MZ2

The von Hippel-Lindau (VHL) tumor suppressor associates with transcription factors elongin-C and elongin-B to form the VHL-elongin-C-elongin-B protein complex and carry out its functions, such as degradation of hypoxia-inducible factors. VHL ligands are used not only to modulate hypoxia-signaling pathways and potentially treat chronic anemia or ischemia but also to form bivalent ligands as proteolysis-targeting chimeras to degrade proteins for potential therapeutic applications. Sensitive and selective VHL-based binding assays are critical for identifying and characterizing VHL ligands with high-throughput screening approaches. VHL ligand-binding assays, such as isothermal titration calorimetry, surface plasmon resonance, and fluorescence polarization assays, are reported but with limitations. Isothermal titration calorimetry requires higher protein concentrations with a lower throughput than fluorescence-based assays do. Surface plasmon resonance requires protein immobilization, which introduces variation and is not suitable for testing a large number of ligands. Fluorescence polarization can be sensitive with high-throughput capability but is susceptible to assay interference, and small-molecule-based fluorescent probes are not available. We developed the first small-molecule-based high-affinity VHL fluorescent probe BODIPY FL VH032 (5), with a K _d of 3.01 nM, for a time-resolved fluorescence resonance energy-transfer assay. This new assay is sensitive, selective, resistant to assay interference, and capable of characterizing VHL ligands with a wide range of affinities. It is also suitable for VHL ligand identification and characterization with high-throughput screening.

The design of proteolysis-targeting chimeras (PROTACs) is a powerful small-molecule approach for inducing protein degradation. PROTACs conjugate a target warhead to an E3 ubiquitin ligase ligand via a linker. Here we examined the impact of derivatizing two different BET bromodomain inhibitors, triazolodiazepine JQ1 and the more potent tetrahydroquinoline I-BET726, via distinct exit vectors, using different polyethylene glycol linkers to VHL ligand VH032. Triazolodiazepine PROTACs exhibited positive cooperativities of ternary complex formation and were more potent degraders than tetrahydroquinoline compounds, which showed negative cooperativities instead. Marked dependency on linker length was observed for BET-degrading and cMyc-driven antiproliferative activities in acute myeloid leukemia cell lines. This work exemplifies as a cautionary tale how a more potent inhibitor does not necessarily generate more potent PROTACs and underscores the key roles played by the conjugation. The provided insights and framework for structure-activity relationships of bivalent degraders are anticipated to have wide future applicability.