Cellular and biochemical consequences of ornithine decarboxylase regulation

Abstract

The polyamines are abundant polycations necessary for eucaryotic cell growth. Ornithine decarboxylase (ODC), the first and often rate-limiting enzyme in polyamine biosynthesis, is responsible for the formation of putrescine, the precursor for polyamine synthesis. ODC is normally very tightly regulated by a complex interaction of control of both synthesis and degradation. Regulation of ODC synthesis allows cells to increase ODC in response to various stimuli; however, the importance of the rapid degradation of ODC in cellular metabolism is less well understood. The studies presented here have compared the parental, HTC cells, which exhibit rapid ODC degradation, to two cell lines, DH23A and HMOA, which express a more stable ODC, to examine the cellular consequences of aberrant ODC expression. Aberrantly elevated ODC in DH23A cells results in a dramatic accumulation of endogenous putrescine, a decrease in cell number and the appearance of apoptotic cells. In contrast, HTC cells, with moderate polyamine contents, can be maintained in exponential growth. Accumulation of similar concentrations of putrescine from an exogenous source mimics the effect of ODC overexpression. This suggests that putrescine is the causative agent or trigger of apoptosis. Accumulation of excess intracellular putrescine inhibits the formation of modified eIF-5A in vivo. eIF-5A is a protein that is post-translationally modified by spermidine. Although the function is unclear, both eIF-5A and the modification by spermidine are necessary for growth in eucaryotic cells. Treatment of DH23A cells with diaminoheptane(DAH), an in vitro inhibitor of eIF-5A modification, both inhibits the modification of eIF-5A in vivo and increases apoptosis. DAH appears to cause a decrease in cell number by acting as a competitive inhibitor of the post-translational modification of eIF-5A by spermidine. Putrescine probably inhibits the formation of modified eIF-5A upstream of the spermidine addition, but not at the level of transcription. These data support the hypothesis that the rapid degradation of ODC may be a protective mechanism to avoid cell toxicity due to putrescine accumulation and suggest that suppression modified eIF-5A formation is one mechanism by which cells may be induced to undergo apoptosis.