Characterization of protein kinase chk1 essential for the cell cycle checkpoint after exposure of human head and neck carcinoma A253 cells to a novel topoisomerase I inhibitor BNP1350

Silicon-containing analogs of camptothecin as anticancer agents

The plant pentacyclic alkaloid camptothecin and its structural analogs were extensively studied. These compounds are interesting due to the antitumor activity associated with their ability to inhibit topoisomerase I in tumor cells. During the last decades of the 20th century, a large number of the silicon-containing camptothecins (silatecans) were synthesized. 7-tert-Butyldimethylsilyl-10-hydroxy-camptothecin (DB-67 or AR-67) has enhanced lipophilicity and demonstrates a antitumor activity superior to its carbon analog. To date, certain silatecans are under clinical trials and their ultimate role in cancer therapy appears promising. In this review, we present chemical methodologies for the synthesis of silicon-containing camptothecins, their chemical properties, biological activity, and results of clinical trials.

Karenitecin (bnp1350) and flavopridol as radiosensitizers in malignant glioma

The poor prognosis of malignant glioma patients highlights the need to develop low toxicity, tumor specific agents with the ability to synergize with proven efficacious treatment modalities, e.g., ionizing irradiation. This paper investigates the potential of BNP1350 (karenitecin), a topoisomerase I-targeting anticancer agent, and flavopridol a cyclin-dependent kinase inhibitor as radiosensitizers at clinically relevant doses in glioblastoma cell lines. A clonogenic survival and apoptosis assays were performed to test the effect of karenitecin (0.1 nM to 10 nM), flavopridol, (50 nM to 500 nM), radiation (1 Gy to 5.5 Gy) and a combination of radiation and karenitecin or radiation and flavopridol on the glioma cell lines T986 and M059K. Cells were stained for cyclins B and D using antibodies followed by flow cytometry. Propidium Iodide staining was used to reveal the various phases of the cell cycle; cyclin staining in the G0/G1 and G2/M phase of the cell cycle was estimated as the Mean Fluorescence Intensity (MFI) after subtracting the MFI recorded by the isotype controls. Results demonstrated that in irradiated cells, pretreatment with karenitecin induced apoptosis, a transient arrest in the G2/M phase of the cell cycle and increased the expression of cyclin B1. Flavopridol treatment also induced apoptosis and a transient block in the G2/M phase of the cell cycle. The combined effects of karenitecin and flavopridol displayed synergistic effects. The unique radiosensitizing activity of orally administrable karenitecin and flavopridol is consistent with continued investigation of these compounds preclinically, as well as in the clinical setting.

Bortezomib activity and in vitro interactions with anthracyclines and cytarabine in acute myeloid leukemia cells are independent of multidrug resistance mechanisms and p53 status

PURPOSE

The proteasome inhibitor bortezomib may be effective in combination with cytarabine and anthracyclines in the treatment of acute myeloid leukemia (AML) by virtue of targeting aberrantly activated NF-kappaB in AML stem cells. We tested whether bortezomib cytotoxicity is affected by multidrug resistance (MDR) proteins expressed in AML cells. We also tested whether bortezomib interactions with cytarabine and anthracyclines are affected by p53, because proteasome inhibition stabilizes p53 and may thus cause cell cycle arrest.

EXPERIMENTAL DESIGN

Bortezomib sensitivity of cell lines overexpressing P-glycoprotein, multidrug resistance protein-1, breast cancer resistance protein and lung resistance protein was studied in the presence and absence of established modulators of these transport proteins. Drug interactions during simultaneous and sequential exposure to bortezomib and anthracyclines or cytarabine in diverse ratios were evaluated by isobologram and combination index analyses in AML cell lines with wild type and inactive p53 and were correlated with cell cycle perturbations induced by bortezomib.

RESULTS

Of the MDR mechanisms studied, only P-glycoprotein conferred resistance to bortezomib, and resistance was only twofold. Interactions between bortezomib and anthracylines and cytarabine changed from antagonistic to additive or synergistic with increasing drug activity levels and were not affected by p53 status.

CONCLUSIONS

MDR proteins and p53 do not affect bortezomib cytotoxicity or in vitro interactions with anthracyclines or cytarabine, but these interactions are concentration-dependent, and this concentration-dependency should be considered in the design of combination regimens.

