosphorylated at Ser139, augmented doxorubicin-induced caspase-3 activation. Treatment of mice with resveratrol, a SIRT1 activator, attenuated doxorubicin-induced cardiac dysfunction, which was associated with a reduction in acetyl-Lys5-H2AX level and a preserved phospho-Ser139-H2AX level.

CONCLUSION

These findings suggest that SIRT1 counteracts doxorubicin-induced cardiotoxicity by mediating H2AX phosphorylation through its deacetylation in cardiomyocytes.

Deubiquitinase USP2 stabilizes the MRE11-RAD50-NBS1 complex at DNA double-strand break sites by counteracting the ubiquitination of NBS1

The MRE11-RAD50-NBS1 (MRN) complex plays essential roles in the cellular response to DNA double-strand breaks (DSBs), which are the most cytotoxic DNA lesions, and is a target of various modifications and controls. Recently, lysine 48-linked ubiquitination of NBS1, resulting in premature disassembly of the MRN complex from DSB sites, was observed in cells lacking RECQL4 helicase activity. However, the role and control of this ubiquitination during the DSB response in cells with intact RECQL4 remain unknown. Here, we showed that USP2 counteracts this ubiquitination and stabilizes the MRN complex during the DSB response. By screening deubiquitinases that increase the stability of the MRN complex in RECQL4-deficient cells, USP2 was identified as a new deubiquitinase that acts at DSB sites to counteract NBS1 ubiquitination. We determined that USP2 is recruited to DSB sites in a manner dependent on ATM, a major checkpoint kinase against DSBs, and stably interacts with NBS1 and RECQL4 in immunoprecipitation experiments. Phosphorylation of two critical residues in the N terminus of USP2 by ATM is required for its recruitment to DSBs and its interaction with RECQL4. While inactivation of USP2 alone does not substantially influence the DSB response, we found that inactivation of USP2 and USP28, another deubiquitinase influencing NBS1 ubiquitination, results in premature disassembly of the MRN complex from DSB sites as well as defects in ATM activation and homologous recombination repair abilities. These results suggest that deubiquitinases counteracting NBS1 ubiquitination are essential for the stable maintenance of the MRN complex and proper cellular response to DSBs.

BRD4-directed super-enhancer organization of transcription repression programs links to chemotherapeutic efficacy in breast cancer

BRD4 is well known for its role in super-enhancer organization and transcription activation of several prominent oncogenes including c-MYC and BCL2 As such, BRD4 inhibitors are being pursued as promising therapeutics for cancer treatment. However, drug resistance also occurs for BRD4-targeted therapies. Here, we report that BRD4 unexpectedly interacts with the LSD1/NuRD complex and colocalizes with this repressive complex on super-enhancers. Integrative genomic and epigenomic analyses indicate that the BRD4/LSD1/NuRD complex restricts the hyperactivation of a cluster of genes that are functionally linked to drug resistance. Intriguingly, treatment of breast cancer cells with a small-molecule inhibitor of BRD4, JQ1, results in no immediate activation of the drug-resistant genes, but long-time treatment or destabilization of LSD1 by PELI1 decommissions the BRD4/LSD1/NuRD complex, leading to resistance to JQ1 as well as to a broad spectrum of therapeutic compounds. Consistently, PELI1 is up-regulated in breast carcinomas, its level is negatively correlated with that of LSD1, and the expression level of the BRD4/LSD1/NuRD complex-restricted genes is strongly correlated with a worse overall survival of breast cancer patients. Together, our study uncovers a functional duality of BRD4 in super-enhancer organization of transcription activation and repression linking to oncogenesis and chemoresistance, respectively, supporting the pursuit of a combined targeting of BRD4 and PELI1 in effective treatment of breast cancer.

The Emerging Roles of Pellino Family in Pattern Recognition Receptor Signaling

The Pellino family is a novel and well-conserved E3 ubiquitin ligase family and consists of Pellino1, Pellino2, and Pellino3. Each family member exhibits a highly conserved structure providing ubiquitin ligase activity without abrogating cell and structure-specific function. In this review, we mainly summarized the crucial roles of the Pellino family in pattern recognition receptor-related signaling pathways: IL-1R signaling, Toll-like signaling, NOD-like signaling, T-cell and B-cell signaling, and cell death-related TNFR signaling. We also summarized the current information of the Pellino family in tumorigenesis, microRNAs, and other phenotypes. Finally, we discussed the outstanding questions of the Pellino family in immunity.

