Proliferation-associated nucleolar antigen P120: a prognostic marker in node-negative breast cancer

BACKGROUND

P120 is a nucleolar proliferation antigen found in rapidly dividing cells and in a variety of malignancies.

METHODS

Our purpose was to determine whether P120 expression is a prognostic factor for patients with node-negative breast cancer by testing pathologic material from 90 patients for P120 immunoreactivity, histologic grade, and estrogen receptors.

RESULTS

P120 was detected in 52 of the 90 specimens (58%). Node-negative cancer patients with tumors that did not express the P120 antigen had a significantly better overall survival rate than node-negative cancer patients with tumors that did express P120 (92% vs 69%; p = 0.035). Histologic studies indicated that 36 tumors were grade I, 28 were grade II, and 26 were grade III. The presence of P120 correlated significantly with the nuclear grade of the tumor: 73% of grade III tumors, 64% of grade II tumors, and 42% of grade I tumors stained positive for P120 (p = 0.033). The correlation between nuclear grade and overall survival rate was also significant (grade 1, 94%; grade II, 79%; grade III, 58%); (p = 0.003). No significant correlation was found between P120 expression and estrogen receptors. Multivariate analysis shows that P120 expression and histologic grade together are the strongest predictors of survival.

CONCLUSIONS

The biologic marker P120 may play an important role in determining which patients with node-negative cancer will benefit most from adjuvant therapy.

Identification of the nuclear and nucleolar localization signals of the protein p120. Interaction with translocation protein B23

The human p120 nucleolar protein is a cell cycle-related protein that peaks during the S phase and has been shown to be associated with a beaded fibrillar structure. To study domains responsible for the nucleolar localization of protein p120, initially deletion mutants were made that defined sequences containing the localization signals; then, fusion genes that were composed of segments of the p120 molecule joined to the N-terminal end of the Escherichia coli beta-galactosidase were constructed. In the absence of the localization signals the beta-galactosidase remained in the cytoplasm. When the identified nuclear localization signal containing the amino acid sequence 99-110 (NAPRGKKRPAPG) was fused to the beta-galactosidase, the protein localized to the nucleus. When only the identified nucleolar localization signal containing the amino acid sequence 40-57 (SKRLSSRARKRAAKRRLG) was fused to the beta-galactosidase, the fusion protein remained in the cytoplasm. When both the nuclear and nucleolar localization signals were fused to the beta-galactosidase it localized predominantly to the nucleolus. Nucleolar protein B23, a putative "shuttle protein," bound to amino acid sequence 24-56 of protein p120. Deletion analysis showed that amino acids 187-215 of protein B23 bound to protein p120. The results suggest that protein B23 may be part of the mechanism of protein targeting to the nucleolus.

Expression of proliferation-associated antigens (PCNA, p120, p145) during the reentry of G0 cells into the cell cycle

Flow cytometric bivariate analysis was used to evaluate the expression of PCNA, p120, and p145 during the G0 reentry of CHO-K1 cells into the cell cycle. CHO-K1 cells were placed in a G0-like state using serum depletion and stimulated to reenter the cell cycle by replating into fresh, serum-containing medium. At discrete intervals after stimulation, replicate samples were stained for either PCNA, p120, or 145; stained for DNA (Coulter DNA-Prep); evaluated on the EPICS Profile I; and analyzed on the EPICS ELITE workstation. PCNA stained less than 10% of the G0 cells; in contrast, however, 30-35% of the G0 cells were positive for p120 and p145. Eight hours after stimulating G0 cells to reenter the cell cycle (during G0/G1), p120 reached 88% positivity, while p145 and PCNA were 63% and 30% positive, respectively. Cells in S phase (12 and 16 h following G0 stimulation) were greater than 90% positive for all three antigens. PCNA had the greatest change throughout the G0 reentry process, both in percentage positive and quantitatively (mean channel fluorescence). This report indicates that all three proliferation-associated antigens studied are differentially expressed during the reentry of G0 cells into the cell cycle. Furthermore, these antigens may be useful in the early detection of G0 recruitment.

Prediction of an rRNA methyltransferase domain in human tumor-specific nucleolar protein P120

Using computer methods for identification of amino acid motifs in sequence databases and multiple alignment, it is shown that human proliferation-associated nucleolar protein P120 contains a putative methyltransferase domain that is conserved in a group of bacterial proteins. It is hypothesized that P120 and the related prokaryotic proteins are rRNA methylases required for division of all types of cells.

Conserved sequences in a class of rare eukaryotic nuclear introns with non-consensus splice sites

Eukaryotic nuclear genomes contain a rare class of pre-mRNA introns with consensus sequence features that differ markedly from most pre-mRNA introns. Four genes have so far been identified that contain one copy each of this rare intron class in addition to several standard introns. These introns and homologous introns from several species were compared to identify conserved sequence elements and to establish consensus sequences for these elements. The only well-conserved elements are found at the 5' and 3' ends of the introns. The 5' splice site sequence is ATATCCTT beginning with the first nucleotide of the intron and is invariant in the introns examined to date. The 3' splice site consensus sequence is YCCAC ending at the last nucleotide of the intron. An almost invariant sequence of TCCTTAAC is also found near the 3' end of the intron (the 3' upstream element). The length of the introns varies between 95 and 2940 nucleotides. The sequence organization of these introns suggests that they represent a variant class of pre-mRNA introns that might be spliced via a spliceosome mechanism employing factors distinct from those used by other pre-mRNA introns. A search of small nuclear RNA (snRNA) sequences for regions complementary to the conserved elements of this rare class of introns found a strong match between U12 snRNA and the 3' upstream element and a weaker match between U11 snRNA and the 5' splice site sequence.

