Major finding: GRP94 chaperone activity maintains HER2 plasma membrane signaling in HER2-overexpressing cells.
Approach: Use of paralog-specific HSP90 inhibitors identified roles of individual paralogs in cancer cells.
Impact: Multiple HSP90 paralogs can regulate a single client protein under different cellular conditions.
The human heat shock protein 90 (HSP90) family of chaperone proteins mediates folding and stability of client proteins and consists of four paralogs (HSP90α, HSP90β, GRP94, and TRAP1) with nonoverlapping functions. Because many HSP90 clients are oncogenic, HSP90 inhibitors are in clinical development. However, most of these ATP-competitive compounds are pan-HSP90 inhibitors, making it impossible to determine the underlying mechanism of any antitumor activity. Given that HSP90 paralogs adopt distinct conformations when bound to ATP, Patel and colleagues screened a purine-scaffold library to identify paralog-selective HSP90 inhibitors. The authors used these paralog-selective inhibitors to parse the roles of HSP90 family members in regulation of the receptor tyrosine kinase HER2, a known HSP90 client that is overexpressed in some cancers. Unexpectedly, selective inhibitors of GRP94, which interacted with a deep hydrophobic cleft only accessible in this HSP90 paralog, reduced steady-state levels of HER2 in HER2-overexpressing breast cancer cells, but not in cells expressing low levels of HER2. Inhibition of HSP90α or HSP90β reduced HER2 levels in both cell types but only regulated cytosolic HER2 levels, whereas GRP94 inhibition specifically regulated HER2 levels at the plasma membrane, which are normally high in HER2-overexpressing cancer cells. A fraction of total cellular GRP94 bound HER2 at the plasma membrane, a surprising finding given that GRP94 localization was thought to be restricted to the endoplasmic reticulum. GRP94 inhibition also specifically blocked signaling downstream of membrane-associated HER2 and was sufficient to kill HER2-overexpressing cells without feedback upregulation of HSP70, a mechanism that is thought to limit the antitumor activity of pan-HSP90 inhibitors. In addition to uncovering unexpected structural and functional features of GRP94, these findings illustrate that HSP90 paralogs can regulate clients in a tumor-specific manner and provide a framework for the development of paralog-specific HSP90 inhibitors for the treatment of specific cancers.
- ©2013 American Association for Cancer Research.
Volume 3, Issue 10
