Proliferation and clonal survival of human lung cancer cells treated with fractionated irradiation in combination with paclitaxel

PURPOSE: This study was performed to determine the effects of a continuous exposure to paclitaxel (taxol) in combination with fractionated irradiation on cell proliferation and survival. METHODS AND MATERIALS: Human lung carcinoma cells (SW1573) were given a daily treatment with 3 Gy of x-rays during 5 days in the continuous presence of 5 nM taxol. The surviving fraction and the total number of cells were determined every 24 h before and immediately after irradiation. RESULTS: Irradiation with 5 x 3 Gy and 5 nM taxol cause approximately the same inhibition of cell proliferation. In combination these treatments have an additional effect and the cell population increases no further after the first 24 h. Whereas the cells become more resistant to taxol after the first 24 h with a minimum survival of 42%, taxol progressively reduces the population of surviving cells in combination with x-rays when the number of fractions increases, up to 25-fold relative to irradiation alone. The enhancement effect of 5 nM taxol is likely to be attributed to an inhibition of the repopulation during fractionated irradiation and not to an increased radiosensitivity. Only after treatment with 10 or 100 nM taxol for 24 h, which is attended with a high cytotoxicity, is moderate radiosensitization observed. CONCLUSION: Taxol, continuously present at a low concentration with little cytotoxicity, causes a progressive reduction of the surviving cell population in combination with fractionated irradiation, mainly by an inhibition of the repopulation of surviving cells between the dose fractions.     

Increased micronucleus frequency in the progeny of irradiated Chinese hamster cells

V79 Chinese hamster cells were irradiated with doses of 1-12 Gy 300 kV X-rays. Their colony-forming ability and the frequency of micronuclei in binucleate cells after treatment with cytochalasin B was determined at various times after irradiation. The frequency of micronuclei determined within the first 24 h after irradiation increased with doses up to 4 Gy and decreased as the dose increased further. Up to 4 Gy there was a close correlation between surviving fraction and the fraction of cells without micronuclei although the surviving fraction was 2-3 times lower than the fraction of cells without micronuclei. Six, 10 or 13 days after irradiation with either 9 or 12 Gy the plating efficiency and the frequency of micronuclei after cytokinesis block with cytochalasin B was determined in the irradiated, but surviving, cells. The delayed plating efficiency of irradiated cells was significantly decreased. The proportion of binucleated cells was in the normal range at 6-13 days after irradiation, indicating that the proliferative activity of irradiated but surviving cells was not depressed at that time. The micronucleus frequency, however, was significantly increased at all times after irradiation. There was little heterogeneity of plating efficiency and micronucleus frequency among 12 clones which had been isolated for irradiated cultures, 3 weeks after 12 Gy.     

Apoptosis in the subependyma of young adult rats after single and fractionated doses of X-rays

Ionizing radiation is commonly used in the treatment of brain tumors but can cause significant damage to surrounding normal brain. The pathogenesis of this damage is uncertain, and understanding the response of potential target cell populations may provide information useful for developing strategies to optimize therapeutic irradiation. In the mammalian forebrain, the subependyma is a mitotically active area that is a source of oligodendrocytes and astrocytes, and it has been hypothesized that depletion of cells from this region could play a role in radiation-induced white matter injury. Using a distinct morphological pattern of nuclear fragmentation and an immunohistochemical method to specifically label the 3'-hydroxyl termini of DNA strand breaks (terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling), we quantified apoptosis in the subependyma in the young adult rat brain after single and fractionated doses of X-rays. Significant increases in apoptotic index (percentage of cells showing apoptosis) were detected 3 h after irradiation, and the peak apoptotic index was detected at 6 h. Six h after irradiation, the dose response for apoptosis was characterized by a steep increase in apoptotic index between 0.5 and 2.0 Gy and a plateau from 2-30 Gy. The fraction of cells susceptible to apoptosis was estimated to be about 40%, and treatment of rats with cycloheximide inhibited apoptosis. When daily 1.5-Gy fractions of X-rays were administered, the first three fractions were equally effective at decreasing the cell population via apoptosis. There was no additional apoptosis or decrease in cellularity in spite of one to four additional doses of X-rays. Those data suggested some input of cells into the subependymal population during fractionated treatment, and subsequent studies showed that there was a significant rise in 5-bromo-2' deoxyuridine labeling index 2-3 days after irradiation, indicating increased cellular proliferation. The proliferative response after depletion of cells via apoptosis may represent the recruitment of a relatively quiescent stem cell population. It is possible that the radiation response of subependymal stem cells and not the apoptotic-sensitive population per se are critical elements in the response of the brain to radiation injury.     

