Mechanism of repression of RNA polymerase I transcription by the retinoblastoma protein

Feedback regulation of c-Myc by ribosomal protein L11

Several ribosomal proteins including L11 have been shown to activate p53 by inhibiting oncoprotein MDM2, leading to inhibition of cell cycle progression. Our recent study showed that L11 also inhibits oncoprotein c-Myc. Overexpression of L11 inhibits c-Myc-induced transcription and cell proliferation, while reduction of endogenous L11 increases these c-Myc activities. Interestingly, L11 is a transcriptional target of c-Myc, thus forming a negative feedback loop. We further showed that L11 competes with coactivator TRRAP for binding to c-Myc through the Myc box II (MB II) and reduces histone H4 acetylation at c-Myc target gene promoters. In addition, L11 appears to regulate c-Myc levels. Knocking down L11 markedly increases the mRNA and protein levels of endogenous c-Myc. These results suggest that L11 also inhibits cell cycle progression by regulating the c-Myc pathway. Here we further discuss the implications of this regulation and questions that this finding raises.

Mitotic occupancy and lineage-specific transcriptional control of rRNA genes by Runx2

Regulation of ribosomal RNA genes is a fundamental process that supports the growth of cells and is tightly coupled with cell differentiation. Although rRNA transcriptional control by RNA polymerase I (Pol I) and associated factors is well studied, the lineage-specific mechanisms governing rRNA expression remain elusive. Runt-related transcription factors Runx1, Runx2 and Runx3 establish and maintain cell identity, and convey phenotypic information through successive cell divisions for regulatory events that determine cell cycle progression or exit in progeny cells. Here we establish that mammalian Runx2 not only controls lineage commitment and cell proliferation by regulating genes transcribed by RNA Pol II, but also acts as a repressor of RNA Pol I mediated rRNA synthesis. Within the condensed mitotic chromosomes we find that Runx2 is retained in large discrete foci at nucleolar organizing regions where rRNA genes reside. These Runx2 chromosomal foci are associated with open chromatin, co-localize with the RNA Pol I transcription factor UBF1, and undergo transition into nucleoli at sites of rRNA synthesis during interphase. Ribosomal RNA transcription and protein synthesis are enhanced by Runx2 deficiency that results from gene ablation or RNA interference, whereas induction of Runx2 specifically and directly represses rDNA promoter activity. Runx2 forms complexes containing the RNA Pol I transcription factors UBF1 and SL1, co-occupies the rRNA gene promoter with these factors in vivo, and affects local chromatin histone modifications at rDNA regulatory regions. Thus Runx2 is a critical mechanistic link between cell fate, proliferation and growth control. Our results suggest that lineage-specific control of ribosomal biogenesis may be a fundamental function of transcription factors that govern cell fate.

MDMX regulation of p53 response to ribosomal stress

Ribosomal stress such as disruption of rRNA biogenesis activates p53 by release of ribosomal proteins from the nucleoli, which bind to MDM2 and inhibit p53 degradation. We found that p53 activation by ribosomal stress requires degradation of MDMX in an MDM2-dependent fashion. Tumor cells overexpressing MDMX are less sensitive to actinomycin D-induced growth arrest due to formation of inactive p53-MDMX complexes. Knockdown of MDMX increases sensitivity to actinomycin D, whereas MDMX overexpression abrogates p53 activation and prevents growth arrest. Furthermore, MDMX expression promotes resistance to the chemotherapeutic agent 5-fluorouracil (5-FU), which at low concentrations activates p53 by inducing ribosomal stress without significant DNA damage signaling. Knockdown of MDMX abrogates HCT116 tumor xenograft formation in nude mice. MDMX overexpression does not accelerate tumor growth but increases resistance to 5-FU treatment in vivo. Therefore, MDMX is an important regulator of p53 response to ribosomal stress and RNA-targeting chemotherapy agents.

Nascent pre-rRNA overexpression correlates with an adverse prognosis in alveolar rhabdomyosarcoma

Comparative expressed sequence hybridization (CESH) is an expression profiling technique which identifies chromosomal regions corresponding to differential gene expression. Here, we observe that various tumor samples including rhabdomyosarcoma show very prominent staining on the short arms of the acrocentric chromosomes suggesting an increase in expression of ribosomal RNA synthesized from the repetitive rDNA of the nucleolar organizer regions located on these chromosomes. Survival analysis showed a correlation with overexpression from this region and a poor prognosis in rhabdomyosarcoma. This phenomenon was studied in an extended set of rhabdomyosarcoma tumor samples using quantitative real-time reverse transcriptase-PCR to quantify levels of pre-rRNA (precursor ribosomal RNA). It was demonstrated first that the strong CESH signals did correspond to a marked increase in pre-rRNA expression and second that high pre-rRNA expression correlated with an adverse prognosis in alveolar subtype rhabdomyosarcoma. In addition, we demonstrate that pre-rRNA expression is significantly correlated with tumor stage. We conclude that measuring expression of pre-rRNA by real-time PCR is a useful prognostic marker in alveolar rhabdomyosarcoma. Furthermore, given that we have observed similar rDNA staining in all cancer types that we have studied by CESH, we propose that pre-rRNA overexpression is a general phenomenon in cancer and that our real-time PCR assay may be applicable as a prognostic marker in other tumor types.

Human tumor suppressor p14ARF negatively regulates rRNA transcription and inhibits UBF1 transcription factor phosphorylation

The nucleolar Arf protein has been shown to regulate cell cycle through both p53-dependent and -independent pathways. In addition to the well-characterized Arf-mdm2-p53 pathway, several partners of Arf have recently been described that could participate in alternative regulation process. Among those is the nucleolar protein B23/NPM, involved in the sequential maturation of rRNA. p19ARF can interact with B23/NPM in high molecular complexes and partially inhibit the cleavage of the 32S rRNA, whereas the human p14ARF protein has been shown to participate in the degradation of NPM/B23 by the proteasome. These data led to define Arf as a negative regulator of ribosomal RNA maturation. Our recent finding that the human p14ARF protein was able to specifically interact with the rRNA promoter in a p53-independent context, led us to analyse in vitro and in vivo the consequences of this interaction. Luciferase assay and pulse-chase experiments demonstrated that the rRNA transcription was strongly reduced upon p14ARF overexpression. Investigations on potential interactions between p14ARF and the transcription machinery proteins demonstrated that the upstream binding factor (UBF), required for the initiation of the transcriptional complex, was a new partner of the p14ARF protein. We next examined the phosphorylation status of UBF as UBF phosphorylation is required to recruit on the promoter factors involved in the transcriptional complex. Upon p14ARF overexpression, UBF was found hypophosphorylated, thus unable to efficiently recruit the transcription complex. Taken together, these data define a new p53-independent pathway that could regulate cell cycle through the negative control of rRNA transcription.

