Fluoride inhibition of the initiation of protein synthesis in the reticulocyte lysate cell-free system

Principles of fluoride toxicity and the cellular response: a review

Fluoride is ubiquitously present throughout the world. It is released from minerals, magmatic gas, and industrial processing, and travels in the atmosphere and water. Exposure to low concentrations of fluoride increases overall oral health. Consequently, many countries add fluoride to their public water supply at 0.7-1.5 ppm. Exposure to high concentrations of fluoride, such as in a laboratory setting often exceeding 100 ppm, results in a wide array of toxicity phenotypes. This includes oxidative stress, organelle damage, and apoptosis in single cells, and skeletal and soft tissue damage in multicellular organisms. The mechanism of fluoride toxicity can be broadly attributed to four mechanisms: inhibition of proteins, organelle disruption, altered pH, and electrolyte imbalance. Recently, there has been renewed concern in the public sector as to whether fluoride is safe at the current exposure levels. In this review, we will focus on the impact of fluoride at the chemical, cellular, and multisystem level, as well as how organisms defend against fluoride. We also address public concerns about fluoride toxicity, including whether fluoride has a significant effect on neurodegeneration, diabetes, and the endocrine system.

The yeast Mig1 transcriptional repressor is dephosphorylated by glucose-dependent and -independent mechanisms

A yeast Saccharomyces cerevisiae Snf1 kinase, an analog of mammalian AMPK, regulates glucose derepression of genes required for utilization of alternative carbon sources through the transcriptional repressor Mig1. It has been suggested that the Glc7-Reg1 phosphatase dephosphorylates Mig1. Here we report that Mig1 is dephosphorylated by Glc7-Reg1 in an apparently glucose-dependent mechanism but also by a mechanism independent of glucose and Glc7-Reg1. In addition to serine/threonine phosphatases another process including tyrosine phosphorylation seems crucial for Mig1 regulation. Taken together, Mig1 dephosphorylation appears to be controlled in a complex manner, in line with the importance for rapid and sensitive regulation upon altered glucose concentrations in the growth medium.

In vitro translation in a hamster brain cell-free system

We have developed a cell-free translation system derived from hamster brain tissue. The optimal incorporation conditions were 160 mM K+, 2 mM Mg2+ and microM spermine. The absence of this latter compound could be compensated only by doubling (to 4 mM) Mg2+ concentration. This system was inhibited by cycloheximide and emetine (elongation inhibitors) as well as NaF and aurintricarboxilic acid (ATA) (initiation inhibitors). The nature of the ATA inhibiting effect was studied by kinetic comparison with cycloheximide and polyribosome sedimentation pattern.

Preparation of a cell-free extract from rat brain which can initiate protein synthesis in vitro

A cell-free protein synthesis system, derived from brains of 3 mo-old male Fischer-344 rats, has been characterized. The optimum conditions for amino acid incorporation in the system were 5 mM magnesium ion and 200 mM potassium ion. Incorporation depended on the addition of ATP, GTP, and an energy-generating system, and was sensitive to addition of the drugs aurintricarboxylic acid and sodium fluoride, inhibitors of initiation of protein synthesis. Both 40S and 80S initiation complexes were labeled in vitro, using [35S]methionine. Such labeling was sensitive to the protein synthesis inhibitors, aurintricarboxylic acid and sodium fluoride. The system, which can initiate protein synthesis, should be of use for examining mechanisms which underlie alterations in rat brain protein synthesis induced by various treatments.

An in vitro amino acid incorporation method for assessing the status of in vivo protein synthesis

A quantitative in vitro amino acid incorporation assay is described which can be used to assess the status of in vivo protein synthesis. The preparation and incubation conditions employed result in constant precursor specific activity and limit amino acid incorporation to completion of nascent peptide chains. Results obtained with this method correlate well with measurements of polyribosome profiles using sucrose gradient centrifugation. The assay is easily applied to a large number of samples, and requires only a fraction of the time and tissue necessary for conventional measures of polysome aggregation. The method has been found suitable for studies of protein synthesis in mouse brain and liver, and in gerbil brain, but not in mouse kidney. Products of in vitro protein synthesis can be separated by standard electrophoretic techniques, allowing a characterization of proteins whose mRNAs are actively translated in vivo.

