The frequency of micronuclei as a measure of cell survival in irradiated cell populations

Mitigation of radiation-induced hematopoietic injury by the polyphenolic acetate 7, 8-diacetoxy-4-methylthiocoumarin in mice

Protection of the hematopoietic system from radiation damage, and/or mitigation of hematopoietic injury are the two major strategies for developing medical countermeasure agents (MCM) to combat radiation-induced lethality. In the present study, we investigated the potential of 7, 8-diacetoxy-4-methylthiocoumarin (DAMTC) to ameliorate radiation-induced hematopoietic damage and the associated mortality following total body irradiation (TBI) in C57BL/6 mice. Administration of DAMTC 24 hours post TBI alleviated TBI-induced myelo-suppression and pancytopenia, by augmenting lymphocytes and WBCs in the peripheral blood of mice, while bone marrow (BM) cellularity was restored through enhanced proliferation of the stem cells. It stimulated multi-lineage expansion and differentiation of myeloid progenitors in the BM and induced proliferation of splenic progenitors thereby, facilitating hematopoietic re-population. DAMTC reduced the radiation-induced apoptotic and mitotic death in the hematopoietic compartment. Recruitment of pro-inflammatory M1 macrophages in spleen contributed to the immune-protection linked to the mitigation of hematopoietic injury. Recovery of the hematopoietic compartment correlated well with mitigation of mortality at a lethal dose of 9 Gy, leading to 80% animal survival. Present study establishes the potential of DAMTC to mitigate radiation-induced injury to the hematopoietic system by stimulating the re-population of stem cells from multiple lineages.

Transient elevation of glycolysis confers radio-resistance by facilitating DNA repair in cells

BACKGROUND

Cancer cells exhibit increased glycolysis for ATP production (the Warburg effect) and macromolecular biosynthesis; it is also linked with therapeutic resistance that is generally associated with compromised respiratory metabolism. Molecular mechanisms underlying radio-resistance linked to elevated glycolysis remain incompletely understood.

METHODS

We stimulated glycolysis using mitochondrial respiratory modifiers (MRMs viz. di-nitro phenol, DNP; Photosan-3, PS3; Methylene blue, MB) in established human cell lines (HEK293, BMG-1 and OCT-1). Glucose utilization and lactate production, levels of glucose transporters and glycolytic enzymes were investigated as indices of glycolysis. Clonogenic survival, DNA repair and cytogenetic damage were studied as parameters of radiation response.

RESULTS

MRMs induced the glycolysis by enhancing the levels of two important regulators of glucose metabolism GLUT-1 and HK-II and resulted in 2 fold increase in glucose consumption and lactate production. This increase in glycolysis resulted in resistance against radiation-induced cell death (clonogenic survival) in different cell lines at an absorbed dose of 5 Gy. Inhibition of glucose uptake and glycolysis (using fasentin, 2-deoxy-D-glucose and 3-bromopyruvate) in DNP treated cells failed to increase the clonogenic survival of irradiated cells, suggesting that radio-resistance linked to inhibition of mitochondrial respiration is glycolysis dependent. Elevated glycolysis also facilitated rejoining of radiation-induced DNA strand breaks by activating both non-homologous end joining (NHEJ) and homologous recombination (HR) pathways of DNA double strand break repair leading to a reduction in radiation-induced cytogenetic damage (micronuclei formation) in these cells.

CONCLUSIONS

These findings suggest that enhanced glycolysis generally observed in cancer cells may be responsible for the radio-resistance, partly by enhancing the repair of DNA damage.

Studies on black tea (Camellia sinensis) extract as a potential antioxidant and a probable radioprotector

Positive health effects of tea (Camellia sinensis) on a wide range of physiological problems and diseases are well known and are in part due to its copious antioxidant content. The effect of black tea extract (BTE), which is rich in polyphenolic antioxidants, against the consequences of radiation exposure has not been properly identified. The functional properties of BTE were analyzed and its radioprotective effect on V79 cells was explored in the present study. BTE scavenged free radicals and inhibited Fenton reaction-mediated 2-deoxyribose degradation and lipid peroxidation in a dose-dependent fashion, establishing its antioxidant properties. The radioprotective effects of BTE on strand break induction in pBR322 plasmid DNA were 100 % at 80 μg/ml and higher. In V79 cells, BTE was effective in decreasing the frequency of radiation-induced micronucleated cells and the yields of reactive oxygen species (ROS) and also in restoring the integrity of cellular mitochondrial membrane potential significantly. BTE exerted maximum protection against radiation-induced damage in V79 at a dose of 5 μg/ml. Due to the functional properties of BTE-flavonoids, which have been identified by HPLC, it is envisaged that the key player in radioprotection is elimination of ROS.

Inhibition of radiation-induced DNA damage by jamun, Syzygium cumini, in the cultured splenocytes of mice exposed to different doses of γ-radiation

The radioprotective property of 50 mg/kg body weight jamun (Syzygium cumini) extract was studied in the cultured splenocytes of mice exposed to 0, 0.5, 1, 2, 3, or 4 Gy of γ-radiation. The spleens of irradiated mice were removed aseptically and the splenocytes were extracted from the individual spleens and cultured. The micronuclei were prepared 72 hours after irradiation in binucleate splenocytes by blocking cytokinesis with cytochalasin-B. Irradiation of mice resulted in a dose-dependent elevation in the micronucleated splenocytes. The exposure of mice not only elevated splenocytes bearing one micronucleus but also cells bearing 2 and multiple (>2) micronuclei indicating induction of complex DNA damage after irradiation. Oral treatment of mice with 50 mg/kg body weight of jamun leaf extract protected against the radiation-induced micronuclei formation. Jamun extract also protected against the formation of 2 and multiple micronuclei indicating repair or inhibition of complex DNA damage. The assessment of lipid peroxidation in mice brain homogenate has indicated a concentration dependent inhibition of lipid peroxidation by jamun extract. Studies in a cell free system revealed that jamun extract inhibited the formation of OH, O(2)-, DPPH, and ABTS(+) free radicals in a concentration dependent manner. Our study demonstrates that jamun extract protected mice against the radiation-induced DNA damage and inhibition of radiation-induced free radical formation may be one of the mechanisms of radioprotection.

