Targeting the molecular chaperone heat shock protein 90 (HSP90): Lessons learned and future directions

Abstract

Due to the critical role of heat shock protein 90 (HSP90) in regulating the stability, activity and intracellular sorting of its client proteins involved in multiple oncogenic processes, HSP90 inhibitors are promising therapeutic agents for cancer treatment. In cancer cells, HSP90 client proteins play a major role in oncogenic signal transduction (i.e., mutant epidermal growth factor receptor), angiogenesis (i.e., vascular endothelial growth factor), anti-apoptosis (i.e., AKT), and metastasis (i.e., matrix metalloproteinase 2 and CD91), processes central to maintaining the cancer phenotype. Thus, HSP90 has emerged as a viable target for antitumor drug development, and several HSP90 inhibitors have transitioned to clinical trials. HSP90 inhibitors include geldanamycin and its derivatives (i.e., tanespimycin, alvespimycin, IPI-504), synthetic and small molecule inhibitors (i.e., AUY922, AT13387, STA9090, MPC3100), other inhibitors of HSP90 and its isoforms (i.e., shepherdin and 5′-N-ethylcarboxamideadenosine). With more than 200 “client” proteins, many of them meta-stable and oncogenic, HSP90 inhibition can affect an array of tumors. Here we review the molecular structure of HSP90, structural features of HSP90 inhibition, pharmacodynamic effects and tumor responses in clinical trials of HSP90 inhibitors. We also discuss lessons learned from completed clinical trials of HSP90 inhibitors, and future directions for these promising therapeutic agents.

Introduction

Heat shock protein of 90 kDa (HSP90) belongs to the heat shock protein family, a functional class of chaperone molecules that are transcriptionally upregulated by heat and other stressors, and thereby, help protect cells against the damaging effects of cellular stress.¹ HSP90 has been highly conserved throughout evolution, is expressed in all eukaryotic cells, and accounts for 1–2% of the total cellular protein load, increasing upon induction from baseline levels to 4–6%.¹ HSP90 facilitates the maturation, stability, activity and intracellular sorting of more than 200 proteins, called “clients” or “client proteins”[1], [2] (a detailed list of HSP90 client proteins is available at http://www.picard.ch/downloads/Hsp90interactors.pdf). HSP90 client proteins may be defined as proteins that bind HSP90 and whose steady-state levels decrease upon exposure to an HSP90 inhibitor.³ Client proteins of HSP90 impact an array of cellular functions that affect health and disease, including natural and acquired immunity, signal transduction, and intracellular movement of proteins.¹ As a molecular chaperone, HSP90 helps nascent proteins adopt their biologically active conformations, correct the conformation of misfolded proteins, and helps incorrigibly misfolded proteins to be removed and degraded by the ubiquitin–proteosome system.¹

The HSP90 molecular structure has three major regions: an amino (N)-terminal domain with an adenosine triphosphate (ATP)-binding and hydrolyzing pocket (with ATPase activity) that regulates client protein folding; a middle domain involved in client protein recognition/binding; and a carboxy (C)-terminal domain which directs HSP90 dimerization.[1], [4] ATP is required for HSP90’s activity. The binding of ATP to HSP90 allows HSP90 to adopt its “closed” conformation, and enables client protein binding/loading. HSP90-bound ATP is then hydrolyzed, and the energy released by ATP hydrolysis enables client protein folding.¹ ATP hydrolysis results in the HSP90 dimer transitioning into its “open” conformation and releasing the client protein. The mechanistic operation of several HSP90 inhibitors involves displacement of ATP, and thus, blockade of HSP90’s activity.¹

Over 20 co-chaperones regulate HSP90 activity. Some of these inhibit HSP90 ATPase activity [such as HSP70/HSP90 organizing protein (HOP), cell division cycle protein 37 (CDC37) and p23] and others enhance it [such as activator of HSP90 ATPase 1 (AHA1) and CPR6]. In general, co-chaperones that inhibit HSP90’s ATPase activity are more likely to be involved in client loading or the formation of mature HSP90 complexes, whereas those that enhance the activity are more likely to be activators of the HSP90 conformational cycle.¹

Multiple isoforms of HSP90 exist and these include HSP90α and HSP90β in the cytoplasm and nucleus, GRP94 in the endoplasmic reticulum, and TRAP1 in the mitochondria. HSP90α is inducible and its functions include stress-induced cytoprotection and cell-cycle regulation, whereas HSP90β is constitutively expressed and is involved in early embryonic development, signal transduction, and long-term cell adaptation.⁵ Due to its generally higher levels than HSP90α, HSP90β is the major form of HSP90 involved in normal cellular functions.