Chk2 is dispensable for p53-mediated G1 arrest but is required for a latent p53-mediated apoptotic response

In response to genotoxic stress, mammalian cells can activate cell cycle checkpoint pathways to arrest the cell for repair of DNA damage or induce apoptosis to eliminate damaged cells. The checkpoint kinase, Chk2, has been implicated in both of these responses and is believed to function in an ataxia telangiectasia (Atm)-dependent manner. We show here that Chk2-/- mouse embryo fibroblasts (MEFs), unlike Atm-/- or p53-/- MEFs, behaved like normal MEFs in manifesting p21 induction and G(1) arrest upon exposure to gamma-irradiation. Therefore, Chk2 is not involved in p53-mediated G(1) arrest. To examine the role of Chk2 in p53-dependent apoptotic response, we used adenovirus E1A-expressing MEFs. We show that Chk2-/- cells, like p53-/- cells, did not undergo DNA damage-induced apoptosis, whereas Atm-/- cells behaved like normal cells in invoking an apoptotic response. Furthermore, this apoptosis could occur in the absence of protein synthesis, suggesting that it is preexisting, or "latent," p53 that mediates this response. We conclude that Chk2 is not involved in Atm- and p53-dependent G(1) arrest, but is involved in the activation of latent p53, independently of Atm, in triggering DNA damage-induced apoptosis.     

Apoptotic pathway induced by diallyl trisulfide in pancreatic cancer cells

AIM:To investigate the effects of diallyl trisulfide(DATS),a garlic-derived organosulfur compound,in pancreatic cancer cells.METHODS:Human pancreatic cancer cells with wildtype p53 gene(Capan-2)and normal pancreatic epithelial cells(H6C7)were cultured in RPMI1640.DATS was prepared at a concentration of 100μmol/L.Cell viability was determined via the methyl thiazolyl tetrazolium assay.Apoptotic cells were detected by TUNEL assay.Cell cycle analysis was performed using flow cytometry.Protein expression was determined by Western blot.Bax and Bcl-2 expression was detected by immunofluorescence.Apoptosis genes and cell cycle were assessed by quantitative real-time polymerase chain reaction.RESULTS:DATS suppressed the viability of cultured human pancreatic cancer cells(Capan-2)by increasing the proportion of cells in the G2/M phase and induced apoptotic cell death.Western blot analysis indicated that DATS enhanced the expression of Fas,p21,p53and cyclin B1,but downregulated the expression of Akt,cyclin D1,MDM2 and Bcl-2.DATS induced cell cycle inhibition which was correlated with elevated levels of cyclin B1 and p21,and reduced levels of cyclin D1 in Capan-2 cells and H6C7 cells.DATS-induced apoptosis was markedly elevated in Capan-2 cells compared with H6C7 cells,and this was correlated with elevated levels of cyclin B1 and p53,and reduced levels of Bcl-2.DATS-induced apoptosis was correlated with downregulation of Bcl-2,Akt and cyclin D1 protein levels,and up-regulation of Bax,Fas,p53 and cyclin B protein levels in Capan-2 cells.CONCLUSION:DATS induces apoptosis of pancreatic cancer cells(Capan-2)and non-tumorigenic pancreatic ductal epithelial cells(H6C7).     

