Up-regulation of ABCG1 is associated with methotrexate resistance in acute lymphoblastic leukemia cells

Acute lymphoblastic leukemia (ALL) is a prevalent hematologic malignancy in children, and methotrexate (MTX) is a widely employed curative treatment. Despite its common use, clinical resistance to MTX is frequently encountered. In this study, an MTX-resistant cell line (Reh-MTXR) was established through a stepwise selection process from the ALL cell line Reh. Comparative analysis revealed that Reh-MTXR cells exhibited resistance to MTX in contrast to the parental Reh cells. RNA-seq analysis identified an upregulation of ATP-binding cassette transporter G1 (ABCG1) in Reh-MTXR cells. Knockdown of ABCG1 in Reh-MTXR cells reversed the MTX-resistant phenotype, while overexpression of ABCG1 in Reh cells conferred resistance to MTX. Mechanistically, the heightened expression of ABCG1 accelerated MTX efflux, leading to a reduced accumulation of MTX polyglutamated metabolites. Notably, the ABCG1 inhibitor benzamil effectively sensitized Reh-MTXR cells to MTX treatment. Moreover, the observed upregulation of ABCG1 in Reh-MTXR cells was not induced by alterations in DNA methylation or histone acetylation. This study provides insight into the mechanistic basis of MTX resistance in ALL and also suggests a potential therapeutic approach for MTX-resistant ALL in the future.

Targeting DNA polymerase β elicits synthetic lethality with mismatch repair deficiency in acute lymphoblastic leukemia

Mismatch repair (MMR) deficiency has been linked to thiopurine resistance and hypermutation in relapsed acute lymphoblastic leukemia (ALL). However, the repair mechanism of thiopurine-induced DNA damage in the absence of MMR remains unclear. Here, we provide evidence that DNA polymerase β (POLB) of base excision repair (BER) pathway plays a critical role in the survival and thiopurine resistance of MMR-deficient ALL cells. In these aggressive resistant ALL cells, POLB depletion and its inhibitor oleanolic acid (OA) treatment result in synthetic lethality with MMR deficiency through increased cellular apurinic/apyrimidinic (AP) sites, DNA strand breaks and apoptosis. POLB depletion increases thiopurine sensitivities of resistant cells, and OA synergizes with thiopurine to kill these cells in ALL cell lines, patient-derived xenograft (PDX) cells and xenograft mouse models. Our findings suggest BER and POLB's roles in the process of repairing thiopurine-induced DNA damage in MMR-deficient ALL cells, and implicate their potentials as therapeutic targets against aggressive ALL progression.

A γ-glutamyl hydrolase lacking the signal peptide confers susceptibility to folates/antifolates in acute lymphoblastic leukemia cells

A key cofactor of several enzymes implicated in DNA synthesis, repair, and methylation, folate has been shown to be required for normal cell growth and replication and is the basis for cancer chemotherapy using antifolates. γ-Glutamyl hydrolase (GGH) catalyzes the removal of γ-polyglutamate tails of folylpoly-/antifolylpoly-γ-glutamates to facilitate their export out of the cell, thereby maintaining the metabolic homeostasis of folates or pharmacological efficacy of antifolates. However, the factors that control or modulate the GGH function are not well understood. In this study, we show that intact GGH is not indispensable for the chemosensitivity and growth of acute lymphoblastic leukemia (ALL) cells, whereas GGH lacking N-terminal signal peptide (GGH^-ΔN ) confers the significant drug resistance of ALL cells to the antifolates MTX and RTX. In addition, ALL cells harboring GGH^-ΔN show high susceptibility to the change in folates, and glycosylation is not responsible for these phenotypes elicited by GGH^-ΔN . Mechanistically, the loss of signal peptide enhances intracellular retention of GGH and its lysosomal disposition. Our findings clearly define the in vivo role of GGH in ALL cells and indicate a novel modulation of the GGH function, suggesting new avenues for ALL treatment in the future.

