A role for genomic instability in cellular radioresistance?

Radioresistance Mechanisms in Prostate Cancer Cell Lines Surviving Ultra-Hypo-Fractionated EBRT: Implications and Possible Clinical Applications

The use of a higher dose per fraction to overcome the high radioresistance of prostate cancer cells has been unsuccessfully proposed. Herein, we present PC3 and DU-145, castration-resistant prostate cancer cell lines that survived a clinically used ultra-higher dose per fraction, namely, radioresistant PC3 and DU-145 cells (PC3RR and DU-145RR). Compared to PC3, PC3RR showed a higher level of aggressive behaviour, with enhanced clonogenic potential, DNA damage repair, migration ability and cancer stem cell features. Furthermore, compared to PC3, PC3RR more efficiently survived further radiation by increasing proliferation and down-regulating pro-apoptotic proteins. No significant changes of the above parameters were described in DU-145RR, suggesting that different prostate cancer cell lines that survive ultra-higher dose per fraction do not display the same grade of aggressive phenotype. Furthermore, both PC3RR and DU-145RR increased antioxidant enzymes and mesenchymal markers. Our data suggest that different molecular mechanisms could be potential targets for future treatments plans based on sequential strategies and synergistic effects of different modalities, possibly in a patient-tailored fashion. Moreover, PC3RR cells displayed an increase in specific markers involved in bone remodeling, indicating that radiotherapy selects a PC3 population capable of migrating to secondary metastatic sites. Finally, PC3RR cells showed a better sensitivity to Docetaxel as compared to native PC3 cells. This suggests that a subset of patients with castration-resistant metastatic disease could benefit from upfront Docetaxel treatment after the failure of radiotherapy.

Chromosome Missegregation as a Modulator of Radiation Sensitivity

Chromosome missegregation over the course of multiple cell divisions, termed chromosomal instability (CIN), is a hallmark of cancer. Multiple causes of CIN have been identified, including defects in the mitotic checkpoint, altered kinetochore-microtubule dynamics, centrosome amplification, and ionizing radiation. Here we review the types, mechanisms, and cellular implications of CIN. We discuss the evidence that CIN can promote tumors, suppress them, or do neither, depending on the rates of chromosome missegregration and the cellular context. Very high rates of chromosome missegregation lead to cell death due to loss of essential chromosomes; thus elevating CIN above a tolerable threshold provides a mechanistic opportunity to promote cancer cell death. Lethal rates of CIN can be achieved by a single insult or through a combination of insults. Because ionizing radiation induces CIN, additional therapies that increase CIN may serve as useful modulators of radiation sensitivity. Ultimately, quantifying the intrinsic CIN in a tumor and modulating this level pharmacologically as well as with radiation may allow for a more rational, personalized radiation therapy prescription, thereby decreasing side effects and increasing local control.

Assessment of γ-H2AX and 53BP1 Foci in Peripheral Blood Lymphocytes to Predict Subclinical Hematotoxicity and Response in Somatostatin Receptor-Targeted Radionuclide Therapy for Advanced Gastroenteropancreatic Neuroendocrine Tumors

BACKGROUND

We aimed to characterize γ-H2AX and 53BP1 foci formation in patients receiving somatostatin receptor-targeted radioligand therapy, and explored its role for predicting treatment-related hematotoxicity, and treatment response.

METHODS

A prospective analysis of double-strand break (DSB) markers was performed in 21 patients with advanced gastroenteropancreatic neuroendocrine tumors. γ-H2AX and 53BP1 foci formation were evaluated in peripheral blood lymphocytes (PBLs) at baseline, +1 h and +24 h after administration of 7.4 GBq (¹⁷⁷Lu)Lu-DOTA-TATE. Hematotoxicity was evaluated using standard hematology. Therapy response was assessed using (⁶⁸Ga)Ga-DOTA-TATE PET/CT before enrollment and after 2 cycles of PRRT according to the volumetric modification of RECIST 1.1.

RESULTS

DSB marker kinetics were heterogeneous among patients. Subclinical hematotoxicity was associated with γ-H2AX and 53BP1 foci formation (e.g., change in platelet count vs change in γ-H2AX⁺ cells between baseline and +1 h (r = -0.6080; p = 0.0045). Patients showing early development of new metastases had less γ-H2AX (p = 0.0125) and less 53BP1 foci per cell at +1 h (p = 0.0289), and demonstrated a distinct kinetic pattern with an absence of DSB marker decrease at +24 h (γ-H2AX: p = 0.0025; 53BP1: p = 0.0008).

CONCLUSIONS

Assessment of γ-H2AX and 53BP1 foci formation in PBLs of patients receiving radioligand therapy may hold promise for predicting subclinical hematotoxicity and early treatment response.