Retention of γH2AX foci as an indication of lethal DNA damage

The application of biological responses of tumours to predict clinical responses to treatment represents a challenging goal with the potential to inform treatment decisions and improve outcome. If tumour cell death is the result of the inability of a cell to repair complex DNA damage, and if γH2AX foci mark sites of unrepaired double-strand breaks, then it may be possible to use residual γH2AX foci to identify treatment-resistant tumour cells early in the course of therapy. This review will highlight some of the evidence that supports the idea that residual γH2AX foci, within certain limitations, may be useful as an early indicator of tumour response to radiotherapy in situ, either alone or in combination with chemotherapy.

Response to high LET radiation 12C (LET, 295 keV/microm) in M5 cells, a radio resistant cell strain derived from Chinese hamster V79 cells

PURPOSE

To study the effects of 12C-beam of 295 keV/microm (57.24 MeV) on M5 and Chinese hamster V79 cells by using cytogenetic assays like micronuclei (MN) induction, chromosomal aberrations (CA) and apoptosis. Additionally, the relative survival of these two cell lines was tested by the colony forming ability of the cells, with a view to understanding the mechanism of cellular damages that lead to difference in cell survival.

MATERIALS AND METHODS

Confluent cells were irradiated with 12C-beam at various doses using 15UD Pelletron accelerator. Cell survival was studied by the colony forming ability of cells. MN assay was done by fluorescent staining. Different types of chromosomal aberrations in metaphase cells were scored at 12 h after irradiation. Apoptosis was measured at different post irradiation times as detected by nuclear fragmentation and DNA ladder was prepared after 48 h of incubation.

RESULTS

Dose-dependent decrease in surviving fractions was found in both the cell lines. However, the surviving fractions were higher in M5 cells in comparison to V79 cells when exposed to the same radiation doses. On the other hand, induced MN frequencies, CA frequencies and apoptosis percentages were less in M5 cells than V79 cells. Very good correlations between surviving fractions and induced MN frequencies or induced total CA or induced apoptosis percentages were obtained in this study.

CONCLUSIONS

The cell strain M5 showed relatively more radio-resistance to 12C-beam compared to Chinese hamster V79 cells in this study. As the MN formation, CA and apoptosis induction were less in M5 cells as compared to parental V79 cells, the higher cell survival in the former could possibly be attributed to their better repairing ability leading to higher cell survival.

Radiation therapy induced micronuclei in cervical cancer—does it have a predictive value for local disease control?

OBJECTIVE

To evaluate the predictive value of serial changes in micronuclei induction during external radiotherapy (EXRT) in cervical cancer with respect to local response at the end of EXRT and local disease free survival (LDFS).

METHODS

Twenty-five patients of squamous cell cancer of the cervix were treated by 50 Gy of EXRT delivered over 5 weeks followed by intracavitary brachytherapy. Serial cytological smears were taken from cervical growth at weekly intervals during the course of EXRT and stained by Giemsa and May-Grunwald's stain. Micronuclei induction were scored as (a) number of cells expressing micronuclei (MN), and (b) total number of micronuclei (TMN) in 1000 tumor cells from each of the serial smears.

RESULTS

A significant rise in micronuclei count was seen for both MN and TMN from pretreatment (week 0) to successive weeks of EXRT. For those having a near total tumor regression by end of EXRT, a significant rise in micronuclei was evident even at the end of first week of EXRT (MN: P = 0.05, TMN: P = 0.04). A superior LDFS was observed in patients showing greater than 50% increment in MN value in the first week (median survival for <50% vs. > or =50% rise: 5 months vs. not reached, P = 0.21), while it reached significance for a similar rise of TMN (median survival <50% vs. > or =50% rise: 5 months vs. not reached, P = 0.04).

CONCLUSIONS

The significant rise of micronuclei at the end of first week of EXRT in cervical cancers as observed from serial cytological smears could predict for a better local response and LDFS.

Genetic damage in exfoliated cells from oral mucosa of individuals exposed to X-rays during panoramic dental radiographies

The genotoxic effects of X-ray emitted during dental panoramic radiography were evaluated in exfoliated cells from oral epithelium through a differentiated protocol of the micronucleus test. Thirty-one healthy individuals agreed to participate in this study and were submitted to this procedure for diagnosis purpose after being requested by the dentist. All of them answered a questionnaire before the examination. Cells were obtained from both sides of the cheek by gentle scrapping with a cervical brush, immediately before the exposure and after 10 days. Cytological preparations were stained according to Feulgen-Rossenbeck reaction and analyzed under light and laser scanning confocal microscopies. Micronuclei, nuclear projections (buds and broken eggs) and degenerative nuclear alterations (condensed chromatin, karyolysis and karyorrhexis) were scored. The frequencies of micronuclei, karyolysis and pycnosis were similar before and after exposure (P > 0.90), whereas the condensation of the chromatin and the karyorrhexis increased significantly after exposure (P < 0.0001). In contrast, both bud and broken egg frequencies were significantly higher before the examination (P < 0.005), suggesting that these structures are associated to the normal epithelium differentiation. The results suggest that the X-ray exposure during panoramic dental radiography induces a cytotoxic effect by increasing apoptosis. We also believe that the score of other nuclear alterations in addition to the micronucleus improves the sensitivity of genotoxic effects detection.