Immediate-Early, Early, and Late Responses to DNA Double Stranded Breaks

Loss or rearrangement of genetic information can result from incorrect responses to DNA double strand breaks (DSBs). The cellular responses to DSBs encompass a range of highly coordinated events designed to detect and respond appropriately to the damage, thereby preserving genomic integrity. In analogy with events occurring during viral infection, we appropriate the terms Immediate-Early, Early, and Late to describe the pre-repair responses to DSBs. A distinguishing feature of the Immediate-Early response is that the large protein condensates that form during the Early and Late response and are resolved upon repair, termed foci, are not visible. The Immediate-Early response encompasses initial lesion sensing, involving poly (ADP-ribose) polymerases (PARPs), KU70/80, and MRN, as well as rapid repair by so-called ‘fast-kinetic’ canonical non-homologous end joining (cNHEJ). Initial binding of PARPs and the KU70/80 complex to breaks appears to be mutually exclusive at easily ligatable DSBs that are repaired efficiently by fast-kinetic cNHEJ; a process that is PARP-, ATM-, 53BP1-, Artemis-, and resection-independent. However, at more complex breaks requiring processing, the Immediate-Early response involving PARPs and the ensuing highly dynamic PARylation (polyADP ribosylation) of many substrates may aid recruitment of both KU70/80 and MRN to DSBs. Complex DSBs rely upon the Early response, largely defined by ATM-dependent focal recruitment of many signalling molecules into large condensates, and regulated by complex chromatin dynamics. Finally, the Late response integrates information from cell cycle phase, chromatin context, and type of DSB to determine appropriate pathway choice. Critical to pathway choice is the recruitment of p53 binding protein 1 (53BP1) and breast cancer associated 1 (BRCA1). However, additional factors recruited throughout the DSB response also impact upon pathway choice, although these remain to be fully characterised. The Late response somehow channels DSBs into the appropriate high-fidelity repair pathway, typically either ‘slow-kinetic’ cNHEJ or homologous recombination (HR). Loss of specific components of the DSB repair machinery results in cells utilising remaining factors to effect repair, but often at the cost of increased mutagenesis. Here we discuss the complex regulation of the Immediate-Early, Early, and Late responses to DSBs proceeding repair itself.

ATM: Functions of ATM Kinase and Its Relevance to Hereditary Tumors

Ataxia-telangiectasia mutated (ATM) functions as a key initiator and coordinator of DNA damage and cellular stress responses. ATM signaling pathways contain many downstream targets that regulate multiple important cellular processes, including DNA damage repair, apoptosis, cell cycle arrest, oxidative sensing, and proliferation. Over the past few decades, associations between germline ATM pathogenic variants and cancer risk have been reported, particularly for breast and pancreatic cancers. In addition, given that ATM plays a critical role in repairing double-strand breaks, inhibiting other DNA repair pathways could be a synthetic lethal approach. Based on this rationale, several DNA damage response inhibitors are currently being tested in ATM-deficient cancers. In this review, we discuss the current knowledge related to the structure of the ATM gene, function of ATM kinase, clinical significance of ATM germline pathogenic variants in patients with hereditary cancers, and ongoing efforts to target ATM for the benefit of cancer patients.

GSTpi reduces DNA damage and cell death by regulating the ubiquitination and nuclear translocation of NBS1