Prospective study correlating P120 antigen expression with established prognostic factors in breast cancer

P120 is a nucleolar proliferation antigen found in rapidly dividing cells and a variety of malignancies. Previous retrospective studies have demonstrated that, when detected in human breast cancer, P120 is associated with a poorer prognosis. To determine whether P120 expression correlates with other prognostic factors in breast cancer, we prospectively analysed pathologic and clinical data from 61 patients. P120 was detected in 40 of the 61 specimens (66%). No significant correlation existed between P120 expression and either tumour size or hormone receptors. A significant correlation was found between P120 expression and histological grade, degree of aneuploidy, S-phase fraction, degree of nodal involvement, and stage of disease. P120 is a biological marker indicative of tumour aggressiveness and may play an important role in determining which patients would most benefit from adjuvant therapy.

Effect of nucleolar P120 expression level on the proliferation capacity of breast cancer cells

Steady-state level of nucleolar P120 protein and P120 mRNA was compared to the doubling time and S-phase fraction in human breast cancer cell lines growing exponentially and in similar cells treated with a single dose of P120 antisense oligodeoxynucleotides. The study included six breast cancer cell lines and one nontransformed breast cell line with doubling times from 1.1 to 5.5 days and with S-phase fractions from 35 to 9%. P120 expression level was determined by densitometric computerized evaluation of protein and mRNA blots and with a quantitative 32P-reverse transcriptase-polymerase chain reaction method developed for small-scale samples. In the slowest growing normal cell line, P120 expression level was only about 10% of the level found in the most rapidly growing cancer cell line. The amount of P120 mRNA was highly correlated with the amount of P120 protein (P = 0.0001), indicating that P120 accumulation is regulated in these cells primarily at a transcriptional level. There was also a significant positive correlation between the level of P120 protein/mRNA and doubling time of cell lines (P = 0.0008) or percentage of S-phase cells (P = 0.210). P120 antisense oligomer treatment decreased the growth rate of cells in a dose-dependent manner, and the inhibition reached 70% at 100 microM concentration. Both P120 mRNA and P120 protein levels were also decreased by approximately 70% in cells treated with 100 microM P120 antisense oligomer. Slowly growing cells exhibited 50% inhibition by treatment at a proportionally lower concentration of P120 antisense oligomer than fast growing cells. This study shows that the expression of P120, measured either at the protein or the mRNA level, correlates with proliferation rate, identifying P120 as a cell proliferation marker.

Effect of staurosporine on MOLT-4 cell progression through G2 and on cytokinesis

Staurosporine (SSP) is an inhibitor of a variety of protein kinases with an especially high affinity towards protein kinase C. Whereas SSP has been shown to halt the cell cycle progression of various normal, nontransformed cell types in G1, most virus transformed or tumor cells are unaffected in G1 but arrest in G2 phase. SSP has also been observed to increase the appearance of cells with higher DNA content, suggestive of endoreduplication, in cultures of tumor cells. Using multivariate flow cytometry (DNA content vs. expression of cyclin B, nuclear p120 protein, or protein reactive with Ki-67 antibody) which makes it possible to discriminate cells with identical DNA content but at different phases of the cycle, we have studied the cell cycle progression of human lymphocytic leukemic MOLT-4 cells in the presence of 0.1 microM SSP. MOLT-4 cells did not arrest in G1 or G2 phase in the presence of the inhibitor. Rather, they failed to undergo cytokinesis, entering G1 phase at higher DNA ploidy (tetraploidy; G1T), and then progressed through ST (rereplication) into G2T and MT. The rates of entrance to G2 and G2T were essentially identical, indicating that the rates of cell progression through S and ST as well as through G2 and G2T, respectively, were similar. Cells entrance to mitosis and mitotic chromatin condensation were also similar at the diploid and tetraploid DNA content level and were unaffected by 0.1 microM SSP. No evidence of growth imbalance (altered protein or RNA to DNA ratio) was observed in the case of tetraploid cells. The data show that, in the case of MOLT-4 cells, all events associated with the chromosome or DNA cycle were unaffected by SSP; the only target of the inhibitor appears to be kinase(s) controlling cytokinesis.

[Ag-NORs, nucleus-nucleolus-associated antigens (Ki67, PCNA, P-105 and p-120): prognostic markers of meningioma recurrence]

The predictive significance of some nucleus-nucleolus associated markers, such as Ag-NOR, Ki67, PCNA, p-120, P-105, for the recurrency of meningiomas was investigated. A retrospective analysis was performed on a series of transitional meningiomas and of their recurrencies. Similar meningiomas but with no recidivism were used as controls. All cases were represented by women between the V and the VI decade of age. Besides, having all tumors presented with a cranial convexity localization total removal had been achieved. Recurrencies had taken place after 4.5 and 7.5 years in ten and two women, respectively. In all cases considered, the tumoral histotype did never present with structural elements in general suggestive of recurrence, such as high degree of cellularity, atypia, nuclear polymorphism, necrosis, appreciable mitotic index. On histological seriated 3 microns thick sections the silver staining technique of nucleolar organizer region-associated proteins (NORs) and the Ki67, PCNA, p-120, P-105 immunostaining ABC technique were applied. For each case 1000 tumoral cells were counted, with evaluation of the number of Ag-NOR dots and the percentage of Ki67, PCNA, p-120, P-105 positive cells. From the analysis, a mean value of Ag-NOR dots resulted of 6.44 +/- 0.65 in primitive meningiomas and of 6.53 +/- 0.88 in their respective recurrencies. In the control tumors the mean value of Ag-NOR dots resulted to be 3.53 +/- 0.55. Such difference between tumors that had repeated and controls that had not, was statistically significant (p < 0.001). For what concerns the expression of immunocytochemical relevant markers the percentage of positive cells, in primitive tumors (P), in recurrencies (R) and in the controls (C) was, namely, as follows: Ki67 (P: 14%, R: 12%, C: 6%); PCNA (P: 38%, R: 37%, C: 8%); and p-120 (P: 57%, R: 62%, C: 12%). The expression of P-105 was but occasional and without significance. From the data described, one can conclude that the evaluation of the whole of the markers considered in transitional meningiomas does predict recidivism.

