Preferential stimulation of ribosomal protein synthesis by insulin and in the absence of ribosomal and messenger ribonucleic acid formation

LARP1 senses free ribosomes to coordinate supply and demand of ribosomal proteins

Terminal oligopyrimidine motif-containing mRNAs (TOPs) encode all ribosomal proteins in mammals and are regulated to tune ribosome synthesis to cell state. Previous studies implicate LARP1 in 40S- or 80S-ribosome complexes that repress and stabilize TOPs. However, a mechanistic understanding of how LARP1 and TOPs interact with these complexes to coordinate TOP outcomes is lacking. Here, we show that LARP1 senses the cellular supply of ribosomes by directly binding non-translating ribosomal subunits. Cryo-EM structures reveal a previously uncharacterized domain of LARP1 bound to and occluding the 40S mRNA channel. Free cytosolic ribosomes induce sequestration of TOPs in repressed 80S-LARP1-TOP complexes independent of alterations in mTOR signaling. Together, this work demonstrates a general ribosome-sensing function of LARP1 that allows it to tune ribosome protein synthesis to cellular demand.

Cell cycle control by the insulin-like growth factor signal: at the crossroad between cell growth and mitotic regulation

In proliferating cells and tissues a number of checkpoints (G1/S and G2/M) preceding cell division (M-phase) require the signal provided by growth factors present in serum. IGFs (I and II) have been demonstrated to constitute key intrinsic components of the peptidic active fraction of mammalian serum. In vivo genetic ablation studies have shown that the cellular signal triggered by the IGFs through their cellular receptors represents a non-replaceable requirement for cell growth and cell cycle progression. Retroactive and current evaluation of published literature sheds light on the intracellular circuitry activated by these factors providing us with a better picture of the pleiotropic mechanistic actions by which IGFs regulate both cell size and mitogenesis under developmental growth as well as in malignant proliferation. The present work aims to summarize the cumulative knowledge learned from the IGF ligands/receptors and their intracellular signaling transducers towards control of cell size and cell-cycle with particular focus to their actionable circuits in human cancer. Furthermore, we bring novel perspectives on key functional discriminants of the IGF growth-mitogenic pathway allowing re-evaluation on some of its signal components based upon established evidences.

PP2Cα positively regulates neuronal insulin signalling and aggravates neuronal insulin resistance

PP2Cα is one of the newly identified isoforms of metal-dependent protein phosphatases (PPM). The role of this phosphatase in neuronal insulin signalling is completely unknown. In the present study, we show insulin-mediated rapid upregulation of a protein of the insulin signalling cascade, PP2Cα, in mouse N2a cells and human SH-SY5Y cells. By contrast, such PP2Cα upregulation is not observed in insulin-resistant conditions despite insulin stimulation. Here, we report that, under insulin-sensitive and insulin-resistant conditions, the translation of PP2Cα was regulated by insulin through c-Jun N-terminal kinase. PP2Cα in turn dephosphorylated a novel inhibitory site of insulin receptor substrate-1 at Ser522 and AMP-activated protein kinase, hence positively regulating neuronal insulin signalling and insulin resistance.

The logic of the Membrane, Magnesium, Mitosis (MMM) model for the regulation of animal cell proliferation

The addition of animal serum or specific protein growth factors to quiescent, serum-deprived vertebrate cells in culture activates a wide variety of biochemical responses within minutes, which are followed in 5-10h by the initiation of DNA synthesis and then by mitosis. The quintessential early and continuing activation step for the increase in DNA synthesis is the increased initiation rate of protein synthesis, which must be continuously maintained throughout the G1 phase for advancement into S. The aggregate of biochemical reactions to growth factors is called the coordinate response, to indicate that many related and unrelated processes are orchestrated to repetitively reproduce cells. It is, however, crucial to recognize that the coordinate response can be induced for one or more rounds of replication by a variety of non-specific and quasi-specific membrane effectors. The logic of considering this framework of events in growth control implied that a single multi-target second messenger plays a central role in coordinating the events of the overall response. The same reasoning suggested that free Mg(2+) is the unifying regulatory element in that response which includes protein kinase pathways, and that the cytoplasmic activity of Mg(2+) increases with the binding of growth factors to their receptors in the cell membrane, or of less specific perturbations of the membrane. Experimental support of this conclusion is presented here and is represented in the MMM model of cell proliferation control.

Central roles of Mg2+ and MgATP2- in the regulation of protein synthesis and cell proliferation: significance for neoplastic transformation

Growth factors are polypeptides that combine with specific membrane receptors on animal cells to stimulate proliferation, but they also stimulate glucose transport, uridine phosphorylation, intermediary metabolism, protein synthesis, and other processes of the coordinate response. There are a variety of nonspecific surface action treatments which stimulate the same set of reactions as the growth factors do, of which protein synthesis is most directly related to the onset of DNA synthesis. Mg(2+) is required for a very wide range of cellular reactions, including all phosphoryl transfers, and its deprivation inhibits all components of the coordinate response that have so far been tested. Growth factors raise the level of free Mg(2+) closer to the optimum for the initiation of protein synthesis. The resulting increase in protein synthesis accelerates progression through G1 to the onset of DNA synthesis and mitosis. None of the other 3 major cellular cations are similarly involved in growth regulation, although internal pH may play an auxiliary role. Almost 10(5) externally bound divalent cations are displaced from membranes for every attached insulin molecule, implying a conformational membrane change that releases enough Mg(2+) from the internal surface of the plasma membrane to account for the increase in free cytosolic Mg(2+). It is proposed that mTOR, the central control point for protein synthesis of the PI 3-K kinase cascade stimulated by insulin, is regulated by MgATP(2-) which varies directly with cytosolic Mg(2+). Other elements of the coordinate response to growth factors such as the increased transport of glucose and phosphorylation of uridine are also dependent upon an increase of Mg(2+). Deprivation of Mg(2+) in neoplastically transformed cultures normalizes their appearance and growth behavior and raises their abnormally low Ca(2+) concentration. Tight packing of the transformed cells at very high saturation density confers the same normalizing effects, which are retained for a few days after subculture at low density. The results suggest that the activity of Mg(2+) within the cell is a central regulator of normal cell growth, and the loss of its membrane-mediated control can account for the neoplastic phenotype.

