Synthesis and turn-over of the replicative Cdc6 protein during the HeLa cell cycle

Significance of DNA Replication Licensing Proteins (MCM2, MCM5 and CDC6), p16 and p63 as Markers of Premalignant Lesions of the Uterine Cervix: Its Usefulness to Predict Malignant Potential

Cervical cancer continues to be a leading cancer among women in many parts of the world. Nation-wide screening with the Pap smear has not been implemented in India due to the lack of adequately trained cytologists. Identification of biomarkers to predict malignant potential of the identified low risk lesions is essential to avoid excessive retesting and follow up. The current study analyzed the expression patterns of DNA replication licensing proteins, proliferation inhibitor protein p16INK4A and tumor suppresser protein p63 in cervical tissues and smears to assess the ability of these proteins to predict progression. Methods: Cervical smears and corresponding tissues were immunostained using mouse monoclonal antibodies against MCM2, MCM5, CDC6, p16 and p63. Smears were treated with a non-ionic surfactant sodium deoxycholate prior to immuno-cytochemistry. The standard ABC method of immunohistochemistry was performed using DAB as the chromogen. The immunostained samples were scored on a 0-3+ scale and staining patterns of smears were compared with those of tissue sections. Sensitivity and specificity for each of these markers were calculated taking histopathology as the gold standard. Result: All the markers were positive in malignant and dysplastic cells. MCM protein expression was found to be up-regulated in LSIL, HSIL and in malignancies to a greater extent than p16 as well as p63. CDC6 protein was preferentially expressed in high grade lesions and in invasive squamous cell carcinomas. A progressive increase in the expression of DNA replication licensing proteins in accordance with the grades of cervical intraepithelial lesion suggests these markers as significant to predict malignant potential of low grade lesions in cervical smears. Conclusion: MCMs and CDC6 can be applied as biomarkers to predict malignant potential of low grade lesions identified in screening programmes and retesting / follow up might be confined to those with high risk lesions alone so that overuse of resources can be safely avoided.

Metabolic Labeling of Protein Antigens with [35S]Methionine

Metabolic labeling of antigens can be achieved by supplementation of the cell culture medium with radioactive amino acid precursors such as [³⁵S]methionine during the incubation period of target cells. In this protocol, intracellular unlabeled methionine levels are reduced by starvation of cells for 0.5-1 h before the addition of labeled [³⁵S]methionine and incubation for 0.5-4 h. Upon completion of the metabolic labeling process, cells can be prepared for immunoprecipitation by lysis or alternatively pelleted and frozen for cell lysate preparations at a later time.

Regulation of S phase

Regulation of DNA replication is critical for accurate and timely dissemination of genomic material to daughter cells. The cell uses a variety of mechanisms to control this aspect of the cell cycle. There are various determinants of origin identification, as well as a large number of proteins required to load replication complexes at these defined genomic regions. A pre-Replication Complex (pre-RC) associates with origins in the G1 phase. This complex includes the Origin Recognition Complex (ORC), which serves to recognize origins, the putative helicase MCM2-7, and other factors important for complex assembly. Following pre-RC loading, a pre-Initiation Complex (pre-IC) builds upon the helicase with factors required for eventual loading of replicative polymerases. The chromatin association of these two complexes is temporally distinct, with pre-RC being inhibited, and pre-IC being activated by cyclin-dependent kinases (Cdks). This regulation is the basis for replication licensing, which allows replication to occur at a specific time once, and only once, per cell cycle. By preventing extra rounds of replication within a cell cycle, or by ensuring the cell cycle cannot progress until the environmental and intracellular conditions are most optimal, cells are able to carry out a successful replication cycle with minimal mutations.

The functional role of Cdc6 in S-G2/M in mammalian cells

The Cdc6 protein is required for licensing of replication origins before the onset of DNA replication in eukaryotic cells. Here, we examined whether Cdc6 has other roles in mammalian cell-cycle progression from S to G2/M phase. Using RNA interference, we showed that depletion of Cdc6 in synchronous G1 cells blocks G1 to S transition, confirming the essential role of Cdc6 in the initiation of DNA replication. In contrast, depletion of Cdc6 in synchronous S-phase cells slowed DNA replication and led to mitotic lethality. The Cdc6-depleted S-phase cells showed fewer newly fired origins; however, established replication forks remained active, even during chromatin condensation. Despite such DNA replication abnormalities, loss of Cdc6 failed to activate Chk1 kinase. These results show that Cdc6 is not only required for G1 origin licensing, but is also crucial for proper S-phase DNA replication that is essential for DNA segregation during mitosis.

p16INK4A, CDC6, and MCM5: predictive biomarkers in cervical preinvasive neoplasia and cervical cancer

AIM

To analyse and compare expression patterns of three potential biomarkers-p16(INK4A), CDC6, and MCM5-and evaluate their use as predictive biomarkers in squamous and glandular cervical preinvasive neoplasia.