Inside the hypoxic tumour: reprogramming of the DDR and radioresistance

The hypoxic tumour is a chaotic landscape of struggle and adaption. Against the adversity of oxygen starvation, hypoxic cancer cells initiate a reprogramming of transcriptional activities, allowing for survival, metastasis and treatment failure. This makes hypoxia a crucial feature of aggressive tumours. Its importance, to cancer and other diseases, was recognised by the award of the 2019 Nobel Prize in Physiology or Medicine for research contributing to our understanding of the cellular response to oxygen deprivation. For cancers with limited treatment options, for example those that rely heavily on radiotherapy, the results of hypoxic adaption are particularly restrictive to treatment success. A fundamental aspect of this hypoxic reprogramming with direct relevance to radioresistance, is the alteration to the DNA damage response, a complex set of intermingling processes that guide the cell (for good or for bad) towards DNA repair or cell death. These alterations, compounded by the fact that oxygen is required to induce damage to DNA during radiotherapy, means that hypoxia represents a persistent obstacle in the treatment of many solid tumours. Considerable research has been done to reverse, correct or diminish hypoxia's power over successful treatment. Though many clinical trials have been performed or are ongoing, particularly in the context of imaging studies and biomarker discovery, this research has yet to inform clinical practice. Indeed, the only hypoxia intervention incorporated into standard of care is the use of the hypoxia-activated prodrug Nimorazole, for head and neck cancer patients in Denmark. Decades of research have allowed us to build a picture of the shift in the DNA repair capabilities of hypoxic cancer cells. A literature consensus tells us that key signal transducers of this response are upregulated, where repair proteins are downregulated. However, a complete understanding of how these alterations lead to radioresistance is yet to come.

The CGL1 (HeLa × Normal Skin Fibroblast) Human Hybrid Cell Line: A History of Ionizing Radiation Induced Effects on Neoplastic Transformation and Novel Future Directions in SNOLAB

Cellular transformation assays have been utilized for many years as powerful in vitro methods for examining neoplastic transformation potential/frequency and mechanisms of carcinogenesis for both chemical and radiological carcinogens. These mouse and human cell based assays are labor intensive but do provide quantitative information on the numbers of neoplastically transformed foci produced after carcinogenic exposure and potential molecular mechanisms involved. Several mouse and human cell systems have been generated to undertake these studies, and they vary in experimental length and endpoint assessment. The CGL1 human cell hybrid neoplastic model is a non-tumorigenic pre-neoplastic cell that was derived from the fusion of HeLa cervical cancer cells and a normal human skin fibroblast. It has been utilized for the several decades to study the carcinogenic/neoplastic transformation potential of a variety of ionizing radiation doses, dose rates and radiation types, including UV, X ray, gamma ray, neutrons, protons and alpha particles. It is unique in that the CGL1 assay has a relatively short assay time of 18-21 days, and rather than relying on morphological endpoints to detect neoplastic transformation utilizes a simple staining method that detects the tumorigenic marker alkaline phosphatase on the neoplastically transformed cells cell surface. In addition to being of human origin, the CGL1 assay is able to detect and quantify the carcinogenic potential of very low doses of ionizing radiation (in the mGy range), and utilizes a neoplastic endpoint (re-expression of alkaline phosphatase) that can be detected on both viable and paraformaldehyde fixed cells. In this article, we review the history of the CGL1 neoplastic transformation model system from its initial development through the wide variety of studies examining the effects of all types of ionizing radiation on neoplastic transformation. In addition, we discuss the potential of the CGL1 model system to investigate the effects of near zero background radiation levels available within the radiation biology lab we have established in SNOLAB.

Global DNA hypomethylation is associated with the development and poor prognosis of tongue squamous cell carcinoma

BACKGROUNDS

Oral cancer is the 4th leading cause of cancer death for males and the top cancer in young adult males in Taiwan. Tongue squamous cell carcinoma (TSCC) is a common oral cancer and generally associated with poor prognosis. Global DNA hypomethylation at the 5 position of cytosine (5mC) is a well-known epigenetic feature of cancer. Therefore, the purpose of this study was to investigate the relationship of the global 5mC content with the tumorigenesis and prognosis of patients with TSCC.

METHODS

The levels of global 5mC were evaluated by immunohistochemistry using tissue microarray slides of 248 surgically resected TSCC and 202 corresponding tumor adjacent normal (TAN) tissues.

RESULTS

We found that the level of 5mC in TSCC (P < 0.001) was significantly decreased as compared to TAN. Among TSCC tissues, decreased levels of 5mC were associated with female gender (P = 0.036). In addition, the global hypomethylation was associated with the poor disease-specific survival in TSCC patients (adjusted hazard ratio: 1.55, P = 0.043), especially for patients in older age group (> 50 years, P = 0.013), with moderate or poor cell differentiation (P = 0.044), early stage of disease (I-II, P = 0.046), small tumor size (T1-T2, P = 0.005), without lymph node involvement (P = 0.041), and ever received postoperative radiotherapy (P = 0.009).

CONCLUSIONS

Global hypomethylation was an independent biomarker for the development and poor prognosis of TSCC.

Cancer stem cells and radioresistance

PURPOSE

Radiation therapy has made significant contributions to cancer treatment. However, despite continuous improvements, tumor recurrence and therapy resistance still occur in a high proportion of patients. One underlying reason for this radioresistance might be attributable to the presence of cancer stem cells (CSC). The purpose of this review is to discuss CSC-specific mechanisms that confer radiation resistance.

CONCLUSIONS

We focus our discussions on breast cancer and glioblastoma multiforme (GBM) and conclude that both CSC-intrinsic and CSC-extrinsic factors as well as adaptive responses in CSC caused by irradiation and microenvironmental changes all make contributions to CSC-mediated radioresistance. Our discussions emphasize CSC as novel therapeutic targets in order to potentiate radiotherapy efficacy.