Radiation response of porcine RPE cells in vitro

PURPOSE: To investigate the antiproliferative effect of ionizing radiation on retinal pigment epithelial (RPE) cells that are supposed to play a major role in the pathogenesis of proliferative vitreoretinopathy (PVR). METHODS: RPE cells from pig eyes were irradiated with doses ranging from 4 to 16 Gy (1 Gray = 1 Joule/kilogram). Cells were counted at 1, 2, 3, and 4 weeks (Experiment 1) or 1, 2, 4, and 6 weeks (Experiment 2) after treatment. In Experiment 3, cells were trypsinized 24 h after radiation and seeded again. Colonies were counted 10 days later, and the surviving fraction was determined. RESULTS: The numbers of cells and colonies were inversely correlated to the doses applied. In Experiment 2, cell numbers of radiated cultures remained stable during the time of follow-up, whereas, in Experiment 1, significant proliferation occurred in treated cultures as well as in controls. This may be due to the higher growing rate that was found in the cultures of Experiment 2, compared to those of Experiment 1, at the time of radiation. In Experiment 3, a D0 value of 0.72 Gy was found. CONCLUSIONS: Proliferation of RPE cells can be suppressed by irradiation in a dose-dependent manner. Therefore, radiotherapy may be useful in the treatment of PVR. Its effect probably depends on the stage or activity of PVR at the time of radiation.     

Variation in radiosensitivity due to cell age and split-dose recovery in polykaryons induced by cytochalasin

We have investigated the properties of an in vitro cell survival assay that uses as its endpoint the ability to form polyploid cells (polykaryons) in the presence of cytochalasin B (CB). The criterion for survival is that a polykaryon-forming unit (PFU) must reach the arbitrary DNA content of at least 16C. The age-dependence of PFU sensitivity to 137Cs irradiation was determined using V79-379A cells synchronized at mitosis. Cells assayed as PFUs demonstrated much less variation in radiosensitivity with age than did clonogens, but the changes in curve shape were qualitatively similar. In both assays mitotic cells yielded an exponential survival curve while that obtained at 5 h (mid-late S) had a marked quadratic component. Owing to the small overall variation in PFU survival with age, at doses greater than about 25 Gy the surviving fraction at 5 h was lower than in mitosis. In both V79-379A and HeLa S3 cells, PFUs demonstrated a capacity for split-dose recovery and yielded recovery ratios at 2.6 at 50 Gy in V79 and 1.5 at 20 Gy in HeLa. Since these ratios were much lower than in clonogens at the same dose, we suggest that this is consistent with an association that we have previously demonstrated between PFU response and the clonogenic initial slope. In an attempt to clarify the DNA lesions to which PFUs may be sensitive, we determined PFU response following exposure to 254-nm UV irradiation. In contrast with ionizing radiation, PFU response to UV was very similar to that of clonogens. This suggests that following UV exposure the absence of cytokinesis in polykaryons may confer less protection than in the case of ionizing radiation, possibly due to fundamental differences in the spectrum of DNA lesions produced.     

