Hypertonic stress response

Mammalian renal inner medullary cells are normally exposed to extremely high NaCl concentrations. Remarkably, under these normal conditions, the high NaCl causes DNA damage and inhibits its repair, yet the cells survive and function both in cell culture and in vivo. The interstitial NaCl concentration in parts of a normal renal medulla can be 500 mM or more, depending on the species. Studies of how the cells survive and function despite this extreme stress have led to the discovery of protective adaptations, including accumulation of large amounts of organic osmolytes, which normalize cell volume and intracellular ionic strength, despite the hypertonicity of the high NaCl. Those adaptations, however, do not prevent DNA damage. High NaCl induces DNA breaks rapidly, and the DNA breaks persist even after the cells become adapted to the high NaCl. The adapted cells proliferate rapidly in cell culture and function adequately in vivo despite the DNA breaks. Both in cell culture and in vivo the breaks are rapidly repaired if the NaCl concentration is lowered. Although acute elevation of NaCl causes transient cell cycle arrest and, when the elevation is too extreme, apoptosis, proliferation of adapted cells is not arrested in culture and apoptosis is not evident either in culture or in vivo. Further, high NaCl impairs activation of several components of the classical DNA damage response such as Mre11, H2AX and Chk1 leading to inhibition of DNA repair. Nevertheless, other regular participants in the DNA damage response, such as Gadd45a, Gadd153, p53, Hsp70, and ATM are still upregulated by high NaCl. How high NaCl causes the DNA breaks and how the cells survive them is conjectural at this point. We discuss possible answers to these questions, based on current knowledge about induction and processing of DNA breaks.     

沉默DAXX基因表达促进人卵巢癌细胞凋亡和衰老

DAXX是Fas死亡结构域相关蛋白(Fas death-associated protein, DAXX),可与Fas死亡受体的死亡域结合.通过激活c-Jun NH2末端激酶通路,DAXX增强Fas介导的凋亡,同时也增强转录生长因子β依赖的凋亡.本研究通过免疫组化检测人正常卵巢组织和卵巢癌组织中DAXX蛋白表达,然后通过Tet-on诱导体系构建DAXX基因沉默的慢病毒,感染卵巢癌细胞后,加入嘌呤霉素筛选稳定表达的OV2008细胞,四环素诱导DAXX基因沉默,通过免疫印迹和定量PCR检测DAXX基因干扰效果,MTT检测细胞增殖,克隆形成实验检测克隆形成能力,免疫印迹和免疫荧光方法检测DNA损伤蛋白的变化,SA-β-gal检测细胞衰老.结果表明,在人正常卵巢组织中DAXX低表达,而在人卵巢癌组织中DAXX高表达,随着四环素浓度的增加DAXX的表达量降低,DAXX基因沉默抑制卵巢癌OV2008细胞的增殖和克隆形成. 免疫印迹结果显示,DAXX基因沉默可诱导DNA损伤相关蛋白p-H2AX和p-CHK2高表达.SA-β-gal检测结果表明,DAXX基因沉默可诱导OV2008细胞发生衰老,免疫印迹检测发现,p21和p27在 DAXX沉默的卵巢癌细胞中高表达.综上结果,DAXX沉默可以抑制卵巢癌细胞的增殖和克隆形成,同时也促进卵巢癌细胞发生DNA损伤和衰老.该研究为卵巢癌的基因治疗提供新的思路.     

Intermittent High Glucose Implements Stress-Induced Senescence in Human Vascular Endothelial Cells: Role of Superoxide Production by NADPH Oxidase

Impaired glucose tolerance characterized by postprandial hyperglycemia, which occurs frequently in elderly persons and represents an important preliminary step in diabetes mellitus, poses an independent risk factor for the development of atherosclerosis. Endothelial cellular senescence is reported to precede atherosclerosis. We reported that continuous high glucose stimulus causes endothelial senescence more markedly than hypertension or dyslipidemia stimulus. In the present study, we evaluated the effect of fluctuating glucose levels on human endothelial senescence. Constant high glucose increased senescence-associated-β-galactosidase(SA-β-gal) activity, a widely used marker for cellular senescence. Interestingly, in intermittent high glucose, this effect was more pronounced as well as increase of p21 and p16^INK4a , senescence related proteins with DNA damage. However, telomerase was not activated and telomere length was not shortened, thus stress-induced senescence was shown. However, constant high glucose activated telomerase and shortened telomere length, which suggested replicative senescence. Intermittent but not constant high glucose strikingly up-regulated the expression of p22^phox, an NADPH oxidase component, increasing superoxide. The small interfering RNA of p22^phox undermined the increase in SA-β-gal activity induced by intermittent high glucose. Conclusively, intermittent high glucose can promote vascular endothelial senescence more than constant high glucose, which is in partially dependent on superoxide overproduction.     

