KB细胞耐药株的建立及其耐药机制的探讨

用对长春新碱(VCR)敏感的KB细胞为亲本,通过诱变剂甲基磺酸乙酯刺激,然后在培养液中加入浓度递增的CVR,得到耐药细胞株KB_V200。此细胞株对VCR的耐受程度约为KB细胞的175倍。对其它抗肿瘤药物如紫杉醇、秋水仙碱和阿霉素等也有不同程度的交叉耐药性。进一步研究表明,KB_V200对³H-VCR的蓄积明显减少,且耐药基因(mdr1)表达增加。钙通道阻滞剂维拉帕米(Ver)可增加KB_V200对³H-VCR的蓄积和对VCR的敏感性。这些结果提示,KB_V200耐药的机制可能是由于mdr1基因表达增加,产生过量的p-糖蛋白,使药物外排增多所致。     

赛赓啶对 KBV200细胞多药抗性的逆转作用

研究赛赓啶对KB_V200细胞多药抗性的逆转作用及逆转机制。在KB_V200细胞,采用MTT法,测出赛赓啶对长春新碱、阿霉素和鬼臼乙叉甙耐药的逆转系数分别为5.5,2.0和1.9,而对5-氟尿嘧啶、美法仑的细胞毒性作用无明显影响,表明赛赓啶为多药抗性逆转剂。荧光分光光度法测定表明,赛赓啶可使KB_V200细胞内阿霉素蓄积量增加。流式细胞荧光测定显示赛赓啶可增加罗丹明123的蓄积并减慢其外排。免疫细胞化学及狭缝杂交表明赛赓啶不影响KB_V200细胞的P-糖蛋白染色深度和 mdr1 RNA 表达水平。以上结果提示赛赓啶的多药抗性逆转机制是抑制P-糖蛋白泵的功能。     

四嗪二甲酰胺对肺癌细胞株A549的体内外作用

为了观察四嗪二甲酰胺(ZGDHu-1)体外抑制肺癌细胞株A549增殖、诱导细胞凋亡和体内抗肿瘤活性的作用及其机制，将不同浓度的ZGDHu-1与A549细胞在体外培养，用台盼蓝染色、SRB法、5′-溴-2′脱氧尿苷-ELISA法，观察ZGDHu-1对A549细胞增殖的作用；用细胞形态学、DNA凝胶电泳、DNA含量及细胞周期分析、Annexin V/PI双标记、Hoechst 33258荧光染色等技术检测细胞凋亡。腹腔注射ZGDHu-1后观察其对裸鼠移植瘤生长的抑制作用。用RT-PCR和流式细胞术观察A549细胞bcl-2，bax，p53基因和蛋白质的表达改变。结果表明，ZGDHu-1能抑制A549细胞的增殖和活力，呈现作用时间和剂量的依赖关系。A549细胞经ZGDHu-1作用后，大部分细胞阻滞于G₂-M期；出现DNA片段化，亚G₁峰显著增加，Annexin V+/PI-表达升高，Hoechst 33258荧光染色后出现凋亡细胞的特征性改变等。ZGDHu-1以10，20及40 mg·kg^-1剂量给裸鼠体内用药14 d后，移植瘤生长抑制率分别为43.7%，56.9%和60.0%。A549细胞经ZGDHu-1作用后，bcl-2基因和蛋白有所下调，但主要是上调bax基因和蛋白，导致bax/bcl-2比值明显增高，p53基因和蛋白表达也上调，均呈现剂量依赖性。ZGDHu-1在体内能明显抑制移植瘤的生长，体外通过诱导细胞凋亡抑制A549细胞增殖，其机制可能与上调bax和p53基因的表达有关。     

紫杉醇诱导人乳腺癌MCF-7细胞周期阻断及凋亡的基因表达谱分析

目的研究紫杉醇诱导人MCF-7细胞周期阻断及凋亡的分子机制。方法用流式细胞仪分析紫杉醇对MCF-7细胞周期变化的影响，用自制的含9 984个已知基因和EST的高密度基因芯片检测MCF-7细胞在不同浓度紫杉醇作用下的基因表达变化。结果MCF-7细胞在100 nmol·L^-1紫杉醇作用24 h，流式细胞仪结果显示77.8%细胞阻断在G₂/M期和1.3%细胞发生凋亡；基因表达谱分析发现：在12.5 nmol·L^-1 (IC₅₀)及100 nmol·L^-1紫杉醇作用下，分别有27及77个基因差异表达。结论紫杉醇可诱导MCF-7细胞周期阻断在G₂/M期并引起部分细胞凋亡，该作用与药物浓度有关。基因表达谱分析显示部分差异表达基因参与细胞微管及骨架结构、细胞周期调控、以及DNA损伤修复和凋亡过程。     