Magnesium: The missing element in molecular views of cell proliferation control

The quantitative study of regulation of cell growth and proliferation began with the development of the technique for monolayer culture of vertebrate cells in the late 1960s. The basic parameters were defined in the early physiological studies, which continued through the next decade. These included specific and non-specific growth factors and the requirement for continuous exposure to such factors through most of the G1 period for progression to S. In the course of this work, the diversity of biochemical responses and the critical role of increased protein synthesis and accumulation for the onset of DNA synthesis were elucidated. In particular, a central role of free cytosolic Mg2+ in direct regulation of protein synthesis and in ancillary processes as a response to membrane perturbation was established. Eventually, the physiological era was superseded by the molecular era beginning in the 1980s. This work focussed on specific receptors for growth factors that entrained a protein kinase cascade, which terminated in a higher frequency of initiation of protein synthesis. However, the molecular studies virtually ignored the key results of the physiological era. Recent studies of the penultimate molecular steps in the regulatory pathway of protein synthesis, however, have supported a model of growth regulation involving membrane perturbation and MgATP2- concentration, results that integrate the findings of the physiological and molecular eras. The resulting relatively simple "membrane, magnesium mitosis" (MMM) model of proliferation control can explain the seeming paradox of the variety of specific and non-specific growth-enhancing treatments that are mediated by the plasma membrane and which bring about a shared, complex but coordinated growth response that drives cell proliferation.

Regulation of ribosomal protein mRNA content and translation in growth-stimulated mouse fibroblasts

When resting (G0) mouse 3T6 fibroblasts are serum stimulated to reenter the cell cycle, the rates of synthesis of rRNA and ribosomal proteins increase, resulting in an increase in ribosome content beginning about 6 h after stimulation. In this study, we monitored the content, metabolism, and translation of ribosomal protein mRNA (rp mRNA) in resting, exponentially growing, and serum-stimulated 3T6 cells. Cloned cDNAs for seven rp mRNAs were used in DNA-excess filter hybridization studies to assay rp mRNA. We found that about 85% of rp mRNA is polyadenylated under all growth conditions. The rate of labeling of rp mRNA relative to total polyadenylated mRNA changed very little after stimulation. The half-life of rp mRNA was about 11 h in resting cells and about 8 h in exponentially growing cells, values which are similar to the half-lives of total mRNA in resting and growing cells (about 9 h). The content of rp mRNA relative to total mRNA was about the same in resting and growing 3T6 cells. Furthermore, the total amount of rp mRNA did not begin to increase until about 6 h after stimulation. Since an increase in rp mRNA content did not appear to be responsible for the increase in ribosomal protein synthesis, we determined the efficiency of translation of rp mRNA under different conditions. We found that about 85% of pulse-labeled rp mRNA was associated with polysomes in exponentially growing cells. In resting cells, however, only about half was associated with polysomes, and about 30% was found in the monosomal fraction. The distribution shifted to that found in growing cells within 3 h after serum stimulation. Similar results were obtained when cells were labeled for 10.5 h. About 70% of total polyadenylated mRNA was in the polysome fraction in all growth states regardless of labeling time, indicating that the shift in mRNA distribution was species specific. These results indicate that the content and metabolism of rp mRNA do not change significantly after growth stimulation. The rate of ribosomal protein synthesis appears to be controlled during the resting-growing transition by an alteration of the efficiency of translation of rp mRNA, possibly at the level of protein synthesis initiation.

Regulation of ribosomal protein mRNA content and translation in growth-stimulated mouse fibroblasts

When resting (G0) mouse 3T6 fibroblasts are serum stimulated to reenter the cell cycle, the rates of synthesis of rRNA and ribosomal proteins increase, resulting in an increase in ribosome content beginning about 6 h after stimulation. In this study, we monitored the content, metabolism, and translation of ribosomal protein mRNA (rp mRNA) in resting, exponentially growing, and serum-stimulated 3T6 cells. Cloned cDNAs for seven rp mRNAs were used in DNA-excess filter hybridization studies to assay rp mRNA. We found that about 85% of rp mRNA is polyadenylated under all growth conditions. The rate of labeling of rp mRNA relative to total polyadenylated mRNA changed very little after stimulation. The half-life of rp mRNA was about 11 h in resting cells and about 8 h in exponentially growing cells, values which are similar to the half-lives of total mRNA in resting and growing cells (about 9 h). The content of rp mRNA relative to total mRNA was about the same in resting and growing 3T6 cells. Furthermore, the total amount of rp mRNA did not begin to increase until about 6 h after stimulation. Since an increase in rp mRNA content did not appear to be responsible for the increase in ribosomal protein synthesis, we determined the efficiency of translation of rp mRNA under different conditions. We found that about 85% of pulse-labeled rp mRNA was associated with polysomes in exponentially growing cells. In resting cells, however, only about half was associated with polysomes, and about 30% was found in the monosomal fraction. The distribution shifted to that found in growing cells within 3 h after serum stimulation. Similar results were obtained when cells were labeled for 10.5 h. About 70% of total polyadenylated mRNA was in the polysome fraction in all growth states regardless of labeling time, indicating that the shift in mRNA distribution was species specific. These results indicate that the content and metabolism of rp mRNA do not change significantly after growth stimulation. The rate of ribosomal protein synthesis appears to be controlled during the resting-growing transition by an alteration of the efficiency of translation of rp mRNA, possibly at the level of protein synthesis initiation.

