Plk1与p21WAF1/CIP1在肝细胞癌中的表达

目的 细胞周期抑制因子p21<'WAF1/CIP1>(p21)可在转录水平抑制Polo-like kinase1(Plk1)基因的表达,本文旨在探讨Plk1和p21蛋白在肝细胞癌中的表达及相关性.方法 应用Westem Blot方法检测10对原发性肝细胞癌和癌旁组织中Plk1和p21蛋白的表达情况.通过将pFlex-21表达质粒瞬时转染人肝癌细胞系HepG2细胞,进一步检测p21过表达对Plk1mRNA和蛋白表达的影响.结果 Plk1在10例肝癌组织中的表达均高于配对癌旁组织,p21蛋白在7对肝癌组织中的表达高于配对癌旁组织.HepG2细胞瞬时表达p21质粒后,Plk1的蛋白表达和mRNA水平均无变化(P>0.05).结论 Plk1和p21在肝细胞癌组织中的表达具有一定的肿瘤特异性.肝癌组织中p21蛋白过表达可能不具有抑制Plk1表达的作用,具体机制还有待于进一步研究.     

Plk1与Survivin蛋白在肝细胞癌中的表达及意义

目的探讨Plk1和凋亡抑制蛋白Survivin在HCC中表达情况,为临床诊断和治疗提供理论依据。方法采用免疫组化法检测100例肝细胞癌标本中Plk1、Survivin的表达。结果⑴肝细胞癌中,Survivin蛋白的表达明显高于癌旁正常组织（P〈0.05）,Survivin在肝细胞癌组织中的表达与患者性别、年龄、肿瘤分化程度、AFP表达及是否侵犯肝被膜均无关联（P〉0.05）。与HBsAg表达有关联（P〈0.05）。⑵肝细胞癌组织中,Plk1阳性表达明显高于癌旁正常组织（P〈0.05）;Plk1在肝细胞癌组织中的表达与患者性别、年龄、AFP表达、HBsAg表达及是否侵犯肝被膜均无关（P〉0.05）,与肿瘤分化程度相关（P〈0.05）。⑶肝细胞癌组织中,Plk1与Survivin表达呈正相关（P〈0.05）。结论 Plk1与Survivin的联合检测为肝细胞癌的诊断和治疗提供新的依据。     

转录因子Sp1在肝细胞癌中的表达

目的通过研究转录因子Sp1在肝细胞癌（HCC）组织中的表达及转化生长因子（TGF）B1对肝癌细胞中Sp1mRNA转录表达的影响,探讨Sp1在HCC中表达的意义及其在细胞凋亡中的作用。方法免疫组织化学方法对51例HCC组织与10例良性病变而切除的正常肝组织中转录因子Sp1进行蛋白检测;体外培养肝癌细胞株HepG2,施加外源性TGFβ1,逆转录一聚合酶链反应方法检测HepG2细胞中Sp1mRNA的表达,并用流式细胞仪检测细胞受到干预后的凋亡情况。结果Sp1蛋白在HCC组织中的表达率明显高于正常肝组织（P〈0．01）;施加外源性TGFβ1与对照组相比HepG2细胞Sp1mRNA的表达显著降低（P〈0．01）,且HepG2细胞凋亡率随着TGF[31浓度的增高而上升。结论Sp1蛋白在HCC中的过表达在肿瘤发生发展中起一定作用;TGFβ1诱导肝癌细胞凋亡,而Sp1基因的抑制在TGFβ1诱导肝癌细胞凋亡中可能起一定作用。     

