Topoisomerase II inhibitors

Role of the Atypical MAPK ERK3 in Cancer Growth and Progression

Extracellular signal-regulated kinase 3 (ERK3) is an atypical mitogen-activated protein kinase (MAPK) whose structural and regulatory features are distinct from those of conventional MAPKs, such as ERK1/2. Since its identification in 1991, the regulation, substrates and functions of ERK3 have remained largely unknown. However, recent years have witnessed a wealth of new findings about ERK3 signaling. Several important biological functions for ERK3 have been revealed, including its role in neuronal morphogenesis, inflammation, metabolism, endothelial cell tube formation and epithelial architecture. In addition, ERK3 has been recently shown to play important roles in cancer cell proliferation, migration, invasion and chemoresistance in multiple types of cancers. Furthermore, accumulating studies have uncovered various molecular mechanisms by which the expression level, protein stability and activity of ERK3 are regulated. In particular, several post-translational modifications (PTMs), including ubiquitination, hydroxylation and phosphorylation, have been shown to regulate the stability and activity of ERK3 protein. In this review, we discuss recent findings regarding biochemical and cellular functions of ERK3, with a main focus on its roles in cancers, as well as the molecular mechanisms of regulating its expression and activity.

Exploration of the Role of the C-Terminal Domain of Human DNA Topoisomerase IIα in Catalytic Activity

Human topoisomerase IIα (TOP2A) is a vital nuclear enzyme involved in resolving knots and tangles in DNA during replication and cell division. TOP2A is a homodimer with a symmetrical, multidomain structure. While the N-terminal and core regions of the protein are well-studied, the C-terminal domain is poorly understood but is involved in enzyme regulation and is predicted to be intrinsically disordered. In addition, it appears to be a major region of post-translational modification and includes several Ser and Thr residues, many of which have not been studied for biochemical effects. Therefore, we generated a series of human TOP2A mutants where we changed specific Ser and Thr residues in the C-terminal domain to Ala, Gly, or Ile residues. We designed, purified, and examined 11 mutant TOP2A enzymes. The amino acid changes were made between positions 1272 and 1525 with 1-7 residues changed per mutant. Several mutants displayed increased levels of DNA cleavage without displaying any change in plasmid DNA relaxation or DNA binding. For example, mutations in the regions 1272-1279, 1324-1343, 1351-1365, and 1374-1377 produced 2-3 times more DNA cleavage in the presence of etoposide than wild-type TOP2A. Further, several mutants displayed changes in relaxation and/or decatenation activity. Together, these results support previous findings that the C-terminal domain of TOP2A influences catalytic activity and interacts with the substrate DNA. Furthermore, we hypothesize that it may be possible to regulate the enzyme by targeting positions in the C-terminal domain. Because the C-terminal domain differs between the two human TOP2 isoforms, this strategy may provide a means for selectively targeting TOP2A for therapeutic inhibition. Additional studies are warranted to explore these results in more detail.

9-Bromo-2,3-diethylbenzo[de]chromene-7,8-dione (MSN54): A novel non-intercalative topoisomerase II catalytic inhibitor

Novel mansonone F derivative MSN54 (9-bromo-2,3-diethylbenzo[de]chromene-7,8-dione) exhibited significant cytotoxicity against twelve human tumor cell lines in vitro, with particularly strong potency against HL-60/MX2 cell line resistant to Topo II poisons. MSN54 was found to have IC₅₀ of 0.69 and 1.43 µM against HL-60 and HL-60/MX2 cells, respectively. The resistance index is 10 times lower than that of the positive control VP-16 (etoposide). Various biological assays confirmed that MSN54 acted as a Topo IIα specific non-intercalative catalytic inhibitor. Furthermore, MSN54 exhibited good antitumor efficacy and low toxicity at a dose of 5 mg/kg in A549 tumor xenograft models. Thus, compound MSN54 is a promising candidate for the development of novel antitumor agents.

