Increased expression of a nucleolar Nop5/Sik family member in metastatic melanoma cells: evidence for its role in nucleolar sizing and function

Ribosomes: An Exciting Avenue in Stem Cell Research

Stem cell research has focused on genomic studies. However, recent evidence has indicated the involvement of epigenetic regulation in determining the fate of stem cells. Ribosomes play a crucial role in epigenetic regulation, and thus, we focused on the role of ribosomes in stem cells. Majority of living organisms possess ribosomes that are involved in the translation of mRNA into proteins and promote cellular proliferation and differentiation. Ribosomes are stable molecular machines that play a role with changes in the levels of RNA during translation. Recent research suggests that specific ribosomes actively regulate gene expression in multiple cell types, such as stem cells. Stem cells have the potential for self-renewal and differentiation into multiple lineages and, thus, require high efficiency of translation. Ribosomes induce cellular transdifferentiation and reprogramming, and disrupted ribosome synthesis affects translation efficiency, thereby hindering stem cell function leading to cell death and differentiation. Stem cell function is regulated by ribosome-mediated control of stem cell-specific gene expression. In this review, we have presented a detailed discourse on the characteristics of ribosomes in stem cells. Understanding ribosome biology in stem cells will provide insights into the regulation of stem cell function and cellular reprogramming.

Epitranscriptomics of cancer

The functional impact of modifications of cellular RNAs, including mRNAs, miRNAs and lncRNAs, is a field of intense study. The role of such modifications in cancer has started to be elucidated. Diverse and sometimes opposite effects of RNA modifications have been reported. Some RNA modifications promote, while others decrease the growth and invasiveness of cancer. The present manuscript reviews the current knowledge on the potential impacts of N6-Methyladenosine, Pseudouridine, Inosine, 2’O-methylation or methylcytidine in cancer’s RNA. It also highlights the remaining questions and provides hints on research avenues and potential therapeutic applications, whereby modulating dynamic RNA modifications may be a new method to treat cancer.

The human box C/D snoRNAs U3 and U8 are required for pre-rRNA processing and tumorigenesis

Small nucleolar RNAs (snoRNAs) are emerging as a novel class of proto-oncogenes and tumor suppressors; their involvement in tumorigenesis remains unclear. The box C/D snoRNAs U3 and U8 are upregulated in breast cancers. Here we characterize the function of human U3 and U8 in ribosome biogenesis, nucleolar structure, and tumorigenesis. We show in breast (MCF-7) and lung (H1944) cancer cells that U3 and U8 are required for pre-rRNA processing reactions leading, respectively, to synthesis of the small and large ribosomal subunits. U3 or U8 depletion triggers a remarkably potent p53-dependent anti-tumor stress response involving the ribosomal proteins uL5 (RPL11) and uL18 (RPL5). Interestingly, the nucleolar structure is more sensitive to perturbations in lung cancer than in breast cancer cells. We reveal in a mouse xenograft model that the tumorigenic potential of cancer cells is reduced in the case of U3 suppression and totally abolished upon U8 depletion. Tumors derived from U3-knockdown cells displayed markedly lower metabolic volume and activity than tumors derived from aggressive control cancer cells. Unexpectedly, metabolic tracer uptake by U3-suppressed tumors appeared more heterogeneous, indicating distinctive tumor growth properties that may reflect non-conventional regulatory functions of U3 (or fragments derived from it) in mRNA metabolism.

