HYPUSINE MODIFICATION IN EUKARYOTIC INITIATION-FACTOR 5A IN RODENT CELLS SELECTED FOR RESISTANCE TO GROWTH-INHIBITION BY ORNITHINE DECARBOXYLASE-INHIBITING DRUGS

Selection of HTC cells in drugs that inhibit ornithine decarboxylase ( ODC) has produced two cell lines, HMOA and DH23A/b, that contain incre ased amounts of more stable ODC. In addition to alterations in ODC, th ese cells appear to produce modified eukaryotic initiation factor 5A ( eIF-5A) at different rates, a reaction that both requires spermidine a nd is essential for proliferation. Alterations to the modification of eIF-5A by spermidine cannot be accounted for by changes in eIF-5A prot ein or modified eIF-5A turnover. Deoxyhypusine synthetase activity is similar in the parental and variant cell lines and is unaltered by gro wth into plateau phase or by spermidine depletion. The increased rate of eIF-5A modification in DH23A/b cells is due to an increased accumul ation of the unmodified eIF-5A precursor. Increased precursor accumula tion is not due to increased eIF-5A transcription, but rather it can b e attributed to a metabolic accumulation caused by growth under condit ions of chronically limiting spermidine. Selection using drugs that in hibit ODC apparently does not cause alterations in the eIF-5A modifica tion pathway. These data support the hypothesis that one of the main e ffects of spermidine depletion is depletion of the modified eIF-5A poo l, and that this is a critical factor in the cytostasis often observed after depletion of cellular polyamines.