The use of inhibitors in studies on protein synthesis

A Quick Guide to Small-Molecule Inhibitors of Eukaryotic Protein Synthesis

Eukaryotic ribosome and cap-dependent translation are attractive targets in the antitumor, antiviral, anti-inflammatory, and antiparasitic therapies. Currently, a broad array of small-molecule drugs is known that specifically inhibit protein synthesis in eukaryotic cells. Many of them are well-studied ribosome-targeting antibiotics that block translocation, the peptidyl transferase center or the polypeptide exit tunnel, modulate the binding of translation machinery components to the ribosome, and induce miscoding, premature termination or stop codon readthrough. Such inhibitors are widely used as anticancer, anthelmintic and antifungal agents in medicine, as well as fungicides in agriculture. Chemicals that affect the accuracy of stop codon recognition are promising drugs for the nonsense suppression therapy of hereditary diseases and restoration of tumor suppressor function in cancer cells. Other compounds inhibit aminoacyl-tRNA synthetases, translation factors, and components of translation-associated signaling pathways, including mTOR kinase. Some of them have antidepressant, immunosuppressive and geroprotective properties. Translation inhibitors are also used in research for gene expression analysis by ribosome profiling, as well as in cell culture techniques. In this article, we review well-studied and less known inhibitors of eukaryotic protein synthesis (with the exception of mitochondrial and plastid translation) classified by their targets and briefly describe the action mechanisms of these compounds. We also present a continuously updated database (http://eupsic.belozersky.msu.ru/) that currently contains information on 370 inhibitors of eukaryotic protein synthesis.

Theoretical Studies on Mechanism of Inactivation of Kanamycin A by 4'-O-Nucleotidyltransferase

This work is focused on mechanistic studies of the transfer of an adenylyl group (Adenoside-5'-monophosfate) from adenosine 5'-triphosphate (ATP) to a OH-4' hydroxyl group of an antibiotic. Using hybrid quantum mechanics/molecular mechanics (QM/MM) techniques, we study the substrate and base-assisted mechanisms of the inactivation process of kanamycin A (KAN) catalyzed by 4'-O-Nucleotidyltransferase [ANT(4')], an active enzyme against almost all aminoglycoside antibiotics. Free energy surfaces, obtained with Free Energy Perturbation methods at the M06-2X/MM level of theory, show that the most favorable reaction path presents a barrier of 12.2 kcal·mol-1 that corresponds to the concerted activation of O4' from KAN by Glu145. In addition, the primary and secondary 18O kinetic isotope effects (KIEs) have been computed for bridge O3α, and non-bridge O1α, O2α, and O5' atoms of ATP. The observed normal 1°-KIE of 1.2% and 2°-KIE of 0.07% for the Glu145-assisted mechanism are in very good agreement with experimentally measured data. Additionally, based on the obtained results, the role of electrostatic and compression effects in enzymatic catalysis is discussed.

Ribosome-Targeting Antibiotics: Modes of Action, Mechanisms of Resistance, and Implications for Drug Design

Genetic information is translated into proteins by the ribosome. Structural studies of the ribosome and of its complexes with factors and inhibitors have provided invaluable information on the mechanism of protein synthesis. Ribosome inhibitors are among the most successful antimicrobial drugs and constitute more than half of all medicines used to treat infections. However, bacterial infections are becoming increasingly difficult to treat because the microbes have developed resistance to the most effective antibiotics, creating a major public health care threat. This has spurred a renewed interest in structure-function studies of protein synthesis inhibitors, and in few cases, compounds have been developed into potent therapeutic agents against drug-resistant pathogens. In this review, we describe the modes of action of many ribosome-targeting antibiotics, highlight the major resistance mechanisms developed by pathogenic bacteria, and discuss recent advances in structure-assisted design of new molecules.

Scalable Device for Automated Microbial Electroporation in a Digital Microfluidic Platform

Electrowetting-on-dielectric (EWD) digital microfluidic laboratory-on-a-chip platforms demonstrate excellent performance in automating labor-intensive protocols. When coupled with an on-chip electroporation capability, these systems hold promise for streamlining cumbersome processes such as multiplex automated genome engineering (MAGE). We integrated a single Ti:Au electroporation electrode into an otherwise standard parallel-plate EWD geometry to enable high-efficiency transformation of Escherichia coli with reporter plasmid DNA in a 200 nL droplet. Test devices exhibited robust operation with more than 10 transformation experiments performed per device without cross-contamination or failure. Despite intrinsic electric-field nonuniformity present in the EP/EWD device, the peak on-chip transformation efficiency was measured to be 8.6 ± 1.0 × 10⁸ cfu·μg^-1 for an average applied electric field strength of 2.25 ± 0.50 kV·mm^-1. Cell survival and transformation fractions at this electroporation pulse strength were found to be 1.5 ± 0.3 and 2.3 ± 0.1%, respectively. Our work expands the EWD toolkit to include on-chip microbial electroporation and opens the possibility of scaling advanced genome engineering methods, like MAGE, into the submicroliter regime.

Conformational Response of 30S-bound IF3 to A-Site Binders Streptomycin and Kanamycin

Aminoglycoside antibiotics are widely used to treat infectious diseases. Among them, streptomycin and kanamycin (and derivatives) are of importance to battle multidrug-resistant (MDR) Mycobacterium tuberculosis. Both drugs bind the small ribosomal subunit (30S) and inhibit protein synthesis. Genetic, structural, and biochemical studies indicate that local and long-range conformational rearrangements of the 30S subunit account for this inhibition. Here, we use intramolecular FRET between the C- and N-terminus domains of the flexible IF3 to monitor real-time perturbations of their binding sites on the 30S platform. Steady and pre-steady state binding experiments show that both aminoglycosides bring IF3 domains apart, promoting an elongated state of the factor. Binding of Initiation Factor IF1 triggers closure of IF3 bound to the 30S complex, while both aminoglycosides revert the IF1-dependent conformation. Our results uncover dynamic perturbations across the 30S subunit, from the A-site to the platform, and suggest that both aminoglycosides could interfere with prokaryotic translation initiation by modulating the interaction between IF3 domains with the 30S platform.

Severely Heat Injured Survivors of E. coli O157:H7 ATCC 43888 Display Variable and Heterogeneous Stress Resistance Behavior

Although minimal food processing strategies aim to eliminate foodborne pathogens and spoilage microorganisms through a combination of mild preservation techniques, little is actually known on the resistance behavior of the small fraction of microorganisms surviving an inimical treatment. In this study, the conduct of severely heat stressed survivors of E. coli O157:H7 ATCC 43888, as an indicator for the low infectious dose foodborne enterohemorrhagic strains, was examined throughout their resuscitation and outgrowth. Despite the fact that these survivors were initially sublethally injured, they were only marginally more sensitive to a subsequent heat treatment and actually much more resistant to a subsequent high hydrostatic pressure (HHP) shock in comparison with unstressed control cells. Throughout further resuscitation, however, their initial HHP resistance rapidly faded out, while their heat resistance increased and surpassed the initial heat resistance of unstressed control cells. Results also indicated that the population eventually emerging from the severely heat stressed survivors heterogeneously consisted of both growing and non-growing cells. Together, these observations provide deeper insights into the particular behavior and heterogeneity of stressed foodborne pathogens in the context of food preservation.