Modest Static Pressure Suppresses Columnar Epithelial Cell Growth in Association with Cell Shape and Cytoskeletal Modifications

Intraluminal pressure elevation can cause degenerative disorders, such as ileus and hydronephrosis, and the threshold is fairly low and constant, 20–30 cm H₂O. We previously devised a novel two-chamber culture system subjecting cells cultured on a semipermeable membrane to increased culture medium height (water pressure up to 60 cm H₂O). Here, we sought to determine how a continuous pressure load of ~30 cm H₂O affects proliferating epithelial cells with special interest in the link with cell morphology. We cultured several different cell lines using the low static pressure-loadable two-chamber system, and examined cell growth, cell cycle, and cell morphology. Madin–Darby canine kidney (MDCK) columnar epithelial cells were growth-suppressed in a manner dependent on static water pressure ranging from 2 to 50 cm H₂O, without cell cycle arrest at any specific phase. Two other types of columnar epithelial cells exhibited similar phenotypes. By contrast, spherical epithelial and mesenchymal cells were not growth-suppressed, even at 50 cm H₂O. Phalloidin staining revealed that 50 cm H₂O pressure load vertically flattened and laterally widened columnar epithelial cells and made actin fiber distribution sparse, without affecting total phalloidin intensity per cell. When the mucosal protectant irsogladine maleate (100 nM) was added to 50-cm-high culture medium, MDCK cells were reduced in volume and their doubling time shortened. Cell proliferation and morphology are known to be regulated by the Hippo signaling pathway. A pressure load of 50 cm H₂O enhanced serine-127 phosphorylation and cytoplasmic retention of YAP, the major constituent of this pathway, suggesting that Hippo pathway was involved in the pressure-induced cell growth suppression. RNA sequencing of MDCK cells showed that a 50 cm H₂O pressure load upregulated keratin 14, an intermediate filament, 12-fold. This upregulation was confirmed at the protein level by immunofluorescence, suggesting a role in cytoskeletal reinforcement. These results provide evidence that cell morphology and the cytoskeleton are closely linked to cell growth. Pathological intraluminal pressure elevation may cause mucosal degeneration by acting directly on this linkage and the Hippo pathway.

Tuning the ribosome: The influence of rRNA modification on eukaryotic ribosome biogenesis and function

rRNAs are extensively modified during their transcription and subsequent maturation in the nucleolus, nucleus and cytoplasm. RNA modifications, which are installed either by snoRNA-guided or by stand-alone enzymes, generally stabilize the structure of the ribosome. However, they also cluster at functionally important sites of the ribosome, such as the peptidyltransferase center and the decoding site, where they facilitate efficient and accurate protein synthesis. The recent identification of sites of substoichiometric 2'-O-methylation and pseudouridylation has overturned the notion that all rRNA modifications are constitutively present on ribosomes, highlighting nucleotide modifications as an important source of ribosomal heterogeneity. While the mechanisms regulating partial modification and the functions of specialized ribosomes are largely unknown, changes in the rRNA modification pattern have been observed in response to environmental changes, during development, and in disease. This suggests that rRNA modifications may contribute to the translational control of gene expression.

Biosynthesis of ribosomal RNA in nucleoli regulates pluripotency and differentiation ability of pluripotent stem cells

Pluripotent stem cells have been shown to have unique nuclear properties, for example, hyperdynamic chromatin and large, condensed nucleoli. However, the contribution of the latter unique nucleolar character to pluripotency has not been well understood. Here, we show that fibrillarin (FBL), a critical methyltransferase for ribosomal RNA (rRNA) processing in nucleoli, is one of the proteins highly expressed in pluripotent embryonic stem (ES) cells. Stable expression of FBL in ES cells prolonged the pluripotent state of mouse ES cells cultured in the absence of leukemia inhibitory factor (LIF). Analyses using deletion mutants and a point mutant revealed that the methyltransferase activity of FBL regulates stem cell pluripotency. Knockdown of this gene led to significant delays in rRNA processing, growth inhibition, and apoptosis in mouse ES cells. Interestingly, both partial knockdown of FBL and treatment with actinomycin D, an inhibitor of rRNA synthesis, induced the expression of differentiation markers in the presence of LIF and promoted stem cell differentiation into neuronal lineages. Moreover, we identified p53 signaling as the regulatory pathway for pluripotency and differentiation of ES cells. These results suggest that proper activity of rRNA production in nucleoli is a novel factor for the regulation of pluripotency and differentiation ability of ES cells.

