The regulation of protein synthesis in mammalian cells by amino acid supply

Amino Acid and protein kinetics in renal failure: an integrated approach

Even apparently healthy patients on dialysis have significant loss of lean body mass. Patients with chronic renal failure without coexisting metabolic acidosis or inflammation have decreased protein turnover, with balanced reduction in protein synthesis and breakdown. However, regional and whole-body protein kinetic studies indicate that hemodialysis (HD) induces net increase in protein breakdown. Whole-body protein turnover studies show that HD is associated with decreased protein synthesis, but proteolysis is not increased. Muscle protein kinetics studies, however, identify enhanced muscle protein breakdown with inadequate compensatory increases in synthesis as the cause of the catabolism. Transmembrane amino acid-transport kinetics studies show that the outward transport is increased more than the inward transport of amino acids during HD. Altered intracellular amino acid transport kinetics and protein turnover during HD could be caused by the loss of amino acids in the dialysate or cytokine activation. Cytokines may be released from peripheral blood mononuclear cells and skeletal muscle during HD. Preliminary evidence indicates that intradialytic increase in cytokines activates the ubiquitin-proteasome pathway. An intradialytic increase in albumin and fibrinogen synthesis is facilitated by interleukin-6 and the constant supply of amino acids derived from skeletal muscle catabolism. Protein anabolism can be induced in end-stage renal disease patients by repletion of amino acids, and perhaps treatment with recombinant human insulin-like growth factor.

Muscle amino acid metabolism and the control of muscle protein turnover in patients with chronic renal failure

Malnutrition is frequently observed in patients with end-stage renal disease. Studies indicate that poor nutritional status plays a major role among factors adversely affecting patients outcome. Therefore prevention and treatment of malnutrition in renal patients is a major issue. In this article the potential mechanisms for alterations in muscle protein metabolism in uremia are explored. Malnutrition has been mainly attributed to inadequate intake of nutrients, superimposed illnesses, or both. However, both clinical and experimental evidence show that uremia per se may adversely affect the control of muscle protein and amino acid metabolism. Available evidence suggests that catabolic factors appear to be distinct for patients at different stages of chronic renal failure and require different modalities of treatments. Both nutritional requirements and the prevalence of malnutrition increase as end-stage renal disease progresses. Muscle protein degradation is increased by metabolic acidosis, which is often found in uremic patients. Another relevant, but less proven cause for increased protein degradation is insulin resistance. Furthermore, specific defects in muscle amino acid metabolism, resistance to growth hormone, insulin-like growth factor 1, or a very low protein intake can reduce muscle protein synthesis. Finally, the hemodialytic procedure per se can stimulate protein breakdown or reduce protein synthesis. All these factors may potentiate the effects of concurrent catabolic illnesses, anorexia, and physical inactivity often found in uremic patients.

Role of amino acid-induced changes in ion fluxes in the regulation of hepatic protein synthesis

Alanine is a powerful stimulator of hepatic protein synthesis whose mechanism of action has not yet been ascertained. The present work aimed to elucidate whether rate changes in ion fluxes accompanying the transport of this amino acid could play a role in the stimulation of protein synthesis. In perfused livers, the utilization of alanine produced a net uptake of K+ of 1.5 mumol/min/liver, a progressively increasing efflux of Ca2+ to reach a maximum of 0.9 mumol/min/liver, and alkalization of the extracellular medium. Inhibition of Na+/K+ exchange by ouabain reversed only the uptake of K+, indicating that this is the main way for the efflux of Na+ cotransported with alanine. In isolated hepatocytes, the uptake of alanine increased the intracellular content of K+ and the cell volume. The following observations suggest that these changes, and not an increased intracellular concentration of Na+, are associated with the stimulation of protein synthesis: 1) Ouabain inhibited the alanine stimulation of L-[3H]-valine incorporation into protein without altering the basal rate of protein labeling; 2) ouabain had no effects on alanine uptake indicating that Na+ influx is not involved in the alanine stimulation of protein synthesis; 3) disruption of Na+ gradient across the plasma membrane by specific ionophores, monensin and gramicidin D, inhibited both basal and alanine-stimulated protein synthesis, but substitution of extracellular Na+ by K+ did not prevent the stimulatory action of alanine. The observation that hypotonic buffer enhanced protein synthesis to the same degree than alanine in liver cells indicates that alanine-induced cell swelling could be sufficient to stimulate protein synthesis.

