Insulin-stimulated alpha-(methyl)aminoisobutyric acid uptake in skeletal muscle. Evidence for a short-term activation of uptake independent of Na+ electrochemical gradient and protein synthesis.

1. The present study was designed to explore the mechanisms by which insulin stimulates system A of amino acid transport in extensor digitorum longus (EDL) muscles, by using a system A analogue, alpha-(methyl)aminoisobutyric acid (MeAIB). 2. Insulin stimulation of MeAIB uptake was noted after only 30 min of incubation and was maximal at 60 min. Kinetics of the insulin effect on MeAIB uptake were characterized by an increased Vmax. without modification of Km for MeAIB. 3. Incubation of EDL muscles with cycloheximide for 90 min did not modify MeAIB uptake in either the presence or the absence of insulin, indicating the independence of insulin action from protein synthesis de novo. Incubations for 180 min with cycloheximide caused a decrease in basal MeAIB uptake; however, the percentage stimulation of amino acid transport by insulin was unaltered. Basal MeAIB uptake was increased by incubation for 180 min, but under these conditions no change in the percentage effect of insulin was found. 4. Ouabain, gramicidin D, or both, markedly decreased basal MeAIB uptake by EDL muscle, but the percentage effect of insulin was unaltered. 5. We conclude that insulin action on amino acid transport through system A in muscle is rapid, is characterized by an increased Vmax., and is independent of protein synthesis de novo and the Na+ electrochemical gradient. Our data are compatible with insulin acting directly on the system A transporter.     

Protein kinase C activators selectively inhibit insulin-stimulated system A transport activity in skeletal muscle at a post-receptor level.

We have investigated the role of phorbol esters on different biological effects induced by insulin in muscle, such as activation of system A transport activity, glucose utilization and insulin receptor function. System A transport activity was measured by monitoring the uptake of the system A-specific analogue alpha-(methyl)aminoisobutyric acid (MeAIB), by intact rat extensor digitorum longus muscle. The addition of 12-O-tetradecanoylphorbol 13-acetate (TPA, 0.5 microM) for 60 or 180 min did not modify basal MeAIB uptake by muscle, suggesting that insulin signalling required to stimulate MeAIB transport does not involve protein kinase C activation. However, TPA added 30 min before insulin (100 nM) markedly inhibited insulin-stimulated MeAIB uptake. The addition of polymyxin B (0.1 mM) or H-7 (1 mM), protein kinase C inhibitors, alone or in combination with TPA leads to impairment of insulin-stimulated MeAIB uptake. This paradoxical pattern is incompatible with a unique action of Polymyxin B or H-7 on protein kinase C activity. Therefore these agents are not suitable tools with which to investigate whether a certain insulin effect is mediated by protein kinase C. TPA did not cause a generalized inhibition of insulin action. Thus both TPA and insulin increased 3-O-methylglucose uptake by muscle, and their effects were not additive. Furthermore, TPA did not modify insulin-stimulated lactate production by muscle. In keeping with this selective modification of insulin action, treatment of muscles with TPA did not modify insulin receptor binding or kinase activities. In conclusion, phorbol esters do not mimic insulin action on system A transport activity; however, they markedly inhibit insulin-stimulated amino acid transport, with no modification of insulin receptor function in rat skeletal muscle. It is suggested that protein kinase C activation causes a selective post-receptor modification on the biochemical pathway by which insulin activates system A amino acid transport in muscle.     

Amino acid regulation of insulin action in isolated adipocytes. Selective ability of amino acids to enhance both insulin sensitivity and maximal insulin responsiveness of the protein synthesis system

