Initial characterization of a HTC cell variant partially resistant to the anti-proliferative effect of ornithine decarboxylase inhibitors

Targeting polyamine metabolism and function in cancer and other hyperproliferative diseases

The polyamines spermidine and spermine and their diamine precursor putrescine are naturally occurring, polycationic alkylamines that are essential for eukaryotic cell growth. The requirement for and the metabolism of polyamines are frequently dysregulated in cancer and other hyperproliferative diseases, thus making polyamine function and metabolism attractive targets for therapeutic intervention. Recent advances in our understanding of polyamine function, metabolic regulation, and differences between normal cells and tumour cells with respect to polyamine biology, have reinforced the interest in this target-rich pathway for drug development.

Human prostatic carcinoma cell lines display altered regulation of polyamine transport in response to polyamine analogs and inhibitors

BACKGROUND

The possibility was investigated that complex homeostatic mechanisms which maintain polyamine pools in prostate-derived tumors may differ from those which are typically seen in other tissues and tumors.

METHODS

Growth sensitivity and various regulatory responses were investigated in three human prostate carcinoma cell lines (LNCaP, DU145, and PC-3) treated with the inhibitor of S-adenosylmethionine decarboxylase CGP-48664 or the polyamine analog N1,N11-diethylnorspermine (DENSPM), both of which are currently undergoing phase I clinical trial.

RESULTS

Prostate tumor cell lines were all similarly growth-inhibited by the inhibitor CGP-48664 (IC50 values, 1-5 microM at 72 hr), but varied considerably in their sensitivity to DENSPM. The rank-order for cell-line growth inhibition by the analog was DU145 > PC-3 > LNCaP, with IC50 values of 1, 30, and 1,000 microM, respectively. Both compounds depleted intracellular polyamine pools to levels which seemed sufficient to account for inhibition of cell growth. While polyamine enzyme regulatory responses to both CGP-48664 and DENSPM were typical of those seen in other cell types, regulation of polyamine transport differed distinctly. Based on Vmax determinations, LNCaP cells failed to upregulate transport in response to CGP-48664, while PC-3 and LNCaP cells failed to downregulate transport in response to DENSPM.

CONCLUSIONS

Relative to other cell lines, polyamine transport in prostate carcinoma cell lines was found to be uniquely insensitive to regulation by polyamines or analogs. Although this did not seem to correlate with growth sensitivity to polyamine analogs in vitro, it should be therapeutically exploitable in in vivo systems.

Hypusine modification in eukaryotic initiation factor 5A in rodent cells selected for resistance to growth inhibition by ornithine decarboxylase-inhibiting drugs

Selection of HTC cells in drugs that inhibit ornithine decarboxylase (ODC) has produced two cell lines, HMOA and DH23A/b, that contain increased amounts of more stable ODC. In addition to alterations in ODC, these cells appear to produce modified eukaryotic initiation factor 5A (eIF-5A) at different rates, a reaction that both requires spermidine and is essential for proliferation. Alterations to the modification of eIF-5A by spermidine cannot be accounted for by changes in eIF-5A protein or modified eIF-5A turnover. Deoxyhypusine synthetase activity is similar in the parental and variant cell lines and is unaltered by growth into plateau phase or by spermidine depletion. The increased rate of eIF-5A modification in DH23A/b cells is due to an increased accumulation of the unmodified eIF-5A precursor. Increased precursor accumulation is not due to increased eIF-5A transcription, but rather it can be attributed to a metabolic accumulation caused by growth under conditions of chronically limiting spermidine. Selection using drugs that inhibit ODC apparently does not cause alterations in the eIF-5A modification pathway. These data support the hypothesis that one of the main effects of spermidine depletion is depletion of the modified eIF-5A pool, and that this is a critical factor in the cytostasis often observed after depletion of cellular polyamines.

Ornithine decarboxylase stability in HMOA and DH23b cells is not due to post-translational truncation of a C-terminal recognition site

The normally labile ornithine decarboxylase (ODC) becomes unusually stable when Cys-441 is replaced with Trp in the variant cell lines HMOA and DH23b. This stable ODC is also observed to have higher mobility on SDS/PAGE. Because previous studies have shown that ODC stability can be achieved when as few as five amino acid residues are removed from its C-terminus, it was suggested that the amino acid substitution in the variant ODC might alter its conformation sufficiently to promote a similar proteolytic loss of a C-terminal degradation signal, resulting in a stable yet active ODC. To examine this mechanism, amino acids in the C-terminal regions of both wild-type and stable (Trp-441) ODC proteins were released, by means of carboxypeptidase-Y digestion, and identified by HPLC. The C-terminal ends were found to be the same, and are as predicted from the cDNA sequence. This study proves that stability of the Trp-441 form of ODC is not simply due to proteolytic removal of a C-terminal proteasome-targeting sequence, thereby implying that the stabilization of this mutant ODC form must result directly from a conformational change associated with the loss of Cys-441.