Single-molecule dynamics of the molecular chaperone trigger factor in living cells

In bacteria, trigger factor (TF) is the molecular chaperone that interacts with the ribosome to assist the folding of nascent polypeptides. Studies in vitro have provided insights into the function and mechanism of TF. Much is to be elucidated, however, about how TF functions in vivo. Here, we use single-molecule tracking, in combination with genetic manipulations, to study the dynamics and function of TF in living E. coli cells. We find that TF, besides interacting with the 70S ribosome, may also bind to ribosomal subunits and form TF-polypeptide complexes that may include DnaK/DnaJ proteins. The TF-70S ribosome interactions are highly dynamic inside cells, with an average residence time of ∼0.2 s. Our results confirm that the signal recognition particle weakens TF's interaction with the 70S ribosome, and further identify that this weakening mainly results from a change in TF's binding to the 70S ribosome, rather than its unbinding. Moreover, using photoconvertible bimolecular fluorescence complementation, we selectively probe TF₂ dimers in the cell and show that TF₂ does not bind to the 70S ribosome but is involved in the post-translational interactions with polypeptides. These findings contribute to the fundamental understanding of molecular chaperones in assisting protein folding in living cells.

The Effect of Kanamycin and Tetracycline on Growth and Photosynthetic Activity of Two Chlorophyte Algae

Antibiotics are routinely used in microalgae culture screening, stock culture maintenance, and genetic transformation. By studying the effect of antibiotics on microalgae growth, we can estimate the least value to inhibit growth of undesired pathogens in algal culture. We studied the effect of kanamycin and tetracycline on the growth and photosynthetic activity of two chlorophyte microalgae, Dictyosphaerium pulchellum and Micractinium pusillum. We measured CFU mL^-1 on agar plates, optical density, fluorescence yields, and photosynthetic inhibition. Our results showed a significant effect of kan and tet on the tested microalgae species except tet, which showed a minor effect on M. pusillum. Both antibiotics are believed to interact with the protein synthesis machinery; hence, the inhibitory effect of the tested antibiotics was further confirmed by isolation and quantification of the whole cell protein. A significant reduction in protein quantity was observed at concentrations more than 5 mg L^-1, except M. pusillum, which showed only a slight reduction in protein quantity even at the maximum tested concentration of tet (30 mg L^-1). This study can further aid in aquaculture industry, for the maintenance of the microalgae stock cultures and it can also help the microalgae genetic engineers in the construction of molecular markers.

Chloramphenicol Derivatives as Antibacterial and Anticancer Agents: Historic Problems and Current Solutions

Chloramphenicol (CAM) is the D-threo isomer of a small molecule, consisting of a p-nitrobenzene ring connected to a dichloroacetyl tail through a 2-amino-1,3-propanediol moiety. CAM displays a broad-spectrum bacteriostatic activity by specifically inhibiting the bacterial protein synthesis. In certain but important cases, it also exhibits bactericidal activity, namely against the three most common causes of meningitis, Haemophilus influenzae, Streptococcus pneumoniae and Neisseria meningitidis. Resistance to CAM has been frequently reported and ascribed to a variety of mechanisms. However, the most important concerns that limit its clinical utility relate to side effects such as neurotoxicity and hematologic disorders. In this review, we present previous and current efforts to synthesize CAM derivatives with improved pharmacological properties. In addition, we highlight potentially broader roles of these derivatives in investigating the plasticity of the ribosomal catalytic center, the main target of CAM.

Comparison of an Enzyme-Linked Immunosorbent Assay with an Immunochromatographic Assay for Detection of Lincomycin in Milk and Honey

An enzyme-linked immunosorbent assay (ELISA) and an immunochromatographic assay were constructed for the detection of lincomycin (LIN) in both milk and honey samples based on the monoclonal antibody named 5F6. The half-maximum inhibition of ELISA was 0.3 ng/mL after optimizing pH and ionic strength conditions; the limit of detection was 0.07 ng/mL. The cross-reactivity with clindamycin was 0.6%. LIN recovery in spiked milk and honey samples ranged from 84.6% to 115.6% with intra-assay coefficient variations of 1.7-25.4% and inter-assay coefficient variations of 2.7-8.9%. The detection limits were estimated as 2.1 µg/L for milk and 2.1 µg/kg for honey samples. The immunochromatographic assay revealed a LIN cut-off value of 10 ng/mL in PBS, 5 ng/mL in milk, and 120 ng/g in honey, and a visual lower detection limit of 2.5 ng/mL, 1 ng/mL and 30 ng/g in PBS, milk and honey, respectively. The immunochromatographic assay is preferred for large-scale practical application for its simpler pretreatment and satisfied sensitivity compared with ELISA assay.

Fighting an old disease with modern tools: characteristics and molecular detection methods of drug-resistant Mycobacterium tuberculosis

Tuberculosis (TB) is an ancient disease, but not a disease of the past. The increasing prevalence of drug-resistant strains of Mycobacterium tuberculosis, the causative agent of TB, demands new measures to combat the situation. Rapid and accurate detection of the pathogen, and its drug susceptibility pattern, is essential for timely initiation of treatment, and ultimately, control of the disease. Molecular-based methods offer a great chance to improve detection of drug-resistant TB; however, their development and usage should be accompanied with a profound understanding of drug resistance mechanisms and circulating M. tuberculosis strains in specific settings, as otherwise, the usefulness of such tests may be limited. This review gives an overview of the history of TB treatment and drug resistance, drug resistance mechanisms for the most commonly used drugs and molecular methods designed to detect drug-resistant strains.

Time-dependent effects of transcription- and translation-halting drugs on the spatial distributions of the Escherichia coli chromosome and ribosomes

Previously observed effects of rifampicin and chloramphenicol indicate that transcription and translation activity strongly affect the coarse spatial organization of the bacterial cytoplasm. Single-cell, time-resolved, quantitative imaging of chromosome and ribosome spatial distributions and ribosome diffusion in live Escherichia coli provides insight into the underlying mechanisms. Monte Carlo simulations of model DNA-ribosome mixtures support a novel nucleoid-ribosome mixing hypothesis. In normal conditions, 70S-polysomes and the chromosomal DNA segregate, while 30S and 50S ribosomal subunits are able to penetrate the nucleoids. Growth conditions and drug treatments determine the partitioning of ribosomes into 70S-polysomes versus free 30S and 50S subunits. Entropic and excluded volume effects then dictate the resulting chromosome and ribosome spatial distributions. Direct observation of radial contraction of the nucleoids 0-5 min after treatment with either transcription- or translation-halting drugs supports the hypothesis that simultaneous transcription, translation, and insertion of proteins into the membrane ('transertion') exerts an expanding force on the chromosomal DNA. Breaking of the DNA-RNA polymerase-mRNA-ribosome-membrane chain in either of two ways causes similar nucleoid contraction on a similar timescale. We suggest that chromosomal expansion due to transertion enables co-transcriptional translation throughout the nucleoids.

