ΔNp73 can modulate the expression of various genes in a p53-independent fashion

DeltaNp73alpha is an isoform of the p53 homologue p73 that lacks an amino-terminal transactivation domain and antagonizes the induction of gene expression by p53. Here, we examined whether DeltaNp73alpha might also modulate cellular transcription in the absence of p53. The expression of DeltaNp73alpha in the p53-/- cell line H1299 reduced the mRNA levels of p21/CDKN1A, but did not affect other p53-responsive genes. Correspondingly, the p21/CDKN1A promoter was downregulated by DeltaNp73alpha in reporter assays, whereas other p53-responsive promoters were not inhibited. To identify additional genes that respond to DeltaNp73alpha in the absence of p53, microarrays carrying 4600 cDNA clones were hybridized. The expression of 30 genes was found to be altered more than threefold by overexpressed DeltaNp73alpha. For instance, DeltaNp73alpha increased the expression of EGR1 and CDC6, whereas it decreased the mRNA levels of c-MYC, cyclin A2/CCNA2, NF-kappaB1, ODC1, and RET finger protein/RFP. Semiquantitative reverse transcription-PCR confirmed these results and further revealed that the influence of DeltaNp73alpha on the regulation of these genes differs from other p73 isoforms and p53. We conclude that the impact of DeltaNp73alpha on gene expression is not limited to p53-responsive genes. Rather, DeltaNp73alpha can regulate the expression of a variety of genes independently of p53.