Chemotherapy and mismatch repair deficiency cooperate to fuel TP53 mutagenesis and ALL relapse

Chemotherapy is a standard treatment for pediatric acute lymphoblastic leukemia (ALL), which sometimes relapses with chemoresistant features. However, whether acquired drug-resistance mutations in relapsed ALL pre-exist or are induced by treatment remains unknown. Here we provide direct evidence of a specific mechanism by which chemotherapy induces drug-resistance-associated mutations leading to relapse. Using genomic and functional analysis of relapsed ALL we show that thiopurine treatment in mismatch repair (MMR)-deficient leukemias induces hotspot TP53 R248Q mutations through a specific mutational signature (thio-dMMR). Clonal evolution analysis reveals sequential MMR inactivation followed by TP53 mutation in some patients with ALL. Acquired TP53 R248Q mutations are associated with on-treatment relapse, poor treatment response and resistance to multiple chemotherapeutic agents, which could be reversed by pharmacological p53 reactivation. Our findings indicate that TP53 R248Q in relapsed ALL originates through synergistic mutagenesis from thiopurine treatment and MMR deficiency and suggest strategies to prevent or treat TP53-mutant relapse.

Abstract 633: Thiopurines and mismatch repair deficiency cooperate to fuel TP53 mutagenesis and ALL relapse

Chemotherapy is curative for most children with acute lymphoblastic leukemia (ALL). Here we provide direct evidence that thiopurine chemotherapeutics can also directly induce drug resistance mutations leading to relapse. Using a large relapsed ALL cohort assembled from Chinese, US and German patients, we found that TP53 R248Q mutations were highly enriched at relapse compared to diagnosis. Relapse-specific TP53 R248Q was associated with the acquisition of MMR deficiency mutations in MSH2, MSH6, or PMS2 and a novel relapse-specific mutational signature. Using isogenic MCF10A cells with or without engineered MSH2 knockout, and the Nalm6 ALL cell line which has native MMR deficiency, we found that this novel signature was caused by a synergistic mutagenic interaction between thiopurine treatment and mismatch repair (MMR) deficiency (called the thio-dMMR signature) that contributes to a hypermutator phenotype and acquisition of TP53 R248Q in residual ALL during remission. Treatment-induced TP53-mutant clones then expand due to broad chemoresistance, leading to eventual relapse. Indeed, thiopurines preferentially induced C>T mutations at the center of NCG trinucleotides, which can lead to TP53 R248Q, and the thiopurine mutation rate was accelerated 2- to 10-fold in MMR-deficient ALL and cell lines. Thiopurine treatment induced C>T mutations preferentially on the transcribed strand, rather than the untranscribed strand, of mRNAs, which further increased the likelihood of TP53 R248Q induction. Further, experimental thiopurine treatment was able to directly induce TP53 R248Q variants in MMR-deficient cultured cells, including Nalm6 and MCF10A MSH2-/-, by activating the thio-dMMR mutational signature, while MMR-proficient MCF10A cells did not experience R248Q induction. The sequential acquisition of MMR deficiency mutations, followed by TP53 mutations, during post-diagnosis ALL evolution was supported by clonal evolution analysis of serial patient samples. p53 R248Q promoted resistance to multiple ALL chemotherapeutic agents, and was associated with on-treatment relapse and poor relapse-treatment response. Our findings indicate that the enrichment of TP53 R248Q in relapsed ALL is due to synergistic mutagenesis from thiopurine treatment and MMR deficiency, followed by selection for TP53 R248Q's chemoresistance phenotype. This suggests that cancer drug resistance mutations may not always pre-exist subclonally at diagnosis, but may be therapy-induced in some patients. Additionally, the qualitative and quantitative mutational signature output of a mutagen (e.g., thiopurines) can vary based on the genetic background. Finally, our findings suggest potential therapeutic strategies, including avoiding thiopurine treatment in MMR-deficient relapses, and therapeutic p53 mutant reactivation, to deal with this genetically-unstable, chemoresistant disease.