Nucleolar and nuclear aberrations in human lox tumor cells following treatment with p120 antisense oligonucleotide ISIS-3466

Previous reports from this laboratory have shown marked cytocidal effects of the ISIS-3466 antisense phosphorothioate oligodeoxynucleotide to the human nucleolar protein p120 on human cancer cell lines in vitro and inhibition of tumor growth in vivo in an i.p/i.p. LOX cell model (L. Perlaky et al. Anti-Cancer Drug Design 8:3-14, 1993). In this study, light and fluorescence microscopy showed that the number of LOX cells in mitosis decreased by 50% after incubation for 4 h in 0.2-0.4 microM antisense oligonucleotide; a 70% reduction in cell number was found from 8-72 h post-treatment. In addition, marked unravelling of nucleolar structures and chromatin fragmentation was found after a 4-h incubation. The nucleolar unravelling occurred in varying degrees ranging from partial unfolding to almost complete separation of the strands of nucleolar residues. Twenty four hours post-treatment, immunofluorescence staining with the anti-p120 monoclonal antibody showed reduced nucleolar protein p120 and translocation of the p120 protein from the nucleoli to the nucleoplasm. Analysis of the mechanisms of the nucleolar unravelling and inhibition of mitosis will provide further understanding of the cytocidal effects of the ISIS-3466 antisense oligonucleotide.

The RHO4a and NUD1 genes on Saccharomyces cerevisiae chromosome XI

Sequence analysis of a 4 kb fragment from the right arm of Saccharomyces cerevisiae chromosome XI, in combination with Northern hybridization experiments revealed the presence of two genes, designated RHO4a and NUD1. The first gene encodes a 32 kDa protein showing significant sequence similarity with members of the ras family. Its 3'-terminal sequence is virtually identical to a sequence published previously as the RHO4 gene [Matsui and Toh-e, Gene 114 (1992), 43-49], which, however, appears to start at an internal ATG codon. The RHO4a sequence also overlaps the 5'-terminal sequence of the RNC1 gene [Chow et al., Nucl. Acids Res. 20 (1992), 5215-5221] proposed to encode the yeast yNucR endo/exonuclease. The remainder of this RNC1 gene overlaps with the 5'-end of the NUD1 gene. However, the RNC1 sequence lacks a portion of 276 bp that in our fragment is part of the intergenic region separating RHO4a and NUD1. From these results we conclude that the proposed RNC1 gene is the result of a cloning artefact and that the yNucR protein is instead encoded by the NUD1 gene.

Differential expression of proliferation-associated molecules in individual micrometastatic carcinoma cells

BACKGROUND

The development of monoclonal antibodies (MAbs) to cytokeratins, which are integral components of the epithelial cytoskeleton, has made possible immunocytochemical detection of epithelial tumor cells. Importantly, this technique allows the detection of epithelial tumor cells that have metastasized from primary adenocarcinomas to secondary sites such as the bone marrow.

PURPOSE

The aim of the study was not only to detect micrometastatic cells in bone marrow, but also to assess the expression of nuclear proliferation markers (Ki-67 and p120) and the erbB2 oncogene (also known as ERBB2) in these cells and, thus, hopefully improve prognostic precision.

METHODS

Bone marrow aspirates were obtained from both sides of the upper iliac crest of 532 patients having definitive diagnoses of either breast or gastrointestinal cancer. The presence of micrometastatic epithelial tumor cells in bone marrow was assayed using the MAb cytokeratin 2 (CK2) to cytokeratin component 18 (CK18), in combination with the alkaline phosphatase-anti-alkaline phosphatase immunostaining technique. After primary screening of all marrow samples with MAb CK2, representative subgroups of CK18+ samples were selected for co-labeling with MAbs either to ErbB (n = 16), ErbB2 (n = 121), Ki-67 (n = 33), or p120 (n = 36) protein. An alternative labeling protocol based on the combination of immunogold and immunoenzymatic techniques was utilized to confirm the results derived from immunoenzymatic double staining.

RESULTS

In total, single CK18-positive tumor cells were detected in 180 (33.8%) of 532 bone marrow aspirates, with few differences among patients with breast or gastrointestinal cancer in TNM stage M0 (i.e., no distant metastasis). In patients with overt metastasis (stage M1), however, the incidence of metastatic cells in marrow increased to 73.7% in breast cancer, 52.5% in gastric cancer, and 39.0% in colon cancer. Whereas expression of Ki-67 or p120 on micrometastatic cells was observed only in 11 (15.9%) of 69 cancer patients analyzed, ErbB2+/CK18+ cells were found in 48 (67.6%) of 71 breast cancer patients and 14 (28.0%) of 50 patients with gastrointestinal cancer (P = .0001). The incidence of ErbB2+/CK18+ cells was positively correlated with the clinical stage of tumor progression.

CONCLUSIONS

The high incidence of ErbB2 expression on micrometastatic breast cancer cells in the bone marrow suggests that these cells might have been positively selected during early stages of metastasis. The majority of these cells appear to be in a dormant state of cell growth.

IMPLICATIONS

Although support from clinical follow-up is still needed, this study demonstrates that, beyond the mere presence of micrometastatic cells in bone marrow, useful prognostic information can be obtained by analysis of additional cell growth markers.

Purification and characterization of a DNA-binding heterodimer of 52 and 100 kDa from HeLa cells

In studies of protein binding to the upstream region of the human proliferation-associated antigen p120 gene, a heterodimer of 52 and 100 kDa proteins was purified from HeLa cells. A 1:1 ratio of p52 and p100 was constant throughout the purification. The heterodimer was localized to cell nuclei, as shown by immunofluorescence. The pI values of the p52 and p100 were 7.8 and 8.6 respectively. The peptide sequences obtained for p52 (QSNKTFNLEKQNHTPRKKHQ and PLRGKQLRVRFAAHSASLTVR) and for p100 (PGGPKPGGGPGLSTPGGHPKPPHRGGGEPPRGRQ and GPGPGQSGPKPPIPPPPPHQQ) were not found in the computer databanks. One p52 peptide sequence, PLRGKQLRVRFA, shows considerable sequence similarity to a conserved motif in topoisomerase II of multiple species. The p52/100 heterodimer bound to different DNA probes. The binding was competed by poly(dI-dC), sonicated salmon sperm DNA, and circular or linearized plasmid DNA. The optimal DNA binding for the heterodimer was at pH 7-9 with low salt. The DNA-binding subunit of the heterodimer was the p100 polypeptide, as shown by u.v.-cross-linking assays and Southwestern blots.

