p73 Function Is Inhibited by Tumor-Derived p53 Mutants in Mammalian Cells

Abstract

The p53 tumor suppressor protein, found mutated in over 50% of all human tumors, is a sequence-specific transcriptional activator. Recent studies have identified a p53 relative, termed p73. We were interested in determining the relative abilities of wild-type and mutant forms of p53 and p73␣ and -␤ isoforms to transactivate various p53-responsive promoters. We show that both p73␣ and p73␤ activate the transcription of reporters containing a number of p53-responsive promoters in the p53-null cell line H1299. However, a number of significant differences were observed between p53 and p73 and even between p73␣ and p73␤. Additionally, a Saccharomyces cerevisiae-based reporter assay revealed a broad array of transcriptional transactivation abilities by both p73 isoforms at 37°C. Recent data have shown that p73 can associate with p53 by the yeast two-hybrid assay. When we examined complex formation in transfected mammalian cells, we found that p73␣ coprecipitates with mutant but not wild-type p53. Since many tumor-derived p53 mutants are capable of inhibiting transactivation by wild-type p53, we tested the effects of two representative hot-spot mutants (R175H and R248W) on p73. By cotransfecting p73␣ along with either p53 mutant and a p53-responsive reporter, we found that both R175H and R248W reduces the transcriptional activity of p73␣. This decrease in transcriptional activity is correlated with the reduced ability of p73␣ to promote apoptosis in the presence of tumorderived p53 mutants. Our data suggest the possibility that in some tumor cells, an outcome of the expression of mutant p53 protein may be to interfere with the endogenous p73 protein.A new gene family whose encoded products show significant sequence similarity to the tumor suppressor protein p53 have been identified (32,33,50,59,60,72). KET, the first to be identified, was cloned from a rat circumvallate taste papilla cDNA library (59). p73, the second identified from a COS cell cDNA library, encodes for at least two splicing variants, p73␣ and p73␤ (32, 33). Finally, the human homolog of KET, referred to as either p51 or p63, encodes at least six isoforms (p63␣/p51B/p73L, p63␤, p63␥/p51A, ⌬Np63␣, ⌬Np63␤, and ⌬Np63␥) that are expressed in a tissue-specific manner and harbor different transactivation potentials (50,60,72). It has been proposed that this family of proteins is ancestral to human p53, in that all show significant amino acid similarity in their C-terminal p53-unrelated extensions to the squid p53 protein (33,59,72).The p53 protein is modular and can be divided into at least four distinct domains: (i) the amino-terminal transcriptional transactivation domain (residues ϳ1 to 70) (6,7,15,55,68), (ii) the PXXP domain (residues ϳ61 to 94) (70), (iii) the sequence-specific DNA binding domain (residues ϳ102 to 292) (1, 27, 51, 71), and (iv) the carboxy-terminal regulatory and tetramerization domains (residues ϳ320 to 393 and ϳ320 to 360, respectively) (3,4,57,71). The various isoforms of p73 and p51/p63 display a modular structure similar to that o...

Mutant p53 subverts p63 control over KLF4 expression in keratinocytes

“…The first is the interplay between p53 and p63/p73: although there is little evidence of heterotetrameric formation of p53-p63 dimers (Davison et al, 1999), several reports have indicated that p63/p73 have an increased affinity for p53 missense mutants (Di Como et al, 1999;Strano et al, 2000;Gaiddon et al, 2001). Specifically, p63 was reported to associate with the p53 mutants of HaCaT cells (Gaiddon et al, 2001).…”

Are interactions with p63 and p73 involved in mutant p53 gain of oncogenic function?

“…Whatever the structural basis for the association of mutant p53 with its homologues, several studies have documented decreased transcriptional activities of p63/p73 and their ability to induce growth suppression and apoptosis when in the presence of mutant p53 (Di Como et al, 1999;Marin et al, 2000;Strano et al, 2000Strano et al, , 2002Gaiddon et al, 2001;Bergamaschi et al, 2003). Importantly, these studies have indicated a good correlation between the efficiency of binding to and inactivation of p63/p73 by mutant p53.…”

“…Despite this it has been shown that a number of p53 mutants can associate with either p73 or p63 as evidenced by co-immunoprecipitation assays when they are either ectopically or endogenously expressed (Di Como et al, 1999;Marin et al, 2000;Strano et al, 2000Strano et al, , 2002Gaiddon et al, 2001;Bergamaschi et al, 2003;Irwin et al, 2003). Both 'conformation' mutants (e.g.…”

Life and death decisions by E2F-1

“…57 Moreover, it has recently been shown that several tumour-derived mutants of p53 that are often retained in cancer are able to bind and inactivate p73, thereby circumventing the necessity to mutate p73 per se. 58 This process reduces chemosensitivity and adds more weight to the fact that E2F-1-induced apoptosis is an important tumour-suppressive mechanism as well a prognostic indicator of therapeutic success. 59,60 Another mechanism of E2F-1-induced apoptosis is via the inhibition of antiapoptotic signalling.…”