The effects of O6-benzylguanine and hypoxia on the cytotoxicity of 1,3-bis(2-chloroethyl)-1-nitrosourea in nitrosourea-resistant SF-763 cells

Radiation-Induced Caspase-8 Mediates p53-Independent Apoptosis in Glioma Cells

Malignant gliomas are almost uniformly fatal and display exquisite radiation resistance. Glioma cells lacking wild-type (WT) p53 function are more susceptible to radiation-induced apoptosis than their isogenic counterparts expressing WT p53. We explored the mechanisms of such apoptosis and found that, in the absence of WT p53, radiation increases caspase-8 expression and activity. Inhibition of caspase-8 expression using caspase-8 antisense or small interfering RNA (siRNA) oligonucleotides partially blocks radiation-induced apoptosis. In contrast, inhibition of the mitochondrial death pathway by expression of Bcl-2 has no effect on radiation-induced caspase-8 activity or apoptosis. Our data indicate that, in contrast to commonly accepted models of p53-dependent radiation-induced apoptosis, in our cell system, radiation relies on caspase-8 activity to help mediate p53-independent cell death. In a system of inducible E2F1 activity, E2F1 activated caspase-8 and, accordingly, decreased cellular viability, effects that were abolished by caspase-8 siRNA. In this model, in the absence of WT p53, p21Cip1 is not induced, and E2F1 activity is sustained and allows transcription and activation of caspase-8. This model may explain why p53 mutations in adult gliomas paradoxically correlate with improved survival and enhanced response to radiation.

PKB/Akt mediates radiosensitization by the signaling inhibitor LY294002 in human malignant gliomas

The phosphoinositide 3-kinase (PI3-kinase) signaling pathway is frequently aberrantly activated in glioblastoma multiforme (GM) by mutation or loss of the 3' phospholipid phosphatase PTEN. PTEN abnormalities result in inappropriate signaling to downstream molecules including protein kinase B (PKB/Akt), and mammalian target of rapamycin (mTOR). PI3-kinase activation increases resistance to radiation-induced cell death; conversely, PI3-kinase inhibition enhances the sensitivity of tumors to radiation. The effects of LY294002, a biochemical inhibitor of PI3-kinase, on the response to radiation were examined in the PTEN mutant glioma cell line U251 MG. Low doses of LY294002 sensitized U251 MG to clinically relevant doses of radiation. In contrast to LY294002, rapamycin, an inhibitor of mTOR, did not result in radiosensitization. We demonstrate that among multiple known targets of LY294002, PI3-kinase is the most likely molecule responsible for LY294002-induced radiosensitization. Furthermore, using a myristoylated PKB/Akt construct, we identified PKB/Akt as the downstream molecule that mediates the synergistic cytotoxicity between LY294002 and radiation. Thus PI3-kinase dysregulation may contribute to the notable radioresistance of GM tumors and inhibition of PKB/Akt offers an excellent target to enhance radiosensitivity.

Delayed repletion of O6-methylguanine-DNA methyltransferase resulting in failure to protect the human glioblastoma cell line SF767 from temozolomide-induced cytotoxicity

OBJECT

Temozolomide (TMZ)-induced O6-methylguanine (MG) DNA lesions, if not removed by MG-DNA methyltransferase (MGMT), mispair with thymine, trigger rounds of futile mismatch repair (MMR), and in glioma cells lead to prolonged G2-M arrest and ultimately cell death. Depletion of MGMT by O6-benzylguanine (BG) sensitizes tumor cells to TMZ, and this combination is currently used in clinical trials. The use of the TMZ+BG combination in gliomas, however, is complicated by the prolonged TMZ-induced G2-M arrest, which may delay activation of poorly defined cell death pathways and allow for MGMT repletion and reversal of toxicity.

METHODS

To address these issues, the actions of TMZ were monitored in DNA MMR-proficient SF767 glioma cells depleted of MGMT by BG, and in cells in which BG was removed at various times after TMZ exposure. In MGMT-depleted cells, TMZ exposure led to DNA single-strand breaks and phosphorylation of cdc2, followed by G2-M arrest, induction of p53/p21, and DNA double-strand breaks. Although DNA single-strand breaks, phosphorylation of cdc2, and G2-M arrest could be reversed by repletion of MGMT up to 5 days after TMZ exposure, TMZ-induced cytotoxicity could only be prevented if MGMT was replenished within 24 hours of the onset of G2-M arrest, and before the creation of DNA double-strand breaks.

CONCLUSIONS

These results indicate that although SF767 glioma cells undergo a prolonged G2-M arrest in response to TMZ, their ability to escape TMZ-induced cytotoxicity by MGMT repletion is limited to an approximately 24-hour period after the onset of G2-M arrest.

