MDM2-C Functions as an E3 Ubiquitin Ligase

Structure and function of MDM2 and MDM4 in health and disease

Both mouse double-minute 2 (MDM2), an E3 ubiquitin ligase, and its closely related paralog, MDM4, which lacks E3 activity, play central roles in cellular homeostasis. MDM-linked dysfunction is associated with an increased risk of oncogenesis, primarily through targeting the tumor suppressor protein p53 for ubiquitination and degradation. Recent studies have revealed multifaceted roles of MDM proteins that are p53 independent with implications for their oncogenic properties. This review aims to provide an overview of MDM2 and MDM4, by assessing gene and protein structure and implications for protein-protein interactions and functions in cell and animal physiology. We also explore MDM2 and MDM4 role(s) in angiogenesis, a critical feature of solid tumor growth and progression. Finally, we discuss the current landscape in the development of MDM2 and MDM4 inhibitors for cancer therapy.

Oncogenic Functions of Alternatively Spliced MDM2-ALT2 Isoform in Retroperitoneal Liposarcoma

Retroperitoneal liposarcoma (RPLPS) is one of the most common histologic subtypes of soft tissue sarcoma (STS). Complete surgical resection remains the mainstay treatment, while the high rate of locoregional recurrence constitutes the predominant cause of mortality. Well-differentiated (WDLPS) and dedifferentiated (DDLPS) liposarcoma are the most frequent subtypes of RPLPS and present amplified MDM2 gene as a hallmark. However, there are few reports evaluating the role of alternatively spliced MDM2 transcripts in RPLPS. In this study, we assessed MDM2-ALT2 expression levels in a cohort of RPLPS patients and evaluated the biological functions of the MDM2-ALT2 isoform in vitro in DDLPS cell lines. Using BaseScope™ and qPCR, we demonstrated that MDM2-Full Length (MDM2-FL) and MDM2-ALT2 expression levels were upregulated in RPLPS patient-derived tissue samples compared to normal adjacent to tumor tissue (NAT). DDLPS cells overexpressing MDM2-FL or MDM2-ALT2 had higher proliferation rates and increased migration and invasion capacities, as well as increased protein levels of p-AKT, mTOR, p70S6K, MMP2, and cJun. Simultaneous overexpression of MDM2-ALT2 and AKT silencing showed that AKT inhibition impaired p-p70S6K and MMP2 protein increased levels and led to significantly decreased proliferation and migration rates compared to cells overexpressing MDM2-ALT2 only. Taken together, our data suggest that MDM2-ALT2 may promote RPLPS progression.

KIF15 knockdown inhibits colorectal cancer proliferation and migration through affecting the ubiquitination modification of NRAS

As one of the most common malignancies, colorectal cancer (CRC) requires a thorough understanding of the mechanisms that promote its development and the discovery of new therapeutic targets. In this study, immunohistochemical staining confirmed significantly higher expression levels of KIF15 in CRC. qPCR and western blot results demonstrated the effective suppression of KIF15 mRNA and protein expression by shKIF15. Downregulation of KIF15 inhibited the proliferation and migration of CRC cells while promoting apoptosis. In addition, evidence from the xenograft experiments in nude mice demonstrated that KIF15 knockdown also suppressed tumor growth. Through bioinformatics analysis, the downstream molecular NRAS and Rac signaling pathway associated with KIF15 were identified. KIF15 knockdown was found to inhibit NRAS expression and disrupt Rac signaling pathway. Moreover, WB and Co-IP assays revealed that KIF15 reduced the ubiquitination modification of NRAS protein by interacting with the E3 ligase MDM2, thereby enhancing NRAS protein stability. Functionally, NRAS knockdown was shown to inhibit cell proliferation and migration. In conclusion, KIF15 promoted CRC progression by regulating NRAS expression and Rac signaling pathway.