Overproduction of stable ornithine decarboxylase and antizyme in the difluoromethylornithine-resistant cell line DH23b

DH23b cells, a variant of the HTC line selected for their resistance to difluoromethylornithine, exhibit defective feedback regulation of ornithine decarboxylase (ODC) stability and polyamine transport, and accumulate ODC protein to > 1000 times normal concentrations. The components of the polyamine feedback regulation system have been examined in an attempt to understand these unusual responses. Southern-blot analysis revealed an amplification (approx. 10-fold) in ODC DNA sequence without any concomitant increase in antizyme. Moreover, the amplified ODC sequence contains a single base substitution that results in the conversion of Cys-441 into Trp. This modification has previously been shown to cause ODC stability in HMOA cells. Although antizyme activity has not been noted in DH23b cells, Western-blot analysis revealed the accumulation of antizyme protein to > 50 times that induced in parental HTC cells. This increase is consistent with a 6-9-fold increase in the half-life of antizyme in these cells, a consequence of the inability of the mutant ODC-antizyme complex to be degraded by 26 S proteasome. Associated with the stabilization of antizyme in both DH23b and HMOA cells is the appearance of two additional forms of antizyme protein with apparent molecular masses of 22 and 18.5 kDa. It is suggested that these result from proteolytic removal of discrete fragments from the N-terminal end of antizyme, perhaps an indication of an initial step in rapid antizyme turnover.

Cloning and expression of two ornithine decarboxylase forms from HMOA cells

In HMOA cells [Mamont, Duchesne, Grove and Tardif (1978) Exp. Cell Res. 115, 387-393] the half-life of ornithine decarboxylase (ODC) is 8-14 h instead of 15 min as in the Hepatoma Tissue Culture parental cells, due to a single amino acid substitution [Miyazaki, Matsufuji, Murakami and Hayashi (1993) Eur. J. Biochem. 214, 837-844]. We demonstrate for the first time that HMOA cells possess two forms of ODC mRNA that are translated into two proteins differing greatly in turnover rates. We have cloned and transfected the cDNAs for the two ODC forms into COS-1 cells for a direct measurement of their turnover rate. The variant ODC form was much more stable than the wild-type protein, with a half-life of 14 h as compared with 2.5 h.

Feedback repression of polyamine transport is mediated by antizyme in mammalian tissue-culture cells

Antizyme, a spermidine-induced protein that binds and stimulates ornithine decarboxylase degradation, is now shown also to mediate the rapid feedback inhibition of polyamine uptake into mammalian cells. Using a cell line (HZ7) transfected with truncated antizyme cDNA, and mutant ornithine decarboxylase cell lines, we demonstrate that this newly discovered action of antizyme is distinct from its role in modulating polyamine biosynthesis.

Single amino-acid replacement is responsible for the stabilization of ornithine decarboxylase in HMOA cells

The half-life of ornithine decarboxylase (ODC) in HMOA cells, a variant cell line derived from hepatoma tissue culture (HTC) cells, is markedly increased compared with that in the parental cell line. In the present study, we examined which of the three relevant factors is responsible for the ODC stabilization in HMOA cells, namely ODC itself, a regulatory protein antizyme and an ODC-degrading activity. SDS/PAGE analysis of radiolabeled ODC revealed that ODC from HMOA cells migrated somewhat faster than that from HTC cells, suggesting that HMOA ODC was structurally altered. Direct sequencing of reverse-transcription/polymerase-chain-reaction (RT-PCR) products of ODC mRNA from HMOA cells revealed a T to G replacement, causing a Cys441-->Trp replacement near the C-terminus. No alteration was found in the whole coding region of antizyme mRNA. An authentic mutant ODC cDNA with the same replacement was transfected and expressed in C55.7 ODC-deficient Chinese hamster ovary cells. Upon cycloheximide treatment, the mutant ODC activity did not decrease appreciably for at least 3 h, whereas wild-type ODC activity decreased with a half-life of 1 h. In-vitro-synthesized mutant ODC with the Cys441-->Trp (or Ala) replacement was also stable in a reticulocyte-lysate ODC-degradation system. Metabolically labeled and purified mouse ODC was degraded in HMOA cell extracts in the presence of ATP and antizyme as rapidly as in HTC cell extracts, indicating that HMOA cells have a normal ODC degrading activity. These results indicated that the single amino acid replacement, Cys441-->Trp, is responsible for the stabilization of ODC in HMOA cells and that Cys441 is important for rapid ODC turnover.

Stable ornithine decarboxylase in a rat hepatoma cell line selected for resistance to alpha-difluoromethylornithine

Ornithine decarboxylase (ODC) is extremely unstable in mammalian cells. This unusual characteristic facilitates rapid fluctuations in the activity of this enzyme in response to variations in its biosynthesis. Unfortunately, very little is known about the mechanism or regulation of this ODC-specific proteolytic pathway. This study describes the production and characterization of a variant of the rat hepatoma HTC cell line that is strikingly deficient in this pathway. This cell variant was induced by selection for growth in stepwise increasing concentrations (up to 10 mM) of the irreversible ODC inhibitor, alpha-difluoromethylornithine (DFMO). Resistance to this inhibitor appears to result from a combination of elevated (10X) ODC biosynthesis and inhibited degradation, producing greater than a 2000-fold increase in the level of ODC protein. In these variant cells (DH23b) inhibition of protein synthesis by cycloheximide did not result in rapid loss of enzyme activity or ODC protein determined by radioimmunoassay. Pulse-chase studies with [35S]methionine confirmed that this enzyme was not preferentially degraded, even when spermidine was added to the media. ODC purified from the variant cells was found to be identical to the control cell enzyme in size, isoelectric point, substrate binding kinetics, and sensitivity to the inhibitor DFMO. Also, as in the control cells, a major fraction of the ODC molecules extracted from DH23b cells was shown to be phosphorylated on a serine residue. The inability to detect physical or kinetic differences between the parent and the variant cell ODC suggests that the unusual stability of ODC in this cell is associated with a defect in a cellular mechanism for ODC-specific degradation.