Synergistic antitumor activity of capecitabine in combination with irinotecan

5-Fluorouracil (5-FU) and capecitabine alone and in combination with irinotecan/oxaliplatin are clinically active in the treatment of colorectal and other solid tumors. Studies of the antitumor activity and toxicity of capecitabine or irinotecan alone and in combination with each other, were compared with 5-FU and raltitrexed in human tumor xenografts of colorectal and squamous cell carcinoma of the head and neck using clinically relevant schedules. Antitumor activity and toxicity were evaluated in nude mice bearing human colon carcinomas of HCT-8 and HT-29 and in head and neck squamous cell carcinomas of A253 and FaDu xenografts using the maximum tolerable dose of single-agent capecitabine, 5-FU, or raltitrexed, or each of the drugs in combination with irinotecan. Mice were treated with capecitabine and irinotecan alone or in combination using 2 different schedules: (1) capecitabine orally once a day for 7 days and a single dose of irinotecan (50 mg/kg intravenously [I.V.]), with each drug alone or in combination, and (2) capecitabine orally 5 days a week for 3 weeks and irinotecan 50 mg/kg (I.V. injection) once a week for 3 weeks, with each drug alone or in combination. For comparative purposes, the antitumor activity of single-agent capecitabine, 5-FU, or raltitrexed, or each drug in combination with irinotecan was carried out at its maximum tolerated dose (MTD) using a 3-week schedule. Results indicated that HT-29 and A253 xenografts were de novo resistant (no cure) to capecitabine and irinotecan alone at the MTD, whereas HCT-8 and FaDu xenografts were relatively more sensitive, yielding 10%-20% cures. The combination of irinotecan/capecitabine was much more active than either drug alone against all 4 tumor models. The cure rates were increased from 0 to 20% in A253 and HT-29 xenografts and from 10%-20% to 80%-100% in HCT-8 and FaDu tumor xenografts, respectively. Irinotecan/capecitabine had clear advantage over irinotecan/5-FU and irinotecan/raltitrexed in efficacy and selectivity in that they were more active and less toxic. The extent of synergy with irinotecan/capecitabine appears to be tumor-dependent and independent of the status of p53 expression. The potential impact of the preclinical results on clinical practice for the use of these drugs in combination needs clinical validation.

Natural products to drugs: natural product derived compounds in clinical trials

Natural product and natural product-derived compounds that are being evaluated in clinical trials or in registration (current 31 December 2004) have been reviewed. Natural product derived drugs launched in the United States of America, Europe and Japan since 1998 and new natural product templates discovered since 1990 are discussed.

Enhanced 7-Ethyl-10-hydroxycamptothecin (SN-38) Lethality by Methylselenocysteine Is Associated with Chk2 Phosphorylation at Threonine-68 and Down-Regulation of Cdc6 Expression

Methylselenocysteine (MSC) is an organic selenium compound in preventative clinical trials involving prostate, lung, and colon carcinoma. We found that methioninase-activated MSC potentiates 7-ethyl-10-hydroxycamptothecin (SN-38)-induced cell lethality in vitro in the p53-defective human head and neck carcinoma A253 cells. Activated MSC increases chk2 phosphorylation at threonine-68 induced by SN-38, with no significant effect on chk1 phosphorylation. Cell cycle arrest induced by SN-38, however, was not abrogated or potentiated by MSC. These results suggest that the enhanced cellular lethality of SN-38 by MSC was not associated with cell cycle regulation pathways. Because chk2, in addition to its role in cell cycle arrest, can induce apoptosis by phosphorylation/activation, we examined whether increased chk2 phosphorylation could induce preapoptotic DNA fragmentation. DNA damage analysis showed that megabase DNA fragmentation is decreased, accompanied by the increased 30 to 300 kilobase pairs of DNA fragmentation after exposure to SN-38 with MSC, compared with SN-38 alone. No significant changes in the amount of DNA fragments were observed in cells treated with SN-38 or MSC alone. Moreover, proteolytic destruction of DNA replication-associated proteins cdc6, MCM2, and cdc25A may induce a DNA damage checkpoint response. The observed down-regulation of DNA replication proteins cdc6, MCM2, and cdc25A after exposure to SN-38 with MSC further indicates a relationship between drug response and DNA damage. Exposure to SN-38 with MSC resulted in a significant increase of poly(ADP-ribose) polymerasecleavage and caspase 3 activation. All together, the data support the hypothesis that enhanced lethality of this combination is associated with increased chk2 phosphorylation at Thr68 and down-regulation of specific DNA replication-associated proteins, which result in poly(ADP-ribose) polymerase cleavage, caspase 3 activation, and the induction of 30 to 300 kilobase pairs of DNA fragmentation.

