INVOLVEMENT OF ORNITHINE DECARBOXYLASE AND POLYAMINES IN GLUCOCORTICOID-INDUCED APOPTOSIS OF RAT THYMOCYTES

Ornithine decarboxylase (ODC), the first and rate-limiting enzyme of p olyamine metabolism, has been shown to be required for entry into and progression through the cell cycle. However, the role of ODC and polya mines in apoptosis remains to be determined. We have examined ODC expr ession and polyamine levels in thymocytes activated to undergo apoptos is by dexamethasone treatment. We have demonstrated a rapid and revers ible induction of ODC (mRNA and activity), as previously reported for the mRNA expression of other ''early'' genes, c-fos, c-jun, and c-myc, in the same experimental model. Surprisingly, polyamine levels dimini shed progressively starting at 2-4 h after dexamethasone treatment, an d spermine was depleted at 8-12 h. This seemed to be relevant since in creasing the intracellular polyamine levels by exogenous spermine admi nistration prevented the DNA ''laddering'' (2-4 h) and the DNA loss fr om the nucleus (8-18 h) due to dexamethasone treatment. Moreover, the activities of spermidine/spermine N-1-acetyltransferase, which control s the cytosolic polyamine interconversion pathway, and of spermidine N -8-acetyltransferase, which regulates the nuclear pool and functions o f polyamines, were measured in apoptotic cells. Spermidine/spermine N- 1-acetyltransferase activity progressively increased and might be resp onsible for spermidine and spermine excretion as acetyl derivatives. I n contrast, spermidine N-8-acetyltransferase activity remained unchang ed. A completely different scenario was observed in proliferating conc anavalin A-treated thymocytes, studied for comparison. In this case, p olyamine levels increased, remaining at high values until 12 h. This i s likely a consequence of the rapid and prolonged induction of ODC (mR NA and activity), accompanied by that of spermidine/spermine N-1-acety ltransferase (mRNA and activity). Spermidine N-8-acetyltransferase act ivity peaked biphasically, indicating its possible involvement in the enhanced gene expression necessary for cell proliferation. Thus, the d ecrease in intracellular polyamine levels may represent a signal to th ymocytes for progression into the apoptotic program.