Consequences of aberrant ornithine decarboxylase regulation in rat hepatoma cells

DH23A cells, an α-difluoromethylornithine (DFMO)–resistant variant of rat hepatoma tissue culture cells (HTC), contain high levels of very stable ornithine decarboxylase (ODC). In the absence of DFMO, the high ODC activity results in a large accumulation of endogenous putrescine. Concomitant with the putrescine increase is a period of cytostasis and a subsequent loss of viable cells. In contrast, HTC cells with a moderate polyamine content can be maintained in exponential growth. This suggests that a moderate polyamine concentration is necessary for both optimal cell growth and survival. The cytoxicity observed in the DH23A cells is apparently not due to byproducts of polyamine oxidation or alterations in steady state intracellular pH or free [Ca²⁺]. It is possible to mimic the effects of high levels of stable ODC by treatment of cells with exogenous putrescine in the presence of DFMO. This suggests that overaccumulation of putrescine is the causative agent in the observed cytotoxicity, although the mechanism is unclear. These data support the hypothesis that downregulation of ODC may be necessary to prevent accumulation of cytotoxic concentrations of the polyamines. © 1994 Wiley-Liss, Inc.     

Effects of partial and sham hepatectomy on ornithine decarboxylase and thymidine kinase activities and mRNA contents

Ornithine decarboxylase and thymidine kinase are enzymes that increase in activity in regenerating liver. We found that both activities and mRNA levels for these enzymes increase significantly after 70% partial hepatectomy in the rat. After sham hepatectomy (laparotomy) there were significant decreases in activity; however, mRNA content was unaltered. Similar decreases in enzyme activity, without changes in mRNA content, were found with pair-feeding, and additional decreases in activity after starvation. In contrast to previous reports of no change in ornithine decarboxylase and thymidine kinase after sham hepatectomy, the present results indicate that decreases occur. This may be mediated by the decrease in food intake after surgery. Dietary factors may be important in the physiologic regulation of these enzymes in the liver.     

Involvement of protein kinase C in the regulation of ornithine decarboxylase mRNA by phorbol esters in rat hepatoma cells

The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulates a rapid increase in ornithine decarboxylase (EC 4.1.1.17; ODC) activity in target cells. Here we demonstrate that this process involves a rapid accumulation of ODC mRNA, which is maximal 3 h after treatment (three- to eightfold greater than control cells) and decays to control levels within 18 h. Stimulation of ODC mRNA by TPA is blocked by phorbol dibutyrate down-regulation of protein kinase C (PKC). ODC mRNA was also induced by the PKC activators, phospholipase C and 1-oleoyl-2-acetyl-rac-glycerol, and blocked by kinase inhibitors (trifluoroperazine, H7, and palmitoyl-L-carnitine), consistent with a requirement for PKC activation in the induction mechanism. However, the non-PKC-specific protein kinase inhibitor HA1004 also suppressed expression of ODC mRNA in response to TPA, under conditions where it did not inhibit PKC, suggesting that additional kinases may be involved in the intracellular signalling process. The stability of the ODC mRNA (control value = 6.2 +/- 1.6 h) is not significantly changed by either TPA (5.7 +/- 0.8 h) or by cycloheximide (6.0 h). These results are inconsistent with any contribution from altered mRNA half-life towards the accumulation of ODC mRNA following treatment with phorbol ester tumor promoters.     

Difluoromethylornithine-induced amplification of ornithine decarboxylase genes in Ehrlich ascites carcinoma cells

Stepwise increments of the concentration of 2-difluoromethylornithine, a mechanism-based irreversible inhibitor of mammalian ornithine decarboxylase (EC 4.1.1.17), resulted in a selection of cultured Ehrlich ascites carcinoma cells capable of growing in the presence of up to 50 mM difluoromethylornithine. Dialyzed extracts of drug-resistant tumor cells exhibited a very high ornithine decarboxylase activity and contained large excess of immunoreactive ornithine decarboxylase protein. Hybridization analyses with cloned complementary DNA revealed that the difluoromethylornithine-resistant tumor cells also expressed mRNA of the enzyme at greatly enhanced rate. The overproduction of ornithine decarboxylase by the tumor cells grown under the pressure of difluoromethylornithine was at least partly attributable to a 10 to 20-fold increase in the total gene dosage of ornithine decarboxylase involving an amplification of several genes of the gene family. The gene amplification developed appeared to be stable, as the gene dosage only slowly (during a period of several months) returned towards the normal level upon the removal of difluoromethylornithine. The overproduction of ornithine decarboxylase was accompanied by an enhanced resistance of the enzyme towards difluoromethylornithine in vitro.     

