PCNA, a multifunctional ring on DNA

CDK-independent role of D-type cyclins in regulating DNA mismatch repair

Although mismatch repair (MMR) is essential for correcting DNA replication errors, it can also recognize other lesions, such as oxidized bases. In G0 and G1, MMR is kept in check through unknown mechanisms as it is error-prone during these cell cycle phases. We show that in mammalian cells, D-type cyclins are recruited to sites of oxidative DNA damage in a PCNA- and p21-dependent manner. D-type cyclins inhibit the proteasomal degradation of p21, which competes with MMR proteins for binding to PCNA, thereby inhibiting MMR. The ability of D-type cyclins to limit MMR is CDK4- and CDK6-independent and is conserved in G0 and G1. At the G1/S transition, the timely, cullin-RING ubiquitin ligase (CRL)-dependent degradation of D-type cyclins and p21 enables MMR activity to efficiently repair DNA replication errors. Persistent expression of D-type cyclins during S-phase inhibits the binding of MMR proteins to PCNA, increases the mutational burden, and promotes microsatellite instability.

Poly-β-thioester-Based Cross-Linked Nanocarrier for Cancer Cell Selectivity over Normal Cells and Cellular Apoptosis by Triggered Release of Parthenolide, an Anticancer Drug

Breast cancer is the most prevalent and aggressive type of cancer, causing high mortality rates in women globally. Many drawbacks and side effects of the current chemotherapy force us to develop a robust chemotherapeutic system that can deal with off-target hazards and selectively combat cancer growth, invasiveness, and cancer-initiating cells. Here, a pH-responsive cross-linked nanocarrier (140-160 nm) endowed with poly-β-thioester functionality (CBAPTL) has been sketched and fabricated for noncovalent firm encapsulation of anticancer drug, parthenolide (PTL) at physiological pH (7.4), which enables sustain release of PTL at relevant endosomal pH (∼5.0-5.3). For this, a bolaamphiphilic molecule integrated with β-thioester and acrylate functionality was synthesized to fabricate the pH-responsive poly-β-thioester-based cross-linked nanocarrier via Michael addition click reactions in water. The poly-β-thioester functionality of CBAPTL hydrolyzes at endosomal acidic conditions, thus leading to the selective release of PTL inside the cancer cell. Cross-linked nanocarriers exhibit high serum stability, dilution insensitivity, and targeted cellular uptake at tumor microenvironment (TME), contrasting normal cells. In vitro study using human MCF-7 breast cancer cells demonstrated that CBAPTL exhibited selective cytotoxicity, reduced clonogenic potential, increased reactive oxygen species (ROS) generation, and arrested the progression of the cell cycle at the G0/G1 phase efficiently. CBAPTL induced apoptosis via downregulating pro-proliferative protein Bcl-2 and upregulating proapoptotic proteins p53, BAD, p21, and cleaved PARP-1. CBAPTL inhibited proliferating signaling by suppressing AKT phosphorylation and p38 expression. CBAPTL also blocked the invasion and migration of MCF-7 cells. CBAPTL effectively inhibits primary and secondary mammosphere formation, thereby preventing cancer-initiating cells' growth. Conversely, CBAPTL has negligible effect on human red blood cells (RBCs) and peripheral blood mononuclear cells (PBMCs). These findings highlight the superior efficacy of CBAPTL compared to PTL alone in suppressing cancer cell growth, inducing apoptosis, and preventing invasiveness of MCF-7 cells. Thus, CBAPTL could be considered a possible selective chemotherapeutic cargo against breast cancer without affecting normal cells.

D-type cyclins regulate DNA mismatch repair in the G1 and S phases of the cell cycle, maintaining genome stability

The large majority of oxidative DNA lesions occurring in the G1 phase of the cell cycle are repaired by base excision repair (BER) rather than mismatch repair (MMR) to avoid long resections that can lead to genomic instability and cell death. However, the molecular mechanisms dictating pathway choice between MMR and BER have remained unknown. Here, we show that, during G1, D-type cyclins are recruited to sites of oxidative DNA damage in a PCNA- and p21-dependent manner. D-type cyclins shield p21 from its two ubiquitin ligases CRL1SKP2 and CRL4CDT2 in a CDK4/6-independent manner. In turn, p21 competes through its PCNA-interacting protein degron with MMR components for their binding to PCNA. This inhibits MMR while not affecting BER. At the G1/S transition, the CRL4AMBRA1-dependent degradation of D-type cyclins renders p21 susceptible to proteolysis. These timely degradation events allow the proper binding of MMR proteins to PCNA, enabling the repair of DNA replication errors. Persistent expression of cyclin D1 during S-phase increases the mutational burden and promotes microsatellite instability. Thus, the expression of D-type cyclins inhibits MMR in G1, whereas their degradation is necessary for proper MMR function in S.

PCNA Ser46-Leu47 residues are crucial in preserving genomic integrity

Proliferating cell nuclear antigen (PCNA) is a maestro of DNA replication. PCNA forms a homotrimer and interacts with various proteins, such as DNA polymerases, DNA ligase I (LIG1), and flap endonuclease 1 (FEN1) for faithful DNA replication. Here, we identify the crucial role of Ser46-Leu47 residues of PCNA in maintaining genomic integrity using in vitro, and cell-based assays and structural prediction. The predicted PCNAΔSL47 structure shows the potential distortion of the central loop and reduced hydrophobicity. PCNAΔSL47 shows a defective interaction with PCNAWT leading to defects in homo-trimerization in vitro. PCNAΔSL47 is defective in the FEN1 and LIG1 interaction. PCNA ubiquitination and DNA-RNA hybrid processing are defective in PCNAΔSL47-expressing cells. Accordingly, PCNAΔSL47-expressing cells exhibit an increased number of single-stranded DNA gaps and higher levels of γH2AX, and sensitivity to DNA-damaging agents, highlighting the importance of PCNA Ser46-Leu47 residues in maintaining genomic integrity.

Targeted degradation of PCNA outperforms stoichiometric inhibition to result in programed cell death

Biodegraders are targeted protein degradation constructs composed of mini-proteins/peptides linked to E3 ligase receptors. We gained deeper insights into their utility by studying Con1-SPOP, a biodegrader against proliferating cell nuclear antigen (PCNA), an oncology target. Con1-SPOP proved pharmacologically superior to its stoichiometric (non-degrading) inhibitor equivalent (Con1-SPOPmut) as it had more potent anti-proliferative effects and uniquely induced DNA damage, cell apoptosis, and necrosis. Proteomics showed that PCNA degradation gave impaired mitotic division and mitochondria dysfunction, effects not seen with the stoichiometric inhibitor. We further showed that doxycycline-induced Con1-SPOP achieved complete tumor growth inhibition in vivo. Intracellular delivery of mRNA encoding Con1-SPOP via lipid nanoparticles (LNPs) depleted endogenous PCNA within hours of application with nanomolar potency. Our results demonstrate the utility of biodegraders as biological tools and highlight target degradation as a more efficacious approach versus stoichiometric inhibition. Once in vivo delivery is optimized, biodegraders may be leveraged as an exciting therapeutic modality.

