Pharmacological inhibition of c-Abl compromises genetic stability and DNA repair in Bcr-Abl-negative cells

Deregulation of whole-transcriptome gene expression in zebrafish (Danio rerio) after chronic exposure to low doses of imatinib mesylate in a complete life cycle study

Imatinib mesylate (IM) is an anticancer drug that belongs to tyrosine kinase inhibitors. We report the results of the first investigation of the chronic exposure of zebrafish (Danio rerio) to IM. The exposure to IM (0.01, 1 and 100 μg/L) was initiated in adult fish and continued through hatching and the offspring generation for seven months. In addition to standard toxicological endpoints, induction of genotoxic effects and whole-genome transcriptome of liver samples of offspring generation of zebrafish were analysed. Exposure to IM did not affect the survival and growth of zebrafish, did not cause any histopathological changes, but it induced a marginal increase in the chromosomal damage in blood cells. The whole-genome transcriptome analyses demonstrated dose-dependent increase in the number of differentially expressed genes with a significantly higher number of deregulated genes in female fish compared to male. Differentially expressed genes included genes involved in response to DNA damage, cell cycle control and regulation of circadian rhythm. Based on the low genotoxic activity and the pattern of the changes in DNA damage responsive genes we consider that at current environmental exposure levels, IM represents low risk for genotoxic effects in aquatic organisms. Exposure to IM also induced deregulation of the expression of genes associated with steroidogenesis and hormone metabolism and function, which indicates hormone-disrupting activity of IM that has not been studied so far. The study provide new information on the potential consequences of chronic exposure to the residues of tyrosine kinase inhibitors, which remain to be further explored.

Evaluation of oxidative stress responses in human circulating blood cells after imatinib mesylate treatment - Implications to its mechanism of action

Imatinib mesylate (IM) is the first developed protein kinase inhibitor and recently it has topped consumption rates among targeted and total anticancer drugs. Although there are indications that IM possesses cyto/genotoxic activities against normal non-target cells as well, there is a lack of information regarding the underlying mechanism involved in those actions. Therefore, we aimed to evaluate the response of human circulating blood cells towards oxidative stress after IM treatment (0.0001–10 µg/mL) in vitro. Based on the results, IM had an influence on all of the oxidative stress parameters tested. Lower concentrations of IM induced an increase of glutathione level, following its decrease at higher IM concentrations indicating impairment in oxidative stress defences. Concomitant to a glutathione decrease, an increase of malondialdehyde and protein carbonyls level was observed indicating oxidative damage of lipids and proteins. The observed effects overlapped with the observed formation of oxidative base damage detected by formamidopyrimidine-DNA glycosylase modified-comet assay indicating that IM managed to induce oxidative DNA damage. Our results provide novelty in their mechanistic approach to IM-induced toxicity in non-target cells and suggest that IM can affect blood cells and induce oxidative stress.

Dasatinib induces DNA damage and activates DNA repair pathways leading to senescence in non-small cell lung cancer cell lines with kinase-inactivating BRAF mutations

Improved therapies are greatly needed for non-small cell lung cancer (NSCLC) that does not harbor targetable kinase mutations or translocations. We previously demonstrated that NSCLC cells that harbor kinase-inactivating BRAF mutations (^KIBRAF) undergo senescence when treated with the multitargeted kinase inhibitor dasatinib. Similarly, treatment with dasatinib resulted in a profound and durable response in a patient with ^KIBRAF NSCLC. However, no canonical pathways explain dasatinib-induced senescence in ^KIBRAF NSCLC. To investigate the underlying mechanism, we used 2 approaches: gene expression and reverse phase protein arrays. Both approaches showed that DNA repair pathways were differentially modulated between ^KIBRAF NSCLC cells and those with wild-type (WT) BRAF. Consistent with these findings, dasatinib induced DNA damage and activated DNA repair pathways leading to senescence only in the ^KIBRAF cells. Moreover, dasatinib-induced senescence was dependent on Chk1 and p21, proteins known to mediate DNA damage-induced senescence. Dasatinib also led to a marked decrease in TAZ but not YAP protein levels. Overexpression of TAZ inhibited dasatinib-induced senescence. To investigate other vulnerabilities in ^KIBRAF NSCLC cells, we compared the sensitivity of these cells with that of ^WTBRAF NSCLC cells to 79 drugs and identified a pattern of sensitivity to EGFR and MEK inhibitors in the ^KIBRAF cells. Clinically approved EGFR and MEK inhibitors, which are better tolerated than dasatinib, could be used to treat ^KIBRAF NSCLC. Our novel finding that dasatinib induced DNA damage and subsequently activated DNA repair pathways leading to senescence in ^KIBRAF NSCLC cells represents a unique vulnerability with potential clinical applications.