迷迭香酸诱导肝癌HepG2细胞凋亡的作用机制研究

摘 要 目的：研究迷迭香酸诱导HepG₂细胞凋亡的作用机制。方法: MTT法检测不同浓度的迷迭香酸作用于HepG₂细胞48 h后,对细胞生长的抑制作用;流式细胞术检测不同浓度迷迭香酸分别作用于HepG₂细胞24 h及48 h后的细胞凋亡;Western Blotting法检测迷迭香酸作用HepG₂细胞48 h对P53和c-Myc蛋白表达的影响。结果: 12.50,25.00,50.00,100.00 μg·ml^-1浓度的迷迭香酸作用48 h后对HepG₂的生长有抑制作用,半抑制浓度（IC₅₀）为43.48 μg·ml^-1。对比不同浓度迷迭香酸作用HepG₂细胞24 h及48 h,当作用48 h时对HepG₂细胞的早期凋亡产生作用。不同浓度迷迭香酸作用于 HepG₂48 h后,c-Myc的表达随迷迭香酸浓度的增加而降低,P53的表达随迷迭香酸浓度的增加而增高。结论:迷迭香酸对HepG₂细胞增殖的抑制作用呈剂量依赖性,可通过对抑癌基因P53的激活及对原癌基因c-Myc蛋白的切割,促进 HepG₂细胞发生凋亡。     

华蟾素诱导人肝癌细胞株HepG2凋亡及其作用机制

为了观察华蟾素体外抑制肝癌细胞株HepG₂增殖、诱导细胞凋亡作用及其机制, 将不同浓度华蟾素作用于HepG₂细胞。采用MTT法观察华蟾素对HepG₂细胞的增殖抑制作用; 用Hoechst 33258染色法观察细胞凋亡的形态学改变; 以流式细胞术观察细胞的凋亡率和细胞周期的变化; 以实时荧光定量PCR技术 (real time-PCR) 和蛋白免疫印迹 (Western blotting) 法检测细胞凋亡相关因子Bcl-2、Bax及p53 mRNA水平和蛋白水平的表达情况。结果表明, 华蟾素对体外培养的肝癌HepG₂细胞有抑制增殖作用, 且具有剂量、时间相关性; 华蟾素可致HepG₂细胞阻滞于G₂/M期, 细胞凋亡率随药物浓度增加亦呈增长趋势; 细胞凋亡相关因子Bax及p53 mRNA和蛋白表达上调, Bcl-2的mRNA和蛋白表达下调。因此, 华蟾素可抑制肝癌细胞株HepG₂的增殖, 诱导肝癌HepG₂细胞凋亡, 其机制可能与调节凋亡相关因子Bcl-2、Bax及p53的表达有关。     

羟基红花黄色素A上调低氧状态下血管内皮细胞中缺氧诱导因子-1α的表达

羟基红花黄色素A(hydroxysafflor yellow A，HSYA)是从菊科植物红花中提取的查尔酮类化合物。本实验观察HSYA对低氧状态(1% O₂)下人脐静脉内皮细胞株(Eahy926)增殖的促进作用及其调节Eahy 926细胞中林希氏基因(von Hippel-Lindau gene，VHL)，抑癌基因p53(tumor suppressor gene p53，p53)和降解缺氧诱导因子-1α(hypoxia-inducible factor 1α，HIF-1α)的作用。采用MTT法测定低氧状态下HSYA(100、 10和1 μmol·L^-1)对Eahy 926细胞增殖率的影响。免疫组织化学方法测定VHL与p53蛋白表达数量和定位。RT-PCR方法检测细胞中VHL、 p53和HIF-1α的转录水平。Western blotting方法检测VHL、 p53和HIF-1α的蛋白表达。与低氧模型对照组相比，HSYA(100 μmol·L^-1)促使细胞增殖率升高，HIF-1α转录和蛋白表达水平显著增强，呈时间依赖性。给药8 h后，VHL与p53的蛋白表达和转录水平均明显降低。HSYA诱导HIF-1α表达增加并提高低氧状态下细胞增殖率的作用，可能通过抑制VHL和p53两种HIF-1α的降解调节因子实现。     

