Distinct mechanisms act in concert to mediate cell cycle arrest

In response to DNA damage, cells arrest at specific stages in the cell cycle. This arrest must fulfill at least 3 requirements: it must be activated promptly; it must be sustained as long as damage is present to prevent loss of genomic information; and after the arrest, cells must re-enter into the appropriate cell cycle phase to ensure proper ploidy. Multiple molecular mechanisms capable of arresting the cell cycle have been identified in mammalian cells; however, it is unknown whether each mechanism meets all 3 requirements or whether they act together to confer specific functions to the arrest. To address this question, we integrated mathematical models describing the cell cycle and the DNA damage signaling networks and tested the contributions of each mechanism to cell cycle arrest and re-entry. Predictions from this model were then tested with quantitative experiments to identify the combined action of arrest mechanisms in irradiated cells. We find that different arrest mechanisms serve indispensable roles in the proper cellular response to DNA damage over time: p53-independent cyclin inactivation confers immediate arrest, whereas p53-dependent cyclin downregulation allows this arrest to be sustained. Additionally, p21-mediated inhibition of cyclin-dependent kinase activity is indispensable for preventing improper cell cycle re-entry and endoreduplication. This work shows that in a complex signaling network, seemingly redundant mechanisms, acting in a concerted fashion, can achieve a specific cellular outcome.     

骨良恶性肿瘤cyclin D1和cdk4的表达及其意义

[目的]研究骨软骨瘤、骨软骨肉瘤和骨肉瘤组织中cyclin D1和cdk4表达及其意义.[方法]采用石蜡包埋切片ABC免疫组化法.[结果]骨软骨瘤cyclin D1(3/16)和cdk4(4/16)阳性率明显低于骨软骨肉瘤(9/13,9/13),和骨肉瘤(5/6,5/6),差异有显著性;cyclin D1和cdk4表达与骨软骨瘤、骨肉瘤和骨肉瘤病理类型有一定关系,但与其发生部位无明显关系.[结论]cyclin D1和cdk4表达特征与骨恶性肿瘤发生发展有较密切关系;检测骨软骨瘤cyclinD4和cdk4表达可能在预防和早期发现骨恶性肿瘤方面有临床意义.     

The Cyclin-Dependent Kinase Ortholog pUL97 of Human Cytomegalovirus Interacts with Cyclins

The human cytomegalovirus (HCMV)-encoded protein kinase, pUL97, is considered a cyclin-dependent kinase (CDK) ortholog, due to shared structural and functional characteristics. The primary mechanism of CDK activation is binding to corresponding cyclins, including cyclin T1, which is the usual regulatory cofactor of CDK9. This study provides evidence of direct interaction between pUL97 and cyclin T1 using yeast two-hybrid and co-immunoprecipitation analyses. Confocal immunofluorescence revealed partial colocalization of pUL97 with cyclin T1 in subnuclear compartments, most pronounced in viral replication centres. The distribution patterns of pUL97 and cyclin T1 were independent of HCMV strain and host cell type. The sequence domain of pUL97 responsible for the interaction with cyclin T1 was between amino acids 231–280. Additional co-immunoprecipitation analyses showed cyclin B1 and cyclin A as further pUL97 interaction partners. Investigation of the pUL97-cyclin T1 interaction in an ATP consumption assay strongly suggested phosphorylation of pUL97 by the CDK9/cyclin T1 complex in a substrate concentration-dependent manner. This is the first demonstration of interaction between a herpesviral CDK ortholog and cellular cyclins.     

人细胞周期素D1和B1真核表达载体的构建及对HeLa细胞的转染

目的：构建人细胞周期素cyclin D1和cyclin B1的真核表达载体,并将其瞬时转染到HeLa细胞株中。方法：以HeLa细胞总RNA为模板,通过RT-PCR扩增cyclin D1和cyclin B1基因编码的cDNA,并将扩增的cDNA片段插入p3XFLAG-CMV~（TM）-14真核表达载体,分别构建p3XFLAG-cyclin D1和p3XFLAG-cyclin B1重组质粒,重组子经酶切分析和测序鉴定后,用脂质体介导的基因瞬时转染法,将重组正确的表达载体转染HeLa细胞,用Western-blot技术检测细胞中FLAG融合蛋白的表达。结果：经酶切鉴定和Western印迹分析证实人cyclin D1和cyclin B1的真核表达载体构建成功,并能在瞬时转染的HeLa细胞中表达分子量大小相符的重组蛋白。结论：成功构建了人cyclin D1和cyclin B1的真核表达载体,为两种细胞周期素及其相关蛋白的功能研究奠定了基础。     