Characterization of the Interaction Between SARS-CoV-2 Membrane Protein (M) and Proliferating Cell Nuclear Antigen (PCNA) as a Potential Therapeutic Target

SARS-CoV-2 is an emerging virus from the Coronaviridae family and is responsible for the ongoing COVID-19 pandemic. In this work, we explored the previously reported SARS-CoV-2 structural membrane protein (M) interaction with human Proliferating Cell Nuclear Antigen (PCNA). The M protein is responsible for maintaining virion shape, and PCNA is a marker of DNA damage which is essential for DNA replication and repair. We validated the M-PCNA interaction through immunoprecipitation, immunofluorescence co-localization, and PLA (Proximity Ligation Assay). In cells infected with SARS-CoV-2 or transfected with M protein, using immunofluorescence and cell fractioning, we documented a reallocation of PCNA from the nucleus to the cytoplasm and the increase of PCNA and γH2AX (another DNA damage marker) expression. We also observed an increase in PCNA and γH2AX expression in the lung of a COVID-19 patient by immunohistochemistry. In addition, the inhibition of PCNA translocation by PCNA I1 and Verdinexor led to a reduction of plaque formation in an in vitro assay. We, therefore, propose that the transport of PCNA to the cytoplasm and its association with M could be a virus strategy to manipulate cell functions and may be considered a target for COVID-19 therapy.

Nuclear DNA replicates during zygote development in Arabidopsis and Torenia fournieri

The progression of the cell cycle is continuous in most cells, but gametes (sperm and egg cells) exhibit an arrest of the cell cycle to await fertilization to form a zygote, which then continues through the subsequent phases to complete cell division. The phase in which gametes of flowering plants arrest has been a matter of debate, since different phases have been reported for the gametes of different species. In this study, we reassessed the phase of cell-cycle arrest in the gametes of two species, Arabidopsis (Arabidopsis thaliana) and Torenia fournieri. We first showed that 4', 6-diamidino-2-phenylindole staining was not feasible to detect changes in gametic nuclear DNA in T. fournieri. Next, using 5-ethynyl-2'-deoxyuridine (EdU) staining that detects DNA replication by labeling the EdU absorbed by deoxyribonucleic acid, we found that the replication of nuclear DNA did not occur during gamete development but during zygote development, revealing that the gametes of these species have a haploid nuclear DNA content before fertilization. We thus propose that gametes in the G1 phase participate in the fertilization event in Arabidopsis and T. fournieri.

Interactional similarities and differences in the protein complex of PCNA and DNA replication factor C between rice and Arabidopsis

BACKGROUND

Proliferating cell nuclear antigen (PCNA), a conserved trimeric ring complex, is loaded onto replication fork through a hetero-pentameric AAA+ ATPase complex termed replication factor C (RFC) to maintain genome stability. Although architectures of PCNA-RFC complex in yeast have been revealed, the functions of PCNA and protein-protein interactions of PCNA-RFC complex in higher plants are not very clear. Here, essential regions mediating interactions between PCNA and RFC subunits in Arabidopsis and rice were investigated via yeast-two-hybrid method and bimolecular fluorescence complementation techniques.

RESULTS

We observed that OsPCNA could interact with all OsRFC subunits, while protein-protein interactions only exist between Arabidopsis RFC2/3/4/5 and AtPCNA1/2. The truncated analyses indicated that the C-terminal of Arabidopsis RFC2/3/4/5 and rice RFC1/2 is essential for binding PCNA while the region of rice RFC3/4/5 mediating interaction with PCNA distributed both at the N- and C-terminal. On the other hand, we found that the C- and N-terminal of Arabidopsis and rice PCNA contribute equally to PCNA-PCNA interaction, and the interdomain connecting loop (IDCL) domain and C-terminal of PCNAs are indispensable for interacting RFC subunits.

CONCLUSIONS

These results indicated that Arabidopsis and rice PCNAs are highly conserved in sequence, structure and pattern of interacting with other PCNA monomer. Nevertheless, there are also significant differences between the Arabidopsis and rice RFC subunits in binding PCNA. Taken together, our results could be helpful for revealing the biological functions of plant RFC-PCNA complex.

Analysis of the in vitro effects of di-(2-ethylhexyl) phthalate exposure on human uterine leiomyoma cells

Uterine leiomyoma is the most common benign tumor type of the female reproductive tract. Despite its high prevalence, the exact pathogenesis of the benign tumor remains unknown. In the present study, the effects of di-(2-ethylhexyl) phthalate (DEHP) on the proliferation and apoptosis rates and expression of inflammatory proteins in human leiomyoma cells were evaluated. The effects of DEHP on cell viability were determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. The effects on apoptosis were evaluated by western blotting, TUNEL assay and Annexin V staining. Western blotting was also performed to evaluate the expression of inflammatory proteins. It was observed that DEHP-treated leiomyoma cells had higher viability, as well as proliferating cell nuclear antigen and B-cell lymphoma 2 protein expression, and lower apoptosis rates compared with the untreated controls. Additionally, hypoxia inducible factor 1α (HIF-1α) and cyclooxygenase-2 (COX-2) expression increased in human leiomyoma cells following DEHP treatment. In conclusion, DEHP promoted cell viability and anti-apoptotic protein expression and induced HIF-1α and COX-2 expression in human leiomyoma cells. These results suggested that DEHP may disrupt mechanisms underlying various processes in human leiomyoma cells. Furthermore, the current study revealed a basic mechanism of action of DEHP in human leiomyoma cells. Further research on the effects of various endocrine disruptors on the pathogenesis of uterine leiomyoma during early development may reveal strategies to prevent this disease.

Exonuclease processivity of archaeal replicative DNA polymerase in association with PCNA is expedited by mismatches in DNA

Family B DNA polymerases comprise polymerase and 3′ −>5′ exonuclease domains, and detect a mismatch in a newly synthesized strand to remove it in cooperation with Proliferating cell nuclear antigen (PCNA), which encircles the DNA to provide a molecular platform for efficient protein–protein and protein–DNA interactions during DNA replication and repair. Once the repair is completed, the enzyme must stop the exonucleolytic process and switch to the polymerase mode. However, the cue to stop the degradation is unclear. We constructed several PCNA mutants and found that the exonuclease reaction was enhanced in the mutants lacking the conserved basic patch, located on the inside surface of PCNA. These mutants may mimic the Pol/PCNA complex processing the mismatched DNA, in which PCNA cannot interact rigidly with the irregularly distributed phosphate groups outside the dsDNA. Indeed, the exonuclease reaction with the wild type PCNA was facilitated by mismatched DNA substrates. PCNA may suppress the exonuclease reaction after the removal of the mismatched nucleotide. PCNA seems to act as a “brake” that stops the exonuclease mode of the DNA polymerase after the removal of a mismatched nucleotide from the substrate DNA, for the prompt switch to the DNA polymerase mode.

Control of Genome Integrity by RFC Complexes; Conductors of PCNA Loading onto and Unloading from Chromatin during DNA Replication

During cell division, genome integrity is maintained by faithful DNA replication during S phase, followed by accurate segregation in mitosis. Many DNA metabolic events linked with DNA replication are also regulated throughout the cell cycle. In eukaryotes, the DNA sliding clamp, proliferating cell nuclear antigen (PCNA), acts on chromatin as a processivity factor for DNA polymerases. Since its discovery, many other PCNA binding partners have been identified that function during DNA replication, repair, recombination, chromatin remodeling, cohesion, and proteolysis in cell-cycle progression. PCNA not only recruits the proteins involved in such events, but it also actively controls their function as chromatin assembles. Therefore, control of PCNA-loading onto chromatin is fundamental for various replication-coupled reactions. PCNA is loaded onto chromatin by PCNA-loading replication factor C (RFC) complexes. Both RFC1-RFC and Ctf18-RFC fundamentally function as PCNA loaders. On the other hand, after DNA synthesis, PCNA must be removed from chromatin by Elg1-RFC. Functional defects in RFC complexes lead to chromosomal abnormalities. In this review, we summarize the structural and functional relationships among RFC complexes, and describe how the regulation of PCNA loading/unloading by RFC complexes contributes to maintaining genome integrity.