DJ-1保护多巴胺能神经元抵抗过氧化氢氧化应激损伤的研究

目的 探讨DJ-1保护多巴胺能神经元免受过氧化氢(H₂O₂)损伤的机制。 方法 使用神经生长因子(NGF)将大鼠嗜铬细胞瘤细胞(PC12细胞)诱导为多巴胺能神经元模型。H₂O₂处理引起细胞氧化应激损伤模型,CCK-8试剂盒检测细胞活性,DHE染色检测细胞内ROS水平。PI / Hoechst染色检测细胞凋亡,蛋白质免疫印迹法(Western blot)检测DJ-1和TH蛋白的表达。构建DJ-1过表达载体,检测DJ-1对H₂O₂中PC12细胞的保护作用及对细胞内活性氧(ROS)的影响。RT-qPCR检测α-synuclein,p53,Bax,Bcl-2的表达变化。 结果 H₂O₂处理可显着降低PC12细胞的活性,H₂O₂处理24 h以上可引起细胞凋亡。H₂O₂处理下调DJ-1蛋白和TH蛋白的表达,并且在RNA水平上α-突触核蛋白的表达增加。另外 p53, Bax和 caspase-3表达增加, Bcl-2表达减少。DJ-1的过表达可以抑制H₂O₂引起的ROS增加,DJ-1可以维持细胞活性,减少H₂O₂的凋亡。在RNA水平抑制α-突触核蛋白,p53,bax,bcl-2的凋亡和抗凋亡基因表达变化。 结论 DJ-1能够抑制H₂O₂引起的 ROS水平升高,减少α-synuclein积累,抑制 p53,凋亡基因如 bax的表达减弱了多巴胺能神经元中H₂O₂诱导的氧化应激损伤。     

基于网络药理学预测三七治疗幽门螺杆菌相关疾病机制及实验验证

目的 基于网络药理学预测三七治疗幽门螺杆菌（Hp）相关疾病机制,研究三七中有效活性成分人参皂苷Rb₃对Hp造成的胃上皮细胞损伤的保护作用及机制。方法 使用Herb数据库收集“三七”的相关预测靶点,使用Gene Cards数据库收集Hp相关疾病的靶点;使用Draw Venn Diagram网站绘制Venn图,得到靶点交集;进行蛋白互作（PPI）网络分析、基因本体（GO）和京都基因与基因组百科全书（KEGG）富集分析。将GES-1细胞分为对照组、模型组及人参皂苷Rb₃低、中和高浓度（1、5、10 μ mol· L^-1）组,人参皂苷Rb₃组使用相应浓度的人参皂苷Rb₃预处理,培养过夜12 h至融合度为70%～80%。Hp悉尼株1 （SS1）按感染复数（MOI） 100加入细胞中制备损伤模型,人参皂苷Rb₃继续给药,共培养48 h。对照组不加SS1,对照组和模型组不加药。改良吉姆萨染色后通过光学显微镜观察细胞形态;结合Hoechst 33342荧光染色和Annexin V/PI双染流式细胞术检测细胞凋亡;试剂盒法检测活性氧（ROS）水平;采用实时荧光定量PCR（qRT-PCR）检测凋亡相关基因TP53、Bax、Bcl-2表达量;Western blotting法检测P53、p-Akt、cleaved/pro-Caspase 9、Bcl-2、Bax、cleaved/pro-Caspase 3的蛋白表达情况。结果 网络药理学结果表明三七治疗Hp相关疾病的靶点共16个,其PPI网络分析得到按度值大小排名前6位靶点为TP53、CASP3、PTGS2、IL6、TNF、IL1β。GO富集分析与KEGG富集分析结果均显示与凋亡相关。与模型组比较,经人参皂苷Rb₃处理后,GES-1细胞的细胞核染色质致密深染,破裂的细胞逐渐减少;Hoechst 33342荧光染色细胞核强荧光数目明显减少;细胞凋亡率显著降低（P＜0.05）;ROS水平显著降低（P＜0.05）;TP53与Bax的mRNA水平显著降低,Bcl-2 mRNA水平显著升高（P＜0.05）; p-Akt、Bcl-2蛋白表达显著升高（P＜0.05）,P53、Bax、cleaved/pro-Caspase 9与cleaved/pro-Caspase 3蛋白表达显著降低（P＜0.05）。结论 三七可能通过包括炎症及凋亡在内的多种途径治疗Hp相关疾病,人参皂苷Rb₃对Hp诱导的胃上皮细胞凋亡发挥显著改善作用,其可能机制是降低氧化应激水平,并调节Akt的磷酸化和P53的表达。     