Impact of radiation therapy fraction size on local control of early glottic carcinoma

PURPOSE: Different radiotherapy fractionation schedules were used over a 10-year period to treat patients with early squamous cell carcinoma of the vocal cords at McGill University. A retrospective analysis was performed to study the effect of fraction size on local control in this group of patients. METHODS AND MATERIALS: A total of 126 previously untreated patients with T1 invasive squamous cell carcinoma of the true vocal cords were irradiated between January 1978 and December 1988 in the Department of Radiation Oncology at McGill University. All patients received megavoltage irradiation, 94 patients received daily fractions > 2 Gy (64 patients received 50 Gy with once-daily 2.5-Gy fractions, and 30 received 65.25 Gy in 29 fractions of 2.25 Gy each), and 32 patients were treated to a dose of 66 Gy in 33 fractions with 2 Gy/fraction. Patients' characteristics of prognostic importance were equally distributed between the two fractionation groups. RESULTS: At a median follow-up of 84 months, the 10-year disease-free survival and overall survival were 76% and 93%, respectively. Local control for patients treated with > 2 Gy fraction was 84%, compared to 65.6% for those treated with 2-Gy fractions (p = 0.026). Among the prognostic factors tested, such as gender, age, stage, anterior and posterior commissure involvement, smoking history, and fraction size, the latter was the only significant predictor of local control for the whole group of patients in univariate (p = 0.041) and multivariate (p = 0.023) analysis. There was no observed difference in the incidence of complications between the two fractionation groups. CONCLUSIONS: From the results of this retrospective review of patients treated with radiotherapy for T1 true vocal cord cancer, and within the range of total doses and overall treatment times used in our patients, it was found that fractionation schedules using daily fraction size > 2 Gy are associated with a better local control than schedules delivering 2 Gy/fraction, with no increase in toxicity.     

Distinguishing small molecular mass differences of proteins by mass spectrometry

This study was undertaken to determine if acidic or basic fibroblast growth factor (FGF1 or FGF2) or vascular endothelial growth factor (VEGF) alters the radiation response of small bowel after total-body irradiation (TBI). Female C3H mice were treated with various doses of angiogenic growth factor administered intravenously 24 h before or 1 h after TBI. Radiation doses ranged from 7 to 18 Gy. End points measured were the number of crypts in three portions of the small bowel, the frequency of apoptosis of crypt cells at various times after TBI, and the LD50/30 (bone marrow syndrome) and LD50/6 (GI syndrome). Fibroblast growth factors alone, without TBI, decreased the number of crypts per circumference significantly. Among the factors tested, FGF2 caused the greatest decline in baseline crypt number. Despite this decrease in the baseline crypt number, after irradiation the number of surviving crypts was greater in animals treated with growth factor. The greatest radioprotection occurred at intermediate doses of growth factor (6 to 18 pg/mouse). Mice treated with FGF1 and FGF2 had crypt survival curves with a slope that was more shallow than that for saline-treated animals, indicating radiation resistance of crypt stem cells in FGF-treated mice. The LD50/6 was increased by approximately 10% for all treatments with angiogenic growth factors, whether given before or after TBI. Apoptosis of crypt cells was maximum at 4 to 8 h after TBI. The cumulative apoptosis was decreased significantly in animals treated with angiogenic growth factors, and the greatest protection against apoptosis was seen in animals treated with FGF2 prior to TBI. All three angiogenic growth factors tested were radioprotective in small bowel whether given 24 h before or 1 h after irradiation. The mechanism of protection is unlikely to involve proliferation of crypt stem cells, but probably does involve prevention of radiation-induced apoptosis or enhanced repair of DNA damage of crypt cells.     

Effects of single doses of irradiation on the expression of resistance-related proteins in murine NIH 3T3 and human lung carcinoma cells

In this report the effects of single doses of ionizing radiation on the mRNA expression of several proteins involved in multiple drug resistance were analyzed. Murine NIH 3T3 cells treated with single doses of 5, 10 and 20 Gy during the time interval from 1.5 to 72 h after irradiation were compared with their corresponding controls at the same points of time. The glutathione S-transferase-pi (GST pi) level was elevated in cells treated with 10 or 20 Gy from 24 to 72 h after irradiation compared with the control. Topoisomerase II alpha and thymidylate synthase were decreased in irradiated cells 24-72 h after exposure. These down-regulations were associated with cellular proliferation, determined by mRNA expression of the proliferation marker histone 3. Irradiated cells exhibited no alteration in the P-glycoprotein or glutathione peroxidase mRNA content. The finding that GST pi mRNA was overexpressed after irradiation was validated by investigations on a human lung carcinoma cell line (LXF 289) on the mRNA and protein level. Thus, our results indicate that irradiation alters the expression of proteins involved in multidrug resistance and may, therefore, play a role in clinical drug response.     