METHODS

Immunocytochemical analysis of p16(INK4A), MCM5, and CDC6 expression was performed on 20 normal, 38 cervical intraepithelial neoplasia 1 (CIN1), 33 CIN2, 46 CIN3, 10 squamous cell carcinoma, 19 cervical glandular intraepithelial neoplasia (cGIN), and 10 adenocarcinoma samples. Staining intensity was assessed using a 0-3 scoring system. p16(INK4A), MCM5, and CDC6 expression was also examined in ThinPrep slides exhibiting mild, moderate, and severe dyskaryosis. Human papillomavirus (HPV) was detected using a modified SYBR green assay. Fluorogenic polymerase chain reaction (PCR) and solution phase PCR were used for specific HPV typing.

RESULTS

All three markers showed a linear correlation between expression and grade of dysplasia. p16(INK4A) and MCM5 protein expression was upregulated in all grades of squamous and glandular dysplasia. CDC6 protein was preferentially expressed in high grade lesions and in invasive squamous cell carcinoma.

CONCLUSION

p16(INK4A) expression was closely associated with high risk HPV infection-all grades of squamous and glandular cervical lesions were immunohistochemically positive. MCM5 staining intensity was independent of high risk HPV infection, highlighting its potential as a biomarker in both HPV dependent and independent cervical dysplasia. CDC6 may be a biomarker of high grade and invasive lesions of the cervix, with limited use in low grade dysplasia. p16(INK4A) was the most reliable marker of cervical dysplasia. Combinations of dysplastic biomarkers may be useful in difficult diagnostic cases.

Quantitation of CDC6 and MCM5 mRNA in cervical intraepithelial neoplasia and invasive squamous cell carcinoma of the cervix

CDC6 and MCM5 play essential roles in eukaryotic DNA replication. Several studies have highlighted the potential of these proteins as molecular markers of dysplastic and malignant cells in histopathological diagnosis. The mode of expression of CDC6 and MCM5 mRNA and their significance in normal, dysplastic and malignant cervical cells remains to be elucidated. Using a quantitative real-time RT PCR assay, we compared CDC6 and MCM5 mRNA expression in normal cervical epithelium, cervical intraepithelial neoplasia and invasive squamous cell carcinoma of the cervix. Our study cohort comprised 20 normal cervical biopsies, 20 CIN3 and eight invasive squamous cell carcinomas. All samples were formalin fixed and paraffin embedded. Total RNA was extracted and analysed for expression of GAPDH, CDC6 and MCM5 using real-time quantitative TaqMan RT-PCR. A linear increase in MCM5 and CDC6 mRNA expression is observed in normal cervix, CIN3 and invasive cervical carcinoma. The overall difference in MCM5 mRNA expression in the normal cervix, CIN3 and invasive cohort groups is highly statistically significant (P=0.001). An increase in CDC6 mRNA expression in CIN3 and invasive cervical squamous cell carcinoma was observed; however, the overall difference between cohort groups was not found to be statistically significant (P=0.104). Increased transcription of MCM5 and CDC6 occurs as a consequence of cervical neoplastic progression. This pattern of increased mRNA expression in CIN3 and invasive cervical carcinoma directly correlates with findings at the phenotypic protein expression level. This study further confirms the importance of MCM5 and CDC6 in malignant transformation and in the pathogenesis of cervical dysplasia.

DNA replication and progression through S phase

Initiation and completion of DNA replication defines the beginning and ending of S phase of the cell cycle. Successful progression through S phase requires that replication be properly regulated and monitored to ensure that the entire genome is duplicated exactly once, without errors, in a timely fashion. Given the immense size and complexity of eukaryotic genomes, this presents a significant challenge for the cell. As a result, DNA replication has evolved into a tightly regulated process involving the coordinated action of numerous factors that function in all phases of the cell cycle. We will review our current understanding of these processes from the formation of prereplicative complexes in preparation for S phase to the series of events that culminate in the loading of DNA polymerases during S phase. We will incorporate structural data from archaeal and bacterial replication proteins and discuss their implications for understanding the mechanism of action of their corresponding eukaryotic homologues. We will also describe the concept of replication licensing which protects against genomic instability by limiting initiation events to once per cell cycle. Lastly, we will review our knowledge of checkpoint pathways that maintain the integrity of stalled forks and relay defects in replication to the rest of the cell cycle.

