Cell type-specific mechanisms of regulating expression of the ornithine decarboxylase gene after growth stimulation

Rapid differentiating colorectal cancer and colorectal polyp using dried blood spot mass spectrometry metabolomic approach

Colorectal cancer (CRC) is the third leading causes of cancer mortality, and the early-stage detection could significantly enhance survival rates. Cancer influences the important metabolic pathways and the changes in metabolite levels had been used in many studies as the potential biomarkers. This study is aimed at screening metabolite biomarkers with CRC diagnosis potentials. The direct infusion mass spectrometry (MS) metabolomic analysis based on dried blood spot was used to distinguish CRC from polyp. The target metabolites were composed of 23 amino acids and 26 acylcarnitines. The 21 metabolites in blood were selected via multivariate analysis. A regression model was established based on parameters C16, Arg, C4/C8, C5/C3, Val, Phe/Tyr, Ala, C4/C3. Tenfold cross validation (CV) method was used to test this model and showed sensitivity of 81.18% and specificity of 83.95%. The metabolomic analysis is a practicable method for CRC detection. The use of direct MS analysis in metabolite screening could be finished in several minutes and served as a higher-throughput method to distinguish CRC and polyps. © 2017 IUBMB Life, 69(5):347-354, 2017.

Inhibition of 12-O-tetradecanoylphorbol-13-acetate-induced inflammatory skin edema and ornithine decarboxylase activity by theaflavin-3,3'-digallate in mouse

Among black tea polyphenols, theaflavins were generally considered to be the most effective in cancer chemoprevention. In this study, we examined the inhibitory effects of black tea polyphenols, including theaflavin (TF-1), a mixture (TF-2) of theaflavin-3-gallate and theaflavin-3'-gallate, theaflavin-3,3'-digallate (TF-3), and the green tea polyphenol (-)-epigallocatechin-3-gallate (EGCG) on 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced edema and ornithine decarboxylase (ODC) activity. Topical application of these polyphenols onto the mouse resulted in inhibition of TPA-induced ear edema and skin epidermal ODC activity. The inhibitory order was as follows: TF-3 > TF-2 approximately equal to EGCG > TF-1. Western and Northern blots indicated that TF-3 significantly reduced the protein and mRNA levels of ODC in TPA-treated mouse skin and NIH 3T3 cells, whereas EGCG showed less activity. EGCG and TF-3 were able to inhibit the ODC enzyme activity in vitro. Furthermore, TF-3 also significantly reduced the basal promoter activity of the ODC gene in NIH 3T3 cells that were transiently transfected with ODC reporter plasmid. These results suggested that TF-3 was a potential inhibitor of ODC activity and TPA-induced edema and might be effective in cancer chemoprevention.

Expression of intestinal ornithine decarboxylase during postnatal development in neonatal rats

Ornithine decarboxylase (ODC) has been shown to play an essential role in intestinal growth and maturation in rats. However, the regulatory mechanisms have not been fully elucidated. We studied the mechanisms of expression of intestinal ODC during postnatal development. Rat small intestinal mucosa was obtained from postnatal days 10, 15, 17, 19, 21, 24 and 30. Intestinal mucosa was assayed for ODC and sucrase activities. In addition, intestinal ODC mRNA, and ODC protein levels were also measured. The results showed that the intestinal sucrase activity was low before postnatal day 19. The sucrase activity then increased steadily from day 19 up to day 30. Intestinal ODC activities remained low from postnatal day 10 to day 17. A sharp increase in ODC activity was noted on day 19, which peaked on day 24 (a 20-fold increase from its low basal level) and declined on day 30. Intestinal ODC proteins followed the same pattern of postnatal expression as that of ODC activity. In contrast, ODC mRNA did not show significant change throughout the study period. The possible mechanisms by which intestinal ODC mRNA levels remain practically unchanged during postnatal development are discussed. We conclude that the ontogenic increase in sucrase activity, a marker for intestinal maturation, occurs at the same time to that of the induction of ODC activity. We also suggest that the induction of intestinal ODC activity during postnatal development is the result of post-transcriptional events or other cellular mechanisms. A better understanding of the regulation of polyamine biosynthesis during postnatal development of the small intestine will provide insights contributing to the maturation of the small intestine.

Antizyme inhibitor is rapidly induced in growth-stimulated mouse fibroblasts and releases ornithine decarboxylase from antizyme suppression

Ornithine decarboxylase (ODC) catalyses the first step in the synthesis of the polyamines putrescine, spermidine and spermine. The polyamines are essential for cell growth, but at elevated levels they may be tumorigenic, toxic, or may induce apoptosis. Therefore, ODC activity is highly regulated. It is induced when cells are stimulated to grow, and it is subjected to feedback inhibition by the polyamines. By causing ribosomal frameshifting, polyamines induce the synthesis of antizyme, a 23-kDa protein, which binds to ODC, inhibits its activity and promotes its degradation by the 26 S proteasome. Antizyme, in turn, is inhibited by antizyme inhibitor (AZI). We describe the cloning of a mouse AZI cDNA, encoding a protein with high homology to mouse ODC. Using purified recombinant proteins, we show that AZI (which has no ODC activity) can release enzymically active ODC from antizyme suppression in vitro. We also show that ODC reactivation takes place in mouse fibroblasts upon transient transfection with an AZI-expressing plasmid construct. Finally we demonstrate that the AZI mRNA content of mouse fibroblasts increases significantly within an hour of growth stimulation, i.e. much earlier than ODC transcripts. Our results indicate that induction of AZI synthesis may represent a means of rescuing ODC molecules that have been inactivated and tagged for degradation by antizyme, when culture conditions improve and polyamine production is needed for cell growth and proliferation.

A binding site for the transcription factor Grainyhead/Nuclear transcription factor-1 contributes to regulation of the Drosophila proliferating cell nuclear antigen gene promoter

The Drosophila proliferating cell nuclear antigen promoter contains multiple transcriptional regulatory elements, including upstream regulatory element (URE), DNA replication-related element, E2F recognition sites, and three common regulatory factor for DNA replication and DNA replication-related element-binding factor genes recognition sites. In nuclear extracts of Drosophila embryos, we detected a protein factor, the URE-binding factor (UREF), that recognizes the nucleotide sequence 5'-AAACCAGTTGGCA located within URE. Analyses in Drosophila Kc cells and transgenic flies revealed that the UREF-binding site plays an important role in promoter activity both in cultured cells and in living flies. A yeast one-hybrid screen using URE as a bait allowed isolation of a cDNA encoding a transcription factor, Grainyhead/nuclear transcription factor-1 (GRH/NTF-1). The nucleotide sequence required for binding to GRH was indistinguishable from that for UREF detected in embryo nuclear extracts. Furthermore, a specific antibody to GRH reacted with UREF in embryo nuclear extracts. From these results we conclude that GRH is identical to UREF. Although GRH has been thought to be involved in regulation of differentiation-related genes, this study demonstrates, for the first time, involvement of a GRH-binding site in regulation of the DNA replication-related proliferating cell nuclear antigen gene.

Role of genomic mechanisms on cAMP-dependent positive inotropism in isolated left atrium of rat

It is well known that beta-adrenoceptor stimulation induces positive inotropism by cAMP-dependent phosphorylation of cardiac calcium channels. Furthermore, hypertrophy of different tissues including the heart have been related to the stimulation of these adrenoceptors via mechanisms coupled to activation of transcription and protein synthesis. Early effects of isoproterenol mediated via this pathway has also been associated to the stimulation of beta-adrenoceptors. However, the effects on the inotropism through genomic mechanisms have not yet been described. Isoproterenol (3 nM to 3 microM) induced a concentration-dependent positive inotropism, in isolated left atrium of male Wistar rats electrically stimulated (0.5 Hz, 5 ms, 30-50% above the threshold voltage), which was antagonized by atenolol (1 microM) and inhibited by a protein kinase A inhibitor, (R)p-cAMPS (10 microM). The inhibitor of transcription, actinomycin D (4 microM), and the protein synthesis inhibitor, cycloheximide (35.5 microM), significantly decreased the positive inotropism induced by isoproterenol. Forskolin (0.1 to 3 microM), an activator of adenylyl cyclase, induced a concentration-dependent positive inotropism which was also inhibited by (R)p-cAMPS, actinomycin D and cycloheximide. In the left atrium of rat, isoproterenol induced a positive inotropism which seems, at least in part, dependent upon intact transcription and protein synthesis, as suggested by the fact that the response was inhibited by the incubation with actinomycin D and cycloheximide. In addition, this genomic effect seems to be mediated by a cAMP-dependent mechanism. As it was inhibited by a protein kinase A inhibitor ((R)p-cAMPS) and similarly to isoproterenol, the positive inotropism induced by forskolin, which increases cytosolic cAMP, was also inhibited by actinomycin D and cycloheximide.