Breast cancer adaptive resistance: HER2 and cancer stem cell repopulation in a heterogeneous tumor society

Purpose

The lethal effects of cancer are associated with the enhanced tumor aggressiveness in recurrent and metastatic lesions that show resistant phenotype to anti-cancer therapy, a major barrier to improving overall survival of cancer patients. The presence of heterogeneous populations of cancer cells within a specific tumor including the tumor-initiating cells or so-called cancer stem cells (CSCs) has linked the acquired resistance (AR, or adaptive resistance). Herein, we discuss the CSC-mediated tumor repopulation in AR of breast cancer in this review.

Methods

We emphasize a dynamic feature of gene induction in tumor cells that undergo long-term treatment, and describe a specific HER2-NF-κB-HER2 pro-survival pathway that can be initiated in breast CSCs upon radiation therapy.

Results

Elucidation of HER2-induced pro-survival networks, specifically the force driving tumor repopulation due to radioresistant CSCs during anticancer therapies, will have a significant impact on the generation of new diagnostic and therapeutic targets to control of recurrent and metastatic breast tumors.

Zoledronic acid significantly enhances radiation‑induced apoptosis against human fibrosarcoma cells by inhibiting radioadaptive signaling

Zoledronic acid (ZOL), a third-generation bisphosphonate, inhibits bone resorption, as well as exhibiting direct antitumor activity. To date, however, the combined effects of ZOL and ionizing radiation (IR) have not been assessed in patients with soft tissue sarcoma. We have, therefore, assessed the combined effects of ZOL and IR in fibrosarcoma cells. HT1080 fibrosarcoma cells were treated with ZOL and/or IR, together or sequentially and the antitumor effects were assessed. We found that ZOL significantly enhanced IR-induced apoptosis, especially when cells were treated with ZOL followed by IR. We, therefore, assessed the detailed mechanism of sequential treatment with ZOL and IR. Cells in G2 and M phases, the most radiosensitive phases of the cell cycle, were not increased by low concentrations of ZOL. However, the levels of expression of Akt, ERK1/2 and NF-κB proteins, all of which are related to radioadaptive resistance, were increased within a short time after irradiation with 3 Gy, and this expression was inhibited by a low concentration of ZOL, which blocked the prenylation of small GTPases. This sequential treatment also increased the generation of reactive oxygen species (ROS). These results suggest that the combination of ZOL with IR may be beneficial in treating patients with soft tissue sarcoma.

Telomere loss, not average telomere length, confers radiosensitivity to TK6-irradiated cells

Many and varied are the proposed mechanisms that lead to resistance to ionizing radiation treatment. Among them, an inverse relationship between telomere length and radioresistance has been recently advanced. Investigating such a relationship in TK6 lymphoblasts, we found that clones originating from cells survived to 4Gy of X-rays showed a significantly higher telomere length when compared with clones grown from untreated cells. The lengthening observed was not attributable to a radiation-induced increase in telomerase activity, as demonstrated by TRAP assay performed in the dose range of 1-10Gy. Given the evidence that TK6 whole population was characterized by heterogeneity in cellular mean telomere length and telomere loss, we tested the hypothesis that a process of selection may favour cells with longer telomeres (more radioresistant cells) following exposure to irradiation. In order to do this 15 independent TK6 clones were selected and characterized for telomere length and loss on the basis of q-FISH and flow-FISH analysis. Among the screened clones four characterized by long telomeres and four characterized by short telomeres were tested for their radiosensitivity by means of clonogenic assay. The results obtained showed that, in our experimental conditions (cellular model, radiation doses) no significant correlation was observed between radiosensitivity and mean telomere lengths, whereas a positive correlation was observed with respect to telomere loss. Overall, these results indicate that telomere loss and not mean telomere length plays a critical role in the phenomenon of radiosensitivity/radioresistance.

Nuclear factor-kappaB and manganese superoxide dismutase mediate adaptive radioresistance in low-dose irradiated mouse skin epithelial cells

Mechanisms governing inducible resistance to ionizing radiation in untransformed epithelial cells pre-exposed to low-dose ionizing radiation (LDIR; </=10 cGy) are not well understood. The present study provides evidence that pre-exposure to 10 cGy X-rays increases clonogenic survival of mouse skin JB6P+ epithelial cells subsequently exposed to 2 Gy doses of gamma-rays. To elucidate the molecular pathways of LDIR-induced adaptive radioresistance, the transcription factor nuclear factor-kappaB (NF-kappaB) and a group of NF-kappaB-related proteins [i.e., p65, manganese superoxide dismutase (MnSOD), phosphorylated extracellular signal-regulated kinase, cyclin B1, and 14-3-3zeta] were identified to be activated as early as 15 min after LDIR. Further analysis revealed that a substantial amount of both 14-3-3zeta and cyclin B1 accumulated in the cytoplasm at 4 to 8 h when cell survival was enhanced. The nuclear 14-3-3zeta and cyclin B1 were reduced and increased at 4 and 24 h, respectively, after LDIR. Using YFP-fusion gene expression vectors, interaction between 14-3-3zeta and cyclin B1 was visualized in living cells, and LDIR enhanced the nuclear translocation of the 14-3-3zeta/cyclin B1 complex. Treatment of JB6P+ cells with the NF-kappaB inhibitor IMD-0354 suppressed LDIR-induced expression of MnSOD, 14-3-3zeta, and cyclin B1 and diminished the adaptive radioresistance. In addition, treatment with small interfering RNA against mouse MnSOD was shown to inhibit the development of LDIR-induced radioresistance. Together, these results show that NF-kappaB, MnSOD, 14-3-3zeta, and cyclin B1 contribute to LDIR-induced adaptive radioresistance in mouse skin epithelial cells.

