RNA干扰Ｃyclin D1基因表达对瘢痕疙瘩成纤维细胞增殖和G1期调控的影响

为研究siRNA干扰瘢痕疙瘩成纤维细胞cyclin D1基因表达,对瘢痕疙瘩成纤维细胞的增殖、细胞周期和G1期调控的影响,构建了靶向cyclin D1的siRNA表达质粒.利用LipofecmmineTM2000转染体外培养的瘢痕疙瘩成纤维细胞,应用荧光定量PCR、RT-PCR检测cyclin D1 mRNA的干扰效果,应用MTT法、流式细胞仪检测细胞增殖和细胞周期的变化,应用免疫组织化学染色检测成纤维细胞中cyclin D1、CDK4、P16、pRb蛋白表达的影响.主要结果如F:a.靶向cyclin D1的特异性siRNA序列可以高效地抑制成纤维细胞cyclin D1基因表达,对照组与实验组在mRNA水平其表达抑制率分别为63.68%和92.83%(P<0.01);b.可以显著抑制瘢痕疙瘩成纤维细胞的增殖,改变细胞周期分布,G0/G1期细胞比例显著高于各对照组(P<0.05),细胞分裂被阻滞;c.免疫组化染色发现,转染72 h后,过表达的cyclin D1、CDK4和pRb蛋白,在瘢痕疙瘩成纤维细胞中均出现了不同程度的表达下调,而低表达的P16则呈上调表现.由上述结果可见,构建的靶向cyclin D1的RNAi表达质粒,可有效地抑制瘢痕疙瘩成纤维细胞cyclin D1基因表达,通过改变Gl期相关周期蛋白的水平,影响G1/S期的进程,显著地抑制成纤维细胞的增殖.     

沉默Smad4基因表达抑制猪卵巢颗粒细胞增殖并使细胞G_1期阻滞

Smad4是TGF-β/Smad信号通路的核心下游信号分子.为探明Smad4基因对猪卵巢颗粒细胞增殖及细胞周期的影响,采用RNA干扰技术,设计并合成猪Smad4基因的靶向小分子干扰RNA,由LipofectamineTMRNAiMix介导转染体外培养的猪卵巢颗粒细胞.应用实时荧光定量PCR检测Smad4mRNA的干扰效果,应用MTT法、流式细胞术检测细胞增殖和细胞周期的变化,同时应用荧光定量PCR检测转染前后CyclinD1、CyclinB、CyclinA2、CDK1、CDK2、CDK4等周期相关基因的mRNA表达量的变化.实验结果显示,靶向猪Smad4的特异性siRNA序列对Smad4mRNA表达的抑制率为79.85%(P0.01);沉默Smad4可以显著抑制猪卵巢颗粒细胞增殖,并且改变细胞周期分布,G0/G1期细胞比例显著高于各对照组(P0.05),S期细胞比例显著低于各对照组(P0.05),细胞分裂被阻滞;转染36h后CyclinD1、CDK1的mRNA表达量显著低于对照组,CyclinA2、CDK2、CDK4极显著低于对照组,CyclinB差异不显著.综上所述,Smad4是影响猪卵巢颗粒细胞增殖及细胞周期进程的重要基因之一.     

过表达Nogo-C对PC12细胞存活及增殖的影响

以PC12细胞为神经元细胞模型,研究Nogo-C对神经元细胞存活及增殖的作用。在PC12细胞中转染过表达Nogo-C,使用G418药物筛选以获得稳定表达的细胞克隆,利用Hoechst33342染色、细胞计数、MTT以及流式细胞仪等技术检测Nogo-C对细胞增殖以及细胞周期的影响。结果表明:(1)Hoechst33342染色未观察到表达Nogo-C的细胞发生明显凋亡;(2)细胞计数及MTT实验观察到转染Nogo-C后的PC12细胞生长增殖活性明显降低;(3)流式细胞仪检测细胞生长周期,正常PC12细胞G1期的百分数为(37.8±7.9)%,S期为(50.4±8.5)%,而转染Nogo-C的PC12细胞G1期为(76.8±4.1)%,S期为(14.7±1.7)%,提示转染Nogo-C的PC12细胞的细胞周期被阻滞在G1期;(4)没有获得稳定表达Nogo-C的PC12细胞模型。实验证明,过表达Nogo-C通过使PC12细胞周期被阻滞在G1期而明显抑制细胞的增殖,但是并不引起细胞的凋亡。     

