Carmustine-resistant cancer cells are sensitized to temozolomide as a result of enhanced mismatch repair during the development of carmustine resistance

Alterations in Cell Motility, Proliferation, and Metabolism in Novel Models of Acquired Temozolomide Resistant Glioblastoma

Glioblastoma (GBM) is an aggressive and incurable tumor of the brain with limited treatment options. Current first-line standard of care is the DNA alkylating agent temozolomide (TMZ), but this treatment strategy adds only ~4 months to median survival due to the rapid development of resistance. While some mechanisms of TMZ resistance have been identified, they are not fully understood. There are few effective strategies to manage therapy resistant GBM, and we lack diverse preclinical models of acquired TMZ resistance in which to test therapeutic strategies on TMZ resistant GBM. In this study, we create and characterize two new GBM cell lines resistant to TMZ in vitro, based on the 8MGBA and 42MGBA cell lines. Analysis of the TMZ resistant (TMZres) variants in conjunction with their parental, sensitive cell lines shows that acquisition of TMZ resistance is accompanied by broad phenotypic changes, including increased proliferation, migration, chromosomal aberrations, and secretion of cytosolic lipids. Importantly, each TMZ resistant model captures a different facet of the “go” (8MGBA-TMZres) or “grow” (42MGBA-TMZres) hypothesis of GBM behavior. These in vitro model systems will be important additions to the available tools for investigators seeking to define molecular mechanisms of acquired TMZ resistance.

Acquired temozolomide resistance in human glioblastoma cell line U251 is caused by mismatch repair deficiency and can be overcome by lomustine

PURPOSE

Glioblastoma multiforme (GBM) is the most common malignant primary brain tumor in adults. While the alkylating agent temozolomide (TMZ) has prolonged overall survival, resistance evolution represents an important clinical problem. Therefore, we studied the effectiveness of radiotherapy and CCNU in an in vitro model of acquired TMZ resistance.

METHODS

We studied the MGMT-methylated GBM cell line U251 and its in vitro derived TMZ-resistant subline, U251/TMZ-R. Cytotoxicity of TMZ, CCNU, and radiation was tested. Both cell lines were analyzed for MGMT promotor status and expression of mismatch repair genes (MMR). The influence of MMR inhibition by cadmium chloride (CdCl₂) on the effects of both drugs was evaluated.

RESULTS

During the resistance evolution process in vitro, U251/TMZ-R developed MMR deficiency, but MGMT status did not change. U251/TMZ-R cells were more resistant to TMZ than parental U251 cells (cell viability: 92.0% in U251/TMZ-R/69.2% in U251; p = 0.032) yet more sensitive to CCNU (56.4%/80.8%; p = 0.023). The effectiveness of radiotherapy was not reduced in the TMZ-resistant cell line. Combination of CCNU and TMZ showed promising results for both cell lines and overcame resistance. CdCl₂-induced MMR deficiency increased cytotoxicity of CCNU.

CONCLUSION

Our results confirm MMR deficiency as a crucial process for resistance evolution to TMZ. MMR-deficient TMZ-resistant GBM cells were particularly sensitive to CCNU and to combined CCNU/TMZ. Effectiveness of radiotherapy was preserved in TMZ-resistant cells. Consequently, CCNU might be preferentially considered as a treatment option for recurrent MGMT-methylated GBM and may even be suitable for prevention of resistance evolution in primary treatment.

Leukemia cells are sensitized to temozolomide, carmustine and melphalan by the inhibition of O6-methylguanine-DNA methyltransferase

The cytotoxicity of the monofunctional alkylator, temozolomide (TMZ), is known to be mediated by mismatch repair (MMR) triggered by O⁶-alkylguanine. By contrast, the cytotoxicity of bifunctional alkylators, including carmustine (BCNU) and melphalan (MEL), depends on interstrand crosslinks formed through O⁶-alkylguanine, which is repaired by nucleotide excision repair and recombination. O⁶-alkylguanine is removed by O⁶-methylguanine-DNA methyltransferase (MGMT). The aim of the present study was to evaluate the cytotoxicity of TMZ, BCNU and MEL in two different leukemic cell lines (HL-60 and MOLT-4) in the context of DNA repair. The transcript levels of MGMT, ERCC1, hMLH1 and hMSH2 were determined using reverse transcription-quantitative polymerase chain reaction. In addition, the proliferation was measured using the trypan blue exclusion assay. Drug sensitivity was found to vary between the two cell lines. Treatment of the cells with TMZ, BCNU or MEL in combination with O⁶-benzylguanine, an MGMT inhibitor, was demonstrated to sensitize the two cell lines to these agents. However, the extent of sensitization was not found to be correlated with the expression levels of MGMT transcripts. Furthermore, the drug sensitivity was also not associated with the transcript levels of ERCC1, hMLH1 and hMSH2. Thus, leukemic cells were sensitized to alkylating agents by the inhibition of MGMT.

