Topoisomerase II and the response to antileukemic therapy

XPO1 Inhibition using Selinexor Synergizes with Chemotherapy in Acute Myeloid Leukemia by Targeting DNA Repair and Restoring Topoisomerase IIα to the Nucleus

PURPOSE

Selinexor, a selective inhibitor of XPO1, is currently being tested as single agent in clinical trials in acute myeloid leukemia (AML). However, considering the molecular complexity of AML, it is unlikely that AML can be cured with monotherapy. Therefore, we asked whether adding already established effective drugs such as topoisomerase (Topo) II inhibitors to selinexor will enhance its anti-leukemic effects in AML.

EXPERIMENTAL DESIGN

The efficacy of combinatorial drug treatment using Topo II inhibitors (idarubicin, daunorubicin, mitoxantrone, etoposide) and selinexor was evaluated in established cellular and animal models of AML.

RESULTS

Concomitant treatment with selinexor and Topo II inhibitors resulted in therapeutic synergy in AML cell lines and patient samples. Using a xenograft MV4-11 AML mouse model, we show that treatment with selinexor and idarubicin significantly prolongs survival of leukemic mice compared with each single therapy.

CONCLUSIONS

Aberrant nuclear export and cytoplasmic localization of Topo IIα has been identified as one of the mechanisms leading to drug resistance in cancer. Here, we show that in a subset of patients with AML that express cytoplasmic Topo IIα, selinexor treatment results in nuclear retention of Topo IIα protein, resulting in increased sensitivity to idarubicin. Selinexor treatment of AML cells resulted in a c-MYC-dependent reduction of DNA damage repair genes (Rad51 and Chk1) mRNA and protein expression and subsequent inhibition of homologous recombination repair and increased sensitivity to Topo II inhibitors. The preclinical data reported here support further clinical studies using selinexor and Topo II inhibitors in combination to treat AML. Clin Cancer Res; 22(24); 6142-52. ©2016 AACR.

Cytotoxic effect and molecular docking of 4-ethoxycarbonylmethyl-1-(piperidin-4-ylcarbonyl)-thiosemicarbazide--a novel topoisomerase II inhibitor

The preliminary cytotoxic effect of 4-ethoxycarbonylmethyl-1-(piperidin-4-ylcarbonyl)-thiosemicarbazide hydrochloride (1)-a potent topoisomerase II inhibitor-was measured using a MTT assay. It was found that the compound decreased the number of viable cells in both estrogen receptor-positive MCF-7 and estrogen receptor-negative MDA-MB-231breast cancer cells, with IC(50) values of 146 ± 2 and 132 ± 2 μM, respectively. To clarify the molecular basis of the inhibitory action of 1, molecular docking studies were carried out. The results suggest that 1 targets the ATP binding pocket.

Substituents on etoposide that interact with human topoisomerase IIalpha in the binary enzyme-drug complex: contributions to etoposide binding and activity

Etoposide is a widely prescribed anticancer agent that stabilizes topoisomerase II-mediated DNA strand breaks. The drug contains a polycyclic ring system (rings A-D), a glycosidic moiety at C4, and a pendant ring (E-ring) at C1. A recent study that focused on yeast topoisomerase II demonstrated that the H15 geminal protons of the etoposide A-ring, the H5 and H8 protons of the B-ring, and the H2', H6', 3'-methoxyl, and 5'-methoxyl protons of the E-ring contact topoisomerase II in the binary enzyme-drug complex [ Wilstermann et al. (2007) Biochemistry 46, 8217-8225 ]. No interactions with the C4 sugar were observed. The present study used DNA cleavage assays, saturation transfer difference [ (1)H] NMR spectroscopy, and enzyme-drug binding studies to further define interactions between etoposide and human topoisomerase IIalpha. Etoposide and three derivatives that lacked the C4 sugar were analyzed. Except for the sugar, 4'-demethyl epipodophyllotoxin is identical to etoposide, epipodophyllotoxin contains a 4'-methoxyl group on the E-ring, and 6,7- O, O-demethylenepipodophyllotoxin replaces the A-ring with a diol. Results suggest that etoposide-topoisomerase IIalpha binding is driven by interactions with the A- and B-rings and potentially by stacking interactions with the E-ring. We propose that the E-ring pocket on the enzyme is confined, because the addition of bulk to this ring adversely affects drug function. The A- and E-rings do not appear to contact DNA in the enzyme-drug-DNA complex. Conversely, the sugar moiety subtly alters DNA interactions. The identification of etoposide substituents that contact topoisomerase IIalpha in the binary complex has predictive value for drug behavior in the enzyme-etoposide-DNA complex.