应用组织芯片检测大肠癌组织中p27和cyclin D1的表达及意义

目的 应用组织芯片检测大肠癌组织中p27和cyclinD1的表达及意义.方法 应用免疫组织化学技术检测人大肠癌组织芯片150芯(包括70例大肠癌组织和5例癌旁组织)中p27和cyclinDl的表达.采用多光谱成像系统对免疫组织化学结果进行图像分析,并用SPSS13.0软件对各组染色分析后的数据做单因素方差分析和SNK(q)检验,检验水准α为0.05.结果 大肠癌组织中P27呈低表达,癌旁组织中呈高表达;大肠癌组织中cyclinD1呈高表达,癌旁组织中cyclinD1呈低表达,2组比较差异均有统计学意义(均P ＜0.05).结论 在大肠癌细胞周期G1/S期调控机制中,cyclinD1作为细胞周期G1／S期调控的正问因子促进细胞增殖;p27具有抑制cyclinD1的作用,从而阻止细胞周期GI/S期的转变,抑制细胞增殖. 统对免疫组织化学结果进行图像分析,并用SPSS13.0软件对各组染色分析后的数据做单因素方差分析和SNK(q)检验,检验水准α为0.05.结果 大肠癌组织中P27呈低表达,癌旁组织中呈高表达;大肠癌组织中cyclinD1呈高表达,癌旁组织中cyclinD1呈低表达,2组比较差异均有统计学意义(均P ＜0.05).结论 在大肠癌细胞周期G1/S期调控机制中,cyclinD1作为细胞周期G1／S期调控的正问因子促进细胞增殖;p27具有抑制cyclinD1的作用,从而阻止细胞周     

D类细胞周期素在细胞周期中的作用

细胞周期进程由周期性表达的细胞周期素和细胞周期素信号性激酶推动完成,CyclinD在G1期中期开始表达,是最早合成的细胞周期素,它与Cdk4或Cdk6形成的活性复合物,不仅可促进pRb的磷酸化,也可隔离P27对cyclinE-Cdk2活性的抑制,使细胞顺利越过G1期进入S期,除此之外,cyclinD过度表达还可抑制细胞增殖,在细胞分化和衰老过程中发挥重要作用。     

抑制VEGF基因表达对乳腺癌细胞细胞周期的影响

目的：研究针对VEGF基因的siRNA（small interferenceRNA）对乳腺癌MCF-7细胞细胞周期的影响。方法：依据Promega公司在网上提供的设计软件,设计针对VEGF基因的siRNA,合成DNA模板,体外转录合成siRNA。脂质体转染法将合成的siRNA转染入MCF-7细胞,以未转染细胞以及错义序列siRNAscr转染细胞为对照。用细胞计数法检测siRNA对MCF-7细胞增殖的影响：流式细胞法检测细胞周期变化,RT—PCR法比较转染前后p21、CyclinDl表达水平的变化,Westemblot检测转染前后磷酸化ERK的表达。结果：细胞计数法结果显示,转染24h后siRNA明显抑制MCF-7细胞增殖,转染48h后,抑制效率稳定。siRNA转染后能有效地抑制MCF-7细胞的增殖,阻滞细胞周期于G0／G1期,S期细胞明显减少,G0／G1期细胞比例逐渐增多;p21mRNA表达显著上调,抑制CyclinD1mRNA及磷酸化ERK蛋白的表达。结论：体外转录合成的siRNA可能通过上调细胞周期蚤白激酶抑制剂p21的表达,下调CyclinDl及磷酸化ERK的表达,将细胞周期阻滞于G0／G1期,从而显著抑制MCF-7细胞的增殖。     

M-CSF上调cyclinD1/D3和CDK2/6的表达促进HeLa细胞增殖

非分泌型巨噬细胞集落刺激因子(M-CSF)的表达在肿瘤的发生发展过程中发挥重要作用,为探讨胞质M-CSF对细胞增殖的影响,采用基因重组技术构建胞内稳定表达M-CSF的HeLa细胞系,以空载体(pCMV/myc/cyto)转染HeLa细胞和未转染HeLa细胞作为对照,MTT法及反义寡核苷酸抑制实验分析M-CSF对细胞增殖的影响,并计算细胞倍增时间,RT-PCR观察胞内M-CSF对G1期细胞周期相关蛋白的影响．结果显示,与对照组比较,转染M-CSF的HeLa细胞倍增时间明显缩短、增殖能力显著增强,M-CSF的特异性反义寡核苷酸能抑制转染M-CSF的HeLa细胞的增殖,且抑制率随着反义寡核苷酸浓度的增高而增强,转染M-CSF 的HeLa细胞的cyclinD1/D3和CDK2/6 mRNA表达显著升高(P < 0.05)．提示：M-CSF可上调cyclinD1/D3和CDK2/6的mRNA表达,促进HeLa细胞的增殖．     