Production of Ribosome Components in Effector CD4+ T Cells Is Accelerated by TCR Stimulation and Coordinated by ERK-MAPK

Effector CD4+ T cells rapidly activate high-level cytokine expression following TCR stimulation. Consistent with accelerated protein production in these cells, global mRNA profiles revealed that, after cytokines, the most impressive cluster of activated genes encode rRNA-maturation factors. Activation of these genes was ERK-MAPK dependent, accompanied by increased rRNA transcription and faster maturation kinetics, and much greater in effector CD4+ T cells than in naive cells. Ribosomal protein subunit (RPS) synthesis was also ERK-MAPK dependent and increased to match rRNA production, but without evident increase in RPS mRNA. Instead, stimulation promoted polysome loading of RPS mRNA via cis-acting, 5'-terminal oligopyrimidines. These results demonstrate how, in response to extracellular signals, effector CD4+ T cells coordinately increase multiple ribosomal components to accommodate burgeoning cytokine production.

The insulin-induced signalling pathway leading to S6 and initiation factor 4E binding protein 1 phosphorylation bifurcates at a rapamycin-sensitive point immediately upstream of p70s6k

Employing specific inhibitors and docking-site mutants of growth factor receptors, recent studies have indicated that the insulin-induced increase in 40S ribosomal protein S6 and initiation factor 4E binding protein 1 (4E-BP1) phosphorylation is mediated by the mTOR/FRAP-p70s6k signal transduction pathway. However, it has not been resolved whether the phosphorylation of both proteins is mediated by p70s6k or whether they reside on parallel pathways which bifurcate upstream of p70s6k. Here we have used either rapamycin-resistant, kinase-dead, or wild-type p70s6k variants to distinguish between these possibilities. The rapamycin-resistant p70s6k, which has high constitutive activity, was able to signal to S6 in the absence of insulin and to prevent the rapamycin-induced block of S6 phosphorylation. This same construct did not increase the basal state of 4E-BP1 phosphorylation or protect it from the rapamycin-induced block in phosphorylation. Unexpectedly, the rapamycin-resistant p70s6k inhibited insulin-induced 4E-BP1 phosphorylation in a dose-dependent manner. This effect was mimicked by the kinase-dead and wild-type p70s6k constructs, which also blocked insulin-induced dissociation of 4E-BP1 from initiation factor 4E. Both the kinase-dead and wild-type constructs also blocked reporter p70s6k activation, although only the kinase-dead p70s6k had a dominant-interfering effect on S6 phosphorylation. Analysis of phosphopeptides from reporter 4E-BP1 and p70s6k revealed that the kinase-dead p70s6k affected the same subset of sites as rapamycin in both proteins. The results demonstrate, for the first time, that activated p70s6k mediates increased S6 phosphorylation in vivo. Furthermore, they show that increased 4E-BP1 phosphorylation is controlled by a parallel signalling pathway that bifurcates immediately upstream of p70s6k, with the two pathways sharing a common rapamycin-sensitive activator.

The differential effects of poly(2-hydroxyethyl methacrylate) and poly(2-hydroxyethyl methacrylate)/poly(caprolactone) polymers on cell proliferation and collagen synthesis by human lung fibroblasts

Because of its chemical versatility and demonstrated biocompatibility, poly(2-hydroxyethyl methacrylate) (pHEMA) has been widely used as a polymer for biomedical applications. Since this hydrophilic material shows a poor interface with cells, blendings with other polymers were done to improve cytocompatibility. In our polymer, the presence of hydrophobic dominions on the material surface, due to the interpenetrating polymerization of pHEMA with poly(caprolactone) (PCL), seems to ameliorate the cytocompatibility in terms of cell adhesion and metabolism. For our experiments, we used IMR-90 human fibroblasts, as these cells strongly regulate DNA, RNA, and protein synthesis as anchorage-dependent variables. Cell attachment on a pHEMA/PCL interpenetrating polymer network was optimal, suggesting a strong adhesion between the cells and the polymer surface. Cell adhesion was weaker on pHEMA, as a significant fraction of the fibroblasts revealed a lack of spreading, with most cells remaining spherical. Moreover, only fibroblasts seeded on pHEMA significantly decreased mRNA synthesis; collagen production and cell shapes ranged from fully flat and proliferating, to minimally spread and nonproliferating. Finally, DNA synthesis, as a measure of cell proliferation, was markedly inhibited in cells cultured on pHEMA but not on pHEMA/PCL. In conclusion, our results suggest that control of cell growth and metabolism by biomedical polymers is based on physicochemical mechanism(s) in which the hydrophilicity/hydrophobicity ratio of the material surfaces may play an important role.

Control of PHAS-I by insulin in 3T3-L1 adipocytes. Synthesis, degradation, and phosphorylation by a rapamycin-sensitive and mitogen-activated protein kinase-independent pathway

