Differentiation-induced radioresistance in muscle cells

DUX4-induced HSATII transcription causes KDM2A/B-PRC1 nuclear foci and impairs DNA damage response

Polycomb repressive complexes regulate developmental gene programs, promote DNA damage repair, and mediate pericentromeric satellite repeat repression. Expression of pericentromeric satellite repeats has been implicated in several cancers and diseases, including facioscapulohumeral dystrophy (FSHD). Here, we show that DUX4-mediated transcription of HSATII regions causes nuclear foci formation of KDM2A/B-PRC1 complexes, resulting in a global loss of PRC1-mediated monoubiquitination of histone H2A. Loss of PRC1-ubiquitin signaling severely impacts DNA damage response. Our data implicate DUX4-activation of HSATII and sequestration of KDM2A/B-PRC1 complexes as a mechanism of regulating epigenetic and DNA repair pathways.

Decline of DNA damage response along with myogenic differentiation

DNA integrity is incessantly confronted to agents inducing DNA lesions. All organisms are equipped with a network of DNA damage response mechanisms that will repair DNA lesions and restore proper cellular activities. Despite DNA repair mechanisms have been revealed in replicating cells, still little is known about how DNA lesions are repaired in postmitotic cells. Muscle fibers are highly specialized postmitotic cells organized in syncytia and they are vulnerable to age-related degeneration and atrophy after radiotherapy treatment. We have studied the DNA repair capacity of muscle fiber nuclei and compared it with the one measured in proliferative myoblasts here. We focused on the DNA repair mechanisms that correct ionizing radiation (IR)-induced lesions, namely the base excision repair, the nonhomologous end joining, and the homologous recombination (HR). We found that in the most differentiated myogenic cells, myotubes, these DNA repair mechanisms present weakened kinetics of recruitment of DNA repair proteins to IR-damaged DNA. For base excision repair and HR, this decline can be linked to reduced steady-state levels of key proteins involved in these processes.

Down-regulation of pro-necroptotic molecules blunts necroptosis during myogenesis

Cell death and differentiation are closely related at the molecular level. Differentiation of skeletal muscle cells attenuates susceptibility to apoptosis. Necroptosis has recently been recognized as a form of regulated cell death but its role in myogenesis has not been studied. This study aimed to compare the sensitivity to TNF-induced necroptosis in skeletal muscle at the undifferentiated (myoblasts) and differentiated (myotubes) stages. Surprisingly, our results showed that TNF-induced necroptosis was blunted during myoblast differentiation. Moreover, our data revealed that the key molecules involved in necroptosis, including receptor-interacting serine/threonine protein kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like protein (MLKL), were significantly down-regulated during myogenic differentiation, resulting in suppression of necroptosis signal transduction in differentiated myotubes. In addition, RIPK1, RIPK3, and MLKL expression levels were significantly lower in the skeletal muscle of adult mice than in newborn mice, suggesting that the susceptibility to necroptosis might be attenuated in differentiated muscle tissue. In conclusion, this study revealed that expression of key molecules involved in necroptosis is down-regulated during muscle differentiation, which results in the differentiation of muscles becoming insensitive to necroptotic cell death.

Irradiation impairs mitochondrial function and skeletal muscle oxidative capacity: significance for metabolic complications in cancer survivors

BACKGROUND

Metabolic complications are highly prevalent in cancer survivors treated with irradiation but the underlying mechanisms remain unknown.

METHODS

Chow or high fat-fed C57Bl/6J mice were irradiated (6Gy) before investigating the impact on whole-body or skeletal muscle metabolism and profiling their lipidomic signature. Using a transgenic mouse model (Tg:Pax7-nGFP), we isolated muscle progenitor cells (satellite cells) and characterised their metabolic functions. We recruited childhood cancer survivors, grouped them based on the use of total body irradiation during their treatment and established their lipidomic profile.

RESULTS

In mice, irradiation delayed body weight gain and impaired fat pads and muscle weights. These changes were associated with impaired whole-body fat oxidation in chow-fed mice and altered ex vivo skeletal muscle fatty acid oxidation, potentially due to a reduction in oxidative fibres and reduced mitochondrial enzyme activity. Irradiation led to fasting hyperglycaemia and impaired glucose uptake in isolated skeletal muscles. Cultured satellite cells from irradiated mice showed decreased fatty acid oxidation and reduced glucose uptake, recapitulating the host metabolic phenotype. Irradiation resulted in a remodelling of lipid species in skeletal muscles, with the extensor digitorum longus muscle being particularly affected. A large number of lipid species were reduced, with several of these species showing a positive correlation with mitochondrial enzymes activity. In cancer survivors exposed to irradiation, we found a similar decrease in systemic levels of most lipid species, and lipid species that increased were positively correlated with insulin resistance (HOMA-IR).

CONCLUSION

Irradiation leads to long-term alterations in body composition, and lipid and carbohydrate metabolism in skeletal muscle, and affects muscle progenitor cells. Such changes result in persistent impairment of metabolic functions, providing a new mechanism for the increased prevalence of metabolic diseases reported in irradiated individuals. In this context, changes in the lipidomic signature in response to irradiation could be of diagnostic value.

Cell Death Response to DNA Damage

The cell death response to DNA damage is discussed in this Perspectives piece with cancer as the backdrop because DNA damaging agents (DDA) are widely used to treat cancer. From decades of clinical results, we learn that DDA have cured some cancers but their toxicity is temporary in most cancers due to emergence of DDA-resistant cancer cells. Investigation of DDA-activated genes, proteins, and pathways, known collectively as the DNA damage response (DDR), has uncovered the inner workings of DDR that protect the genome to sustain life. Paradoxically, however, DDR can also activate death. Current knowledge on DDA-activated death and hypotheses for how DDR may determine when and where to execute death are discussed. Given that cancer cells suffer from DDR defects, which account for their initial sensitivity to DDA, future therapeutic development may exploit those cancer-specific DDR defects to selectively create death-inducing DNA lesions, without using DDA, to kill DDA-resistant cancers.

Stretching magnitude-dependent inactivation of AKT by ROS led to enhanced p53 mitochondrial translocation and myoblast apoptosis

Previously, we had shown that high magnitude stretch (HMS), rather than low magnitude stretch (LMS), induced significant apoptosis of skeletal muscle C2C12 myoblasts. However, the molecular mechanism remains obscure. In this study, we found that p53 protein accumulated in the nucleus of LMS-loaded cells, whereas it translocated into mitochondria of HMS-loaded cells. Knocking down endogenous p53 by shRNA abrogated HMS-induced apoptosis. Furthermore, we demonstrated that overaccumulation of reactive oxygen species (ROS) during HMS-inactivated AKT that was activated in LMS-treated cells, which accounted for the distinct p53 subcellular localizations under HMS and LMS. Blocking ROS generation by N-acetylcysteine (NAC) or overexpressing constitutively active AKT vector (CA-AKT) inhibited HMS-incurred p53 mitochondrial translocation and promoted its nuclear targeting. Moreover, both NAC and CA-AKT significantly attenuated HMS-induced C2C12 apoptosis. Finally, we found that Ser389 phosphorylation of p53 was a downstream event of ROS-inactivated AKT pathway, which was critical to p53 mitochondrial trafficking during HMS stimuli. Transfecting p53-shRNA C2C12s with the mutant p53 (S389A) that was unable to target p53 to mitochondria underwent significantly lower apoptosis than transfection with wild-type p53. Altogether, our study uncovered that mitochondrial localization of p53, resulting from p53 Ser389 phosphorylation through ROS-inactivated AKT pathway, prompted C2C12 myoblast apoptosis during HMS stimulation.