Acetylation of UBF changes during the cell cycle and regulates the interaction of UBF with RNA polymerase I

The upstream binding factor UBF, an activator of RNA polymerase I transcription, is posttranslationally modified by phosphorylation and acetylation. We found that in NIH3T3 cells, UBF is acetylated in S-phase but not in G₁-phase. To assess the role of acetylation in regulation of UBF activity, we have established an NIH3T3 cell line that inducibly overexpresses HDAC1. Both in vivo and in vitro, HDAC1 efficiently hypoacetylates UBF. Immunoprecipitation with antibodies against the Pol I-associated factor PAF53 co-precipitated UBF in mock cells but not in cells overexpressing HDAC1. Pull-down experiments showed that acetylation of UBF augments the interaction with Pol I. Consistent with acetylation of UBF being important for association of PAF53 and recruitment of Pol I, the level of Pol I associated with rDNA and pre-rRNA synthesis were reduced in cells overexpressing HDAC1. The results suggest that acetylation and deacetylation of UBF regulate rRNA synthesis during cell cycle progression.

Ribosomes and marrow failure: coincidental association or molecular paradigm?

Gene products mutated in the inherited bone marrow failure syndromes dyskeratosis congenita (DC), cartilage-hair hypoplasia (CHH), Diamond-Blackfan anemia (DBA), and Shwachman-Diamond syndrome (SDS) are all predicted to be involved in different aspects of ribosome synthesis. At this moment, however, it is unclear whether this link indicates a causal relationship. Although defective ribosome synthesis may contribute to each of these bone marrow failure syndromes (and perhaps others), precisely which feature of each disease is a consequence of failure to produce adequate amounts of ribosomes is obscured by the tendency of each gene product to have extraribosomal functions. Delineation of the precise role of each gene product in ribosomal biogenesis and in hematopoietic development may have both therapeutic and prognostic importance and perhaps even direct the search for new bone marrow failure genes.

Involvement of the transcriptional factor E2F1 in the regulation of the rRNA promoter

p16INK4a-pRB-E2F and ARF-MDM2-p53 are two major tumor suppressor networks involved in cell proliferation control. The nucleolar ARF protein binds to MDM2 to activate the growth suppressive functions of p53, but can also exert its tumor suppressor activity independently of p53, through mechanisms involving other regulators: in that manner, p14ARF has been shown to inhibit the transcriptional activity of E2F1 in vitro, suggesting that the two pathways intersect with one another. More recently, ARF has been shown to inhibit ribosomal RNA processing, and to specifically interact with the rRNA promoter, suggesting a role in the regulation of both maturation and transcription processes. We show here that E2F1 can bind the rRNA promoter and modulate its activity through the interaction with two E2F1-binding sequences we have identified. The regulation of ribosome biogenesis appears as a major p53-independent process, which involves both ARF and E2F1 to control cell proliferation.

Direct regulation of rRNA transcription by fibroblast growth factor 2

Fibroblast growth factor 2 (FGF-2), which is highly expressed in developing tissues and malignant cells, regulates cell growth, differentiation, and migration. Five isoforms (18 to approximately 34 kDa) of FGF-2 are derived from alternative initiation codons of a single mRNA. The 18-kDa FGF-2 isoform is released from cells by a nonclassical secretory pathway and regulates gene expression by binding to cell surface receptors. This isoform also localizes to the nucleolus, raising the possibility that it may directly regulate ribosome biogenesis, a rate-limiting process in cell growth. Although several growth factors have been shown to accumulate in the nucleolus, their function and mechanism of action remain unclear. Here we show that 18-kDa FGF-2 interacts with upstream binding factor (UBF), an architectural transcription factor essential for rRNA transcription. The maximal activation of rRNA transcription in vitro by 18-kDa FGF-2 requires UBF. The 18-kDa FGF-2 localizes to rRNA genes and is necessary for the full activation of pre-rRNA synthesis in vivo. Our results demonstrate that 18-kDa FGF-2 directly regulates rRNA transcription.

Nuclear tumor suppressors in space and time

Numerous studies have identified key binding partners and functional activities of nuclear tumor-suppressor proteins such as the retinoblastoma protein, p53 and BRCA1. Historically, less attention has been given to the subnuclear locations of these proteins. Here, we describe several recent studies that promote the view that regulated association with subcompartments of the nucleus is inherent to tumor-suppressor function.

PTEN represses RNA Polymerase I transcription by disrupting the SL1 complex

PTEN is a tumor suppressor whose function is frequently lost in human cancer. It possesses a lipid phosphatase activity that represses the activation of PI3 kinase/Akt signaling, leading to decreased cell growth, proliferation, and survival. The potential for PTEN to regulate transcription of the large rRNAs by RNA polymerase I (RNA Pol I) was investigated. As increased synthesis of rRNAs is a hallmark of neoplastic transformation, the ability of PTEN to control the transcription of rRNAs might be crucial for its tumor suppressor function. The expression of PTEN in PTEN-deficient cells represses RNA Pol I transcription, while decreasing PTEN expression enhances transcription. PTEN-mediated repression requires its lipid phosphatase activity and is independent of the p53 status of the cell. This event can be uncoupled from PTEN's ability to regulate the cell cycle. RNA Pol I is regulated through PI3 kinase/Akt/mammalian target of rapamycin/S6 kinase, and the expression of constitutively activated S6 kinase is able to abrogate transcription repression by PTEN. No change in the expression of the RNA Pol I transcription components, upstream binding factor or SL1, was observed upon PTEN expression. However, chromatin immunoprecipitation assays demonstrate that PTEN differentially reduces the occupancy of the SL1 subunits on the rRNA gene promoter. Furthermore, PTEN induces dissociation of the SL1 subunits. Together, these results demonstrate that PTEN represses RNA Pol I transcription through a novel mechanism that involves disruption of the SL1 complex.

CAP-1A is a novel linker that binds clathrin and the voltage-gated sodium channel Na(v)1.8

The voltage-gated sodium channel Na(v)1.8 produces a tetrodotoxin-resistant current and plays a key role in nociception. Annexin II/p11 binds to Na(v)1.8 and facilitates insertion of the channel within the cell membrane. However, the mechanisms responsible for removal of specific channels from the cell membrane have not been studied. We have identified a novel protein, clathrin-associated protein-1A (CAP-1A), which contains distinct domains that bind Na(v)1.8 and clathrin. CAP-1A is abundantly expressed in DRG neurons and colocalizes with Na(v)1.8 and can form a multiprotein complex with Na(v)1.8 and clathrin. Coexpression of CAP-1A and Na(v)1.8 in DRG neurons reduces Na(v)1.8 current density by approximately 50% without affecting the endogenous or recombinant tetrodotoxin-sensitive currents. This effect of CAP-1A is blocked by bafilomycin A1 treatment of transfected DRG neurons. CAP-1A thus is the first example of an adapter protein that links clathrin and a sodium channel and may regulate Na(v)1.8 channel density at the cell surface.

mTOR function in skeletal muscle hypertrophy: increased ribosomal RNA via cell cycle regulators

The purpose of this study was to identify the potential downstream functions associated with mammalian target of rapamycin (mTOR) signaling during myotube hypertrophy. Terminally differentiated myotubes were serum stimulated for 3, 6, 12, 24, and 48 h. This treatment resulted in significant myotube hypertrophy (protein/DNA) and increased RNA content (RNA/DNA) with no changes in DNA content or indices of cell proliferation. During myotube hypertrophy, the increase in RNA content was accompanied by an increase in tumor suppressor protein retinoblastoma (Rb) phosphorylation and a corresponding increase in the availability of the ribosomal DNA transcription factor upstream binding factor (UBF). Serum stimulation also induced an increase in cyclin D1 protein expression in the differentiated myotubes with a concomitant increase in cyclin D1-dependent cyclin-dependent kinase (CDK)-4 activity toward Rb. The increases in myotube hypertrophy and RNA content were blocked by rapamycin treatment, which also prevented the increase in cyclin D1 protein expression, CDK-4 activity, Rb phosphorylation, and the increase in UBF availability. Our findings demonstrate that activation of mTOR is necessary for myotube hypertrophy and suggest that the role of mTOR is in part to modulate cyclin D1-dependent CDK-4 activity in the regulation of Rb and ribosomal RNA synthesis. On the basis of these results, we propose that common molecular mechanisms contribute to the regulation of myotube hypertrophy and growth during the G1 phase of the cell cycle.