Transforming growth factor-β1 activates ΔNp63/c-Myc to promote oral squamous cell carcinoma

OBJECTIVE

During the development of oral squamous cell carcinoma (OSCC), the transformed epithelial cells undergo increased proliferation resulting in tumor growth and invasion. Interestingly, throughout all phases of differentiation and progression to OSCC, transforming growth factor-β1 (TGF)-β1 induces cell cycle arrest or apoptosis; however, the role of TGF-β1 in promoting cancer cell proliferation has not been explored in detail. The purpose of this study was to identify the effect of TGF-β1 on OSCC cell proliferation.

STUDY DESIGN

Using both human OSCC samples and cell lines (UMSCC38 and UMSCC11B), we assessed protein, mRNA, gene expression, and protein-DNA interactions during OSCC progression.

RESULTS

Our results showed that TGF-β1 increased OSCC cell proliferation by upregulating the expression of ΔNp63 and c-Myc oncogenes. Although the basal OSCC cell proliferation is sustained by activating ΔNp63, increased induction of c-Myc causes unregulated OSCC cell proliferation. Following induction of the cell cycle by ΔNp63 and c-Myc, cancer cells that halt c-Myc activity undergo epithelial mesenchymal transition or invasion while those that continue to express ΔNp63/c-Myc undergo unlimited progression through the cell cycle.

CONCLUSIONS

OSCC proliferation is manifested by the induction of c-Myc in response to TGF-β1 signaling, which is essential for OSCC growth. Our data highlight the potential role of TGF-β1 in the induction of cancer progression and invasion of OSCC.

Harmine induces apoptosis and inhibits tumor cell proliferation, migration and invasion through down-regulation of cyclooxygenase-2 expression in gastric cancer

Cyclooxygenase-2 (COX-2) plays an important role in the carcinogenesis and progression of gastric cancer. Harmine is reported as a promising drug candidate for cancer therapy; however, effects and action mechanism of harmine on the human gastric cancer cells remain unclear. This study evaluated the anti-tumor effects of harmine on human gastric cancer both in vitro and in vivo. The cell proliferation was determined using MTT colorimetric assay. Apoptosis was measured by DAPI staining and flow cytometry analysis. The wound healing and transwell invasion assays were performed to evaluate the effects of harmine on the migration and invasion of gastric cancer cells. The expression of COX-2, proliferating cell nuclear antigen (PCNA), Bcl-2, Bax and matrix metalloproteinase-2 (MMP-2) was detected by Western blot analysis. Our results showed that harmine significantly inhibited cellular proliferation, migration, invasion and induced apoptosis in vitro, as well as inhibited tumor growth in vivo. In addition, harmine significantly inhibited the expression of COX-2, PCNA, Bcl-2 and MMP-2 as well as increased Bax expression in gastric cancer cells. These results collectively indicate that harmine induces apoptosis and inhibits proliferation, migration and invasion of human gastric cancer cells, which may be mediated by down-regulation of COX-2 expression.

Germinated brown rice and its bioactives modulate the activity of uterine cells in oophorectomised rats as evidenced by gross cytohistological and immunohistochemical changes

Background

Germinated brown rice (GBR) is gaining momentum in the area of biomedical research due to its increased use as a nutraceutical for the management of diseases. The effect of GBR on the reproductive organs of oophorectomised rats was studied using the gross, cytological, histological and immunohistochemical changes, with the aim of reducing atrophy and dryness of the genital organs in menopause.

Methods

Experimental rats were divided into eight groups of six rats per group. Groups 1, 2 and 3 (sham-operated (SH), oophorectomised without treatment (OVX) and oophorectomised treated with 0.2 mg/kg oestrogen, respectively) served as the controls. The groups 4,5,6,7 and 8 were treated with 20 mg/kg Remifemin, 200 mg/kg of GBR, ASG, oryzanol and GABA, respectively. All treatments were administered orally, once daily for 8 weeks. Vaginal smear cytology was done at the 7^th week on all the rats. The weight and dimensions of the uterus and vagina were determined after sacrifice of the rats. Uterine and vaginal tissues were taken for histology and Immunohistochemical examinations.

Results

GBR and its bioactives treated groups significantly increased the weight and length of both the uterus and the vagina when compared to Oophorectomised non-treated group (OVX-non-treated) (p < 0.05). Significant changes were observed in the ratio of cornified epithelial cells and number of leucocytes in the vaginal cytology between the oophorectomised non-treated and treated groups. There was also an increase in the luminal and glandular epithelial cells activity in the treated compared with the untreated groups histologically. Immunohistochemical staining showed specific proliferating cell nuclear antigen (PCNA) in the luminal and glandular epithelium of the treated groups, which was absent in the OVX-non-treated group. GBR improved the length and weight of the uterus and also increased the number of glandular and luminal cells epithelia of the vagina.

Conclusion

GBR and its bioactives could be a potential alternative in improving reproductive system atrophy, dryness and discomfort during menopause.

Targeting the EGFR/PCNA signaling suppresses tumor growth of triple-negative breast cancer cells with cell-penetrating PCNA peptides

Tyrosine 211 (Y211) phosphorylation of proliferation cell nuclear antigen (PCNA) coincides with pronounced cancer cell proliferation and correlates with poor survival of breast cancer patients. In epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI)-resistant cells, both nuclear EGFR (nEGFR) expression and PCNA Y211 phosphorylation are increased. Moreover, the resistance to EGFR TKI is a major clinical problem in treating EGFR-overexpressing triple-negative breast cancer (TNBC). Thus, effective treatment to combat resistance is urgently needed. Here, we show that treatment of cell-penetrating PCNA peptide (CPPP) inhibits growth and induces apoptosis of human TNBC cells. The Y211F CPPP specifically targets EGFR and competes directly for PCNA tyrosine Y211 phosphorylation and prevents nEGFR from binding PCNA in vivo; it also suppresses tumor growth by sensitizing EGFR TKI resistant cells, which have enhanced nEGFR function and abrogated classical EGFR membrane signaling. Furthermore, we identify an active motif of CPPP, RFLNFF (RF6 CPPP), which is necessary and sufficient to inhibit TKI-resistant TNBC cell growth of orthotopic implanted tumor in mice. Finally, the activity of its synthetic retro-inverted derivative, D-RF6 CPPP, on an equimolar basis, is more potent than RF6 CPPP. Our study reveals a drug candidate with translational potential for the future development of safe and effective therapeutic for EGFR TKI resistance in TNBC.

Jasmonate controls leaf growth by repressing cell proliferation and the onset of endoreduplication while maintaining a potential stand-by mode

Phytohormones regulate plant growth from cell division to organ development. Jasmonates (JAs) are signaling molecules that have been implicated in stress-induced responses. However, they have also been shown to inhibit plant growth, but the mechanisms are not well understood. The effects of methyl jasmonate (MeJA) on leaf growth regulation were investigated in Arabidopsis (Arabidopsis thaliana) mutants altered in JA synthesis and perception, allene oxide synthase and coi1-16B (for coronatine insensitive1), respectively. We show that MeJA inhibits leaf growth through the JA receptor COI1 by reducing both cell number and size. Further investigations using flow cytometry analyses allowed us to evaluate ploidy levels and to monitor cell cycle progression in leaves and cotyledons of Arabidopsis and/or Nicotiana benthamiana at different stages of development. Additionally, a novel global transcription profiling analysis involving continuous treatment with MeJA was carried out to identify the molecular players whose expression is regulated during leaf development by this hormone and COI1. The results of these studies revealed that MeJA delays the switch from the mitotic cell cycle to the endoreduplication cycle, which accompanies cell expansion, in a COI1-dependent manner and inhibits the mitotic cycle itself, arresting cells in G1 phase prior to the S-phase transition. Significantly, we show that MeJA activates critical regulators of endoreduplication and affects the expression of key determinants of DNA replication. Our discoveries also suggest that MeJA may contribute to the maintenance of a cellular "stand-by mode" by keeping the expression of ribosomal genes at an elevated level. Finally, we propose a novel model for MeJA-regulated COI1-dependent leaf growth inhibition.