Response of renal inner medullary epithelial cells to osmotic stress

As part of the urinary concentrating mechanism, renal inner medullary epithelial (IME) cells are normally exposed to variable and often very high interstitial levels of NaCl and urea, yet they survive and function. We have been studying the mechanisms involved, using an established cell line (mIMCD3). Acute increase of NaCl or urea from 300 to >500 mOsmol/kg causes cell cycle delay and apoptosis. High NaCl, but not high urea, causes DNA double strand breaks. At 500-600 mOsmol/kg inhibition of DNA replication following high NaCl depends on activation of the tumor suppressor protein, p53, and provides time for DNA repair. If p53 expression is suppressed, cells continue to replicate DNA, and many of those cells die. At higher levels of NaCl (>650 mOsmol/kg) the mitochondria rapidly depolarize and most cells die within a few hours despite a high level of p53 protein (which, however, is less phosphorylated than at 500 mOsmol/kg). Since the levels of NaCl and urea that kill mIMCD3 cells are much lower than those that exist in vivo, we investigated the difference, using early passage mouse IME cells under various conditions. Passage 2 IME cells survive higher levels of NaCl and urea than do mIMCD3 cells, but still not levels as high as in vivo. However, when the osmolality is increased linearly over 20 h, as occurs in vivo, rather than as a single step, cell survival increases to levels close to those found in vivo. We conclude that a more gradual increase in osmolality provides time for accumulation of organic osmolytes and activation of heat shock protein, previously known to be important for cell survival.     

Living with DNA Breaks is an Everyday Reality for Cells Adapted to High NaCl

A rapid, coordinated response to DNA breaks, including activation of cell cycle checkpoints and initiation of accurate DNA repair is believed to be necessary to maintain genomic integrity and prevent accumulation of mutations. That is why it was so unexpected to discover recently that in the mouse renal inner medulla the otherwise healthy cells contain numerous DNA breaks, yet they survive and function adequately. The DNA breaks in the renal inner medulla are caused by the high NaCl concentrations to which the cells are constantly exposed as a consequence of the urinary concentrating mechanism. Cells adapted to high NaCl in cell culture also contain many DNA breaks. The DNA breaks do not trigger cell cycle arrest or cause apoptosis, and the cells safely proliferate rapidly despite their presence. Further, high NaCl inhibits the activity of key components of the classical DNA damage response such as Mre11, chk1 and H2AX. In order to explain why the DNA breaks do not cause disabling mutations, oncogenic transformations and/or apoptosis we speculate that in the presence of high NaCl there might be alternative DNA damage response pathways or special ways of coping with DNA damage.     

Living with DNA breaks is an everyday reality for cells adapted to high NaCl

A rapid, coordinated response to DNA breaks, including activation of cell cycle checkpoints and initiation of accurate DNA repair is believed to be necessary to maintain genomic integrity and prevent accumulation of mutations. That is why it was so unexpected to discover recently that in the mouse renal inner medulla the otherwise healthy cells contain numerous DNA breaks, yet they survive and function adequately. The DNA breaks in the renal inner medulla are caused by the high NaCl concentrations to which the cells are constantly exposed as a consequence of the urinary concentrating mechanism. Cells adapted to high NaCl in cell culture also contain many DNA breaks. The DNA breaks do not trigger cell cycle arrest or cause apoptosis, and the cells safely proliferate rapidly despite their presence. Further, high NaCl inhibits the activity of key components of the classical DNA damage response such as Mre11, chk1 and H2AX. In order to explain why the DNA breaks do not cause disabling mutations, oncogenic transformations and/or apoptosis we speculate that in the presence of high NaCl there might be alternative DNA damage response pathways or special ways of coping with DNA damage.     

