UC Riverside

Polyamines are aliphatic polycations existing in all living organisms and are essential for life. Most organisms synthesize three types of polyamines: putrescine, spermidine, and spermine. Putrescine is the product of the rate-limiting reaction of ornithine decarboxylase (ODC) and the amino acid ornithine. Spermidine and spermine are downstream metabolites sequentially derived from putrescine. In a recent landmark colon cancer chemopreventative clinical trial researchers found that combining alpha-difluoromethylornithine (DFMO), a selective ODC suicide inhibitor, and sulindac, a nonsteroidal anti-inflammatory drug, led to a 70% reduction of recurrence of all adenomas and a 90% reduction of recurrence of advanced and/or multiple adenomas over a 3-year treatment. This study shows that polyamine metabolic enzymes are attractive targets for possible chemotherapeutics. Our previous research has shown that stable ODC overexpression led to increased ODC activity, elevated intracellular putrescine levels with concomitant losses of spermidine and spermine, and increased spermidine/spermine-N1-acetyltransferase (SSAT) activity. Following a genome-wide array analysis on the effects of ODC overexpression in mammalian cell lines, several cell cycle-related genes were found to be upregulated. We investigated whether changes in polyamine levels, polyamine metabolism, and gene expression following stable ODC overexpression could be replicated following treatment with exogenously supplied putrescine. We observed that intracellular putrescine levels following treatment with exogenous putrescine accumulated to levels observed during stable ODC overexpression. These putrescine-mediated changes in intracellular putrescine pools also led to increased mRNA levels and activity of SSAT and to increased gene expression of ID1, c-Jun, and c-Fos in a time- and dose-dependent manner. We also determined that the observed increases in gene expression were mediated by the accumulation of intracellular putrescine independent from the activation of key, rate-limiting polyamine metabolic enzymes. Changes in intracellular polyamine pools, polyamine metabolism, and gene expression following the administration of exogenously supplied putrescine closely resembled the changes observed following stable ODC overexpression.