Proapoptotic Activities of Protein Disulfide Isomerase (PDI) and PDIA3 Protein, a Role of the Bcl-2 Protein Bak

Guoping Zhao,Huayi Lu,Chi Li

doi:10.1074/jbc.m114.619353

Abstract

Protein disulfide isomerase (PDI) family proteins are classified as enzymatic chaperones for reconstructing misfolded proteins. Previous studies have shown that several PDI members possess potential proapoptotic functions. However, the detailed molecular mechanisms of PDI-mediated apoptosis are not completely known. In this study, we investigated how two members of PDI family, PDI and PDIA3, modulate apoptotic signaling. Inhibiting PDI and PDIA3 activities pharmacologically alleviates apoptosis induced by various apoptotic stimuli. Although a decrease of PDIA3 expression alleviates apoptotic responses, overexpression of PDIA3 exacerbates apoptotic signaling. Importantly, Bak, but not Bax, is essential for PDIA3-induced proapoptotic signaling. Furthermore, both purified PDI and PDIA3 proteins induce Bak-dependent, but not Bax-dependent, mitochondrial outer membrane permeabilization in vitro, probably through triggering Bak oligomerization on mitochondria. Our results suggest that both of PDI and PDIA3 possess Bak-dependent proapoptotic function through inducing mitochondrial outer membrane permeabilization, which provides a new mechanism linking ER chaperone proteins and apoptotic signaling.

Highlights

Protein disulfide isomerase (PDI) family members are chaperones involved in apoptotic signaling through unclear mechanisms
Cytotoxic Effects of Apoptotic Stimuli Depend on PDI and PDIA3 Catalytic Activities—To explore the role of PDI family members in apoptotic signaling, we first investigated whether PDI and PDIA3 catalytic activities to facilitate protein folding are essential to mediate apoptotic responses
The thiomuscimol-inactive analog muscimol failed to decrease cell death and caspase-3/7 activation induced by apoptotic stimuli, indicating that the effects of thiomuscimol on cytotoxicity is due to PDI and PDIA3 catalytic activities (Fig. 1, E and F)

Summary

Background

Protein disulfide isomerase (PDI) family members are chaperones involved in apoptotic signaling through unclear mechanisms. Our results suggest that both of PDI and PDIA3 possess Bak-dependent proapoptotic function through inducing mitochondrial outer membrane permeabilization, which provides a new mechanism linking ER chaperone proteins and apoptotic signaling. A recent study demonstrates that inhibiting PDI activities in rat brain cells suppresses apoptosis induced by misfolded huntingtin and amyloid precursor protein/␤ amyloid protein [18]. This proapoptotic function of PDI is distinct from ER stress-mediated canonical apoptosis pathways [18]. Our data suggest that both PDI and PDIA3 trigger MOMP through activating Bak, providing a new mechanism linking the unfolded protein response and apoptotic signaling

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION