Polyribosomes and control of protein synthesis: effects of sodium fluoride and temperature of reticulocytes

The acid test of fluoride: how pH modulates toxicity

Background

It is not known why the ameloblasts responsible for dental enamel formation are uniquely sensitive to fluoride (F⁻). Herein, we present a novel theory with supporting data to show that the low pH environment of maturating stage ameloblasts enhances their sensitivity to a given dose of F⁻. Enamel formation is initiated in a neutral pH environment (secretory stage); however, the pH can fall to below 6.0 as most of the mineral precipitates (maturation stage). Low pH can facilitate entry of F⁻ into cells. Here, we asked if F⁻ was more toxic at low pH, as measured by increased cell stress and decreased cell function.

Methodology/Principal Findings

Treatment of ameloblast-derived LS8 cells with F⁻ at low pH reduced the threshold dose of F⁻ required to phosphorylate stress-related proteins, PERK, eIF2α, JNK and c-jun. To assess protein secretion, LS8 cells were stably transduced with a secreted reporter, Gaussia luciferase, and secretion was quantified as a function of F⁻ dose and pH. Luciferase secretion significantly decreased within 2 hr of F⁻ treatment at low pH versus neutral pH, indicating increased functional toxicity. Rats given 100 ppm F⁻ in their drinking water exhibited increased stress-mediated phosphorylation of eIF2α in maturation stage ameloblasts (pH<6.0) as compared to secretory stage ameloblasts (pH∼7.2). Intriguingly, F⁻-treated rats demonstrated a striking decrease in transcripts expressed during the maturation stage of enamel development (Klk4 and Amtn). In contrast, the expression of secretory stage genes, AmelX, Ambn, Enam and Mmp20, was unaffected.

Conclusions

The low pH environment of maturation stage ameloblasts facilitates the uptake of F⁻, causing increased cell stress that compromises ameloblast function, resulting in dental fluorosis.

Fluoride induces endoplasmic reticulum stress and inhibits protein synthesis and secretion

BACKGROUND

Exposure to excessive amounts of fluoride (F(-)) causes dental fluorosis in susceptible individuals; however, the mechanism of F(-)-induced toxicity is unclear. Previously, we have shown that high-dose F(-) activates the unfolded protein response (UPR) in ameloblasts that are responsible for dental enamel formation. The UPR is a signaling pathway responsible for either alleviating endoplasmic reticulum (ER) stress or for inducing apoptosis of the stressed cells.

OBJECTIVES

In this study we determined if low-dose F(-) causes ER stress and activates the UPR, and we also determined whether F(-) interferes with the secretion of proteins from the ER.

METHODS

We stably transfected the ameloblast-derived LS8 cell line with secreted alkaline phosphatase (SEAP) and determined activity and localization of SEAP and F(-)-mediated induction of UPR proteins. Also, incisors from mice given drinking water containing various concentrations of F(-) were examined for eucaryotic initiation factor-2, subunit alpha (eIF2alpha) phosphorylation.

RESULTS

We found that F(-) decreases the extracellular secretion of SEAP in a linear, dose-dependent manner. We also found a corresponding increase in the intracellular accumulation of SEAP after exposure to F(-). These changes are associated with the induction of UPR proteins such as the molecular chaperone BiP and phosphorylation of the UPR sensor PKR-like ER kinase, and its substrate, eIF2alpha. Importantly, F(-)-induced phosphorylation of eIF2alphawas confirmed in vivo.

CONCLUSIONS

These data suggest that F(-) initiates an ER stress response in ameloblasts that interferes with protein synthesis and secretion. Consequently, ameloblast function during enamel development may be impaired, and this may culminate in dental fluorosis.

Folding of the nascent peptide chain into a biologically active protein

The refolding of denatured proteins with complete sequences may not be fast enough to account for the in vivo folding of growing peptide chains during biosynthesis. As some peptide fragments have secondary structures not unlike those of the corresponding segments in the intact molecules and native disulfide bonds of some proteins can form cotranslationally, it is suggested that the folding of the nascent chain begins early during synthesis. However, further adjustments may be necessary during chain elongation and after posttranslational modifications of the completed peptide chain to generate the native conformation of a biologically active protein.

