Caspase-8 Regulates Endoplasmic Reticulum Stress-Induced Necroptosis Independent of the Apoptosis Pathway in Auditory Cells.

Kishino Kishino,Jike Jike,Hayashi Hayashi,Hidai Hidai,Nomura Nomura,Maeda Maeda,Oshima Oshima

doi:10.3390/ijms20235896

Kishino Kishino, Jike Jike + Show 5 more

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https://doi.org/10.3390/ijms20235896

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Abstract

The aim of this study is to elucidate the detailed mechanism of endoplasmic reticulum (ER) stress-induced auditory cell death based on the function of the initiator caspases and molecular complex of necroptosis. Here, we demonstrated that ER stress initiates not only caspase-9-dependent intrinsic apoptosis along with caspase-3, but also receptor-interacting serine/threonine kinase (RIPK)1-dependent necroptosis in auditory cells. We observed the ultrastructural characteristics of both apoptosis and necroptosis in tunicamycin-treated cells under transmission electron microscopy (TEM). We demonstrated that ER stress-induced necroptosis was dependent on the induction of RIPK1, negatively regulated by caspase-8 in auditory cells. Our data suggested that ER stress-induced intrinsic apoptosis depends on the induction of caspase-9 along with caspase-3 in auditory cells. The results of this study reveal that necroptosis could exist for the alternative backup cell death route of apoptosis in auditory cells under ER stress. Interestingly, our data results in a surge in the recognition that therapies aimed at the inner ear protection effect by caspase inhibitors like zVAD-fmk might arrest apoptosis but can also have the unanticipated effect of promoting necroptosis. Thus, RIPK1-dependent necroptosis would be a new therapeutic target for the treatment of sensorineural hearing loss due to ER stress.

Highlights

The endoplasmic reticulum (ER) is the central cellular organelle responsible for synthesis and maturation of transmembrane and secretory proteins [1,2]
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The data are shown as means ± S.D. of three or more independent studies (*** p < 0.001 compared to the 0 h group, determined using unpaired Student’s t-test). (e–h) Representative Western blots showing the expressions of IRE1-α, XBP1s, and cleaved/full-length caspase-3 in tunicamycin-treated cells (50 μg/mL for 48 h). β-actin was included as the loading control

Summary

Introduction

The endoplasmic reticulum (ER) is the central cellular organelle responsible for synthesis and maturation of transmembrane and secretory proteins [1,2]. Under ER stress, unfolded protein response (UPR), the adaptive system conserved from yeast to eukaryotic cells, is activated to restore ER homeostasis [4]. Sustained ER stress leads to cell death [5]. Accumulating evidence suggests that ER stress-induced cellular dysfunction and death is involved in the pathogenesis of several human diseases, such as neurodegenerative diseases, psychiatric diseases, and aging [6,7,8]. It has been reported that ER stress results in acute sensorineural hearing loss or age-related hearing loss, inducing apoptosis in auditory cells [9,10,11]. It is clinically difficult to attribute the hearing loss to apoptosis (programmed cell death) because the hearing level varies widely in the treatment process

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