Role of Ape1 and base excision repair in the radioresponse and heat-radiosensitization of HeLa Cells

BACKGROUND

The mechanism by which heat sensitizes mammalian cells to ionizing radiation remains to be elucidated. We determined whether base excision repair (BER) is involved in heat-radiosensitization and report novel findings that provide insight regarding the role of BER in the radiation response of HeLa cells.

MATERIALS AND METHODS

An siRNA approach was utilized to suppress expression of AP endonuclease (Ape1), a critical enzyme of BER. Clonogenic survival curves were obtained for HeLa cells expressing normal or reduced Ape1 content and which had been irradiated, and these were compared to survival curves from cells that were irradiated prior to hyperthermia treatment.

RESULTS

The amount of heat-radiosensitization observed in Ape1-suppressed cells was similar to or slightly greater than that observed in cells expressing near-normal levels of Ape1. Interestingly, we also found that for unheated HeLa cells, suppressed expression of Ape1 resulted in enhanced resistance to X-rays.

CONCLUSION

The data suggest that Ape1, and therefore BER, is not involved in heat-radiosensitization. However, the observation that suppressed expression of Ape1 results in enhanced radioresistance supports the notion that BER may be detrimental to the survival of irradiated cells.

A tool for enhancement and scoring of DNA repair foci

Upon induction of DNA double-strand breaks (DSBs), Mre11 and Rad50 proteins of the Mre11 DNA repair complex accumulate at the sites of DSBs and form discrete nuclear foci. Precision in scoring of Mre11/Rad50-containing foci depends upon detection of those foci, some of which have a fluorescence staining intensity that is too close to the fluorescence staining intensity of the remaining Mre11 and Rad50 proteins that have not been incorporated into foci. Human U-1 melanoma cells in exponential growth were irradiated with various doses of X-rays (0-12 Gy) to induce the formation of repair foci. Four hours after irradiation, cells were simultaneously labeled for Mre11 and Rad50 proteins, using a two-color immunofluorescence staining technique. Laser scanning confocal microscopy was used to collect the composite images of randomly selected cell nuclei. Intensity correlation analysis (ICA) of equally intense fluorescence signals from Mre11 and Rad50 proteins was performed to obtain the regions with correlated pixels. ICA permitted enhanced detection of low level fluorescence of Mre11/Rad50 foci ("hidden" foci) that can be barely detected upon imaging of only one protein. For example, while imaging of only one protein (either Mre11 or Rad50) in the nucleus of a 6 Gy-irradiated cell revealed 9 foci, imaging of two proteins with ICA revealed 11 foci. ICA permitted an evaluation of the dose dependence of nuclear foci in cells irradiated with various doses of X-rays, with focus formation increasing up to a dose of 6 Gy. Our data accumulated using two-color immunofluorescence staining of Mre11 and Rad50 proteins and ICA of these two target proteins provide a basis for enhanced detection and accuracy in the scoring of DNA repair foci.

Ovarian hormone modulation of radiation-induced cataractogenesis: dose-response studies

PURPOSE

Epidemiologic data on the effects of female sex hormones in cataract formation are conflicting. With the use of a rat model of radiation-induced cataractogenesis, it was found that estrogen can either enhance or inhibit the progression of radiation cataracts, depending on when the hormone is administered. The present study was performed to further define radiation-hormone interactions during cataractogenesis.

METHODS

In one experiment, rats were left ovary-intact or ovariectomized and were then irradiated with 2.5, 5, 10, or 15 Gy to one eye. In another experiment, ovariectomized rats were treated continuously with three different doses of estradiol through a slow-release capsule implanted subcutaneously, after which one eye was irradiated with 15 Gy. In all animals, cataract formation was followed by slit lamp examination at regular intervals.

RESULTS

Latency to identification of cataracts decreased exponentially with increasing radiation dose. The presence of ovaries enhanced cataractogenesis when the eye was irradiated with 15 Gy, but there was no difference between ovary-intact and ovariectomized rats that were irradiated at lower doses. In ovariectomized rats irradiated with 15 Gy, estradiol increased the rate of progression of cataracts in a dose-dependent manner. The rate of cataract progression increased linearly with increasing estradiol dose; there was no sign of saturation at high estradiol doses, as would be expected from a receptor-mediated effect.