Using the number and concentration of amino acids in Dulbecco's modified Eagle's medium as reference (DMEM = 100%), we found that a maximally effective concentration of insulin (10 ng/ml) stimulated protein synthesis by 125% over basal rate in the presence of 50% amino acids (EC50 = 19%), but by only 48% in amino acid-free buffer. Moreover, time course experiments revealed that amino acid regulation of insulin action was very rapid (t1/2 of 9.5 min) and readily reversible (less than 30 min). This effect was specific in that basal rates of protein synthesis were unaltered by amino acids. A second effect of amino acids was to markedly enhance insulin sensitivity of the protein synthesis system in a dose-dependent manner. Thus, the half-maximally effective concentrations of insulin required to stimulate protein synthesis fell from 0.43 to 0.25 to 0.15 ng/ml in the presence of 0, 50, and 150% amino acids. Neither insulin sensitivity nor maximal insulin responsiveness of the glucose transport system was altered by amino acids, nor did amino acids affect the insulin binding capacity of cells. When we divided the 14 amino acids found in DMEM into two groups, we found that one group of 7 amino acids had little or no effect on insulin sensitivity or responsiveness, whereas the other group was fully active (a 157% increase in insulin responsiveness, ED50 of 0.21 ng/ml versus a 68% increase, ED50 of 0.51 ng/ml, with no amino acids). Isoleucine and serine together increased both insulin sensitivity and responsiveness to 60-70% of that seen with the full complement of amino acids. In conclusion: 1) amino acids modulate insulin action by enhancing maximal insulin responsiveness and insulin sensitivity of the protein synthesis system, and the regulatory site of amino acid action appears to be distal to the common signal pathway, within the insulin action-protein synthesis cascade, and 2) the effects of amino acids are specific, in that basal rates of protein synthesis are unaffected, only certain amino acids influence insulin action, and amino acids fail to alter insulin binding or the insulin-responsive glucose transport system. These studies, together with those in the companion paper, demonstrate that the pleiotropic actions of insulin on enhancing glucose uptake and protein synthesis are mediated through divergent pathways that can be independently regulated.(ABSTRACT TRUNCATED AT 400 WORDS)     

Inhibition of intracellular proteolysis in muscle cultures by multiplication-stimulating activity. Comparison of effects of multiplication-stimulating activity and insulin on proteolysis, protein synthesis, amino acid uptake, and sugar transport

The effects of the insulin-like growth factor, multiplication-stimulating activity (MSA), on chick myotube cultures were investigated. In serum-free media, MSA at levels reported to be present in fetal serum (5 ng/ml) significantly inhibited overall rates of protein degradation and stimulated protein synthesis and amino acid uptake. Half-maximal effects on protein degradation (-30%), synthesis (+25%), and amino acid uptake (+50%) occurred at approximately 0.05 micrograms/ml. In contrast, 10(2)-10(3)-fold higher concentrations (5 micrograms/ml) were required to stimulate transport of the glucose analog 2-deoxyglucose. The results indicate that MSA is an effective anabolic agent regulating protein metabolism and amino acid uptake, but not sugar transport in these cells. Parallel studies conducted with insulin demonstrated similar size effects on protein metabolism and amino acid uptake in serum-free media. However, unlike MSA, insulin levels (10(-2) units/ml) well in excess of its normal physiological range were required to produce significant effects. In addition, the relative sensitivity of sugar transport with respect to protein metabolic effects differed for insulin and MSA. Thus, 2-deoxyglucose transport was approximately 10 times more sensitive to insulin than protein synthesis, proteolysis, or amino acid uptake in contrast to MSA where the reverse was true. However, despite the relatively higher sensitivity of sugar transport to insulin, supraphysiological levels (10(-3) units/ml) of this hormone were still required for significant stimulation. These results suggest a generally low insulin sensitivity in cultured chick myotubes relative to adult tissues. In contrast, the effects of MSA are consistent with a possible role of this or similar factors in regulating growth and development of embryonic muscle.     

Effect of sugars on amino acid transport by symbiotic Chlorella

Symbiotic Chlorella F36-ZK isolated from Paramecium bursaria F36 has constitutive amino acid transport systems, whereas free-living Chlorella does not. We found that in symbiotic algae, the rate of serine (Ser) uptake increased in the presence of glucose (Glc) and non-metabolisable analogues, whilst incorporation of Ser into protein was not affected. The activation did not involve new protein synthesis and was enhanced under alkaline conditions. An increase in the rate of Ser transport resulted from Glc treatment even when pulsed for only 1min at low concentrations (EC(50)=3muM). No uptake of Glc itself was observed in F36-ZK. Thus, the transport signal appears to be transmitted via a glucose sensing and signalling pathway. Many Glc-related compounds also increased the rate of Ser uptake without an additive effect, suggesting recognition of these sugars by the same receptor and providing some insight into features of the structure-activity relationship. Ser uptake by F36-ZK is inhibited by Ca(2+), which is typically considered to be a positive modulator of amino acid uptake. Given that Glc restored Ser uptake from inhibition by Ca(2+), we propose that this compound is possibly involved in regulation of amino acid transport in this symbiotic relationship.     