Citation Format: Fan Yang, Samuel W. Brady, Huiying Sun, Chao Tang, Lijuan Du, Malwine Barz, Xiaotu Ma, Yao Chen, Houshun Fang, Xiaomeng Li, Pandurang Kolekar, Omkar Pathak, Jiaoyang Cai, Lixia Ding, Tianyi Wang, Arend von Stackelberg, Shuhong Shen, Caiwen Duan, Cornelia Eckert, Hongzhuan Chen, Yu Liu, Jeffery M. Klco, Hui Li, Benshang Li, Jinghui Zhang, Renate Kirschner-Schwabe, Bin-Bing S. Zhou. Thiopurines and mismatch repair deficiency cooperate to fuel TP53 mutagenesis and ALL relapse [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 633.

Bromodomains and Extra-Terminal (BET) Inhibitor JQ1 Suppresses Proliferation of Acute Lymphocytic Leukemia by Inhibiting c-Myc-Mediated Glycolysis

Background

Acute lymphocytic leukemia (ALL) is a common blood cancer which induces high mortality in children. Bromodomains and extra-terminal (BET) protein inhibitors, such as JQ1 and ARV-825, are promising cancer therapeutic agents that can be used by targeting c-Myc. A recent work reported that JQ1 effectively attenuates ALL in vitro by suppressing cell proliferation and accelerating apoptosis. The purpose of this research was to probe into the potential mechanism of how JQ1 inhibits ALL cell proliferation in vitro.

Material/Methods

Cell viability of ALL cells were measured by CTG after treatment by JQ1. Cell cycle analysis was done by EdU and PI staining. Cell apoptosis was assessed by Annexin V/PI staining. Glycolysis was detected using Seahorse and LC-MS kits. The expression of glycolytic rate-limiting enzymes was assessed by RNA-seq, qRT-PCR, and Western blot.

Results

JQ1 suppressed cell proliferation by arresting the cell cycle and inducing the apoptosis of acute lymphocytic leukemia cells. JQ1 inhibited cell proliferation of B-ALL cells by restraining glycolysis. Conversely, the cell cycle block of B-ALL cells induced by JQ1 was partially abolished after pretreatment with 2-Deoxy-D-glucose (2-DG), an inhibitor of glycolysis. Furthermore, JQ1 restrained the glycolysis of B-ALL cell lines by remarkably downregulating the rate-limiting enzymes of glycolysis, such as hexokinase 2, phosphofructokinase, and lactate dehydrogenase A. Moreover, the cell cycle arrest was reversed in B-ALL cells with overexpressed c-Myc treated by JQ1, which is involved in the enhancement of glycolysis.

Conclusions

The BET inhibitor JQ1 suppresses the proliferation of ALL by inhibiting c-Myc-mediated glycolysis, thus providing a new strategy for the treatment of ALL.

Therapy-induced mutations drive the genomic landscape of relapsed acute lymphoblastic leukemia

To study the mechanisms of relapse in acute lymphoblastic leukemia (ALL), we performed whole-genome sequencing of 103 diagnosis-relapse-germline trios and ultra-deep sequencing of 208 serial samples in 16 patients. Relapse-specific somatic alterations were enriched in 12 genes (NR3C1, NR3C2, TP53, NT5C2, FPGS, CREBBP, MSH2, MSH6, PMS2, WHSC1, PRPS1, and PRPS2) involved in drug response. Their prevalence was 17% in very early relapse (<9 months from diagnosis), 65% in early relapse (9-36 months), and 32% in late relapse (>36 months) groups. Convergent evolution, in which multiple subclones harbor mutations in the same drug resistance gene, was observed in 6 relapses and confirmed by single-cell sequencing in 1 case. Mathematical modeling and mutational signature analysis indicated that early relapse resistance acquisition was frequently a 2-step process in which a persistent clone survived initial therapy and later acquired bona fide resistance mutations during therapy. In contrast, very early relapses arose from preexisting resistant clone(s). Two novel relapse-specific mutational signatures, one of which was caused by thiopurine treatment based on in vitro drug exposure experiments, were identified in early and late relapses but were absent from 2540 pan-cancer diagnosis samples and 129 non-ALL relapses. The novel signatures were detected in 27% of relapsed ALLs and were responsible for 46% of acquired resistance mutations in NT5C2, PRPS1, NR3C1, and TP53. These results suggest that chemotherapy-induced drug resistance mutations facilitate a subset of pediatric ALL relapses.