Inhibition of c-Abl kinase activity renders cancer cells highly sensitive to mitoxantrone

Although c-Abl has increasingly emerged as a key player in the DNA damage response, its role in this context is far from clear. We studied the effect of inhibition of c-Abl kinase activity by imatinib with chemotherapy drugs and found a striking difference in cell survival after combined mitoxantrone (MX) and imatinib treatment compared to a panel of other chemotherapy drugs. The combinatory treatment induced apoptosis in HeLa cells and other cancer cell lines but not in primary fibroblasts. The difference in MX and doxorubicin was related to significant augmentation of DNA damage. Transcriptionally active p53 accumulated in cells in which human papillomavirus E6 normally degrades p53. The combination treatment resulted in caspase activation and apoptosis, but this effect did not depend on either p53 or p73 activity. Despite increased p53 activity, the cells arrested in G2 phase became defective in this checkpoint, allowing cell cycle progression. The effect after MX treatment depended partially on c-Abl: Short interfering RNA knockdown of c-Abl rendered HeLa cells less sensitive to MX. The effect of imatinib was decreased by c-Abl siRNA suggesting a role for catalytically inactive c-Abl in the death cascade. These findings indicate that MX has a unique cytotoxic effect when the kinase activity of c-Abl is inhibited. The treatment results in increased DNA damage and c-Abl–dependent apoptosis, which may offer new possibilities for potentiation of cancer chemotherapy.

RITA can induce cell death in p53-defective cells independently of p53 function via activation of JNK/SAPK and p38

Significant advances have been made in the development of small molecules blocking the p53/MDM2 interaction. The Mdm2 inhibitor Nutlin-3 is restricted to tumors carrying wtp53. In contrast, RITA, a compound that binds p53, has recently been shown also to restore transcriptional functions of mtp53. As more than 50% of solid tumors carry p53 mutations, RITA promises to be a more effective therapeutic strategy than Nutlin-3. We investigated effects of RITA on apoptosis, cell cycle and induction of 45 p53 target genes in a panel of 14 cell lines from different tumor entities with different p53 status as well as primary lymphocytes and fibroblasts. Nine cell strains expressed wtp53, four harbored mtp53, and three were characterized by the loss of p53 protein. A significant induction of cell death upon RITA was observed in 7 of 16 cell lines. The nonmalignant cells in our panel were substantially less sensitive. We found that in contrast to Nultin-3, RITA is capable to induce cell death not only in tumor cells harboring wtp53 and mtp53 but also in p53-null cells. Importantly, whereas p53 has a central role for RITA-mediated effects in wtp53 cells, neither p53 nor p63 or p73 were essential for the RITA response in mtp53 or p53-null cells in our panel demonstrating that besides the known p53-dependent action of RITA in wtp53 cells, RITA can induce cell death also independently of p53 in cells harboring defective p53. We identified an important role of both p38 and JNK/SAPK for sensitivity to RITA in these cells leading to a typical caspase- and BAX/BAK-dependent mitochondrial apoptosis. In conclusion, our data demonstrate that RITA can induce apoptosis through p38 and JNK/SAPK not only in tumor cells harboring wtp53 and mtp53 but also in p53-null cells, making RITA an interesting tumor-selective drug.