Resveratrol Induced Premature Senescence Is Associated with DNA Damage Mediated SIRT1 and SIRT2 Down-Regulation

The natural polyphenolic compound resveratrol (3,4,5-trihydroxy-trans-stilbene) has broad spectrum health beneficial activities including antioxidant, anti-inflammatory, anti-aging, anti-cancer, cardioprotective, and neuroprotective effects. Remarkably, resveratrol also induces apoptosis and cellular senescence in primary and cancer cells. Resveratrol’s anti-aging effects both in vitro and in vivo attributed to activation of a (NAD)-dependent histone deacetylase family member sirtuin-1 (SIRT1) protein. In mammals seven members (SIRT1-7) of sirtuin family have been identified. Among those, SIRT1 is the most extensively studied with perceptive effects on mammalian physiology and suppression of the diseases of aging. Yet no data has specified the role of sirtuins, under conditions where resveratrol treatment induces senescence. Current study was undertaken to investigate the effects of resveratrol in human primary dermal fibroblasts (BJ) and to clarify the role of sirtuin family members in particular SIRT1 and SIRT2 that are known to be involved in cellular stress responses and cell cycle, respectively. Here, we show that resveratrol decreases proliferation of BJ cells in a time and dose dependent manner. In addition the increase in senescence associated β-galactosidase (SA-β-gal) activity and methylated H3K9-me indicate the induction of premature senescence. A significant increase in phosphorylation of γ-H2AX, a surrogate of DNA double strand breaks, as well as in levels of p53, p21^CIP1 and p16^INK4A is also detected. Interestingly, at concentrations where resveratrol induced premature senescence we show a significant decrease in SIRT1 and SIRT2 levels by Western Blot and quantitative RT-PCR analysis. Conversely inhibition of SIRT1 and SIRT2 via siRNA or sirtinol treatment also induced senescence in BJ fibroblasts associated with increased SA-β-gal activity, γ-H2AX phosphorylation and p53, p21^CIP1 and p16^INK4A levels. Interestingly DNA damaging agent doxorubicin also induced senescence in BJ fibroblasts associated with decreased SIRT1/2 levels. In conclusion our data reveal that resveratrol induced premature senescence is associated with SIRT1 and SIRT2 down regulation in human dermal fibroblasts. Here we suggest that the concomitant decline in SIRT1/2 expression in response to resveratrol treatment may be a cause for induction of senescence, which is most likely mediated by a regulatory mechanism activated by DNA damage response.     

High Glucose Induced Alteration of SIRTs in Endothelial Cells Causes Rapid Aging in a p300 and FOXO Regulated Pathway

In diabetes, some of the cellular changes are similar to aging. We hypothesized that hyperglycemia accelerates aging-like changes in the endothelial cells (ECs) and tissues leading to structural and functional damage. We investigated glucose-induced aging in 3 types of ECs using senescence associated β-gal (SA β-gal) staining and cell morphology. Alterations of sirtuins (SIRTs) and their downstream mediator FOXO and oxidative stress were investigated. Relationship of such alteration with histone acetylase (HAT) p300 was examined. Similar examinations were performed in tissues of diabetic animals. ECs in high glucose (HG) showed evidence of early senescence as demonstrated by increased SA β-gal positivity and reduced replicative capacities. These alterations were pronounced in microvascular ECs. They developed an irregular and hypertrophic phenotype. Such changes were associated with decreased SIRT (1–7) mRNA expressions. We also found that p300 and SIRT1 regulate each other in such process, as silencing one led to increase of the others’ expression. Furthermore, HG caused reduction in FOXO1’s DNA binding ability and antioxidant target gene expressions. Chemically induced increased SIRT1 activity and p300 knockdown corrected these abnormalities slowing aging-like changes. Diabetic animals showed increased cellular senescence in renal glomerulus and retinal blood vessels along with reduced SIRT1 mRNA expressions in these tissues. Data from this study demonstrated that hyperglycemia accelerates aging-like process in the vascular ECs and such process is mediated via downregulation of SIRT1, causing reduction of mitochondrial antioxidant enzyme in a p300 and FOXO1 mediated pathway.     

The PI3K/Akt/mTOR Pathway Is Implicated in the Premature Senescence of Primary Human Endothelial Cells Exposed to Chronic Radiation

The etiology of radiation-induced cardiovascular disease (CVD) after chronic exposure to low doses of ionizing radiation is only marginally understood. We have previously shown that a chronic low-dose rate exposure (4.1 mGy/h) causes human umbilical vein endothelial cells (HUVECs) to prematurely senesce. We now show that a dose rate of 2.4 mGy/h is also able to trigger premature senescence in HUVECs, primarily indicated by a loss of growth potential and the appearance of the senescence-associated markers ß-galactosidase (SA-ß-gal) and p21. In contrast, a lower dose rate of 1.4 mGy/h was not sufficient to inhibit cellular growth or increase SA-ß-gal-staining despite an increased expression of p21. We used reverse phase protein arrays and triplex Isotope Coded Protein Labeling with LC-ESI-MS/MS to study the proteomic changes associated with chronic radiation-induced senescence. Both technologies identified inactivation of the PI3K/Akt/mTOR pathway accompanying premature senescence. In addition, expression of proteins involved in cytoskeletal structure and EIF2 signaling was reduced. Age-related diseases such as CVD have been previously associated with increased endothelial cell senescence. We postulate that a similar endothelial aging may contribute to the increased rate of CVD seen in populations chronically exposed to low-dose-rate radiation.     

