Spontaneous and radiation-induced differentiationof fibroblasts

Management of Graves' ophthalmopathy by radiotherapy: A literature review

Orbital radiotherapy for Graves' ophthalmopathy is an example of non-oncological radiotherapy. First introduced in the 1930s, this treatment has become widely used since the 1980s with several studies showing proof of both effectiveness and safety: a decrease of soft tissue involvement in 70 to 80% of patients and an improvement of ocular mobility in 30 to 80% of patients. Nowadays, it's one of the second line treatment options recognized by the European Group on Graves' orbitopathy in the management of a moderate to severe and active disease after failure of glucocorticoids. In that setting, orbital radiotherapy should be combined with glucocorticoids. To our knowledge, there are no practical recommendations on how orbital radiotherapy should be planned and conducted for Graves' ophthalmopathy. Optimal dose is not defined however the most frequent regimen consists of 20Gy in ten fractions of 2Gy, though other options may yield better results. Lastly, the use of modern technique of radiotherapy such as intensity-modulated radiation therapy may allow a better sparing of organs at risk compared to three-dimensional radiotherapy using lateral opposing fields.

The Effects of Ionising and Non-Ionising Electromagnetic Radiation on Extracellular Matrix Proteins

Exposure to sub-lethal doses of ionising and non-ionising electromagnetic radiation can impact human health and well-being as a consequence of, for example, the side effects of radiotherapy (therapeutic X-ray exposure) and accelerated skin ageing (chronic exposure to ultraviolet radiation: UVR). Whilst attention has focused primarily on the interaction of electromagnetic radiation with cells and cellular components, radiation-induced damage to long-lived extracellular matrix (ECM) proteins has the potential to profoundly affect tissue structure, composition and function. This review focuses on the current understanding of the biological effects of ionising and non-ionising radiation on the ECM of breast stroma and skin dermis, respectively. Although there is some experimental evidence for radiation-induced damage to ECM proteins, compared with the well-characterised impact of radiation exposure on cell biology, the structural, functional, and ultimately clinical consequences of ECM irradiation remain poorly defined.

Radiation Induces Valvular Interstitial Cell Calcific Response in an in vitro Model of Calcific Aortic Valve Disease

Background: Mediastinal ionizing radiotherapy is associated with an increased risk of valvular disease, which demonstrates pathological hallmarks similar to calcific aortic valve disease (CAVD). Despite advances in radiotherapy techniques, the prevalence of comorbidities such as radiation-associated valvular disease is still increasing due to improved survival of patients receiving radiotherapy. However, the mechanisms of radiation-associated valvular disease are largely unknown. CAVD is considered to be an actively regulated disease process, mainly controlled by valvular interstitial cells (VICs). We hypothesize that radiation exposure catalyzes the calcific response of VICs and, therefore, contributes to the development of radiation-associated valvular disease.

Methods and Results: To delineate the relationship between radiation and VIC behavior (morphology, calcification, and matrix turnover), two different in vitro models were established: (1) VICs were cultured two-dimensional (2D) on coverslips in control medium (CM) or osteogenic medium (OM) and irradiated with 0, 2, 4, 8, or 16 Gray (Gy); and (2) three-dimensional (3D) hydrogel system was designed, loaded with VICs and exposed to 0, 4, or 16 Gy of radiation. In both models, a dose-dependent decrease in cell viability and proliferation was observed in CM and OM. Radiation exposure caused myofibroblast-like morphological changes and differentiation of VICs, as characterized by decreased αSMA expression. Calcification, as defined by increased alkaline phosphatase activity, was mostly present in the 2D irradiated VICs exposed to 4 Gy, while after exposure to higher doses VICs acquired a unique giant fibroblast-like cell morphology. Finally, matrix turnover was significantly affected by radiation exposure in the 3D irradiated VICs, as shown by decreased collagen staining and increased MMP-2 and MMP-9 activity.

Conclusions: The presented work demonstrates that radiation exposure enhances the calcific response in VICs, a hallmark of CAVD. In addition, high radiation exposure induces differentiation of VICs into a terminally differentiated giant-cell fibroblast. Further studies are essential to elucidate the underlying mechanisms of these radiation-induced valvular changes.

Gene Expression Profiles Reveal Extracellular Matrix and Inflammatory Signaling in Radiation-Induced Premature Differentiation of Human Fibroblast in vitro

Purpose

Fibroblasts are considered to play a major role in the development of fibrotic reaction after radiotherapy and premature radiation-induced differentiation has been proposed as a cellular basis. The purpose was to relate gene expression profiles to radiation-induced phenotypic changes of human skin fibroblasts relevant for radiogenic fibrosis.

Materials and Methods

Exponentially growing or confluent human skin fibroblast strains were irradiated in vitro with 1–3 fractions of 4 Gy X-rays. The differentiated phenotype was detected by cytomorphological scoring and immunofluorescence microscopy. Microarray analysis was performed on Human Genome U133 plus2.0 microarrays (Affymetrix) with JMP Genomics software, and pathway analysis with Reactome R-package. The expression levels and kinetics of selected genes were validated with quantitative real-time PCR (qPCR) and Western blotting.

Results

Irradiation of exponentially growing fibroblast with 1 × 4 Gy resulted in phenotypic differentiation over a 5-day period. This was accompanied by downregulation of cell cycle-related genes and upregulation of collagen and other extracellular matrix (ECM)-related genes. Pathway analysis confirmed inactivation of proliferation and upregulation of ECM- and glycosaminoglycan (GAG)-related pathways. Furthermore, pathways related to inflammatory reactions were upregulated, and potential induction and signaling mechanisms were identified. Fractionated irradiation (3 × 4 Gy) of confluent cultures according to a previously published protocol for predicting the risk of fibrosis after radiotherapy showed similar downregulation but differences in upregulated genes and pathways.

Conclusion

Gene expression profiles after irradiation of exponentially growing cells were related to radiation-induced differentiation and inflammatory reactions, and potential signaling mechanisms. Upregulated pathways by different irradiation protocols may reflect different aspects of the fibrogenic process thus providing a model system for further hypothesis-based studies of radiation-induced fibrogenesis.