Glutathione S-transferase pi (GSTpi) is an important phase II detoxifying enzyme that participates in various physiological processes, such as antioxidant, detoxification, and signal transduction. The high expression level of GSTpi has been reported to be related to drug-resistant and anti-inflammatory and it functioned via its non-catalytic ligandin. However, the previous protection mechanism of GSTpi in DNA damage has not been addressed so far. Nijmegen breakage syndrome 1 (NBS1) is one of the most important sensor proteins to detect damaged DNA. Here, we investigated the interaction between GSTpi and NBS1 in HEK-293 T cells and human breast adenocarcinoma cells during DNA damage. Our results showed that overexpression of GSTpi in cells by transfecting DNA vector decreased the DNA damage level after methyl methanesulfonate (MMS) or adriamycin (ADR) treatment. We found that cytosolic GSTpi could increase NBS1 ubiquitin-mediated degradation in unstimulated cells, which suggested that GSTpi could maintain the basal level of NBS1 during normal conditions. In response to DNA damage, GSTpi can be phosphorylated in Ser184 and inhibit the ubiquitination degradation of NBS1 mediated by Skp2 to recover NBS1 protein level. Phosphorylated GSTpi can further enhance NBS1 nuclear translocation to activate the ATM-Chk2-p53 signaling pathway. Finally, GSTpi blocked the cell cycle in the G2/M phase to allow more time for DNA damage repair. Thus, our finding revealed the novel mechanism of GSTpi via its Ser184 phosphorylation to protect cells from cell death during DNA damage and it enriches the function of GSTpi in drug resistance.

Damsel in distress calling on her knights: Illuminating the pioneering role of E3 ubiquitin ligases in guarding the genome integrity

The maintenance of genomic integrity is of utmost importance for the organisms to survive and to accurately inherit traits to their progenies. Any kind of DNA damage either due to defect in DNA duplication and/ or uncontrolled cell division or intracellular insults or environment radiation can result in gene mutation, chromosomal aberration and ultimately genomic instability, which may cause several diseases including cancers. Therefore, cells have evolved machineries for the surveillance of genomic integrity. Enormous exciting studies in the past indicate that ubiquitination (a posttranslational modification of proteins) plays a crucial role in maintaining the genomic integrity by diverse ways. In fact, various E3 ubiquitin ligases catalyse ubiquitination of key proteins to control their central role during cell cycle, DNA damage response (DDR) and DNA repair. Some E3 ligases promote genomic instability while others prevent it, deregulation of both of which leads to several malignancies. In this review, we consolidate the recent findings wherein the role of ubiquitination in conferring genome integrity is highlighted. We also discuss the latest discoveries on the mechanisms utilized by various E3 ligases to preserve genomic stability, with a focus on their actions during cell cycle progression and different types of DNA damage response as well as repair pathways.

PELI1 promotes radiotherapy sensitivity by inhibiting noncanonical NF-κB in esophageal squamous cancer

The low sensitivity of radiotherapy is the main cause of tumor tolerance against ionizing radiation (IR). However, the molecular mechanisms by which radiosensitivity is controlled remain elusive. Here, we observed that high expression of pellino E3 ubiquitin protein ligase 1 (PELI1) was correlated with improved prognosis in human esophageal squamous cell carcinoma stage III patients that received adjuvant radiotherapy. Moreover, we found PELI1‐mediated IR‐induced tumor cell apoptosis in vivo and in vitro. Mechanistically, PELI1 mediated the lysine 48 (Lys48)–linked polyubiquitination and degradation of NF‐κB–inducing kinase (NIK; also known as MAP3K14), the master kinase of the noncanonical NF‐κB pathway, thereby inhibiting IR‐induced activation of the noncanonical NF‐κB signaling pathway during radiotherapy. As a consequence, PELI1 inhibited the noncanonical NF‐κB–induced expression of the anti‐apoptotic gene BCL2 like 1 (Bclxl; also known as BCL2L1), leading to an enhancement of the IR‐induced apoptosis signaling pathway and ultimately promoting IR‐induced apoptosis in tumor cells. Therefore, Bclxl or NIK knockdown abolished the apoptosis‐resistant effect in PELI1‐knockdown tumor cells after radiotherapy. These findings establish PELI1 as a critical tumor intrinsic regulator in controlling the sensitivity of tumor cells to radiotherapy through modulating IR‐induced noncanonical NF‐κB expression.

Stable maintenance of the Mre11-Rad50-Nbs1 complex is sufficient to restore the DNA double-strand break response in cells lacking RecQL4 helicase activity