AICAr suppresses cell proliferation by inducing NTP and dNTP pool imbalances in acute lymphoblastic leukemia cells

It has been shown that 5-amino-4-imidazolecarboxamide riboside (AICAr) can inhibit cell proliferation and induce apoptosis in childhood acute lymphoblastic leukemia (ALL) cells. Although AICAr could regulate cellular energy metabolism by activating AMPK, the cytotoxic mechanisms of AICAr are still unclear. Here, we knocked out TP53 or PRKAA1 gene (encoding AMPKα1) in NALM-6 and Reh cells by using the clustered regularly interspaced short palindromic repeats/Cas9 system and found that AICAr-induced proliferation inhibition was independent of AMPK activation but dependent on p53. Liquid chromatography-mass spectrometry analysis of nucleotide metabolites indicated that AICAr caused an increase in adenosine triphosphate, deoxyadenosine triphosphate, and deoxyguanosine triphosphate levels by up-regulating purine biosynthesis, while AICAr led to a decrease in cytidine triphosphate, uridine triphosphate, deoxycytidine triphosphate, and deoxythymidine triphosphate levels because of reduced phosphoribosyl pyrophosphate production, which consequently impaired the pyrimidine biosynthesis. Ribonucleoside triphosphate (NTP) pool imbalances suppressed the rRNA transcription efficiency. Furthermore, deoxy-ribonucleoside triphosphate (dNTP) pool imbalances induced DNA replication stress and DNA double-strand breaks, followed by cell cycle arrest and apoptosis in ALL cells. Exogenous uridine could rebalance the NTP and dNTP pools by supplementing pyrimidine and then attenuate AICAr-induced cytotoxicity. Our data indicate that RNA transcription inhibition and DNA replication stress induced by NTP and dNTP pool imbalances might play a key role in AICAr-mediated cytotoxic effects on ALL cells, suggesting a potential clinical application of AICAr in future ALL therapy.-Du, L., Yang, F., Fang, H., Sun, H., Chen, Y., Xu, Y., Li, H., Zheng, L., Zhou, B.-B. S. AICAr suppresses cell proliferation by inducing NTP and dNTP pool imbalances in acute lymphoblastic leukemia cells.

Increase of PRPP enhances chemosensitivity of PRPS1 mutant acute lymphoblastic leukemia cells to 5-Fluorouracil

Relapse‐specific mutations in phosphoribosyl pyrophosphate synthetase 1 (PRPS1), a rate‐limiting purine biosynthesis enzyme, confer significant drug resistances to combination chemotherapy in acute lymphoblastic leukemia (ALL). It is of particular interest to identify drugs to overcome these resistances. In this study, we found that PRPS1 mutant ALL cells specifically showed more chemosensitivity to 5‐Fluorouracil (5‐FU) than control cells, attributed to increased apoptosis of PRPS1 mutant cells by 5‐FU. Mechanistically, PRPS1 mutants increase the level of intracellular phosphoribosyl pyrophosphate (PRPP), which causes the apt conversion of 5‐FU to FUMP and FUTP in Reh cells, to promote 5‐FU‐induced DNA damage and apoptosis. Our study not only provides mechanistic rationale for re‐targeting drug resistant cells in ALL, but also implicates that ALL patients who harbor relapse‐specific mutations of PRPS1 might benefit from 5‐FU‐based chemotherapy in clinical settings.

HPRT1 activity loss is associated with resistance to thiopurine in ALL

Acute lymphoblastic leukemia (ALL) is an aggressive hematological tumor resulting from the malignant transformation of lymphoid progenitors. Thiopurine is a widely used drug in the maintaining treatment of ALL. After a period of chemotherapy, 20% of pediatric patients and over 50% of adult patients will relapse. To investigate the mechanisms of drug resistance in vitro, we established the thiopurine resistant cell lines Reh-6MPR (6-MP Resistant cell) and Reh-6TGR (6-TG Resistant cell) by stepwise selection of the ALL cell line Reh. Cell viability assay revealed that 6MPR and 6TGR cells were almost 1000-fold more resistant to thiopurine comparing with the control Reh cells, and thiopurine conversion was significantly impaired in the resistant cells. Mechanistically, a same novel hypoxanthine phosphoribosyl transferase 1 (HPRT1) mutation c.495_496insA (p.V165fs) was found by whole exome sequencing in both resistant cells. The HPRT1 mutation dramaticly decreased the production of [¹³C₅,¹⁵N₄]-IMP from [¹³C₅,¹⁵N₄]-hypoxanthine (HX), showed a loss-of-funciton mechanism. Notably, re-expression the wildtype HPRT1 in Reh-6MPR cell can reverse the drug resistance and thiopurine conversion in Reh-6MPR cells. These results highlight the importance of HPRT1's activity in thiopurine resistance.