CONCLUSIONS

Ovarian hormones enhance radiation-induced cataract formation; hormone supplementation experiments indicate that estrogen is responsible for this effect. The data suggest that the enhancing effect of estradiol is not mediated by its receptor, but this requires further study.

NF-kappaB-mediated HER2 overexpression in radiation-adaptive resistance

The molecular mechanisms governing acquired tumor resistance during radiotherapy remain to be elucidated. In breast cancer patients, overexpression of HER2 (human epidermal growth factor receptor 2) is correlated with aggressive tumor growth and increased recurrence. In the present study, we demonstrate that HER2 expression can be induced by radiation in breast cancer cells with a low basal level of HER2. Furthermore, HER2-postive tumors occur at a much higher frequency in recurrent invasive breast cancer (59%) compared to the primary tumors (41%). Interestingly, NF-kappaB is required for radiation-induced HER2 transactivation. HER2 was found to be co-activated with basal and radiation-induced NF-kappaB activity in radioresistant but not radiosensitive breast cancer cell lines after long-term radiation exposure, indicating that NF-kappaB-mediated HER2 overexpression is involved in radiation-induced repopulation in heterogeneous tumors. Finally, we found that inhibition of HER2 resensitizes the resistant cell lines to radiation. Since HER2 is shown to activate NF-kappaB, our data suggest a loop-like HER2-NF-kappaB-HER2 pathway in radiation-induced adaptive resistance in breast cancer cells.

Estrogen protects against radiation-induced cataractogenesis

Cataractogenesis is a complication of radiotherapy when the eye is included in the treatment field. Low doses of densely ionizing space radiation may also result in an increased risk of cataracts in astronauts. We previously reported that estrogen (17-beta-estradiol), when administered to ovariectomized rats commencing 1 week before gamma irradiation of the eye and continuously thereafter, results in a significant increase in the rate and incidence of cataract formation and a decreased latent period compared to an ovariectomized control group. We therefore concluded that estrogen accelerates progression of radiation-induced opacification. We now show that estrogen, if administered continuously, but commencing after irradiation, protects against radiation cataractogenesis. Both the rate of progression and incidence of cataracts were greatly reduced in ovariectomized rats that received estrogen treatment after irradiation compared to ovariectomized rats. As in our previous study, estradiol administered 1 week prior to irradiation at the time of ovariectomy and throughout the period of observation produced an enhanced rate of cataract progression. Estrogen administered for only 1 week prior to irradiation had no effect on the rate of progression but resulted in a slight reduction in the incidence. We conclude that estrogen may enhance or protect against radiation cataractogenesis, depending on when it is administered relative to the time of irradiation, and may differentially modulate the initiation and progression phases of cataractogenesis. These data have important implications for astronauts and radiotherapy patients.

Recent initiatives for radiation oncology resident education in radiation and cancer biology

Nearly all residents from accredited radiation oncology residency programs in the United States are required to take the American College of Radiology (ACR) In-Training examination each year. The test is comprised of three sections: Clinical Radiation Oncology, Radiological Physics, and Radiation (and Cancer) Biology. Here we provide an update on changes to the biology portion of the ACR exam. We also discuss the availability and use of the ACR and biology practice exams as assessment and teaching tools for both the instructors of radiation and cancer biology and the residents they teach.