Nuclear factor-kappaB p65 inhibits mitogen-activated protein kinase signaling pathway in radioresistant breast cancer cells

The molecular mechanism by which tumor cells increase their resistance to therapeutic radiation remains to be elucidated. We have previously reported that activation of nuclear factor-kappaB (NF-kappaB) is causally associated with the enhanced cell survival of MCF+FIR cells derived from breast cancer MCF-7 cells after chronic exposure to fractionated ionizing radiation. The aim of the present study was to reveal the context of NF-kappaB pathways in the adaptive radioresistance. Using cell lines isolated from MCF+FIR populations, we found that the elevated NF-kappaB activity was correlated with enhanced clonogenic survival, and increased NF-kappaB subunit p65 levels were associated with a decrease in phosphorylation of mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase (MEK)/ERK in all radioresistant MCF+FIR cell lines. Further irradiation with 30 fractions of radiation also inhibited MEK/ERK phosphorylation in paired cell lines of MCF+FIR and parental MCF-7 cells. Activation of ataxia-telangiectasia mutated (ATM) protein, a sensor to radiation-induced DNA damage, was elevated with increased interaction with NF-kappaB subunits p65 and p50. The interaction between p65 and MEK was also enhanced in the presence of activated ATM. In contrast, both interaction and nuclear translocation of p65/ERK were reduced. Inhibition of NF-kappaB by overexpression of mutant IkappaB increased ERK phosphorylation. In addition, MEK/ERK inhibitor (PD98059) reduced the interaction between p65 and ERK. Taken together, these results suggest that NF-kappaB inhibits ERK activation to enhance cell survival during the development of tumor adaptive radioresistance.

Radiation-induced genomic instability is associated with DNA methylation changes in cultured human keratinocytes

The mechanism by which radiation-induced genomic instability is initiated, propagated and effected is currently under intense scrutiny. We have investigated the potential role of altered genomic methylation patterns in the cellular response to irradiation and have found evidence for widespread dysregulation of CpG methylation persisting up to 20 population doublings post-irradiation. Similar effects are seen with cells treated with medium from irradiated cells (the 'bystander effect') rather than subjected to direct irradiation. Using an arbitrarily primed methylation sensitive PCR screening method we have demonstrated that irradiation causes reproducible alterations in the methylation profile of a human keratinocyte cell line, HPV-G, and have further characterised one of these sequences as being a member of a retrotransposon element derived sequence family on chromosome 7; MLT1A. Multiple changes were also detected in the screen, which indicate that although the response of cells is predominantly hypermethylation, specific hypomethylation occurs as well. Sequence specific changes are also reported in the methylation of the pericentromeric SAT2 satellite sequence. This is the first demonstration that irradiation results in the induction of heritable methylation changes in mammalian cells, and provides a link between the various non-radiological instigators of genomic instability, the perpetuation of the unstable state and several of its manifestations.

Ionizing radiation induces delayed hyperrecombination in Mammalian cells

Exposure to ionizing radiation can result in delayed effects that can be detected in the progeny of an irradiated cell multiple generations after the initial exposure. These effects are described under the rubric of radiation-induced genomic instability and encompass multiple genotoxic endpoints. We have developed a green fluorescence protein (GFP)-based assay and demonstrated that ionizing radiation induces genomic instability in human RKO-derived cells and in human hamster hybrid GM10115 cells, manifested as increased homologous recombination (HR). Up to 10% of cells cultured after irradiation produce mixed GFP(+/-) colonies indicative of delayed HR or, in the case of RKO-derived cells, mutation and deletion. Consistent with prior studies, delayed chromosomal instability correlated with delayed reproductive cell death. In contrast, cells displaying delayed HR showed no evidence of delayed reproductive cell death, and there was no correlation between delayed chromosomal instability and delayed HR, indicating that these forms of genome instability arise by distinct mechanisms. Because delayed hyperrecombination can be induced at doses of ionizing radiation that are not associated with significantly reduced cell viability, these data may have important implications for assessment of radiation risk and understanding the mechanisms of radiation carcinogenesis.

Radiation Research Society. 1952-2002. Historical and current highlights in radiation biology: has anything important been learned by irradiating cells?