MIF与JNK1在原发性肝细胞癌中的表达及相关性研究

目的研究巨噬细胞移动抑制因子(MIF)和c-Jun氨基末端蛋白激酶1(JNK1)蛋白在肝细胞癌(HCC)组织和HepG2肝癌细胞中的表达,并探讨二者的相互关系。方法选择有乙肝肝硬化基础的HCC患者52例,取手术切除的癌组织及相应的癌旁组织为实验组,另选取20例肝血管瘤、肝破裂手术切除的肝组织标本作正常对照,应用免疫组织化学法检测MIF、JNK1和pJNK1的表达;原位杂交法检测MIF mRNA和JNK1 mRNA表达。将不同浓度的rMIF分别与HepG2肝癌细胞和L-O2人正常肝细胞共培养,通过Western blotting法检测JNK1和pJNK1蛋白的表达,通过RT-PCR检测相应蛋白在核酸水平的表达。结果 MIF、JNK1、pJNK1和MIF mRNA、JNK1 mRNA在HCC癌组织和癌旁组织中的阳性表达率均明显高于正常肝组织(P均0.05);经不同浓度的rMIF刺激后,HepG2肝癌细胞中pJNK1和JNK1mRNA的相对表达量均呈浓度依赖性升高,尤以rMIF浓度为200 ng/ml升高最明显,与L-O2细胞比较,差异有统计学意义(P0.05)。结论 MIF、MIF mRNA、JNK1、JNK1 mRNA、pJNK1在HCC组织和HepG2肝癌细胞中均呈高表达,MIF参与HCC进展,可能是通过激活JNK1信号通路来实现的。     

小干扰RNA沉默巨噬细胞移动抑制因子基因对肝癌细胞p27表达的影响

目的 观察巨噬细胞移动抑制因子(MIF)和细胞周期调控因子p27在肝细胞癌中表达的相互关系,探讨小干扰RNA(siRNA)沉默MIF基因对肝癌细胞p27表达的影响.方法 免疫组织化学法和荧光定量PCR法检测MIF、p27的蛋白和mRNA在肝癌及其癌旁组织中的表达情况.化学合成MIF siRNA和对照siRNA,脂质体法转染肝癌细胞PLC和Hep3B,荧光定量PCR法检测MIF和p27 mRNA在实验组及对照组中的表达情况.根据不同资料分别采用X2检验、Logistic回归分析或单因素方差分析.结果 MIF蛋白及其mRNA在肝癌组织中过表达,在癌旁组织中低表达;p27蛋白及其mRNA在癌组织中低表达,在癌旁组织中高表达.Logistic回归分析提示MIF为肝癌发生的危险因素,p27为保护因素.MIF mRNA在肝癌细胞株中过表达(F=61.036,P＜0.01),p27 mRNA在正常肝细胞L02中高表达(F=529.853,P＜0.01).经MIFsiRNA转染后,MIF mRNA在PLC及Hep3B中的表达水平降低,并且呈剂量依赖关系(F值分别为320.1和201.2,P值均＜0.01);p27 mRNA伴随MIF mRNA的降低而增加(F值分别为419.4和459.9,P值均＜0.01).结论 MIF在肝细胞癌中过表达,MIF siRNA能特异性抑制其在肝癌细胞中的表达;MIF可能参与了p27基因表达的调控.     

Stathmin/Oncoprotein 18(Op18)在肝细胞癌中的表达及其意义

目的探讨Stathmin／oncoprotein18（Op18）mRNA和蛋白在肝细胞癌中的表达程度及其与肝癌临床病理特征的关系。方法应用RT—PCR技术检测33例肝癌、相应癌旁组织和肝癌细胞株SMMC7721和HepG2中Stathmin mRNA的水平，同时应用免疫组织化学方法检测了上述组织和细胞株中Stathmin蛋白的表达。结果肝癌组织Stathmin mRNA的表达水平高于相应的癌旁组织（P〈0．05）。肝癌组织Stathmin蛋白的阳性表达率为51．5％，癌旁组织为6％，阳性染色位于胞质内。两种肝癌细胞株中亦有Stathmin表达。Stathmin过表达与肿瘤大小、门脉受侵、TNM分期及Edmondson分级有关（P〈0．05）。结论Stathmin在肝癌中过表达，可能与肝癌的发生及进展有关。     

