The effect of transport system A and N amino acids and of nerve and epidermal growth factors on the induction of ornithine decarboxylase activity

Asparagine as a signal for glutamine sufficiency via asparagine synthetase: a fresh evidence-based framework in physiology and oncology

Among the 20 proteinogenic amino acids, glutamine (GLN) and asparagine (ASN) represent a unique cohort in containing a terminal amide in their side chain, and share a direct metabolic relationship, with glutamine generating asparagine through the ATP-dependent asparagine synthetase (ASNS) reaction. Circulating glutamine levels and metabolic flux through cells and tissues greatly exceed those for asparagine, and "glutamine addiction" in cancer has likewise received considerable attention. However, historic and recent evidence collectively suggest that in spite of its modest presence, asparagine plays an outsized regulatory role in cellular function. Here, we present a unifying evidence-based hypothesis that the amides constitute a regulatory signaling circuit, with glutamine as a driver and asparagine as a second messenger that allosterically regulates key biochemical and physiological functions, particularly cell growth and survival. Specifically, it is proposed that ASNS serves as a sensor of substrate sufficiency for S-phase entry and progression in proliferating cells. ASNS-generated asparagine serves as a subsequent second messenger that modulates the activity of key regulatory proteins and promotes survival in the face of cellular stress, and serves as a feed-forward driver of S-phase progression in cell growth. We propose that this signaling pathway be termed the amide signaling circuit (ASC) in homage to the SLC1A5-encoded ASCT2 that transports both glutamine and asparagine in a bidirectional manner, and has been implicated in the pathogenesis of a broad spectrum of human cancers. Support for the ASC model is provided by the recent discovery that glutamine is sensed in primary cilia via ASNS during metabolic stress.

Role of Polyamines and Hypusine in β Cells and Diabetes Pathogenesis

The polyamines-putrescine, spermidine, and spermine-are polycationic, low molecular weight amines with cellular functions primarily related to mRNA translation and cell proliferation. Polyamines partly exert their effects via the hypusine pathway, wherein the polyamine spermidine provides the aminobutyl moiety to allow posttranslational modification of the translation factor eIF5A with the rare amino acid hypusine (hydroxy putrescine lysine). The "hypusinated" eIF5A (eIF5Ahyp) is considered to be the active form of the translation factor necessary for the translation of mRNAs associated with stress and inflammation. Recently, it has been demonstrated that activity of the polyamines-hypusine circuit in insulin-producing islet β cells contributes to diabetes pathogenesis under conditions of inflammation. Elevated levels of polyamines are reported in both exocrine and endocrine cells of the pancreas, which may contribute to endoplasmic reticulum stress, oxidative stress, inflammatory response, and autophagy. In this review, we have summarized the existing research on polyamine-hypusine metabolism in the context of β-cell function and diabetes pathogenesis.

Antizyme (AZ) regulates intestinal cell growth independent of polyamines

Since antizyme (AZ) is known to inhibit cell proliferation and to increase apoptosis, the question arises as to whether these effects occur independently of polyamines. Intestinal epithelial cells (IEC-6) were grown in control medium and medium containing 5 mM difluoromethylornithine (DFMO) to inhibit ODC, DFMO + 5 µM spermidine (SPD), DFMO + 5 µM spermine (SPM), or DFMO + 10 µM putrescine (PUT) for 4 days and various parameters of growth were measured along with AZ levels. Cell counts were significantly decreased and mean doubling times were significantly increased by DFMO. Putrescine restored growth in the presence of DFMO. However, both SPD and SPM when added with DFMO caused a much greater inhibition of growth than did DFMO alone, and both of these polyamines caused a dramatic increase in AZ. The addition of SPD or SPM to media containing DFMO + PUT significantly inhibited growth and caused a significant increase in AZ. IEC-6 cells transfected with AZ-siRNA grew more than twice as rapidly as either control cells or those incubated with DFMO, indicating that removal of AZ increases growth in cells in which polyamine synthesis is inhibited as well as in control cells. In a separate experiment, the addition of SPD increased AZ levels and inhibited growth of cells incubated with DFMO by 50%. The addition of 10 mM asparagine (ASN) prevented the increase in AZ and restored growth to control levels. These results show that cell growth in the presence or absence of ODC activity and in the presence or absence of polyamines depends only on the levels of AZ. Therefore, the effects of AZ on cell growth are independent of polyamines.

Spermidine, a sensor for antizyme 1 expression regulates intracellular polyamine homeostasis

Although intracellular polyamine levels are highly regulated, it is unclear whether intracellular putrescine (PUT), spermidine (SPD), or spermine (SPM) levels act as a sensor to regulate their synthesis or uptake. Polyamines have been shown to induce AZ1 expression through a unique +1 frameshifting mechanism. However, under physiological conditions which particular polyamine induces AZ1, and thereby ODC activity, is unknown due to their inter-conversion. In this study we demonstrate that SPD regulates AZ1 expression under physiological conditions in IEC-6 cells. PUT and SPD showed potent induction of AZ1 within 4 h in serum-starved confluent cells grown in DMEM (control) medium. Unlike control cells, PUT failed to induce AZ1 in cells grown in DFMO containing medium; however, SPD caused a robust AZ1 induction in these cells. SPM showed very little effect on AZ1 expression in both the control and polyamine-depleted cells. Only SPD induced AZ1 when S-adenosylmethionine decarboxylase (SAMDC) and/or ODC were inhibited. Surprisingly, addition of DENSpm along with DFMO restored AZ1 induction by putrescine in polyamine-depleted cells suggesting that the increased SSAT activity in response to DENSpm converted SPM to SPD, leading to the expression of AZ1. This study shows that intracellular SPD levels controls AZ1 synthesis.