Correlation of micronuclei-induction with the cell survival in HeLa cells treated with a base analogue, azidothymidine (AZT) before exposure to different doses of gamma-radiation

The effect of 0.1 microM azidothymidine (AZT) a pyrimidine analogue has been studied on the growth kinetics, cell survival and micronuclei formation in HeLa cells exposed to 0, 0.25, 0.5, 1, 2 and 3 Gy of 60Co gamma-radiation. The AZT pretreatment resulted in a significant decline in the cell growth kinetics, cell survival and cell proliferation indices when compared with the PBS+irradiation group at 20, 30 and 40 h post-irradiation. Conversely, the frequency of micronucleated binucleate cells (MNBNC) elevated in a dose dependent manner in both PBS+irradiation and AZT+irradiation group. This elevation in MNBNC-induction was significantly higher in the latter when compared with the former group at all post-irradiation scoring time periods studied. The dose-response relationship for micronuclei induction for both the PBS+irrradiation and AZT+irradiation groups was linear. The biological response was studied by correlating the cell survival with MNBNC-induction. The cell survival and MNBNC-induction showed a close but inverse relationship and this relationship gave a best fit on the linear quadratic model.

Cellular uptake, localization and photodynamic effects of haematoporphyrin derivative in human glioma and squamous carcinoma cell lines

Uptake, intracellular concentration, localization and photodynamic effects of a haematoporphyrin derivative (HpD, Photosan-3) were compared in human glioma (BMG-1, wild-type p53) and squamous carcinoma (4451, mutated p53) cell lines. Concentration and time dependence of cellular uptake of HpD was assayed from methanol extracts and whole cell suspension spectroscopy, while localization was studied by fluorescence microscopy-based image analysis. Colony-forming ability, apoptosis, cell-cycle progression and cytogenetic damage (micronuclei formation) were investigated as parameters of photodynamic response following irradiation with red light. BMG-1 cells were more sensitive to the photodynamic treatment than 4451 cells, although the 4451 cells accumulated a higher amount of HpD and did not differ significantly from BMG-1 cells with respect to intracellular localization. Photodynamically-induced cytogenetic damage and apoptosis were considerably higher in BMG-1 cells as compared to 4451 cells. The present results strongly suggest that manifestation of the photodynamically-induced lesions in the form of cytogenetic damage and apoptosis are among the important determinants of cellular sensitivity to HpD-PDT besides the photodynamic dose (intracellular concentration of the photosensitizer and the light dose).

The genotoxic and cytotoxic effects of ribavirin in rat bone marrow

The genotoxic and cytotoxic effects of the antiviral drug, ribavirin, was studied in rat bone marrow by employing the micronucleus assay. Ribavirin in doses of 10, 15, 20, 30, 50, 75, 100 and 200 mg/kg, and cyclophosphamide (CP) 40 mg/kg (only for sex-difference study) were injected intraperitoneally. Bone marrow was collected at 24 h and 48 h following the injection. To evaluate the recovery, the bone marrow was also sampled at 72 h from 20, 100 and 200 mg/kg treated rats. The micronucleus assay was conducted according to the standard procedure. Ribavirin elevated the incidence of micronuclei (except 10 mg/kg) in erythrocytes (P<0.01). The micronucleated polychromatic erythrocytes showed the initial steep increase at 15 and 20 mg/kg dose level, then with the gradual increase, possibly due to the limited metabolism and action of higher doses. The incidence of micronucleated normochromatic erythrocytes was not dose dependent. The effect was more at 48 h than 24 h due to prolonged toxicity of the drug or its metabolites, and by 72 h, recovery was observed even though the genotoxicity was significant. The PCE% decreased as the dose was increased up to 75 mg/kg, then without much difference between two higher doses. Only 100 mg/kg ribavirin and CP showed more toxicity on male rats. Cytotoxicity was seen due to hindered erythropoiesis or cell destruction. Our findings suggest that ribavirin is genotoxic and cytotoxic agent for rat bone marrow.

Selective radioprotection of hepatocytes by systemic and portal vein infusions of amifostine in a rat liver tumor model

PURPOSE

The tolerance of the liver to radiation is too low to permit an effective dose to be delivered to patients who have diffuse intrahepatic cancer. In this study we evaluated whether systemic or portal venous administration of the aminothiol compound, amifostine, could protect the normal liver from the effects of ionizing radiation without compromising tumor cell kill in a rat liver tumor model.

METHODS AND MATERIALS

Rats implanted with liver tumors were infused with 200 mg/kg amifostine over 15 min via the femoral or portal vein. The livers were irradiated with a single 6-Gy fraction 15-20 min after the termination of amifostine infusion. Protection of the liver was assessed by an in vitro hepatocyte micronucleus assay and tumor protection by an in vivo-in vitro clonogenic survival assay. Tissue levels of the active metabolite, free WR-1065, were determined in the tumor and in the normal liver using a specific HPLC assay with electrochemical detection.

RESULTS

After a 6-Gy fraction, the frequency of hepatocyte micronuclei after administration of saline, systemic amifostine, and portal venous amifostine was 18.7+/-1%, 6.8+/-1%, and 9.9+/-2%, respectively, corresponding to a radiation equivalent effect of 6 Gy +/- 0.5, 1.8 Gy +/- 0.3, and 2.5 Gy +/- 1.3, respectively. Both amifostine conditions showed considerably less radiation effect than saline-treated controls (p < 0.01); the two amifostine conditions did not differ (p = 0.3). The surviving fraction of tumor cells was not affected by amifostine treatment and was 0.03+/-0.02 and 0.05+/-0.03 for systemic and portal venous delivery and 0.06+/-0.02 for control animals (ANOVA analysis showed no significant difference of the means p = 0.34). Portal venous delivery produced significantly less WR-1065 in the tumor compared to systemic administration (54 microM +/- 36 vs. 343 microM +/- 88, respectively, p = 0.03).

