p53 must be competent for transcriptional regulation to suppress tumor formation

Abstract

In vitro studies suggest that effective tumor suppression by p53 requires multiple domains to execute transcriptiondependent and transcription-independent functions. We generated a mutant p53 allele in mice, p53 W25QL26S (p53 QS ), containing an inactive transactivation domain to evaluate the importance of transactivation for p53-mediated tumor suppression. Recently, we discovered that the allele also contains a valine substitution for alanine at codon 135, which borders the DNA-binding domain. We found that p53QSval135 bound to chromatin albeit less well than p53 QSala135, but both were equally deficient in transcriptional regulation, apoptosis induction in mouse embryo fibroblasts (MEFs), and suppression of tumor formation by E1A, Ha-Ras transformed MEFs. p53QSval135 mice and p53-null mice exhibited identical tumor development kinetics and spectra in spontaneous and oncogene-initiated tumorigenicity assays, when tested in a homo-and heterozygous configuration. The p53QSval135 allele did not have dominant negative functions and behaved as a null allele. Taken together, these data indicate that effective tumor suppression requires the transcriptional regulation function of p53, and they suggest that transactivation independent functions of p53 are unlikely to contribute significantly to tumor suppression in vivo.

Consequences of DJ-1 upregulation following p53 loss and cell transformation

“…We observed a significant enhancement in DJ-1 level in p53 À/À MEFs (six folds) as compared with p53 þ / þ MEFs, suggesting that p53 exerts an inhibitory effect on DJ-1 protein expression ( Figure 1a). We found a similar increase of DJ-1 in MEFs harboring a transcriptional mutant of p53, which is impaired in transactivation of p53 target genes (p53 QS MEFs) (Figure 1a) (Nister et al, 2005). Therefore, p53-mediated inhibition of DJ-1 is likely exerted by a transcriptional target of p53.…”

Mdm2 and Mdm4 Loss Regulates Distinct p53 Activities

“…The cells were maintained and characterized as populations and are hereafter referred to as TSD2, TSD4, and TS cells, respectively. The p53A135V allele resembles a p53-null allele in vivo (30). In cells, f80% of p53A135V resides in the cytoplasm in a Representative tumors arising in p53 À/À Mdm2 À/À Mdm4 À/À mice.…”

Mouse bites dogma: how mouse models are changing our views of how P53 is regulated in vivo

“…We have yet to observe induction of cell cycle arrest, apoptosis, or suppression of xenograft formation by p53QSA or p53QSV. 81 p53QSA and p53QSV are, as expected, extremely stable proteins (T 1/2 48 h). There is, however, one very important difference between P53QSA and P53QSV proteins.…”

“…81 Unfortunately, a second study did not attempt to produce mice expressing p53QS, but did generate differentiated ES cells and reconstituted thymus expressing this allele. 78 The data from both our studies and those of Chao et al, 78 as well as our more recent comparisons of the p53QS alleles with Ala135 (p53QSA) or Val135 (p53QSV), 81 show that the p53QS mutations themselves generate p53 molecules devoid of detectable transcriptional activity, ability to induce cell cycle arrest or apoptosis, or suppress tumor formation under the conditions tested. In stark contrast to this conclusion, data presented at the Dunedin meeting showed that an independently generated p53QS with Ala135 is embryonic lethal in heterozygotes.…”

“…81 We carefully compared the properties of endogenous p53QSV with exogenously expressed p53QSA (introduced into p53-null MEFs by lentiviral infection, and clones expressing the same levels of P53QSV and P53QSA proteins were identified and analyzed). We detected no transactivation activity of either mutant on genes such as p21, mdm2, and Puma, but, consistent with the results from the Attardi lab, we did detect bax transactivation by our p53QSA.…”