Small nucleolar RNAs in cancer

Non-coding RNAs (ncRNAs) are important regulatory molecules involved in various physiological and cellular processes. Alterations of ncRNAs, particularly microRNAs, play crucial roles in tumorigenesis. Accumulating evidence indicates that small nucleolar RNAs (snoRNAs), another large class of small ncRNAs, are gaining prominence and more actively involved in carcinogenesis than previously thought. Some snoRNAs exhibit differential expression patterns in a variety of human cancers and demonstrate capability to affect cell transformation, tumorigenesis, and metastasis. We are beginning to comprehend the functional repercussions of snoRNAs in the development and progression of malignancy. In this review, we will describe current studies that have shed new light on the functions of snoRNAs in carcinogenesis and the potential applications for cancer diagnosis and therapy.

B-chromosome ribosomal DNA is functional in the grasshopper Eyprepocnemis plorans

B-chromosomes are frequently argued to be genetically inert elements, but activity for some particular genes has been reported, especially for ribosomal RNA (rRNA) genes whose expression can easily be detected at the cytological level by the visualization of their phenotypic expression, i.e., the nucleolus. The B(24) chromosome in the grasshopper Eyprepocnemis plorans frequently shows a nucleolus attached to it during meiotic prophase I. Here we show the presence of rRNA transcripts that unequivocally came from the B(24) chromosome. To detect these transcripts, we designed primers specifically anchoring at the ITS-2 region, so that the reverse primer was complementary to the B chromosome DNA sequence including a differential adenine insertion being absent in the ITS2 of A chromosomes. PCR analysis carried out on genomic DNA showed amplification in B-carrying males but not in B-lacking ones. PCR analyses performed on complementary DNA showed amplification in about half of B-carrying males. Joint cytological and molecular analysis performed on 34 B-carrying males showed a close correspondence between the presence of B-specific transcripts and of nucleoli attached to the B chromosome. In addition, the molecular analysis revealed activity of the B chromosome rDNA in 10 out of the 13 B-carrying females analysed. Our results suggest that the nucleoli attached to B chromosomes are actively formed by expression of the rDNA carried by them, and not by recruitment of nucleolar materials formed in A chromosome nucleolar organizing regions. Therefore, B-chromosome rDNA in E. plorans is functional since it is actively transcribed to form the nucleolus attached to the B chromosome. This demonstrates that some heterochromatic B chromosomes can harbour functional genes.

Morphological Changes of Mammalian Nucleoli during Spermatogenesis and Their Possible Role in the Chromatoid Body Assembling

Chromatoid body (CB) is a typical cytoplasmic organelle of germ cells, and it seems to be involved in RNA/protein accumulation for later germ-cell differentiation. Despite most of the events in mammals spermatogenesis had been widely described in the past decades and the increase in the studies related to the CB molecular composition and physiology, the origins and functions of this important structure of male germ cells are still unclear. The aims of this study were to describe the nucleolar cycle and also to find some relationship between the nucleolar organization and the CB assembling during the spermatogenesis in mammals. Cytochemical and cytogenetics analysis showed nucleolar fragmentation in post-pachytene spermatocytes and nucleolar reorganization in post-meiotic spermatids. Significant difference in the number and in the size of nucleoli between spermatogonia and round spermatids, as well as differences in the nucleolar position within the nucleus were also observed. Ultrastructural analysis showed the CB assembling in the cytoplasm of primary spermatocytes and the nucleolar fragmentation occurring at the same time. In conclusion our results suggest that the CB may play important roles during the spermatogenesis process in mammals and that its origin may be related to the nucleolar cycle during the meiotic cell cycle.