Human DNA damage checkpoints and their relevance to soft tissue sarcoma

Soft tissue sarcoma (STS) is a malignant neoplasm, arising in mesenchymal tissues, that is difficult to treat clinically because it can be highly resistant to chemo-radiotherapy. At present, the mechanism of that resistance remains unclear. Cell cycle checkpoints engender strict control of cell proliferation, arresting the cell cycle to provide time for repair or apoptosis when DNA damage is induced by unprogrammed extrinsic events. These pathways involve at least two checkpoints: one at the G1/S transition and one at the G2/M transition. The p53 gene, which is mutated in several malignant tumors, plays an important role in DNA repair at the G1/S transition; however, there is little information on the G2/M checkpoint in STS. In the present study, several proteins (phospho-p53, -cdc25, -cdc2, -Chk1 and -Chk2) involved in checkpoint pathways were investigated using immunohistochemistry in STS specimens. Most STSs maintain a well-preserved G2/M checkpoint despite the loss of the G1/S checkpoint (phospho-p53: 4.9% (2/41); -cdc25: 41% (17/41); -cdc2: 61% (25/41); -Chk1: 29% (12/41); -Chk2: 46% (19/41)). Furthermore, in a postoperative chemotherapy case the number of cells positive for phospho-cdc25 and -Chk2 was higher in a recurrent tumor than in the primary tumor (n = 7, P = 0.046 < 0.05, Wilcoxon signed-ranks test). These findings indicate that the G2/M checkpoint pathway is well preserved and might contribute to the chemotherapeutic resistance associated with STS.

Homocamptothecins: potent topoisomerase I inhibitors and promising anticancer drugs

Homocamptothecins (hCPTs) represent a new generation of antitumor agents targeting DNA topoisomerase I. The expanded seven-membered lactone E-ring that characterizes hCPTs enhances the plasma stability of the drug and reinforces the inhibition of topoisomerase I compared with conventional six-membered CPTs. hCPTs are more efficient than the CPTs at promoting cleavage at T/G sites and induce additional cleavage at C/G sites. Compound BN80765 and its difluoro analogue diflomotecan (DN80915) are potent cytotoxic agents and efficiently induce apoptosis in tumor cells. They display strong antiproliferative activities against specific tumor types. Diflomotecan is remarkably efficient at inhibiting the growth of human colon cancer cells in vivo and, administered orally, it also shows superior activities against human prostate cancers compared with the benchmark products topotecan (TPT) and irinotecan (IRT). Diflomotecan has entered phase I clinical testing and antitumor activity has been observed in patients. This 9,10-difluoro-hCPTs derivative is one of the most promising new members of the 'tecan' family. This review summarizes the recent discoveries in the topoisomerase I field and presents the different camptothecin (CPT) analogues currently evaluated as anticancer agents. The specific properties of hCPTs are highlighted.

Chk1 signaling pathways that mediated G(2)M checkpoint in relation to the cellular resistance to the novel topoisomerase I poison BNP1350

A novel karenitecin, BNP1350, is a topoisomerase I-targeting anticancer agent with significant antitumor activity in vitro and in vivo. A BNP1350-resistant human head and neck carcinoma A253 cell line, denoted A253/BNPR, was developed. The A253/BNPR cell line was approximately 9-fold resistant to BNP1350 and 4-fold cross-resistant to another topoisomerase I inhibitor SN-38, the active metabolite of irinotecan. After drug treatment with equimolar concentrations of BNP1350 (0.7 microM) for 2h, activation of the DNA double-strand break repair protein complexes was similar in the two cell lines, suggesting that DNA dsb repair is not attributable to resistance to BNP1350 in the A253/BNPR cells. Cell cycle analysis indicates that the A253 cell line accumulated primarily in S phase, but G(2) phase accumulation was observed in the A253/BNPR cell line at 48 h after drug removal. Elevated chk1 phosphorylation at Ser(345) following DNA damage induced by BNP1350 was accompanied by G(2) accumulation in the A253/BNPR cell line, while exposure to equimolar concentrations of BNP1350 (0.7 microM) induced S-phase arrest and no increased phosphorylation of chk1 at Ser(345) in the A253 cell line. Under the same conditions, increased chk1 activity was observed in the A253/BNPR cell line, but not in the A253 cell line. Moreover, stimulated binding of 14-3-3 proteins to chk1 was observed in BNP1350-treated A253/BNPR cells. To confirm relationship between chk1 expression/phosphorylation and drug resistance to topo I poisons, we examined the effects of chk1 or chk2 antisense oligonucleotides on the cellular growth inhibition. Chk1 antisense oligonucleotide can sensitize the A253/BNPR cells to killing by topo I inhibitor BNP1350, but no significant sensitization of BNP1350-induced growth inhibition was observed in the drug-sensitive cell line. Chk2 antisense oligonucleotide has only a small sensitization effect on BNP1350-induced growth inhibition in both cell lines. The data indicate that the chk1 signaling pathways that mediate cell cycle checkpoint are associated with cellular resistance to BNP1350 in the A253/BNPR cell line.

