A role for ornithine in the regulation of putrescine accumulation and ornithine decarboxylase activity in Reuber H35 hepatoma cells

Structural and Biochemical Characterization of MppQ, an L-Enduracididine Biosynthetic Enzyme from Streptomyces hygroscopicus

MppQ is an enzyme of unknown function from Streptomyces hygroscopicus (ShMppQ) that operates in the biosynthesis of the nonproteinogenic amino acid L-enduracididine (L-End). Since L-End is a component of several peptides showing activity against antibiotic-resistant pathogens, understanding its biosynthetic pathway could facilitate the development of chemoenzymatic routes to novel antibiotics. Herein, we report on the crystal structures of ShMppQ complexed with pyridoxal-5'-phosphate (PLP) and pyridoxamine-5'-phosphate (PMP). ShMppQ is similar to fold-type I PLP-dependent aminotransferases like aspartate aminotransferase. The tertiary structure of ShMppQ is composed of an N-terminal extension, a large domain, and a small domain. The active site is placed at the junction of the large and small domains and includes residues from both protomers of the homodimer. We also report the first functional characterization of MppQ, which we incubated with the enzymatically produced 2-ketoenduracidine and observed the conversion to L-End, establishing ShMppQ as the final enzyme in L-End biosynthesis. Additionally, we have observed that MppQ has a relatively high affinity for 2-keto-5-guanidinovaleric acid (i.e., 2-ketoarginine), a shunt product of MppP, indicating the potential role of MppQ in increasing the efficiency of L-End biosynthesis by converting 2-ketoarginine back to the starting material, l-arginine. A panel of potential amino-donor substrates was tested for the transamination activity against a saturating concentration of 2-ketoarginine in end-point assays. Most l-Arg was produced with l-ornithine as the donor substrate. Steady-state kinetic analysis of the transamination reaction with l-Orn and 2-ketoarginine shows that the kinetic constants are in line with those for the amino donor substrate of other fold-type I aminotransferases.

Metabolomics analysis reveals heavy metal copper-induced cytotoxicity in HT-29 human colon cancer cells

LC-MS based metabolomics analysis reveals heavy metal copper-induced cytotoxicity in a human intestinal cell line, HT-29.

Rat colon ornithine and arginine metabolism: coordinated effects after proliferative stimuli

Ornithine decarboxylase (ODC) catalyzes the first step in the polyamine biosynthetic pathway, a highly regulated pathway in which activity increases during rapid growth. Other enzymes also metabolize ornithine, and in hepatomas, rate of growth correlates with decreased activity of these other enzymes, which thus channels more ornithine to polyamine biosynthesis. Ornithine is produced from arginase cleavage of arginine, which also serves as the precursor for nitric oxide production. To study whether short-term coordination of ornithine and arginine metabolism exists in rat colon, ODC, ornithine aminotransferase (OAT), arginase, ornithine, arginine, and polyamine levels were measured after two stimuli (refeeding and/or deoxycholate exposure) known to synergistically induce ODC activity. Increased ODC activity was accompanied by increased putrescine levels, whereas OAT and arginase activity were reduced by either treatment, accompanied by an increase in both arginine and ornithine levels. These results indicate a rapid reciprocal change in ODC, OAT, and arginase activity in response to refeeding or deoxycholate. The accompanying increases in ornithine and arginine concentration are likely to contribute to increased flux through the polyamine and nitric oxide biosynthetic pathways in vivo.

