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Ornithine decarboxylase as a target for chemoprevention
A E Pegg1, L M Shantz, C S Coleman
1Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Milton S. Hershey Medical Center, Hershey 17033, USA.
Abstract:
l-Ornithine decarboxylase (ODC) is essential for polyamine synthesis and growth in mammalian cells; it provides putrescine that is usually converted into the higher polyamines, spermidine and spermine. Many highly specific and potent inhibitors of ODC are based on the lead compound alpha-difluoromethylornithine (DFMO), which is an enzyme-activated irreversible inhibitor. DFMO is accepted as a substrate by ODC and is decarboxylated, leading to the formation of a highly reactive species that forms a covalent adduct with either cysteine-360 (90%) or lysine-69 (10%). Both modifications inactivate the enzyme. ODC activity is normally very highly regulated at both transcriptional and post-transcriptional levels according to the growth state of the cell and the intracellular polyamine content. Experimental over-production of ODC can be caused by either transfection with plasmids containing the ODC cDNA with part of the 5'-untranslated region (5'UTR) deleted under the control of a very strong viral promoter, or transfection of plasmids that cause the overproduction of eIF-4E, reported to be a limiting factor in the translation of mRNAs with extensive secondary structures in the 5'UTR. In both cases, unregulated overexpression of ODC transforms NIH 3T3 cells to a neoplastic state. Along with studies showing that many tumor promoters increase ODC activity and that a number of preneoplastic conditions and tumor samples show high levels of ODC, these results suggest that ODC may act as an oncogene in an appropriate background. This provides a rationale for the possible use of ODC inhibitors as chemopreventive agents.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
l-Ornithine decarboxylase (ODC) is crucial for cell growth and polyamine synthesis. Inhibiting ODC, an enzyme linked to cancer, shows potential for chemoprevention strategies.
Area of Science:
- Biochemistry
- Molecular Biology
- Oncology
Background:
- l-Ornithine decarboxylase (ODC) is vital for synthesizing putrescine, a precursor to spermidine and spermine, essential for mammalian cell growth.
- ODC activity is tightly regulated by cellular growth and polyamine levels at transcriptional and post-transcriptional levels.
- Alpha-difluoromethylornithine (DFMO) is a potent, enzyme-activated irreversible inhibitor of ODC.
Purpose of the Study:
- To investigate the role of ODC in cell transformation and its potential as a chemopreventive target.
- To understand the mechanism of DFMO inhibition of ODC.
Main Methods:
- DFMO was studied as a substrate for ODC, leading to the formation of a reactive intermediate.
- The mechanism of ODC inactivation by DFMO, including covalent adduct formation with cysteine-360 and lysine-69, was analyzed.
- Experimental ODC overexpression was induced via plasmid transfection to study its effect on NIH 3T3 cells.
Main Results:
- DFMO acts as a substrate, forming a reactive species that covalently modifies ODC at Cys-360 (90%) or Lys-69 (10%), leading to enzyme inactivation.
- Unregulated overexpression of ODC, induced by specific genetic manipulations, transformed NIH 3T3 cells into a neoplastic state.
- Elevated ODC activity is observed in tumor promoters, preneoplastic conditions, and tumor samples.
Conclusions:
- ODC can function as an oncogene in a suitable cellular context.
- The findings support the potential use of ODC inhibitors, like DFMO, as chemopreventive agents against cancer.