5,7,3'-Trihydroxy-3,4'-dimethoxyflavone inhibits the tubulin polymerization and activates the sphingomyelin pathway

Flavonoids are polyphenolic compounds which display a vast array of biological activities and are among the most promising anti-cancer agents. The derivative of quercetin, 5,7,3'-trihydroxy-3,4'-dimethoxyflavone (THDF), is a natural flavonoid that inhibits cell proliferation and induces apoptosis in human leukemia cells. Here we show that THDF induces cell-cycle arrest in the M phase and inhibits tubulin polymerization. This was associated with the accumulation of cyclin B1 and p21(Cip1) , changes in the phosphorylation status of cyclin B1, Cdk1, Cdc25C, and MPM-2, and activation of the acidic sphingomyelinase (ASMase). Moreover, desipramine attenuated THDF-mediated cell death, indicating a crucial role of ASMase in the mechanism of cell death. In vivo studies on the athymic nude mouse xenograft model also confirmed that THDF inhibits growth of human leukemia cells and suggest that this compound may have therapeutic value.     

High-throughput tissue microarray analysis of G1-cyclin alterations in classical Hodgkin's lymphoma indicates overexpression of cyclin E1

Deregulation of G1-cyclins (CCN) plays a key role in the pathogenesis of many human malignancies, including non-Hodgkin's lymphomas (NHLs). In contrast to NHL, little is known about phenotypic and genotypic changes in the regulation of the cell cycle in classical Hodgkin's lymphoma (cHL). To facilitate analysis of aberrant gene expression in cHL, a lymphoma tissue microarray (TMA) containing 752 cores of 330 different cHL samples was constructed. Direct comparison of Epstein-Barr virus (EBV) latent membrane protein 1 (LMP-1) expression in Hodgkin's and Reed-Sternberg (HRS) cells on conventional full sections with the corresponding duplicate/triplicate tumour cores on the TMA showed a concordance of 100%, indicating that cHL-TMA is a reliable and representative method for evaluating gene expression profiles in situ. Using TMA technology, protein expression and gene amplification of different G1-CCNs in cHL were analysed. Among the G1-CCNs analysed, cyclin E (CCNE) was expressed in 212/253 cases (84%). In most of the individual tumours, over 75% of the HRS cells stained positive for CCNE, suggesting that CCNE is overexpressed in cHL. This overexpression was not due to CCNE gene amplification, as judged by fluorescence in situ hybridization, and did not correlate with EBV infection, as assessed by the expression of LMP-1. Thus, the overexpression of CCNE could be caused by profound changes in HRS cell-cycle regulation that could contribute to the malignant phenotype.     

前列腺素E1抑制血管平滑肌细胞增殖的作用

目的 从细胞周期蛋白和凋亡相关基因探讨前列腺素E₁(PGE₁)对血管平滑肌细胞(VSMC)增殖的调控作用。方法 采用培养的新生牛主动脉VSMC，以胎牛血清及白介素1(IL-1)分别作为促增殖剂，测定不同浓度PGE₁对VSMC增殖、细胞内周期蛋白d₁ mRNA表达、p53及Bcl-2基因表达和培养液内NO含量的影响。结果 PGE₁浓度依赖性抑制胎牛血清促VSMC增殖作用；显著减少细胞周期蛋白d₁ mRNA含量；浓度依赖性诱导IL 1孵育下VSMC产生大量NO, 同时抑制VSMC增殖；并促进p53基因表达，下调bcl-2基因，诱导细胞凋亡。结论 PGE₁具有抑制VSMC增殖的作用，其机制可能与其阻滞VSMC生长有关；在炎性因子存在情况下，PGE₁可诱导VSMC内大量产生NO，抑制VSMC增殖，并加速VSMC凋亡。     