MGMT promoter methylation status and prognosis of patients with primary or recurrent glioblastoma treated with carmustine wafers

The prognostic role of O(6)-methylguanine-DNA methyltransferase (MGMT) promoter methylation in glioblastoma patients treated with carmustine (BCNU) wafer implantation is unclear. Here, we report on a retrospective study of 47 patients with either newly diagnosed (30 patients) or recurrent (17 patients) glioblastoma (WHO grade IV) treated with BCNU (bis-chloroethylnitrosourea) wafers. Thirteen of the newly diagnosed patients received local BCNU and irradiation only (first-line BCNU), while 17 patients additionally received concomitant and adjuvant temozolomide (TMZ) radiochemotherapy (first-line BCNU + TMZ). Of the 17 patients treated for recurrent glioblastoma (second-line BCNU), 16 had received radiotherapy with concomitant and adjuvant TMZ as an initial treatment. Median overall survival (OS) did not significantly differ between 19 patients with MGMT promoter methylated tumors when compared to 28 patients with unmethylated tumors (18.9 vs 15.0 months; p = 0.1054). In the first-line BCNU + TMZ group, MGMT promoter methylation was associated with longer OS (21.0 vs 11.1 months, p = 0.0127), while no significant survival differences were detected in the other two subgroups. Progression-free survival did not significantly differ between patients with and without MGMT promoter methylated tumors in the entire patient cohort or any of the three subgroups. The first-line BCNU + TMZ group showed no significant difference in OS when compared to the first-line BCNU group (18.9 vs 14.7 months), but tended to have more therapy-related adverse effects (53% vs 24%, p = 0.105). In summary, MGMT promoter methylation showed a non-significant trend toward longer survival in our patient cohort. The combination of TMZ radiochemotherapy with local delivery of BCNU did not provide a significant survival benefit compared to local BCNU alone, but was associated with a higher rate of adverse effects. Owing to the small number of patients investigated, however, these findings would need to be corroborated in larger patient cohorts.

Carmustine-induced phosphatidylserine translocation in the erythrocyte membrane

The nitrosourea alkylating agent, carmustine, is used as chemotherapeutic drug in several malignancies. The substance triggers tumor cell apoptosis. Side effects of carmustine include myelotoxicity with anemia. At least in theory, anemia could partly be due to stimulation of eryptosis, the suicidal death of erythrocytes, characterized by cell shrinkage and breakdown of phosphatidylserine asymmetry of the cell membrane with phosphatidylserine exposure at the erythrocyte surface. Stimulators of eryptosis include increase of cytosolic Ca²⁺ activity ([Ca²⁺]i). The present study tested whether carmustine triggers eryptosis. To this end [Ca²⁺]i was estimated from Fluo3 fluorescence, cell volume from forward scatter, phosphatidylserine exposure from annexin V binding, and hemolysis from hemoglobin release. As a result a 48 h exposure to carmustine (≥25 µM) significantly increased [Ca²⁺]i, decreased forward scatter and increased annexin V binding. The effect on annexin V binding was significantly blunted in the absence of extracellular Ca²⁺. In conclusion, carmustine stimulates eryptosis at least partially by increasing cytosolic Ca²⁺ activity.

Inhibition of STAT3 reverses drug resistance acquired in temozolomide-resistant human glioma cells

The alkylating agent temozolomide (TMZ) is an effective drug used for the treatment of malignant gliomas. However, tumor relapse combined with the development of drug resistance remains a significant problem. To clarify the mechanism of the resistance of glioma cells to TMZ chemotherapy, TMZ-resistant glioma cell lines (TR cells) were generated using U373 and U251 human glioma cells, and TMZ-resistance was confirmed via viability and apoptosis assays. The TMZ-resistance of TR cells was not associated with the TMZ-resistance molecule O(6)-methylguanine-DNA-methyltransferase. Notably, the expression level of signal transducers and activators of transcription 3 (STAT3) and serine 727-phosphorylated STAT3 (pSTAT3-Ser727) was highly increased in TR cells, while that of 705-phosphorylated STAT3 (pSTAT3-Tyr705) was decreased. The inhibition of STAT3 expression by small interfering RNA enhanced TR cell TMZ sensitivity. These results suggest that STAT3 contributes to TMZ-resistance in gliomas and is a potential target for the reversal of TMZ-resistance in patients with a recurrent glioma.

Bifunctional DNA alkylator 1,3-bis(2-chloroethyl)-1-nitrosourea activates the ATR-Chk1 pathway independently of the mismatch repair pathway

The presence of DNA damage initiates signaling through the ataxia-telangiectasia mutated kinase (ATM) and the ATM- and the Rad3-related kinase (ATR), which phosphorylate, thus activating, the checkpoint kinases (Chk) 1 and 2, which leads to cell cycle arrest. The bifunctional DNA alkylator 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) is cytotoxic primarily by inducing DNA monoadducts and ultimately, interstrand cross-links, which block DNA replication. In this study, we investigated the activation of the ATR-Chk1 pathway in response to BCNU treatment and the dependence of this response on the DNA mismatch repair (MMR) capacity. Medulloblastoma cells were exposed to low and moderate doses of BCNU, and the effects on this DNA damage signaling pathway were examined. In response to BCNU, Chk1 was found to be phosphorylated at serine 345 and exhibited increased kinase activity. Caffeine and wortmannin, which are broad-spectrum inhibitors of ATM and ATR, reduced this phosphorylation. Cell cycle analysis further revealed an accumulation of cells in the S phase in response to BCNU, an effect that was attenuated by caffeine. Small interfering RNA knockdown of ATR also reduced Chk1 phosphorylation after exposure to BCNU. However, knockdown of ATM had no effect on the observed Chk1 phosphorylation, suggesting that ATR was primarily responsible for Chk1 activation. Analysis of Chk1 activation in cells deficient in MMR proteins MutLalpha or MutSalpha indicated that the DNA damage response induced by BCNU was independent of the MMR apparatus. This MMR-independent activation seems to be the result of DNA interstrand cross-link formation.