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Updated: May 2, 2026
Malachite Green Assay for the Discovery of Heat-Shock Protein 90 Inhibitors
Published on: January 20, 2023
mTOR inhibition potentiates HSP90 inhibitor activity via cessation of HSP synthesis
Jaime Acquaviva1, Suqin He, Jim Sang
1Authors' Affiliation: Synta Pharmaceuticals Corporation, Lexington, Massachusetts.
Unlabelled:
Because of their pleiotropic effects on critical oncoproteins, inhibitors of HSP90 represent a promising new class of therapeutic agents for the treatment of human cancer. However, pharmacologic inactivation of HSP90 subsequently triggers a heat shock response that may mitigate the full therapeutic benefit of these compounds. To overcome this limitation, a clinically feasible method was sought to block HSP synthesis induced by the potent HSP90 inhibitor ganetespib. An immunoassay screen of 322 late-stage or clinically approved drugs was performed to uncover compounds that could block upregulation of the stress-inducible HSP70 that results as a consequence of HSP90 blockade. Interestingly, inhibitors of the phosphoinositide 3-kinase (PI3K)/mTOR class counteracted ganetespib-induced HSP70 upregulation at both the gene and protein level by suppressing nuclear translocation of heat shock factor 1 (HSF1), the dominant transcription factor controlling cellular stress responses. This effect was conserved across multiple tumor types and was found to be regulated, in part, by mTOR-dependent translational activity. Pretreatment with cycloheximide, PI3K/mTOR inhibitor, or an inhibitor of eIF4E (a translation initiation factor and downstream effector of mTOR) all reduced ganetespib-mediated nuclear HSF1 accumulation, indicating that mTOR blockade confers a negative regulatory effect on HSF1 activity. Moreover, combined therapy regimens with mTOR or dual PI3K/mTOR inhibitors potentiated the antitumor efficacy of ganetespib in multiple in vivo models.
Implications:
Collectively these data identify a novel strategy to optimize the therapeutic potential of HSP90 inhibitors.
Insights
Combining HSP90 inhibitors with PI3K/mTOR inhibitors can enhance cancer therapy. This strategy blocks heat shock protein synthesis, a common resistance mechanism, leading to improved antitumor efficacy in preclinical models.
Area of Science:
- Oncology
- Molecular Biology
- Drug Discovery
Background:
- Heat shock protein 90 (HSP90) inhibitors are promising cancer therapeutics due to their effects on oncoproteins.
- Pharmacologic HSP90 inhibition can trigger a compensatory heat shock response, limiting therapeutic benefit.
- Blocking HSP synthesis is crucial to maximize the efficacy of HSP90 inhibitors like ganetespib.
Purpose of the Study:
- To identify clinically feasible methods to block heat shock protein (HSP) synthesis induced by HSP90 inhibitors.
- To overcome therapeutic limitations caused by the heat shock response following HSP90 blockade.
Main Methods:
- An immunoassay screen of 322 drugs identified compounds that inhibit HSP70 upregulation after HSP90 inhibition.
- Investigated the mechanism by which phosphoinositide 3-kinase (PI3K)/mTOR inhibitors counteract HSP70 upregulation.
- Assessed the role of heat shock factor 1 (HSF1) nuclear translocation and mTOR-dependent translational activity.
- Evaluated combined therapy efficacy in preclinical cancer models.
Main Results:
- PI3K/mTOR inhibitors effectively counteracted ganetespib-induced HSP70 upregulation at gene and protein levels.
- This effect was mediated by suppressing the nuclear translocation of heat shock factor 1 (HSF1).
- mTOR blockade negatively regulated HSF1 activity, partly through translational control via eIF4E.
- Combined therapy with mTOR or dual PI3K/mTOR inhibitors potentiated ganetespib's antitumor efficacy in vivo.
Conclusions:
- A novel strategy combining HSP90 inhibitors with PI3K/mTOR inhibitors optimizes therapeutic potential.
- Blocking HSF1 nuclear translocation via mTOR inhibition is key to enhancing HSP90 inhibitor efficacy.
- This approach offers a promising avenue for improving cancer treatment outcomes.
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