Roles of TRAF2 and TRAF3 in Epstein-Barr virus latent membrane protein 1-induced alternative NF-κB activation

Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1)-induced NF-κB activation is essential for EBV-transformed B cell survival. LMP1 has two C-terminal cytoplasmic domains referred to as C-Terminal Activation Regions (CTAR) 1 and 2 that activate the alternative and canonical NF-κB pathways, respectively. While CTAR2 activates TRAF6, IKKβ and IKKγ-dependent canonical NF-κB pathway, CTAR1 interacts with TRAF2 and TRAF3 and activates NIK and IKKα-dependent alternative NF-κB pathway involving p100 processing into functional p52. Using IKKα−/−, IKKβ−/−, IKKγ−/−, TRAF2−/−, TRAF3−/−, TRAF6−/−, and NIKaly/aly mouse embryonic fibroblasts (MEFs), potential roles of these proteins in LMP1-induced alternative NF-κB activation were investigated. Deficiency in IKKα or functional NIK, but not in IKKβ, IKKγ, or TRAF6, severely impaired LMP1-induced p100 processing. Notably, p100 was constitutively processed in TRAF2−/− or TRAF3−/− MEFs independently of LMP1 suggesting that TRAF2 or TRAF3 may play a regulatory role in p100 processing. Subsequently, TRAF2 or TRAF3 over-expression in HEK293 cells significantly blocked LMP1-induced p100 processing. The LMP1 CTAR1 expression in 293HEK cells activated the alternative p65/p52 complex while CTAR2 failed to do so. Taken together, LMP1 activates alternative NF-κB pathway through functional NIK and IKKα that is regulated by TRAF2 or TRAF3.