Global Protein Shotgun Expression Profiling of Proliferating MCF-7 Breast Cancer Cells

Protein expression becomes altered in breast epithelium during malignant transformation. Knowledge of these perturbations should provide insight into the molecular basis of breast cancer, as well as reveal possible new therapeutic targets. To this end, we have performed an extensive comparative proteomic survey of global protein expression patterns in proliferating MCF-7 breast cancer cells and normal human mammary epithelial cells using gel-free shotgun tandem mass spectrometry. Pathophysiological alterations associated with the malignant breast cancer phenotype were detected, including differences in the apparent levels of key regulators of the cell cycle, signal transduction, apoptosis, transcriptional regulation, and cell metabolism.

RNA polymerases I and III, growth control and cancer

Transcription of rRNA and tRNA genes by RNA polymerases I and III is essential for sustained protein synthesis and is therefore a fundamental determinant of the capacity of a cell to grow. When cell growth is not required, this transcription is repressed by retinoblastoma protein, p53 and ARF. However, inactivation of these tumour suppressors in cancers deregulates RNA polymerases I and III, and oncoproteins such as Myc can stimulate these systems further. Such events might have a significant impact on the growth potential of tumours.

Regulation of Upstream Binding Factor 1 Activity by Insulin-like Growth Factor I Receptor Signaling

The upstream binding factor 1 (UBF1) is one of the proteins in a complex that regulates the activity of RNA polymerase I, which controls the rate of ribosomal RNA (rRNA) synthesis. We have shown previously that insulin receptor substrate-1 (IRS-1) can translocate to the nuclei and nucleoli of cells and bind UBF1. We report here that activation of the type I insulin-like growth factor receptor (IGF-IR) by IGF-I increases transcription from the ribosomal DNA (rDNA) promoter in both myeloid cells and mouse fibroblasts. The increased activity of the rDNA promoter is accompanied by increased phosphorylation of UBF1, a requirement for UBF1 activation. Phosphorylation occurs on a number of UBF1 peptides, most prominently on the highly acidic, serine-rich C terminus. In myeloid cells (but not in mouse embryo fibroblasts) IRS-1 signaling stabilizes the levels of UBF1 protein. These findings demonstrate that IGF-IR signaling can increase the activity of UBF1 and transcription from the rDNA promoter, providing one explanation for the reported effects of the IGF/IRS-1 axis on cell and body size in animals and cells in culture.

Nucleolar size and activity are related to pRb and p53 status in human breast cancer

Cell proliferation is tightly coordinated with cell growth. The oncosuppressor proteins pRb and p53 may exert a key role in coupling growth and proliferation by controlling both ribosome biogenesis and cell cycle progression. In the present study we evaluated the relationship between the pRb and p53 status and rRNA transcriptional activity in histological sections of 343 human primary breast carcinomas. Ribosomal biogenesis was quantified by morphometric analysis of silver-stained interphase nucleolar organizer regions (AgNORs). pRb and p53 status was assessed by immunohistochemistry. Twenty-four tumors were considered to be pRb deleted, 260 tumors showed a phosphorylated-pRb labeling index (LI) up to 25%, and 55 tumors an LI >25%. Tumors with deleted pRb or phosphorylated-pRb-LI > or =25% were characterized by significantly greater mean AgNOR area values than those with unaltered pRb (p<0.001). In the 71 tumors with mutated p53 the NOR area mean value was greater than in the 272 tumors with normal p53 (p<0.001). Our results demonstrate, for the first time in vivo, that pRb and p53 status is related to the ribosome biogenesis rate and suggest that in tumors with altered pRb and p53 function the up-regulation of rRNA synthesis may always assure an adequate growth to cancer cells with uncontrolled cell cycle progression.

Ribosomal protein L23 activates p53 by inhibiting MDM2 function in response to ribosomal perturbation but not to translation inhibition

The p53-MDM2 feedback loop is vital for cell growth control and is subjected to multiple regulations in response to various stress signals. Here we report another regulator of this loop. Using an immunoaffinity method, we purified an MDM2-associated protein complex that contains the ribosomal protein L23. L23 interacted with MDM2, forming a complex independent of the 80S ribosome and polysome. The interaction of L23 with MDM2 was enhanced by treatment with actinomycin D but not by gamma-irradiation, leading to p53 activation. This activation was inhibited by small interfering RNA against L23. Ectopic expression of L23 reduced MDM2-mediated p53 ubiquitination and also induced p53 activity and G(1) arrest in p53-proficient U2OS cells but not in p53-deficient Saos-2 cells. These results reveal that L23 is another regulator of the p53-MDM2 feedback regulation.

The prognostic value of the AgNOR parameter in human breast cancer depends on the pRb and p53 status

BACKGROUND

The amount of argyrophilic nucleolar organiser regions (AgNORs) represents a cell kinetics parameter used in tumour pathology for prognostic purposes. AgNOR expression is directly related to the rate of ribosome biogenesis, which has been recently shown to be controlled also by the tumour suppressor proteins pRb and p53.

AIMS

To ascertain the relative prognostic value of AgNORs and of pRb and p53 expression in breast carcinoma.

METHODS

This study was carried out on 335 human primary breast carcinomas with a median follow up time of 108 months. AgNORs were measured by morphometric analysis on sections that had been selectively silver stained; expression of p53 and phosphorylated and non-phosphorylated pRb forms was assessed by immunohistochemistry.

RESULTS

Patients were divided into groups with low (249) and high (86) AgNOR values; with normal (267) and mutated p53 (68); and with normal (256) and hyperphosphorylated or deleted pRb (79). Univariate analysis of disease free survival showed that AgNORs and the status of pRb and p53 were significantly related to the patients' clinical outcome. However, among the four groups characterised by different pRb and p53 status, the AgNOR parameter was not capable of distinguishing subgroups of patients with different clinical outcomes.

CONCLUSIONS

These findings indicate that the prognostic value of the AgNOR parameter depends on the status of the tumour suppressor proteins pRb and p53, and it cannot be ascribed to the relation between AgNORs and the cell proliferation rate.