Proteasomal inhibition stabilizes topoisomerase IIalpha protein and reverses resistance to the topoisomerase II poison ethonafide (AMP-53, 6-ethoxyazonafide)

Multiple myeloma (MM) is an incurable malignancy of plasma cells. Although multiple myeloma patients often respond to initial therapy, the majority of patients will relapse with disease that is refractory to further drug treatment. Thus, new therapeutic strategies are needed. One common mechanism of acquired drug resistance involves a reduction in the expression or function of the drug target. We hypothesized that the cytotoxic activity of topoisomerase II (topo II) poisons could be enhanced, and drug resistance overcome, by increasing the expression and activity of the drug target, topo II in myeloma cells. To test this hypothesis, we evaluated the cytotoxicity of the anthracene-containing topo II poison, ethonafide (AMP-53/6-ethoxyazonafide), in combination with the proteasome inhibitor bortezomib (PS-341/Velcade). Combination drug activity studies were done in 8226/S myeloma cells and its drug resistant subclone, 8226/Dox1V. We found that a 24-h treatment of cells with bortezomib maximally increased topo IIalpha protein expression and activity, and consistently increased the cytotoxicity of ethonafide in the 8226/S and 8226/Dox1V cell lines. This increase in cytotoxicity corresponded to an increase in DNA double-strand breaks, as measured by the neutral comet assay. Therefore, increasing topo IIalpha expression through inhibition of proteasomal degradation increased DNA double-strand breaks and enhanced the cytotoxicity of the topo II poison ethonafide. These data suggest that bortezomib-mediated stabilization of topo IIalpha expression may potentiate the cytotoxic activity of topo II poisons and thereby, provide a strategy to circumvent drug resistance.

Topoisomerase II - drug interaction domains: identification of substituents on etoposide that interact with the enzyme

Etoposide is one of the most successful chemotherapeutic agents used for the treatment of human cancers. The drug kills cells by inhibiting the ability of topoisomerase II to ligate nucleic acids that it cleaves during the double-stranded DNA passage reaction. Etoposide is composed of a polycyclic ring system (rings A-D), a glycosidic moiety at the C4 position, and a pendent ring (E-ring) at the C1 position. Although drug-enzyme contacts, as opposed to drug-DNA interactions, mediate the entry of etoposide into the topoisomerase II-drug-DNA complex, the substituents on etoposide that interact with the enzyme have not been identified. Therefore, saturation transfer difference [1H]-nuclear magnetic resonance spectroscopy and protein-drug competition binding assays were employed to define the groups on etoposide that associate with yeast topoisomerase II and human topoisomerase IIalpha. Results indicate that the geminal protons of the A-ring, the H5 and H8 protons of the B-ring, and the H2' and H6' protons and the 3'- and 5'-methoxyl protons of the pendent E-ring interact with both enzymes in the binary protein-ligand complexes. In contrast, no significant nuclear Overhauser enhancement signals arising from the C-ring, the D-ring, or the C4 glycosidic moiety were observed with either enzyme, suggesting that there is limited or no contact between these portions of etoposide and topoisomerase II in the binary complex. The functional importance of E-ring substituents was confirmed by topoisomerase II-mediated DNA cleavage assays.

Mutation of cysteine residue 455 to alanine in human topoisomerase IIalpha confers hypersensitivity to quinones: enhancing DNA scission by closing the N-terminal protein gate

Several quinone-based metabolites of industrial and environmental toxins are potent topoisomerase II poisons. These compounds act by adducting the protein, and previous studies suggest that they increase levels of enzyme-associated DNA strand breaks by at least two potential mechanisms. Quinones act directly on the DNA cleavage-ligation equilibrium of topoisomerase II by inhibiting the rate of ligation. They also block the N-terminal gate of the protein, thereby stabilizing topoisomerase II in its "closed clamp" form and trapping DNA in the central annulus of the enzyme. It has been proposed that this latter activity enhances DNA cleavage by increasing the population of enzyme molecules with DNA in their active sites, but a causal relationship has not been established. In order to more fully characterize the mechanistic basis for quinone action against topoisomerase II, the present study characterized the sensitivity of human topoisomerase IIalpha carrying a Cys455-->Ala mutation (top2alphaC455A) toward quinones. Cys455 was identified as a site of quinone adduction by mass spectrometry. The mutant enzyme was approximately 1.5-2-fold hypersensitive to 1,4-benzoquinone and the polychlorinated biphenyl quinone 4'Cl-2,5pQ, but it displayed wild-type sensitivity to traditional topoisomerase II poisons. The ability of 1,4-benzoquinone to inhibit DNA ligation mediated by top2alphaC455A was similar to that of wild-type topoisomerase IIalpha. However, the quinone induced approximately 3 times the level of clamp closure with the mutant enzyme. These findings strongly support the hypothesis that the ability of quinones to block the N-terminal gate of the type II enzyme contributes to their actions as topoisomerase II poisons.