In vitro microbial culture models and their application in drug development

Drug development faces its nemesis in the form of drug resistance. The rate of bacterial resistance to antibiotics, or tumor resistance to chemotherapy decisively depends on the surrounding heterogeneous tissue. However, in vitro drug testing is almost exclusively done in well stirred, homogeneous environments. Recent advancements in microfluidics and microfabrication introduce opportunities to develop in vitro culture models that mimic the complex in vivo tissue environment. In this review, we will first discuss the design principles underlying such models. Then we will demonstrate two types of microfluidic devices that combine stressor gradients, cell motility, large population of competing/cooperative cells and time varying dosage of drugs. By incorporating ideas from how natural selection and evolution move drug resistance forward, we show that drug resistance can occur at much greater rates than in well-stirred environments. Finally, we will discuss the future direction of in vitro microbial culture models and how to extend the lessons learned from microbial systems to eukaryotic cells.

The deadly connection between endoplasmic reticulum, Ca2+, protein synthesis, and the endoplasmic reticulum stress response in malignant glioma cells

BACKGROUND

The endoplasmic reticulum (ER) is involved in Ca(2+) signaling and protein processing. Accumulation of unfolded proteins following ER Ca(2+) depletion triggers the ER stress response (ERSR), which facilitates protein folding and removal of damaged proteins and can induce cell death. Unfolded proteins bind to chaperones, such as the glucose-regulated protein (GRP)78 and cause the release of GRP78-repressed proteins executing ERSR.

METHODS

Several glioma cell lines and primary astrocytes were used to analyze ERSR using standard western blots, reverse transcription-PCR, viability assays, and single cell Ca(2+) imaging.

RESULTS

ERSR induction with thapsigargin results in a more intense ERSR associated with a larger loss of ER Ca(2+), activation of ER-associated caspases (4/12) and caspase 3, and a higher rate of malignant glioma cell death than in normal glial cells. Malignant glioma cells have higher levels of protein synthesis and expression of the translocon (a component of the ribosomal complex, guiding protein entry in the ER), the activity of which is associated with the loss of ER Ca(2+). Our experiments confirm increased expression of the translocon in malignant glioma cells. In addition, blockade of the ribosome-translocon complex with agents differently affecting translocon Ca(2+) permeability causes opposite effects on ERSR deployment and death of malignant glioma cells.

CONCLUSIONS

Excessive ER Ca(2+) loss due to translocon activity appears to be responsible for the enhancement of ERSR, leading to the death of glioma cells. The results reveal a characteristic of malignant glioma cells that could be exploited to develop new therapeutic strategies to treat incurable glial malignancies.

Blasticidin S inhibits translation by trapping deformed tRNA on the ribosome

The antibiotic blasticidin S (BlaS) is a potent inhibitor of protein synthesis in bacteria and eukaryotes. We have determined a 3.4-Å crystal structure of BlaS bound to a 70S⋅tRNA ribosome complex and performed biochemical and single-molecule FRET experiments to determine the mechanism of action of the antibiotic. We find that BlaS enhances tRNA binding to the P site of the large ribosomal subunit and slows down spontaneous intersubunit rotation in pretranslocation ribosomes. However, the antibiotic has negligible effect on elongation factor G catalyzed translocation of tRNA and mRNA. The crystal structure of the antibiotic-ribosome complex reveals that BlaS impedes protein synthesis through a unique mechanism by bending the 3' terminus of the P-site tRNA toward the A site of the large ribosomal subunit. Biochemical experiments demonstrate that stabilization of the deformed conformation of the P-site tRNA by BlaS strongly inhibits peptidyl-tRNA hydrolysis by release factors and, to a lesser extent, peptide bond formation.

RETRACTED: Marine actinobacterial metabolites: current status and future perspectives

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). This article has been retracted at the request of the Editor. Authors and Editor agreed to retract this article because substantial parts of the text were copied from the following sources without proper attribution: Lam, K.S. (2006), Discovery of novel metabolites from marine actinomycetes. Current Opinion in Microbiology 9(3), pp. 245–251; Subramani, R., Aalbersberg, W. (2012), Marine actinomycetes: An ongoing source of novel bioactive metabolites. Microbiological Research 167(10), pp. 571–580; Dharmaraj, S. (2010), Marine Streptomyces as a novel source of bioactive substances. World Journal of Microbiology and Biotechnology 26(12), pp. 2123–2139. The authors apologize for this oversight and any inconvenience caused.

Efficacy of end-only-functionalized oligo(arylene-ethynylene)s in killing bacterial biofilms

Cationic end-only-functionalized oligo(arylene-ethynylene)s (EO-OPEs) have recently been found to be broad-spectrum and effective antimicrobial agents because of their unique structure and optical properties. In this study, we investigated their potential use for preventing and reducing Escherichia coli (E. coli) biofilms. The Calgary biofilm device (CBD) was used to form bacterial biofilms of E. coli; in these studies, the minimum inhibitory concentration (MIC) and the minimum biofilm eradication concentration (MBEC) were determined. E. coli biofilms uniformly grow on pegs of the CBD device lid. The MIC values determined for EO-OPEs are comparable to those found for standard antibiotics such as kanamycin (MIC = 11.2 μg/mL). About 10-30 times the concentration of EO-OPEs was required to eradicate E. coli biofilms and prevent regrowth in the dark. Near-UV irradiation of EO-OPEs enhanced their efficacy in killing biofilms.

The insect peptide coprisin prevents Clostridium difficile-mediated acute inflammation and mucosal damage through selective antimicrobial activity

Clostridium difficile-associated diarrhea and pseudomembranous colitis are typically treated with vancomycin or metronidazole, but recent increases in relapse incidence and the emergence of drug-resistant strains of C. difficile indicate the need for new antibiotics. We previously isolated coprisin, an antibacterial peptide from Copris tripartitus, a Korean dung beetle, and identified a nine-amino-acid peptide in the α-helical region of it (LLCIALRKK) that had antimicrobial activity (J.-S. Hwang et al., Int. J. Pept., 2009, doi:10.1155/2009/136284). Here, we examined whether treatment with a coprisin analogue (a disulfide dimer of the nine peptides) prevented inflammation and mucosal damage in a mouse model of acute gut inflammation established by administration of antibiotics followed by C. difficile infection. In this model, coprisin treatment significantly ameliorated body weight decreases, improved the survival rate, and decreased mucosal damage and proinflammatory cytokine production. In contrast, the coprisin analogue had no apparent antibiotic activity against commensal bacteria, including Lactobacillus and Bifidobacterium, which are known to inhibit the colonization of C. difficile. The exposure of C. difficile to the coprisin analogue caused a marked increase in nuclear propidium iodide (PI) staining, indicating membrane damage; the staining levels were similar to those seen with bacteria treated with a positive control for membrane disruption (EDTA). In contrast, coprisin analogue treatment did not trigger increases in the nuclear PI staining of Bifidobacterium thermophilum. This observation suggests that the antibiotic activity of the coprisin analogue may occur through specific membrane disruption of C. difficile. Thus, these results indicate that the coprisin analogue may prove useful as a therapeutic agent for C. difficile infection-associated inflammatory diarrhea and pseudomembranous colitis.