Effect of Estrogen on Radiation-Induced Cataractogenesis

Cataractogenesis is a widely reported late effect that is observed in patients receiving total-body irradiation (TBI) prior to bone marrow transplantation or radiotherapy for ocular or head and neck cancers. Recent studies indicate that estrogens may protect against age-related and drug-induced cataracts. Moreover, other reports suggest that estrogen possesses antioxidant properties. Since the effect of estrogen on radiation cataractogenesis is unknown, we wished to determine whether estrogen modulates radiation-induced opacification of the lens. Intact or ovariectomized Sprague-Dawley rats were treated with either 17-beta-estradiol or an empty silastic capsule. The right orbit was then irradiated with either 10 or 15 Gy of (60)Co gamma rays using a Leksell Gamma Knife, and lenses were examined at various times postirradiation with a slit lamp or evaluated for light transmission. We found that for ovariectomized rats irradiated with 15 Gy, the lens opacity and the incidence of cataract formation in the estradiol-treated group were significantly increased compared to the control group at the end of the 25-week period of observation. Cataract incidence was also high in irradiated eyes of ovary-intact animals at 25 weeks postirradiation but was greatly reduced in the ovariectomized control group, with less than half of irradiated eyes showing evidence of cataractogenesis. Thus, after irradiation with 15 Gy of gamma rays, estrogen increased the incidence of cataract formation. We also observed that although the incidence of cataract formation in rats irradiated with 10 Gy and receiving continuous estrogen treatment was not altered compared to rats in the control group that did not receive estrogen, the latent period for posterior subcapsular cataract formation decreased and the severity of the anterior cataract increased. Taken together, our data suggest that estrogen accelerates progression of radiation-induced opacification.

A Radiation-Induced Acute Apoptosis Involving TP53 and BAX Precedes the Delayed Apoptosis and Neoplastic Transformation of CGL1 Human Hybrid Cells

Exposing CGL1 (HeLa x fibroblast) hybrid cells to 7 Gy of X rays results in the onset of a delayed apoptosis in the progeny of the cells 10 to 12 cell divisions postirradiation that correlates with the emergence of neoplastically transformed foci. The delayed apoptosis begins around day 8 postirradiation and lasts for 11 days. We now demonstrate that the delayed apoptosis is also characterized by the appearance of approximately 50-kb apoptotic DNA fragments and caspase 3 activation postirradiation. In addition, we confirm that stabilization of TP53 and transactivation of pro-apoptosis BAX also occurs during the delayed apoptosis and show that anti-apoptosis BCL-X(L) is down-regulated. To test whether the delayed apoptosis was due to a nonfunctional acute TP53 damage response in CGL1 cells, studies of acute apoptosis were completed. After irradiation, CGL1 cells underwent an acute wave of apoptosis that involves TP53 stabilization, transactivation of BAX gene expression, and a rapid caspase activation that ends by 96 h postirradiation. In addition, the acute onset of apoptosis correlates with transactivation of a standard wild-type TP53-responsive reporter (pG13-CAT) in CGL1 cells after radiation exposure. We propose that the onset of the delayed apoptosis is not the result of a nonfunctional acute TP53 damage response pathway but rather is a consequence of X-ray-induced genomic instability arising in the distant progeny of the irradiated cells.

Mechanism of radiosensitization by hyperthermia (> or = 43 degrees C) as derived from studies with DNA repair defective mutant cell lines

All biochemical and cytogenetic data on radiosensitization by heat treatment at and above 43 degrees C indicate that inhibition of DNA repair plays a central role. There are several DNA repair pathways involved in restoration of damage after ionising irradiation and the kinetics of all of them are affected by heat shock. This, however, does not imply that the inhibition of each of these pathways is relevant to the effect of heat on cellular radiosensitivity. The current review evaluates the available data on heat radiosensitization in mutant or knockout cell lines defective in various DNA repair proteins and/or pathways. The data show that thermal inhibition of the non-homologous end-joining pathway (NHEJ) plays no role in heat radiosensitization. Furthermore, limited data suggest that the homologous recombination pathway may also not be a major heat target. By deduction, it is suggested that inhibition of base damage repair (BER) could be the crucial step in radiosensitization by heat. While a lack of mutant cell lines and redundancy of the BER pathway have hampered efforts toward a conclusive study, biochemical and correlative evidence support this hypothesis.

