Human myeloma cells acquire resistance to difluoromethylornithine without overproducing ornithine decarboxylase

Automated fluorescent genomic sequencing as applied to the methylation analysis of the human ornithine decarboxylase gene

A genomic sequencing method for an automated DNA sequencer was developed. The method described here is an improved version of the previously published protocol, which utilizes bisulfite-induced modification of genomic DNA. In our method, the modified DNA is purified without a time-consuming dialysis, and the subsequent 2-step DNA amplification is carried out with one biotinylated primer in order to separate and isolate the strands of the product with the aid of streptavidin-coated magnetic beads. The strands are then sequenced with fluorescent primers and automated DNA sequencer. This provides means to determine reliably the methylation status of cytosines as well as the degree of methylation in a given CpG, site of the target sequence. The method was successfully applied to analyze the promoter region and the 11th exon of the human ornithine decarboxylase ODC gene in various human myeloma cell lines. The study revealed a totally unmethylated promoter region in every cell line studied, whereas the protein coding region appeared to be extensively methylated, although a dexamethasone resistant cell line displayed demethylation in certain CpG sequences. Also, a previously unknown ODC allele was detected.

Certain changes in ornithine decarboxylase gene methylation accompany gene amplification

The ornithine decarboxylase (ODC; EC 4.1.1.17) gene in parental, dexamethasone-resistant and 2-difluoromethylornithine (DFMO)-resistant human IgG-myeloma-cell lines was studied with the aid of methylation-sensitive restriction endonucleases and probes recognizing different parts of the gene. In all cell lines the promoter region of the ODC gene appeared to be heavily methylated, whereas the first long intron was unmethylated. Methylation analyses of several clones from the parental cell line revealed that these cells are heterogeneous with respect to the methylation status of the ODC gene, whereas all clones from DFMO-resistant cell lines displayed the same methylation pattern. Two of the parental clones represented a hypomethylated type very close to that exclusively found among the DFMO-resistant clones with ODC gene amplification. This typical methylation pattern was due to decreased methylation of a few CCGG sequences in the 3'-flanking region of the gene. It is possible that this kind of hypomethylation favours the initiation of the gene-amplification process in certain individual cells. This hypothesis was supported by the finding that no hypomethylation was present in the ODC gene of another human myeloma cell line that had acquired resistance to DFMO without gene amplification. In a dexamethasone-resistant cell line that overproduced ODC mRNA at normal gene dosage there were some minor differences between the methylation pattern of the ODC gene of different clones, but no such hypomethylation could be found in clones from the parental cell line. In dexamethasone-resistant cells the ODC gene was hypomethylated around the two HpaII sites and three CfoI sites in the coding region and also, as well as in cells with amplified ODC sequences, in the 3'-flanking region of the gene. Some hypomethylation in the distant 5'-flanking region was also observed.

Levels of ornithine decarboxylase genomic sequences, heterogeneous nuclear RNA and mRNA in human myeloma cells resistant to alpha-difluoromethylornithine

We previously isolated and characterized a human myeloma cell line overproducing ornithine decarboxylase (ODC) due to gene amplification [Leinonen, Alhonen-Hongisto, Laine, Jänne & Jänne (1987) Biochem. J. 242, 199-203]. We have now employed the PCR combined with reverse transcription to determine semiquantitatively ODC gene dosage and the amounts of heterogeneous nuclear (hn) RNA and of mature mRNA of the enzyme in parental and alpha-difluoromethylornithine-resistant human myeloma cells. Experiments with dilution series revealed that the ODC gene copy number and the amount of both hnRNA and mRNA were increased to the same extent (about 100-fold) in the resistant cells. Similar dot-blot analyses of ODC-specific genomic DNA and total RNA indicated that the ODC gene copy number was increased by a factor of 380 and the amount of ODC mRNA by a factor of 700. Our results indicate that the PCR combined with reverse transcription is at least as useful as blot analyses to give semiquantitative assessment of the amounts of specific DNA or RNA sequences. In addition, the use of the PCR enables the analysis of minute sample amounts in extremely short time.