Differential stimulation of S-adenosylmethionine decarboxylase by difluoromethylornithine in the rat colon and small intestine

The effects of chronic inhibition of ornithine decarboxylase (ODC) by the specific inhibitor difluoromethylornithine (DFMO) in the rat colon and small intestine on mucosal contents of polyamines, decarboxylated S-adenosylmethionine (decarboxylated AdoMet) and S-adenosylmethionine decarboxylase (AdoMet decarboxylase) activity were studied. Administration of 1% DFMO in the drinking water for 10 or 15 weeks resulted in inhibition of ODC and decreases in intracellular putrescine and spermidine contents in both proximal and distal segments of small intestine and colon. At both time points DFMO administration resulted in a dramatic stimulation of AdoMet decarboxylase activity and a rise in decarboxylated AdoMet content in the proximal and distal small-intestinal segments compared with controls, which was not seen in either colonic segment of DFMO-treated animals. This differential stimulation of AdoMet decarboxylase by DFMO in the small intestine and colon could not be entirely explained on the basis of differences in polyamine contents, which are known to regulate this enzyme activity. Kinetic and inhibition studies of AdoMet decarboxylase in control small and large intestine revealed that: (1) there was no difference in Vmax. values between the tissues; (2) the Km for AdoMet was higher in the small intestine than in the colon; and (3) the Ki for product inhibition by decarboxylated AdoMet was higher in the small intestine than in the colon. These results suggest that the differential stimulation of AdoMet decarboxylase by DFMO in the small intestine and colon may be due to different isoenzymes and could play a significant role in the regulation of polyamine contents throughout the gut.

Prevention of rapid intracellular degradation of ODC by a carboxyl-terminal truncation

Ornithine decarboxylase (ODC) was converted from a protein with a short intracellular half-life in mammalian cells to a stable protein by truncating 37 residues at its carboxyl terminus. Cells expressing wild-type protein lost ODC activity with a half-life of approximately 1 hour. Cells expressing the truncated protein, however, retained full activity for at least 4 hours. Pulse-chase experiments in which immunoprecipitation and gel electrophoresis were used confirmed the stabilizing effect of the truncation. Thus, a carboxyl-terminal domain is responsible for the rapid intracellular degradation of murine ODC.

Feedback control of ornithine decarboxylase expression by polyamines. Analysis of ornithine decarboxylase mRNA distribution in polysome profiles and of translation of this mRNA in vitro

Cell growth and differentiation require the presence of optimal concentrations of polyamines. Ornithine decarboxylase (ODC) catalyses the first and rate-controlling step in polyamine synthesis. In studies using cultures of Ehrlich ascites-tumour cells, we have shown that the expression of ODC is subject to feedback regulation by the polyamines. A decrease in the cellular polyamine concentration results in a compensatory increase in the synthesis of ODC, whereas an increase in polyamine concentration results in suppression of ODC synthesis. These changes in ODC synthesis were attributed to changes in the efficiency of ODC mRNA translation, because the steady-state amount of ODC mRNA remained constant. We now show that the number of ribosomes associated with ODC mRNA is low, and that the increase in ODC mRNA translation takes place without a shift in the distribution of ODC mRNA towards larger polysomes. This finding indicates that the polyamines regulate the efficiency of ODC mRNA translation by co-ordinately affecting the rates of initiation and elongation. By analysing ODC mRNA translation in vitro, using a rabbit reticulocyte lysate, polyadenylated RNA from a cell line with an amplified ODC gene, and a monospecific anti-ODC antibody, we also show that spermidine, but not putrescine, exerts a direct regulatory effect on ODC synthesis.

Fluorinated analogues of spermidine as substrates of spermine synthase

A series of mono- and geminal difluorinated analogues of spermidine (4-azaoctane-1,8-diamine) have been tested as potential substrates of partially purified rat hepatoma (HTC) cell or pure bovine spleen spermine synthase (EC 2.5.1.22). Substitution of the hydrogen atoms of the methylene group at position 7 by one or two fluorine atoms decreases 8-fold and 160-fold the apparent Km values for the HTC cell enzyme respectively. Similarly, the Km values of 7-monofluoro and 7,7-difluorospermidine for the pure bovine enzyme are reduced 8-fold and 100-fold respectively, in comparison with spermidine. Di-, but not monofluoro substitution, in the 6-position causes a 5-fold reduction in the affinity for the HTC cell enzyme. Gem-fluorine substitution in the 2-position abolishes substrate capability. In addition to their high affinity for spermine synthase, 7-monofluorospermidine and 7,7-difluorospermidine cause substrate inhibition. This phenomenon, which is more pronounced in the case of the difluorinated analogues is pH-dependent. These enzymatic findings are discussed with regard to the protonation sites of the spermidine analogues, determined by potentiometric titration, which vary as a function of the number and position of the fluorine substituents relative to the basic amino groups.

Ehrlich ascites tumour cells become refractory to alpha-difluoromethylornithine at a certain stage of growth

When Ehrlich ascites tumour cells are induced to proliferate by serum stimulation, the ornithine decarboxylase (ODC) activity increases rapidly and reaches two to three peaks during the first 24 h. Inhibition of the first peak in ODC activity (occurring at 4 h) by adding alpha-difluoromethylornithine (DFMO) within 2 h of serum stimulation, results in maximal growth inhibition. Under these conditions, similar degrees of polyamine depletion are achieved. When DFMO is added 3 h after seeding, however, enough polyamines have already accumulated during the initial burst in ODC activity to reduce the antiproliferative effect of the drug. The antiproliferative effect is further reduced when DFMO is added 6 h after seeding. When DFMO is added 23 h after seeding, i.e. after maximal accumulation of polyamines, there is no inhibition of cell proliferation. These findings are important to consider both when designing experimental as well as clinical regimens for this drug.

