Hsp90 Complex Research Articles

Selective Inhibition of hsp90 Paralogs: Uncovering the Role of Helix 1 in Grp94-Selective Ligand Binding.

Grp94 is the endoplasmic reticulum paralog of the hsp90 family of chaperones, which have been targeted for therapeutic intervention via their highly conserved ATP binding sites. The design of paralog-selective inhibitors relies on understanding the protein structural elements that drive higher affinity in selective inhibitors. Here, we determined the structures of Grp94 and Hsp90 in complex with the Grp94-selective inhibitor PU-H36, and of Grp94 with the non-selective inhibitor PU-H71. In Grp94, PU-H36 derives its higher affinity by utilizing Site 2, a Grp94-specific side pocket adjoining the ATP binding cavity, but in Hsp90 PU-H36 occupies Site 1, a side pocket that is accessible in all paralogs with which it makes lower affinity interactions. The structure of Grp94 in complex with PU-H71 shows only Site 1 binding. While changes in the conformation of helices 4 and 5 in the N-terminal domain occur when ligands bind to Site 1 of both Hsp90 and Grp94, large conformational shifts that also involve helix 1 are associated with the engagement of the Site 2 pocket in Grp94 only. Site 2 in Hsp90 is blocked and its helix 1 conformation is insensitive to ligand binding. To understand the role of helix 1 in ligand selectivity, we tested the binding of PU-H36 and other Grp94-selective ligands to chimeric Grp94/Hsp90 constructs. These studies show that helix 1 is the major determinant of selectivity for Site 2 targeted ligands and also influences the rate of ATPase activity in Hsp90 paralogs.

Selective inhibition of hsp90 paralogs: Uncovering the role of helix 1 in Grp94-selective ligand binding.

Grp94 is the endoplasmic reticulum paralog of the hsp90 family of chaperones, which have been targeted for therapeutic intervention via their highly conserved ATP binding sites. The design of paralog-selective inhibitors relies on understanding the protein structural elements that drive higher affinity in selective inhibitors. Here, we determined the structures of Grp94 and Hsp90 in complex with the Grp94-selective inhibitor PU-H36, and of Grp94 with the non-selective inhibitor PU-H71. In Grp94, PU-H36 derives its higher affinity by utilizing Site 2, a Grp94-specific side pocket adjoining the ATP binding cavity, but in Hsp90 PU-H36 occupies Site 1, a side pocket that is accessible in all paralogs with which it makes lower affinity interactions. The structure of Grp94 in complex with PU-H71 shows only Site 1 binding. While changes in the conformation of helices 4 and 5 in the N-terminal domain occur when ligands bind to Site 1 of both Hsp90 and Grp94, large conformational shifts that also involve helix 1 are associated with the engagement of the Site 2 pocket in Grp94 only. Site 2 in Hsp90 is blocked and its helix 1 conformation is insensitive to ligand binding. To understand the role of helix 1 in ligand selectivity, we tested the binding of PU-H36 and other Grp94-selective ligands to chimeric Grp94/Hsp90 constructs. These studies show that helix 1 is the major determinant of selectivity for Site 2 targeted ligands, and also influences the rate of ATPase activity in Hsp90 paralogs.

Abstract 6053: Mechanism of action of tumor-selective, chaperone-mediated protein degraders (CHAMPs)