The effect of antisense p120 construct on p120 expression and cell proliferation in human breast cancer MCF-7 cells

Malignant transformation of NIH3T3 cells was observed by transfection with the pSVX vector containing a sense human p120 cDNA construct (pSVX120). Subsequent transfection of these transformed cells with a dexamethasone inducible antisense p120 construct (pMSG021) markedly reduced the expression of human p120 and the growth rate of these transformed cells (Perklaky et al., Cancer Res., (1992) 52, 428-436). In the present study, a human breast cancer cell line (MCF-7) which expresses the p120 protein was transfected by electroporation with a pSVX plasmid-construct containing the antisense p120 cDNA (pSVX021). Clones containing the pSVX021 construct were selected and analyzed for expression of p120 mRNA, protein and growth characteristics. The expression of the p120 protein was inhibited by 44% in the antisense-transfected MCF-7pSVX021 cells; a 56% inhibition of cell-growth and a reduced colony formation in soft agarose were also observed. The growth of MCF-7 cells transfected with the p120 antisense construct was reduced by 93% in nude mice.

Growth inhibition of human tumor cell lines by antisense oligonucleotides designed to inhibit p120 expression

The human nucleolar antigen p120 was detected with an anti-p120 monoclonal antibody (MAbp120) in most human malignant tumors (Freeman et al., Cancer Research, 48, 1244-1251, 1988). Stable transfection of the sense p120 cDNA caused malignant transformation of NIH/3T3 cells in vitro, and the antisense p120 constructs markedly delayed the growth of these transformed cells (Perlaky et al., Cancer Research, 52, 428-436, 1992). Several p120 antisense phosphorothioate oligonucleotides designed to hybridize with different regions of the p120 sequence were screened on human tumor cell lines in vitro. Marked growth inhibition of HeLa, LOX and HRCC cell lines was found, particularly with antisense p120 oligonucleotide ISIS 3466 in combination with N-[1-(2,3-dioleyloxy)propyl]-N,N,N-trimethylammonium chloride (DOTMA); oligonucleotide ISIS 3466 is complementary to a non-translated region at the 3' end of the molecule. Preliminary in vivo studies on human LOX ascites tumor in nude mice showed marked inhibitory effects on tumor growth by the antisense oligonucleotide ISIS 3466 in the presence of DOTMA when treated on alternate days.

[In vivo and ex vivo expression of nucleolar proliferation-associated antigens (p120, B23) in the prostate]

Expression of two nucleolar antigens, p120 and B23, was studied in a prostatic carcinoma cell line (LNCaP) and in frozen and paraffin embedded tissue sections of 40 benign and malignant prostatic lesions. The percentage of p120 negative G0/G1 phase cells rose significantly during transition from exponential to plateau growth phase in vitro (from 9% to 32%). In contrast, B23 was equally expressed throughout different cell cycle and growth phases. Thus, nucleoli of almost all stromal and epithelial cells were stained by B23 in tissue sections. P120, however, selectively stained nucleoli of proliferating prostatic epithelium. Whereas 88% (13/15) of benign hyperplasia were p120 negative this was the case in only 24% (6/25) of carcinomas. Using microwave procedure both MoAbs reacted in paraffin sections, but the percentage of p120 negative cases doubled. A routine application to formalin fixed and paraffin embedded tissue cannot be recommended thus far.

Cell cycle regulated expression of nucleolar antigen P120 in normal and transformed human fibroblasts

Normal and SV40 virus-transformed WI-38 human lung fibroblasts were serum starved and refed, or synchronized by double thymidine block and released from the block. At different time points in the cell cycle, steady state levels of P120 mRNA and P120 protein content of the cells were determined by densitometric scans of Northern and Western blots. At the same time points, [3H]thymidine uptake was measured and flow cytometric analysis performed for DNA content and P120 antigen staining. Levels of P120 protein and P120 mRNA were approximately 4 times greater in non-synchronous, exponentially growing transformed cells than in similarly growing normal cells. Early G1-phase cells, synchronized either with serum deprivation or with metabolic block, contained only a trace amount of P120 protein and mRNA. The P120 gene was transcribed early in G1 and P120 protein synthesis initiated in middle G1. A dramatic increase of P120 protein level occurred in S-phase with a corresponding mRNA peak preceding the P120 protein peak. These results indicate that P120 is overexpressed in transformed WI-38 cells and that P120 is temporally regulated during the cell cycle of both transformed and normal fibroblasts. The dramatic increase in P120 protein expression at the G1 to S boundary suggests that P120 may play a role in the regulation of cell cycle and increased nucleolar activity that is associated with cell proliferation.

In-vitro recombination in rad and rnc mutants of Saccharomyces cerevisiae

Extracts of S. cerevisiae cells can catalyze homologous recombination between plasmids in vitro. Extracts prepared from rad50, rad52 or rad54 disruption mutants all have reduced recombinational activity compared to wild-type. The rad52 and rad54 extracts are more impaired in the recombination of plasmids containing double-strand breaks than of intact plasmids, whereas rad50 extracts are deficient equally for both types of substrate. The nuclease RhoNuc (previously designated yNucR), encoded by the RNC1 (previously designated NUC2) gene and regulated by the RAD52 gene, is not required for recombination when one substrate is single-stranded but is essential for the majority of recombination events when both substrates are double-stranded. Furthermore, elimination of this nuclease restores recombination in rad52 extracts to levels comparable to those in wild-type extracts.