Chain-fluorinated polyamines as tumor markers--IV. Comparison of 2-fluoroputrescine and 2,2-difluoroputrescine as substrates of spermidine synthase in vitro and in vivo

1. 2-Fluoroputrescine has a high affinity for spermidine synthase (Km 12 microM) and obeys normal Michaelis-Menten kinetics. 2. The only product of the spermidine synthase-catalysed aminopropylation of 2-fluoroputrescine is 6-fluorospermidine. Formation of the isomeric 7-fluorospermidine could not be detected. 3. 2,2-Difluoroputrescine has even a higher affinity for spermidine synthase than putrescine and 2-fluoroputrescine; however, at concentrations greater than 25 microM one observes inhibition of the aminopropylation reaction. 4. Competition experiments between putrescine and 2,2-difluoroputrescine revealed mixed type inhibition. 5. HTC cells in suspension culture incorporated only small amounts of 2-fluoroputrescine, and even less in the case of 2,2-difluoroputrescine, if they were exposed to 10 microM concentrations of these diamines for up to 24 hr. However, in the presence of 0.5 mM DFMO, a concentration not sufficient to decrease cell growth significantly, but sufficient to decrease cellular putrescine and spermidine concentrations, the uptake of the chain-fluorinated diamines and their transformation into the fluorinated polyamine analogues was dramatically enhanced. In comparison with the difluoro analogues the accumulation rate of monofluoropolyamines was greater by a factor of about two. 6. 6-Fluorospermidine and 6-fluorospermine could be detected in significant quantities in nearly all tissues of mice 48 hr after a single dose (500 mg/kg) of 2-fluoroputrescine. In an analogous experiment with 2,2-difluoroputrescine, the formation of chain-fluorinated polyamines was considerably smaller. 7. Pretreatment of Lewis lung carcinoma bearing C57BL mice with alpha-difluoromethylornithine enhanced the incorporation of 2-fluoroputrescine into all organs, except the brain. Tumor and small intestines showed by far the highest accumulation of 6-fluoropolyamines. 8. Under identical experimental conditions the accumulation of chain-fluorinated polyamines in tumor tissue was more than twice as high with 2-fluoroputrescine as precursor than with the same dose of 2,2-difluoroputrescine. In normal tissues the difference between the uptake of 2-fluoroputrescine and 2,2-difluoroputrescine was usually even greater. 9. From the fact that the accumulation of 6-fluoropolyamines is less selective in tumors than that of 6,6-difluoropolyamines, and from the lower detection sensitivity due to its lower fluorine content, we conclude that 2,2-difluoroputrescine is more advantageous as a tumor marker than 2-fluoroputrescine for detection with 19F-NMR spectroscopy.(ABSTRACT TRUNCATED AT 400 WORDS)

A clonal derivative of tunicamycin-resistant Chinese hamster ovary cells with increased N-acetylglucosamine-phosphate transferase activity has altered asparagine-linked glycosylation

A population of Chinese hamster ovary (CHO) cells resistant to the antibiotic tunicamycin (TM) had previously been isolated (Criscuolo, B.A., and Krag, S.S. (1982) J. Cell Biol. 94:586-591) by a stepwise selection procedure using progressive increments of TM added to the medium. TM inhibits asparagine-linked glycoprotein biosynthesis by blocking the transfer of N-acetylglucosamine-1-phosphate from the sugar nucleotide UDP-N-acetylglucosamine to the isoprenoid lipid carrier, dolichyl phosphate. Four clonal derivatives were isolated from the TM-resistant population in the presence of 27 micrograms TM/ml and were found to overproduce the N-acetylglucosamine-phosphate transferase activity to the same extent (approximately 15-fold compared to wild-type cells). One of these clones, 3E11, was greater than 550-fold more resistant to TM than wild-type cells. The resistance phenotype remained during at least 2.5 months of growth in the absence of TM. 3E11 cells exhibited chromosomal translocations, but no homogeneously staining regions (HSR) or double minute chromosomes. The N-acetylglucosamine-phosphate transferase activity in 3E11 cells was membrane-associated and was inhibited by TM. A 140,000-dalton membrane protein and at least four other membrane proteins were enriched in 3E11 cells. Mannosylphosphoryldolichol synthase and glucosylphosphoryldolichol synthase activities were not elevated in membranes prepared from 3E11 cells. Asparagine-linked glycosylation was altered such that 3E11 cells synthesized primarily a truncated oligosaccharide, Man5GlcNAc2, perhaps due to the reduced amount of mannosylphosphoryldolichol relative to wild-type cells.

Polyamines in normal and cancer cells

Based on available evidence, it appears that polyamines are critical for proliferation of both normal and transformed cells. Although the requirement of polyamines for DNA replication and cell proliferation is established, the molecular events in which the polyamines are essential are yet unknown. Furthermore, transformed and cancer cells, possibly because of their higher proliferative rate, appear to be more dependent on polyamine metabolism than their normal counterparts. This has been shown by the in vivo response of tumor models and human tumor xenografts in nude mice to polyamine depletion by DFMO. Although there has been associated toxicity to the host, the inhibition of cell proliferation has been higher in the implanted tumors than in the host. DFMO, a specific irreversible inhibitor of ODC, has been used extensively in studies which have shed light on the role of polyamines in cell proliferation and differentiation. DFMO has shown interesting anti-tumor effect in a number of experimental tumor models. Currently, DFMO clinical trials are being completed, and it will be of interest to see whether this polyamine inhibitor, or other newer polyamine analogs and inhibitors, will find a place in the treatment of neoplastic disorders.

