Response to pulsed dose rate and low dose rate irradiation with and without mild hyperthermia using human breast carcinoma cell lines.

The purpose of this study was to establish whether a pulsed dose rate (PDR) treatment of 1.5 Gy given every 3 h in combination with 41 degrees C mild hyperthermia or a continuous low dose rate (LDR) treatment with mild hyperthermia could radiosensitize two isogenic human breast carcinoma cell lines in comparison to pulsed dose rate or low dose rate irradiation alone. The radiation resistant cell line was derived from the parental cell line and was transfected to over-express DNA polymerase beta. The end-points assessed were the survival of the cells using the clonogenic assay, the amount of residual DSB(s) using the comet assay and gene expression of polymerase beta using RT-PCR. Results showed that the PDR and LDR treatments combined with mild hyperthermia caused significant radiosensitization when compared to PDR and LDR irradiation alone in terms of the clonogenic and comet assays with both cell lines. RT-PCR results showed that polymerase beta levels of expression were not elevated in response to these treatments, implying that this polymerase may not be involved in sub-lethal damage repair or thermal radiosensitization. These results suggest a potential clinical advantage when combining LDR or PDR with hyperthermia, since they indicate that hyperthermia is an effective radiosensitizer.     

Comparison of radiosensitization by 41 degrees C hyperthermia during low dose rate irradiation and during pulsed simulated low dose rate irradiation in human glioma cells

PURPOSE: Long duration mild hyperthermia has been shown to be an effective radiosensitizer when given concurrently with low dose rate irradiation. Pulsed simulated low dose rate (PSLDR) is now being used clinically, and we have set out to determine whether concurrent mild hyperthermia can be an effective radiosensitizer for the PSLDR protocol. MATERIALS AND METHODS: Human glioma cells (U-87MG) were grown to plateau phase and treated in plateau phase in order to minimize cell cycle redistribution during protracted treatments. Low dose rate (LDR) irradiation and 41 degrees C hyperthermia were delivered by having a radium irradiator inside a temperature-controlled incubator. PSLDR was given using a 150 kVp X-ray unit and maintaining the cells at 41 degrees C between irradiations. The duration of irradiation and concurrent heating depended on total dose and extended up to 48 h. RESULTS: When 41 degrees C hyperthermia was given currently with LDR or PSLDR, the thermal enhancement ratios (TER) were about the same if the average dose rate for PSLDR was the same as for LDR. At higher average dose rates for PSLDR the TERs became less. CONCLUSIONS: Our data show that concurrent mild hyperthermia can be an effective sensitizer for PSLDR. This sensitization can be as effective as for LDR if the same average dose rate is used and the TER increases with decreasing dose rate. Thus mild hyperthermia combined with PSLDR may be an effective clinical protocol.     

Response to pulsed dose rate irradiation with and without mild hyperthermia using tumour and normal cell lines

To determine whether pulsed dose rate irradiation in combination with mild hyperthermia could radiosensitize cells in comparison to pulsed dose rate irradiation alone, human ovarian carcinoma (A2780s, cisplatin- and radiation-sensitive, and A2780cp, cisplatin- and radiation-resistant) and human fibroblast (AG1522) cell lines were used. Cells were irradiated in vitro using two fraction sizes, 0.53Gy given every hour and 1.6Gy given every 3h, with an overall average dose rate of 0.53Gy/h. The data showed that 40°C hyperthermia did not radiosensitize any of the cell lines for the 0.53Gy every 1h fractionation scheme. In addition, mild hyperthermia radiosensitized both carcinoma cell lines when using the 1.6Gy fraction size for all doses tested in the A2780s and at higher doses in the A2780cp, but not the normal cell line. These results suggest a potential clinical advantage when using the 1.6Gy fraction size with 40°C mild hyperthermia, since hyperthermia radiosensitized the carcinoma cells but not the normal cells.     

Response to pulsed dose rate irradiation with and without mild hyperthermia using tumour and normal cell lines.

