Linkage between O6-methylguanine-DNA methyltransferase (O6-MT) activity and cellular resistance to antitumour nitrosoureas in cultured rat brain tumour cell strains

We have examined O6-methylguanine-DNA methyltransferase (O6-MT) activity of rat brain tumour cell strains with reference to cellular resistance to antitumour nitrosoureas, 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (nimustine, ACNU) and methyl-6-[3-(2-chloroethyl)-3-nitrosoureido]-6-deoxy-alpha-D-glucopyrano side (ramustine, MCNU). The values of O6-MT activity were 52 and 160 fmol/mg protein extract in 9L and C6 rat brain tumour cells, respectively; while HeLa S3 cells, as a methyl excision repair positive (Mer+) cell strain, revealed a rather high value of 488 fmol/mg. 9L cells indicative of a low O6-MT activity showed 13 microM for ACNU and 18 microM for MCNU at a 10% survival dose (SD10), determined by a clonogenic cell assay as an index of cellular resistance. In contrast to this, C6 cells revealed a SD10 value of 67 microM and 36 microM for ACNU and MCNU, respectively, indicating higher resistance than 9L cells. HeLa S3 cells showed the highest SD10 value as follows: 84 microM for ACNU and 73 microM for MCNU. The relationship between the O6-MT activity and the cellular resistance was almost linear, with relatively resistant cell lines exhibiting the higher levels of the O6-MT activity. This correlation between the O6-MT activity and the cellular resistance to nitrosoureas as ACNU and MCNU was not observed among other antitumour drugs, which included bleomycin (BLM), neocarzinostatin (NCS), cis-diamminedichloroplatinum (II) (CDDP), and etoposide (VP-16) in clinical use for brain tumour chemotherapy. This indicates that O6-MT activity can be an indicator of cellular resistance to antitumour nitrosoureas in the chemotherapy of brain tumours.

[Augmentation of TNF- and lymphotoxin-mediated cytotoxic effect in the combined use of ACNU and involvement of oxygen free radicals]

Roles of oxygen free radicals in recombinant human TNF- and human lymphotoxin (LT)-mediated cytotoxicity have been examined. Nimustine (ACNU), which inhibits glutathione reductase, and buthionine sulphoximine (BSO), an inhibitor of glutathione (GSH) synthesis, were used to modify the steady-state level of intracellular H2O2. TNF-mediated cytotoxicity was augmented when ACNU was added simultaneously to target L cells or Meth A tumor cells. Similar augmented effect was observed when TNF or LT was added to ACNU-treated target cells. However, the addition of GSH nullified the augmentation of TNF-mediated cytotoxicity to ACNU-treated Meth A tumor cells. Meth A tumor cells were pretreated with BSO for 24 hr, and thereafter TNF or LT was added in the presence or the absence of BSO. The cytotoxic effect of TNF and LT was augmented by the treatment of the cell with BSO or simultaneous addition of BSO. High degree of the augmentation was obtained when the pretreatment with BSO and further addition of BSO were combined. These results suggest that oxygen free radicals are closely involved in TNF- and LT-mediated cytotoxicity and the modulation of intracellular GSH level alters the degree of the cytotoxicity of these cytotoxins.

Sensitivity of human malignant intracranial tumors against MCNU in vitro in comparison to ACNU and BCNU

An in vitro chemosensitivity test was carried out in 50 specimen of human malignant intracranial tumors. Aim of the study was to evaluate the proportion of sensitivity against MCNU (Ranomustine) in comparison to ACNU and BCNU. 47 tests were evaluable. Mean viability of the specimen was 83.3 +/- 18.7%, mean plating efficiency was 0.068 +/- 0.051%. 9/47 settings revealed sensitivity against MCNU in vitro (ACNU: 10/47; BCNU: 16/46). There was no advantage of MCNU concerning age or sex of the patients. Brain metastases seemed to be slightly more frequent sensitive against MCNU than primary brain tumors. Cross resistance between ACNU, BCNU and MCNU was rather high. The results of this in vitro series do not encourage a clinical trial of MCNU as an alternative to the commonly used nitrosoureas.

[Enhancement of the effect of X-irradiation against cultured human glioblastoma cells by pretreatment with ACNU]

Human glioblastoma A-7 (GB A-7) cells can apparently recover from potentially lethal X-irradiation. The authors, using a colony-forming assay, studied the influence of pretreatment with 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) on the effectiveness of X-irradiation against GB A-7 cells grown in monolayers and as multicellular spheroids. Pre-exposure to ACNU inhibited the recovery of irradiated GB A-7 cells. In monolayer cells, the combination treatment was most effective when ACNU was applied 2 to 8 hours prior to irradiation, and the larger the X-ray dose, the more potent the effect. ACNU pretreatment was more effective against large spheroids (enhancement ratio 1.86) than against small ones (1.34). Large spheroids showed necrosis, whereas small ones did not. Isobolographic analysis disclosed that the effect of combining X-irradiation and ACNU is within an additive envelope at the surviving fraction of 10(-2), while supra-additive at the surviving fraction of 10(-3). These results suggest that the potency of X-irradiation is augmented by ACNU pretreatment through an interactive mechanism. Further, suppression of recovery from X-ray induced potentially lethal damage was influenced by the presence of necrosis.