Transforming growth factor-β activates c-Myc to promote palatal growth

During palatogenesis, the palatal mesenchyme undergoes increased cell proliferation resulting in palatal growth, elevation and fusion of the two palatal shelves. Interestingly, the palatal mesenchyme expresses all three transforming growth factor (TGF) β isoforms (1, 2, and 3) throughout these steps of palatogenesis. However, the role of TGFβ in promoting proliferation of palatal mesenchymal cells has never been explored. The purpose of this study was to identify the effect of TGFβ on human embryonic palatal mesenchymal (HEPM) cell proliferation. Our results showed that all isoforms of TGFβ, especially TGFβ3, increased HEPM cell proliferation by up-regulating the expression of cyclins and cyclin-dependent kinases as well as c-Myc oncogene. TGFβ activated both Smad-dependent and Smad-independent pathways to induce c-Myc gene expression. Furthermore, TBE1 is the only functional Smad binding element (SBE) in the c-Myc promoter and Smad4, activated by TGFβ, binds to the TBE1 to induce c-Myc gene activity. We conclude that HEPM proliferation is manifested by the induction of c-Myc in response to TGFβ signaling, which is essential for complete palatal confluency. Our data highlights the potential role of TGFβ as a therapeutic molecule to correct cleft palate by promoting growth.

Downregulation of miR-101 in gastric cancer correlates with cyclooxygenase-2 overexpression and tumor growth

Cyclooxygenase-2 (COX-2) plays an important role in the carcinogenesis and progression of gastric cancer. It has been demonstrated that COX-2 overexpression depends on different cellular pathways, involving both transcriptional and post-transcriptional regulation. MicroRNAs (miRNAs) are small, noncoding RNAs that function as post-transcriptional regulators. Here, we characterize miR-101 expression and its role in the regulation of COX-2 expression, which in turn, will provide us with additional insights into the potential therapeutic benefits of exogenous miR-101 for treatment of gastric cancer. Our results showed that miR-101 levels in gastric cancer tissues were significantly lower than those in the matched normal tissue (P < 0.01). Furthermore, lower levels of miR-101 were associated with increased tumor invasion and lymph node metastasis (P < 0.05). We also found an inverse correlation between miR-101 and COX-2 expression in both gastric cancer specimens and cell lines. Significant decreases in COX-2 mRNA and COX-2 levels were observed in the pre-miR-101-infected gastric cancer cells. One possible mechanism of interaction is that miR-101 inhibited COX-2 expression by directly binding to the 3'-UTR of COX-2 mRNA. Overexpression of miR-101 in gastric cancer cell lines also inhibited cell proliferation and induced apoptosis in vitro, as well as inhibiting tumor growth in vivo. These results collectively indicate that miR-101 may function as a tumor suppressor in gastric cancer, with COX-2 as a direct target.

Targeting of colorectal cancer growth, metastasis, and anti-apoptosis in BALB/c nude mice via APRIL siRNA

A proliferation-inducing ligand (APRIL) is overexpressed in most tumor cells and tissues, especially in tumors of the alimentary system, such as colorectal cancer (CRC), gastric cancer, and liver cancer. RNA interference (RNAi) has been proved to be a powerful tool for gene knockdown and holds great promise for the treatment of cancer. In this study, the efficacy of RNAi targeting APRIL was analyzed via relevant experiments on human CRC xenografted in BALB/c nude mice. Both the mRNA and protein levels of APRIL were examined after intratumoral injection of APRIL small interfering RNA (siRNA). Meanwhile, pathological tools were utilized to observe the alterations on the aspects of proliferation, metastasis, apoptosis and cellular necrosis by means of detecting proliferating cell nuclear antigen, Ki-67, MMP-2, MMP-9, TIMP-3, TIMP-4, Bcl-2, Bax and Bcl-xL of CRC. In addition, terminal deoxyribonucleotidyl transferase-mediated dUTP-digoxigenin nick end-labeling (TUNEL) and hematoxylin and eosin staining were also conducted to examine cell apoptosis and necrosis. It was found that grafted human colorectal tumor growth and metastasis were obviously inhibited while tumor cell apoptosis and necrosis were induced after in vivo APRIL siRNA injection into nude mice. The data indicated that silencing of the APRIL gene using RNAi may serve as a novel therapeutic strategy for treatment of CRC.

Role of p21 in SP600125-induced cell cycle arrest, endoreduplication, and apoptosis

The anti-cancer effect of the c-Jun N-terminal kinase (JNK) inhibitor SP600125 has been well evaluated in human cancer cells. However the role of p21 in SP600125-mediated G(2)/M distribution is not fully understood. Our results showed that the transcriptional activation of p21 by SP600125 is mediated through the proximal regions of multiple Sp1 sites in the p21 promoter following ERK-dependent phosphorylation of Sp1. In this process, p21 induces endoreduplication through the inhibition of cyclin E/Cdk2 activity at 24 h but does not directly regulate cyclin B1/Cdc2 activity. Furthermore, SP600125 induces the phosphorylation of p21 at Thr 145 through the PI3K/Akt pathway. Akt-mediated phosphorylation of p21 and protection of apoptosis are completely abolished by inhibitors of PI3K and Akt. In summary using time points, we identified the dual functions of p21 as an inhibitor of cell-cycle progression at 24 h and as an anti-apoptotic factor at 48 h.

Molecular modeling and expression of the Litopenaeus vannamei proliferating cell nuclear antigen (PCNA) after white spot syndrome virus shrimp infection

Proliferating cell nuclear antigen (PCNA) is the eukaryotic sliding clamp that tethers DNA polymerase to DNA during replication. The full-length cDNA of the Pacific white shrimp Litopenaeus vannamei PCNA (LvPCNA) was cloned and encoded a protein of 260 amino acids that is highly similar to other Crustacean PCNAs. The theoretical shrimp PCNA structure has all the domains that are necessary for its interaction with template DNA and DNA polymerase. RT-PCR analysis showed that LvPCNA is expressed mainly in muscle and hemocytes and much less in hepatopancreas and gills. LvPCNA mRNA levels are not statistically different in muscle from healthy and challenged shrimp with the white spot syndrome virus (WSSV). In contrast, the mRNA levels of the viral DNA polymerase show a biphasic pattern with expression at 6 h post-infection and later at 24 and 48 h. These results suggest that in shrimp muscle LvPCNA levels are steadily kept to allow viral replication and that WSSV DNA polymerase (WSSV-DNApol) is more responsive towards later stages of infection. More knowledge of the DNA replication machinery would result in a better understanding of the mechanism and components of viral replication, since the WSSV genome does not have all the components required for assembly of a fully functional replisome.

