Human ΔNp73 regulates a dominant negative feedback loop for TAp73 and p53

Abstract

Inactivation of the tumour suppressor p53 is the most common defect in cancer cells. p53 is a sequence specific transcription factor that is activated in response to various forms of genotoxic stress to induce cell cycle arrest and apoptosis. Induction of p53 is subjected to complex and strict control through several pathways, as it will often determine cellular fate. The p73 protein shares strong structural and functional similarities with p53 such as the potential to activate p53 responsive genes and the ability to induce apoptosis. In addition to alternative splicing at the carboxyl terminus which yields several p73 isoforms, a p73 variant lacking the Nterminal transactivation domain (DNp73) was described in mice. In this study, we report the cloning and characterisation of the human DNp73 isoforms, their regulation by p53 and their possible role in carcinogenesis. As in mice, human DNp73 lacks the transactivation domain and starts with an alternative exon (exon 3'). Its expression is driven by a second promoter located in a genomic region upstream of this exon, supporting the idea of two independently regulated proteins, derived from the same gene. As anticipated, DNp73 is capable of regulating TAp73 and p53 function since it is able to block their transactivation activity and their ability to induce apoptosis. Interestingly, expression of the DNp73 is strongly upregulated by the TA isoforms and by p53, thus creating a feedback loop that tightly regulates the function of TAp73 and more importantly of p53. The regulation of DNp73 is exerted through a p53 responsive element located on the DN promoter. Expression of DNp73 not only regulates the function of p53 and TAp73 but also shuts off its own expression, once again finely regulating the whole system. Our data also suggest that increased expression of DNp73, functionally inactivating p53, could be involved in tumorogenesis. An extensive analysis of the expression pattern of DNp73 in primary tumours would clarify this issue. Cell Death and Differentiation (2001) 8, 1213 ± 1223.

Regulation of telomerase activity by the p53 family member p73

“…We and other groups have recently demonstrated that tumor cells frequently overexpress N-terminally truncated p73 isoforms that result from abberant splicing or the use of an alternative intronic promoter (Fillippovich et al, 2001;Grob et al, 2001;Kartasheva et al, 2002;Stiewe et al, 2002a, b;Zaika et al, 2002;Pu¨tzer et al, 2003). These DNp73 proteins act as dominant-negative inhibitors of the proapoptotic p53 family members by formation of transactivation-defective heteromeric complexes (Grob et al, 2001;Kartasheva et al, 2002;Stiewe et al, 2002a). Such a dominant-negative function of DNp73 has also been observed in this study with respect to hTERT regulation.…”

“…In contrast to downregulation of hTERT by differentiation or proapoptotic stress signals, hTERT is typically activated in immortal cells and established tumors . We and other groups have recently demonstrated that tumor cells frequently overexpress N-terminally truncated p73 isoforms that result from abberant splicing or the use of an alternative intronic promoter (Fillippovich et al, 2001;Grob et al, 2001;Kartasheva et al, 2002;Stiewe et al, 2002a, b;Zaika et al, 2002;Pu¨tzer et al, 2003). These DNp73 proteins act as dominant-negative inhibitors of the proapoptotic p53 family members by formation of transactivation-defective heteromeric complexes (Grob et al, 2001;Kartasheva et al, 2002;Stiewe et al, 2002a).…”

p73 and caspase-cleaved p73 fragments localize to mitochondria and augment TRAIL-induced apoptosis

“…Each isoform is also subject to extensive alternative splicing at the 3 0 -end of mRNA, resulting in up to 24 transcripts, able to promote or repress apoptosis to different extents (De Laurenzi et al, 1998;Yang et al, 2000). The proteins originating from the second promoter, DNp73, have a dominant negative activity on the tumor suppressor functions of both TAp73 and p53 and this regulatory loop has been implicated in tumorigenesis (Grob et al, 2001).…”

ΔNp73 stabilises TAp73 proteins but compromises their function due to inhibitory hetero-oligomer formation

“…[9][10][11][12] They act as powerful inhibitors of p53 and TAp73 since they retain their DNA-binding and tetramerization competence. [7][8][9]11,13,14 In cultured cells, DNp73 abrogates the suppressive activity of p53 and TAp73 and inactivates their ability to induce apoptosis and cell cycle arrest. 9 DNp73 overexpression results in malignant transformation of immortalized NIH3T3 fibroblasts 8 and also promotes immortalization in primary cells and cooperates with oncogenic Ras in driving their transformation in vivo.…”

“…Also, the relative longevity of DNp73 versus TAp73 proteins, when in isolation, supports previous anecdotal observations that DNp73 proteins are more stable than TAp73 in tumors. 13,19 We next tested whether DNp73 proteins can induce stabilization of TAp73 proteins. H1299 cells were transfected with either TAp73a or b alone, or cotransfected with the same amount of TAp73a or b plus increasing amounts of DNp73a or stabilization, possibly because the levels of DNp73a are often higher than DNp73 which might in turn be related to the longer half life of DNp73a compared to b.…”