REGULATION OF MDM2 EXPRESSION BY P53 - ALTERNATIVE PROMOTERS PRODUCE TRANSCRIPTS WITH NONIDENTICAL TRANSLATION POTENTIAL

The mdm2 proto oncogene product binds to the p53 tumor suppressor prot ein and inhibits its ability to trans-activate target genes. One such target gene is mdm2 itself, which is therefore considered a component of a p53 negative feedback loop. Two tandem p53-binding motifs residin g within the first intron of the murine mdm2 gene confer upon it p53-m ediated activation. We now report that in murine cells p53 activates a n internal mdm2 promoter (P-2) located near the 3' end of intron 1, re sulting in mRNA whose transcription starts within exon 2. P-2 is activ ated by p53 within artificial constructs, as well as within the contex t of the chromosomal mdm2 gene. Activation follows either the introduc tion of overexpressed wild-type p53 into cells or the induction of end ogenous wild-type p53 by ionizing radiation. The upstream, constitutiv e (P-1) mdm2 promoter is only mildly affected by p53, if at all. The p 53-derived mdm2 transcripts lack exon 1 and a few nucleotides from exo n 2. As the first in-frame AUG of mdm2 is located within exon 3, the t wo types of mdm2 transcripts should possess similar coding potentials. Nevertheless, in vitro conditions, where each of these transcripts yi elds a distinct translation profile, reflect the differential usage of translation initiation codons. Initiation of translation at internal AUG codons, which occurs also in vivo, gives rise to MDM2 polypeptides incapable of binding to p53. In vitro translation profiles of the var ious mdm2 transcripts could be manipulated by changing the amounts of input RNA. Thus, p53 can modulate both the amount and the nature of MD M2 polypeptides through activation of the internal P-2 promoter.