MAD1 and c-MYC regulate UBF and rDNA transcription during granulocyte differentiation

The regulation of cell mass (cell growth) is often tightly coupled to the cell division cycle (cell proliferation). Ribosome biogenesis and the control of rDNA transcription through RNA polymerase I are known to be critical determinants of cell growth. Here we show that granulocytic cells deficient in the c-MYC antagonist MAD1 display increased cell volume, rDNA transcription and protein synthesis. MAD1 repressed and c-MYC activated rDNA transcription in nuclear run-on assays. Repression of rDNA transcription by MAD1 was associated with its ability to interact directly with the promoter of upstream binding factor (UBF), an rDNA regulatory factor. Conversely, c-MYC activated transcription from the UBF promoter. Using siRNA, UBF was shown to be required for c-MYC-induced rDNA transcription. These data demonstrate that MAD1 and c-MYC reciprocally regulate rDNA transcription, providing a mechanism for coordination of ribosome biogenesis and cell growth under conditions of sustained growth inhibition such as granulocyte differentiation.

Regulation of ribosomal RNA gene expression in porcine oocytes

In vitro production (IVP) of porcine embryos including in vitro maturation (IVM) of oocytes followed by in vitro fertilization (IVF) and in vitro culture (IVC) of the resultant embryos may result in live offspring, but it is still associated with great inefficiencies probably due to incomplete cytoplasmic maturation of the oocytes in vitro. Therefore, fundamental knowledge on the regulation of transcription during the oocyte growth phase when the messengers and protein synthetic machinery necessary for oocyte developmental competence are formed, is of great importance. In mammals, synthesis of RNA, up to 60-70% of which is ribosomal (rRNA), increases during oocyte growth and reaches a peak at the beginning of follicular antrum formation. In oocytes at the end of the growth phase, acquisition of full meiotic competence coincides with a markedly decreased rRNA transcriptional activity in the gametes. Our recent studies on the porcine oocyte growth phase have revealed a deeper molecular and biological insight into the complex regulation of rRNA transcription at different stages of follicular development. The data indicate that the so-called pocket protein, p130, is involved in the down-regulation of rRNA transcription at the end of the oocyte growth phase through an inhibition of the action of upstream binding factor (UBF). The latter protein is necessary for the function of RNA polymerase I (RNA Pol I), which is the actual enzyme driving rRNA gene transcription. Moreover, rRNA transcription also appears to be down-regulated by a decrease in the expression of mRNA encoding PAF53, an RNA Pol I-associated factor also required for the polymerase to exert its action. At the ultrastructural level, these molecular changes are paralleled by marginalization of the fibrillar centres of the oocyte nucleolus followed by compaction of the nucleolus into an inactive sphere of fibrils.

Role of pescadillo and upstream binding factor in the proliferation and differentiation of murine myeloid cells

Pescadillo (PES1) and the upstream binding factor (UBF1) play a role in ribosome biogenesis, which regulates cell size, an important component of cell proliferation. We have investigated the effects of PES1 and UBF1 on the growth and differentiation of cell lines derived from 32D cells, an interleukin-3 (IL-3)-dependent murine myeloid cell line. Parental 32D cells and 32D IGF-IR cells (expressing increased levels of the type 1 insulin-like growth factor I [IGF-I] receptor [IGF-IR]) do not express insulin receptor substrate 1 (IRS-1) or IRS-2. 32D IGF-IR cells differentiate when the cells are shifted from IL-3 to IGF-I. Ectopic expression of IRS-1 inhibits differentiation and transforms 32D IGF-IR cells into a tumor-forming cell line. We found that PES1 and UBF1 increased cell size and/or altered the cell cycle distribution of 32D-derived cells but failed to make them IL-3 independent. PES1 and UBF1 also failed to inhibit the differentiation program initiated by the activation of the IGF-IR, which is blocked by IRS-1. 32D IGF-IR cells expressing PES1 or UBF1 differentiate into granulocytes like their parental cells. In contrast, PES1 and UBF1 can transform mouse embryo fibroblasts that have high levels of endogenous IRS-1 and are not prone to differentiation. Our results provide a model for one of the theories of myeloid leukemia, in which both a stimulus of proliferation and a block of differentiation are required for leukemia development.

RNA polymerase III transcription and cancer

RNA polymerase (pol) III synthesizes a range of essential products, including tRNA, 5S rRNA and 7SL RNA, which are required for protein synthesis and trafficking. High rates of pol III transcription are necessary for cells to sustain growth. A wide range of transformed and tumour cell types have been shown to express elevated levels of pol III products. This review will summarize what is known about the mechanisms responsible for this deregulation. Some transforming agents have been shown to stimulate expression of the pol III-specific transcription factors TFIIIB or TFIIIC2. In addition, TFIIIB is bound and activated by several oncogenic proteins, including c-Myc. Conversely, TFIIIB interacts in healthy cells with the tumour suppressors RB and p53. Indeed, the ability to limit pol III transcription through TFIIIB may contribute to their growth-suppression capacities. The function of p53 and/or RB is compromised in most if not all transformed cells; the resultant derepression of TFIIIB may provide an almost universal route to deregulate pol III transcription in cancers. In addition to effects on protein synthesis and growth, there is a precedent for a pol III product having oncogenic activity.

The role of translation in neoplastic transformation from a pathologist's point of view

Increased cell proliferation, which is a hallmark of aggressive malignant neoplasms, requires a general increase in protein synthesis and a specific increase in the synthesis of replication-promoting proteins. Transient increase in the general protein synthesis rate, as well as preferential translation of specific mRNAs coding for growth promoting proteins (e.g. cyclin D1), takes place during normal mitogenic response. A number of extensively studied growth signal transduction pathways (Ras, PI3K, MAPK, mTOR-dependent pathways) activate the function and expression of various components of the translational machinery. In abnormal situations, constitutive activation of signal transduction pathways (e.g. oncogenic activation of Ras or Myc) leads to continuous upregulation of key elements of translational machinery. On the other hand, tumor suppressor genes (p53, pRb) downregulate ribosomal and tRNA synthesis, and their inactivation results in uncontrolled production of these translational components. During recent years, a significant effort has been dedicated to determining whether expression of translation factors is increased in human tumors using clinical biopsy specimens. The results of these studies indicate that expression of particular translation initiation factors is not always increased in human neoplasms. The pattern of expression is characteristic for a particular tumor type. For example, eIF-4E is usually increased in bronchioloalveolar carcinomas but not in squamous cell carcinomas of the lung. Interestingly, in certain highly proliferative and aggressive neoplasms (e.g. squamous cell carcinoma of the lung, melanoma), the expression of eIF-4E is barely detectable. These findings suggest that mechanisms for increasing general protein synthesis in various neoplasms differ significantly. Finally, the possibility of qualitative alterations in the translational machinery, rather than a simple increase in the activity of its components, is discussed along with the possibility of targeting those qualitative differences for tumor therapy.

