BRCA1-mediated ubiquitination inhibits topoisomerase II alpha activity in response to oxidative stress

GINS1 promotes the proliferation and migration of glioma cells through USP15-mediated deubiquitination of TOP2A

GINS1 is a GINS complex subunit that functions along with the MCM2-7 complex and Cdc45 in eukaryotic DNA replication. Despite the significance of the GINS complex in the switch between quiescence and proliferation of glioma cells inside and outside the perinecrotic niche, the biological functions and the underlying mechanism of GINS1 remain unclear. Unlike in normal cells and tissues, GINS1 expression level was significantly upregulated in glioma cells and tissues. High expression of GINS1 predicted an advanced clinical grade and a poor survival. Functional assays revealed that GINS1 aggravated glioma malignant phenotypes in vitro and in vivo. Mechanistically, this study identified that GINS1 physically interacts with TOP2A. GINS1 promotes glioma cell proliferation and migration through USP15-mediated deubiquitination of TOP2A protein. Our results delineate the clinical significance of GINS1 in glioma and the regulatory mechanisms involved in glioma cell proliferation and migration. This work provides potential therapeutic targets for glioma treatment.

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GINS1 expression was upregulated and associated with poor clinical outcome in glioma

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GINS1 aggravated glioma malignant phenotypes in vitro and in vivo

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GINS1 physically interacts with TOP2A

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GINS1 regulates glioma progression through USP15-mediated deubiquitination of TOP2A

DNA and RNA Binding Proteins: From Motifs to Roles in Cancer

DNA and RNA binding proteins (DRBPs) are a broad class of molecules that regulate numerous cellular processes across all living organisms, creating intricate dynamic multilevel networks to control nucleotide metabolism and gene expression. These interactions are highly regulated, and dysregulation contributes to the development of a variety of diseases, including cancer. An increasing number of proteins with DNA and/or RNA binding activities have been identified in recent years, and it is important to understand how their activities are related to the molecular mechanisms of cancer. In addition, many of these proteins have overlapping functions, and it is therefore essential to analyze not only the loss of function of individual factors, but also to group abnormalities into specific types of activities in regard to particular cancer types. In this review, we summarize the classes of DNA-binding, RNA-binding, and DRBPs, drawing particular attention to the similarities and differences between these protein classes. We also perform a cross-search analysis of relevant protein databases, together with our own pipeline, to identify DRBPs involved in cancer. We discuss the most common DRBPs and how they are related to specific cancers, reviewing their biochemical, molecular biological, and cellular properties to highlight their functions and potential as targets for treatment.

Regulation of topoisomerase II stability and activity by ubiquitination and SUMOylation: clinical implications for cancer chemotherapy

DNA topoisomerases II (TOP2) are peculiar enzymes (TOP2α and TOP2β) that modulate the conformation of DNA by momentarily breaking double-stranded DNA to allow another strand to pass through, and then rejoins the DNA phosphodiester backbone. TOP2α and TOP2β play vital roles in nearly all events involving DNA metabolism, including DNA transcription, replication, repair, and chromatin remodeling. Beyond these vital functions, TOP2 enzymes are therapeutic targets for various anticancer drugs, termed TOP2 poisons, such as teniposide, etoposide, and doxorubicin. These drugs exert their antitumor activity by inhibiting the activity of TOP2-DNA cleavage complexes (TOP2ccs) containing DNA double-strand breaks (DSBs), subsequently leading to the degradation of TOP2 by the 26S proteasome, thereby exposing the DSBs and eliciting a DNA damage response. Failure of the DSBs to be appropriately repaired leads to genomic instability. Due to this mechanism, patients treated with TOP2-based drugs have a high incidence of secondary malignancies and cardiotoxicity. While the cytotoxicity associated with TOP2 poisons appears to be TOP2α-dependent, the DNA sequence rearrangements and formation of DSBs appear to be mediated primarily through TOP2β inhibition, likely due to the differential degradation patterns of TOP2α and TOP2β. Research over the past few decades has shown that under various conditions, the ubiquitin-proteasome system (UPS) and the SUMOylation pathway are primarily responsible for regulating the stability and activity of TOP2 and are therefore critical regulators of the therapeutic effect of TOP2-targeting drugs. In this review, we summarize the current progress on the regulation of TOP2α and TOP2β by ubiquitination and SUMOylation. By fully elucidating the basic biology of these essential and complex molecular mechanisms, better strategies may be developed to improve the therapeutic efficacy of TOP2 poisons and minimize the risks of therapy-related secondary malignancy.