Use of the Leksell Gamma Knife for localized small field lens irradiation in rodents

For most basic radiobiological research applications involving irradiation of small animals, it is difficult to achieve the same high precision dose distribution realized with human radiotherapy. The precision for irradiations performed with standard radiotherapy equipment is +/-2 mm in each dimension, and is adequate for most human treatment applications. For small animals such as rodents, whose organs and tissue structures may be an order of magnitude smaller than those of humans, the corresponding precision required is closer to +/-0.2 mm, if comparisons or extrapolations are to be made to human data. The Leksell Gamma Knife is a high precision radiosurgery irradiator, with precision in each dimension not exceeding 0.5 mm, and overall precision of 0.7 mm. It has recently been utilized to treat ocular melanoma and induce targeted lesions in the brains of small animals. This paper describes the dosimetry and a technique for performing irradiation of a single rat eye and lens with the Gamma Knife while allowing the contralateral eye and lens of the same rat to serve as the "control". The dosimetry was performed with a phantom in vitro utilizing a pinpoint ion chamber and thermoluminescent dosimeters, and verified by Monte Carlo simulations. We found that the contralateral eye received less than 5% of the administered dose for a 15 Gy exposure to the targeted eye. In addition, after 15 Gy irradiation 15 out of 16 animals developed cataracts in the irradiated target eyes, while 0 out of 16 contralateral eyes developed cataracts over a 6-month period of observation. Experiments at 5 and 10 Gy also confirmed the lack of cataractogenesis in the contralateral eye. Our results validate the use of the Gamma Knife for cataract studies in rodents, and confirmed the precision and utility of the instrument as a small animal irradiator for translational radiobiology experiments.

Intracellular redistribution and modification of proteins of the Mre11/Rad50/Nbs1 DNA repair complex following irradiation and heat-shock

Mre11, Rad50, and Nbs1form a tight complex which is homogeneously distributed throughout the nuclei of mammalian cells. However, after irradiation, the Mre11/Rad50/Nbs1 (M/R/N) complex rapidly migrates to sites of double strand breaks (DSBs), forming foci which remain until DSB repair is complete. Mre11 and Rad50 play direct roles in DSB repair, while Nbs1 appears to be involved in damage signaling. Hyperthermia sensitizes mammalian cells to ionizing radiation. Radiosensitization by heat shock is believed to be mediated by an inhibition of DSB repair. While the mechanism of inhibition of repair by heat shock remains to be elucidated, recent reports suggest that the M/R/N complex may be a target for inhibition of DSB repair and radiosensitization by heat. We now demonstrate that when human U-1 melanoma cells are heated at 42.5 or 45.5 degrees C, Mre11, Rad50, and Nbs1 are rapidly translocated from the nucleus to the cytoplasm. Interestingly, when cells were exposed to ionizing radiation (12 Gy of X-rays) prior to heat treatment, the extent and kinetics of translocation were increased when nuclear and cytoplasmic fractions of protein were analyzed immediately after treatment. The kinetics of the translocation and subsequent relocalization back into the nucleus when cells were incubated at 37 degrees C from 30 min to 7 h following treatment were different for each protein, which suggests that the proteins redistribute independently. However, a significant fraction of the translocated proteins exist as a triple complex in the cytoplasm. Treatment with leptomycin B (LMB) inhibits the translocation of Mre11, Rad50, and Nbs1 to the cytoplasm, leading us to speculate that the relocalization of the proteins to the cytoplasm occurs via CRM1-mediated nuclear export. In addition, while Nbs1 is rapidly phosphorylated in the nuclei of irradiated cells and is critical for a normal DNA damage response, we have found that Nbs1 is rapidly phosphorylated in the cytoplasm, but not in the nucleus, of heated irradiated cells. The phosphorylation of cytoplasmic Nbs1, which cannot be inhibited by wortmannin, appears to be a unique post-translational modification in heated, irradiated cells, and coupled with our novel observations that Mre11, Rad50, and Nbs1 translocate to the cytoplasm, lend further support for a role of the M/R/N complex in thermal radiosensitization and inhibition of DSB repair.