p53 and p14 increase sensitivity of gastric cells to H. pylori-induced apoptosis

H. Pylori infection of the gastric mucosa is associated with increased epithelial cell apoptosis. In vitro, interferon- and TNF- have been shown to increase the sensitivity of cells to apoptosis induced by H. Pylori. The p53 tumor suppressor gene is frequently mutated in many cancers, including gastric cancer. Since p53 protein can induce apoptosis, we sought to determine whether or not p53 increases the ability of gastric epithelial cells to undergo apoptosis in response to H. Pylori-induced cell injury. Human gastric epithelial cell lines, AGS (p53 wild-type) cells and AGS cells infected with HPV E6 gene (AGS-E6) to inactivate p53 were exposed to H. Pylori. The p53, p21, and p14^ARF proteins were measured in gastric epithelial cells by immunoelectrophoresis. Gastric epithelial cell apoptosis was measured by DNA end-labeling assay (TUNEL) and subG₀ cell fractions using flow cytometry, and by agarose gel electrophoresis of DNA. Exposure to H. Pylori increased the levels of p53, p21, and p14^ARF proteins two fold in AGS cells. Gastric AGS cells with fragmented DNA increased from 1.1% to 68% in after exposure to H. Pylori for 24 hr. However, AGS-E6 cells were relatively resistant to apoptosis induced by H. Pylori (only 15% of cells underwent apoptosis). In additional experiments, mouse embryonic fibroblasts (MEFs) were used to further investigate the role of ARF in stabilizing p53 after exposure to H. Pylori. Wild-type and p19^ARF–/– MEFs were exposed to H. Pylori and evaluated for activation of p53, p19^ARF, and apoptosis. As with AGS cells, H. Pylori stimulated a 2-fold increase in p53 and p19^ARF in wild-type MEFs; however, there was no increase in p53 in ARF-null MEFs. H. Pylori easily stimulated apoptosis in wild-type MEFs, although, the absence of p19^ARF significantly reduced the ability of H. Pylori to induce apoptosis in these cells. Activation of ARF by H. Pylori is important in stabilizing p53 resulting in increased apoptosis. Thus, inactivation of either ARF or p53 in gastric cells may reduce their ability to undergo apoptosis in response to injury induced by H. Pylori.     

Adenovirus-mediated p53 gene transfer sensitizes hepatocellular carcinoma cells to heavy-ion radiation

Background The purpose of this study was to investigate whether adenovirus-mediated p53 transfer could sensitize hepatocellular carcinoma to heavy-ion irradiation. Methods HepG2 cells were preexposed to a ¹²C⁶⁺ beam, and then infected with replication-deficient adenovirus recombinant vectors containing human wild-type p53 (AdCMV-p53) (¹²C⁶⁺ irradiation + AdCMV-p53 infection). The survival fraction was determined by clonogenic assay. The cell cycle, cell apoptosis, and p53 expression were monitored by flow cytometric analysis. Results p53 expression in ¹²C⁶⁺ irradiation + AdCMV-p53 infection groups was markedly higher than that in ¹²C⁶⁺ irradiation only groups (P < 0.05), suggesting that the preexposure to the ¹²C⁶⁺ beam promoted the expression of exogenous p53 in HepG2 cells infected with AdCMV-p53 only. The G₁-phase arrest and cell apoptosis in the ¹²C⁶⁺ irradiation + AdCMV-p53 infection groups were significantly more than those in the ¹²C⁶⁺ irradiated groups (P < 0.05). The survival fractions of the ¹²C⁶⁺ irradiation + AdCMV-p53 infection groups decreased by 30%–49% compared with those of the ¹²C⁶⁺ beam-irradiated only groups (P < 0.05). Conclusions Adenovirus-mediated p53 gene transfer can promote G₁-phase arrest and cell apoptosis, thus sensitizing hepatocellular carcinoma cells to heavy-ion irradiation.     

Mutant p53 drives metastasis and overcomes growth arrest/senescence in pancreatic cancer

TP53 mutation occurs in 50–75% of human pancreatic ductal adenocarcinomas (PDAC) following an initiating activating mutation in the KRAS gene. These p53 mutations frequently result in expression of a stable protein, p53^R175H, rather than complete loss of protein expression. In this study we elucidate the functions of mutant p53 (Trp53^R172H), compared to knockout p53 (Trp53^fl), in a mouse model of PDAC. First we find that although Kras^G12D is one of the major oncogenic drivers of PDAC, most Kras^G12D-expressing pancreatic cells are selectively lost from the tissue, and those that remain form premalignant lesions. Loss, or mutation, of Trp53 allows retention of the Kras^G12D-expressing cells and drives rapid progression of these premalignant lesions to PDAC. This progression is consistent with failed growth arrest and/or senescence of premalignant lesions, since a mutant of p53, p53^R172P, which can still induce p21 and cell cycle arrest, is resistant to PDAC formation. Second, we find that despite similar kinetics of primary tumor formation, mutant p53^R172H, as compared with genetic loss of p53, specifically promotes metastasis. Moreover, only mutant p53^R172H-expressing tumor cells exhibit invasive activity in an in vitro assay. Importantly, in human PDAC, p53 accumulation significantly correlates with lymph node metastasis. In summary, by using ‘knock-in’ mutations of Trp53 we have identified two critical acquired functions of a stably expressed mutant form of p53 that drive PDAC; first, an escape from Kras^G12D-induced senescence/growth arrest and second, the promotion of metastasis.     