Radiotherapy-induced heart disease: a review of the literature

Radiotherapy as one of the four pillars of cancer therapy plays a critical role in the multimodal treatment of thoracic cancers. Due to significant improvements in overall cancer survival, radiotherapy-induced heart disease (RIHD) has become an increasingly recognized adverse reaction which contributes to major radiation-associated toxicities including non-malignant death. This is especially relevant for patients suffering from diseases with excellent prognosis such as breast cancer or Hodgkin’s lymphoma, since RIHD may occur decades after radiotherapy. Preclinical studies have enriched our knowledge of many potential mechanisms by which thoracic radiotherapy induces heart injury. Epidemiological findings in humans reveal that irradiation might increase the risk of cardiac disease at even lower doses than previously assumed. Recent preclinical studies have identified non-invasive methods for evaluation of RIHD. Furthermore, potential options preventing or at least attenuating RIHD have been developed. Ongoing research may enrich our limited knowledge about biological mechanisms of RIHD, identify non-invasive early detection biomarkers and investigate potential treatment options that might attenuate or prevent these unwanted side effects. Here, we present a comprehensive review about the published literature regarding clinical manifestation and pathological alterations in RIHD. Biological mechanisms and treatment options are outlined, and challenges in RIHD treatment are summarized.

Radiotherapy for Active Thyroid Eye Disease

PURPOSE

To critically review the published literature on orbital radiotherapy as a treatment modality for thyroid eye disease (TED).

METHODS

A systematic review and analysis of the relevant published literature was performed.

RESULTS

Thyroid eye disease is an autoimmune condition that is amenable to treatments that modulate the immune response, including orbital radiotherapy (ORT). Ideal candidates for ORT are patients in the early, active phase of TED with moderate to severe, or rapidly progressive, disease, including patients with significant motility deficits and compressive optic neuropathy. Patients with progressive strabismus may also benefit. Patients with mild or inactive disease will not benefit from ORT when compared with the natural history of the disease. Orbital radiotherapy should generally be used in conjunction with corticosteroid therapy, with response to corticosteroids demonstrating the immunomodulatory therapeutic potential of ORT. When treating TED-compressive optic neuropathy, ORT may help obviate the need for urgent surgical decompression, or postpone it until the stable, inactive phase of the disease. Orbital radiotherapy treatment doses should approach 20 Gy in most cases, but lower doses may be considered in younger patients without significant dysmotility. The safety profile of ORT is well established, and side effects are minimal in appropriately selected patients.

CONCLUSIONS

Radiotherapy is a safe and effective treatment for active TED in appropriately selected patients.

Impact of Ionizing Radiation on the Cardiovascular System: A Review

Radiation therapy has become one of the main forms of treatment for various types of cancers. Cancer patients previously treated with high doses of radiation are at a greater risk to develop cardiovascular complications later in life. The heart can receive varying doses of radiation depending on the type of therapy and can even reach doses in the range of 17 Gy. Multiple studies have highlighted the role of oxidative stress and inflammation in radiation-induced cardiovascular damage. Doses of ionizing radiation below 200 mGy, however, have been shown to have beneficial effects in some experimental models of radiation-induced damage, but low-dose effects in the heart is still debated. Low-dose radiation may promote heart health and reduce damage from oxidative stress and inflammation, however there are few studies focusing on the impact of low-dose radiation on the heart. In this review, we summarize recent studies from animal models and human data focusing on the effects and mechanism(s) of action of radiation-induced damage to the heart, as well as the effects of high and low doses of radiation and dose rates.

Basics of Radiation Biology When Treating Hyperproliferative Benign Diseases

For decades, low- and moderate-dose radiation therapy (RT) has been shown to exert a beneficial therapeutic effect in a multitude of non-malignant conditions including painful degenerative muscoloskeletal and hyperproliferative disorders. Dupuytren and Ledderhose diseases are benign fibroproliferative diseases of the hand/foot with fibrotic nodules and fascial cords, which determine debilitating contractures and deformities of fingers/toes, while keloids are exuberant scar formations following burn damage, surgery, and trauma. Although RT has become an established and effective option in the management of these diseases, experimental studies to illustrate cellular composites and factors involved remain to be elucidated. More recent findings, however, indicate the involvement of radiation-sensitive targets like mitotic fibroblasts/myofibroblasts as well as inflammatory cells. Radiation-related molecular mechanisms affecting these target cells include the production of free radicals to hamper proliferative activity and interference with growth factors and cytokines. Moreover, an impairment of activated immune cells involved in both myofibroblast proliferative and inflammatory processes may further contribute to the clinical effects. We here aim at briefly describing mechanisms contributing to a modulation of proliferative and inflammatory processes and to summarize current concepts of treating hyperproliferative diseases by low and moderate doses of ionizing radiation.

Radiogenomics: A systems biology approach to understanding genetic risk factors for radiotherapy toxicity?

Adverse reactions in normal tissue after radiotherapy (RT) limit the dose that can be given to tumour cells. Since 80% of individual variation in clinical response is estimated to be caused by patient-related factors, identifying these factors might allow prediction of patients with increased risk of developing severe reactions. While inactivation of cell renewal is considered a major cause of toxicity in early-reacting normal tissues, complex interactions involving multiple cell types, cytokines, and hypoxia seem important for late reactions. Here, we review 'omics' approaches such as screening of genetic polymorphisms or gene expression analysis, and assess the potential of epigenetic factors, posttranslational modification, signal transduction, and metabolism. Furthermore, functional assays have suggested possible associations with clinical risk of adverse reaction. Pathway analysis incorporating different 'omics' approaches may be more efficient in identifying critical pathways than pathway analysis based on single 'omics' data sets. Integrating these pathways with functional assays may be powerful in identifying multiple subgroups of RT patients characterised by different mechanisms. Thus 'omics' and functional approaches may synergise if they are integrated into radiogenomics 'systems biology' to facilitate the goal of individualised radiotherapy.

Cellular Pathways in Response to Ionizing Radiation and Their Targetability for Tumor Radiosensitization

During the last few decades, improvements in the planning and application of radiotherapy in combination with surgery and chemotherapy resulted in increased survival rates of tumor patients. However, the success of radiotherapy is impaired by two reasons: firstly, the radioresistance of tumor cells and, secondly, the radiation-induced damage of normal tissue cells located in the field of ionizing radiation. These limitations demand the development of drugs for either radiosensitization of tumor cells or radioprotection of normal tissue cells. In order to identify potential targets, a detailed understanding of the cellular pathways involved in radiation response is an absolute requirement. This review describes the most important pathways of radioresponse and several key target proteins for radiosensitization.