Structure and biochemical characterization of proliferating cellular nuclear antigen from a parasitic protozoon

Proliferating cellular nuclear antigen (PCNA) is a toroidal-shaped protein that is involved in cell-cycle control, DNA replication and DNA repair. Parasitic protozoa are early-diverged eukaryotes that are responsible for neglected diseases. In this work, a PCNA from a parasitic protozoon was identified, cloned and biochemically characterized and its crystal structure was determined. Structural and biochemical studies demonstrate that PCNA from Entamoeba histolytica assembles as a homotrimer that is able to interact with and stimulate the activity of a PCNA-interacting peptide-motif protein from E. histolytica, EhDNAligI. The data indicate a conservation of the biochemical mechanisms of PCNA-mediated interactions between metazoa, yeast and parasitic protozoa.

Proliferating cell nuclear antigen in the cytoplasm interacts with components of glycolysis and cancer

Proliferating cell nuclear antigen (PCNA) is involved in a wide range of functions in the nucleus. However, a substantial amount of PCNA is also present in the cytoplasm, although their function is unknown. Here we show, through Far-Western blotting and mass spectrometry, that PCNA is associated with several cytoplasmic oncoproteins, including elongation factor, malate dehydrogenase, and peptidyl-prolyl isomerase. Surprisingly, PCNA is also associated with six glycolytic enzymes that are involved in the regulation of steps 4-9 in the glycolysis pathway.

Identification of differentially expressed genes at two key endosperm development stages using two maize inbreds with large and small grain and integration with detected QTL for grain weight

Maize endosperm accounts for more than 80% of the grain weight. Cell division and grain filling are the two key stages for endosperm development. Previous studies showed that gene expression during differential stages in endosperm development is greatly different. However, information on systematic identification and characterization of the differentially expressed genes between the two stages are limited. In this study, suppression subtractive hybridization (SSH) was used to generate four subtracted cDNA libraries for the two stages using two maize inbreds with large and small grain. Totally, 4,784 differentially expressed sequence tags (ESTs) were sequenced and 902 were non-redundant, which consisted of 344 unique ESTs. Among them 192 had high sequence similarity to the GenBank entries and represent diverse of functional categories, such as metabolism, cell growth/division, transcription, signal transduction, protein destination/storage, protein synthesis and others. The expression patterns of 75.7% SSH-derived cDNAs were confirmed by reverse Northern blot and semi-quantitative reverse transcription polymerase chain reaction, and exhibited the similar results (75.0%). Genes differentially expressed between two key stages for the two inbreds were involved in diverse physiological process pathway, which might be responsible for the formation of grain weight. 43.8% (70 of the 160 unique ESTs) of the identified ESTs were assigned to 39 chromosome bins distributed over all ten maize chromosomes. Eleven ESTs were found to co-localize with previous detected QTLs for grain weight, which might be considered as the candidate genes of grain weight for further study.

Identification, mRNA expression and characterization of proliferating cell nuclear antigen gene from Chinese mitten crab Eriocheir japonica sinensis

The sliding clamp proliferating cell nuclear antigen (PCNA) plays important roles in nucleic acid metabolism. In this work, we isolated a PCNA gene (designated as EjsPCNA, accession: FJ483830) by rapid amplification of cDNA ends approach from the Chinese mitten crab Eriocheir japonica sinensis. The full-length cDNA of EjsPCNA consists of 1123 nucleotides with an open-reading frame of 780bp encoding 259 amino acids (28.62kDa) and containing an interdomain connecting loop, C-terminal tail, and center loop. Sequence alignment, phylogenetic analyses, and structure comparison revealed that EjsPCNA is a member of the PCNA family. Real-time RT-PCR results indicate that EjsPCNA is expressed throughout three developmental stages. EjsPCNA mRNA expression levels at the first crab stage are significantly higher than that of the other two stages. Present data showed that the expression levels of EjsPCNA in E. j. sinensis are likely related to proliferation activity of tissues, and suggested that EjsPCNA gene is probably involved in the crabs' early developmental regulation.

Antiproliferative effect of peripheral benzodiazepine receptor antagonist PK11195 in rat mammary tumor cells

This study aims to establish the antiproliferative effects of PK11195, a peripheral benzodiazepine receptor antagonist (PBR) in rat mammary tumor cells. Breast tumors were induced by administration of a carcinogen, dimethylbenz[a]anthracene to 50-day-old female rats maintained on a standard AIN-76A diet with casein as the protein source. The tumors were developed approximately after 120 days. The tumors were of grade I (20%), grade II (60%), and grade III (20%). The tumors were isolated and cultured in DMEM/F12 media with supplements. We characterized the properties of the isolated cells and study the effect of PK11195 on those cells. We were successful in growing breast tumor cells up to 30 passages for cellular characterization. These cells had high reactivity with Ki-67 and PCNA antibodies suggesting high proliferation rate. These cells were highly invasive as evident by matrigel invading ability. Furthermore, these cells acquired a positive response for CD-31 and VEGF antibodies suggesting angiogenic potential, and also possessed migrating ability/motility as evident by the wound healing properties. These cells expressed elevated levels of PBR, a cancer promoting gene. The proliferation, invasion and migration appear to decrease when treated with PK11195, a PBR antagonist. Furthermore, PK11195 treatment caused an increase in apoptosis as evident by increase in the levels of annexin V. However, the inhibition of cell proliferation by PK11195 was counteracted by Ro5-4864, a PBR agonist. Thus, PBR antagonist may be a potential therapeutic agent for the control of aggressiveness of breast cancer.

Analysis of the role of PCNA-DNA contacts during clamp loading

BACKGROUND

Sliding clamps, such as Proliferating Cell Nuclear Antigen (PCNA) in eukaryotes, are ring-shaped protein complexes that encircle DNA and enable highly processive DNA replication by serving as docking sites for DNA polymerases. In an ATP-dependent reaction, clamp loader complexes, such as the Replication Factor-C (RFC) complex in eukaryotes, open the clamp and load it around primer-template DNA.

RESULTS

We built a model of RFC bound to PCNA and DNA based on existing crystal structures of clamp loaders. This model suggests that DNA would enter the clamp at an angle during clamp loading, thereby interacting with positively charged residues in the center of PCNA. We show that simultaneous mutation of Lys 20, Lys 77, Arg 80, and Arg 149, which interact with DNA in the RFC-PCNA-DNA model, compromises the ability of yeast PCNA to stimulate the DNA-dependent ATPase activity of RFC when the DNA is long enough to extend through the clamp. Fluorescence anisotropy binding experiments show that the inability of the mutant clamp proteins to stimulate RFC ATPase activity is likely caused by reduction in the affinity of the RFC-PCNA complex for DNA. We obtained several crystal forms of yeast PCNA-DNA complexes, measuring X-ray diffraction data to 3.0 A resolution for one such complex. The resulting electron density maps show that DNA is bound in a tilted orientation relative to PCNA, but makes different contacts than those implicated in clamp loading. Because of apparent partial disorder in the DNA, we restricted refinement of the DNA to a rigid body model. This result contrasts with previous analysis of a bacterial clamp bound to DNA, where the DNA was well resolved.

CONCLUSION

Mutational analysis of PCNA suggests that positively charged residues in the center of the clamp create a binding surface that makes contact with DNA. Disruption of this positive surface, which had not previously been implicated in clamp loading function, reduces RFC ATPase activity in the presence of DNA, most likely by reducing the affinity of RFC and PCNA for DNA. The interaction of DNA is not, however, restricted to one orientation, as indicated by analysis of the PCNA-DNA co-crystals.

