Dual functionality of MDM2 in PROTACs expands the horizons of targeted protein degradation

Abstract

The evolution of targeted protein degradation (TPD) has been significantly propelled by the advent of proteolysis-targeting chimeras (PROTACs), which utilize heterobifunctional molecules to facilitate the ubiquitination-mediated degradation of previously "undruggable" proteins. Mouse double minute 2 (MDM2), which is often overexpressed in various diseases and plays a crucial role in regulating key pathways like p53, emerges as an exemplary candidate for therapeutic exploitation within the TPD realm, serving both as an intrinsic E3 ligase and as a direct protein of interest (POI). By harnessing MDM2's inherent E3 ligase activity, PROTACs have been designed to efficiently degrade specific POIs, achieving substantial success in both in vitro and in vivo studies. Alternatively, PROTACs have been developed to directly target MDM2 itself, offering new approaches for therapeutic intervention. Recent research has yielded valuable strategies for optimizing MDM2-harnessing and MDM2-targeted PROTAC designs, concentrating on warhead selection of POI, linker length and composition optimization, and the choice among various E3 ligases and their corresponding recruiters. These advancements not only broaden the scope of PROTAC technologies but also expedite the development of MDM2-based therapies, inspiring approaches for disease treatment.

Keywords: Bridged PROTAC; Homo-PROTAC; MDM2; PROTAC; TPD.

Fig. 1

Two protein degradation pathways. A Ubiquitin–proteasome system. This system is the primary pathway for selective protein degradation in eukaryotic cells. It targets short-lived, misfolded, or damaged proteins for destruction in a tightly regulated process. The 26S proteasome, a barrel-shaped protease complex, recognizes ubiquitinated proteins, removes the ubiquitin tags via deubiquitinases, and unfolds and degrades substrates into short peptides. B Autophagy-lysosome pathway. Lysosomes are membrane-bound organelles containing hydrolytic enzymes that degrade macromolecules via autophagy or endocytosis. The key roles of lysosomes include the degradation of long-lived proteins, the clearance of aggregated proteins, and the recycling of organelles

Fig. 2

Targeted Protein Degradation Strategies. A The UPS-based degraders. PROTACs are bifunctional molecules that recruit the E3 ligase to POI, marking it for ubiquitination and subsequent proteasomal degradation. Molecular Glues can induce or stabilize interactions between proteins, leading to POI degradation. HyTTC employs hydrophobic tags attached to POI, mimicking misfolded proteins and triggering POI degradation by cellular quality control mechanisms [35]. B Endosome-lysosome-based degraders. LYTACs utilize ligand binding to LTRs, such as CI-MPR and ASGPR, mediating ex-POI degradation. Whereas CI-MPR is ubiquitously expressed in all human tissues, ASGPR is only expressed in the liver. AbTACs utilize a recombinant bispecific antibody to recruit the m-POI and the m-E3 ligase, RNF43 [35]. The POI is likely degraded via the lysosomes, but not by the proteasomes. However, the exact mechanisms remain to be established. C Autophagy-lysosome-based degraders. ATTECs simultaneously bind LC3 and the POI, while an AUTOTAC molecule binds p62 and the POI [35]. The binding induces the formation of autophagosomes, and subsequent fusion between autophagosomes and lysosomes leads to the POI degradation. LTRs, lysosome-targeting receptors; CI-MPR, cation-independent mannose 6-phosphate receptor; ASGPR, asialoglycoprotein receptor; Ex-POI, extracellular-POI; m-POI, membrane-bound protein; m-E3 ligase, membrane-bound E3 ligase; RNF43, ring finger protein 43; LC3, microtubule-associated protein 1 light-chain 3

Fig. 3

MDM2 Sequence and Its Function: p53 and Beyond. A MDM2 sequence landmark. MDM2 is located on chromosome 12q15 and spans about 37 kb of genomic DNA. MDM2 is composed of 491 amino acids and contains three major functional regions: N-terminal p53-binding domain, central acidic domain, and C-terminal RING Finger domain. B MDM2 basic functions. With specific PTM, including multisites phosphorylation and acetylation, MDM2 is activated to recognize and regulate the functions of corresponding signal effectors via either facilitating ubiquitination or occupying their functional domains. Therefore, maintaining the physiological functions of MDM2 ensures cellular homeostasis, balanced stress responses, and tumor suppression

Fig. 4

MDM2-harnessing PROTACs and molecular glue. Four distinct types of MDM2-harnessing PROTACs are classified by their mechanisms of recruiting MDM2. These recruitment strategies are as follows: A Peptide-based PROTACs. Representatives of this group include ARTC and optimized DSARTC designed to degrade AR and AR-V7. B Small molecule-based PROTACs. Instances include X1-4P-MDM2 and A1874 engineered to degrade DYRK1A and BRD4, respectively. C ANM-based PROTACs. They are exemplified by AS1411-S3-1 and AS1411-V7t1, degrading STAT3 and VEGF165, respectively. D Ori-based PROTACs. They are presented by Ori-JQ1-3, degrading BRD4. Another type of degraders utilizing UPS: E Molecular glues. VNPP433-3β is a prime example of them that specifically facilitates the interaction between MDM2 and AR/AR-V7

Fig. 5

The MDM2-targeted PROTACs. Two types of MDM2-targeted PROTACs are classified by the selection of E3 ligases promoting MDM2 degradation. A MDM2-targeted PROTACs. Representatives of this group include ursane-thalidomide-based PROTAC, GAA PROTAC: V10, KT-253 and MD-4251, which recruit either CRBN or VHL as E3 ligase for MDM2 ubiquitination. B Homo-PROTACs. Instances include Nutlin-3-based Homo-PROTAC, HomoAS1411, and Ori-Ori, inducing MDM2 autoubiquitination and suicide in different recruiting mechanisms