Strategies for p53 Activation and Targeted Inhibitors of the p53-Mdm2/MdmX Interaction

Abstract

p53 is a tumor suppressor gene and is regarded as one of the most crucial genes in protecting humans against cancer. The protein Mdm2 and its homolog MdmX serve as negative regulators of p53. In nearly half of cancer cells, there is an overexpression of Mdm2 and MdmX, which inhibit p53 activity. Furthermore, Mdm2's E3 ubiquitin ligase activity promotes the ubiquitination and degradation of p53. Therefore, blocking the interaction between p53 and Mdm2/MdmX to prevent the degradation of wild-type p53 is an effective strategy for inhibiting tumor growth. This paper primarily discusses the regulatory relationship between p53, MdmX and Mdm2, and provides a review of the current status of p53-Mdm2/MdmX inhibitors. It aims to offer a theoretical foundation and research direction for the future discovery and design of targeted inhibitors against the p53-Mdm2/MdmX interaction.

Keywords: Mdm2; MdmX; cancer; inhibitors; p53.

Figure 1

The structure of human p53 and its physiological functions. (A) The N-terminal region of p53 (residues 1–101) contains two transcription activation domains, TAD1 and TAD2, along with a proline-rich region PP (residues 61–94), featuring five PXXP motifs, which are crucial for its apoptotic function. Additionally, a nuclear export signal (NES) is located between residues 11 and 27. The central core domain (amino acids 102–292) includes the sequence-specific DNA binding domain (DBD). Most missense mutations in the p53 gene occur within this domain, which harbors a highly conserved subregion. The C-terminal region of p53 (residues 292–393) consists of a flexible linker (residues 292–324) that connects the core domain to the tetramerization domain (Tet, residues 325–355) and the C-terminal regulatory domain (CTD, residues 363–393). The C-terminal region also contains both NES and nuclear localization signal (NLS) sequences. (B) In response to stimuli such as DNA damage, hypoxia, and ribosomal stress, the p53 pathway is activated in cells. In cases of moderate damage, cancer cells repair through p21 mediation, while severe damage triggers the activation of apoptotic genes. In normal cells, the pathway primarily supports repair processes and plays a crucial role throughout various phases of the cell lifecycle, including cell division, differentiation, metabolism, and more.

Figure 2

The structures of Mdm2 and MdmX. (A) The Mdm2 structure includes the N-terminal p53 binding domain (residues 18–101), a central acidic domain (residues 237–288). It also contains a zinc finger motif (residues 289–331) involved in ribosomal protein interactions. The C-terminal region of Mdm2 contains a RING (Really Interesting New Gene) domain (residues 436–482) with E3 ubiquitin ligase activity, while the C-terminal tail (residues 485–491) regulates the RING motif by promoting Mdm2 homodimer formation and Mdm2-MdmX heterodimerization. (B) MdmX shares structural similarities with Mdm2, containing an N-terminal p53 binding domain (residues 19–102), a central acidic region (residues 215–255), a zinc finger region (residues 290–332), and a C-terminal RING domain (residues 467–483). MdmX lacks NLS, NES, and nucleolar localization signal (NoLS) sequences but features a unique WWW motif (W, residues 190–210) that inhibits its interaction with p53. In contrast to Mdm2, the RING domain of MdmX does not exhibit E3 ligase activity. Abbreviations: C′, C-terminal; F, phenylalanine; L, leucine; N′, N-terminal; W, tryptophan.

Figure 3

The regulatory loop between p53 and Mdm2/MdmX. Under non-stress conditions, p53 levels are maintained at low levels primarily through binding to its key negative regulators, Mdm2 and MdmX. In response to stress stimuli, post-translational modifications (PTMs), such as acetylation (Ac) or phosphorylation (p) of checkpoint kinases CHK1 and CHK2, are induced by the activation of upstream kinases, Ataxia Telangiectasia Mutated (ATM) and Ataxia Telangiectasia and Rad3-related protein (ATR). These modifications stabilize and activate p53 by preventing its binding to Mdm2. Subsequently, heat shock proteins (HSPs) aid in the proper folding of p53 monomers, leading to the formation of its active tetrameric form, which binds to DNA for gene transcription. During this activation process, p53 inhibition can occur through the removal of PTMs: the histone deacetylase (HDAC) sirtuin 1 (SirT1) induces deacetylation of p53 and promotes its ubiquitination by Mdm2. Additionally, p53 induces the expression of another negative regulator, WIP1/PPM1D, a phosphatase 1, which destabilizes p53 by dephosphorylating serine 15 on p53 and dephosphorylating Mdm2, thereby promoting the stabilization of Mdm2.

Figure 4

Comparison of the crystal structures of Mdm2 complexes with their different ligands. (A) Comparison of the binding modes of Nutlin-2 (green) and p53p (cyan) with Mdm2 proteins (PDB ID: 1RV1 and 1YCR); (B) Comparison of the binding modes between Nutlin-3a (purple) and RG7112 (pink) with Mdm2 proteins (PDB ID: 4J3E and 4IPF).

Figure 5

Chemical structures of Mdm2 inhibitors in clinical trials that have either started or are currently recruiting patients.

Figure 6

The MdmX inhibitors and Mdm2/MdmX dual inhibitors.