Cell and Molecular Biology of Nucleolar Assembly and Disassembly

Nucleoli disassemble in prophase of the metazoan mitotic cycle, and they begin their reassembly (nucleologenesis) in late anaphase?early telophase. Nucleolar disassembly and reassembly were obvious to the early cytologists of the eighteenth and nineteenth centuries, and although this has lead to a plethora of literature describing these events, our understanding of the molecular mechanisms regulating nucleolar assembly and disassembly has expanded immensely just within the last 10-15 years. We briefly survey the findings of nineteenth-century cytologists on nucleolar assembly and disassembly, followed by the work of Heitz and McClintock on nucleolar organizers. A primer review of nucleolar structure and functions precedes detailed descriptions of modern molecular and microscopic studies of nucleolar assembly and disassembly. Nucleologenesis is concurrent with the reinitiation of rDNA transcription in telophase. The perichromosomal sheath, prenucleolar bodies, and nucleolar-derived foci serve as repositories for nucleolar processing components used in the previous interphase. Disassembly of the perichromosomal sheath along with the dynamic movements and compositional changes of the prenucleolar bodies and nucleolus-derived foci coincide with reactivation of rDNA synthesis within the chromosomal nucleolar organizers during telophase. Nucleologenesis is considered in various model organisms to provide breadth to our understanding. Nucleolar disassembly occurs at the onset of mitosis primarily as a result of the mitosis-specific phosphorylation of Pol I transcription factors and processing components. Although we have learned much regarding nucleolar assembly and disassembly, many questions still remain, and these questions are as vibrant for us today as early questions were for nineteenth- and early twentieth-century cytologists.

Immunolocalization of upstream binding factor and pocket protein p130 during final stages of bovine oocyte growth

The aim of this study was to describe the dynamic changes in the localization of the key nucleolar protein markers, fibrillarin, B23/nucleophosmin, C23/nucleolin, protein Nopp140, during the final stages of bovine oocyte growth. All these proteins were present in the large reticulated nucleoli of oocytes from the small-size category follicles (<1 mm). The entire nucleolus exhibited strong positivity for UBF (upstream binding factor, RNA polymerase I-specific transcription initiation factor), which displayed a dotted staining pattern. In contrast, protein p130 was diffusely distributed throughout the nucleus and excluded from nucleoli. In oocytes approaching the late period of growth (2-3-mm follicles), UBF localization shifted to the nucleolar periphery. Double staining of UBF-p130 revealed a gradual accumulation of p130 at the periphery shell around the nucleolus. In fully grown oocytes (>3-mm follicles), all studied nucleolar proteins were detected in the small compact nucleoli. The cap structure, attached to the compact nucleolus surface, was positive for UBF and PAF53 (subunit of RNA polymerase I). The UBF-positive cap showed a close structural association with p130. It is concluded that, during the process of oocyte nucleolus compaction, UBF and PAF53, proteins involved in the rDNA transcription, are segregated from fibrillarin and Nopp140, proteins essential for early steps of pre-rRNA processing. The observed changes may reflect the transition from pre-rRNA synthesis to pre-rRNA processing as an analysis of the relative abundance of the developmentally important gene transcripts confirmed. In addition, discovered structural association between UBF and p130 suggests a role for pocket proteins in ribosomal gene silencing in mammalian oocytes.

Regulation of Ribosomal RNA Synthesis During the Final Phases of Porcine Oocyte Growth

In porcine oocytes, acquisition of meiotic competence coincides with a decrease of general transcriptional activity at the end of the oocyte growth phase and, specifically, of ribosomal RNA (rRNA) synthesis in the nucleolus. The present study investigated the regulation of rRNA synthesis during porcine oocyte growth. Localization and expression of components involved in regulation of the rRNA synthesis (the RNA polymerase I-associated factor PAF53, upstream binding factor [UBF], and the pocket proteins p130 and pRb) were assessed by immunocytochemistry and semiquantitative reverse transcription-polymerase chain reaction and correlated with ultrastructural analysis and autoradiography following [3H]uridine incubation in growing and fully grown porcine oocytes. In addition, meiotic resumption, ultrastructure, and expression of p130, UBF, and PAF53 were analyzed in growing and fully grown porcine oocytes cultured with 100 microM butyrolactone I (BL-I), a potent inhibitor of cyclin-dependent kinases, to gain insight concerning the regulation of rRNA transcription during meiotic arrest. Immunocytochemical analysis demonstrated that p130 became colocalized with UBF and PAF53 and that the intensity of the PAF53 labeling decreased toward the end of the oocyte growth phase. These data suggest that the decrease in rRNA synthesis is regulated through inhibition of UBF by p130 as well as by decreased availability of PAF53. Moreover, expression of mRNA encoding PAF53 was decreased at the end of the oocyte growth phase. At the morphological level, these events coincided with inactivation of the nucleolus, as visualized by the transformation of the fibrillogranular nucleolus to an electron-dense fibrillar sphere with remnants of the fibrillar centers at the surface. Meiotic inhibition with 100 microM BL-I had a detrimental effect on the ability of porcine oocytes to resume meiosis and on nucleolus morphology, resulting in a lack of RNA synthetic capability as the fibrillar components, where rRNA transcription and initial processing occur, condensed or even disintegrated.

Calmodulin binds to the C terminus of sodium channels Nav1.4 and Nav1.6 and differentially modulates their functional properties

Modulation of voltage-gated sodium channels (VGSC) can have a major impact on cell excitability. Analysis of calmodulin (CaM) binding to GST-fusion proteins containing the C-terminal domains of Nav1.1-Nav1.9 indicates that some of the tetrodotoxin-sensitive VGSC isoforms, including NaV1.4 and NaV1.6, are able to bind CaM in a calcium-independent manner. Here we demonstrate that association with CaM is important for functional expression of NaV1.4 and NaV1.6 VGSCs. Disrupting the interaction between CaM and the C terminus of NaV1.4 and NaV1.6 channels reduced current amplitude by 99 and 62%, respectively. Overexpression of CaM increased the current generated by Nav1.4 and Nav1.6 C-terminal mutant constructs that exhibited intermediate current densities and intermediate binding affinities for CaM, demonstrating that this effect on current density was directly dependent on the ability of the C terminus to bind CaM. In addition to the effects on current density, calmodulin also was able to modulate the inactivation kinetics of Nav1.6, but not Nav1.4, currents in a calcium-dependent manner. Our data demonstrate that CaM can regulate the properties of VGSCs via calcium-dependent and calcium-independent mechanisms and suggest that modulation of neuronal sodium channels may play a role in calcium-dependent neuronal plasticity.

Calmodulin binds to the C terminus of sodium channels Nav1.4 and Nav1.6 and differentially modulates their functional properties

Modulation of voltage-gated sodium channels (VGSC) can have a major impact on cell excitability. Analysis of calmodulin (CaM) binding to GST-fusion proteins containing the C-terminal domains of Nav1.1-Nav1.9 indicates that some of the tetrodotoxin-sensitive VGSC isoforms, including NaV1.4 and NaV1.6, are able to bind CaM in a calcium-independent manner. Here we demonstrate that association with CaM is important for functional expression of NaV1.4 and NaV1.6 VGSCs. Disrupting the interaction between CaM and the C terminus of NaV1.4 and NaV1.6 channels reduced current amplitude by 99 and 62%, respectively. Overexpression of CaM increased the current generated by Nav1.4 and Nav1.6 C-terminal mutant constructs that exhibited intermediate current densities and intermediate binding affinities for CaM, demonstrating that this effect on current density was directly dependent on the ability of the C terminus to bind CaM. In addition to the effects on current density, calmodulin also was able to modulate the inactivation kinetics of Nav1.6, but not Nav1.4, currents in a calcium-dependent manner. Our data demonstrate that CaM can regulate the properties of VGSCs via calcium-dependent and calcium-independent mechanisms and suggest that modulation of neuronal sodium channels may play a role in calcium-dependent neuronal plasticity.

