Alteration of cell cycle in cervical tumor associated with human papillomavirus: cyclin-dependent kinase inhibitors

The ability of viral oncoproteins to subvert cell cycle checkpoints may constitute a mechanism by which viral oncoproteins induce genetic instability. HPV 16 E6 and E7 disrupt cell cycle checkpoints, particularly affecting nearly all cyclin-dependent kinase inhibitors linked to the G1- and G2- checkpoints, in each case by means of a different mechanism. HPV 16 E7 shows homology with the pRb binding sites of cyclin D1, which consequently releases E2F. In addition, E7 directly binds to p21, and releases PCNA and other S-phase promoting genes. In turn, released E2F activates cyclin E, and cyclin E accelerates p27 proteolysis as a function of the antagonistic reaction of its own inhibitor. The induction of p16 expression is assumed to be indirectly associated with E7, which is upregulated only after prolonged inactivation of Rb. HPV 16 E6 decreased the fidelity of multiple checkpoints controlling both entry into and exit from mitosis, with the mechanism of p53 inactivation. In addition, HPV 16 E6 increased the sensitivity to chemically induced S-phase premature mitosis and decreased mitotic spindle assembly checkpoint function. Alongside the impressive advances made in the understanding of the molecular mechanisms, which HPV disrupts, the validity of these conclusions should be evaluated in the diagnostic and prognostic fields.     

Centrosomes,genomic instability,and cervical carcinogenesis

High-risk human papillomavirus (HPV)-associated carcinogenesis of the uterine cervix is a particularly useful model to study basic mechanisms of genomic instability in cancer. Cervical carcinogenesis is associated with the expression of two high-risk HPV-encoded oncoproteins, E6 and E7. Aneuploidy, the most frequent form of genomic instability in human carcinomas, develops as early as in nonmalignant cervical precursor lesions. In addition, cervical neoplasia is frequently associated with abnormal multipolar mitotic figures, suggesting disturbances of the cell-division process as a mechanism for chromosome segregation defects. Spindle poles are formed by centrosomes, and the high-risk HPV E6 and E7 oncoproteins can each induce abnormal centrosome numbers. These two HPV oncoproteins, however, induce centrosome abnormalities through fundamentally different mechanisms and, presumably, with different functional consequences. High-risk HPV E7, which targets the pRB tumor suppressor pathway, can provoke abnormal centrosome duplication in phenotypically normal cells. On the contrary, cells expressing the HPV E6 oncoprotein, which inactivates p53, accumulate abnormal numbers of centrosomes in parallel with multinucleation and nuclear atypia. These two pathways are not mutually exclusive, since co-expression of HPV E6 and E7 has synergistic effects on centrosome abnormalities and chromosomal instability. Taken together, these findings support the general model in which chromosomal instability arises as a direct consequence of oncogenic insults and can develop at early stages of tumor progression.     

Variable levels of chromosomal instability and mitotic spindle checkpoint defects in breast cancer

Cytogenetic analyses have revealed that many aneuploid breast cancers have cell-to-cell variations of chromosome copy numbers, suggesting that these neoplasms have instability of chromosome numbers. To directly test for possible chromosomal instability in this disease, we used fluorescent in situ hybridization to monitor copy numbers of multiple chromosomes in cultures of replicating breast cancer-derived cell lines and nonmalignant breast epithelial cells. While most (7 of 9) breast cancer cell lines tested are highly unstable with regard to chromosome copy numbers, others (2 of 9 cell lines) have a moderate level of instability that is higher than the "background" level of normal mammary epithelial cells and MCF-10A cells, but significantly less than that seen in the highly unstable breast cancer cell lines. To evaluate the potential role of a defective mitotic spindle checkpoint as a cause of this chromosomal instability, we used flow cytometry to monitor the response of cells to nocodazole-induced mitotic spindle damage. All cell lines with high levels of chromosomal instability have defective mitotic spindle checkpoints, whereas the cell lines with moderate levels of chromosomal instability (and the stable normal mammary cells and MCF10A cells) arrest in G(2) when challenged with nocodazole. Notably, the extent of mitotic spindle checkpoint deficiency and chromosome numerical instability in these cells is unrelated to the presence or absence of p53 mutations. Our results provide direct evidence for chromosomal instability in breast cancer and show that this instability occurs at variable levels among cells from different cancers, perhaps reflecting different functional classes of chromosomal instability. High levels of chromosomal instability are likely related to defective mitotic checkpoints but not to p53 mutations.     

The conserved protein kinase Ipl1 regulates microtubule binding to kinetochores in budding yeast

Chromosome segregation depends on kinetochores, the structures that mediate chromosome attachment to the mitotic spindle. We isolated mutants in IPL1, which encodes a protein kinase, in a screen for budding yeast mutants that have defects in sister chromatid separation and segregation. Cytological tests show that ipl1 mutants can separate sister chromatids but are defective in chromosome segregation. Kinetochores assembled in extracts from ipl1 mutants show altered binding to microtubules. Ipl1p phosphorylates the kinetochore component Ndc10p in vitro and we propose that Ipl1p regulates kinetochore function via Ndc10p phosphorylation. Ipl1p localizes to the mitotic spindle and its levels are regulated during the cell cycle. This pattern of localization and regulation is similar to that of Ipl1p homologs in higher eukaryotes, such as the human aurora2 protein. Because aurora2 has been implicated in oncogenesis, defects in kinetochore function may contribute to genetic instability in human tumors.     