Effect of thyroxine on pancreatic digestive enzymes and ornithine decarboxylase gene expression in neonatal rats

BACKGROUND

Thyroxine has been shown to play a role in the development of exocrine pancreatic enzymes in neonatal rats.

METHODS

To further evaluate the regulatory mechanisms for thyroxine in pancreatic development, we examined the changes in the expression of pancreatic enzymes (amylase and trypsinogen) and ornithine decarboxylase (ODC) genes following daily injection of thyroxine for 5 and 10 days to neonatal rats (5 days old).

RESULTS

Total pancreatic proteins and DNA contents as well as the activity of ODC and exocrine enzymes were significantly increased after 5 and 10 days of thyroxine treatment. These increases were associated with parallel alterations (to three to fourfold rise) in steady-state mRNA levels of both amylase and trypsinogen. In contrast, thyroxine only produced a 57-68% increase in steady-state ODC mRNA levels.

CONCLUSIONS

These data suggest that thyroxine stimulated the express of amylase and trypsinogen genes partly due to increased transcriptional rate and/ or decreased mRNA turnover. Thyroxine also stimulated ODC gene expression. However, the stimulatory mechanisms may involve transnational or posttranslational regulation of ODC and are independent of thyroxine effects.

Localization of the 12-O-tetradecanoylphorbol-13-acetate response of the human ornithine decarboxylase promoter to the TATA box

In a previous study, we narrowed the region of the human ornithine decarboxylase (ODC) promoter responsive to 12-O-tetradecanoylphorbol-13-acetate (TPA) to nt -42 to +54 around the transcription initiation site (Kim YJ, Pan H, Verma AK, Mol Carcinog 10:169-179, 1994). Here we report defining the role of the TATA box in TPA-induced transcription from the -42/+54 ODC promoter fragment. A transversion mutation at the third position of the TATA box (TATAAGT-->TAAAAGT) reduced TPA responsiveness of the reporter construct -42/+54 ODC-Luc by 49%. Electrophoretic mobility shift assays (EMSAs) using HeLa cell nuclear protein extracts revealed no differences in the binding pattern between the natural -42/+54 ODC promoter element and the -42/+54 ODC promoter element containing the T-->A mutation. However, antibodies to the general transcription factor TFIIB disrupted the DNA-protein complexes normally formed with the -42/+54 ODC promoter element in EMSAs. A consensus TATA box oligonucleotide formed two bands, with the faster mobility band displaying enhanced binding with nuclear protein extracts from TPA treated cells. Furthermore, incubation of HeLa cell nuclear extracts with an oligonucleotide containing the ODC TATA box also caused formation of two specific bands in EMSA. Both bands exhibited augmented binding to nuclear proteins from TPA-treated cells. Introduction of the T-->A transversion mutation in the ODC TATA oligonucleotide eliminated binding of the faster migrating band formed with the natural ODC TATA oligonucleotide. These results indicate that TPA modulation of the general transcription machinery may play a role in the TPA-activated transcription of the human ODC promoter.

Polyamine metabolism of enterocyte-like Caco-2 cells after exposure to Phaseolus vulgaris lectin

The effect of Phaseolus vulgaris isolectin E4 on polyamine concentrations and ornithine decarboxylase activity of proliferating and differentiating Caco-2 cells was investigated. Values of putrescine, spermidine, and spermine in control cells were highest during the early phase of proliferative cell growth and lowest in the stationary phase. Phytohaemagglutinin E4 significantly increased cellular polyamine values during the late proliferative phase of cell growth. Ornithine decarboxylase activity was high during intensive proliferation and growth, but was lower when proliferation slowed down or ceased. Exposure of Caco-2 cells in the early proliferative phase of cell growth to increasing concentrations of the potent intestinal growth factor phytohaemagglutinin E4 greatly stimulated enzyme activity. In contrast, the activity of ornithine decarboxylase was not stimulated in Caco-2 cells of the late proliferative phase nor was there any increase in the enzyme activity in differentiating and fully differentiated cells of the stationary phase. Accordingly, when proliferating Caco-2 cells possessed the highest ornithine decarboxylase activity, the polyamine values were also at their highest. During differentiation, as the ornithine decarboxylase activity fell close to zero, polyamine values also decreased. In the early proliferative phase of cell growth ornithine decarboxylase activity coincided with DNA synthesis in cells exposed to Phaseolus vulgaris isolectin E4. These findings with Caco-2 cells were similar to those found in brush border cells of the rat small intestine.