Thrombin stimulates vascular smooth muscle cell polyamine synthesis by inducing cationic amino acid transporter and ornithine decarboxylase gene expression

Thrombin, a serine protease, is a potent mitogen for vascular smooth muscle cells (SMCs), but its mechanism of action is not known. Since L-ornithine is metabolized to growth-stimulatory polyamines, we examined whether thrombin regulates the transcellular transport and metabolism of L-ornithine by vascular SMCs. Treatment of SMCs with thrombin initially (0 to 2 hours) decreased L-ornithine uptake, whereas longer exposures (6 to 24 hours) progressively increased transport. Kinetic studies indicated that thrombin-induced inhibition was associated with a decrease in affinity for L-ornithine, whereas stimulation was mediated by an increase in transport capacity. Thrombin induced the expression of both cationic amino acid transporter (CAT)-1 and CAT-2 mRNA. Furthermore, thrombin stimulated L-ornithine metabolism by inducing ornithine decarboxylase (ODC) mRNA expression and activity. The stimulatory effect of thrombin on both L-ornithine transport and ODC activity was reversed by hirudin, a thrombin inhibitor, and was mimicked by a 14-amino acid thrombin receptor-activating peptide. Thrombin also markedly increased the capacity of SMCs to generate putrescine, a polyamine, from extracellular L-ornithine. The thrombin-mediated increase in putrescine production was reversed by N(G)-methyl-L-arginine, a competitive inhibitor of cationic amino acid transport, or by alpha-difluoromethylornithine (DFMO), an ODC inhibitor. DFMO also inhibited thrombin-induced SMC proliferation. These results demonstrate that thrombin stimulates polyamine synthesis by inducing CAT and ODC gene expression and that thrombin-stimulated SMC proliferation is dependent on polyamine formation. The ability of thrombin to upregulate L-ornithine transport and direct its metabolism to growth-stimulatory polyamines may contribute to postangioplasty restenosis and atherosclerotic lesion formation.

Lysophosphatidylcholine regulates cationic amino acid transport and metabolism in vascular smooth muscle cells. Role in polyamine biosynthesis

Lysophosphatidylcholine (lyso-PC) is a major component of atherogenic lipids that stimulate vascular smooth muscle cell (SMC) proliferation. Because cationic amino acids are metabolized to growth-stimulatory polyamines, we examined whether lyso-PC regulates the transcellular transport and metabolism of cationic amino acids by vascular SMC. Treatment of SMC with lyso-PC initially (0-2 h) decreased cationic amino acid uptake, whereas longer exposures (6-24 h) progressively increased transport. Kinetic studies indicated that lyso-PC-induced inhibition was associated with a decrease in affinity for cationic amino acids, but the stimulation was mediated by an increase in transport capacity. Lyso-PC strongly induced the expression of cationic amino acid transporter-2 mRNA while modestly elevating the level of cationic amino acid transporter-1 mRNA. In addition, lyso-PC stimulated intracellular cationic amino acid metabolism by inducing ornithine decarboxylase activity and mRNA expression and also by inducing arginase activity in vascular SMC. In contrast, lyso-PC inhibited the catabolism of L-arginine to nitric oxide by blocking inducible nitric oxide synthase expression. Lyso-PC increased markedly the capacity of SMC to generate putrescine, a polyamine, from extracellular L-ornithine and L-arginine. The lyso-PC-mediated increase in the production of putrescine was reversed by NG-methyl-L-arginine, a competitive inhibitor of cationic amino acid transport, or by alpha-difluoromethylornithine, an ornithine decarboxylase inhibitor. The formation of putrescine from L-arginine was also prevented by arginase inhibitor NG-hydroxy-L-arginine. These results demonstrate that lyso-PC stimulates polyamine synthesis in vascular SMC by inducing the expression of the genes that regulate both the transport and metabolism of cationic amino acids. The actions of lyso-PC in stimulating cationic amino acid uptake and directing their metabolism to growth-stimulatory polyamines while simultaneously inhibiting the synthesis of antiproliferative NO, may contribute to lyso-PC-induced SMC proliferation and atherosclerotic lesion formation.