Polyamine-mediated turnover of ornithine decarboxylase in Chinese-hamster ovary cells

We have used Chinese-hamster ovary (CHO) cells maintained in a chemically defined medium to study the regulation of ornithine decarboxylase (ODC) by polyamines. Cells maintained in the defined medium had no detectable putrescine, and approx. 1-3 units of ODC activity/10(6) cells, where 1 unit corresponds to 1 nmol of substrate decarboxylated in 30 min. The defined medium is ornithine-deficient, thus limiting the exogenous substrate for ODC, and subsequently decreasing intracellular polyamine accumulation. Restoration of intracellular putrescine and increased formation of spermidine by addition of exogenous ornithine or putrescine led to a marked decrease in ODC activity, which was paralleled by a decrease in a alpha-DL-difluoromethyl[3,4-3H]ornithine (DFMO)-binding protein of Mr approx. 53,000, which is precipitable with anti-ODC antibody. Calculation of DFMO binding per unit of activity showed no change in the specific activity of the enzyme. We identified [35S]methionine-labelled peptides corresponding to ODC by immunoprecipitation of radiolabeled whole cell proteins. Only one protein was precipitated, of Mr approx. 53 000, which co-migrated with the DFMO-binding protein. Immunoprecipitation of radiolabelled proteins from cells incubated in the presence of exogenous ornithine indicated that the observed activity decrease was not due to an inhibition of ODC protein synthesis. Analysis of immunoprecipitable ODC protein from cells that had been pulse-labelled with [35S]methionine, and then treated for 5 h with 100 microM-ornithine, -putrescine or -spermidine, revealed a distinct disappearance of labelled ODC protein after restoration of intracellular polyamine pools. No detectable turnover of ODC was observed in the absence of exogenous polyamine treatment. These data support the hypothesis that ODC protein, and subsequent activity, is regulated by intracellular polyamine content through mechanisms that influence turnover of the enzyme.

Posttranscriptional regulation of ornithine decarboxylase activity

We have used a Chinese hamster ovary cell line (DF3) that overproduces ornithine decarboxylase (ODC) to examine various parameters in the cell cycle-dependent regulation of this enzyme. Under a variety of conditions, alterations in the activity of ODC were accompanied by parallel changes in the levels of the protein, as measured by immunologically cross-reactive material (CRM). While putrescine has been known to suppress the induction of ODC, we have found that in DF3 cells 10(-4)M ornithine completely suppresses ODC activity. We also show that the levels of ODC mRNA are not modulated when the levels of ODC activity and CRM change drastically. The data can be interpreted in terms of models involving either an effect of putrescine on the translation of ODC mRNA, or on the activity of a relatively specific protease with ODC as its target.

Ornithine decarboxylase inhibitors increase the cellular content of the enzyme: implications for translational regulation

Ehrlich ascites tumor cells grown in the presence of inhibitors of ornithine decarboxylase (EC 4.1.1.17) exhibited an elevated content of this enzyme. The increase could not solely be explained by a decrease in the degradation rate of the enzyme. Instead a stimulation of enzyme synthesis, probably mediated via the polyamine-depleting properties of the inhibitors, is suggested. The enhancement of cellular ornithine decarboxylase content was not accompanied by any significant changes in the amount of ornithine decarboxylase mRNA, indicating a regulation at the level of translation.

Accumulation of ornithine decarboxylase-antizyme complex in HMOA cells

A new method was developed for the assay of ornithine decarboxylase (ODC)-antizyme complex, in which alpha-difluoromethylornithine (DFMO)-inactivated ODC was used to release active ODC competitively from the complex. ODC-antizyme complex was present in the extracts of hepatoma tissue-culture (HTC) cells and of ODC-stabilized variant HMOA cells, in much larger amounts in the latter. Cellular amounts of the complex fluctuated after a change of medium in a similar manner in HTC and HMOA cells, increasing during the period of ODC decay. After treatment with cycloheximide, the decay of ODC-antizyme complex in HMOA cells was more rapid than the decay of free ODC, but it was much slower than the decay of free ODC or complexed ODC in HTC cells. Administration of putrescine caused a rapid increase in the amount of ODC-antizyme complex in both HTC and HMOA cells, but nevertheless the decay of total ODC (free ODC plus ODC-antizyme complex) was more rapid with putrescine than with cycloheximide. These results suggested the possibility that ODC is degraded through complex-formation with antizyme. In contrast with complexed antizyme, free antizyme was not stabilized in HMOA cells.

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.