N-Heterocycle based Degraders (PROTACs) Manifesting Anticancer Efficacy: Recent Advances

Proteolysis Targeting Chimeras (PROTACs) technology has emerged as a promising strategy for the treatment of undruggable therapeutic targets. Researchers have invested a great effort in developing druggable PROTACs; however, the problems associated with PROTACs, including poor solubility, metabolic stability, cell permeability, and pharmacokinetic profile, restrict their clinical utility. Thus, there is a pressing need to expand the size of the armory of PROTACs which will escalate the chances of pinpointing new PROTACs with optimum pharmacokinetic and pharmacodynamics properties. N- heterocycle is a class of organic frameworks that have been widely explored to construct new and novel PROTACs. This review provides an overview of recent efforts of medicinal chemists to develop N-heterocycle-based PROTACs as effective cancer therapeutics. Specifically, the recent endeavors centred on the discovery of PROTACs have been delved into various classes based on the E3 ligase they target (MDM2, IAP, CRBN, and other E3 ligases). Mechanistic insights revealed during the biological assessment of recently furnished Nheterocyclic- based PROTACs constructed via the utilization of ligands for various E3 ligases have been discussed.

DAB2IP suppresses tumor malignancy by inhibiting GRP75-driven p53 ubiquitination in colon cancer

Increasing evidence has shown that DAB2IP acts as a tumor suppressor and plays an inhibitory role in many signals associated with tumorigenesis. However, the underlying mechanism of this function remains unclear. Our study shows that DAB2IP was positively associated with a good prognosis in patients with colorectal cancer and wild-type p53 expression. An in vitro assay showed that DAB2IP elicited potent tumor-suppressive effects by inhibiting cell invasiveness and colony formation and promoting cell apoptosis in wild-type p53 colon cancer cells. In addition, DAB2IP improved the stability of wild-type p53 by inhibiting its degradation in a ubiquitin-proteasome-dependent manner. Using mass spectrometry profiling, we revealed that DAB2IP and p53 interacted with the ubiquitin ligase-related protein GRP75. Mechanistically, DAB2IP is competitively bound to GRP75, thus reducing GRP75-driven p53 ubiquitination and degradation. Moreover, the Ras-GAP domain was required for the DAB2IP-GRP75 interaction and DAB2IP-mediated p53 ubiquitination. Finally, animal experiments revealed that DAB2IP inhibited tumor progression in vivo. In conclusion, our study presents a novel function of DAB2IP in GRP75-driven wild-type p53 degradation, providing new insight into DAB2IP-induced tumor suppression and a novel molecular interpretation of the p53 pathway.

circNUDT21 promotes bladder cancer progression by modulating the miR-16-1-3p/MDM2/p53 axis

Bladder cancer (BC) is a common genitourinary malignancy. This study investigated the regulatory effects of an exonic circRNA, circNUDT21, in the progression of BC. The circNUDT21 level was overexpressed in BC tissues and cell lines as compared to normal controls. Overexpression and silencing of circNUDT21 promoted and inhibited, respectively, the proliferative and invasive abilities of BC cells. Mechanistical analysis showed that circNUDT21 acted as a miR-16-1-3p sponge and that MDM2 was a potential downstream target of miR-16-1-3p. We further verified that overexpression of circNUDT21 was associated with elevated MDM2 and reduced p53 expression. CircNUDT21 promoted BC progression by acting as a sponge of miR-16-1-3p to activate the miR-16-1-3p/MDM2/p53 axis. These findings suggest that circNUDT21 functions as an oncogenic circRNA and may be a potential therapy target for BC.

Regulation of p53 by E3s

More than 40 years of research on p53 have given us tremendous knowledge about this protein. Today we know that p53 plays a role in different biological processes such as proliferation, invasion, pluripotency, metabolism, cell cycle control, ROS (reactive oxygen species) production, apoptosis, inflammation and autophagy. In the nucleus, p53 functions as a bona-fide transcription factor which activates and represses transcription of a number of target genes. In the cytoplasm, p53 can interact with proteins of the apoptotic machinery and by this also induces cell death. Despite being so important for the fate of the cell, expression levels of p53 are kept low in unstressed cells and the protein is largely inactive. The reason for the low expression level is that p53 is efficiently degraded by the ubiquitin-proteasome system and the vast inactivity of the tumor suppressor protein under normal growth conditions is due to the absence of activating and the presence of inactivating posttranslational modifications. E3s are important enzymes for these processes as they decorate p53 with ubiquitin and small ubiquitin-like proteins and by this control p53 degradation, stability and its subcellular localization. In this review, we provide an overview about E3s that target p53 and discuss the connection between p53, E3s and tumorigenesis.