Short double-stranded RNAs of specific sequence activate ribosomal TAK1-D and induce a global inhibition of translation

We have previously shown that short double-stranded RNAs of specific sequence induce phosphorylation in the activation loop of splicing variant D of the transforming growth factor beta-activated protein kinase 1 (TAK1-D). Here, we further characterize this novel function of TAK1-D and the mechanisms of this dsRNA-triggered phenomenon. Using a dominant negative TAK1-D mutant we demonstrate that TAK1-D activation is functionally required to trigger the activation of p38 MAP kinase and c-JUN terminal kinase and to induce cell death in NCI-H460 cells. While total TAK1-D protein was found in the cytoplasm as well as in the ribosomal fraction, activated TAK1-D phosphorylated on T184 and T187 in the activation loop was found to be exclusively associated with the 80S ribosome. The association of TAK1-D with the ribosome suggests an involvement in translation-dependent signaling and we demonstrate here that dsRNA-mediated activation of TAK1-D leads to a downregulation of mRNA translation. In addition, we show that TAK1-D is also phosphorylated after the induction of ribotoxic stress. Our data indicate that TAK1-D plays a role in the signaling events triggered by selected types of ribotoxic stress.

Ribosome-translocon complex mediates calcium leakage from endoplasmic reticulum stores

Under resting conditions, the endoplasmic reticulum (ER) intraluminal free calcium concentration ([Ca(2+)](ER)) reflects a balance between active uptake by Ca(2+)-ATPases and passive efflux via 'leak channels'. Despite their physiological importance and ubiquitous leak pathway mechanism, very little is known about the molecular nature of these channels. As it has been suggested that the open translocon pore complex of the ER is permeable to ions and neutral molecules, we hypothesized that the ribosome-bound translocon would be permeable to calcium after treatment with puromycin, a translation inhibitor that specifically releases polypeptide chains. At this time, the translocon channel is left open. We measured the fluctuations in cytoplasmic and luminal calcium concentrations using fluorescent dyes (fura-2 and magfura-2, respectively). The calcium release induced by thapsigargin (a Ca(2+)-ATPase inhibitor) was lower after puromycin treatment. Puromycin also reduced the [Ca(2+)](ER) level when perfused into the medium, but was ineffective after anisomycin pre-treatment (an inhibitor of the peptidyl transferase). Puromycin had a similar effect in the presence of heparin and ryanodine. This puromycin-evoked [Ca(2+)](ER) decrease was specific to the translocon. We conclude that the translocon complex is a major calcium leak channel. This work reveals a new role for the translocon which is involved in the control of the [Ca(2+)](ER) and could therefore supervise many physiological processes, including gene expression and apoptosis.

Essential role of ribosomal protein L11 in mediating growth inhibition-induced p53 activation

The ribosomal protein L11 binds to and suppresses the E3 ligase function of HDM2, thus activating p53. Despite being abundant as a component of the 60S large ribosomal subunit, L11 does not induce p53 under normal growth conditions. In search of mechanisms controlling L11-HDM2 interaction, we found that the induction of p53 under growth inhibitory conditions, such as low dose of actinomycin D or serum depletion, can be significantly attenuated by knocking down L11, indicating the importance of L11 in mediating these growth inhibitory signals to p53. We show that L11 is not regulated by transcription or protein stability and its level remains relatively constant during serum starvation. However, serum starvation induces translocation of L11 from the nucleolus to the nucleoplasm, where it participates in a complex with HDM2. We propose that the nucleolus acts as a barrier to prevent L11 interacting with HDM2 during normal growth. Growth inhibition, presumably through suppression of rRNA production in the nucleolus, facilitates translocation of L11 to the nucleoplasm, thus activating p53 through inhibiting HDM2.

Polyamines affect diversely the antibiotic potency: insight gained from kinetic studies of the blasticidin S AND spiramycin interactions with functional ribosomes

The effects of spermine on peptidyltransferase inhibition by an aminohexosylcytosine nucleoside, blasticidin S, and by a macrolide, spiramycin, were investigated in a model system derived from Escherichia coli, in which a peptide bond is formed between puromycin and AcPhe-tRNA bound at the P-site of poly(U)-programmed ribosomes. Kinetics revealed that blasticidin S, after a transient phase of interference with the A-site, is slowly accommodated near to the P-site so that peptide bond is still formed but with a lower catalytic rate constant. At high concentrations of blasticidin S (>10 x K(i)), a second drug molecule binds to a weaker binding site on ribosomes, and this may account for the onset of a subsequent mixed-noncompetitive inhibition phase. Spermine enhances the blasticidin S inhibitory effect by facilitating the drug accommodation to both sites. On the other hand, spiramycin (A) was found competing with puromycin for the A-site of AcPhe-tRNA.poly(U).70 S ribosomal complex (C) via a two-step mechanism, according to which the fast formation of the encounter complex CA is followed by a slow isomerization to a tighter complex, termed C(*)A. In contrast to that observed with blasticidin S, spermine reduced spiramycin potency by decreasing the formation and stability of complex C(*)A. Polyamine effects on drug binding were more pronounced when a mixture of spermine and spermidine was used, instead of spermine alone. Our kinetic results correlate well with cross-linking and crystallographic data and suggest that polyamines bound at the vicinity of the antibiotic binding pockets modulate diversely the interaction of these drugs with ribosomes.

Effect of polyamines on the inhibition of peptidyltransferase by antibiotics: revisiting the mechanism of chloramphenicol action

Chloramphenicol is thought to interfere competitively with the binding of the aminoacyl-tRNA 3'-terminus to ribosomal A-site. However, noncompetitive or mixed-noncompetitive inhibition, often observed to be dependent on chloramphenicol concentration and ionic conditions, leaves some doubt about the precise mode of action. Here, we examine further the inhibition effect of chloramphenicol, using a model system derived from Escherichia coli in which a peptide bond is formed between puromycin and AcPhe-tRNA bound at the P-site of poly(U)-programmed ribosomes, under ionic conditions (6 mM Mg2+, 100 mM NH4+, 100 microM spermine) more closely resembling the physiological status. Kinetics reveal that chloramphenicol (I) reacts rapidly with AcPhe-tRNA.poly(U).70S ribosomal complex (C) to form the encounter complex CI which is then isomerized slowly to a more tight complex, C*I. A similar inhibition pattern is observed, if complex C modified by a photoreactive analogue of spermine, reacts in buffer free of spermine. Spermine, either reversibly interacting with or covalently attached to ribosomes, enhances the peptidyltransferase activity and increases the chloramphenicol potency, without affecting the isomerization step. As indicated by photoaffinity labeling, the peptidyltransferase center at which chloramphenicol binds, is one of the preferred cross-linking sites for polyamines. This fact may explain the effect of spermine on chloramphenicol binding to ribosomes.

Co-translational targeting and translocation of the amino terminus of opsin across the endoplasmic membrane requires GTP but not ATP

The tight coupling between ongoing translation and translocation across the mammalian endoplasmic reticulum has made it difficult to determine the requirements that are specific for translocation. We have developed an in vitro assay that faithfully mimics the co-translational targeting and translocation of the amino terminus of opsin without ongoing translation. Using this system we demonstrate that this post-translational targeting and translocation requires nucleotide triphosphates but not cytosolic proteins. The addition of GTP alone was sufficient to fully restore targeting. The addition of ATP was not specifically required, and non-hydrolyzable analogs of ATP that blocked 90% of the ATPase activity also had no inhibitory effect on translocation.