Induction of biphasic DNA double strand breaks and activation of multiple repair protein complexes by DNA topoisomerase I drug 7-ethyl-10-hydroxy-camptothecin

Camptothecins demonstrate a broad spectrum of antitumor activity. Although they are known to trap DNA topoisomerase I on DNA, form cleavable complexes, and generate DNA breaks upon collision with DNA or RNA polymerases, the precise mechanisms predictive for antitumor activity remain to be identified. Recent studies using panels of colorectal and breast cancer cell lines indicate that events downstream of cleavable complexes are more relevant. In this study, we chose SN-38, an active metabolite of irinotecan, to characterize DNA double strand breaks and repair mechanisms induced by this type of drugs using a human head and neck squamous cell carcinoma cell line A253. The results showed that 2-h exposure of cells to an IC(50) concentration of SN-38 induces biphasic DNA double-strand break (DSBs): an immediate phase, which was greatly reduced within 8 h, and a lagging phase, culminating 24 h after drug removal. Three DNA double-strand break repair protein complexes were activated: DNA-dependent protein kinase (DNA-PK), NBS1-MRE11-RAD50, and BRCA1. Aphidicolin, a DNA polymerase inhibitor, abolished both phase I DSBs and the activation of repair protein complexes, suggesting that they resulted from the collision between the cleavable complex and DNA polymerase of S-phase cells. This is in contrast to ionizing radiation-induced activation of DNA-PK and NBS1-MRE11-RAD50 complexes that occur predominantly among non-S-phase cells. The trigger for phase II DSBs cannot be abolished by aphidicolin. The data also indicate that DNA fragments in the size of 50 to 200 kilobases were detected in the lagging phase. This suggests that the late DNA DSBs were associated with apoptotic cell death.

Cloning and characterization of liver-specific isoform of Chk1 gene from rat

We have isolated and characterized an isoform of protein kinase Chk1 gene from rat liver and a rat liver cDNA library by 5'-rapid amplification of cDNA ends. The gene (Cil) contains the C-terminal region of the Chk1 gene, but the 5'-end is derived from a sequence in the intron of Chk1 preceding the C-terminal domain by differential RNA splicing. The kinase domain of Chk1 gene is absent in this isoform. Tissue RNA and protein blot analyses indicated that Cil was specifically expressed only in rat liver, and its expression increased with liver development. Expression of Cil was found to be reduced in three rat hepatoma cell lines examined. A promoter trap experiment suggested that a promoter was located in the intron preceding the C-terminal domain of Chk1, and transcription from this novel promoter generated the new 5' noncoding exon of Cil. Thus Cil was generated by both alternate promoter usage and differential RNA splicing. UV irradiation induced caffeine-sensitive phosphorylation of both Chk1 and Cil at Ser-345 in Chk1 and its equivalent site in Cil, implying a role for ATR kinase in the phosphorylation of both proteins. We demonstrated the interaction between the kinase domain of Chk1 and Cil using a yeast two-hybrid assay and pull-down technique. In contrast to the effect of Chk1, Cil was found to decrease the transactivating function of p53, and the S63A mutation of Cil abolished this effect. These results suggest that Cil may serve as a dominant negative competitor of Chk1 as suggested previously.