Regulation of intestinal mucosal growth by amino acids

Amino acids, especially glutamine (GLN) have been known for many years to stimulate the growth of small intestinal mucosa. Polyamines are also required for optimal mucosal growth, and the inhibition of ornithine decarboxylase (ODC), the first rate-limiting enzyme in polyamine synthesis, blocks growth. Certain amino acids, primarily asparagine (ASN) and GLN stimulate ODC activity in a solution of physiological salts. More importantly, their presence is also required before growth factors and hormones such as epidermal growth factor and insulin are able to increase ODC activity. ODC activity is inhibited by antizyme-1 (AZ) whose synthesis is stimulated by polyamines, thus, providing a negative feedback regulation of the enzyme. In the absence of amino acids mammalian target of rapamycin complex 1 (mTORC1) is inhibited, whereas, mTORC2 is stimulated leading to the inhibition of global protein synthesis but increasing the synthesis of AZ via a cap-independent mechanism. These data, therefore, explain why ASN or GLN is essential for the activation of ODC. Interestingly, in a number of papers, AZ has been shown to inhibit cell proliferation, stimulate apoptosis, or increase autophagy. Each of these activities results in decreased cellular growth. AZ binds to and accelerates the degradation of ODC and other proteins shown to regulate proliferation and cell death, such as Aurora-A, Cyclin D1, and Smad1. The correlation between the stimulation of ODC activity and the absence of AZ as influenced by amino acids is high. Not only do amino acids such as ASN and GLN stimulate ODC while inhibiting AZ synthesis, but also amino acids such as lysine, valine, and ornithine, which inhibit ODC activity, increase the synthesis of AZ. The question remaining to be answered is whether AZ inhibits growth directly or whether it acts by decreasing the availability of polyamines to the dividing cells. In either case, evidence strongly suggests that the regulation of AZ synthesis is the mechanism through which amino acids influence the growth of intestinal mucosa. This brief article reviews the experiments leading to the information presented above. We also present evidence from the literature that AZ acts directly to inhibit cell proliferation and increase the rate of apoptosis. Finally, we discuss future experiments that will determine the role of AZ in the regulation of intestinal mucosal growth by amino acids.

Amino acids regulate expression of antizyme-1 to modulate ornithine decarboxylase activity

In a glucose-salt solution (Earle's balanced salt solution), asparagine (Asn) stimulates ornithine decarboxylase (ODC) activity in a dose-dependent manner, and the addition of epidermal growth factor (EGF) potentiates the effect of Asn. However, EGF alone fails to activate ODC. Thus, the mechanism by which Asn activates ODC is important for understanding the regulation of ODC activity. Asn reduced antizyme-1 (AZ1) mRNA and protein. Among the amino acids tested, Asn and glutamine (Gln) effectively inhibited AZ1 expression, suggesting a differential role for amino acids in the regulation of ODC activity. Asn decreased the putrescine-induced AZ1 translation. The absence of amino acids increased the binding of eukaryotic initiation factor 4E-binding protein (4EBP1) to 5'-mRNA cap and thereby inhibited global protein synthesis. Asn failed to prevent the binding of 4EBP1 to mRNA, and the bound 4EBP1 was unphosphorylated, suggesting the involvement of the mammalian target of rapamycin (mTOR) in the regulation of AZ1 synthesis. Rapamycin treatment (4 h) failed to alter the expression of AZ1. However, extending the treatment (24 h) allowed expression in the presence of amino acids, indicating that AZ1 is expressed when TORC1 signaling is decreased. This suggests the involvement of cap-independent translation. However, transient inhibition of mTORC2 by PP242 completely abolished the phosphorylation of 4EBP1 and decreased basal as well as putrescine-induced AZ1 expression. Asn decreased the phosphorylation of mTOR-Ser(2448) and AKT-Ser(473), suggesting the inhibition of mTORC2. In the absence of amino acids, mTORC1 is inhibited, whereas mTORC2 is activated, leading to the inhibition of global protein synthesis and increased AZ1 synthesis via a cap-independent mechanism.

Mitochondrial localization of antizyme is determined by context-dependent alternative utilization of two AUG initiation codons

Ornithine decarboxylase-antizyme (Az), a polyamine-induced protein that targets ornithine decarboxylase (ODC) to rapid degradation, is synthesized as two isoforms. Studies performed in vitro indicated that the 29 and 24.5 kDa isoforms originate from translation initiation at two alternative initiation codons. Using transient transfections we demonstrate here that also in cells the two isoforms are synthesized from two AUG codons with the second being utilized more efficiently. The more efficient utilization of the second AUG is due to its location within a better sequence context for translation initiation. By using immunostaining we demonstrate that only the less expressed long form of Az is localized in the mitochondria. Moreover, this long isoform of Az and not the more efficiently expressed short isoform is imported into mitochondria in an in vitro uptake assay. Our data therefore demonstrate that a single Az transcript gives rise to two Az proteins with different N-terminal sequence and that the longer Az form containing a potential N-terminal mitochondrial localization signal is transported to mitochondria.

Impaired system A amino acid transport mimics the catabolic effects of acid in L6 cells

BACKGROUND

Metabolic acidaemia stimulates protein catabolism in skeletal muscle cells, leading to muscle wasting. As this occurs without decreasing cytosolic pH, the initial signal is unclear. A possible explanation is that extracellular pH acts on solute transporters at the cell surface, inhibiting nutrient influx.

DESIGN

Influx through glucose and Pi transporters and System A amino acid transporters into L6 skeletal muscle cells was assessed using 3H-2-deoxyglucose (2-DG), 33Pi and 14C-methylaminoisobutyrate (MeAIB), respectively. Protein degradation (PD) was assessed from 14C efflux from cells prelabelled with 14C-Phe. Branched-chain amino acids and Phe were assayed by selective fluorimetric assays.