DNA damage and eIF4G1 in breast cancer cells reprogram translation for survival and DNA repair mRNAs

The cellular response to DNA damage is mediated through multiple pathways that regulate and coordinate DNA repair, cell cycle arrest, and cell death. We show that the DNA damage response (DDR) induced by ionizing radiation (IR) is coordinated in breast cancer cells by selective mRNA translation mediated by high levels of translation initiation factor eIF4G1 (eukaryotic initiation factor 4γ1). Increased expression of eIF4G1, common in breast cancers, was found to selectively increase translation of mRNAs involved in cell survival and the DDR, preventing autophagy and apoptosis [Survivin, hypoxia inducible factor 1α (HIF1α), X-linked inhibitor of apoptosis (XIAP)], promoting cell cycle arrest [growth arrest and DNA damage protein 45a (GADD45a), protein 53 (p53), ATR-interacting protein (ATRIP), Check point kinase 1 (Chk1)] and DNA repair [p53 binding protein 1 (53BP1), breast cancer associated proteins 1, 2 (BRCA1/2), Poly-ADP ribose polymerase (PARP), replication factor c2–5 (Rfc2-5), ataxia telangiectasia mutated gene 1 (ATM), meiotic recombination protein 11 (MRE-11), and others]. Reduced expression of eIF4G1, but not its homolog eIF4G2, greatly sensitizes cells to DNA damage by IR, induces cell death by both apoptosis and autophagy, and significantly delays resolution of DNA damage foci with little reduction of overall protein synthesis. Although some mRNAs selectively translated by higher levels of eIF4G1 were found to use internal ribosome entry site (IRES)-mediated alternate translation, most do not. The latter group shows significantly reduced dependence on eIF4E for translation, facilitated by an enhanced requirement for eIF4G1. Increased expression of eIF4G1 therefore promotes specialized translation of survival, growth arrest, and DDR mRNAs that are important in cell survival and DNA repair following genotoxic DNA damage.     

Dissection of the genetic programs of p53-mediated G1 growth arrest and apoptosis: blocking p53-induced apoptosis unmasks G1 arrest

Employing the myeloblastic leukemia M1 cell line, which does not express endogenous p53, and genetically engineered variants, it was recently shown that activation of p53, using a p53 temperature- sensitive mutant transgene (p53ts), resulted in rapid apoptosis that was delayed by high level ectopic expression of bcl-2. In this report, advantage has been taken of these M1 variants to investigate the relationship between p53-mediated G1 arrest and apoptosis. Flow cytometric cell cycle analysis has provided evidence that activation of wild-type (wt) p53 function in M1 cells resulted in the induction of G1 growth arrest; this was clearly seen in the M1p53/bcl-2 cells because of the delay in apoptosis that unmasked p53-induced G1 growth arrest. This finding was further corroborated at the molecular level by analysis of the expression and function of key cell cycle regulatory genes in M1p53 versus M1p53/bcl-2 cells after the activation of wt p53 function; events that take place at early times during the p53-induced G1 arrest occur in both the M1p53 and the M1p53/bcl-2 cells, whereas later events occur only in the M1p53/bcl-2 cells, which undergo delayed apoptosis, thereby allowing the cells to complete G1 arrest. Finally, it was observed that a spectrum of p53 target genes implicated in p53- induced growth suppression and apoptosis were similarly regulated, either induced (gadd45, waf1, mdm2, and bax) or suppressed (c-myc and bcl-2), after activation of wt p53 function in M1p53 and M1p53/bcl-2 cells. Taken together, these findings show that wt p53 can simultaneously induce the genetic programs of both G1 growth arrest and apoptosis within the same cell type, in which the genetic program of cell death can proceed in either G1-arrested (M1p53/bcl-2) or cycling (M1p53) cells. These findings increase our understanding of the functions of p53 as a tumor suppressor and how alterations in these functions could contribute to malignancy.     