Hedgehog信号通路在人肝细胞癌细胞系及肝癌组织中的表达与意义

目的探讨Hedgehog信号通路在人肝细胞癌细胞系及Shh基因在肝癌组织中的表达及意义。方法采用半定量RT-PCR法检测Hep3B和HCC-LM3细胞中Shh、Ihh、Ptch、Smo、Gli mRNA及21例肝细胞癌癌组织和癌旁肝组织中Shh mRNA的表达状况。结果Shh、Ihh、Ptch、Smo、Gli mRNA在两个肝癌细胞系中均有表达。其中,Shh、Gli1 mRNA在Hep3B细胞中的表达强度显著高于HCC-LM3细胞,而Gli2和Gli3的表达强度正相反;Ihh、Smo mRNA在Hep3B细胞中的表达强度高于HCC-LM3细胞,但两者间无显著性差异;Ptch mRNA在两个细胞系细胞中的表达无显著性差异;Shh mRNA在肝细胞癌癌组织中的阳性检出率为57.1%（12/21）,在癌旁组织中为4.8%（1/21）,两者间有显著性差异（P〈0.05）。其中,6例高分化、8例中分化和7例低分化肝细胞癌中Shh mRNA的阳性检出率分别为83.3%（5/6）、62.5%（5/8）和28.6%（2/7）,表现为随肝细胞癌由高到低的分化而呈显著性下降的趋势（P〈0.05）;Shh mRNA的表达强度在肝细胞癌癌组织之间、癌与癌旁组织之间无显著性差异。结论Hedgehog信号通路的异常激活参与了肝细胞癌的发生发展过程,Shh mRNA在部分高中分化肝细胞癌中表达上调,这或许为探讨肝癌的发生机制、早期诊断及治疗提供了新的依据。     

LZTS1蛋白在肝组织及HepG2细胞中的表达

目的探讨LZTS1蛋白在正常肝、肝细胞癌组织及癌旁慢性肝纤维化、肝硬化组织中的表达,以及在人肝癌细胞系HepG2中的表达。方法选择我院正常人肝组织、原发性肝细胞癌和癌旁慢性肝纤维化、肝硬化组织标本共50例,采用免疫组化方法检测不同肝组织样本中LZTS1蛋白,光镜下观察LZTSI阳性细胞表达定位和分布,按表达面积和表达强度半定量计分。同时培养HepG2人肝癌细胞系,免疫印迹和细胞免疫组化方法检测HepG2细胞中内源性LZTS1的表达,并以正常肝细胞L02为对照。SPSS统计分析各组间的表达差异。结果LZTSI蛋白在正常胆管上皮胞浆中表达。肝细胞包膜和胞浆中均表达,且阳性细胞呈规律性分布。LZTS1蛋白在肝细胞癌组织中14／20例（70％）表达强阳性,5／20例（25％）中等表达;癌旁肝硬化结节中10／17例（58．8％）强阳性表达,6／17（35．3％）例中等阳性表达;慢性肝纤维化组织中1／3（33．3％）例弱阳性,2／3例（66．7％）中等阳性。20例HCC组织中有17例为HBV—HCC,其中13例（76．5％）LZTSI蛋白呈强阳性表达。结论LZTS1蛋白主要表达于正常胆管上皮和肝细胞胞浆,且在肝腺泡中呈规律性分布。在肝癌、肝硬化组织和HepG2细胞中表达增高。     

丙型肝炎病毒1b基因型核心蛋白对HepG2细胞Bax与Bcl-2基因表达的影响

目的研究丙型肝炎病毒（HCV）1b基因型核心蛋白（C）对HepG2细胞B细胞淋巴瘤-2基因（Bcl-2）与Bcl-2相关X蛋白（Bax）表达的影响,以探索1b型HCV C蛋白与HepG2细胞凋亡的关系。方法利用RT-PCR扩增出HCV-1b-C基因,经双酶切后连接pcDNA3.1（-）,成功构建真核表达载体pcDNA3.1（-）/HCV-1b-C。利用脂质体转染HepG2细胞,RT-PCR及Western Blot检测其mRNA及蛋白的表达,RT-PCR及Western Blot检测转染成功后HCV-1b-C对HepG2细胞Bax与Bcl-2表达的影响,并设转染空质粒组及未处理组作对照。结果成功构建真核表达载体pcDNA3.1（-）/HCV-1b-C;瞬时转染HepG2细胞,成功表达HCV C mRNA及蛋白;转染C基因组的Bax的mRNA及蛋白相对表达量减少,与转染空质粒组及未处理组比较差异均有统计学意义（P〈0.01）;转染C基因组的Bcl-2的mRNA及蛋白相对表达量增多,与转染空质粒组及未处理组比较差异均有统计学意义（P〈0.01）。结论 1b基因型HCV C蛋白转染HepG2细胞会导致Bax表达减少及Bcl-2表达增多,降低Bax/Bcl-2比值,可能是抑制HepG2细胞凋亡的机制之一。     