The Chk1-Cdc25C regulation is involved in sensitizing A253 cells to a novel topoisomerase I inhibitor BNP1350 by bax gene transfer

Promotion of apoptosis may potentiate the sensitivity of tumor cells to chemotherapeutic agents, thus improving the efficacy of cancer treatment. The transfection of the proapoptotic bax gene, which results in the overexpression of bax protein, augments the growth inhibition of A253 cells by BNP1350. Increased drug response was associated with the induction of DNA fragmentation in the size of 30-200 Kb, generating a cleaved fragment of 18 kDa from full-length 21 kDa bax and the cleavage of PARP. A253/vec cells treated with 0.07 microM(IC50) of BNP1350 accumulated in G2 phase at 24 h after drug removal. In contrast, A253/Bax cells treated with an equimolar concentration of BNP1350 primarily displayed a G1 phase accumulation with a concurrent decrease in G2 phase. Certain cell cycle regulatory protein expression and activities were altered following drug exposure in both cell lines under similar conditions. Cdk2- and cdc2-associated H1 kinase activities were markedly increased in the A253/Bax cell line with marginal increased activity in the A253/vec cell line. A chk1 activity assay was performed with GST-cdc25C (200-256) or GST-cdc25C(S216A) (200-256) fusion proteins as the substrate. Increased chk1 activity was observed in the A253/vec cell line, with little change in the A253/Bax cell line, when exposed to equimolar concentrations of BNP1350 (0.07 microM). A Western blot of immunoprecipitated chk1 indicated that increased chk1 phosphorylation following DNA damage induced by BNP1350 was accompanied by the observed G2 accumulation in the A253/vec cell line, while only a slight increase in chk1 phosphorylation was seen in the A253/Bax cell line. A decreased expression of cdc25C was observed in the BNP1350-treated A253/Bax cells, but not in the A253/vec cell line. Following exposure to BNP1350, increased binding of 14-3-3 proteins to chk1 occurred in both cell lines, with more being observed in the A253/vec cell line. The data have shown that inhibition of the chk1 pathway accompanied by the abrogation of G2 arrest is involved in sensitizing A253 cells to BNP1350 by bax gene transfer. These findings suggest that bax gene transfer sensitizes A253 cells to BNP1350 through apoptosis promoting and G2/M DNA damage checkpoint regulatory pathways.

ATR-mediated checkpoint pathways regulate phosphorylation and activation of human Chk1

Chk1 is an evolutionarily conserved protein kinase that regulates cell cycle progression in response to checkpoint activation. In this study, we demonstrated that agents that block DNA replication or cause certain forms of DNA damage induce the phosphorylation of human Chk1. The phosphorylated form of Chk1 possessed higher intrinsic protein kinase activity and eluted more quickly on gel filtration columns. Serines 317 and 345 were identified as sites of phosphorylation in vivo, and ATR (the ATM- and Rad3-related protein kinase) phosphorylated both of these sites in vitro. Furthermore, phosphorylation of Chk1 on serines 317 and 345 in vivo was ATR dependent. Mutants of Chk1 containing alanine in place of serines 317 and 345 were poorly activated in response to replication blocks or genotoxic stress in vivo, were poorly phosphorylated by ATR in vitro, and were not found in faster-eluting fractions by gel filtration. These findings demonstrate that the activation of Chk1 in response to replication blocks and certain forms of genotoxic stress involves phosphorylation of serines 317 and 345. In addition, this study implicates ATR as a direct upstream activator of Chk1 in human cells.

Recombinogenic flap ligation pathway for intrinsic repair of topoisomerase IB-induced double-strand breaks

Topoisomerase IB catalyzes recombinogenic DNA strand transfer reactions in vitro and in vivo. Here we characterize a new pathway of topoisomerase-mediated DNA ligation in vitro (flap ligation) in which vaccinia virus topoisomerase bound to a blunt-end DNA joins the covalently held strand to a 5' resected end of a duplex DNA containing a 3' tail. The joining reaction occurs with high efficiency when the sequence of the 3' tail is complementary to that of the scissile strand immediately 5' of the cleavage site. A 6-nucleotide segment of complementarity suffices for efficient flap ligation. Invasion of the flap into the duplex apparently occurs while topoisomerase remains bound to DNA, thereby implying a conformational flexibility of the topoisomerase clamp around the DNA target site. The 3' flap acceptor DNA mimics a processed end in the double-strand-break-repair recombination pathway. Our findings suggest that topoisomerase-induced breaks may be rectified by flap ligation, with ensuing genomic deletions or translocations.