Negative feedback-defective PRPS1 mutants drive thiopurine resistance in relapsed childhood ALL

Relapse is the leading cause of mortality in children with acute lymphoblastic leukemia (ALL). Among chemotherapeutics, thiopurines are key drugs in ALL combination therapy. Using whole-exome sequencing, we identified relapse-specific mutations in the phosphoribosyl pyrophosphate synthetase 1 gene (PRPS1), which encodes a rate-limiting purine biosynthesis enzyme, in 24/358 (6.7%) relapsed childhood B cell ALL (B-ALL) cases. All individuals who harbored PRPS1 mutations relapsed early during treatment, and mutated ALL clones expanded exponentially before clinical relapse. Our functional analyses of PRPS1 mutants uncovered a new chemotherapy-resistance mechanism involving reduced feedback inhibition of de novo purine biosynthesis and competitive inhibition of thiopurine activation. Notably, the de novo purine synthesis inhibitor lometrexol effectively abrogated PRPS1 mutant-driven drug resistance. These results highlight the importance of constitutive activation of the de novo purine synthesis pathway in thiopurine resistance, and they offer therapeutic strategies for the treatment of relapsed and thiopurine-resistant ALL.

Delineation of a cellular hierarchy in lung cancer reveals an oncofetal antigen expressed on tumor-initiating cells

Poorly differentiated tumors in non-small cell lung cancer (NSCLC) have been associated with shorter patient survival and shorter time to recurrence following treatment. Here, we integrate multiple experimental models with clinicopathologic analysis of patient tumors to delineate a cellular hierarchy in NSCLC. We show that the oncofetal protein 5T4 is expressed on tumor-initiating cells and associated with worse clinical outcome in NSCLC. Coexpression of 5T4 and factors involved in the epithelial-to-mesenchymal transition were observed in undifferentiated but not in differentiated tumor cells. Despite heterogeneous expression of 5T4 in NSCLC patient-derived xenografts, treatment with an anti-5T4 antibody-drug conjugate resulted in complete and sustained tumor regression. Thus, the aggressive growth of heterogeneous solid tumors can be blocked by therapeutic agents that target a subpopulation of cells near the top of the cellular hierarchy.

Abstract 475: Delineation of a cellular hierarchy in lung cancer reveals an oncofetal antigen expressed on tumor-initiating cells

Poorly differentiated tumors in non-small cell lung cancer (NSCLC) are refractory to chemotherapy and associated with short survival time. Here we integrate multiple experimental models with clinicopathological analysis of patient tumors to delineate a cellular hierarchy in NSCLC. We demonstrate that the oncofetal protein 5T4 is expressed on tumor-initiating cells and associated with worse clinical outcome in NSCLC. Coexpression of 5T4 and factors involved in the epithelial-to-mesenchymal transition was observed in undifferentiated but not in differentiated tumor cells. Despite heterogeneous expression of 5T4 in NSCLC patient-derived xenografts, treatment with an anti-5T4 antibody-drug conjugate resulted in complete and sustained tumor regression. Thus, the aggressive growth of heterogeneous solid tumors can be blocked by therapeutic agents that target a subpopulation of cells near the top of the cellular hierarchy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 475. doi:10.1158/1538-7445.AM2011-475

Tumour-initiating cells: challenges and opportunities for anticancer drug discovery

The hypothesis that cancer is driven by tumour-initiating cells (popularly known as cancer stem cells) has recently attracted a great deal of attention, owing to the promise of a novel cellular target for the treatment of haematopoietic and solid malignancies. Furthermore, it seems that tumour-initiating cells might be resistant to many conventional cancer therapies, which might explain the limitations of these agents in curing human malignancies. Although much work is still needed to identify and characterize tumour-initiating cells, efforts are now being directed towards identifying therapeutic strategies that could target these cells. This Review considers recent advances in the cancer stem cell field, focusing on the challenges and opportunities for anticancer drug discovery.