CONCLUSIONS

Both systemic and portal venous administration of amifostine effectively protect hepatocytes from ionizing radiation without compromising tumor cell kill in a clinically relevant animal model. These findings suggest that amifostine may be a selective normal tissue radioprotectant in liver cancer and that regional/portal infusions may be preferable to systemic dosing.

Effect of acyclovir on the radiation-induced micronuclei and cell death

Treatment of HeLa cells with 0.1 microM Acyclovir [9-(2-hydroxyethoxymethyl)guanine] (ACV) before exposure to 0, 0.25, 0.5, 1, 2 and 3 Gy of gamma-radiation resulted in a dose-dependent decline in the growth kinetics and cell proliferation indices at 20, 30 and 40 h post-irradiation when compared with the PBS+irradiation group. These results were reflected in the cell survival, which declined in a dose-dependent manner and the surviving fraction of cells was significantly lower in ACV+irradiation group than that of PBS+irradiation group. The effect of ACV+1 Gy irradiation was almost similar to PBS+3 Gy irradiation suggesting an enhancement of the radiation effect by ACV pretreatment. The frequency of micronuclei increased in a dose-dependent manner at all the post-irradiation time periods in both PBS+irradiation and ACV+irradiation group and it was significantly elevated in the latter when compared with the former group. The dose-response for both groups was linear. The surviving fraction of HeLa cells declined with the increasing MN frequency and a close linear quadratic correlation between cell survival and micronuclei-induction was observed.

Correlation between cell survival and micronuclei-induction in HeLa cells treated with adriamycin after exposure to various doses of gamma-radiation

The effect of 10 microg/ml of adriamycin (doxorubicin) post-treatment was studied in HeLa cells exposed to 0, 0.5, 1, 2 and 3 Gy of gamma radiation. The survival of HeLa cells declined in a dose dependent manner in both irradiation+PBS and irradiation+ADR groups. Treatment of adriamycin immediately after irradiation resulted in a significant decline in the cell survival. The surviving fraction of HeLa cells reduced to 0.61 after exposure to 0. 5 Gy in the irradiation+ADR group, whereas a similar effect (i.e. surviving fraction of 0.61) was obtained for 3 Gy in the irradiation+PBS group. In contrast, the frequency of micronuclei increased in a dose dependent manner in both irradiation+PBS and irradiation+ADR groups. A significant elevation in the frequency of micronuclei was observed in the latter when compared with the former group. The dose response for both groups was linear quadratic. The cell proliferation indices also showed a dose dependent decline in both the groups. The decline in the cell proliferation was significantly higher in the irradiation+ADR group when compared with the irradiation+PBS group. A close correlation between the cell survival and micronuclei induction was observed in both groups, where the cell survival declined with the elevation in the micronuclei frequency. The relationship between cell survival and micronuclei induction was linear quadratic.

Correlation between cell survival and micronuclei formation in V79 cells treated with vindesine before exposure to different doses of gamma-radiation

Effect of 20 nM vindesine sulphate (VDS) treatment was studied on cell survival, growth kinetics and micronuclei induction in V79 cells exposed to 0-300 cGy of gamma-radiation at 16, 22 and 28 h post-irradiation. Treatment of V79 cells with VDS before exposure to different doses of gamma radiation resulted in a significant decline in cell survival and growth kinetic when compared with the concurrent PBS+irradiation group. The decline in cell survival and growth kinetics was dose related. Similarly, the cell proliferation indices also declined in a dose dependent manner in both PBS+irradiation and VDS+irradiation groups and this decline was higher in VDS+irradiation group in comparison with the PBS+irradiation group. In contrast, the frequency of micronuclei increased in a dose related manner in both PBS+irradiation and VDS+irradiation groups. However, the frequency of micronuclei was significantly greater in the VDS+irradiation group when compared to the PBS+irradiation group at all the post-irradiation time periods studied and the dose response for both groups was linear for all the scoring time periods. The biological response was determined by plotting surviving fraction and micronuclei frequencies on X- and Y-axes, respectively. The plot between surviving fraction and micronuclei induction showed a close correlation. The surviving fraction of V79 cells reduced with the increasing frequency of micronuclei in both groups and the relationship between micronuclei induction and cell survival could be fitted on a linear quadratic model.

Correlation between cell survival, micronuclei-induction, and LDH activity in V79 cells treated with teniposide (VM-26) before exposure to different doses of gamma radiation

The influence of teniposide (VM-26) treatment was studied on the radiation-induced alterations in cell survival, micronuclei (MN) formation and lactate dehydrogenase (LDH) release in V79 cells. Treatment of V79 cells with 10 nM teniposide before exposure to different doses of gamma radiation resulted in a significant decline in the cell survival when compared with the PBS + irradiation group. The decline in cell survival was dose related. The cell proliferation indices also declined in a dose-dependent manner in both PBS + irradiation and VM-26 + irradiation groups. The decline was higher in the VM-26 + irradiation group in comparison with the PBS + irradiation group. In contrast, the frequency of micronuclei increased in a dose-related manner in both PBS + irradiation and VM-26 + irradiation groups. However, the frequency of micronuclei was significantly greater in the latter group when compared with the former group at all the post-irradiation time periods studied. The LDH contents increased in a dose-dependent manner in both PBS + irradiation and VM-26 + irradiation groups at all the post-irradiation time periods evaluated. This elevation in LDH contents was significantly greater in the VM-26 + irradiation group in comparison with the PBS + irradiation group.