Identification of Genomic Alterations Associated with the Aggressiveness of Pancreatic Cancer Using an Ultra-High-Resolution CGH Array

Background: Genomic alterations present in pancreatic adenocarcinoma have been described only partially. In addition, the relations between these alterations and the aggressiveness of the phenotype remain unknown. Methods: Genomic DNA and total RNA from 5 pancreatic cell lines, of which 2 have an aggressive phenotype and are gemcitabine-resistant (Mia-Paca2 and Panc-1), and 3 less aggressive and gemcitabine-sensitive (Capan-1, Capan-2 and BxPC3), have been purified. DNA abnormalities have been analyzed using an ultra-high-resolution CGH array and mRNA expression was studied with an Affymetrix GeneChip expression array. Results: We identified 573 amplified and 30 deleted genes common to all 5 cell lines. Some of them have already been described, whereas other genes, implicated in signal transduction, apoptosis, cell cycle or cell migration, are described for the first time as being related to this cancer. Comparison of genomic abnormalities between the 2 most aggressive and the 3 less aggressive cell lines led to the identification of 368 genes specifically amplified in the aggressive cell lines. However, no specific gene deletion seems to be associated with the aggressive phenotype. Conclusion: Using a high-resolution approach, we could precisely describe the genomic alterations associated with pancreatic adenocarcinoma and determine those associated with an aggressive phenotype.     

A role for c-Abl in cell senescence and spontaneous immortalization

c-Abl is a proto-oncogene that is essential for mouse development and tissue homeostasis. Misregulation of c-Abl, as seen in the constitutively active BCR-ABL, is the leading cause of human chronic myeloid leukemia. However, how the Abl proteins execute their functions still remains largely unknown. Here, we report an important role for c-Abl in replicative senescence and immortalization by regulating the expression of two tumor suppressors that induce cellular senescence, p53 and p16^INK4a. Using primary mouse embryonic fibroblasts (MEFs), we show that c-Abl^−/− cells were more resistant to immortalization than wildtype cells using a standard 3T3 or 3T9 protocol. We could only immortalize three out of nine c-Abl^−/− MEF cultures even when we increased the number of starting cells. This resistance was attributed to premature senescence and reduced survival in senescent c-Abl^−/− cells due to an increase in p16^INK4a and p53 expression. Deleting p53 allows c-Abl^−/−p53^−/− MEFs to bypass senescence to be spontaneously immortalized. Cell immortalization, but not senescence, was generally accompanied by mutations in p53 in both wildtype and c-Abl^−/− MEFs, although the spectrum is different from that of human tumors. The role for c-Abl in regulating cell senescence and immortalization might explain some of the developmental defects in c-Abl^−/− mice and how BCR-ABL transforms cells.

Electronic supplementary material

The online version of this article (doi:10.1007/s11357-012-9452-4) contains supplementary material, which is available to authorized users.     

IL-12-dependent innate immunity arrests endothelial cells in G0-G1 phase by a p21(Cip1/Waf1)-mediated mechanism

Innate immunity may activate paracrine circuits able to entail vascular system in the onset and progression of several chronic degenerative diseases. In particular, interleukin (IL)-12 triggers a genetic program in lymphomononuclear cells characterized by the production of interferon-γ and specific chemokines resulting in an angiostatic activity. The aim of this study is to identify molecules involved in the regulation of cell cycle in endothelial cells co-cultured with IL-12-stimulated lymphomonuclear cells. By using a transwell mediated co-culture system we demonstrated that IL-12-stimulated lymphomonuclear cells induce an arrest of endothelial cells cycle in G1, which is mainly mediated by the up-regulation of p21^Cip1/Waf1, an inhibitor of cyclin kinases. This effect requires the activation of STAT1, PKCδ and p38 MAPK, while p53 is ineffective. In accordance, siRNA-dependent silencing of these molecules in endothelial cells inhibited the increase of p21^Cip1/Waf1 and the modification in cell cycle promoted by IL-12-stimulated lymphomonuclear cells. These results indicate that the angiostatic action of IL-12-stimulated lymphomononuclear cells may lie in the capability to arrest endothelial cells in G1 phase through a mechanisms mainly based on the specific up-regulation of p21^Cip1/Waf1 induced by the combined activity of STAT1, PKCδ and p38 MAPK.     

Costimulation with interleukin-4 and interleukin-10 induces mast cell apoptosis and cell-cycle arrest: the role of p53 and the mitochondrion