Bioflavonoids as poisons of human topoisomerase II alpha and II beta

Bioflavonoids are human dietary components that have been linked to the prevention of cancer in adults and the generation of specific types of leukemia in infants. While these compounds have a broad range of cellular activities, many of their genotoxic effects have been attributed to their actions as topoisomerase II poisons. However, the activities of bioflavonoids against the individual isoforms of human topoisomerase II have not been analyzed. Therefore, we characterized the activity and mechanism of action of three major classes of bioflavonoids, flavones, flavonols, and isoflavones, against human topoisomerase IIalpha and IIbeta. Genistein was the most active bioflavonoid tested and stimulated enzyme-mediated DNA cleavage approximately 10-fold. Generally, compounds were more active against topoisomerase IIbeta. DNA cleavage with both enzyme isoforms required a 5-OH and a 4'-OH and was enhanced by the presence of additional hydroxyl groups on the pendant ring. Competition DNA cleavage and topoisomerase II binding studies indicate that the 5-OH group plays an important role in mediating genistein binding, while the 4'-OH moiety contributes primarily to bioflavonoid function. Bioflavonoids do not require redox cycling for activity and function primarily by inhibiting enzyme-mediated DNA ligation. Mutagenesis studies suggest that the TOPRIM region of topoisomerase II plays a role in genistein binding. Finally, flavones, flavonols, and isoflavones with activity against purified topoisomerase IIalpha and IIbeta enhanced DNA cleavage by both isoforms in human CEM leukemia cells. These data support the hypothesis that bioflavonoids function as topoisomerase II poisons in humans and provide a framework for further analysis of these important dietary components.

Quinone-induced enhancement of DNA cleavage by human topoisomerase IIalpha: adduction of cysteine residues 392 and 405

Several quinone-based metabolites of drugs and environmental toxins are potent topoisomerase II poisons. These compounds act by adducting the protein and appear to increase levels of enzyme-DNA cleavage complexes by at least two potentially independent mechanisms. Treatment of topoisomerase IIalpha with quinones inhibits DNA religation and blocks the N-terminal gate of the protein by cross-linking its two protomer subunits. It is not known whether these two effects result from adduction of quinone to the same amino acid residue(s) in topoisomerase IIalpha or whether they are mediated by modification of separate residues. Therefore, this study identified amino acid residues in human topoisomerase IIalpha that are modified by quinones and determined their role in the actions of these compounds as topoisomerase II poisons. Four cysteine residues were identified by mass spectrometry as sites of quinone adduction: Cys170, Cys392, Cys405, and Cys455. Mutations (Cys --> Ala) were individually generated at each position. Only mutations at Cys392 or Cys405 reduced sensitivity ( approximately 50% resistance) to benzoquinone. Top2alphaC392A and top2alphaC405A displayed faster rates ( approximately 2-fold) of DNA religation than wild-type topoisomerase IIalpha in the presence of the quinone. In contrast, as determined by DNA binding, protein clamp closing, and protomer cross-linking experiments, mutations at Cys392 and Cys405 did not affect the ability of benzoquinone to block the N-terminal gate of topoisomerase IIalpha. These findings indicate that adduction of Cys392 and Cys405 is important for the actions of quinones against the enzyme and increases levels of cleavage complexes primarily by inhibiting DNA religation.

Activation of the unfolded protein response is necessary and sufficient for reducing topoisomerase IIalpha protein levels and decreasing sensitivity to topoisomerase-targeted drugs

A wide range of chemotherapeutic agents has been identified that are active against solid tumors. However, resistance remains an important obstacle to the development of curative regimens. Whereas much attention has been paid to acquired drug resistance, a variety of physiological pathways also have been described that reduce the sensitivity of previously untreated tumors to cytotoxic antitumor agents. Treatment of cells with pharmacological agents that alter the environment of the endoplasmic reticulum (ER) and activate the unfolded protein response (UPR) can render cells resistant to topoisomerase II poisons. We describe experiments showing that activation of the mammalian ER stress response is both necessary and sufficient to decrease topoisomerase IIalpha protein levels and to render cells resistant to etoposide, a topoisomerase II-targeting drug. This is not caused by the elevated levels of BiP that are a hallmark of this response, because a cell line that has been engineered to overexpress BiP does not show increased resistance to etoposide. The UPR was shown to be required for altered drug sensitivity, because the BiP-overexpressing cell line, which is unable to activate the UPR, did not show decreased topoisomerase II levels or increased resistance to etoposide in response to stress conditions. The transient overexpression of an unfolded protein activated the UPR and led to the concomitant loss of topoisomerase IIalpha protein from the cells, demonstrating that UPR activation is sufficient for the changes in topoisomerase II levels that had been observed previously with pharmacological induction of the UPR.