The increment of micronucleus frequency in cervical carcinoma during irradiation in vivo and its prognostic value for tumour radiocurability

A potential usefulness of micronucleus assay for prediction of tumour radiosensitivity has been tested in 64 patients with advanced stage (II B-IV B) cervical carcinoma treated by radiotherapy. The study of cellular radiosensitivity in vitro was conducted in parallel with the study of cellular damage after tumour irradiation in vivo. Radiosensitivity of in vitro cultured primary cells isolated from tumour biopsies taken before radiotherapy was evaluated using cytokinesis-block micronucleus assay. Frequency of micronuclei per binucleated cell (MN/BNC) at 2 Gy was used as a measure of radiosensitivity. Radiation sensitivity in vivo was expressed as per cent increment of micronucleus frequency in cells isolated from biopsy taken after 20 Gy (external irradiation, 10 x 2 Gy) over the pre-treatment spontaneous micronucleus level and was called MN20. Very low correlation (r = 0.324) was observed between micronucleus frequency in vitro and in vivo. Although micronucleus frequency at 2 Gy differed widely between tumours evaluated (mean MN/BNC was 0.224; range 0.08-0.416), no significant correlation was observed between this parameter and clinical outcome. The average increment of micronucleus frequency after 20 Gy amounted to 193% of spontaneous level (range 60-610%) and was independent of spontaneous micronucleation before radiotherapy. In contrast to in vitro results, these from in vivo assay seem to have a predictive value for radiotherapy of cervix cancer. The micronucleus increment in vivo that reached at least 117.5% of pretreatment value (first quartile for MN20 data set) correlated significantly with better tumour local control (P < 0.008) and overall survival (P < 0.045). Our results suggest that evaluation of increment of micronucleus frequency during radiotherapy (after fixed tested dose of 20 Gy) offers a potentially valuable approach to predicting individual radioresponsiveness and may be helpful for individualization of treatment strategy in advanced stage cervical cancer.

How does radiation kill cells?

Recent advances in the understanding of intracellular signaling after genotoxic injury have led to a better understanding of the pathways that influence radiation-induced cell death. Particular progress has been made in defining molecular controls of apoptosis and radiation-induced cell cycle arrest, as well as the possible role of telomerase activity in stabilizing DNA breaks.

Response of quiescent and total tumor cells in solid tumors to neutrons with various cadmium ratios

PURPOSE

Response of quiescent (Q) and total tumor cells in solid tumors to neutron irradiation with three different cadmium (Cd) ratios was examined. The role of Q cells in tumor control was also discussed.

METHODS AND MATERIALS

C3H/He mice bearing SCC VII tumors received continuous administration of 5-bromo-2'-deoxyuridine (BrdU) for 5 days using implanted mini-osmotic pumps to label all proliferating (P) cells. Thirty minutes after intraperitoneal injection of sodium borocaptate-10B (BSH), or 3 h after oral administration of dl-p-boronophenylalanine-10B (BPA), the tumors were irradiated with neutrons, or those without 10B-compounds were irradiated with gamma rays. This neutron irradiation was performed using neutrons with three different cadmium (Cd) ratios. The tumors were then excised, minced, and trypsinized. The tumor cell suspensions were incubated with cytochalasin-B (a cytokinesis-blocker), and the micronucleus (MN) frequency in cells without BrdU labeling (Q cells) was determined using immunofluorescence staining for BrdU. The MN frequency in total (P + Q) tumor cells was determined from tumors that were not pretreated with BrdU. The sensitivity to neutrons was evaluated in terms of the frequency of induced micronuclei in binuclear tumor cells (MN frequency).

RESULTS

Without 10B-compounds, the MN frequency in Q cells was lower than that in the total cell population. The sensitivity difference between total and Q cells was reduced by neutron irradiation. Relative biological effectiveness (RBE) of neutrons compared with gamma rays was larger in Q cells than in total cells, and the RBE values for low-Cd-ratio neutrons tended to be larger than those for high-Cd-ratio neutrons. With 10B-compounds, MN frequency for each cell population was increased, especially for total cells. This increase in MN frequency was marked when high-Cd-ratio neutrons were used. BPA increased the MN frequency for total tumor cells more than BSH. Nevertheless, the sensitivity of Q cells treated with BPA was lower than that in BSH-treated Q cells. This tendency was clearly observed in high-Cd-ratio neutrons.

CONCLUSION

From the viewpoint of enhancing the Q-cell sensitivity, tumors should be irradiated with high-Cd-ratio neutrons after BSH administration. However, normal tissue reaction remains to be examined because of its low tumor-to-normal tissue and tumor-to-blood biodistribution ratios.

Modification of the response of a quiescent cell population within a murine solid tumour to boron neutron capture irradiation: studies with nicotinamide and hyperthermia