Nucleolar Sik-similar protein (Sik-SP) is required for the maintenance of uterine estrogen signaling mechanism via ERα

Sik-similar protein (Sik-SP), a small nucleolar ribonucleoprotein, has been shown to be primarily involved in ribosome biogenesis. However, its role in the hormone-directed nuclear receptor signaling is largely unknown. Here, we provide novel evidence that Sik-SP is required for appropriate regulation of estrogen receptor (ER)α-mediated estradiol-17β (E2)-dependent uterine physiologic responses in mice. Studies by Western blotting using the newly developed antibodies for Sik-SP showed that this protein is up-regulated in both the ovariectomized wild-type and ERα null uteri by E2. Immunohistochemical analyses in uterine sections showed that this protein is induced in the epithelial and stromal cells. Coimmunoprecipitation studies revealed that E2 directs molecular interaction between Sik-SP and ERα. Furthermore, gel-mobility shift and chromatin immunoprecipitation analyses provided evidence that Sik-SP is recruited with ERα to estrogen-responsive uterine gene promoters. Overexpression of Sik-SP in vitro demonstrated a role for Sik-SP in cellular growth and viability. In a primary uterine epithelial-stromal coculture system, E2 exhibited early induction of Sik-SP in both the epithelial and stromal cells. Interestingly, suppression of Sik-SP in this coculture model, for the stromal but not epithelial cells, caused perturbation of E2-dependent proliferation in the epithelial cell layer. Similarly, in vivo uterine suppression of Sik-SP also caused inhibition of epithelial cell proliferation and aberrant prolongation of water imbibition in the late phase by E2. Finally, studies showed that Sik-SP is physiologically important during the onset of implantation by E2. In conclusion, Sik-SP, an early E2-responsive nucleolar protein, is necessary to induce E2-dependent ERα-mediated appropriate physiologic responses in the uterus.

Nucleolar cycle and chromatoid body formation: is there a relationship between these two processes during spermatogenesis of Dendropsophus minutus (Amphibia, Anura)?

The goals of this study were to monitor the nucleolar material distribution during Dendropsophus minutus spermatogenesis using cytological and cytochemical techniques and ultrastructural analysis, as well as to compare the nucleolar material distribution to the formation of the chromatoid body (CB) in the germ epithelium of this amphibian species. Nucleolar fragmentation occurred during the pachytene of prophase I and nucleolus reorganization occurred in the early spermatid nucleus. The area of the spermatogonia nucleolus was significantly larger than that of the earlier spermatid nucleolus. Ultrastructural analysis showed an accumulation of nuages in the spermatogonia cytoplasm, which form the CB before nucleolar fragmentation. The CB was observed in association with mitochondrial clusters in the cytoplasm of primary spermatocytes, as well as in those of earlier spermatids. In conclusion, the nucleolus seems to be related to CB formation during spermatogenesis of D. minutus, because, at the moment of nucleolus fragmentation in the primary spermatocytes, the CB area reaches a considerable size and is able to execute its important functions during spermatogenesis. The reorganized nucleolus of the earlier spermatids has a smaller area due to several factors, among them the probable migration of nucleolar fragments from the nucleus to the cytoplasm, and plays a part in the CB chemical composition.

Quantitative analysis of NOR expression in a B chromosome of the grasshopper Eyprepocnemis plorans

The B24 chromosome in the Torrox population of the grasshopper Eyprepocnemis plorans is recurrently attached to a nucleolus in diplotene cells, indicating the activity of its distally located ribosomal DNA (rDNA). The frequency of males expressing the B chromosome nucleolus organizer region (B-NOR) almost doubled in 4 years. The likelihood of expressing the B-NOR increased with the B number and, in males expressing it, about 20% of their cells showed a nucleolus attached to the B. When active, the B-NOR contributed more than 25% of total cell nucleolar area (NA). Within males expressing the B-NOR, total cell NA did not differ between cells showing the active or inactive B-NOR, suggesting that total cell NA is tightly regulated in this species. However, this parameter tended to increase in this population from 1999 to 2004, in parallel to the neutralization process which is taking place in this population. Finally, an analysis of A chromosome NOR interdependence for activity revealed a positive correlation among autosomes but a negative correlation between autosomes and the X chromosome, the manifestation of which depends on B-NOR activity. These results are discussed in the context of the nucleolus as a sensor of the stress caused by parasitic B chromosomes.

