Studies on the role of protein synthesis and of sodium on the regulation of ornithine decarboxylase activity

Refractive Index Sensing for Measuring Single Cell Growth

Accessing cell growth on adhesive substrates is critical for identifying biophysical properties of cells and their therapeutic response to drug therapies. However, optical techniques have low sensitivity, and their reliability varies with cell type, whereas microfluidic technologies rely on cell suspension. In this paper, we introduced a plasmonic functional assay platform that can precisely measure cell weight and the dynamic change in real-time for adherent cells. Possessing this ability, our platform can determine growth rates of individual cells within only 10 min to map the growth profile of populations in short time intervals. The platform could successfully determine heterogeneity within the growth profile of populations and assess subpopulations exhibiting distinct growth profiles. As a proof of principle, we investigated the growth profile of MCF-7 cells and the effect of two intracellular metabolisms critical for their proliferation. We first investigated the negative effect of serum starvation on cell growth. We then studied ornithine decarboxylase (ODC) activity, a key enzyme which is involved in proliferation, and degraded under low osmolarity that inhibits cell growth. We successfully determined the significant distinction between growth profiles of MCF-7 cells and their ODC-overproducing variants that possess strong resistance to the negative effects of low osmolarity. We also demonstrated that an exogenous parameter, putrescine, could rescue cells from ODC inhibition under hypoosmotic conditions. In addition to the ability of accessing intracellular activities through ex vivo measurements, our platform could also determine therapeutic behaviors of cancer cells in response to drug treatments. Here, we investigated difluoromethylornithine (DFMO), which has antitumor effects on MCF-7 cells by inhibiting ODC activity. We successfully demonstrated the susceptibility of MCF-7 cells to such drug treatment, while its DFMO-resistant subpopulation could survive in the presence of this antigrowth agent. By rapidly determining cell growth kinetics in small samples, our plasmonic platform may be of broad use to basic research and clinical applications.

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.

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.

Rapid and regulated degradation of ornithine decarboxylase

Images

Asparagine markedly induces the expression of ornithine decarboxylase gene in transformed mammalian cells but not in their untransformed counterparts

We have previously shown that asparagine alone induces a 10-15-fold increase in ornithine decarboxylase (ODC) mRNA level in DF-40 mouse neuroblastoma cells. The induction is due to an accumulation of ODC mRNA through a post-transcriptional stabilization mechanism (Chen, Z.P. and Chen, K.Y. (1992) J. Biol. Chem., 267, 6946-6951). In the present study we showed that asparagine induced ODC gene expression in v-Ha-ras-transformed 3T3 (ras-3T3) cells but not in 3T3 cells. Other growth related genes including c-src, c-ras, and c-fos were not affected by asparagine in ras-3T3 cells. Southern blot analysis indicated that the pronounced asparagine effect was not due to ODC gene amplification in ras-3T3 cells. The effect of asparagine on the induction of ODC mRNA could account for the significant increases in the ODC activity in ras-3T3 cells. We also examined the effect of asparagine on ODC gene expression in human KD cells and their transformed counterparts. Our findings strongly suggest that the induction of ODC mRNA by asparagine may represent a component of an altered growth regulatory program associated most prominently with cell transformation.

Mechanisms of dramatic fluctuations of ornithine decarboxylase activity upon tonicity changes in primary cultured rat hepatocytes

We investigated the mechanisms underlying the marked induction of ornithine decarboxylase (ODC) activity by hypotonic treatment and its rapid decay upon reversal to isotonicity in primary cultures of adult rat hepatocytes. Upon hypotonic treatment, ODC synthesis rate increased progressively whereas the amount of ODC mRNA increased only about twofold. In addition, ODC was stabilized severalfold. ODC activity rapidly decreased upon restoration of isotonicity, owing to immediate and nearly complete suppression of ODC synthesis and 3-6-fold stimulation of ODC decay. The stimulation of ODC decay caused by restoration of isotonicity was mostly independent of time and protein synthesis. ODC decay was also stimulated by putrescine, even under hypotonic conditions, depending on time and new protein synthesis. Restoration of isotonicity and putrescine treatment together caused a synergistic stimulation of ODC decay, confirming that these act by different mechanisms.

Translational control mechanism of ornithine decarboxylase by asparagine and putrescine in primary cultured hepatocytes

Asparagine stimulated the translation of ornithine decarboxylase (ODC) mRNA more than 10-fold in cultured hepatocytes which had been pretreated with glucagon in simple salt/glucose medium. Putrescine suppressed the increase in the rate of ODC synthesis caused by asparagine without significant change in the amount of ODC mRNA, suggesting that putrescine inhibited the effect of asparagine at least in part at the level of translation. Polysomal distribution of ODC mRNA was analyzed to examine the site of translational regulation by these effectors. In uninduced hepatocytes, most of the ODC mRNA was sedimented slightly after the 40 S ribosomal subunit. This ODC mRNA was sequestered from translational machinery since it was not shifted to the polysome fraction when peptide elongation was specifically inhibited by a low concentration of cycloheximide. In asparagine-treated cells, 40% of total ODC mRNA was in the polysomal fraction and formed heavier polysomes, indicating that asparagine stimulated both recruitment of ODC mRNA from the untranslatable pool and the initiation steps of translation. Putrescine did not change the distribution pattern of ODC mRNA on polysomes significantly. Thus, 30% of ODC mRNA remained on polysomes even when ODC synthesis was completely inhibited by putrescine. Paradoxically more than 70% of ODC mRNA was shifted into polysomes by putrescine in the presence of low concentrations of cycloheximide. These results, together with changes in the polysome profile, suggested that putrescine nonspecifically stimulated the recruitment of ODC mRNA from the untranslatable pool, whereas it specifically inhibited its translation at both the initiation and the elongation steps.