Resveratrol Induces Premature Senescence in Lung Cancer Cells via ROS-Mediated DNA Damage

Resveratrol (RV) is a natural component of red wine and grapes that has been shown to be a potential chemopreventive and anticancer agent. However, the molecular mechanisms underlying RV''s anticancer and chemopreventive effects are incompletely understood. Here we show that RV treatment inhibits the clonogenic growth of non-small cell lung cancer (NSCLC) cells in a dose-dependent manner. Interestingly, the tumor-suppressive effect of low dose RV was not associated with any significant changes in the expression of cleaved PARP and activated caspase-3, suggesting that low dose RV treatment may suppress tumor cell growth via an apoptosis-independent mechanism. Subsequent studies reveal that low dose RV treatment induces a significant increase in senescence-associated β–galactosidase (SA-β-gal) staining and elevated expression of p53 and p21 in NSCLC cells. Furthermore, we show that RV-induced suppression of lung cancer cell growth is associated with a decrease in the expression of EF1A. These results suggest that RV may exert its anticancer and chemopreventive effects through the induction of premature senescence. Mechanistically, RV-induced premature senescence correlates with increased DNA double strand breaks (DSBs) and reactive oxygen species (ROS) production in lung cancer cells. Inhibition of ROS production by N-acetylcysteine (NAC) attenuates RV-induced DNA DSBs and premature senescence. Furthermore, we show that RV treatment markedly induces NAPDH oxidase-5 (Nox5) expression in both A549 and H460 cells, suggesting that RV may increase ROS generation in lung cancer cells through upregulating Nox5 expression. Together, these findings demonstrate that low dose RV treatment inhibits lung cancer cell growth via a previously unappreciated mechanism, namely the induction of premature senescence through ROS-mediated DNA damage.     

Mitochondrial dysfunction is a possible cause of accelerated senescence of mesothelial cells exposed to high glucose

High glucose has been found to accelerate cell senescence in vitro. The exact mechanism of this effect is, however, still poorly understood. In this paper we show that human peritoneal mesothelial cells (HPMCs) propagated under high (30 mM) glucose were characterized by higher density of DNA double-strand breaks than cells exposed to standard (5 mM) glucose concentration. Under both low and high glucose conditions, the vast majority of DNA damage localized to non-telomeric regions of the genome. Moreover, exposure to high glucose resulted in increased accumulation of lipofuscin, increased production of superoxides and peroxides as well as reduced mitochondrial membrane potential and increased mitochondrial mass. Treatment of cells with the free radical scavenger PBN partially rescued the premature senescence caused by high glucose. Together, these results indicate that high glucose may accelerate senescence of HPMCs by impairing mitochondrial function, resulting in overproduction of reactive oxygen species and extensive DNA damage.     

Lipid peroxidation and the subsequent cell death transmitting from ferroptotic cells to neighboring cells

Ferroptosis regulated cell death due to the iron-dependent accumulation of lipid peroxide. Ferroptosis is known to constitute the pathology of ischemic diseases, neurodegenerative diseases, and steatohepatitis and also works as a suppressing mechanism against cancer. However, how ferroptotic cells affect surrounding cells remains elusive. We herein report the transfer phenomenon of lipid peroxidation and cell death from ferroptotic cells to nearby cells that are not exposed to ferroptotic inducers (FINs). While primary mouse embryonic fibroblasts (MEFs) and NIH3T3 cells contained senescence-associated β-galactosidase (SA-β-gal)-positive cells, they were decreased upon induction of ferroptosis with FINs. The SA-β-gal decrease was inhibited by ferroptotic inhibitors and knockdown of Atg7, pointing to the involvement of lipid peroxidation and activated autophagosome formation during ferroptosis. A transfer of cell culture medium of cells treated with FINs, type 1 or 2, caused the reduction in SA-β-gal-positive cells in recipient cells that had not been exposed to FINs. Real-time imaging of Kusabira Orange-marked reporter MEFs cocultured with ferroptotic cells showed the generation of lipid peroxide and deaths of the reporter cells. These results indicate that lipid peroxidation and its aftereffects propagate from ferroptotic cells to surrounding cells, even when the surrounding cells are not exposed to FINs. Ferroptotic cells are not merely dying cells but also work as signal transmitters inducing a chain of further ferroptosis.Subject terms: Cell death, Autophagy     