Analyses of the combination of 6-MP and dasatinib in cell culture

A major tenet of cancer therapeutics is that combinations of anticancer agents with different mechanisms of action and different toxicities may be effective treatment regimens. Evaluation of additivity/synergy in cell culture may be used to identify drug combination opportunities and to assess risk of additive/synergistic toxicity. The combination of 6-mercaptopurine and dasatinib was assessed for additivity/synergy using the combination index (CI) method and a response surface method in six human tumor cell lines including MCF-7 and MDA-MB‑468 breast cancer, NCI-H23 and NCI-H460 non‑small cell lung cancer, and A498 and 786-O renal cell cancer, based on two experimental end‑points: ATP content and colony formation. Clonal colony formation by human bone marrow CFU-GM was used to assess risk of enhanced toxicity. The concentration ranges tested for each drug were selected to encompass the clinical Cmax concentrations. The combination regimens were found to be additive to sub‑additive by both methods of data analysis, but synergy was not detected. The non-small cell lung cancer cell lines were the most responsive among the tumor lines tested and the renal cell carcinoma lines were the least responsive. The bone marrows CFU-GM were more sensitive to the combination regimens than were the tumor cell lines. Based upon these data, it appears that the possibility of enhanced efficacy from combining 6-mercaptopurine (6-MP) and dasatinib would be associated with increased risk of severe bone marrow toxicity, so the combination is unlikely to provide a therapeutic advantage for treating solid tumor patients where adequate bone marrow function must be preserved.

Imatinib sensitivity in BCR-ABL1-positive chronic myeloid leukemia cells is regulated by the remaining normal ABL1 allele

Chronic myeloid leukemia in chronic phase (CML-CP) cells that harbor oncogenic BCR-ABL1 and normal ABL1 allele often become resistant to the ABL1 kinase inhibitor imatinib. Here, we report that loss of the remaining normal ABL1 allele in these tumors, which results from cryptic interstitial deletion in 9q34 in patients who did not achieve a complete cytogenetic remission (CCyR) during treatment, engenders a novel unexpected mechanism of imatinib resistance. BCR-ABL1-positive Abl1(-/-) leukemia cells were refractory to imatinib as indicated by persistent BCR-ABL1-mediated tyrosine phosphorylation, lack of BCR-ABL1 protein degradation, increased cell survival, and clonogenic activity. Expression of ABL1 kinase, but not a kinase-dead mutant, restored the antileukemic effects of imatinib in ABL1-negative chronic myelogenous leukemia (CML) cells and in BCR-ABL1-positive Abl1(-/-) murine leukemia cells. The intracellular concentration of imatinib and expression of its transporters were not affected, although proteins involved in BCR-ABL1 degradation were downregulated in Abl1(-/-) cells. Furthermore, 12 genes associated with imatinib resistance were favorably deregulated in Abl1(-/-) leukemia. Taken together, our results indicate that loss of the normal ABL1 kinase may serve as a key prognostic factor that exerts major impact on CML treatment outcomes.

c-Abl downregulates the slow phase of double-strand break repair

c-Abl tyrosine kinase is activated by agents that induce double-strand DNA breaks (DSBs) and interacts with key components of the DNA damage response and of the DSB repair machinery. However, the functional significance of c-Abl in these processes, remained unclear. In this study, we demonstrate, using comet assay and pulsed-field gel electrophoresis, that c-Abl inhibited the repair of DSBs induced by ionizing radiation, particularly during the second and slow phase of DSB repair. Pharmacological inhibition of c-Abl and c-Abl depletion by siRNA-mediated knockdown resulted in higher DSB rejoining. c-Abl null MEFs exhibited higher DSB rejoining compared with cells reconstituted for c-Abl expression. Abrogation of c-Abl kinase activation resulted in higher H2AX phosphorylation levels and higher numbers of post-irradiation γH2AX foci, consistent with a role of c-Abl in DSB repair regulation. In conjunction with these findings, transient abrogation of c-Abl activity resulted in increased cellular radioresistance. Our findings suggest a novel function for c-Abl in inhibition of the slow phase of DSB repair.

