Ornithine decarboxylase in difluoromethylornithine-resistant mouse lymphoma cells. Two-dimensional gel analysis of synthesis and turnover

Isolation and characterization of human breast adenocarcinoma cells made resistant to alpha-difluoromethylornithine

Human breast adenocarcinoma cells MCF-7 were selected for resistance to ornithine decarboxylase (ODC) inhibitor, alpha-difluoromethylornithine (DFMO). Stepwise increments of the concentration of DFMO resulted in selection of MCF-7 cells that were capable of growing in the presence of 1.0 mM DFMO. This capacity was associated with a 10-fold increase in ODC activity and marked enhancement in the synthesis rate of ODC protein as verified by a 2-hr [35S]methionine labeling of cellular proteins followed by immunoprecipitation and SDS-PAGE. The resistant cells had much higher concentration of putrescine, spermidine, and spermine than the control cells. A 25-fold increase in ED50 (effective dose causing 50% inhibition) for the antiproliferative action of DFMO in these resistant cells was observed. The susceptibility of wild-type and resistant cell lines to other inhibitors of the polyamine biosynthetic pathway and adriamycin is also reported.

Growth-inhibitory effect of a high glucose concentration on osteoblast-like cells

Impaired bone formation resulting from a decline of osteoblast activity has been implicated in the pathogenesis of diabetic osteopenia. We examined the effects of high glucose concentration alone, independent of insulin deficiency, on the growth of a human osteoblast-like cell line (MG-63). Sustained exposure to high glucose for 7 days inhibited cell growth in a concentration-dependent manner up to 49.5 mmol/L, as compared with cells cultured with a normal glucose concentration (5.5 mmol/L) or a high mannitol concentration (an iso-osmolar control). Glucose (49.5 mmol/L) attenuated the increment either in DNA content or in [3H]thymidine incorporation induced by insulin-like growth factor I (IGF-I). The IGF-I-induced increase of ornithine decarboxylase (ODC) activity, which plays an important role in cell growth, was also attenuated. The half-life of ODC protein was not shortened by the high glucose culture, but the intracellular content of putrescine (an end product of ODC) was significantly decreased. These changes did not occur in the high mannitol culture, strongly suggesting a specific effect of glucose. In summary, our observations suggest that a high glucose concentration significantly impairs the proliferative response of osteoblastic cells to IGF-I and that the defective cell function caused by sustained exposure to high glucose levels might contribute to impaired bone formation in patients with diabetic osteopenia.

Phenotypic features of breast cancer cells overexpressing ornithine-decarboxylase

Polyamines (PA) have been shown to be critical mediators of estradiol-induced breast cancer cell proliferation. This finding suggests that constitutive activation of the PA pathway may promote tumor progression, possibly leading to hormone independence. To test this hypothesis, we transfected hormone-responsive MCF-7 breast cancer cells with a complementary DNA coding for ornithine-decarboxylase (ODC), the first rate-limiting enzyme in PA biosynthesis. Marked ODC overexpression observed in stably transfected clones was associated with a selective increase in cellular putrescine content, while spermidine and spermine levels were not altered. ODC-overexpressing MCF-7 cells were resistant to the antiproliferative effects of low but not high concentrations of the enzyme inhibitor, alpha-difluoromethylornithine. In agreement with our hypothesis, sensitivity to the growth-promoting action of estradiol was reduced by approximately one third (P < 0.001) in ODC-overexpressing MCF-7 cells compared with vector-only transfected clones. Basal growth under anchorage-dependent conditions was only marginally increased by ODC overexpression (P = 0.048), while clonogenicity in soft agar was actually reduced. These data suggest that activation of PA biosynthesis may contribute in part to the acquisition of estrogen independence by breast cancer cells. Since only putrescine content was increased as a result of ODC overexpression, these data may underestimate the overall influence of the PA pathway on breast cancer phenotype.