C3H/He mice bearing SCC VII tumours received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps, to label all proliferating (P) cells. 20 min after intraperitoneal injection of sodium borocaptate-10B (BSH), or 3 h after oral administration of dl-p-boronophenylalanine-10B (BPA), the tumours were irradiated with thermal neutrons. To modify the uptake dose of 10B, nicotinamide (NA) was intraperitoneally injected 60 min before the administration of 10B-compounds and/or the tumours were heated to 41.5 degrees C for 20 min immediately before irradiation. After irradiation, the tumours were excised, minced and trypsinized. The tumour cell suspensions were then incubated with cytochalasin-B (a cytokinesis-blocker). The micronucleus (MN) frequency in cells not BrdU-labelled (quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. With or without the administration of 10B-compounds, the sensitivity of Q cells was lower than that of total (P + Q) tumour cells. With thermal neutron irradiation in the presence of either BPA or BSH, the MN frequency in each cell population was increased. A greater increase in the MN frequency of total tumour cells was observed after thermal neutron irradiation in the presence of BPA than in the presence of BSH. The distribution of 10B from BPA into tumour cells was thought to be more dependent on the uptake ability of the tumour cells than that from BSH. Sufficient quantity of 10B from these two 10B-compounds to cause a highly lethal event inside the cancer cell with thermal neutron irradiation could not be delivered to Q cells. When NA and/or heat treatment were combined with 10B-compound administration, NA increased MN frequency in the BSH treated total cells, and heat treatment elevated MN frequency in Q cells. From the viewpoint of cell kill effect, the combined treatment with nicotinamide and heat treatment was more useful than treatment with either nicotinamide or heat treatment alone, not only in the total tumour cells but also in the Q cells.

Sources of variability in the determination of micronuclei in irradiated peripheral blood lymphocytes

Following a dose of 4 Gy 250 kVp X-rays to the blood lymphocytes from three healthy donors the frequency of micronuclei was enumerated by three scorers. Eight slides were prepared from each blood sample with 1000 cytokinesis-blocked binucleate cells being scored for micronuclei and a recount undertaken 4 weeks later. Seven possible sources of variation were investigated: differences between donors, scorers, the four stages of sample preparation and time. A complete factorial analysis indicated that the major sources of variation were differences between donors and differences between scorers. One scorer identified fewer micronuclei than did the other two scorers in all slides of the irradiated blood samples and gave wide and inconsistent results for baseline counts. These results suggest that the performance of scorers should be screened before undertaking analysis of micronuclei. The most striking result, however, was the consistent performance of the scorers on the repeat counts compared with the very large differences between the scorers.

Determination of radiosensitivity in established and primary squamous cell carcinoma cultures using the micronucleus assay

In this study, the cytokinesis-block micronucleus assay (CBMN) was used to measure radiosensitivity in three established cell lines (SCC-61, V175 and V134) and 10 primary cell cultures of squamous cell carcinoma (SCC) of the head and neck. Assessment involved optimisation of the assay to determine cytochalasin-B (CB) concentration and sampling time postirradiation. A much closer correlation between dose-response data measured in the clonogenic and micronucleus assays was found when the micronucleus assay was performed under standardised conditions for each cell line (2 micrograms/ml CB: 48 h postirradiation) instead of predetermined optimised assay conditions. This indicates that, for these SCC cell lines, the CBMN assay may be able to predict in vitro radiosensitivity. To be of clinical use in predicting radiosensitivity, the CBMN assay also needs to be evaluated with primary cell cultures. In this study, no relationship between micronucleus frequency at 2 or 6 Gy and patient clinical outcome 12 months following surgery and radiotherapy was seen. Similarly, no association between patient outcome and tumour stage, nodal stage and histology was observed. These CBMN assay data from the primary cell cultures are presently inconclusive as a measure of patient tumour radiosensitivity.

Dose-response of X-irradiated human and equine lymphocytes

We have investigated and compared DNA damage and cell killing induced in human and equine lymphocytes after in vitro X-irradiation. Our data show that the cytogenetic and the lethality effects are both greater in equine lymphocytes, but that the difference is wider for lethality. The ratios between doses inducing the same effect are 1.3, 1.7 and 9.4 for the number of binucleated cells with micronuclei, micronucleus frequency in binucleated cells and DNA synthesis inhibition, respectively. The very different radiosensitivity observed for the two mammalian species encourages us to use their lymphocytes in cell radiobiology studies.

Micronucleus induction in 10 human tumour cells after high- and low-dose radiation

A number of data measuring survival of animal or human cells to low LET ionizing radiation have demonstrated that these cells may be hypersensitive to doses below 1 Gy, possibly due to the absence of an inducible repair mechanism, which is observed at higher doses. The production of micronuclei (MN) in cells exposed to ionizing radiation reflects genotoxic damage. Moreover, the micronucleus assay is sensitive to low radiation doses. We have exposed 10 human tumour cell lines to doses ranging between 0.12 and 4 Gy. Using cytochalasin B to block the cells in a binucleate phase, we have scored the fraction of binucleate cells (BNC) expressing MN, as well as the number of MN per BNC, as a function of gamma-ray dose. Experimental points were fitted with a binomial equation. Doses from 1 to 4 Gy were fitted separately from those below 1 Gy, and the initial slopes after both fits were compared. Taken together, the initial slopes of MN induction after low-dose (LD) irradiation were not different from those after high-dose (HD) irradiation. Only in one cell line was a significant increase in MN production detected after LD irradiation. This cell line had the shallowest linear term after HD irradiation. It appeared that the likeliness of expressing hypersensitivity at LD was correlated with the quadratic term of MN induction at HD, which does not contradict an inducible repair hypothesis. However, the failure of observation of a significant hypersensitivity at LD for nine cell lines, and the high variability of response at LD suggests that this occasional effect may be influenced by other factors as well.