Effects of heat shock on the Mre11/Rad50/Nbs1 complex in irradiated or unirradiated cells

The mechanism by which hyperthermia sensitizes mammalian cells to ionizing radiation remains to be elucidated, but an overwhelming amount of circumstantial evidence suggests that heat radiosensitization might be mediated by inhibition of double-strand break repair, particularly after exposure of irradiated cells to heat treatments in excess of about 43 degrees C. In mammalian cells, double-strand break repair usually occurs via two pathways, non-homologous end-joining and homologous recombination. Several reports suggest a role for non-homologous end-joining in heat radiosensitization, while others implicate homologous recombination as a target. However, cell lines that are compromised in either the non-homologous end-joining or homologous recombination pathway are still capable of being radiosensitized, suggesting that heat affects both pathways. Indeed, several of the proteins involved in one or both of these pathways have been observed to undergo alterations or translocation after unirradiated or irradiated cells are exposed to heat shock. The work summarized in this review implicates proteins of the Mre11/Rad50/Nbs1 complex as targets for heat radiosensitization.

Education and training for radiation scientists: radiation research program and American Society of Therapeutic Radiology and Oncology Workshop, Bethesda, Maryland, May 12-14, 2003

Current and potential shortfalls in the number of radiation scientists stand in sharp contrast to the emerging scientific opportunities and the need for new knowledge to address issues of cancer survivorship and radiological and nuclear terrorism. In response to these challenges, workshops organized by the Radiation Research Program (RRP), National Cancer Institute (NCI) (Radiat. Res. 157, 204-223, 2002; Radiat. Res. 159, 812-834, 2003), and National Institute of Allergy and Infectious Diseases (NIAID) (Nature, 421, 787, 2003) have engaged experts from a range of federal agencies, academia and industry. This workshop, Education and Training for Radiation Scientists, addressed the need to establish a sustainable pool of expertise and talent for a wide range of activities and careers related to radiation biology, oncology and epidemiology. Although fundamental radiation chemistry and physics are also critical to radiation sciences, this workshop did not address workforce needs in these areas. The recommendations include: (1) Establish a National Council of Radiation Sciences to develop a strategy for increasing the number of radiation scientists. The strategy includes NIH training grants, interagency cooperation, interinstitutional collaboration among universities, and active involvement of all stakeholders. (2) Create new and expanded training programs with sustained funding. These may take the form of regional Centers of Excellence for Radiation Sciences. (3) Continue and broaden educational efforts of the American Society for Therapeutic Radiology and Oncology (ASTRO), the American Association for Cancer Research (AACR), the Radiological Society of North America (RSNA), and the Radiation Research Society (RRS). (4) Foster education and training in the radiation sciences for the range of career opportunities including radiation oncology, radiation biology, radiation epidemiology, radiation safety, health/government policy, and industrial research. (5) Educate other scientists and the general public on the quantitative, basic, molecular, translational and applied aspects of radiation sciences.

The non-homologous end-joining pathway is not involved in the radiosensitization of mammalian cells by heat shock

A synergistic increase in cell killing is observed when a heat-shock is administered prior to, during, or immediately after exposure to ionizing radiation (IR). This phenomenon, known as heat-radiosensitization, is believed to be mediated by inhibition of repair of radiation-induced double strand breaks (DSB) when cells are exposed to temperatures above 42 degrees C. However, the mechanism by which heat inhibits DSB repair is unclear. The bulk of radiation-induced DSBs are repaired via the non-homologous end-joining pathway (NHEJ). Several reports indicate that the Ku70 and Ku80 subunits of the mammalian DNA-dependent protein kinase (DNA-PK), a complex involved in NHEJ, appear to be susceptible to a heat-induced loss of DNA-binding activity, with Ku80 representing the heat-sensitive component. Since the heat-induced loss and subsequent recovery of Ku-DNA binding activity correlates well with heat-radiosensitization, a role for Ku80 and NHEJ in heat-radiosensitization has been proposed. However, direct evidence implicating Ku80 (and NHEJ) in heat-radiosensitization has been indeterminate. In this study, we demonstrate that equitoxic heat treatments at 42.5-45.5 degrees C induce a similar amount of aggregation of Ku80 in human U-1 melanoma cells. These data suggest that the time-temperature-dependent relationship between heat lethality and Ku80 aggregation are similar. However, the aggregation/disaggregation of Ku80 and its transient or permanent inactivation is unrelated to heat-radiosensitization. When survival curves were obtained for irradiated or irradiated and heated Ku80(-/-) mouse embryo fibroblasts (MEFs) and compared with survival curves obtained for wild-type (WT) cells, we found that heat-radiosensitization was not reduced in the Ku80(-/-) cells, but actually increased. Thus, our findings indicate that Ku80 is not essential for heat-radiosensitization. Non-involvement of Ku-dependent or Ku-independent NHEJ pathways in heat-radiosensitization was confirmed by comparing clonogenic survival between DNA ligase IV-defective and WT human cells. Our data therefore implicate homologous recombination in inhibition of repair of radiation-induced DSBs and as a target for heat-radiosensitization.