S-adenosylmethionine decarboxylase as target of chemotherapy

Although ornithine decarboxylase under most conditions is the rate-controlling enzyme of polyamine biosynthesis and thus the most logical target for chemical intervention, the inhibition of the enzyme triggers a series of compensatory reactions all aimed to circumvent the inhibition. These include secondary induction of adenosylmethionine decarboxylase, enhanced accumulation of extracellular polyamines and an overproduction of ornithine decarboxylase resulting from enhanced expression and gene amplification. Thus chemotherapy based on an intervention of polyamine formation has also to be directed to reactions other than the decarboxylation of ornithine. Adenosylmethionine decarboxylase is the second natural target for chemotherapy. Virtually all effective inhibitors of this enzyme are members of the family of bis(guanylhydrazones). Small modifications, such as increased hydrophobicity at the glyoxal portion of the parent compound glyoxal bis(guanylhydrazone), greatly enhance the inhibition of adenosylmethionine decarboxylase and diminish the undesirable inhibition of diamine oxidase. However, although ethylglyoxal and propylglyoxal bis(guanylhydrazone) appear to utilize the putative polyamine carrier for their cellular entry, their cellular accumulation, in contrast to that of glyoxal and methylglyoxal bis(guanylhydrazone), is not stimulated by putrescine and spermidine deprivation produced by inhibitors of ornithine decarboxylase. It is obvious that the cellular accumulation of each of the bis(guanylhydrazones) is determined by their different efflux rates: GBG and MGBG are effectively retained whereas EGBG is rapidly excreted by the tumor cells. GBG and MGBG, but possibly not EGBG, behave as mitochondrial poisons and rapidly produce extensive morphological damage of the mitochondria. The bis(guanylhydrazones) likewise inhibit carnitine-dependent mitochondrial oxidation of long-chain fatty acids, competitively in respect to carnitine. It is possible that this inhibition has something to do with the mitochondrial damage, as carnitine protects tumor cells from the early mitochondrial damage produced by MGBG. Carnitine also protects experimental animals from MGBG-induced acute toxicity and death.

Molecular mechanism for the regulation of hepatic ornithine decarboxylase

A single injection of thioacetamide into starved rats induced a 40- to 100-fold increase in hepatic ODC activity. However, immunotitratable ODC protein increased by only 5-fold because of the presence of significant amounts of inactive ODC protein in the liver of untreated starved rats. Polysomal ODC-mRNA activity also increased only 5-fold, a significant amount being present in control liver. Furthermore, the peak of polysomal ODC-mRNA activity preceded that of ODC activity or ODC protein by several hours. These results indicate that the enzyme induction is due not only to increase in polysomal ODC-mRNA activity, but also to some translational and/or post-translational regulation. Exogenously administered diamines or polyamines cause rapid decay of ODC activity and induce antizyme that binds to ODC and inactivates it. Another protein factor, antizyme inhibitor, was found in the liver of thioacetamide-treated or protein-fed rats. Antizyme inhibitor binds to antizyme and reactivates ODC in the ODC-antizyme complex. A small, but significant, amount of antizyme was found in the liver of starved rats. Only small amounts of ODC-antizyme complex were detected in rat liver and cultured hepatocytes, even during the period of rapid ODC decay caused by exogenously added diamines. On the other hand, the complex was present in HTC cells and more especially in ODC-stabilized HMOA cells, even under physiological conditions. On addition of 10(-2) M putrescine, the amount of complex first increased and then decreased in both types of cells. Decay of total ODC activity (free plus complexed ODC) was more rapid with putrescine than with cycloheximide. These results suggest that antizyme plays an essential role in the degradation of ODC by a catalytic effect both in the presence and absence of exogenous putrescine and that antizyme inhibitor stabilizes ODC by removing antizyme.

Ornithine decarboxylase as target of chemotherapy

Ornithine decarboxylase is a key enzyme in polyamine synthesis and growth of mammalian cells. In this chapter I review recent reports on the purification and properties of the pure enzyme, and on the localization, synthesis and regulation of the enzyme in the cell. The use of monospecific antibodies, radiolabeled irreversible inhibitors and cDNA clones for studying enzyme localization, turnover and regulation, is briefly described. This first part is meant to serve as a basis for the analysis of ornithine decarboxylase as a target of chemotherapy. A selection of the most potent inhibitors of ornithine decarboxylase is presented and the effects of some of these in cell culture, in animals and in the clinical setting are reviewed.

Metabolism of N8-monoacetylspermidine in rat hepatoma cells. Investigation of its effect on the activity of L-ornithine decarboxylase

Recent evidence has indicated a role for the acetyl derivatives of polyamines, particularly N8-monoacetylspermidine, as activators of L-ornithine decarboxylase in rat hepatoma tissue culture (HTC) cells. This is in contrast with the well-described negative regulatory control of ornithine decarboxylase exerted by their non-acetylated counterparts. Because of the possibility of a rapid extracellular and intracellular catabolism of the acetyl derivatives of polyamines, the metabolism of N8-monoacetylspermidine and its effect on HTC cell ornithine decarboxylase have been investigated, under conditions which eliminate its extracellular catabolism. Differing from previous reports, we demonstrate that N8-monoacetylspermidine does not elevate ornithine decarboxylase activity when added at low concentrations to the culture medium of HTC cells. Higher concentrations decrease ornithine decarboxylase activity in a dose-dependent manner. This effect cannot be unambiguously attributed to the effect of the acetyl derivative itself, because of the presence in situ of a very active N8-monoacetylspermidine deacetylase, which generates spermidine intracellularly.

Polyamine metabolism and function

Polyamines are ubiquitous organic cations of low molecular weight. The content of these amines is closely regulated by the cell according to the state of growth. The reactions responsible for the biosynthesis and interconversion of the polyamines and their precursor putrescine are described and the means by which polyamine content can be varied in response to exogenous stimuli are discussed. The role of polyamines in the cell cycle, cell division, tissue growth, and differentiation is considered. Recent studies using highly specific inhibitors of polyamine biosynthesis such as alpha-difluoromethylornithine to prevent accumulation of polyamines have indicated that the synthesis of polyamines is intimately associated with these processes. Such inhibitors have great potential for investigation of the cellular role of polyamines.