Abstract HSP90 mediates the folding of many important cancer-associated proteins, but it can also direct its substrates towards degradation via the ubiquitin-proteasome system. Furthermore, in tumor tissues, HSP90 complexes are in an activated state relative to normal tissues, and small molecule HSP90 inhibitors display unique tumor-selective pharmacokinetics. To take advantage of these attributes, we have developed a novel targeted protein degradation technology, termed Chaperone-Mediated Protein Degradation (CHAMP), and present here an in-depth characterization of the CHAMP mechanism of action. Initially, from a chemical library of greater than 1000 linkered HSP90 binders, hetero-bifunctional CHAMPs were synthesized in which target protein binders and HSP90 binders were covalently coupled together by short linkers. The resulting compounds were screened for target protein degradation and cancer cell cytotoxicity to identify promising leads for further optimization. We found that CHAMPs can degrade a wide variety of target proteins. This included proteins that are known to be regulated by HSP90, such as transcription factor BRD4 or ERK5 kinase. However, proteins that are normally independent of HSP90 function can also be degraded, including mutated KRAS and SHP2 phosphatase. In vitro, CHAMP treatment of cells resulted in formation of a ternary complex between the target protein, CHAMP compound and HSP90. Moreover, an X-ray crystal structure was solved for a mKRAS-CHAMP-HSP90 ternary complex. CHAMP-mediated degradation required both the target- and HSP90-binding moieties to be covalently coupled and involved ubiquitination of the target protein. Multiple ubiquitin E3 ligases were present in ternary complexes, and depending on the target protein, NEDD8 inhibition or CRISPR knockouts of individual E3 ligases could suppress proteasome-dependent target degradation. In vivo, irrespective of target, CHAMPs displayed prolonged exposure in tumors relative to plasma and normal tissues, resulting in prolonged target degradation in tumors and strong tumor growth inhibition at well-tolerated doses. CHAMP technology can be applied to a diversity of cancer-associated targets and has potential advantages relative to other protein degradation approaches, including an improved safety margin due to preferential accumulation in tumor tissues. Citation Format: Xiangcai Yang, Yan Dai, Qinglin Ding, Feng Du, Jinhua Li, Chuhe Liu, Chunyang Lv, Liang Ma, Thomas L. Prince, Yuetong Sun, Mingkai Wang, Rong Wang, Yaya Wang, Zhiyong Wang, Min Wu, Mengmeng Xu, Zimo Yang, Long Ye, Wei Yin, Chenghao Ying, Haoxin Zhou, Guoqiang Wang, Weiwen Ying, Kevin P. Foley. Mechanism of action of tumor-selective, chaperone-mediated protein degraders (CHAMPs) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6053.

Structural dynamics of RAF1-HSP90-CDC37 and HSP90 complexes reveal asymmetric client interactions and key structural elements

RAF kinases are integral to the RAS-MAPK signaling pathway, and proper RAF1 folding relies on its interaction with the chaperone HSP90 and the cochaperone CDC37. Understanding the intricate molecular interactions governing RAF1 folding is crucial for comprehending this process. Here, we present a cryo-EM structure of the closed-state RAF1-HSP90-CDC37 complex, where the C-lobe of the RAF1 kinase domain binds to one side of the HSP90 dimer, and an unfolded N-lobe segment of the RAF1 kinase domain threads through the center of the HSP90 dimer. CDC37 binds to the kinase C-lobe, mimicking the N-lobe with its HxNI motif. We also describe structures of HSP90 dimers without RAF1 and CDC37, displaying only N-terminal and middle domains, which we term the semi-open state. Employing 1 μs atomistic simulations, energetic decomposition, and comparative structural analysis, we elucidate the dynamics and interactions within these complexes. Our quantitative analysis reveals that CDC37 bridges the HSP90-RAF1 interaction, RAF1 binds HSP90 asymmetrically, and that HSP90 structural elements engage RAF1’s unfolded region. Additionally, N- and C-terminal interactions stabilize HSP90 dimers, and molecular interactions in HSP90 dimers rearrange between the closed and semi-open states. Our findings provide valuable insight into the contributions of HSP90 and CDC37 in mediating client folding.

J-domain Proteins form Binary Complexes with Hsp90 and Ternary Complexes with Hsp90 and Hsp70

Hsp90 and Hsp70 are highly conserved molecular chaperones that help maintain proteostasis by participating in protein folding, unfolding, remodeling and activation of proteins. Both chaperones are also important for cellular recovery following environmental stresses. Hsp90 and Hsp70 function collaboratively for the remodeling and activation of some client proteins. Previous studies using E. coli and S. cerevisiae showed that residues in the Hsp90 middle domain directly interact with a region in the Hsp70 nucleotide binding domain, in the same region known to bind J-domain proteins. Importantly, J-domain proteins facilitate and stabilize the interaction between Hsp90 and Hsp70 both in E. coli and S. cerevisiae. To further explore the role of J-domain proteins in protein reactivation, we tested the hypothesis that J-domain proteins participate in the collaboration between Hsp90 and Hsp70 by simultaneously interacting with Hsp90 and Hsp70. Using E. coli Hsp90, Hsp70 (DnaK), and a J-domain protein (CbpA), we detected a ternary complex containing all three proteins. The interaction involved the J-domain of CbpA, the DnaK binding region of E. coli Hsp90, and the J-domain protein binding region of DnaK where Hsp90 also binds. Additionally, results show that E. coli Hsp90 interacts with E. coli J-domain proteins, DnaJ and CbpA, and that yeast Hsp90, Hsp82, interacts with a yeast J-domain protein, Ydj1. Together these results suggest that the complexes may be transient intermediates in the pathway of collaborative protein remodeling by Hsp90 and Hsp70.