The proper cellular response to DNA double-strand breaks (DSBs) is critical for maintaining the integrity of the genome. RecQL4, a DNA helicase of which mutations are associated with Rothmund-Thomson syndrome (RTS), is required for the DNA DSB response. However, the mechanism by which RecQL4 performs these essential roles in the DSB response remains unknown. Here, we show that RecQL4 and its helicase activity are required for maintaining the stability of the Mre11-Rad50-Nbs1 (MRN) complex on DSB sites during a DSB response. We found using immunocytochemistry and live-cell imaging that the MRN complex is prematurely disassembled from DSB sites in a manner dependent upon Skp2-mediated ubiquitination of Nbs1 in RecQL4-defective cells. This early disassembly of the MRN complex could be prevented by altering the ubiquitination site of Nbs1 or by expressing a deubiquitinase, Usp28, which sufficiently restored homologous recombination repair and ATM, a major checkpoint kinase against DNA DSBs, activation abilities in RTS, and RecQL4-depleted cells. These results suggest that the essential role of RecQL4 in the DSB response is to maintain the stability of the MRN complex on DSB sites and that defects in the DSB response in cells of patients with RTS can be recovered by controlling the stability of the MRN complex.

Chemotherapy of HER2- and MDM2-Enriched Breast Cancer Subtypes Induces Homologous Recombination DNA Repair and Chemoresistance

Simple Summary

MDM2 is a protein responsible for negative regulation of the p53 tumor suppressor. In addition, MDM2 exhibits chaperone-like properties similar to the HSP90 molecular chaperone. Multiple studies revealed that MDM2 is deeply involved in cancer development and progression. Some recently published results indicate that the role of MDM2 in DNA repair inhibition is more complex than previously thought. We show that MDM2 is directly involved in the homologous recombination DNA repair, and its chaperone-like activity is crucial for this function. The DNA repair inhibition is a result of inefficient MDM2 dissociation from the NBN protein complex. When cancer cells are treated with chemotherapy, MDM2 can be easily released from the interaction and degraded, resulting in effective homologous recombination DNA repair, which translates into the acquisition of a chemoresistant phenotype by the tumor. This knowledge may allow for identification of the patients that are at particular risk of tumor chemoresistance.

Abstract

Analyzing the TCGA breast cancer database, we discovered that patients with the HER2 cancer subtype and overexpression of MDM2 exhibited decreased post-treatment survival. Inhibition of MDM2 expression in the SKBR3 cell line (HER2 subtype) diminished the survival of cancer cells treated with doxorubicin, etoposide, and camptothecin. Moreover, we demonstrated that inhibition of MDM2 expression diminished DNA repair by homologous recombination (HR) and sensitized SKBR3 cells to a PARP inhibitor, olaparib. In H1299 (TP53^−/−) cells treated with neocarzinostatin (NCS), overexpression of MDM2 WT or E3-dead MDM2 C478S variant stimulated the NCS-dependent phosphorylation of ATM, NBN, and BRCA1, proteins involved in HR DNA repair. However, overexpression of chaperone-dead MDM2 K454A variant diminished phosphorylation of these proteins as well as the HR DNA repair. Moreover, we demonstrated that, upon NCS treatment, MDM2 K454A interacted with NBN more efficiently than MDM2 WT and that MDM2 WT was degraded more efficiently than MDM2 K454A. Using a proliferation assay, we showed that overexpression of MDM2 WT, but not MDM2 K454A, led to acquisition of resistance to NCS. The presented results indicate that, following chemotherapy, MDM2 WT was released from MDM2-NBN complex and efficiently degraded, hence allowing extensive HR DNA repair leading to the acquisition of chemoresistance by cancer cells.

Overexpression of Taspase 1 Predicts Poor Prognosis in Patients with Hepatocellular Carcinoma

Background

Taspase 1 (TASP1) is a highly conserved protease involved in site-specific proteolysis. Existing researches have revealed a link between TASP1 expression and carcinogenesis. However, limited data are available regarding the prognosis and functions of TASP1 in hepatocellular carcinoma (HCC).

Methods

Western Blotting and qRT-PCR were employed to evaluate the level of TASP1 in HCC cell lines and clinical specimens. TASP1 expression was further calculated in clinical specimens by immunohistochemistry and the mRNA level of TASP1 in HCC was analyzed using Oncomine and UALCAN databases. The TASP1 promoter methylation modification was shown via MEXPRESS and UALCAN. The association between TASP1 expression and postoperative prognosis was evaluated using Kaplan–Meier and Cox regression analysis in clinical patients. The effect of TASP1 on HCC prognosis was analyzed via Kaplan-Meier plotter, GEPIA and UALCAN. Additionally, the regulators, kinases, miRNA and transcription factor targets of TASP1 were identified using LinkedOmics. Moreover, cBioPortal was used to detect the genetic alteration of TASP1. Finally, TIMER was utilized to assess the relation between TASP1 and the immune cell infiltration, whereas the correlation of TASP1 with three immune factors was detected through TISIDB.