Mesenchymal stem cells are resistant to carbon ion radiotherapy

Mesenchymal stem cells (MSCs) participate in regeneration of tissues damaged by ionizing radiation. However, radiation can damage MSCs themselves. Here we show that cellular morphology, adhesion and migration abilities were not measurably altered by photon or carbon ion irradiation. The potential for differentiation was unaffected by either form of radiation, and established MSC surface markers were found to be stably expressed irrespective of radiation treatment. MSCs were able to efficiently repair DNA double strand breaks induced by both high-dose photon and carbon ion radiation. We have shown for the first time that MSCs are relatively resistant to therapeutic carbon ion radiotherapy. Additionally, this form of radiation did not markedly alter the defining stem cell properties or the expression of established surface markers in MSCs.

Cholesteatoma-associated fibroblasts modulate epithelial growth and differentiation through KGF/FGF7 secretion

The keratinocyte growth factor (KGF/FGF7), produced by stromal cells, is a key paracrine mediator of epithelial proliferation, differentiation and migration. Expression of the growth factor is increased in wound healing and in hyperproliferative epithelial diseases, as a consequence of the activation of dermal fibroblasts by the inflammatory microenvironment. The middle ear cholesteatoma, an aural epidermal pathology characterized by keratinocyte hyperproliferation and chronic inflammation, may represent a model condition to study the epithelial-mesenchymal interactions. To develop an in vitro model for this disease, we isolated and characterized human primary cultures of fibroblasts associated with the cholesteatoma lesion, analyzing their secretory behaviour and degree of differentiation or activation. Compared to the perilesional or control normal fibroblasts, all cultures derived from cholesteatoma tissues were less proliferating and more differentiated and their highly variable activated phenotype correlated with the secretion of KGF as well as of metalloproteases 2 and 9. Culture supernatants collected from the cholesteatoma-associated fibroblasts were able to increase the proliferation and differentiation of human keratinocytes assessed by the expression of Ki67 and keratin-1 markers. The single crucial contribution of the KGF released by fibroblasts on the keratinocyte biological response was shown by the specific, although partial, block induced by inhibiting the KGF receptor or by immunoneutralizing the growth factor. Altogether, these results suggest that the activation of the stromal fibroblasts present in the pathological tissue, and the consequent increased secretion of KGF, play a crucial role in the deregulation of the epidermal proliferation and differentiation.

Stromal responses to fractionated radiotherapy

PURPOSE

Recently, several landmark randomized trials were published that justify the use of alternative fractionation schemes, e.g., hypofractionation, in adjuvant applications of whole-breast radiotherapy following breast-conserving surgery. We are studying effects of fractionated photon radiotherapy on stromal cell biology to understand how fractionation parameters influence the cellular microenvironment.

METHODS AND MATERIALS

Three-dimensional (3-D) collagen matrices, fibroblasts, and transforming growth factor beta 1 (TGF-β1) were combined to model microenvironmental components of mammary stroma. We explored the effects of fractionation schemes on collagen matrix stiffness and fibroblast activation using this culture model. Samples were exposed to 6 MV X-rays from a linear accelerator in daily fraction sizes of 90, 180 and 360 cGy over three days in a manner consistent with irradiation exposure during radiotherapy.

RESULTS

Fibroblast-cell activation and collagen sample stiffness both increased over time for all samples, but marked changes were noted when samples were irradiated and/or stimulated with growth factors in relation to the magnitude of the stimulus. We found a significant reduction in fibroblast proliferation and activation with fraction size but a modest and irreversible increase in matrix stiffness as the dose increased. Overall, larger fraction sizes reduced conditions leading to the formation of a reactive stroma.

CONCLUSION

There is a significant reduction in fibroblast activation and a modest increase in matrix stiffness with increasing fraction size over a 72-hour observation time in 3-D cultures modeling mammary stroma. However, expanded in vitro studies with more mammary components are needed to evaluate the net effects of stromal reactivity to radiotherapy. Our results suggest that the stromal cell microenvironment is an important consideration when optimizing fractionation schedules.

Effects of radiotherapy fractionation on breast stromal activity

We study the dynamics of tumor cell progression as growth factors and ionizing radiation (IR) combine to modify cellular microenvironments. Breast tumor growth depends on the behavior of cancer cells in their microenvironment, and both components are affected by IR fractionation parameters. TGF-β1 promotes differentiation of fibroblasts to myofibroblasts, which stiffens the extracellular matrix (ECM) and promotes malignant cell phenotypes. IR generates reactive oxygen species (ROS) that damages and inactivates cells thus controlling proliferation. The effects of TGF-β1 and IR at various fraction sizes on ECM stiffness and fibroblast differenation are studied using MRC-5 fibroblasts in 3-D collagen cultures.

Stromal mediation of radiation carcinogenesis

Ionizing radiation is a well-established carcinogen in human breast and rodent mammary gland. This review addresses evidence that radiation elicits the critical stromal context for cancer, affecting not only frequency but the type of cancer. Recent data from the breast tumors of women treated with radiation therapy and the cellular mechanisms evident in experimental models suggest that radiation effects on stromal-epithelial interactions and tissue composition are a major determinant of cancer development.

X-irradiation-induced cell cycle delay and DNA double-strand breaks in the murine osteoblastic cell line OCT-1

Radiation response of bone cells, especially the bone-forming osteoblasts, is an important issue for radiotherapy in young age. A radiation-induced cell cycle arrest may enhance or accelerate osteoblastic differentiation. To analyze radiation response of osteoblastic cells, the correlation between DNA double-strand break induction (DSB), cell cycle alterations and gene expression modifications after X-irradiation was investigated in the osteoblast-like cell line OCT-1. As marker of the cellular response to DSB, the temporal appearance of gamma-H2AX foci after X-irradiation was visualized. Gene expression profiles of the key cell cycle regulatory protein p21 (CDKN1A), and the most abundant growth factor in human bone, transforming growth factor beta 1 (TGF-beta1) were recorded using quantitative real-time reverse transcription PCR (qRT-PCR). The distribution of cells in the cell cycle phases G1, S and G2 was determined by propidium iodide (PI) staining and flow cytometry. Initial studies show a strong dose dependency in the number of gamma-H2AX foci shortly after X-irradiation. Exposure to 1 Gy yields approximately 36 small foci in OCT-1 cells after 30 min that became larger after 1 h of incubation; after 24 h most of the foci had disappeared. X-rays provoked a dose-dependent arrest in G2 phase of the cell cycle, accompanied by a dose-dependent gene expression regulation for p21 and TGF-beta1. As TGF-beta1 is known to affect osteoblast differentiation, matrix formation and mineralization, modulation of its expression could influence the expression of the main osteogenic transcription factor Runx2 (Cbfa1) and other osteoblast differentiation markers.