没药甾酮对H2O2损伤PC12细胞的保护作用

探讨没药甾酮(guggulsterone)对氧化应激损伤PC12细胞的保护作用。以过氧化氢(hydrogen peroxide，H₂O₂)损伤PC12细胞为氧化应激损伤模型， 维生素E为对照， 采用四甲基偶氮唑蓝［3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyl-tetrazolium bromide，MTT］法检测细胞增殖状况； 试剂盒检测乳酸脱氢酶(lactate dehydrogenase,LDH)及一氧化氮(nitric oxide，NO)的释放； DCFH法和Fura 2-AM法检测细胞内活性氧(reactive oxygen species，ROS)和Ca²⁺的含量； 碘化丙啶(propidium iodide，PI)染色流式细胞术(flow cytometry，FCM)检测细胞凋亡； 罗丹明123(rhodamine 123，Rh 123)染色FCM检测细胞线粒体膜电位(mitochondrial membrane protential，MMP)。结果表明， 没药甾酮(0.1～10 μmol·L^-1)可使200 μmol·L^-1 H₂O₂作用24 h后的PC12细胞生长抑制率下降； 细胞外LDH和NO， 细胞内ROS和Ca²⁺含量降低； 明显抑制200 μmol·L^-1 H₂O₂作用12 h后诱导的PC12细胞凋亡和线粒体膜电位降低作用，没药甾酮(0.1～10 μmol·L^-1)使细胞凋亡率由24.3%下降至18.4%、 15.9%、 11.8%。实验结果表明, 没药甾酮对氧化应激损伤PC12细胞具有保护作用， 其机制可能为降低细胞内ROS含量， 进而抑制LDH和NO释放， 降低细胞内Ca²⁺含量， 升高线粒体膜电位,减少细胞凋亡。     

苦参碱与3种抗肿瘤药物联合作用对KBV200耐药细胞株细胞周期的影响

目的:探讨苦参碱分别与3种常用抗肿瘤药物长春新碱(VCR)、阿霉素(ADM)、顺铂(DDP)合用对KBV200耐药细胞株细胞周期的影响。方法:将培养后的KBV耐药细胞分为对照组、抗肿瘤药物组及其与苦参碱合用组,采用流式细胞仪检测、分析各组细胞周期的变化及凋亡情况。结果:与抗肿瘤药物单用组比较,苦参碱与VCR、ADM合用组细胞处于不同周期的细胞比例及凋亡数均无明显变化;苦参碱与DDP合用组G0~G1期细胞数下降、G2~M期细胞数升高。结论:推测苦参碱逆转KBV200细胞对VCR和ADM的耐药作用可能与细胞周期及凋亡无关;苦参碱能轻度增强DDP的细胞毒作用。     

儿茶素对耐药人口腔表皮样癌细胞KBV200凋亡的影响

目的　研究两种儿茶素ECG、EGCG对耐药人口腔表皮样癌细胞KBV2 0 0的促凋亡作用。方法　MTT法检测药物对细胞的毒性作用 ,用形态学观察法、TUNUL法、琼脂糖凝胶电泳及流式细胞仪PI染色法观察细胞凋亡。结果　 30mg·L-1ECG或 8mg·L-1EGCG联合 0 0 2 4mg·L-1长春新碱 (VCR)可用形态学观察、免疫组化法、琼脂糖凝胶电泳法见到细胞凋亡改变 ,而对照组无此类改变或改变不明显。结论　两种儿茶素在低细胞毒剂量时可促进长春新碱对耐药口腔表皮样癌细胞KBV2 0 0的细胞凋亡作用。     