A patient-derived iPSC model revealed oxidative stress increases facioscapulohumeral muscular dystrophy-causative DUX4

Double homeobox 4 (DUX4), the causative gene of facioscapulohumeral muscular dystrophy (FSHD), is ectopically expressed in the skeletal muscle cells of FSHD patients because of chromatin relaxation at 4q35. The diminished heterochromatic state at 4q35 is caused by either large genome contractions [FSHD type 1 (FSHD1)] or mutations in genes encoding chromatin regulators, such as SMCHD1 [FSHD type 2 (FSHD2)]. However, the mechanism by which DUX4 expression is regulated remains largely unknown. Here, using a myocyte model developed from patient-derived induced pluripotent stem cells, we determined that DUX4 expression was increased by oxidative stress (OS), a common environmental stress in skeletal muscle, in both FSHD1 and FSHD2 myocytes. We generated FSHD2-derived isogenic control clones with SMCHD1 mutation corrected by clustered regularly interspaced short palindromic repeats (CRISPR)/ CRISPR associated 9 (Cas9) and homologous recombination and found in the myocytes obtained from these clones that DUX4 basal expression and the OS-induced upregulation were markedly suppressed due to an increase in the heterochromatic state at 4q35. We further found that DNA damage response (DDR) was involved in OS-induced DUX4 increase and identified ataxia-telangiectasia mutated, a DDR regulator, as a mediator of this effect. Our results suggest that the relaxed chromatin state in FSHD muscle cells permits aberrant access of OS-induced DDR signaling, thus increasing DUX4 expression. These results suggest OS could represent an environmental risk factor that promotes FSHD progression.

Chondrocytes differentiated from human induced pluripotent stem cells: Response to ionizing radiation

Purpose

Data on the response of chondrocytes differentiated from hiPSCs (hiPSC-DCHs) to ionizing radiation (IR) are lacking. The aim of present study was to assess DNA damage response (DDR) mechanisms of IR-treated hiPSC-DCHs.

Methods and materials

The following IR-response characteristics in irradiated hiPSC-DCHs were assessed: 1) the kinetics of DNA DSB formation; 2) activation of major DNA repair mechanisms; 3) cell cycle changes and 4) reactive oxygen species (ROS), level of key markers of apoptosis and senescence.

Results

DNA DSBs were observed in 30% of the hiPSC-DCHs overall, and in 60% after high-dose (> 2 Gy) IR. Nevertheless, these cells displayed efficient DNA repair mechanisms, which reduced the DSBs over time until it reached 30% by activating key genes involved in homologous recombination and non-homologous end joining mechanisms. As similar to mature chondrocytes, irradiated hiPSC-DCH cells revealed accumulation of cells in G2 phase. Overall, the hiPSC-DCH cells were characterized by low levels of ROS, cPARP and high levels of senescence.

Conclusions

The chondrocyte-like cells derived from hiPSC demonstrated features characteristic of both mature chondrocytes and “parental” hiPSCs. The main difference between hiPSC-derived chondrocytes and hiPSCs and mature chondrocytes appears to be the more efficient DDR mechanism of hiPSC-DCHs. The unique properties of these cells suggest that they could potentially be used safely in regenerative medicine if these preliminary findings are confirmed in future studies.

Cleavage of caspase-12 at Asp94, mediated by endoplasmic reticulum stress (ERS), contributes to stretch-induced apoptosis of myoblasts

Mechanical overloading can lead to skeletal muscle damage instead of remodeling. This is attributed to the excessive apoptosis of myoblasts, mechanism of which remains to be elucidated. The present study aimed to investigate the involvement of endoplasmic reticulum stress (ERS) and caspase-12 in mediating the stretch-induced apoptosis of myoblasts. Myoblast apoptosis was evaluated by Hoechst staining, DNA fragmentation assay, Annexin V binding, and propidium iodide staining, as well as caspase-3 and poly-ADP-ribose polymerase 1 cleavage. First, our results showed that apoptosis was elevated in a time-dependent manner when myoblasts were subjected to cyclic mechanical stretch (CMS) for 12, 24, and 36 hr. Concomitantly, CMS triggered the ERS and caspase-12 cleavage; ERS inhibitor GSK 2606414 suppressed the CMS-induced cleavage of caspase-12 and myoblast apoptosis. Silencing caspase-12 attenuated the apoptosis of myoblasts under CMS. Furthermore, CMS-induced myoblast apoptosis was partially recovered by overexpressing wild-type caspase-12 in caspase-12-silenced myoblasts. In contrast, overexpressing mutant caspase-12 (D94N), which cannot be cleaved into the active caspase-12 fragments, failed to accomplish the same effect. Finally, C2C12 overexpressing truncated caspase-12 segment (TC-casp12-D94), which starts from Asp94 and ends at Asn419, underwent apoptosis under both static and stretched conditions. Interestingly, C2C12 myoblasts seemed to be resistant to stretch-induced apoptosis upon low-serum-induced differentiation. In conclusion, our study provided evidence that caspase-12 cleavage at Asp94, induced by ERS under mechanical stimuli, is the key molecule in initiating the stretch-triggered apoptosis of myoblasts.

Ionizing Radiation Potentiates High-Fat Diet-Induced Insulin Resistance and Reprograms Skeletal Muscle and Adipose Progenitor Cells

Exposure to ionizing radiation increases the risk of chronic metabolic disorders such as insulin resistance and type 2 diabetes later in life. We hypothesized that irradiation reprograms the epigenome of metabolic progenitor cells, which could account for impaired metabolism after cancer treatment. C57Bl/6 mice were treated with a single dose of irradiation and subjected to high-fat diet (HFD). RNA sequencing and reduced representation bisulfite sequencing were used to create transcriptomic and epigenomic profiles of preadipocytes and skeletal muscle satellite cells collected from irradiated mice. Mice subjected to total body irradiation showed alterations in glucose metabolism and, when challenged with HFD, marked hyperinsulinemia. Insulin signaling was chronically disrupted in skeletal muscle and adipose progenitor cells collected from irradiated mice and differentiated in culture. Epigenomic profiling of skeletal muscle and adipose progenitor cells from irradiated animals revealed substantial DNA methylation changes, notably for genes regulating the cell cycle, glucose/lipid metabolism, and expression of epigenetic modifiers. Our results show that total body irradiation alters intracellular signaling and epigenetic pathways regulating cell proliferation and differentiation of skeletal muscle and adipose progenitor cells and provide a possible mechanism by which irradiation used in cancer treatment increases the risk for metabolic disease later in life.

Cell cycle-dependent resolution of DNA double-strand breaks

DNA double strand breaks (DSBs) elicit prompt activation of DNA damage response (DDR), which arrests cell-cycle either in G₁/S or G₂/M in order to avoid entering S and M phase with damaged DNAs. Since mammalian tissues contain both proliferating and quiescent cells, there might be fundamental difference in DDR between proliferating and quiescent cells (or G₀-arrested). To investigate these differences, we studied recruitment of DSB repair factors and resolution of DNA lesions induced at site-specific DSBs in asynchronously proliferating, G₀-, or G₁-arrested cells. Strikingly, DSBs occurring in G₀ quiescent cells are not repaired and maintain a sustained activation of the p53-pathway. Conversely, re-entry into cell cycle of damaged G₀-arrested cells, occurs with a delayed clearance of DNA repair factors initially recruited to DSBs, indicating an inefficient repair when compared to DSBs induced in asynchronously proliferating or G₁-synchronized cells. Moreover, we found that initial recognition of DSBs and assembly of DSB factors is largely similar in asynchronously proliferating, G₀-, or G₁-synchronized cells. Our study thereby demonstrates that repair and resolution of DSBs is strongly dependent on the cell-cycle state.

