Many heat shock proteins (HSP) are overexpressed in cancers and correlate with a poor disease prognosis.  Small molecule HSP90 inhibitors are an attractive candidate for treatment of cancer because it has been shown that the inhibition of HSP90 can lead to a reversal of the disease phenotype.  These small molecules are designed to bind the ATP-binding domain and obstruct the HSP90 chaperone function.  Despite the promise of these small molecule inhibitors, little information has been gathered regarding the distinct HSP90 paralogs: HSP90α, HSP90β, GRP94 and Trap-1.  This is particularly important when noting that each paralog adopts a different structural conformation when ATP substrate is bound.

In a recent study, researchers looked to identify HSP90 paralog-specific inhibitor ligands and assess the significance of this in cancer.

A purine-scaffold compound library was screened and inhibitor molecules with unique selectivity for GRP94 and others for HSP90α were identified.  This group then further investigated the structure of GRP94 when bound to PU-H54, a GRP94-specific ligand.  They noted distinct structural features in the ATP-binding pocket: Phe199 is swung away from the binding-pocket, revealing a deep hydrophobic cleft.  The X2-Ar functional group on PU-H54 takes on a backward bend conformation, which allows it to insert into the hydrophobic cleft and be stabilized by GRP94 residue contacts.  These unique structural characteristics were not seen in the crystal structure of HSP90α bound PU-H54.

Having confirmed that there is in fact selectivity between paralog HSP90s, this group moved on to test this finding ‘in vivo’.  They studied the effect of two cellular pathways mediated by GRP94 and found dose-dependent inhibition when in the presence of GRP94-specific ligands.   Similarly, two cellular pathways affected by HSP90α inhibition were observed in the presence of HSP90α-specific ligands and the expected effect was seen.  Conversely, when each of these pathways was observed in the presence of their non-specific ligand, no effect was seen.

HSP90 paralogs are known to regulate the trafficking and stability of HER2, a cancer-driving protein.  This study showed that when in the presence of GRP94 inhibitors, the level of HER2 decreases.  However, this was only seen in the SKBr3 breast cancer cell line, the cell line with high HER2 expression levels.  The cell line with low levels of HER2, MCF7, showed no change in HER2 expression in the presence of GRP94 inhibitor.  In contrast, expression of HER2 in both of the cell types was sensitive to inhibition of both Hsp90α and Hsp90β together.  This group also found that the inhibition of GRP94 decreases the viability of SKBr3 cells.   This effect was also seen in other breast cancer cell lines overexpressing HER2.

This study is significant because it identifies multiple avenues for therapeutic selectivity, which is particularly valuable in cancer.  One has the ability to selectively target a certain type of cancer, just by choosing a specific type of purine-scaffold inhibitor.  When the ideal circumstance is to selectively kill the cancerous cells, and leave the normal healthy cells alone, this can be a very advantageous tool.  However, this study also puts an emphasis on the variability of cancer, and thus scientists have to keep this in mind when researching this disease.

The original research paper was published in:

Paralog-selective Hsp90 inhibitors define tumor-specific regulation of HER2.

StressMarq’s Hsp90beta Antibody was used in this study.