HDAC inhibitors potentiate the activity of the BCR/ABL kinase inhibitor KW-2449 in imatinib-sensitive or -resistant BCR/ABL+ leukemia cells in vitro and in vivo

PURPOSE

The purpose of this study was to determine whether histone deacetylase (HDAC) inhibitors (HDACI) such as vorinostat or entinostat (SNDX-275) could increase the lethality of the dual Bcr/Abl-Aurora kinase inhibitor KW-2449 in various Bcr/Abl(+) human leukemia cells, including those resistant to imatinib mesylate (IM).

EXPERIMENTAL DESIGN

Bcr/Abl(+) chronic myelogenous leukemia (CML) and acute lymphoblastic leukemia (ALL) cells, including those resistant to IM (T315I, E255K), were exposed to KW-2449 in the presence or absence of vorinostat or SNDX-275, after which apoptosis and effects on signaling pathways were examined. In vivo studies combining HDACIs and KW2449 were carried out by using a systemic IM-resistant ALL xenograft model.

RESULTS

Coadministration of HDACIs synergistically increased KW-2449 lethality in vitro in multiple CML and Ph(+) ALL cell types including human IM resistant cells (e.g., BV-173/E255K and Adult/T315I). Combined treatment resulted in inactivation of Bcr/Abl and downstream targets (e.g., STAT5 and CRKL), as well as increased reactive oxygen species (ROS) generation and DNA damage (γH2A.X). The latter events and cell death were significantly attenuated by free radical scavengers (TBAP). Increased lethality was also observed in primary CD34(+) cells from patients with CML, but not in normal CD34(+) cells. Finally, minimally active vorinostat or SNDX275 doses markedly increased KW2449 antitumor effects and significantly prolonged the survival of murine xenografts bearing IM-resistant ALL cells (BV173/E255K).

CONCLUSIONS

HDACIs increase KW-2449 lethality in Bcr/Abl(+) cells in association with inhibition of Bcr/Abl, generation of ROS, and induction of DNA damage. This strategy preferentially targets primary Bcr/Abl(+) hematopoietic cells and exhibits enhanced in vivo activity. Combining KW-2449 with HDACIs warrants attention in IM-resistant Bcr/Abl(+) leukemias.

Low pesticide rates may hasten the evolution of resistance by increasing mutation frequencies

At very low pesticide rates, a certain low proportion of pests may receive a sublethal dose, are highly stressed by the pesticide and yet survive. Stress is a general enhancer of mutation rates. Thus, the survivors are likely to have more than normal mutations, which might include mutations leading to pesticide resistance, both for multifactorial (polygenic, gene amplification, sequential allelic mutations) and for major gene resistance. Management strategies should consider how to eliminate the subpopulation of pests with the high mutation rates, but the best strategy is probably to avoid too low application rates of pesticides from the outset.

A positive role for c-Abl in Atm and Atr activation in DNA damage response

DNA damage triggers Atm- and/or Atr-dependent signaling pathways to control cell cycle progression, apoptosis, and DNA repair. However, how Atm and Atr are activated is not fully understood. One of the downstream targets of Atm is non-receptor tyrosine kinase c-Abl, which is phosphorylated and activated by Atm. The current view is that c-Abl relays pro-apoptotic signals from Atm to p73 and p53. Here we show that c-Abl deficiency resulted in a broad spectrum of defects in cell response to genotoxic stress, including activation of Chk1 and Chk2, activation of p53, nuclear foci formation, apoptosis, and DNA repair, suggesting that c-Abl might also act upstream of the DNA damage-activated signaling cascades in addition to its role in p73 and p53 regulation. Indeed, we found that c-Abl is required for proper activation of both Atm and Atr. c-Abl is bound to the chromatin and shows enhanced interaction with Atm and Atr in response to DNA damage. c-Abl can phosphorylate Atr on Y291 and Y310 and this phosphorylation appears to have a positive role in Atr activation under genotoxic stress. These findings suggest that Atm-mediated c-Abl activation in cell response to double-stranded DNA breaks might facilitate the activation of both Atm and Atr to regulate their downstream cellular events.