Links between innate immunity and normal tissue radiobiology

The body senses "danger" from "damaged self" molecules through members of the same receptor superfamily it uses for microbial "non-self", triggering canonical signaling pathways that lead to the generation of acute inflammatory responses. For this reason, the biology of normal tissue responses to moderate and clinically relevant doses of radiation is inextricably connected to innate immunity. The complex sequence of inflammatory events that ensues causes further cell and tissue damage to eliminate potential invaders but also leads to cytoprotective responses that limit the spread of damage and to wound healing through tissue regeneration or replacement. These sequential processes are orchestrated through multiple feedback control mechanisms involving cyclical production of free radicals and cytokines that are common to both radiation and immune signaling. This requires a concerted effort by resident tissue and inflammatory cell types, with macrophages apparently leading the way. The initial response to moderate doses of radiation therefore feeds into a pro-inflammatory paradigm whose eventual outcome is critically dependent upon the properties of the immune cells that are involved in tissue damage, regeneration and repair and that are in part under genetic influence. Importantly, these canonical pathways provide targets for interventions aimed at modifying normal tissue radiation responses. In this review, we examine areas of intersection between innate immunity and normal tissue radiobiology.

Factors to consider in the use of stem cells for pharmaceutic drug development and for chemical safety assessment

Given the reality of the inadequacies of current concepts of the mechanisms of chemical toxicities, of the various assays to predict toxicities from current molecular, biochemical, in vitro and animal bioassays, and of the failure to generate efficacious and safe chemicals for medicines, food supplements, industrial, consumer and agricultural chemicals, the recent NAS Report, "Toxicity Testing in the 21st Century: A Vision and a Strategy", has drawn attention to a renewed examination of what needs to be done to improve our current approach for better assessment of potential risk to human health. This "Commentary" provides a major paradigm challenge to the current concepts of how chemicals induce toxicities and how these various mechanisms of toxicities can contribute to the pathogenesis of some human diseases, such as birth defects and cancer. In concordance with the NAS Report to take "... advantage of the on-going revolution in biology and biotechnology", this "Commentary" supports the use of human embryonic and adult stem cells, grown in vitro under simulated "in vivo niche conditions". The human being should be viewed "as greater than the sum of its parts". Homeostatic control of the "emergent properties" of the human hierarchy, needed to maintain human health, requires complex integration of endogenous and exogenous signaling molecules that control cell proliferation, differentiation, apoptosis and senescence of stem, progenitor and differentiated cells. Currently, in vitro toxicity assays (mutagenesis, cytotoxicity, epigenetic modulation), done on 2-dimensional primary rodent or human cells (which are always mixtures of cells), on immortalized or tumorigenic rodent or human cell lines do not represent normal human cells in vivo [which do not grow on plastic and which are in micro-environments representing 3 dimensions and constantly interacting factors]. In addition, with the known genetic, gender, and developmental state of cells in vivo, any in vitro toxicity assay will need to mimic these conditions in vitro. More specifically, while tissues contain a few stem cells, many progenitor/transit cells and terminally differentiated cells, it should be obvious that both embryonic and adult stem cells would be critical "target" cells for toxicity testing. The ultimate potential for in vitro testing of human stem cells will to try to mimic a 3-D in vitro micro-environment on multiple "organ-specific and multiple genotypic/gender "adult stem cells. The role of stem cells in many chronic diseases, such as cancer, birth defects, and possibly adult diseases after pre-natal and early post-natal exposures (Barker hypothesis), demands toxicity studies of stem cells. While alteration of gene expression ("toxico-epigenomics") is a legitimate endpoint of these toxicity studies, alteration of the quantity of stem cells during development must be serious considered. If the future utility of human stem cells proves to be valid, the elimination of less relevant, expensive and time-consuming rodent and 2-D human in vitro assays will be eliminated.

Radiation carcinogenesis in context: how do irradiated tissues become tumors?

It is clear from experimental studies that genotype is an important determinant of cancer susceptibility in general, and for radiation carcinogenesis specifically. It has become increasingly clear that genotype influences not only the ability to cope with DNA damage but also influences the cooperation of other tissues, like the vasculature and immune system, necessary for the establishment of cancer. Our experimental data and that of others suggest that the carcinogenic action of ionizing radiation (IR) can also be considered a two-compartment problem: while IR can alter genomic sequence as a result of DNA damage, it can also induce signals that alter multicellular interactions and phenotypes that underpin carcinogenesis. Rather than being accessory or secondary to genetic damage, we propose that such non-targeted radiation effects create the critical context that promotes cancer development. This review focuses on experimental studies that clearly define molecular mechanisms by which cell interactions contribute to cancer in different organs, and addresses how non-targeted radiation effects may similarly act though the microenvironment. The definition of non-targeted radiation effects and their dose dependence could modify the current paradigms for radiation risk assessment since radiation non-targeted effects, unlike DNA damage, are amenable to intervention. The implications of this perspective in terms of reducing cancer risk after exposure are discussed.

Cell cycle arrest and aberration yield in normal human fibroblasts. I. Effects of X‐rays and 195 MeV u−1C ions

PURPOSE

To examine the relationship between cell proliferation and the expression of chromosomal damage in normal human skin fibroblasts after X-ray and particle irradiation.

MATERIALS AND METHODS

Confluent G0/G1 AG1522B cells were exposed to X-rays or 195MeV u(-1) C ions with a linear energy transfer of 16.6 keV microm(-1) in the dose range 1-4 Gy. Directly after irradiation, cells were reseeded at a low density in medium containing 5-bromo-2'-deoxyuridine. At multiple time points post-irradiation, the cumulative BrdU-labelling index, mitotic index and aberration frequency were measured. Based on these data, the total amount of damage induced within the entire cell population was estimated by means of mathematical analysis.