The fission yeast DASH complex is essential for satisfying the spindle assembly checkpoint induced by defects in the inner-kinetochore proteins

Spindle assembly checkpoint (SAC) is an evolutionarily conserved surveillance system for chromosome missegregation. We isolated fission yeast Hos2, a component of the Dam1/DASH complex, as a multicopy suppressor of temperature-sensitive (ts) growth of nnf1-495 mutant that exhibits the minichromosome instability (mis) phenotype, producing lethal aneuploids without prominent mitotic delay. It remains elusive why SAC is satisfied in mis mutants despite the occurrence of missegregation. We found that Hos2 binds to the inner-kinetochore regions in both prometaphase and metaphase. Hos2 is essential for kinetochore localization of Dis1, a microtubule (MT) associated Dis1/XMAP215/TOG family protein that is required for proper MT dynamics. Cells lacking DASH exhibit cold-sensitive (cs) growth with the defective in sister-chromatid disjoining (dis) phenotype, which is characterized by hyper-condensed sister-chromatid pairs and elongated spindle MTs. Although DASH-deficient cells are viable at high temperatures, DASH-deletion transforms all the inner-kinetochore mis mutants so far tested into a constitutively active state of SAC, leading to the dis phenotype. We also discovered that Hos2 over-expression commonly suppresses growth retardation in a variety of inner-kinetochore mutants. These genetic interactions highlight the DASH-action(s) in satisfying SAC when aneuploids are formed during mitosis in the inner-kinetochore-defective mis mutants.     

共表达HPV16L1、L2、E6、E7蛋白的重组非复制型痘苗病毒构建及鉴定

目的构建共表达人乳头瘤病毒16型(HPV16)L1、L2、E6、E7蛋白的非复制型重组痘苗病毒人用疫苗株。方法以痘苗病毒为载体、利用同源重组技术筛选共表达HPV16L1、L2、E6、E7蛋白的重组痘苗病毒并对其进行鉴定。结果该病毒在CEF细胞上连续传至第15代,经斑点杂交结果表明重组病毒基因组中有L1、L2、E6、E7基因插入;经WesternBlot检测,重组病毒能稳定表达HPV16L1、L2、E6、E7蛋白。结论非复制型重组痘苗病毒NTVJE6E7CKL1L2可作为预防和治疗HPV16相关肿瘤及其癌前病变候选疫苗。     

Nucleotide and amino acid sequence variation in the L1 and E7 open reading frames of human papillomavirus type 6 and type 16.

Human papillomavirus (HPV) type 6 and type 16 DNA sequence variants were found by partially sequencing the L1 and E7 open reading frames, using templates generated with the polymerase chain reaction. Identical variants were found in patients from widely separated locations, such as the United States, the Philippines, and India. The same sequence variants of HPV 16 were found in women with invasive cervical carcinoma and in women with no evidence of disease. Variation in the predicted amino acid sequences of the HPV 16 L1 and E7 proteins was found. A single nucleotide change at position 6433 was found in about 50% of the HPV 16 DNAs, resulting in a change in predicted amino acid sequence from threonine to alanine at the equivalent position in the L1 protein. Predicted amino acid changes were found in the HPV 16 E7 proteins at amino acid positions 28, 29, and 47. Variation at these positions could affect known properties of the E7 protein, including binding to the retinoblastoma protein.     

Oligomerization of the E5 protein of human papillomavirus type 16 occurs through multiple hydrophobic regions

The high risk forms of human papillomavirus (HPV) (primarily types 16 and 18) are the leading cause of cervical cancer worldwide. Infection results in expression of three oncoproteins, E5, E6, and E7, the latter two being of predominant importance in maintaining a transformed state of the host epithelial cell. While little is known about the role(s) of the HPV E5, the bovine papillomavirus type 1 (BPV1) E5 protein has been well characterized. A study of HPV16 E5 was performed, focusing on the protein's ability to self-interact, its ability to bind to the 16-kDa subunit of the vacuolar H(+)-ATPase (16K), and its cellular localization. As has been previously shown for BPV1 E5, we found that HPV16 E5 is also capable of self-interaction and binding to 16K. Further, we examined which portions of the HPV16 E5 protein were involved in these interactions using progressive deletions of putative transmembrane helices of the protein. All of the E5 deletion mutants tested bound to full-length E5 as well as to 16K, suggesting that these protein-protein interactions are based on hydrophobic interactions. The majority of E5 expressed in HEK 293-T7 cells was perinuclear but did not appear to localize to the cis/medial-Golgi, in contrast to previous reports for both HPV16 E5 and BPV1 E5.     