PHAS-I levels increased 8-fold as 3T3-L1 fibroblasts differentiated into adipocytes and acquired sensitivity to insulin. Insulin increased PHAS-I protein (3.3-fold after 2 days), the rate of PHAS-I synthesis (3-fold after 1 h), and the half-life of the protein (from 1.5 to 2.5 days). Insulin also increased the phosphorylation of PHAS-I and promoted dissociation of the PHAS-I eukaryotic initiation factor-4E (eIF-4E) complex, effects that were maximal within 10 min. With recombinant [H6]PHAS-I as substrate, mitogen-activated protein (MAP) kinase was the only insulin-stimulated PHAS-I kinase detected after fractionation of extracts by Mono Q chromatography; however, MAP kinase did not readily phosphorylate [H6]PHAS-I when the [H6]PHAS-I.eIF-4E complex was the substrate. Thus, while MAP kinase may phosphorylate free PHAS-I, it is not sufficient to dissociate the complex. Moreover, rapamycin attenuated the stimulation of PHAS-I phosphorylation by insulin and markedly inhibited dissociation of PHAS-I.eIF-4E, without decreasing MAP kinase activity. Rapamycin abolished the effects of insulin on increasing phosphorylation of ribosomal protein S6 and on activating p70S6K. The MAP kinase kinase inhibitor, PD 098059, markedly decreased MAP kinase activation by insulin, but it did not change PHAS-I phosphorylation or the association of PHAS-I with eIF-4E. In summary, insulin increases the expression of PHAS-I and promotes phosphorylation of multiple sites in the protein via multiple transduction pathways, one of which is rapamycin-sensitive and independent of MAP kinase. Rapamycin may inhibit translation initiation by increasing PHAS-I binding to eIF-4E.

Vertebrate mRNAs with a 5'-terminal pyrimidine tract are candidates for translational repression in quiescent cells: characterization of the translational cis-regulatory element

The translation of mammalian ribosomal protein (rp) mRNAs is selectively repressed in nongrowing cells. This response is mediated through a regulatory element residing in the 5' untranslated region of these mRNAs and includes a 5' terminal oligopyrimidine tract (5' TOP). To further characterize the translational cis-regulatory element, we monitored the translational behavior of various endogenous and heterologous mRNAs or hybrid transcripts derived from transfected chimeric genes. The translational efficiency of these mRNAs was assessed in cells that either were growing normally or were growth arrested under various physiological conditions. Our experiments have yielded the following results: (i) the translation of mammalian rp mRNAs is properly regulated in amphibian cells, and likewise, amphibian rp mRNA is regulated in mammalian cells, indicating that all of the elements required for translation control of rp mRNAs are conserved among vertebrate classes; (ii) selective translational control is not confined to rp mRNAs, as mRNAs encoding the naturally occurring ubiquitin-rp fusion protein and elongation factor 1 alpha, which contain a 5' TOP, also conform this mode of regulation; (iii) rat rpP2 mRNA contains only five pyrimidines in its 5' TOP, yet this mRNA is translationally controlled in the same fashion as other rp mRNAs with a 5' TOP of eight or more pyrimidines; (iv) full manifestation of this mode of regulation seems to require both the 5' TOP and sequences immediately downstream; and (v) an intact translational regulatory element from rpL32 mRNA fails to exert its regulatory properties even when preceded by a single A residue.

Vertebrate mRNAs with a 5'-terminal pyrimidine tract are candidates for translational repression in quiescent cells: characterization of the translational cis-regulatory element

The translation of mammalian ribosomal protein (rp) mRNAs is selectively repressed in nongrowing cells. This response is mediated through a regulatory element residing in the 5' untranslated region of these mRNAs and includes a 5' terminal oligopyrimidine tract (5' TOP). To further characterize the translational cis-regulatory element, we monitored the translational behavior of various endogenous and heterologous mRNAs or hybrid transcripts derived from transfected chimeric genes. The translational efficiency of these mRNAs was assessed in cells that either were growing normally or were growth arrested under various physiological conditions. Our experiments have yielded the following results: (i) the translation of mammalian rp mRNAs is properly regulated in amphibian cells, and likewise, amphibian rp mRNA is regulated in mammalian cells, indicating that all of the elements required for translation control of rp mRNAs are conserved among vertebrate classes; (ii) selective translational control is not confined to rp mRNAs, as mRNAs encoding the naturally occurring ubiquitin-rp fusion protein and elongation factor 1 alpha, which contain a 5' TOP, also conform this mode of regulation; (iii) rat rpP2 mRNA contains only five pyrimidines in its 5' TOP, yet this mRNA is translationally controlled in the same fashion as other rp mRNAs with a 5' TOP of eight or more pyrimidines; (iv) full manifestation of this mode of regulation seems to require both the 5' TOP and sequences immediately downstream; and (v) an intact translational regulatory element from rpL32 mRNA fails to exert its regulatory properties even when preceded by a single A residue.

Translational regulation of ribosomal protein synthesis in Xenopus cultured cells: mRNA relocation between polysomes and RNP during nutritional shifts

Translational control of ribosomal protein mRNA was analyzed in a Xenopus cell line during growth-rate changes induced by serum deprivation and readdition. After being transferred into serum-free medium, the cells rapidly decrease their DNA, RNA and protein synthesis, while addition of serum to the culture after a few hours of deprivation causes a rapid recovery. During these growth-rate changes, we observed a shift in ribosomal protein mRNA distribution between polysomes and RNP. The proportion of mRNA on polysomes for the four ribosomal proteins analyzed changed from 70-80% during rapid growth to 25-35% during the downshift and back to 70-80% after the upshift. Northern blot analysis showed that ribosomal protein mRNA level was constant during the shifts even in the presence of the transcriptional inhibitor actinomycin D. This indicates that the distribution changes were due to a reversible transfer of ribosomal protein mRNA between polysomes and RNP without altering mRNA stability. We have also compared the kinetics of ribosomal protein mRNA distribution changes with the kinetics of the changes in the partition of ribosomes between free monomers and polysomes. The results obtained show that the change in ribosomal protein mRNA localization is very fast, allowing short-term adjustments of ribosome synthesis rate. Moreover, our observations are consistent with the hypothesis that the amount of free ribosomes present in the cell could affect ribosomal protein mRNA utilization.

