Transactivation-deficient ΔTA-p73 Inhibits p53 by Direct Competition for DNA Binding

The p53 family member p73 displays significant structural and functional homology to p53. However, instead of mutational inactivation, overexpression of wild-type p73 has been reported in various tumor types compared with normal tissues, arguing against a classical tumor suppressor function. Recently, N-terminally truncated, transactivation-deficient p73 isoforms (ΔTA-p73) have been identified as a second class of p73 proteins. Because overexpression of p73 in tumors includes ΔTA-p73, we further characterized these novel p73 isoforms. We show that ΔTA-p73 retains DNA-binding competence but lacks transactivation functions, resulting in an inability to induce growth arrest and apoptosis. Importantly, ΔTA-p73 acts as a dominant-negative inhibitor of p53 and full-length p73 (TA-p73). We demonstrate that inhibition of p53 involves competition for DNA binding, whereas TA-p73 can be inhibited by direct protein-protein interaction. Further, we show that up-regulation of endogenous p73 just like ectopic overexpression of ΔTA-p73 confers resistance to p53-mediated apoptosis induced by the chemotherapeutic agent H-7. Because inhibition of p53 is a common theme in human cancer, our data strongly support a role of ΔTA-p73 expression for tumor formation.