RESULTS

While acid (pH 7.1) had little immediate effect on 2-DG or 33Pi influx, exposure to pH 7.1 rapidly inhibited MeAIB influx. To determine whether System A inhibition was sufficient to trigger PD, it was blocked at pH 7.5 by a saturating dose (10 mmol L(-1)) of nonmetabolisable substrate (MeAIB). Like acid, this increased PD and decreased total protein. It also mimicked the decreases in protein synthesis, DNA synthesis, glucose transport and glycolysis, and depletion of branched-chain amino acids and Phe, which are induced in L6 by acid. The onset of inhibition of PD by an extracellular Gln load was retarded at pH 7.1, and stimulation of PD by acid was negligible if PD had already been stimulated by Gln depletion. The stimulatory effect of MeAIB on PD was selectively blunted by an excess of Gln, whereas the inhibitory effect of Gln on PD was blocked by excess MeAIB.

CONCLUSIONS

The similarity of changes in response to MeAIB and acid implies that these share a common intracellular signalling pathway triggered by inhibition of System A. Even though System A is only a minor contributor to total Gln influx in L6 cells, it is suggested that blockade of System A with acid or MeAIB induces a catabolic state by denying Gln access to a key intracellular regulatory site.

Effect of low-protein diets on growth performance and body composition of broiler chicks

Three experiments were conducted to investigate effects of dietary manipulations to improve growth performance and whole-body composition of broiler chicks fed low-protein diets supplemented with crystalline amino acids. In all experiments, male chicks (1 d old) were fed a common corn-soybean meal diet (23% CP) for 7 d and subsequently allotted to treatment diets in a completely randomized design (10 chicks per floor pen, six replications). Chicks had free access to the isoenergetic diets (3,200 kcal MEn/kg) for 2 wk, after which chicks were weighed and then fasted for 24 h, and the whole-body DM, N, and ether extract contents of two chicks per pen (and six baseline chicks) were determined. In Experiment 1, Gln or Asn replaced 1% triammonium citrate in the low-protein diet (19% CP). In Experiments 2 and 3, dietary concentrations of crystalline essential and nonessential amino acids, respectively, were increased incrementally in the low-protein diets (19 to 20% CP). In all experiments, chicks fed low-protein diets grew slower, used feed less efficiently, and retained less N and more ether extract than chicks fed the control diets (P < or = 0.05), despite additions of crystalline Gln or Asn and despite increased dietary concentrations of crystalline essential and nonessential amino acids. Chicks fed low-protein diets excreted less N (P < 0.001) than did chicks fed the high-protein diets, and N excretion increased linearly (P < 0.001) with N intake. In summary, low-protein diets failed to support equal growth performance to that of high-protein control diets.

Increased translation efficiency and antizyme-dependent stabilization of ornithine decarboxylase in amino acid-supplemented human colon adenocarcinoma cells, Caco-2

The mechanisms of the response of ornithine decarboxylase(ODC), the rate-limiting enzyme in polyamine biosynthesis, to amino acid supplementation were studied in the human colon adenocarcinoma cell line, Caco-2. Supplementation of serum-deprived, subconfluent Caco-2 cells with any one of a series of amino acids (10 mM) resultedin increased ODC activity, reaching a maximum of approx. 12.5-fold after approx. 4 h, over control cells either not supplemented or supplemented with iso-osmolar D-mannitol. Glycine, L-asparagine and L-serine, as well as their D-enantiomers, were the strongest effectors and acted in a concentration-dependent manner; millimolar concentrations of most of these amino acids being sufficient to significantly increase ODC activity. In contrast, supplementation with D-methionine, L-lysine, L-aspartate or L-glutamate had little or no effect on ODC activity, whereas supplemental L-methionine, L-arginine, L-ornithine or L-cysteine was inhibitory. Polyamine assays showed that the putrescine content of cells varied in accordance with the changes in ODC activity. Western-blot and Northern-blot analyses revealed specifically increased levels of ODC protein but not mRNA,respectively, in response to supplementation with an ODC-inducing amino acid. Suppression of the increase in cycloheximide-treated cellsconfirmed a requirement for protein synthesis. Pulse-labelling of cellswith [(35)S]methionine showed a 3-fold increase in thesynthesis of ODC protein after 4 h of supplementation with glycineor L-serine. Supplemental glycine also augmented, reversibly, the half-life of ODC by almost 4-fold and simultaneously decreased the activity of putrescine-induced free antizyme. These results suggest that translational, but not transcriptional, regulation of ODC takes part in ODC induction by amino acids in Caco-2 cells. However, it also appears to occur in concert with decreased enzyme in activation and/or degradation.

Interaction of asparagine and EGF in the regulation of ornithine decarboxylase in IEC-6 cells

Our laboratory has shown that asparagine (ASN) stimulates both ornithine decarboxylase (ODC) activity and gene expression in an intestinal epithelial cell line (IEC-6). The effect of ASN is specific, and other A- and N-system amino acids are almost as effective as ASN when added alone. In the present study, epidermal growth factor (EGF) was unable to increase ODC activity in cells maintained in a salt-glucose solution (Earle's balanced salt solution). However, the addition of ASN (10 mM) in the presence of EGF (30 ng/ml) increased the activity of ODC 0.5- to 4-fold over that stimulated by ASN alone. EGF also showed induction of ODC with glutamine and alpha-aminoisobutyric acid, but ODC induction was maximum with ASN and EGF. Thus the mechanism of the interaction between ASN and EGF is important for understanding the regulation of ODC under physiological conditions. Therefore, we examined the expression of the ODC gene and those for several protooncogenes under the same conditions. Increased expression of the genes for c-Jun and c-Fos but not for ODC occurred with EGF alone. The addition of ASN did not further increase the expression of the protooncogenes, but the combination of EGF and ASN further increased the expression of ODC over that of ASN alone. Western analysis showed no significant difference in the level of ODC protein in Earle's balanced salt solution, ASN, EGF, or EGF plus ASN. Addition of cycloheximide during ASN and ASN plus EGF treatment completely inhibited ODC activity without affecting the level of ODC protein. These results indicated that 1) the increased expression of protooncogenes in response to EGF is independent of increases in ODC activity and 2) potentiation between EGF and ASN on ODC activity may not be due to increased gene transcription but to posttranslational regulation and the requirement of ongoing protein synthesis involving a specific factor dependent on ASN.