Phase I study of fotemustine in pediatric patients with refractory brain tumors

BACKGROUND

Fotemustine is a nitrosourea with theoretic and preclinical advantages over the original analogs, carmustine and lomustine, in the treatment of brain tumors. This is the first pediatric Phase I study of fotemustine.

METHODS

Patients younger than 21 with recurrent/resistant brain tumors were enrolled in a conventional Phase I study. Fotemustine was administered intravenously every 3 weeks at increasing dose levels starting at 100 mg/m(2). Toxicity and response data were monitored closely.

RESULTS

Fifteen evaluable patients entered the study and received a total of 45 courses of fotemustine (dose range, 100-175 mg/m(2)). Myelosuppression was observed, with the dose-limiting toxicity being Grade 4 neutropenia and thrombocytopenia. Toxicity was delayed and cumulative. The maximum tolerated dose was 150 mg/m(2) every 3 weeks. There were three documented radiologic responses (20% of patients) comprising one partial response and two minor responses in patients with a sarcoma, medulloblastoma, and ependymoma, respectively.

CONCLUSIONS

Fotemustine administered at a dose of 150 mg/m(2) every 3 weeks is well tolerated in children and has antitumor activity in several brain tumors. This is the first dedicated Phase I study of a single agent nitrosourea in a pediatric population. More comparative studies should be undertaken to define the optimum nitrosourea analog for use in children with brain tumors.

Transcriptional activation of TRADD mediates p53-independent radiation-induced apoptosis of glioma cells

Survival of patients with Glioblastoma Multiforme (GM), a highly malignant brain tumor, remains poor despite concerted efforts to improve therapy. The median survival of patients with GM has remained approximately 1 year regardless of the therapeutic approach. Since radiation therapy is the most effective adjuvant therapy for GM and nearly half of GM tumors harbor p53 mutations, we sought to identify genes that mediate p53-independent apoptosis of GM cells in response to ionizing radiation. Using broad-scale gene expression analysis we found that following radiation treatment, TRADD expression was induced in a uniquely radiosensitive GM cell line but not in radioresistant GM cell lines. TRADD over-expression killed GM cells and activated NF-kappa B. We found that blocking the TRADD-mediated pathway using a dominant-negative mutant of FADD (FADD-DN) enhanced radiation resistance of GM cells, as reflected in both susceptibility to apoptosis and clonogenic survival following irradiation. Conversely, stable expression of exogenous TRADD enhanced radiation-induced apoptosis of GM cell lines, reflecting the biological significance of TRADD regulation in p53-independent apoptosis. These findings generate interest in utilizing TRADD in gene therapy for GM tumors, particularly in light of its dual function of directly inducing rapid apoptosis and sensitizing GM cells to standard anti-neoplastic therapy.

Inhibition of DNA repair as a means of increasing the antitumor activity of DNA reactive agents

Chemotherapeutic alkylnitrosoureas (BCNU, CCNU, streptozotocin) and alkyltriazenes (DTIC, temozolomide) produce a cytotoxic lesion at the O(6)-position of guanine. The DNA repair protein, O(6)-alkylguanine-DNA alkyltransferase removes damage from the O(6)-position in a single-step mechanism without co-factors. There is extensive evidence that this protein is one of the most important factors contributing to alkylnitrosourea and alkyltriazene treatment failure. There is an inverse correlation between the level of this protein and the sensitivity of cells to the cytotoxic effects of O(6)-alkylating agents. Attempts have been made to modulate AGT activity using anti-sense technology, methylating agents, O(6)-alkylguanines, and O(6)-benzylguanine analogs. O(6)-Benzylguanine and its analogs are clearly the most potent direct inactivators of the AGT protein. The mechanism involves O(6)-benzylguanine acting as a low-molecular weight substrate with transfer of the benzyl group to the cysteine residue within the active site of the repair protein. Pretreatment of cells with non-toxic doses of O(6)-benzylguanine results in an increase in the sensitivity to O(6)-alkylating agents. Animal studies revealed that the therapeutic index of BCNU increased when administered in combination with O(6)-benzylguanine. This drug is currently in phase I clinical trials. Evidence from animal studies indicates that myelosuppression may be the dose-limiting toxicity, thus, efforts are aimed at improving the therapeutic index by the stable expression of O(6)-benzylguanine-resistant AGT proteins into targeted normal tissue such as bone marrow. The successful modulation of alkyltransferases brings on an exciting new era for alkylnitrosoureas and alkyltriazenes.