The permeability of the endoplasmic reticulum is dynamically coupled to protein synthesis

Proteins synthesized by the rough endoplasmic reticulum (RER) co-translationally cross the membrane through the pore of a ribosome-bound translocon (RBT) complex. Although this pore is also permeable to small molecules, it is generally thought that barriers to their permeation prevent the cyclical process of protein translation from affecting the permeability of the RER. We tested this hypothesis by culturing Chinese hamster ovary-S cells with inhibitors of protein translation that affect the occupancy of RBTs by nascent proteins and then permeabilizing the plasma membrane and measuring the permeability of the RER to a small molecule, 4-methyl-umbelliferyl-alpha-d-glucopyranoside (4-MalphaG). The premature or normal release of nascent proteins by puromycin or pactamycin, respectively, increased the permeability of the RER to 4-MalphaG by 20-30%. In contrast, inhibition of elongation and the release of nascent proteins by cycloheximide did not increase the permeability, but it prevented the increase in permeability by pactamycin. We conclude that the permeability of the RER is coupled to protein translation by a simple gating mechanism whereby a nascent protein blocks the pore of a RBT during translation, but after release of the nascent protein the pore is permeable to small molecules as long as an empty ribosome remains bound to the translocon.

Basal and physiological Ca(2+) leak from the endoplasmic reticulum of pancreatic acinar cells. Second messenger-activated channels and translocons

We have studied the Ca(2+) leak pathways in the endoplasmic reticulum of pancreatic acinar cells by directly measuring Ca(2+) in the endoplasmic reticulum ([Ca(2+)](ER)). Cytosolic Ca(2+) ([Ca(2+)](C)) was clamped to the resting level by a BAPTA-Ca(2+) mixture. Administration of cholecystokinin within the physiological concentration range caused a graded decrease of [Ca(2+)](ER), and the rate of Ca(2+) release generated by 10 pm cholecystokinin is at least 3x as fast as the basal Ca(2+) leak revealed by inhibition of the endoplasmic reticulum Ca(2+)-ATPase. Acetylcholine also evokes a dose-dependent decrease of [Ca(2+)](ER), with an EC(50) of 0.98 +/- 0.06 microm. Inhibition of receptors for inositol 1,4,5-trisphosphate (IP(3)) by heparin or flunarizine blocks the effect of acetylcholine but only partly blocks the effect of cholecystokinin. 8-NH(2) cyclic ADP-ribose (20 microm) inhibits the action of cholecystokinin, but not of acetylcholine(.) The basal Ca(2+) leak from the endoplasmic reticulum is not blocked by antagonists of the IP(3) receptor, the ryanodine receptor, or the receptor for nicotinic acid adenine dinucleotide phosphate. However, treatment with puromycin (0.1-1 mm) to remove nascent polypeptides from ribosomes increases Ca(2+) leak from the endoplasmic reticulum by a mechanism independent of the endoplasmic reticulum Ca(2+) pumps and of the receptors for IP(3) or ryanodine.

Translocon pores in the endoplasmic reticulum are permeable to a neutral, polar molecule

The pore of the translocon complex in the endoplasmic reticulum (ER) is large enough to be permeated by small molecules, but it is generally believed that permeation is prevented by a barrier at the luminal end of the pore. We tested the hypothesis that 4-methylumbelliferyl alpha-d-glucopyranoside (4MalphaG), a small, neutral dye molecule, cannot permeate an empty translocon pore by measuring its activation by an ER resident alpha-glucosidase, which is dependent on entry into the ER. The basal entry of dye into the ER of broken Chinese hamster ovary-S cells was remarkably high, and it was increased by the addition of puromycin, which purges translocon pores of nascent polypeptides, creating additional empty pores. The basal and puromycin-dependent entries of 4MalphaG were mediated by a common, salt-sensitive pathway that was partially blocked by spermine. A similar activation of 4MalphaG was observed in nystatin-perforated cells, indicating that the entry of 4MalphaG into the ER did not result simply from the loss of cytosolic factors in broken cells. We reject the hypothesis and conclude that a small, neutral molecule can permeate the empty pore of a translocon complex, and we propose that translationally inactive, ribosome-bound translocons could provide a pathway for small molecules to cross the ER membrane.

The structural basis for the action of the antibiotics tetracycline, pactamycin, and hygromycin B on the 30S ribosomal subunit

We have used the recently determined atomic structure of the 30S ribosomal subunit to determine the structures of its complexes with the antibiotics tetracycline, pactamycin, and hygromycin B. The antibiotics bind to discrete sites on the 30S subunit in a manner consistent with much but not all biochemical data. For each of these antibiotics, interactions with the 30S subunit suggest a mechanism for its effects on ribosome function.

Estradiol regulates estrogen receptor mRNA stability

Previous studies suggest that post-transcriptional events play an important role in estrogen-induced loss of estrogen receptor expression. The present study shows that treatment of MCF-7 cells with estradiol resulted in a six-fold decrease in estrogen receptor mRNA half-life from 4 h in control cells to 40 min in estradiol treated cells. To determine the role of protein synthesis in the regulation of estrogen receptor mRNA stability, several translational inhibitors were utilized. Pactamycin and puromycin, which prevent ribosome association with mRNA, inhibited the effect of estradiol on receptor mRNA stability, whereas cycloheximide, which has no effect on ribosome association with mRNA, had no effect on estradiol regulation of estrogen receptor mRNA stability. In control cells, the total cellular content of estrogen receptor mRNA was associated with high molecular weight polyribosomes. Treatment with estradiol resulted in a 70% decrease in estrogen receptor mRNA associated with polyribosomes but had no effect on the polyribosome distribution of estrogen receptor mRNA. In an in vitro degradation assay, polyribosomes isolated from estradiol-treated cells degraded ER mRNA faster than polyribosomes isolated from control cells. The nuclease activity associated with the polysome fraction appeared to be Mg2+ independent and inhibited by RNasin. Freeze-thawing and heating at 90 degrees C for 10 min resulted in the loss of nuclease activity. These studies suggest that an estrogen-regulated nuclease activity associated with ribosomes alters the stability of estrogen receptor mRNA.

Translational control of terminal oligopyrimidine mRNAs requires a specific regulator

Terminal oligopyrimidine (TOP) mRNAs are a group of messengers translationally regulated according to the growth status of the cell. Two hypotheses have been proposed for the mechanism of the regulation: (i) there is a specific translational regulator which can reversibly alter TOP-mRNA structure, (ii) a component of the general translational apparatus can specifically affect the translation of TOP-mRNAs. To verify one of the two hypotheses we induced a partial inhibition of translation initiation in Xenopus cultured cells and analyzed the effect on TOP-mRNA translation. Our results suggest that a specific regulator is necessary to explain the translational control of these of mRNAs.