DNA Damage Response and DNA Repair in Skeletal Myocytes From a Mouse Model of Spinal Muscular Atrophy

We studied DNA damage response (DDR) and DNA repair capacities of skeletal muscle cells from a mouse model of infantile spinal muscular atrophy (SMA) caused by loss-of-function mutation of survival of motor neuron (Smn). Primary myocyte cultures derived from skeletal muscle satellite cells of neonatal control and mutant SMN mice had similar myotube length, myonuclei, satellite cell marker Pax7 and differentiated myotube marker myosin, and acetylcholine receptor clustering. DNA damage was induced in differentiated skeletal myotubes by γ-irradiation, etoposide, and methyl methanesulfonate (MMS). Unexposed control and SMA myotubes had stable genome integrity. After γ-irradiation and etoposide, myotubes repaired most DNA damage equally. Control and mutant myotubes exposed to MMS exhibited equivalent DNA damage without repair. Control and SMA myotube nuclei contained DDR proteins phospho-p53 and phospho-H2AX foci that, with DNA damage, dispersed and then re-formed similarly after recovery. We conclude that mouse primary satellite cell-derived myotubes effectively respond to and repair DNA strand-breaks, while DNA alkylation repair is underrepresented. Morphological differentiation, genome stability, genome sensor, and DNA strand-break repair potential are preserved in mouse SMA myocytes; thus, reduced SMN does not interfere with myocyte differentiation, genome integrity, and DNA repair, and faulty DNA repair is unlikely pathogenic in SMA.

TOPOISOMERASE1α Acts through Two Distinct Mechanisms to Regulate Stele and Columella Stem Cell Maintenance

TOPOISOMERASE1 (TOP1), which releases DNA torsional stress generated during replication through its DNA relaxation activity, plays vital roles in animal and plant development. In Arabidopsis (Arabidopsis thaliana), TOP1 is encoded by two paralogous genes (TOP1α and TOP1β), of which TOP1α displays specific developmental functions that are critical for the maintenance of shoot and floral stem cells. Here, we show that maintenance of two different populations of root stem cells is also dependent on TOP1α-specific developmental functions, which are exerted through two distinct novel mechanisms. In the proximal root meristem, the DNA relaxation activity of TOP1α is critical to ensure genome integrity and survival of stele stem cells (SSCs). Loss of TOP1α function triggers DNA double-strand breaks in S-phase SSCs and results in their death, which can be partially reversed by the replenishment of SSCs mediated by ETHYLENE RESPONSE FACTOR115 In the quiescent center and root cap meristem, TOP1α is epistatic to RETINOBLASTOMA-RELATED (RBR) in the maintenance of undifferentiated state and the number of columella stem cells (CSCs). Loss of TOP1α function in either wild-type or RBR RNAi plants leads to differentiation of CSCs, whereas overexpression of TOP1α mimics and further enhances the effect of RBR reduction that increases the number of CSCs Taken together, these findings provide important mechanistic insights into understanding stem cell maintenance in plants.

The concept of radiation-enhanced stem cell differentiation

BACKGROUND

Efficient stem cell differentiation is considered to be the holy grail of regenerative medicine. Pursuing the most productive method of directed differentiation has been the subject of numerous studies, resulting in the development of many effective protocols. However, the necessity for further improvement in differentiation efficiency remains. This review contains a description of molecular processes underlying the response of stem cells to ionizing radiation, indicating its potential application in differentiation procedures. In the first part, the radiation-induced damage response in various types of stem cells is described. Second, the role of the p53 protein in embryonic and adult stem cells is highlighted. Last, the hypothesis on the mitochondrial involvement in stem cell development including its response to ionizing radiation is presented.

CONCLUSIONS

In summary, despite the many threats of ionizing radiation concerning genomic instability, subjecting cells to the appropriate dosage of ionizing radiation may become a useful method for enhancing directed differentiation in certain stem cell types.

p53 suppresses muscle differentiation at the myogenin step in response to genotoxic stress

Acute muscle injury and physiological stress from chronic muscle diseases and aging lead to impairment of skeletal muscle function. This raises the question of whether p53, a cellular stress sensor, regulates muscle tissue repair under stress conditions. By investigating muscle differentiation in the presence of genotoxic stress, we discovered that p53 binds directly to the myogenin promoter and represses transcription of myogenin, a member of the MyoD family of transcription factors that plays a critical role in driving terminal muscle differentiation. This reduction of myogenin protein is observed in G1-arrested cells and leads to decreased expression of late but not early differentiation markers. In response to acute genotoxic stress, p53-mediated repression of myogenin reduces post-mitotic nuclear abnormalities in terminally differentiated cells. This study reveals a mechanistic link previously unknown between p53 and muscle differentiation, and suggests new avenues for managing p53-mediated stress responses in chronic muscle diseases or during muscle aging.

SUV39H1 downregulation induces deheterochromatinization of satellite regions and senescence after exposure to ionizing radiation

While the majority of cancer patients are exposed to ionizing radiation during diagnostic and therapeutic procedures, age-dependent differences in radiation sensitivity are not yet well understood. Radiation sensitivity is characterized by the appearance of side effects to radiation therapy, such as secondary malignancies, developmental deficits, and compromised immune function. However, the knowledge of the molecular mechanisms that trigger these side effects is incomplete. Here we used an in vitro system and showed that low-senescent normal human diploid fibroblasts (WI-38) senesce in response to 5 Gy IR, while highly senescent cultures do not show changes in cell cycle regulation and only a slight increase in the percentage of senescent cells. Our study shows that this is associated with changes in the expression of genes responsible for cell cycle progression, apoptosis, DNA repair, and aging, as well as transcriptional and epigenetic regulators. Furthermore, we propose a role of the downregulation of SUV39H1 expression, a histone methyltransferase that specifically trimethylates H3K9, and the corresponding reduction in H3K9me3 levels in the establishment of IR-induced senescence.

Stem cells: the pursuit of genomic stability

Stem cells harbor significant potential for regenerative medicine as well as basic and clinical translational research. Prior to harnessing their reparative nature for degenerative diseases, concerns regarding their genetic integrity and mutation acquisition need to be addressed. Here we review pluripotent and multipotent stem cell response to DNA damage including differences in DNA repair kinetics, specific repair pathways (homologous recombination vs. non-homologous end joining), and apoptotic sensitivity. We also describe DNA damage and repair strategies during reprogramming and discuss potential genotoxic agents that can reduce the inherent risk for teratoma formation and mutation accumulation. Ensuring genomic stability in stem cell lines is required to achieve the quality control standards for safe clinical application.

The effect of radiation dose on mouse skeletal muscle remodeling

Background

The purpose of this study was to determine the effect of two clinically relevant radiation doses on the susceptibility of mouse skeletal muscle to remodeling.

Materials and methods.