The mechanical roles of chaperones.

Protein folding under force is an integral source of generating mechanical energy in various cellular processes, ranging from protein translation to degradation. Although chaperones are well known to interact with proteins under mechanical force, how they respond to force and control cellular energetics remains unknown. To address this question, we introduce novel real-time covalent-magnetic-tweezers technology to apply physiological force pulses on client proteins, keeping the chaperones unperturbed. Interestingly we found that chaperones, under physiological constraints, can work in a different way than its previously known function. For example, TF and DsbA like tunnel associated chaperones, otherwise acting as a holdase in absence of force, accelerate the protein folding under force. Similarly, a well described foldase Hsp70 complex (DnaKJE) fails to accelerate the folding process under mechanical constraints. However, individual components of this complex (DnaJ, DnaK) showed similar holdase properties, both in the absence and presence of force. Thus, our study reveals a novel concept of a spatially distributed mechanical behavior of chaperones, i.e. tunnel-associated chaperones can accelerate the folding of translocating polypeptides and thus reduce the overall energy required for the translation or translocation process, while their cytosolic counterparts (PDI, Thioredoxin or DnaKJE) fail to do so. This behavior may have evolved to minimize the energetic cost of various biological processes.

Unique interface and dynamics of the complex of HSP90 with a specialized cochaperone AIPL1

Photoreceptor phosphodiesterase PDE6 is central for visual signal transduction. Maturation of PDE6 depends on a specialized chaperone complex of HSP90 with aryl hydrocarbon receptor-interacting protein-like 1 (AIPL1). Disruption of PDE6 maturation underlies a severe form of retina degeneration. Here, we report a 3.9Å cryoelectron microscopy (cryo-EM) structure of the complex of HSP90 with AIPL1. This structure reveals a unique interaction of the FK506-binding protein (FKBP)-like domain of AIPL1 with HSP90 at its dimer interface. Unusually, the N terminus AIPL1 inserts into the HSP90 lumen in a manner that was observed previously for HSP90 clients. Deletion of the 7 N-terminal residues of AIPL1 decreased its ability to cochaperone PDE6. Multi-body refinement of the cryo-EM data indicated large swing-like movements of AIPL1-FKBP. Modeling the complex of HSP90 with AIPL1 using crosslinking constraints indicated proximity of the mobile tetratricopeptide repeat (TPR) domain with the C-terminal domain of HSP90. Our study establishes a framework for future structural studies of PDE6 maturation.

HSP90-CDC37-PP5 forms a structural platform for kinase dephosphorylation

Activation of client protein kinases by the HSP90 molecular chaperone system is affected by phosphorylation at multiple sites on HSP90, the kinase-specific co-chaperone CDC37, and the kinase client itself. Removal of regulatory phosphorylation from client kinases and their release from the HSP90-CDC37 system depends on the Ser/Thr phosphatase PP5, which associates with HSP90 via its N-terminal TPR domain. Here, we present the cryoEM structure of the oncogenic protein kinase client BRAFV600E bound to HSP90-CDC37, showing how the V600E mutation favours BRAF association with HSP90-CDC37. Structures of HSP90-CDC37-BRAFV600E complexes with PP5 in autoinhibited and activated conformations, together with proteomic analysis of its phosphatase activity on BRAFV600E and CRAF, reveal how PP5 is activated by recruitment to HSP90 complexes. PP5 comprehensively dephosphorylates client proteins, removing interaction sites for regulatory partners such as 14-3-3 proteins and thus performing a ‘factory reset’ of the kinase prior to release.