Expression of cell adhesion molecule 1 in malignant pleural mesothelioma as a cause of efficient adhesion and growth on mesothelium

Cell adhesion molecule 1 (CADM1), formerly referred to as SgIGSF, TSLC1, or Necl-2, has been characterized as a mast-cell adhesion molecule that mediates efficient interactions with mesothelial cells. Here, we examined whether CADM1 might be involved in the diffuse tumor growth over the pleural surface that characterizes malignant pleural mesothelioma (MPM). Immunohistochemical and western blot analyses revealed that 14 (25%) of 57 MPMs expressed the full-length form of CADM1 on the cell membrane, but non-neoplastic mesothelial cells did not express it at all. The majority of available MPM cell lines also expressed the full-length form of CADM1. We compared CADM1-positive and -negative MPM cells in culture within soft agar and in coculture on mesothelial or fibroblastic monolayers. Within soft agar, CADM1-negative MPM cells were capable of forming colonies, whereas CADM1-positive cells were not, suggesting that CADM1 is a potential tumor suppressor of MPM, consistent with the past characterization of this molecule in other types of tumors. However, in coculture on mesothelial cell monolayers lacking full-length CADM1, CADM1-positive MPM cells spread more widely and grew more quickly, whereas the CADM1-negative cells piled up. Transfection of the CADM1-negative cells with CADM1 cDNA caused them to behave like the CADM1-positive cells, with faster, more widespread growth. These phenotypic differences were not detectable in cocultures on lung fibroblastic monolayers, in which all MPM cells grew much more slowly than on mesothelial cells, irrespective of CADM1 positivity. CADM1 thus appears to mediate efficient adhesion and growth of MPM cells specifically on mesothelial cells, probably via trans-heterophilic binding, and thus may be involved in the manifestation of a considerable subset of MPMs as diffusely growing tumors disseminated over the pleural surface.

Nucleolus size variation during meiosis and NOR activity of a B chromosome in the grasshopper Eyprepocnemis plorans

The number of nucleoli and nucleolar area were measured in meiotic cells from males of the grasshopper Eyprepocnemis plorans collected in three natural populations. Number of nucleoli per cell showed no significant correlation among cells in different meiotic stages, but there was strong positive correlation for nucleolar area between leptotene and interkinesis cells in individuals from distant populations (Salobreña in Spain, and Smir in Morocco). No correlation was, however, observed for both parameters between the meiotic stages analysed in individuals from the population of Torrox (Spain). The number of nucleoli at leptotene was about double the number at interkinesis, as expected from the double ploidy level at leptotene and the corresponding double number of rDNA clusters. Leptotene nucleolar area, however, was about fourfold that in interkinesis, presumably due to higher requirements for ribosome biogenesis in meiosis I than meiosis II. In Torrox, diplotene cells showed a lower number of nucleoli but larger nucleolar area than in leptotene cells, suggesting an increase in nucleolus size during prophase I. Significant differences were found among populations for nucleolar area but not for number of nucleoli, the smallest nucleolar area being observed in Torrox, which is the population harbouring the most parasitic B chromosome variant. No clear effects on nucleolar area or number of nucleoli were associated with the B-chromosome number. However, B-chromosome effects on the nucleolar area were apparent in the Torrox population when data were analysed with respect to a B-chromosome odd-even pattern in leptotene and interkinesis cells. However, in diplotene cells no odd-even pattern was observed for both nucleolar parameters, suggesting that the increase in nucleolar size from leptotene to diplotene dilutes the leptotene odd-even pattern. The rDNA distally located in the B chromosome was associated with a nucleolus in 6.5% out of the 247 diplotene cells analysed. The implications of these findings are discussed in the context of B chromosomes as stress-causing genome parasites and the nucleolus as a sensor of stress.