Enhanced Fructose Utilization Mediated by SLC2A5 Is a Unique Metabolic Feature of Acute Myeloid Leukemia with Therapeutic Potential

Rapidly proliferating leukemic progenitor cells consume substantial glucose, which may lead to glucose insufficiency in bone marrow. We show that acute myeloid leukemia (AML) cells are prone to fructose utilization with an upregulated fructose transporter GLUT5, which compensates for glucose deficiency. Notably, AML patients with upregulated transcription of the GLUT5-encoding gene SLC2A5 or increased fructose utilization have poor outcomes. Pharmacological blockage of fructose uptake ameliorates leukemic phenotypes and potentiates the cytotoxicity of the antileukemic agent, Ara-C. In conclusion, this study highlights enhanced fructose utilization as a metabolic feature of AML and a potential therapeutic target.

PCL–F68–PCL/PLGA–PEG–PLGA mixed micelles mediated delivery of mitoxantrone for reversing multidrug resistant in breast cancer

Mitoxantrone-loaded PCL–F68–PCL/PLGA–PEG–PLGA mixed micelles for reversing multidrug resistant in breast cancer.

Neurologic complications of chemotherapy and other newer and experimental approaches

Neurologic complications of conventional cytototxic agents as well as those from monoclonal antibodies and targeted therapies are increasingly observed in patients with cancer. The major categories are represented by alkylating agents (platinum compounds, ifosfamide, procarbazine, thiotepa), mitotic spindle inhibitors (vinca alkaloids, taxanes, etoposide, teniposide), proteasome inhibitors (bortezomib), antibiotics, antimetabolites, thalidomide, lenalidomide, topoisomerase inhibitors, interferon-α, hormones, bevacizumab, trastuzumab, and small tyrosine kinase inhibitors. Peripheral neuropathy is a common adverse effect of a number of chemotherapeutic drugs and often represents a critical factor limiting an adequate dose-intensity of chemotherapy. Regarding the central nervous system (CNS), it is vulnerable to many forms of toxicity from chemotherapeutic agents, including encephalopathy syndromes and confusional states, seizures, headache, cerebrovascular complications, visual loss, cerebellar syndromes, and myelopathy. For a given drug, the occurrence of CNS toxicity depends on several factors, including the total dose, route of administration, presence of structural brain lesions, exposure to prior or concurrent irradiation, and interactions with other drugs. However, many of the neurotoxic reactions are rare and idiosyncratic, and remain unpredictable. Several forms of neuroprotection and rehabilitation are being investigated. Last, the so-called "chemobrain" is an emerging issue, as it is a model of a subtle of and long-lasting damage to neuronal structures from some antineoplastic agents.

Bullied no more: when and how DNA shoves proteins around

The predominant protein-centric perspective in protein-DNA-binding studies assumes that the protein drives the interaction. Research focuses on protein structural motifs, electrostatic surfaces and contact potentials, while DNA is often ignored as a passive polymer to be manipulated. Recent studies of DNA topology, the supercoiling, knotting, and linking of the helices, have shown that DNA has the capability to be an active participant in its transactions. DNA topology-induced structural and geometric changes can drive, or at least strongly influence, the interactions between protein and DNA. Deformations of the B-form structure arise from both the considerable elastic energy arising from supercoiling and from the electrostatic energy. Here, we discuss how these energies are harnessed for topology-driven, sequence-specific deformations that can allow DNA to direct its own metabolism.

Neurological complications of chemotherapy to the central nervous system

One of the most common complications of chemotherapeutic drugs is toxicity to the central nervous system (CNS). This toxicity can manifest in many ways, including encephalopathy syndromes and confusional states, seizure activity, headache, cerebrovascular complications and stroke, visual loss, cerebellar dysfunction, and spinal cord damage with myelopathy. For many drugs, the toxicity is related to route of administration and cumulative dose, and can vary from brief, transient episodes to more severe, chronic sequelae. However, the neurotoxicity can be idiosyncratic and unpredictable in some cases. Among the antimetabolite drugs, methotrexate, 5-fluorouracil, and cytosine arabinoside are most likely to cause CNS toxicity. Of the alkylating agent chemotherapeutic drugs, the nitrosoureas (e.g., BCNU) and cisplatin most frequently cause toxicity to the CNS, especially when given via the intra-arterial route. Ifosfamide is also likely to cause neurotoxicity at high intravenous doses. Other alkylating agents, such as busulfan, cyclophosphamide, procarbazine, and temozolomide, are better tolerated by the CNS at moderate doses. The retinoid drugs are known to cause severe headaches at high doses. l-Asparaginase can induce an encephalopathy syndrome, as well as cerebrovascular complications such as stroke.