Structure and experimental uses of arthropod venom proteins

In summary, the initial studies conducted thus far into the components of venoms of parasitic wasps and other arthropods have already yielded a number of interesting properties of the proteins therein. These properties have already offered the possibilities of additional principles operating in the evolution of venoms. That so many unexpected rewards have already surfaced with the relatively little experimental digging conducted thus far generates great anticipation that indeed there remains a pharmacological gold mine awaiting to be discovered in components of the other insect venoms as yet unmined by science.

Messenger RNA decay of macrophage colony-stimulating factor in human ovarian carcinomas in vitro

OBJECTIVE

Recently, the importance of the macrophage colony-stimulating factor (CSF-1) and its receptor (encoded by the c-fms proto-oncogene) in patients with epithelial ovarian cancer has been recognized. Overexpression of CSF-1 denotes poor prognosis. Macrophage colony-stimulating factor may be one of a group of inflammatory cytokines, whose 3' untranslated region (UTR) contains AU-rich stretches and whose expression may be largely dependent on mRNA decay. The purposes of this study were to investigate the effect of protein synthesis inhibition on CSF-1 transcript expression, to help determine whether there is a role for labile intermediary proteins in the regulation of CSF-1 expression, and to explore the transcriptional or post-transcriptional mechanisms that could underlie such overexpression of CSF-1 transcripts by protein synthesis inhibitors. Although regulation of CSF-1 gene expression has been investigated in hematopoietic cells, such studies have not been carried out in any epithelial cancer.

METHODS

Northern blot analyses for CSF-1 expression were performed on total cellular RNA extracted from primary and established ovarian cancer cell lines in the absence or presence of proteins synthesis inhibitors with different modes of action. The probe for the AU-rich exon 10 of CSF-1 was prepared by amplification of the terminal 143 bp of the 3' UTR of human CSF-1 by polymerase chain reaction. Transcription rates were assessed in the presence or absence of cycloheximide (CH) in nuclei of ovarian cancer cells by run-off analyses. The effect of CH on CSF-1 mRNA half-life was measured by actinomycin D chase experiments in SKOV3 cells.

RESULTS

We demonstrate that CSF-1 mRNA was expressed by all of a panel of primary and established ovarian cancer cell lines. There are at least three CSF-1 transcripts expressed by ovarian cancer cells; we demonstrate that the 4.2-kb CSF-1 transcript contains exon-6 sequences, which are spliced from the 3.4- and 1.9-kb transcripts, whereas both the 4.2- and 3.4-kb CSF-1 transcripts contain the 3' AU-rich exon 10. Treatment with several different protein synthesis inhibitors resulted in marked overexpression of CSF-1 transcript levels, suggesting a potential role for labile proteins in the regulation of CSF-1 expression. The predominant effect of CH is on the two CSF-1 transcripts that contain exon 10. Although CH does not change the rate of CSF-1 gene transcription, measurement of CSF-1 mRNA stability reveals a prolongation of CSF-1 transcript half-life by CH from 4.5 hours to significantly greater than 6 hours in ovarian cancer cells.

CONCLUSIONS

Augmented CSF-1 transcript stability underlies the marked overexpression of CSF-1 seen with protein synthesis inhibitors. Our results suggest the involvement of a labile regulatory protein that contributes to CSF-1 mRNA decay in ovarian cancer cells. Our data suggest further that exon 10 (not the spliced exon-6 sequences) of the CSF-1 transcript may contain an instability determinant.

Post-transcriptional regulation of c-fms proto-oncogene expression by dexamethasone and of CSF-1 in human breast carcinomas in vitro

The c-fms proto-oncogene encodes the receptor for a hematopoietic growth factor, CSF-1. Recently, the importance of c-fms and its ligand CSF-1 in malignancies of non-hematopoietic origin, such as breast, ovarian, endometrial, pulmonary, and trophoblastic cancers has been recognized. We have previously shown that glucocorticoids induce a large increase in c-fms mRNA and protein levels in breast carcinoma cell lines. In this report, we investigate the mechanism underlying such c-fms overexpression by dexamethasone. We show that dexamethasone treatment of two breast carcinoma cell lines (BT20-c-fms expressor, and SKBR3-co-expressor of both c-fms and CSF-1) does not increase the rate of c-fms gene transcription, suggesting a post-transcriptional mechanism of regulation of c-fms expression by dexamethasone. The effect of protein synthesis inhibition was studied to help determine whether there was a role for intermediary regulatory proteins in the regulation of c-fms expression. We find that several protein synthesis inhibitors interfere with dexamethasone induction of c-fms transcripts, suggesting the existence of regulatory proteins. These regulatory proteins do not appear to be constitutively expressed, as we show no effect of protein synthesis inhibition on c-fms transcript expression in resting BT20 cells. These findings suggest that the putative regulatory proteins are induced by dexamethasone. Furthermore, the addition of a protein synthesis inhibitor, pactamycin, to dexamethasone-treated BT20 cells results in a decrease in c-fms mRNA stability.(ABSTRACT TRUNCATED AT 250 WORDS)

Reactivation of apolipoprotein II gene transcription by cycloheximide reveals two steps in the deactivation of estrogen receptor-mediated transcription

In this report, we describe apolipoprotein II (apoII) gene expression in cell lines derived by stable expression of the chicken estrogen receptor in LMH chicken hepatoma cells. In cell lines expressing high levels of receptor (LMH/2A), apoII gene expression is increased by estrogen 300-fold compared with levels in the receptor-deficient parent LMH line. LMH/2A cells show apoII mRNA induction and turnover kinetics similar to those in chicken liver. Inhibition of protein synthesis with cycloheximide (CHX) or puromycin following estrogen withdrawal superinduces apoII mRNA without affecting apoII mRNA stability. Superinduction is due to an estrogen-independent reactivation of apoII gene transcription. The apoII gene can be reactivated by CHX for up to 24 h following hormone withdrawal, suggesting that the gene is in a repressed yet transcriptionally competent state. These results reveal two distinct events necessary for termination of estrogen receptor-mediated transcription. The first event, removal of hormone, is sufficient to stop transcription when translation is ongoing. The second event is revealed by the CHX-induced superinduction of apoII mRNA following hormone withdrawal. This superinduction suggests that deactivation of estrogen receptor-mediated transcription requires a labile protein. Furthermore, reactivation of apoII gene expression by CHX and estrogen is additive, suggesting that estrogen is unable to overcome repression completely. Thus, a labile protein may act to repress estrogen receptor-mediated transcription of the apoII gene.