Translational control during amino acid starvation

Amino acid starvation of mammalian cells results in a pronounced fall in the overall rate of protein synthesis. This is associated with increased phosphorylation of the alpha-subunit of the initiation factor eIF-2, which in turn impairs the activity of the guanine nucleotide exchange factor, eIF-2B. Similar mechanisms have now been found to operate in the yeast, Saccharomyces cerevisiae, where the major physiological result is to circumvent the lack of external amino acids by promoting the translation of a transcription factor, GCN4, that facilitates the expression of a number of enzymes required for amino acid biosynthesis. This article reviews current knowledge of these mechanisms in both mammalian and yeast cells and identifies questions still requiring elucidation.

The pleiotypic response to amino acid deprivation is the result of interactions between components of the glycolysis and protein synthesis pathways

Several diverse metabolic events become compromised when mammalian cells are made deficient in essential amino acids or when charging of their tRNA is blocked by amino acid analogs. This rapid general demise of cell function can be due to inhibition of phosphofructokinase (PFK) by uncharged tRNA. It has now been demonstrated that when tRNA is added to PFK in an assay dependent upon the reassociation of inactive, dissociated enzyme subunits, nanomolar concentrations cause complete inhibition. The model for control suggests that charged tRNA becomes associated with EF-1, which is specific for aminoacyl-tRNAs and is present in sufficiently high concentrations in cells to sequester that charged forms from an inhibitory role. Support for this model include: (1) the rapid onset of inhibition of glycolysis and glucose uptake upon amino acid deficiency; (2) the unique role of the product of PFK activity, fructose-1,6-diphosphate, in reactions of peptide chain initiation, particularly its role as a co-factor for purified eIF-2B, the GDP/GTP exchange factor; (3) the correlations of this interaction with the cellular and molecular lesions of insulin insufficiency; (4) the recognition that the anomalous role of high concentrations of cAMP as a stimulant of peptide chain initiation in energy depleted or gel-filtered cell lysates correlates with its stimulatory action on PFK as an analog for the positive effector, adenosine-5'-monophosphate; and (5) the role of fructose-1,6-diphosphate in the formation of glyceraldehyde-3-phosphate, a substrate for synthesis of ribose-5-phosphate via the non-oxidative portion of the pentose phosphate pathway, which, as a precursor of phosphoribosylpyrophosphate, is essential for nucleic acid synthesis.

Evidence for a role of phosphofructokinase and tRNA in the polyribosome disaggregation of amino acid deficiency

The activity of rabbit muscle phosphofructokinase was inhibited by transfer ribonucleic acid. This inhibition was reduced by inclusion of an amino-acyl-tRNA charging system. The results are discussed in terms of the loss of ATP in amino acid deprived cells and in the critical role of fructose 1,6-diphosphate in peptide chain initiation.

Specific regulation of the gene expression of some pancreatic enzymes during postnatal development and weaning in the calf

The construction of cDNA library from calf pancreas allowed us to examine the mRNA levels of four pancreatic hydrolases (chymotrypsin, lipase, trypsin and amylase) during postnatal development in preruminant and ruminant animals. The lack of parallel variations in the levels of the enzyme specific activities suggested that protein synthesis was not coordinately regulated. In preruminant calves, the change in chymotrypsin and lipase mRNA concentrations (0-28 day period) and in trypsin mRNA concentrations (0-119 day period) was opposite to that in the corresponding specific activities. In contrast, both the activity and mRNA profiles of amylase during the latter period, on the one hand, and those of chymotrypsin and lipase during the 28-119 day period, on the other hand, were comparable. However, the extent to which the specific activity and mRNA concentration of each enzyme were increased did not necessarily coincide. The observed changes in mRNA levels probably resulted from some transcriptional control of the gene expression and/or variation in mRNA stability. Moreover, a translational regulation of the messengers could explain the existence of non-parallel mRNA and specific activity profiles. In sharp contrast with the multiple control of protein synthesis during postnatal development in preruminant calves, weaning was found to induce the same increase in enzyme activity and corresponding mRNA for each of the four pancreatic enzymes, suggesting that pretranslational modulation of gene expression was mainly, if not exclusively, concerned.

Amino acid metabolism in uremia

The uremic syndrome is multifactorial, and affects most tissues and organs. Disturbances in protein and amino acid metabolism may play important roles, especially in chronic uremia, either directly or by production of toxic metabolites, with resultant negative nitrogen (N) balance, muscle wasting, reduced protein synthesis, and characteristically abnormal intracellular free amino acid concentrations. There are also grossly abnormal amino acid levels in the plasma of uremic patients, e.g., increases in conjugated amino acids, high levels of several nonessential and low levels of essential amino acids. The ratios of tyrosine/phenylalanine and of valine/glycine are decreased. The low tryptophan levels may contribute to encephalopathy as a result of an imbalance in neurotransmitter synthesis. Citrulline is found in excess; the explanation is unresolved. There are elevated concentrations of the sulfur-containing amino acids: cystine, taurine, cystathionine, and homocysteine. Excess of the latter is implicated in the atherogenesis of renal failure. Disturbed metabolism and interorgan exchange of amino acids in the uremic state explains some of the abnormalities in tissue and plasma concentrations of individual amino acids. Enzymatic defects are involved in the disturbed metabolism of branched chain amino acids (BCAA), with possible antagonism among them, which impairs growth and amino acid utilization. Carbohydrate intolerance, associated with insensitivity of peripheral tissues to insulin and hyperinsulinemia, elicits decreased plasma BCAA. Protein synthesis rates in normal and pathological conditions are more closely related to the intracellular amino acid pool than to plasma amino acid levels. Concentrations of individual amino acids in the plasma pool are poor indicators of their intracellular concentrations. Muscle contains the largest pool of protein and free amino acids in the body. In chronic renal failure patients, the intracellular concentrations of valine, threonine, lysine, and carnosine are low. With low protein diets and in hemodialysis, serine, tyrosine, and taurine often are also low. The low taurine may be related to fatigue and to uremic cardiomyopathies. The commonly used amino acid supplements generally fail to correct the intracellular amino acid deficits. A "New Formula" has been developed to correct these intracellular amino acid abnormalities, and to supplement a low protein diet. It provides more valine than leucine, increased tyrosine and threonine, and less histidine, leucine, isoleucine, lysine, methionine, and phenylalanine than in formulas customarily used for patients with chronic renal failure. It is uncertain whether other ap

Physiological stresses inhibit guanine-nucleotide-exchange factor in Ehrlich cells

Previously, we have shown that phosphorylation of the eukaryotic initiation factor eIF-2 alpha increases under several physiological stresses in which protein synthesis is inhibited in Ehrlich ascites tumor cells. As phosphorylated eIF-2 [eIF-2(alpha P)] is a potent inhibitor of guanine nucleotide exchange factor (GEF), it seemed likely that it was responsible for the inhibition. We have assayed GEF activity levels in extracts prepared from Ehrlich cells exposed to three such stresses, namely heat shock, serum deprivation and glutamine deprivation. Activity was estimated by the ability of GEF to enhance the release of [alpha-32P]GDP from purified eIF-2 [a modification of the reticulocyte lysate assay of Matts, R. L. & London, I. M. (1984) J. Biol. Chem. 259, 6708]. GEF activity was reduced from control values in extracts of heat-shocked cells and serum-deprived cells, concomitant with increased eIF-2 alpha phosphorylation. Inhibition of GEF activity in heat-shocked and serum-deprived cells was reversed to control levels by increasing the concentration of purified eIF-2.GDP added as substrate in the GEF assay. Since we have shown elsewhere that eIF-2(alpha P).GDP inhibits GEF by competition with eIF-2.GDP, the complete reversal of inhibition of GEF activity in heat-shocked and serum-deprived cells indicates that inhibition is due solely to phosphorylation of eIF-2 alpha. In glutamine-deprived cells phosphorylation of eIF-2 alpha was increased modestly and GEF activity was reduced but GEF activity could not be fully reversed by addition of eIF-2.GDP, suggesting that GEF may also be regulated in other ways. There are greater amounts of GEF relative to eIF-2 in Ehrlich cells (approximately 50%) compared with rabbit reticulocytes (approximately 20%). This explains the efficient rates of protein synthesis in control Ehrlich cells even though they have 30% of their eIF-2 phosphorylated which is enough to inhibit GEF and initiation in reticulocytes completely but only enough to trap approximately 60% of the GEF in Ehrlich cells.