Selection of tunicamycin-resistant Chinese hamster ovary cells with increased N-acetylglucosaminyltransferase activity

Chinese hamster ovary (CHO) cells resistant to the antibiotic tunicamycin (TM) have been isolated by a stepwise selection procedure with progressive increments of TM added to the medium. TM inhibits asparagine-linked glycoprotein biosynthesis by blocking the transfer of N-acetylglucosamine-1-phosphate from UDP-N-acetylglucosamine to the lipid carrier. The TM-resistant cells exhibited a 200-fold increase in their LD50 for TM and were morphologically distinct from the parental cells. The rate of asparagine-linked glycoprotein biosynthesis was the same for wild-type and TM-resistant cells. Membrane preparations from TM-resistant cells cultured for 16 d in the absence of TM had a 15-fold increase in the specific activity of the UDP-N-acetylglucosamine:dolichol phosphate N-acetylglucosamine-1-phosphate transferase as compared to membranes of wild-type cells. The products of the in vitro assay were N-acetylglucosaminylpyrophosphoryl-lipid and N,N'-diacetylchitobiosylpyrophosphoryl-lipid for membranes from both TM-resistant and wild-type cells. The transferase activity present in membrane preparations from wild-type of TM-resistant cells was inhibited by comparable levels of TM. The data presented are consistent with overproduction of enzyme as the mechanism of resistance in these variant CHO cells.

Accumulation of decarboxylated S-adenosyl-L-methionine in mammalian cells as a consequence of the inhibition of putrescine biosynthesis

Biological transmethylation reactions and polyamine biosynthesis share the substrate S-adenosyl-L-methionine. Under normal conditions, decarboxylated S-adenosyl-L-methionine, the aminopropyl donor for polyamine biosynthesis, does not accumulate because of its rapid utilization in spermidine and spermine synthesis. Alteration of polyamine synthesis by DL-alpha-difluoromethylornithine, an enzyme-activated irreversible inhibitor of L-ornithine decarboxylase, leads to a striking accumulation of decarboxylated S-adenosyl-L-methionine in rat hepatoma cells cultured in vitro and in rat ventral prostate. This increase is due both to lack of putrescine and spermidine for the aminopropyltransferase reactions and to the elevation of S-adenosyl-L-methionine decarboxylase activity. The biological implications of accumulation of decarboxylated S-adenosyl-L-methionine are discussed with regard to the regulation of S-adenosyl-L-methionine decarboxylase activity and to the antiproliferative effects of DL-alpha-difluoromethylornithine.

Regulation of tRNA methyltransferase activities by spermidine and putrescine. Inhibition of polyamine synthesis and tRNA methylation by alpha-methylornithine or 1,3-diaminopropan-2-ol in Dictyostelium

Inhibitors of polyamine synthesis (alpha-methylornithine and 1,3-diaminopropan-2-ol) were used to study the relationship between polyamine synthesis and specific methylations of tRNA in Dictyostelium discoideum during vegetative growth. Polyamine concentrations were found to be 10 mM for putrescine, 1.6 mM for spermidine and 7 mM for 1,3-diaminopropane throughout the growth stage. On treatment of growing amoebae with alpha-methylornithine or with 1,3-diaminopropan-2-ol (each at 5 mM), the syntheses of putrescine, spermidine and 1,3-diaminopropane were arrested within 4h. After polyamine synthesis had ceased, the incorporation of methyl groups into tRNA was considerably decreased under conditions that had no effect on the incorporation of uridine into tRNA, or on net syntheses of protein and of DNA. The following nucleosides in tRNA were concerned: 1 methyladenosine, 5-methylcytidine, 7-methylguanosine, 2-methylguanosine, N2N2-dimethylguanosine and 5-methyluridine (ribosylthymine). The corresponding tRNA methyltransferases, determined in Mg2+-free enzyme extracts, proved to be inactive unless polyamines were added. Putrescine and/or spermidine at concentrations of 10 mM or 1-2 mM respectively stimulate the transmethylation reaction in vitro to a maximal rate and to an optimal extent at exactly the same concentrations as found in vegetative cells. In contrast, 1,3-diaminopropane, which is formed from spermidine, does not affect the methylation of tRNA in vitro at physiological concentrations. Putrescine and/or spermidine stabilize the tRNA methyltransferases in crude extracts in the presence but not in the absence of the substrate tRNA. The results support the view that S-adenosylmethionine-dependent transmethylation reactions can be regulated by alterations of polyamine concentrations in vivo.

Differences between tissues in response of S-adenosylmethionine decarboxylase to administration of polyamines

1. Administration of spermidine or sym-norspermidine decreased the activity of AdoMet (S-adenosylmethionine) decarboxylase in extracts prepared from rat liver, Kidney, psoas, diaphragm, soleus and small intestine, but not heart. The decline in psoas, diaphragm and soleus was much greater than that in liver and kidney. The difference in sensitivity to spermidine could not be explained by changes in the uptake and accumulation of the polyamine, because much higher contents were found in liver and kidney that in diaphragm and psoas. 2. Spermidine administration also led to a substantial increase in putrescine in all tissues examined. However, the rise in putrescine was not responsible for the decline in AdoMet decarboxylase activity, since norspermidine, which cannot form putrescine, also produced the decline. Also, administration of putrescine or 1,3-diaminopropane did not decrease AdoMet decarboxylase. 3. The decline in skeletal-muscle AdoMet decarboxylase activity in response to spermidine may be due to an increased rate of degradation of the enzyme protein. The t1/2 (half-time) for the decline in activity after inhibition of protein synthesis by cycloheximide was almost halved in the psoas of spermidine-treated rats. Spermidine treatment did not change the t1/2 in liver. 4. These results raise the possibility that there are at least two different forms of AdoMet decarboxylase and that the enzyme from psoas or diaphragm differs from that in liver. Additional support for this hypothesis was obtained by comparing the activation by putrescine of AdoMet decarboxylase from these tissues. The liver enzyme was stimulated 10-fold, but the muscle enzyme was stimulated 30-fold.