The demonstration of heritable lethal mutations in the progeny of x-ray-irradiated CHO cells by micronucleus count in clone cells

PURPOSE: Low doses of ionizing radiation reduce the growth rates of clones following irradiation of the progenitor cells. Such reductions of clone growth have been proven by means of measurements of clone size distributions. The medians of such distributions can be used to quantify the radiation damage. Prolongations of generation times and cell death as result of heritable lethal mutations have been discussed as causes for the reduction of clone growth. MATERIAL AND METHODS: The cell number of a clone of hypotetraploid CHO-cells was compared to the frequency of micronucleated binucleated cells in the same clone using the cytokinesis-block-micronucleus method. The dose dependent reduction of clone sizes is measured by the difference of the medians (after log transformation) of the clone size distributions. RESULTS: At cytochalasin-B concentrations of 1 microgram/ml and after an incubation time of 16 h a yield of binucleated cells of about 50% was obtained. Median clone size differences as a measure of clonal radiation damage increased linearly with incubation times of 76, 100, 124, and 240 h following irradiation with 3, 5, 7, and 12 Gy. The frequency of binucleated clone cells with micronuclei strongly increased with decreasing clone size by a factor up to 20 following irradiation with 3, 5, and 7 Gy. The frequency of micronucleated binucleated clone cells was found to be independent of incubation time after irradiation. CONCLUSION: Radiation induced clone size reductions result from cell losses caused by intraclonal expression of micronuclei which have its origin in heritable lethal mutations. Measurements of clone size distributions can be done automatically. They can serve as predictive test for determination of median cell loss rates of surviving cell clones.     

The ratio of initial/residual DNA damage predicts intrinsic radiosensitivity in seven cervix carcinoma cell lines

The single-cell gel electrophoresis (comet) assay was used to measure radiation-produced DNA double-strand breaks (dsbs) in a series of seven cervical tumour cell lines (ME180, HT3, C33A, C41, SiHa, MS751 and CaSki). The proportion of DNA dsbs was measured immediately after radiation treatment (initial damage) and 16 h later after incubation at 37 degrees C (residual damage). Linear dose-response curves were seen for initial (slopes 0.23-0.66) and residual (slopes 0.16-0.87) DNA dsbs. Neither of the slopes of the linear regression analysis on the initial and on the residual DNA dsbs dose-response curves (range 0-80 Gy) correlated with SF2 (surviving fraction at 2 Gy) measured after high- (HDR) or low-dose-rate (LDR) irradiation. An association was evident between SF2 after HDR and LDR irradiation and the ratio of the absolute level of initial and residual damage after a single dose of 60 Gy. However, a significant correlation was found between HDR (r= -0.78, P = 0.04) and LDR (r = -0.86, P = 0.03) SF2 values and the ratio of the slopes of the initial and residual DNA dsbs dose-response curves (range 0.47-0.99), representing the fraction of DNA damage remaining. These results indicate that the neutral comet assay can be used to predict radiosensitivity of cervical tumour cell lines by assessing the ratio of initial and residual DNA dsbs.     

Response of dermal fibroblast cultures from patients with unusually severe responses to radiotherapy and from ataxia telangiectasia heterozygotes to fractionated radiation

We examined the cytotoxic effects of radiation delivered in daily fractions at clinically relevant doses in plateau phase cultures of skin fibroblast cell strains derived from ataxia telangiectasia (AT) heterozygotes, patients with unusually sensitive responses to radiotherapy, apparently normal patients, and cell bank controls. A gradual linear reduction in surviving fraction versus total dose was observed in the control group, comprised of apparently normal individuals and one patient with a normal clinical response to radiotherapy, after exposure to daily fractions of 2.0 Gy. There was a much steeper decline in surviving fraction among the AT heterozygotes and the group with sensitive responses to radiotherapy, such that after six daily fractions of 2.0 Gy (12 Gy total dose), the mean surviving fraction of the control group was significantly different from that of the AT heterozygotes (P = 0.0009) and that of the patients with unusually sensitive responses to radiotherapy (P = 0.0002). We propose that this assay may be a useful means of identifying cell strains from AT heterozygotes. Based on these results, the hypothesis is discussed that patients who suffer unusually sensitive clinical reactions to radiotherapy may be AT heterozygotes.     

