Changes in gene expression in response to polyamine depletion indicates selective stabilization of mRNAs

We used differential display analysis to identify mRNAs responsive to changes in polyamine synthesis. As an overproducing model we used the kidneys of transgenic hybrid mice overexpressing ornithine decarboxylase and S-adenosylmethionine decarboxylase, two key enzymes in polyamine biosynthesis. To identify mRNAs that respond to polyamine starvation, we treated Rat-2 cells with alpha-difluoromethylornithine, a specific inhibitor of polyamine biosynthesis. We isolated 41 partial cDNA clones, representing 37 differentially expressed mRNAs. Of these, 15 have similarity with known genes, five appear to be similar to reported expressed sequence tags and seventeen clones were novel sequences. Of the 35 mRNAs expressed differentially after alpha-difluoromethylornithine treatment, 26 were up-regulated. The expression of only three mRNAs was altered in the transgenic animals and all three were down-regulated. Determination of mRNA half-life of three of the mRNAs up-regulated in response to polyamine depletion revealed that the accumulation results from stabilization of the messages. Because most of the transcripts identified from Rat-2 cells suffering polyamine starvation were accumulated, we conclude that polyamine depletion, while blocking cell growth, is stabilizing mRNAs. This may be due to the lack of spermidine for post-translational modification of the eukaryotic initiation factor 5A, which plays a major role in mRNA turnover. The coupling of mRNA stabilization with cell-growth arrest in response to polyamine starvation provides cells with an economical way to resume growth after recovery from polyamine deficiency.