NEI-01-Induced Arginine Deprivation Has Potent Activity Against Acute Myeloid Leukemia Cells Both In Vitro and In Vivo.

Recent studies have revealed that targeting amino acid metabolic enzymes is a promising strategy in cancer therapy. Acute myeloid leukemia (AML) downregulates the expression of argininosuccinate synthase (ASS1), a recognized rate-limiting enzyme for arginine synthesis, and yet displays a critical dependence on extracellular arginine for survival and proliferation. This dependence on extracellular arginine, also known as arginine auxotrophy, suggests that arginine deprivation would be a treatment strategy for AML. NEI-01, a novel arginine-depleting enzyme, is capable of binding to serum albumin to extend its circulating half-life, leading to a potent anticancer activity. Here we reported the preclinical activity of NEI-01 in arginine auxotrophic AMLs. NEI-01 efficiently depleted arginine both in vitro and in vivo NEI-01-induced arginine deprivation was cytotoxic to arginine auxotrophic AML cells through induction of cell-cycle arrest and apoptosis. Furthermore, the potent anti-leukemia activities of NEI-01 were observed in three different types of mouse models including human cell line-derived xenograft, mouse cell line-derived homografts in syngeneic mice and patient-derived xenograft. This preclinical data provide strong evidence to support the potential use of NEI-01 as a therapeutic approach in AML treatment.

A modified arginine-depleting enzyme NEI-01 inhibits growth of pancreatic cancer cells.

Arginine deprivation cancer therapy targets certain types of malignancies with positive result in many studies and clinical trials. NEI-01 was designed as a novel arginine-depleting enzyme comprising an albumin binding domain capable of binding to human serum albumin to lengthen its half-life. In the present work, NEI-01 is shown to bind to serum albumin from various species, including mice, rat and human. Single intraperitoneal administration of NEI-01 to mice reduced plasma arginine to undetectable level for at least 9 days. Treatment of NEI-01 specifically inhibited cell viability of MIA PaCa-2 and PANC-1 cancer cell lines, which were ASS1 negative. Using a human pancreatic mouse xenograft model, NEI-01 treatment significantly reduced tumor volume and weight. Our data provides proof of principle for a cancer treatment strategy using NEI-01.

DNA damage during the spindle-assembly checkpoint degrades CDC25A, inhibits cyclin-CDC2 complexes, and reverses cells to interphase.

Cell cycle checkpoints that monitor DNA damage and spindle assembly are essential for the maintenance of genetic integrity, and drugs that target these checkpoints are important chemotherapeutic agents. We have examined how cells respond to DNA damage while the spindle-assembly checkpoint is activated. Single cell electrophoresis and phosphorylation of histone H2AX indicated that several chemotherapeutic agents could induce DNA damage during mitotic block. DNA damage during mitotic block triggered CDC2 inactivation, histone H3 dephosphorylation, and chromosome decondensation. Cells did not progress into G1 but seemed to retract to a G2-like state containing 4N DNA content, with stabilized cyclin A and cyclin B1 binding to Thr14/Tyr15-phosphorylated CDC2. The loss of mitotic cells was not due to cell death because there was no discernible effect on caspase-3 activation, DNA fragmentation, or viability. Extensive DNA damage during mitotic block inactivated cyclin B1-CDC2 and prevented G1 entry when the block was removed. The mitotic DNA damage responses were independent of p53 and pRb, but they were dependent on ATM. CDC25A that accumulated during mitosis was rapidly destroyed after DNA damage in an ATM-dependent manner. Ectopic expression of CDC25A or nonphosphorylatable CDC2 effectively inhibited the dephosphorylation of histone H3 after DNA damage. Hence, although spindle disruption and DNA damage provide conflicting signals to regulate CDC2, the negative regulation by the DNA damage checkpoint could overcome the positive regulation by the spindle-assembly checkpoint.

Protein tyrosine phosphatase receptor type Z is inactivated by ligand-induced oligomerization.

Receptor-type protein tyrosine phosphatases (RPTPs) are considered to transduce extracellular signals across the membrane through changes in their PTP activity, however, our understanding of the regulatory mechanism is still limited. Here, we show that pleiotrophin (PTN), a natural ligand for protein tyrosine phosphatase receptor type Z (Ptprz) (also called PTPzeta/RPTPbeta), inactivates Ptprz through oligomerization and increases the tyrosine phosphorylation of substrates for Ptprz, G protein-coupled receptor kinase-interactor 1 (Git1) and membrane associated guanylate kinase, WW and PDZ domain containing 1 (Magi1). Oligomerization of Ptprz by an artificial dimerizer or polyclonal antibodies against its extracellular region also leads to inactivation, indicating that Ptprz is active in the monomeric form and inactivated by ligand-induced oligomerization.