Recent advances in cancer therapy targeting proteins involved in DNA double-strand break repair

Damage to genetic material represents a persistent and ubiquitous threat to genomic stability. Once DNA damage is detected, a multifaceted signaling network is activated that halts the cell cycle, initiates repair, and in some instances induces apoptotic cell death. In this article, we will review DNA damage surveillance networks, which maintain the stability of our genome, and discuss the efforts underway to identify chemotherapeutic compounds targeting the core components of DNA double-strand breaks (DSB) response pathway. The majority of tumor cells have defects in maintaining genomic stability owing to the loss of an appropriate response to DNA damage. New anticancer agents are exploiting this vulnerability of cancer cells to enhance therapeutic indexes, with limited normal tissue toxicity. Recently inhibitors of the checkpoint kinases Chk1 and Chk2 have been shown to sensitize tumor cells to DNA damaging agents. In addition, the treatment of BRCA1- or BRCA2-deficient tumor cells with poly(ADP-ribose) polymerase (PARP) inhibitors also leads to specific tumor killing. Due to the numerous roles of p53 in genomic stability and its defects in many human cancers, therapeutic agents that restore p53 activity in tumors are the subject of multiple clinical trials. In this article we highlight the proteins mentioned above and catalog several additional players in the DNA damage response pathway, including ATM, DNA-PK, and the MRN complex, which might be amenable to pharmacological interventions and lead to new approaches to sensitize cancer cells to radio- and chemotherapy. The challenge is how to identify those patients most receptive to these treatments.

Modulation of Notch signaling by antibodies specific for the extracellular negative regulatory region of NOTCH3

The Notch pathway regulates the development of many tissues and cell types and is involved in a variety of human diseases, making it an attractive potential therapeutic target. This promise has been limited by the absence of potent inhibitors or agonists that are specific for individual human Notch receptors (NOTCH1-4). Using an unbiased functional screening, we identified monoclonal antibodies that specifically inhibit or induce activating proteolytic cleavages in NOTCH3. Remarkably, the most potent inhibitory and activating antibodies bind to overlapping epitopes within a juxtamembrane negative regulatory region that protects NOTCH3 from proteolysis and activation in its resting autoinhibited state. The inhibitory antibodies revert phenotypes conveyed on 293T cells by NOTCH3 signaling, such as increased cellular proliferation, survival, and motility, whereas the activating antibody mimics some of the effects of ligand-induced Notch activation. These findings provide insights into the mechanisms of Notch autoinhibition and activation and pave the way for the further development of specific antibody-based modulators of the Notch receptors, which are likely to be of utility in a wide range of experimental and therapeutic settings.

Targeting ADAM-mediated ligand cleavage to inhibit HER3 and EGFR pathways in non-small cell lung cancer

We describe here the existence of a heregulin-HER3 autocrine loop, and the contribution of heregulin-dependent, HER2-mediated HER3 activation to gefitinib insensitivity in non-small cell lung cancer (NSCLC). ADAM17 protein, a major ErbB ligand sheddase, is upregulated in NSCLC and is required not only for heregulin-dependent HER3 signaling, but also for EGFR ligand-dependent signaling in NSCLC cell lines. A selective ADAM inhibitor, INCB3619, prevents the processing and activation of multiple ErbB ligands, including heregulin. In addition, INCB3619 inhibits gefitinib-resistant HER3 signaling and enhances gefitinib inhibition of EGFR signaling in NSCLC. These results show that ADAM inhibition affects multiple ErbB pathways in NSCLC and thus offers an excellent opportunity for pharmacological intervention, either alone or in combination with other drugs.

