Activation of mammalian ribosomal gene transcription requires phosphorylation of the nucleolar transcription factor UBF

Synthesis of the ribosomal RNA precursor in human cells: mechanisms, factors and regulation

The ribosomal RNA precursor (pre-rRNA) comprises three of the four ribosomal RNAs and is synthesized by RNA polymerase (Pol) I. Here, we describe the mechanisms of Pol I transcription in human cells with a focus on recent insights gained from structure-function analyses. The comparison of Pol I-specific structural and functional features with those of other Pols and with the excessively studied yeast system distinguishes organism-specific from general traits. We explain the organization of the genomic rDNA loci in human cells, describe the Pol I transcription cycle regarding structural changes in the enzyme and the roles of human Pol I subunits, and depict human rDNA transcription factors and their function on a mechanistic level. We disentangle information gained by direct investigation from what had apparently been deduced from studies of the yeast enzymes. Finally, we provide information about how Pol I mutations may contribute to developmental diseases, and why Pol I is a target for new cancer treatment strategies, since increased rRNA synthesis was correlated with rapidly expanding cell populations.

Consecutive bouts of electrical stimulation-induced contractions alter ribosome biogenesis in rat skeletal muscle

Ribosome biogenesis has been implicated in resistance exercise training (RET)-induced skeletal muscle hypertrophy. However, it is unclear how increasing bouts of RET affects ribosome content and biogenesis. This was investigated in the present study using simulated RET where rat skeletal muscle is subjected to increasing bouts of electrical stimulation. Sprague-Dawley rats were randomly assigned to the following seven groups: sedentary for 5 days (SED) or 6 wk (SED_6w), resistance-exercise trained with 1 bout (1B), 2 bouts (2B), 3 bouts (3B), 6 bouts (6B), and 18 bouts (18B). RET was simulated on the right gastrocnemius muscle by transcutaneous electric stimulation under isoflurane anesthesia, and a RET bout was given 3 times a week. Rats in 1B, 2B, and 3B groups showed increased 45S precursor (pre-) rRNA and 18S+28S rRNA content per muscle weight and elevated mRNA levels of c- myc and upstream binding factor (UBF). Increases in phosphorylated UBF and total cyclin D1 protein level were observed 48 h after RET; the former increased as a function of RET duration. In 3B, 6B, and 18B groups, the 18S+28S rRNA content per muscle weight was kept unchanged, and 45S pre-rRNA, cyclin D1, and phosphorylated UBF levels in 18B were lower than those in 3B. These results suggest that RET activates ribosome biogenesis and increases ribosome content through modulation of UBF and cyclin D1 activity at its early phase. Additional bouts of RET may not lead to a further increase in ribosome content per muscle weight through possibly the attenuation of transcription process.

NEW & NOTEWORTHY Ribosome biogenesis has been implicated in resistance exercise training-induced skeletal muscle hypertrophy. However, it remains unclear how this is influenced by the volume of repeated bouts of resistance exercise training. Using resistance exercise training model with rat skeletal muscle, we provide evidence that ribosome biogenesis is stimulated by the initial few bouts of resistance exercise training with no additional effect of further increase in the exercise bout.

The C-terminal region of Net1 is an activator of RNA polymerase I transcription with conserved features from yeast to human

RNA polymerase I (Pol I) synthesizes ribosomal RNA (rRNA) in all eukaryotes, accounting for the major part of transcriptional activity in proliferating cells. Although basal Pol I transcription factors have been characterized in diverse organisms, the molecular basis of the robust rRNA production in vivo remains largely unknown. In S. cerevisiae, the multifunctional Net1 protein was reported to stimulate Pol I transcription. We found that the Pol I-stimulating function can be attributed to the very C-terminal region (CTR) of Net1. The CTR was required for normal cell growth and Pol I recruitment to rRNA genes in vivo and sufficient to promote Pol I transcription in vitro. Similarity with the acidic tail region of mammalian Pol I transcription factor UBF, which could partly functionally substitute for the CTR, suggests conserved roles for CTR-like domains in Pol I transcription from yeast to human.

The production of ribosomes, cellular factories of protein synthesis, is an essential process driving proliferation and cell growth. Ribosome biogenesis is controlled at the level of synthesis of its components, ribosomal proteins and ribosomal RNA. In eukaryotes, RNA polymerase I is dedicated to transcribe the ribosomal RNA. RNA polymerase I has been identified as a potential target for cell proliferation inhibition. Here we describe the C-terminal region of Net1 as an activator of RNA polymerase I transcription in baker’s yeast. In the absence of this activator RNA polymerase I transcription is downregulated and cell proliferation is strongly impaired. Strikingly, this activator might be conserved in human cells, which points to a general mechanism. Our discovery will help to gain a better understanding of the molecular basis of ribosomal RNA synthesis and may have implications in developing strategies to control cellular growth.

Impact of resistance exercise on ribosome biogenesis is acutely regulated by post-exercise recovery strategies

Muscle hypertrophy occurs following increased protein synthesis, which requires activation of the ribosomal complex. Additionally, increased translational capacity via elevated ribosomal RNA (rRNA) synthesis has also been implicated in resistance training-induced skeletal muscle hypertrophy. The time course of ribosome biogenesis following resistance exercise (RE) and the impact exerted by differing recovery strategies remains unknown. In the present study, the activation of transcriptional regulators, the expression levels of pre-rRNA, and mature rRNA components were measured through 48 h after a single-bout RE. In addition, the effects of either low-intensity cycling (active recovery, ACT) or a cold-water immersion (CWI) recovery strategy were compared. Nine male subjects performed two bouts of high-load RE randomized to be followed by 10 min of either ACT or CWI. Muscle biopsies were collected before RE and at 2, 24, and 48 h after RE. RE increased the phosphorylation of the p38-MNK1-eIF4E axis, an effect only evident with ACT recovery. Downstream, cyclin D1 protein, total eIF4E, upstream binding factor 1 (UBF1), and c-Myc proteins were all increased only after RE with ACT. This corresponded with elevated abundance of the pre-rRNAs (45S, ITS-28S, ITS-5.8S, and ETS-18S) from 24 h after RE with ACT. In conclusion, coordinated upstream signaling and activation of transcriptional factors stimulated pre-rRNA expression after RE. CWI, as a recovery strategy, markedly blunted these events, suggesting that suppressed ribosome biogenesis may be one factor contributing to the impaired hypertrophic response observed when CWI is used regularly after exercise.