p53 dependent and independent sensitivity to oxaliplatin of colon cancer cells

Oxaliplatin is an efficient chemotherapeutic agent used for the treatment of metastatic human colon cancer, but cancer cells are frequently resistant. The aim of this study was to analyse the underlying mechanisms in a panel of 10 human colorectal cancer cell lines submitted to a short (2h) oxaliplatin treatment period, accordingly to the usual therapeutic procedure in humans. Sensitivity to oxaliplatin was a characteristic of p53 wild-type colon cancer cells. In contrast, all p53-mutated cell lines had a high IC50 to oxaliplatin, with the exception of the V9P cell line. Exposure to oxaliplatin resulted in G0/G1 arrest in p53 wild-type cell lines, and in S phase in p53-mutated cell lines. In our treatment conditions, no DNA accumulation in sub G0/G1 phase, no caspase-3 activation nor PARP cleavage were detected after oxaliplatin treatment, except for the V9P cell line. The major role of the p53-p21 pathway in oxaliplatin sensitivity was confirmed in the p53 wild-type HCT116 cell line, using siRNA duplex, and knockdown of the TAp73 protein also enhanced resistance to oxaliplatin in this cell line. Surprisingly, siRNA duplex invalidation revealed a residual effect of the mutant p53 protein in p53-mutated cell lines. Persistent sensitivity to oxaliplatin of the p53-mutated V9P cell line was associated with oxalipatin-induced apoptosis but TAp73 was not the responsible alternative pathway.     

鬼臼毒素衍生物CIP-36诱导KBV200细胞凋亡

目的：研究鬼臼毒素衍生物CIP-36对多药耐药人口腔鳞状上皮癌细胞KBV200的抗肿瘤活性及其作用机制。方法：MTT法考察CIP-36对KBV200体外增殖的抑制作用;Giemsa染色、DNA ladder和流式细胞仪等方法进行细胞凋亡检测;免疫荧光法观察CIP-36对细胞骨架的作用;western-blot法检测CIP-36对KBV200细胞P-gp表达的影响。结果：CIP-36对KBV200细胞有明显的抑制作用,IC50值为（2.06±0.38）μmol/L,能够诱导细胞产生凋亡小体和DNAladder。流式细胞检测到了细胞凋亡峰,并观察到细胞周期出现S/G2＋M期阻滞。Western-blot结果显示P-gp表达降低,并且观察到CIP-36可破坏KBV200细胞的细胞骨架。结论：CIP-36可能通过降低P-gp的表达,破坏细胞骨架等多靶点克服KBV200细胞株的多药耐药性。     

Induction of apoptosis and expression of apoptosis related genes in human epithelial carcinoma cells by Helicobacter pylori VacA toxin.

Virulence factors produced by Helicobacter pylori have been known to be associated with serious gastroduodenal diseases. The aims of this study were to clarify the apoptosis-inducing properties of vacuolating cytotoxin (VacA) and examine the expression of apoptosis related proteins in human epithelial carcinoma cells expressing (AGS) or lacking (Kato III) p53. The midregion VacA homolog from H. pylori strain Q35 (Korean isolate) was cloned, expressed and sequenced. Recombinant VacA (VacA(418-799)) inhibited cell growth and induced apoptosis in gastric epithelial cells. Treatment with VacA(418-799) resulted in morphological changes and DNA fragmentation. Cell cycle analysis revealed subdiploid cells suggesting apoptosis, which was confirmed by the activation of caspase-3 and cleavage of PARP. VacA(418-799) also mediated a prolongation of the cell cycle progression in G1 phase. Furthermore, VacA(418-799) increased the expression of p53, p21(waf1/cip1) and Bax in AGS cells, but not in Kato III cells and did not affect the phosphorylation of Rb in both cell lines. These results indicate that recombinant VacA of H. pylori induces apoptosis in both Kato III and AGS cells, regardless of p53 status and suggest that VacA(418-799) mediate the development of gastric diseases through cell cycle arrest in the G1 phase. VacA(418-799) induction of apoptosis is associated with up-regulation of p53, p21(waf1/cip1), Bax in AGS cells and activation of caspase-3 in both cell lines.     