Platelet-derived growth factor regulates vascular smooth muscle cell proliferation by inducing cationic amino acid transporter gene expression

Since recent studies demonstrated that platelet-derived growth factor (PDGF) induces vascular smooth muscle cell (SMC) proliferation by stimulating polyamine synthesis, we examined whether the transcellular transport of L-ornithine, the cationic amino acid precursor of polyamines, could regulate the mitogenic response of PDGF. Treatment of SMC with PDGF stimulated DNA and putrescine synthesis, and this was enhanced further by increasing the extracellular concentration of L-ornithine. The potentiating effect of L-ornithine was reversed by the competitive inhibitor of cationic amino acid transport, methyl-L-arginine, or by preventing putrescine formation with alpha-difluoromethylornithine. Cationic amino acid uptake by SMC was Na+-independent and was mediated by both a high and low affinity carrier system. Treatment of SMC with PDGF initially (0-2 h) decreased basic amino acid transport, while longer exposures (6-24 h) progressively increased uptake. Kinetic studies indicated that PDGF-induced inhibition was associated with a decrease in affinity for cationic amino acids, while the stimulation was mediated by an increase in transport capacity. Endogenous PDGF released by collagen-activated platelets likewise up-regulated cationic amino acid transport in SMC. Reverse transcriptase-polymerase chain reaction detected the presence of mRNA encoding two distinct cationic amino acid transporter (CAT) proteins, CAT-1 and CAT-2B. Treatment of SMC with PDGF strongly induced the expression CAT-2B mRNA and modestly elevated the level of CAT-1 mRNA. These results demonstrate that PDGF-induced polyamine synthesis and SMC mitogenesis are dependent on the transcellular transport of L-ornithine. The capacity of PDGF to up-regulate the transport of L-ornithine by inducing the expression of the genes for CAT-1 and CAT-2B may modulate its mitogenic effect by providing SMC with the necessary intracellular precursor for polyamine biosynthesis.

Arginine and ornithine metabolizing enzymes in testosterone-induced hypertrophic mouse kidney

Administration of testosterone to female mice causes hypertrophy of their kidneys with spectacular induction of ornithine decarboxylase and significant increase in the level of putrescine. We tried to find out whether testosterone treatment affects also the renal activities of enzymes participating in the formation and utilization of ornithine, specifically arginase and ornithine aminotransferase, and whether they are dependent on putrescine level. Swiss, CFW, DBA2 or F1 (CFW x DBA2) female and male mice were injected with testosterone (125 mg/kg) or CB 3717 (100 mg/kg). DFMO was applied in the drinking water. The activities of the enzymes were determined 24 hr or 5 days after administration of CB 3717 or testosterone, respectively. Renal activities of ornithine decarboxylase (ODC), arginase and ornithine aminotransferase (OAT) were found to be sex-differentiated. The highest activity of ODC was characteristic for the kidneys of males, whereas those of arginase and OAT for the kidneys of females. In the kidneys of testosterone-treated female mice a decrease (50%) of OAT, and a significant increase of arginase activities (up to 200%), were observed. In the males these changes were less pronounced. DFMO, which completely inhibited the activity of renal ODC, did not influence significantly the testosterone-induced arginase and the testosterone-decreased OAT. Arginase and OAT, in contrast to ODC, were not changed in CB 3717-induced hyperplastic kidney. The study showed testosterone-induced differential changes in the activity of two enzymes involved in ornithine biosynthesis and catabolism which accompanied ODC induction in female mouse kidney.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective putrescine export is regulated by insulin and ornithine in Reuber H35 hepatoma cells