Overproduction of S-adenosylmethionine decarboxylase in ethylglyoxal-bis(guanylhydrazone)-resistant mouse FM3A cells

A variant cell line, termed SAM-1, which overproduced S-adenosylmethionine decarboxylase (AdoMetDC), was isolated by treatment of mouse FM3A cells with N-methyl-N'-nitro-N-nitrosoguanidine and subsequent incubation with ethylglyoxal bis(guanylhydrazone), an inhibitor of the enzyme. The cells were resistant to ethylglyoxal bis(guanylhydrazone), and showed AdoMetDC activity approximately five-times higher than control cells. The rate of AdoMetDC synthesis and the amount of AdoMetDC existing in SAM-1 cells were about five-times those in control cells. The amount of AdoMetDC mRNA existing in SAM-1 cells was five-times more than that in control cells. The amount of 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine, an irreversible inhibitor of AdoMetDC, necessary to inhibit cell growth was also five-times more in SAM-1 cells than in control cells. However, the following were the same in both SAM-1 and control cells; the amount of genomic DNA for AdoMetDC, the size and nucleotide sequence of 5' untranslated region of AdoMetDC mRNA, the deduced amino acid sequence (334 residues) from the nucleotide sequence of AdoMetDC cDNA and the degradation rate (t1/2 = about 4 h) of AdoMetDC. In addition, AdoMetDC mRNA in control cells was slightly more stable than that in SAM-1 cells. The results indicate that the overproduction of AdoMetDC in SAM-1 cells was caused by the increase of AdoMetDC mRNA. The variant cell line is convenient for studying the regulation of AdoMetDC and the physiological function of polyamines.

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.

Poly(A+)RNA levels of growth-, differentiation- and transformation-associated genes in the progressive development of hepatocellular carcinoma in the rat

The development of chemically induced hepatocellular carcinoma in the rat proceeds through a series of premalignant changes that may ultimately progress to a primary malignant tumor. Using the selection technique based on diminished binding of preneoplastic hepatocytes to tissue culture plates precoated with asialofetuin, we have isolated poly(A+)RNA from early preneoplastic foci as well as preneoplastic persistent nodules and primary hepatocellular carcinoma induced by the Solt-Farber protocol in the Fischer rat. The steady-state poly(A+)RNA levels of genes traditionally associated with growth, differentiation and/or transformation were then determined to address the question of their temporal expression in the multistep nature of cancer development. Ornithine decarboxylase- and P53-specific transcripts did not significantly change in preneoplastic foci but were increased in later-stage preneoplastic nodules and hepatocellular carcinoma. Albumin-specific transcripts were decreased in all hepatocellular carcinoma but there was no consistent coordinated increase in alpha-fetoprotein-specific transcripts. c-myc and raf transcripts increased at the very early preneoplastic foci stage and continued to increase throughout the neoplastic process. No L-myc or N-myc transcripts could be detected in any RNA sample. c-Ha-ras-specific transcripts were essentially unaltered in all RNA samples whereas no c-Ki-ras or N-ras transcripts could be detected throughout the neoplastic process. In addition, no dominant-acting transforming mutations in the ras gene family were detected by DNA transfection experiments using NIH/3T3 cells.

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.

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.