Polyamine regulation of ornithine decarboxylase and its antizyme in intestinal epithelial cells

Ornithine decarboxylase (ODC) is feedback regulated by polyamines. ODC antizyme mediates this process by forming a complex with ODC and enhancing its degradation. It has been reported that polyamines induce ODC antizyme and inhibit ODC activity. Since exogenous polyamines can be converted to each other after they are taken up into cells, we used an inhibitor of S-adenosylmethionine decarboxylase, diethylglyoxal bis(guanylhydrazone) (DEGBG), to block the synthesis of spermidine and spermine from putrescine and investigated the specific roles of individual polyamines in the regulation of ODC in intestinal epithelial crypt (IEC-6) cells. We found that putrescine, spermidine, and spermine inhibited ODC activity stimulated by serum to 85, 46, and 0% of control, respectively, in the presence of DEGBG. ODC activity increased in DEGBG-treated cells, despite high intracellular putrescine levels. Although exogenous spermidine and spermine reduced ODC activity of DEGBG-treated cells close to control levels, spermine was more effective than spermidine. Exogenous putrescine was much less effective in inducing antizyme than spermidine or spermine. High putrescine levels in DEGBG-treated cells did not induce ODC antizyme when intracellular spermidine and spermine levels were low. The decay of ODC activity and reduction of ODC protein levels were not accompanied by induction of antizyme in the presence of DEGBG. Our results indicate that spermine is the most, and putrescine the least, effective polyamine in regulating ODC activity, and upregulation of antizyme is not required for the degradation of ODC protein.     

Differentiation associated changes in thymidylate synthetase and thymidine kinase activities in intestinal cells.

Proliferative and mature intestinal cells of the jejunum and colon of rat, colon of man, and the surface cells of neoplastic colon lesions of man were assayed for thymidylate synthetase and thymidine kinase activities. Cells from the proliferative region of rat jejunal mucosa were found to have higher enzyme activities than cells from the non-proliferative region. Thymidylate synthetase activity was observed to decrease as cells migrated from base to upper crypt, whereas thymidine kinase activity increased during crypt migration and then declined as cells migrated onto villi. Thymidine kinase activity also remained elevated longer than thymidylate synthetase during cell migration in colonic mucosa of rat and man. High thymidine kinase: thymidylate synthetase ratios similar to those observed in flat mucosa before cells become fully mature were found in cells removed from expanding neoplastic lesions of man.     

Structural basis for the dual thymidine and thymidylate kinase activity of herpes thymidine kinases

Crystal structures of equine herpesvirus type-4 thymidine kinase (EHV4-TK) in complex with (i). thymidine and ADP, (ii). thymidine and SO(4) and the bisubstrate analogs, (iii). TP(4)A, and (iv). TP(5)A have been solved. Additionally, the structure of herpes simplex virus type-1 thymidine kinase (HSV1-TK) in complex with TP(5)A has been determined. These are the first structures of nucleoside kinases revealing conformational transitions upon binding of bisubstrate analogs. The structural basis for the dual thymidine and thymidylate kinase activity of these TKs is elucidated. While the active sites of HSV1-TK and EHV4-TK resemble one another, notable differences are observed in the Lid regions and in the way the enzymes bind the base of the phosphoryl-acceptor. The latter difference could partly explain the higher activity of EHV4-TK toward the prodrug ganciclovir.     

Similarities in ornithine decarboxylase regulation in intact and enucleated 3T3 cells

The role of the cytoplasm in the regulation of ornithine decarboxylase (ODC) has been examined in enucleated 3T3 cells (cytoplasts). ODC activity can be increased 15–25-fold by a cytoplasmic mechanism(s) in enucleates prepared from growing cells by treatment which lead to 50–75-fold increases in intact growing cells. Since the enzyme activity simultaneously becomes less stable in these cytoplasts as in whole cells, it appears this increase is due either to activation of pre-existing enzyme or increased synthesis. A biphasic increase during the first 20 h after stimulus is seen in cytoplasts prepared from growing cells which have been stimulated for the prior 5 h. The second increase in activity does not appear to be due to decreased degradation or conversion to a more active form. These results are analogous to those reported previously for intact growing cells in experiments which employed metabolic inhibitors, and similarly suggest that there is cytoplasmic control of ODC synthesis. Medium polyamines reduce ODC activity in cytoplasts with kinetics and characteristics similar to those previously reported for intact cells. These data are also most consistent with regulation of synthesis at the translational level.     

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

We investigated the ability of intracellular ornithine to alter both the biosynthesis of putrescine and the activity of ornithine decarboxylase in Reuber H35 hepatoma cells in culture incubated with 12-O-tetradecanoylphorbol 13-acetate (TPA). In confluent cultures of H35 cells, the addition of TPA (1.6 μM) caused the activity of ornithine decarboxylase to increase by more than 100-fold within 4 h. When exogenous ornithine (0.1–1.0 mM) was added to the culture medium with TPA, a marked dose-dependent increase in the production of putrescine was observed. The activity of ornithine decarboxylase in the same cultures incubated with ornithine decreased in a similar dose-dependent manner. The addition of arginine (0.1–1.0 mM) (but not lysine or histidine) to the H35 cells in culture concomitant with TPA also led to a relative increase in putrescine biosynthesis and a decrease in ornithine decarboxylase activity compared to cultures not receiving the amino acids. A similar response to exogenous ornithine and TPA was observed in a series of less confluent rapidly growing cultures which were in culture for a shorter period of time. The confluent cultures possessed a basal level of arginase (55 units/mg protein) which increased approx. 2-fold upon treatment with TPA. The intracellular concentration of ornithine in the unstimulated cells was in the order of 0.02–0.03 mM. Upon incubation of the cells with exogenous ornithine or arginine, the intracellular pools of these amino acids increased 4- to 8-fold.     