Targeting Ligand Cleavage to Inhibit the ErbB Pathway in Cancer

ADAMs (a disintegrin and metalloproteases) are zinc-dependent transmembrane metalloproteases that shed the extracellular domains of membrane-bound growth factors, cytokines, and receptors. Recently, ADAMs have emerged in ErbB signaling pathways as sheddases or multiple ErbB ligands. As the ErbB pathway is a validated target for anticancer drugs, upstream activators of ErbB ligands, their sheddases, become new drug targets in the ErbB pathway. We have identified selective small molecule inhibitors of ADAM proteases that block shedding and activation of multiple ErbB ligands, and we are planning to test the compounds in the clinic.

ADAM proteases, ErbB pathways and cancer

A disintegrin and metalloproteases (ADAMs) are zinc-dependent trans-membrane metalloproteases that shed the extracellular domains of membrane-bound growth factors, cytokines and receptors. Key functions of ADAMs have emerged in ErbB signalling pathways as being sheddases for multiple ErbB ligands. As the ErbB pathway is a validated target for anti-cancer drugs, the upstream activators of ErbB ligands, their sheddases, now enter the spotlight as new drug targets in the ErbB pathway. ADAMs are involved not only in tumour cell proliferation but also in angiogenesis and metastasis. Therefore, strategies targeting ADAMs might be an important complement to existing anti-ErbB approaches.

Inhibition of Chk1 by the G2 DNA damage checkpoint inhibitor isogranulatimide

Inhibitors of the G(2) DNA damage checkpoint can selectively sensitize cancer cells with mutated p53 to killing by DNA-damaging agents. Isogranulatimide is a G(2) checkpoint inhibitor containing a unique indole/maleimide/imidazole skeleton identified in a phenotypic cell-based screen; however, the mechanism of action of isogranulatimide is unknown. Using natural and synthetic isogranulatimide analogues, we show that the imide nitrogen and a basic nitrogen at position 14 or 15 in the imidazole ring are important for checkpoint inhibition. Isogranulatimide shows structural resemblance to the aglycon of UCN-01, a potent bisindolemaleimide inhibitor of protein kinase C beta (IC(50), 0.001 micromol/L) and of the checkpoint kinase Chk1 (IC(50), 0.007 micromol/L). In vitro kinase assays show that isogranulatimide inhibits Chk1 (IC(50), 0.1 micromol/L) but not protein kinase C beta. Of 13 additional protein kinases tested, isogranulatimide significantly inhibits only glycogen synthase kinase-3beta (IC(50), 0.5 micromol/L). We determined the crystal structure of the Chk1 catalytic domain complexed with isogranulatimide. Like UCN-01, isogranulatimide binds in the ATP-binding pocket of Chk1 and hydrogen bonds with the backbone carbonyl oxygen of Glu(85) and the amide nitrogen of Cys(87). Unlike UCN-01, the basic N15 of isogranulatimide interacts with Glu(17), causing a conformation change in the kinase glycine-rich loop that may contribute importantly to inhibition. The mechanism by which isogranulatimide inhibits Chk1 and its favorable kinase selectivity profile make it a promising candidate for modulating checkpoint responses in tumors for therapeutic benefit.

Inhibition of Chk1 by the G2 DNA damage checkpoint inhibitor isogranulatimide

Inhibitors of the G2 DNA damage checkpoint can selectively sensitize cancer cells with mutated p53 to killing by DNA-damaging agents. Isogranulatimide is a G2 checkpoint inhibitor containing a unique indole/maleimide/imidazole skeleton identified in a phenotypic cell-based screen; however, the mechanism of action of isogranulatimide is unknown. Using natural and synthetic isogranulatimide analogues, we show that the imide nitrogen and a basic nitrogen at position 14 or 15 in the imidazole ring are important for checkpoint inhibition. Isogranulatimide shows structural resemblance to the aglycon of UCN-01, a potent bisindolemaleimide inhibitor of protein kinase Cβ (IC50, 0.001 μmol/L) and of the checkpoint kinase Chk1 (IC50, 0.007 μmol/L). In vitro kinase assays show that isogranulatimide inhibits Chk1 (IC50, 0.1 μmol/L) but not protein kinase Cβ. Of 13 additional protein kinases tested, isogranulatimide significantly inhibits only glycogen synthase kinase-3β (IC50, 0.5 μmol/L). We determined the crystal structure of the Chk1 catalytic domain complexed with isogranulatimide. Like UCN-01, isogranulatimide binds in the ATP-binding pocket of Chk1 and hydrogen bonds with the backbone carbonyl oxygen of Glu85 and the amide nitrogen of Cys87. Unlike UCN-01, the basic N15 of isogranulatimide interacts with Glu17, causing a conformation change in the kinase glycine-rich loop that may contribute importantly to inhibition. The mechanism by which isogranulatimide inhibits Chk1 and its favorable kinase selectivity profile make it a promising candidate for modulating checkpoint responses in tumors for therapeutic benefit.