鼻咽癌相关基因NGX6对鼻咽癌细胞周期的影响

为了探讨鼻咽癌（NPC）候选抑瘤基因NGX6对NPC细胞的细胞周期进程及细胞周期素的影响,阐明它的作用机制,通过建立稳定表达NGX6的鼻咽癌HNE1细胞株,采用细胞免疫组织化学,流式细胞仪检测与分析细胞周期及细胞周期素的改变,用western blot验证它对细胞周期的影响。结果显示稳定表达NGX6的HNE1细胞较对照组细胞周期中G0/G1期比值明显增加,而S期比例减少。细胞凋亡率无明显变化。流式细胞仪检测发现cyclinD1、A和E的表达明显减少,以cyclinD1的改变最为明显。Western blot检测也发现cyclinD1的表达明显下调。以上结果说明NGX6主要通过下调cyclinD1的表达,延缓细胞周期的G1→S的进程,从而抑制NPC细胞的过度增殖。     

乙肝病毒X蛋白通过β-catenin通路调控肝癌细胞增殖、细胞周期的研究

乙肝病毒X蛋白(HBx)是由乙型肝炎病毒(HBV)DNAX基因编码的蛋白,具有促癌作用且与乙肝相关肝癌的发生有关,但其发挥促癌作用的机制尚不清楚.在乙肝相关肝癌中,β-连环蛋白(β-catenin)通路过度激活并介导了促进细胞增殖、加速细胞周期的效应.为了观察HBx是否通过β-catenin通路调控肝癌细胞增殖、细胞周期,本实验培养肝癌Huh7细胞株并分为对照组、转染HBx质粒的HBx组、转染HBx并用β-catenin抑制剂ICG-001处理的HBx+ICG-001组、转染HBx并用β-catenin抑制剂XAV939处理的HBx+XAV939组,检测细胞增殖活力OD490nm、细胞周期及生存素(survivin)、B淋巴细胞瘤-2基因(Bcl-2)、细胞周期蛋白D1(CyclinD1)、c-myc、β-catenin、磷酸型GSK-3β(p-GSK-3β)的表达量.结果显示,HBx组的OD490nm水平、细胞周期S期、G2/M期的比例、survivin、bcl-2、cyclinD1、c-myc、β-catenin、p-GSK-3β的表达量均高于对照组,细胞周期G0/G1期的比例低于对照组(P＜0.05);HBx+ICG-001 组、HBx+XAV939组的OD490nm水平、细胞周期S期、G2/M期的比例、survivin、bcl-2、cyclinD1、c-myc、β-catenin、p-GSK-3β的表达量均低于HBx组,细胞周期G0/G1期的比例高于HBx组(P＜0.05).结果表明,HBx促进肝癌细胞增殖、加速细胞周期的作用与激活β-catenin通路有关.本研究的创新之处在于阐明HBx促进肝癌细胞增殖、加速细胞周期的分子机制,激活β-catenin通路是介导HBx上述作用的分子机制之一,这也为今后乙肝相关肝癌的防治提供了新思路、新靶点.     

缺血缺氧对体外培养星形胶质细胞细胞活化和细胞周期的影响

目的观察缺血缺氧损伤对星形胶质细胞细胞活化和细胞周期的影响。方法用流式细胞仪及BrdU掺入法检测缺血缺氧后不同时间点星形胶质细胞细胞周期变化和细胞的增殖活力;用荧光免疫细胞化学技术测定胶质细胞纤维酸性蛋白(GFAP)及细胞周期蛋白cyclinD1的表达水平。结果体外缺血缺氧损伤后星形胶质细胞S期较正常组明显增高,6h达高峰,BrdU掺入法显示损伤后6h星形胶质细胞的增殖活力最高,而随后S期细胞数目及细胞增殖活力都呈下降趋势。在缺血缺氧早期,GFAP阳性染色增强,6h最高;缺血缺氧12h后GFAP阳性染色变弱,而cyclinD1的表达在损伤后逐渐增加,在24h时达高峰。结论缺血缺氧损伤激活星形胶质细胞,使其进入新的细胞周期,出现细胞的增殖反应;cyclinD1参与了损伤后星形胶质细胞的修复和增殖;细胞周期事件与星形胶质细胞的增殖活化密切相关。     

