X Chromosome-linked Inhibitor of Apoptosis Regulates Cell Death Induction by Proapoptotic Receptor Agonists

Avi Ashkenazi,Kerry Zobel,J.Michael Elliott,Domagoj Vucic,Justin M. Scheer,Eugene Varfolomeev,Bruno Alicke,Kristina West,Harvey Wong,Wayne J. Fairbrother,Stephen E. Gould

doi:10.1074/jbc.m109.040139

Abstract

Proapoptotic receptor agonists cause cellular demise through the activation of the extrinsic and intrinsic apoptotic pathways. Inhibitor of apoptosis (IAP) proteins block apoptosis induced by diverse stimuli. Here, we demonstrate that IAP antagonists in combination with Fas ligand (FasL) or the death receptor 5 (DR5) agonist antibody synergistically stimulate death in cancer cells and inhibit tumor growth. Single-agent activity of IAP antagonists relies on tumor necrosis factor-alpha signaling. By contrast, blockade of tumor necrosis factor-alpha does not affect the synergistic activity of IAP antagonists with FasL or DR5 agonist antibody. In most cancer cells, proapoptotic receptor agonist-induced cell death depends on amplifying the apoptotic signal via caspase-8-mediated activation of Bid and subsequent activation of the caspase-9-dependent mitochondrial apoptotic pathway. In the investigated cancer cell lines, induction of apoptosis by FasL or DR5 agonist antibody can be inhibited by knockdown of Bid. However, knockdown of X chromosome-linked IAP (XIAP) or antagonism of XIAP allows FasL or DR5 agonist antibody to induce activation of effector caspases efficiently without the need for mitochondrial amplification of the apoptotic signal and thus rescues the effect of Bid knockdown in these cells.

Highlights

3 (BH3)3 proteins, leading to neutralization of the antiapoptotic proteins, such as Bcl-2, Bcl-xL, or Mcl-1, activation of proapoptotic proteins Bax and Bak, and subsequent disruption of mitochondrial membrane potential [7]
To elucidate the mechanism of combined apoptotic activity of Inhibitor of apoptosis (IAP) antagonists and proapoptotic receptor agonists, we examined the activity of the IAP antagonist BV6 in combination with Fas ligand (FasL) or the death receptor 5 (DR5) agonist antibody, in cultured cancer cells and in a tumor xenograft model
Proapoptotic Activity of IAP Antagonists and DR5 Agonist Antibody or FasL Does Not Depend on TNF␣—Cell death induction by IAP antagonists depends on TNF␣-mediated signaling and de novo protein synthesis (28 –31)

Summary

Introduction

3 (BH3)3 proteins, leading to neutralization of the antiapoptotic proteins, such as Bcl-2, Bcl-xL, or Mcl-1, activation of proapoptotic proteins Bax and Bak, and subsequent disruption of mitochondrial membrane potential [7]. Combination of IAP antagonist BV6 and FasL or DR5 agonist antibody prominently enhanced PARA-induced activation of caspase-9 and caspases-3/7 (Fig. 2, D and E). BV6 and DR5 agonist antibody synergistically induced apoptosis and caspase-3 activation in MDA-MB-231-X1.1 cells (Fig. 3A and supplemental Fig. S2).

Results

Conclusion