Around 30 years ago, a very prominent molecular biologist confidently proclaimed that nothing of fundamental importance has ever been learned by irradiating cells! The poor man obviously did not know about discoveries such as DNA repair, mutagenesis, connections between mutagenesis and carcinogenesis, genomic instability, transposable genetic elements, cell cycle checkpoints, or lines of evidence historically linking the genetic material with nucleic acids, or origins of the subject of oxidative stress in organisms, to name a few things of fundamental importance learned by irradiating cells that were well known even at that time. Early radiation studies were, quite naturally, phenomenological. They led to the realization that radiations could cause pronounced biological effects. This was followed by an accelerating expansion of investigations of the nature of these radiobiological phenomena, the beginnings of studies aimed toward better understanding the underlying mechanisms, and a better appreciation of the far-reaching implications for biology, and for society in general. Areas of principal importance included acute tissue and tumor responses for applications in medicine, whole-body radiation effects in plants and animals, radiation genetics and cytogenetics, mutagenesis, carcinogenesis, cellular radiation responses including cell reproductive death, cell cycle effects and checkpoint responses, underlying molecular targets leading to biological effects, DNA repair, and the genetic control of radiosensitivity. This review summarizes some of the highlights in these areas, and points to numerous examples where indeed, many things of considerable fundamental importance have been learned by irradiating cells.

HOXD3 enhances motility and invasiveness through the TGF-beta-dependent and -independent pathways in A549 cells

Homeobox genes regulate sets of genes that determine cellular fates in embryonic morphogenesis and maintenance of adult tissue architecture by regulating cellular motility and cell-cell interactions. Our previous studies showed that a specific member, HOXD3, when overexpressed, upregulates integrin beta3 expression in human erythroleukemia HEL cells and lung cancer A549 cells, and enhances their motility and invasiveness. We performed a microarray study of over 7075 genes to determine the mechanisms underlying the HOXD3-enhanced motility and invasiveness in A549 cells. RT-PCR-based tracking gene analyses highlighted a set of TGF-beta-upregulated genes, which included matrix metalloproteinase-2, syndecan-1, CD44, and TGF-beta-induced 68 kDa protein. Exogenous TGF-beta also caused this pattern of upregulation in A549 cells and enhanced their migratory and invasive activity, confirming the involvement of TGF-beta signaling. However, HOXD3 reduced the expression of TGF-beta-independent genes coding for desmosomal components such as desmoglein, desmoplakin and plakoglobin which are known to suppress tumor invasion and metastasis. These results suggest that HOXD3 enhances the invasive and metastatic potential of cancer cells through the TGF-beta-dependent and -independent pathways.

Genomic instability: potential contributions to tumour and normal tissue response, and second tumours, after radiotherapy

PURPOSE

Induced genomic instability generally refers to a type of damage which is transmissible down cell generations, and which results in a persistently enhanced frequency of de novo mutations, chromosomal abnormalities or lethality in a significant fraction of the descendant cell population. The potential contribution of induced genomic instability to tumour and normal tissue response, and second tumours, after radiotherapy, is explored.

RESULTS

The phenomenon of spontaneous genomic instability is well known in some rare genetic diseases (e.g. Gorlin's syndrome), and there is evidence in such cases that it can lead to a greater propensity for carcinogenesis (with shortened latency) which is enhanced after irradiation. It is unclear what role induced genomic instability plays in the response of normal individuals, but persistent chromosomal instability has been detected in vivo in lymphocytes and keratinocytes from irradiated normal individuals. Such induced genomic instability might play some role in tumour response in a subset of tumours with specific defects in damage response genes, but again its contribution to radiocurability in the majority of cancer patients is unclear. In normal tissues, genomic instability induced in wild-type cells leading to delayed cell death might contribute to more severe or prolonged early reactions as a consequence of increased cell loss, a longer time required for recovery, and greater residual injury. In tumours, induced genomic instability reflected in delayed reductions in clonogenic capacity might contribute to the radiosensitivity of primary tumours, and also to a lower incidence, longer latency and slower growth rate of recurrences and metastases.

CONCLUSIONS

The evidence which is reviewed shows that there is little information at present to support these propositions, but what exists is consistent with their expectations. Also, it is not yet clear to what extent mutations associated with genomic instability, particularly gene polymorphisms, or other low penetrant gene mutations, contribute to the recognized spectrum of normal tissue radiosensitivity amongst cancer patients, or in the general population. Tests for such genetic modifications may help in the search for more accurate prognostic markers of response, which hopefully could be used in addition to other strategies to further improve the outcome for cancer patients given radiotherapy.

Tumor specific modulation of KU70/80 DNA binding activity in breast and bladder human tumor biopsies