Results

TASP1 expression was increased in HCC cell lines and HCC tissues. CNV and DNA methylation of TASP1 were changed. Survival analysis revealed that high TASP1 expression was correlated with overall survival (OS). Functional network analysis about TASP1 in HCC showed that the double-strand break repair, peptidyl-threonine modification, spindle organization, peptidyl-lysine modification and microtubule-based movement were modulated. Furthermore, TASP1 expression revealed puissant relation to the infiltration of immune cells and three immune factors in HCC.

Conclusion

These data indicate that TASP1 may act as a potential prognostic marker in HCC and regulate HCC via multiple mechanisms.

Regulation of tamoxifen sensitivity by the PLAC8/MAPK pathway axis is antagonized by curcumin-induced protein stability change

Tamoxifen resistance remains the major obstacle to the estrogen receptor positive breast cancer endocrine therapy. Placenta-specific 8 (PLAC8) has been implicated in epithelial-mesenchymal transition and tumorigenesis. However, the molecular mechanisms underlying PLAC8 function in the context of tamoxifen resistance are unclear. Curcumin has attracted considerable attention in the last decades. It is isolated from Curcuma longa and has beneficial effects in cancer therapy. We studied this property by using MCF-7 and tamoxifen-resistant breast cancer cells (MCF-7/TAM) cell lines. PLAC8 can regulate MCF-7/TAM cell drug sensitivity through the MAPK/ERK pathway and shows the potential effects of curcumin or as a possible druggable target against tamoxifen failure.

MRN complex is an essential effector of DNA damage repair

Genome stability can be threatened by both endogenous and exogenous agents. Organisms have evolved numerous mechanisms to repair DNA damage, including homologous recombination (HR) and non-homologous end joining (NHEJ). Among the factors associated with DNA repair, the MRE11-RAD50-NBS1 (MRN) complex (MRE11-RAD50-XRS2 in Saccharomyces cerevisiae) plays important roles not only in DNA damage recognition and signaling but also in subsequent HR or NHEJ repair. Upon detecting DNA damage, the MRN complex activates signaling molecules, such as the protein kinase ataxia-telangiectasia mutated (ATM), to trigger a broad DNA damage response, including cell cycle arrest. The nuclease activity of the MRN complex is responsible for DNA end resection, which guides DNA repair to HR in the presence of sister chromatids. The MRN complex is also involved in NHEJ, and has a species-specific role in hairpin repair. This review focuses on the structure of the MRN complex and its function in DNA damage repair.

Advances in synthetic lethality for cancer therapy: cellular mechanism and clinical translation

Synthetic lethality is a lethal phenomenon in which the occurrence of a single genetic event is tolerable for cell survival, whereas the co-occurrence of multiple genetic events results in cell death. The main obstacle for synthetic lethality lies in the tumor biology heterogeneity and complexity, the inadequate understanding of synthetic lethal interactions, drug resistance, and the challenges regarding screening and clinical translation. Recently, DNA damage response inhibitors are being tested in various trials with promising results. This review will describe the current challenges, development, and opportunities for synthetic lethality in cancer therapy. The characterization of potential synthetic lethal interactions and novel technologies to develop a more effective targeted drug for cancer patients will be explored. Furthermore, this review will discuss the clinical development and drug resistance mechanisms of synthetic lethality in cancer therapy. The ultimate goal of this review is to guide clinicians at selecting patients that will receive the maximum benefits of DNA damage response inhibitors for cancer therapy.