On the stimulation by insulin of tryptophan transport across the blood-brain barrier

Following previous studies showing that in vivo insulin administration increases brain tryptophan levels, we have tested the effect of insulin on tryptophan uptake by isolated bovine brain capillaries, which represent the in vitro equivalent of the blood-brain barrier. In the presence of insulin and Na+ ions, the uptake of 14C-labelled tryptophan was significantly increased with respect to controls, this increase being essentially due to a higher affinity of the transport system for the amino acid, while the Vmax was not affected. Insulin increased also, to a similar extent, the uptake of alpha-methylaminoisobutyrate in the presence of Na+ ions, while the uptake of beta-aminobicyclo(2.2.1)heptane carboxylic acid was not affected. Addition of phloretine, or of anti-insulin antibodies, as well as omission of Na+ ions from the buffer abolished the effect of insulin. Insulin appears therefore to increase specifically the substrate affinity of the A-system for neutral amino acid transport, without exerting any influence on the L-system. The absence of the A-system from the luminal side of the microvessels, and the high insulin concentrations needed, raise however some problems as to the physiological significance of this effect.     

The effects of bioregulators upon amino acid transport and protein synthesis in isolated rat hepatocytes

Isolated rat hepatocytes prepared by an enzyme perfusion technique possess a functional amino acid transport system and retain the capacity to synthesize protein. Amino acid transport was studied using the non-metabolizable amino acid analog alpha-aminoisobutyric acid. The transport process was time, temperature and concentration dependent. Similarly, leucine incorporation into protein was time and temperature dependent being optimal at 3m degrees C. Amino acid, fetal calf serum, growth hormone and glucose all produced small, reproducible increases in protein synthesis rates. Bovine serum albumin diminished the uptake of alpha-aminoisobutyric acid and leucine incorporation into protein. The amino acid content on either side of the cell membrane was found to affect transport into or out of the cellular compartment (transconcentration effects). High cell concentrations decreased transport and protein synthesis as a result of isotopic dilution of labelled amino acids with those released by the hepatocytes. This was consistent with the capacity of naturally occurring amino aicds to compete with alpha-aminoisobutyric acid for uptake into the hepatocyte. In order to define more precisely the effects of bioregulators on transport and protein synthesis it will be necessary to define and subfractionate cellular compartments and proteins which are the specific targets of cellular regulation.     

Regulation of amino acid transport and protein metabolism in myotubes derived from chicken muscle satellite cells by insulin-like growth factor-I

The effects of insulin and insulin-like growth factor-I (IGF-I) on amino acid transport and protein metabolism were compared in myotubes derived from chicken breast muscle satellite cells. Protein synthesis was assessed by continuous labelling with [³H]-tyrosine. Protein degradation was estimated by the release of trichloroacetic acid (TCA) soluble radioactivity by cells which had been previously labelled with [³H]-tyrosine for 3 days. Amino acid transport was measured in myotubes incubated in Dulbecco's modified Eagle's medium (DMEM) 0.5% bovine serum albumin (BSA) with or without insulin or IGF-I. Subsequent [³H]-aminoisobutyric acid (AIB) uptake was then measured in amino acid-free medium. IGF-I was more efficient than insulin at equimolar concentration (3.2 nmol/l) in stimulating protein synthesis (127 and 113% of basal, respectively) and inhibiting protein degradation (32% and 13% inhibition of protein degradation following 4 h incubation). Half maximal effective concentrations for stimulation of AIB uptake were 0.27 ± 0.03 nmol/l and 34.8 ± 3.1 nmol/l for IGF-I and insulin respectively, with maximal stimulation of about 340% of basal. Cycloheximide (3.6 μmol/l) diminished IGF-I-stimulated AIB uptake by 55%. Chicken growth hormone had no effect on basal AIB uptake in these cells and neither glucagon nor dexamethasone had an effect on basal or IGF-I-stimulated AIB uptake. This study demonstrates an anabolic effect for IGF-I in myotubes derived from primary chicken satellite cells which is mediated by the type I IGF receptor, since the cation-independent mannose 6-phosphate receptor does not bind IGF-II in chicken cells. © 1993 Wiley-Liss, Inc.     

