Dynamically Shaping Chaperones. Allosteric Modulators of HSP90 Family as Regulatory Tools of Cell Metabolism in Neoplastic Progression

Abstract

Molecular chaperones have recently emerged as fundamental regulators of salient biological routines, including metabolic adaptations to environmental changes. Yet, many of the molecular mechanisms at the basis of their functions are still unknown or at least uncertain. This is in part due to the lack of chemical tools that can interact with the chaperones to induce measurable functional perturbations. In this context, the use of small molecules as modulators of protein functions has proven relevant for the investigation of a number of biomolecular systems. Herein, we focus on the functions, interactions and signaling pathways of the HSP90 family of molecular chaperones as possible targets for the discovery of new molecular entities aimed at tuning their activity and interactions. HSP90 and its mitochondrial paralog, TRAP1, regulate the activity of crucial metabolic circuitries, making cells capable of efficiently using available energy sources, with relevant implications both in healthy conditions and in a variety of disease states and especially cancer. The design of small-molecules targeting the chaperone cycle of HSP90 and able to inhibit or stimulate the activity of the protein can provide opportunities to finely dissect their biochemical activities and to obtain lead compounds to develop novel, mechanism-based drugs.

Keywords: ATP-competitive inhibitors; HSP90; TRAP1; allosteric inhibitor; anti-neoplastic strategies; chaperones; mitochondria; tumor metabolism.

Figure 1

HSP90 and TRAP1 chaperones are involved in several pro-neoplastic biological processes. Many clients of HSP90 and TRAP1 (in yellow and orange, respectively) are proteins that regulate key tumorigenic processes. GLUT1, glucose transporter 1; SDH, succinate dehydrogenase; COX, cytochrome c oxidase; VEGF, vascular endothelial growth factor; MMP2, matrix metallopeptidase 2; HIF1α, hypoxia-inducible factor 1-alpha; hTERT, human telomerase reverse transcriptase; ERK 1/2, extracellular signal-regulated kinase 1/2; HER2, human epidermal growth factor receptor 2; ALK, anaplastic lymphoma kinase; CDK, cyclin-dependent kinase; CyP-D, cyclophilin D; Epha2, ephrin type-A receptor 2; IFIT3; Interferon induced protein with tetratricopeptide repeats 3.

Figure 2

Schematic representation of the HSP90 conformational cycle. HSP90 domains are shown in different hues and colored blue [ATP-bound, ADP-bound or naked N-terminal domain (NTD)], green [middle domain (MD)] and orange [C-terminal domain (CTD)]. Binding of the co-chaperone activator of Hsp90 ATPase homolog 1 (AHA1) or allosteric HSP90 activators promote the formation of the closed state and increase the rate of ATP hydrolysis. The co-chaperones HOP (also known as STIP1) and cell division cycle 37 homolog (CDC37) and HSP90 inhibitors exert an opposite effect by preventing the structural changes necessary for the completion of the cycle. Prostaglandin E synthase 3 (p23; also known as PTGES3) slows the HSP90 ATPase cycle by stabilizing the twisted state.

Figure 3

Pro-neoplastic effects of the mitochondrial chaperone TRAP1 in tumor cells. TRAP1 downregulates OXPHOS by inhibiting both succinate dehydrogenase (SDH) and cytochrome c oxidase (aka complex II and complex IV of the mitochondrial respiratory chain, respectively). SDH down-regulation is enhanced by ERK1/2-dependent phosphorylation of TRAP1 and leads to an increase in intracellular succinate concentration. In turn, succinate inhibits the dioxygenase families PHD (prolyl hydroxylase), preventing proteasomal degradation of the pro-neoplastic transcription factor HIF1α, TET (10–11 translocation family of methylcytosine hydroxylases) and KDMs (histone lysine demethylases), thus prompting alterations in gene expression through epigenetic modifications. TRAP1-dependent down-modulation of Complex IV activity involves inhibition of mitochondrial c-Src. TRAP1 also protects from cell death by inhibiting the opening of the permeability transition pore (PTP) and interacts with the PTP regulator cyclophilin D (CyP-D). IMS, intermembrane space; M, methylation; P, phosphorylation; U, ubiquitination.

Figure 4

Two orientations of the predicted 3D structure of HSP90 with one allosteric activator bound. (A) The full-length structure of the complex, with the allosteric activator displayed as yellow van der Waals spheres. (B) Zoom on the structure of the activator and the contacting Hsp90 residues, together with its structure formula. The compound and the structure derive from simulations described in Sattin et al. (149), run starting from structure 2cg9.pdb (C) The 3D structures of Zebrafish and Human TRAP1, respectively from 6d14.pdb and 4z1i.pdb.