To determine whether pulsed dose rate irradiation in combination with mild hyperthermia could radiosensitize cells in comparison to pulsed dose rate irradiation alone, human ovarian carcinoma (A2780s, cisplatin- and radiation-sensitive, and A2780cp, cisplatin- and radiation-resistant) and human fibroblast (AG1522) cell lines were used. Cells were irradiated in vitro using two fraction sizes, 0.53 Gy given every hour and 1.6 Gy given every 3h, with an overall average dose rate of 0.53 Gy/h. The data showed that 40 degrees C hyperthermia did not radiosensitize any of the cell lines for the 0.53 Gy every 1 h fractionation scheme. In addition, mild hyperthermia radiosensitized both carcinoma cell lines when using the 1.6 Gy fraction size for all doses tested in the A2780s and at higher doses in the A2780cp, but not the normal cell line. These results suggest a potential clinical advantage when using the 1.6 Gy fraction size with 40 degrees C mild hyperthermia, since hyperthermia radiosensitized the carcinoma cells but not the normal cells.     

Response of glioblastoma cell lines to low dose rate irradiation

Glioblastoma U251 and U87 cells irradiated with single fraction high dose rate radiation (1.1 Gy/min) were relatively insensitive to inactivation of colony forming ability, similar to other glioblastoma cell lines. Initial rates of cell kill with continuous low dose rate irradiation (0.075 Gy/hr to 0.49 Gy/hr) were low, but at times greater than 20 hours and with dose rates of 0.25 Gy/hr or higher, the rate of cell kill increased. Population doubling times for these cell lines were about 24 hours, suggesting that cell cycle redistribution may be responsible for the increased sensitivity. DNA histograms obtained by flow cytometry support this hypothesis, with cells accumulating in the G2 and M phases of the cell cycle. These results suggest that low dose rate irradiation may be effective in treating glioblastomas. Optimization of time intervals between radiation treatments as well as dose rates used for glioblastoma patients may be influenced by these findings, resulting in better integration of continuous low-dose-rate irradiation (radioactive antibodies and implants) and high-dose-rate irradiation (fractionated external beam) into therapeutic programs.     

Cell killing, radiosensitization and cell cycle redistribution induced by chronic hypoxia

Some of the biological changes associated with extreme hypoxia at 37 degrees C (less than 10 ppm pO2) were examined in Chinese hamster V79 cells. Specifically, extreme hypoxia caused an initial decrease in plating efficiency to 55% in 4 hr after the onset of hypoxia. Beyond this time, the decline in plating efficiency was more gradual reaching 35% of control at 20 hr. Flow microfluorimetry (FMF) studies, in which cells are sorted on the basis of DNA content and then assayed for viability, demonstrated that mid S phase cells were most sensitive to chronic hypoxia, with surviving fraction equal to 2.5% at 20 hr. Furthermore, the viability of G1 and G2/M cells, after 20 hr of hypoxic storage, was also reduced to 20 and 7.6%, respectively. Hypoxia also caused alterations in the cell cycle distribution of initially asynchronous cells, as determined by dual parameter FMF measurements of both cellular DNA content and incorporated BudR. In particular, G2/M cells completed mitosis, while G1 cells showed little or no movement. Lastly, cells stored in chronic hypoxia displayed an enhanced radiosensitivity when compared to acutely hypoxic cells. Possible reasons for these observations are discussed.     

The effects of 41 degrees C hyperthermia on the DNA repair protein, MRE11, correlate with radiosensitization in four human tumor cell lines.