Requirement for protein synthesis in the regulation of protein breakdown in cultured hepatoma cells.

The modes of action of insulin and of inhibitors of protein synthesis on the degradation of labeled cellular proteins have been studied in cultured hepatoma (HTC) cells. Protein breakdown is accelerated upon the deprivation of serum (normally present in the culture medium), and this enhancement is inhibited by either insulin or cycloheximide. An exception is a limited class of rapidly turning over cellular proteins, the degradation of which is not influenced by insulin or cycloheximide. Alternative hypotheses to explain the relationship of protein synthesis to the regulation of protein breakdown, viz., control by the levels of precursors of protein synthesis, regulation by the state of the ribosome cycle, or requirement for a product of protein synthesis, have been examined. Protein breakdown was not influenced by amino acid deprivation, and measurements of valyl-tRNA levels in HTC cells subjected to various experimental conditions showed no correlation between the levels of charged tRNAVal and the rates of protein degradation. Three different inhibitors of protein synthesis (puromycin, pactamycin, and cycloheximide) suppressed enhanced protein breakdown in a similar fashion. A direct relationship was found between the respective potencies of these drugs to inhibit protein synthesis and to block enhanced protein breakdown. When cycloheximide and insulin were added following a prior incubation of HTC cells in a serum-free medium, protein breakdown was maximally suppressed within 15-30 min. Actinomycin D inhibited protein breakdown only after a time lag of about 90 min. It is suggested that the regulation of protein breakdown in hepatoma cells requires the continuous formation of a product of protein synthesis, in a manner analogous to the mode of the control of this process in bacteria.     

Characteristics and development of leucine transport activity in the scutellum of germinating barley grain

Scutella separated from grains of Himalaya barley after germination for 3 days rapidly took up l-leucine from aerated incubation media; with 1 millimolar leucine the rate varied between 4 and 14 micromoles per gram per hour and the pH optimum was at 3.5 to 5, both depending on buffer composition and prewashing time. The rate of the uptake increased with increasing concentration of leucine in a complex manner, which could be interpreted as multiphasic kinetics with apparent K(m) values of 3.4 and 15.5 millimolar below and above 3 millimolar leucine, respectively. The uptake took place against a concentration difference (highest estimated ratio 270: 1) and was strongly inhibited by dinitrophenol. Uptake was apparently due to active transport requiring metabolic energy.The development of the uptake activity during germination was studied using Pirkka barley. A low activity was present in the scutella of ungerminated grains. It began to increase after 6 hours imbibition, and the increase was biphasic, the major changes occurring during days 0 to 3 and 4 to 6. The total increase was about 20-fold.The regulation of the development was studied by allowing separated embryos to germinate on agar gel. The increase of uptake activity was strongly inhibited by inhibitors of RNA or protein synthesis. Increase did not require the presence of the embryo proper, and was not affected by gibberellic or abscisic acid. Removal of the endosperm greatly accelerated the increase of uptake activity, and the presence of 5 or 20 millimolar glutamine counteracted the removal of the endosperm. The results suggest that the availability of glutamine or amino acids in general in the endosperm may regulate the development or the activity of the transport system.     

Glucose transport by cultured human fibroblasts: regulation by phorbol esters and insulin.

The regulation of 3-O-methyl-D-glucose (OMG) uptake by insulin and phorbol esters was studied in cultured human skin fibroblasts. Insulin rapidly stimulated OMG uptake through a mechanism independent of new protein synthesis. Maximal insulin effect was reached in 30 min and remained constant up to 12 h. The protein kinase C activators 12-O-tetradecanoyl phorbol 13-acetate (TPA) and phorbol 12,13-dibutyrate (PdBU) promoted an initial rapid stimulation followed by a secondary long-term rise of OMG influx. This latter effect of phorbol esters on OMG influx began after 1 h, reached a maximum in 12-15 h, and was prevented by the simultaneous addition of protein synthesis inhibitors, suggesting that phorbol esters increased the synthesis of new glucose transporters. In accord with this interpretation, phorbol esters, but not insulin, increased mRNA levels for two distinct glucose transporters (GLUT1 and GLUT3) in human fibroblasts. Both the rapid and the long-term effects of phorbol esters on OMG influx were dose-dependent and half-maximal stimulations occurred at 15 nM for both PdBU and TPA. Kinetic analysis of OMG uptake indicated that both effects of phorbol esters were associated with an increase in the Vmax of the transport process, with no significant changes of the Km (4-6 mM). These results suggest that, in human fibroblasts, phorbol esters, unlike insulin, produce a long-term stimulation of OMG uptake, which is dependent upon protein synthesis and is associated with increased levels of GLUT1 and GLUT3 mRNA.     

