DEVELOPMENTAL EXPRESSION OF AUF1 AND HUR, 2 C-MYC MESSENGER-RNA BINDING-PROTEINS

Several proteins that may regulate c-myc mRNA posttranscriptionally we re previously isolated and characterized. Two of them, HuR and AUF1, b ind specifically to the 3' untranslated region (UTR) of c-myc mRNA. Be cause c-myc is regulated post-transcriptionally in various mouse tissu es, including quiescent tissues, fetal liver and regenerating liver, w e investigated whether HuR and AUF1 expression was also regulated in t hese tissues. Concerning AUF1, we analysed the expression of various m RNA and protein isoforms, We discovered a new AUF1 mRNA variant with a long AU-rich 3' UTR. We show that AUF1 expression, regardless of the RNA isoform considered, and HuR mRNA expression parallel C-myc express ion in quiescent tissues and during liver development; their expressio n is high in lymphoid tissues and fetal liver and low in adult liver. However, no upregulation of HuR or AUF1 accompanies the upregulation o f c-myc mRNA following partial hepatectomy. We discuss our results in relation to the current hypothesis that HuR and AUF1 act as mRNA desta bilizing factors.