A region of antisense RNA from human p120 cDNA with high homology to mouse p120 cDNA inhibits NIH 3T3 proliferation

The human nucleolar p120 protein is a proliferation-associated antigen which is expressed in G1 and peaks during the early S phase of the cell cycle. Overexpression of the human p120 protein caused the transformation of NIH 3T3 cells and expression of an antisense p120 construct inhibited the growth of NIH 3T3 cells (Perlaky et al., Cancer Res., 52:428-436, 1992). The middle region of the antisense p120 RNA was found to be almost as inhibitory as the full length antisense construct but the 5' and 3' antisense portions did not affect NIH 3T3 cell proliferation. After the mouse p120 complementary DNA was cloned and sequenced, comparison with the human p120 complementary DNA showed a striking conservation of 85% of the nucleotide sequence and 96% of the amino acid sequence. The two ends of the p120 molecule had less homology in their nucleotide and amino acid sequences. Based on this homology, the observed inhibitory effects of the middle portion of antisense human p120 RNA may be related to suppression of mouse p120 expression by RNA:RNA duplex formation. The high evolutionary conservation of the middle region suggests it has a critical role for the function of this protein.

Yeast RNC1 encodes a chimeric protein, RhoNUC, with a human rho motif and deoxyribonuclease activity

The yeast Saccharomyces cerevisiae contains an endoexonuclease yNucR that has been implicated in both recombination and repair. We describe the isolation and characterization of the corresponding gene. Within the predicted N-terminal half of the protein there is extensive homology (approximately 50%) with human rho genes, which are related to the ras oncogene, particularly in the proposed GTP-binding region. The C-terminal region, which is related to the Escherichia coli recC protein, presumably encodes the endoexonuclease activity. The yNucR may thus represent a new class of GTP-binding proteins. Because of the chimeric nature of the polypeptide, this protein is renamed RhoNUC (rather than the original yNucR) and the gene is RNC1 for Rho-associated-NuClease. Over expression of the gene leads to altered cell growth and nuclear morphology. We propose that the gene plays an important role in cell development as well as DNA repair/recombination.

Antisense-mediated specific inhibition of P120 protein expression prevents G1- to S-phase transition

A pentadecadeoxyribonucleotide (5'-AAAGCCCCCCACCAC), complementary to a splice junction site of mRNA for human proliferation-associated nucleolar protein P120, inhibited expression of the P120 gene and the mitogen-induced proliferation of human lymphocytes. The inhibition of P120 gene expression and proliferation was concentration dependent and reached 90% at 200 microM, as measured by [3H]thymidine uptake and by densitometric scanning of Northern (mRNA) and Western (protein) blots of P120. Inhibition was not observed in cells treated with the correspondent nonsense oligomer. P120 antisense oligomer treatment prevented S-phase entry of mitogen-stimulated lymphocytes, as determined by flow cytometric analysis, but did not block G0-G1 transition assessed by morphological blast transformation and induction of [3H]uridine incorporation. Results of this study suggest that P120 expression may be required for the upregulation of nucleolar function necessary for cell proliferation.

Cell cycle-related expression of p120 nucleolar antigen in normal human lymphocytes and in cells of HL-60 and MOLT-4 leukemic lines: effects of methotrexate, camptothecin, and teniposide

Expression of the proliferation-associated nucleolar antigen p120 was studied by flow cytometry in human quiescent and phytohemagglutinin-stimulated lymphocytes, as well as in human lymphocytic (MOLT-4) and promyelocytic (HL-60) cell lines. Bivariate analysis of p120 and DNA content made it possible to correlate p120 expression with cell position in the cycle. Proliferating lymphocytes and MOLT-4 and HL-60 cells had a similar pattern of p120 expression. Populations of G1 cells, in all three cell types, were very heterogenous with respect to p120, and a threshold in G1 was observed. The cells with a p120 level below the threshold value did not enter S phase. An increase in p120 was observed during progression through S phase, and the antigen was maximally expressed in G2 cells. The p120/DNA content ratio, however, was highest in late G1 cells (G1B) and was declining during S and G2. The data thus suggest that p120 may be degraded during mitosis and that the postmitotic cells inherit little, if any, of this protein; the antigen then accumulates predominantly during G1, and must reach a threshold level to enable the cells to enter S phase. Antigen p120 could not be detected in noncycling lymphocytes nor in HL-60 cells induced to myeloid differentiation by growth in the presence of dimethyl sulfoxide. Treatment of MOLT-4 cells with pharmacological concentrations of methotrexate, camptothecin, or teniposide induced cell arrest in S or G2; expression of p120 in the arrested cells was unchanged from that of untreated MOLT-4 controls at the same phase of the cycle. The level of p120 was minimal in MOLT-4 or HL-60 cells arrested in M phase by vinblastine, but vinblastine had no effect on p120 fluorescence of interphase cells. Camptothecin or teniposide induced apoptosis selectively in S phase of HL-60 cells; apoptotic cells from camptothecin-treated cultures, however, despite the marked nucleolysis, still expressed p120. The data on the drug-treated cells indicate that the p120 level in tumors of patients may be used as a marker of tumor/malignancy even in clinical samples obtained during treatment.

Effect of ionic strength in immunocytochemical detection of the proliferation associated nuclear antigens p120, PCNA, and the protein reacting with Ki-67 antibody

This study was aimed at revealing whether or not ionic interactions between the epitope of the antigen detected by Ki-67 antibody, or the proliferation-associated proteins PCNA or p120, and neighboring cellular constituents impede detectability of these antigens in HL-60 cells by indirect immunofluorescence assay. To this end, the ionic strength (NaCl concentration) of the solutions in which cells were suspended during their fixation with 0.5% paraformaldehyde was increased, to up to 1.65 M NaCl, to weaken the intra- and/or intermolecular ionic interactions during the process of crosslinking, and the cells were then immunostained. Fluorescence of cells reacting with Ki-67 antibody was maximally increased after their treatment with 1.15 M NaCl; the average increase was nearly 110% above the level seen with the standard methodology utilizing 0.15 M NaCl. The increase was greater for cells in the G1 phase of the cell cycle compared to cells in S or G2. Fluorescence of cells stained with the PCNA antibody was maximally enhanced after cell treatment with 0.65 M NaCl. The enhancement, however, varied depending on the source of the antibody; it was nearly 200% in the case of the antibody provided by Boehringer and over 100% by DAKO. Detection of the nucleolar antigen p120 was not significantly affected by 0.65-1.65 M NaCl. The data indicate that ionic interactions between cellular constituents indeed play a role in masking the epitope of PCNA and the antigen detected by Ki-67.(ABSTRACT TRUNCATED AT 250 WORDS)