Polyamines: from molecular biology to clinical applications

The polyamines putrescine, spermidine and spermine represent a group of naturally occurring compounds exerting a bewildering number of biological effects, yet despite several decades of intensive research work, their exact physiological function remains obscure. Chemically these compounds are organic aliphatic cations with two (putrescine), three (spermidine) or four (spermine) amino or amino groups that are fully protonated at physiological pH values. Early studies showed that the polyamines are closely connected to the proliferation of animal cells. Their biosynthesis is accomplished by a concerted action of four different enzymes: ornithine decarboxylase, adenosylmethionine decarboxylase, spermidine synthase and spermine synthase. Out of these four enzyme, the two decarboxylases represent unique mammalian enzymes with an extremely short half life and dramatic inducibility in response to growth promoting stimuli. The regulation of ornithine decarboxylase, and to some extent also that of adenosylmethionine decarboxylase, is complex, showing features that do not always fit into the generally accepted rules of molecular biology. The development and introduction of specific inhibitors to the biosynthetic enzymes of the polyamines have revealed that an undisturbed synthesis of the polyamines is a prerequisite for animal cell proliferation to occur. The biosynthesis of the polyamines thus offers a meaningful target for the treatment of certain hyperproliferative diseases, most notably cancer. Although most experimental cancer models responds strikingly to treatment with polyamine antimetabolites--namely, inhibitors of various polyamine synthesizing enzymes--a real breakthrough in the treatment of human cancer has not yet occurred. It is, however, highly likely that the concept is viable. An especially interesting approach is the chemoprevention of cancer with polyamine antimetabolites, a process that appears to work in many experimental animal models. Meanwhile, the inhibition of polyamine accumulation has shown great promise in the treatment of human parasitic diseases, such as African trypanosomiasis.

Human ornithine decarboxylase(ODC)-encoding gene: cloning and expression in ODC-deficient CHO cells

We have cloned a full-length human ornithine decarboxylase (ODC)-encoding gene from a genomic library of human myeloma cells which overproduce ODC due to a selective gene amplification. Correct expression of the cloned gene was assessed by transfecting it into a Chinese hamster ovary (CHO) cell mutant devoid of ODC activity. Transfection with a 10-kb BamHI DNA fragment of the genomic clone, conferred ODC activity to the recipient cells and relieved them of dependence on exogenous polyamines for growth. A set of 40 transformants was isolated, eight of which were further characterized. The transfected ODC gene appeared to be hypomethylated at the cytosine residues in the sequence CpG. The transfectants were all responsive to serum stimulation, but showed different levels of ODC expression depending on both copy number and integration site of the transfected ODC gene. ODC serum induction in the transfectants was sensitive to cycloheximide and polyamine additions, and the half-life of the enzyme was very short, like that in normal CHO cells. These results suggest that the human ODC gene we transfected contains all the elements needed for normal control of ODC expression.

Characterization of difluoromethylornithine-resistant mouse and human tumour cell lines

Four mouse and two human tumour cell lines resistant to alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase (ODC), were analysed for the activities of polyamine-biosynthetic and -biodegradative enzymes as well as for cellular polyamine contents. In all but one of these cell lines the resistance to DFMO was based on an overproduction of ODC. In a human myeloma cell line the resistance was based on a greatly enhanced arginase activity. Except for one L1210 variant cell line, all the resistant cell lines contained elevated S-adenosylmethionine decarboxylase activity. Similarly, all the resistant mouse, but not human, cell lines displayed enhanced spermidine and spermine synthase activities. Arginase activity was detected only in human cell lines. In both DFMO-resistant cell lines the activity of arginase was strikingly elevated. Of the biodegradative enzymes, polyamine oxidase activity was readily detectable in all mouse cells, but no measurable activity was found in the human cells. Spermidine/spermine N1-acetyltransferase activity was elevated in three out of four resistant mouse cell lines. Even though the concentration of spermidine was usually lower in the overproducer cells, this was compensated by an increased content of spermine. The two resistant human myeloma cells contained intracellular ornithine concentrations that were from more than 5 to more than 20 times higher than those in the parental cells.

