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The Johns Hopkins Oncology Center, Bunting-Blaustein Cancer Research Building, Baltimore, Maryland 21231 [Y. W., W. D., T. M. S., A. H., R. A. C.], and Department of Pharmaceutical Sciences, Wayne State University, Detroit, Michigan 48202 [P. M. W.]
| ABSTRACT |
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5-fold induction of PAO mRNA and a >3-fold induction of PAO activity. These results demonstrate that in at least one cell type, PAO is up-regulated in response to polyamine analogue exposure. The PAO clone described here should provide a useful tool, which will facilitate the dissection of the role of polyamine catabolism in normal growth and in response to the antitumor polyamine analogues. | Introduction |
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In addition to regulating the biosynthetic enzymes, some of the most effective antitumor polyamine analogues were found to profoundly increase the catabolism of polyamines (7) . In some instances, it appears that the activity of polyamine catabolism is directly associated with the activity of the analogues (2 , 8 , 9) .
Polyamine catabolism is mediated by the activity of two enzymes acting sequentially. The first and generally rate-limiting enzyme is SSAT (10) .3 This enzyme catalyzes the addition of an acetyl group to the N1-position of either spermidine or spermine. The acetylated polyamine then becomes the preferred substrate for the activity of PAO, a flavin adenine dinucleotide-dependent oxidase that results in production of 3-acetamido propanal, H2O2, and either spermidine or putrescine, depending on the starting polyamine (11 , 12) . It should be noted that unmodified spermine is also a substrate of PAO (11) .
As stated above, the polyamine catabolic pathway has been implicated in the sensitivity of several tumor types to many antitumor polyamine analogues. This has been particularly true for SSAT. Several studies (2 , 7 , 13) have demonstrated that the activity of SSAT can increase several thousand-fold in response to exposure to polyamine analogues. However, it has been thought previously (12) that the activity of PAO was constitutive and primarily limited by the availability of the acetylated substrate. Although there are recent cloning reports of yeast and plant PAOs (14 , 15) , the direct study of the regulation of mammalian PAO has been hampered by the fact that no clone of the mammalian enzyme has been available. In this study, we report the cloning and characterization of human PAO and demonstrate that the expression of the enzyme can be significantly induced by exposure to the antitumor polyamine analogues. The results presented here suggest another mechanism by which various tumor types can respond differentially to exposure to the antitumor polyamine analogues.
| Materials and Methods |
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-32P]dCTP and [
-35S]methionine) were supplied by Amersham Pharmacia Biotech (Piscataway, NJ). The TnT-coupled wheat germ extract system was purchased from Promega (Madison, WI). The TA cloning kit was purchased from Invitrogen (Carlsbad, CA). Trizol total RNA reagent was from Life Technologies, Inc. (Rockville, MD). Advantage cDNA Polymerase Mix system and a retroviral placenta cDNA library were from Clontech Laboratories, Inc. (Palo Alto, CA). Restriction and DNA-modifying enzymes were purchased from New England Biolabs, Inc. (Beverly, MA), Life Technologies, Inc., and Sigma Chemical Co. (St. Louis, MO). Life Technologies, Inc. synthesized all of the oligomers used in the experiments. The DNA sequencing was performed with a Perkin-Elmer ABI Automated DNA Sequencer. N1,N4-bis(2,3-butadienyl)-1,4-butanediamine (MDL 72,527) was kindly supplied by Dr. Eugene Gerner (University of Arizona, Tucson, AZ). Other chemicals came from Sigma Chemical Co., Roche Molecular Biochemicals (Indianapolis, IN), Bio-Rad (Hercules, CA), Aldrich Chemical Company, Inc. (Milwaukee, WI), and J. T. Baker, Inc. (Phillipsburg, NJ).
Cloning of Human PAO.