Reversible changes in the nuclear lamina induced by hyperthermia

The nuclear matrix (NM) has been identified as a potential target for heat-induced cell killing. Previous studies have shown that heat-shock may significantly modulate lamin B content. Since changes in NM structure have often been accompanied by changes in protein composition, we investigated whether hyperthermia induced changes in nuclear lamina (NL) structure in non-tolerant and thermotolerant cells, and the implications of these changes on cell survival. Using indirect immunofluorescence techniques and confocal microscopy, we found that heating cells at 42 or 45.5 degrees C caused invaginations and other distortions of the peripheral NL. While hyperthermia did not alter the number or structure of internal lamin B foci, heat-induced alterations to the peripheral NL were dose-dependent. Interestingly, NL structure recovered with time after heating in cells that were destined to live or die. Thermotolerant cells heated at 45.5 degrees C showed similar initial changes in the NL compared to non-tolerant cells, but recovery occurred much faster. Taken together, these results suggest that the amount of initial damage to the peripheral NL is not correlated with heat-induced cell killing. However, the possibility that an increased rate of recovery might confer a survival advantage cannot be discounted.

Toward a national consensus: teaching radiobiology to radiation oncology residents

PURPOSE

The ASTRO Joint Working Group on Radiobiology Teaching, a committee composed of members having affiliations with several national radiation oncology and biology-related societies and organizations, commissioned a survey designed to address issues of manpower, curriculum standardization, and instructor feedback as they relate to resident training in radiation biology.

METHODS AND MATERIALS

Radiation biology instructors at U.S. radiation oncology training programs were identified and asked to respond to a comprehensive electronic questionnaire dealing with instructor educational background, radiation biology course content, and sources of feedback with respect to curriculum planning and resident performance on standardized radiation biology examinations.

RESULTS

Eighty-five radiation biology instructors were identified, representing 73 radiation oncology residency training programs. A total of 52 analyzable responses to the questionnaire were received, corresponding to a response rate of 61.2%.

CONCLUSION

There is a decreasing supply of instructors qualified to teach classic, and to some extent, clinical, radiobiology to radiation oncology residents. Additionally, those instructors with classic training in radiobiology are less likely to be comfortable teaching cancer molecular biology or other topics in cancer biology. Thus, a gap exists in teaching the whole complement of cancer and radiobiology curricula, particularly in those programs in which the sole responsibility for teaching falls to one faculty member (50% of training programs are in this category). On average, the percentage of total teaching time devoted to classic radiobiology (50%), clinical radiobiology (30%), and molecular and cancer biology (20%) is appropriate, relative to the current makeup of the board examination. Nevertheless large variability exists between training programs with respect to the total number of contact hours per complete radiobiology course (ranging from approximately 10 to >50 h). A number of lecture topics, particularly in clinical radiobiology, are covered by fewer than 60% of training programs. A sizeable minority of radiation biology instructors are dissatisfied with the feedback they receive with respect to both course content and the performance of their residents on standardized radiobiology examinations administered by the American College of Radiology and/or the American Board of Radiology.

