In silico identification and computational analysis of the nucleotide binding site in the C-terminal domain of Hsp90

Recent advances toward the development of Hsp90 C-terminal inhibitors

Heat shock protein 90 (Hsp90) is a dynamic protein which serves to ensure proper folding of nascent client proteins, regulate transcriptional responses to environmental stress and guide misfolded and damaged proteins to destruction via ubiquitin proteasome pathway. Recent advances in the field of Hsp90 have been made through development of isoform selective inhibitors, Hsp90 C-terminal inhibitors and disruption of protein-protein interactions. These approaches have led to alleviation of adverse off-target effects caused by pan-inhibition of Hsp90 using N-terminal inhibitors. In this review, we provide an overview of relevant advances on targeting the Hsp90 C-terminal Domain (CTD) and the development of Hsp90 C-terminal inhibitors (CTIs) since 2015.

Discovery of novel Hsp90 C-terminal domain inhibitors that disrupt co-chaperone binding

Heat shock protein 90 (Hsp90) is an essential molecular chaperone that performs vital stress-related and housekeeping functions in cells and is a current therapeutic target for diseases such as cancers. Particularly, the development of Hsp90 C-terminal domain (CTD) inhibitors is highly desirable as inhibitors that target the N-terminal nucleotide-binding domain may cause unwanted biological effects. Herein, we report on the discovery of two drug-like novel Hsp90 CTD inhibitors by using virtual screening and intrinsic protein fluorescence quenching binding assays, paving the way for future development of new therapies that employ molecular chaperone inhibitors.

Modulation of Human Hsp90α Conformational Dynamics by Allosteric Ligand Interaction at the C-Terminal Domain

Recent years have seen heat shock protein 90 kDa (Hsp90) attract significant interest as a viable drug target, particularly for cancer. To date, designed inhibitors that target the ATPase domain demonstrate potent anti-proliferative effects, but have failed clinical trials due to high levels of associated toxicity. To circumvent this, the focus has shifted away from the ATPase domain. One option involves modulation of the protein through allosteric activation/inhibition. Here, we propose a novel approach: we use previously obtained information via residue perturbation scanning coupled with dynamic residue network analysis to identify allosteric drug targeting sites for inhibitor docking. We probe the open conformation of human Hsp90α for druggable sites that overlap with these allosteric control elements, and identify three putative natural compound allosteric modulators: Cephalostatin 17, 20(29)-Lupene-3β-isoferulate and 3'-Bromorubrolide F. We assess the allosteric potential of these ligands by examining their effect on the conformational dynamics of the protein. We find evidence for the selective allosteric activation and inhibition of Hsp90's conformational transition toward the closed state in response to ligand binding and shed valuable insight to further the understanding of allosteric drug design and Hsp90's complex allosteric mechanism of action.

The in silico identification of potent anti-cancer agents by targeting the ATP binding site of the N-domain of HSP90

To identify new HSP90 inhibitors, the ATP binding site of the N-domain of HSP90 was targeted by molecular docking of a library of 23,129,083 compounds (from the ZINC database) to the ATP binding site of the N-domain of HSP90. Structure-based virtual screen (SBVS) was performed using idock software on the istar web platform. Based on idock binding energies, 40 molecules were considered as HSP90 inhibitors. In the next step, the 40 molecules and the compound AT13387 (Onalespib) were docked to the XJX binding site using AutoDock Vina software. By comparing the binding energies of the 40 molecules selected with compound AT13387, 26 molecules were selected. By applying the rule of five, eight molecules were selected as hit compounds. The interactions of these eight compounds with the XJX binding site were obtained and investigated, and two-dimensional interaction maps were provided for the others. Finally, computing the toxicity of these compounds with the ProTox-II webserver shows that three compounds, namely ZINC89453765, ZINC23918431 and ZINC12414793, can be considered as good HSP90 inhibitors. These compounds are inactive for nuclear receptor signalling and stress response pathways including heat shock response, so do not have the limitations of common HSP90 inhibitors. They are also inactive for hepatotoxicity, carcinogenicity, immunotoxicity, mutagenicity and cytotoxicity.