Topoisomerase poisons differentially activate DNA damage checkpoints through ataxia-telangiectasia mutated-dependent and -independent mechanisms.

Camptothecin and Adriamycin are clinically important inhibitors for topoisomerase (Topo) I and Topo II, respectively. The ataxia-telangiectasia mutated (ATM) product is essential for ionizing radiation-induced DNA damage responses, but the role of ATM in Topo poisons-induced checkpoints remains unresolved. We found that distinct mechanisms are involved in the activation of different cell cycle checkpoints at different concentrations of Adriamycin and camptothecin. Adriamycin promotes the G(1) checkpoint through activation of the p53-p21(CIP1/WAF1) pathway and decrease of pRb phosphorylation. Phosphorylation of p53(Ser20) after Adriamycin treatment is ATM dependent, but is not required for the full activation of p53. The G(1) checkpoint is dependent on ATM at low doses but not at high doses of Adriamycin. In contrast, the Adriamycin-induced G(2) checkpoint is independent on ATM but sensitive to caffeine. Adriamycin inhibits histone H3(Ser10) phosphorylation through inhibitory phosphorylation of CDC2 at low doses and down-regulation of cyclin B1 at high doses. The camptothecin-induced intra-S checkpoint is partially dependent on ATM, and is associated with inhibitory phosphorylation of cyclin-dependent kinase 2 and reduction of BrdUrd incorporation after mid-S phase. Finally, apoptosis associated with high doses of Adriamycin or camptothecin is not influenced by the absence of ATM. These data indicate that the involvement of ATM following treatment with Topo poisons differs extensively with dosage and for different cell cycle checkpoints.

Co-inhibition of polo-like kinase 1 and Aurora kinases promotes mitotic catastrophe.

Mitosis is choreographed by a number of protein kinases including polo-like kinases and Aurora kinases. As these kinases are frequently dysregulated in cancers, small-molecule inhibitors have been developed for targeted anticancer therapies. Given that PLK1 and Aurora kinases possess both unique functions as well as co-regulate multiple mitotic events, whether pharmacological inhibition of these kinases together can enhance mitotic catastrophe remains an outstanding issue to be determined. Using concentrations of inhibitors that did not induce severe mitotic defects on their own, we found that both the metaphase arrest and mitotic slippage induced by inhibitors targeting Aurora A and Aurora B (MK-5108 and Barasertib respectively) were enhanced by a PLK1 inhibitor (BI 2536). We found that PLK1 is overexpressed in cells from nasopharyngeal carcinoma, a highly invasive cancer with poor prognosis, in comparison to normal nasopharyngeal epithelial cells. Nasopharyngeal carcinoma cells were more sensitive to BI 2536 as a single agent and co-inhibition with Aurora kinases than normal cells. These observations underscore the mechanism and potential benefits of targeting PLK1 and Aurora kinases to induce mitotic catastrophe in cancer cells.

Pharmacological inactivation of CHK1 and WEE1 induces mitotic catastrophe in nasopharyngeal carcinoma cells.

Nasopharyngeal carcinoma (NPC) is a rare but highly invasive cancer. As radiotherapy is the primary treatment for NPC, this offers a rationale to investigate if uncoupling the DNA damage responses can sensitize this cancer type. The G2 DNA damage checkpoint is controlled by a cascade of protein kinases: ATM/ATR, which phosphorylates CHK1/CHK2, which in turn phosphorylates WEE1. A number of small molecule inhibitors have been developed against these kinases as potential therapeutic agents. Here we demonstrated that compare to that in immortalized nasopharyngeal epithelial cells, ATR, CHK1, and WEE1 were overexpressed in NPC cell lines. Inhibitors of these kinases were unable to promote extensive mitotic catastrophe in ionizing radiation-treated NPC cells, indicating that they are not very effective radiosensitizer for this cancer. In the absence of prior irradiation, however, mitotic catastrophe could be induced with inhibitors against CHK1 (AZD7762) or WEE1 (MK-1775). NPC cells were more sensitive to WEE1 inactivation than nasopharyngeal epithelial cells. Targeting CHK1 and WEE1 together induced more extensive mitotic catastrophe than the individual components alone. Taken together, our results show that NPC cells depend on CHK1 and WEE1 activity for growth and that inhibitors of these kinases may serve as potential therapeutics for NPC.

PARP1 is overexpressed in nasopharyngeal carcinoma and its inhibition enhances radiotherapy.