Alterations in muscle morphology and regulatory signaling were examined in tibialis anterior and gastrocnemius muscles after radiation doses that differed in total biological effective dose (BED). Female C57BL/6 (8-wk) mice were randomly assigned to non-irradiated control, four fractionated doses of 4 Gy (4x4 Gy; BED 37 Gy), or a single 16 Gy dose (16 Gy; BED 100 Gy). Mice were sacrificed 2 weeks after the initial radiation exposure.

Results

The 16 Gy, but not 4x4 Gy, decreased total muscle protein and RNA content. Related to muscle regeneration, both 16 Gy and 4x4 Gy increased the incidence of central nuclei containing myofibers, but only 16 Gy increased the extracellular matrix volume. However, only 4x4 Gy increased muscle 4-hydroxynonenal expression. While both 16 Gy and 4x4 Gy decreased IIB myofiber mean cross-sectional area (CSA), only 16 Gy decreased IIA myofiber CSA. 16 Gy increased the incidence of small diameter IIA and IIB myofibers, while 4x4 Gy only increased the incidence of small diameter IIB myofibers. Both treatments decreased the frequency and CSA of low succinate dehydrogenase activity (SDH) fibers. Only 16 Gy increased the incidence of small diameter myofibers having high SDH activity. Neither treatment altered muscle signaling related to protein turnover or oxidative metabolism.

Conclusions

Collectively, these results demonstrate that radiation dose differentially affects muscle remodeling, and these effects appear to be related to fiber type and oxidative metabolism.

Redox or death: checking on fetal myogenesis

Reporting in this issue of Developmental Cell, L'honoré et al. (2014) show that a network composed of Pitx2/Pitx3 and downstream antioxidant enzymes protects differentiating skeletal muscle from excessive reactive oxygen species production during fetal myogenesis. Genetic deficiency of Pitx2/Pitx3 results in irreversible oxidative DNA damage and apoptosis, impairing skeletal muscle development.

A tissue graft model of DNA damage response in the normal and malignant human prostate

PURPOSE

DNA damage responses are relevant to prostate cancer initiation, progression and treatment. Few models of the normal and malignant human prostate that maintain stromal-epithelial interactions in vivo exist in which to study DNA damage responses. We evaluated the feasibility of maintaining tissue slice grafts at subcutaneous vs subrenal capsular sites in RAG2(-/-)γC(-/-) mice to study the DNA damage responses of normal and malignant glands.

MATERIALS AND METHODS

We compared the take rate and histology of tissue slice grafts from fresh, precision cut surgical specimens that were maintained for 1 to 4 weeks in subcutaneous vs subrenal capsular sites. Induction of γH2AX, p53, ATM and apoptosis was evaluated as a measure of the DNA damage response after irradiation.

RESULTS

The take rate of subcutaneous tissue slice grafts was higher than typically reported but lower than at the subrenal capsular site. Subcutaneous tissue slice grafts frequently showed basal cell hyperplasia, squamous metaplasia and cystic atrophy, and cancer did not survive. In contrast, normal and malignant histology was well maintained in subrenal capsular tissue slice grafts. Regardless of implantation site the induction of γH2AX and ATM occurred in tissue slice graft epithelium 1 hour after irradiation and decreased to basal level by 24 hours, indicating DNA damage recognition and repair. As observed previously in prostatic ex vivo models, p53 was not activated. Notably, tumor but not normal cells responded to irradiation by undergoing apoptosis.

CONCLUSIONS

To our knowledge this is the first study of DNA damage responses in a patient derived prostate tissue graft model. The subrenal capsular site of RAG2(-/-)γC(-/-) mice optimally maintains normal and malignant histology and function, permitting novel studies of DNA damage responses in a physiological context.

Phosphorylation of histone H2AX in the mouse brain from development to senescence

Phosphorylation of the histone H2AX (γH2AX form) is an early response to DNA damage and a marker of aging and disease in several cells and tissues outside the nervous system. Little is known about in vivo phosphorylation of H2AX in neurons, although it was suggested that γH2AX is an early marker of neuronal endangerment thus opening the possibility to target it as a neuroprotective strategy. After experimental labeling of DNA-synthesizing cells with 5-bromo-2-deoxyuridine (BrdU), we studied the brain occurrence of γH2AX in developing, postnatal, adult and senescent (2 years) mice by light and electron microscopic immunocytochemistry and Western blotting. Focal and/or diffuse γH2AX immunostaining appears in interkinetic nuclei, mitotic chromosomes, and apoptotic nuclei. Immunoreactivity is mainly associated with neurogenetic areas, i.e., the subventricular zone (SVZ) of telencephalon, the cerebellar cortex, and, albeit to a much lesser extent, the subgranular zone of the hippocampal dentate gyrus. In addition, γH2AX is highly expressed in the adult and senescent cerebral cortex, particularly the piriform cortex. Double labeling experiments demonstrate that γH2AX in neurogenetic brain areas is temporally and functionally related to proliferation and apoptosis of neuronal precursors, i.e., the type C transit amplifying cells (SVZ) and the granule cell precursors (cerebellum). Conversely, γH2AX-immunoreactive cortical neurons incorporating the S phase-label BrdU do not express the proliferation marker phosphorylated histone H3, indicating that these postmitotic cells undergo a significant DNA damage response. Our study paves the way for a better comprehension of the role of H2AX phosphorylation in the normal brain, and offers additional data to design novel strategies for the protection of neuronal precursors and mature neurons in central nervous system (CNS) degenerative diseases.

Maintaining genome stability in the nervous system

Active maintenance of genome stability is a prerequisite for the development and function of the nervous system. The high replication index during neurogenesis and the long life of mature neurons highlight the need for efficient cellular programs to safeguard genetic fidelity. Multiple DNA damage response pathways ensure that replication stress and other types of DNA lesions, such as oxidative damage, do not affect neural homeostasis. Numerous human neurologic syndromes result from defective DNA damage signaling and compromised genome integrity. These syndromes can involve different neuropathology, which highlights the diverse maintenance roles that are required for genome stability in the nervous system. Understanding how DNA damage signaling pathways promote neural development and preserve homeostasis is essential for understanding fundamental brain function.

Deficiency in repair of the mitochondrial genome sensitizes proliferating myoblasts to oxidative damage

Reactive oxygen species (ROS), generated as a by-product of mitochondrial oxidative phosphorylation, are particularly damaging to the genome of skeletal muscle because of their high oxygen consumption. Proliferating myoblasts play a key role during muscle regeneration by undergoing myogenic differentiation to fuse and restore damaged muscle. This process is severely impaired during aging and in muscular dystrophies. In this study, we investigated the role of oxidatively damaged DNA and its repair in the mitochondrial genome of proliferating skeletal muscle progenitor myoblasts cells and their terminally differentiated product, myotubes. Using the C2C12 cell line as a well-established model for skeletal muscle differentiation, we show that myoblasts are highly sensitive to ROS-mediated DNA damage, particularly in the mitochondrial genome, due to deficiency in 5’ end processing at the DNA strand breaks. Ectopic expression of the mitochondrial-specific 5’ exonuclease, EXOG, a key DNA base excision/single strand break repair (BER/SSBR) enzyme, in myoblasts but not in myotubes, improves the cell’s resistance to oxidative challenge. We linked loss of myoblast viability by activation of apoptosis with deficiency in the repair of the mitochondrial genome. Moreover, the process of myoblast differentiation increases mitochondrial biogenesis and the level of total glutathione. We speculate that our data may provide a mechanistic explanation for depletion of proliferating muscle precursor cells during the development of sarcopenia, and skeletal muscle dystrophies.