Camellia oil and its distillate fractions effectively inhibit the spontaneous metastasis of mouse melanoma BL6 cells

We previously reported that daily intraperitoneal injections of oleamide weakly inhibits the spontaneous metastasis of BL6 cells by blocking the gap junction-mediated intercellular communications (GJIC) of connexin 26 (Cx26). In the present study, we tested camellia oil, olive oil and cottonseed oil which are rich in oleamide-like oleic acid for their inhibitory potency on Cx26-mediated GJIC and spontaneous metastasis of BL6 cells. We found that camellia oil, olive oil and cottonseed oil, and their distillate fractions inhibited Cx26-mediated GJIC. We also showed that daily intraperitoneal injection of camellia oil and its distillate fractions more potently inhibited spontaneous lung metastasis of BL6 cells than oleamide. Moreover, a daily oral administration of camellia oil distillate fraction effectively inhibited spontaneous metastasis. Notably, even camellia Tempura-oil, a commercially available food, weakly inhibited the spontaneous metastasis of BL6 cells. Since these oils are used as foods and are quite safe, we propose that they could be used as supplements to protect patients from lung metastasis of melanomas.

Cloning of a soluble isoform of the SgIGSF adhesion molecule that binds the extracellular domain of the membrane-bound isoform

SgIGSF (spermatogenic immunoglobulin superfamily) is a recently identified intercellular adhesion molecule of the immunoglobulin superfamily. In a mast-cell cDNA library, we found a clone that resulted from the retention of intron 7 within the mature SgIGSF message. This clone was predicted to encode a soluble isoform of SgIGSF (sSgIGSF) with 336 amino-acid residues because its open reading frame ended just before the transmembrane domain. We constructed a plasmid expressing sSgIGSF fused to the human IgG Fc fragment at its C-terminus (sSgIGSF-Fc), and transfected it into COS-7 cells. The fusion protein was readily detectable in the culture supernatant. Solid-phase binding assay showed that sSgIGSF interacted directly the extracellular domain of membrane-bound SgIGSF (mSgIGSF). We next examined whether this interaction inhibited homophilic binding of mSgIGSF by aggregation assays using L cells that did not express mSgIGSF. A stable L-cell clone that overexpressed mSgIGSF aggregated with each other but not with mock-transfected L cells, indicating that a homophilic interaction of mSgIGSF mediated the aggregation. Addition of sSgIGSF-Fc inhibited the aggregation of L cells overexpressing mSgIGSF in a dose-dependent manner. Moreover, FACScan analyses revealed the specific binding of sSgIGSF-Fc to mSgIGSF expressed in L cells. Binding of sSgIGSF-Fc to mSgIGSF appeared to inhibit homophilic interactions of mSgIGSF.

A new enrichment approach identifies genes that alter cell cycle progression in Saccharomyces cerevisiae

Mechanisms that coordinate cell growth with division are thought to determine the timing of initiation of cell division and to limit overall cell proliferation. To identify genes involved in this process in Saccharomyces cerevisiae, we describe a method that does not rely on cell size alterations or resistance to pheromone. Instead, our approach was based on the cell surface deposition of the Flo1p protein in cells having passed START. We found that over-expression of HXT11 (which encodes a plasma membrane transporter), PPE1 (coding for a protein methyl esterase), or SIK1 (which encodes a protein involved in rRNA processing) shortened the duration of the G1 phase of the cell cycle, prior to the initiation of DNA replication. In addition, we found that, although SIK1 was not part of a mitotic checkpoint, SIK1 over-expression caused spindle orientation defects and sensitized G2/M checkpoint mutant cells. Thus, unlike HXT11 and PPE1, SIK1 over-expression is also associated with mitotic functions. Overall, we used a novel enrichment approach and identified genes that were not previously associated with cell cycle progression. This approach can be extended to other organisms.