Interrelation between the charge isoforms of mammalian ornithine decarboxylase

Ornithine decarboxylase (ODC) isolated from a variety of tissues has been separated, using DEAE ion-exchange chromatography, into multiple peaks of activity that appear to be related to control of this enzyme stability. Reports of these charge isoforms in current literature are generally unclear as to whether these represent a covalent posttranslational modification or merely an alteration in structural conformation or association. In this study we investigated the relationship of this form separation to the degree of enzyme polymerization, interaction with other proteins and buffer components, and the multiple isoelectric forms of this enzyme noted in denaturing concentrations of urea. High-performance chromatography techniques were used to demonstrate that two of the major enzyme forms, ODC I and II, are really monomers of the enzyme, while minor peaks of activity frequently observed to elute after ODC II contain various dimeric enzyme states. Pyridoxal 5'-phosphate (0.05 mM) added to isolated enzyme preparations composed of I and II monomers induced the formation of I and II dimers as well as a mixed I-II dimer. All three dimer forms were observed to be natural components of freshly isolated crude cell homogenates. The charge distinction between the monomer forms I and II was found to be maintained during ion-exchange chromatography in the presence of 8 M urea, and the enzyme isoforms demonstrated distinct bands on isoelectric focusing gels run in the presence of 9 M urea. Thus, although some of the multiple ornithine decarboxylase forms identified by ion-exchange chromatography of crude mammalian cell homogenates are related to enzyme conformation, the two major forms are distinctly charged protein states that can be visualized using two-dimensional gel electrophoresis of highly purified samples.

Phorbol myristate acetate inhibits growth in S49 cells: isolation of resistant variants

We have used S49 mouse lymphoma cells to study phorbol ester effects on growth. Treatment of wild-type (wt) cells with phorbol 12-myristate 13-acetate (PMA) results in growth arrest within 72 hr. We have selected variants that are resistant to PMA-induced growth arrest, based on a selection in the presence of 10 nM PMA. We have characterized one of these variants, termed 21.1, in detail. The 21.1 and wt cells contain similar levels of protein kinase C (PKC) as determined by [3H]phorbol 12,13-dibutyrate ([3H]PDBu) binding. Treatment of both wt and 21.1 cells with PMA results in translocation of PKC to the membrane, suggesting that the coupling between PKC and an immediate biological response is intact. PMA treatment leads to the phosphorylation of many similar proteins in wild-type and 21.1 cells. However, in the 21.1 cells there is a prominent substrate of approximately 70 kilodaltons (kD) which is no longer phosphorylated after PMA treatment. In wild-type cells ornithine decarboxylase (ODC) activity and mRNA levels are decreased within 1 hr of PMA treatment. Likewise, ODC levels are decreased in the 21.1 cells after exposure to PMA even though PMA only slightly modulates the growth of these cells. The 21.1 cells represent a unique line with a dominant phenotype in which ODC expression is uncoupled from the growth state of the cell. These cells may represent a good model system in which to examine the steps involved in phorbol ester growth regulation in S49 cells.

A rat monoclonal antibody which interacts with mammalian ornithine decarboxylase at an epitope involved in phosphorylation

Ornithine decarboxylase was purified from androgen-treated mouse kidney to homogeneity and high specific activity. The purified enzyme was utilized for production and screening of rat monoclonal and polyclonal antibodies. A rat monoclonal antibody was isolated which was capable of immunoprecipitation of native mouse kidney ornithine decarboxylase activity or the [3H]difluoromethylornithine-inactivated enzyme. Phosphorylation of mouse ornithine decarboxylase by casein kinase-II prior to immunoprecipitation led to complete loss of the epitope recognized by the monoclonal antibody but did not alter recognition by polyclonal antibody. Mammalian ornithine decarboxylase activity obtained from several species, in crude or partially purified extracts, was subjected to quantitative immunoprecipitation with monoclonal and polyclonal antibody. Polyclonal antibody immunoprecipitated all of the ornithine decarboxylase activity from every extract tested, while monoclonal antibody was capable of only limited immunoprecipitation (60-80%). Due to the inability of the monoclonal antibody to recognize ornithine decarboxylase phosphorylated in vitro by casein kinase-II and the partial immunoprecipitation of ornithine decarboxylase activity from cell extracts, a portion of the ornithine decarboxylase molecule population must exist in a phosphorylated state. This immunological evidence further confirms existing data that the enzyme exists in at least two distinct forms.