Plk1在胰腺癌中的表达及临床意义

目的 研究胰腺癌组织标本及细胞系中Plk1的表达及临床意义。方法 应用免疫组化EnVision两步法检测33例胰腺癌及其癌旁组织、3例胰腺浆液性囊腺瘤、3例黏液性囊腺瘤、5例慢性胰腺炎的石蜡标本中Plk1的表达。用半定量RT—PCR检测5例新鲜胰腺癌组织及其癌旁组织中Plk1 mRNA的表达。用Westernblot检测两株胰腺癌细胞PANC1和CFPAC1中Plk1表达。结果 胰腺癌组织中Plk1的阳性表达率为72、7％（24／33），其表达与胰腺癌患者的年龄、性别、临床分期、组织学分级、淋巴结转移、肿瘤的部位无显著相关（P〉0.05）。癌旁组织、慢性胰腺炎和良性胰腺肿瘤组织中Plk1的表达均为阴性。5例新鲜胰腺癌组织中都有Plk1mRNA的高表达，两株胰腺癌细胞中也有Plk1蛋白的表达。结论 Plk1在胰腺癌中的表达具有一定肿瘤特异性，有可能成为胰腺癌诊治的理想靶标。     

Polo-like kinase (Plk)1 depletion induces apoptosis in cancer cells

Elevated expression of mammalian polo-like kinase (Plk)1 occurs in many different types of cancers, and Plk1 has been proposed as a novel diagnostic marker for several tumors. We used the recently developed vector-based small interfering RNA technique to specifically deplete Plk1 in cancer cells. We found that Plk1 depletion dramatically inhibited cell proliferation, decreased viability, and resulted in cell-cycle arrest with 4 N DNA content. The formation of dumbbell-like chromatin structure suggests the inability of these cells to completely separate the sister chromatids at the onset of anaphase. Plk1 depletion induced apoptosis, as indicated by the appearance of subgenomic DNA in fluorescence-activated cell-sorter (FACS) profiles, the activation of caspase 3, and the formation of fragmented nuclei. Plk1-depletion-induced apoptosis was partially reversed by cotransfection of nondegradable mouse Plk1 constructs. In addition, the p53 pathway was shown to be involved in Plk1-depletion-induced apoptosis. DNA damage occurred in Plk1-depleted cells and inhibition of ATM strongly potentiated the lethality of Plk1 depletion. Although p53 is stabilized in Plk1-depleted cells, DNA damage also occurs in p53(-/-) cells. These data support the notion that disruption of Plk1 function could be an important application in cancer therapy.     

靶向Plk1的siRNA对肝癌细胞凋亡的影响

目的:观察靶向作用于Plk1的siRNA对肝癌细胞系BCL-7402细胞中Plk1基因和p53基因表达及细胞凋亡的影响,探求靶向该基因的治疗在肝癌基因治疗中的可行性及效应.方法:设计合成两对靶向作用于Plk1基因的双链siRNA序列,分别命名为siRNA1和siRNA2,应用脂质体法将其转入BCL-7402细胞中.实验...     

Transcriptional upregulation of p21/WAF/Cip1 in myeloid leukemic blasts expressing AML1-ETO

An inducible model for conditional expression of AML1-ETO in myeloid U-937 cells was generated previously to determine cellular effects of AML1-ETO and to identify target genes. Induction of AML1-ETO expression in U-937 resulted in reduced cell growth, G1 arrest and apoptosis. Microarray analysis showed more genes up-regulated than down-regulated (180 vs. 69). Clustering of AML1-ETO-positive and -negative cell lines was possible based on these differentially expressed genes. p21/WAF/Cip1 (CDKN1A) was up-regulated 4.6-fold upon induction of AML1-ETO which was confirmed in additional experiments. Knock-down of AML1-ETO by siRNA could reduce p21/WAF/Cip1 expression in Kasumi-1 cells. mRNA expression analysis of p21/WAF/Cip1 in a large cohort of acute myeloid leukemia patients demonstrated a significantly higher expression in AML1-ETO-positive leukemia. The increased expression of p21/WAF/Cip1 in primary leukemic blasts suggests that elevated p21/WAF/Cip1 levels may contribute to specific features observed in AML1-ETO positive leukemia.     