Effects of cyclic AMP and fluoride on phosphorylation of ribosomal protein S6 and on protein synthesis in rabbit reticulocytes

The effects of N6,O2-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP) and sodium fluoride on the phosphorylation of ribosomal proteins S6 and on protein synthesis were examined. Rabbit reticulocytes were incubated in a nutritional medium containing 32Pi in the presence and absence of Bt2cAMP (1mM) and 3-isobutyl-1-methyl-xanthine (1mM). In the control cells, four phosphorylated derivatives of S6 were observed, with most of the radioactivity in the monophosphorylated form. Upon addition of cyclic nucleotide, a twofold increase in the phosphorylation of ribosomal protein S6 was observed. This was accompanied by an increase of radioactive phosphate in the diphosphorylated derivative. No alteration in protein synthesis was observed upon addition of cAMP and analogues of cAMP in conjunction with 3-isobutyl-1-methyl-xanthine or theophylline. The effects of sodium fluoride on phosphorylation of S6 and on protein synthesis were examined also. At 5 mM sodium fluoride, protein synthesis was inhibited by 85%. A 2.5-fold increase in the phosphorylation of ribosomal protein S6 was observed with an accumulation of 32Pi in the diphosphorylated, triphosphorylated and tetraphosphorylated derivatives. Inhibition of protein synthesis coincided with an increase in the more highly phosphorylated derivatives, whereas an increase of radioactive phosphate in the diphosphorylated derivative could not be correlated with an alteration in globin synthesis.

Characterisation of protein synthesis in cell-free extracts from different mammalian cells by their sensitivity to inhibitors of polypeptide-chain initiation

Plasma cells and reticulocytes are differentiated mammalian cell systems specialized in the synthesis of distinct proteins. To study whether a cell specificity of polypeptide-chain initiation in such cell systems can be detected through inhibitors, the sensitivity to several drugs interfering with initiation was compared in cell-free systems with S-30 extracts from these cells. The following was indicated by the experiments: (1) under the selected conditions, the different cell-free systems are comparable with respect to their activity of initiation, and that aurintricarboxylic acid inhibits mRNA binding to the ribosome in plasma cell tumours the same as in reticulocytes. (2) The sensitivity of translation of endogenous mRNAs in reticulocytes and plasma cell tumours to inhibitors of mRNA binding to the ribosome, aurintricarboxylic acid and the homopolynucleotides poly(U) and poly(A), are different. (3) Inhibition of the succeeding reactions of peptide-chain initiation by sodium fluoride and pactamycin was not selective. (4) In both cell systems translation of poly(U) is equally sensitive to aurintricarboxylic acid. (5) In extracts from TEPC 15 myeloma cells synthesis of immunoglobulin L-chains compared with that of the other myeloma proteins is partially resistant to aurintricarboxylic acid, whereas in reticulocytes, no differential sensitivities of individual proteins could be observed. The different susceptibility of the mRNA binding reaction in plasma cell tumours and reticulocytes suggests that the predominant mRNAs of these cells have different affinities to ribosome binding sites.

Protein biosynthesis in a cell-free system from bovine retina

A cell-free extract active in the incorporation of [3H]leucine into protein has been prepared from bovine retina. This system may be useful in identifying early precursors and transported forms of the visual pigment apoprotein. The system completed a minimum of 50% of the preexisting nascent polypeptide chains. 20% of the labelled product accumulated in the soluble fraction of the extract. Gel electrophoresis revealed that this fraction contained six major labelled polypeptide species, each of which correspond to a major unlabelled soluble polypeptide. Some completed polypeptides were retained in the particulate fraction of the extract; several of these were resolved on gel electrophoresis and were of very high specific radioactivity. An antiserum to rod outer segment disc proteins precipitated several labelled species from each fraction. The most interesting of these is a polypeptide of molecular weight 41 000 which was found only in the particulate fraction.