Transient induction of ornithine decarboxylase mRNA in rat hepatoma cells treated with 12-O-tetradecanoylphorbol-13-acetate

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.

Association of protein kinase C activation with induction of ornithine decarboxylase in murine but not human keratinocyte cultures

The goal of this study was to compare the response of mouse epidermal keratinocytes (MEKs) and human epidermal keratinocytes (HEKs) to 12-O-tetradecanoylphorbol-13-acetate (TPA) with respect to the activation and downregulation of protein kinase C (PKC), the expression of c-jun and c-fos, and the expression and induction of ornithine decarboxylase (ODC) activity. Keratinocytes from adult CD-1 mice and from discarded adult human skin were grown in primary culture in a high-calcium serum-free medium that supported proliferation and differentiation. Immunoblotting of freshly isolated and cultured MEKs and HEKs for isozymes of protein kinase C revealed that fresh HEKs contained PKC alpha, PKC beta, and PKC delta; no PKC gamma, PKC epsilon, or PKC zeta were detected. In fresh MEKs, PKC alpha, PKC beta, PKC delta, and PKC zeta were observed, but not PKC gamma or PKC epsilon. After 2 wk in culture, the isozyme profiles of MEKs and HEKs were similar except that PKC gamma was noticeably present in HEK cultures. Activation of partially purified total PKC by TPA was similar in freshly isolated and cultured MEKs and HEKs, indicating that the two species were similar in this regard and that 2 wk of culture did not alter this characteristic. When MEK and HEK cultures were treated with TPA for 3 h, less than 30% of the control level of PKC activity was detected, indicating that TPA-induced downregulation of PKC was similar in MEKs and HEKs. After treatment with TPA, MEK cultures produced a large induction of both c-jun and c-fos mRNA by 60 min, as determined by northern blot analysis, and a large induction of ODC mRNA and enzyme activity by 6 h. TPA treatment of cultured HEKs, however, did not induce ODC activity; in fact, less activity, compared with that of control cultures, was observed. Northern blot analysis also revealed no increase in c-jun, c-fos, and ODC mRNA in HEKs. However, c-jun and c-fos mRNA and both ODC mRNA and enzyme activity were induced in HEKs fed growth factors after several days of deprivation. This suggests that the lack of ODC induction by TPA in HEKs is probably due to species differences in downstream steps in PKC signal transduction.

Glucocorticoids stimulate ornithine decarboxylase gene expression in pancreatic AR42J cells

The effects of dexamethasone on ornithine decarboxylase gene expression were examined in rat pancreatic AR42J cells. Dexamethasone increased ornithine decarboxylase activity and messenger RNA (mRNA) concentrations in a time-dependent manner, with a maximal effect at 12 hours (207% +/- 63% and 327% +/- 34% of control, respectively; n = 5). Ornithine decarboxylase mRNA levels returned to control values at 48 hours, whereas ornithine decarboxylase activity was decreased to 41% +/- 8% of control (n = 3). Dexamethasone induction of ornithine decarboxylase mRNA was dose dependent, with half-maximal effects at 10(-8) mol/L (210% +/- 20% of control; n = 4) and maximal effects at 10(-7) mol/L (327% +/- 26% of control; n = 4). The glucocorticoid antagonist RU 38486 blocked the dexamethasone effects in a dose-dependent manner, with maximal effects occurring at 10(-7) mol/L (120% +/- 18% of control; n = 3). When protein synthesis was blocked by addition of cycloheximide, ornithine decarboxylase mRNA levels remained unchanged in response to glucocorticoids, indicating a primary effect of dexamethasone. Furthermore, cycloheximide by itself had no significant effect on ornithine decarboxylase mRNA levels. Inhibition of transcription with actinomycin D showed a half-life for ornithine decarboxylase mRNA of approximately 240 minutes. Ornithine decarboxylase mRNA stability was not affected by dexamethasone pretreatment for 12 hours. Therefore, these data suggest that dexamethasone regulates ODC gene expression via glucocorticoid receptor-mediated gene transcription. Furthermore, translational mechanisms seem to be involved in glucocorticoid-regulated ornithine decarboxylase induction.