Induction of polyamine limitation in Chinese hamster ovary cells by alpha-methylornithine

Chinese hamster ovary (CHO) cells in culture were limited for polyamines through the use of alpha-methylornithine (alpha MO), a competitive inhibitor of ornithine decarboxylase. Initial exposure of the cells to the inhibitor caused growth rate and intracellular polyamine content to decline continuously. Reseeding the alpha MO-treated cells into medium containing the inhibitor resulted in steady-state (exponential) growth at cell densities below 5 x 10(3) cells/cm2, at a rate approximately twofold slower than untreated cells. Under these conditions, putrescine and spermidine were undetectable and spermine remained relatively constant at a level approximately half that found in untreated cells. Addition of exogenous putrescine elevated the polyamine content and stimulated the growth of alpha MO-treated cultures. Thus, growth rate correlated with polyamine content in the alpha MO-treated cells. The growth of reseeded, alpha MO-treated cells became nonexponential at a density (5 x 10(3) cells/cm2) far below that at which untreated cells departed from exponential growth (1 x 10(5) cells/cm2). Medium obtained from high density, alpha MO-treated cultures inhibited the growth of cells at low density in the presence of alpha MO. Doubling the concentration of the defined components of conditioned medium did not markedly affect its capacity to inhibit growth. However, dialysis completely not markedly affect its capacity to inhibit growth. However, dialysis completely removed the inhibitory activity from conditioned medium. The results imply that a low molecular weight inhibitor of growth is produced by polyamine-limited cells. This is a variable that must be controlled in studies with polyamine-limited animal cells. Morphological studies indicated that subcellular organelles, including mitochondria, were largely unaffected by treatment with alpha MO. The maintenance of mitochondrial integrity in the presence of alpha MO demonstrates that the swelling of mitochondria observed previously in cells treated with methylglyoxal bis(guanylhydrazone) was not due to polyamine limitation. alpha MO-treated cells did, however, accumulate numerous cytoplasmic vacuoles. The identity of these vacuoles and their relationship to cellular physiology is not yet understood.

Indirect evidence for a strict negative control of S-adenosyl-L-methionine decarboxylase by spermidine in rat hepatoma cells

1. Direct or indirect inhibitors of l-ornithine decarboxylase (EC 4.1.1.17), structurally related or unrelated to l-ornithine, including dl-alpha-difluoromethylornithine, alpha-methylornithine and 1,3-diaminopropane, used alone or in combination, decreased polyamine concentrations in rat hepatoma tissue culture (HTC) cells and increased S-adenosyl-l-methionine decarboxylase activity (EC 4.1.1.50). 2. Comparison of the catalytic properties of S-adenosyl-l-methionine from cells with elevated and normal activities revealed no apparent modification of the catalytic site as judged by affinity for the substrate, stimulation by di- and tri-amines and inhibition by methylglyoxal bis-(guanylhydrazone). 3. Actinomycin D and cycloheximide, and RNA and a proteinsynthesis inhibitor respectively, blocked the increase of S-adenosyl-l-methionine decarboxylase activity elicited by alpha-difluoromethylornithine. In polyamine-depleted cells the apparent half-life of elevated S-adenosyl-l-methionine decarboxylase activity, determined by inhibition of protein synthesis, was 2.5-fold longer than in control cells. The present results suggest that elevation of S-adenosyl-l-methionine decarboxylase activity by alpha-difluoromethylornithine is due to stabilization of the enzyme. 4. Restoration of the normal intracellular putrescine content, by addition of putrescine to the medium of polyamine-deficient cells, transiently increased S-adenosyl-l-methionine decarboxylase activity. Thereafter, intracellular conversion of putrescine into spermidine was accompanied by inactivation of the enzyme at a rate that was similar to that found on addition of spermidine itself. No relationship between total intracellular spermine content and S-adenosyl-l-methionine decarboxylase activity could be established. 5. Addition of 1mm-1,3-diaminopropane to polyamine-deficient cells did not cause a decrease in the activity of S-adenosyl-l-methionine decarboxylase, whereas addition of 1,5-diaminopentane (cadaverine) did. 1,3-Diamino-N-(3-aminopropyl)propane did not accumulate in cells treated with alpha-difluoromethylornithine and 1,3-diaminopropane, whereas addition of 1,5-diaminopentane led to the accumulation of 1,5-diamino-N-(3-aminopropyl)pentane. 1,3-Diamino-N-(3-aminopropyl)propane (10mum) was as effective as spermidine in decreasing S-adenosyl-l-methionine decarboxylase activity. Thus effectiveness of a diamine in decreasing enzyme activity is related to its capability of being converted into a closely structurally related homologue of spermidine by spermidine synthase. 6. The spermidine site of action appears to be post-translational since (a) the spermidine-induced decrease of S-adenosyl-l-methionine activity was not prevented by actinomycin D and (b) spermidine in the presence of cycloheximide led to a synergistic inactivation of the enzyme with a decay rate that progressively approached control values. Altogether these results are indirect evidence for a strict negative control of S-adenosyl-l-methionine decarboxylase by spermidine and substantiate previous findings [Mamont, Duchesne, Grove & Tardif (1978) Exp. Cell Res.115, 387-393]. Spermidine appears to act on some processes involved in denaturation and/or degradation of the enzyme protein. Putrescine appears to decrease the rate of these processes. The physiological significance of the regulatory control of S-adenosyl-l-methionine decarboxylase is discussed.