Characterization of novel phorbol ester- and serum-responsive sequences of the rat ornithine decarboxylase gene promoter

Ornithine decarboxylase (ODC), the key regulatory enzyme in mammalian polyamine biosynthesis, is rapidly induced by mitogens and tumor promoters. We used transient expression assays and DNA-protein binding studies to examine the regulation of ODC promoter activity by phorbol esters and serum growth factors. A fragment of the ODC 5' flanking region (nt-1156 to +13) was sufficient to confer 12-O-tetradecanoylphorbol-13-acetate (TPA)-responsive expression to a luciferase reporter gene when transfected into H35 cells. However, induction by TPA was not observed in Rat2 fibroblasts, although refeeding of serum-starved Rat2 cells with fresh serum-containing medium rapidly induced a fivefold to sixfold increase in ODC promoter activity, maximal about 8 h after refeeding. Deletion analysis demonstrated that several sequences contributed to basal ODC promoter activity but that nt -92 to +13 was sufficient for induction by TPA or by serum. This sequence lacked canonical TPA-responsive elements, and an activator protein-1 (AP-1) consensus oligonucleotide failed to compete effectively for proteins binding to this region. Two of four protein complexes observed by gel-shift analysis of nt -92 to +13 were competitively inhibited by wild-type but not mutant oligonucleotides encompassing a variant cyclic AMP-response element (CRE) (ODC nt -50 to -42); however, a consensus CRE did not compete. Mutagenesis of this site demonstrated that it contributes to basal expression of the ODC promoter but not to TPA or serum responsiveness. Thus, we conclude that the proximal ODC promoter (nt -92 to +13) responds to TPA and serum stimulation in a cell-type-specific manner that is not mediated by canonical AP-1 elements.

Regulation of ornithine decarboxylase during cell growth. Changes in the stability and translatability of the mRNA, and in the turnover of the protein

When Ehrlich ascites tumor cells were stimulated to grow, their ornithine decarboxylase (ODC) activity increased 20- to 30-fold. The increase in ODC mRNA content was one order of magnitude less during the corresponding period. Likewise, the subsequent changes in ODC activity failed to show proportionality to those of the ODC mRNA content. The changes in ODC activity were not attributable to changes in ODC turnover, even though the half-life of the enzyme decreased from 56 min during the period of increasing, to 36 min during the period of decreasing ODC activity. There was no evidence of an activation-inactivation-cycle for the enzyme. In view of these findings it appears that ODC mRNA alterations are amplified mainly at the translational level. The biphasic change in ODC mRNA content was partly attributable to a change in turnover of the message, as determined after inhibition of transcription with actinomycin D. Thus, the ODC mRNA half-life was estimated to decrease from 8.7 h during the period of increasing ODC activity to 4.0 h during the period of decreasing ODC activity. Despite the inhibition of transcription by actinomycin D, there was a marked superinduction of ODC activity. Our data demonstrate that the regulation of ODC expression is a complex phenomenon, involving controls at many levels.

Regulation of rat ornithine decarboxylase promoter activity by binding of transcription factor Sp1

Ornithine decarboxylase (ODC) is the rate-limiting enzyme of polyamine biosynthesis. We investigated the transcriptional regulation of the rat ODC gene using transient expression assays. The 5'-flanking region (-1156 to +13) of the ODC gene was sufficient to mediate strong basal expression of a luciferase reporter. Sequences between -345 and -93 contributed to basal promoter activity. This region, containing five potential Sp1 binding sites, was analyzed by electrophoretic mobility shift assays. Three specific DNA-protein complexes were identified using H35 nuclear extracts and the -345/-93 ODC probe. Binding to all three was eliminated by competition with an oligonucleotide containing an Sp1 binding site, but not by a mutant Sp1 oligonucleotide. Preincubation with an antibody against Sp1 supershifted complexes associated with one or more of Sp1 binding sites 1-4 as well as with site 5. DNase I footprinting revealed two protected regions: PR-I (-92 to -130) and PR-II (-304 to -332). PR-I contains a putative binding site for Sp1 that was protected by recombinant Sp1 protein. Transfection studies in Schneider SL2 cells demonstrated that the ODC promoter is transactivated up to 350-fold by Sp1 and that this transactivation is dependent on the presence of Sp1 binding sites 1-4. Thus, although the ODC promoter binds multiple nuclear proteins, Sp1 or a related protein appears to be a critical determinant of ODC transcription, possibly through cooperative interactions between Sp1 and additional transcription factors.