Egr1 promotes growth and survival of prostate cancer cells. Identification of novel Egr1 target genes

In the majority of aggressive tumorigenic prostate cancer cells, the transcription factor Egr1 is overexpressed. We provide new insights of Egr1 involvement in proliferation and survival of TRAMP C2 prostate cancer cells by the identification of several new target genes controlling growth, cell cycle progression, and apoptosis such as cyclin D2, P19ink4d, and Fas. Egr1 regulation of these genes, identified by Affymetrix microarray, was confirmed by real-time PCR, immunoblot, and chromatin immunoprecipitation assays. Furthermore we also showed that Egr1 is responsible for cyclin D2 overexpression in tumorigenic DU145 human prostate cells. The regulation of these genes by Egr1 was demonstrated using Egr1 antisense oligonucleotides that further implicated Egr1 in resistance to apoptotic signals. One mechanism was illustrated by the ability of Egr1 to inhibit CD95 (Fas/Apo) expression, leading to insensitivity to FasL. The results provide a mechanistic basis for the oncogenic role of Egr1 in TRAMP C2 prostate cancer cells.

Localization of the PP2A B56gamma regulatory subunit at the Golgi complex: possible role in vesicle transport and migration

The BL6 subline was derived from the F10 line, which was derived from the B16 mouse melanoma cell line. BL6 cells are more invasive than F10 cells and differ genetically from F10 cells by an alteration of the gene encoding the B56gamma regulatory subunit of protein phosphatase 2A (PP2A). This alteration results in the transcription of mRNA encoding a truncated variant of the B56gamma1 isoform (Deltagamma1). When F10 cells were stained with a polyclonal antibody that recognizes three B56gamma isoforms, B56gamma1, B56gamma2, and B56gamma3, the immunofluorescent signals co-localized well with the cis-Golgi marker proteins. When BL6 cells were fractionated in a sucrose gradient, B56gamma1 and B56gamma2, but not B56gamma3, were present in the Golgi-enriched fraction. This fraction also contained the catalytic subunit of PP2A. FLAG-tagged Deltagamma1 preferentially localized to the trans-Golgi area rather than the cis-Golgi. This localization was the same as that of FLAG-tagged B56gamma1. NIH3T3 cells stably expressing Deltagamma1 transported a mutant viral protein from the endoplasmic reticulum to the plasma membrane much faster than wild-type cells. Their directional migration, as assessed by the advance of cells into a cell-free area, was also elevated. As Deltagamma1 reduces the activity of the B56gamma-containing PP2A holoenzymes, these results suggest that the normal holoenzymes suppress vesicle transport and that Deltagamma1 might increase the invasive ability of BL6 cells by activating Golgi function.

A truncated isoform of the protein phosphatase 2A B56gamma regulatory subunit may promote genetic instability and cause tumor progression

F10, a subline of the B16 mouse melanoma cell line, is itself the parent of the more metastatic BL6 line. BL6 cells differ from F10 cells by an alteration of the gene encoding the B56gamma regulatory subunit of protein phosphatase 2A (PP2A), which results in the expression of a truncated variant of the subunit (Deltagamma1). PP2A is involved in regulating the cell-cycle checkpoint and we found that the checkpoint in BL6 cells is aberrant when the Deltagamma1 protein is expressed. That is, although Deltagamma1 protein levels in cultured BL6 cells are low and these cells do not show an altered checkpoint on gamma-irradiation, irradiated footpad BL6 tumor cells show both a marked increase in Deltagamma1 levels and more extensive polyploidy and less apoptosis than F10 cells. These observations were reproduced with Deltagamma1 gene-transfected F10 cells (F10(Deltagamma1)). Deltagamma1 expression and an aberrant checkpoint are also associated with a higher metastatic ability because irradiated F10(Deltagamma1) tumors metastasized much more frequently than F10 tumors, which rarely metastasized whether irradiated or not. Nonirradiated F10(Deltagamma1) tumors, which do not express Deltagamma1 protein, had similarly low rates of metastasis. The greater metastatic ability of irradiated F10(Deltagamma1) tumors also correlated with the acquisition of many more genomic alterations. Thus, it seems that Deltagamma1 expression may damage the checkpoint, which may then allow the acquisition of genetic alterations that promote metastasis. These observations support the notion that mechanisms promoting the genetic instability of tumors could also aid tumor progression from the nonmetastatic to the metastatic state.