Nasopharyngeal carcinoma is a rare but highly invasive cancer. As options of agents for effective combination chemoradiotherapy for advanced nasopharyngeal carcinoma are limited, novel therapeutic approaches are desperately needed. The ubiquitin ligase CHFR is known to target PARP1 for degradation and is epigenetically inactivated in nasopharyngeal carcinoma. We present evidence that PARP1 protein is indeed overexpressed in nasopharyngeal carcinoma cells in comparison with immortalized normal nasopharyngeal epithelial cells. Tissue microarray analysis also indicated that PARP1 protein is significantly elevated in primary nasopharyngeal carcinoma tissues, with strong correlation with all stages of nasopharyngeal carcinoma development. We found that the PARP inhibitor AZD2281 (olaparib) increased DNA damage, cell-cycle arrest, and apoptosis in nasopharyngeal carcinoma cells challenged with ionizing radiation or temozolomide. Isobologram analysis confirmed that the cytotoxicity triggered by AZD2281 and DNA-damaging agents was synergistic. Finally, AZD2281 also enhanced the tumor-inhibitory effects of ionizing radiation in animal xenograft models. These observations implicate that PARP1 overexpression is an early event in nasopharyngeal carcinoma development and provide a molecular basis of using PARP inhibitors to potentiate treatment of nasopharyngeal carcinoma with radio- and chemotherapy.

Inhibition of Eg5 acts synergistically with checkpoint abrogation in promoting mitotic catastrophe.

The G(2) DNA damage checkpoint is activated by genotoxic agents and is particularly important for cancer therapies. Overriding the checkpoint can trigger precocious entry into mitosis, causing cells to undergo mitotic catastrophe. But some checkpoint-abrogated cells can remain viable and progress into G(1) phase, which may contribute to further genome instability. Our previous studies reveal that the effectiveness of the spindle assembly checkpoint and the duration of mitosis are pivotal determinants of mitotic catastrophe after checkpoint abrogation. In this study, we tested the hypothesis whether mitotic catastrophe could be enhanced by combining genotoxic stress, checkpoint abrogation, and the inhibition of the mitotic kinesin protein Eg5. We found that mitotic catastrophe induced by ionizing radiation and a CHK1 inhibitor (UCN-01) was exacerbated after Eg5 was inhibited with either siRNAs or monastrol. The combination of DNA damage, UCN-01, and monastrol sensitized cancer cells that were normally resistant to checkpoint abrogation. Importantly, a relatively low concentration of monastrol, alone not sufficient in causing mitotic arrest, was already effective in promoting mitotic catastrophe. These experiments suggest that it is possible to use sublethal concentrations of Eg5 inhibitors in combination with G(2) DNA damage checkpoint abrogation as an effective therapeutic approach.

Inhibitory phosphorylation of cyclin-dependent kinase 1 as a compensatory mechanism for mitosis exit.

The current paradigm states that exit from mitosis is triggered by the ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C) acting in concert with an activator called CDC20. While this has been well established for a number of systems, the evidence of a critical role of CDC20 in somatic cells is not unequivocal. In this study, we reexamined whether mitotic exit can occur properly after CDC20 is depleted. Using single-cell analysis, we found that CDC20 depletion with small interfering RNAs (siRNAs) significantly impaired the degradation of APC/C substrates and delayed mitotic exit in various cancer cell lines. The recruitment of cyclin B1 to the core APC/C was defective after CDC20 downregulation. Nevertheless, CDC20-depleted cells were still able to complete mitosis, albeit requiring twice the normal time. Intriguingly, a high level of cyclin-dependent kinase 1 (CDK1)-inhibitory phosphorylation was induced during mitotic exit in CDC20-depleted cells. The expression of an siRNA-resistant CDC20 rescued both the mitotic exit delay and the CDK1-inhibitory phosphorylation. Moreover, the expression of a nonphosphorylatable CDK1 mutant or the downregulation of WEE1 and MYT1 abolished mitotic exit in CDC20-depleted cells. These findings indicate that, in the absence of sufficient APC/C activity, an alternative mechanism that utilized the classic inhibitory phosphorylation of CDK1 could mediate mitotic exit.

Determinants of mitotic catastrophe on abrogation of the G2 DNA damage checkpoint by UCN-01.

Genotoxic stress such as ionizing radiation halts entry into mitosis by activation of the G(2) DNA damage checkpoint. The CHK1 inhibitor 7-hydroxystaurosporine (UCN-01) can bypass the checkpoint and induce unscheduled mitosis in irradiated cells. Precisely, how cells behave following checkpoint abrogation remains to be defined. In this study, we tracked the fates of individual cells after checkpoint abrogation, focusing in particular on whether they undergo mitotic catastrophe. Surprisingly, while a subset of UCN-01-treated cells were immediately eliminated during the first mitosis after checkpoint abrogation, about half remained viable and progressed into G(1). Both the delay of mitotic entry and the level of mitotic catastrophe were dependent on the dose of radiation. Although the level of mitotic catastrophe was specific for different cell lines, it could be promoted by extending the mitosis. In supporting this idea, weakening of the spindle-assembly checkpoint, by either depleting MAD2 or overexpressing the MAD2-binding protein p31(comet), suppressed mitotic catastrophe. Conversely, delaying of mitotic exit by depleting either p31(comet) or CDC20 tipped the balance toward mitotic catastrophe. These results underscore the interplay between the level of DNA damage and the effectiveness of the spindle-assembly checkpoint in determining whether checkpoint-abrogated cells are eliminated during mitosis.