Regulation of human ornithine decarboxylase expression following prolonged quiescence: role for the c-Myc/Max protein complex

WI-38 cells can remain quiescent for long periods of time and still be induced to reenter the cell cycle by the addition of fresh serum. However, the longer these cells remain growth arrested, the more time they require to enter S phase. This prolongation of the prereplicative phase has been localized to a point early in G1, after the induction of "immediate early" G1 genes such as c-fos and c-jun but before maximal expression of "early" G1 genes such as ornithine decarboxylase (ODC). Understanding the molecular basis for ODC mRNA induction can therefore provide information about the molecular events which regulate the progression of cells out of long-term quiescence into G1 and subsequently into DNA synthesis. Studies utilizing electrophoretic mobility shift assays (EMSA) of nuclear extracts from short- and long-term quiescent WI-38 cells identified a region of the human ODC promoter at -491 bp to -474 bp which exhibited a protein binding pattern that correlated with the temporal pattern of ODC mRNA expression. The presence of a CACGTG element within this fragment, studies with antibodies against c-Myc and Max, the use of purified recombinant c-Myc protein in the mobility shift assay, and antisense studies suggest that these proteins can specifically bind this portion of the human ODC promoter in a manner consistent with growth-associated modulation of the expression of ODC and other early G1 genes following prolonged quiescence. These studies suggest a role for the c-Myc/Max protein complex in regulating events involved in the progression of cells out of long-term quiescence into G1 and subsequently into S.

Estradiol control of ornithine decarboxylase mRNA, enzyme activity, and polyamine levels in MCF-7 breast cancer cells: therapeutic implications

Previous studies have shown that natural polyamines-putrescine, spermidine, and spermine-play a key role in the mechanism of action of estrogens in breast cancer. Ornithine decarboxylase (ODC) is the first enzyme of the polyamine biosynthetic pathway. To examine estrogenic regulation of polyamine biosynthesis in breast cancer, we measured ODC mRNA, ODC activity, and polyamine levels in G1 synchronized MCF-7 cells. ODC mRNA and activity increased four-fold over that of cells in G1 phase between 8 to 16 h after the addition of estradiol. Polyamine levels showed a sharp increase by 8 h after the addition of estradiol and decreased by 12 h. We further examined whether synthetic homologs of putrescine or spermidine could replace natural polyamines in supporting MCF-7 cell growth. Treatment of MCF-7 cells with 1 mM difluoromethylornithine (DFMO), an inhibitor of ODC, suppressed putrescine, spermidine, and spermine levels by 74, 78, and 10%, respectively, within 48 h. Cells treated with DFMO for 48 h were supplemented with either putrescine or its homologs or spermidine or its homologs. Diaminopropane, diaminobutane (putrescine), and diaminopentane were capable of fully or partially reversing the growth inhibitory effects of DFMO, whereas diaminoethane had no significant effect. Among a series of triamines, H2N(CH2)nNH(CH2)3NH2 (where n = 2 to 8; abbreviated as APn n = 4 for spermidine, or AP4), spermidine was most effective in reversing the effects of DFMO, whereas compounds with shorter or longer methylene bridging regions were less effective. AP8 was ineffective in reversing the growth inhibitory effects of DFMO. At 10 microM concentration, AP8 also inhibited DNA synthesis by 66%, as measured by [3H]-thymidine incorporation. These data show that MCF-7 cells have a strong requirement for polyamines for their growth and that estradiol stimulates the polyamine cascade by inducing the ODC mRNA level. Our results also suggest that polyamine homologs such as AP8 might be potentially useful in breast cancer therapy.