Targeting DNA checkpoint kinases in cancer therapy

The DNA damage response includes not only cell cycle arrest and apoptosis, but also direct activation of DNA repair networks. Four DNA checkpoint kinases ATM, ATR, Chk1 and Chk2 have been identified in the mammalian DNA damage response signal transduction pathway. In this article, we review and discuss current knowledge and thinking about checkpoint kinases, and their potential as cancer drug targets. Particular emphasis is given to various therapeutic hypotheses and their promise for improving current cancer therapies.

Drug discovery targeting Chk1 and Chk2 kinases

The DNA damage response includes not only checkpoint and apoptosis, but also direct activation of DNA repair networks. Downstream in the DNA damage response pathway are Chk1, an essential checkpoint kinase, and Chk2, which plays a critical role in p53-dependent apoptosis. Chk1 inhibition is expected to lead to chemosensitization of tumors, while Chk2 inhibition could protect normal sensitive tissues from some chemotherapeutic agents. Drugs targeting Chk1 and Chk2 have the potential to significantly improve the therapeutic window of DNA damaging agents available in the clinic.

Structural Basis for Chk1 Inhibition by UCN-01

Chk1 is a serine-threonine kinase that plays an important role in the DNA damage response, including G(2)/M cell cycle control. UCN-01 (7-hydroxystaurosporine), currently in clinical trials, has recently been shown to be a potent Chk1 inhibitor that abrogates the G(2)/M checkpoint induced by DNA-damaging agents. To understand the structural basis of Chk1 inhibition by UCN-01, we determined the crystal structure of the Chk1 kinase domain in complex with UCN-01. Chk1 structures with staurosporine and its analog SB-218078 were also determined. All three compounds bind in the ATP-binding pocket of Chk1, producing only slight changes in the protein conformation. Selectivity of UCN-01 toward Chk1 over cyclin-dependent kinases can be explained by the presence of a hydroxyl group in the lactam moiety interacting with the ATP-binding pocket. Hydrophobic interactions and hydrogen-bonding interactions were observed in the structures between UCN-01 and the Chk1 kinase domain. The high structural complementarity of these interactions is consistent with the potency and selectivity of UCN-01.

Chfr regulates a mitotic stress pathway through its RING-finger domain with ubiquitin ligase activity

Resistance to chemotherapy targeting microtubules could be partially because of the delay in chromosome condensation and segregation during mitosis. The Chfr pathway has been defined recently, and its activation causes a delay in chromosome condensation in response to mitotic stress. Because Chfr contains a RING-finger domain, we tested whether Chfr inhibits chromosome condensation through an ubiquitin (ubiquitin)-dependent pathway. In the presence of purified E1, Ubc4, or Ubc5, and ubiquitin, Chfr catalyzes its own ubiquitination in vitro, an activity requiring the RING domain. In vivo, overexpressed Chfr but not a RING domain mutant is spontaneously ubiquitinated. Our studies with DLD1 cells stably expressing wild-type Chfr and Chfr lacking the RING domain indicated that the RING-finger deletion mutant was defective in inhibiting chromosome condensation after Taxol treatment. In addition, Chfr expression increases the survival rate after Taxol treatment, an activity requiring the RING domain. Preliminary studies indicate that Chfr expression is cell cycle regulated and is dependent on its ubiquitin ligase activity. It is very likely that the Chfr-mediated ubiquitin-dependent pathway is a critical component of the response to mitotic stress.