RESULTS

Both types of radiation exposure exert a pronounced effect on the cell cycle progression of fibroblasts. They result in delayed entry of cells into S-phase and into the first mitosis, and cause a dramatic reduction in mitotic activity. Measurement of chromosomal damage in first-cycle cells at multiple time points post-irradiation shows that the frequencies of aberrant cells and aberrations increase with time up to twofold for the lower doses. However, for the higher doses, this effect is less pronounced or even disappears. When the data for the whole cell population are analysed, it becomes evident that only a few damaged fibroblasts can progress to the first mitosis, a response attributable at least in part to a long-term arrest of injured cells in the initial G0/G1-phase. As observed in other investigations, the effectiveness of 195 MeV u(-1) C ions was similar or slightly higher than X-rays for all endpoints studied leading to a relative biological effectiveness in the range 1.0-1.4.

CONCLUSIONS

Cell cycle arrests affect the aberration yield observable in normal human fibroblasts at mitosis. The data obtained for the cell population as a whole reveal that injured cells are rapidly removed from the mitotically active population through a chronic cell cycle arrest, which is consistent with other studies that indicate that this response is a specific strategy of fibroblasts to minimize the fixation and propagation of genetic alterations.

DNA damage in telomeres and mitochondria during cellular senescence: is there a connection?

Cellular senescence is the ultimate and irreversible loss of replicative capacity occurring in primary somatic cell culture. It is triggered as a stereotypic response to unrepaired nuclear DNA damage or to uncapped telomeres. In addition to a direct role of nuclear DNA double-strand breaks as inducer of a DNA damage response, two more subtle types of DNA damage induced by physiological levels of reactive oxygen species (ROS) can have a significant impact on cellular senescence: Firstly, it has been established that telomere shortening, which is the major contributor to telomere uncapping, is stress dependent and largely caused by a telomere-specific DNA single-strand break repair inefficiency. Secondly, mitochondrial DNA (mtDNA) damage is closely interrelated with mitochondrial ROS production, and this might also play a causal role for cellular senescence. Improvement of mitochondrial function results in less telomeric damage and slower telomere shortening, while telomere-dependent growth arrest is associated with increased mitochondrial dysfunction. Moreover, telomerase, the enzyme complex that is known to re-elongate shortened telomeres, also appears to have functions independent of telomeres that protect against oxidative stress. Together, these data suggest a self-amplifying cycle between mitochondrial and telomeric DNA damage during cellular senescence.

Energetic heavy ions accelerate differentiation in the descendants of irradiated normal human diploid fibroblasts

Ionizing radiation-induced genomic instability has been demonstrated in a variety of endpoints such as delayed reproductive death, chromosome instability and mutations, which occurs in the progeny of survivors many generations after the initial insult. Dependence of these effects on the linear energy transfer (LET) of the radiation is incompletely characterized; however, our previous work has shown that delayed reductions in clonogenicity can be most pronounced at LET of 108 keV/microm. To gain insight into potential cellular mechanisms involved in LET-dependent delayed loss of clonogenicity, we investigated morphological changes in colonies arising from normal human diploid fibroblasts exposed to gamma-rays or energetic carbon ions (108 keV/microm). Exposure of confluent cultures to carbon ions was 4-fold more effective at inactivating cellular clonogenic potential and produced more abortive colonies containing reduced number of cells per colony than gamma-rays. Second, colonies were assessed for clonal morphotypic heterogeneity. The yield of differentiated cells was elevated in a dose- and LET-dependent fashion in clonogenic colonies, whereas differentiated cells predominated to a comparable extent irrespective of radiation type or dose in abortive colonies. The incidence of giant or multinucleated cells was also increased but much less frequent than that of differentiated cells. Collectively, our results indicate that carbon ions facilitate differentiation more effectively than gamma-rays as a major response in the progeny of irradiated fibroblasts. Accelerated differentiation may account, at least in part, for dose- and LET-dependent delayed loss of clonogenicity in normal human diploid cells, and could be a defensive mechanism that minimizes further expansion of aberrant cells.

TGFbeta-1 dependent fast stimulation of ATM and p53 phosphorylation following exposure to ionizing radiation does not involve TGFbeta-receptor I signalling

BACKGROUND AND PURPOSE

It has been proposed that radiation induced stimulation of ATM and downstream components involves activation of TGFbeta-1 and that this may be due to TGFbeta-1-receptor I-Smad signalling. Therefore, the aim of this study was to clarify the distinct role of TGFbeta-1-receptor I-Smad signalling in mediating ATM activity following radiation exposure.

MATERIALS AND METHODS

A549 cells were stably transfected with a conditionally regulatable TGFbeta-1 antisense construct (Tet-on-system) to test clonogenic activity following irradiation. Phosphorylation profile of ATM, p53, and chk2 was determined in non-cycling, serum-starved cells by immunoblotting. Likewise, A549 wild type cells were used to identify cell cycle distribution as a function of irradiation with or without pretreatment with CMK, a specific inhibitor of furin protease involved in activation of latent TGFbeta-1. Furthermore Western and immunoblot analyses were performed on serum-starved cells to investigate the dependence of ATM- and p53-stimulation on TGFbeta-1-receptor I-Smad signalling by applying a specific TGFbeta-1-receptor I inhibitor.

RESULTS

Knock down of TGFbeta-1 by an antisense construct significantly increased clonogenic cell survival following exposure to ionizing radiation. Likewise, CMK treatment diminished the radiation induced G1 arrest of A549 cells. Moreover, both TGFbeta-1-knock down as well as CMK treatment inhibited the fast post-radiation phosphorylation of ATM, p53, and chk2. However, as shown by the use of a specific inhibitor TGFbeta-1-receptor I-Smad signalling was not involved in this fast activation of ATM and p53.

CONCLUSIONS

We confirm that TGFbeta-1 plays a critical role in the stimulation of ATM- and p53 signalling in irradiated cells. However, this fast stimulation seems not to be dependent on activation of TGFbeta-1-receptor I-Smad signalling as recently proposed.

Interrelation amongst differentiation, senescence and genetic instability in long-term cultures of fibroblasts exposed to different radiation qualities

BACKGROUND AND PURPOSE

The goal of the present study was to investigate aging and genetic instability in the progeny of human fibroblasts exposed to X-rays and carbon ions.

MATERIALS AND METHODS

Following irradiation, cells were regularly subcultured until senescence. At selected time-points BrdU-labelling index, expression of cell cycle related proteins, cell differentiation pattern and chromosome aberrations were assessed.