Sp1 is essential and its position is important for p120 gene transcription: a 35 bp juxtaposed positive regulatory element enhances transcription 2.5 fold

Human proliferating cell nucleolar antigen p120 is expressed in tumor cells in the early G1 phase of the cell cycle. Deletion analyses of the essential cis-acting region -537/-278 showed that a 58 bp sequence from -457 to -400 is an important cis-acting element. An Sp1 transcription factor binds to the sequence AGAGGCGGGG (-425 to -416) within the -458/-400 cis-acting region. Deletion of the Sp1 binding sequence eliminated transcription. Substitution of the Sp1 box(-437/-406), containing the Sp1 recognition site, for the entire cis-acting region (-537/-278) restored transcription only at a very low level (18%). Deletion of the -537/-278 cis-acting region followed by substitutions showed that the Sp1 box (-437/-406) stimulated transcription 2.4 fold, when juxtaposed and downstream of a 35 bp (-472 GGGCGAGCGTAAGTTCCGGGTGCGGCGGCCGACTA -438) positive regulatory cis-element (PRE) over that by substitution of the Sp1 box alone. When the -406/-278 sequence was downstream of the PRE-Sp1 box, transcription was stimulated 4.4 fold over that produced by substitution of the Sp1 box alone. These results suggest that Sp1 is essential and its proper position in the 5' flanking sequence, juxtaposed and down stream of a 35 bp positive regulatory sequence, is required for efficient transcription.

DNA footprint enhancement using tandem binding sites

A concatenated DNA fragment containing a five-repeat binding site was used for DNase I footprinting. Under the same conditions, the tandem repeat assay greatly enhanced the DNA footprint as compared with a native DNA sequence with only one binding site. This technique provides an approach for improving poor DNA footprints.

Purification of a group of HeLa nuclear proteins that bind to a regulatory element (-1430/-1327) of the human proliferating cell nucleolar protein P120 gene

A group of proteins was purified from HeLa nuclear extract by DNA affinity chromatography which bind to an important regulatory element (-1430/-1327) of the P120 gene. The DNA binding activity was enriched 1075 fold. By silver staining three major polypeptides (50, 40, 37 kDa) were detected in the purified fraction. The band shift assay and the southwestern assay showed that the 50 kDa protein (P50) binds to the F1 (-1430/-1327) DNA fragment. The binding specificity of the group of proteins with F1 DNA in the presence of non-specific competitor DNA is much higher than that of P50 alone. On the basis of molecular weight and specific antibody binding, the 37 kDa protein appears to be the B23 nucleolar protein.

Nucleolar protein P120 and its targeting for cancer chemotherapy

Identification of the G1-P120 antigen with the aid of the monoclonal antibody to its "human-specific epitope" has resulted in rapid development of information on its molecular biology. With the monoclonal antibody, it rapidly became possible to identify and subsequently sequence its cDNA and with cDNA clones to isolate and sequence its genomic DNA. It was demonstrated that the protein had 4 major domains: a basic domain, an acidic domain, a hydrophobic and methionine-rich domain and a domain rich in cysteine and proline residues. In addition to a nuclear recognition signal, the epitope region is juxtaposed to phosphorylation sites. The epitope region contains the sequence Gln-Ala-Ala-Ala-Gly-Ile-Asn-Trp which is unique to the human P120 molecule; this may be a site for drug attack either by analogs to the region or by novel constructs based on antisense oligonucleotides. When tumor cells were transfected with antisense constructs of the P120 gene, growth rates were markedly reduced. 3T3 cells transformed by transfection with the P120 gene reverted to a nontransformed state by subsequent transfection and activation of a P120 antisense construct. Opportunities for control of malignant cells with antisense oligonucleotides are currently under study.

Prognostic significance of proliferation associated nucleolar antigen P120 in human breast carcinoma

Nucleolar antigen P120 is detected in rapidly proliferating cells but not in normal resting cells or in many benign and slowly growing malignant tumors. The objective of the study was to determine whether the expression of P120 in breast cancer correlated with histopathological or biological properties associated with prognosis. In this retrospective study, 120 primary breast tumors were analyzed for P120; 114 of these tumors were also stained for the erbB-2 protein. Immunopositive staining was correlated with patient survival, nodal status, estrogen receptor levels, and number of mitoses. Sixty-nine % (83 of 120) of the tumors were positive for P120; 25% (28 of 114) stained positively for erbB-2. Of the 28 erbB-2 positive tumors 26 were also positive for the P120 protein. Forty-six % (55 of 120) of the specimens were from patients who later died from recurrent breast cancer; P120 was detected in 89% (49 of 55) of these specimens. In 52% of the survivors the P120 protein was also expressed. P120 negative tumors were highly correlative with survival (P = 0.0001); 84% (32 of 37) of patients with P120 negative tumors survived more than 7 years without evidence of recurrent disease. Multivariate analysis showed that the worst prognosis was for patients who had tumor positive nodes and expressed P120 (P = 0.0001); death occurred in 73% (30 of 41) of these patients. For the node negative patients who did not express P120, 5-year survival was 90% (19 of 21 patients); 5-year survival for the node negative patients who expressed P120 was significantly less (67%; 28 of 42 patients). Patients with P120 negative tumors had a good prognosis, irrespective of their nodal status. In this group, survival of node negative patients was 86% (18 of 21) and for those with positive nodes survival was 82% (13 of 16). A poor prognosis was found for patients with intense erbB-2 stained tumors (5 of 7 patients died). Weak staining of erbB-2 tumors (21 specimens) was not correlated with patient survival. Compared to P120 negative tumors, P120 positive tumors had greater numbers of mitoses (9.06 versus 6.65) and an almost 2-fold increase in the occurrence of positive nodes (one of every 4.67 versus one of every 8.81). The number of P120 positive tumors was greater in estrogen receptor positive tumors (75%) than in estrogen negative tumors (54%).(ABSTRACT TRUNCATED AT 250 WORDS)

