OBATOCLAX and ABT-737 Induce ER Stress Responses in Human Melanoma Cells that Limit Induction of Apoptosis

David Wroblewski,Xu Dong Zhang,Margaret L Farrelly,Chen Chen Jiang,Amanda Croft,Peter Hersey,Andreas Villunger

doi:10.1371/journal.pone.0084073

Abstract

Anti-apoptotic Bcl-2 family proteins, in particular, Mcl-1, are known to play a critical role in resistance of human melanoma cells to induction of apoptosis by endoplasmic reticulum stress and other agents. The present study examined whether the BH3 mimetics, Obatoclax and ABT-737, which inhibit multiple anti-apoptotic Bcl-2 family proteins, would overcome resistance to apoptosis. We report that both agents induced a strong unfolded protein response (UPR) and that RNAi knockdown of UPR signalling proteins ATF6, IRE1α and XBP-1 inhibited Mcl-1 upregulation and increased sensitivity to the agents. These results demonstrate that inhibition of anti-apoptotic Bcl-2 proteins by Obatoclax and ABT-737 appears to elicit a protective feedback response in melanoma cells, by upregulation of Mcl-1 via induction of the UPR. We also report that Obatoclax, but not ABT-737, strongly induces autophagy, which appears to play a role in determining melanoma sensitivity to the agents.

Highlights

Induction of apoptosis is an important mediator of cell death in response to a number of treatments such as chemotherapy and targeted therapies
The above results indicate that treatment of melanoma cells with the BH3 mimetics Obatoclax and ABT-737 elicits a number of cellular responses related to inhibition of antiapoptotic Bcl-2 family proteins, resulting in the induction of apoptosis
Inhibition of endoplasmic reticulum (ER) stress responses by small interfering RNA (siRNA) knockdown of three proteins involved in the unfolded protein response (UPR) - ATF6, IRE1α and XBP-1 - confirmed the role of ER stress in the Obatoclax- and ABT-737-induced upregulation of myeloid cell leukemia-1 (Mcl-1), resulting in increased sensitivity to the agents

Summary

Introduction

Induction of apoptosis is an important mediator of cell death in response to a number of treatments such as chemotherapy and targeted therapies. Metastatic melanoma, in particular, has proven resistant to treatment with a variety of chemotherapeutic and biological agents [2]. This is believed to be largely due to activation of survival signalling pathways and upregulation of anti-apoptotic Bcl-2 family proteins [3,4]. Among the latter, the myeloid cell leukemia-1 (Mcl-1) protein plays a dominant role in resistance of melanoma to apoptosis. Upregulation of Mcl-1 was critical for protection of melanoma cells against endoplasmic reticulum (ER) stress-induced apoptosis [7] and survival of melanoma cells treated with the proteasome inhibitor Bortezomib [9]

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Results

Conclusion