DNA polymerases and mutagenesis in human cancers

DNA polymerases (Pols) act as key players in DNA metabolism. These enzymes are the only biological macromolecules able to duplicate the genetic information stored in the DNA and are absolutely required every time this information has to be copied, as during DNA replication or during DNA repair, when lost or damaged DNA sequences have to be replaced with "original" or "correct" copies. In each DNA repair pathway one or more specific Pols are required. A feature of mammalian DNA repair pathways is their redundancy. The failure of one of these pathways can be compensated by another one. However, several DNA lesions require a specific repair pathway for error free repair. In many tumors one or more DNA repair pathways are affected, leading to error prone repair of some kind of lesions by alternatives routes, thus leading to accumulation of mutations and contributing to genomic instability, a common feature of cancer cell. In this chapter, we present the role of each Pol in genome maintenance and highlight the connections between the malfunctioning of these enzymes and cancer progress.

Effects of the special extract ERr 731 from Rheum rhaponticum on estrogen-regulated targets in the uterotrophy model of ovariectomized rats

A recent clinical study with a two-year application of the extract ERr 731 from Rheum rhaponticum demonstrated its efficacy and potentially suggested it safety regarding unwanted endometrial side effects. The aim of the present study is to provide experimental proof for the latter observation in a preclinical experimental animal model by assessing dose-dependent effects of ERr 731 - either alone or in combination with estradiol (E2) - on growth and proliferation in the uterus of ovariectomized (ovx) rats. ERr 731 was given in a dose corresponding to human therapeutic application and additionally in three pharmacologically relevant doses. In addition to uterine wet weight, this study examines the effects of ERr 731 on the uterine mRNA expression of the proliferation marker Ki67, proliferating cell nuclear antigen (PCNA), insulin-like growth factor-1 (IGF-1), type 1 IGF receptor (IGF-1R), the two estrogen receptor (ER) subtypes alpha and beta (ERalpha and ERbeta) and the estrogen-responsive gene complement C3 (C3). ERr 731 did neither stimulate an uterotrophic response in the uterotrophic assay with ovx rats nor stimulate or modulate the expression of genes associated with proliferation. In combination with E2, ERr 731 reduced the E2-induced uterine growth stimulation. These observations were further substantiated by the expression pattern of genes related to proliferation control, in view of the fact that the E2-induced elevation of Ki67 mRNA and PCNA protein levels in the uterus were counteracted by simultaneous treatment of the animals with ERr 731. In conclusion, the experimental findings presented here provide further evidence for the safety of ERr 731 towards unwanted uterine and endometrial proliferative events in response to ERr 731 and support observations from recent clinical trials.

Vaccinia virus G8R protein: a structural ortholog of proliferating cell nuclear antigen (PCNA)

Background

Eukaryotic DNA replication involves the synthesis of both a DNA leading and lagging strand, the latter requiring several additional proteins including flap endonuclease (FEN-1) and proliferating cell nuclear antigen (PCNA) in order to remove RNA primers used in the synthesis of Okazaki fragments. Poxviruses are complex viruses (dsDNA genomes) that infect eukaryotes, but surprisingly little is known about the process of DNA replication. Given our previous results that the vaccinia virus (VACV) G5R protein may be structurally similar to a FEN-1-like protein and a recent finding that poxviruses encode a primase function, we undertook a series of in silico analyses to identify whether VACV also encodes a PCNA-like protein.

Results

An InterProScan of all VACV proteins using the JIPS software package was used to identify any PCNA-like proteins. The VACV G8R protein was identified as the only vaccinia protein that contained a PCNA-like sliding clamp motif. The VACV G8R protein plays a role in poxvirus late transcription and is known to interact with several other poxvirus proteins including itself. The secondary and tertiary structure of the VACV G8R protein was predicted and compared to the secondary and tertiary structure of both human and yeast PCNA proteins, and a high degree of similarity between all three proteins was noted.

Conclusions

The structure of the VACV G8R protein is predicted to closely resemble the eukaryotic PCNA protein; it possesses several other features including a conserved ubiquitylation and SUMOylation site that suggest that, like its counterpart in T4 bacteriophage (gp45), it may function as a sliding clamp ushering transcription factors to RNA polymerase during late transcription.

Inhibition of COX-2 and activation of peroxisome proliferator-activated receptor gamma synergistically inhibits proliferation and induces apoptosis of human pancreatic carcinoma cells

Although inhibition of cyclooxygenase-2 (COX-2) or activation of peroxisome proliferators-activated receptor gamma (PPAR-gamma) leads to growth inhibition in malignancies, the synergistic anti-tumor effects of combination of COX-2 inhibitor (NS-398) and PPAR-gamma agonist (rosiglitazone) on the human pancreatic cancer cells remains unknown. Here, we evaluated the effects of NS-398 and/or rosiglitazone on the cell proliferation and apoptosis in a pancreatic cancer cell line, SW1990. NS-398 and rosiglitazone decreased cell proliferation in a dose- and time-dependent manner. Proliferating cell nuclear antigen (PCNA) labeling index significantly decreased in the cells treated with either NS-398 or rosiglitazone. Both NS-398 and rosiglitazone alone induced apoptotic cell death of SW1990. The combination of NS-398 and rosiglitazone exerted synergistic effects on proliferation inhibition, and apoptosis induction in SW1990 cells, with down-regulation of Bcl-2 and up-regulation of Bax expression. Our results indicate that simultaneous targeting of COX-2 and PPAR-gamma inhibits pancreatic cancer development more effectively than targeting each molecule alone.

Arabidopsis thaliana Y-family DNA polymerase eta catalyses translesion synthesis and interacts functionally with PCNA2

Upon blockage of chromosomal replication by DNA lesions, Y-family polymerases interact with monoubiquitylated proliferating cell nuclear antigen (PCNA) to catalyse translesion synthesis (TLS) and restore replication fork progression. Here, we assessed the roles of Arabidopsis thaliana POLH, which encodes a homologue of Y-family polymerase eta (Poleta), PCNA1 and PCNA2 in TLS-mediated UV resistance. A T-DNA insertion in POLH sensitized the growth of roots and whole plants to UV radiation, indicating that AtPoleta contributes to UV resistance. POLH alone did not complement the UV sensitivity conferred by deletion of yeast RAD30, which encodes Poleta, although AtPoleta exhibited cyclobutane dimer bypass activity in vitro, and interacted with yeast PCNA, as well as with Arabidopsis PCNA1 and PCNA2. Co-expression of POLH and PCNA2, but not PCNA1, restored normal UV resistance and mutation kinetics in the rad30 mutant. A single residue difference at site 201, which lies adjacent to the residue (lysine 164) ubiquitylated in PCNA, appeared responsible for the inability of PCNA1 to function with AtPoleta in UV-treated yeast. PCNA-interacting protein boxes and an ubiquitin-binding motif in AtPoleta were found to be required for the restoration of UV resistance in the rad30 mutant by POLH and PCNA2. These observations indicate that AtPoleta can catalyse TLS past UV-induced DNA damage, and links the biological activity of AtPoleta in UV-irradiated cells to PCNA2 and PCNA- and ubiquitin-binding motifs in AtPoleta.