中药"金叶败毒制剂"对人巨细胞病毒感染细胞病变及细胞周期蛋白表达影响的研究

目的研究中药金叶败毒制剂对人巨细胞病毒(HCMV)感染细胞细胞病变及细胞周期蛋白(Cyclins)表达的影响.方法中药金叶败毒制剂、西药更昔洛韦(药物对照组)与HCMV AD169毒株在体外感染的人胚肺成纤维上皮细胞(humanembryo lung fibrobla st,HEL)共同培养,于12小时及48小时分别用光镜、电镜观察致细胞病变(cytopathologiceffect s,CPE)及其超微结构的改变;用Western Blot检测金叶败毒制剂、更昔洛韦干预HCMV169感染组、HCMV1 69感染细胞、正常培养细胞中Cyclin D、E、A、B1的表达.结果 (1)培养12小时,仅病毒感染组在电镜下可观察到细胞肿胀,其余各组未见明显病变;(2)培养48小时,与HCMV169感染组相比,药物干预HCMV169感染HEL两组细胞中,光镜及电镜CPE程度较轻,正常组细胞无CPE;(3)细胞培养12及48小时,病毒组CyclinE、CyclinD的表达为最强,而中药及西药组次之,正常组未见表达,(P均＜0.05),但各组CyclinA、CyclinB均未见明显表达.结论在HEL细胞受HCMV1 69感染导致细胞病变过程中,Cyclins发生了导致细胞周期进程的改变,而中药金叶败毒制剂、西药更昔洛韦能改善HCMV169细胞感染细胞周期的进程,起到延缓CPE的作用.     

表皮生长因子促进真皮成纤维细胞生长的机制

目的探讨表皮生长因子(EGF)对真皮成纤维细胞中cyclinD1和CDK-4表达的影响,从细胞周期的角度认识EGF促进真皮成纤维细胞生长的机制。方法研究对象为大鼠真皮成纤维细胞,用含100ml/L胎牛血清的DMEM,加入50mg/L的EGF培养SD大鼠的真皮成纤维细胞,通过MTT检测、流式细胞仪分析观察细胞的生长状态,免疫组化检测cyclinD1和CDK-4的表达结合流式细胞仪分析来观察细胞的周期变化。结果MTT检测(加药前0.37±0.011,加药后0.55±0.008)、流式细胞仪结果(加药前9.1,加药后14.7)都显示50mg/L的EGF能显著促进真皮成纤维细胞的生长增殖,免疫组化结果显示,两者一致,加药后阳性明显增强。结论50mg/L的EGF对真皮成纤维细胞的生长有极大地促进作用,促进了细胞周期蛋白cyclinD1和CDK-4的表达增强,细胞G1期变短,增殖能力增强。     

Interaction of retinoblastoma protein and D cyclins during cell-growth inhibition by hexamethylenebisacetamide in TM2H mouse epithelial cells

To explore the regulation and function of D-type cyclins in breast cancer cells, the mouse mammary hyperplastic epithelial cell line TM2H was treated with 5 mM hexamethylenebisacetamide (HMBA), a polar differentiation factor. The resulting growth-inhibitory effect of HMBA was completely reversible and was analyzed in terms of percent cells in G₁; association of D-type cyclins with cyclin-dependent kinase (cdk) 4 and cdk6; G₁ kinase activity; association of retinoblastoma protein (pRb) and phosphorylated pRb with D-type cyclins; and association of p16^INK4a, p15^INK4b, and p27^Kip1 with cdk4 and cdk6. Synchronized TM2H cells were examined at 0, 3, 5, 9, 12, and 24 h after exposure to 5 mM HMBA. Inhibition of DNA synthesis, as measured by thymidine uptake, was first observed at 5 h (40%) and peaked at 24 h (80%). Flow cytometry at 9 h showed treated cells to be in G₁ arrest. Western blot analysis showed weakly detectable cyclin D1 but readily detectable cyclin D2 and D3 proteins at 0 h; thereafter, cyclin D2 and D3 protein levels remained higher while cyclin D1 levels declined significantly in treated versus untreated cells. By 5 h (early G₁), HMBA had markedly inhibited cdk4 and cdk6 kinase activity (67% and 75%, respectively) in treated versus untreated cells. By 9 and 12 h, pRb levels had increased 3.4-fold in treated versus untreated cells. At 5 h, cyclin D–associated pRb was totally hypophosphorylated in treated cells and hyperphosphorylated in untreated cells. The levels of pRb associated with cyclin D2 and D3 increased 2.89-fold and 4.6-fold, respectively, in treated versus untreated cells. At 5 h, treated cells showed a fivefold increase in cdk4-associated p27^Kip1 and, at 9 h, a fourfold increase in cdk6-associated p27^Kip1 over control levels. In confirmation of these data, HMBA was found to inhibit the growth of Rb-positive Du/145Rb cells but not their Rb-negative parental Du/145 cells. The data suggest that HMBA-induced growth inhibition is due to multifactorial mechanisms involving decreases in total cyclin D1 and inhibition of cdk4 and cdk6 kinase activities through elevation of levels of cdk4- and cdk6-associated p27^Kip1 and concomitant increases in hypophosphorylated pRb and stable cyclin D2/pRb and cyclin D3/pRb complexes that help maintain pRb in a functional state. Mol. Carcinog. 22:128–143, 1998. © 1998 Wiley-Liss, Inc.