UPF1 promotes chemoresistance to oxaliplatin through regulation of TOP2A activity and maintenance of stemness in colorectal cancer

UPF1 is proved to dysregulate in multiple tumors and influence carcinogenesis. However, the role of UPF1 in oxaliplatin resistance in colorectal cancer (CRC) remains unknown. In our study, UPF1 is upregulated in CRC in mRNA and protein levels and overexpression of UPF1 predicts a poor overall survival (OS) and recurrence-free survival (RFS) in CRC patients and is an independent risk factor for recurrence. UPF1 promotes chemoresistance to oxaliplatin in vitro and in vivo. UPF1-induced oxaliplatin resistance can be associated with interaction between zinc finger of UPF1 and Toprim of TOP2A and increasing phosphorylated TOP2A in a SMG1-dependent manner. Moreover, UPF1 maintains stemness in a TOP2A-dependent manner in CRC. Taken together, UPF1 was overexpressed and predicted a poor prognosis in CRC. UPF1 enhanced chemoresistance to oxaliplatin in CRC, which may result from regulation of TOP2A activity and maintenance of stemness. Our findings could provide a new therapy strategy for chemoresistance to oxaliplatin in CRC patients.

The deubiquitylase USP15 regulates topoisomerase II alpha to maintain genome integrity

Ubiquitin-specific protease 15 (USP15) is a widely expressed deubiquitylase that has been implicated in diverse cellular processes in cancer. Here we identify topoisomerase II (TOP2A) as a novel protein that is regulated by USP15. TOP2A accumulates during G2 and functions to decatenate intertwined sister chromatids at prophase, ensuring the replicated genome can be accurately divided into daughter cells at anaphase. We show that USP15 is required for TOP2A accumulation, and that USP15 depletion leads to the formation of anaphase chromosome bridges. These bridges fail to decatenate, and at mitotic exit form micronuclei that are indicative of genome instability. We also describe the cell cycle-dependent behaviour for two major isoforms of USP15, which differ by a short serine-rich insertion that is retained in isoform-1 but not in isoform-2. Although USP15 is predominantly cytoplasmic in interphase, we show that both isoforms move into the nucleus at prophase, but that isoform-1 is phosphorylated on its unique S229 residue at mitotic entry. The micronuclei phenotype we observe on USP15 depletion can be rescued by either USP15 isoform and requires USP15 catalytic activity. Importantly, however, an S229D phospho-mimetic mutant of USP15 isoform-1 cannot rescue either the micronuclei phenotype, or accumulation of TOP2A. Thus, S229 phosphorylation selectively abrogates this role of USP15 in maintaining genome integrity in an isoform-specific manner. Finally, we show that USP15 isoform-1 is preferentially upregulated in a panel of non-small cell lung cancer cell lines, and propose that isoform imbalance may contribute to genome instability in cancer. Our data provide the first example of isoform-specific deubiquitylase phospho-regulation and reveal a novel role for USP15 in guarding genome integrity.

RNF168 and USP10 regulate topoisomerase IIα function via opposing effects on its ubiquitylation

Topoisomerase IIα (TOP2α) is essential for chromosomal condensation and segregation, as well as genomic integrity. Here we report that RNF168, an E3 ligase mutated in the human RIDDLE syndrome, interacts with TOP2α and mediates its ubiquitylation. RNF168 deficiency impairs decatenation activity of TOP2α and promotes mitotic abnormalities and defective chromosomal segregation. Our data also indicate that RNF168 deficiency, including in human breast cancer cell lines, confers resistance to the anti-cancer drug and TOP2 inhibitor etoposide. We also identify USP10 as a deubiquitylase that negatively regulates TOP2α ubiquitylation and restrains its chromatin association. These findings provide a mechanistic link between the RNF168/USP10 axis and TOP2α ubiquitylation and function, and suggest a role for RNF168 in the response to anti-cancer chemotherapeutics that target TOP2.