Molecular cloning and characterization of proliferating cell nuclear antigen (PCNA) from Chinese shrimp Fenneropenaeus chinensis

The proliferating cell nuclear antigen gene was cloned from Fenneropenaeus chinensis (FcPCNA). The full-length cDNA sequence of FcPCNA encodes 260 amino acids showing high identity with PCNAs reported in other species. FcPCNA expressed especially high in proliferating tissues of shrimp such as haematopoietic tissue (HPT) and ovary. In order to understand the response of HPT to bacteria and virus challenge, mRNA level of FcPCNA in HPT was analyzed after shrimp were challenged by Vibrio anguillarum and white spot syndrome virus (WSSV). FcPCNA expression in HPT of shrimp was responsive to WSSV and Vibrio challenge, but different expression profiles were obtained after challenge by these two pathogens. The data provide additional information to understand the defense mechanisms of shrimp against virus and bacteria.

Development of the retinotectal system in the direct-developing frog Eleutherodactylus coqui in comparison with other anurans

BACKGROUND

Frogs primitively have a biphasic life history with an aquatic larva (tadpole) and a usually terrestrial adult. However, direct developing frogs of the genus Eleutherodactylus have lost a free living larval stage. Many larval structures never form during development of Eleutherodactylus, while limbs, spinal cord, and an adult-like cranial musculoskeletal system develop precociously.

RESULTS

Here, I compare growth and differentiation of the retina and tectum and development of early axon tracts in the brain between Eleutherodactylus coqui and the biphasically developing frogs Discoglossus pictus, Physalaemus pustulosus, and Xenopus laevis using morphometry, immunohistochemical detection of proliferating cell nuclear antigen (PCNA) and acetylated tubulin, biocytin tracing, and in situ hybridization for NeuroD. Findings of the present study indicate that retinotectal development was greatly altered during evolution of Eleutherodactlyus mostly due to acceleration of cell proliferation and growth in retina and tectum. However, differentiation of retina, tectum, and fiber tracts in the embryonic brain proceed along a conserved slower schedule and remain temporally coordinated with each other in E. coqui.

CONCLUSION

These findings reveal a mosaic pattern of changes in the development of the central nervous system (CNS) during evolution of the direct developing genus Eleutherodactylus. Whereas differentiation events in directly interconnected parts of the CNS such as retina, tectum, and brain tracts remained coordinated presumably due to their interdependent development, they were dissociated from proliferation control and from differentiation events in other parts of the CNS such as the spinal cord. This suggests that mosaic evolutionary changes reflect the modular character of CNS development.

Asbestos Induces Nitric Oxide Synthesis in Mesothelioma Cells via Rho Signaling Inhibition

We have observed that in three human malignant mesothelioma cell lines, crocidolite asbestos induced the activation of the transcription factor NF-kappaB and the synthesis of nitric oxide (NO) by inhibiting the RhoA signaling pathway. The incubation with crocidolite decreased the level of GTP-bound RhoA and the activity of Rho-dependent kinase, and induced the activation of Akt/PKB and IkBalpha kinase, leading to the nuclear translocation of NF-kappaB. The effects of crocidolite fibers on NF-kappaB activation and NO synthesis were mimicked by Y27632 (an inhibitor of the Rho-dependent kinases) and toxin B (an inhibitor of RhoA GTPase activity), while they were reverted by mevalonic acid, the product of 3-hydroxy-3-methylglutaryl coenzyme A (HMGCoA) reductase. Furthermore, crocidolite, similarly to mevastatin, inhibited the synthesis of cholesterol and ubiquinone and the prenylation of RhoA: these effects were prevented in the presence of mevalonic acid. This suggests that crocidolite fibers might inhibit the synthesis of isoprenoid molecules at the level of the HMGCoA reductase reaction or of an upstream step, thus impairing the prenylation and subsequent activation of RhoA. Akt can stimulate NO synthesis via a double mechanism: it can activate the inducible NO synthase via the NF-kappaB pathway and the endothelial NO synthase via a direct phosphorylation. Our results suggest that crocidolite increases the NO levels in mesothelioma cells by modulating both NO synthase isoforms.

A cancer-associated PCNA expressed in breast cancer has implications as a potential biomarker

Two isoforms of proliferating cell nuclear antigen (PCNA) have been observed in breast cancer cells. Commercially available antibodies to PCNA recognize both isoforms and, therefore, cannot differentiate between the PCNA isoforms in malignant and nonmalignant breast epithelial cells and tissues. We have developed a unique antibody that specifically detects a PCNA isoform (caPCNA) associated with breast cancer epithelial cells grown in culture and breast-tumor tissues. Immunostaining studies using this antibody suggest that the caPCNA isoform may be useful as a marker of breast cancer and that the caPCNA-specific antibody could potentially serve as a highly effective detector of malignancy. We also report here that the caPCNA isoform functions in breast cancer-cell DNA replication and interacts with DNA polymerase delta. Our studies indicate that the caPCNA isoform may be a previously uncharacterized detector of breast cancer.

Genetic analysis of ionizing radiation-induced mutagenesis in Saccharomyces cerevisiae reveals TransLesion Synthesis (TLS) independent of PCNA K164 SUMOylation and ubiquitination

Ionizing radiation-induced mutagenesis (IR-IM) underlies a basis for radiation associated carcinogenesis as well as resistance to radiation therapy. This process was examined in Saccharomyces cerevisiae using an array of isogenic DNA repair deficient mutants. Mutations inactivating homologous recombination (rad51, 52, 54) or nucleotide excision repair (rad1, rad10, rad4) caused elevated IR-IM whereas inactivation of TransLesion Synthesis (TLS: rad6) caused severely defective IR-IM. Of the mutations inactivating TLS polymerases, rev3 and rev1 caused equally severe defects in IR-IM whereas rad30 did not significantly affect the process. The effects of the rev3, rev1, and rad6 mutations on IR-IM were epistatic, suggesting the requirement of both polymerase zeta and Rev1p in IR-IM related TLS. Although PCNA K164 SUMOylation/ubiquitination is a proposed prerequisite for TLS, the IR-IM defect of a rev3 or a rad6 mutant was worse than and epistatic to the pol30K164R mutant, a mutant in which the PCNA had been mutated to abolish such modifications. These results suggested that IR-IM related TLS occurs in the absence of PCNA K164 modification. Further analysis of a mutant simultaneously defective in SUMOylation and mono-ubiquitination (rad18 siz1) revealed that these modifications redundantly affected TLS as well as NHEJ. A genetic model based on these observations is proposed.

Interaction between PCNA and diubiquitinated Mcm10 is essential for cell growth in budding yeast

The minichromosome maintenance protein 10 (Mcm10) is an evolutionarily conserved factor that is essential for replication initiation and elongation. Mcm10 is part of the eukaryotic replication fork and interacts with a variety of proteins, including the Mcm2-7 helicase and DNA polymerase alpha/primase complexes. A motif search revealed a match to the proliferating cell nuclear antigen (PCNA)-interacting protein (PIP) box in Mcm10. Here, we demonstrate a direct interaction between Mcm10 and PCNA that is alleviated by mutations in conserved residues of the PIP box. Interestingly, only the diubiquitinated form of Mcm10 binds to PCNA. Diubiquitination of Mcm10 is cell cycle regulated; it first appears in late G(1) and persists throughout S phase. During this time, diubiquitinated Mcm10 is associated with chromatin, suggesting a direct role in DNA replication. Surprisingly, a Y245A substitution in the PIP box of Mcm10 that inhibits the interaction with PCNA abolishes cell proliferation. This severe-growth phenotype, which has not been observed for analogous mutations in other PCNA-interacting proteins, is rescued by a compensatory mutation in PCNA that restores interaction with Mcm10-Y245A. Taken together, our results suggest that diubiquitinated Mcm10 interacts with PCNA to facilitate an essential step in DNA elongation.