Effect of cell cycle inhibitor p19ARF on senescence of human diploid cell

To investigate the effect of cell cycle inhibitor p19ARF on replicative senescence of human diploid cell, recombinant p19ARF eukaryotic expression vector was constructed and p19ARF gene was transfected into human diploid fibroblasts (WI-38 cells) by liposome-mediated transfection for overexpression. Then, the effects of p19ARF on replicative senescence of WI-38 cells were observed. The results re- vealed that, compared with control cells, the WI-38 cells in which p19ARF gene was introduced showed significant up-regulation of p53 and p21 expression level, decrease of cell generation by 10 12 generations, decline of cell growth rate with cell cycle being arrested at G1 phase, increase of positive rate of senescent marker SA-β-gal staining, and decrease of mitochondrial membrane potential. The morphology of the transfected fibroblasts presented the characteristics changes similar to senescent cells. These results indicated that high expression of p19ARF may promote the senescent process of human diploid cells.     

Accelerated cellular senescence in degenerate intervertebral discs: a possible role in the pathogenesis of intervertebral disc degeneration

Current evidence implicates intervertebral disc degeneration as a major cause of low back pain, although its pathogenesis is poorly understood. Numerous characteristic features of disc degeneration mimic those seen during ageing but appear to occur at an accelerated rate. We hypothesised that this is due to accelerated cellular senescence, which causes fundamental changes in the ability of disc cells to maintain the intervertebral disc (IVD) matrix, thus leading to IVD degeneration. Cells isolated from non-degenerate and degenerate human tissue were assessed for mean telomere length, senescence-associated β-galactosidase (SA-β-gal), and replicative potential. Expression of P16 ^INK4A (increased in cellular senescence) was also investigated in IVD tissue by means of immunohistochemistry. RNA from tissue and cultured cells was used for real-time polymerase chain reaction analysis for matrix metalloproteinase-13, ADAMTS 5 (a disintegrin and metalloprotease with thrombospondin motifs 5), and P16 ^INK4A . Mean telomere length decreased with age in cells from non-degenerate tissue and also decreased with progressive stages of degeneration. In non-degenerate discs, there was an age-related increase in cellular expression of P16 ^INK4A . Cells from degenerate discs (even from young patients) exhibited increased expression of P16 ^INK4A , increased SA-β-gal staining, and a decrease in replicative potential. Importantly, there was a positive correlation between P16 ^INK4A and matrix-degrading enzyme gene expression. Our findings indicate that disc cell senescence occurs in vivo and is accelerated in IVD degeneration. Furthermore, the senescent phenotype is associated with increased catabolism, implicating cellular senescence in the pathogenesis of IVD degeneration.     

Escherichia coli Producing Colibactin Triggers Premature and Transmissible Senescence in Mammalian Cells

Cellular senescence is an irreversible state of proliferation arrest evoked by a myriad of stresses including oncogene activation, telomere shortening/dysfunction and genotoxic insults. It has been associated with tumor activation, immune suppression and aging, owing to the secretion of proinflammatory mediators. The bacterial genotoxin colibactin, encoded by the pks genomic island is frequently harboured by Escherichia coli strains of the B2 phylogenetic group. Mammalian cells exposed to live pks+ bacteria exhibit DNA-double strand breaks (DSB) and undergo cell-cycle arrest and death. Here we show that cells that survive the acute bacterial infection with pks+ E. coli display hallmarks of cellular senescence: chronic DSB, prolonged cell-cycle arrest, enhanced senescence-associated β-galactosidase (SA-β-Gal) activity, expansion of promyelocytic leukemia nuclear foci and senescence-associated heterochromatin foci. This was accompanied by reactive oxygen species production and pro-inflammatory cytokines, chemokines and proteases secretion. These mediators were able to trigger DSB and enhanced SA-β-Gal activity in bystander recipient cells treated with conditioned medium from senescent cells. Furthermore, these senescent cells promoted the growth of human tumor cells. In conclusion, the present data demonstrated that the E. coli genotoxin colibactin induces cellular senescence and subsequently propel bystander genotoxic and oncogenic effects.     