Imatinib mesylate induces cisplatin hypersensitivity in Bcr-Abl+ cells by differential modulation of p53 transcriptional and proapoptotic activity

Imatinib is highly effective in inducing remission in chronic myelogenous leukemia (CML). However, complete eradication of the malignant clone by imatinib is rare. We investigated the efficacy of combining imatinib with cisplatin. Inhibition of Bcr-Abl by imatinib induced a hypersensitive phenotype both in Bcr-Abl(+) cell lines and in CD34(+) cells from CML patients. Importantly, cisplatin sensitivity of leukemic cells harboring an inactive Bcr-Abl greatly exceeded that of Bcr-Abl(-) parental cells. The cisplatin response of Bcr-Abl(+) cells treated with imatinib was characterized by an impaired G(2)-M arrest and by rapid induction of mitochondrial cell death after the first passage through G(2). Imatinib abrogated ATM activation on cisplatin selectively in Bcr-Abl(+) cells. As a consequence, phosphorylation of p53 on Ser(15) and its activity as a transcription factor was significantly diminished. Furthermore, p53 accumulated predominantly in the cytoplasm in Bcr-Abl(+) cells treated with imatinib and cisplatin. Silencing of p53 significantly reduced sensitivity to cisplatin in imatinib-treated Bcr-Abl(+) cells, indicating that p53 retains its proapoptotic activity. Simultaneous downregulation of Bcl-x(L) was an additional requirement for cisplatin hypersensitivity, as p53-dependent cell death could be antagonized by exogenous Bcl-x(L). We conclude that imatinib sensitizes Bcr-Abl(+) cells to cisplatin by simultaneous inhibition of p53 transactivation, induction of p53 accumulation predominantly in the cytoplasm, and reduction of Bcl-x(L).

Clonal hematopoiesis in Philadelphia chromosome-negative bone marrow cells of chronic myeloid leukemia patients receiving dasatinib

A clonal cytogenetic abnormality was observed in Philadelphia chromosome-negative bone marrow cells of 6/27 chronic myeloid leukemia patients (+8 in 4, -7 in 1, and 20q- in 1) with dasatinib-induced remissions. The X-linked human androgen receptor gene assay demonstrated clonality in one additional patient. Single nucleotide polymorphism array analysis revealed somatic uniparental disomy involving chromosome 17(p12-pter) in another patient. The TP53 gene had a 5' splice site deletion of exon 6 that caused alternative splicing, frame shifting and introduction of a premature stop codon. After three years, no patient developed myelodysplastic syndrome or acute myeloid leukemia.

Dasatinib sensitizes primary chronic lymphocytic leukaemia lymphocytes to chlorambucil and fludarabine in vitro

The dual c-abl/Src kinase inhibitor, dasatinib, utilized to treat chronic myeloid leukaemia (CML) when used at clinically attainable sublethal concentrations, synergistically sensitized primary chronic lymphocytic leukaemia (CLL) lymphocytes to chlorambucil and fludarabine. In contrast, dasatinib alone demonstrated toxicity to CLL lymphocytes at concentrations that are generally not clinically attainable. Dasatinib resistance and poorer dasatinib-mediated sensitization to chlorambucil and fludarabine was associated with higher expression of c-abl protein levels. In contrast, chlorambucil and fludarabine resistance correlated with basal p53 protein levels. Moreover, Western blot analysis after in vitro treatment of primary CLL lymphocytes with dasatinib, chlorambucil and/or fludarabine, showed that dasatinib: (i) inhibited c-abl function (e.g. downregulation of c-abl protein levels and decreased the phosphorylation of a c-abl downstream target, Dok2), (ii) decreased chlorambucil/fludarabine induced accumulation of p53 protein levels, (iii) altered the response to chlorambucil/fludarabine induced DNA-damage as evidenced by an increase in chlorambucil/fludarabine-induced H2AX phosphorylation, and (iv) accentuated the c-abl downregulation induced by chlorambucil/fludarabine. Our results suggest that dasatinib in combination with chlorambucil or fludarabine may improve the therapy of CLL.