The response to DNA damage during differentiation: pathways and consequences

Damage to genomic DNA triggers a prompt set of signaling events known as the DNA damage response (DDR) which coordinates DNA repair, cell cycle arrest and ultimately cell death or senescence. Although activation of adequate DNA damage signaling and repair systems depends on the type of lesion and the cell-cycle phase in which it occurs, emerging evidence indicates that DNA repair and DDR function differently in different cellular contexts. Depending on the time maintenance and function of a specific cell type the risk of accumulating DNA damage may vary. For instance, damage to stem cells if not repaired can lead to mutation amplification or propagation through the processes of self-renewal and differentiation, respectively, whereas damage to post-mitotic cells can affect mostly tissue homeostasis. Stem cells are therefore expected to address DNA damage differently from their somatic counterparts. In this review the information available on the common and distinct mechanisms of control of genome integrity utilized by different cell types along the self-renewal/differentiation program will be reviewed, with special emphasis on their roles in the prevention of aging and disease.

Long-term muscle-derived cell culture: multipotency and susceptibility to cell death stimuli

Improvement in the yield of adult organism stem cells, and the ability to manage their differentiation and survival potential are the major goals in their application in regenerative medicine and in the adult stem cell research. We have demonstrated that adult rabbit muscle-derived cell lines with an unlimited proliferative potential in vitro can differentiate into myogenic, osteogenic, adipogenic and neurogenic lineages. Studies of cell survival in vitro showed that differentiated cells, except neurogenic ones, are more resistant to apoptosis inducers compared to proliferating cells. Resistance to death signals correlated with the level of protein kinase AKT phosphorylation. Skeletal muscle-derived cell lines can be multipurpose tools in therapy. Enhanced resistance of differentiated cells to certain types of damage shows their potential for long-term survival and maintenance in an organism. This article was published online on 29 January 2013. An error was subsequently identified. This notice is included in the online and print versions to indicate that both have been corrected 6 March 2013.

Maintenance of genomic integrity after DNA double strand breaks in the human prostate and seminal vesicle epithelium: the best and the worst

Prostate cancer is one of the most frequent cancer types in men, and its incidence is steadily increasing. On the other hand, primary seminal vesicle carcinomas are extremely rare with less than 60 cases reported worldwide. Therefore the difference in cancer incidence has been estimated to be more than a 100,000-fold. This is astonishing, as both tissues share similar epithelial structure and hormonal cues. Clearly, the two epithelia differ substantially in the maintenance of genomic integrity, possibly due to inherent differences in their DNA damage burden and DNA damage signaling. The DNA damage response evoked by DNA double strand breaks may be relevant, as their faulty repair has been implicated in the formation of common genomic rearrangements such as TMPRSS2-ERG fusions during prostate carcinogenesis. Here, we review DNA damaging processes of both tissues with an emphasis on inflammation and androgen signaling. We discuss how benign prostate and seminal vesicle epithelia respond to acute DNA damage, focusing on the canonical DNA double strand break-induced ATM-pathway, p53 and DNA damage induced checkpoints. We propose that the prostate might be more prone to the accumulation of genetic aberrations during epithelial regeneration than seminal vesicles due to a weaker ability to enforce DNA damage checkpoints.

DNA damage response by single-strand breaks in terminally differentiated muscle cells and the control of muscle integrity

DNA single-strand breaks (SSB) formation coordinates the myogenic program, and defects in SSB repair in post-mitotic cells have been associated with human diseases. However, the DNA damage response by SSB in terminally differentiated cells has not been explored yet. Here we show that mouse post-mitotic muscle cells accumulate SSB after alkylation damage, but they are extraordinarily resistant to the killing effects of a variety of SSB-inducers. We demonstrate that, upon SSB induction, phosphorylation of H2AX occurs in myotubes and is largely ataxia telangiectasia mutated (ATM)-dependent. However, the DNA damage signaling cascade downstream of ATM is defective as shown by lack of p53 increase and phosphorylation at serine 18 (human serine 15). The stabilization of p53 by nutlin-3 was ineffective in activating the cell death pathway, indicating that the resistance to SSB inducers is due to defective p53 downstream signaling. The induction of specific types of damage is required to activate the cell death program in myotubes. Besides the topoisomerase inhibitor doxorubicin known for its cardiotoxicity, we show that the mitochondria-specific inhibitor menadione is able to activate p53 and to kill effectively myotubes. Cell killing is p53-dependent as demonstrated by full protection of myotubes lacking p53, but there is a restriction of p53-activated genes. This new information may have important therapeutic implications in the prevention of muscle cell toxicity.

Inhibitors of tyrosine phosphatases and apoptosis reprogram lineage-marked differentiated muscle to myogenic progenitor cells

Muscle regeneration declines with aging and myopathies, and reprogramming of differentiated muscle cells to their progenitors can serve as a robust source of therapeutic cells. Here, we used the Cre-Lox method to specifically label postmitotic primary multinucleated myotubes and then utilized small molecule inhibitors of tyrosine phosphatases and apoptosis to dedifferentiate these myotubes into proliferating myogenic cells, without gene overexpression. The reprogrammed, fusion competent, muscle precursor cells contributed to muscle regeneration in vitro and in vivo and were unequivocally distinguished from reactivated reserve cells because of the lineage marking method. The small molecule inhibitors downregulated cell cycle inhibitors and chromatin remodeling factors known to promote and maintain the cell fate of myotubes, facilitating cell fate reversal. Our findings enhance understanding of cell-fate determination and create novel therapeutic approaches for improved muscle repair.

Terminally differentiated astrocytes lack DNA damage response signaling and are radioresistant but retain DNA repair proficiency

The impact and consequences of damage generation into genomic DNA, especially in the form of DNA double-strand breaks, and of the DNA-damage response (DDR) pathways that are promptly activated, have been elucidated in great detail. Most of this research, however, has been performed on proliferating, often cancerous, cell lines. In a mammalian body, the majority of cells are terminally differentiated (TD), and derives from a small pool of self-renewing somatic stem cells. Here, we comparatively studied DDR signaling and radiosensitivity in neural stem cells (NSC) and their TD-descendants, astrocytes - the predominant cells in the mammalian brain. Astrocytes have important roles in brain physiology, development and plasticity. We discovered that NSC activate canonical DDR upon exposure to ionizing radiation. Strikingly, astrocytes proved radioresistant, lacked functional DDR signaling, with key DDR genes such as ATM being repressed at the transcriptional level. Nevertheless, astrocytes retain the expression of non-homologous end-joining (NHEJ) genes and indeed they are DNA repair proficient. Unlike in NSC, in astrocytes DNA-PK seems to be the PI3K-like protein kinase responsible for γH2AX signal generation upon DNA damage. We also demonstrate the lack of functional DDR signaling activation in vivo in astrocytes of irradiated adult mouse brains, although adjacent neurons activate the DDR.