Formation of the triple helix of type I procollagen in cellulo. Temperature-dependent kinetics support a model based on cis in equilibrium trans isomerization of peptide bonds

The kinetics of triple-helix formation in type I procollagen at 37 degrees C in cellulo have been found to agree with predictions from the following model: triple-helix formation is initiated after completion of the synthesis of the procollagen polypeptide chains and after the chains associate to form interchain disulfide bonds within the C-propeptide; triple-helix formation propagates from this single nucleation site toward the N terminus of the molecule, interrupted by the random occurrence of peptide bonds in the cis configuration; cis-trans isomerization controls the rate of triple-helix formation. This model predicts that the activation energy of the rate-limiting process should be strongly positive. However, studies of triple-helix formation in vitro using thermally denatured material have shown only a low, or even negative, dependence of the rate on temperature in the physiological range. Here we report the temperature dependence of the rate of triple-helix formation in cellulo and a novel procedure for analyzing the resulting data to give an estimate of the Arrhenius activation energy of the rate-controlling process. It was found that this rate showed a strong, positive dependence on temperature, as expected, and that the activation energy was in satisfactory agreement with independent direct determinations of this parameter for cis-trans isomerizations. These findings lend further support to the model of triple-helix formation described above.

Is the major Drosophila heat shock protein present in cells that have not been heat shocked?

When eukaryotic cells are exposed to elevated temperatures they respond by vigorously synthesizing a small group of proteins called the heat shock proteins. An essential element in defining the role of these proteins is determining whether they are unique to a stressed state or are also found in healthy, rapidly growing cells at normal temperatures. To date, there have been conflicting reports concerning the major heat-induced protein of Drosophila cells, HSP 70. We report the development of monoclonal antibodies specific for this protein. These antibodies were used to assay HSP 70 in cells incubated under different culture conditions. The protein was detectable in cells maintained at normal temperatures, but only when immunological techniques were pushed to the limits of their sensitivity. To test for the possibility that these cells contain a reservoir of protein in a cryptic antigenic state (i.e., waiting posttranslational modification for use at high temperature), we treated cells with cycloheximide or actinomycin D immediately before heat shock. HSP 70 was not detected in these cells. Finally, we tested for the presence of a reservoir of inactive messages by using a high stringency hybridization of 32P-labeled cloned gene sequences to electrophoretically separated RNAs. Although HSP 70 mRNA was detectable in rapidly growing cells, it was present at less than 1/1,000th the level achieved after induction.

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.

Evidence for control of protein synthesis in HeLa cells via the elongation rate

The effects of fresh medium and serum on protein synthesis in suspension-cultured HeLa cells after growth to high cell density (> 5 x 10(5) cells/ml) were studied. Cells which were resuspended in fresh medium plus serum and grown for 24 hours (control) were compared with cells grown for 2 hours after resuspension (stimulated). The spectrum of proteins being synthesized by control and stimulated cells does not appear to be grossly different; that is, the weight and number average molecular weights of newly synthesized whole-cell protein are about the same in both cultures. Also, no significant differences were observed in the number of ribosomes per polysome or in the fraction of total ribosomes in polysomes. However, the transit times (combined elongation and termination times) were found to differ significantly; the average transit time for control cells was 2.24 minutes, while the average transit time for stimulated cells was 1.26 minutes. (An appendex evaluating the methodology involved in measuring the transit time is included.) In aggreement with the difference in transit time, the absolute rate of protein synthesis in stimulated cells was approximately 1.8 times the rate measured in control cells. These data are taken as evidence that under certain conditions, the rate of elongation and/or termination of polypeptide chains limits the overall rate of translation, and that cells can respond to growth conditions by changing the elongation and/or termination rate of protein synthesis.

The initiation and elongation steps in protein synthesis: relative rates in Chinese hamster ovary cells during and after hyperthermic and hypothermic shocks

The relative rates of the initiation and elongation phases of protein synthesis have been determined in heat- and cold-shocked CHO cells from measurements of the incorporation of 35S-methionine into N-terminal and internal positions of growing peptides by a modified Edman degradation. When the cells are shifted from 37 degrees C to temperatures between 10 degrees C and 34 degrees C, the rate of initiation is at first reduced more extensively than that of elongation. After 20 to 30 minutes at the lower temperature, however, the cells undergo a metabolic adjustment which includes increasing the rate of initiation until it corresponds to the rate of elongation at that temperature. Calculated apparent energies of activation for initiation and elongation are in reasonable agreement with those determined in other mammalian cells. When the cooled cells are returned to 37 degrees C, the rates of initiation and elongation recover immediately but do not exceed the control values. Exposure to elevated temperatures (43 degrees C) causes an immediate cessation of initiation and thus a delayed inhibition of elongation; upon return to 37 degrees C, the rate of initiation is transiently elevated above the control rate, and the rate of elongation returns to the control rate after a 2- to 3-minute delay. Hence, a factor which leads to supranormal rates of initiation may accumulate at high but not at low temperatures.