Regulation of development of leucine uptake activity by glutamine in the scutellum of germinating barley grain

Scutella from ungerminated grains of barley (Hordeum vulgare L. cv Pirkka) take up leucine at a slow rate, which increases rapidly during germination. When endosperms were removed from the grains after imbibition for 4 hours or after germination for 12 or 72 hours, the increase in the rate of leucine uptake was greatly accelerated during subsequent incubation of the embryos or scutella. These increases were rapidly inhibited by cordycepin and cycloheximide, suggesting that protein synthesis, probably synthesis of the carrier protein, was required for the development of the uptake activity.

In separated embryos or scutella, the increases in the leucine uptake activity were inhibited by glutamine. The inhibitions caused by glutamine and cycloheximide were not additive, suggesting that glutamine did not interfere with the function of the carrier but repressed its synthesis. Glutamine did not inhibit the simultaneous increase in peptide uptake; in this respect, its effect was specific for leucine uptake, which appears to be due to a general amino acid uptake system.

Some other protein amino acids also inhibited the increase in leucine uptake without inhibiting the increase in peptide uptake. However, these effects were smaller than that of glutamine.

These results suggest that the transfer of leucine (and other amino acids) from the endosperm to the seedling in a germinating barley grain is regulated at the uptake step by repression of the synthesis of the amino acid carrier protein by glutamine and—possibly to a lesser extent—by some other amino acids taken up from the endosperm.

     

Inhibition of metabolic effects of growth hormone by various inhibitors of cyclic nucleotide phosphodiesterase

When isolated diaphragms of hypophysectomized rats were incubated with bovine growth hormone in the presence of the cyclic nucleotide inhibitors theophylline, quinine and papaverine, the stimulatory effects of the hormone on leucine incorporation into protein, α-aminoisobutyric acid and 3-O-methylglucose transport were suppressed or abolished entirely. The degree of suppression of the hormone effects appeared to correlate with the extent of glycogenolysis caused by the drugs. Thoephylline also rapidly reversed the stimulation of protein synthesis and amino acid and sugar transport produced by growth hormone. When protein synthesis and transport were stimulated by preincubation of the diaphragm with growth hormone, the subsequent addition of theophylline to the medium inhibited the hormonal effects on protein synthesis and sugar transport within 15 min and the effect on amino acid transport within 60 min. These results may mean that the rapid in vitro effects of growth hormone on protein synthesis and membrane transport in rat diaphragm muscle are mediated by a reduction in the cellular level of cyclic AMP or some other nucleotide.Attempts to block the action of growth hormone on 3-O-methylglucose transport by preincubation of the diaphragm with high concentrations (10 mM) of cyclic GMP, cyclic UMP, cyclic TMP and cyclic CMP were unsuccessful. Also an effort was made to mimic the action of growth hormone on sugar transport by incubating the diaphragm with high concentrations of imidazole and histamine, agents known to activate cyclic nucleotide phosphodiesterase. Slight stimulatory effects were obtained, but they could not be correlated with any certainty to the actions of imidazole and histamine on phosphodiesterase.Like growth hormone, insulin also stimulates protein synthesis and amino acid and sugar transport in the isolated rat diaphragm. However, the actions of insulin on these processes were not abolished by theophylline, suggesting some basic difference in the mode of action of these two hormones on protein synthesis and membrane transport in muscle.     

