EFFECTS OF POLYAMINES, POLYAMINE ANALOGS, AND INHIBITORS OF PROTEIN-SYNTHESIS ON SPERMIDINE-SPERMINE N-1-ACETYLTRANSFERASE GENE-EXPRESSION

The key polyamine catabolizing enzyme spermidine-spermine N-1-acetyltr ansferase (SSAT) is among the few genes known to be inducible by the n atural polyamines. Certain polyamine analogs markedly exaggerate this response and thus provide useful tools for studying the underlying reg ulatory mechanisms, As shown here, the analog which most potently indu ces SSAT activity, N-1,N-11-diethylnorspermine (DENSPM), increases SSA T mRNA in MALME-3M human melanoma cells to a maximum of >20-fold and i mmunodetectable SSAT protein to >300-fold. By comparison, the natural polyamine spermine is far less effective, increasing SSAT mRNA by simi lar to 3-fold and protein by similar to 7-fold, In particular, the dif ference in mRNA accumulation by spermine and the analog was shown to b e due to differential effects on both gene transcription and mRNA stab ilization. Although the analog DENSPM has been regarded as the most po tent inducer of SSAT activity and mRNA, we now report that inhibitors of protein synthesis are capable of increasing SSAT mRNA to nearly com parable levels. Inhibitor-induced accumulation in SSAT mRNA was shown to involve increased gene transcription and mRNA stabilization; This s uggests that, under basal conditions, SSAT gene expression is suppress ed by a labile protein (or proteins). While induction of SSAT mRNA by inhibitors of protein synthesis only occurred at concentrations which blocked protein synthesis, that by DENSPM took place at concentrations which did not. The combination of either protein inhibitor with DENSP M or spermine produced an additive increase in SSAT mRNA. Taken togeth er, these findings suggest the involvement of two separate but possibl y converging pathways in the regulation of SSAT mRNA, one mediated by polyamines and their analogs and the other mediated by a labile repres sor of SSAT gene transcription and/or mRNA stabilization. In addition to its apparent regulatory importance, induction of SSAT mRNA by inhib itors of protein synthesis represents a potentially useful system for studying the posttranscriptional regulation of this interesting gene.