Molecular analysis of the nerve growth factor inducible ornithine decarboxylase gene in PC12 cells

In an effort to understand molecular mechanisms by which nerve growth factor (NGF) regulates gene expression, we have isolated a full-length rat cDNA clone encoding ornithine decarboxylase (ODC) and utilized this probe to identify and examine the transcriptionally active, NGF inducible ODC gene in rat PC12 cells. This same gene is also responsive to epidermal growth factor, basic fibroblasts growth factor, and dibutyryl cAMP. Primer extension analysis demonstrates that both basal and NGF induced transcription of the ODC gene utilize the same major transcriptional start site, demonstrating that NGF acts to increase transcriptional activity at the basal start site as opposed to unmasking an alternative, stronger start site. Functional promoter analysis reveals the presence of a constitutive core promoter residing between positions -201 and +390, relative to the start site of transcription. Additional analyses reveal that sequences in the region -7800 to +2257 are insufficient to mediate NGF induced transcriptional activation, demonstrating that at least some of the regulatory sequences necessary for NGF mediated transcriptional induction of the ODC gene must reside at relatively enormous distances from the transcriptional start site. Such a long distance transcriptional regulatory mechanism is unique when compared with other NGF responsive genes that have been similarly analyzed.

Effects of sialagogues on ornithine decarboxylase induction and proto-oncogene expression in murine parotid gland

The mechanism of a sialagogue-induced increase in ornithine decarboxylase (ODC) activity and the expressions of proto-oncogenes in murine parotid gland were investigated by use of isoproterenol (IPR), carbachol (CC), and methoxamine (MTX). The results were as follows: (1) The three sialagogues had similar effects on the parotid in vivo (mouse parotid after a single injection of IPR) and/or in vitro (rat parotid explants cultured on siliconized lens paper floating on 199 medium containing IPR, CC, or MTX), the order of their effectiveness being IPR > CC > MTX. (2) Northern/dot and Western blot analyses revealed that the sialagogues elevated the steady-state levels of ODC mRNA and ODC protein to maxima at two h and six h, respectively, after stimulation. The increases were roughly proportional to those in ODC activity, suggesting that sialagogue-dependent enzyme induction is regulated at the transcriptional level. (3) The mRNAs of four of nine proto-oncogenes examined showed sialagogue-dependent increases to maxima at 30 min (c-fos) or 60 min (c-jun, c-myc, and c-src) after the beginning of stimulation. These increases were all transient, with the levels returning to the control values (without sialagogue) within 60 min. (4) The IPR-dependent elevations of ODC activity and the mRNAs of ODC, c-fos, and c-jun were inhibited by monensin, but not by polymyxin B. On the other hand, the CC-dependent increases in these parameters were inhibited by polymyxin B but not by monensin.(ABSTRACT TRUNCATED AT 250 WORDS)

TSH is able to induce cell cycle-related gene expression in rat thyroid cell

Rat thyroid cells in culture (FRTL-5 strain) require thyrotropic hormone (TSH) for growth. TSH alone in serum free medium is able to induce DNA synthesis of FRTL-5 cells. DNA synthesis occurs 18-20 hours following TSH stimulation of quiescent cells. Here we demonstrate that two sets of genes, related to the entry of cells in the S phase, are induced by TSH: 1) immediate early genes, such as c-jun and a gene coding for a zinc-finger protein Xrox 20/Egr2, both having a pattern of expression similar to the c-fos oncogene; 2) early delayed genes such as ornithine decarboxylase (ODC), 2F-1, a gene that shows a strong similarity in aminoacid sequence to a mitochondrial ADP/ATP carrier, and the asparagine synthetase gene (TS11). Furthermore, an increased expression of the histone H3 gene, a typical marker of S phase, has been observed in TSH-treated FRTL-5 cells.

A new perspective on ornithine decarboxylase regulation: prevention of polyamine toxicity is the overriding theme

The polyamines are essential cellular components for growth. Control of a key regulated enzyme of polyamine biosynthesis, ornithine decarboxylase (ODC), as a function of growth, is an area of intense interest. A unique regulatory property of ODC is the short half-life of the protein, which has been suggested to be an important factor in rapid activation of polyamine biosynthesis after cells are mitogenically stimulated. In this paper, it is argued that the biological significance of the short half-life of ODC is unrelated to the rate of its induction to a new steady state by growth factors, which is in fact limited by the relatively long half-life of the ODC mRNA. Instead, I suggest that the rapid turnover of ODC protein becomes of significance when cells cease growth and expeditious downregulation of the enzyme is important in preventing polyamine overproduction, which would result in cytotoxicity in the arrested cells. Although mitogenic activation of ODC expression has been studied extensively, there is very little known about the mechanisms controlling downregulation of polyamine biosynthesis during the arrest of animal cell growth. These considerations suggest that this would be a fertile area of future inquiry.

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.