OBJECTIVE: The aim of this study was to determine the mechanism by which interleukin (IL)-4 + IL-10 costimulation regulates mast cell numbers to maintain immune homeostasis. MATERIALS AND METHODS: We employed mouse bone marrow-derived mast cells (BMMC) to measure the effects of IL-4 + IL-10 on survival and cell-cycle progression. p53-Deficient, bax-deficient, and bcl-2 transgenic BMMC were compared to wild-type cells to determine the role of these proteins in apoptosis. The molecular regulation of apoptosis and cell-cycle progression was investigated using flow cytometric analysis, RNase protection, and Western blotting. RESULTS: IL-4 + IL-10 induced BMMC apoptosis and arrest. Apoptosis was p53-dependent. Cell death was accompanied by loss of mitochondrial membrane potential, the importance of which was demonstrated by resistance to IL-4 + IL-10-mediated cell death when Bax was deleted or Bcl-2 was overexpressed. Those cells not killed by apoptosis demonstrated a p53-independent G1 cell-cycle arrest. Apoptosis and arrest may be explained by reduced IL-3 receptor signaling. CONCLUSION: Costimulation with IL-4 + IL-10 partly controls mast cell homeostasis through a delayed apoptosis and arrest program that is induced by a blockade of IL-3 receptor signaling. The delay in these negative effects would allow the protective effects of mast cell activation to occur for several days.     

DDB2 decides cell fate following DNA damage

The xeroderma pigmentosum complementation group E (XP-E) gene product damaged-DNA binding protein 2 (DDB2) plays important roles in nucleotide excision repair (NER). Previously, we showed that DDB2 participates in NER by regulating the level of p21^Waf1/Cip1. Here we show that the p21^Waf1/Cip1 -regulatory function of DDB2 plays a central role in defining the response (apoptosis or arrest) to DNA damage. The DDB2-deficient cells are resistant to apoptosis in response to a variety of DNA-damaging agents, despite activation of p53 and the pro-apoptotic genes. Instead, these cells undergo cell cycle arrest. Also, the DDB2-deficient cells are resistant to E2F1-induced apoptosis. The resistance to apoptosis of the DDB2-deficient cells is caused by an increased accumulation of p21^Waf1/Cip1 after DNA damage. We provide evidence that DDB2 targets p21^Waf1/Cip1 for proteolysis. The resistance to apoptosis in DDB2-deficient cells also involves Mdm2 in a manner that is distinct from the p53-regulatory activity of Mdm2. Our results provide evidence for a new regulatory loop involving the NER protein DDB2, Mdm2, and p21^Waf1/Cip1 that is critical in deciding cell fate (apoptosis or arrest) upon DNA damage.     

From the Cover: Trim24 targets endogenous p53 for degradation

Numerous studies focus on the tumor suppressor p53 as a protector of genomic stability, mediator of cell cycle arrest and apoptosis, and target of mutation in 50% of all human cancers. The vast majority of information on p53, its protein-interaction partners and regulation, comes from studies of tumor-derived, cultured cells where p53 and its regulatory controls may be mutated or dysfunctional. To address regulation of endogenous p53 in normal cells, we created a mouse and stem cell model by knock-in (KI) of a tandem-affinity-purification (TAP) epitope at the endogenous Trp-53 locus. Mass spectrometry of TAP-purified p53-complexes from embryonic stem cells revealed Tripartite-motif protein 24 (Trim24), a previously unknown partner of p53. Mutation of TRIM24 homolog, bonus, in Drosophila led to apoptosis, which could be rescued by p53-depletion. These in vivo analyses establish TRIM24/bonus as a pathway that negatively regulates p53 in Drosophila. The Trim24-p53 link is evolutionarily conserved, as TRIM24 depletion in human breast cancer cells caused p53-dependent, spontaneous apoptosis. We found that Trim24 ubiquitylates and negatively regulates p53 levels, suggesting Trim24 as a therapeutic target to restore tumor suppression by p53.     

替莫唑胺对人胶质瘤细胞系U251细胞的增殖影响

目的探讨替莫唑胺（TMZ）对人胶质瘤细胞系U251细胞的杀伤作用及其作用机理。方法将人胶质瘤细胞系U251细胞分成空白对照组、TMZ组和常用化疗药物组,TMZ组下设10、25、50、100、200、400μmol／L组,常用化疗药物组（ADM、MTX、VCR、DDP）各药物浓度均为其IC50值。各组分别于作用后24、48、72h观察细胞形态学改变,用MTT比色法检测细胞活力,流式细胞仪检测细胞周期和凋亡率。结果①TMZ组各浓度对U251细胞的抑制率分别为4．23％、7．43％、31．63％、64．53％、82．18％、86．15％,呈良好的时间—剂量效应关系,其1％为81μmol,初始抑制浓度为50μmol;②ADM、MTX、VCR、DDP对U251细胞的抑制率分别为39．27％．44．59％、54．56％、55．28％,VCR、DDP两药的抑制作用明显优于ADM、MTX（P均〈0．01）,与TMZ的杀伤作用相当（P〉0．05）;③流式细胞仪检测显示,U251细胞经TMZ作用后出现G2／M期阻滞,但促凋亡作用不强。结论TMZ对胶质瘤细胞有明显的杀伤作用,并呈时间和剂量依赖性;TMZ可使胶质瘤细胞阻滞于G2／M期,但其促凋亡作用不强;替奠唑胺对U251细胞的杀伤作用优于其他4种常用化疗药物。     