Effects of DL-alpha-difluoromethylornithine on Leishmania donovani promastigotes

Alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase, has been demonstrated to be an effective agent against a variety of parasitic protozoa but not against Leishmania spp. In this report, we show that Leishmania donovani promastigotes in continuous culture are sensitive to the growth inhibitory and cytotoxic effects of DFMO. Incubation of the promastigotes with DFMO obliterates intracellular putrescine pools and depletes spermidine concentrations, which correlates with the onset of growth inhibition. The effects of DFMO on the growth and the intracellular polyamine pools can be reversed completely by the addition of 10 microM putrescine to the culture medium. These results suggest that the treatment of leishmaniasis may be amenable to chemotherapeutic manipulation by DFMO.

An inhibitor of ornithine decarboxylase in lactogen-deprived Nb2 node rat lymphoma cells

A previous study has shown that the activity of ornithine decarboxylase in cultured Nb2 node rat lymphoma cells falls to undetectable levels when cells become quiescent following incubation in lactogen (prolactin)-deficient medium. In the present study, it was found that addition of extracts of the lactogen-deprived, quiescent cells to extracts of log-phase cells markedly reduced the ornithine decarboxylase activity of the latter, the inhibitory activity being proportional to the amount of quiescent cell extract added. Evidence is presented that the ornithine decarboxylase-inhibitory activity in the quiescent cell extracts is due to an antizyme-like, polypeptide factor with an Mr of approx. 28,000. The activity of the inhibitor appears to be directed rather specifically against ornithine decarboxylase, since the activities of S-adenosylmethionine decarboxylase, thymidine kinase and uridine kinase were not affected. The Nb2 cell ornithine decarboxylase inhibitor may have an important role in modulating the cellular levels of ornithine decarboxylase as they change in response to the withdrawal and restoration of extracellular mitogenic lactogens.

Effects of human interferon and alpha-difluoromethylornithine on T47D cells

The effect of human interferon (IFN) and alpha-difluoromethylornithine (DFMO), an enzyme-activated irreversible inhibitor of eukaryotic ornithine decarboxylase, on the rate of DNA synthesis and the increase of ornithine decarboxylase activity of T47D cells was examined. It was found that IFN or DFMO alone causes little or appreciable inhibition of the [3H]thymidine incorporation, respectively. Each of the drugs alone has a significant inhibitory effect on ornithine decarboxylase activity. Combination of the two drugs has a synergistic effect and eliminates completely the [3H]thymidine incorporation and the activity of ornithine decarboxylase. The biological implication of IFN and DFMO is discussed with regard to the regulation of ornithine decarboxylase activity and the antiproliferative effects of the two drugs.

Different factors possibly involved in post-translational regulation of ornithine decarboxylase activity

Present results concern a microsome-bound enzymatic system which has been recognized as responsible for the rapid inactivation in vitro of ornithine decarboxylase (ODC). Two different models have been investigated: a) rat liver after a single thioacetamide administration, and b) the 3924 A Morris hepatoma. In both these models we observed variations in the microsome-bound ODC-inactivating capacity. In parallel, changes in ODC properties were observed. The possibility of a causal relationship between the two events is discussed. The actual role of the microsome-bound ODC-inactivating system, in ODC activity regulation in vivo cannot be established, but it remains as a fairly plausible working hypothesis.