Morphologic changes in the rat enamel organ following a single intraperitoneal injection of sodium fluoride

Enamel organs in developing teeth of young rats were studied after single intraperitoneal injections of a high dose of sodium fluoride (60 mg NaF/kg body wt.). The study employed primarily light microscopy, but electron microscopy was used to clarify some of the light microscopic findings. The pathogenesis of the fluoride-induced changes was followed during 72 h. Cellular changes were consistently found in the molars, but were never seen in the incisors. In the maxillary molars, ameloblastic injury was most commonly seen on the mesial surfaces of the cusps. One hour after injection, the most prominent findings were swollen mitochondria in the secretory ameloblasts and cleft formations between the ameloblasts and the enamel matrix. The clefts were filled with a stippled material. Some of the clefts gradually expanded to cystic cavities. The stippled material began to calcify after 24 h and formed small, darkly stained globules. After 72 h dearranged ameloblasts were found as islands intermingled with calcified rounded structures in the stellate reticulum. In stratum intermedium numerous atypic autophagic vacuoles appeared 2 h after injection. No light microscopic changes were observed in the postsecretory ameloblasts.

Faster synthesis and slower degradation of liver protein during developmental growth

A study is presented of the liver protein gain during the early stages of postnatal development. Fractional rates of protein synthesis and degradation were determined in vivo in livers of 4-day-old mice. At this age, liver protein accumulated at a rate of 18% per day. Synthesis was measured after the injection of massive amounts of radioactive leucine. Degradation was extimated as the balance between synthesis and accumulation of stable liver proteins, or from the disappearance of radioactivity from liver protein previously labelled by the administration of NaH14CO3. We found that the neonatal livers: (1) synthesize 139% as much protein per unit time and unit mass as adult tissue, which is accounted for by a higher ribosome concentration (synthesis per mg of RNA was the same); (2) retain 39% of the newly synthesized protein as stable liver components (compared with 48% in adult mice); (3) degrade protein at 56% of the rate in the adult liver. This lower rate of degradation is quantitatively the most significant difference between the growing and non-growing liver.

The polypeptide chain growth rate in amino acid-starved Escherichia coli determined by a novel method

The proteins synthesized by arginine-requiring Escherichia coli during growth or arginine starvation were characterized by polyacrylamide gel electrophoresis in sodium dodecyl sulfate to give size distributions. The proteins made during amino acid starvation were smaller than those made by growing cells. This was true for otherwise isogenic rel- ("relaxed") and rel+ ("stringent") bacteria. Also using electrophoretic profiles, the peptide chain growth rate was estimated by a novel method based on comparison of theoretically predicted and observed kinetics of pulse labeling protein chains of different sizes. During arginine starvation, the rate was 2--5 amino acids/s for both rel- and rel+ cells, compared to 20 amino acids/s for growing cells. The results rule out chain growth-rate differences as an aspect of the "relaxed" phenomenon.

Regulation of translation in rabbit reticulocytes and mouse L-cells; comparison of the effects of temperature

Various parameters of protein synthesis were analyzed in rabbit reticulocytes exposed to various temperatures for up to five hours. Between 10 degrees C and 40 degrees C total protein synthesis exhibited two different apparent activation energies (36 kcal/mole, 10-24 degrees C; 22 kcal/mole, 24-40 degrees C), as did protein elongation and release (35 kcal/mole, 10-25 degrees C; 12 kcal/mole, 25-40 degrees C). However, the level of polysomes remained essentially unchanged between 0 degrees C and 42 degrees C which implies that the activation energy for polypeptide initiation is quite similar to that for elongation and is also biphasic. This situation is different from that in cultured mouse L-cells where the polysome level is dependent on temperatures. Nevertheless, reticulocytes and L-cells appear to be similar in their temperature dependence of initiation and in their rate of elongation (5-6 amino acids/second at 36 degrees C.

Control of haemoglobin synthesis. The effects of iron deprivation, cobalt and temperature on the rate and extent of globin synthesis in reticulocytes