Enhancer-mediated activation of a growth-regulated promoter

We have demonstrated that the collagen alpha 2 type 1 promoter inserted in an expression vector, behaves as a growth-regulated promoter, which is consistent with previous observations that collagen synthesis is growth-regulated in vivo. In contrast, the activity of the H2-K or the simian virus 40 early promoters does not seem to be affected by the rate of cell proliferation. The insertion of a polyoma enhancer 5' or 3' to the collagen transcription unit activates the collagen alpha 2 type 1 promoter, by a threefold greater factor in slowly growing cells compared to cells growing exponentially. These results show that enhancers can also function in slowly proliferating cells and activate the normally low activity of a promoter in these cells.

Key role of L-alanine in the control of hepatic protein synthesis

We investigated the effects of administration of single amino acids to starved rats on the regulation of protein synthesis in the liver. Of all the amino acids tested, only alanine, ornithine and proline promoted statistically significant increases in the extent of hepatic polyribosome aggregation. The most effective of these was alanine, whose effect of promoting polyribosomal aggregation was accompanied by a decrease in the polypeptide-chain elongation time. The following observations indicate that alanine plays an important physiological role in the regulation of hepatic protein synthesis. Alanine was the amino acid showing the largest decrease in hepatic content in the transition from high (fed) to low (starved) rates of protein synthesis. The administration of glucose or pyruvate is also effective in increasing liver protein synthesis in starved rats, and their effects were accompanied by an increased hepatic alanine content. An increase in hepatic ornithine content does not lead to an increased protein synthesis, unless it is accompanied by an increase of alanine. The effect of alanine is observed either in vivo, in rats pretreated with cycloserine to prevent its transamination, or in isolated liver cells under conditions in which its metabolic transformation is fully impeded.

A (re)initiation-dependent cell-free protein-synthesis system from mouse erythroleukemia cells

Cultured mouse erythroleukemia cells (MEL cells) can be induced in vivo to erythroid differentiation which is marked by the onset of globin mRNA and haemoglobin synthesis. When these cells are briefly exposed to hypertonic growth medium prior to lysis, the resulting post-mitochondrial supernatants show a high in vitro protein-synthesis activity. Amino acid incorporation is linear up to 60 min; more than 80% of this is due to (re)initiation, as shown by the inhibition with edeine. Extracts from induced cells reach only a third of overall incorporation as compared to extracts from uninduced cells. This reduction of the protein-synthesizing capacity is also observed in vivo. Polyacrylamide gel electrophoresis shows that extracts from uninduced cells faithfully translate their endogenous mRNA, whereas in extracts from induced cells, non-globin protein synthesis is reduced and globin is preferentially synthesized. Haemin (40 microM) as well as purified eukaryotic initiation factor 2 (eIF-2) from rabbit reticulocytes enhance amino acid incorporation in both kinds of extracts, which suggests that both uninduced and induced MEL cells contain a haemin-controlled eIF-2 alpha kinase. This system should be useful for studying the mechanisms controlling protein synthesis in a nucleated differentiating cell.

Regulation of polypeptide chain initiation and activity of initiation factor eIF-2 in Chinese-hamster-ovary cell mutants containing temperature-sensitive aminoacyl-tRNA synthetases

The regulation of polypeptide chain initiation has been investigated in extracts from a number of well-characterized Chinese hamster ovary (CHO) cell mutants containing different temperature-sensitive aminoacyl-tRNA synthetases. These cells exhibit a large decline in the rate of initiation when cultures are shifted from the permissive temperature of 34 degrees C to the non-permissive temperature of 39.5 degrees C. During a brief incubation with [35S]Met-tRNAMetf or [35S]methionine, formation of initiation complexes on native 40S ribosomal subunits and 80S ribosomes is severely impaired in extracts from the mutant cell lines exposed to 39.5 degrees C. Wild-type cells exposed to 39.5 degrees C do not show any inhibition of protein synthesis or initiation complex formation. Inhibition of formation of 40S initiation complexes in the extracts from mutant cells, incubated at the non-permissive temperature, is shown to be independent of possible changes in mRNA binding or the rate of polypeptide chain elongation and is not due to any decrease in the total amount of initiation factor eIF-2 present. However, assays of eIF-2 X GTP X Met-tRNAMetf ternary complex formation in postribosomal supernatants from the temperature-sensitive mutants reveal a marked defect in the activity of eIF-2 after exposure of the cells to 39.5 degrees C and addition of exogenous eIF-2 to cell-free protein-synthesizing systems from cells incubated at 34 degrees C and 39.5 degrees C eliminates the difference in activity between them. The activity of the initiation factor itself is not directly temperature-sensitive in the mutant CHO cells. The results suggest that the activity of aminoacyl-tRNA synthetases can affect the ability of eIF-2 to bind Met-tRNAMetf and form 40S initiation complexes in intact cells, indicating a regulatory link between polypeptide chain elongation and chain initiation.