The role of glutamine, serum and energy factors in growth of enterocyte-like cell lines

BACKGROUND

Glutamine is routinely added to most cell cultures. Glutamine has been found to be the preferential nutrient to the rapidly replicating intestinal mucosa, but whether this is a metabolic effect or due to other properties of this amino acid is not determined. To study the importance of glutamine on the growth of two enterocyte-like cell lines, the effects of depriving the media or supplementing it with glutamine were assessed in media with different serum and energy supplements.

METHODS

CaCo-2 and HT-29 cells were grown in serum-free medium, with fetal bovine or synthetic serum, and with or without glucose or galactose. The glutamine content was varied between 0 and 4 mM. All growth assays were performed in triplicate by counting in a hemocytometer.

RESULTS

Both cell lines were dependent of serum factors for growth, but displayed distinct requirements on glutamine supplementation. Glutamine was an obligate supplement with dose-dependent correlation to growth (r = 0.87, p < 0.01) for CaCo-2 cells cultured in synthetic, but not in fetal bovine serum. In HT-29 cells, the correlation between glutamine and growth was significant (r = 0.68, p < 0.05) only in fetal bovine serum in the absence of galactose.

CONCLUSION

This study shows that glutamine has different growth stimulating effects on two enterocyte-like cell lines studied. This could reflect different modes of action of glutamine on proliferation and differentiation in an enterocyte cell population.

Tumor cell invasion of basement membrane in vitro is regulated by amino acids

Because most cancer deaths result from disseminated disease, understanding the regulation of tumor invasion and metastasis is a central theme in tumor cell biology. Interactions between extracellular matrices (ECM) and cellular microenvironment play a crucial role in this process. We have tested selected amino acids and polyamines for their ability to regulate RL95-2 cell invasion through both intact human amniotic basement membrane and a novel human ECM (Amgel). Three major systems for neutral amino acid transport, systems L, A, and ASC, are operational in these neoplastic cells. Amino acids entering the cell via transport system A or N, i.e., (methyl amino)-isobutyrate (MeAIB) or Asn, markedly enhanced invasiveness of these human adenocarcinoma cells as measured by a standard 72-hr amnion or Amgel invasion assay. Addition of 2-amino-2-norborane carboxylic acid (BCH; 1 mM), a model substrate of the L transport system, caused a significant decrease in invasive activity when tested in the Amgel assay. Interestingly, Val lowers steady-state levels of MeAIB uptake and blocks the increase in cell invasion elicited by MeAIB. At the same time, these amino acids do not influence cell proliferation activity. Neither the charged amino acid Lys or Asp (not transported by A/N/L systems) nor the polyamines putrescine, spermidine, or spermine modulate invasiveness under similar experimental conditions. Moreover, the observed time-dependent stimulation of system A activity (cellular influx of MeAIB) by substrate depletion is prevented by the addition of actinomycin D (5 microM) or cycloheximide (100 microM), suggesting the involvement of de novo RNA and protein synthesis events in these processes. MeAIB treatment of tumor cells selectively increased the activities of key invasion-associated type IV collagenases/gelatinases. These results indicate that in the absence of defined regulators (growth factors or hormones), certain amino acids may contribute to the epigenetic control of human tumor cell invasion and, by extension, metastasis. We propose that amino acids, acting via specific signaling pathways, modulate phenotypic cell behavior by modulating the levels of key regulatory enzymatic proteins.

Stable intracellular acidification upon polyamine depletion induced by alpha-difluoromethylornithine or N1,N12-bis(ethyl)spermine in L1210 leukaemia cells

Polyamines play major roles in ionic and osmotic regulation, but their exact involvement in specific ion transport processes is poorly defined. Treatment of L1210 mouse leukaemia cells with either 5 mM alpha-difluoromethylornithine (DFMO), a suicide substrate of ornithine decarboxylase, or 25 microM N1,N12-bis(ethyl)spermine (BE-3-4-3), a dysfunctional polyamine analogue, caused a stable decreased in intracellular pH (pHi) by 0.1-0.4 unit from steady-state control values between 7.4 and 7.6, as measured either by partition of a weak acid or with a fluorescent pH-sensitive probe. This effect was not related to cell growth status or differences in metabolic acid generation, and was observed in either the presence or absence of HCO3-. Exogenous spermidine (10-25 microM) or putrescine (25-50 microM) fully reversed DFMO- or BE-3-4-3-induced acidification within 2 and 8 h respectively. Recovery of pHi in L1210 cells after a nigericin- or NH4(+)-mediated acid load in HCO3(-)-free buffers was mediated by Na+/H+ antiporter activity, in addition to a minor Na(+)-independent and amiloride-insensitive pathway. Decreased steady-state pHi was maintained in polyamine-depleted L1210 cells after recovery from acid stress. Moreover, the pHi-dependence of the rate of Na(+)-dependent H+ extrusion after an acid stress was altered by DFMO and BE-3-4-3, resulting in a set-point which was lower by 0.25-0.30 pH unit in polyamine-depleted cells. On the other hand, neither the rate nor the magnitude of Na+/H(+)-exchanger-mediated alkalinization induced by hypertonic shock was decreased by polyamine depletion. Thus polyamine depletion induces a persistent defect in pHi homeostasis which is due, at least in part, to a stable decrease in the pHi set-point of the Na+/H+ exchanger.