Acute resistance exercise activates rapamycin-sensitive and -insensitive mechanisms that control translational activity and capacity in skeletal muscle

KEY POINTS

Ribosome biogenesis is the primary determinant of translational capacity, but its regulation in skeletal muscle following acute resistance exercise is poorly understood. Resistance exercise increases muscle protein synthesis acutely, and muscle mass with training, but the role of translational capacity in these processes is unclear. Here, we show that acute resistance exercise activated pathways controlling translational activity and capacity through both rapamycin-sensitive and -insensitive mechanisms. Transcription factor c-Myc and its downstream targets, which are known to regulate ribosome biogenesis in other cell types, were upregulated after resistance exercise in a rapamycin-independent manner and may play a role in determining translational capacity in skeletal muscle. Local inhibition of myostatin was also not affected by rapamycin and may contribute to the rapamycin-independent effects of resistance exercise.

ABSTRACT

This study aimed to determine (1) the effect of acute resistance exercise on mechanisms of ribosome biogenesis, and (2) the impact of mammalian target of rapamycin on ribosome biogenesis, and muscle protein synthesis (MPS) and degradation. Female F344BN rats underwent unilateral electrical stimulation of the sciatic nerve to mimic resistance exercise in the tibialis anterior (TA) muscle. TA muscles were collected at intervals over the 36 h of exercise recovery (REx); separate groups of animals were administered rapamycin pre-exercise (REx+Rapamycin). Resistance exercise led to a prolonged (6-36 h) elevation (30-50%) of MPS that was fully blocked by rapamycin at 6 h but only partially at 18 h. REx also altered pathways that regulate protein homeostasis and mRNA translation in a manner that was both rapamycin-sensitive (proteasome activity; phosphorylation of S6K1 and rpS6) and rapamycin-insensitive (phosphorylation of eEF2, ERK1/2 and UBF; gene expression of the myostatin target Mighty as well as c-Myc and its targets involved in ribosome biogenesis). The role of c-Myc was tested in vitro using the inhibitor 10058-F4, which, over time, decreased basal RNA and MPS in a dose-dependent manner (correlation of RNA and MPS, r(2) = 0.98), even though it had no effect on the acute stimulation of protein synthesis. In conclusion, acute resistance exercise stimulated rapamycin-sensitive and -insensitive mechanisms that regulate translation activity and capacity.

Human papillomavirus 16 oncoprotein E7 stimulates UBF1-mediated rDNA gene transcription, inhibiting a p53-independent activity of p14ARF

High-risk human papillomavirus oncoproteins E6 and E7 play a major role in HPV-related cancers. One of the main functions of E7 is the degradation of pRb, while E6 promotes the degradation of p53, inactivating the p14^ARF-p53 pathway. pRb and p14^ARF can repress ribosomal DNA (rDNA) transcription in part by targeting the Upstream Binding Factor 1 (UBF1), a key factor in the activation of RNA polymerase I machinery. We showed, through ectopic expression and siRNA silencing of p14^ARF and/or E7, that E7 stimulates UBF1-mediated rDNA gene transcription, partly because of increased levels of phosphorylated UBF1, preventing the inhibitory function of p14^ARF. Unexpectedly, activation of rDNA gene transcription was higher in cells co-expressing p14^ARF and E7, compared to cells expressing E7 alone. We did not find a difference in P-UBF1 levels that could explain this data. However, p14^ARF expression induced E7 to accumulate into the nucleolus, where rDNA transcription takes place, providing an opportunity for E7 to interact with nucleolar proteins involved in this process. GST-pull down and co-immunoprecipitation assays showed interactions between p14^ARF, UBF1 and E7, although p14^ARF and E7 are not able to directly interact. Co-expression of a pRb-binding-deficient mutant (E7C24G) and p14^ARF resulted in EC24G nucleolar accumulation, but not in a significant higher activation of rDNA transcription, suggesting that the inactivation of pRb is involved in this phenomenon. Thus, p14^ARF fails to prevent E7-mediated UBF1 phosphorylation, but could facilitate nucleolar pRb inactivation by targeting E7 to the nucleolus. While others have reported that p19^ARF, the mouse homologue of p14^ARF, inhibits some functions of E7, we showed that E7 inhibits a p53-independent function of p14^ARF. These results point to a mutually functional interaction between p14^ARF and E7 that might partly explain why the sustained p14^ARF expression observed in most cervical pre-malignant lesions and malignancies may be ineffective.

A RUNX2-HDAC1 co-repressor complex regulates rRNA gene expression by modulating UBF acetylation

The osteogenic and oncogenic transcription factor RUNX2 downregulates the RNA polymerase I (RNA Pol I)-mediated transcription of rRNAs and changes histone modifications associated with the rDNA repeat. However, the mechanisms by which RUNX2 suppresses rRNA transcription are not well understood. RUNX2 cofactors such as histone deacetylases (HDACs) play a key role in chromatin remodeling and regulation of gene transcription. Here, we show that RUNX2 recruits HDAC1 to the rDNA repeats in osseous cells. This recruitment alters the histone modifications associated with active rRNA-encoding genes and causes deacetylation of the protein upstream binding factor (UBF, also known as UBTF). Downregulation of RUNX2 expression reduces the localization of HDAC1 to the nucleolar periphery and also decreases the association between HDAC1 and UBF. Functionally, depletion of HDAC1 relieves the RUNX2-mediated repression of rRNA-encoding genes and concomitantly increases cell proliferation and global protein synthesis in osseous cells. Our findings collectively identify a RUNX2-HDAC1-dependent mechanism for the regulation of rRNA-encoding genes and suggest that there is plasticity to RUNX2-mediated epigenetic control, which is mediated through selective mitotic exclusion of co-regulatory factors.