A novel connection between Trypanosoma brucei mitochondrial proteins TbTim17 and TbTRAP1 is discovered using Biotinylation Identification (BioID)

The protein translocase of the mitochondrial inner membrane in Trypanosoma brucei, TbTIM17, forms a modular complex in association with several other trypanosome-specific proteins. To identify transiently interacting proximal partner(s) of TbTim17, we used Biotinylation Identification (BioID) by expressing a modified biotin ligase-TbTim17 (BirA∗-TbTim17) fusion protein in T.brucei. BirA∗-TbTim17 was targeted to mitochondria and assembled in the TbTIM complex. In the presence of biotin, BirA∗-TbTim17 biotinylated several mitochondrial proteins. Interestingly, TbHsp84/TbTRAP1, a mitochondrial Hsp90 homolog, was identified as the highest enriched biotinylated proteins. We validated that interaction and colocalization of TbTim17 and TbHsp84 in T.brucei mitochondria by coimmunoprecipitation analysis and confocal microscopy, respectively. TbTim17 association with TbTRAP1 increased several folds during denaturation/renaturation of mitochondrial proteins invitro, suggesting TbTRAP1 acts as a chaperone for TbTim17 refolding. We demonstrated that knockdown of TbTRAP1 reduced cell growth and decreased the levels of the TbTIM17, TbTim62, and mitochondrial (m)Hsp70 complexes. However, ATPase, VDAC, and Atom69 complexes were minimally affected. Additionally, the steady state levels of TbTim17, TbTim62, and mHsp70 were reduced significantly, but Atom69, ATPase β, and RBP16 were mostly unaltered due to TbTRAP1 knockdown. Quantitative proteomics analysis also showed significant reduction of TbTim62 along with a few other mitochondrial proteins due to TbTRAP1 knockdown. Finally, TbTRAP1 depletion did not hamper the import of the ectopically expressed TbTim17-2xMyc into mitochondria but reduced its assembly into the TbTIM17 complex, indicating TbTRAP1 is critical for assembly of TbTim17. This is the first report showing the role of TRAP1 in the TIM complex assembly in eukaryotes.

79 (PB069) - Tumor-selective, chaperone-mediated protein degradation (CHAMP) of the bromodomain transcription factor BRD4

Background: The HSP90 chaperone complex has an important role in facilitating folding of newly synthesized substrate proteins, but can also mediate degradation of misfolded proteins through interaction with ubiquitin E3 ligases. Additionally, the HSP90 complex is often activated in cancer cells, which is associated with HSP90-binding agents displaying unusual tumor-selective pharmacokinetics in vivo. Taking advantage of these characteristics of the HSP90 chaperone complex, chaperone-mediated protein degraders (CHAMPs) are hetero-bifunctional small molecules that bind to a target protein of interest and direct its interaction with the HSP90 chaperone complex, thereby inducing degradation of the target protein via the ubiquitin proteasome system.

Role of Heat Shock Protein Influencing Bioactive Compounds From Mangrove Tropical Estuarine Microalgae for Enhancement of Copepod Egg Production in Culture System

In silico investigations of the natural bioactive compounds in the microalgae from mangrove tropical estuaries showed an influence on heat shock protein -70 production. Incorporation of algae with such compounds in the diet of copepod high density culture might lead to enhanced egg production. For this study, the structure of the ligands (bioactive compounds from microalgae in the region of the mangrove estuary) and X-ray crystal structure of hsp-70 complex was taken from PDB (3P9Y) with a resolution of 2.10 Å. The molecular docking study was performed using GOLD software. In the present study, a total of ten bioactive compounds showed good molecular interaction with hsp-70 protein. Among these bioactive compounds, Quercetin from the microalga, Chlamydomonas eugametos exhibited the highest molecular interaction and this compound is potential for enhancement of hsp-70 protein compared to other bioactive compounds and is considered a good nutrient enrichment for copepod culture as well as enhancement of hsp-70 protein against ROS and adverse environmental conditions. Successful high density copepod culture might lead to scaling up of hatchery rearing of marine finfish larvae.

Conformational Dynamics and Mechanisms of Client Protein Integration into the Hsp90 Chaperone Controlled by Allosteric Interactions of Regulatory Switches: Perturbation-Based Network Approach for Mutational Profiling of the Hsp90 Binding and Allostery.