Amino acid transport systems modulate human tumor cell growth and invasion: a working hypothesis

Interactions between the extracellular matrix (ECM) and the neoplastic cells they envelop are thought to play a fundamental role in those cells' ability to invade, one of the key events in the metastatic cascade. Cellular transport of amino acids, in turn, is known to be mediated by functionally distinct membrane transport systems and is modulated by substrate bioavailability in the microenvironment. We postulate that certain advantages enjoyed by a neoplastic cell population over their normal counterparts (for example, increased proliferating capability and invasiveness across ECM barriers) are linked to changes in the cells' differential control of amino acid transport (aaT) via host ECM-tumor cell generated signals. Our studies suggest that active transport of neutral amino acids modulates a cells' functional behavior among phenotypically distinct human transformed cell types, irrespective of whether they are categorized as a sarcoma, melanoma, or carcinoma. We present preliminary laboratory evidence which has lead us to formulate a series of working hypotheses as follows: 1. aaT systems operating in both non-transformed and transformed human cells exhibit differential transport kinetics; 2. adaptive regulation of certain amino acids via cell-specific aaT systems alters a cell's ability to invade human ECM; and 3. aaT induction involves changes both at the cellular and molecular levels. This report, therefore, provides experimental support, and suggests a possible mechanism, to explain how neutral amino acids, acting as nutrient signalling factors (along with other biologic elements) within the cell milieu, have the capability of regulating the phenotypic nature of human neoplastic cells.

Role of calcium in the regulation of ornithine decarboxylase enzyme activity in mouse colon cancer cells

Activity of ornithine decarboxylase (ODC), one of the rate-limiting enzymes in the pathway of polyamine biosynthesis, is regulated by various factors. In this study, we examined the role of Ca2+ in the regulation of ODC enzyme activity in mouse colon cancer cells (MC-26). KCl, a membrane-depolarizing agent that opens the voltage-dependent Ca(2+)-channel to increase intracellular Ca2+, decreased serum-induced ODC enzyme activity in MC-26 cells in a dose-dependent, reversible fashion. Both verapamil and nifedipine, inhibitors of the L-type voltage-dependent Ca(2+)-channel, decreased serum-induced ODC enzyme activity. W-7, a calmodulin inhibitor, decreased ODC enzyme activity in a dose-dependent, reversible fashion while trifluoperazine, another calmodulin inhibitor, failed to affect ODC enzyme activity in MC-26 cells. Our findings indicate that intracellular Ca2+ participates in the regulatory mechanism of ODC enzyme activity in MC-26 cells, although the exact role of Ca2+ is still unclear.

Treatment of acute diarrhea with oral rehydration solutions containing glutamine

OBJECTIVE

In a randomized, double-blind, controlled clinical trial we compared the efficacy of adding 90 mmol/L L-glutamine to the standard World Health Organization oral rehydration salts (WHO-ORS).

SUBJECTS

One hundred twenty male infants, > 1 month and < 1 year of age, with acute non-cholera diarrhea and dehydration were randomly assigned to one of the two treatment groups.

METHODS

Patients were kept in a metabolic unit where body weight, ORS, water and food intake, as well as stool, urine and vomitus output were recorded at 6-hour intervals. Laboratory evaluation, including blood gases and electrolytes, were monitored during hospitalization.

RESULTS

Diarrheal stool output, duration of diarrhea and volume of ORS required to achieve and maintain hydration was not significantly different between the treatment groups.

CONCLUSION

This study demonstrated that a glutamine-based ORS did not provide any additional therapeutic advantage over the standard WHO-ORS during treatment of dehydration in infants with acute non-cholera diarrhea.

Regulatory and molecular aspects of mammalian amino acid transport

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Hyperosmolarity leads to an increase in derepressed system A activity in the renal epithelial cell line NBL-1

Hyperosmolarity induced an increase in Na(+)-dependent L-alanine uptake in confluent monolayers of the established renal epithelial cell line NBL-1. This induction was attributable to system A and was only seen when the cells had been previously deprived of amino acids in the culture medium to derepress system A activity. It was additive to the adaptive regulation induction, and both were inhibited by cycloheximide. However, the hyperosmolarity effect was inhibited by colcemid (an inhibitor of microtubular function), but adaptive regulation was not. Otherwise, when cell monolayers were incubated in a control medium, basal Na(+)-dependent L-alanine uptake mediated by system B0 decreased. The results of this study show that: (i) system A activity was not induced by cell shrinkage and subsequent swelling due to extracellular hyperosmolarity when cells were incubated in control medium; (ii) previous expression of system A activity induced by amino acid starvation seems to be a prerequisite for further induction due to hyperosmolarity; and (iii) the effects of adaptive regulation and hyperosmotic stress are mediated by different mechanisms.

Sodium-dependent co-transported analogues of glucose stimulate ornithine decarboxylase mRNA expression in LLC-PK1 cells

Non-metabolizable analogues of glucose, including 1-O-methyl alpha-D-glucopyranoside (alpha MDG), that are co-transported with Na+ increase the specific activity of ornithine decarboxylase (ODC) in LLC-PK1 cells [Lundgren and Vacca (1990) Am. J. Physiol. 259, C647-C653]. The present study examines the effect of alpha MDG on LLC-PK1-cell ODC mRNA expression. The relative concentration of ODC mRNA in cells incubated in Earle's balanced salts solution minus glucose (EBSS--G) plus 3 mM alpha MDG was 5-6-fold higher than the concentration of ODC mRNA in cells incubated in either EBSS--G alone or in EBSS--G plus 3 mM 3-O-methyl-D-glucopyranose, a non-metabolizable analogue of glucose that is taken up by a passive carrier-mediated glucose transporter. Actinomycin D and cycloheximide completely blocked the increase in ODC activity induced by alpha MDG. Actinomycin D was also a potent inhibitor of ODC mRNA expression by alpha MDG. Cycloheximide had very little effect on the ability of this sugar to increase ODC mRNA. The relative concentration of ODC mRNA increased as a function of the incubation time in EBSS--G plus alpha MDG. The amount of ODC mRNA also increased as a function of the concentration of alpha MDG in EBSS--G. The addition of phlorizin (100 microM) to EBSS--G prevented alpha MDG from increasing ODC mRNA in LLC-PK1 cells. Phlorizin did not prevent phorbol 12-myristate 13-acetate (PMA) from enhancing LLC-PK1-cell ODC mRNA expression. The positive effect of both alpha MDG and TPA on ODC mRNA expression was suppressed when cells were incubated in hypertonic EBSS--G. From these results it is suggested that the uptake of Na(+)-dependent cotransported sugars increase ODC activity by enhancing ODC gene transcription and that this process may be dependent on cell volume expansion.