Identification of DHX33 as a mediator of rRNA synthesis and cell growth

In this report, we employed a lentiviral RNA interference screen to discover nucleolar DEAD/DEAH-box helicases involved in RNA polymerase I (Pol I)-mediated transcriptional activity. Our screen identified DHX33 as an important modulator of 47S rRNA transcription. We show that DHX33 is a cell cycle-regulated nucleolar protein that associates with ribosomal DNA (rDNA) loci, where it interacts with the RNA Pol I transcription factor upstream binding factor (UBF). DHX33 knockdown decreased the association of Pol I with rDNA and caused a dramatic decrease in levels of rRNA synthesis. Wild-type DHX33 overexpression, but not a DNA binding-defective mutant, enhanced 47S rRNA synthesis by promoting the association of RNA polymerase I with rDNA loci. In addition, an NTPase-defective DHX33 mutant (K94R) acted as a dominant negative mutant, inhibiting endogenous rRNA synthesis. Moreover, DHX33 deficiency in primary human fibroblasts triggered a nucleolar p53 stress response, resulting in an attenuation of proliferation. Thus, we show the mechanistic importance of DHX33 in rRNA transcription and proliferation.

Nuclear-localized calcineurin homologous protein CHP1 interacts with upstream binding factor and inhibits ribosomal RNA synthesis

Calcineurin homologous protein 1 (CHP1) is a widely expressed, 22-kDa myristoylated EF-hand Ca(2+)-binding protein that shares a high degree of similarity with the regulatory B subunit of calcineurin (65%) and with calmodulin (59%). CHP1 localizes to the plasma membrane, the Golgi apparatus, and the nucleus and functions to regulate trafficking of early secretory vesicles, activation of T cells, and expression and transport of the Na-H exchanger NHE1. Although CHP1 contains nuclear export signals, whether its nuclear and cytoplasmic localization is regulated and has distinct functions remain unknown. We show that CHP1 is predominantly in the nucleus in quiescent fibroblasts, is translocated to cytoplasmic compartments with growth medium, and that translocation is inhibited by mutations in the nuclear export motifs. In a screen for proteins co-precipitating with CHP1 in quiescent cells we identified the upstream binding factor UBF, a DNA-binding protein and component of the RNA polymerase I complex regulating RNA synthesis. The CHP1-UBF interaction is restricted to the nucleus and inhibited by Ca(2+). Nuclear retention of CHP1 attenuates the abundance of UBF in the nucleolus and inhibits RNA synthesis when quiescent cells are transferred to growth medium. These data show UBF as a newly identified CHP1-binding protein and regulation of RNA synthesis as a newly identified function for nuclear-localized CHP1, which is distinct from CHP1 functions in the cytosol.

The regulation of protein synthesis in cancer

Translational control of cancer is a multifaceted process, involving alterations in translation factor levels and activities that are unique to the different types of cancers and the different stages of disease. Translational alterations in cancer include adaptations of the tumor itself, of the tumor microenvironment, an integral component in disease, and adaptations that occur as cancer progresses from development to local disease and ultimately to metastatic disease. Adaptations include the overexpression and increased activity of specific translation factors, the physical or functional loss of translation regulatory components, increased production of ribosomes, selective mRNA translation, and alteration of signal transduction pathways to permit unfettered activation of protein synthesis. There is intense clinical interest to capitalize on the emerging new understanding of translational control in cancer by targeting specific components of the translation apparatus that are altered in disease for the development of specific cancer therapeutics. Clinical trial data are nascent but encouraging, suggesting that translational control constitutes an important new area for drug development in human cancer.

Dual regulation of upstream binding factor 1 levels by IRS-1 and ERKs in IGF-1-receptor signaling

The Upstream Binding Factor 1 (UBF1) is a nucleolar protein that participates in the regulation of RNA polymerase I activity and ribosomal RNA (rRNA) synthesis. In 32D myeloid cells expressing the type 1 insulin-like growth factor receptor (IGF-IR), the UBF1 protein (but not its mRNA) is down regulated when the cells are shifted from Interleukin-3 (IL-3) to IGF-1. Ectopic expression of insulin receptor substrate-1 (IRS-1) in these cells inhibits the down-regulation of UBF1. We now show that the stability of UBF1 in 32D-derived cells requires also a signal from the extracellular regulated kinases (ERKs). When ERKs signaling is defective, as in cells over-expressing the insulin receptor (InR) or selected mutants of the IGF-1R, UBF1 is down-regulated, even in the presence of IRS-1. The down-regulation is corrected by the expression of an activated Ha-ras, which stimulates ERKs activity. Mutations at threonines 117 and 201 of UBF1, known to be phosphorylated by ERKs, cause its down-regulation. However, when IRS-2, instead of IRS-1, is ectopically expressed in 32D InR cells, ERKs phosphorylation is increased and UBF is stabilized. Taken together, these results indicate that in 32D-derived myeloid cells expressing either the IGF-IR or the InR, UBF1 levels are regulated by signaling from both IRS proteins and ERKs.

