The Human Papillomavirus E6 Oncoprotein Targets USP15 and TRIM25 To Suppress RIG-I-Mediated Innate Immune Signaling

J Virol. 2018 Feb 26;92(6):e01737-17. doi: 10.1128/JVI.01737-17. Print 2018 Mar 15.

Abstract

Retinoic acid-inducible gene I (RIG-I) is a key pattern recognition receptor that senses viral RNA and interacts with the mitochondrial adaptor MAVS, triggering a signaling cascade that results in the production of type I interferons (IFNs). This signaling axis is initiated by K63-linked ubiquitination of RIG-I mediated by the E3 ubiquitin ligase TRIM25, which promotes the interaction of RIG-I with MAVS. USP15 was recently identified as an upstream regulator of TRIM25, stabilizing the enzyme through removal of degradative K48-linked polyubiquitin, ultimately promoting RIG-I-dependent cytokine responses. Here, we show that the E6 oncoprotein of human papillomavirus type 16 (HPV16) as well as of other HPV types form a complex with TRIM25 and USP15 in human cells. In the presence of E6, the K48-linked ubiquitination of TRIM25 was markedly increased, and in line with this, TRIM25 degradation was enhanced. Our results further showed that E6 inhibited the TRIM25-mediated K63-linked ubiquitination of RIG-I and its CARD-dependent interaction with MAVS. HPV16 E6, but not E7, suppressed the RIG-I-mediated induction of IFN-β, chemokines, and IFN-stimulated genes (ISGs). Finally, CRISPR-Cas9 gene targeting in human keratinocytes showed that the TRIM25-RIG-I-MAVS triad is important for eliciting an antiviral immune response to HPV16 infection. Our study thus identifies a novel immune escape mechanism that is conserved among different HPV strains and further indicates that the RIG-I signaling pathway plays an important role in the innate immune response to HPV infection.IMPORTANCE Persistent infection and tumorigenesis by HPVs are known to require viral manipulation of a variety of cellular processes, including those involved in innate immune responses. Here, we show that the HPV E6 oncoprotein antagonizes the activation of the cytoplasmic innate immune sensor RIG-I by targeting its upstream regulatory enzymes TRIM25 and USP15. We further show that the RIG-I signaling cascade is important for an antiviral innate immune response to HPV16 infection, providing evidence that RIG-I, whose role in sensing RNA virus infections has been well characterized, also plays a crucial role in the antiviral host response to small DNA viruses of the Papillomaviridae family.

Keywords: RIG-I; innate immunity; interferons; papillomavirus.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • DEAD Box Protein 58 / genetics
  • DEAD Box Protein 58 / immunology*
  • HEK293 Cells
  • Human papillomavirus 6 / genetics
  • Human papillomavirus 6 / immunology*
  • Humans
  • Immunity, Innate*
  • Keratinocytes / immunology*
  • Keratinocytes / pathology
  • Keratinocytes / virology
  • Oncogene Proteins, Viral / genetics
  • Oncogene Proteins, Viral / immunology*
  • Papillomavirus Infections / genetics
  • Papillomavirus Infections / immunology*
  • Papillomavirus Infections / pathology
  • Receptors, Immunologic
  • Signal Transduction / genetics
  • Signal Transduction / immunology*
  • Transcription Factors / genetics
  • Transcription Factors / immunology*
  • Tripartite Motif Proteins / genetics
  • Tripartite Motif Proteins / immunology*
  • Ubiquitin-Protein Ligases / genetics
  • Ubiquitin-Protein Ligases / immunology*
  • Ubiquitin-Specific Proteases / genetics
  • Ubiquitin-Specific Proteases / immunology*

Substances

  • E6 protein, Human papillomavirus type 6
  • Oncogene Proteins, Viral
  • Receptors, Immunologic
  • Transcription Factors
  • Tripartite Motif Proteins
  • TRIM25 protein, human
  • Ubiquitin-Protein Ligases
  • USP15 protein, human
  • Ubiquitin-Specific Proteases
  • RIGI protein, human
  • DEAD Box Protein 58