Effects of short-term treatment with prednisolone and calcitriol on bone and mineral metabolism in normal men

PURPOSE: This study was conducted to clarify the relationship among the frequencies of micronuclei (MN) and apoptosis, and clonogenic cell survival after irradiation. MATERIALS AND METHODS: The frequencies of MN and apoptosis were compared in the surviving fraction in three human tumour cell lines and two rodent cell lines at various irradiation doses. RESULTS: The SHIN-3, DU-145 and CHO-K1 cells showed dose-dependent increases of MN per binucleate cell and an excellent correlation between the MN frequency and surviving fraction after irradiation. The F9 and COLO 320DM cells did not show this correlation. The number of apoptotic cells increased according to the increase in radiation dose in the F9 and COLO 320DM cells, but not in the SHIN-3, DU-145 or CHO-K1 cells. CONCLUSIONS: The detection of the MN frequency alone is insufficient to measure cellular intrinsic radiosensitivity. The simultaneous use of the MN assay and the detection of apoptotic cells would be more reliable as a method for predicting cell survival after radiation.     

Inhibition and activation of the thyroidal axis by the adrenal axis in vertebrates

PURPOSE: To determine the long-term effects of 75.6- and 81.0-Gy doses of three-dimensional conformal radiation therapy in a dose-escalation study in patients with stage T2c-T3 prostatic cancer. MATERIALS AND METHODS: Fifty patients received an initial 75.6-Gy dose, and the dose in 46 patients was subsequently escalated to 81.0-Gy. Median follow-up was 60 and 40 months, respectively. RESULTS: The rates of effects of acute toxicity during the course of treatment were similar for both dose levels. Among the 96 patients, the rate of grade 2 morbidities necessitating medication to relieve acute symptoms was 17% (16 patients) for rectal and 36% (35 patients) for urinary morbidities. All other patients had either no or grade 1 morbidities. Fourteen patients (15%) developed late grade 2 rectal morbidities. There were no differences in 5-year actuarial rates of late grade 2 rectal or urinary morbidities among patients who received 75.6 Gy versus those who received 81.0 Gy. One patient treated with 81.0 Gy developed a grade 3 urethral stricture, which was resolved with dilatation. CONCLUSION: Tumor dose escalation beyond conventional radiation doses for localized prostatic cancer is feasible when delivered with three-dimensional conformal radiation therapy, with no increase in morbidity in normal tissue.     

A neurophysiological study in children and adolescents with Crigler-Najjar syndrome type I

BACKGROUND: Paclitaxel, a natural product from Taxus brevifolia, is a microtubule stabilizing agent, which has been shown to block different cells in the G2/M phase of the cell cycle and consequently, to modulate their radioresponsiveness. Our aim was to test the cytotoxic and radiosensitizing potential of paclitaxel, with respect to different gynecological tumors with varying radiosensitivities. MATERIAL AND METHOD: We performed clonogenic assays and flow cytometry on 2 cell lines, MCF-7 (breast) and CaSki (cervix) cells, and on 2 primary ovarian tumor samples (OC-I and OC-II). The cells were irradiated with 200 kV X-rays, radiation doses of up to 8 Gy were applied either as single doses or in 2 Gy fractions. Paclitaxel concentrations varied from 0.07 to 700 nM, incubation times varied from 3 to 120 h. RESULTS: Paclitaxel alone changed the cell cycle distribution of the cells tested and was cytotoxic in a time and concentration dependent manner. When combined with radiation, most schedules resulted in additive effects of the combined treatments. However, for MCF-7 cells, when 7 nM paclitaxel, applied 24 h before irradiation, were combined with fractionated irradiation a supra-additive effect with a SER of 1.2 was found. For CaSki cells, under comparable conditions the SER was 1.13 but the effects were not statistically significant. CONCLUSION: Under specific conditions, paclitaxel exerted a weak radiosensitizing effect on breast and cervical carcinoma cells. A therapeutic gain may be possible on the basis of an optimal paclitaxel/radiation scheduling.     