Skeletal muscle apoptotic response to physical activity: potential mechanisms for protection

Apoptosis is a highly conserved type of cell death that plays a critical role in tissue homeostasis and disease-associated processes. Skeletal muscle is unique with respect to apoptotic processes, given its multinucleated morphology and its apoptosis-associated differences related to muscle and (or) fiber type as well as mitochondrial content and (or) subtype. Elevated apoptotic signaling has been reported in skeletal muscle during aging, stress-induced states, and disease; a phenomenon that plays a role in muscle dysfunction, degradation, and atrophy. Exercise is a strong physiological stimulus that can influence a number of extracellular and intracellular signaling pathways, which may directly or indirectly influence apoptotic processes in skeletal muscle. In general, acute strenuous and eccentric exercise are associated with a proapoptotic phenotype and increased DNA fragmentation (a hallmark of apoptosis), whereas regular exercise training or activity is associated with an antiapoptotic environment and reduced DNA fragmentation in skeletal muscle. Interestingly, the protective effect of regular activity on skeletal muscle apoptotic processes has been observed in healthy, aged, stress-induced, and diseased rodent models. Several mechanisms for this protective response have been proposed, including altered anti- and proapoptotic protein expression, increased mitochondrial biogenesis and improved mitochondrial function, and reduced reactive oxygen species generation and (or) enhanced antioxidant status. Given the current literature, we propose that regular physical activity may represent an effective strategy to decrease apoptotic signaling, and possibly muscle wasting and dysfunction, during aging and disease.

Cell death-resistance of differentiated myotubes is associated with enhanced anti-apoptotic mechanisms compared to myoblasts

Skeletal muscle atrophy is associated with elevated apoptosis while muscle differentiation results in apoptosis resistance, indicating that the role of apoptosis in skeletal muscle is multifaceted. The objective of this study was to investigate mechanisms underlying apoptosis susceptibility in proliferating myoblasts compared to differentiated myotubes and we hypothesized that cell death-resistance in differentiated myotubes is mediated by enhanced anti-apoptotic pathways. C(2)C(12) myoblasts and myotubes were treated with H(2)O(2) or staurosporine (Stsp) to induce cell death. H(2)O(2) and Stsp induced DNA fragmentation in more than 50% of myoblasts, but in myotubes less than 10% of nuclei showed apoptotic changes. Mitochondrial membrane potential dissipation was detected with H(2)O(2) and Stsp in myoblasts, while this response was greatly diminished in myotubes. Caspase-3 activity was 10-fold higher in myotubes compared to myoblasts, and Stsp caused a significant caspase-3 induction in both. However, exposure to H(2)O(2) did not lead to caspase-3 activation in myoblasts, and only to a modest induction in myotubes. A similar response was observed for caspase-2, -8 and -9. Abundance of caspase-inhibitors (apoptosis repressor with caspase recruitment domain (ARC), and heat shock protein (HSP) 70 and -25 was significantly higher in myotubes compared to myoblasts, and in addition ARC was suppressed in response to Stsp in myotubes. Moreover, increased expression of HSPs in myoblasts attenuated cell death in response to H(2)O(2) and Stsp. Protein abundance of the pro-apoptotic protein endonuclease G (EndoG) and apoptosis-inducing factor (AIF) was higher in myotubes compared to myoblasts. These results show that resistance to apoptosis in myotubes is increased despite high levels of pro-apoptotic signaling mechanisms, and we suggest that this protective effect is mediated by enhanced anti-caspase mechanisms.

Effect of hypothermia on doxorubicin-induced cardiac myoblast signaling and cell death

BACKGROUND

Anthracyclines (AC) are useful chemotherapeutic agents whose principal limitation is cardiac toxicity, which may progress to heart failure, transplantation or even death. We have shown that this toxicity involves oxidative stress-induced activation of the DNA damage pathway. Hypothermia has been shown to be protective against other diseases involving oxidative stress but has not been studied in models of AC toxicity.

METHODS

In the current experiments, H9C2 cardiac myoblasts were treated with varying concentrations of the AC doxorubicin (DOX) during normothermia (37°C) or mild hypothermia (35°C). Total cell death was assayed using trypan blue exclusion and apoptosis by terminal deoxynucleotidyl transferase-mediated deoxyuridine-biotin nick end labeling (TUNEL) staining. Oxidative stress was assayed using the fluorescent indicator 2'7'-dichlorofluorescein diacetate. DNA damage pathway activation was assayed by immunostaining for H2AX and p53. Mitochondrial membrane potential was assayed by JC-1 staining.

RESULTS

At all concentrations of DOX examined (1, 2.5 and 5 μM), hypothermia reduced oxidative stress, activation of H2AX and p53, loss of mitochondrial membrane potential and total and apoptotic cell death (P=.001-.03 for each observation).

CONCLUSIONS

The reduction of oxidative stress-induced activation of the DNA damage pathway and consequent cell death by mild hypothermia supports a possible protective role to reduce the clinical impact of DOX-induced cardiac toxicity. Such an approach may allow expanded use of these effective chemotherapeutic agents to increase cancer cure rates.

Positive regulation of DNA double strand break repair activity during differentiation of long life span cells: the example of adipogenesis

Little information is available on the ability of terminally differentiated cells to efficiently repair DNA double strand breaks (DSBs), and one might reasonably speculate that efficient DNA repair of these threatening DNA lesions, is needed in cells of long life span with no or limited regeneration from precursor. Few tissues are available besides neurons that allow the study of DNA DSBs repair activity in very long-lived cells. Adipocytes represent a suitable model since it is generally admitted that there is a very slow turnover of adipocytes in adult. Using both Pulse Field Gel Electrophoresis (PFGE) and the disappearance of the phosphorylated form of the histone variant H2AX, we demonstrated that the ability to repair DSBs is increased during adipocyte differentiation using the murine pre-adipocyte cell line, 3T3F442A. In mammalian cells, DSBs are mainly repaired by the non-homologous end-joining pathway (NHEJ) that relies on the DNA dependent protein kinase (DNA-PK) activity. During the first 24 h following the commitment into adipogenesis, we show an increase in the expression and activity of the catalytic sub-unit of the DNA-PK complex, DNA-PKcs. The increased in DNA DSBs repair activity observed in adipocytes was due to the increase in DNA-PK activity as shown by the use of DNA-PK inhibitor or sub-clones of 3T3F442A deficient in DNA-PKcs using long term RNA interference. Interestingly, the up-regulation of DNA-PK does not regulate the differentiation program itself. Finally, similar positive regulation of DNA-PKcs expression and activity was observed during differentiation of primary culture of pre-adipocytes isolated from human sub-cutaneous adipose tissue.

Our results show that DNA DSBs repair activity is up regulated during the early commitment into adipogenesis due to an up-regulation of DNA-PK expression and activity. In opposition to the general view that DNA DSBs repair is decreased during differentiation, our results demonstrate that an up-regulation of this process might be observed in post-mitotic long-lived cells.

Repair system of 7, 8-dihydro-8-oxoguanine as a defense line against carcinogenesis

Reactive oxygen species (ROS) are essentially harmful for living organisms, including human beings. It is well known that ROS-induced damage of cellular components may lead to human diseases, such as inflammatory diseases, degenerative diseases, or cancer. In particular, oxidative DNA damage is premutagenic, and thus, the generation of DNA damage and the failure of its removal are critical events for tumorigenesis or carcinogenesis. To prevent this disadvantage, living organisms have defense mechanisms against ROS-induced gene instability. Studies of 8-oxo-Gua and its main repair enzyme, 8-oxoguanine DNA glycosylase 1 (OGG1), are informative and useful, because 8-oxo-Gua is commonly observed in DNA, and OGG1 enzymes exist in a wide variety of living organisms. The importance of OGG1 was confirmed by polymorphism analyses and studies using knockout mice. Moreover, analyses of the influences of environmental factors on DNA damage and repair systems have confirmed the effects of heavy metals on 8-oxo-Gua formation and OGG1 expression. These studies revealed that the 8-oxo-Gua repair system is crucial for the prevention of mutation-related diseases, such as cancer. In this review, the advances in this field during the last two decades are described.