Molecular genetics of ornithine decarboxylase in human tumor cells

As the molecular biology of mammalian ornithine decarboxylase is coming of age, more and more interesting features of this unique protein are being uncovered. Ornithine decarboxylase belongs to those 20 or so enzymes or binding proteins, the genes for which are easily amplified under suitable selection pressure. This also applies to the human enzyme. Gene amplification of ornithine decarboxylase is not the only means to acquire resistance to inhibitors of the enzyme, as its overproduction can occur through an enhanced transcription or even through a more efficient translation of normal mRNA amounts. The resistance in human tumors can likewise be acquired by activating other enzymes, such as arginase. In contrast to the multigene family in mouse, it appears that in the human genome only two ornithine decarboxylase genes are present mapping to the chromosomes 2 and 7. Out of these, at least the sequences in the short arm of chromosome 2 are transcriptionally active and amplifiable. Human ornithine decarboxylase also belongs to those proteins which show a positive correlation between gene hypomethylation and expression. The genes of human ornithine decarboxylase are methylated to varying extents and distinct hypomethylation is seen in certain malignant cells, most notably human lymphatic leukemia cells. The human ornithine decarboxylase gene is easily transferrable into other mammalian cells in which it is efficiently expressed. Some indirect evidence seems to indicate that overproduction of ornithine decarboxylase may confer a growth advantage to mammalian cells. A further piece of information suggesting an important role for the enzyme is the fact that the structure of the ornithine decarboxylase gene is extremely well conserved during evolution. This does not apply only to the coding region but also to the overall organization of the gene itself.

The role of polyamines in interferon and retinoic acid mediated synergistic antiproliferative action

Retinoic acid alone has no effect on the human breast cancer cell line BT-20 but can amplify the antiproliferative action of interferon-gamma (IFN-gamma). In our system ornithine decarboxylase (ODC) activity correlates well with growth rate; it was investigated whether the antiproliferative effects of IFN-gamma and IFN-gamma plus retinoic acid could be attributed to suppression of ODC activity. The ODC inhibitor difluoromethylornithine (DFMO), which is active as a single agent did not enhance growth inhibition induced by the biological response modifiers. The substitution of the BT-20 cells with putrescine, the product of the enzymatic reaction mediated by ODC, reversed DFMO induced antiproliferative action. On the other hand putrescine did not affect the proliferation of BT-20 cells treated with interferon alone or in combination with retinoic acid.

Excretion of acetylated and free polyamines by polyamine depleted Chinese hamster ovary cells

1. Cultured Chinese hamster ovary cells (CHO) and their ornithine decarboxylase deficient mutant cells (C55.7) were found to excrete small amounts of N8-acetylspermidine and free polyamines, putrescine and spermidine into the culture medium. 2. The concentration of N8-acetylspermidine in the control cells was 2-3% of that of spermidine. In the medium, however, the amount of N8-acetylspermidine was about 2-fold that of spermidine and 2- to 3-fold higher than the intracellular amount. N1-acetylspermidine or acetylated spermine were never detected in the cells or in the media. 3. Confluent CHO cells treated with 2 mM difluoromethylornithine stopped the excretion when the intracellular spermidine concentration had decreased to 20% of control while there was no decrease in spermine concentration. At low cell density, neither polyamine depleted CHO cells nor the C55.7 cells excreted any polyamines into the culture media.

Cadaverine supplementation during a chronic exposure to difluoromethylornithine allows an overexpression, but prevents gene amplification, of ornithine decarboxylase in L1210 mouse leukaemia cells

We recently selected a variant mouse L1210 leukaemia-cell line overproducing ornithine decarboxylase (ODC) (EC 4.1.1.17) as a result of chronic exposure to 2-difluoromethylornithine (DFMO) in the presence of micromolar concentrations of cadaverine. These cells, now grown for more than 2 years in the presence of DFMO and cadaverine, continued to accumulate ODC-specific mRNA in an amount 30-50 times higher than that in the parental cells, yet showing practically no changes in the gene dosage for the enzyme. However, analysis of the genomic DNA with the isoschizomeric restriction enzymes HpaII and MspI revealed that the ODC sequences in the overproducer cells were hypomethylated in comparison with the parental cells. The natural polyamines (putrescine, spermidine and spermine) were almost totally replaced by cadaverine and aminopropylcadaverine. Omission of cadaverine from the culture medium, but leaving 10 mM-DFMO, resulted in an about 10-fold ODC gene amplification within a few weeks. The accumulation of ODC mRNA was enhanced by the same factor. Concomitantly, the content of the natural polyamines was almost normalized, representing about 65% of that found in the parental cells. The present results suggest that, under a given selection pressure, an overproduction of the target gene product may be primarily based on an enhanced transcriptional activity, possibly associated with hypomethylation and, if not sufficient, a secondary amplification of the active gene occurs.