Insulin-like growth factor I receptor signaling and nuclear translocation of insulin receptor substrates 1 and 2

The insulin receptor substrate 1 (IRS-1) can translocate to the nuclei and nucleoli of several types of cells. Nuclear translocation can be induced by an activated insulin-like growth factor 1 receptor (IGF-IR), and by certain oncogenes, such as the Simian virus 40 T antigen and v-src. We have asked whether IRS-2 could also translocate to the nuclei. In addition, we have studied the effects of functional mutations in the IGF-IR on nuclear translocation of IRS proteins. IRS-2 translocates to the nuclei of mouse embryo fibroblasts expressing the IGF-IR, but, at variance with IRS-1, does not translocate in cells expressing the Simian virus 40 T antigen. Mutations in the tyrosine kinase domain of the IGF-IR abrogate translocation of the IRS proteins. Other mutations in the IGF-IR, which do not interfere with its mitogenicity but inhibit its transforming capacity, result in a decrease in translocation, especially to the nucleoli. Nuclear IRS-1 and IRS-2 interact with the upstream binding factor, which is a key regulator of RNA polymerase I activity and, therefore, rRNA synthesis. In 32D cells, wild-type, but not mutant, IRS-1 causes a significant activation of the ribosomal DNA promoter. The interaction of nuclear IRS proteins with upstream binding factor 1 constitutes the first direct link of these proteins with the ribosomal DNA transcription machinery.

Does the ribosome translate cancer?

Ribosome biogenesis and translation control are essential cellular processes that are governed at numerous levels. Several tumour suppressors and proto-oncogenes have been found either to affect the formation of the mature ribosome or to regulate the activity of proteins known as translation factors. Disruption in one or more of the steps that control protein biosynthesis has been associated with alterations in the cell cycle and regulation of cell growth. Therefore, certain tumour suppressors and proto-oncogenes might regulate malignant progression by altering the protein synthesis machinery. Although many studies have correlated deregulation of protein biosynthesis with cancer, it remains to be established whether this translates directly into an increase in cancer susceptibility, and under what circumstances.

Nucleolar Arf tumor suppressor inhibits ribosomal RNA processing

The p19(Arf) tumor suppressor, a nucleolar protein, binds to Mdm2 to induce p53-dependent cell cycle arrest. Arf also prevents the proliferation of cells lacking Mdm2 and p53, albeit less efficiently. We show that p19(Arf) inhibits production of ribosomal RNA, retarding processing of 47/45S and 32S precursors. These effects correlate with but do not strictly depend upon inhibition of rRNA biosynthesis or cell cycle arrest, are not mimicked by p53, and require neither p53 nor Mdm2. Arf mutants lacking conserved amino acid residues 2-14 do not block rRNA synthesis and processing or inhibit cell proliferation. Evolution may have linked a primordial nucleolar Arf function to Mdm2 and p53, creating a more efficient checkpoint-signaling pathway for coordinating ribosomal biogenesis and cell cycle progression.

Interview with the retinoblastoma family members: do they help each other?

The ultimate destiny of a cell to undergo division, differentiation, survival, and death results from an intricate balance between multiple regulators including oncogenes, tumor suppressor genes, and cell cycle associated proteins. Deregulation of the cell cycle machinery switches the phenotype from a normal cell to a cancerous cell. Fundamental alterations of tumor suppressor genes may result in an unregulated cell cycle with the accumulation of mutations and eventual neoplastic transformation. As such, one may define cancer as a genetic disease of the cell cycle. In this review, we will emphasize our current understanding of how the cell cycle machinery maintains cellular homeostasis by studying the consequences of its deregulation.

Rrn3 phosphorylation is a regulatory checkpoint for ribosome biogenesis

Cycloheximide inhibits ribosomal DNA (rDNA) transcription in vivo. The mouse homologue of yeast Rrn3, a polymerase-associated transcription initiation factor, can complement extracts from cycloheximide-treated mammalian cells. Cycloheximide inhibits the phosphorylation of Rrn3 and causes its dissociation from RNA polymerase I. Rrn3 interacts with the rpa43 subunit of RNA polymerase I, and treatment with cycloheximide inhibits the formation of a Rrn3.rpa43 complex in vivo. Rrn3 produced in Sf9 cells but not in bacteria interacts with rpa43 in vitro, and such interaction is dependent upon the phosphorylation state of Rrn3. Significantly, neither dephosphorylated Rrn3 nor Rrn3 produced in Escherichia coli can restore transcription by extracts from cycloheximide-treated cells. These results suggest that the phosphorylation state of Rrn3 regulates rDNA transcription by determining the steady-state concentration of the Rrn3.RNA polymerase I complex within the nucleolus.

Upstream binding factor up-regulated in hepatocellular carcinoma is related to the survival and cisplatin-sensitivity of cancer cells

Upstream binding factor (UBF) is an RNA polymerase I-specific transcription factor. By representational difference analysis, Northern blot, and cDNA array analysis, up-regulation of UBF was detected in 12 of 17 clinical hepatocellular carcinoma samples comparing to the paired normal liver tissues. Introduction of UBF in human lung fibroblast cells that do not express UBF resulted in an accelerated rate of cell growth; on the other hand, antisense oligodeoxynucleotides (ODNs) treatment of UBF-expressing hepatoma cell lines reduced the level of UBF protein, suppressed the colony formation capacity of these cells on soft agarose, and finally caused cell death. Annexin V binding analysis suggested that anti-UBF ODN-caused cell death might involve weak apoptosis, however, DNA laddering and cleavage of poly (ADP-ribose) polymerase were not observed in these ODN-treated cells. Expression profiling of the anti-UBF ODN-treated cells using a human cDNA array revealed that the expression of 30 genes was altered in response to the inhibition of UBF expression. Notably, UBF expression could increase the cell sensitivity to the chemotherapeutic reagent cis-diaminedichloroplatinum (II). We proposed that UBF is fundamental to the survival of cells expressing the gene, and is potential as a target for screening anti-cancer drugs and an indicator in selecting chemotherapeutic reagents.

Retinoblastoma protein partners

Studies of the retinoblastoma gene (Rb) have shown that its protein product (pRb) acts to restrict cell proliferation, inhibit apoptosis, and promote cell differentiation. The frequent mutation of the Rb gene, and the functional inactivation of pRb in tumor cells, have spurred interest in the mechanism of pRb action. Recently, much attention has focused on pRb's role in the regulation of the E2F transcription factor. However, biochemical studies have suggested that E2F is only one of many pRb-targets and, to date, at least 110 cellular proteins have been reported to associate with pRb. The plethora of pRb-binding proteins raises several important questions. How many functions does pRb possess, which of these functions are important for development, and which contribute to tumor suppression? The goal of this review is to summarize the current literature of pRb-associated proteins.

