RELIEF OF ORNITHINE DECARBOXYLASE MESSENGER-RNA TRANSLATIONAL REPRESSION INDUCED BY ALTERNATIVE SPLICING OF ITS 5'-UNTRANSLATED REGION

The ornithine decarboxylase enzyme (ODC) is the key regulator of polya mine synthesis and is a member of the cellular proto-oncogene family, Its expression becomes constitutively activated by carcinogens, viruse s, and oncogenes. ODC mRNA has a long 5' untranslated region that coul d be important in the regulation of enzyme levels by affecting transla tion, To test this hypothesis, we have determined the role of this reg ion on the constitutive ODC hyperexpression measured in AR4-2J cells, an azaserine-induced, tumor-derived pancreatic acinar cell line, Const ruction of expression vectors in which ODC 5' leader sequence was plac ed flanking the chloramphenicol acetyltransferase reporter gene allowe d us to identify three AR4-2J-specific, different alternatively splice d ODC 5' leaders, The 5' ends of exons 2 and 3 were lengthened by 17 a nd 13 bases, respectively, Translation performed in a cell-free system as well as in COS7 transient transfection experiments demonstrated th at AR4-2J isoforms induce a strong increase in the rate of translation . These results provide evidence that alternative splicing observed in tumoral cells, coupled with translation regulation, relieves the tran slation repression mediated by the long and structured 5' untranslated region of the ODC proto-oncogene.