Modification of the ATM/ATR directed DNA damage response state with aging and long after hepatocyte senescence induction in vivo

The cellular DNA damage response (DDR) entails the activation of ATM, ATR and/or DNA PK protein kinases that causes modifications of proteins including Chk1, Chk2 and 53BP1, aggregation of DDR proteins into foci, and activation of p53. The DDR is thought to be required for initiation and maintenance of cellular senescence. Potentially senescent cells with DNA damage foci occur in large numbers in vivo with many diseases, but, with the exception of mammalian dermis, there is little evidence for that with normal aging. After experimental induction of cellular senescence in the livers of juvenile mice, there was robust expression of DDR markers in hepatocytes at 1 week; however, by 7 weeks, activation of ATM/ATR kinase targets was limited, although cells with DNA damage foci were present. An analysis of hepatocytes of aged, 22-month-old mice, not experimentally exposed to genotoxins, showed limited activation of ATM/ATR targets, though high numbers of cells with DNA damage foci were found, similar to that seen many weeks after artificial senescence induction in young mice. Based on senescence heterochromatin and SA ss Gal assays of the 22-month-old mouse liver, more than 20% of hepatocytes were potentially senescent, though only some components of the DDR were enriched.

TGF-beta1-induced cardiac myofibroblasts are nonproliferating functional cells carrying DNA damages

TGF-beta1 induces differentiation and total inhibition of cardiac MyoFb cell division and DNA synthesis. These effects of TGF-beta1 are irreversible. Inhibition of MyoFb proliferation is accompanied with the expression of Smad1, Mad1, p15Ink4B and total inhibition of telomerase activity. Surprisingly, TGF-beta1-activated MyoFbs are growth-arrested not only at G1-phase but also at S-phase of the cell cycle. Staining with TUNEL indicates that these cells carry DNA damages. However, the absolute majority of MyoFbs are non-apoptotic cells as established with two apoptosis-specific methods, flow cytometry and caspase-dependent cleavage of cytokeratin 18. Expression in MyoFbs of proliferative cell nuclear antigen even in the absence of serum confirms that these MyoFbs perform repair of DNA damages. These results suggest that TGF-beta1-activated MyoFbs can be growth-arrested by two checkpoints, the G1/S checkpoint, which prevents cells from entering S-phase and the intra-S checkpoint, which is activated by encountering DNA damage during the S phase or by unrepaired damage that escapes the G1/S checkpoint. Despite carrying of the DNA damages TGF-beta1-activated MyoFbs are highly functional cells producing lysyl oxidase and contracting the collagen matrix.

DNA damage and cellular differentiation: more questions than responses

Studies on DNA damage responses in proliferating cells have revealed the relationship between sensing and repair of the DNA lesions and the regulation of the cell cycle, leading to the discovery and molecular characterization of the DNA damage-activated cell cycle checkpoints. Much less is known about the DNA damage response in progenitors of differentiated cells, in which cell cycle arrest is a critical signal to trigger the differentiation program, and in terminally differentiated cells, which are typically post-mitotic. How DNA lesions are detected, processed and repaired in these cells, the functional impact of DNA damage on transcription of differentiation-specific genes, how these events are coordinated at the molecular level, the consequence of defective DNA damage response on tissue-specific functions and its potential relationship with age-related diseases are currently open questions. In particular the biological complexity inherent to the global genome reprogramming of tissue progenitors, such as embryonic or adult stem cells, suggests the importance of an accurate DNA damage response at the transcription level in these cells to ensure the genomic integrity of regenerating tissues.

Terminally differentiated muscle cells are defective in base excision DNA repair and hypersensitive to oxygen injury

The differentiation of skeletal myoblasts is characterized by permanent withdrawal from the cell cycle and fusion into multinucleated myotubes. Muscle cell survival is critically dependent on the ability of cells to respond to oxidative stress. Base excision repair (BER) is the main repair mechanism of oxidative DNA damage. In this study, we compared the levels of endogenous oxidative DNA damage and BER capacity of mouse proliferating myoblasts and their differentiated counterpart, the myotubes. Changes in the expression of oxidative stress marker genes during differentiation, together with an increase in 8-hydroxyguanine DNA levels in terminally differentiated cells, suggested that reactive oxygen species are produced during this process. The repair of 2-deoxyribonolactone, which is exclusively processed by long-patch BER, was impaired in cell extracts from myotubes. The repair of a natural abasic site (a preferred substrate for short-patch BER) also was delayed. The defect in BER of terminally differentiated muscle cells was ascribed to the nearly complete lack of DNA ligase I and to the strong down-regulation of XRCC1 with subsequent destabilization of DNA ligase IIIalpha. The attenuation of BER in myotubes was associated with significant accumulation of DNA damage as detected by increased DNA single-strand breaks and phosphorylated H2AX nuclear foci upon exposure to hydrogen peroxide. We propose that in skeletal muscle exacerbated by free radical injury, the accumulation of DNA repair intermediates, due to attenuated BER, might contribute to myofiber degeneration as seen in sarcopenia and many muscle disorders.

DNA damage response mediators MDC1 and 53BP1: constitutive activation and aberrant loss in breast and lung cancer, but not in testicular germ cell tumours

MDC1 and 53BP1 are critical components of the DNA damage response (DDR) machinery that protects genome integrity and guards against cancer, yet the tissue expression patterns and involvement of these two DDR adaptors/mediators in human tumours remain largely unknown. Here we optimized immunohistochemical analyses of human 53BP1 and MDC1 proteins in situ and identified their virtually ubiquitous expression, both in proliferating and quiescent, differentiated tissues. Focus formation by 53BP1 and/or MDC1 in human spermatogenesis and subsets of breast and lung carcinomas indicated physiological and 'pathological' activation of the DDR, respectively. Furthermore, aberrant reduction or lack of either protein in significant proportions of carcinomas supported the candidacy of 53BP1 and MDC1 for tumour suppressors. Contrary to carcinomas, almost no activation or loss of MDC1 or 53BP1 were found among testicular germ-cell tumours (TGCTs), a tumour type with unique biology and exceptionally low incidence of p53 mutations. Such concomitant presence (in carcinomas) or absence (in TGCTs) of DDR activation and DDR aberrations supports the roles of MDC1 and 53BP1 within the ATM/ATR-regulated checkpoint network which, when activated, provides an early anti-cancer barrier the pressure of which selects for DDR defects such as p53 mutations or loss of 53BP1/MDC1 during cancer progression.

Muscle cachexia is regulated by a p53-PW1/Peg3-dependent pathway

Muscle wasting (cachexia) is an incurable complication associated with chronic infection and cancers that leads to an overall poor prognosis for recovery. Tumor necrosis factor-alpha (TNFalpha) is a key inflammatory cytokine associated with cachexia. TNFalpha inhibits myogenic differentiation and skeletal muscle regeneration through downstream effectors of the p53 cell death pathway including PW1/Peg3, bax, and caspases. We report that p53 is required for the TNFalpha-mediated inhibition of myogenesis in vitro and contributes to muscle wasting in response to tumor load in vivo. We further demonstrate that PW1 and p53 participate in a positive feedback regulatory loop in vitro. Consistent with this observation, we find that the number of PW1-expressing stem cells in skeletal muscle declines significantly in p53 nullizygous mice. Furthermore, gene transfer of a dominant-negative form of PW1 into muscle tissue in vivo blocks myofiber atrophy in response to tumor load. Taken together, these results show a novel role for p53 in mediating muscle stem cell behavior and muscle atrophy, and point to new targets for the therapeutic treatment of muscle wasting.