Amino acid uptake and protein synthesis in preimplanatation mouse embryos

Amino acid uptake and cycloheximide inhibitable incorporation into protein are demonstrable in follicular ova, unfertilized eggs, and in mouse embryos ranging from the 1-cell to the late blastocyst stages. The rates of uptake and incorporation are low and relatively constant until the early blastocyst (day 3) stage of development when they increase 3- to 9-fold. The rate of uptake continues to increase during the fourth day (late blastocyst stage) of development, but, despite embryonic growth, incorporation into protein remains constant. By exposing embryos to leucine and lysine at different concentrations, it is possible to dissociate the processes of uptake and incorporation into protein from one another and to use the latter as a measure of protein synthesis. Taking the number of embryonic cells into account, it is postulated that the period between 8- to 16-cell stage (day 2) and the early blastocyst stage is the only one in which the synthesis of major amounts of protein based on new messenger RNA synthesis is occurring.Leucine and lysine uptake take place by a facilitated process, although lysine transport is not readily saturated. Inhibitors of energy metabolism do not significantly affect amino acid uptake, but they do inhibit protein synthesis. However, since the rate of transport is highly temperature sensitive (Q₁₀ ? 3) and leucine is accumulated against a concentration gradient, active amino acid transport appears to be present.     

Paradoxical effects of protein synthesis inhibitors on uridine uptake in cultured cells: Possible role of uncharged tRNA in regulating metabolism

Previous studies (J. Biol. Chem, 253: 99–105, 1978) showed that thyrotropin-releasing hormone (TRH) acutely stimulated uridine uptake in pituitary cell (GH₄C₁) cultures. Studies on the role of protein synthesis in this response to TRH led to the finding that an inhibitor of ribosomal translation, cycloheximide, also stimulated uridine uptake acutely. Studies reported here attempt to determine the mechanism of cycloheximide action and whether cycloheximide and hormone stimulation of uridine uptake occurred by similar pathways. The experiments presented indicate that: (1) seven inhibitors of ribosomal translation stimulated uridine uptake; (2) in contrast, inhibition of protein synthesis at tRNA aminoacylation resulted in reduced rates of uridine uptake; (3) inhibition of tRNA aminoacylation blocked cycloheximide but not TRH stimulation of uptake; (4) cycloheximide stimulation of uptake was restricted to amino acid-depleted cultures; (5) amino acid supplementation stimulated uridine uptake with a time-course identical to that of cycloheximide; (6) cycloheximide and amino acid supplementation promoted reacylation of cellular tRNAs in amino acid-depleted cultures; and (7) cycloheximide stimulation of uridine uptake resulted from enhanced nucleoside phosphorylation rather than increased uridine transport. We conclude that cycloheximide and amino acid stimulation of uridine phosphorylation may be mediated through a common pathway involving the extent of amino-acylation of cellular tRNAs. Furthermore, cycloheximide and TRH stimulate uridine phosphorylation by pathways that are distinguishable. It is apparent that not all cellular effects of cycloheximde can be attributed solely to inhibition of the synthesis of proteins.     

Amino acid transport in normal and Rous sarcoma virus-transformed chicken embryo fibroblasts.

A study was made of the transport of a variety of amino acids by uninfected and Rous sarcoma virus-infected chicken embryo fibroblasts. Following a period of amino acid starvation, transformed, but not normal cells, showed increased levels of transport for alpha-aminoisobutyric acid, proline and alanine, three amino acids which are transported primarily by the A transport system. There was no starvation-induced increase in the transport of leucine, phenylalanine, lysine, or cycloleucine. In the absence of starvation, normal and transformed cells exhibited comparable rates of amino acid transport. Cycloheximide was able to block the increase in uptake. The enhanced uptake was characterized by an increase in Vmax for transport and little change in Km. The data demonstrate that an alteration in the regulation of the A amino acid transport system is an early event in malignant transformation by Rous sarcoma virus. However, since this alteration in made manifest only following a period of starvation, our findings suggest that increased amino acid uptake does not play a role in generating the other manifestations of the transformed state seen in cell culture.     

Amino acid transport in placental slices mechanism of increased accumulation by prolonged incubation

The accumulation of α-aminoisobutyric acid by placental slices is increased dramatically upon prior incubation of the slices in amino acid-free, buffered saline. This increase is inhibited by inhibitors of protein synthesis and is accompanied by an increased $\text{V}$ for the transport process. While alternative explanations are discussed, these data suggest that the incubation effect may be mediated through an increase in the number of available transport sites which are synthesized during the incubation period. Incubation with an amino acid mixture diminishes the increase as well as general protein synthesis, suggesting that a reduced availability of amino acids may initiate compensatory changes in the synthesis of cellular transport proteins.