鬼臼毒素衍生物CIP-36诱导KBV200细胞凋亡

目的研究鬼臼毒素衍生物CIP-36对多药耐药人口腔鳞状上皮癌细胞KBV200的抗肿瘤活性及其作用机制。方法MTT法考察CIP-36对KBV200体外增殖的抑制作用;Giemsa染色、DNAladder和流式细胞仪等方法进行细胞凋亡检测;免疫荧光法观察CIP-36对细胞骨架的作用;West-ernblot法检测CIP-36对KBV200细胞P-glycoprotein表达的影响。结果CIP-36对KBV200细胞有明显的抑制作用,IC50值为(2.06±0.38)μmol.L-1,能够诱导细胞产生凋亡小体和DNAladder。流式细胞检测到了细胞凋亡峰,并观察到细胞周期出现S/G2+M期阻滞。Westernblot结果显示P-gp表达降低,并且观察到CIP-36可破坏KBV200细胞的细胞骨架。结论CIP-36可能通过降低P-gp的表达,破坏细胞骨架等多靶点克服KBV200细胞株的多药耐药性。     

Cell cycle arrest and proapoptotic effects of the anticancer cyclodepsipeptide serratamolide (AT514) are independent of p53 status in breast cancer cells

In a search for new anticancer agents, we have identified serratamolide (AT514), a cyclodepsipeptide from Serratia marcescens 2170 that induces cell cycle arrest and apoptosis in various cancer cell lines. A cell viability assay showed that the concentrations that cause 50% inhibition (IC50) in human cancer cell lines range from 5.6 to 11.5 microM depending on the cell line. Flow cytometry analysis revealed that AT514 caused cell cycle arrest in G0/G1 or cell death, depending on the cell type and the length of time for which the cells were exposed to the drug. Subsequent studies revealed that AT514-induced cell death is caused by apoptosis, as indicated by caspases activation (8, 9, 2 and 3) and cleavage of poly (ADP-ribose) polymerase (PARP), release of cytochrome c and apoptosis inducing factor (AIF) from mitochondria, and the appearance of apoptotic bodies and DNA laddering. Alterations in protein levels of Bcl-2 family members might be involved in the mitochondrial disruption observed. AT514 induced p53 accumulation in wild-type p53 cells but cell death was observed in both deficient and wild-type p53 cells. Our results indicate that AT514 induces cell cycle arrest and apoptosis in breast cancer cells irrespectively of p53 status, suggesting that it might represent a potential new chemotherapeutic agent.     

Expression of p53-family members and associated target molecules in breast cancer cell lines in response to vincristine treatment

As the antimitotic agent vincristine (VCR) has been reported to induce a weak p53 response in some studies, we hypothesised that p73 and p63, the recently described p53 homologues, may replace p53 in triggering apoptosis or cell cycle arrest effectors in VCR-treated cell lines. To address this issue, we measured p53, p73 and p63 mRNA and protein levels in two VCR-treated breast cancer cell lines, one p53-proficient (MCF7) and the other p53-deficient (MDA-MB157). We found an increase of p53 mRNA and protein levels in VCR-treated MCF7 cells, while, as expected, no p53 protein was detected in VCR-treated MDA-MB157 cells. Surprisingly, the p73 mRNA and protein expression levels decreased in both cell lines during VCR treatment, whereas p63 protein levels remained unchanged. In both cell lines, up-regulations of the canonical p53-target genes, such as p21 and GADD45, were consistently observed. We conclude that, in response to VCR treatment: (1) p53 is markedly induced in MCF7 cells, with the same extent than after DNA damaging drugs treatments; and (2) p63 is not involved, while p73 expression is down-regulated regardless of the p53 status of the cell lines. Our results therefore suggest the involvement of a fourth member of the p53 gene family, or the use of another pathway able to trigger canonical p53-target genes in response to VCR in p53-deficient cells.     

The involvement of p53 in paraquat-induced apoptosis in human lung epithelial-like cells

To investigate the possible role of p53 in the progression of paraquat-induced apoptosis, the authors used two cell lines that were wild-type p53-expressing human lung epithelial-like cell line (L132) and a p53-deficient human promyelocytic leukemia cell line (U937) and explored the linkage between p53, DNA damage, and apoptosis. Following paraquat exposure to L132 cells, the percentage of S-phase cells decreased significantly and the expression of p53 protein increased, suggesting that entry into S phase from G1 phase was blocked. U937 cells showed complete resistance to paraquat, although paraquat-evoked initial single-stranded DNA breaks was shown equally in either L132 or U937 cells, as assessed by single-cell gel electrophoresis. U937 and L132 cells die normally with similar kinetics when exposed to tumor necrosis factor in the presence of cycloheximide, indicating that their capacity to undergo p53-independent mechanisms of inducing apoptosis has an equal rate. These results suggest that paraquat-induced DNA damage caused G1 arrest and apoptosis only in L132 cells, and that p53 protein accumulation is required for the induction of apoptosis by paraquat.