Cultured Reuber H35 rat hepatoma cells under highly viable serum-free conditions were found to selectively export putrescine from inside the cell into the culture medium, but not spermidine, spermine, or their acetylated derivatives. Even untreated cells, with very low intracellular putrescine levels, constitutively exported significant amounts of only putrescine for a 12 h period. Administration of the phorbol ester TPA (12-O-tetradecanoylphorbol 13-acetate) which markedly elevates ornithine decarboxylase (ODC), did not potentiate putrescine export over what was measured in the unstimulated cultures. However, addition of 1 mM ornithine to the cultures resulted in increased intracellular putrescine (maximum at 4 h) with a marked concomitant increase in putrescine export between 0 and 8 h, after which putrescine export again stopped. Treatment with 10(-7) M insulin yielded intracellular putrescine levels that remained elevated for 36 along with a continuous and more rapid export of putrescine over the same 36 h time period. When insulin and ornithine were administered together, even higher levels of intracellular putrescine and putrescine export were observed, with putrescine efflux proceeding over the 36 h time-course at the highest observed rates of 1.5 (0-12 h) and 1.0 (12-36 h) nmol/mg total protein per h. Exposure to DFMO, an inhibitor of ODC, depleted intracellular putrescine stores and effectively suppressed putrescine export. There was not a positive correlation between the time-dependent decreases in the intracellular putrescine concentrations and the respective alterations in the rate of putrescine export under a variety of conditions. Furthermore, the drug verapamil was capable of completely inhibiting putrescine export (IC50 approx. 1 microM) without any change in the level of intracellular putrescine. This data was not consistent with the involvement of simple diffusion of putrescine through the membrane as the major mechanism for putrescine export. The potential mechanisms involved in putrescine export and the role of this process in regulating intracellular polyamine levels, as well as, possible functions of extracellular putrescine are discussed.

The effects of O6-benzylguanine and hypoxia on the cytotoxicity of 1,3-bis(2-chloroethyl)-1-nitrosourea in nitrosourea-resistant SF-763 cells

O6-Alkylguanine-DNA alkyltransferase (AGT) activity is associated with resistance of brain tumor cell lines to the cytotoxic effects of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). SF-763 cells exhibit high AGT activity and are resistant to BCNU. In this study, we compared the effects of the AGT inhibitor O6-benzylguanine (BG) on the cytotoxicity of BCNU in oxic and hypoxic SF-763 cells; we also measured AGT activity, ornithine decarboxylase (ODC) activity, and polyamine levels to determine if there was any correlation with cell survival as determined by colony-forming efficiency assay. Exponentially growing monolayer cells were pretreated with 10 microM BG for 2 h under oxic or hypoxic (95% nitrogen/5% CO2) conditions and then exposed to graded concentrations of BCNU for 1 h. BG significantly lowered AGT activity but had no cytotoxic effect in oxic or hypoxic cells; hypoxia alone was not cytotoxic. The cytotoxicity of BCNU was 4 times higher in BG-treated hypoxic cells than in oxic cells treated with BCNU alone; the BCNU doses required for a 1-log cell kill were 75 and 300 microM, respectively. ODC activity was lowered by hypoxia alone but was not significantly affected by BG in either hypoxic or oxic cells. Polyamine levels were not significantly affected by hypoxia or BG. These results indicate that pretreatment with BG dramatically lowers AGT activity and increases the cytotoxicity of BCNU in both oxic and hypoxic SF-763 cells. The mechanism of this enhanced cytotoxicity is apparently unrelated to ODC activity or polyamine levels.

Persistence of the circadian variation and altered response to hepatectomy of hepatic ornithine decarboxylase activity with malignant tumor burden

We measured the effect of MC-26 mouse colon cancers (of different sizes) on the circadian rhythm of hepatic ornithine decarboxylase (ODC) activity and hepatic ODC activity during the 24 hr after 60% hepatectomy. Tumor-free control mice showed a normal circadian rhythm of ODC activity with the highest levels at 1100 hr and the lowest levels at 2300 hr. The amplitude of the rhythm was diminished significantly in mice with a large tumor burden (3% of their body weight), and hepatic ODC activity was significantly less than in the tumor-free mice at every point during the 24 hr of the study. In mice with "early" tumors (0.3% of body weight), basal activity of ODC was normal and there was no reactive increase in activity following hepatectomy. In contrast, mice with "late" (3% of body weight) tumors had significantly lower basal ODC activities and the increase in ODC activity following hepatectomy was prolonged and exaggerated. We concluded that tumor burden is associated with abnormal ODC activity and that these differences are exaggerated after hepatectomy. Furthermore, although average ODC concentrations in tumor-bearing mice fell precipitously, the circadian rhythm in hepatic ODC persisted. This finding indicates early recognition by the host of tumor presence, which has a profound negative regulatory effect on hepatic ODC. Apparently, this effect does not impinge on circadian control mechanisms, indicating that these signals act independently.