Preoperative plasma transcript AA454543 level is an independent prognostic factor for hepatocellular carcinoma after partial hepatectomy.

BACKGROUND: We have previously reported that tissue expression levels of transcript AA454543 in hepatocellular carcinoma (HCC) are significantly higher than those of normal livers, livers with cirrhosis, and livers with hepatitis. In addition, a higher level of transcript AA454543 in tumor tissues is associated with poor prognosis. We aim to examine whether quantitative measurement of preoperative plasma transcript AA454543 can provide similar prognostic information. PATIENTS AND METHODS: Blood samples were obtained from 84 HCC patients before surgery. Real-time quantitative reverse transcription-polymerase chain reaction, using TaqMan system, was employed to measure plasma transcript AA454543 and alpha-fetoprotein (AFP) RNA levels. We assessed their prediction power in prognosis using univariate and multivariate analyses. RESULTS: High plasma transcript AA454543 RNA levels were associated with poor overall survival (log-rank test, P < .01). Patients with different plasma AFP RNA levels revealed no difference in overall survival (log-rank test, P = .88). By multivariate Cox regression analysis, plasma transcript AA454543 RNA level (hazard ratio = 4.8, P < .01) and tumor stage (hazard ratio = 1.7, P < .01) were determined to be independent risk factors for the prediction of overall survival. CONCLUSION: Preoperative plasma transcript AA454543 RNA level can provide prognostic information for HCC patients receiving curative partial hepatectomy.

The relative contribution of CHK1 and CHK2 to Adriamycin-induced checkpoint.

Topoisomerase II poisons like Adriamycin (ADR, doxorubicin) are clinically important chemotherapeutic agents. Adriamycin-induced DNA damage checkpoint activates ATM and ATR, which could in turn inhibit the cell cycle engine through either CHK1 or CHK2. In this study, we characterized whether CHK1 or CHK2 is required for Adriamycin-induced checkpoint. We found that both CHK1 and CHK2 were phosphorylated after Adriamycin treatment. Several lines of evidence from dominant-negative mutants, short hairpin RNA (shRNA), and knockout cells indicated that CHK1, but not CHK2, is critical for Adriamycin-induced cell cycle arrest. Disruption of CHK1 function bypassed the checkpoint, as manifested by the increase in CDC25A, activation of CDC2, increase in histone H3 phosphorylation, and reduction in cell survival after Adriamycin treatment. In contrast, CHK2 is dispensable for Adriamycin-induced responses. Finally, we found that CHK1 was upregulated in primary hepatocellular carcinoma (HCC), albeit as an inactive form. The presence of a stockpile of dormant CHK1 in cancer cells may have important implications for treatments like topoisomerase II poisons. Collectively, the available data underscore the pivotal role of CHK1 in checkpoint responses to a variety of stresses.

Differential contribution of inhibitory phosphorylation of CDC2 and CDK2 for unperturbed cell cycle control and DNA integrity checkpoints.

Inhibition of cyclin-dependent kinases (CDKs) by Thr14/Tyr15 phosphorylation is critical for normal cell cycle progression and is a converging event for several cell cycle checkpoints. In this study, we compared the relative contribution of inhibitory phosphorylation for cyclin A/B1-CDC2 and cyclin A/E-CDK2 complexes. We found that inhibitory phosphorylation plays a major role in the regulation of CDC2 but only a minor role for CDK2 during the unperturbed cell cycle of HeLa cells. The relative importance of inhibitory phosphorylation of CDC2 and CDK2 may reflect their distinct cellular functions. Despite this, expression of nonphosphorylation mutants of both CDC2 and CDK2 triggered unscheduled histone H3 phosphorylation early in the cell cycle and was cytotoxic. DNA damage by a radiomimetic drug or replication block by hydroxyurea stimulated a buildup of cyclin B1 but was accompanied by an increase of inhibitory phosphorylation of CDC2. After DNA damage and replication block, all cyclin-CDK pairs that control S phase and mitosis were to different degrees inhibited by phosphorylation. Ectopic expression of nonphosphorylated CDC2 stimulated DNA replication, histone H3 phosphorylation, and cell division even after DNA damage. Similarly, a nonphosphorylation mutant of CDK2, but not CDK4, disrupted the G2 DNA damage checkpoint. Finally, CDC25A, CDC25B, a dominant-negative CHK1, but not CDC25C or a dominant-negative WEE1, stimulated histone H3 phosphorylation after DNA damage. These data suggest differential contributions for the various regulators of Thr14/Tyr15 phosphorylation in normal cell cycle and during the DNA damage checkpoint.