Biologic basis of radiation therapy

The biologic effects of ionizing radiation are well understood. The limitations of radiation therapy time-dose schemes typically used in veterinary medicine are also well understood. Before expensive and potentially toxic combinations of treatment, such as radiation combined with chemotherapy or radiation combined with hyperthermia, can be fully understood, the effect of optimizing the manner in which radiation itself is administered must first be defined. This will only occur after a sufficient period of observation after improvement of the radiation time-dose schemes in use today. Also, when evaluating historic data regarding the response of canine and feline tumors to irradiation, the time-dose scheme used must be considered. Many papers were published based on coarsely fractionated schemes using large doses per fraction and relatively low total doses. Thus, the response rates published must be tempered by the fact that it may be possible to obtain better tumor control rates using smaller doses per fraction and a larger total dose.

p53-dependent G1 arrest and p53-independent apoptosis influence the radiobiologic response of glioblastoma

PURPOSE

Loss of the p53 tumor suppressor gene has been associated with tumor progression, disease relapse, poor response to antineoplastic therapy, and poor prognosis in many malignancies. We have investigated the contribution of p53-mediated radiation-induced apoptosis and G1 arrest to the well described radiation resistance of glioblastoma multiforme (GM) cells.

METHODS AND MATERIALS

Radiation survival in vitro was quantitated using linear quadratic and repair-saturation mathematical models. Isogenic derivatives of glioblastoma cells differing only in their p53 status were generated using a retroviral vector expressing a dominant negative mutant of p53. Radiation-induced apoptosis was assayed by Fluorescence-activated cell sorter (FACS) analysis, terminal deoxynucleotide transferase labeling technique, and chromatin morphology. Cells were synchronized in early G1 phase and mitotic and labeling indices were measured.

RESULTS

Radiation-induced apoptosis of GM cells was independent of functional wild-type p53 (wt p53). Decreased susceptibility to radiation-induced apoptosis was associated with lower alpha values characterizing the shoulder of the clonogenic radiation survival curve. Using isogenic GM cells differing only in their p53 activity, we found that a p53-mediated function, radiation-induced G1 arrest, could also influence the value of alpha and clonogenic radiation resistance. Inactivation of wt p53 function by a dominant negative mutant of p53 resulted in a significantly diminished alpha value with no alteration in cellular susceptibility to radiation-induced apoptosis. The clonal derivative U87-LUX.8 expressing a functional wt p53 had an alpha (Gy-1) value of 0.609, whereas the isogenic clonal derivative U87-175.4 lacking wt p53 function had an alpha (Gy-1) value of 0.175.

CONCLUSION

We conclude that two distinct cellular responses to radiation, p53-independent apoptosis and p53-dependent G1-arrest, influence radiobiological parameters that characterize the radiation response of glioblastoma cells. Further understanding of the molecular basis of GM radiation resistance will lead to improvement in existing therapeutic modalities and to the development of novel treatment approaches.

O6-benzylguanine enhances the sensitivity of a glioma xenograft with low O6-alkylguanine-DNA alkyltransferase activity to temozolomide and BCNU

The effect of the O6-alkylguanine-DNA alkyltransferase (AGT) inhibitor, O6-benzylguanine (O6-BG), on the anti-tumour activity of 8-carbamoyl-3-methylimidazo [5,1-d]-1,2,3,5-tetrazine-4(3H)-one (temozolomide) or 1,3-bis(2-chloroethyl)-nitrosourea (BCNU) was evaluated in athymic mice bearing subcutaneous (s.c.) human glioma (U87MG) xenografts. The activity of AGT in U87MG xenografts was 4.3 +/- 1.5 fmol mg-1 protein (mean +/- s.d). These xenografts were inherently sensitive to treatment with alkylating compounds alone, with non-toxic doses of temozolomide (35 mg kg-1) or BCNU (10 mg kg-1) producing tumour growth delays of 23.3 and 11.8 days respectively. O6-BG (40 mg kg-1) did not inhibit tumour growth when administered alone, but was found to enhance significantly the anti-tumour activity of temozolomide or BCNU when administered 1 h before therapy (P < 0.002, Mann-Whitney test). AGT activity measured 24 h after the administration of 40 mg kg-1 O6-BG, was only 0.9 +/- 0.2 fmol mg-1 protein. These results are in contrast to previous studies in vitro with tumour cell lines of low AGT activity (< 15 fmol mg-1 protein), in which the cytotoxicity of temozolomide or BCNU was unaffected by AGT depletion.