Understanding the allosteric mechanisms of the Hsp90 chaperone interactions with cochaperones and client protein clientele is fundamental to dissect activation and regulation of many proteins. In this work, atomistic simulations are combined with perturbation-based approaches and dynamic network modeling for a comparative mutational profiling of the Hsp90 binding and allosteric interaction networks in the three Hsp90 maturation complexes with FKBP51 and P23 cochaperones and the glucocorticoid receptor (GR) client. The conformational dynamics signatures of the Hsp90 complexes and dynamics fluctuation analysis revealed how the intrinsic plasticity of the Hsp90 dimer can be modulated by cochaperones and client proteins to stabilize the closed dimer state required at the maturation stage of the ATPase cycle. In silico deep mutational scanning of the protein residues characterized the hot spots of protein stability and binding affinity in the Hsp90 complexes, showing that binding hot spots may often coincide with the regulatory centers that modulate dynamic allostery in the Hsp90 dimer. We introduce a perturbation-based network approach for mutational scanning of allosteric residue potentials and characterize allosteric switch clusters that control mechanism of cochaperone-dependent client recognition and remodeling by the Hsp90 chaperone. The results revealed a conserved network of allosteric switches in the Hsp90 complexes that allow cochaperones and GR protein to become integrated into the Hsp90 system by anchoring to the conformational switch points in the functional Hsp90 regions. This study suggests that the Hsp90 binding and allostery may operate under a regulatory mechanism in which activation or repression of the Hsp90 activity can be pre-encoded in the allosterically regulated Hsp90 dimer motions. By binding directly to the conformational switch centers on the Hsp90, cochaperones and interacting proteins can efficiently modulate the allosteric interactions and long-range communications required for client remodeling and activation.

Recognition of BRAF by CDC37 and Re-Evaluation of the Activation Mechanism for the Class 2 BRAF-L597R Mutant.

The kinome specific co-chaperone, CDC37 (cell division cycle 37), is responsible for delivering BRAF (B-Rapidly Accelerated Fibrosarcoma) to the Hsp90 (heat shock protein 90) complex, where it is then translocated to the RAS (protooncogene product p21) complex at the plasma membrane for RAS mediated dimerization and subsequent activation. We identify a bipartite interaction between CDC37 and BRAF and delimitate the essential structural elements of CDC37 involved in BRAF recognition. We find an extended and conserved CDC37 motif, 20HPNID---SL--W31, responsible for recognizing the C-lobe of BRAF kinase domain, while the c-terminal domain of CDC37 is responsible for the second of the bipartite interaction with BRAF. We show that dimerization of BRAF, independent of nucleotide binding, can act as a potent signal that prevents CDC37 recognition and discuss the implications of mutations in BRAF and the consequences on signaling in a clinical setting, particularly for class 2 BRAF mutations.

Multivalent protein–protein interactions are pivotal regulators of eukaryotic Hsp70 complexes

Heat shock protein 70 (Hsp70) is a molecular chaperone and central regulator of protein homeostasis (proteostasis). Paramount to this role is Hsp70’s binding to client proteins and co-chaperones to produce distinct complexes, such that understanding the protein–protein interactions (PPIs) of Hsp70 is foundational to describing its function and dysfunction in disease. Mounting evidence suggests that these PPIs include both “canonical” interactions, which are universally conserved, and “non-canonical” (or “secondary”) contacts that seem to have emerged in eukaryotes. These two categories of interactions involve discrete binding surfaces, such that some clients and co-chaperones engage Hsp70 with at least two points of contact. While the contributions of canonical interactions to chaperone function are becoming increasingly clear, it can be challenging to deconvolute the roles of secondary interactions. Here, we review what is known about non-canonical contacts and highlight examples where their contributions have been parsed, giving rise to a model in which Hsp70’s secondary contacts are not simply sites of additional avidity but are necessary and sufficient to impart unique functions. From this perspective, we propose that further exploration of non-canonical contacts will generate important insights into the evolution of Hsp70 systems and inspire new approaches for developing small molecules that tune Hsp70-mediated proteostasis.