PURPOSE: The goal of this study was to determine if reduced availability of the DNA repair protein, MRE11, for the repair of damaged DNA is a basis for thermal radiosensitization induced by moderate hyperthermia. To test this hypothesis, we measured the total amount of MRE11 DNA repair protein and its heat-induced alterations in four human tumor cell lines requiring different heating times at 41 degrees C to induce measurable radiosensitization. MATERIALS AND METHODS: Human colon adenocarcinoma cell lines (NSY42129, HT29 and HCT15) and HeLa cells were used as the test system. Cells were irradiated immediately after completion of hyperthermia. MRE11 levels in whole cell extract, nuclear extract and cytoplasmic extracts were measured by Western blotting. The nuclear and cytoplasmic extracts were separated by TX100 solubility. The subcellular localization of MRE11 was determined by immunofluorescence staining. RESULTS: The results show that for the human tumor cell lines studied, the larger the endogenous amount of MRE11 protein per cell, the longer the heating time at 41 degrees C required for inducing measurable radiosensitization in that cell line. Further, the residual nuclear MRE11 protein level, measured in the nuclear extract and in the cytoplasmic extract as a function of heating time, both correlated with the thermal enhancement ratio (TER). CONCLUSIONS: These observations are consistent with the possibility that delocalization of MRE11 from the nucleus is a critical step in the radiosensitization by moderate hyperthermia.     

Comparison between fractionated high dose rate irradiation and continuous low dose rate irradiation in spheroids

Recent interest in clinical brachytherapy focuses on the possible radiobiological equivalence between fractionated high dose rate (HDR) and continuous low dose rate (LDR) irradiations. This study is designed to compare the radiobiological effects between the two in vitro using multicellular spheroids of human tumor. Both HDR and LDR irradiations were delivered by 137Cs source, the dose rates of which were as 1.18 Gy/min and 5.5 mGy/min, respectively. Fractionated HDR irradiation of various fraction sizes was applied twice a day. We found that: (1) The fractionated HDR irradiation (8 Gy/2 fr/day) was more effective radiobiologically than continuous LDR irradiation (8 Gy/day) and the ratio of radiobiological effects of these irradiations was estimated as 0.82, based on the 50% spheroid cure dose (SCD50); (2) the radiobiological effectiveness was independent of the fraction size of HDR irradiation administrated, and the repair of sublethal damage (SLD) was absent, suggesting that the sparing effect of fractionated HDR irradiations was absent in spheroids. Our findings could provide important information for the clinical usage of the fractionated HDR radiotherapy to replace continuous LDR radiotherapy.     

Lack of interferon beta-induced radiosensitization in four out of five human glioblastoma cell lines

PURPOSE: To investigate the potential of interferon beta to enhance the cytotoxic activity of ionizing irradiation against glioma cells, and to elucidate the possible mechanisms responsible for conflicting clinical results. METHODS AND MATERIALS: Five glioblastoma cell lines (U87MG, U118MG, U373MG, MO59K, MO59J) with different radiosensitivity and genetic background were used. Experiments were performed in exponentially growing cultures, and cell survival was measured by a colony-forming assay. Cells were incubated with natural interferon beta (n-IFN-beta; 30-3000 IU/mL) for 24 h followed by single dose irradiation with 1 to 6 Gy of gamma-rays. RESULTS: Significant differences in n-IFN-beta sensitivity were found. The cell lines also differed in their radiation sensitivity, and there was no correlation between the n-IFN-beta and the radiation sensitivity. In three of five cell lines, the interaction of n-IFN-beta and irradiation was infra-additive; in one cell line, it was additive. For MO59J cells only, which are NHEJ-deficient, supra-additivity was observed. CONCLUSION: Our results confirm the remarkable heterogeneity that is characteristic of malignant glioma. The combined effect of n-IFN-beta and radiation was mostly infra-additive or additive; synergistic interaction might occur in tumor cells that already have acquired repair deficiencies because of their genetic instability, as shown for the MO59J cell line.     