Basonuclin regulates a subset of ribosomal RNA genes in HaCaT cells

Basonuclin (Bnc1), a cell-type-specific ribosomal RNA (rRNA) gene regulator, is expressed mainly in keratinocytes of stratified epithelium and gametogenic cells of testis and ovary. Previously, basonuclin was shown in vitro to interact with rRNA gene (rDNA) promoter at three highly conserved sites. Basonuclin's high affinity binding site overlaps with the binding site of a dedicated and ubiquitous Pol I transcription regulator, UBF, suggesting that their binding might interfere with each other if they bind to the same promoter. Knocking-down basonuclin in mouse oocytes eliminated approximately one quarter of RNA polymerase I (Pol I) transcription foci, without affecting the BrU incorporation of the remaining ones, suggesting that basonuclin might regulate a subset of rDNA. Here we show, via chromatin immunoprecipitation (ChIP), that basonuclin is associated with rDNA promoters in HaCaT cells, a spontaneously established human keratinocyte line. Immunoprecipitation data suggest that basonuclin is in a complex that also contains the subunits of Pol I (RPA194, RPA116), but not UBF. Knocking-down basonuclin in HaCaT cells partially impairs the association of RPA194 to rDNA promoter, but not that of UBF. Basonuclin-deficiency also reduces the amount of 47S pre-rRNA, but this effect can be seen only after cell-proliferation related rRNA synthesis has subsided at a higher cell density. DNA sequence of basonuclin-bound rDNA promoters shows single nucleotide polymorphisms (SNPs) that differ from those associated with UBF-bound promoters, suggesting that basonuclin and UBF interact with different subsets of promoters. In conclusion, our results demonstrate basonuclin's functional association with rDNA promoters and its interaction with Pol I in vivo. Our data also suggest that basonuclin-Pol I complex transcribes a subset of rDNA.

Downregulation of the upstream binding factor1 by glycogen synthase kinase3beta in myeloid cells induced to differentiate

The upstream binding factor 1 (UBF1), one of the proteins that regulate the activity of RNA polymerase I, is downregulated in 32D myeloid cells induced to differentiate into granulocytes, either by the type 1 insulin-like growth factor (IGF-1) or the granulocytic colony stimulating factor (G-CSF). Downregulation of UBF1 is largely due to protein degradation, while mRNA levels are not affected. Inhibition of UBF1 degradation by lithium chloride (LiCl)and lactacystin suggest a role of glycogen synthase kinase beta (GSK3beta) in a proteasome-dependent degradation of UBF. GSK3beta phosphorylates in vitro and in vivo the UBF protein, which has five putative motifs for phosphorylation by GSK3beta. Elimination and/or mutations of these motifs stabilize the UBF1 protein even in cells induced to differentiate. Conversely, a stably transfected, constitutively active GSK3beta accelerates the downregulation of UBF1. We show further that activation of the differentiating protein C/EPBalpha in 32D cells transformed by the oncogenic BCR/ABL protein causes downregulation of UBF1. Finally, inhibition of differentiation of myeloid cells by a dominant negative mutant of Stat3 stabilizes the UBF1 protein, while rapamycin-induced differentiation of myeloid cells downregulates UBF1 levels. Taken together, our results indicate that the induction of granulocytic differentiation in 32D murine myeloid cells causes the degradation of UBF1, via GSK3beta and the proteasome pathway.

Mass spectrometric identification of phosphorylation sites of rRNA transcription factor upstream binding factor

rRNA transcription is a fundamental requirement for all cellular growth processes and is activated by the phosphorylation of the upstream binding factor (UBF) in response to growth stimulation. Even though it is well known that phosphorylation of UBF is required for its activation and is a key step in activation of rRNA transcription, as yet, there has been no direct mapping of the UBF phosphorylation sites. The results of the present studies employed sophisticated nano-flow HPLC-microelectrospray-ionization tandem mass spectrometry (nHPLC-μESI-MS/MS) coupled with immobilized metal affinity chromatography (IMAC) and computer database searching algorithms to identify 10 phosphorylation sites on UBF at serines 273, 336, 364, 389, 412, 433, 484, 546, 584, and 638. We then carried out functional analysis of two of these sites, serines 389 and 584. Serine-alanine substitution mutations of 389 (S389A) abrogated rRNA transcription in vitro and in vivo, whereas mutation of serine 584 (S584A) reduced transcription in vivo but not in vitro. In contrast, serine-glutamate mutation of 389 (S389E) restored transcriptional activity. Moreover, S389A abolished UBF-SL1 interaction in vitro, while S389E partially restored UBF-SL1 interaction. Taken together, the results of these studies suggest that growth factor stimulation induces an increase in rRNA transcriptional activity via phosphorylation of UBF at serine 389 in part by facilitating a rate-limiting step in the recruitment of RNA polymerase I: i.e., recruitment of SL1. Moreover, studies provide critical new data regarding multiple additional UBF phosphorylation sites that will require further characterization by the field.

[The negative regulation of ribosome biogenesis: a new Arf-dependent pathway controlling cell proliferation?]

The nucleolar Arf protein has initially been shown to regulate cell cycle through the so-called Arf-mdm2-p53 pathway. In addition to this well characterized pathway, convergent data published since 2000 indicate that Arf can inhibit cell proliferation in absence of p53, suggesting the existence of a p53-independent pathway. Several partners have recently been described that could participate in an alternative regulatory process. Recent results show that : (1) Arf binds the rDNA promoter to inhibit the transcription of the 47S rRNA precursor and (2) Arf interacts with the nucleophosmin/B23 protein to negatively regulate rRNA maturation, it is assumed that the tumour suppressor may downregulate the cell cycle progression through the control of ribosome biogenesis, thus resulting in completion of cell cycle arrest.

Nucleolar protein upstream binding factor is sequestered into adenovirus DNA replication centres during infection without affecting RNA polymerase I location or ablating rRNA synthesis

When human adenovirus infects human cells there is disruption of rRNA biogenesis. This report examines the effect of adenovirus infection on the nucleolar protein, upstream binding factor (UBF) which plays a major role in regulating rRNA synthesis. We determined that early after infection, UBF associates with the replication of viral DNA, preferentially associating with the ends of the linear viral genome, and that addition of anti-UBF serum to in vitro replication assays markedly reduced viral DNA replication. Regions of UBF important to these observations are also established. Interestingly, sequestering the majority of UBF from the nucleolus did not lead to the ablation of rRNA synthesis or the sequestration of RNA pol I. In infected cells the bulk of RNA synthesis was RNA pol I associated and distinct from the location of most of the detectable UBF. We propose that UBF plays a role in viral DNA replication, further strengthening the role of nucleolar antigens in the adenovirus life cycle.