DNA topoisomerase II alpha: a favorable prognostic factor in colorectal caner

PURPOSE

There is a lack of study concerning expression of Topoisomerase IIα (Topo IIα) and long-term results in colorectal cancer patients. We aimed to investigate the relationship between expression of Topo IIα and clinicopathological parameters including overall survival in colorectal cancer.

METHODS

Paraffin-fixed specimens from a large prospective cohort of colorectal cancer patients who had been followed up for 4 years were assayed immunohistochemically.

RESULTS

Of 490 colorectal cancer patients accessible for Topo IIα expression, expression of Topo IIα was scored as (-) in 4 (0.8%) patients, (+) in 41 (8.4%) patients, (++) in 396 (80.8%) patients, and (+++) in 49 (10.0%) patients. Overexpression of Topo IIα was found to be related with lower T stage (p = 0.042), lower N stage (p = 0.038), and a lower incidence of recurrence with nearly significance (p = 0.053). Kaplan-Meier analyses showed that overexpression of Topo IIα was related with prolonged overall survival (p = 0.022) and disease-free survival (p = 0.036). Multivariate analyses showed that elevated serum CEA (p < 0.001), elevated serum CA199 (p = 0.002), poor differentiation (p = 0.001), advanced Dukes stage (p < 0.001), and lower expression of Topo IIα (p = 0.017) were independent predictive factors for poor prognosis.

CONCLUSIONS

Topo IIα expression is a valuable prognostic indicator for colorectal cancer and would be useful in treatment selection for early colorectal cancer and malignant colorectal polyps resected under endoscopy, especially when it is used in combination with serum CEA, CA199, and differentiation.

The use of divalent metal ions by type II topoisomerases

Type II topoisomerases are essential enzymes that regulate DNA under- and overwinding and remove knots and tangles from the genetic material. In order to carry out their critical physiological functions, these enzymes utilize a double-stranded DNA passage mechanism that requires them to generate a transient double-stranded break. Consequently, while necessary for cell survival, type II topoisomerases also have the capacity to fragment the genome. This feature of the prokaryotic and eukaryotic enzymes, respectively, is exploited to treat a variety of bacterial infections and cancers in humans. All type II topoisomerases require divalent metal ions for catalytic function. These metal ions function in two separate active sites and are necessary for the ATPase and DNA cleavage/ligation activities of the enzymes. ATPase activity is required for the strand passage process and utilizes the metal-dependent binding and hydrolysis of ATP to drive structural rearrangements in the protein. Both the DNA cleavage and ligation activities of type II topoisomerases require divalent metal ions and appear to utilize a novel variant of the canonical two-metal-ion phosphotransferase/hydrolase mechanism to facilitate these reactions. This article will focus primarily on eukaryotic type II topoisomerases and the roles of metal ions in the catalytic functions of these enzymes.

The DNA cleavage reaction of topoisomerase II: wolf in sheep's clothing

Topoisomerase II is an essential enzyme that is required for virtually every process that requires movement of DNA within the nucleus or the opening of the double helix. This enzyme helps to regulate DNA under- and overwinding and removes knots and tangles from the genetic material. In order to carry out its critical physiological functions, topoisomerase II generates transient double-stranded breaks in DNA. Consequently, while necessary for cell survival, the enzyme also has the capacity to fragment the genome. The DNA cleavage/ligation reaction of topoisomerase II is the target for some of the most successful anticancer drugs currently in clinical use. However, this same reaction also is believed to trigger chromosomal translocations that are associated with specific types of leukemia. This article will familiarize the reader with the DNA cleavage/ligation reaction of topoisomerase II and other aspects of its catalytic cycle. In addition, it will discuss the interaction of the enzyme with anticancer drugs and the mechanisms by which these agents increase levels of topoisomerase II-generated DNA strand breaks. Finally, it will describe dietary and environmental agents that enhance DNA cleavage mediated by the enzyme.