Preparation and properties of an improved cell-free protein synthesis system from mammalian liver

A cell-free protein synthesis system derived from mouse liver has been developed which faithfully translates endogenous and exogenous mRNA. The system is based on the unfractionated post-mitochondrial supernatant. The main measures taken to improve the activity of the system were: the use of high levels (30 mM) of creatine phosphate as an energy-generating system to counteract a hyperactive nucleoside triphosphatase activity in the extracts, the choice of homgenisation buffer, and the use of potassium acetate rather than KCl in the assay. The system exhibits a high initial rate of amino acid incorporation, and reinitiates translation on endogenous mRNA. Added tobacco mosaic virus (TMV) RNA is faithfully translated into full-length products at a rate of 60-80 amino acid residues per min at 30 degrees. The rate of overall amino acid incorporation slows after about 20 min and eventually ceases due to a failure in the re-initiation of translation, and not because of degradation of mRNA. Over a limited period of time, this improved cell-free translation system is comparable in activity to other eukaryotic systems generated to date, and should be useful in studies of the control of translation rates in mammalian liver.

Influence of starvation and total protein deprivation on cardiac mRNA levels

The effect of starvation and of protein-deprivation on the extractable amount of cardiac mRNA was investigated in male rats. Cardiac mRNA was determined by either (a) isolation of cardiac mRNA by SDS-Phenol/oligo-dT-cellulose, or by (b) hybridization of cardiac mRNA to 3H-Poly(U). During starvation (1-6 days) the extractable amount of cardiac microsomal RNA decreased from 870 micrograms/g heart (controls) to 606 micrograms/g (3 days) and to 547 micrograms/g (6 days), the extractable amount of mRNA fell from 28.6 micrograms/g heart (controls) to 18.7 micrograms/g (3 days) and to 14.5 micrograms/g (6 days). When a normocaloric but protein-deficient diet was fed, the decreases in cardiac microsomal RNA and mRNA were qualitatively similar, but slightly less severe. An analysis of the intracellular distribution of cardiac microsomal RNA and mRNA in the hearts of normal animals and of animals starved or fed a protein-deficient diet indicates that during starvation cardiac mRNA does not accumulate in the cell sap, but gets rapidly degraded. In the refeeding period, mRNA is transported from the nucleus to the cytoplasm and engages in polyribosome formation. The specific mRNA species coding for the major myofibrillar cardiac proteins are affected to a similar extent by these changes during starvation/protein-deprivation and refeeding.

Failure of leucine to stimulate protein synthesis in vivo

The effect of 100 mumol of leucine on protein synthesis in several tissues was assessed in the intact rat. Leucine had no immediate effect on protein synthesis in gastrocnemius muscle, heart, jejunal serosa, jejunal mucosa or liver in rats which were fed, starved for 2 days or deprived of dietary protein for 9 days. Leucine treatment for 1 h also failed to stimulate protein synthesis in tissues of 2-day-starved animals.

The effects of haem on translational control of protein synthesis in cell-free extracts from fed and lysine-derived Ehrlich ascites tumour cells

1. Addition of haem to cell-free extracts from Ehrlich ascites tumour cells stimulates protein synthesis only in extracts from cells previously incubated in nutritionally complete conditions. Extracts from amino-acid-deprived cells do not respond to haem. The stimulation of protein synthesis in fed cell extracts is due to increased initiation on endogenous mRNA mediated by an increase in the levels of 40-S-subunit X Met-tRNA initiation complexes. Extracts from starved cells exhibit a defect in 40-S initiation complex formation which cannot be overcome by haem. 2. Experiments to test for the presence of an inhibitor of initiation in Ehrlich cell extracts by monitoring effects on translation in haem-supplemented reticulocyte lysates have revealed that extracts from both fed and starved cells contain one or more inhibitory activities which shut off protein synthesis, dissagregate polysomes and reduce the level of 40-S initiation complexes in the lysate. Extracts from starved cells are more inhibitory for protein synthesis than those from fed cells. 3. Initiation factor eIF-2 is phosphorylated by an endogenous Ehrlich cell protein kinase in vitro, but this occurs to the same extent in extracts from fed and starved cells. 4. We propose a possible model for the role of eIF-2 in the control of protein synthesis by amino acid supply in Ehrlich cells.