Independent actions of asparagine and high levels of free Ca2+ in the induction of ornithine decarboxylase

During growth stimulation of cells, Ca2+ and amino acids of the A, ASC and N transport systems are important for the induction of ornithine decarboxylase (ODC, L-ornithine carboxylase, EC 4.1.1.17). In order to clarify the relationship between Ca2+ and amino acids, we studied the induction of ODC by asparagine under three different Ca2+ states in H-35 rat hepatoma cells. First, in normal cells, extracellular Ca2+ above 0.1 mM and 10 mM asparagine separately stimulated ODC activity and their effects were approximately additive. In these normal cells, asparagine could act in the absence of medium Ca2+. TMB-8, a sequestered-Ca2+ release antagonist, had no effect on ODC induction whilst the asparagine action is sensitive to treatment with W7, a Ca-calmodulin antagonist, or lanthanum, a Ca2+ antagonist. Secondly, in cells treated with 0.5 mM EGTA in Ca(2+)-free medium, the asparagine action on ODC induction was blocked but the inhibition could be reversed by the addition of Ca2+ to the medium. Thirdly, ionomycin treatment in the absence of medium Ca2+ did not block the asparagine effect. Furthermore, in ionomycin-treated cells, the presence of high levels of medium Ca2+ increased ODC activity, but this increase was additive to, and could not replace, the action of asparagine. Our results indicate that the asparagine action does not depend on an increase of intracellular free-Ca2+.

The induction of ornithine decarboxylase by ornithine takes place at post-transcriptional level in perifused Ehrlich carcinoma cells

The increase in ODC activity during perifusion of Ehrlich carcinoma cells with 0.5 mM ornithine correlates with an increase in 'de novo' synthetized ODC protein. ODC synthesis was followed by immunoprecipitation of equal quantities of 35S-labelled proteins after 10, 20 and 30 min of labelling. In addition, the rate of 'de novo' protein synthesis is very much elevated in cells perifused with saline buffer supplemented with 0.5 mM ornithine than in cells perifused with the saline buffer only. In spite of the higher specific ODC activity observed in cells perifused with saline buffer plus 0.5 mM ornithine respect to cells perifused with only saline buffer for 3.5 h, no elevation in ODC mRNA was observed when the cells were perifused in the presence of 0.5 mM ornithine.

Ornithine decarboxylase activity in mouse tumour tissue in response to refeeding and diet components

Ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase activity (SAMD) were measured in tumour tissue in mice during periods of starvation (24 h) and refeeding. Starvation led to a 60% reduction in tumour ODC activity. Refeeding normalised the activity within 4 h. Restitution in ODC activity, representing de novo enzyme synthesis, preceded DNA resynthesis. SAMD activity continued to fall along the increase in ODC activity during refeeding, while difluoro-methyl-ornithine (DFMO) caused a compensatory increase in SAMD activity as expected. A fall and regain in ODC activity was associated with inhibition and regrowth of the tumour. Starvation-refeeding was not related to any decrease in tumour polyamine concentrations, while systemic DFMO blockade was. Glucose stimulated ODC when refed orally, but not when given systemically. Tumour ODC activity was not decreased in refed mice by anti-insulin, a procedure that antagonised insulin's bioactivity. Exogenous insulin did not stimulate tumour ODC activity. Our results suggest that gastrointestinal metabolism of carbohydrates stimulates the release of a factor, which initiates both ODC activity and DNA synthesis in tumour cells. This factor was not insulin.

Regulation by 1,4-diamines of the ornithine decarboxylase activity induced by ornithine in perifused tumor cells

Ornithine decarboxylase (ODC) activity of Ehrlich carcinoma cells was increased more than 36-fold after being maintained for 3.5 hr in vitro in a special chamber which allowed continuous perifusion with 0.5 mM ornithine; if incubated in vitro without perifusion the ODC activity was, of course, only 9-fold by the same concentration of ornithine. Ornithine withdrawal from the perifusion medium resulted in a decay of enzyme activity observed after 90 min; this decay was prevented by addition of 55 microM pyridoxal to the medium. The 1,4-diamines putrescine, spermidine, spermine, agmatine, histamine, serotonin, tryptamine, chlorpheniramine and harmaline at 55 microM strongly suppressed ODC induction by 0.5 mM ornithine in perifused Ehrlich ascites cells. Methyl derivatives also behave as strong inhibitors of ODC induction. On the contrary, N-acetylation paralleled with a decrease in the inhibition capacity: 55 microM N-acetyl putrescine, N-acetyl serotonin or N-omega-acetylhistamine suppressed ODC induction by ornithine in 66, 64 and 19%, respectively. The addition to the perifusion medium of the same concentrations of 1,3-diamines (1,3-diaminopropane, 1,3-diamino-2-propanol or the alkaloid gramine) as well as 1,5-diamines (1,5-diaminopentane and the antihistamic doxylamine or cimetidine) failed to suppress the induction of ODC activity by ornithine. Interestingly, 1,4-benzenediamine, which strongly inhibits ODC activity when the induced enzyme is assayed in its presence, did not suppress the induction of the enzyme when both 0.5 mM ornithine and 55 microM 1,4-benzenediamine were present in the perifusion medium. The inhibitory capacity in down-regulating ODC is not due to differences in the diamine uptake by the cells. The results suggest that the N-N distance (6A) and the charge of one amino group are important chemical characteristics for regulatory effects.