Casein kinase 2 associates with initiation-competent RNA polymerase I and has multiple roles in ribosomal DNA transcription

Mammalian RNA polymerase I (Pol I) complexes contain a number of associated factors, some with undefined regulatory roles in transcription. We demonstrate that casein kinase 2 (CK2) in human cells is associated specifically only with the initiation-competent Pol Ibeta isoform and not with Pol Ialpha. Chromatin immunoprecipitation analysis places CK2 at the ribosomal DNA (rDNA) promoter in vivo. Pol Ibeta-associated CK2 can phosphorylate topoisomerase IIalpha in Pol Ibeta, activator upstream binding factor (UBF), and selectivity factor 1 (SL1) subunit TAFI110. A potent and selective CK2 inhibitor, 3,8-dibromo-7-hydroxy-4-methylchromen-2-one, limits in vitro transcription to a single round, suggesting a role for CK2 in reinitiation. Phosphorylation of UBF by CK2 increases SL1-dependent stabilization of UBF at the rDNA promoter, providing a molecular mechanism for the stimulatory effect of CK2 on UBF activation of transcription. These positive effects of CK2 in Pol I transcription contrast to that wrought by CK2 phosphorylation of TAFI110, which prevents SL1 binding to rDNA, thereby abrogating the ability of SL1 to nucleate preinitiation complex (PIC) formation. Thus, CK2 has the potential to regulate Pol I transcription at multiple levels, in PIC formation, activation, and reinitiation of transcription.

CK2-mediated stimulation of Pol I transcription by stabilization of UBF-SL1 interaction

High levels of rRNA synthesis by RNA polymerase I are important for cell growth and proliferation. In vitro studies have indicated that the formation of a stable complex between the HMG box factor [Upstream binding factor (UBF)] and SL1 at the rRNA gene promoter is necessary to direct multiple rounds of Pol I transcription initiation. The recruitment of SL1 to the promoter occurs through protein interactions with UBF and is regulated by phosphorylation of UBF. Here we show that the protein kinase CK2 co-immunoprecipitates with the Pol I complex and is associated with the rRNA gene promoter. Inhibition of CK2 kinase activity reduces Pol I transcription in cultured cells and in vitro. Significantly, CK2 regulates the interaction between UBF and SL1 by counteracting the inhibitory effect of HMG boxes five and six through the phosphorylation of specific serines located at the C-terminus of UBF. Transcription reactions with immobilized templates indicate that phosphorylation of CK2 phosphoacceptor sites in the C-terminal domain of UBF is important for promoting multiple rounds of Pol I transcription. These data demonstrate that CK2 is recruited to the rRNA gene promoter and directly regulates Pol I transcription re-initiation by stabilizing the association between UBF and SL1.

UBF activates RNA polymerase I transcription by stimulating promoter escape

Ribosomal RNA gene transcription by RNA polymerase I (Pol I) is the driving force behind ribosome biogenesis, vital to cell growth and proliferation. The key activator of Pol I transcription, UBF, has been proposed to act by facilitating recruitment of Pol I and essential basal factor SL1 to rDNA promoters. However, we found no evidence that UBF could stimulate recruitment or stabilization of the pre-initiation complex (PIC) in reconstituted transcription assays. In this, UBF is fundamentally different from archetypal activators of transcription. Our data imply that UBF exerts its stimulatory effect on RNA synthesis, after PIC formation, promoter opening and first phosphodiester bond formation and before elongation. We provide evidence to suggest that UBF activates transcription in the transition between initiation and elongation, at promoter escape by Pol I. This novel role for UBF in promoter escape would allow control of rRNA synthesis at active rDNA repeats, independent of and complementary to the promoter-specific targeting of SL1 and Pol I during PIC assembly. We posit that stimulation of promoter escape could be a general mechanism of activator function.

The RNA polymerase I transcription machinery

The rRNAs constitute the catalytic and structural components of the ribosome, the protein synthesis machinery of cells. The level of rRNA synthesis, mediated by Pol I (RNA polymerase I), therefore has a major impact on the life and destiny of a cell. In order to elucidate how cells achieve the stringent control of Pol I transcription, matching the supply of rRNA to demand under different cellular growth conditions, it is essential to understand the components and mechanics of the Pol I transcription machinery. In this review, we discuss: (i) the molecular composition and functions of the Pol I enzyme complex and the two main Pol I transcription factors, SL1 (selectivity factor 1) and UBF (upstream binding factor); (ii) the interplay between these factors during pre-initiation complex formation at the rDNA promoter in mammalian cells; and (iii) the cellular control of the Pol I transcription machinery.

Growth factor signaling regulates elongation of RNA polymerase I transcription in mammals via UBF phosphorylation and r-chromatin remodeling

Synthesis of the 45S rRNA by RNA polymerase I limits cell growth. Knowledge of the mechanism of its regulation is therefore key to understanding growth control. rRNA transcription is believed to be regulated solely at initiation/promoter release. However, we found that stimulation of endogenous 45S rRNA synthesis by epidermal growth factor (EGF) and serum failed to induce an increase in RNA polymerase I engagement on the rRNA genes, despite robust enhancement of 45S rRNA synthesis. Further, endogenous transcription elongation rates were measured and found to be directly proportional to 45S rRNA synthesis. Thus, elongation is a rate-limiting step for rRNA synthesis in vivo. ERK phosphorylation of the HMG boxes of UBF, an RNA polymerase I factor essential for transcription enhancement, was shown to directly regulate elongation by inducing the remodeling of ribosomal gene chromatin. The data suggest a mechanism for coordinating the cotranscriptional assembly of preribosomal particles.

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.