Regulation of transcription by translational components in coupled translation-transcription cell-free system

A coupled translation-transcription cell-free system was established from eukaryotic cells. The biosynthetic activity of this coupled system closely resembles the synthetic behavior of cells in vivo, and exhibits regulatory phenomena similar to that of intact cells. The translational system consists of rabbit reticulocyte lysate, or its components fractionated by centrifugation. The transcriptional portion consists of cockerel liver nuclei. Incorporation of amino acids into protein by the coupled system is linear for hours. Similarly, transcription in the coupled system is continuous for hours and is proportional with time. More than 90% of the transcriptional products are secreted into the incubation medium. The components of the translational system influence and regulate transcriptional activities. In the presence of ribosomes the nuclei transcribe mostly poly(A)+ RNA with alpha-amanitin sensitivity consistent with activation of RNA polymerase II. Hybrid selection experiments demonstrate authentic preproalbumin mRNA among the transcriptional products. The putative mRNA secreted into the medium in the coupled system is found on polysomes, indicating translation of de novo synthesized message. Addition of excess reticulocyte mRNP to the medium of the coupled system results in transcription of primarily ribosomal RNA, 5S RNA, and tRNA, the products of RNA polymerases I and III. These activities closely imitate the behavior of liver in vivo under conditions of nutritional shifts or hormonal influences. The coupled system transcribes, processes, and transports substantial quantities of RNA, about 1.6 micrograms/10(6) nuclei/h. Thus, a coupled system has been established that lends itself to the exploration of regulatory interactions of cell components as it appears to closely resemble the in vivo situation.

Sequences mediating the translation of mouse S16 ribosomal protein mRNA during myoblast differentiation and in vitro and possible control points for the in vitro translation

The translation of ribosomal protein (r-protein) mRNAs is generally inefficient and regulated during the differentiation of mouse myoblasts into fibers. In this discussion we show that the first 31 nucleotides of the S16 r-protein mRNA, when located at the 5' end of the mRNA, are sufficient to impart the translational properties of an r-protein mRNA to the SV-GALK mRNA, which is normally translated efficiently in both myoblasts and fibers. If the same S16 sequences are located within the interior of the 5'-untranslated region of the SV-GALK mRNA, however, they do not impart the translational properties of an r-protein mRNA to the SV-GALK mRNA. The translation of mouse r-protein mRNAs was examined in vitro to help elucidate the mechanisms controlling their translation. Mouse r-protein mRNAs are inefficiently translated in rabbit reticulocyte extracts, and the same sequences that mediate their inefficient and regulated translation during myoblast differentiation also mediate their inefficient translation in a position-dependent manner in reticulocyte extracts. To determine whether the subpolysomal r-protein mRNAs that are not actively translated in vivo are capable of translation, subpolysomal RNA was translated in reticulocyte extracts. The subpolysomal r-protein mRNAs are just as capable of translation as are polysomal mRNAs. To help identify the initiation factors and/or the steps in the initiation pathway that mediate the inefficient translation of r-protein mRNAs, reticulocyte extracts were supplemented with purified initiation factors. Only eIF-4F, the cap-binding complex, and eIF-3, which is involved in subunit dissociation and interacts with eIF-4F during initiation, stimulated the translation of r-protein mRNA. These experiments, along with m7GDP inhibition studies, suggest that eIF-4F and/or eIF-3, or the steps mediated by these factors, mediate the inefficient translation in reticulocyte extracts and raise the possibility that these steps also control the regulated translation of r-protein mRNAs during myoblast differentiation.

Ribosomal protein genes are overexpressed in colorectal cancer: isolation of a cDNA clone encoding the human S3 ribosomal protein

We have isolated a cDNA clone encoding the human S3 ribosomal protein from a normal human colon cDNA library. The clone was identified as one of many that detected genes whose level of expression was increased in adenocarcinoma of the colon relative to normal colonic mucosa. Increased levels of the S3 transcript were present in the tumors of all eight patients examined. Moreover, the S3 mRNA was also more abundant in 7 of 10 adenomatous polyps, the presumed precursor of carcinoma. Additional studies demonstrated that increased levels of mRNAs encoding several other ribosomal proteins, including S6, S8, S12, L5, and P0, were present in colorectal tumors and polyps. These results suggest that there is increased synthesis of ribosomes in colorectal tumors and that this increase is an early event in colon neoplasia.

Ribosomal protein genes are overexpressed in colorectal cancer: isolation of a cDNA clone encoding the human S3 ribosomal protein

We have isolated a cDNA clone encoding the human S3 ribosomal protein from a normal human colon cDNA library. The clone was identified as one of many that detected genes whose level of expression was increased in adenocarcinoma of the colon relative to normal colonic mucosa. Increased levels of the S3 transcript were present in the tumors of all eight patients examined. Moreover, the S3 mRNA was also more abundant in 7 of 10 adenomatous polyps, the presumed precursor of carcinoma. Additional studies demonstrated that increased levels of mRNAs encoding several other ribosomal proteins, including S6, S8, S12, L5, and P0, were present in colorectal tumors and polyps. These results suggest that there is increased synthesis of ribosomes in colorectal tumors and that this increase is an early event in colon neoplasia.

Oligopyrimidine tract at the 5' end of mammalian ribosomal protein mRNAs is required for their translational control

Mammalian ribosomal protein (rp) mRNAs are subject to translational control, as illustrated by their selective release from polyribosomes in growth-arrested cells and their underrepresentation in polysomes in normally growing cells. In the present experiments, we have examined whether the translational control of rp mRNAs is attributable to the distinctive features of their 5' untranslated region, in particular to the oligopyrimidine tract adjacent to the cap structure. Murine lymphosarcoma cells were transfected with chimeric genes consisting of selected regions of rp mRNA fused to non-rp mRNA segments, and the translational efficiency of the resulting chimeric mRNAs was assessed in cells that either were growing normally or were growth-arrested by glucocorticoid treatment. We observed that translational control of rpL32 mRNA was abolished when its 5' untranslated region was replaced by that of beta-actin. At the same time, human growth hormone (hGH) mRNA acquired the typical behavior of rp mRNAs when it was preceded by the first 61 nucleotides of rpL30 mRNA or the first 29 nucleotides of rpS16 mRNA. Moreover, the translational control of rpS16-hGH mRNA was abolished by the substitution of purines into the pyrimidine tract or by shortening it from eight to six residues with a concomitant cytidine----uridine change at the 5' terminus. These results indicate that the 5'-terminal pyrimidine tract plays a critical role in the translational control mechanism. Possible factors that might interact with this translational cis regulatory element are discussed.

