Triptolide, a HSP90 middle domain inhibitor, induces apoptosis in triple manner.

Frederick Zhehao Zhang,Derek Hoi-Hang Ho,Roger Hoi-Fung Wong

doi:10.18632/oncotarget.24737

Frederick Zhehao Zhang, Derek Hoi-Hang Ho + Show 1 more

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https://doi.org/10.18632/oncotarget.24737

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Abstract

Triptolide (TL) is a potent anti-tumor, anti-inflammatory and immunosuppressive natural compound. Mechanistic studies revealed that TL inhibits tumor growth and triggers programmed cell death. Studies further suggested that TL inhibits heat shock response in cancer cells to induce apoptosis. HSP90β is the major component of heat shock response and is overexpressed in different types of cancers. Given almost all identified HSP90β inhibitors are either N or C-terminal inhibitors, small molecules attacking cysteine(s) in the middle domain might represent a new class of inhibitors. In the current study, we showed that TL inhibits HSP90β in triple manner. Characterization suggests that TL inhibits ATPase activity by preventing ATP binding thus blunts the chaperone activity. TL disrupts HSP90β-CDC37 (co-chaperone) complex through middle domain Cys366 of HSP90β and causes kinase client protein degradation. At the cellular level, the TL-mediated decrease in CDK4 protein levels in HeLa cells causes reduced phosphorylation of Rb resulting in cell cycle arrest at the G1 phase. Furthermore, our results demonstrated that TL triggers programmed cell death in an HSP90β-dependent manner as knockdown of HSP90β further sensitized TL-mediated cell cycle arrest and apoptotic effect. Surprisingly, our data showed that TL is the first drug to be reported to induce site-specific phosphorylation of HSP90β to drive apoptosome formation in the early phase of the treatment.In summary, our study established that TL is a novel middle domain HSP90β inhibitor with bi-phasic multi-mechanistic inhibition. The unique regulatory mechanism of TL on HSP90β makes it an effective inhibitor.

Highlights

Molecular chaperones facilitate proper protein folding and allow protein complex to assemble effectively [1]
Since TL was reported to trigger cellular apoptosis through HSP70 inhibition, we would like to investigate if the apoptotic effect of TL was mediated by HSP90β, the critical molecular chaperone with anti-apoptotic activities
ATPase activity is crucial to the function of HSP90β chaperone activity as HSP90β dimerization and co-chaperone recruitment are facilitated by its activity

Summary

Introduction

Molecular chaperones facilitate proper protein folding and allow protein complex to assemble effectively [1]. The function of molecular chaperones is critical to cellular survivals. Chaperones are highly expressed and heat shock responses are constitutively activated in cancer cells. Different chaperones, such as HSP90β, HSP70 and HSP27, were reported to have oncogenic functions [3]. HSP90β is the major component of heat shock response and is overexpressed in different types of cancers. HSP90β is a critical factor for cancer cell survival and oncogene addiction. C-terminal domain of HSP90β is responsible for dimerization and tetratricopeptide repeats (TPR) co-chaperone recognition. HSP90β and its co-chaperones work in concert to regulate the conformation and activity www.oncotarget.com of a large variety of signalling molecules, transcription factors and cytoskeletons in response to different cellular stresses [7]

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