Reactivation of apolipoprotein II gene transcription by cycloheximide reveals two steps in the deactivation of estrogen receptor-mediated transcription

In this report, we describe apolipoprotein II (apoII) gene expression in cell lines derived by stable expression of the chicken estrogen receptor in LMH chicken hepatoma cells. In cell lines expressing high levels of receptor (LMH/2A), apoII gene expression is increased by estrogen 300-fold compared with levels in the receptor-deficient parent LMH line. LMH/2A cells show apoII mRNA induction and turnover kinetics similar to those in chicken liver. Inhibition of protein synthesis with cycloheximide (CHX) or puromycin following estrogen withdrawal superinduces apoII mRNA without affecting apoII mRNA stability. Superinduction is due to an estrogen-independent reactivation of apoII gene transcription. The apoII gene can be reactivated by CHX for up to 24 h following hormone withdrawal, suggesting that the gene is in a repressed yet transcriptionally competent state. These results reveal two distinct events necessary for termination of estrogen receptor-mediated transcription. The first event, removal of hormone, is sufficient to stop transcription when translation is ongoing. The second event is revealed by the CHX-induced superinduction of apoII mRNA following hormone withdrawal. This superinduction suggests that deactivation of estrogen receptor-mediated transcription requires a labile protein. Furthermore, reactivation of apoII gene expression by CHX and estrogen is additive, suggesting that estrogen is unable to overcome repression completely. Thus, a labile protein may act to repress estrogen receptor-mediated transcription of the apoII gene.

Identification of a thymidylate synthase ribonucleoprotein complex in human colon cancer cells

Translation of thymidylate synthase (TS) mRNA is controlled by its own protein product, TS, in an autoregulatory manner. Direct binding of TS protein to two different cis-acting elements on the TS mRNA is associated with this translational regulation. In this study, an immunoprecipitation-reverse transcription-PCR technique was used to identify a TS ribonucleoprotein (RNP) complex in cultured human colon cancer cells. Using antibodies specific for TS protein, we show that TS is complexed in vivo with its own TS RNA. Furthermore, evidence demonstrating a direct interaction between the mRNA of the nuclear oncogene c-myc and TS protein is presented.

Identification of a thymidylate synthase ribonucleoprotein complex in human colon cancer cells

Translation of thymidylate synthase (TS) mRNA is controlled by its own protein product, TS, in an autoregulatory manner. Direct binding of TS protein to two different cis-acting elements on the TS mRNA is associated with this translational regulation. In this study, an immunoprecipitation-reverse transcription-PCR technique was used to identify a TS ribonucleoprotein (RNP) complex in cultured human colon cancer cells. Using antibodies specific for TS protein, we show that TS is complexed in vivo with its own TS RNA. Furthermore, evidence demonstrating a direct interaction between the mRNA of the nuclear oncogene c-myc and TS protein is presented.

Nonsense codons in human beta-globin mRNA result in the production of mRNA degradation products

Human beta zero-thalassemic beta-globin genes harboring either a frameshift or a nonsense mutation that results in the premature termination of beta-globin mRNA translation have been previously introduced into the germ line of mice (S.-K. Lim, J.J. Mullins, C.-M. Chen, K. Gross, and L.E. Maquat, EMBO J. 8:2613-2619, 1989). Each transgene produces properly processed albeit abnormally unstable mRNA as well as several smaller RNAs in erythroid cells. These smaller RNAs are detected only in the cytoplasm and, relative to mRNA, are longer-lived and are missing sequences from either exon I or exons I and II. In this communication, we show by using genetics and S1 nuclease transcript mapping that the premature termination of beta-globin mRNA translation is mechanistically required for the abnormal RNA metabolism. We also provide evidence that generation of the smaller RNAs is a cytoplasmic process: the 5' ends of intron 1-containing pre-mRNAs were normal, the rates of removal of introns 1 and 2 were normal, and studies inhibiting RNA synthesis with actinomycin D demonstrated a precursor-product relationship between full-length mRNA and the smaller RNAs. In vivo, about 50% of the full-length species that undergo decay are degraded to the smaller RNAs and the rest are degraded to undetectable products. Exposure of erythroid cells that expressed a normal human beta-globin transgene to either cycloheximide or puromycin did not result in the generation of the smaller RNAs. Therefore, a drug-induced reduction in cellular protein synthesis does not reproduce this aspect of cytoplasmic mRNA metabolism. These data suggest that the premature termination of beta-globin mRNA translation in either exon I or exon II results in the cytoplasmic generation of discrete mRNA degradation products that are missing sequences from exon I or exons I and II. Since these degradation products appear to be the same for all nonsense codons tested, there is no correlation between the position of translation termination and the sites of nucleolytic cleavage.

Nonsense codons in human beta-globin mRNA result in the production of mRNA degradation products

Human beta zero-thalassemic beta-globin genes harboring either a frameshift or a nonsense mutation that results in the premature termination of beta-globin mRNA translation have been previously introduced into the germ line of mice (S.-K. Lim, J.J. Mullins, C.-M. Chen, K. Gross, and L.E. Maquat, EMBO J. 8:2613-2619, 1989). Each transgene produces properly processed albeit abnormally unstable mRNA as well as several smaller RNAs in erythroid cells. These smaller RNAs are detected only in the cytoplasm and, relative to mRNA, are longer-lived and are missing sequences from either exon I or exons I and II. In this communication, we show by using genetics and S1 nuclease transcript mapping that the premature termination of beta-globin mRNA translation is mechanistically required for the abnormal RNA metabolism. We also provide evidence that generation of the smaller RNAs is a cytoplasmic process: the 5' ends of intron 1-containing pre-mRNAs were normal, the rates of removal of introns 1 and 2 were normal, and studies inhibiting RNA synthesis with actinomycin D demonstrated a precursor-product relationship between full-length mRNA and the smaller RNAs. In vivo, about 50% of the full-length species that undergo decay are degraded to the smaller RNAs and the rest are degraded to undetectable products. Exposure of erythroid cells that expressed a normal human beta-globin transgene to either cycloheximide or puromycin did not result in the generation of the smaller RNAs. Therefore, a drug-induced reduction in cellular protein synthesis does not reproduce this aspect of cytoplasmic mRNA metabolism. These data suggest that the premature termination of beta-globin mRNA translation in either exon I or exon II results in the cytoplasmic generation of discrete mRNA degradation products that are missing sequences from exon I or exons I and II. Since these degradation products appear to be the same for all nonsense codons tested, there is no correlation between the position of translation termination and the sites of nucleolytic cleavage.

A protein-conducting channel in the endoplasmic reticulum

The existence of a protein-conducting channel in the endoplasmic reticulum membrane was demonstrated by electrophysiological techniques. Pancreatic rough microsome (RM) vesicles were fused to one side (cis) of a planar lipid bilayer separating two aqueous compartments of 50 mM salt. This exposed the cytoplasmic surface of the RMs, with its attached ribosomes, to the cis chamber. Addition of 100 microM puromycin to the cis side caused a large increase in membrane conductance, presumably the result of puromycin-induced clearance of nascent protein chains from the lumen of protein-conducting channels. When puromycin was added at low concentrations (0.33 microM), single channels of 220 pS were observed. These closed when the salt concentration was raised to levels at which ribosomes detach from the membrane (150-400 mM), indicating that the attached ribosome keeps the channel in an open conformation. A mechanism for a complete cycle of opening and closing of the protein-conducting channel is suggested.

Role of protein synthesis in the accumulation of ferritin mRNA during exposure of cells to iron

The iron-induced biosynthesis of ferritin is regulated at the translational level via multiple mechanism. A prolonged exposure of cells to iron leads to a marked increase in ferritin mRNA levels caused by stabilization of the message. Here we show that this stabilization requires the synthesis de novo of an iron-inducible protein factor.

