Mutant p53 rescue and modulation of p53 redox state.

The p53 tumor suppressor is a key regulator of cell growth and survival upon various forms of cellular stress. p53 is a redox-regulated transcription factor that binds specifically to DNA and activates transcription of target genes. The core domain of p53 holds a zinc atom that protects p53 from oxidation and is critical for DNA binding. A large fraction of human tumors carry p53 mutation, allowing evasion of apoptosis and tumor progression. Restoration of wild type p53 expression triggers rapid elimination of tumors in vivo. This makes mutant p53 an attractive target for novel cancer therapy. Small molecules have been identified that reactivate mutant p53 and induce apoptosis in tumor cells. Interestingly, several of these compounds share the ability to target thiols and affect the redox state of p53, indicating that this is critical for mutant p53 rescue. The identification of a common chemical activity among mutant p53-targeting compounds will facilitate the design of even more potent and selective mutant p53-targeting drugs for improved cancer therapy in the future.

PRIMA-1 reactivates mutant p53 by covalent binding to the core domain.

Restoration of wild-type p53 expression triggers cell death and eliminates tumors in vivo. The identification of mutant p53-reactivating small molecules such as PRIMA-1 opens possibilities for the development of more efficient anticancer drugs. Although the biological effects of PRIMA-1 are well demonstrated, little is known about its molecular mechanism of action. We show here that PRIMA-1 is converted to compounds that form adducts with thiols in mutant p53. Covalent modification of mutant p53 per se is sufficient to induce apoptosis in tumor cells. These findings might facilitate the design of more potent and specific mutant p53-targeting anticancer drugs.

PRIMA-1MET inhibits growth of mouse tumors carrying mutant p53.

Reactivation of the tumor suppressor activity to mutant p53 should trigger massive apoptosis and eliminate tumors. The low molecular weight compounds PRIMA-1 and the structural analog PRIMA-1MET reactivate human mutant p53 in vitro and suppress growth of human tumor xenografts in SCID mice. However, little is known about their effect on mouse mutant p53 in mouse tumor cells. We have examined the effect of PRIMA-1MET on mouse sarcomas, mammary carcinomas and chemically induced fibrosarcomas. PRIMA-1MET showed potent growth suppression in mutant p53-carrying mouse tumors in vitro and a significant anti-tumor effect in syngeneic mice in vivo. These results demonstrate that PRIMA-1MET targets mouse tumors carrying mutant p53 and provide strong support for the anti-tumor efficiency of PRIMA-1MET in vivo.

Mutant p53 targeting by the low molecular weight compound STIMA-1.

Reactivation of mutant p53 in human tumor cells should induce cell death by apoptosis and thus eliminate the tumor. Several small molecules that reactivate mutant p53 have been identified. Here we show that STIMA-1, a low molecular weight compound with some structural similarities to the previously identified molecule CP-31398, can stimulate mutant p53 DNA binding in vitro and induce expression of p53 target proteins and trigger apoptosis in mutant p53-expressing human tumor cells. Human diploid fibroblasts are significantly more resistant to STIMA-1 than mutant or wild type p53-carrying tumor cells. STIMA-1 may provide new insights into possible mechanisms of mutant p53 reactivation and thus facilitate the development of novel anticancer drugs that target mutant p53-carrying tumors.

In vitro and in vivo cytotoxic effects of PRIMA-1 on hepatocellular carcinoma cells expressing mutant p53ser249.

Hepatocellular carcinoma (HCC) is highly lethal due to limited curative options. In high-incidence regions, such as parts of Africa and Southeastern Asia, >50% of cases carry an AGG to AGT mutation at codon 249 of the TP53 gene, considered as a 'signature' of mutagenesis by aflatoxins. The protein product, p53ser249, may represent a therapeutic target for HCC. The small molecule p53 reactivation and induction of massive apoptosis (PRIMA)-1 has been shown to induce apoptosis in tumour cells by reactivating the transactivation capacity of some p53 mutants. In this study, we have investigated the cytotoxic effects of PRIMA-1 on HCC cells expressing p53ser249. In p53-null Hep3B cells, over-expression of p53ser249 or p53gln248 by stable transfection increased the cytotoxicity of PRIMA-1 at 50 muM. Furthermore, PRIMA-1 treatment delayed the growth of p53ser249-expressing Hep3B cells xenografted in severe combined immunodeficiency mice. However, PRIMA-1 did not restore wild-type DNA binding and transactivation activities to p53ser249 or to p53gln248 in Hep3B cells. Moreover, in PLC/PRF/5, a HCC cell line constitutively expressing p53ser249, small interfering RNA (siRNA) silencing of the mutant increased the cytotoxic effect of PRIMA-1. These apparently contradictory effects can be reconciled by proposing that p53ser249 exerts a gain-of-function effect, which favours the survival of HCC cells. Thus, both inhibition of this effect by PRIMA-1 and removal of the mutant by siRNA can lead to the decrease of survival capacity of HCC cells.