Proteasome inhibition triggers the formation of TRAIL receptor 2 platforms for caspase-8 activation that accumulate in the cytosol

Christian T Hellwig,Carol Hanna,Markus Rehm,Lydia Dyck,Cathrin Hagenlocher,M Eugenia Delgado,Marion Macfarlane,Claudia Langlais,Kelvin Cain,Alexandra Mack,Alexa Wentges,David Dinsdale,Josip Skoko

doi:10.1038/s41418-021-00843-7

Abstract

Cancer cells that are resistant to Bax/Bak-dependent intrinsic apoptosis can be eliminated by proteasome inhibition. Here, we show that proteasome inhibition induces the formation of high molecular weight platforms in the cytosol that serve to activate caspase-8. The activation complexes contain Fas-associated death domain (FADD) and receptor-interacting serine/threonine-protein kinase 1 (RIPK1). Furthermore, the complexes contain TRAIL-receptor 2 (TRAIL-R2) but not TRAIL-receptor 1 (TRAIL-R1). While RIPK1 inhibition or depletion did not affect proteasome inhibitor-induced cell death, TRAIL-R2 was found essential for efficient caspase-8 activation, since the loss of TRAIL-R2 expression abrogated caspase processing, significantly reduced cell death, and promoted cell re-growth after drug washout. Overall, our study provides novel insight into the mechanisms by which proteasome inhibition eliminates otherwise apoptosis-resistant cells, and highlights the crucial role of a ligand-independent but TRAIL-R2-dependent activation mechanism for caspase-8 in this scenario.

Highlights

The activation of initiator caspase-8 by induced proximity requires the formation of complexes that recruit and locally accumulate procaspase-8 [1, 2]
Caspase-8 is the apical caspase activated upon proteasome inhibition in Bax/Bak deficient cells Studying cells incapable of triggering apoptotic Bax/Bak pore formation (HCT-116 (Bax/Bak)−/−), we noted that lack of Bax/Bak substantially reduced or prevented cell death in response to common chemotherapeutics (5-fluorouracil, cisplatin) or the ER stress-inducing agent tunicamycin
Here, we report that proteasome inhibition triggers TRAIL-R2dependent apoptosis in cells incapable of activating the mitochondrial apoptosis pathway

Summary

Introduction

The activation of initiator caspase-8 by induced proximity requires the formation of complexes that recruit and locally accumulate procaspase-8 [1, 2]. On the cytosolic receptor sides, the caspase-8 recruiting adapter protein FADD binds to form the death-inducing signaling complex (DISC) that activates caspase-8 [3]. Proteasome inhibition eliminates cells that cannot trigger Bax/Bak-dependent apoptosis, and caspase-8 stabilization is probably crucial to directly activate caspase-3 and drive apoptosis execution by type I signaling [11]. Since proteasome inhibitors such as bortezomib and carfilzomib are nowadays commonly used as anti-cancer therapeutics [13], we here set out to obtain mechanistic insight into the process of caspase-8 activation, as well as its regulation and relevance for apoptosis susceptibility under conditions of proteasome inhibition

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Results

Conclusion