Human topoisomerase IIalpha uses a two-metal-ion mechanism for DNA cleavage

The DNA cleavage reaction of human topoisomerase IIα is critical to all of the physiological and pharmacological functions of the protein. While it has long been known that the type II enzyme requires a divalent metal ion in order to cleave DNA, the role of the cation in this process is not known. To resolve this fundamental issue, the present study utilized a series of divalent metal ions with varying thiophilicities in conjunction with DNA cleavage substrates that replaced the 3′-bridging oxygen of the scissile bond with a sulfur atom (i.e. 3′-bridging phosphorothiolates). Rates and levels of DNA scission were greatly enhanced when thiophilic metal ions were included in reactions that utilized sulfur-containing substrates. Based on these results and those of reactions that employed divalent cation mixtures, we propose that topoisomerase IIα mediates DNA cleavage via a two-metal-ion mechanism. In this model, one of the metal ions makes a critical interaction with the 3′-bridging atom of the scissile phosphate. This interaction greatly accelerates rates of enzyme-mediated DNA cleavage, and most likely is needed to stabilize the leaving 3′-oxygen.

Using 3'-bridging phosphorothiolates to isolate the forward DNA cleavage reaction of human topoisomerase IIalpha

The ability to cleave DNA is critical to the cellular and pharmacological functions of human type II topoisomerases. However, the low level of cleavage at equilibrium and the tight coupling of the cleavage and ligation reactions make it difficult to characterize the mechanism by which these enzymes cut DNA. Therefore, to establish a system that isolates topoisomerase II-mediated DNA scission from ligation, oligonucleotide substrates were developed that contained a 3'-bridging phosphorothiolate at the scissile bond. Scission of these substrates generates a 3'-terminal -SH moiety that is a poor nucleophile relative to the normal 3'-terminal -OH group. Consequently, topoisomerase II cannot efficiently ligate phosphorothiolate substrates once they are cleaved. The characteristics of topoisomerase IIalpha-mediated cleavage of phosphorothiolate oligonucleotides were identical to those seen with wild-type substrates, except that no ligation was observed. This unidirectional accumulation of cleavage complexes provided critical information regarding coordination of the protomer subunits of topoisomerase IIalpha and the mechanism of action of topoisomerase II poisons. Results indicate that the two enzyme subunits are partially coordinated and that cleavage at one scissile bond increases the degree of cleavage at the other. Furthermore, anticancer drugs such as etoposide and amsacrine that strongly inhibit topoisomerase II-mediated DNA ligation have little effect on the forward scission reaction. In contrast, abasic sites that increase levels of cleavage complexes without affecting ligation stimulate the forward rate of scission. Phosphorothiolate substrates provide significant advantages over traditional "suicide substrates" and should be valuable for future studies on DNA scission and the topoisomerase II-DNA cleavage complex.

Resistance to chemotherapy in cancer: a complex and integrated cellular response

Inherent and acquired resistance pathways account for the high rate of failure in cancer chemotherapy. The mechanisms or pathways mediating resistance may be classified as pharmacokinetic (i.e. alter intratumour drug exposue) or pharmacodynamic (i.e. failure to elicit cytotoxicity). More often than not, the resistant phenotype is characterised by alterations in multiple pathways. Consequently, the pathways may act synergistically or generate a broad spectrum of resistance to anticancer drugs. There has been a great deal of systematic characterisation of drug resistance in vitro. However, translating this greater understanding into clinical efficacy has rarely been achieved. This review explores the phenomenon of drug resistance in cancer and highlights the gap between in vitro and in vivo observations. This gap presents a major obstacle in overcoming drug resistance and restoring sensitivity to chemotherapy.