The importance of the specific Z-DNA structure and polyamines in carcinogenesis: fact or fiction

In this work some aspects of carcinogenesis are given. The importance of the emergence of Z or H DNA structure in the gene, or in the flanking gene sequences for the gene deletion and unusual gene recombination, is discussed. Some considerations on the role of selective pressure (of polyamines, of Mg2+, of the various levels of topoisomerase II, and of ATP) in the process of oncogene amplification, are given too.

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.

Regulation of ornithine decarboxylase mRNA by phorbol esters and insulin in normal and C-kinase-deficient rat hepatoma cells

Tumor-promoting phorbol esters and insulin produce similar effects in Reuber H35 rat hepatoma cell proliferation, including increased ornithine decarboxylase (ODC) enzyme activity, DNA synthesis, and mitogenesis. We investigated ODC mRNA accumulation in cells treated with either insulin or 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Both agents caused rapid accumulation of ODC mRNA: for TPA, it was maximal 3 hr after treatment (4-6-fold greater than control cells) and returned quickly to control levels; for insulin, it was significantly longer, continuing to increase for at least 6 hr. Simultaneous treatment with TPA and insulin led to additive effects on ODC mRNA. Induction of ODC by TPA was blocked by down-regulation or inhibition of protein kinase C (PKC), consistent with a PKC-mediated mechanism. In contrast, PKC down-regulation had little effect on ODC induction by insulin. Furthermore, although both agents stimulated ribosomal S6 protein phosphorylation in cells containing normal amounts of PKC, the response to TPA was abolished in PKC-depleted cells; the effect of insulin was only slightly inhibited. TPA caused a rapid redistribution of essentially all of the PKC activity from the cytosolic to the membrane fraction of the cells, whereas insulin had no effect on PKC distribution. These results suggest that although insulin and TPA share some common cytoplasmic signalling pathways, their effects on phosphorylation of nuclear proteins and transcription of ODC may be mediated by distinct factors.

Effect of adrenergic and Ca2+ antagonists on increased ornithine decarboxylase expression in regenerating rat liver

Partial hepatectomy (PH) (70% resection) causes within 4 hr an accumulation of ornithine decarboxylase (EC 4.1.1.17, ODC) mRNAs concomitant with an increase in ODC activity, maximum values being observed at 8 and 16 hr, respectively. In the early hours of hepatic regeneration, enhancement of transcriptional-rate of ODC gene, demonstrated by nuclear run-on analysis, can account for the accumulation of ODC mRNAs. The involvement of catecholamines in these processes is demonstrated by using prazosin and propranolol, specific antagonists of alpha 1 and beta adrenoceptors, respectively. Prazosin reduces almost completely the rise of ODC activity at 4 hr, without affecting mRNA levels. At 16 hr, enzyme activity and mRNAs increase, however, over the values observed in regenerating liver of prazosin-untreated animals. These findings suggest that alpha 1-receptor activation triggers positive control signals for ODC gene expression at the early time of liver regeneration and, on the contrary, negative signals at later times by mainly post-transcriptional and transcriptional mechanisms, respectively. Propranolol reduces similarly the initial 4 hr-rise of ODC activity. These results indicate that activation of both alpha 1- and beta-adrenoceptors causes the large increase in ODC activity. Pharmacological manipulation of intracellular Ca2+ levels by verapamil, a Ca2(+)-channel blocker, or neomycin, an inhibitor of Ca2+ release from endogenous stores, diminishes ODC activity at 4 and 16 hr after PH. ODC mRNA levels, which are not modified at 4 hr, increase over the values of partially hepatectomized rat liver at 16 hr. Trifluoperazine inhibits both ODC activity and mRNA accumulation at the times studied. As a working hypothesis it is proposed that Ca2(+)-mediated processes induced by catecholamines are involved in ODC gene expression during the prereplicative phase of liver regeneration.

Differential effects of 12-O-tetradecanoylphorbol 13-acetate on cytochrome P-450-dependent monooxygenase activities in rat hepatoma cells: induction of P-450I and suppression of P-450II