Retracted: Smad4 gene silencing enhances the chemosensitivity of human lymphoma cells to adriamycin via inhibition of the activation of transforming growth factor β signaling pathway

There are diverse investigations focused on the therapies of lymphoma. Our research was taken to identify the effects of lentiviral-mediated Smad4 gene silencing on chemosensitivity of human lymphoma cells to adriamycin (ADM) via transforming growth factor β (TGFβ) signaling pathway. Raji/ADM cells were cultured and infected with lentiviral particles Smad4-short hairpin (shRNA) and control-shRNA. Then, the messenger RNA (mRNA) and protein levels of TGFβ signaling pathway–related factors (Smad4, Smad3, cyclinE, cyclinD1, and p21) in Raji/ADM cells were determined. The effect of Smad4-shRNA on cell viability, invasion and migration, and apoptosis were also detected. Compared with the Raji group, increased mRNA and protein levels of Smad4, Smad3, cyclinE, cyclinD1, enhanced cell proliferation, migration and invasion as well as decreased mRNA, and protein levels of p21 and cell apoptosis rate were found in the Raji/ADM and control-shRNA groups. However, Smad4 gene silencing resulted in decreased mRNA and protein levels of Smad4, Smad3, cyclinE, and cyclinD1 along with inhibited cell proliferation, migration and invasion but increased expression of p21 together with cell apoptosis. Collectively, Smad4 gene silencing can inhibit the activation of TGFβ signaling pathway, thereby enhancing the chemosensitivity of human lymphoma cells to ADM.     

Smad7 induces G0/G1 cell cycle arrest in mesenchymal cells by inhibiting the expression of G1 cyclins

The major Smad pathways serve in regulating the expression of genes downstream of TGFbeta signals. In this study, we examined the effects of sustained Smad7 expression in cultured cells. Interestingly, Smad7 caused various mesenchymal cells, including NIH3T3 fibroblast and ST2 bone-marrow stromal cells, to undergo a marked morphological alteration into a flattened cell shape, but kept them alive for as long as 60 days. Furthermore, Smad7 arrested the proliferation of the cells even before they reached confluence. These cells became quiescent in G0/G1 phase and accumulated a hypophosphorylated form of retinoblastoma. The cytostatic effect of Smad7 was closely associated with a preceding decrease in the levels of G1 cyclins, such as cyclin D1 and cyclin E. Accordingly, ectopic cyclin E was able to overcome the Smad7-induced arrest of proliferation. These results indicate that Smad7 functions upstream of G1 cyclins and suggest a novel role for Smad7 as an antiproliferative factor. In contrast to the growth of mesenchymal cells, that of epithelial cells was little susceptible to Smad7. The present findings raise the possibility that a link between Smad7 and the G1 to S phase transition may also contribute to the cell cycle control by certain Smad7-inducing stimuli in a cell-type-dependent fashion.     

Inhibition of benzo(a)pyrene-induced cell cycle progression by all-trans retinoic acid partly through cyclin D1/E2F-1 pathway in human embryo lung fibroblasts

Benzo(a)pyrene [B(a)P] is a potent environmental carcinogen, which induces cell cycle changes. All-trans retinoic acid (ATRA) is a promising agent in prevention and treatment of human cancers. In the present study, we investigated the inhibition of B(a)P-induced cell cycle progression by ATRA in human embryo lung fibroblast (HELF). Our results showed that after treatment with B(a)P, the expression of cyclin D1 and E2F-1 were both increased significantly in HELF. There were almost no changes of CDK4 and E2F-4 expression by treatment with B(a)P. As expected, pretreatment with ATRA could efficiently decrease B(a)P-induced overexpression of cyclin D1 and E2F-1. In a further study, we stably transfected antisense cyclin D1 and antisense CDK4 plasmid into HELF. The inhibition of cyclin D1 expression and the inhibition of CDK4 expression significantly impaired the B(a)P-induced overexpression of E2F-1 respectively. Pretreatment with ATRA, cells expressing antisense cyclinD1 or antisense CDK4 showed a lesser decrease of B(a)P-induced overexpression of E2F-1 compared with similarly treated HELF. Furthermore, flow cytometry analysis showed that B(a)P promoted cell cycle progression from G(1) phase to S phase, while pretreatment with ATRA could inhibit B(a)P-induced cell cycle progression by an accumulation of cells in the G(1) phase. It was suggested that ATRA could block B(a)P-induced cell cycle promotion partly through the cyclin D1/E2F-1 pathway in HELF.     