Transformation by the simian virus 40 T antigen is regulated by IGF-I receptor and IRS-1 signaling

Previous work has shown that the Simian Virus 40 T antigen (T antigen) cannot transform mouse embryo fibroblasts (MEFs) that do not express the type 1 insulin-like growth factor receptor (IGF-IR). We have now investigated the mechanism(s) by which the transforming activity of T antigen is affected by IGF-IR signaling. We demonstrate that transformation by T antigen of MEFs and several other cell lines requires an insulin receptor substrate-1 (IRS-1) phosphorylated on tyrosines. If IRS-1 is not expressed, or is serine phosphorylated or otherwise inactive, T antigen fails to transform cells in culture. For instance, while T antigen cannot transform 32D myeloid cells (that do not express IRS-1), its transforming activity is restored by the expression of a wild-type IRS-1, but not of an IRS-1 mutated at the PI3K binding sites. The importance of IRS-1 activation of PI3K in T-antigen transformation is supported by the finding that a constitutively activated p110 subunit of PI3K, a target of IRS-1, overcomes the inability of T antigen to transform MEFs with a serine phosphorylated IRS-1. Taken together, these results indicate that the IRS-1/PI3K signaling is one of the mechanisms regulating transformation by the SV40 T antigen. We propose that the requirement for a tyrosyl-phosphorylated IRS-1 provides a mechanism to explain the failure of T antigen to transform MEFs with deleted IGF-IR genes.

The insulin-like growth factor-I receptor as a target for cancer therapy

This review examines the rationale for targeting the insulin-like growth factor (IGF)-I receptor in the therapy of human tumours and their metastases. The rationale is based on two crucial findings: 1) in experimental animals, normal cells are only partially affected by the deletion of the IGF-I receptor, whereas tumour cells undergo apoptosis when the IGF-I receptor is downregulated; and 2) cells with a deleted IGF-I receptor are refractory to transformation by viral and cellular oncogenes. This review focuses on the mechanisms underlying the experimental findings, and discusses the possibility of extrapolating the results obtained in animals to the cure of human tumours.

Functional significance of type 1 insulin-like growth factor-mediated nuclear translocation of the insulin receptor substrate-1 and beta-catenin

Previous work has shown that the transcriptional regulator beta-catenin can translocate to the nuclei when cells are stimulated with the type 1 insulin-like growth factor (IGF-1). We show by immunocoprecipitation and by confocal microscopy that beta-catenin binds to and co-localizes with the insulin receptor substrate-1 (IRS-1), a docking protein for both the insulin and the IGF-1 receptors. IRS-1 is required for IGF-1-mediated nuclear translocation of beta-catenin, resulting in the activation of the beta-catenin target genes. IGF-1-mediated nuclear translocation of beta-catenin is facilitated by the nuclear translocation of IRS-1. Both IRS-1 and beta-catenin are recruited to the cyclin D1 promoter, an established target for beta-catenin, but only IRS-1 is recruited to the ribosomal DNA (rDNA) promoter. UBF proteins (known to interact with both IRS-1 and beta-catenin) are also detectable in the cyclin D1 and rDNA promoters. These results indicate that IRS-1 (activated by the IGF-1 receptor) is one of several proteins that regulate the subcellular localization and activity of beta-catenin. The ability of IRS-1 to localize to both RNA polymerase II (with beta-catenin) and RNA polymerase I-regulated promoters suggest an explanation for the effect of IRS-1 on both cell growth in size and cell proliferation. This possibility is supported by the demonstration that enforced nuclear localization of IRS-1 causes nuclear translocation of beta-catenin and transformation of normal mouse embryo fibroblasts (colony formation in soft agar).

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.

Activation of RNA polymerase I transcription by hepatitis C virus core protein

The hepatitis C virus (HCV) core protein has been implicated in the transregulation of various RNA polymerase (Pol) II dependent genes as well as in the control of cellular growth and proliferation. In this study, we show that the core protein, whether individually expressed or produced as part of the HCV viral polyprotein, is the only viral product that has the potential to activate RNA Pol I transcription. Deletion analysis demonstrated that the fragment containing the N-terminal 1-156 residues, but not the 1-122 residues, of HCV core protein confers the same level of transactivation activity as the full-length protein. Moreover, the integrity of the Ser(116) and Arg(117) residues of HCV core protein was found to be critical for its transregulatory functions. We used DNA affinity chromatography to analyze the human ribosomal RNA promoter associated transcription machinery, and the results indicated that recruitment of the upstream binding factor and RNA Pol I to the ribosomal RNA promoter is enhanced in the presence of HCV core protein. Additionally, the HCV core protein mediated activation of ribosomal RNA transcription is accompanied by the hyperphosphorylation of upstream binding factor on serine residues, but not on threonine residues. Moreover, HCV core protein is present within the RNA Pol I multiprotein complex, indicating its direct involvement in facilitating the formation of a functional transcription complex. Protein-protein interaction studies further indicated that HCV core protein can associate with the selectivity factor (SL1) via direct contact with a specific component, TATA-binding protein (TBP). Additionally, the HCV core protein in cooperation with TBP is able to activate RNA Pol II and Pol III mediated transcription, in addition to RNA Pol I transcription. Thus, the results of this study suggest that HCV has evolved a mechanism to deregulate all three nuclear transcription systems, partly through targeting of the common transcription factor, TBP. Notably, the ability of the HCV core protein to upregulate RNA Pol I and Pol III transcription supports its active role in promoting cell growth, proliferation, and the progression of liver carcinogenesis during HCV infection.