RESULTS

After exposure, an immediate cell cycle arrest occurred followed by a period of a few weeks where premature differentiation and senescence were observed. In all cultures cycling cells expressing low levels of cell cycle inhibiting proteins were present and finally dominated the populations. About 5months after exposure, the cellular and molecular changes attributed to differentiation and senescence reappeared and persisted. Concurrently, genetic instability was observed, but the aberration yields and types differed between repeated experiments. The descendants of cells exposed to carbon ions did not senesce earlier and displayed a similar rate of genetic instability as the X-ray progeny. For high doses an impaired cell cycle regulation and extended life span was observed, but finally cell proliferation ceased in all populations.

CONCLUSIONS

The descendants of irradiated fibroblasts undergo stepwise senescence and differentiation. Genetic instability is frequent and an extension of the life span may occur.

Epigenetic control of cellular senescence in disease: opportunities for therapeutic intervention

Understanding how senescence is established and maintained is an important area of study both for normal cell physiology and in tumourigenesis. Modifications to N-terminal tails of histone proteins, which can lead to chromatin remodelling, appear to be key to the regulation of the senescence phenotype. Epigenetic mechanisms such as modification of histone proteins have been shown to be sufficient to regulate gene expression levels and specific gene promoters can become epigenetically altered at senescence. This suggests that epigenetic mechanisms are important in senescence and further suggests epigenetic deregulation could play an important role in the bypass of senescence and the acquisition of a tumourigenic phenotype. Tumour suppressor proteins and cellular senescence are intimately linked and such proteins are now known to regulate gene expression through chromatin remodelling, again suggesting a link between chromatin modification and cellular senescence. Telomere dynamics and the expression of the telomerase genes are also both implicitly linked to senescence and tumourigenesis, and epigenetic deregulation of the telomerase gene promoters has been identified as a possible mechanism for the activation of telomere maintenance mechanisms in cancer. Recent studies have also suggested that epigenetic deregulation in stem cells could play an important role in carcinogenesis, and new models have been suggested for the attainment of tumourigenesis and bypass of senescence. Overall, proper regulation of the chromatin environment is suggested to have an important role in the senescence pathway, such that its deregulation could lead to tumourigenesis.

Oxygen free radicals in cell senescence: are they signal transducers?

Oxygen free radicals have a major impact on senescence of primary human cells. In replicative senescence, which is induced by uncapping of telomeres, the rate of telomere shortening is largely determined by telomere-specific accumulation of DNA damage induced by reactive oxygen species (ROS). More intense ROS-generating stressors can induce premature senescence via generation of telomere-independent DNA damage. Interestingly, ROS levels were also elevated when premature senescence was triggered by pathways downstream or independent of DNA damage. This has led to the suggestion that ROS generation could be a specific component of the signalling pathways inducing senescence. However, the available data are compatible with the concept that senescence is triggered as a DNA damage response. ROS appear to be involved as inducers of DNA damage rather than as specific signalling molecules. The upregulation of ROS production often seen in premature senescence might be related to retrograde response initiated by mitochondria.

Preventing or reducing late side effects of radiation therapy: radiobiology meets molecular pathology

Radiation therapy has curative or palliative potential in roughly half of all incident solid tumours, and offers organ and function preservation in most cases. Unfortunately, early and late toxicity limits the deliverable intensity of radiotherapy, and might affect the long-term health-related quality of life of the patient. Recent progress in molecular pathology and normal-tissue radiobiology has improved the mechanistic understanding of late normal-tissue effects and shifted the focus from initial-damage induction to damage recognition and tissue remodelling. This stimulates research into new pharmacological strategies for preventing or reducing the side effects of radiation therapy.

Methoxyamine potentiates iododeoxyuridine-induced radiosensitization by altering cell cycle kinetics and enhancing senescence

We previously reported that methoxyamine (an inhibitor of base excision repair) potentiates iododeoxyuridine (IUdR)-induced radiosensitization in human tumor cells. In this study, we investigated the potential mechanisms of this enhanced cell death. Human colorectal carcinoma RKO cells were exposed to IUdR (3 micromol/L) and/or methoxyamine (3 mmol/L) for 48 hours before ionizing radiation (5 Gy). We found that IUdR/methoxyamine altered cell cycle kinetics and led to an increased G1 population but a decreased S population before ionizing radiation. Immediately following ionizing radiation (up to 6 hours), IUdR/methoxyamine-pretreated cells showed a stringent G1-S checkpoint but an insufficient G2-M checkpoint, whereas a prolonged G1 arrest, containing 2CG1 and 4CG1 cells, was found at later times up to 72 hours. Levels of cell cycle-specific markers [p21, p27, cyclin A, cyclin B1, and pcdc2(Y15)] and DNA damage signaling proteins [gammaH2AX, pChk1(S317), and pChk2(T68)] supported these altered cell cycle kinetics. Interestingly, we found that IUdR/methoxyamine pretreatment reduced ionizing radiation-induced apoptosis. Additionally, the extent of cell death through necrosis or autophagy seemed similar in all (IUdR +/- methoxyamine + ionizing radiation) treatment groups. However, a larger population of senescence-activated beta-galactosidase-positive cells was seen in IUdR/methoxyamine/ionizing radiation-treated cells, which was correlated with the increased activation of the senescence factors p53 and pRb. These data indicate that IUdR/methoxyamine pretreatment enhanced the effects of ionizing radiation by causing a prolonged G1 cell cycle arrest and by promoting stress-induced premature senescence. Thus, senescence, a novel ionizing radiation-induced tumor suppression pathway, may be effectively targeted by IUdR/methoxyamine pretreatment, resulting in an improved therapeutic gain for ionizing radiation.

Bystander-induced differentiation: a major response to targeted irradiation of a urothelial explant model

A ureter primary explant technique, using porcine tissue sections was developed to study bystander effects under in vivo like conditions where dividing and differentiated cells are present. Targeted irradiations of ureter tissue fragments were performed with the Gray Cancer Institute charged particle microbeam at a single location (2 microm precision) with 10 3He2+ particles (5 MeV; LET 70 keV/microm). After irradiation the ureter tissue section was incubated for 7 days allowing explant outgrowth to be formed. Differentiation was estimated using antibodies to Uroplakin III, a specific marker of terminal urothelial differentiation. Even although only a single region of the tissue section was targeted, thousands of additional cells were found to undergo bystander-induced differentiation in the explant outgrowth. This resulted in an overall increase in the fraction of differentiated cells from 63.5+/-5.4% to 76.6+/-5.6%. These changes are much greater than that observed for the induction of damage in this model. One interpretation of these results is that in the tissue environment, differentiation is a much more significant response to targeted irradiation and potentially a protective mechanism.