The Hsp90 Chaperone Regulates p38 Activation During ER Stress

Diseases such as diabetes, Alzheimer’s disease, and Parkinson’s disease can be characterized by the presence of endoplasmic reticulum stress. The endoplasmic reticulum is an organelle that plays a major role in protein synthesis, but an accumulation of misfolded proteins within the ER lumen causes the organelle to release a stress signal through the unfolded protein response (UPR). Despite being present in many disorders, there are alternative signaling routes within the UPR branches that are not fully understood. Perk dimerization initiates one of three main signaling cascades in the UPR by phosphorylating eIF2α and causing a cell‐wide pause in mRNA translation; however, a study has shown that outside of the established pathway, activation of Perk results in p38 activation, which induces apoptosis during prolonged ER stress. The mechanism responsible for p38 activation is still unknown. One route for elucidating the regulation of p38 activation during ER stress is the Hsp90 chaperone. The Hsp90 chaperone complex sequesters p38 in the cytoplasm and prevents its auto‐activation under normal conditions, so we hypothesized that Perk activation may mediate post‐translational modification of the Hsp90 complex in a manner that would release p38, allowing it to activate. In this study, Tunicamycin‐induced ER stress and Hsp90 inhibition were applied to BHK21 fibroblasts, a cell line in which ER stress has been well characterized, and dissociation of the p38‐Hsp90 complex was measured through immunoblot and co‐immunoprecipitation analysis. Initial immunoblot results from 50nM and 200nM of tunicamycin reconfirm that moderate doses of tunicamycin induced p38 activation in a dose dependent manner. Co‐immunoprecipitation of Hsp90 reconfirmed that tunicamycin‐induced ER stress causes p38 to dissociate from Hsp90. Previous experiments with co‐immunoprecipitation of p38 by Jingyi Chen revealed that treatment with 50nM of tunicamycin resulted in 0.3 decrease in fold induction after 15 minutes. Unexpectedly, we found that inhibition of Hsp90 causes it to exhibit increased binding of p38 with an peak average fold increase of 7 after treatment with 5uM of Hsp90‐I (17‐AAG) for 30 minutes, which suggests Hsp90 as a potential regulator for p38. Because p38 plays an important role in the induction of many intracellular signaling pathways, learning ways to regulate its actions within the context of ER stress may be beneficial in mediating symptoms of disease.

Advances towards Understanding the Mechanism of Action of the Hsp90 Complex.

Hsp90 (Heat Shock Protein 90) is an ATP (Adenosine triphosphate) molecular chaperone responsible for the activation and maturation of client proteins. The mechanism by which Hsp90 achieves such activation, involving structurally diverse client proteins, has remained enigmatic. However, recent advances using structural techniques, together with advances in biochemical studies, have not only defined the chaperone cycle but have shed light on its mechanism of action. Hsp90 hydrolysis of ATP by each protomer may not be simultaneous and may be dependent on the specific client protein and co-chaperone complex involved. Surprisingly, Hsp90 appears to remodel client proteins, acting as a means by which the structure of the client protein is modified to allow its subsequent refolding to an active state, in the case of kinases, or by making the client protein competent for hormone binding, as in the case of the GR (glucocorticoid receptor). This review looks at selected examples of client proteins, such as CDK4 (cyclin-dependent kinase 4) and GR, which are activated according to the so-called ‘remodelling hypothesis’ for their activation. A detailed description of these activation mechanisms is paramount to understanding how Hsp90-associated diseases develop.

Dual Targeting Strategies on Histone Deacetylase 6 (HDAC6) and Heat Shock Protein 90 (Hsp90).

The design of multi-target drugs acting simultaneously on multiple signaling pathways is a growing field in medicinal chemistry, especially for the treatment of complex diseases, such as cancer. Histone deacetylase 6 (HDAC6) is an established anticancer drug target involved in tumor cells transformation. Being an epigenetic enzyme at the interplay of many biological processes, HDAC6 has become an attractive target for polypharmacology studies aimed at improving the therapeutic efficacy of anticancer drugs. For example, the molecular chaperone Heat shock protein 90 (Hsp90) is a substrate of HDAC6 deacetylation, and several lines of evidence demonstrate that simultaneous inhibition of HDAC6 and Hsp90 promotes synergistic antitumor effects on different cancer cell lines, highlighting the potential benefits of developing a single molecule endowed with multi-target activity. This review will summarize the complex interplay between HDAC6 and Hsp90, providing also useful hints for multi-target drug design and discovery approaches in this field. To this end, crystallographic structures of HDAC6 and Hsp90 complexes will be extensively reviewed in light of discussing binding pockets features and pharmacophore requirements and providing useful guidelines for the design of dual inhibitors. The few examples of multi-target inhibitors obtained so far, mostly based on chimeric approaches, will be summarized and put into context. Finally, the main features of HDAC6 and Hsp90 inhibitors will be compared, and ligand- and structure-based strategies potentially useful for the development of small molecular weight dual inhibitors will be proposed and discussed.