Identification of new classes among acute myelogenous leukaemias with normal karyotype using gene expression profiling

Conventional cytogenetic analysis currently stratifies acute myelogenous leukaemia (AML) into prognostically relevant groups. However, approximately 50% of adult AMLs have normal cytogenetics (NC-AMLs), and represent a heterogeneous and poorly understood group. We analysed gene expression in 55 AML samples including 53 cases from adult patients with NC-AML (n = 36), trisomy 8, t(15;17), t(8;21), t(11;19), 7q deletion, and two cell lines using 9000-gene DNA microarrays. Global hierarchical clustering showed that NC-AMLs are a heterogeneous group. Supervised analysis distinguished two subgroups of NC-AML: one subgroup constituted a homogeneous NC cluster ('pure NC-AML'), and the other NC-AMLs were close to the AML cases with translocations ('translocation like'). Gene expression signatures were also derived for patients with trisomy 8, as well as FLT3 and MLL gene duplications. Importantly, samples from 24 NC-AML patients who could be evaluated for clinical outcome were analysed. In all, 43 genes that discriminated two classes of patients with significantly different prognosis were identified. The poor prognosis class contained a majority of 'pure NC-AMLs', whereas the 'translocation-like' AMLs were in the good prognosis class. Discriminator genes included genes involved in drug resistance (TOP2B), protein transport (MTX2, SLC35A2), and cell signalling (MAPK1, PRKAB2). Our results demonstrate the transcriptional heterogeneity of NC-AMLs, and suggest the existence of 'translocation-like' NC-AMLs and of a gene expression signature that may predict response to chemotherapy.

Modification of topoisomerase genes copy number in newly diagnosed childhood acute lymphoblastic leukemia

Topoisomerase genes were analyzed at both DNA and RNA levels in 25 cases of newly diagnosed childhood acute lymphoblastic leukemia (ALL). The results of molecular analysis were compared to risk group classification of children in order to identify molecular characteristics associated with response to therapy. At diagnosis, allelic imbalance at topo-isomerase IIalpha (TOP2A) gene locus was found in 75% of informative cases whereas topoisomerase I and IIbeta gene loci are altered in none or only one case, respectively. By semi-quantitative Polymerase chain reaction, we found a 2.5 to 8-fold TOP2A gene amplification in 72% of the children, which was correlated to gene overexpression in every case. These results show that TOP2A gene amplification is a frequent event in ALL at diagnosis. Interestingly, we also identified a small population of children that do not present TOP2A gene amplification or gene overexpression and who are significantly associated with very high risk classified patients showing glucocorticoid resistance. In conclusion, characterization of TOP2A gene status in childhood ALL at diagnosis provides useful complementary information for risk assessment.

Drug resistance mechanisms in acute leukemia

Markers of anticancer drug resistance are predictive of treatment response and outcome in patients with acute myeloid leukemia. Immunologic detection of the drug efflux pumps, P-glycoprotein (Pgp) and multidrug resistance-associated protein 1 (MRP1), correlate with functional assays of drug resistance. These accumulation defects also appear operable in acute lymphoblastic leukemia. Many of the efflux pumps identified share significant structural homology with the large superfamily of ATP-binding cassette transporters. Other markers such as lung-resistance protein, bcl-2, and breast cancer-resistance protein, have been described in acute myeloid leukemia patients although their pathophysiology and clinical relevance are less clear and the methodology for their quantification are not well standardized. Preclinical studies have shown that small molecules capable of reversing efflux can restore drug sensitivity in resistant tumor models. Although initial clinical studies were limited by both potency and specificity of the reverser, later studies with more effective reversers have in many instances been limited by pharmacokinetic interactions exacerbating the clinical toxicities of chemotherapy. Although one large randomized study has demonstrated a proven survival advantage without increased toxicity using cyclosporine, the inconsistent results with other modulators raise doubt as to the utility and overall strategy of using drug efflux blockers in patients with established Pgp overexpression. Many of these patients have additional resistance mechanisms, and achieving meaningful clinical responses will likely require more complex clinical strategies. Preventing or delaying development of drug resistance in chemosensitive patients represents another therapeutic strategy to be tested.