We have studied the effects of the tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA) on cytochrome P-450-dependent monooxygenase activities in several differentiated and dedifferentiated Reuber rat hepatoma cell lines using aryl hydrocarbon (benzo[a]pyrene) hydroxylase (AHH), ethoxyresorufin O-deethylase (EROD), and aldrin epoxidase (AE) as test systems. The following results were obtained: (1) Exposure of cultures to 400 nM TPA for 18-24 h increased AHH activities in the differentiated lines 2sFou, H41IEC3/G- and Fao as well as in the dedifferentiated line 5L, 1.5-2.5-fold. The phorbol ester did not affect AHH activity in the dedifferentiated line H5. (2) EROD, a marker for P-450I, was induced by the phorbol ester to a similar degree as AHH. (3) A monoclonal antibody directed against P-450I strongly inhibited the AHH activity induced by TPA. (4) The onset of AHH or EROD induction by TPA was much later than that elicited by benz[a]anthracene. (6) In contrast to the induction of AHH and EROD, TPA decreased AE activity, a marker for P-450II, by about 50% in all the cell lines containing this monooxygenase activity. (7) The half-maximum-effect concentration of TPA for inducing or suppressing AHH and AE, respectively, was approximately 20 nM. (8) TPA did not interfere with AHH induction by benz[a]anthracene. However, the phorbol ester moderately decreased AHH induction and markedly suppressed AE induction by dexamethasone. The results indicate that TPA simultaneously induces P-450I and suppresses P-450II forms in rat hepatoma cells. P-450I induction by TPA in these cells did not appear to depend on their status of differentiation. Furthermore, the results suggest that the mechanism of P-450I induction by TPA differs from that elicited by polycyclic aromatic hydrocarbons or glucocorticoids.

Regulation of mouse ornithine decarboxylase activity by cell growth, serum and tetradecanoyl phorbol acetate is governed primarily by sequences within the coding region of the gene

To determine the genetic elements required for modulation of ornithine decarboxylase (ODC) activity in response to cell growth or treatment with serum or with tetradecanoyl phorbol acetate, ODC-deficient cells were transfected with a series of recombinant DNAs encoding mouse ODC. All of the transfected cells expressing an intact mouse ODC protein displayed regulation of ODC activity, including those expressing a construct deprived of all ODC-specific sequence information except the protein-coding region. ODC mRNA changed much less than enzymatic activity. A mutation of the protein-coding region that converted ODC from an unstable to a stable intracellular protein attenuated the regulatory response. We conclude that post-transcriptional events associated with ODC degradation dominate the response to these stimuli.

Effects of S-adenosyl-1,8-diamino-3-thio-octane and S-methyl-5'-methylthioadenosine on polyamine synthesis in Ehrlich ascites-tumour cells

The rate-limiting enzymes in polyamine biosynthesis, ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (AdoMetDC), are negatively regulated by the polyamines spermidine and spermine. In the present work the spermidine synthase inhibitor S-adenosyl-1,8-diamino-3-thio-octane (AdoDATO) and the spermine synthase inhibitor S-methyl-5'-methylthioadenosine (MMTA) were used to evaluate the regulatory role of the individual polyamines. Treatment of Ehrlich ascites-tumour cells with AdoDATO caused a marked decrease in spermidine content together with an accumulation of putrescine and spermine. Treatment with MMTA, on the other hand, gave rise to a marked decrease in spermine, with a simultaneous accumulation of spermidine. A dramatic increase in the activity of AdoMetDC, but not of ODC, was observed in MMTA-treated cells. This increase appears to be unrelated to the decrease in spermine content, because a similar rise in AdoMetDC activity was obtained when AdoDATO was given in addition to MMTA, in which case the spermine content remained largely unchanged. Instead, we show that the increase in AdoMetDC activity is mainly due to stabilization of the enzyme, probably by binding of MMTA. Treatment with AdoDATO had no effects on the activities of ODC and AdoMetDC, even though it caused a precipitous decrease in spermidine content. The expected decrease in spermidine-mediated suppression of ODC and AdoMetDC was most probably counteracted by the simultaneous increase in spermine. The combination of AdoDATO and MMTA caused a transient rise in ODC activity. Concomitant with this rise, the putrescine and spermidine contents increased, whereas that of spermine remained virtually unchanged. The increase in ODC activity was due to increased synthesis of the enzyme. There were no major effects on the amount of AdoMetDC mRNA by treatment with the inhibitors, alone or in combination. However, the synthesis of AdoMetDC was slightly stimulated in cells treated with MMTA or AdoDATO plus MMTA. The present study demonstrates that regulation of neither ODC nor AdoMetDC is a direct function of the polyamine structure. Instead, it appears that the biosynthesis of the polyamines is feedback-regulated by the various polyamines at many different levels.