CADM1 is essential for KSHV-encoded vGPCR-and vFLIP-mediated chronic NF-κB activation

Abstract

Approximately 12% of all human cancers worldwide are caused by infections with oncogenic viruses. Kaposi's sarcoma herpesvirus/human herpesvirus 8 (KSHV/HHV8) is one of the oncogenic viruses responsible for human cancers, including Kaposi’s sarcoma (KS), Primary Effusion Lymphoma (PEL), and the lymphoproliferative disorder multicentric Castleman’s disease (MCD). Chronic inflammation mediated by KSHV infection plays a decisive role in the development and survival of these cancers. NF-κB, a family of transcription factors regulating inflammation, cell survival, and proliferation, is persistently activated in KSHV-infected cells. The KSHV latent and lytic expressing oncogenes involved in NF-κB activation are vFLIP/K13 and vGPCR, respectively. However, the mechanisms by which NF-κB is activated by vFLIP and vGPCR are poorly understood. In this study, we have found that a host molecule, Cell Adhesion Molecule 1 (CADM1), is robustly upregulated in KSHV-infected PBMCs and KSHV-associated PEL cells. Further investigation determined that both vFLIP and vGPCR interacted with CADM1. The PDZ binding motif localized at the carboxyl terminus of CADM1 is essential for both vGPCR and vFLIP to maintain chronic NF-κB activation. Membrane lipid raft associated CADM1 interaction with vFLIP is critical for the initiation of IKK kinase complex and NF-κB activation in the PEL cells. In addition, CADM1 played essential roles in the survival of KSHV-associated PEL cells. These data indicate that CADM1 plays key roles in the activation of NF-κB pathways during latent and lytic phases of the KSHV life cycle and the survival of KSHV-infected cells.

Regulation of KSHV Latency and Lytic Reactivation

“…K15, a transmembrane protein expressed at low levels during latency, contributes to NF-κB activation by recruiting a complex composed of NF-κB-inducing kinase (NIK) (another NF-κB stimulatory factor), IKKα, and IKKβ, which results in the serine 536 phosphorylation of p65 [ 33 , 34 ]. vFLIP forms a complex with cell adhesion molecule 1 (CADM1) and NEMO at the plasma membrane to promote IKK activation and subsequent NF-κB-mediated restriction of lytic reactivation [ 35 ]. Additionally, K15 and vFLIP stimulate mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1) protease activity, a primary driver of NF-κB activation in lymphocytes.…”

The interplay between EBV and KSHV viral products and NF-κB pathway in oncogenesis

“…'In vitro' and xenograft models have been revealed KSHV proteins vFLIP and K15 exploit MALT1 to promote NF-κB activation, driving latency, survival and growth in PEL cell lines [100]. KSHV G protein-coupled receptor (vGPCR) has been emerged as a candidate for KSHV oncogenesis properties and another NF-κB-activating protein [101]. Azzi et al showed the participation of vGPCR in NF-κBrelated cell survival as well as production of proinflammatory cytokines in both PEL cells and KS patients [102].…”

“…KSHV-GPCR may drive Pak1, as an upstream stimulus of NF-κB activity during the cellular transformation in KS tumors [105]. A recent research revealed the crucial effect of cell adhesion molecule 1 (CADM1; involved in cell signaling and tumorigenesis) in survival of PEL cells and KSHV-induced tumorigenesis through chronic activation of NF-κB [101]. vFLIP and vGPCR targeted a special motif at carboxyl terminus of CADM1, maintaining chronic activation of NF-κB.…”

“…KSHV G protein-coupled receptor (vGPCR) has been emerged as a candidate for KSHV oncogenesis properties and another NF-κB-activating protein [ 101 ]. Azzi et al showed the participation of vGPCR in NF-κB-related cell survival as well as production of proinflammatory cytokines in both PEL cells and KS patients [ 102 ].…”

Novel functions of Kaposi’s sarcoma herpesvirus viral FLICE inhibitory protein

“…Viral FLICE inhibitory protein (vFLIP or K13), is a latently expressed gene that was originally identified as an inhibitor of apoptosis, due to the presence of tandem death effector domains 8,9 . vFLIP is a potent activator of NFκB signaling and this activity is dependent on interaction with IKKߛ 10-12 . vFLIP has also been shown to promote NFκB signaling through upregulation of IKKߝ and CADM1 and inhibition of the SAP18/HDAC1 complex resulting in activation of NFκB via acetylation of p65 [13][14][15] . NFκB signaling is required for the virus to maintain latency, as chemical inhibition of this signaling pathway has been shown to promote lytic replication 16,17 .…”