Sister-chromatid exchanges (SCEs), cell survival and mutation in HeLa s3 cells with different sensitivity to alkylating agents; evidence that SCE induction and cell survival or mutation induction are dissociable

We previously isolated N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-resistant cells, MR from HeLa S3 Mer- cells. In the present study, we have isolated 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU)-resistant cells, ACr. The MR cells had only a little O6-methylguanine-DNA methyltransferase (MT) activity, while the ACr cells had increased MT activity and also became resistant to the cytotoxic effect of MNNG. We compared the induction of sister-chromatid exchanges (SCEs), cell survival and mutation in these HeLa S3 cells with different sensitivity to MNNG. The ACr cells were much more resistant than the parental HeLa S3 Mer- cells to cytotoxicity, mutagenicity and SCE induction by MNNG, showing a positive correlation between SCE induction and cell killing or mutation. In contrast, this positive relationship was not observed between HeLa S3 Mer- and MR cells. These results suggest that O6-methylguanine (O6-MeG) is involved in the induction of the biological effects of MNNG such as cytotoxicity, mutagenicity and SCEs, and also indicate that SCE induction does not always correlate with cell killing and mutation.

5-Azacytidine-induced recovery of O6-alkylguanine--DNA alkyltransferase activity in mouse Ha821 cells

Mouse Ha821 cells, Harvey murine sarcoma virus-transformed NIH3T3 cells, have extremely low O6-alkylguanine--DNA alkyltransferase (O6AGTase) activity and are hypersensitive to an anti-tumor chloroethylating agent 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU). The treatment of Ha821 cells with a DNA demethylating agent, 5-azacytidine, resulted in an increase in the frequency of ACNU-resistant cell clones. All randomly isolated ACNU-resistant cell clones were found to have O6AGTase activity comparable to the level of the parental NIH3T3 cells. These results suggest that reversible loss of O6AGTase activity in Ha821 cells is caused at least in part by inactivation of the O6AGTase gene due to methylation of cytosine in the gene.

[Chemotherapeutic strategy in rat brain tumor cells resistant to ACNU using an in vitro colony formation assay]

Nitrosourea compounds have been widely used in the chemotherapy of malignant brain tumors, because of their blood-brain barrier permeability. However, drug resistance to nitrosoureas has been recently a major concern. Using an in vitro colony formation assay, intrinsic and acquired resistances to an anticancer nitrosourea, 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU), were analyzed in rat 9L and C6 glioma cells. 9L and C6 cells were treated with varying doses of ACNU for 2 hours. Ten days after, the cells were fixed and stained with crystal violet. Colonies consisting more than 50 cells were counted. The survival fraction following treatment is the ratios of colony efficiency of treated cells to the colony efficiency of untreated control cells. The dose-response curve for ACNU indicated the existence of a shoulder (Dq, quasithreshold dose) at doses and an exponential cell-killing at higher doses with D0(37% survival dose). Based on dose-response curves corresponding to multitarget single-hit model, 9L cells showed 7.4 microM, 2.9 microM, and 14 microM at Dq, D0, and SD10 (10% survival dose) values, respectively, whereas C6 cells showed respective values of 6.4 microM, 30 microM, and 75 microM. 9L cells had significantly less intrinsic resistance to ACNU than C6 cells at the p less than 0.005 level by a covariance analysis of the curves. As with changes of drug susceptibility after ACNU treatment, both parent cells were treated every other day (1, 5, and 10 repeated times) with various doses up to approximately 1% survival dose of the parent cells.(ABSTRACT TRUNCATED AT 250 WORDS)

[Antitumor efficacy of FK 973 on malignant glioma cells]

FK 973, a new substituted dihydrobenzoxazine, was obtained by chemical modification of a novel antibiotic which was isolated from the fermentation products of streptomyces sandaensis No. 6897. FK 973 had cytotoxic effects against in vitro cultured human and murine glioma cells. The concentration of FK 973 required to inhibit cell growth by 50% was 0.06-5 micrograms/ml, after 2-day exposure of this drug against human glioblastoma (ONS-6, 12, 23, and ONS-12/ACNU), human medulloblastoma (ONS-76, 81), human neuroblastoma (ST), and murine glioblastoma (RSV-M glioma). FK 973 showed antitumor efficacy in the meningeal gliomatosis models by RSV-M glioma cells. The median survival time (MST) of models treated by FK 973 (i.t.) was 30 days. However, the MST of control group was 23 days. In the in vitro neurotoxicity test, FK 973 proved to be slightly more toxic than ACNU and MTX, but it had no crucial problems, compared with ADM.