Two cell-cycle regulated SET-domain proteins interact with proliferating cell nuclear antigen (PCNA) in Arabidopsis

The proliferating cell nuclear antigen (PCNA) functions as a sliding clamp for DNA polymerase, and is thus a key actor in DNA replication. It is also involved in DNA repair, maintenance of heterochromatic regions throughout replication, cell cycle regulation and programmed cell death. Identification of PCNA partners is therefore necessary for understanding these processes. Here we identify two Arabidopsis SET-domain proteins that interact with PCNA: ATXR5 and ATXR6. A truncated ATXR5Deltaex2, incapable of interacting with PCNA, also occurs in planta. ATXR6, upregulated during the S phase, is upregulated by AtE2F transcription factors, suggesting that it is required for S-phase progression. The two proteins differ in their subcellular localization: ATXR5 has a dual localization in plastids and in the nucleus, whereas ATXR6 is solely nuclear. This indicates that the two proteins may play different roles in plant cells. However, overexpression of either ATXR5 or ATXR6 causes male sterility because of the degeneration of defined cell types. Taken together, our results suggest that both proteins may play a role in the cell cycle or DNA replication, and that the activity of ATXR5 may be regulated via its subcellular localization.

Cooperative fluctuations point to the dimerization interface of p53 core domain

Elastic network models are used for investigation of the p53 core domain functional dynamics. Global modes of motion indicate high positive correlations for residue fluctuations across the A-B interface, which are not observed at the B-C interface. Major hinge formation is observed at the A-B interface upon dimerization indicating stability of the A-B dimer. These findings imply A-B as the native dimerization interface, whereas B-C is the crystal interface. The A-B dimer exhibits an opening-closing motion about DNA, supporting the previously suggested clamp-like model of nonspecific DNA binding followed by diffusion. Monomer A has limited positive correlations with DNA, while monomer B exhibits high positive correlations with DNA in the functionally significant slow modes. Thus, monomer B might seem to maintain the stability of the dimer-DNA complex by forming the relatively fixed arm of the dimer clamp, whereas the other arm of the clamp, monomer A, might allow sliding via continuous association/dissociation mechanisms.

Cell proliferation marker MCM2, but not Ki67, is helpful for distinguishing between minimally invasive follicular carcinoma and follicular adenoma of the thyroid

AIMS

To compare cell proliferation markers, minichromosome maintenance protein 2 (MCM2) and Ki67, in minimally invasive follicular carcinoma (MIFC) and follicular adenoma (FA) of the thyroid and among MIFCs with different diagnostic criteria.

METHODS AND RESULTS

Twenty-two MIFCs and 20 FAs were immunohistochemically stained for MCM2 and Ki67. The MIFCs were subdivided into six Group 1 tumours with both capsular and vascular invasions, seven Group 2 tumours with vascular invasion only and nine Group 3 tumours with capsular invasion only. The MCM2 and Ki67 indices were calculated, counting more than 1000 tumour cells in the most frequently positive areas. In total and Groups 1-3 MIFCs and in FAs, the average MCM2 index was 26.7 +/- 11.0, 28.4 +/- 8.6, 26.3 +/- 14.8, 25.9 +/- 8.4 and 10.7 +/- 4.5, respectively, whereas the average Ki67 index was 2.07 +/- 1.65, 1.93 +/- 2.02, 2.49 +/-1.38, 1.84 +/- 1.5 and 1.78 +/- 0.92, respectively. There was a significant difference in the MCM2 index, but not in the Ki67 index, between each category of MIFCs and FA (P < 0.01). However, neither the MCM2 index nor the Ki67 index showed a statistically significant difference among the subgroups of MIFC.

CONCLUSIONS

MCM2, but not Ki67, is a helpful marker for differentiating MIFC from FA. The tumour cell proliferative activity supports the histological criteria based on diagnosing MIFC by either capsular or vascular invasion only.

Grow-ING, Age-ING and Die-ING: ING proteins link cancer, senescence and apoptosis

The INhibitor of Growth (ING) family of plant homeodomain (PHD) proteins induce apoptosis and regulate gene expression through stress-inducible binding of phospholipids with subsequent nuclear and nucleolar localization. Relocalization occurs concomitantly with interaction with a subset of nuclear proteins, including PCNA, p53 and several regulators of acetylation such as the p300/CBP and PCAF histone acetyltransferases (HATs), as well as the histone deacetylases HDAC1 and hSir2. These interactions alter the localized state of chromatin compaction, subsequently affecting the expression of subsets of genes, including those associated with the stress response (Hsp70), apoptosis (Bax, MDM2) and cell cycle regulation (p21WAF1, cyclin B) in a cell- and tissue-specific manner. The expression levels and subcellular localization of ING proteins are altered in a significant number of human cancer types, while the expression of ING isoforms changes during cellular aging, suggesting that ING proteins may play a role in linking cellular transformation and replicative senescence. The variety of functions attributed to ING proteins suggest that this tumor suppressor serves to link the disparate processes of cell cycle regulation, cell suicide and cellular aging through epigenetic regulation of gene expression. This review examines recent findings in the ING field with a focus on the functions of protein-protein interactions involving ING family members and the mechanisms by which these interactions facilitate the various roles that ING proteins play in tumorigenesis, apoptosis and senescence.

Stoichiometric complex formation by proliferating cell nuclear antigen (PCNA) and its interacting protein: purification and crystallization of the DNA polymerase and PCNA monomer mutant complex from Pyrococcus furiosus

Replicative DNA polymerase interacts with processivity factors, the beta-subunit of DNA polymerase III or proliferating cell nuclear antigen (PCNA), in order to function with a long template DNA. The archaeal replicative DNA polymerase from Pyrococcus furiosus interacts with PCNA via its PCNA-interacting protein (PIP) motif at the C-terminus. The PCNA homotrimeric ring contains one PIP interacting site on each monomer and since the ring can accommodate up to three molecules simultaneously, formation of a stable stoichiometric complex of PCNA with its interacting protein has been difficult to control in vitro. A stable complex of the DNA polymerase with PCNA, using a PCNA monomer mutant, has been purified and crystallized. The best ordered crystal diffracted to 3.0 A resolution using synchrotron radiation. The crystals belong to space group P2(1)2(1)2, with unit-cell parameters a = 225.3, b = 123.3, c = 91.3 A.

The characteristics of astrocytomas and oligodendrogliomas are caused by two distinct and interchangeable signaling formats

Chronic platelet-derived growth factor (PDGF) signaling in glial progenitors leads to the formation of oligodendrogliomas in mice, whereas chronic combined Ras and Akt signaling leads to astrocytomas. Different histologies of these tumors imply that the pathways activated by these two oncogenic stimulations are different, and that the apparent lineage of the tumor cells may result from specific signaling activity. Therefore, we have investigated the signaling effects of PDGF in culture and in gliomas in vivo. In culture, PDGF transiently activates ERK1/2 and Akt, and subsequently elevates p21 and PCNA expression similar to chronic PDGF autocrine signaling in cultured astrocytes and PDGF-induced oligodendrogliomas in vivo. Culture experiments show that autocrine PDGF stimulation, and combined active Ras and Akt generate signaling patterns that are in some ways mutually exclusive. Furthermore, forced Akt activity in the context of chronic PDGF stimulation results in cells with an astrocytic differentiation pattern both in culture and in vivo. These data imply that these two interconvertible signaling motifs are distinct in mice and lead to gliomas resembling the two major glioma histologies found in humans. The ability of signaling activity to convert tumor cells from one lineage to another presents a mechanism for the development of tumors apparently comprised of cells from multiple lineages.