Small-molecule MDM2 inhibitor LQFM030-induced apoptosis in p53-null K562 chronic myeloid leukemia cells

The p53 family member p73 in the regulation of cell stress response

During oncogenesis, cells become unrestrictedly proliferative thereby altering the tissue homeostasis and resulting in subsequent hyperplasia. This process is paralleled by resumption of cell cycle, aberrant DNA repair and blunting the apoptotic program in response to DNA damage. In most human cancers these processes are associated with malfunctioning of tumor suppressor p53. Intriguingly, in some cases two other members of the p53 family of proteins, transcription factors p63 and p73, can compensate for loss of p53. Although both p63 and p73 can bind the same DNA sequences as p53 and their transcriptionally active isoforms are able to regulate the expression of p53-dependent genes, the strongest overlap with p53 functions was detected for p73. Surprisingly, unlike p53, the p73 is rarely lost or mutated in cancers. On the contrary, its inactive isoforms are often overexpressed in cancer. In this review, we discuss several lines of evidence that cancer cells develop various mechanisms to repress p73-mediated cell death. Moreover, p73 isoforms may promote cancer growth by enhancing an anti-oxidative response, the Warburg effect and by repressing senescence. Thus, we speculate that the role of p73 in tumorigenesis can be ambivalent and hence, requires new therapeutic strategies that would specifically repress the oncogenic functions of p73, while keeping its tumor suppressive properties intact.

The roles and regulation of MDM2 and MDMX: it is not just about p53

In this review, Klein et al. discuss the p53-independent roles of MDM2 and MDMX. First, they review the structural and functional features of MDM2 and MDMX proteins separately and together that could be relevant to their p53-independent activities. Following this, they summarize how these two proteins are regulated and how they can function in cells that lack p53.

Most well studied as proteins that restrain the p53 tumor suppressor protein, MDM2 and MDMX have rich lives outside of their relationship to p53. There is much to learn about how these two proteins are regulated and how they can function in cells that lack p53. Regulation of MDM2 and MDMX, which takes place at the level of transcription, post-transcription, and protein modification, can be very intricate and is context-dependent. Equally complex are the myriad roles that these two proteins play in cells that lack wild-type p53; while many of these independent outcomes are consistent with oncogenic transformation, in some settings their functions could also be tumor suppressive. Since numerous small molecules that affect MDM2 and MDMX have been developed for therapeutic outcomes, most if not all designed to prevent their restraint of p53, it will be essential to understand how these diverse molecules might affect the p53-independent activities of MDM2 and MDMX.

Ctnnb1 transcriptional upregulation compensates for Mdm2/p53-mediated β-catenin degradation in neutrophils following cardioembolic stroke

BACKGROUND

A better understanding of the mechanism(s) underlying cardioembolic stroke can promote recovery and reduce the risk of recurrent embolisms.

METHODS

Peripheral blood mononuclear cell (PBMC) gene expression datasets from cardioembolic patients and healthy controls were obtained from the Gene Expression Omnibus (GEO) database (GSE58294). The Limma software package was utilized to identify differentially-expressed genes (DEGs). Protein-protein interaction (PPI) analysis of the DEGs was performed using STRING. A weighted gene co-expression network analysis (WGCNA) was used to build a gene co-expression network. In vitro experiments assessed the effects on neutrophils exposed to oxygen and glucose-deprived (OGD) cortical neurons. An in vivo murine model of thromboembolic stroke was constructed through thrombin injection to examine effects on circulating neutrophils. Mechanistic in vitro studies were conducted using the proteasome inhibitor MG132, the p53-Mdm2 binding inhibitor Nutlin-3a, Mdm2 small-interfering RNA (siRNA), and Ctnnb1 siRNA.

RESULTS

DEG analysis identified 44 upregulated and 66 downregulated genes in cardioembolic stroke PBMCs. PPI analysis of these DEGs yielded one eight-node protein module with β-catenin (CTNNB1) as the central hub protein. Integration of the DEGs with WGCNA-derived hub genes revealed the key hub DEGs CTNNB1 and mouse double minute 2 (MDM2). Follow-up experiments revealed Mdm2, p53, and phospho-β-catenin upregulation in neutrophils exposed to OGD neurons in vitro and following thromboembolic stroke in vivo. Mechanistic studies revealed that neutrophils transcriptionally upregulate Ctnnb1 expression to compensate for Mdm2/p53-mediated β-catenin degradation induced by exposure to OGD neurons, thereby promoting neutrophil survival.

CONCLUSION

Compensatory Ctnnb1 transcriptional upregulation in neutrophils induced by ischemic neuron exposure may be involved in promoting neutrophil survival following cardioembolic stroke.

Targeting USP7-Mediated Deubiquitination of MDM2/MDMX-p53 Pathway for Cancer Therapy: Are We There Yet?

The p53 tumor suppressor protein and its major negative regulators MDM2 and MDMX oncoproteins form the MDM2/MDMX-p53 circuitry, which plays critical roles in regulating cancer cell growth, proliferation, cell cycle progression, apoptosis, senescence, angiogenesis, and immune response. Recent studies have shown that the stabilities of p53, MDM2, and MDMX are tightly controlled by the ubiquitin-proteasome system. Ubiquitin specific protease 7 (USP7), one of the most studied deubiquitinating enzymes plays a crucial role in protecting MDM2 and MDMX from ubiquitination-mediated proteasomal degradation. USP7 is overexpressed in human cancers and contributes to cancer initiation and progression. USP7 inhibition promotes the degradation of MDM2 and MDMX, activates the p53 signaling, and causes cell cycle arrest and apoptosis, making USP7 a potential target for cancer therapy. Several small-molecule inhibitors of USP7 have been developed and shown promising efficacy in preclinical settings. In the present review, we focus on recent advances in the understanding of the USP7-MDM2/MDMX-p53 network in human cancers as well as the discovery and development of USP7 inhibitors for cancer therapy.