Trimetazidine alone or in combination with gemcitabine and/or abraxane decreased cell viability,migration and ATP levels and induced apoptosis of human pancreatic cells

BackgroundTrimetazidine (TMZ) is an anti-ischemic agent that can inhibit the fatty acid oxidation. It has been stated that inhibition of fatty acid oxidation may be an acceptable approach to cancer treatment.MethodsWe examined the effects of TMZ alone or together with abraxane (ABX) and/or gemcitabine (GEM) on cell viability, apoptosis, adhesion, migration and ATP levels of human pancreatic cancer cell line PANC-1.ResultsTMZ significantly reduced the cell viability at higher concentrations. Lower cell viability values were found in cells co-treated with TMZ + GEM, TMZ + ABX and GEM + ABX. The combined treatment of TMZ with ABX and/or GEM significantly increased the apoptosis rates. The highest percentages of apoptosis were found in TMZ + ABX or TMZ + ABX + GEM treatments. TMZ alone or together with ABX and/or GEM significantly reduced the ATP levels. The lowest migration rates were also found at TMZ + ABX and TMZ + ABX + GEM treatments.ConclusionsOur study is the first study to indicate that TMZ can induce cytotoxicity and apoptosis and reduce migration and ATP levels, especially in cells co-treated with ABX and/or GEM. A combination strategy based on inhibition of fatty acid oxidation and anticancer drugs may be more effective in the treatment of pancreatic cancers.     

Biological effects of low-dose-rate irradiation of pancreatic carcinoma cells in vitro using ~(125)I seeds

AIM:To determine the mechanism of the radiationinduced biological effects of 125I seeds on pancreatic carcinoma cells in vitro.METHODS:SW1990 and PANC-1 pancreatic cancer cell lines were cultured in DMEM in a suitable environment.Gray’s model of iodine-125(125I)seed irradiation was used.In vitro,exponential phase SW1990,and PANC-1cells were exposed to 0,2,4,6,and 8 Gy using 125I radioactive seeds,with an initial dose rate of 12.13c Gy/h.A clonogenic survival experiment was performed to observe the ability of the cells to maintain their clonogenic capacity and to form colonies.Cell-cycle and apoptosis analyses were conducted to detect the apoptosis percentage in the SW1990 and PANC-1 cells.DNA synthesis was measured via a tritiated thymidine(3H-Td R)incorporation experiment.After continuous low-dose-rate irradiation with 125I radioactive seeds,the survival fractions at 2 Gy(SF2),percentage apoptosis,and cell cycle phases of the SW1990 and PANC-1 pancreatic cancer cell lines were calculated and compared.RESULTS:The survival fractions of the PANC-1 andSW1990 cells irradiated with 125I seeds decreased exponentially as the dose increased.No significant difference in SF2 was observed between SW1990 and PANC-1 cells(0.766±0.063 vs 0.729±0.045,P0.05).The 125I seeds induced a higher percentage of apoptosis than that observed in the control in both the SW1990and PANC-1 cells.The rate of apoptosis increased with increasing radiation dosage.The percentage of apoptosis was slightly higher in the SW1990 cells than in the PANC-1 cells.Dose-dependent G2/M cellcycle arrest was observed after 125I seed irradiation,with a peak value at 6 Gy.As the dose increased,the percentage of G2/M cell cycle arrest increased in both cell lines,whereas the rate of DNA incorporation decreased.In the 3H-Td R incorporation experiment,the dosimetry results of both the SW1990 and PANC-1cells decreased as the radiation dose increased,with a minimum at 6 Gy.There were no significant differences in the dosimetry results of the two cell lines when they were exposed to the same dose of radiation.CONCLUSION:The pancreatic cancer cell-killing effects induced by 125I radioactive seeds mainly occurred via apoptosis and G2/M cell cycle arrest.