The Ku70/80 heterodimer is the regulatory subunit of the DNA-dependent protein kinase (DNA-PK) and its DNA-binding activity mediates DNA double-strand breaks repair. Although Ku80 was recently proposed as a caretaker gene involved in the control of genome integrity, no data are available on Ku70/80 DNA-binding activity in human tumors. Heterodimer DNA-binding activity and protein expression were assayed by electrophoretic-mobility-shift-assay (EMSA) and Western blot analysis, in nuclear and cytoplasmic extracts from eight breast, seven bladder primary tumors and three metastatic nodes from breast cancers. Corresponding normal tissues of the same patients were used as controls. Ten out of 15 tumors showed nuclear Ku-binding activity 3-10 times higher than in the normal tissues, irrespective of bladder or breast origin. Conversely, in 5/15 primary tumors and in all the metastatic nodes analysed, nuclear Ku-activity was 1.5-4.5-fold lower than in the corresponding normal tissues. Cytoplasmic heterodimer activity significantly differed between tumor and normal tissues, displaying a 2-10-fold increase in neoplastic tissues. Three different patterns combining both Ku expression and activity with tumor characteristics were identified. In low aggressive breast tumors p70/p80 proteins were expressed in tumor but not in normal tissues. The heterodimer binding-activity matched the protein levels. In non-invasive bladder carcinomas no significant differences in protein expression between tumor and the corresponding normal tissues were found, however heterodimer binding-activity was increased in tumor samples. In breast and bladder tumors, at the advanced stage and in node metastases, the binding activity was strongly reduced in tumor biopsies, however no differences were demonstrated between normal and tumor protein levels. Our results suggest a different modulation of Ku70/80 DNA-binding activity in human neoplastic tissues, possibly related to tumor progression. Findings provide further data on tissue-specific protein expression and post-translational regulation of heterodimer activity.

A role for chromosomal instability in the development of and selection for radioresistant cell variants

Chromosome instability is a common occurrence in tumour cells. We examined the hypothesis that the elevated rate of mutation formation in unstable cells can lead to the development of clones of cells that are resistant to the cancer therapy. To test this hypothesis, we compared chromosome instability to radiation sensitivity in 30 independently isolated clones of GM10115 human-hamster hybrid cells. There was a broader distribution of radiosensitivity and a higher mean SF(2)in chromosomally unstable clones. Cytogenetic and DNA double-strand break rejoining assays suggest that sensitivity was a function of DNA repair efficiency. In the unstable population, the more radioresistant clones also had significantly lower plating efficiencies. These observations suggest that chromosome instability in GM10115 cells can lead to the development of cell variants that are more resistant to radiation. In addition, these results suggest that the process of chromosome breakage and recombination that accompanies chromosome instability might provide some selective pressure for more radioresistant variants.

Relative effectiveness of HZE iron-56 particles for the induction of cytogenetic damage in vivo

One of the risks of prolonged manned space flight is the exposure of astronauts to radiation from galactic cosmic rays, which contain heavy ions such as (56)Fe. To study the effects of such exposures, experiments were conducted at the Brookhaven National Laboratory by exposing Wistar rats to high-mass, high-Z, high-energy (HZE) particles using the Alternating Gradient Synchrotron (AGS). The biological effectiveness of (56)Fe ions (1000 MeV/nucleon) relative to low-LET gamma rays and high-LET alpha particles for the induction of chromosome damage and micronuclei was determined. The mitotic index and the frequency of chromosome aberrations were evaluated in bone marrow cells, and the frequency of micronuclei was measured in cells isolated from the trachea and the deep lung. A marked delay in the entry of cells into mitosis was induced in the bone marrow cells that decreased as a function of time after the exposure. The frequencies of chromatid aberrations and micronuclei increased as linear functions of dose. The frequency of chromosome aberrations induced by HZE particles was about 3.2 times higher than that observed after exposure to (60)Co gamma rays. The frequency of micronuclei in rat lung fibroblasts, lung epithelial cells, and tracheal epithelial cells increased linearly, with slopes of 7 x 10(-4), 12 x 10(-4), and 11 x 10(-4) micronuclei/binucleated cell cGy(-1), respectively. When genetic damage induced by radiation from (56)Fe ions was compared to that from exposure to (60)Co gamma rays, (56)Fe-ion radiation was between 0.9 and 3.3 times more effective than (60)Co gamma rays. However, the HZE-particle exposures were only 10-20% as effective as radon in producing micronuclei in either deep lung or tracheal epithelial cells. Using microdosimetric techniques, we estimated that 32 cells were hit by delta rays for each cell that was traversed by the primary HZE (56)Fe particle. These calculations and the observed low relative effectiveness of the exposure to HZE particles suggest that at least part of the cytogenetic damage measured was caused by the delta rays. Much of the energy deposited by the primary HZE particles may result in cell killing and may therefore be "wasted" as far as production of detectable micronuclei is concerned. The role of wasted energy in studies of cancer induction may be important in risk estimates for exposure to HZE particles.

Perpetuating radiation-induced chromosomal instability

Chromosomal instability is characterized by an increased rate of chromatid and chromosome rearrangements occurring multiple generations after exposure to ionizing radiation. Although chromosomal instability has been observed in many different cell types after exposure to ionizing radiation, the mechanisms perpetuating the manifestation of this instability phenotype are unknown. This review explores the current evidence concerning the role of dicentric-mediated bridge-breakage-refusion cycles and the role of recombination at interstitial telomere bands in mediating chromosomal instability. In addition, the contribution of genetic and epigenetic factors are discussed with regard to the predisposition of cells to chromosomal instability.