Gli proteins up-regulate the expression of basonuclin in Basal cell carcinoma

Tumorigenesis is frequently accompanied by enhanced rRNA transcription, but the signaling mechanisms responsible for such enhancement remain unclear. Here, we report evidence suggesting a novel link between deregulated Hedgehog signaling and the augmented rRNA transcription in cancer. Aberrant activation of the Hedgehog pathway in keratinocytes is a hallmark of basal cell carcinoma (BCC), the most common cancer in light-skinned individuals. We show that Gli proteins, downstream effectors of the Hedgehog pathway, increase expression of a novel rRNA gene (rDNA) transcription factor, basonuclin, whose expression is markedly elevated in BCCs. The promoter of the human basonuclin gene contains a Gli-binding site, which is required for Gli protein binding and transcriptional activation. We show also that the level of 47S pre-rRNA is much higher in BCCs than in normal epidermis, suggesting an accelerated rRNA transcription in the neoplastic cells. Within BCC, those cells expressing the highest level of basonuclin also exhibit the greatest increase in 47S pre-rRNA, consistent with a role for basonuclin in increasing rRNA transcription in these cells. Our data suggest that Hedgehog-Gli pathway enhances rRNA transcription in BCC by increasing basonuclin gene expression.

Role of pescadillo in the transformation and immortalization of mammalian cells

The murine and human homologs of the zebrafish pescadillo protein (Pes1 and PES1, respectively) play important roles in ribosome biogenesis and DNA replication. We investigated the effect of Pes1 on the growth of mouse embryo (3T3-like) fibroblasts and conditionally immortalized human fibroblasts expressing the SV40 T antigen (AR5 cells). Increased expression of Pes1 causes transformation of mouse and human fibroblasts in culture (colony formation in soft agar). Although Pes1 can replace the SV40 T antigen in inducing colony formation in soft agar, it cannot substitute the T antigen in the immortalization of human fibroblasts, indicating that it distinguishes between the two functions. As the biological effects of Pes1 are similar to those of the insulin receptor substrate-1 (IRS-1), we investigated the interactions of Pes1 with IRS-1 itself and with the SV40 T antigen. The Pes1 protein (which localizes to the nuclei and nucleoli of cells) interacts with both IRS-1 and the SV40 T antigen, and markedly decreases the interaction of T antigen with p53. Taken together, these results suggest mechanisms for the ability of Pes1 to transform cells, and its failure to immortalize them.

The Treacher Collins syndrome (TCOF1) gene product is involved in ribosomal DNA gene transcription by interacting with upstream binding factor

Treacher Collins syndrome (TCS) is an autosomal dominant disorder characterized by an abnormality of craniofacial development that arises during early embryogenesis. TCS is caused by mutations in the gene TCOF1, which encodes the nucleolar phosphoprotein treacle. Even though the genetic alterations causing TCS have been uncovered, the mechanism underlying its pathogenesis and the function of treacle remain unknown. Here, we show that treacle is involved in ribosomal DNA gene transcription by interacting with upstream binding factor (UBF). Immunofluorescence labeling shows treacle and UBF colocalize to specific nucleolar organizer regions and cosegregate within nucleolar caps of actinomycin d-treated HeLa cells. Biochemical analysis shows the association of treacle and UBF with chromatin. Immunoprecipitation and the yeast two-hybrid system both suggest physical interaction of the two nucleolar phosphoproteins. Down-regulation of treacle expression using specific short interfering RNA results in inhibition of ribosomal DNA transcription and cell growth. A similar correlation is observed in Tcof(+/-) mouse embryos that exhibit craniofacial defects and growth retardation. Thus, treacle haploinsufficiency in TCS patients might result in abnormal development caused by inadequate ribosomal RNA production in the prefusion neural folds during the early stages of embryogenesis. The elucidation of a physiological function of treacle provides important information of relevance to the molecular dissection of the biochemical pathology of TCS.

Control of cell size through phosphorylation of upstream binding factor 1 by nuclear phosphatidylinositol 3-kinase

The insulin-like growth factor I/insulin receptor substrate 1 axis controls, in a nonredundant way, approximately 50% of cell and body size in animals from Drosophila to mice and in cells in culture. Although other factors may also intervene, cell size is strongly dependent on ribosome biogenesis, which is under the control of RNA polymerase I activity. We have previously shown that insulin receptor substrate 1 (IRS-1) translocates to the nuclei and nucleoli, where it binds to the upstream binding factor (UBF) 1, a regulator of RNA polymerase I activity. Activation of UBF1 requires its phosphorylation. However, IRS-1 is not a kinase, and we searched for an intermediate kinase that can phosphorylate UBF1. We demonstrate here that IRS-1 binds also to the phosphatidylinositol 3-kinase (PI3-K) subunits in nuclear extracts, and that the p110 subunit of PI3-K directly phosphorylates and activates UBF1, an exclusively nucleolar protein. The interaction of IRS-1, PI3-K, and UBF1 in the nucleoli provides one of the mechanisms for the effects of IRS-1 on cell and body size.

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 IGF-1 receptor in cancer biology

The type 1 insulin-like growth factor receptor (IGF-1R) plays an important role in the establishment and maintenance of the transformed phenotype. It also has a strong antiapoptotic activity and has a significant influence on the control of cell and body size. Downregulation of the IGF-1R leads to massive apoptosis of cancer cells. These characteristics make it an attractive target for anticancer 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.

mTOR-dependent regulation of ribosomal gene transcription requires S6K1 and is mediated by phosphorylation of the carboxy-terminal activation domain of the nucleolar transcription factor UBF

Mammalian target of rapamycin (mTOR) is a key regulator of cell growth acting via two independent targets, ribosomal protein S6 kinase 1 (S6K1) and 4EBP1. While each is known to regulate translational efficiency, the mechanism by which they control cell growth remains unclear. In addition to increased initiation of translation, the accelerated synthesis and accumulation of ribosomes are fundamental for efficient cell growth and proliferation. Using the mTOR inhibitor rapamycin, we show that mTOR is required for the rapid and sustained serum-induced activation of 45S ribosomal gene transcription (rDNA transcription), a major rate-limiting step in ribosome biogenesis and cellular growth. Expression of a constitutively active, rapamycin-insensitive mutant of S6K1 stimulated rDNA transcription in the absence of serum and rescued rapamycin repression of rDNA transcription. Moreover, overexpression of a dominant-negative S6K1 mutant repressed transcription in exponentially growing NIH 3T3 cells. Rapamycin treatment led to a rapid dephosphorylation of the carboxy-terminal activation domain of the rDNA transcription factor, UBF, which significantly reduced its ability to associate with the basal rDNA transcription factor SL-1. Rapamycin-mediated repression of rDNA transcription was rescued by purified recombinant phosphorylated UBF and endogenous UBF from exponentially growing NIH 3T3 cells but not by hypophosphorylated UBF from cells treated with rapamycin or dephosphorylated recombinant UBF. Thus, mTOR plays a critical role in the regulation of ribosome biogenesis via a mechanism that requires S6K1 activation and phosphorylation of UBF.