Quantifying intracellular radioresponse diversity in irradiated sandwich cultures via micronucleus expression

Determining the degree of diversity in therapeutic sensitivity exhibited by a tumour population is of considerable clinical importance. In addition to being a contributor to radiation resistance, diversity is the basis for variation in sensitivity over the course of treatment. To study intrapopulation diversity in radiosensitivity following gamma-irradiation (2 Gy), distributions of the number of micronuclei/binucleate cell were obtained for human cervix carcinoma sandwich populations. Cell-to-cell diversity in radioresponse (micronucleus expression) was quantified using the overdispersion index ((variance/mean)--1). As measured by this index, the radioresponse diversity of sandwich cultures sharply increased after introduction of oxygen/nutrients to the cultures, mimicking tumour reperfusion. In addition, a strong correlation was found between this measure of diversity and the extent to which the fraction of cells without micronuclei exceeds that expected from a Poisson distribution. This correlation indicates that for a diverse population there can be a significant departure of the aggregate population sensitivity (determined, for instance, by log-survival in a clonogenic assay) from that inferable from simply averaging per-cell sensitivities (reflected, e.g. by mean number of chromosome aberrations/cell). Our experimental results suggest a model attributing diversity in a population to its being a mixture of distinct subpopulations, each biologically homogeneous with respect to micronucleus expression, and each contributing an individual Poisson-distributed micronucleus response. We demonstrate how such radiodiversity may be quantified and show that reoxygenation of a microenvironmentally heterogeneous population leads to an increase in its radiobiological diversity.

Lack of a correlation between micronucleus formation and radiosensitivity in established and primary cultures of human tumours

The radiation-induced genotoxic damage in three established cell lines and 15 primary cultures of human malignant melanoma and ovarian carcinoma showing different radiosensitivity was tested by the cytokinesis-block micronucleus assay. A dose-related increase in micronucleus frequency was observed in all the cell systems. The mean number of micronuclei per Gy of ionising radiation per binucleated cell was respectively 0.44 +/- 0.0075 and 0.43 +/- 0.04 for M14 and JR8 malignant melanoma cell lines and 0.19 +/- 0.013 for the A2780 ovarian cancer cell line. The number of micronuclei did not rank the cell lines in the same order of radiosensitivity as clonogenic cell survival, which showed a surviving fraction at 2 Gy of 0.38 +/- 0.02 for JR8, 0.34 +/- 0.05 for M14 and 0.22 +/- 0.007 for A2780. As regards primary tumour cultures, no correlation was observed between micronucleus induction and surviving fraction at 2 Gy. In conclusion, the discrepancy we observed between micronucleus formation and cell death raises doubts about the potential of the micronucleus assay as a preclinical means to predict radiosensitivity.

Isolation and preliminary characterisation of an X-ray-sensitive mammalian mutant cell line (WMXRS-1)

Mammalian cell lines that are sensitive to particular genotoxic agents have proved the most effective starting point for the cloning of human DNA-repair genes. After ethyl methanesulphonate mutagenesis of the parent murine fibroblast L-cell line, a new mammalian X-ray-sensitive cell line (WMXRS-1) was isolated. For selection of the mutant, a novel detection method was used: putative X-ray-sensitive clones were identified by their lack of incorporation of the DNA precursor, bromodeoxyuridine, after irradiation. The WMXRS-1 cell line was collaterally sensitive to ultraviolet radiation and some other agents known to be removed from DNA by the nucleotide excision repair pathway, but not to bleomycin or hydrogen peroxide. In relation to the wild-type strain, WMXRS-1 showed a similar pattern of induction of micronuclei up to an X-ray dose of 4 Gray and a similar DNA double-strand break (dsb) induction profile. The overall level of dsb rejoining was the same in the parent and mutant lines. However, WMXRS-1 demonstrated a reduced initial rate of dsb-rejoining, perhaps accounting for its radiosensitivity. WMXRS-1 also showed a greater G2 cell cycle phase accumulation after treatment with mitomycin-C. The cross-sensitivity profile and strand-break rejoining deficiency phenotype of WMXRS-1 is unique amongst previously characterised mammalian mutant cell lines.

The cytochalasin-B micronucleus/kinetochore assay in vitro: studies with 10 suspected aneugens

An in vitro micronucleus assay in low passage Chinese hamster Luc2 cells capable of detecting numerical and structural chromosome changes was developed. Chromosome loss was inferred by indirect visualisation of human CREST antikinetochore antibodies bound to centromeres in chemically-induced micronuclei of cytochalasin-B arrested binucleated cells. The assay was used to evaluate 10 chemicals which had been selected for their known or suspected effects upon various components of the cell-division apparatus. These chemicals were colchicine (COL), vinblastine (VBL), thiabendazole (TBZ), chloral hydrate (CH), thimerosal (TM), diazepam (DZ), pyrimethamine (PYR), hydroquinone (HQ), cadmium chloride (CdCl2) and econazole nitrate (EZ). Mitomycin-C (MMC) was used as a positive control for the induction of micronuclei. 8 of the core chemicals induced micronuclei in Chinese hamster Luc2 cells. 4 of the chemicals (COL, VBL, TBZ, CH) increased levels of micronuclei which were positive for kinetochore antibody labelling and hence chromosome loss. 3 of the chemicals (DZ, PYR, HQ) and the positive control (MMC) increased the levels of Mn which were negative for kinetochore antibody labelling. The results with TM were equivocal and EN was negative. The results of these studies suggest that the cytochalasin-B Mn/k assay is a cost-effective, simple and rapid alternative to classical cytogenetic assays for the detection of chemically induced aneuploidy.

Micronucleus formation in human tumour cells: lack of correlation with radiosensitivity

The micronucleus (MN) test has been carefully characterized in four human tumour cell lines of widely differing radiosensitivity. Two radioresistant bladder carcinoma cell lines (MGH-U1 and RT112), one sensitive medulloblastoma cell line (D283MED) and a sensitive neuroblastoma cell line (HX142) were used. The number of MN per Gy of ionising radiation was 0.13 for HX142, 0.17 for D283MED, 0.21 for RT112 and 0.26 for MGH-U1. This does not rank the cell lines in the same order of radiosensitivity as clonogenic cell survival where the surviving fraction at 2 Gy (SF2) was 0.11 for HX142, 0.2 for D283MED, 0.62 for RT112 and 0.53 for MGH-U1. This discrepancy between MN formation and cell death leaves doubt as to the potential usefulness of the MN test as a rapid assay of radiosensitivity but it has potential implications for the mechanistic basis of radiosensitivity in these cells.