Paclitaxel enhances in vitro radiosensitivity of squamous carcinoma cell lines of the head and neck

Squamous cell carcinoma of the head and neck is the fourth most common cancer in the United States, and therapy for very advanced cases is relatively ineffective. Paclitaxel has activity against cancers of the breast, lung, prostate, cervix, and ovary. The activity of paclitaxel for squamous cell carcinoma of the head and neck is less certain, and results of its radiosensitization properties have been variable. The radiation responses of two squamous carcinomas, SCC-9 (oropharynx) and HEP-2 (larynx), were examined to determine the radiosensitizing potential of paclitaxel. In vitro exposures for 24 and 48 h with paclitaxel concentrations of 10(-4) to 6 x 10(-2) microg/ml were followed by irradiation of 0.1-10 Gy. Percent survival was calculated by colony count, and the paclitaxel-radiation interaction was quantitated by the median effect principle and the combination index method of Chou and Talalay. The paclitaxel-radiation combination resulted in multiphasic interactions in both 24 and 48 h paclitaxel pretreatment in SCC-9 and HEP-2 cell lines. In general there was slight synergism [combination index (CI) <1] at low dose-low effect levels (e.g., at a paclitaxel concentration of 0.002 microg/ml or lower and radiation of 0.1-0.3 Gy), moderate antagonism (CI >1) at median dose ranges and strong synergism (CI <1) at high dose ranges (e.g., at a paclitaxel concentration of 0.012-0.06 microg/ml and radiation doses of 3-10 Gy), especially at a surviving fraction of <0.1, which is therapeutically relevant. The median effect principle and combination index method provided a simple way to quantitate the synergism or antagonism of a paclitaxel-radiation interaction under various conditions. This analysis demonstrated that paclitaxel-radiation synergy exists at doses that are readily achievable in the clinical scenario for both agents and that greater synergy occurred at high dose-high effect levels. These results suggest that the combination of both therapies should be explored further in clinical trials assessing the treatment of squamous cell carcinomas of the head and neck.     

Might intrinsic radioresistance of human tumour cells be induced by radiation?

Survival measurements were made on six human tumour cell lines in vitro after irradiation with single doses of X rays. Doses up to 5 Gy were used giving surviving fractions down to 20%, but the majority of the measurements were made at doses < 1 Gy. These six cell lines have very different intrinsic radiosensitivities: HT29, Be11, and RT112 are radioresistant with surviving fractions at 2 Gy (SF2) between 60 and 74%, while MeWo, SW48, and HX142 are radiosensitive (SF2 = 3-29%). For all the cell lines, response over the dose range 2-5 Gy showed a good fit to a Linear-Quadratic (LQ) model. However, HT29, Be11, and RT112 cells showed a significant increase in X-ray radiosensitivity at doses below < 1 Gy compared with the prediction extrapolated from a LQ model fitted to the data at higher doses. The LQ model also slightly underpredicted the effect of low-dose X rays in MeWo cells, but the response of SW48 and HX142 cells was well described by the LQ model at all doses, with no evidence of increased low-dose effectiveness. The most plausible explanation for this phenomenon is that it reflects an induced radioresistance so that low doses of X-rays in vitro are more effective per Gy than higher doses, because only at higher doses is there sufficient damage to trigger repair systems or other radioprotective mechanisms. It follows that variation in the amount of inducible radioresistance might explain, in part, differences in intrinsic radiosensitivity above > 1 Gy between cell lines: cells would be intrinsically radiosensitive because they have a diminished inducible response.     