PCR (Advantage cDNA Polymerase Mix system) was used to clone human PAO cDNA. PCR was performed with a gene-specific primer pair (5'-CGCCGCTCGCCGCAGACTTACTTC-3' and 5'-AAAGCTACAGGGCCAGGTCTGAAG-3') and cDNA from a human placenta library. The PCR products were then cloned into pCR2.1 vector (pCR2.1/PAOhx). To construct the pPAOh1 plasmid, the cDNA insert in pCR2.1/PAOh1 was removed by cutting with HindIII and EcoRV and then inserting the resultant fragment into pcDNA3.1(-) vector in the same restriction sites.
In Vitro Transcription and Translation.
In vitro transcription and translation reactions were performed with the TnT-coupled wheat germ extract system. Parallel reactions were prepared by adding an unlabeled amino acid mixture to one reaction and a [35S]methionine containing amino acid mixture to the other, according to the supplied protocol. Vector pcDNA3.1 and pPAOh1 were linearized by SalI restriction and served as the templates. The labeled translation reactions products were separated by 10% SDS-PAGE, and radioactivity on the labeled PAO band was determined by Phosphor image analysis using Image Quant software (Molecular Dynamics, Sunnyvale, CA).
RNA Purification and Northern Blot Assay.
Total cellular RNA from the NCI H157 cell line was extracted using Trizol total RNA reagent according to the protocol from the manufacturer. Total RNA (20 µg) was separated on a denaturing 1.5% agarose gel containing 6% formaldehyde, transferred to Zetaprobe membrane (Bio-Rad), and hybridized with a random primer-labeled pPAOh1 cDNA as the probe. Blots were washed and reprobed with an 18S ribosomal cDNA probe as a loading control.
Determination of PAO Enzyme Activity.
The cultured H157 cells with or without treatment of 10 µM N1, N''-bis(ethyl)norspermine (BENSpm) were homogenized with a Dounce tissue homogenizer in ice-cold 0.083 M sodium borate buffer (pH 9.0). The PAO activity in homogenates was assayed by the method of Suzuki et al. (16)
, which measures the H2O2 formed due to oxidation of spermine by converting homovanillic acid into a highly fluorescent compound in the presence of horseradish peroxidase. The samples were prepared in a 600-µl reaction containing 83 mM sodium borate buffer (pH 9.0), 0.04 mg of horseradish peroxidase, 100 µl of cell homogenate, 0.1 mg of homovanillic acid, and 250 µM spermine. Before the addition of homovanillic acid and spermine, the tubes were preincubated for 20 min with shaking at 37°C to remove endogenous substrates of H2O2-producing enzymes. After preincubation, homovanillic acid and spermine were added, and the reactions were incubated for 1 h at 37°C. The enzyme activity was stopped by the addition of 2.0 ml of 0.1 M NaOH solution. The fluorescence intensity was measured with excitation at 323 nm and emission at 426 nm. Background fluorescence was determined by addition of the spermine substrate into the reaction mixture only after inactivation of the enzyme by NaOH. Protein content of the cellular homogenate was determined using the Bio-Rad protein assay kit (Bio-Rad). One unit of PAO activity in cell homogenate was defined as that amount that transformed 1 pmol spermine/mg cell protein/60 min at 37°C. For determination of PAO activity in the product of the unlabeled TnT reaction, 10 µl of TnT reaction were used in the place of the homogenate. The PAO activity in the TnT reactions was represented by the fluorescent compound formed within the 1-h incubation. In some tests, the inhibitor for monoamine oxidase (pargyline), diamine oxidase (semicarbazide), or polyamine oxidase (MDL-72527) was used at the final concentration of 1.0 mM, 0.1 mM, and 0.25 mM, respectively. The concentrations chosen for each inhibitor were based on studies published previously (12)
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Determination of Km.
The apparent Km of PAO using spermine as a substrate was determined using the TnT-produced protein described above. Concentrations of spermine used were similar to those reported by Libby and Porter (11)
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| Results |
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62 kDa after denaturing PAGE, consistent with the expected size of the open reading frame (Fig. 2C)
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18 µM (Fig. 2D)
Effects of BENSpm Treatment on PAO mRNA Expression and Enzyme Activity in NCI H157 Cells.
