Programmed cell death is a vital biological process for multicellular organisms to maintain cellular homeostasis, which is regulated in a complex manner. Over the past several years, apart from apoptosis, which is the principal mechanism of caspase-dependent cell death, research on non-apoptotic forms of programmed cell death has gained momentum. p53 is a well characterized tumor suppressor that controls cell proliferation and apoptosis and has also been linked to non-apoptotic, non-canonical cell death mechanisms. p53 impacts these non-canonical forms of cell death through transcriptional regulation of its downstream targets, as well as direct interactions with key players involved in these mechanisms, in a cell type- or tissue context-dependent manner. In this review article, we summarize and discuss the involvement of p53 in several non-canonical modes of cell death, including caspase-independent apoptosis (CIA), ferroptosis, necroptosis, autophagic cell death, mitotic catastrophe, paraptosis, and pyroptosis, as well as its role in efferocytosis which is the process of clearing dead or dying cells.

Accumulation of missense mutant p53 (mutp53) in cancers promotes malignant progression. DNAJA1, a member of HSP40 (also known as J-domain proteins: JDPs), is shown to prevent misfolded or conformational mutp53 from proteasomal degradation. Given frequent addiction of cancers to oncogenic mutp53, depleting mutp53 by DNAJA1 inhibition is a promising approach for cancer therapy. However, there is no clinically available inhibitor for DNAJA1. Our in silico molecular docking study with a natural compound-derived small molecule library identified a plumbagin derivative, PLIHZ (plumbagin–isoniazid analog), as a potential compound binding to the J domain of DNAJA1. PLIHZ efficiently reduced the levels of DNAJA1 and several conformational mutp53 with minimal impact on DNA contact mutp53 and wild-type p53 (wtp53). An analog, called PLTFBH, which showed a similar activity to PLIHZ in reducing DNAJA1 and mutp53 levels, inhibited migration of cancer cells specifically carrying conformational mutp53, but not DNA contact mutp53, p53 null, and wtp53, which was attenuated by depletion of DNAJA1 or mutp53. Moreover, PLTFBH reduced levels of multiple other HSP40/JDPs with tyrosine 7 (Y7) and/or tyrosine 8 (Y8) but failed to deplete DNAJA1 mutants with alanine substitution of these amino acids. Our study suggests PLTFBH as a potential inhibitor for multiple HSP40/JDPs.

The tumor suppressor p53 induces cell cycle arrest and/or apoptosis by transactivating numerous downstream target genes and also translocating to the mitochondrial outer membrane.

MDM4 (Murine Double Minute 4), also known as MDMX, is a crucial negative regulator of the tumor suppressor p53. MDM4 heterodimerizes with MDM2 to enhance MDM2-mediated ubiquitination and degradation of p53, thereby promoting tumorigenesis. Beyond its canonical role in inhibiting p53 activity, recent studies have revealed diverse p53-independent functions. MDM4 interacts with various proteins, including p73, E2F1, casein kinase 1α, PPARα, and TRIM21 to regulate cell cycle progression, β-catenin-mediated pre-leukemic progression, and ferroptosis independent of p53. In addition, MDM4 functions independently of both p53 and MDM2 by interacting with proteins, such as SMAD family members 3/4, retinoblastoma protein (pRB), p21, Nbs1 (also known as Nibrin), mTOR complex 1 (mTORC1), and the Polycomb Repressive Complexes (PRCs) complex, to control cell proliferation and survival, as well as protein degradation, double-strand break (DSB) repair, and replication fork progression. Intriguingly, multiple studies suggest that MDM4 exhibits oncogenic activity independent of p53; however, other reports highlight a potential tumor-suppressive role for MDM4 in the absence of p53. Thus, MDM4’s functions extend well beyond the canonical p53–MDM2 axis. A deeper understanding of MDM4 biology may facilitate the development of novel targeted therapies for various cancers.

MTBP is implicated in cell cycle progression, DNA replication, and cancer metastasis. However, the function of MTBP remains enigmatic and is dependent on cellular contexts and its cellular localization. To understand the in vivo physiological role of MTBP, it is important to generate Mtbp knockout mice. However, complete deletion of the Mtbp gene in mice results in early embryonic lethality, while its heterozygous deletion shows modest biological phenotypes, including enhanced cancer metastasis. To overcome this and better characterize the in vivo physiological function of MTBP, we, for the first time, generated mice that carry an Mtbp hypomorphic allele (Mtbp^H) in which Mtbp protein is expressed at approximately 30% of that in the wild-type allele. We treated wild-type, Mtbp^+/−, and Mtbp^H/− mice with a liver carcinogen, diethylnitrosamine (DEN), and found that the Mtbp^H/− mice showed worse overall survival when compared to the wild-type mice. Consistent with previous reports using human liver cancer cells, mouse embryonic fibroblasts (MEFs) from the Mtbp^H/− mice showed an increase in the nuclear localization of p-Erk1/2 and migratory potential. Thus, Mtbp^H/− mice and cells from Mtbp^H/− mice are valuable to understand the in vivo physiological role of Mtbp and validate the diverse functions of MTBP that have been observed in human cells.