GST-hPlk2融合蛋白表达载体的构建及其在原核细胞中的表达

目的 构建GST-hPlk2融合蛋白表达载体,并在原核细胞大肠埃希菌(E.coli)中诱导表达.方法 从人HEK293细胞中提取mRNA,反转录为cDNA.用PCR方法扩增出hPlk2基因全长,通过BamH Ⅰ和Xho Ⅰ酶切位点将其定向插入pGEX-4T-2载体中,构建原核表达质粒pGEX-4T-2-hPlk2,并转化E.coli DH5α,筛选阳性重组子,通过限制性内切酶酶切电泳鉴定和DNA序列测定正确后,转入Ecoli BL21中,经异丙基硫代β-D半乳糖苷大量诱导表达,SDS-PAGE电泳和Western blot鉴定.结果 酶切电泳及测序结果证明,成功构建了原核表达质粒GST-hPlk2,并用Western blot方法证实了GST-hPlk2融合蛋白的表达.结论 成功构建了GST-hPlk2原核表达载体,并证实了其在原核细胞E coli中的表达,为进一步纯化Plk2及研究其结构与功能提供了前提基础.     

Absence of tumor-specific over-expression of Polo-like kinase 1 (Plk1) in major non-Hodgkin lymphoma and relatively low expression of Plk1 in nasal NK/T cell lymphoma

Based on the presence of the tumor-specific over-expression of Plk1 (polo-like kinases) in various malignancies, we examined Plk1 expression in nine cases of reactive follicular hyperplasia (RFH), 42 of diffuse large B cell lymphoma (DLBCL), 16 of follicular lymphoma (FL), and 10 of nasal NK/T lymphoma. There was no significant difference in the Plk1-positive cell percentage between RFH and DLBCL. The Plk1-positive cell percentage ranged from 6 to 20% with a median of 12.9% in DLBCL. In FL, Plk1-positivity was at most 7%. Plk1-positivity in nasal NK/T cell lymphoma (4.7–14.1% with a median of 9.2%) was significantly higher than that of FL and tended to be lower than DLBCL (p < 0.001, p = 0.05, respectively). Although a strong correlation between positive cell percentages for Plk1 and Ki-67 in these three lymphomas specified Plk1 as a proliferation marker (r = 0.83–0.91), the Plk1-positive cell percentage relative to the other proliferation markers tended to be particularly low in nasal NK/T cell lymphoma. In 41 cases of DLBCL, the positive cell percentages of Plk1 and Ki-67 were both correlated with overall survival. The 4-year overall survival rates by Kaplan–Meier analysis for Plk1-negative and positive patients were 80 and 38%, respectively (p = 0.02).     

Polo-like kinase 1 phosphorylation of p150Glued facilitates nuclear envelope breakdown during prophase

Nuclear envelope breakdown (NEBD) is an essential step during the G2/M transition in higher eukaryotic cells. Increasing evidence supports the notion that both microtubules and microtubule-associated motor proteins are critical regulators of NEBD. Although it has been described that p150^Glued, the major component of the dynein/dynactin complex, localizes in the nuclear envelope during prophase, the exact role of p150^Glued and its regulation during NEBD are largely elusive. Polo-like kinase 1 (Plk1), the best characterized Ser/Thr kinase, is involved in mitotic entry in several systems; however, the targets of Plk1 during NEBD are unknown. Herein, we show that in mammalian cells both Plk1 and p150^Glued regulate NEBD and that Plk1 interacts with and phosphoryates p150^Glued during NEBD at prophase. Using various approaches, we showed that Plk1 phosphorylates p150^Glued at Ser-179 and that the pS179 epitope is generated at the nuclear envelope of prophase cells. Significantly, Plk1-mediated phosphorylation of p150^Glued at Ser-179 positively regulates its accumulation at the nuclear envelope during prophase. Finally, we found that cells expressing the Plk1-unphosphorylatable mutant (p150^Glued-S179A) arrest at G2, as indicated by reduced NEBD, increased levels of cyclin B and phospho-H3, but a decreased level of Cdc2 kinase activity. Taking these data together, we conclude that Plk1 phosphorylation of p150^Glued might be one major pathway of NEBD regulation.     