Monoclonal antibodies to human tumor nucleolar antigens: probes for studying biological function and determining clinical significance

Using novel strategies, monoclonal antibodies (MAbs) were developed to two proliferation-associated nucleolar antigens. These two new antigens, termed P145 and P120, according to their molecular weights, were found in a broad range of tumor tissues. Overall, P120 was more restricted in its distribution than P145 because it was absent from many benign tumors and some malignant tumors. Using the MAbs as probes, antigens P145 and P120 were localized to separate nucleolar components. P145 was mainly associated with high molecular weight nucleolar RNPs. Antigen P120 was localized to a novel beaded microfibril found in a nucleolar residue fraction. DNAse and high salt treatment of this fraction provided optimal extraction of the P120 antigen. P145 and P120 differed from other proliferation-associated nuclear/nucleolar antigens identified by autoimmune sera and by other MAbs. In PHA stimulated lymphocytes, both antigens were expressed in early G1 prior to or concurrent with increased RNA Pol I transcription. Microinjection of tumor cells with the P120 MAb inhibited cell proliferation and blocked the appearance of nucleolar pleomorphism that is characteristic of tumor cells. Clinical studies showed that P145 was not detected in normal bone marrow but was expressed in leukemic marrows (AML); the percentage of P145 positive cells correlated with percentage of malignant blasts (Raza et al., Am. Soc. Hematology, 1990). Antigen P145 was detected in 10 of 39 marrows from patients in complete remission. This result suggested that these cells have a higher proliferative potential than normal stem cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of the human cell proliferation-associated nucleolar protein p120

The human cell proliferation-associated nucleolar protein p120 was found in a variety of human cancer specimens but not in most normal resting cells. Polyclonal antibodies raised against bacterially expressed p120 were used to immunoprecipitate the p120 protein isolated from 32P-labeled HeLa cells. The p120 protein was phosphorylated at serine, threonine and tyrosine residues. A tryptic peptide map showed it contained three labeled peptides. One of these peptides comigrated with a p120 peptide phosphorylated in vitro by casein kinase II. This peptide was phosphorylated in vitro both at Ser-181 and Thr-185. This region is juxtaposed to the epitope site recognized by the anti-p120 monoclonal antibody.

Expression of human proliferation-associated nucleolar antigen p120

The amount of human nucleolar protein p120 is small or nondetectable in most normal resting cells but increases greatly in some actively proliferating cells. The time course was compared for accumulation of mRNAs for p120, c-myc, and histones in phytohemagglutinin-stimulated peripheral blood mononuclear cells. The mRNAs for p120 and c-myc were undetectable in unstimulated peripheral blood mononuclear cells; phytohemagglutinin stimulation increased the p120 mRNA level within 2 h, and it reached a maximum within 24-48 h. The c-myc mRNA level increased within 4 h and had a biphasic maximum at 4 h and 24 h. Indirect immunofluorescence staining and Western blot analysis showed an increase in the level of p120 protein in the nucleolus following the increase in its mRNA level, which suggests that p120 expression is transcriptionally controlled. Both p120 mRNA and c-myc mRNA levels were significantly decreased in 12-O-tetradecanoyl-phorbol-13-acetate-differentiated HL-60 leukemic cells and in confluent normal immortalized human fibroblasts (WSI). These data indicate that p120 is expressed very early in the G1 phase of the cell cycle and suggest that it has an important function in the cell cycle.

Genomic structure of the human proliferating cell nucleolar protein P120

A gene for human proliferating cell nucleolar protein p120 has been isolated from a human genomic library using p120 cDNA as a probe. The gene spanned 12 kilobase pairs and was composed of 15 exons and 14 introns. Unusual splice junction sequences, (AT) and (AC), instead of (GT) and (AG), respectively, were located at the splice sites for intron F. The 5' flanking region was analyzed for putative cis-acting factor binding sites. The region contained a TATA-like sequence and the CCAAT box. CAT assays indicated that the region -2532 /+ 102 was necessary for transcription. Two specific areas at -1444/-1224 and -537/-278 were shown to be important for transcription. Two "GC" boxes and two "GC"-rich sequences were observed Other sites upstream of these sites were analyzed for homology to other gene control regions.

mRNA levels for human nucleolar protein P120 in tumor and nontumor cells

A monoclonal antibody to a human tumor nucleolar 120 kD protein was developed by Freeman et al. (Cancer Res. 48: 1244-1251, 1988). Its complementary DNA (cDNA) has been isolated and sequenced (Fonagy et al., submitted). To determine the relative messenger RNA (mRNA) level for protein p120, cellular mRNA was extracted, slot-blotted onto nitrocellulose filters, and hybridized to radioactive p120 cDNA fragments. Human tumor cells contained 15-60 times more p120 mRNA than human term placenta. The rat Novikoff hepatoma ascites cell mRNA hybridized to the p120 cDNA probes, but the p120 monoclonal antibody did not react with the Novikoff hepatoma proteins. Novikoff hepatoma mRNA contained 8 times as much p120 mRNA as normal rat liver. As a control, a cDNA was used for protein B23, an abundant nucleolar protein; there were 3.5, 29, and 14 times more B23 mRNA than p120 mRNA in normal rat liver, Novikoff hepatoma ascites cells, and HeLa cells, respectively. Whereas the increased levels of the mRNA and protein B23 reflect increased activity of the nucleolus for any increment of nucleolar function, the increased levels of p120 mRNA and the p120 protein reflect the activity of the G1 phase of the cell cycle. The elevated level of p120 mRNA in tumors may reflect the heightened G1 cascade in transformed cells.

