Nuclear receptor antagonists are used to treat various diseases, but the precise antagonist mechanisms differ among receptors and compounds. Understanding the interplay between ligand-receptor interactions and transcriptional outcomes is critical. The nuclear receptor pregnane X receptor (PXR) is activated by many medicinal compounds and upregulates drug metabolism genes in response, decreasing efficacy and/or increasing toxicity of drugs. Co-administered PXR antagonists could reduce these effects, but such compounds have only recently been identified, and molecular elements governing their actions remain largely unknown. Here, we show chemically similar PXR ligands with three distinct activities (agonist, antagonist, and inverse agonist) that are altered by PXR mutations. These diverging activities are linked to ligand-induced changes at the intersection of ligand, receptor ligand-binding pocket, and receptor surface where transcriptional coregulators are recruited. We also find that antagonists can act by multiple mechanisms regarding coregulator recruitment, highlighting the complexity of ligand-receptor interactions that influence transcriptional activity.
Copyright © 2025 The Author(s). Published by Elsevier Inc. All rights reserved.

Proteolysis-targeting chimeras (PROTACs) containing a target protein ligand linked to an E3 ubiquitin ligase ligand induce target protein degradation through E3 recruitment. Most PROTACs bind a surface cleft of the protein of interest rather than a buried pocket. Using the nuclear receptor PXR, we previously described the inherent difficulties of PROTAC targeting via a deep solvent-inaccessible ligand binding pocket. Here, we discover that the CRBN-dependent MDM2 PROTAC MD-224 is a potent PXR degrader that achieves its activity from binding adjacent to the ligand-binding pocket. Furthermore, because the proximal region is a structural feature common among nuclear receptors, MD-224 also targets additional receptors for proteasomal degradation. Using structure- and activity-guided medicinal chemistry, we ablated MDM2 degradation and generated MD-224 analogs with activities skewed toward different receptors. Thus, we describe (1) PROTAC repurposing as a potential route of degrader discovery and (2) nuclear receptor-targeted degradation through a noncanonical binding site.
© 2025. The Author(s).

Proteolysis-targeting chimeras (PROTACs) are heterobifunctional molecules containing a ligand for a protein of interest linked to an E3 ubiquitin ligase ligand that induce protein degradation through E3 recruitment to the target protein. Small changes in PROTAC linkers can have drastic consequences, including loss of degradation activity, but the structural mechanisms governing such changes are unclear. To study this phenomenon, we screened PROTACs of diverse targeting modalities and identified dTAG-13 as an activator of the xenobiotic-sensing pregnane X receptor (PXR), which promiscuously binds various ligands. Characterization of dTAG-13 analogs and precursors revealed interplay between the PXR-binding moiety, linker, and E3 ligand that altered PXR activity without inducing degradation. A crystal structure of PXR ligand binding domain bound to a precursor ligand showed ligand-induced binding pocket distortions and a linker-punctured tunnel to the protein exterior at a region incompatible with E3 complex formation, highlighting the effects of linker environment on PROTAC activity.
Copyright © 2024 Elsevier Inc. All rights reserved.

Pregnane X receptor (PXR) is a ligand-activated transcription factor that binds diverse compounds and upregulates drug metabolism machinery in response. PXR activation is detrimental to drug efficacy and safety because it reduces active drug concentrations and increases reactive metabolites, leading to toxicity and/or drug-drug interactions. Thus, effort must be expended in drug development pipelines to assess PXR activation by lead candidates and chemically modify agonists to reduce PXR liabilities while maintaining on-target potencies. Coadministration of drugs with PXR antagonists could prevent PXR-mediated metabolism events, but such compounds are rare and may themselves be converted to agonists by metabolic enzymes or PXR mutations. Here, we report the design, synthesis, optimization, and biological validation of proteolysis targeting chimeras that induce PXR degradation through E3 ubiquitin ligase recruitment. PXR degradation blocks agonist-induced gene expression and enhances anticancer effects of the chemotherapy paclitaxel, a known PXR agonist and substrate of downstream metabolic enzymes.

Identifying individual functional B cell receptors (BCRs) is common, but two-dimensional analysis of B cell frequency versus BCR potency would delineate both quantity and quality of antigen-specific memory B cells. We efficiently determine quantitative BCR neutralizing activities using a single-cell-derived antibody supernatant analysis (SCAN) workflow and develop a frequency-potency algorithm to estimate B cell frequencies at various neutralizing activity or binding affinity cutoffs. In an HIV-1 fusion peptide (FP) immunization study, frequency-potency curves elucidate the quantity and quality of FP-specific immunoglobulin G (IgG)+ memory B cells for different animals, time points, and antibody lineages at single-cell resolution. The BCR neutralizing activities are mainly determined by their affinities to soluble envelope trimer. Frequency analysis definitively demonstrates dominant neutralizing antibody lineages. These findings establish SCAN and frequency-potency analyses as promising approaches for general B cell analysis and monoclonal antibody (mAb) discovery. They also provide specific rationales for HIV-1 FP-directed vaccine optimization.
Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.