Extended life span of human endometrial stromal cells transfected with cloned origin-defective, temperature-sensitive simian virus 40

Human endometrial stromal cells transfected with an origin-defective, temperature-sensitive simian virus 40 recombinant plasmid are dependent on T-antigen function for proliferation and at the permissive temperature have an extended life span in culture. Southern blot analysis indicates that the transfected gene is present in low copy number, possibly at a single integration site. Normal stromal cells are capable of 10 to 20 population doublings in culture. Transfected cultures have been carried at the permissive temperature to 80 population doublings before crisis. In the multistep model of malignant transformation of human cells, these cells represent one of the earliest stages: extended but finite life span. We have used these cells to investigate alterations in signal transduction that may be responsible for this early stage of transformation caused by the large T antigen. Temperature shift experiments indicate that the expression of ornithine decarboxylase (ODC) but not of c-fos is altered by the large T antigen. Induction of c-fos by serum or 12-O-tetradecanoylphorbol-13-acetate is independent of temperature. However, in the transfected cells, the induction of ODC by asparagine or serum is greatly enhanced at the permissive temperature. This result indicates that the large T antigen acts downstream of c-fos but upstream of ODC expression in the signal-transducing cascade.

Nonmetabolizable glucose analogues and ornithine decarboxylase expression in LLC-PK1 cells

This report examines the effect of nonmetabolizable glucose analogues on ornithine decarboxylase (ODC) activity in LLC-PK1 cells. The addition of Na(+)-dependent cotransported glucose analogues, 1-O-methyl-alpha-D-glucopyranoside (alpha-MDG) and 1-O-methyl-beta-D-glucopyranoside, to Earle's balanced salt solution minus glucose (EBSS-G) increased ODC activity five- to sevenfold above basal levels. The passive carrier-mediated transported glucose analogue 3-O-methyl-D-glucopyranose had very little effect on enzyme activity. alpha-MDG increased ODC activity in quiescent but not growing cells. ODC activity increased as a function of both the incubation time in EBSS-G + alpha-MDG and the concentration of alpha-MDG in EBSS-G. Phlorizin significantly reduced the level of enzyme activity induced by alpha-MDG. ODC expression by alpha-MDG was reduced in cells incubated in hypertonic EBSS-G + alpha-MDG. Enzyme activity, in the absence of extracellular organic substrates, was markedly elevated in cells incubated in hypotonic media. It is suggested that an influx of Na+ and/or an increase in cell volume elevates one or more signal transducers that regulate ODC expression.

Glucose elevates ornithine decarboxylase expression in Vero cells

The addition of Earle's balanced salt solution (EBSS) of amino acids that are transported by a Na+-dependent cotransport system was not required by Vero cells for ornithine decarboxylase (ODC:EC 4.1.1.17) amplification. Vero cell ODC activity was elevated tenfold above basal levels when confluent cells were incubated for 5 hr in EBSS alone. ODC activity increased as a function of the incubation time in EBSS and was not elevated above basal enzyme levels when cells were incubated in EBSS minus glucose. ODC expression increased as a function of the glucose concentration in EBSS, with 20 mM glucose producing a 90-fold increase in ODC activity. ODC expression is more responsive to glucose in high-density quiescent cultures than in low-density growing cultures. Enhanced ODC expression by glucose depended on Na+ and K+ concentrations. The specific activity of ODC was also elevated above basal levels when mannose or fructose replaced glucose in EBSS. The addition of alanine or asparagine to EBSS enhanced ODC activity above levels obtained with EBSS containing standard (5.5 mM) glucose concentrations. In the absence of glucose, alanine was more effective than asparagine in enhancing ODC expression. These results suggest that the transport of amino acids is not an absolute requirement for Vero cell ODC expression and that ODC expression is linked to changes in cellular energetics and/or ion fluxes.

Induction of ornithine decarboxylase in adult rat hippocampal slices

Several factors involved in the regulation of ornithine decarboxylase (ODC) activity in adult rat brain tissue have been identified by using the in vitro hippocampal slice preparation. The same amino acids that have previously been reported to induce ODC in tissue culture, i.e., asparagine and glutamine, were found to produce a concentration- and time-dependent increase in ODC activity that reached a 100 fold the control value after 6 h of incubation. The effect of asparagine was totally blocked by inhibition of either protein or RNA synthesis, suggesting that the inducing amino acids increase ODC activity by stimulating the transcription of genes directly or indirectly regulating ODC activity. The effect of the inducing amino acids was potentiated by a variety of factors which by themselves did not modify ODC activity. In particular, opioid peptides markedly potentiated the effect of asparagine. Although the opiate antagonists naloxone and naltrexone totally blocked the effects of the opioid peptides on ODC induction, they also produced an inhibition of the asparagine-mediated increase in ODC activity. Other factors like dibutyryl cyclic AMP and insulin also potentiated the effects of asparagine on ODC activity. These results provide the first description of ODC induction in an in vitro preparation of adult brain tissue and indicate that the hippocampal slice preparation could be used to study the molecular mechanisms which regulate the expression and activity of ODC in the adult central nervous system. Moreover the data suggest possible mechanisms which may be involved in the induction of ODC in hippocampus by seizure activity.