Presence of ornithine decarboxylase antizyme in primary cultured hepatocytes of the frog Xenopus laevis

Ornithine decarboxylase (ODC; EC 4.1.1.17) could be induced in primary cultured hepatocytes of the frog, Xenopus laevis, by a hypotonic treatment. Addition of 10 mM putrescine caused a rapid decay of preinduced ODC after a lag period of 30 min. The putrescine-induced ODC decay was faster than the ODC decay in the presence of cycloheximide. Simultaneous addition of cycloheximide blocked the putrescine-induced acceleration of ODC decay, indicating an involvement of protein synthesis. Addition of putrescine to normal medium caused complete loss of ODC activity in 2 h and then ODC-inhibitory activity appeared and progressively increased. The inhibitory factor was non-dialysable and temperature-sensitive and showed a time-independent and stoichiometric pattern of ODC inhibition. On the basis of these observations the inhibitory factor was identified as ODC antizyme. These results indicated that in frog hepatocytes, like in mammalian cells and tissues, ODC is under negative feedback regulation mediated by antizyme.

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.

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.

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.

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

The induction of ornithine decarboxylase (EC 4.1.1.17) (ODC) by amino acids and by the peptide hormones nerve growth factor (NGF) and epidermal growth factor (EGF) in salts-glucose media has been studied. Only those neutral amino acids taken into the cell via one of the Na+ dependent transport systems stimulate ODC activity. Asparagine and the nonmetabolizable alpha-amino-isobutyric acid (AIB) were used as representatives of this class of inducing amino acids, and their intracellular concentrations were related to the levels of ODC induced. A threshold intracellular concentration of asparagine or AIB has to be attained before ODC can be induced. Further slight increases in intracellular concentrations of asparagine or AIB produce disproportionately large increases of ODC, resulting in a sigmoidal curve of ODC induction. These results, and the fact that the decrease in ODC levels caused by valine is associated with a concurrent decrease in the intracellular level of the inducing amino acid, suggest that the intracellular amino acid level is causally related to the induction of ornithine decarboxylase. Glutamic acid, EGF, and NGF do not induce ODC except in the presence of an inducing amino acid. They act synergistically with the inducing amino acid and produce higher ODC levels at the same intracellular concentration of the inducing amino acid.

Regulation of polyamine biosynthesis by antizyme and some recent developments relating the induction of polyamine biosynthesis to cell growth. Review

This review considers the role of antizyme, of amino acids and of protein synthesis in the regulation of polyamine biosynthesis. The ornithine decarboxylase of eukaryotic cells and of Escherichia coli can be non-competitively inhibited by proteins, termed antizymes, which are induced by di- and poly- amines. Some antizymes have been purified to homogeneity and have been shown to be structurally unique to the cell of origin. Yet, the E. coli antizyme and the rat liver antizyme cross react and inhibit each other's biosynthetic decarboxylases. These results indicate that aspects of the control of polyamine biosynthesis have been highly conserved throughout evolution. Evidence for the physiological role of the antizyme in mammalian cells rests upon its identification in normal uninduced cells, upon the inverse relationship that exists between antizyme and ornithine decarboxylase as well as upon the existence of the complex of ornithine decarboxylase and antizyme in vivo. Furthermore, the antizyme has been shown to be highly specific; its Keq for ornithine decarboxylase is 1.4 X 10(11) M-1. In addition, mammalian cells contain an anti-antizyme, a protein that specifically binds to the antizyme of an ornithine decarboxylase-antizyme complex and liberates free ornithine decarboxylase from the complex. In E. coli, in which polyamine biosynthesis is mediated both by ornithine decarboxylase and by arginine decarboxylase, three proteins (one acidic and two basic) have been purified, each of which inhibits both these enzymes. They do not inhibit the biodegradative ornithine and arginine decarboxylases nor lysine decarboxylase. The two basic inhibitors have been shown to correspond to the ribosomal proteins S20/L26 and L34, respectively. The relationship of the acidic antizyme to other known E. coli proteins remains to be determined. In mammalian cells, ornithine decarboxylase can be induced by a broad spectrum of compounds. These range from hormones and growth factors to natural amino acids such as asparagine and to non-metabolizable amino acid analogues such as alpha-amino-isobutyric acid. The amino acids that induce ornithine decarboxylase as well as those that promote polyamine uptake utilize the sodium dependent A and N transport systems. Consequently, they act in concert and increase intracellular polyamine levels by both mechanisms. The induction of ornithine decarboxylase by growth factors, such as NGF, EGF, and PDGF as well as by insulin requires the presence of these same amino acids and does not occur in their absence. However, the inducing amino acid need not be incorporated into protein nor covalently modified.(ABSTRACT TRUNCATED AT 400 WORDS)

Sodium ion concentrations in uterine flushings from "implanting" and "delayed implanting" mice

The uteri of mice were flushed with isotonic mannitol (1) on the day of implantation during normal pregnancy or (2) during delay of implantation. The Na+ concentration in flushings was reduced during diapause but had increased by 65% 24 h after progesterone-treated ovariectomized mice received estrogen injections. The results suggest that the apparent metabolic quiescence of diapausing blastocysts may, in part, be a consequence of lower (Na+) in the uterus during delay of implantation.

Non-metabolizable amino acids are potent stimulators of hepatic and renal ornithine decarboxylase activity

The non-metabolizable amino acids alpha-aminoisobutyric acid (AIB) and cycloleucine and the poorly metabolizable amino acid D-alanine potently stimulated hepatic ornithine decarboxylase (ODC) activity in starved rats. The stimulation by AIB was shown to have several of the characteristics of stimulation by a protein meal and occurred in hypophysectomized animals. AIB also stimulated renal, but not brain or heart, ODC activity.