Expression of HPV16 E5 produces enlarged nuclei and polyploidy through endoreplication

Anogenital cancers and head and neck cancers are causally associated with infection by high-risk human papillomavirus (HPV). The mechanism by which high-risk HPVs contribute to oncogenesis is poorly understood. HPV16 encodes three genes (HPV16 E5, E6, and E7) that can transform cells when expressed independently. HPV16 E6 and E7 have well-described roles causing genomic instability and unregulated cell cycle progression. The role of HPV16 E5 in cell transformation remains to be elucidated. Expression of HPV16 E5 results in enlarged, polyploid nuclei that are dependent on the level and duration of HPV16 E5 expression. Live cell imaging data indicate that these changes do not arise from cell-cell fusion or failed cytokinesis. The increase in nuclear size is a continual process that requires DNA synthesis. We conclude that HPV16 E5 produces polyploid cells by endoreplication. These findings provide insight into how HPV16 E5 can contribute to cell transformation.     

宫颈癌组织中人乳头瘤病毒16型E7蛋白致癌机理初探

目的 研究宫颈癌组织中人乳头瘤病毒（ＨＰＶ）１６－Ｅ７蛋白对视网膜母细胞瘤基因（Ｒｅｔｉｎｏｂｌａｓｔｏｍａ）Ｒｂ蛋白及Ｅ２Ｆ－１的作用的机制,探讨ＨＰＶ１６－Ｅ７蛋白与宫颈癌发生的关系。方法 采用聚合酶链反应检测宫颈癌及正常宫颈组织中ＨＰＶ１６感染等,用蛋白印迹技术对ＨＰＶ１６ ＤＮＡ阳性的宫颈癌组织中是否存在ＨＰＶ１６－Ｅ７蛋白和Ｒ６蛋白－Ｅ２Ｆ－１形成的复合物进行检测。正常宫颈组织作为对照,     

The E7 oncoprotein of high-risk human papillomavirus type 16 enters the nucleus via a nonclassical Ran-dependent pathway

E7, the major transforming protein of high-risk human papillomavirus (HPV), type 16, binds and inactivates the retinoblastoma protein (pRb), and the Rb-related proteins p107 and p130. HPV16 E7 is a nuclear protein lacking a classical basic nuclear localization signal. In this study we investigated the nuclear import of HPV16 E7 oncoprotein in digitonin-permeabilized HeLa cells. HPV16 E7 nuclear import was independent of pRb, as an E7(DeltaDLYC) variant defective in pRb binding was imported into the nuclei of digitonin-permeabilized cells as efficiently as wild-type E7 in the presence of exogenous cytosol. Interestingly, we discovered that HPV16 E7 is imported into the nuclei via a novel pathway different from those mediated by Kap alpha2beta1 heterodimers, Kap beta1, or Kap beta2. Nuclear accumulation of E7 required Ran and was not inhibited by the RanG19V-GTP variant, an inhibitor of Kap beta mediated import pathways. Together the data suggest that HPV16 E7 translocates through the nuclear pores via a nonclassical Ran-dependent pathway, independent of the main cytosolic Kap beta import receptors.     

人乳头瘤病毒多肽与人免疫球蛋白G融合表达

目的 将人乳头瘤病毒16型(Human papillomavirus type 16，HPV-16)的晚期表达蛋白E7上的抗原24肽(从第38位氨基酸到第61位氨基6病毒感染防治酸)与人免疫球蛋白G的重链恒定区融合表达，并以此融合蛋白作为抗原，可能为HPV-1提供免疫治疗方法。方法 利用PCR方法分别扩增HPV-16 E7(38-61)24肽的DNA片段和人免疫球蛋白G的重链恒定区DNA片段，并构建到pEV21a表达载体上，转化入E．coli中表达，利用SDS-聚丙烯酰胺凝胶电泳(SDS-PAGE)和蛋白质免疫印迹(Western-blotting)的方法对表达结果进行鉴定。结果 构建的表达载体HPV16E7e／hIgGHCCR-pET21a经酶切鉴定和测序显示序列正确；通过SDS-PAGE和Western-blotting的鉴定，重组融合蛋白Mr约40000，表达量可占菌体蛋白的20％左右。结论 成功构建HPV16-E7的抗原多肽片段和人免疫球蛋白G的重链恒定区的融合蛋白，并可在E．coli中高效表达。     

Glucocorticoid-dependent transformation by human papillomavirus type 16 E7 coding and 3' noncoding sequences.

The establishment of transformation of primary rodent cells by human papillomavirus (HPV) type 16 DNA requires glucocorticoid hormones (Pater et al., Nature 335, 832-835, 1988). Here we provide evidence by mutational analysis that, in the context of the hormone-regulated HPV 16 promoter/enhancer, the only protein coding sequences of HPV 16 required are those of the E7 gene. Moreover, additional sequences adjacent to the 3' end of E7 coding sequences are also essential for the establishment of the transformed phenotype. Splice donor sites, especially an E7 ORF 3' proximal one, are implicated for this cis-acting function, since specific deletion mutations of these splice sites greatly or completely reduced the frequency of transformation and the level of E7 RNA.