Radiation induced chromosomal instability in human T-lymphocytes

Chromosomal instability in proliferating mammalian cells is characterized by a persistent increase of chromosomal aberrations and rearrangements occurring de novo during successive cell generations. Recent results from many laboratories using a variety of cells and cytogenetic end points show that this phenotype can be induced by low as well as high LET irradiation. A typical feature of chromosomal instability in primary human G0-lymphocytes exposed to gamma-irradiation at both high dose rate (45 Gy h-1) and low dose rate (0.024 Gy h-1) is the appearance of novel aberrations in the clonal progeny of the irradiated cell, many generations after the exposure. The same phenotype was observed in lymphocytes that were allowed to recover for 5 days in G0 after the radiation exposure, as well as in hprt-mutant T cell clones. These results demonstrate that neither the acute genotoxic stress caused by high dose rate as compared to low dose rate irradiation, nor a hypothesized conflict between mitogen induced growth stimulation and growth arrest due to radiation damage, seem to be critical conditions for the development chromosomal instability in these cells. In contrast to observations in other cells, no evidence of a persistent decrease of cloning ability was observed in the progeny of radiation-exposed human lymphocytes, and no alteration was observed in their sensitivity to a second radiation exposure. Furthermore, the frequency of CA-repeat length variation at three loci was not increased in the progeny of X-irradiated T cells as compared to non-irradiated cells, which indicates that microsatellite instability is not part of the chromosomal instability phenotype in human T-lymphocytes.

Elevated recombination in immortal human cells is mediated by HsRAD51 recombinase

Normal diploid cells have a limited replicative potential in culture, with progressively increasing interdivision time. Rarely, cell lines arise which can divide indefinitely; like tumor cells, such "immortal" lines display frequent chromosomal aberrations which may reflect high rates of recombination. Recombination frequencies within a plasmid substrate were 3.5-fold higher in nine immortal human cell lines than in six untransformed cell strains. Expression of HsRAD51, a human homolog of the yeast RAD51 and Escherichia coli recA recombinase genes, was 4.5-fold higher in immortal cell lines than in mortal cells. Stable transformation of human fibroblasts with simian virus 40 large T antigen prior to cell immortalization increased both chromosomal recombination and the level of HsRAD51 transcripts by two- to fivefold. T-antigen induction of recombination was efficiently blocked by introduction of HsRAD51 antisense (but not control) oligonucleotides spanning the initiation codon, implying that HsRAD51 expression mediates augmented recombination. Since p53 binds and inactivates HsRAD51, T-antigen-p53 association may block such inactivation and liberate HsRAD51. Upregulation of HsRAD51 transcripts in T-antigen-transformed and other immortal cells suggests that recombinase activation can also occur at the RNA level and may facilitate cell transformation to immortality.

The effect of different TP53 mutations on the chromosomal stability of a human colonic adenoma derived cell line with endogenous wild type TP53 activity, before and after DNA damage

We examined the effect of loss of wild type TP53 activity on the chromosomal stability of a human colonic adenoma derived cell line (designated AA/Cl) by studying transfected variants which express different TP53 mutations. Using gross chromosomal aberrations as a measure of instability, we studied metaphase spreads of a vector control cell line (AA/PCMV) and variants expressing the 143(Val-Ala) mutation, which retain endogenous wild type TP53 activity, or the 273(Arg-His) TP53 mutation, which acts as a dominant negative. It was found that the proportion of cells with more than 4% aberrations was significantly greater in the AA/273p53/B cell line (an approximate 5-Fold increase) than in the vector control or the AA/143p53/A cell line. To investigate whether loss of TP53 dependent checkpoints also predisposed the cells to accumulate persistent chromosomal aberrations after DNA damage, cells were exposed to 5 Gy gamma radiation. Regardless of TP53 status, cells with radiation induced chromosomal damage were eliminated through a TP53 independent mechanism, suggesting that loss of TP53 activity did not permit the survival of these cells. In contrast, when exposed to low level gamma radiation (0.2 Gy), decreased wild type TP53 function and/or expression of mutant TP53 protein led to increased radioresistance (both in the non-dominant as well as the dominant mutant expressing cell lines). These findings suggest that loss of TP53 activity and/or acquisition of specific TP53 mutations can increase chromosomal instability and resistance to low level DNA damage in human colonic adenoma cells. This study emphasises the different biological consequences of individual TP53 mutations on the genotype of premalignant colorectal epithelial cells and subsequent implications for tumorigenic progression.

The tumor suppressor p53 modifies mutational processes in a human lymphoblastoid cell line

Abnormalities in the p53 gene play an important role in genomic instability and tumorigenesis. Our previous work showed that p53 status is correlated with differential mutability in two closely related human lymphoblastoid cell lines, TK6 and WTK1. WTK1 cells, which contain a mutation in p53 (p53Ile237) show a remarkably increased mutability, larger genetic alterations at the thymidine kinase locus (tk), an increased ability to catalyze recombination, and a delay in the onset of apoptosis after X-irradiation, compared to TK6 (p53 +/+). In the present study, we demonstrate that after transfection and subsequent overexpression of the known dominant negative mutant p53 Ala143 allele (mp53Ala143) in TK6, there were significantly enhanced spontaneous and X-ray-induced mutant frequencies at the tk locus, and delayed onset of X-ray-induced apoptosis, to a similar extent as in WTK1. In addition, high protein expression of mp53Ala143 in transfectants was correlated with both increased mutation frequency and altered apoptosis kinetics. Similar results were obtained with p53 Ile237 transfection into TK6. Our observations indicate that the product of the p53 gene affects mutational processes. We hypothesize that p53 dysfunction can lead to increased mutagenicity at the endogenous tk gene in human lymphoblastoid cell lines either through delayed apoptosis in response to DNA damage or by mediating increased recombination.