Identification of a human ribosomal protein mRNA with increased expression in colorectal tumours

A human ribosomal protein cDNA was selected from a normal colon cDNA library on the basis of overexpression in familial adenomatous polyposis. Nucleotide sequence analysis was used to identify this cDNA as corresponding to the human equivalent of the rat ribosomal protein L31 (HL31). We have quantified the expression of HL31 mRNA in colorectal tumours and found overexpression in 23 out of 23 cases. Our results indicate that HL31 is associated with a malfunction of normal growth regulatory mechanisms in these tumours, and suggest a role for HL31 in proliferation and neoplasia.

Avian reovirus mRNAs are nonfunctional in infected mouse cells: translational basis for virus host-range restriction

Avian reovirus S1133 penetrates and uncoats in suspension cultures of mouse L cells. The multiple species of viral transcripts are produced in the cytoplasm of the infected cell, but they fail to associate with polysomes, consistent with the absence of viral protein synthesis. The selective block in avian virus mRNA translation is not overcome by coinfection with mammalian reovirus type 3, which replicates in mouse L cells, or by hypertonic shock or exposure to a low concentration of cycloheximide. Although the avian viral transcripts are inactive in vivo, RNA extracted from infected, nonpermissive L cells directs the synthesis of a normal spectrum of viral proteins in rabbit reticulocyte lysates. These results indicate that avian viral transcription is not restricted in mouse cells and that viral replication is prevented at the level of initiation of protein synthesis.

The synthesis of ribosomal proteins S16 and L32 is not autogenously regulated during mouse myoblast differentiation

A series of mouse myoblast cell lines was constructed that contain 1 to 34 extra copies of either the S16 or the L32 ribosomal protein (r-protein) gene. The metabolism of the S16 and L32 r-proteins and mRNAs was examined in myoblasts and fibers of these cell lines to determine whether the synthesis of these r-proteins is autogenously regulated. The incorporation of extra copies of these r-protein genes into the genome resulted in the accumulation of the corresponding mRNAs to levels that were directly proportional to the gene copy number. The levels of the overproduced mRNAs decreased after the differentiation of mouse myoblasts into fibers in parallel to the decrease in the levels of the endogenous r-protein mRNAs. These results indicate that the synthesis of these r-proteins is not autogenously regulated at the level of transcription, RNA processing, or mRNA stability. To determine whether the synthesis of these r-proteins is regulated at the level of translation, the translational efficiencies of the overproduced mRNAs were inferred from their distribution in polysomal gradients. The translational efficiencies of these overproduced r-protein mRNAs in myoblasts are similar to those of the endogenous r-protein mRNAs. After myoblast differentiation, the translational efficiencies of the overproduced r-protein mRNAs decrease exactly like those of the endogenous r-protein mRNAs. Examination of the synthesis and stability of r-proteins in one of the L32-overproducing cell lines demonstrated that the overproduced L32 r-protein degrades shortly after its synthesis. The synthesis and stability of the other r-proteins were unaffected in this cell line. Thus, the synthesis of S16 and L32 r-proteins is not autogenously regulated at any level in either myoblasts or fibers.

Glucocorticoids selectively inhibit translation of ribosomal protein mRNAs in P1798 lymphosarcoma cells

When P1798 murine lymphosarcoma cells are exposed to 10(-7) M dexamethasone, there is a dramatic inhibition of rRNA synthesis, which is completely reversible when the hormone is withdrawn. In the present experiments we examined whether dexamethasone treatment causes any alteration in the accumulation or utilization of mRNAs that encode ribosomal proteins (rp mRNAs). No effect on the accumulation of six different rp mRNAs was detected. However, the translation of five of six rp mRNAs was selectively inhibited in the presence of the hormone, as judged by a substantial decrease in ribosomal loading. Normal translation of rp mRNA was resumed within a few hours after hormone withdrawal. In untreated or fully recovered cells, the distribution of rp mRNAs between polyribosomes and free ribonucleoprotein is distinctly bimodal, suggesting that rp mRNAs are subject to a particular form of translational control in which they are either translationally inactive or fully loaded with ribosomes. A possible relationship between this mode of translational control and the selective suppression of rp mRNA translation by glucocorticoids is discussed.

The synthesis of ribosomal proteins S16 and L32 is not autogenously regulated during mouse myoblast differentiation

A series of mouse myoblast cell lines was constructed that contain 1 to 34 extra copies of either the S16 or the L32 ribosomal protein (r-protein) gene. The metabolism of the S16 and L32 r-proteins and mRNAs was examined in myoblasts and fibers of these cell lines to determine whether the synthesis of these r-proteins is autogenously regulated. The incorporation of extra copies of these r-protein genes into the genome resulted in the accumulation of the corresponding mRNAs to levels that were directly proportional to the gene copy number. The levels of the overproduced mRNAs decreased after the differentiation of mouse myoblasts into fibers in parallel to the decrease in the levels of the endogenous r-protein mRNAs. These results indicate that the synthesis of these r-proteins is not autogenously regulated at the level of transcription, RNA processing, or mRNA stability. To determine whether the synthesis of these r-proteins is regulated at the level of translation, the translational efficiencies of the overproduced mRNAs were inferred from their distribution in polysomal gradients. The translational efficiencies of these overproduced r-protein mRNAs in myoblasts are similar to those of the endogenous r-protein mRNAs. After myoblast differentiation, the translational efficiencies of the overproduced r-protein mRNAs decrease exactly like those of the endogenous r-protein mRNAs. Examination of the synthesis and stability of r-proteins in one of the L32-overproducing cell lines demonstrated that the overproduced L32 r-protein degrades shortly after its synthesis. The synthesis and stability of the other r-proteins were unaffected in this cell line. Thus, the synthesis of S16 and L32 r-proteins is not autogenously regulated at any level in either myoblasts or fibers.