Toward a consensus on radiobiology teaching to radiation oncology residents

There are approximately 82 radiation oncology residency programs in the United States, which provide training opportunities for about 400 residents. All accredited radiation oncology residency programs must have at least one basic scientist on the faculty, and it is these individuals who often assume, wholly or in part, the responsibility of teaching radiation and cancer biology to radiation oncology residents in preparation for the American College of Radiology (ACR) In-Training Examination in Radiation Oncology and the American Board of Radiology (ABR) written examinations. In response to a perceived lack of uniformity in radiation and cancer biology curricula currently being taught to residents and a perceived lack of guidance for instructors in formulating course content for this population, a special session was presented at the Forty-eighth Annual Radiation Research Society meeting on April 23, 2001. The session, entitled "Toward a Consensus on Radiobiology Teaching to Radiation Oncology Residents", was focused on issues related to teaching radiobiology to radiation oncology residents and targeted for individuals who actively teach radiation and cancer biology as well as coordinators of residency training programs. The speakers addressed current challenges and future problems facing instructors and programs. Among these were lack of feedback on resident performance on ABR and ACR written examinations and on course content, uncertainty about what topics residents must know to pass the ABR examination, and, in the near future, a reduction (due to retirement) of instructors qualified to teach radiobiology. This article provides a synopsis of the information that was presented during that session, offers a glimpse into how the ABR and ACR examinations are prepared and details of the content of past and future examinations, and summarizes the activities of the Joint Working Group on Radiobiology Teaching which was formed to educate instructors, to establish a consensus for course curricula, and to improve the overall quality of resident teaching.

Heat-induced aggregation of XRCC5 (Ku80) in nontolerant and thermotolerant cells

XRCC5 (also known as Ku80) is a component of the DNA-dependent protein kinase (DNA-PK), existing as a heterodimer with G22P1 (also known as Ku70). DNA-PK is involved in the nonhomologous end-joining (NHEJ) pathway of DNA double-strand break (DSB) repair, and kinase activity is dependent upon interaction of the Ku subunits with the resultant DNA ends. Nuclear XRCC5 is normally extractable with non-ionic detergent; it is found in the soluble cytoplasmic fraction after nuclear isolation with Triton X-100. In this study, we found that heating at 45.5 degrees C causes a decreased extractability of XRCC5 from the nuclei of human U-1 melanoma or HeLa cells. Such decreases in extractability are indicative of protein aggregation within nuclei. Recovery of extractability of XRCC5 to that of unheated control cells was observed after incubation at 37 degrees C after heat shock. The decrease in extractability and the kinetics of recovery were dependent on dose, although the decrease in extractability reached a plateau after heating for 15 min or more. Thermotolerant U-1 cells also showed decreased extractability of XRCC5, but to a lesser degree compared to nontolerant cells. When a comparable initial reduction of extractability of XRCC5 was induced in both thermotolerant and nontolerant cells, the kinetics of recovery was nearly identical. The kinetics of recovery of the extractability of XRCC5 was different from that of total nuclear protein in nontolerant cells; recovery of extractability of XRCC5 occurred faster initially and returned to the level in unheated cells faster than total nuclear protein. Similar results were obtained for thermotolerant cells, with differences between the initial recovery of the extractability of XRCC5 and total protein being particularly evident after longer heating times. Heat has been shown to inactivate XRCC5. We speculate that inactivation of XRCC5 after heat shock results from protein aggregation, and that changes in XRCC5 may, in part, lead to inhibition of DSB repair through inactivation of the NHEJ pathway.

Translocation of MRE11 from the nucleus to the cytoplasm as a mechanism of radiosensitization by heat