Micronuclei expression in tumors as a test for radiation sensitivity

Dose-effect curves were determined for the frequency of micronuclei and impairment of cell clonogenicity from two types of tumours of different sensitivity irradiated in situ. Micronucleated cells were measured 24, 48 and 72 h after treatment. The quantitative relationships between cell reproductive death and the induction of micronuclei are the same for both tumours.

Energy linked modifications of the radiation response in a human cerebral glioma cell line

Effects of cellular energy metabolism on the radiation response of a cell derived from a human cerebral glioma have been studied under conditions of energy limitation produced by the presence of inhibitors of respiratory metabolism (KCN) and glycolysis (glucose analogues such as 2-DG, 5-TG, and 3-0-MG). Radiation 60Co induced DNA repair (Unscheduled DNA Synthesis) and micronuclei formation were studied as measures of radiation response. Glycolysis (lactate production) and levels of adenine and related nucleotides (UTP, GTP, ATP etc.) were measured as parameters of energy metabolism. Two 2-DG (5 mM) inhibited DNA repair and increased micronuclei frequency both in the presence and absence of respiration (KCN, 2 mM). Under similar experimental conditions, the presence of 2-DG also significantly reduced the cellular energy status. Five-TG and 3-0-MG on the other hand, neither significantly altered the energy status (sigma XTP) nor influenced the radiation response under respiratory proficient conditions. The results can be explained on the basis of a model postulating differential energy linked modulations of the repair and fixation processes acting on DNA lesions. Implications of the present results for the radiotherapy of brain tumors are discussed.

Modification of the radiation induced damage by 2-deoxy-D-glucose in organ cultures of human cerebral gliomas

Effects of a glucose antimetabolite, 2-deoxy-D-glucose (2-DG), on the gamma ray induced radiation damage have been studied in organ cultures of human cerebral gliomas. Percentage of cells with micronuclei (M-fraction) was used to assay the radiation damage. Experimental data indicate the following results. Untreated cerebral gliomas show considerable spatial heterogeneity in M-fraction; In spite of this heterogeneity, increases in M-fraction induced by gamma rays can be clearly observed, if multiple and randomly selected explants are analyzed for each group; The radiation induced M-fraction in different gliomas varies over a wide range; Presence of 2-DG (5 mM) for 4 h after irradiation leads to an increase in the radiation induced M-fraction in the majority of tumors, while in a smaller number (congruent to 25%) a decrease is observed under similar conditions. These results can be explained on the basis of a model postulating differential effects of 2-DG on the energy linked modulations of the processes of repair and fixation of DNA damage, which competitively influence the formation of micronuclei.

Measurement of micronuclei by cytokinesis-block method in cultured Chinese hamster cells: comparison with types and rates of chromosome aberrations

The cytokinesis-block method of Fenech and Morley (1985) has been tested for the enumeration and characterization of micronuclei in exponentially growing Chinese hamster cells in culture. The consistent dose-response relations were obtained in cultures treated with mitomycin C, caffeine and colcemid. Comparison with the chromosome aberration frequencies indicated that approximately 30% of the acentric chromosomes are expressed as micronuclei in the mitomycin C and caffeine treated cells. The size distribution of the micronuclei suggested that the base-line frequency of micronuclei is mainly a reflection of mitotic dysfunctions rather than chromosome structural aberrations.

A critical comparison of the micronucleus yield from high and low LET irradiation of plateau-phase cell populations

Experimental data are presented which cast doubt on the usefulness of micronucleus assays as a quantitative measure of radiation damage. Synchronised (G1) Syrian hamster fibroblasts (BHK21 C13) were exposed to doses of gamma- or neutron radiation which yielded equivalent survival response. The cultures were examined at intervals during a 120-h post-irradiation incubation, for the appearance of micronuclei. Dose-response curves for the micronucleus yield constructed at a single sample time of 30 h were compared with those for the peak yield, irrespective of sampling time at each dose. When the total production of acentric fragments was compared with the peak yield of micronuclei no clear correlation could be seen. Qualitative hypotheses have been suggested to account for the various features of the data. The production and expression of micronuclei has been found to be a very complicated relationship and is a warning against too simplistic an interpretation of micronuclei data.

Potential methods for predicting tumor radiocurability

Several predictors of tumor radiocurability are already integrated into clinical practice, for example, tumor size, gross morphology (that is, infiltrative or exophytic), histologic type and grade. These are nonspecific and relatively imprecise. The aim of research into predictive assays is not only to refine the discrimination of existing predictors, but also to suggest specific experimental approaches for overcoming tumor radioresistance in individual patients. Two broad categories of predictive assays can be defined: direct and indirect measurement of tumor cell survival and/or repair capability following irradiation, and measurement of cellular and extracellular parameters affecting radiosensitivity. The ongoing research at The University of Texas M. D. Anderson Hospital is overviewed to illustrate potential methods for predicting radiocurability.

Radiation-dose dependence of the formation of micronuclei in misonidazole treated cell cultures

The effect of misonidazole at different concentrations on the anoxic radiation sensitivity of Chinese hamster cells was investigated using the frequency of radiation induced micronuclei as criterion. The result indicates that, in a high radiation dose region, sensitization with a dose modifying factor of about 1.9 and 1.3 occurs after treatment with the substance at a concentration of 8 and 0.2 mmol/l, respectively. In a low dose region the corresponding values were 1.7 and 0.8. It was concluded that high concentration of the substance in combination with high radiation doses are most beneficial.