Glucocorticoids selectively inhibit translation of ribosomal protein mRNAs in P1798 lymphosarcoma cells

When P1798 murine lymphosarcoma cells are exposed to 10(-7) M dexamethasone, there is a dramatic inhibition of rRNA synthesis, which is completely reversible when the hormone is withdrawn. In the present experiments we examined whether dexamethasone treatment causes any alteration in the accumulation or utilization of mRNAs that encode ribosomal proteins (rp mRNAs). No effect on the accumulation of six different rp mRNAs was detected. However, the translation of five of six rp mRNAs was selectively inhibited in the presence of the hormone, as judged by a substantial decrease in ribosomal loading. Normal translation of rp mRNA was resumed within a few hours after hormone withdrawal. In untreated or fully recovered cells, the distribution of rp mRNAs between polyribosomes and free ribonucleoprotein is distinctly bimodal, suggesting that rp mRNAs are subject to a particular form of translational control in which they are either translationally inactive or fully loaded with ribosomes. A possible relationship between this mode of translational control and the selective suppression of rp mRNA translation by glucocorticoids is discussed.

Translational control of ribosomal protein synthesis during early Dictyostelium discoideum development

Throughout the developmental program of Dictyostelium discoideum there are substantial changes in the rates of both ribosome utilization and rRNA transcription and processing. We examined the regulation of ribosomal protein (r-protein) gene expression and found that, at the start of development, expression of these genes was drastically and specifically reduced by a block to translational initiation. An apparently separate event signals a sudden decrease in the relative amount of r-protein mRNA at about 10 h of development, a time when aggregated amoebae are forming tight cell-cell contacts. For the first 9 h of development, the relative amount of r-protein mRNA remained essentially unchanged and comparable to levels detected in growing cells. While the r-protein mRNAs were almost fully loaded on polysomes during vegetative growth, they were specifically excluded from polysomes at the start of development. The translational block was not the result of irreversible structural changes which inactivate the r-protein mRNAs since they remained translatable both in vitro, in wheat germ extracts, and in vivo, where they were recruited onto polysomes in the presence of the elongation inhibitor cycloheximide. In addition, precise measurements of poly(A) tail lengths on individual hybrid-selected mRNA species showed that there is no difference in the poly(A) tail length of r-protein mRNA isolated from growing cells and 1-h developing cells. Therefore, changes in translational efficiency cannot be attributed to cleavage of poly(A) tails.

Control by insulin and insulin-related growth factor 1 of protein synthesis in a cell-free translational system from chick-embryo fibroblasts

Insulin and insulin-related growth factor 1 (IGF-1) increase by 1.5-1.6-fold the rate of [3H]leucine incorporation into protein in primary monolayer cultures of chick-embryo fibroblasts (CEF); half-maximal hormone concentrations are 10 and 0.25 nM respectively. To investigate the mechanism of this effect, a rapid method is used to prepare a lysate from CEF which is active in protein synthesis. Lysate derived from cells treated for 30-150 min with insulin synthesized protein at 1.8-3.0-fold greater rate than did controls; the increased rate persisted for 20 min in vitro. Pactamycin (0.5 microM), an inhibitor of peptide-chain initiation, inhibited protein synthesis by 50% in lysates derived from insulin-treated and control cells. Thus insulin and IGF-1 cause an increase in the protein-synthesis rate in vivo, which persists in cell-free protein-synthesizing lysates of CEF.

Translational control of ribosomal protein synthesis during early Dictyostelium discoideum development

Throughout the developmental program of Dictyostelium discoideum there are substantial changes in the rates of both ribosome utilization and rRNA transcription and processing. We examined the regulation of ribosomal protein (r-protein) gene expression and found that, at the start of development, expression of these genes was drastically and specifically reduced by a block to translational initiation. An apparently separate event signals a sudden decrease in the relative amount of r-protein mRNA at about 10 h of development, a time when aggregated amoebae are forming tight cell-cell contacts. For the first 9 h of development, the relative amount of r-protein mRNA remained essentially unchanged and comparable to levels detected in growing cells. While the r-protein mRNAs were almost fully loaded on polysomes during vegetative growth, they were specifically excluded from polysomes at the start of development. The translational block was not the result of irreversible structural changes which inactivate the r-protein mRNAs since they remained translatable both in vitro, in wheat germ extracts, and in vivo, where they were recruited onto polysomes in the presence of the elongation inhibitor cycloheximide. In addition, precise measurements of poly(A) tail lengths on individual hybrid-selected mRNA species showed that there is no difference in the poly(A) tail length of r-protein mRNA isolated from growing cells and 1-h developing cells. Therefore, changes in translational efficiency cannot be attributed to cleavage of poly(A) tails.