Effects of p21Waf1/Cip1/Sdi1 on cellular gene expression: implications for carcinogenesis, senescence, and age-related diseases

Induction of cyclin-dependent kinase inhibitor p21(Waf1/Cip1/Sdi1) triggers cell growth arrest associated with senescence and damage response. Overexpression of p21 from an inducible promoter in a human cell line induces growth arrest and phenotypic features of senescence. cDNA array hybridization showed that p21 expression selectively inhibits a set of genes involved in mitosis, DNA replication, segregation, and repair. The kinetics of inhibition of these genes on p21 induction parallels the onset of growth arrest, and their reexpression on release from p21 precedes the reentry of cells into cell cycle, indicating that inhibition of cell-cycle progression genes is a mechanism of p21-induced growth arrest. p21 also up-regulates multiple genes that have been associated with senescence or implicated in age-related diseases, including atherosclerosis, Alzheimer's disease, amyloidosis, and arthritis. Most of the tested p21-induced genes were not activated in cells that had been growth arrested by serum starvation, but some genes were induced in both forms of growth arrest. Several p21-induced genes encode secreted proteins with paracrine effects on cell growth and apoptosis. In agreement with the overexpression of such proteins, conditioned media from p21-induced cells were found to have antiapoptotic and mitogenic activity. These results suggest that the effects of p21 induction on gene expression in senescent cells may contribute to the pathogenesis of cancer and age-related diseases.     

Circadian variation in expression of G1 phase cyclins D1 and E and cyclin-dependent kinase inhibitors p16 and p21 in human bowel mucosa

AIM: To evaluate whether the cellular proliferation rate in the large bowel epithelial cells is characterized by circadian rhythm. METHODS: Between January 2003 and December 2004, twenty patients who were diagnosed as suffering from primary, resectable, non-metastatic adenocarcinoma of the lower rectum, infiltrating the sphincter mechanism, underwent abdominoperineal resection, total mesorectal excision and permanent left iliac colostomy. In formalin-fixed and paraffin-embedded biopsy specimens obtained from the colostomy mucosa every six hours (00:00, 06:00, 12:00, 18:00 and 24:00), we studied the expression of G1 phase cyclins (D1 and E) as well as the expression of the G1 phase cyclin-dependent kinase (CDK) inhibitors p16 and p21 as indicators of cell cycle progression in colonic epithelial cells using immunohistochemical methods. RESULTS: The expression of both cyclins showed a similar circadian fashion obtaining their lowest and highest values at 00:00 and 18:00, respectively (P< 0.001). A circadian rhythm in the expression of CDK inhibitor proteins p16 and p21 was also observed, with the lowest levels obtained at 12:00 and 18:00 (P< 0.001), respectively. When the complexes cyclins D1-p21 and E-p21 were examined, the expression of the cyclins was adversely correlated to the p21 expression throughout the day. When the complexes the cyclins D1-p16 and E-p16 were examined, high levels of p16 expression were correlated to low levels of cyclin expression at 00:00, 06:00 and 24:00. Meanwhile, the highest expression levels of both cyclins were correlated to high levels of p16 expression at 18:00. CONCLUSION: Colonic epithelial cells seem to enter the G1 phase of the cell cycle during afternoon (between 12:00 and 18:00) with the highest rates obtained at 18:00. From a clinical point of view, the present results suggest that G1-phase specific anticancer therapies in afternoon might maximize their anti-tumor effect while minimizing toxicity.     

Role for Plk1 phosphorylation of Hbo1 in regulation of replication licensing

In a search for Polo-like kinase 1 (Plk1)-interacting proteins using a yeast two-hybrid system, we have identified histone acetyltransferase binding to the origin recognition complex 1 (Hbo1) as a potential Plk1 target. Here, we show that the interaction between Plk1 and Hbo1 is mitosis-specific and that Plk1 phosphorylates Hbo1 on Ser-57 in vitro and in vivo. During mitosis, Cdk1 phosphorylates Hbo1 on Thr-85/88, creating a docking site for Plk1 to be recruited. Significantly, the overexpression of Hbo1 mutated at the Plk1 phosphorylation site (S57A) leads to cell-cycle arrest in the G₁/S phase, inhibition of chromatin loading of the minichromosome maintenance (Mcm) complex, and a reduced DNA replication rate. Similarly, Hbo1 depletion results in decreased DNA replication and a failure of Mcm complex binding to chromatin, both of which can be partially rescued by the ectopic expression of WT Hbo1 but not Hbo1-S57A. These results suggest that Plk1 phosphorylation of Hbo1 may be required for prereplicative complex (pre-RC) formation and DNA replication licensing.