In silico identification and analysis of the binding site for aminocoumarin type inhibitors in the C-terminal domain of Hsp90

Hsp90 contains two Nucleotide Binding Sites (NBS): one each in its N-terminal domain (NTD) and C-terminal domain (CTD), respectively. Previously we used computational techniques to locate a nucleotide-binding site in the CTD. Nucleotide binding at this site stabilized the structurally labile region within this domain, thus providing a rationale for increased resistance to thermal denaturation and proteolysis. A scan for ligand-binding sites in CTD revealed four potential sites with the requisite volume to accommodate aminocoumarins and -derived inhibitors. Only one of these reproducibly formed docked complexes with inhibitors and showed excellent interactions with residues lining the site. Fortuitously, it was identical to the aforementioned nucleotide-binding site thus providing an explanation for the reported direct competition between inhibitors and nucleotides. Further studies with carefully chosen inhibitors and some inactive analogues provided an explanation for the known Structure-Activity Relationships (SAR) of aminocoumarin and -derived inhibitors. We also performed similar studies of the NTD to discern the reason(s) for its inability to bind aminocoumarins, given the family resemblance to prokaryotic Top-IV and Gyr-B. Our studies permitted the identification of the putative inhibitor binding site in the CTD, an explanation for increased resistance to thermal denaturation and proteolysis upon inhibitor binding as well as direct competition with ATP.

Discovery of new molecular entities able to strongly interfere with Hsp90 C-terminal domain

Heat shock protein 90 (Hsp90) is an ATP dependent molecular chaperone deeply involved in the complex network of cellular signaling governing some key functions, such as cell proliferation and survival, invasion and angiogenesis. Over the past years the N-terminal protein domain has been fully investigated as attractive strategy against cancer, but despite the many efforts lavished in the field, none of the N-terminal binders (termed "classical inhibitors"), currently in clinical trials, have yet successfully reached the market, because of the detrimental heat shock response (HSR) that showed to induce; thus, recently, the selective inhibition of Hsp90 C-terminal domain has powerfully emerged as a more promising alternative strategy for anti-cancer therapy, not eliciting this cell rescue cascade. However, the structural complexity of the target protein and, mostly, the lack of a co-crystal structure of C-terminal domain-ligand, essential to drive the identification of new hits, represent the largest hurdles in the development of new selective C-terminal inhibitors. Continuing our investigations on the identification of new anticancer drug candidates, by using an orthogonal screening approach, here we describe two new potent C-terminal inhibitors able to induce cancer cell death and a considerable down-regulation of Hsp90 client oncoproteins, without triggering the undesired heat shock response.

Computational Modeling of the Hsp90 Interactions with Cochaperones and Small-Molecule Inhibitors

Allosteric interactions of the molecular chaperone Hsp90 with a diverse array of cochaperones and client proteins, such as protein kinases and transcription factors, allow for efficient molecular communication in signal transduction networks. Deregulation of pathways involving these proteins is commonly associated with cancer pathologies and allosteric inhibition of oncogenic clients by targeting Hsp90 provides a powerful therapeutic strategy in cancer research. We review several validated computational approaches and tools used in the studies of the Hsp90 interactions with proteins and small molecules. These methods include experimentally guided docking to predict Hs90-protein interactions, molecular and binding free energy simulations to analyze Hsp90 binding with small molecules, and structure-based network modeling to evaluate allosteric interactions and communications in the Hsp90 regulatory complexes. Through the lens of allosteric-centric view on Hsp90 function and regulation, we discuss newly emerging computational tools that link protein structure modeling with biophysical simulations and network-based systems biology approaches.

Targeting the Hsp90 C-terminal domain to induce allosteric inhibition and selective client downregulation

BACKGROUND

Inhibition of Hsp90 is desirable due to potential downregulation of oncogenic clients. Early generation inhibitors bind to the N-terminal domain (NTD) but C-terminal domain (CTD) inhibitors are a promising class because they do not induce a heat shock response. Here we present a new structural class of CTD binding molecules with a unique allosteric inhibition mechanism.

METHODS

A hit molecule, NSC145366, and structurally similar probes were assessed for inhibition of Hsp90 activities. A ligand-binding model was proposed indicating a novel Hsp90 CTD binding site. Client protein downregulation was also determined.

RESULTS

NSC145366 interacts with the Hsp90 CTD and has anti-proliferative activity in tumor cell lines (GI₅₀=0.2-1.9μM). NSC145366 increases Hsp90 oligomerization resulting in allosteric inhibition of NTD ATPase activity (IC₅₀=119μM) but does not compete with NTD or CTD-ATP binding. Treatment of LNCaP prostate tumor cells resulted in selective client protein downregulation including AR and BRCA1 but without a heat shock response. Analogs had similar potencies in ATPase and chaperone activity assays and variable effects on oligomerization. In silico modeling predicted a binding site at the CTD dimer interface distinct from the nucleotide-binding site.

CONCLUSIONS

A set of symmetrical scaffold molecules with bisphenol A cores induced allosteric inhibition of Hsp90. Experimental evidence and molecular modeling suggest that the binding site is independent of the CTD-ATP site and consistent with unique induction of allosteric effects.

GENERAL SIGNIFICANCE

Allosteric inhibition of Hsp90 via a mechanism used by the NSC145366-based probes is a promising avenue for selective oncogenic client downregulation.