Radioresistance, chemoresistance, and apoptosis resistance. The past, present, and future

A wide variety of cancer therapy resistance mechanisms are available, with perhaps surprising overlap between causes of chemotherapy and radiation resistance. As previously discussed, most of the widely used chemotherapeutic agents and gamma-radiation utilize apoptosis as the common death pathway, thereby making apoptosis and the ability to manipulate apoptosis of integral importance to the future of cancer therapy. This is evidenced by the information explosion on this topic in the last 5 years. Obviously much remains to be learned. However, with the aforementioned cooperation of the oncology team and a greater understanding of the molecular mechanisms behind cancer and cancer therapy resistance, response rates, survival times, the quality of life of patients enduring antineoplastic therapies, and the importance of comparative oncology can only increase. We look forward to the findings in the coming years on this incredibly clinically relevant topic with great fondness and uncertainty.

The mutation rate and cancer

The selection of advantageous mutations underlies tumorigenesis. The growth of a tumor is therefore a form of evolution at the somatic level, in which the population is comprised of individual cells within the tumor. Models of tumorigenesis have considered the relative importance of mutation and selection. We show that selection is more important than an increased mutation rate in the growth of a tumor. Some cancers may acquire a "mutator phenotype," probably leading to faster growth, but mutator phenotypes are not necessary for carcinogenesis.

The p53 gene as a modifier of intrinsic radiosensitivity: implications for radiotherapy

BACKGROUND AND PURPOSE

Experimental studies have implicated the normal or "wild type' p53 protein (i.e. WTp53) in the cellular response to ionizing radiation and other DNA damaging agents. Whether altered WTp53 protein function can lead to changes in cellular radiosensitivity and/or clinical radiocurability remains an area of ongoing study. In this review, we describe the potential implications of altered WTp53 protein function in normal and tumour cells as it relates to clinical radiotherapy, and describe novel treatment strategies designed to re-institute WTp53 protein function as a means of sensitizing cells to ionizing radiation.

METHODS AND MATERIALS

A number of experimental and clinical studies are critically reviewed with respect to the role of the p53 protein as a determinant of cellular oncogenesis, genomic stability, apoptosis, DNA repair and radioresponse in normal and transformed mammalian cells.

RESULTS

In normal fibroblasts, exposure to ionizing radiation leads to a G1 cell cycle delay (i.e. a "G1 checkpoint') as a result of WTp53 mediated inhibition of G1-cyclin-kinase and retinoblastoma (pRb) protein function. The G1 checkpoint response is absent in tumour cells which express a mutant form of the p53 protein (i.e. MTp53), leading to acquired radioresistance in vitro. Depending on the cell type studied, this increase in cellular radiation survival can be mediated through decreased radiation-induced apoptosis, or altered kinetics of the radiation-induced G1 checkpoint. Recent biochemical studies support an indirect role for the p53 protein in both nucleotide excision and recombinational DNA repair pathways. However, based on clinicopathologic data, it remains unclear as to whether WTp53 protein function can predict for human tumour radiocurability and normal tissue radioresponse.

CONCLUSIONS

Alterations in cell cycle control secondary to aberrant WTp53 protein function may be clinically significant if they lead to the acquisition of mutant cellular phenotypes, including the radioresistant phenotype. Pre-clinical studies suggest that these phenotypes may be reversed using adenovirus-mediated gene therapy or pharmacologic strategies designed to re-institute WTp53 protein function. Our analysis of the published data strongly argues for the use of functional assays for the determination of WTp53 protein function in studies which attempt to correlate normal and tumour tissue radioresponse with p53 genotype, or p53 protein expression.

Effects of ionizing- and UV B-radiation on proteins controlling cell cycle progression in human cells: comparison of the MCF-7 adenocarcinoma and the SCL-2 squamous cell carcinoma cell line

MCF-7 and SCL-2 cells were irradiated with UV B-radiation or with 137Cs gamma-radiation, in order to investigate cell cycle checkpoint control mechanisms. Effects of both qualities of radiation were investigated for the two cell lines in regard to p53 protein levels, and alterations in Cdk1 (cyclin dependent kinase 1) and Cdk2 phosphorylation were monitored. SCL-2 cells constitutively overexpressed a form of p53 protein whose abundance remained unchanged after irradiation, whereas MCF-7 cells expressed wild type p53 whose abundance increased after irradiation. Accordingly, MCF-7 cells showed a strong G1 phase arrest, whereas SCL-2 cells were only delayed in S phase (after UV B-irradiation) and arrested in G2 phase (after gamma-irradiation and UV B-irradiation), as monitored by flow cytometry. In MCF-7 cells increased p53 levels were observed for up to 30 h after gamma-irradiation and up to 20 h after UV B-irradiation. Only in SCL-2 cells was there a significant radiation induced inactivation of Cdk1 by hyperphosphorylation. This effect was prevented by culturing cells in the presence of caffeine after irradiation. After UV B-irradiation the inactivation of Cdk1 was less pronounced and only partially diminished in the presence of caffeine. No alteration in Cdk2 phosphorylation was observed after irradiation in either cell line.