Regulation of ribosome biogenesis within the nucleolus

Ribosome biogenesis is both necessary for cellular adaptation, growth, and proliferation as well as a major energetic and biosynthetic demand upon cells. For these reasons, ribosome biogenesis requires precise regulation to balance supply and demand. The complexity of ribosome biogenesis gives rise to many steps and opportunities where regulation could take place. For trans-acting factors involved in ribosome biogenesis in the nucleolus, there may be a dynamic coordination, both spatially and temporally, that regulates their functions from the transcription of rDNA to the assembly and export of preribosomal particles. Here we summarize most of the described regulations on ribosome biogenesis in the nucleolus. However, these may represent only a small fraction of a larger picture. Further studies are required to determine the initial signals, signal transduction pathways utilized, and the specific targets of these regulatory modifications and how these are used to control ribosome biogenesis as a whole.

Direct interaction with contactin targets voltage-gated sodium channel Na(v)1.9/NaN to the cell membrane

The mechanisms that target various sodium channels within different regions of the neuronal membrane, which they endow with different physiological properties, are not yet understood. To examine this issue we studied the voltage-gated sodium channel Na(v)1.9/NaN, which is preferentially expressed in small sensory neurons of dorsal root ganglia and trigeminal ganglia and the nonmyelinated axons that arise from them. Our results show that the cell adhesion molecule contactin binds directly to Na(v)1.9/NaN and recruits tenascin to the protein complex in vitro. Na(v)1.9/NaN and contactin co-immunoprecipitate from dorsal root ganglia and transfected Chinese hamster ovary cell line, and co-localize in the C-type neuron soma and along nonmyelinated C-fibers and at nerve endings in the skin. Co-transfection of Chinese hamster ovary cells with Na(v)1.9/NaN and contactin enhances the surface expression of the sodium channel over that of Na(v)1.9/NaN alone. Thus contactin binds directly to Na(v)1.9/NaN and participates in the surface localization of this channel along nonmyelinated axons.

Phosphorylation of UBF at serine 388 is required for interaction with RNA polymerase I and activation of rDNA transcription

Modulation of the activity of the upstream binding factor (UBF) plays a key role in cell cycle-dependent regulation of rRNA synthesis. Activation of rDNA transcription on serum stimulation requires phosphorylation of UBF at serine 484 by G(1)-specific cyclin-dependent kinase (cdk)/cyclin complexes. After G(1) progression UBF is phosphorylated at serine 388 by cdk2/cyclin E and cdk2/cyclin A. Conversion of serine 388 to glycine abolishes UBF activity, whereas substitution by aspartate enhances the transactivating function of UBF. Protein-protein interaction studies reveal that phosphorylation at serine 388 is required for the interaction between RNA polymerase I and UBF. The results suggest that phosphorylation of UBF represents a powerful means of modulating the assembly of the transcription initiation complex in a proliferation- and cell cycle-dependent fashion.

The role of acetylation in rDNA transcription

Treatment of NIH 3T3 cells with trichostatin A (TSA), an inhibitor of histone deacetylase (HDAC), resulted in a dose-dependent increase in transcription from a rDNA reporter and from endogenous rRNA genes. Chromatin immunoprecipitation using anti-acetyl-histone H4 antibodies demonstrated a direct effect of TSA on the acetylation state of the ribosomal chromatin. TSA did not reverse inhibition of transcription from the rDNA reporter by retinoblastoma (Rb) protein, suggesting that the main mechanism by which Rb blocks rDNA transcription may not involve recruitment of deacetylases to rDNA chromatin. Overexpression of histone transacetylases p300, CBP and PCAF stimulated transcription in transfected NIH 3T3 cells. Recombinant p300, but not PCAF, stimulated rDNA transcription in vitro in the absence of nucleosomes, suggesting that the stimulation of rDNA transcription by TSA might have a chromatin-independent component. We found that the rDNA transcription factor UBF was acetylated in vivo. Finally, we also demonstrated the nucleolar localization of CBP. Our results suggest that the organization of ribosomal chromatin of higher eukaryotes is not static and that acetylation may be involved in affecting these dynamic changes directly through histone acetylation and/or through acetylation of UBF or one of the other components of rDNA transcription.

pRb2/p130 and p107 control cell growth by multiple strategies and in association with different compartments within the nucleus

It has been recently reported that retinoblastoma family proteins suppress cell growth by regulating not only E2F-dependent mRNA transcription but also rRNA and tRNA transcription and, through HDAC1 recruitment, chromatin packaging. In the present study we report data showing that these various control strategies are correlated, at least in part, with nuclear compartmentalization of retinoblastoma proteins. In a first series of experiments, we showed that pRb2/p130 and p107 are not evenly distributed within the nucleus and that cell cycle-dependent binding with E2F4 changes also as a function of their subnuclear localization. Namely, in the nucleoplasm pRb2/p130-E2F4 complexes are more numerous during G0/G1 while in the nucleolus they increase in S phase. Partially different functions for p107 are suggested since p107-E2F4 complexes in the nucleoplasm are more numerous is S phase with respect to G0/G1 and no cell cycle change is observed in the nucleolus. In a second series of experiments we showed that pRb2/p130, p107, E2F4, and pRb2/p130-HDAC1 complexes are all inner nuclear matrix-associated proteins and localize to sites different from pRb/p105 ones. We provide further evidence of multiple and partially distinct retinoblastoma protein family functional roles during cell cycle. Moreover, our data support emerging evidence for functional interrelationships between nuclear structure and gene expression.

Overlapping functions of the pRb family in the regulation of rRNA synthesis

The "pocket" proteins pRb, p107, and p130 are a family of negative growth regulators. Previous studies have demonstrated that overexpression of pRb can repress transcription by RNA polymerase (Pol) I. To assess whether pRb performs this role under physiological conditions, we have examined pre-rRNA levels in cells from mice lacking either pRb alone or combinations of the three pocket proteins. Pol I transcription was unaffected in pRb-knockout fibroblasts, but specific disruption of the entire pRb family deregulated rRNA synthesis. Further analysis showed that p130 shares with pRb the ability to repress Pol I transcription, whereas p107 is ineffective in this system. Production of rRNA is abnormally elevated in Rb(-/-) p130(-/-) fibroblasts. Furthermore, overexpression of p130 can inhibit an rRNA promoter both in vitro and in vivo. This reflects an ability of p130 to bind and inactivate the upstream binding factor, UBF. The data imply that rRNA synthesis in living cells is subject to redundant control by endogenous pRb and p130.