Sequential changes in ornithine decarboxylase thymidine kinase, and other enzyme activities in regenerating liver in rats on controlled feeding schedules.

The changes in activity of five enzymes including ornithine decarboxylase (ODC), tyrosine aminotransferase (TAT), thymidine kinase (TK), ornithine aminotransferase (OAT) and serine dehydratase (SDH) in the early stage of the regenerating rat liver have been studied under a controlled feeding and lighting schedule. The first three enzyme activities were stimulated sequentially by partial hepatectomy. The earliest response was observed in ODC activity. A significant increase in this enzyme activity was observed at 2 hrs and the maximal level was at 4 hrs after the operation. TAT began to increase at 4 hrs and the maximal level was at 8 hrs. The TK activity was induced at about 24 hrs and the highest value was at 48 hrs after partial hepatectomy.A significant decrease in OAT activity was observed at 24 hrs after the operation and subsequently. Although a decrease in SDH activity was also observed this decrease did not seem to correlate directly with the regeneration process, since a lowered level of the enzyme activity was also found in the sham operated group.     

Nucleotide sequence reveals overlap between T4 phage genes encoding dihydrofolate reductase and thymidylate synthase

We have determined the nucleotide sequence of a 1075-base-pair HindIII fragment of the T4 phage genome. This fragment contains the structural gene (frd) for dihydrofolate reductase and part of the gene (td) encoding thymidylate synthase. The fragment contains a 579-base-pair open reading frame, encoding a 193-residue polypeptide with a calculated mass of 21,603 Da, in agreement with our reported subunit molecular mass of 23,000. The deduced amino acid sequence shows partial homology with other dihydrofolate reductases, with most of the identities lying in regions known to be involved in substrate binding and catalysis. The 3' end of the coding strand overlaps the coding region for thymidylate synthase; the sequence - ATGA -includes an opal terminator for the frd gene and an initiating triplet for the td gene. The deduced amino acid sequence from this initiating ATG is identical, for the first 20 residues, with the NH2-terminal 20 residues reported for the td protein (M. Belfort , A. Moelleken , G. F. Maley , and F. Maley (1983) J. Biol. Chem. 258, 2045-2051). The sequenced HindIII fragment was transferred into a high expression plasmid vector for large scale production of homogeneous T4 dihydrofolate reductase. The experimentally determined sequence of 20 residues at the NH2-terminus of this protein is identical with that deduced from the nucleotide sequence for T4 dihydrofolate reductase.     

Phosphorylation of ornithine decarboxylase by casein kinase II from RAW264 cells

Casein kinase II and ornithine decarboxylase were purified from a virally-transformed macrophage-like cell line, RAW264. The addition of casein kinase II to a reaction mixture containing [tau-32P]GTP, Mg++, and ornithine decarboxylase led to the phosphorylation of a 55,000 dalton protein band in the purified preparation of ornithine decarboxylase. Stoichiometric estimates indicated that casein kinase II incorporated 0.15 mole of phosphate per mole of ornithine decarboxylase, which was increased to 0.3 mole/per mole in the presence of spermine. The apparent Km and Vmax values for the casein kinase II-mediated phosphorylation of ornithine decarboxylase were 0.36 microM and 62.5 nmol/min./mg kinase. The addition of spermine to the reaction did not alter the Km but increased the Vmax to 100 nmol/min./mg kinase. The phosphorylation of ornithine decarboxylase by casein kinase II affected neither the rate of maximal ornithine decarboxylase activity nor the affinity of the enzyme for ornithine.     

Close relation between fetal thymidine kinase and thymidylate kinase activities in breast cancer

In human breast cancers, assays on thymidine kinase activity revealed the synthesis of large amounts of d-TTP. This fact suggested the presence of thymidylate kinase closely associated with thymidine kinase. Results obtained with experimental tumors were quite different. These tumors appeared inadequate for the study on thymidine metabolism in mammary cancers.     

Alterations in creatine kinase, ornithine decarboxylase, and transglutaminase during muscle regeneration

Creatine kinase (CK), transglutaminase (TGase) and ornithine decarboxylase (ODC), enzymes implicated in the regulation of growth processes, were studied during muscle regeneration subsequent to the injection of bupivacaine into rat tibialis anterior. Within 2 days, the percent BB isozyme of CK detected in the muscle was elevated 70-fold coincident with a marked decrease in total CK activity. The MB isozyme also increased and was 15-fold of control at 4-5 days postinjection. TGase activity was increased significantly to greater than 2-fold of control within 2 days of injection and significantly decreased at days 3 through 7 compared to controls. ODC activity was elevated significantly to 2- to 3-fold of control from 2-7 days after injection. These results suggest an early alteration in the expression of a coordinated battery of genes in this model of muscle degeneration-regeneration. The increased expression of MB and BB isozymes of CK in various human neuromuscular diseases may be a manifestation of an ongoing process of degeneration-regeneration.