Understanding the complexity of p53 in a new era of tumor suppression

Abstract

p53 was discovered 45 years ago as an SV40 large T antigen binding protein, coded by the most frequently mutated TP53 gene in human cancers. As a transcription factor, p53 is tightly regulated by a rich network of post-translational modifications to execute its diverse functions in tumor suppression. Although early studies established p53-mediated cell-cycle arrest, apoptosis, and senescence as the classic barriers in cancer development, a growing number of new functions of p53 have been discovered and the scope of p53-mediated anti-tumor activity is largely expanded. Here, we review the complexity of different layers of p53 regulation, and the recent advance of the p53 pathway in metabolism, ferroptosis, immunity, and others that contribute to tumor suppression. We also discuss the challenge regarding how to activate p53 function specifically effective in inhibiting tumor growth without harming normal homeostasis for cancer therapy.

Keywords: MDM2; MDMX; apoptosis; cancer treatment; cell competition; cell-cycle arrest; ferroptosis; genome stability; immunity; metabolism; metastasis; p53; p53 mutation; p63; p73; senescence; stem cell dynamics; targeting p53; tumor suppression.

Figure 1.. Timeline of research in the p53 field over the past 45 years.

This figure shows the number of publications recorded in PubMed every five years since 1979, along with key discoveries about p53. Due to space constraints, many excellent studies cannot be included here. LFS, Li-Fraumeni syndrome; PTM, post-translational modification; GOF, gain-of-function; NDD, neurodegenerative disease; iPSC, induced pluripotent stem cell; TAD2, transactivation domain 2; KR, lysine-to-arginine mutation.

Figure 2.. Regulation of p53.

The expression and activity of p53 are controlled by multilayered regulation at the DNA, RNA, and protein levels. At the DNA level, SNPs (e.g. P72R) and mutations (e.g. R273H) may occur in the p53 gene. p53 possesses two promoters, which can be methylated and silenced. The transcription of p53 gene is activated or suppressed by various TFs (e.g. HOXA5). At the RNA level, the cellular localization, stability, and translation of p53 mRNA are modulated by RNA-binding proteins (e.g. TIA1) and ncRNAs (e.g. miR-380–5p). p53 pre-mRNA and mRNA can undergo alternative splicing and alternative translation, respectively. At the protein level, p53 folding, stability, cellular localization, DNA binding, transactivation ability, and target selection are primarily mediated by post-translational modifications (e.g. ubiquitination, phosphorylation, and acetylation) and cofactors (e.g. MDM2, MDMX, and CBP). Diverse stress signals (e.g. DNA damage) can activate p53, and its activity as a TF is highly dynamic. p53 also exhibits TF-independent function in cytoplasm (e.g. promoting apoptosis via interacting with Bcl-XL). P1 and P2, promoter 1 and 2; SNP, single nucleotide polymorphism; E3, E3 ubiquitin ligase; TF, transcription factor; Me, methylation; Ub, ubiquitination; P, phosphorylation; Ac, acetylation.

Figure 3.. Functions and physiopathological roles of p53.

p53 exhibits diverse and complex functions: classical functions (including inducing cell-cycle arrest, apoptosis, and senescence, and maintaining genome stability) and other functions (such as mediating metabolism, ferroptosis, stem cell dynamics, cell competition, metastasis, and immunity). Due to its wide array of functions, p53 plays a crucial role in numerous physiological processes (e.g., reproduction, development, regeneration, repair, and aging) and pathological disorders (like neurodegenerative disease, radiation sickness, chemotherapeutic toxicity, ischemic injury, metabolic disease, and cancer). The black curves illustrate how specific functions of p53 contribute to its role in linked physiological or pathological processes.

Figure 4.. p53 and cancer hallmarks.

Activity of WT p53 antagonizes all the hallmarks of cancer, as depicted in the surrounding ovals. In contrast, alterations in p53, including repression, mutation, and deletion, promote these hallmarks. It is partially adapted from reference.

Figure 5.. Targeting p53 in cancer.

Various methods have been developed to target p53 for tumor treatment. In tumors retaining WT p53, RG7388, APG-115, KRT-232, and ALRN-6924 are used to disrupt the PPIs between p53 and MDM2 or MDMX, while RITA, tenovin-6, ML364, and UNC0379 target other negative regulators of p53. Activation of p53 can be utilized in cyclotherapy to protect normal cells, or in combination with other treatments for synergistic tumor eradication. WT p53 can be misfolded into a pseudo-mutant conformation, which may be reversed with appropriate drugs. Downstream targets of p53 are also potential therapeutic targets to partially reactivate the p53 signaling pathway. In tumors containing p53 missense mutations, APR-246, COTI-2, ATO, and PAT are capable of restoring the WT conformation of many p53 mutants. Specific agents such as PhiKan083, PK7088, PC14586, KG13, and MS78 target the p53 Y220C mutation, while ZMC1 is used for the p53 R175H mutant. Additionally, genome editing may be useful in correcting p53 gene mutations. NSC59984, ganetespib, MCB-613, and nanoreceptors are able to degrade mutant p53. ONYX-015, an oncolytic virus, specifically kills tumor cells with p53 mutations. Agents like ReACp53 and ADH-6 resolve the aggregation of mutant p53, partially restoring WT p53 functions. Antibodies like P1C1TM and H2-scDb, which recognize neoantigens derived from mutant p53, mediate tumor cell elimination by immune cells. p53MVA and p53-SLP are p53 vaccines used in immunotherapy. Mutant p53 neoantigens are also useful for developing adoptive cell therapies. Mutant p53 DNA fragments and proteins (including their aggregates) can be utilized for tumor diagnosis and prognosis. In tumors with p53 nonsense mutations, G418, 2,6-DAP, CC-90009, and NMDI14 can induce the readthrough of p53 mutant mRNAs, or inhibiting NMD. In p53-null tumors, delivery of p53 protein, mRNA, and DNA may restore p53 expression and eliminate tumor cells. LOF, loss-of-function; DNE, dominant-negative effect; GOF, gain-of-function; NR, negative regulator of p53.