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.

Cardiac hypertrophy: a matter of translation

1. Left ventricular hypertrophy (LVH) of the heart is an adaptive response to sustained increases in blood pressure and hormone imbalances. Left ventricular hypertrophy is associated with programmed responses at the molecular and biochemical level in different subsets of cardiac cells, including the cardiac muscle cells (cardiomyocytes), fibroblasts, conductive tissue and coronary vasculature. 2. Regardless of the initiating cause, the actual increase in chamber enlargement is, in each case, due to an increase in size of a pre-existing cardiomyocyte population, with little or no change in their number; a process referred to as cellular hypertrophy. 3. An accelerated rate of global protein synthesis is the primary mechanism by which protein accumulation increases during cardiomyocyte hypertrophy. In turn, increased rates of synthesis are a result of increased translational rates of existing ribosomes (translational efficiency) and/or synthesis and recruitment of additional ribosomes (translational capacity). 4. The present review examines the relative importance of translational capacity and translational efficiency in the response of myocytes to acute and chronic demands for increased protein synthesis and the role of these mechanisms in the development of LVH.

Nuclear translocation of insulin receptor substrate-1 by the insulin receptor in mouse embryo fibroblasts

Translocation of the insulin receptor substrate-1 (IRS-1) to the nuclei has been reported to occur in cells stimulated by insulin-like growth factor-1 (IGF-I) or expressing certain viral and cellular oncogenes. We show here that insulin can also induce nuclear translocation of IRS-1 in mouse embryo fibroblasts (MEF), that do not express the type 1 insulin-like growth factor receptor (IGF-IR). Only the A isoform of the insulin receptor (IR) can induce IRS-1 nuclear translocation, which is significant when the receptor is over-expressed. At physiological receptor levels, translocation occurs only in a fraction of cells, and only at high concentrations of ligand.

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.

Interaction of casein kinase II with ribosomal protein L22 of Drosophila melanogaster

The ubiquitous eukaryotic protein kinase CKII (casein kinase II) has been found to interact with a number of cellular proteins, either through the catalytic subunit or the regulatory subunit. Using the yeast two-hybrid screening method, we found that the catalytic subunit of Drosophila melanogaster CKII (DmCKII) interacts with Drosophila ribosomal protein L22 (rpL22). This interaction was also observed in vitro with a glutathione-S-transferase (GST)-rpL22 fusion protein. The predicted full-length Drosophila rpL22 protein has an N-terminal extension rich in alanine, lysine, and proline that appears to be unique to Drosophila. Deletion mapping revealed that the conserved core of rpL22 is responsible for the interaction with CKII. Moreover, purified DmCKII can phosphorylate a GST-L22 fusion protein at the C-terminal end, suggesting that this protein may be a substrate of CKII in Drosophila.

Translational control: implications for skeletal muscle hypertrophy

Skeletal muscle hypertrophy is characterized, in part, by increases in protein mass per fiber. This increased accumulation of protein results from a net increase in protein synthesis relative to breakdown. Increases in rates of protein synthesis (translation) have been reported across different models of resistance exercise and across all species studied. However, although an increase in protein synthesis after exercise is reported commonly, the mechanisms underlying this response are not understood clearly. Therefore, the aim of the current review was to select areas of research within which translational control has been well-studied. The logic is that the mechanisms described in this review have the potential to contribute to the changes seen in protein synthesis after high-resistance exercise. The field of translational control has seen rapid growth in the past 5 to 10 years and although attempts have been made to include all contributing studies, apologies are given from the start because many have undoubtedly been overlooked.

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.

Werner protein is a target of DNA-dependent protein kinase in vivo and in vitro, and its catalytic activities are regulated by phosphorylation

Human Werner Syndrome is characterized by early onset of aging, elevated chromosomal instability, and a high incidence of cancer. Werner protein (WRN) is a member of the recQ gene family, but unlike other members of the recQ family, it contains a unique 3'-->5' exonuclease activity. We have reported previously that human Ku heterodimer interacts physically with WRN and functionally stimulates WRN exonuclease activity. Because Ku and DNA-PKcs, the catalytic subunit of DNA-dependent protein kinase (DNA-PK), form a complex at DNA ends, we have now explored the possibility of functional modulation of WRN exonuclease activity by DNA-PK. We find that although DNA-PKcs alone does not affect the WRN exonuclease activity, the additional presence of Ku mediates a marked inhibition of it. The inhibition of WRN exonuclease by DNA-PKcs requires the kinase activity of DNA-PKcs. WRN is a target for DNA-PKcs phosphorylation, and this phosphorylation requires the presence of Ku. We also find that treatment of recombinant WRN with a Ser/Thr phosphatase enhances WRN exonuclease and helicase activities and that WRN catalytic activity can be inhibited by rephosphorylation of WRN with DNA-PK. Thus, the level of phosphorylation of WRN appears to regulate its catalytic activities. WRN forms a complex, both in vitro and in vivo, with DNA-PKC. WRN is phosphorylated in vivo after treatment of cells with DNA-damaging agents in a pathway that requires DNA-PKcs. Thus, WRN protein is a target for DNA-PK phosphorylation in vitro and in vivo, and this phosphorylation may be a way of regulating its different catalytic activities, possibly in the repair of DNA dsb.