The human papillomavirus E6 protein targets apoptosis-inducing factor (AIF) for degradation

Masaru Shimada,Kenji Okuda,Takao Kinjo,Nobuhisa Mizuki,Dennis M Klinman,Akio Yamashita,Manami Saito,Motohide Ichino

doi:10.1038/s41598-020-71134-3

Abstract

Oncoprotein E6 of high-risk human papillomavirus (HPV) plays a critical role in inducing cell immortalization and malignancy. E6 downregulates caspase-dependent pathway through the degradation of p53. However, the effect of HPV E6 on other pathways is still under investigation. In the present study, we found that HPV E6 directly binds to all three forms (precursor, mature, and apoptotic) of apoptosis-inducing factor (AIF) and co-localizes with apoptotic AIF. This binding induced MG132-sensitive reduction of AIF expression in the presence of E6 derived from HPV16 (16E6), a cancer-causing type of HPV. Conversely, E6 derived from a non-cancer-causing type of HPV, HPV6 (6E6), did not reduce the levels of AIF despite its interaction with AIF. Flow cytometric analysis revealed that 16E6, but not 6E6, suppressed apoptotic AIF-induced chromatin degradation (an indicator of caspase-independent apoptosis) and staurosporine (STS, a protein kinase inhibitor)-induced apoptosis. AIF knockdown reduced STS-induced apoptosis in both of 16E6-expressing and 6E6-expressing cells; however, the reduction in 16E6-expressing cells was lower than that in 6E6-expressing cells. These findings indicate that 16E6, but not 6E6, blocks AIF-mediated apoptosis, and that AIF may represent a novel therapeutic target for HPV-induced cervical cancer.

Highlights

Oncoprotein E6 of high-risk human papillomavirus (HPV) plays a critical role in inducing cell immortalization and malignancy
E6 from high-risk HPV plays an important role in HPV-induced cancer; we detected cellular proteins that interact with HPV16 E6 (16E6) using affinity purification mass spectrometry. 293TT cells were transfected with p16E6-streptavidin-binding protein (SBP) and HPV16 E6-binding cellular proteins were pulled down, purified, and analyzed by mass spectrometry. pGFP-SBP was used as a control
This study demonstrated that HPV E6 binds to apoptosis-inducing factor (AIF), altering its activity

Summary

Introduction

Oncoprotein E6 of high-risk human papillomavirus (HPV) plays a critical role in inducing cell immortalization and malignancy. AIF knockdown reduced STS-induced apoptosis in both of 16E6-expressing and 6E6-expressing cells; the reduction in 16E6-expressing cells was lower than that in 6E6-expressing cells These findings indicate that 16E6, but not 6E6, blocks AIF-mediated apoptosis, and that AIF may represent a novel therapeutic target for HPV-induced cervical cancer. E6 in high-risk HPV types is a key oncoprotein that induces cell immortalization and malignancy. E6 has been identified to interact with many cellular proteins which may support its oncogenic activity Some of these proteins bind to the E6 protein of low-risk HPV types. High-risk E6 binds more strongly to p300/CBP11, and only high-risk E6 protein binds to E6BP/ERC-5512, MCM713, c-Myc[14], and paxillin[15], which are strongly associated with cell transformation and apoptosis

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Conclusion