Wound healing morbidity in STS patients treated with preoperative radiotherapy in relation to in vitro skin fibroblast radiosensitivity, proliferative capacity and TGF-β activity

BACKGROUND AND PURPOSE

In a recent study, we demonstrated that the ability of dermal fibroblasts, obtained from soft tissue sarcoma (STS) patients, to undergo initial division in vitro following radiation exposure correlated with the development of wound healing morbidity in the patients following their treatment with preoperative radiotherapy. Transforming growth factor beta (TGF-beta) is thought to play an important role in fibroblast proliferation and radiosensitivity both of which may impact on wound healing. Thus, in this study we examined the interrelationship between TGF-beta activity, radiosensitivity and proliferation of cultured fibroblasts and the wound healing response of STS patients after preoperative radiotherapy to provide a validation cohort for our previous study and to investigate mechanisms.

PATIENTS AND METHODS

Skin fibroblasts were established from skin biopsies of 46 STS patients. The treatment group consisted of 28 patients who received preoperative radiotherapy. Eighteen patients constituted a control group who were either irradiated postoperatively or did not receive radiation treatment. Fibroblast cultures were subjected to the colony forming and cytokinesis-blocked binucleation assays (low dose rate: approximately 0.02 Gy/min) and TGF-beta assays (high dose-rate: approximately 1.06 Gy/min) following gamma-irradiation. Fibroblast radiosensitivity and initial proliferative ability were represented by the surviving fraction at 2.4 Gy (SF(2.4)) and binucleation index (BNI), respectively. Active and total TGF-beta levels in fibroblast cultures were determined using a biological assay. Wound healing complication (WHC), defined as the requirement for further surgery or prolonged deep wound packing, was the clinical endpoint examined.

RESULTS

Of the 28 patients treated with preoperative radiotherapy, 8 (29%) had wound healing difficulties. Fibroblasts from patients who developed WHC showed a trend to retain a significantly higher initial proliferative ability after irradiation compared with those from individuals in the treatment group with normal wound healing, consistent with the results of our previous study. No link was observed between fibroblast radiosensitivity and WHC. Neither active nor total TGF-beta levels in cultures were significantly affected by irradiation. Fibroblast proliferation in unirradiated and irradiated cultures, as well as radiosensitivity, was not influenced by TGF-beta content. TGF-beta expression in fibroblast cultures did not reflect wound healing morbidity.

CONCLUSIONS

These data are consistent with our previous study and combined the results suggest that in vitro fibroblast proliferation after irradiation may be a useful predictor of wound healing morbidity in STS patients treated with preoperative radiotherapy. TGF-beta levels in culture do not predict WHC, suggesting that the role of TGF-beta in wound healing is likely controlled by other in vivo factors.

Radiation and the microenvironment - tumorigenesis and therapy

Radiation rapidly and persistently alters the soluble and insoluble components of the tissue microenvironment. This affects the cell phenotype, tissue composition and the physical interactions and signalling between cells. These alterations in the microenvironment can contribute to carcinogenesis and alter the tissue response to anticancer therapy. Examples of these responses and their implications are discussed with a view to therapeutic intervention.

Human cell senescence as a DNA damage response

It has been established that telomere-dependent replicative senescence of human fibroblasts is stress-dependent. First, it was shown that telomere shortening, which is a major contributor to telomere uncapping, is stress-dependent to a significant degree. Second, the signalling pathway connecting telomere uncapping and replicative senescence appears to be the same as the one that is activated by DNA damage: uncapped telomeres activate signalling cascades involving the protein kinases ATM, ATR and, possibly, DNA-PK. Furthermore, phosphorylation of histone H2A.X facilitates the formation of DNA damage foci around uncapped telomeres, and this in turn activates downstream kinases Chk1 and Chk2 and, eventually, p53. It appears that this signalling pathway has to be maintained in order to keep cells in a senescent state. Thus, cellular senescence can be regarded as a permanently maintained DNA damage response state. This suggests that antibodies against DNA damage foci components might be useful markers for senescent cells in vivo.

[Frequency of terminally differentiated fibroblasts in the synovial membrane of rheumatoid arthritis patients]

The metabolic activation of synovial fibroblasts (SF) and their expression of matrix degrading enzymes and inflammatory cytokines contributes to the pathology of rheumatoid arthritis (RA). It is remarkable that SF of RA patients do not proliferate at higher rates when compared to SF of other patients, but they are resistant to apotposis inducing signals. The chronic inflammation in RA causes fibrosis of the synovial tissue and fibrosis has been associated with terminal differentiation. Therefore we investigated if there are increased numbers of terminally differentiated fibroblasts in the RA synovium and if there is a correlation between terminal differentiation of SF and increased levels of expression of interleukins and matrix metalloproteinases. We analyzed specimen of four RA patients, two patients with osteoarthritis (OA) and two healthy donors suffering from joint injuries. By use of RT-PCR techniques we examined mRNA expression of two genes in SF which are associated with terminal differentiation, p16INK4a and p21-cip. In addition, we labelled differentiated fibroblasts using the SA-beta-galaktosidase assay and investigated differences in protein expression patterns of factor PIVa and the tropomyosin 1 and 2 molecules. We report that the number of terminally differentiated fibrolasts are not increased in the synovial membrane of RA patients. On the contrary we show that the synovia of the much younger patients has higher levels of terminally differentiated fibroblasts. Consequently, the fibrosis of synovial tissues in RA patients at later stages of disorder is not associated with proliferation and differentiation of the fibroblasts but rather a consequence of chronic inflammation.