A detailed examination of the kinetics of protein synthesis in rabbit reticulocytes in the presence of the iron chelating agent 2,2'-dipyridyl showed that between 30 degrees C and 42 degrees C there were characteristically two distinct phases of protein synthesis. An initial phase (I), in which no inhibition of protein synthesis was apparent, was followed by a gradual decline in the rate of protein synthesis leading to the second phase (II) in which protein synthesis occurred at a linear but inhibited rate for extended periods. In contrast, below 30 degrees C, incubation in the presence of dipyridyl caused no inhibition of protein synthesis. Between 30 degrees C and 42 degrees C the duration and amount of protein synthesis occurring in phase I before the onset of inhibition were inversely related of the inhibition as was the final rate of incorporation in phase II. During phase II, a partial reversal of the inhibition caused by dipyridyl was obtained by lowering the incubation temperature. This resulted in a burst of protein synthesis at the uninhibited rate until the amount of protein synthesis reached the same level as that in reticulocytes maintained continuously with dipyridyl at the lower incubation temperature. This burst of synthesis was observed in reticulocytes which had been held in phase II for as long as 90 min. It was also possible to reverse the inhibition by addition of haemin to cells in phase II. At any particular incubation temperature, a fixed number of rounds of protein synthesis had to occur before the onset of phase II became apparent. By the use of puromycin we showed that this was not a requirement for the synthesis of globin or of any other protein. We believe that this critical amount of protein synthesis reflects the residual ability of reticulocytes to initiate new protein chains in the absence of concurrent haem synthesis. Reticulocytes preincubated in the presence of cobaltous ions showed almost no inhibition of protein synthesis upon subsequent incubation with dipyridyl. The results are compared to those obtained in reticulocyte lysates and are discussed in terms of current theories to account for control of protein chain initiation by haemin.

Effect of reduced temperatures on protein synthesis in mouse L cells

The rate of incorporation of leucine into protein, the rate of polypeptide elongation and termination, and the relative quantity and size of polysomes were analyzed in mouse L cells grown in suspension culture at various temperatures between 0 degrees C and 36 degrees C. Between 10 degrees C and 36 degrees C protein synthesis exhibited two different apparent activation energies (39 kcal/mole, 10-25 degrees C; 14 kcal/mole, 25-36 degrees C), whereas elongation and termination had only one (16 kcal/mole). Below 36 degrees C, the polysome level and size decreased, reaching a minimum of 30% of the control 36 degrees C values at 10 degrees C; below 10 degrees C the level increased again back to control values at 0 degrees C. The polysome decline was time dependent, requiring about 5 hr to reach the equilibrium value. This decline is completely reversible within 60 min, even in the presence of 4 mug/ml of actinomycin D, and even after 15 hr of incubation at the lower temperature. The results suggest that polypeptide initiation is rate limiting, particularly below 25 degrees C; whereas above this temperature, elongation or perhaps some other process may be limiting. These results are quite different from those obtained for E. coli and rabbit reticulocyte protein synthesis.

Nerve-muscle interaction in vitro. Role of acetylcholine

Nerve and muscle cells from clonal lines interact in vitro, resulting in the association on the muscle surface of an area of increased acetylcholine sensitivity with a site of nerve-muscle contact. This localization of acetylcholine sensitivity on the muscle cell to a site of contact between nerve and muscle was found to occur when acetylcholine receptors on the muscle had been blocked with alpha-neurotoxin. Localization was also found to occur when the nerve cell had been prevented from releasing acetylcholine. It is concluded that neither the presence of active acetylcholine receptors on the muscle, nor the release of acetylcholine from the nerve, was required for the events leading to the localization of acetylcholine sensitivity in vitro.

Monoribosomal attachment to messenger ribonucleic acid in sodium fluoride-treated rabbit reticulocytes

Mild proteolysis with Pronase selectively dissociates ribosomes not attached to mRNA into subunits; ribosomes attached to mRNA remain intact. A portion of monoribosomes from reticulocytes incubated with NaF resisted proteolytic dissociation. Recovery of mRNA from monoribosomes of NaF-treated reticulocytes therefore may be explained by persistent attachment of some monoribosomes to mRNA.

Evidence for a proposed initiation complex for protein synthesis in reticulocyte polyribosome profiles

Polysomes consisting of 40S ribosomal subunits attached to monosomes, disomes, trisomes, and tetrasomes have been isolated for the first time by the use of isokinetic sucrose density gradients. They are formed upon incubation of rabbit reticulocyte lysate in the presence of 10 mM NaF. It is proposed that these structures represent transient initiation complexes for protein synthesis.

Differentiation of mammalian somatic cells: DNA and hemoglobin synthesis in fetal mouse yolk sac erythroid cells

The relationship between the synthesis and replication of DNA and the capacity to synthesize specialized protein on a stable messenger RNA has been examined in yolk sac erythroid cells differentiating in vivo in fetal mice. Evidence is presented that by day 11 of gestation, at least 98 per cent of yolk sac erythroid cells are synthesizing hemoglobin. These cells are shown to incorporate (3)H-thymidine and to replicate, with more than twofold increase in cell number between days 10 and 11. Division of these cells continues through day 13. Hemoglobin formation in these cells is resistant to actinomycin D from day 10. During this period, ribosomal content per cell decreases by more than a factor of 2. These data indicate that in yolk sac erythroid cells, hemoglobin formation proceeds independent of continued RNA formation in cells capable of DNA synthesis and replication.