The level of substrate ornithine can alter polyamine-dependent DNA synthesis following phorbolester stimulation of cultured hepatoma cells

Although the precise intracellular function(s) of the polyamines remain incompletely defined, a myraid of evidence now shows that the polyamines must accumulate or be maintained at a specific intracellular concentration in order for all mammalian cells to grow or divide. The initial step in polyamine biosynthesis normally involves the decarboxylation of ornithine by the enzyme ornithine decarboxylase (ODCase E.C. 4.1.1.17) to yield putrescine. Increases in the steady-state level of intracellular ornithine have been reported to markedly alter the accumulation of the polyamines following stimulation of Reuber H35 Hepatoma cells with 12-O-tetradecanoylphorbol-beta-acetate (TPA) in the presence of serum (Wu and Byus: (Biochem. Biophys. Acta 804:89-99, 1984); Wu et al.: (Cancer Res. 41:3384-3391, 1981). We wished to determine whether or not incubation of H35 hepatoma cells with exogenous ornithine would result in a stimulation of DNA synthesis following treatment with the mitogens TPA and insulin. For these studies, H35 cells were maintained under serum-free conditions for 2-3 days in order to obtain synchronous cultures suitable for analysis of the level of DNA synthesis. Cultures treated in this manner were highly viable, maintained similar growth rates, and possessed the equivalent levels of intracellular ornithine and polyamines as the serum-containing cultures. Arginine levels, however, were approximately twofold higher following culture under serum-restricted conditions for 3 days. The addition of exogenous ornithine (0.5 mM) was accompanied by a 4-5-fold increase in intracellular steady-state ornithine levels and by a 6-8-fold increase in the presence of TPA and ornithine. In a manner identical to the serum-containing cultures (Wu and Byus (1984] the addition of TPA and exogenous ornithine to the serum-free cells caused a dose-dependent increase in intracellular putrescine (up to 5-fold) and a concomitant decrease in ODC activity in comparison to stimulation with TPA alone. The addition of TPA led to a 3-5-fold increase in the incorporation of tritiated thymidine into DNA. In the presence of exogenous ornithine, TPA-induced DNA synthesis was further stimulated more than twofold in a dose-dependent manner. Insulin (10(-10)-10(-8) M) proved to be more efficacious as a mitogen in the H35 cells and led to greater stimulation of DNA synthesis than TPA. Insulin alone also resulted in a higher steady-state level of ornithine and putrescine in comparison with TPA alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Ornithine aminotransferase activity, tissue ornithine concentrations and polyamine metabolism

1. Inactivation of L-ornithine:2-oxoacid aminotransferase (OAT) by 5-fluoromethylornithine (5FMOrn), a specific inactivator of OAT, causes a great elevation of tissue ornithine (Orn) concentrations. 2. Inhibition of L-ornithine decarboxylase (ODC) by 2-difluoromethylornithine (DFMO) had no effect on Orn concentrations. 3. The combined administration of 5FMOrn and DFMO produced a 2- to 3-fold greater enhancement of tissue Orn concentrations than treatment with 5FMOrn alone. 4. The increase of tissue Orn concentrations had a long-lasting enhancing effect on polyamine metabolism. 5. In the brain this could be demonstrated by the elevation of putrescine and spermidine concentrations and the increase of spermidine turnover rate. 6. In visceral organs polyamine concentrations were not elevated because polyamines can be eliminated by transport. 7. In line with this notion is the fact that urinary polyamine excretion was increased for several days, even after a single dose of 5FMOrn. 8. Inhibitors of 4-aminobutyric acid:2-oxoglutarate aminotransferase which are also inactivators of OAT had the same effect on polyamine excretion as 5FMOrn.