Intracellular receptor binding and nuclear transport of nerve growth factor in intact cells and a cell-free system

Nuclear uptake of 125I-labeled nerve growth factor (NGF) by cells that either express or do not express the cell surface receptor was tested using intact cells and a cell-free system. Intracellular and consequently nuclear uptake of NGF in intact cells was dependent on the presence of surface NGF receptor, whereas nuclear uptake in a cell-free system did not correlate with cell surface receptor expression. In the cell-free system, nuclear transport was inhibited when NGF receptor was being actively synthesized. Preincubation of intact cells with unlabeled NGF, cycloheximide, puromycin, or actinomycin D increased nuclear uptake up to threefold. The data suggest that, in intact cells, NGF transported into the cell via the surface receptors is also bound by the NGF receptor being synthesized in the cytoplasm. NGF taken up by the nucleus inhibited transcription of ribosomal RNA genes by 70% and, in turn, inhibited cell proliferation by 60%. A direct effect of NGF on transcription is discussed.

Metabolic processes involved in repair of Escherichia coli cells damaged by exposure to acid mine water

Escherichia coli was stressed by exposure to filter-sterilized acid mine water. Synthetic processes required for repair of sublethally injured survivors were studied by the addition of specific metabolic inhibitors to a resuscitation broth. Inhibitors of protein, RNA, DNA, lipid, and peptidoglycan synthesis as well as uncouplers and inhibitors of electron transport and ATPase activity were used. Acid mine water injury was severe, causing damage to the outer and cytoplasmic membranes. Repair of sublethally injured cells required protein, RNA, and lipid synthesis as well as a proton motive force.

Evidence for rapid metabolic turnover of hyaluronate synthetase in Swarm rat chondrosarcoma chondrocytes

Synthesis of [3H]hyaluronate from [6-3H]glucosamine was investigated in cultures of Swarm rat chondrosarcoma chondrocytes treated with various concentrations (0.1 microM-0.1 mM) of cycloheximide for various times. Concentrations greater than 1 microM inhibited protein synthesis by greater than 90%. Hyaluronate synthesis was decreased, with a t1/2 for 50% inhibition of 80-120 min, depending on the concentration of cycloheximide present. Similar experiments using [1-3H]glucose as a precursor label gave similar results. Experiments using [6-3H]glucosamine as a precursor label and 0.18 mM-puromycin to inhibit protein synthesis inhibited hyaluronate synthesis (t1/2 = 82 min) with similar kinetics to cycloheximide-induced inhibition. Cultures incubated with 3.6 microM-cycloheximide for up to 9 h and supplemented with p-nitrophenyl beta-D-xyloside during the last 75 min of treatment showed increased synthesis of [3H,35S]chondroitin sulphate, demonstrating that UDP-hexose precursors for glycosaminoglycan synthesis are not rapidly depleted on blockage of protein synthesis. Rapid metabolic turnover of hyaluronate synthetase is the most likely cause for decreased hyaluronate synthesis in chondrocytes in which protein synthesis is inhibited.

Enhanced transcription of c-myc in bursal lymphoma cells requires continuous protein synthesis

In several bursal lymphoma cell lines in which c-myc transcription is regulated by avian leukosis virus (ALV) long terminal repeat (LTR) sequences, protein synthesis inhibition decreases the transcriptional activity of c-myc as well as other LTR driven viral genes. This decrease in transcription is associated with a change in the chromatin structure of c-myc, as measured by deoxyribonuclease I (DNase I) hypersensitivity, and a shift of transcription from the LTR to the normal c-myc promoter. In contrast, cycloheximide had little or no effect on the transcription of LTR driven genes in infected chicken embryo fibroblasts treated with the drug. These results suggest that a labile, cell type-specific protein may interact with the retroviral LTR and regulate transcription of genes under LTR control. Further, the results demonstrate that the increase in intracellular concentration of c-myc RNA induced by cycloheximide treatment of normal cells is the result of stabilization of this message.

Local protein synthesizing activity in axonal fields regenerating in vitro

The goldfish retinal explant system of Landreth and Agranoff was used to study endogenous protein synthesizing activity of retinal ganglion cell axons regenerating in culture. Light and electron microscopic examination of axonal fields showed that axons were free of nonneural cell investment. Decentralized axons were incubated with a mixture of tritiated amino acids, and direct quantitative microanalysis of protein and tritium radioactivity was carried out on individual axonal fields. Our findings showed that radioactive amino acids were incorporated into axonal protein in a manner that inhibited significantly by cycloheximide, but not by chloramphenicol. Decentralized axons appeared to maintain their viability for at least 3-4 h. Axonal fields maintaining their central connections to the explant incorporated 3H-amino acids at an apparent rate that was similar to decentralized axonal fields. Labeled material transported into axonal fields from ganglion cell bodies appeared in significant amounts after a delay of 2-3 h. Fluorographic patterns of axonal proteins after labeling with either 3H-amino acids or [35S]methionine and separated by microelectrophoresis indicated that primarily tubulin and, to a lesser extent, actin were labeled. Our findings indicate that goldfish retinal ganglion cell axons regenerating in vitro exhibit measureable endogenous protein-synthesizing activity.

Localization of the puromycin binding site on the large ribosomal subunit of Escherichia coli by immunoelectron microscopy

By using immunoelectron microscopy, we have localized the binding site on 50S Escherichia coli ribosomal subunits for puromycin, an antibiotic that interacts with the ribosomal peptidyltransferase center. This was achieved by affinity-labeling 50S subunits with N-bromoacetyl puromycin and treating the labeled subunits with an antibody specific for the N6,N6-dimethyladenosine moiety of puromycin. The position of the puromycin binding site was then revealed by localizing the attachment sites of the IgG molecules on the surfaces of the 50S subunits under the electron microscope: it was located at the interface between the subunits, on and around the wider lateral protuberance of the 50S subunit. This localizes directly the peptidyl transferase center on the surface of the large ribosomal subunit.

Lincomycin increases synthetic rate and periplasmic pool size for cholera toxin

Increased enterotoxigenicity of Vibrio cholerae 569B grown with low concentrations of lincomycin, previously described in terms of increased extracellular biological activity (capillary permeability factor and fluid accumulation in ligated rabbit ileal loops), was further characterized. Polyacrylamide gel electrophoresis and single radial immunodiffusion showed that lincomycin-stimulated cells produced increased molar quantities of cholera toxin (CT) both extra- and intracellularly. The intracellular CT was released in comparable amounts by sonication, deoxycholate extraction, and polymyxin B treatment. Polymyxin B release of CT was nearly complete under conditions wherein only 6% of total cellular beta-galactosidase was released, implying a periplasmic pool of CT in stimulated cells. No intracellular choleragenoid (CT subunit B) was found in stimulated cells by polymyxin B release. No proteolysis of 14C-labeled CT was detected after prolonged incubation with sonicated nonstimulated cultures or sonicated concentrated cells. These data support the conclusion that the stimulatory effect of lincomycin involves an increase in the rate of synthesis of the CT molecule, and argue against alternative models involving inhibition of putative normal degradation of CT, increased release of otherwise cell-bound CT, or activation of inactive, or less active, forms of CT.