Proteasomal inhibition potentiates drugs targeting DNA topoisomerase II

The reaction mechanism of DNA topoisomerase II (TOP2) involves a covalent double-strand break intermediate in which the enzyme is coupled to DNA via a 5′-phosphotyrosyl bond. This normally transient enzyme-bridged break is stabilised by drugs such as mitoxantrone, mAMSA, etoposide, doxorubicin, epirubicin and idarubicin, which are referred to as TOP2 poisons. Removal of topoisomerase II by the proteasome is involved in the repair of these lesions. In K562 cells, inhibiting the proteasome with MG132 significantly potentiated the growth inhibition by these six drugs that target topoisomerase II, and the highest level of potentiation was observed with mitoxantrone. Mitoxantrone also showed the greatest potentiation by MG132 in three Nalm 6 cell lines with differing levels of TOP2A or TOP2B. Mitoxantrone was also potentiated by the clinically used proteasome inhibitor PS341 (Velcade). We have also shown that proteasome inhibition with MG132 in K562 cells reduces the rate of removal of mitoxantrone or etoposide stabilised topoisomerase complexes from DNA, suggesting a possible mechanism for the potentiation of topoisomerase II drugs by proteasomal inhibition.

PTEN stabilizes TOP2A and regulates the DNA decatenation

PTEN is a powerful tumor suppressor that antagonizes the cytoplasmic PI3K-AKT pathway and suppresses cellular proliferation. PTEN also plays a role in the maintenance of genomic stability in the nucleus. Here we report that PTEN facilitates DNA decatenation and controls a decatenation checkpoint. Catenations of DNA formed during replication are decatenated by DNA topoisomerase II (TOP2), and this process is actively monitored by a decatenation checkpoint in G2 phase. We found that PTEN deficient cells form ultra-fine bridges (UFBs) during anaphase and these bridges are generated as a result of insufficient decatenation. We show that PTEN is physically associated with a decatenation enzyme TOP2A and that PTEN influences its stability through OTUD3 deubiquitinase. In the presence of PTEN, ubiquitination of TOP2A is inhibited by OTUD3. Deletion or deficiency of PTEN leads to down regulation of TOP2A, dysfunction of the decatenation checkpoint and incomplete DNA decatenation in G2 and M phases. We propose that PTEN controls DNA decatenation to maintain genomic stability and integrity.

Topoisomerase IIα in chromosome instability and personalized cancer therapy

Genome instability is a hallmark of cancer cells. Chromosome instability (CIN), which is often mutually exclusive from hypermutation genotypes, represents a distinct subtype of genome instability. Hypermutations in cancer cells are due to defects in DNA repair genes, but the cause of CIN is still elusive. However, because of the extensive chromosomal abnormalities associated with CIN, its cause is likely a defect in a network of genes that regulate mitotic checkpoints and chromosomal organization and segregation. Emerging evidence has shown that the chromosomal decatenation checkpoint, which is critical for chromatin untangling and packing during genetic material duplication, is defective in cancer cells with CIN. The decatenation checkpoint is known to be regulated by a family of enzymes called topoisomerases. Among them, the gene encoding topoisomerase IIα (TOP2A) is commonly altered at both gene copy number and gene expression level in cancer cells. Thus, abnormal alterations of TOP2A, its interacting proteins, and its modifications may play a critical role in CIN in human cancers. Clinically, a large arsenal of topoisomerase inhibitors have been used to suppress DNA replication in cancer. However, they often lead to the secondary development of leukemia because of their effect on the chromosomal decatenation checkpoint. Therefore, topoisomerase drugs must be used judiciously and administered on an individual basis. In this review, we highlight the biological function of TOP2A in chromosome segregation and the mechanisms that regulate this enzyme's expression and activity. We also review the roles of TOP2A and related proteins in human cancers, and raise a perspective for how to target TOP2A in personalized cancer therapy.