Inflammation-associated events in liver nuclei during acute-phase reaction

Isolated liver nucleoli from rats undergoing turpentine-induced inflammation (acute-phase reaction) synthesize rRNA at a rate significantly higher than normal. This increase is associated with, and possibly preceded by, an enhanced methylation of RNA, which further increases when rRNA synthesis has reached a plateau level. Five hours after turpentine treatment, before clear activation of RNA synthesis and methylations, the nucleocytoplasmic transport of rRNA (largely 40S and 60S subunits) and the related ATPase activity of isolated nuclei are significantly increased. Apparently, posttranscriptional control is affected before transcription of rRNA during the onset of the acute-phase reaction: both kinds of events eventually contribute to the expansion of the ribosome population which occurs in the liver cells from rats undergoing an inflammatory process. All these processes are activated before the liver starts the synthesis of acute-phase proteins.

Precise localization of m6A in Rous sarcoma virus RNA reveals clustering of methylation sites: implications for RNA processing

N6-methyladenosine (m6A) residues are present as internal base modifications in most higher eucaryotic mRNAs; however, the biological function of this modification is not known. We describe a method for localizing and quantitating m6A within a large RNA molecule, the genomic RNA of Rous sarcoma virus. Specific fragments of 32P-labeled Rous sarcoma virus RNA were isolated by hybridization with complementary DNA restriction fragments spanning nucleotides 6185 to 8050. RNA was digested with RNase and finger-printed, and individual oligonucleotides were analyzed for the presence of m6A by paper electrophoresis and thin-layer chromatography. With this technique, seven sites of methylation in this region of the Rous sarcoma virus genome were localized at nucleotides 6394, 6447, 6507, 6718, 7414, 7424, and 8014. Further, m6A was observed at two additional sites whose nucleotide assignments remain ambiguous. A clustering of two or more m6A residues was seen at three positions within the RNA analyzed. Modification at certain sites was found to be heterogeneous, in that different molecules of RNA appeared to be methylated differently. Previous studies have determined that methylation occurs only in the sequences Gm6AC and Am6AC. We observed a high frequency of methylation at PuGm6ACU sequences. The possible involvement of m6A in RNA splicing events is discussed.

Recognition of the ribosomal RNA structures by purified nucleolar RNA methyltransferase

Previously we have isolated the specific RNA methyltransferase from the nucleoli of Ehrlich ascites tumor cells. The purified enzyme was found to be specific for methylation of C5 position of cytosine residue in ribosomal RNA in vitro (Obara, 1982b). In the present study, we have investigated the recognition mechanisms of RNA structure by this enzyme from the points of view of both primary and secondary structures. Analysis of in vitro methylation product by ribonuclease T1 digestion indicated the methylation-site(s) was limited to a certain number of nonanucleotide. The next experiments with either Sl nuclease or actinomycin D and ethidium bromide suggested that the enzyme modified only cytidine residue in or located close to the double stranded part of RNA. On the other hand, the characterization of analogues of cytidine residue in the RNA at molecular level showed that the methylation of rRNA was inhibited by either cytidine, CDP or CTP, but little inhibition was observed in the presence of cytosine, 5-methylcytidine and CMP.

Methylation patterns of tRNA at different concentrations of dimethylsulphate

The methylation patterns produced in E. coli B tRNA by various concentrations of dimethylsulphate were found to differ with a predominant formation of 7-methylguanine and 1-methyladenine at low concentrations and of a methylated compound not yet identified at high concentrations of methylating agent. The analysis by Scatchard plot of dimethylsulphate interaction with the nucleic acid suggested the presence of high and low affinity sites.

[Participation of 5'-deoxyadenosyl-B12 in regulating methylation of tRNA]

The effects of different forms of cobalamines on the activities of tRNA-methylases of Zajdela ascite hepatoma were studied. Of six cobamides studied 5'-deoxyadenosyl-B12 and factor B containing as a ligand HSO3 in the concentrations of 2.4-10(-5) and 4.8-10(-5) M inhibited the tRNA-methylase activity by 21% and 15% correspondingly. The inhibitory effect of 5'-deoxyadenosyl-B12 is probably dependent on the adenosyl part of the molecule. 5'deoxyadenosyl-B12 exerted a selective effect of Zajdela ascite hepatoma tRNA-methylases, inhibiting largely the activity of 5-methyl cytosine methylase during the methylation of the E. coli K12W6 tRNA and yeast tRNA1 Val.

Effects of estradiol on uterine ribonucleic acid metabolism. Assessment of transfer ribonucleic acid methylation

Immature rats treated with estradiol for selected periods of time demonstrated both increased methylation of uterine transfer ribonucleic acid (tRNA) and methylase activities. Whereas the former parameter was assessed by incubating whole uteri with [methyl-14C]methionine and measuring the incorporation of isotope into the tRNA, methylase activity was obtained by measuring the rate of incorporation of methyl groups from S-adenosyl[methyl-14C]methionine into heterologous tRNA (Escherichia coli B) in the presence of uterine cytosol preparations (100,000g supernatants). Although increased methylation of tRNA during the estrogen response was demonstrated, additional studies indicated that these results were largely attributable to an increased rate of synthesis of tRNA rather than gross changes in either the type or amount of methylated constituents present. Evidence in this regard included the inability of estrogen treatment of alter significantly the (a) resulting patterns of methyl-14C-methylated constituents of uterine tRNA, (b) the extent ot which [2-14C]guanine residues, incorporated into tRNA, become methylated, (c) the extent of methylation of precursor tRNA in the absence of tRNA synthesis, and (d) the types of methylase activities expressed in vitro.

NUCLEOLUS: A CENTER OF RNA METHYLATION

The ubiquitously distributed complex of enzymes, the RNA methylases, the apparent function of which is the alteration of the structure of transfer RNA at the macromolecular level by the introduction of methyl groups into the component bases, are concentrated in the nucleolus, an organelle previously implicated in RNA synthesis.