Apparent ornithine decarboxylase activity, measured by 14CO2 trapping, after frozen storage of rat tissue and rat tissue supernatants

Ornithine decarboxylase (ODC) activity of rat tissues was measured by the standard 14CO2 trapping method after frozen storage (-60 or -70 degrees C) of the tissues or their 105,000g supernatants. True ODC activity was determined by two methods: (a) addition of the inhibitors alpha-difluoromethylornithine (DFMO), a specific irreversible inhibitor of ODC, or aminooxyacetate (AOA), an inhibitor that blocks the decarboxylation of ornithine by mitochondrial enzymes; and (b) chromatographic analysis of the reaction products. In the frozen supernatants of liver and spleen, ODC activity changed only slightly after 1 day but increased 29 and 14%, respectively, by 30 days; activity in kidney supernatant decreased 17% after 1 day and remained near that level at 30 days. Kidney and spleen ODC activity was inhibited 90-100% by DFMO, but apparent liver ODC activity was inhibited only 60-75%. In the supernatant prepared from tissue stored frozen for 1 day, apparent ODC activity in liver increased 500% over that activity in the freshly prepared supernatant; at 23 days, apparent activity increased 755% for liver and 121% for kidney. After 23 days, DFMO did not inhibit apparent ODC activity in supernatants from frozen liver and inhibited ODC in frozen kidney by only 49%. With AOA, the ODC activities of the fresh and frozen supernatants were similar, indicating that the large increase in apparent ODC activity in frozen tissue was due to artifacts from the metabolism of ornithine via the mitochondrial pathway. HPLC analysis of the reaction products resulting from the incubation of uniformly labeled [14C]ornithine with the fresh and frozen preparations indicated no increase in putrescine with the frozen preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

Possible role of the membrane Na+/H+ antiport in ornithine decarboxylase induction by L-asparagine

Amino acids of transport systems A and N play certain important role in cell activation. For example, the presence of these amino acids is essential in the induction of ornithine decarboxylase by growth factors and hormones. At mM concentrations, each of these amino acids, particularly L-asparagine, can also induce the enzyme without being further metabolized or incorporated into proteins. We have reported that the addition of 10 mM L-asparagine to quiescent Reuber's H-35 rat hepatoma cells caused an immediate and transient increase in intracellular pH. Here we report that concomitant with the intracellular alkalinization was an increase in H+ extrusion which was amiloride-sensitive and Na+-dependent. The induction of ornithine decarboxylase by L-asparagine was also amiloride-sensitive.

Modulation of cellular heat sensitivity by specific amino acids

When either plateau-phase or exponentially growing Chinese hamster ovary (CHO) cells are incubated in amino acid-free medium, the cells become sensitized to killing by heat. For cells deprived of amino acids for 12 h survival decreases from 1 X 10(-2) for controls to 1 X 10(-6) for the deprived cells, following heating at 45 degrees C for 38 min. The survival of these sensitized cells is rapidly increased by the addition of a single amino acid just prior to heating. Of the 21 amino acids which are added in purified form to make McCoy's 5a medium, 12 show no protective effect, four have a small protective effect, and either alanine, asparagine, glutamine, serine, or theronine raise survival to a level similar to that of the control cells. The nonmetabolizable alanine analogue, 2-aminoisobutyric acid (AIB), increases survival of amino acid-deprived cells as effectively as each member of the group of five listed above, suggesting that metabolic conversion of the amino acids is not required for their protective effect. The data suggest that an increase in the intracellular concentrations of specific amino acids, independent of any change in cellular ATP content or the rate of protein synthesis, enables these cells to become quickly more resistant to killing by heat. We also conclude that the amino acid concentrations in poorly vascularized regions of some tumors should be considered, along with the oxygen, glucose, and proton concentrations, as factors which determine cellular survival following hyperthermia.

Effects of putrescine on synthesis and degradation of ornithine decarboxylase in primary cultured hepatocytes

Changes in both synthesis rate and degradation rate of ornithine decarboxylase (ODC) were pursued in primary cultures of adult rat hepatocytes during the process of ODC induction caused by asparagine and glucagon and also during the process of rapid ODC decay caused by putrescine. The synthesis rate of ODC was determined by [35S]methionine incorporation into the enzyme, which was separated afterwards by immunoprecipitation and sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The degradation rate of ODC was determined by following the decay of prelabeled ODC. The enzyme induction caused by asparagine (10 mM) and glucagon (1 microM) was due both to an increase in the synthesis rate and to a decrease in the degradation rate. Addition of 10 mM putrescine caused a rapid decay of ODC activity, which was faster than ODC decay in the presence of cycloheximide. This rapid decay in ODC activity was accompanied by slightly slower decay in ODC protein, which was due both to partial suppression of ODC synthesis and to several fold acceleration of ODC degradation.

Induction of ornithine decarboxylase activity by insulin and growth factors is mediated by amino acids

The polypeptide growth factors, nerve growth factor, epidermal growth factor, and platelet-derived growth factor, as well as insulin do not induce ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) unless a minimal concentration of an ornithine decarboxylase-inducing amino acid, such as asparagine, is present in the medium. The effects of the growth factors were studied in appropriately responsive cell lines: pheochromocytoma (PC12) cells for nerve and epidermal growth factors, fibroblasts (NIH 3T3) for platelet-derived growth factor, and fibroblasts and hepatoma (KRC-7) cells for insulin. The nonmetabolizable amino acid analog alpha-aminoisobutyric acid can replace asparagine, indicating that the covalent modification of the inducing amino acid is not necessary for the induction of ornithine decarboxylase by these growth factors. For the same intracellular concentration of the inducing amino acid, the presence of the growth factors induces higher levels of ornithine decarboxylase. The evidence indicates that these growth factors do not induce ornithine decarboxylase by raising the intracellular concentration of amino acids but rather act synergistically with the inducing amino acid. Evidence is provided that the induction of polyamine-dependent growth by these growth factors is mediated by amino acids. The relationship of these results to the A and N amino acid transport systems and to the Na+ influxes in relation to growth is discussed.