ATM augments nuclear stabilization of DYRK2 by inhibiting MDM2 in the apoptotic response to DNA damage

The tumor suppressor p53 is a transcription factor that regulates cell cycle, DNA repair, senescence, and apoptosis in response to DNA damage. Phosphorylation of p53 at Ser-46 is indispensable for the commitment to apoptotic cell death. A previous study has shown that upon exposure to genotoxic stress, DYRK2 translocates into the nucleus and phosphorylates p53 at Ser-46, thereby inducing apoptosis. However, less is known about mechanisms responsible for intracellular control of DYRK2. Here we show the functional nuclear localization signal at N-terminal domain of DYRK2. Under normal conditions, nuclear and not cytoplasmic DYRK2 is ubiquitinated by MDM2, resulting in its constitutive degradation. In the presence of proteasome inhibitors, we detected a stable complex of DYRK2 with MDM2 at the nucleus. Upon exposure to genotoxic stress, ATM phosphorylates DYRK2 at Thr-33 and Ser-369, which enables DYRK2 to escape from degradation by dissociation from MDM2 and to induce the kinase activity toward p53 at Ser-46 in the nucleus. These findings indicate that ATM controls stability and pro-apoptotic function of DYRK2 in response to DNA damage.

The E3 ubiquitin-ligase Bmi1/Ring1A controls the proteasomal degradation of Top2alpha cleavage complex - a potentially new drug target

Background

The topoisomerases Top1, Top2α and Top2β are important molecular targets for antitumor drugs, which specifically poison Top1 or Top2 isomers. While it was previously demonstrated that poisoned Top1 and Top2β are subject to proteasomal degradation, this phenomena was not demonstrated for Top2α.

Methodology/Principal Findings

We show here that Top2α is subject to drug induced proteasomal degradation as well, although at a lower rate than Top2β. Using an siRNA screen we identified Bmi1 and Ring1A as subunits of an E3 ubiquitin ligase involved in this process. We show that silencing of Bmi1 inhibits drug-induced Top2α degradation, increases the persistence of Top2α-DNA cleavage complex, and increases Top2 drug efficacy. The Bmi1/Ring1A ligase ubiquitinates Top2α in-vitro and cellular overexpression of Bmi1 increases drug induced Top2α ubiquitination. A small-molecular weight compound, identified in a screen for inhibitors of Bmi1/Ring1A ubiquitination activity, also prevents Top2α ubiquitination and drug-induced Top2α degradation. This ubiquitination inhibitor increases the efficacy of topoisomerase 2 poisons in a synergistic manner.

Conclusions/Significance

The discovery that poisoned Top2α is undergoing proteasomal degradation combined with the involvement of Bmi1/Ring1A, allowed us to identify a small molecule that inhibits the degradation process. The Bmi1/Ring1A inhibitor sensitizes cells to Top2 drugs, suggesting that this type of drug combination will have a beneficial therapeutic outcome. As Bmi1 is also a known oncogene, elevated in numerous types of cancer, the identified Bmi1/Ring1A ubiquitin ligase inhibitors can also be potentially used to directly target the oncogenic properties of Bmi1.

Nuclear trafficking of pro-apoptotic kinases in response to DNA damage

The cellular response to genotoxic stress includes cell-cycle arrest, activation of DNA repair and induction of apoptosis. However, the signals that determine cell fate are largely unknown. Recent studies have shown that several pro-apoptotic kinases, including protein kinase C (PKC)delta, Abelson murine leukemia viral oncogene homolog 1 (c-Abl) and dual-specificity tyrosine-phosphorylation-regulated kinase 2 (DYRK2), undergo nuclear-cytoplasmic shuttling in response to DNA damage. Importantly, whereas precise regulation for the shuttling of these kinases remains uncertain, this mechanism has consequences for induction of apoptosis and implies that proper localization is central to the function of pro-apoptotic kinases. This review highlights recent progress demonstrating that the nuclear targeting of kinases is a novel and essential regulatory mechanism that directly influences the induction of apoptosis in response to DNA damage. The potential implications for novel therapies are also discussed.