The Tocotrienol-Rich Fraction Is Superior to Tocopherol in Promoting Myogenic Differentiation in the Prevention of Replicative Senescence of Myoblasts

Aging results in a loss of muscle mass and strength. Myoblasts play an important role in maintaining muscle mass through regenerative processes, which are impaired during aging. Vitamin E potentially ameliorates age-related phenotypes. Hence, this study aimed to determine the effects of the tocotrienol-rich fraction (TRF) and α-tocopherol (ATF) in protecting myoblasts from replicative senescence and promoting myogenic differentiation. Primary human myoblasts were cultured into young and senescent stages and were then treated with TRF or ATF for 24 h, followed by an analysis of cell proliferation, senescence biomarkers, cellular morphology and differentiation. Our data showed that replicative senescence impaired the normal regenerative processes of myoblasts, resulting in changes in cellular morphology, cell proliferation, senescence-associated β-galactosidase (SA-β-gal) expression, myogenic differentiation and myogenic regulatory factors (MRFs) expression. Treatment with both TRF and ATF was beneficial to senescent myoblasts in reclaiming the morphology of young cells, improved cell viability and decreased SA-β-gal expression. However, only TRF treatment increased BrdU incorporation in senescent myoblasts, as well as promoted myogenic differentiation through the modulation of MRFs at the mRNA and protein levels. MYOD1 and MYOG gene expression and myogenin protein expression were modulated in the early phases of myogenic differentiation. In conclusion, the tocotrienol-rich fraction is superior to α-tocopherol in ameliorating replicative senescence-related aberration and promoting differentiation via modulation of MRFs expression, indicating vitamin E potential in modulating replicative senescence of myoblasts.     

Telomere Fragment Induced Amnion Cell Senescence: A Contributor to Parturition?

Oxidative stress (OS)-induced senescence of the amniochorion has been associated with parturition at term. We investigated whether telomere fragments shed into the amniotic fluid (AF) correlated with labor status and tested if exogenous telomere fragments (T-oligos) could induce human and murine amnion cell senescence. In a cross-sectional clinical study, AF telomere fragment concentrations quantitated by a validated real-time PCR assay were higher in women in labor at term compared to those not in labor. In vitro treatment of primary human amnion epithelial cells with 40 μM T-oligos ([TTAGGG]₂) that mimic telomere fragments, activated p38MAPK, produced senescence-associated (SA) β-gal staining and increased interleukin (IL)-6 and IL-8 production compared to cells treated with complementary DNA sequences (Cont-oligos, [AATCCC]₂). T-oligos injected into the uteri of pregnant CD1 mice on day 14 of gestation, led to increased p38MAPK, SA-β-gal (SA β-gal) staining in murine amniotic sacs and higher AF IL-8 levels on day 18, compared to saline treated controls. In summary, term labor AF samples had higher telomere fragments than term not in labor AF. In vitro and in situ telomere fragments increased human and murine amnion p38MAPK, senescence and inflammatory cytokines. We propose that telomere fragments released from senescent fetal cells are indicative of fetal cell aging. Based on our data, these telomere fragments cause oxidative stress associated damages to the term amniotic sac and force them to release other DAMPS, which, in turn, provide a sterile immune response that may be one of the many inflammatory signals required to initiate parturition at term.     

ATM-deficient human fibroblast cells are resistant to low levels of DNA double-strand break induced apoptosis and subsequently undergo drug-induced premature senescence

DNA DSBs are induced by IR or radiomimetic drugs such as doxorubicin. It has been indicated that cells from ataxia-telangiectasia patients are highly sensitive to radiation due to defects in DNA repair, but whether they have impairment in apoptosis has not been fully elucidated. A-T cells showed increased sensitivity to high levels of DNA damage, however, they were more resistant to low doses. Normal cells treated with combination of KU55933, a specific ATM kinase inhibitor, and doxorubicin showed increased resistance as they do in a similar manner to A-T cells. A-T cells have higher viability but more DNA breaks, in addition, the activations of p53 and apoptotic proteins (Bax and caspase-3) were deficient, but Akt expression was enhanced. A-T cells subsequently underwent premature senescence after treatment with a low dose of doxorubicin, which was confirmed by G2 accumulation, senescent morphology, and SA-β-gal positive until 15 days repair incubation. Finally, A-T cells are radio-resistant at low doses due to its defectiveness in detecting DNA damage and apoptosis, but the accumulation of DNA damage leads cells to premature senescence.