Potentiation of temozolomide and BCNU cytotoxicity by O(6)-benzylguanine: a comparative study in vitro

Depletion of the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) with O(6)-benzylguanine (O(6)-BG) has been widely shown to enhance 1,3-bis(2-chloroethyl)-nitrosourea (BCNU) activity. This study aimed to determine whether temozolomide, a methylating imidazotetrazinone, would similarly benefit from combination with O(6)-BG. Seven human cell lines were examined with AGT activities ranging from <6 fmol mg-1 protein to >700 fmol mg-1 protein. Comparisons with BCNU were made on both single and multiple dosing schedules, since temozolomide cytotoxicity is highly schedule dependent. In single-dose potentiation studies, cells were preincubated with 100 microM O(6)-BG for 1 h, a treatment found to deplete AGT activity by >90% for 24 h. No potentiation of either temozolomide or BCNU cytotoxicity was observed in two glioblastoma cell lines with <6 fmol mg-1 protein AGT. In all other cell lines studied potentiation of BCNU toxicity by O(6)-BG was between 1.6- and 2.3-fold and exceeded that of temozolomide (1.1- to 1.7-fold). The magnitude of this potentiation was unrelated to AGT activity and the relative potentiation of temozolomide and BCNU cytotoxicity was found to be highly variable between cell lines. In multiple dosing studies two colorectal cell lines (Mawi and LS174T) were treated with temozolomide or BCNU at 24 h intervals for up to 5 days, with or without either 100 microM O(6)-BG for 1 h or 1 microM O(6)-BG for 24 h, commencing 1 h before alkylating treatment. Extended treatment with 1 microM O(6)-BG produced greater potentiation than intermittent treatment with 100 microM O(6)-BG. Potentiation of temozolomide cytotoxicity increased linearly in Mawi with each subsequent dosing: from 1.4-fold (day 1) to 4.2-fold (day 5) with continuous 1 microM O(6)-BG. In contrast, no potentiation was observed in LS174T, a cell line that would appear to be 'tolerant' of methylation. Potentiation of BNCU cytotoxicity increased in both cell lines with repeat dosing, although the rate of increase was less than that observed with temozolomide and continuous 1 microM O(6)-BG in Mawi. These results suggest that repeat dosing of an AGT inhibitor and temozolomide may have a clinical role in the treatment of tumours that exhibit AGT-mediated resistance.

Contribution of O6-methylguanine-DNA methyltransferase to resistance to 1,3-(2-chloroethyl)-1-nitrosourea in human brain tumor-derived cell lines

To assess the possible role of the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) in resistance of brain neoplasms to the clinically important chloroethylating agent 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), we quantitated MGMT activity, BCNU survival, and the effect of ablating MGMT activity on the sensitivity of 14 human medulloblastoma- and glioma-derived cell lines. BCNU resistance, measured as 10% survival dose (LD10), differed eightfold among the lines. Elimination of measurable MGMT activity with the substrate analogue inhibitor O6-benzylguanine (O6-BG) revealed a variable but limited contribution of MGMT to survival. In no case did O6-BG reduce LD10 by more than 3.4-fold. In contrast, O6-BG reduced the LD10 for N-methyl-N'-nitro-N-nitrosoguanidine up to 31-fold in the same cell lines (Bobola MS, Blank A, Berger MS, Silber JR, Mol Carcinog 13:70-80, 1995). Variability in BCNU survival, manifested as a sevenfold range of LD10, persists after measurable MGMT was eliminated, indicating that another mechanism or mechanisms is operating to limit cytotoxicity. Cells alkylated while suspended in growth medium are more resistant to BCNU and display less dependence on MGMT than cells treated while proliferating on a plastic substratum. When alkylated in suspension, most of the lines are either unresponsive to O6-BG or contain a subpopulation that did not respond to O6-BG. Our results demonstrate that BCNU resistance is multifactorial and that MGMT makes a modest contribution to resistance in our lines.

A pial window model for the intracranial study of human glioma microvascular function

A new model for human brain tumor uses the intracranial placement of tumor xenografts under transparent glass cranial windows in nude rats, which require no immunosuppression for tumor engraftment. Adult male nude rats underwent implantation of human anaplastic astrocytomas (D-54 MG in 10 rats, D-317 MG in 11 rats). The tumors were placed on the pial surface of the left cerebral hemisphere under a glass cranial window overlying the cranium. Six control animals underwent cranial window placement alone. Tumor volumes were estimated from direct measurements of tumor dimensions, revealing a mean doubling time of 1.58 days for the D-54 MG tumors and 2.62 days for the D-317 MG tumors. When tumor volume estimates reached 35 mm3, photomicrographs revealed tumor vasculature in each tumor cell line that was distinct from both the other xenograft and the normal brain parenchyma. Qualitative differences in vascular appearance were supported by length/density coefficient calculations in each study group, with D-317 MG demonstrating the highest vascular density. Vessel caliber tended to be smaller in D-54 MG tumors than in D-317 MG tumors. Laser-Doppler measurements of local blood flow in tumors and normal parenchyma revealed significantly lower blood flow in both tumor cell lines than in control brain. Evaluation of leukocyte/endothelial cell interactions indicated more leukocyte rolling in D-54 MG tumors than in D-317 MG tumors; no evidence of this cell interaction was found in normal pial vasculature. This model allows direct serial inspection of human brain tumor growth and vascular function in an experimental animal and could be used to study tumor vascular and inflammatory responses to a variety of therapeutic manipulations.