Molecular insights into the maturation of phosphodiesterase 6 by the specialized chaperone complex of HSP90 with AIPL1

Phosphodiesterase 6 (PDE6) is a key effector enzyme in vertebrate phototransduction, and its maturation and function are known to critically depend on a specialized chaperone, aryl hydrocarbon receptor-interacting protein-like 1 (AIPL1). Defects in PDE6 and AIPL1 underlie several severe retinal diseases, including retinitis pigmentosa and Leber congenital amaurosis. Here, we characterize the complex of AIPL1 with HSP90 and demonstrate its essential role in promoting the functional conformation of nascent PDE6. Our analysis suggests that AIPL1 preferentially binds to HSP90 in the closed state with a stoichiometry of 1:2, with the tetratricopeptide repeat domain and the tetratricopeptide repeat helix 7 extension of AIPL1 being the main contributors to the AIPL1/HSP90 interface. We demonstrate that mutations of these determinants markedly diminished both the affinity of AIPL1 for HSP90 and the ability of AIPL1 to cochaperone the maturation of PDE6 in a heterologous expression system. In addition, the FK506-binding protein (FKBP) domain of AIPL1 encloses a unique prenyl-binding site that anchors AIPL1 to posttranslational lipid modifications of PDE6. A mouse model with rod PDE6 lacking farnesylation of its PDE6A subunit revealed normal expression, trafficking, and signaling of the enzyme. Furthermore, AIPL1 was unexpectedly capable of inducing the maturation of unprenylated cone PDE6C, whereas mutant AIPL1 deficient in prenyl binding competently cochaperoned prenylated PDE6C. Thus, we conclude neither sequestration of the prenyl modifications is required for PDE6 maturation to proceed, nor is the FKBP-lipid interaction involved in the conformational switch of the enzyme into the functional state.

Extracellular HSP90 Machineries Build Tumor Microenvironment and Boost Cancer Progression.

HSP90 is released by cancer cells in the tumor microenvironment where it associates with different co-chaperones generating complexes with specific functions, ranging from folding and activation of extracellular clients to the stimulation of cell surface receptors. Emerging data indicate that these functions are essential for tumor growth and progression. The understanding of the exact composition of extracellular HSP90 complexes and the molecular mechanisms at the basis of their functions in the tumor microenvironment may represent the first step to design innovative diagnostic tools and new effective therapies. Here we review the impact of extracellular HSP90 complexes on cancer cell signaling and behavior.

Abstract 971: Chaperone-mediated protein degradation (CHAMP): A novel technology for tumor-targeted protein degradation

Abstract The HSP90 chaperone mediates folding of many important client proteins and mutated oncoproteins, but can also direct its substrates towards degradation by the ubiquitin-proteasome system. Further, in tumor tissues, HSP90 complexes are in a highly activated state relative to normal tissues, which results in HSP90-binding small molecule compounds displaying unique tumor-selective pharmacokinetics. In order to take advantage of these attributes of HSP90 to degrade the transcription factor BRD4 in a tumor-selective fashion, hetero-bifunctional small molecule compounds were synthesized, termed chaperone-mediated protein degraders (CHAMPs), that chemically induced proximity between BRD4 and HSP90. In vitro, treatment of MV-4-11 leukemia cells with a BRD4-CHAMP compound resulted in formation of a BRD4:CHAMP:HSP90 ternary complex and subsequent proteasome-dependent BRD4 degradation and inhibition of cell proliferation. CHAMPs were identified that induced selective degradation of BRD4 and displayed only minimal effects on other HSP90-regulated client proteins. While BRD4 is a known HSP90 client protein, non-HSP90 clients such as mutated KRAS could also be degraded using this approach. In vivo, in the MV-4-11 mouse xenograft model, a selective BRD4-CHAMP compound displayed prolonged pharmacokinetics in tumors relative to plasma and normal tissues, with its tumor concentration remaining above the in vitro cytotoxicity EC50 value for at least 7 days following a single dose. This resulted in greater than 90% and prolonged degradation of BRD4 in tumors, as well as decreased expression of the BRD4-regulated MYC gene. Treatment with BRD4-CHAMP at its MTD resulted in dramatic tumor regressions after a single dose and the majority of tumors were undetectable following 3 weekly doses. In contrast, daily dosing of a clinical stage pan-BET inhibitor at its MTD resulted in stable to progressive disease, indicating that BRD4-CHAMP has a greatly improved therapeutic index in this model. CHAMP technology has a number of advantages relative to other targeted protein degradation approaches, such as PROTAC, including an improved safety margin due to selective accumulation in tumor tissues. Citation Format: Kevin P. Foley, Long Ye, Mingkai Wang, Chenghao Ying, Wei Yin, Lingjie Zhang, Weiwen Ying. Chaperone-mediated protein degradation (CHAMP): A novel technology for tumor-targeted protein degradation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 971.