Sensitization of rat 9L gliosarcoma cells to low dose rate irradiation by long duration 41 degrees C hyperthermia

Modification of survival by long duration, 41 degrees C hyperthermia in combination with low dose rate radiation (0.5 Gy/h) was determined in rat 9L gliosarcoma cells. Cells were exposed to radiation in a manner that simulated continuous irradiation at a dose rate relevant to clinical brachytherapy. High dose rate X-irradiation was fractionated in 1.0-Gy fractions at 2-h intervals (FLDRI). Previous studies had demonstrated that 9L cells exposed to FLDRI with these parameters have survival characteristics that are equivalent to continuous low dose rate irradiation. Cells exposed to 41 degrees C throughout FLDRI were sensitized significantly (thermal enhancement ratio of 2.07) compared with cells irradiated at 37 degrees C. Incubation for 24 h at 41 degrees C before and/or after FLDRI at either 37 degrees C or 41 degrees C did not increase the slope of the radiation survival curves but did reduce the shoulder. Similarly, heating at 43 degrees C for 30 or 60 min before and/or after irradiation at 0.5 Gy/h also did not enhance cell sensitivity. Survival of cells after irradiation at high dose rate (60 Gy/h) was independent of the temperature during irradiation. Preheat at 41 degrees C for 24 h did not sensitize cells to high dose rate irradiation by increasing the slope of the survival curve, although a loss of shoulder was observed. Sensitization of cells heated at 43 degrees C for 30 or 60 min before high dose rate irradiation was expressed as classical slope modification. Our results demonstrate that 41 degrees C heating during FLDRI greatly sensitizes cells to radiation-induced killing for exposure durations up to 36 h. Heating 9L cells at 41 degrees C or 43 degrees C adjacent to FLDRI at 0.5 Gy/h resulted in no additional enhancement of terminal sensitivity, although shoulder modification was observed. The sensitization by simultaneous heating described above occurred even though thermotolerance developed during extended incubation at 41 degrees C. These in vitro data demonstrate that simultaneous protracted heating at modest temperatures could greatly enhance the cytotoxic effects of low dose rate interstitial irradiation and could be of significance in clinical application.     

Combined continuous ultra low dose rate irradiation and radiofrequency hyperthermia in the C3H mouse

Intradermally inoculated RIF tumors and normal skin of male C3H mice were implanted with variable activities of Iodine 125 seeds or dummy seeds enclosed in Vicryl sutures, and subjected to 0, 1, 2 or 3 local radiofrequency heat treatments for 30 minutes. Each treatment raised the tumor volume to 44 degrees C. Gastrointestinal toxicity (assessed by weight change), skin reaction, tumor growth delay, and tumor cure were assessed. Neither radiofrequency treatment alone or continuous ultra low dose rate irradiation up to 16,000 rad over 180 days alone was successful in curing these tumors; however, combined modality treatments employing doses as low as 8,000 rad over 180 days plus two radiofrequency treatments did effect cures. Gastrointestinal toxicity was best correlated with hyperthermia treatments, while skin reactions secondary to hyperthermia were prolonged by continuous ultra low dose rate irradiation. Implications for clinical usage are discussed.     

Interactions between hyperthermia and irradiation in two human lymphoblastic leukemia cell lines in vitro

Thermal radiosensitization was studied in two human T-cell acute lymphoblastic leukemia cell lines (JM and MOLT3) with regard to heat-irradiation sequence and heating duration. In MOLT3 thermal radiosensitization was maximal when 43.5 degrees C hyperthermia immediately preceded or followed irradiation; at 41.5 degrees C, radiosensitization was maximal with hyperthermia immediately before or up to 3 h after irradiation. In JM, enhancement of radiation killing was unexpectedly maximal when 41.5 or 43.5 degrees C hyperthermia preceded irradiation by 2 to 4 h. Thermal radiosensitization increased exponentially with increasing duration of heating at 41.5 degrees C for at least 3 h in MOLT3. In contrast, in JM, radiosensitization increased exponentially for 1.6 h but additional heating (up to 3 h net heating) had no appreciable further effect on radiation killing. For JM, repair of single and double stranded DNA breaks was investigated using alkaline and neutral elution techniques to determine whether the unusual results regarding heat-irradiation sequencing were related to effects of heat on repair of DNA damage. These studies were unable to detect significant differences in repair of single or double stranded DNA breaks between unheated control cells and cells heated at 41.5 degrees C for 1 h ending 4 h before irradiation. The direct cytotoxicity of hyperthermia was also studied in both cell lines.     