Intracellular glutathione levels in human colon cancer cells naturally resistant to cross-linking agents

Correlation between sensitivity to two cross-linking agents, 1-(4-amino-2-methylpyridine-5-yl)-methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU) and cisplatin (DDP), and intracellular glutathione (GSH) level was investigated for two naturally drug-resistant human colon cancer cell lines in comparison with two drug-sensitive human leukemia cell lines. As a result, no appreciable correlation was observed between them. We also studied the possibility that DL-buthionine-S,R-sulfoximine (BSO), an inhibitor of GSH biosynthesis, can sensitize the cancer cells to these anticancer agents via depletion of intracellular GSH. It was found that BSO potentiated ACNU cytotoxicity against human leukemia K562 cells and DDP cytotoxicity against K562 and human colon cancer WiDr cells. It indicates that cancer cells with higher GSH level are more effectively sensitized by BSO regardless of degree of their intrinsic sensitivity to these anticancer agents. These results suggest that intracellular GSH level is not a common mechanism for natural resistance to cross-linking agents in human colon cancer cells but one of the determinants of sensitivity to these anticancer agents of GSH-rich cells.

Importance of antitumor immunity for complete cure of highly drug-sensitive leukemia in mice

Seven transplantable leukemia lines were established from spontaneous leukemias and screened for 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-1-(chloroethyl)-1-nitrosourea hydrochloride (ACNU) sensitivity in DDD mice. Three of them were classified as highly sensitive, two as sensitive and two as resistant to ACNU. A highly sensitive line, DL812, was extensively characterized from a therapeutic point of view. DL812 cells were so invasive as to produce enlargement of spleens and lymph nodes but not local tumors when injected s.c., markedly sensitive to in vitro ACNU exposure and moderately immunogenic. The invasion process of DL812 cells differed with the status of host immunity. Advanced DL812 leukemias were macroscopically completely cured with normalization of spleen and lymph node sizes 3-7 days after an i.p. injection of 0.5 mg ACNU, but more ACNU-resistant leukemias with splenomegaly and enlarged lymph nodes recurred thereafter. Recurring DL812 cells were approximately four times more resistant to in vitro ACNU exposure but maintained similar immunogenicity as compared to the original ones. Permanent cures of advanced leukemias were achieved by ACNU treatment plus subsequent adoptive transfer of immune splenocytes in 15% of diseased mice. The results suggest the importance of host antitumor immunity for permanent cures of highly drug-sensitive leukemias, overcoming drug resistance and relapse.

Microspectrofluorometric evaluation of single- and double-stranded DNA in short-term cultured human glioma cells

Seven cerebral gliomas in short-term culture were studied by microspectrofluorometry and acridine orange staining to assess their nuclear content of single- and double-stranded DNA. Benign gliomas showed a diploid DNA pattern, whereas malignant gliomas revealed a higher frequency of DNA aneuploidy and hyperploidy. The content of single-stranded DNA remained relatively low in benign gliomas; however, that in malignant gliomas varied widely. After ACNU treatment, the double-stranded DNA histograms showed S-phase-specific accumulation in all cases but 1 with increasing concentrations of ACNU. The single-stranded DNA content decreased considerably in 2 cases, which responded well to chemotherapy and showed clinical amelioration.

[Circumvention of ACNU-resistance in rat glioma cells by pretreatment with O6-methylguanine]

The chemotherapy of malignant brain tumors has been, only partially successful yet. Recently major concern is drug resistance, one of possible mechanisms of such drug resistance stems from inducible repair enzyme, especially in case of chloroethylnitrosoureas as ACNU or BCNU. We examined the changes of acquired resistance to ACNU in rat glioma cells by pretreatment with O6-methylguanine, which is a substrate for O6-methylguanine methyltransferase. ACNU-resistant (9L/AC) cells had established after 10 times treatments of ACNU. 9L/AC cells were pretreated with 2 mM O6-methylguanine for 2 hours, and subsequently challenged with increasing doses of ACNU for 2 hours. In vitro colony formation assay the survival fraction of 9L and 9L/AC cells ranged from 0.39 to 0.63 by 2-hour reaction of 1-3 mM O6-methylguanine. Based on the dose-response curve for ACNU in 9L/AC cells, by O6-methylguanine pretreatment (2 mM), ACNU-resistance decreased markedly to one-third, one-fifth, and one-two hundredth at 12, 24, 36 microM ACNU, respectively. In contrast, the survival of 9L cells against ACNU was similar under O6-methylguanine pretreatment or nontreatment condition. Therefore, ACNU-resistance is considerably related to DNA repair enzyme induction, and the substrates may potentiate the cell-killing effect of ACNU in the resistant glioma cells.

DNA lability induced by nimustine and ramustine in rat glioma cells

The DNA labile sites induced by two nitrosoureas, nimustine (ACNU) and ramustine (MCNU) synthesised in Japan, have been examined in highly reiterated DNA sequences of rat glioma cells. Reiterated fragments of 167 and 203 base pairs (bp), obtained after Hind III and Hae III restriction endonuclease digestion of rat glioma cells DNA, were used as target DNA sequences to determine the labile sites. In vitro reaction with ACNU and MCNU resulted in scission products corresponding to the locations of guanine. Subsequent piperidine hydrolysis produced more frequent breaks of the phosphodiester bonds at guanine positions, thus forming alkali-labile sites.