Postmitotic differentiation of rat lung fibroblasts: induction by bleomycin and effect on prolyl 4-hydroxylase

The cytostatic drug bleomycin (BLM) induces pulmonary fibrosis as its main side effect. Fibroblasts in fibrotic foci are the main cellular source for extracellular matrix accumulation that typifies fibrosis. In vitro studies demonstrated the ability of cytotoxic drugs to induce terminal differentiation of fibroblasts. These postmitotic cells are very active in regard to production of collagens. The present study was addressed to investigate the potential of BLM to induce terminal differentiation of rat lung fibroblasts in vitro and the consequences for collagen production and for the expression and activity of the collagen modifying enzyme prolyl 4-hydroxylase (P4H). The BLM effects were compared with those of mitomycin C (MMC), another cytotoxic agent with known potential for initiation of postmitotic differentiation of fibrobasts. BLM induced postmitotic differentiation of rat lung fibroblasts. The capacity of the cells to form clones was diminished by BLM or MMC in a concentration dependent manner. Both drugs initiated the formation of an increasing number of postmitotic cell clones. The postmitotic differentiation was accompanied by an increase in total collagen production by the cells. Administration of BLM to cultures of lung fibroblasts at concentrations of 1 or 10 mU/ml resulted in an increase of the collagen amount to about the 1.5-fold and 1.6-fold of controls, respectively. Treatment of fibroblasts with MMC elevated the collagen level to about the 2-fold. P4H activity and P4Halpha mRNA levels in cells exposed to BLM or MMC were found to be increased. We conclude that terminally differentiated fibroblasts might be part of the heterogeneous population of fibroblast-like cells in fibrotic foci responsible for the increased production of collagen during the fibrotic phase of the development of pulmonary fibrosis.

A Sense of Danger from Radiation1

Tissue damage caused by exposure to pathogens, chemicals and physical agents such as ionizing radiation triggers production of generic "danger" signals that mobilize the innate and acquired immune system to deal with the intrusion and effect tissue repair with the goal of maintaining the integrity of the tissue and the body. Ionizing radiation appears to do the same, but less is known about the role of "danger" signals in tissue responses to this agent. This review deals with the nature of putative "danger" signals that may be generated by exposure to ionizing radiation and their significance. There are a number of potential consequences of "danger" signaling in response to radiation exposure. "Danger" signals could mediate the pathogenesis of, or recovery from, radiation damage. They could alter intrinsic cellular radiosensitivity or initiate radioadaptive responses to subsequent exposure. They may spread outside the locally damaged site and mediate bystander or "out-of-field" radiation effects. Finally, an important aspect of classical "danger" signals is that they link initial nonspecific immune responses in a pathological site to the development of specific adaptive immunity. Interestingly, in the case of radiation, there is little evidence that "danger" signals efficiently translate radiation-induced tumor cell death into the generation of tumor-specific immunity or normal tissue damage into autoimmunity. The suggestion is that radiation-induced "danger" signals may be inadequate in this respect or that radiation interferes with the generation of specific immunity. There are many issues that need to be resolved regarding "danger" signaling after exposure to ionizing radiation. Evidence of their importance is, in some areas, scant, but the issues are worthy of consideration, if for no other reason than that manipulation of these pathways has the potential to improve the therapeutic benefit of radiation therapy. This article focuses on how normal tissues and tumors sense and respond to danger from ionizing radiation, on the nature of the signals that are sent, and on the impact on the eventual consequences of exposure.

An in silico investigation into the causes of telomere length heterogeneity and its implications for the Hayflick limit

In telomerase-negative cell populations the mean telomere length (TL) decreases with increasing population doubling number (PD). A critically small TL is believed to stop cell proliferation at a cell-, age- and species-specific PD thus defining the Hayflick limit. However, positively skewed TL distributions are broad compared to differences between initial and final mean TL and strongly overlap at middle and late PD, which is inconsistent with a limiting role of TL. We used computer-assisted modelling to define what set of premises may account for the above. Our model incorporates the following concepts. DNA end replication problem: telomeres loose 1 shortening unit (SU) upon each cell division. Free radical-caused TL decrease: telomeres experience random events resulting in the loss of a random SU number within a remaining TL. Stochasticity of gene expression and cell differentiation: cells experience random events inducing mitoses or committing cells to proliferation arrest, the latter option requiring a specified number of mitoses to be passed. Cells whose TL reaches 1SU cannot divide. The proliferation kinetics of such virtual cells conforms to the transition probability model of cell cycle. When no committing events occur and at realistic SU estimates of the initial TL, maximal PD values far exceed the Hayflick limit observed in normal cells and are consistent with the crisis stage entered by transformed cells that have surpassed the Hayflick limit. At intermediate PD, symmetrical TL distributions are yielded. Upon introduction of committing events making the ratio of the rates of proliferating and committing events (P/C) range from 1.10 to 1.25, TL distributions at intermediate PD become positively skewed, and virtual cell clones show bimodal size distributions. At P/C as high as 1.25 the majority of virtual cells at maximal PD contain telomeres with TL>1SU. A 10% increase in P/C within the 1.10-1.25 range produces a two-fold increase in the maximal PD, which can reach values of up to 25 observed in rodent and some human cells. Increasing the number of committed mitoses from 0 to 10 can increases PD to about 50 observed in human fibroblasts. Introduction of the random TL breakage makes the shapes of TL distributions quite dissimilar from those observed in real cells.

CONCLUSIONS

Telomere length decrease is a correlate of cell proliferation that cannot alone account for the Hayflick limit, which primarily depends on parameters of cell population kinetics. Free radical damage influences the Hayflick limit not through TL but rather by affecting the ratio of the rates of events that commit cells to mitoses or to proliferation arrest.

From Weismann's theory to present day gerontology 1889-2003

From Weismann's theory to present day gerontology--Weismann's theory was based on the concept that through natural selection the division potential of somatic cells become finite thus limiting the regeneration of the soma and the life span of the organism. Indeed, the somatic cells of some animals have a finite division potential but what became apparent is that the implications for aging are more complex. Experiments showed that at each cell division the genetic information received by each daughter cell differs; cells are this way progressively modified through division creating a functional drift that is responsible in part for the continuous modifications going on in the organism from its very beginning to its extinction. Comparative biology showed that the finite or the infinite division potential of somatic cells has a complex connotation with developmental characteristics of the respective organism with implications for longevity that are far from being understood.

Replicative senescence and the art of counting

The idea that aging is largely the result of (endogenous) stress appears to be at odds with the concept of biological 'clocks', which seem to programme and terminate cellular aging processes. Here, data are reviewed that show that telomeres, the major clock identified in human cells so far, do in fact measure stress and damage accumulation much more than simple mitotic time. Telomere shortening is significantly stress-dependent due to a telomere-specific damage repair deficiency. This identifies telomere-driven human cell replicative senescence as a stress response with high potential importance for human aging.