Response of cells of human origin, normal and malignant, to acute and low dose rate irradiation

Dose response curves were obtained for normal human fibroblasts and for several cell lines derived from human tumors, including melanomas and an osteosarcoma. Most of the tumor lines are similar in radiosensitivity to the normal fibroblasts, except for the melanoma lines, which are significantly more resistant. The two melanoma lines differ, one being much more radioresistant than the other. Potentially lethal damage repair (PLDR) has been studied in these cell lines as well. The extent of PLDR does not appear to correlate with radioresistance; for example, the most resistant melanoma line shows very little repair of PLD. In addition, the normal fibroblasts repair PLD at least as well as any of the tumor derived lines, which casts doubts on the wisdom of introducing into clinical practice inhibitors of PLD until a clear differential between normal tissues and tumors has been demonstrated in vivo. Low dose-rate studies with normal human fibroblasts indicate a smaller dose-rate effect than for most established cell lines of rodent origin. Indeed, in the human cells studied, the effect of sublethal damage repair is quantitatively similar to the repair of potentially lethal damage. Dose response curves for acute and protracted exposures have been obtained for cells derived from patients with cancer-prone syndromes including ataxia telangiectasia (AT) and Bloom's syndrome. Both cell lines are much more radiosensitive than normal human fibroblasts; the AT cells show a dose-rate effect, while Bloom's syndrome cells do not.     

Relationship between DNA repair capacity and resistance to genotoxins in four human cell lines

We have developed fast, reliable and simple fluorescent method to assess and compare repair capacity of cells. To this end plasmid pEGFP containing the gene for the enhanced green fluorescent protein was damaged in vitro by genotoxic agents and introduced into cells by transfection. The repair capacity of the cells was determined from the number of fluorescent cells counted with a fluorescent microscope 24 h after transfection. The ability of four human tumor cell lines--HEK293, HeLa, Namalwa and K562 to repair DNA lesions inflicted by cis-diamminedichloroplatinum(II), UV light, 8-methoxypsoralen and 4',5'-8-trimethylpsoralen were determined and compared to the survival rates of the cells after treatment with the same genotoxic agents. In most but not all cases, there was a good correlation between repair capacity and cell survival. This finding indicates that the DNA repair capacity could be used as a biomarker in risk assessment and/or drug resistance assays.     

Single extreme low dose/low dose rate irradiation causes alteration in lifespan and genome instability in primary human cells

To investigate the long-term biological effect of extreme low dose ionising radiation, we irradiated normal human fibroblasts (HFLIII) with carbon ions (290 MeV u(-1), 70 keV microm(-1)) and gamma-rays at 1 mGy (total dose) once at a low dose rate (1 mGy 6-8 h(-1)), and observed the cell growth kinetics up to 5 months by continuous culturing. The growth of carbon-irradiated cells started to slow down considerably sooner than that of non-irradiated cells before reaching senescence. In contrast, cells irradiated with gamma-rays under similar conditions did not show significant deviation from the non-irradiated cells. A DNA double strand break (DSB) marker, gamma-H2AX foci, and a DSB repair marker, phosphorylated DNA-PKcs foci, increased in number when non-irradiated cells reached several passages before senescence. A single low dose/low dose rate carbon ion exposure further raised the numbers of these markers. Furthermore, the numbers of foci for these two markers were significantly reduced after the cells became fully senescent. Our results indicate that high linear energy transfer (LET) radiation (carbon ions) causes different effects than low LET radiation (gamma-rays) even at very low doses and that a single low dose of heavy ion irradiation can affect the stability of the genome many generations after irradiation.     