DNA looping in the RNA polymerase I enhancesome is the result of non-cooperative in-phase bending by two UBF molecules

The so-called upstream binding factor (UBF) is required for the initial step in formation of an RNA polymerase I initiation complex. This function of UBF correlates with its ability to induce the ribosomal enhancesome, a structure which resembles in its mass and DNA content the nucleosome of chromatin. DNA looping in the enhancesome is probably the result of six in-phase bends induced by the HMG boxes of a UBF dimer. Here we show that insertion/deletion mutations in the basic peptide linker lying between the N-terminal dimerisation domain and the first HMG box of Xenopus UBF prevent the DNA looping characteristic of the enhancesome. Using these mutants we demonstrate that (i) the enhancesome structure does not depend on tethering of the entering and exiting DNA duplexes, (ii) UBF monomers induce hemi-enhancesomes, bending the DNA by 175 +/- 24 degrees and (iii) two hemi-enhancesomes are precisely phased by UBF dimerisation. We use this and previous data to refine the existing enhancesome model and show that HMG boxes 1 and 2 of UBF lie head-to-head along the DNA.

Fibroblast Growth Factor Homologous Factor 1B Binds to the C Terminus of the Tetrodotoxin-resistant Sodium Channel rNav1.9a (NaN)

In this study we demonstrate a direct interaction between a cytosolic fibroblast growth factor family member and a sodium channel. A yeast two-hybrid screen for proteins that associate with the cytoplasmic domains of the tetrodotoxin-resistant sodium channel rNa(v)1.9a (NaN) led to the identification of fibroblast growth factor homologous factor 1B (FHF1B), a member of the fibroblast growth factor family, as an interacting partner of rNa(v)1.9a. FHF1B selectively interacts with the C-terminal region but not the other four intracellular segments of rNa(v)1.9a. FHF1B binds directly to the C-terminal polypeptide of rNa(v)1.9a both in vitro and in mammalian cell lines. The N-terminal 5-77 amino acid residues of FHF1B are essential for binding to rNa(v)1.9a. FHF1B did not interact with C termini of two other sodium channels hNa(v)1.7a (hNaNE) and rNa(v)1.8a (SNS), which share 50% similarity to the C-terminal polypeptide of rNa(v)1.9a. FHF1B is the first growth factor found to bind specifically to a sodium channel. Although the functional significance of this interaction is not clear, FHF1B may affect the rNa(v)1.9a channel directly or by recruiting other proteins to the channel complex. Alternatively, it is possible that rNa(v)1.9a may help deliver this factor to the cell membrane, where it exerts its function.

Acetylation of TAF(I)68, a subunit of TIF-IB/SL1, activates RNA polymerase I transcription

Mammalian rRNA genes are preceded by a terminator element that is recognized by the transcription termination factor TTF-I. In exploring the functional significance of the promoter-proximal terminator, we found that TTF-I associates with the p300/CBP-associated factor PCAF, suggesting that TTF-I may target histone acetyltransferase to the rDNA promoter. We demonstrate that PCAF acetylates TAF(I)68, the second largest subunit of the TATA box-binding protein (TBP)-containing factor TIF-IB/SL1, and acetylation enhances binding of TAF(I)68 to the rDNA promoter. Moreover, PCAF stimulates RNA polymerase I (Pol I) transcription in a reconstituted in vitro system. Consistent with acetylation of TIF-IB/SL1 being required for rDNA transcription, the NAD(+)-dependent histone deacetylase mSir2a deacetylates TAF(I)68 and represses Pol I transcription. The results demonstrate that acetylation of the basal Pol I transcription machinery has functional consequences and suggest that reversible acetylation of TIF-IB/SL1 may be an effective means to regulate rDNA transcription in response to external signals.

Function of basonuclin in increasing transcription of the ribosomal RNA genes during mouse oogenesis

Active protein synthesis during early oogenesis requires accelerated transcription of ribosomal RNA genes (rDNAs). In response to this demand, rDNAs are amplified more than 1000-fold early in Xenopus oogenesis. Here, we report evidence that rDNA is not amplified in mouse oocytes, but these cells may instead employ the zinc-finger protein basonuclin, a putative rDNA transcription factor, to enhance rRNA synthesis. This conclusion is based on observations that basonuclin is localized in the nucleolus in the mouse oocyte early in its growth phase, when rRNA transcription is highly active; and that the binding sites of basonuclin zinc fingers on the human and mouse rDNA promoters are homologous. In a co-transfection assay, basonuclin can elevate transcription from an rDNA promoter, and its zinc-finger domain can inhibit RNA polymerase I transcription, as detected by a run-on assay, in growing mouse oocytes.

The transcript release factor PTRF augments ribosomal gene transcription by facilitating reinitiation of RNA polymerase I

Termination of murine rDNA transcription by RNA polymerase I (Pol I) requires pausing of Pol I by terminator-bound TTF-I (transcription termination factor for Pol I), followed by dissociation of the ternary complex by PTRF (Pol I and transcript release factor). To examine the functional correlation between transcription termination and initiation, we have compared transcription on terminator-containing and terminator-less rDNA templates. We demonstrate that terminated RNA molecules are more efficiently synthesized than run-off transcripts, indicating that termination facilitates reinitiation. Transcriptional enhancement is observed in multiple- but not single-round transcription assays measuring either promoter-dependent or promoter-independent Pol I transcription. Increased synthesis of terminated transcripts is observed in crude extracts but not in a PTRF-free reconstituted transcription system, indicating that PTRF-mediated release of pre-rRNA is responsible for transcriptional enhancement. Consistent with PTRF serving an important role in modulating the efficiency of rRNA synthesis, PTRF exhibits pronounced charge heterogeneity, is phosphorylated at multiple sites and fractionates into transcriptionally active and inactive forms. The results suggest that regulation of PTRF activity may be an as yet unrecognized means to control the efficiency of ribosomal RNA synthesis.

Retinoblastoma protein disrupts interactions required for RNA polymerase III transcription

The retinoblastoma protein (RB) has been shown to suppress RNA polymerase (Pol) III transcription in vivo (R. J. White, D. Trouche, K. Martin, S. P. Jackson, and T. Kouzarides, Nature 382:88-90, 1996). This regulation involves interaction with TFIIIB, a multisubunit factor that is required for the expression of all Pol III templates (C. G. C. Larminie, C. A. Cairns, R. Mital, K. Martin, T. Kouzarides, S. P. Jackson, and R. J. White, EMBO J. 16:2061-2071, 1997; W.-M. Chu, Z. Wang, R. G. Roeder, and C. W. Schmid, J. Biol. Chem. 272:14755-14761, 1997). However, it has not been established why RB binding to TFIIIB results in transcriptional repression. For several Pol II-transcribed genes, RB has been shown to inhibit expression by recruiting histone deacetylases, which are thought to decrease promoter accessibility. We present evidence that histone deacetylases exert a negative effect on Pol III activity in vivo. However, RB remains able to regulate Pol III transcription in the presence of the histone deacetylase inhibitor trichostatin A. Instead, RB represses by disrupting interactions between TFIIIB and other components of the basal Pol III transcription apparatus. Recruitment of TFIIIB to most class III genes requires its binding to TFIIIC2, but this can be blocked by RB. In addition, RB disrupts the interaction between TFIIIB and Pol III that is essential for transcription. The ability of RB to inhibit these key interactions can explain its action as a potent repressor of class III gene expression.