Hepatocytes with extensive telomere deprotection and fusion remain viable and regenerate liver mass through endoreduplication

We report that mouse liver cells are highly resistant to extensive telomere dysfunction. In proliferating cells, telomere dysfunction results in chromosome end fusions, a DNA damage signal, and apoptosis or senescence. To determine the consequences of telomere dysfunction in noncycling cells, we used conditional deletion of the telomeric protein TRF2 in hepatocytes. TRF2 loss resulted in telomeric accumulation of gamma-H2AX and frequent telomere fusions, indicating telomere deprotection. However, there was no induction of p53 or apoptosis, and liver function appeared unaffected. Furthermore, the loss of TRF2 did not compromise liver regeneration after partial hepatectomy. Remarkably, liver regeneration occurred without cell division involving endoreduplication and cell growth, thereby circumventing the chromosome segregation problems associated with telomere fusions. We conclude that nondividing hepatocytes can maintain and regenerate liver function despite substantial loss of telomere integrity.

The CHEK2 gene and inherited breast cancer susceptibility

Checkpoint kinase 2 (CHEK2, Chk2) emerges as an important signal transducer of cellular responses to DNA damage and a candidate tumor suppressor whose defects contribute to molecular pathogenesis of diverse types of human malignancies, both sporadic and hereditary. Here, we briefly outline the molecular properties, regulation and physiological role of CHEK2, and review in more detail its defects that predispose to tumors, with particular emphasis on familial breast cancer. The frequency, penetrance and epidemiological as well as clinical significance of the two most studied breast cancer-predisposing variants of the CHEK2 gene, 1100delC and I157T, are highlighted in more depth, and additional CHEK2 mutations and their cancer relevance are discussed as well. These recent findings are considered also from a broader perspective of CHEK2 as the integral component of the ataxia telangiectasia-mutated-CHEK2-p53 pathway within the genome integrity maintenance system and a barrier against tumor progression. Finally, the potential value of information about the CHEK2 status in family counseling and optimizition of individualized cancer treatment is discussed.

DNA damage is an early event in doxorubicin-induced cardiac myocyte death

Anthracyclines are antitumor agents the main clinical limitation of which is cardiac toxicity. The mechanism of this cardiotoxicity is thought to be related to generation of oxidative stress, causing lethal injury to cardiac myocytes. Although protein and lipid oxidation have been documented in anthracycline-treated cardiac myocytes, DNA damage has not been directly demonstrated. This study was undertaken to determine whether anthracyclines induce cardiac myocyte DNA damage and whether this damage is linked to a signaling pathway culminating in cell death. H9c2 cardiac myocytes were treated with the anthracycline doxorubicin at clinically relevant concentrations, and DNA damage was assessed using the alkaline comet assay. Doxorubicin induced DNA damage, as shown by a significant increase in the mean tail moment above control, an effect ameliorated by inclusion of a free radical scavenger. Repair of DNA damage was incomplete after doxorubicin treatment in contrast to the complete repair observed in H2O2-treated myocytes after removal of the agent. Immunoblot analysis revealed that p53 activation occurred subsequent in time to DNA damage. By a fluorescent assay, doxorubicin induced loss of mitochondrial membrane potential after p53 activation. Chemical inhibition of p53 prevented doxorubicin-induced cell death and loss of mitochondrial membrane potential without preventing DNA damage, indicating that DNA damage was proximal in the events leading from doxorubicin treatment to cardiac myocyte death. Specific doxorubicin-induced DNA lesions included oxidized pyrimidines and 8-hydroxyguanine. DNA damage therefore appears to play an important early role in anthracycline-induced lethal cardiac myocyte injury through a pathway involving p53 and the mitochondria.

Impaired elimination of DNA double-strand break-containing lymphocytes in ataxia telangiectasia and Nijmegen breakage syndrome

The repair of DNA double-strand breaks is critical for genome integrity and tumor suppression. Here we show that following treatment with the DNA-intercalating agent actinomycin D (ActD), normal quiescent T cells accumulate double-strand breaks and die, whereas T cells from ataxia telangiectasia (AT) and Nijmegen breakage syndrome (NBS) patients are resistant to this death pathway despite a comparable amount of DNA damage. We demonstrate that the ActD-induced death pathway in quiescent T lymphocytes follows DNA damage and H2AX phosphorylation, is ATM- and NBS1-dependent and due to p53-mediated cellular apoptosis. In response to genotoxic 2-Gy gamma-irradiation, on the other hand, quiescent T cells from normal donors survive following complete resolution of the damage thus induced. T cells from AT and NBS patients also survive, but retain foci of phosphorylated H2AX due to a subtle double-strand break (DSB) repair defect. A common consequence of these two genetic defects in the DSB response is the apparent tolerance of cells containing DNA breaks. We suggest that this tolerance makes a major contribution to the oncogenic risk of patients with chromosome instability syndromes.

The ATM-mediated DNA-damage response: taking shape

Cellular responses to DNA damage are crucial for maintaining homeostasis and preventing the development of cancer. Our understanding of the DNA-damage response has evolved: whereas previously the focus was on DNA repair, we now appreciate that the response to DNA lesions involves a complex, highly branched signaling network. Defects in this response lead to severely debilitating, cancer-predisposing "genomic instability syndromes". Double strand breaks (DSBs) in DNA are potent triggers of the DNA-damage response, which is why they are used to study this pathway. The chief transducer of the DSB signal is the nuclear protein kinase ataxia-telangiectasia mutated (ATM). Genetic, biochemical and structural studies have recently provided insights into the ATM-mediated DSB response, reshaping our view of this signaling pathway while raising new questions.

DNp73α protects myogenic cells from apoptosis

The P73 gene is transcribed from two promoters, P1 and P2, that direct the expression of multiple transactivation competent (TA) and dominant negative (DN) isoforms. TAp73 transcription factors mediate cell cycle arrest and/or apoptosis in response to DNA damage and are involved in developmental processes. P73 mRNA levels increase and the P1p73 promoter is upregulated during myogenic differentiation of C2C12 skeletal muscle satellite cells. The DNp73 proteins act as trans-repressors of p53- and p73-dependent transcription, and possess both antiapoptotic and pro-proliferative potential. Here, we show that DNp73alpha is expressed in proliferating C2C12 myoblasts, rapidly accumulates in differentiating myocytes and remains elevated in C2C12 myotubes. By combining transactivation assays and chromatin immunoprecipitation analysis, we could show that the upregulation of the P2p73 promoter during myogenic differentiation is mediated by the coordinated recruitment and activity of MyoD and p53/p73. Abrogation of DNp73 expression by specific siRNA led to a strong potentiation of the spontaneous apoptosis of C2C12 myoblasts induced to differentiate. Finally, unlike TAp73 that contributes to DNA damage-induced apoptosis of myotubes, endogenous DNp73 mediates the relative resistance of differentiated myotubes to DNA damage. Altogether, our findings identify DNp73alpha as an important target in designing strategies aimed at the potentiation of the regenerative potential of skeletal satellite cells.