Studies of gene transcription and translation in regenerating rat liver

Specific transcriptional and translational products associated with regenerating liver were analyzed by differential hybridization to a cDNA library and by two-dimensional electrophoresis of hepatic proteins, respectively. Comparisons of approximately 800 soluble and 800 particulate liver proteins from normal and 70% partially hepatectomized Fischer rats resulted in the identification of only three apparently unique polypeptides in 70% partially hepatectomized livers, although many quantitative changes were observed. A subset of these quantitative changes were also observed after sham operation. A cDNA library was generated from polyadenylated RNA isolated 18 hr post-70% partial hepatectomy. Comparative analysis of 6,000 transformants with single-stranded cDNA probes prepared from 18 hr post-70% partial hepatectomy and sham-operated animals identified three clones whose sequences were preferentially expressed 4- to 6-fold 18 hr post-70% partial hepatectomy. Southern blot analysis of one clone, REG-A, showed no homology to albumin, alpha-fetoprotein, three different forms of cytochrome P-450, ornithine decarboxylase, globin, or to a putative tumor promotion associated gene called PRO-2. A single, REG-A specific 2.5 kb band was identified by Northern blot analysis of liver samples. REG-A expression was increased 2-fold 18 hr postsham operation; 4-fold 18 hr post-70% partial hepatectomy and following chronic 2,3,7,8-tetrachlorodibenzo-p-dioxin or phenobarbital treatment. REG-A expression returned to control levels 1 week after 70% partial hepatectomy. Furthermore, expression of REG-A was reduced in chemically induced preneoplastic nodules and in primary and transplantable hepatomas. Hybrid selection studies indicated that the REG-A sequence selected a mRNA(s) species, that in an in vitro translation assay, produced two major polypeptides of 21,000 and 25,000 molecular weight with a pI of 6.9. Thus, these data support the hypothesis that liver regeneration is characterized by quantitative changes in genes normally expressed at low levels in the Go hepatocyte and is not the result of major qualitative changes in gene expression.

Intrinsic and extrinsic factors in estrogen action in human breast cancer: role of polyamines and pituitary factors

Although polyamines are important in regulating proliferation of mammalian cells, their role in hormone induction of cell growth has not been delineated. In the estradiol-responsive human breast cancer cell line, T-47D clone 11, estradiol (10(-10) M) was able to stimulate cell proliferation and the activity of ornithine decarboxylase (ODC), the first and rate-limiting enzyme in the biosynthesis of polyamines. alpha-Difluoromethylornithine (DFMO), a specific inhibitor of ODC, blocked the estradiol-induced cell proliferation and ODC activity. Exogenous addition of putrescine, the natural product of ODC, rescued the inhibitory effect of DFMO. In addition, DFMO abolished the estradiol-induced growth of several other estrogen-responsive human breast cancer cell lines but did not affect the growth of hormone-independent cell lines. Further, a serum factor was found to be required for estradiol to exert its effect. To gain insight into the nature of this and possibly other extrinsic factors involved, the effect of estradiol on the proliferation of T-47D cells transplanted into athymic nude mouse was evaluated. In this in vivo system, estradiol alone produced only moderate growth of the human breast tumor. The simultaneous transplantation of a prolactin (PRL)- and growth hormone (GH)-secreting rat pituitary tumor or normal rat pituitary glands at a different site dramatically potentiated the effect of estradiol on the growth of the breast tumor xenograft. Purified PRL or GH were without effect, indicating that the active pituitary factor is neither PRL nor GH. Further, conditioned medium from rat pituitary tumor cells potentiated the mitogenic effect of estradiol on T-47D and several other estrogen receptor-positive human breast cancer cell lines in vitro under serum-free condition. In conclusion, we have identified both intrinsic (polyamines) and extrinsic (pituitary/serum) factors that are importance for estrogen to exert its mitogenic action. The next goal will be to elucidate the mechanisms of action of these molecules in the modulation of estrogen action.

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.

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

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

Polyamine biosynthetic enzymes as targets in cancer chemotherapy

In this chapter we focus attention on recent developments in the biosynthesis of putrescine, spermidine and spermine and their linkage to salvage pathways of methionine and adenine nucleotide synthesis. We describe the use of specific inhibitors of the polyamine biosynthetic enzymes for studying the role of polyamines in cell growth and division as well as in cell differentiation. Some novel findings are presented which suggest that part of the inhibitory action that polyamine synthesis inhibitors exert on DNA synthesis may be due to the accumulation of ADP and ATP. We show that polyamine synthesis inhibitors are capable of inducing terminal differentiation of neoplastic cells to forms with no further proliferative potential, and briefly discuss the potential use of this approach in cancer chemotherapy.