PAO has frequently been described as a constitutively expressed protein. To test this hypothesis, we exposed NCI H157 cells to 10 µM BENSpm for 24 h. This time and concentration were chosen because BENSpm has demonstrated the ability to highly induce SSAT in H157 cells and produce H2O2-related apoptosis (3)
. BENSpm exposure resulted in
5-fold increase in PAO message (Fig. 3A)
and a >3-fold increase in PAO activity (Fig. 3B)
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| Discussion |
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Previously (12) , it was assumed that PAO was expressed in a constitutive manner or only slowly induced. In this study, it is clearly demonstrated that this is not always the case. The significant induction of PAO message and activity in the analogue-treated human non-small cell lung carcinoma cells clearly demonstrates that PAO can be up-regulated within 24 h in a manner similar to that observed for SSAT. The induction of PAO activity correlates well with the message level, suggesting that the major regulation of PAO activity is at the transcriptional level. This appears to be in contrast to SSAT induction, where post-transcriptional regulation plays a large role in the regulation of SSAT expression (19 , 20) . It will be necessary to examine several other cell types to determine how common the induction of PAO is in response to analogues or other stimuli. If, as is the case for SSAT, PAO demonstrates a cell type-specific induction response to stimuli, it is possible that PAO may play a significant role in the sensitivity of cells to the antitumor polyamine analogues.
It is also important to note that alternative splicing may have a role in how PAO is regulated. The significance of the multiple splice variants is not known and will take additional experimentation to address. Although pPAOh1 represents a longer clone than was predicted by Babbage, the submitting author of the genomic sequence AL121675, it is possible that even longer splice variants may exist that were not represented in our PCR clones. The multiple splice variants observed and those suggested by Babbage are consistent with the results of Libby and Porter (11) who identified two isozymes of PAO in the murine leukemia line, L1210. Further study will be required to determine the number and significance of the possible splice variants and resulting isozymes.
We have previously demonstrated that in NCI H157 cells, the induction of SSAT is associated with apoptosis. Additionally, we found that the onset of apoptosis could be delayed after treatment with the PAO inhibitor, MDL 72,527, suggesting that the H2O2 produced by PAO had a role in producing the toxic effects of BENSpm in NCI H157. However, in subsequent work by others (9) the addition of the PAO inhibitor had no effect on the cytotoxicity produced by BENSpm. This apparent contradiction in results may be the result of different levels of PAO activity expressed among the different cell types.
The current results suggest that a substantial increase in PAO mRNA after BENSpm treatment is a result of increased transcription of the PAO gene. However, because it is also possible that analogue treatment may alter message stability, nuclear run-on assays must be performed to confirm an increase in PAO transcription.
PAO is the terminal enzymatic step in polyamine catabolism. Although, previous studies have indicated that PAO is constitutively expressed, the data presented here with the newly cloned human PAO demonstrate that expression of PAO can be positively modulated by cellular exposure to a cytotoxic polyamine analogue. These results suggest that the sensitivity of tumor cells to the polyamine analogues may result from a combination of inducible SSAT and PAO activity. The availability of the human PAO clone will make further study of this important enzyme considerably easier.
| FOOTNOTES |
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1 Supported by NIH Grants CA51085, CA85509, and CA58184. ![]()
2 To whom requests for reprints should be addressed, at The Johns Hopkins Oncology Center, Bunting-Blaustein Cancer Research Building, Room 551, 1650 Orleans Street, Baltimore, MD 21231. E-mail: rcasero{at}jhmi.edu ![]()
3 The abbreviations used are: SSAT, spermidine/spermine N1-acetyltransferase; PAO, polyamine oxidase; BENSpm, N1,N''-bis(ethyl)norspermine. ![]()
4 The nucleotide sequence reported in this paper has been submitted to the GenBank and has the assigned accession no. AY033889. ![]()
Received 5/ 2/01. Accepted 5/24/01.
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