Developmental expression of AUF1 and HuR, two c-myc mRNA binding proteins.

Several proteins that may regulate c-myc mRNA post-transcriptionally were previously isolated and characterized. Two of them, HuR and AUF1, bind specifically to the 3' untranslated region (UTR) of c-myc mRNA. Because c-myc is regulated post-transcriptionally in various mouse tissues, including quiescent tissues, fetal liver and regenerating liver, we investigated whether HuR and AUF1 expression was also regulated in these tissues. Concerning AUF1, we analysed the expression of various mRNA 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 expression in quiescent tissues and during liver development; their expression is high in lymphoid tissues and fetal liver and low in adult liver. However, no upregulation of HuR or AUF1 accompanies the upregulation of c-myc mRNA following partial hepatectomy. We discuss our results in relation to the current hypothesis that HuR and AUF1 act as mRNA destabilizing factors.