Effects of Δ40p53, an isoform of p53 lacking the N-terminus, on transactivation capacity of the tumor suppressor protein p53

Background: The p53 protein is expressed as multiple isoforms that differ in their N- and C-terminus due to alternative splicing, promoter or codon initiation usage. Delta 40p53 lacks the first 39 residues containing the main transcriptional activation domain, resulting from initiation of translation at AUG +40 in fully spliced p53 mRNA or in a specific variant mRNA retaining intron 2. Overexpression of Delta 40p53 antagonizes wild-type p53 in vitro. However, animal models of Delta 40p53 in mouse or Zebrafish have shown complex phenotypes suggestive of p53-dependent growth suppressive effects. Methods: We have co-transfected expression vectors for p53 and Delta 40p53 in p53-null cell lines Saos-2 and H1299 to show that Delta 40p53 forms mixed oligomers with p53 that bind to DNA and modulate the transcription of a generic p53-dependent reporter gene. Results: In H1299 cells, co-expression of the two proteins induced a decrease in transcription with amplitude that depended upon the predicted composition of the hetero-tetramer. In Saos-2, a paradoxical effect was observed, with a small increase in activity for hetero-tetramers predicted to contain 1 or 2 monomers of Delta 40p53 and a decrease at higher Delta 40p53/p53 ratios. In this cell line, co-transfection of Delta 40p53 prevented Hdm2-mediated degradation of p53. Conclusion: Delta 40p53 modulates transcriptional activity by interfering with the binding of Hdm2 to hetero-tetramers containing both Delta 40p53 and p53. These results provide a basis for growth suppressive effects in animal models co-expressing roughly similar levels of p53 and Delta 40p53.