Cell cycle perturbations in cisplatin-sensitive and resistant human ovarian carcinoma cells following treatment with cisplatin and low dose rate irradiation

Purpose: To investigate cell cycle pertubations in plateau-phase human ovarian carcinoma cells following treatment with cisplatin, low dose-rate irradiation (LDRI), or combined cisplatin and LDRI, in order to understand cell cycle mechanisms by which these two treatment modalities interact. Methods: Human ovarian carcinoma cells sensitive (A2780) and resistant (2780^CP) to cisplatin were grown to plateau phase and given protracted cisplatin treatments (A2780 0.7 and 2 μg/ml; 2780^CP 5 and 15 μg/ml) and/or LDRI (0.41 cGy/min). Cell cycle distribution following treatment was determined by two-parameter flow cytometry, based on bromodeoxyuridine (BrdU) uptake and DNA content using propidium iodide staining.Results: The cisplatin-sensitive A2780 cells exposed to cisplatin alone for up to 28 h showed depletion of the G1 fraction and accumulation in S-phase, although the percentage of S-phase cells actively incorporating BrdU dropped to almost zero. The cisplatin-resistant 2780^CP cells exposed to cisplatin alone showed a G1 arrest when exposed to 15 μg/ml, but not when exposed to 5 μg/ml. LDRI alone caused little cell cycle redistribution different from controls in either cell line. When LDRI was combined with cisplatin, no significant cell cycle redistribution was observed, apart from a decline in the actively incorporating S-phase fraction. Conclusions: Cisplatin caused A2780 cells to accumulate in nonincorporating S-phase, with no evidence of G1 arrest. Cisplatin-resistant 2780^CP cells showed a G1 block when exposed to a high enough cisplatin concentration. This could indicate a mechanism of cisplatin resistance in these cells. LDRI alone or in combination with cisplatin did not result in significant cell cycle redistribution. Received: 22 December 1995 / Accepted: 28 September 1996     

Iododeoxyuridine incorporation and radiosensitization in three human tumor cell lines.

Iododeoxyuridine is a halogenated pyrimidine and non-hypoxic cell radiosensitizer currently being used in clinical trials. The amount of radiosensitization by IdUrd is related to the amount of incorporation of the drug into a cell's DNA. These experiments were carried out in three human tumor cell lines (lung, glioma, and melanoma) in monolayer culture exposed to concentrations of IdUrd from 0.1-10 microM for one and three cell cycles before irradiation to determine incorporation and sensitization as a function of drug exposure. Except for the lung cell line, which required greater than 1 microM IdUrd, these cells demonstrate radiosensitization when exposed to 0.1 microM or greater of IdUrd. Maximum sensitization occurred at 10 microM IdUrd for all the cell lines at three cell cycles. The percent thymidine replacement by IdUrd increased with increasing concentrations, but was cell line dependent. Maximum percent replacement occurred at 10 microM at three cell cycles for all the cell lines: lung = 22.4%, glioma = 32.0%, and melanoma = 39.1%. The relationships between percent thymidine replacement and sensitization are not identical across these human tumor cell lines. If IdUrd is going to be a successful radiosensitizer in clinical trials, sustained plasma levels of 10 microM or greater for at least three cell cycles should be achieved during irradiation. This may be best accomplished with repeated short exposures to IdUrd (three cell cycles or approximately 4 days in these cell lines) every 1-2 weeks during radiation. Measurements of thymidine replacement in a tumor biopsy should be attempted prior to radiation to develop a predictive assay for radiosensitization.     

Single-dose and fractionated irradiation of four human lung cancer cell lines in vitro.