Functional analysis of bacterial artificial chromosomes in mammalian cells: mouse Cdc6 is associated with the mitotic spindle apparatus

Bacterial artificial chromosomes (BACs) provide a well-characterized resource for studying the functional organization of genes and other large chromosomal domains. To facilitate functional studies in cell cultures, we have developed a simple approach for generating stable cell lines with variable copy numbers of any BAC. Here we describe hamster cell lines with BAC transgenes that express mouse Cdc6 at levels that correlate with BAC copy number; show that mouse Cdc6 is regulated normally during the cell cycle, binds chromatin, and is degraded during apoptosis; and report a novel fraction of Cdc6 that associates with the spindle apparatus during mitosis. With RNA interference to assess genetic complementation by BAC alleles, this system will facilitate functional studies on large chromosomal domains at variable copy number in mammalian cell models.

Enigmatic variations: divergent modes of regulating eukaryotic DNA replication

Proteins involved in DNA replication are conserved from yeast to mammals, suggesting that the mechanism was established at an early stage of eukaryotic evolution. In spite of this common origin, recent findings have revealed surprising variations in how replication initiation is controlled, implying that a conserved mechanism has not necessarily resulted in regulatory conservation.

At the crossroads of SUMO and NF-kappaB

BACKGROUND

Recognition of pathogens by immune receptors leads to activation of macrophages, dendritic cells, and lymphocytes. Signals are communicated to enhance expression of target molecules such as cytokines and adhesion molecules, depending on activation of various inducible transcription factors, among which the family NF-kappaB transcription factors plays an evolutionarily conserved and critical role. Classical activation of NF-kappaB involves phosphorylation, polyubiquitination and subsequent degradation of the inhibitor molecules of NF-kappaB, referred to as IkappaB. Modification of IkappaBalpha, one of the mammalian IkappaB isoforms, with the small ubiquitin-like modifier (SUMO) results its protection from degradation.

PRESENTATION OF THE HYPOTHESIS

SUMO-IkappaBalpha localizes in the nucleus. The nuclear SUMO-IkappaBalpha pool may be dynamic. SUMO-IkappaBalpha functions as synergy control factor.

TESTING THE HYPOTHESIS

Immunoprecipitation from cellular fractions, 35S methionine pulse-chase, and FRET assays should reveal the localization of SUMO-IkappaBalpha and the dynamics of the pool. Expression of SUMOylation defective IkappaBalpha in an IkappaBalpha -/- background should yield insights into the function of SUMO-IkappaBalpha.

IMPLICATION OF THE HYPOTHESIS

IkappaBalpha contains the required SUMOylation motif but IkappaBbeta does not. The suggested study would provide evidence whether or not IkappaBalpha and IkappaBbeta can substitute each other. In addition, the suggested assays would reveal a possible redundancy in controlling transcriptional activity of NF-kappaB.

Expression and phosphorylation of the replication regulator protein geminin

It has been described that the replication regulator protein geminin is rapidly degraded at the end of mitosis and newly expressed at the beginning of the next S phase in the metazoan cell cycle. We have performed experiments to investigate the synthesis of geminin in cycling human HeLa cells. The levels of geminin-mRNA vary only modestly during the cell cycle with a 2-3-fold higher mRNA level at the G1/S phase transition, whereas newly synthesized geminin can only be detected in post-G1 phases. Surprisingly, geminin, once synthesized, does not remain stable, but is turned over during S phase with a half-life of 3-4h. We also show that geminin becomes phosphorylated as S phase proceeds and identify by MALDI mass spectrometry two specific major phosphorylation sites.

Interaction and assembly of murine pre-replicative complex proteins in yeast and mouse cells

Eukaryotic cells coordinate chromosome duplication by the assembly of protein complexes at origins of DNA replication by sequential binding of member proteins of the origin recognition complex (ORC), CDC6, and minichromosome maintenance (MCM) proteins. These pre-replicative complexes (pre-RCs) are activated by cyclin-dependent kinases and DBF4/CDC7 kinase. Here, we carried out a comprehensive yeast two-hybrid screen to establish sequential interactions between two individual proteins of the mouse pre-RC that are probably required for the initiation of DNA replication. The studies revealed multiple interactions among ORC subunits and MCM proteins as well as interactions between individual ORC and MCM proteins. In particular CDC6 was found to bind strongly to ORC1 and ORC2, and to MCM7 proteins. DBF4 interacts with the subunits of ORC as well as with MCM proteins. It was also demonstrated that CDC7 binds to different ORC and MCM proteins. CDC45 interacts with ORC1 and ORC6, and weakly with MCM3, -6, and -7. The three subunits of the single-stranded DNA binding protein RPA show interactions with various ORC subunits as well as with several MCM proteins. The data obtained by yeast two-hybrid analysis were paradigmatically confirmed in synchronized murine FM3A cells by immunoprecipitation of the interacting partners. Some of the interactions were found to be cell-cycle-dependent; however, most of them were cell-cycle-independent. Altogether, 90 protein-protein interactions were detected in this study, 52 of them were found for the first time in any eukaryotic pre-RC. These data may help to understand the complex interplay of the components of the mouse pre-RC and should allow us to refine its structural architecture as well as its assembly in real time.