Inhibition of the mitogen activated protein (MAP) kinase cascade potentiates cell killing by low dose ionizing radiation in A431 human squamous carcinoma cells

The molecular mechanism(s) by which tumor cells survive after exposure to ionizing radiation are not fully understood. Exposure of A431 cells to low doses of radiation (1 Gy) caused prolonged activations of the mitogen activated protein (MAP) kinase and stress activated protein (SAP) kinase pathways, and induced p21(Cip-1/WAF1) via a MAP kinase dependent mechanism. In contrast, higher doses of radiation (6 Gy) caused a much weaker activation of the MAP kinase cascade, but a similar degree of SAP kinase cascade activation. In the presence of MAP kinase blockade by the specific MEK1 inhibitor (PD98059) the basal activity of the SAP kinase pathway was enhanced twofold, and the ability of a 1 Gy radiation exposure to activate the SAP kinase pathway was increased approximately sixfold 60 min after irradiation. In the presence of MAP kinase blockade by PD98059 the ability of a single 1 Gy exposure to cause double stranded DNA breaks (TUNEL assay) was enhanced at least threefold over the following 24-48 h. The increase in DNA damage within 48 h was also mirrored by a similar decrease in A431 cell growth as judged by MTT assays over the next 4-8 days following radiation exposure. This report demonstrates that the MAP kinase cascade is a key cytoprotective pathway in A431 human squamous carcinoma cells which is activated in response to clinically used doses of ionizing radiation. Inhibition of this pathway potentiates the ability of low dose radiation exposure to induce cell death in vitro.     

New developments in hematopoietic stem cell expansion

In the present study we investigated the effects of various doses of gamma-irradiation, followed by induction of granulocytic differentiation with all-trans-retinoic acid (ATRA), on proliferative rate, differentiation capability and oxidative metabolism of leukaemic cells from two different myeloid leukaemia cell lines, HL-60 and PLB-985. Regarding the effects of such combined treatment on the proliferative capabilities of HL-60 and PLB-985 cell lines, we showed that their growth kinetics were similar after 2 Gy gamma-irradiation combined with ATRA. However, with doses >2 Gy, the behaviour of the cell lines differed largely. Indeed, HL-60 appeared to be more radiosensitive than PLB-985 regarding cell viability and proliferation. Besides, whatever dose of irradiation (2, 5 or 10 Gy) was applied, ATRA was still able to induce differentiation of HL-60 and PLB-985 into granulocytes that retained the capacity to produce superoxide anion. The results of these in vitro studies suggest that leukaemia cell lines retain their ability to respond to ATRA, a granulocytic-differentiating inducer following high doses of irradiation. This may have implications for the use of radiation therapy in combination with ATRA for the treatment of extramedullary infiltrations of myeloid leukaemias in humans.     

Comparison between the in vitro intrinsic radiation sensitivity of human soft tissue sarcoma and breast cancer cell lines

The purpose of this study is to evaluate the radiation sensitivity of human soft tissue sarcoma cell lines in vitro and to compare with that of human breast carcinoma and glioblastoma cell lines. The intrinsic radiation sensitivity parameters of seven human soft tissue sarcomas and eight breast carcinoma cell lines were investigated in vitro by clonogenic assays for single-dose irradiation under aerobic conditions on cells in exponential phase of growth. The results for sarcoma cell lines showed that the mean surviving fraction at 2 Gy (SF2) was 0.39 (SD +/- 0.09) with a range of 0.24 to 0.53, and the average mean inactivation dose (MID) was 1.92 (SD +/- 0.35) range from 1.36 Gy to 2.49 Gy. These values were not different from that of breast cell lines examined concurrently and using the same experimental methods (mean SF2 0.38, SD +/- 0.09; MID 1.9 Gy, SD +/- 0.37). However radiobiological parameters of nine karyotyped human malignant glioma cell lines determined earlier in this laboratory were significantly higher (mean SF2 0.50 +/- 0.14; mean MID 2.61 +/- 0.60). In conclusion, the data presented here do not support the view that cells of sarcomas show unusual radiation resistance. To the extent that the in vitro determined cellular radiation sensitivity reflects the tumor response in vivo, the success rate for radiation applied against sarcoma and breast carcinoma of comparable size could be similar.