Faster ribosome synthesis induced by elevated aortic pressure in rat heart

An increase in aortic pressure from 60 to 120 mmHg accelerated ribosomal protein synthesis in rat hearts during 1 or 2 h of labeling with 0.4 mM [3H]phenylalanine. When hearts were perfused with buffer that contained 20 mM glucose and normal plasma concentrations of 19 other amino acids without added insulin, ribosomal protein synthesis relative to the rate of total protein synthesis increased from approximately 0.22 to 0.36 and 0.30 as aortic pressure was raised from 60 to 120 mmHg during 1 or 2 h of labeling, respectively. With the addition of insulin, the relative rate of ribosomal protein synthesis averaged 0.33 at an aortic pressure of 60 mmHg and increased to 0.42 when aortic pressure was raised to 120 mmHg. These results indicate that elevation of aortic pressure has a preferential effect on synthesis of new ribosomes. This response appears to be an early and physiologically significant event in cardiac hypertrophy.

Diabetes-induced alterations in the translational activity of specific messenger ribonucleic acids isolated from rat hearts

During diabetes mellitus, total proteins and ribonucleic acids are significantly decreased in the rat heart, and these parameters can be increased by insulin administration. To determine whether all ribonucleic acids are equally sensitive to insulin, we examined the influence of this hormone on individual translatable ribonucleic acids. Cardiac ribonucleic acid prepared from control, untreated, and insulin-treated diabetic animals was translated in vitro in the presence of [35S]methionine. The radiolabeled peptides were separated by two-dimensional gel electrophoresis and were analyzed by fluorometry. We found that diabetes induces both qualitative and quantitative changes in the predominance of a few specific translatable messenger ribonucleic acid species. The translation of 11 messenger ribonucleic acid species was significantly decreased and that of eight messenger ribonucleic acid species was significantly increased in diabetic preparation. Twelve of the 19 translation products were quantified by digital matrix photometry: three labeled peptides were observed only when cardiac ribonucleic acid from diabetic animals was added to the cell-free translation system, four new peptides appeared when cardiac ribonucleic acid from control animals was added, and although the remaining five peptides were translated in vitro after either control or diabetic ribonucleic acid was added, their relative predominance was altered 2- to 200-fold. When translation products coded for by messenger ribonucleic acids prepared from either diabetic or hypothyroid hearts were compared, we found that most of the alterations induced by diabetes were also induced by hypothyroidism.(ABSTRACT TRUNCATED AT 250 WORDS)

Faster protein and ribosome synthesis in thyroxine-induced hypertrophy of rat heart

Rates of protein synthesis and degradation were measured in hearts from normal and thyroxine-injected rats that were perfused as working preparations with Krebs-Henseleit bicarbonate buffer containing 400 microU insulin/ml, 2 mM lactate, 10 mM glucose, and normal plasma concentrations of amino acids. Hearts were perfused after four daily injections (1 microgram/g body wt) of thyroxine. Protein synthesis was 24% greater in hypertrophying hearts compared with controls; ribosomal RNA content increased 25%. In addition, the proportion of total RNA in free ribosomal subunits in hypertrophying hearts was unchanged from perfused hearts of control rats and from unperfused normal hearts. These results indicated that increased protein synthetic machinery as monitored by content of ribosomes, rather than more efficient initiation or elongation of peptide chains, accounted for the faster rate of protein synthesis in hypertrophying hearts. Rates of protein degradation were the same in hearts from thyroxine-injected and control animals. When rates of ribosome production were measured in vitro at various times after a single injection of thyroxine in vivo, faster ribosome synthesis was detected within 8 h; no change in the rate of total protein synthesis occurred after a single injection of thyroxine. These studies indicated that accelerated ribosome formation was an early and quantitatively important factor in cardiac hypertrophy.

The effects of oestradiol-17 beta on the synthesis and modification of ribosomal proteins in the uterus of the immature rat

The effects of oestrogen on the incorporation of newly-made ribosomal proteins into the ribosomes of the immature rat uterus has been investigated. Different newly-made proteins were shown to enter ribosomes at different rates and there was some evidence that the hormone exerted differential effects. Oestradiol also stimulated the phosphorylation of ribosomal protein S6 but the effect could be explained by hormone-induced changes in the precursor pools.

Regulation of ribosome synthesis in Tetrahymena pyriformis. 2. Coordination of synthesis of ribosomal proteins and ribosomal RNA during nutritional shift-up

The addition of nutrients to long-time-starved cells of Tetrahymena pyriformis leads to a 50-60-fold increase in the rate of synthesis of ribosomal proteins (r-proteins). This is achieved by a 6-fold increase in the relative rate of r-protein synthesis and a 8-10-fold increase in the rate of total protein synthesis. Synthesis of r-proteins constitutes one third of total cellular protein synthesis 2-4 h after refeeding and the absolute rate of r-protein synthesis is approximately three-times greater than in exponentially growing cells. The synthesis of the individual r-proteins is coordinately regulated during a nutritional shift-up, and de novo synthesized r-proteins are stable. Addition of actinomycin D prevents the increase in the rate of r-protein synthesis. The rates of synthesis of rRNA and r-protein increase in concert, implying coordinate regulation. Furthermore, a comparison of the observed accumulation of r-proteins with the predicted accumulation based on the accumulation of rRNA suggests that rRNA and r-protein are synthesized in a stoichiometrically balanced way during the entire refeeding period.

Effect of lipopolysaccharide stimulation on the transcription and translation of messenger RNA for cell surface immunoglobulin M

Analysis of mu-specific mRNA in the B cell tumor line, BCL1, shows that the cells contain predominantly mRNA for mu chain of membrane-bound immunoglobulin M (IgM) (2.7 kb, mu m mRNA). Stimulation of the cells to Ig secretion by lipopolysaccharide (LPS) results in a 6-12 fold increase in amount of mRNA for the mu chain of secreted IgM (2.4 kb mu s mRNA). The increase in mu s mRNA is accompanied by a 3-4-fold increase in mu m mRNA. The rate of mu chain synthesis of membrane IgM in LPS-stimulated cells is, however, reduced by at least twofold, suggesting that both transcriptional and translational regulatory events are involved in the induction of B lymphocytes to secretion.