Zhu, W-G., Seno, J. D., Beck, B. D. and Dynlacht, J. R. Translocation of MRE11 from the Nucleus to the Cytoplasm as a Mechanism of Radiosensitization by Heat. Radiat. Res. 156, 95-102 (2001).Hyperthermia sensitizes mammalian cells to ionizing radiation, presumably by inhibiting the repair of radiation-induced double-strand breaks (DSBs). However, the mechanism by which heat inhibits DSB repair is unclear. The nuclear protein MRE11 is a component of a multi-protein complex involved in nonhomologous end joining (NHEJ) of radiation-induced DSBs. Using one-dimensional sodium dodecylsulfate polyacrylamide gel electrophoresis and Western blotting, we found that MRE11 is translocated from the nucleus to the cytoplasm when human U-1 melanoma or HeLa cells are heated for 15 min at 45.5 degrees C or when cells are heated after irradiation with 12 Gy of X rays. No such translocation is observed in unheated irradiated cells. The kinetics of migration of MRE11 to the cytoplasm was dependent upon whether the heated cells were irradiated, while the magnitude of redistribution of MRE11 was dependent upon post-treatment incubation time at 37 degrees C. Cytoplasmic MRE11 content reached a maximum 2-4 h after heating; the increase was not due to new protein synthesis. Partial recovery of nuclear MRE11 content was observed when heated cells or heated irradiated cells were incubated for up to 7 h at 37 degrees C after treatment. Western blotting results showing translocation of MRE11 from the nucleus to the cytoplasm after heating and irradiation were confirmed using confocal microscopy and immunofluorescence staining of fixed cells. Our data suggest that radiosensitization by heat may be caused, at least in part, by translocation of the DNA repair protein MRE11 from the nucleus to the cytoplasm.

Different patterns of DNA fragmentation and degradation of nuclear matrix proteins during apoptosis induced by radiation, hyperthermia or etoposide

Several nuclear matrix proteins are substrates for proteolytic cleavage during apoptosis. Using Western blotting, the temporal patterns of cleavage of three nuclear matrix proteins (lamin B, NUMA and the nucleoporin TPR) were compared in HL60 cells induced to undergo apoptosis after irradiation, heat shock or treatment with etoposide. Flow cytometry was used to compare the kinetics of post-cleavage degradation of lamin B, NUMA and TPR after irradiation, and to correlate DNA fragmentation with protein degradation in cells induced to undergo apoptosis with different agents. During radiation-induced apoptosis, cleavage and subsequent degradation of lamin B, NUMA and TPR occurred with different kinetics. Low-molecular-weight DNA fragmentation occurred subsequent to the initiation of NUMA cleavage, coincided with lamin B cleavage, but occurred before more extensive degradation of lamin B and NUMA. A similar sequence was observed for cells treated with etoposide. However, during heat-induced apoptosis, cleavage of lamin B and NUMA occurred much sooner compared to other agents, with NUMA cleaved into multiple fragments within 15 min after heating. We conclude that the hierarchical sequence and kinetics of degradative events contributing to nuclear disassembly during apoptosis are highly dependent on the inducing agent. Furthermore, the nuclear pore complex, like the nuclear lamina and internal nuclear matrix, is a target for proteolytic cleavage.

Heat-induced modulation of lamin B content in two different cell lines

Previous work in our laboratory indicates that the nuclear matrix protein lamin B is a "prompt" heat shock protein, which increases significantly when human U-1 melanoma and HeLa cells are exposed to 45.5 degrees C for 5-40 min. Using Western blotting, we found that the lamin B content in U-1 and HeLa cells increased to a greater extent during post-heat incubation at 37 degrees C than during the heat dose itself. When HeLa cells were heated at 45.5 degrees C for 30 min, and then incubated at 37 degrees C for up to 7 h, lamin B content was increased significantly (1.69-fold maximum increase at 3 h) compared to unincubated heated cells. Also, thermotolerant HeLa cells showed a greater increase (up to 1.72-fold) in lamin B content during subsequent heating compared to nontolerant cells. The increase in lamin B content in thermotolerant cells, or when heated cells were incubated at 37 degrees C, was also observed in U-1 cells. HeLa cells heated in the presence of glycerol (a heat protector) showed a 1.21-1.72-fold increase in lamin B content compared to cells heated for 10-30 min without glycerol. In contrast, lamin B content decreased 1.23-1.85-fold when cells were heated for 10-30 min in the presence of procaine (a heat sensitizer) compared to cells heated without procaine. These data suggest that lamin B may play an important role in the heat shock response, and that modulation of lamin B content by heat sensitizers or protectors may play a role in regulation of heat sensitivity.