Effect of sodium butyrate on cell proliferation and cell cycle in porcine intestinal epithelial (IPEC-J2) cells

Conflicting results have been reported that butyrate in normal piglets leads either to an increase or to a decrease of jejunal villus length, implying a possible effect on the proliferation of enterocytes. No definitive study was found for the biological effects of butyrate in porcine jejunal epithelial cells. The present study used IPEC-J2 cells, a non-transformed jejunal epithelial line to evaluate the direct effects of sodium butyrate on cell proliferation, cell cycle regulation, and apoptosis. Low concentrations (0.5 and 1 mM) of butyrate had no effect on cell proliferation. However, at 5 and 10 mM, sodium butyrate significantly decreased cell viability, accompanied by reduced levels of p-mTOR and PCNA protein. Sodium butyrate, in a dose-dependent manner, induced cell cycle arrest in G0/G1 phase and reduced the numbers of cells in S phase. In addition, relative expression of p21, p27, and pro-apoptosis bak genes, and protein levels of p21Waf1/Cip1, p27Kip1, cyclinD3, CDK4, and Cleave-caspase3 were increased by higher concentrations of sodium butyrate (1, 5, 10 mM), and the levels of cyclinD1 and CDK6 were reduced by 5 and 10 mM butyrate. Butyrate increased the phosphorylated form of the signaling molecule p38 and phosphorylated JNK. In conclusion, the present in vitro study indicated that sodium butyrate inhibited the proliferation of IPEC-J2 cells by inducing cell cycle arrest in the G0/G1 phase of cell cycles and by increasing apoptosis at high concentrations.     

Synchronized prostate cancer cells for studying androgen regulated events in cell cycle progression from G1 into S phase

Androgen-ablation is a most commonly prescribed treatment for metastatic prostate cancer but it is not curative. Development of new strategies for treatment of prostate cancer is limited partly by a lack of full understanding of the mechanism by which androgen regulates prostate cancer cell proliferation. This is due, mainly, to the limitations in currently available experimental models to distinguish androgen/androgen receptor (AR)-induced events specific to proliferation from those that are required for cell viability. We have, therefore, developed an experimental model system in which both androgen-sensitive (LNCaP) and androgen-independent (DU145) prostate cancer cells can be reversibly blocked in G(0)/G(1) phase of cell cycle by isoleucine deprivation without affecting their viability. Pulse-labeling studies with (3)H-thymidine indicated that isoleucine-deprivation caused LNCaP and DU145 cells to arrest at a point in G(1) phase which is 12-15 and 6-8 h, respectively, before the start of S phase and that their progression into S phase was dependent on serum factors. Furthermore, LNCaP, but not DU145, cells required AR activity for progression from G(1) into S phase. Western blot analysis of the cell extracts prepared at regular intervals following release from isoleucine-block revealed remarkable differences in the expression of cyclin E, p21(Cip1), p27(Kip1), and Rb at the protein level between LNCaP and DU145 cells during progression from G(1) into S phase. However, in both cell types Cdk-2 activity associated with cyclin E and cyclin A showed an increase only when the cells transited from G(1) into S phase. These observations were further corroborated by studies using exponentially growing cells that were enriched in specific phases of the cell cycle by centrifugal elutriation. These studies demonstrate usefulness of the isoleucine-deprivation method for synchronization of androgen-sensitive and androgen-independent prostate cancer cells, and for examining the role of androgen and AR in progression of androgen-sensitive prostate cancer cells from G(1) into S phase.     

Role of p53 tumor suppressor in ageing: regulation of transient cell cycle arrest and terminal senescence.

