Activation of Noncanonical NF-κB Signaling by the Oncoprotein Tio

NF-kappa B transcription factors are key regulators of cellular proliferation and frequently contribute to oncogenesis. The herpes-viral oncoprotein Tio, which promotes growth transformation of human T cells in a recombinant herpesvirus saimiri background, potently induces canonical NF-kappa B signaling through membrane recruitment of the ubiquitin ligase tumor necrosis factor receptor-associated factor 6 (TRAF6). Here, we show that, in addition to Tio-TRAF6 interaction, the Tio-induced canonical NF-kappa B signal requires the presence of the regulatory subunit of the inhibitor of kappa B kinase (IKK) complex, NF-kappa B essential modulator (NEMO), and the activity of its key kinase, IKK beta, to up-regulate expression of endogenous cellular inhibitor of apoptosis 2 (cIAP2) and interleukin 8 (IL-8) proteins. Dependent on TRAF6 and NEMO, Tio enhances the expression of the noncanonical NF-kappa B proteins, p100 and RelB. Independent of TRAF6 and NEMO, Tio mediates stabilization of the noncanonical kinase, NF-kappa B-inducing kinase (NIK). Concomitantly, Tio induces efficient processing of the p100 precursor molecule to its active form, p52, as well as DNA binding of nuclear p52 and RelB. In human T cells transformed by infection with a Tio-recombinant virus, sustained expression of p100, RelB, and cIAP2 depends on IKK beta activity, yet processing to p52 remains largely unaffected by IKK beta inhibition. However, long term inhibition of IKK beta disrupts the continuous growth of the transformed cells and induces cell death. Hence, the Tio oncoprotein triggers noncanonical NF-kappa B signaling through NEMO-dependent up-regulation of p100 precursor and RelB, as well as through NEMO-independent generation of p52 effector.