Kinesin light chain 4 as a new target for lung cancer chemoresistance via targeted inhibition of checkpoint kinases in the DNA repair network

The poor therapeutic efficacy of non-small cell lung cancer (NSCLC) is partly attributed to the acquisition of chemoresistance. To investigate the mechanism underlying this resistance, we examined the potential link between kinesin light chain 4 (KLC4), which we have previously reported to be associated with radioresistance in NSCLC, and sensitivity to chemotherapy in human lung cancer cell lines. KLC4 protein levels in lung cancer cells correlated with the degree of chemoresistance to cisplatin treatment. Furthermore, KLC4 silencing enhanced the cytotoxic effect of cisplatin by promoting DNA double-strand breaks and apoptosis. These effects were mediated by interaction with the checkpoint kinase CHK2, as KLC4 knockdown increased CHK2 activation, which was further enhanced in combination with cisplatin treatment. In addition, KLC4 and CHEK2 expression levels showed negative correlation in lung tumor samples from patients, and KLC4 overexpression correlated negatively with survival. Our results indicate a novel link between the KLC4 and CHK2 pathways regulating DNA damage response in chemoresistance, and highlight KLC4 as a candidate for developing lung cancer-specific drugs and customized targeted molecular therapy.

Clinical potential of ATM inhibitors

The protein defective in the human genetic disorder ataxia-telangiectasia, ATM, plays a central role in responding to DNA double strand breaks and other lesions to protect the genome against DNA damage and in this way minimize the risk of mutations that can lead to abnormal cellular behaviour. Its function in normal cells is to protect the cell against genotoxic stress but inadvertently it can assist cancer cells by providing resistance against chemotherapeutic agents and thus favouring tumour growth and survival. However, it is now evident that ATM also functions in a DNA damage-independent fashion to protect the cell against other forms of stress such as oxidative and nutrient stress and this non-canonical mechanism may also be relevant to cancer susceptibility in individuals who lack a functional ATM gene. Thus the use of ATM inhibitors to combat resistance in tumours may extend beyond a role for this protein in the DNA damage response. Here, we provide some background on ATM and its activation and investigate the efficacy of ATM inhibitors in treating cancer.

Beyond reversal: ubiquitin and ubiquitin-like proteases and the orchestration of the DNA double strand break repair response

The cellular response to genotoxic DNA double strand breaks (DSBs) uses a multitude of post-translational modifications to localise, modulate and ultimately clear DNA repair factors in a timely and accurate manner. Ubiquitination is well established as vital to the DSB response, with a carefully co-ordinated pathway of histone ubiquitination events being a central component of DSB signalling. Other ubiquitin-like modifiers (Ubl) including SUMO and NEDD8 have since been identified as playing important roles in DSB repair. In the last five years ∼20 additional Ub/Ubl proteases have been implicated in the DSB response. The number of proteases identified highlights the complexity of the Ub/Ubl signal present at DSBs. Ub/Ubl proteases regulate turnover, activity and protein-protein interactions of DSB repair factors both catalytically and non-catalytically. This not only ensures efficient repair of breaks but has a role in channelling repair into the correct DSB repair sub-pathways. Ultimately Ub/Ubl proteases have essential roles in maintaining genomic stability. Given that deficiencies in many Ub/Ubl proteases promotes sensitivity to DNA damaging chemotherapies, they could be attractive targets for cancer treatment.

ATM in DNA repair in cancer

Alterations in DNA damage response (DDR) pathways are hallmarks of cancer. Incorrect repair of DNA lesions often leads to genomic instability. Ataxia telangiectasia mutated (ATM), a core component of the DNA repair system, is activated to enhance the homologous recombination (HR) repair pathway upon DNA double-strand breaks. Although ATM signaling has been widely studied in different types of cancer, its research is still lacking compared with other DDR-involved molecules such as PARP and ATR. There is still a vast research opportunity for the development of ATM inhibitors as anticancer agents. Here, we focus on the recent findings of ATM signaling in DNA repair of cancer. Previous studies have identified several partners of ATM, some of which promote ATM signaling, while others have the opposite effect. ATM inhibitors, including KU-55933, KU-60019, KU-59403, CP-466722, AZ31, AZ32, AZD0156, and AZD1390, have been evaluated for their antitumor effects. It has been revealed that ATM inhibition increases a cancer cell's sensitivity to radiotherapy. Moreover, the combination with PARP or ATR inhibitors has synergistic lethality in some cancers. Of note, among these ATM inhibitors, AZD0156 and AZD1390 achieve potent and highly selective ATM kinase inhibition and have an excellent ability to penetrate the blood-brain barrier. Currently, AZD0156 and AZD1390 are under investigation in phase I clinical trials. Taken together, targeting ATM may be a promising strategy for cancer treatment. Hence, further development of ATM inhibitors is urgently needed in cancer research.