In this study we investigated the function of p53 as a regulator of cell cycle progression in cycling and senescent cells. Using the conditional temperature-sensitive (ts) mutant we could prevent the detrimental effect of constitutive expression of high levels of wt p53 protein. High levels of wt p53 inhibited cell proliferation by blocking the cells to progress from G1 to S phase of the cell cycle. Flow cytometric analysis revelaed a maintenance of G1 cell population for a longer time depending on the prolonged expression of wt p53 protein. The p53 mediated inhibition of cell proliferation and of the cycle was reversible. However, a spontaneous increase of wt p53 occurring in ageing normal human MRC-5 fibroblasts was associated with irreversible reduction of proliferative potential. The accumulation of G1 cells was detected by flow cytometry. By the measurement of DNA content it is not possible to discriminate between cells arrested in G1 and G0 phase, therefore, the expression of G1 markers was determined. Analysis of the expression of distinct cell cycle regulators revealed that quiescent MRC-5 cells were in G0 phase. Our results indicate that cell cycle arrest occurring in senescent cells is associated with the G0 transition.     

LTF基因在鼻咽癌细胞系中生物学功能的初步研究

目的:初步探讨野生型LTF基因在鼻咽癌细胞系中的生物学功能。方法:野生型LTF导入鼻咽癌细胞系,G418筛选,RT-PCR和Western-blotting分别在mRNA和蛋白质水平进行验证,得到稳定表达LTF基因的鼻咽癌细胞系。流式细胞术、平板克隆形成实验和MTT法分别检测细胞周期、细胞的克隆形成能力和细胞生长曲线。结果:成功导入LTF并稳定表达的鼻咽癌细胞系,G0-G1期细胞百分比例明显增加(72.01%vs 62.31%),G2-M期细胞百分比例减少(6.26%vs 10.81%);克隆形成能力降低(39.5%vs 59.7%),体外瘤细胞增殖能力降低(P0.05)。结论:LTF基因可阻滞细胞周期、抑制鼻咽癌细胞系的增殖能力和克隆形成率,同时为进一步的体内试验研究奠定基础。     

PDK1 regulates cell proliferation and cell cycle progression through control of cyclin D1 and p27Kip1 expression

PDK1 (3-phosphoinositide-dependent protein kinase 1) is a key mediator of signaling by phosphoinositide 3-kinase. To gain insight into the physiological importance of PDK1 in cell proliferation and cell cycle control, we established immortalized mouse embryonic fibroblasts (MEFs) from mice homozygous for a "floxed" allele of Pdk1 and from wild-type mice. Introduction of Cre recombinase by retrovirus-mediated gene transfer resulted in the depletion of PDK1 in Pdk1(lox/lox) MEFs but not in Pdk1(+/+) MEFs. The insulin-like growth factor-1-induced phosphorylation of various downstream effectors of PDK1, including Akt, glycogen synthase kinase 3, ribosomal protein S6, and p70 S6 kinase, was markedly inhibited in the PDK1-depleted (Pdk1-KO) MEFs. The rate of serum-induced cell proliferation was reduced; progression of the cell cycle from the G(0)-G(1) phase to the S phase was delayed, and cell cycle progression at G(2)-M phase was impaired in Pdk1-KO MEFs. These cells also manifested an increased level of p27(Kip1) expression and a reduced level of cyclin D1 expression during cell cycle progression. The defect in cell cycle progression from the G(0)-G(1) to the S phase in Pdk1-KO MEFs was rescued by forced expression of cyclin D1, whereas rescue of the defect in G(2)-M progression in these cells required both overexpression of cyclin D1 and depletion of p27(Kip1) by RNA interference. These data indicate that PDK1 plays an important role in cell proliferation and cell cycle progression by controlling the expression of both cyclin D1 and p27(Kip1).     

A new compound which reversibly arrests T lymphocyte cell cycle near the G1/S boundary

A novel cell cycle blocking agent profoundly suppressed the proliferation of mitogen-stimulated T lymphocytes. The carboxythiazole derivative arrested cells in the G1 phase of the cell cycle but did not inhibit the induction of cell surface receptors for either interleukin-2 or transferrin. The uncoupling of transferrin receptor expression from DNA synthesis indicated that a previously undefined restriction point in the cell cycle has been identified which occurs after transferrin receptor expression in late G1 and just prior to the initiation of DNA replication in S phase. T cells incubated in an inhibitory dose of the carboxythiazole derivative resumed cell cycle progression subsequent to its removal, indicating that the compound reversibly arrests cells at the late G1 restriction point. In contrast to other techniques which have been inefficient in achieving T cell synchronization, T cells released from the block mediated by the carboxythiazole compound progress through S phase with a considerable degree of synchrony.