DNA Topology and Topoisomerases: Teaching a "Knotty" Subject

DNA is essentially an extremely long double-stranded rope in which the two strands are wound about one another. As a result, topological properties of the genetic material, including DNA underwinding and overwinding, knotting, and tangling, profoundly influence virtually every major nucleic acid process. Despite the importance of DNA topology, it is a conceptionally difficult subject to teach, because it requires students to visualize three-dimensional relationships. This article will familiarize the reader with the concept of DNA topology and offer practical approaches and demonstrations to teaching this "knotty" subject in the classroom. Furthermore, it will discuss topoisomerases, the enzymes that regulate the topological state of DNA in the cell. These ubiquitous enzymes perform a number of critical cellular functions by generating transient breaks in the double helix. During this catalytic event, topoisomerases maintain genomic stability by forming covalent phosphotyrosyl bonds between active site residues and the newly generated DNA termini. Topoisomerases are essential for cell survival. However, because they cleave the genetic material, these enzymes also have the potential to fragment the genome. This latter feature of topoisomerases is exploited by some of the most widely prescribed anticancer and antibacterial drugs currently in clinical use. Finally, in addition to curing cancer, topoisomerase action also has been linked to the induction of specific types of leukemia.

DNA topoisomerases I and II in human mature sperm cells: characterization and unique properties

BACKGROUND

The condensed state of the human sperm's chromatin is essential for the compact structure of the spermatozoon head, which is important for the fertilization process. The enzymes DNA topoisomerases (topo I and topo II) are responsible for the topological structure of the chromatin in somatic cells. The activities and the characterization of topoisomerases in mature human sperm cells have not been previously investigated.

METHODS

Sperm cells were purified from the semen of healthy donors by standard procedures and assays measuring the activities, protein size and sensitivity to inhibitors of topoisomerases were performed.

RESULTS

Topo I and topo II DNA relaxation activities are present in nuclear extracts derived from human sperm. The sperm topo I activity is inhibited by camptothecin, similarly to the somatic enzyme. An 85 kDa sperm protein, compared with the 100 kDa somatic topo IB enzyme, reacted with anti-human topo I antibody. Sperm topo II lacks the DNA decatenation activity of the somatic enzyme and a 97 kDa protein, compared with the 170 kDa somatic topo IIalpha enzyme, was detected with anti-human topo II antibody. Sperm nuclear extracts contained inhibitors of somatic topo II decatenation activity.

CONCLUSIONS

Topoisomerase I and II activities as well as topo I and topo II proteins are present in mature human sperm cells. These enzymes possess unique properties compared with their somatic counterparts.

Does topoisomerase II specifically recognize and cleave hairpins, cruciforms and crossovers of DNA?

DNA topoisomerase II is an enzyme that specializes in DNA disentanglement. It catalyzes the interconversion of DNA between different topological states. This event requires the passage of one duplex through another one via a transient double-strand break. Topoisomerase II is able to process any type of DNA, including structures such as DNA juxtapositions (crossovers), DNA hairpins or cruciforms, which are recognized with high specificity. In this review, we focused our attention on topoisomerase II recognizing DNA substrates that possess particular geometries. A strong cleavage site, as we identified in pBR322 DNA in the presence of ellipticine (site 22), appears to be characterized by a cruciform structure formed from two stable hairpins. The same sequence could also constitute a four-way junction structure stabilized by interactions involving ATC sequences. The latter have been shown to be able to promote Holliday junctions. We reviewed the recent literature that deals with the preferential recognition of crossovers by various topoisomerases. The single molecule relaxation experiments have demonstrated the differential abilities of the topoisomerases to recognize crossovers. It appears that enzymes, which distinguish the chirality of the crossovers, possess specialized domains dedicated to this function. We also stress that the formation of crossovers is dependent on the presence of adequate stabilizing sequences. Investigation of the impact of such structures on enzyme activity is important in order to both improve our knowledge of the mechanism of action of the topoisomerase II and to develop new inhibitors of this enzyme.