Four established human lung cancer cell lines were exposed to single-dose irradiation. The survival curves of 2 small cell lung carcinomas (SCLC) were characterized by a limited capacity for repair with small and moderate shoulders with extrapolation numbers (n) of 1.05 and 1.60 respectively. Two non-small cell lung carcinoma (NSCLC) cell lines, one squamous cell (SQCLC) and one large cell (LCLC) had large shoulders with n-values of 73 and 15 respectively. The radiosensitivity when measured as D0 did not, however, differ as much from cell line to cell line, with values from 1.22 to 1.65. The surviving fraction after 2 Gy (SF2) was 0.24 and 0.42 respectively in the SCLC cell lines and 0.90 and 0.88 respectively in the NSCLC cell lines. Fractionated irradiation delivered according to 3 different schedules was also investigated. All the schedules delivered a total dose of 10 Gy in 5 days and were applied in 1, 2, and 5 Gy dose fractions respectively. Survival followed the pattern found after single-dose irradiation; it was lowest in the SCLC cell line with the lowest SF and highest in the two NSCLC cell lines. In the SCLC cell lines all schedules were approximately equally efficient. In the LCLC and in the SQCLC cell lines, the 5 Gy schedule killed more cells than the 1 and 2 Gy schedules. The results indicate that the size of the shoulder of the survival curve is essential when choosing the most tumoricidal fractionation schedule.     

Increased tumour response of a murine fibrosarcoma to low temperature hyperthermia and low dose rate brachytherapy

The present animal tumour study was carried out to determine the effectiveness of low temperature hyperthermia combined with low dose rate radiation based on the cell culture studies of our laboratory and others that demonstrated a significant radiosensitization obtained by low temperature hyperthermia and low dose rate radiation. Well-oxygenated murine fibrosarcoma Meth-A tumours growing in Balb/c mice were treated with heat (41d`C tumour temperature) by immersion of the tumour-bearing leg in a waterbath concurrently with low dose rate radiation. Radiation was delivered using <sup>192</sup>Ir interstitial implantation at absolute dose rates of 0.416–0.542 Gy/h. The effect of heat alone on tumour growth and normal tissue was minimal. Tumour growth delay following 30 Gy radiation was 4.9 days. Significant delay in tumour growth was observed with the addition of low temperature hyperthermia delivered concurrently. Enhancement in radiation response was seen with increasing duration of heat treatment; tumour growth delays were 9.5 days following 4h heat (41d`C) treatment and 16 days following 6 h treatment. Three sessions of fractionated hyperthermia 4 h/day during the course of low dose-rate radiation significantly delayed tumour growth to 18.6 days. The results indicate that fractionated heat treatment in conjunction with low dose rate radiation has potential for improving tumour response without adversely affecting normal tissue reaction. This in vivo study represents an extension of the cell culture data and provides further radiobiological basis for the combined use of low temperature hyperthermia and low dose rate radiation.     

DNA polymerase beta expression differences in selected human tumors and cell lines.

A long-standing question in cancer biology has been the extent to which DNA repair may be altered during the process of carcinogenesis. We have shown recently that DNA polymerase beta (beta-pol) provides a rate-determining function during in vitro repair of abasic sites by one of the mammalian DNA base excision repair pathways. Therefore, altered expression of beta-pol during carcinogenesis could alter base excision repair and, consequently, be critical to the integrity of the mammalian genome. We examined the expression of beta-pol in several cell lines and human adenocarcinomas using a quantitative immunoblotting method. In cell lines from normal breast or colon, the level of beta-pol was approximately 1 ng/mg cell extract, whereas in all of the breast and colon adenocarcinoma cell lines tested, a higher level of beta-pol was observed. In tissue samples, colon adenocarcinomas had a higher level of beta-pol than adjacent normal mucosa. Breast adenocarcinomas exhibited a wide range of beta-pol expression: one tumor had a much higher level of beta-pol (286 ng/mg cell extract) than adjacent normal breast tissue, whereas another tumor had the same level of beta-pol as adjacent normal tissue. Differences in beta-pol expression level, from normal to elevated, were also observed with prostate adenocarcinomas. All kidney adenocarcinomas tested had a slightly lower beta-pol level than adjacent normal tissue. This study reveals that the base excision repair enzyme DNA polymerase beta is up-regulated in some types of adenocarcinomas and cell lines, but not in others.