Exclusive Ubiquitination and Sumoylation on Overlapping Lysine Residues Mediate NF-κB Activation by the Human T-Cell Leukemia VirusTax Oncoprotein

Abstract

The transcription factor NF-B is critical for the induction of cancer, including adult T-cell leukemia, which is linked to infection by human T-cell leukemia virus type 1 and the expression of its regulatory protein Tax. Although activation of the NF-B pathway by Tax involves its interaction with the regulatory subunit of the IB kinase (IKK) complex, NEMO/IKK␥, the mechanism by which Tax activates specific cellular genes in the nucleus remains unknown. Here, we demonstrate that the attachment of SUMO-1 to Tax regulates its localization in nuclear bodies and the recruitment of both the RelA subunit of NF-B and free IKK␥ in these nuclear structures. However, this sumoylation step is not sufficient for the activation of the NF-B pathway by Tax. This activity requires the prior ubiquitination and colocalization of ubiquitinated Tax with IKK complexes in the cytoplasm and the subsequent migration of the RelA subunit of NF-B to the nucleus. Thus, the ubiquitination and sumoylation of Tax function in concert to result in the migration of RelA to the nucleus and its accumulation with IKK␥ in nuclear bodies for activation of gene expression. These modifications may result in targets for the treatment of adult T-cell leukemia.The human T-cell leukemia virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia and a virus-associated neurodegenerative disease, HTLV-1-associated myelopathy/tropical spastic paraparesis (31,36,47). These two diseases have been linked to the expression of the HTLV-1 regulatory protein Tax (10,13,29,41). Tax is a potent transcriptional activator of viral genes as well as specific cellular genes and has a clear oncogenic potential since Tax is able to transform T lymphocytes and fibroblasts and induces tumors in transgenic mice (19). A substantial part of the oncogenic properties of Tax is associated with its ability to activate the expression of cellular genes that control T-cell proliferation and differentiation by inducing constitutive activation of the NF-B pathway (2, 38).In nonstimulated cells, inactive NF-B complexes composed of p50 and RelA heterodimers are retained in the cytoplasm by NF-B inhibitors of the IB family (15,23,40). Tax activation of the NF-B pathway involves its interaction with the regulatory subunit of the IB kinase (IKK) complex, NEMO/IKK␥, leading to the activation of the two catalytic subunits, IKK␣ and IKK␤ (8,14). Activation of the IB kinase complex by Tax determines the phosphorylation of the IB proteins, leading to their ubiquitination and degradation by the proteasome and to the migration of the NF-B complexes to the nucleus (18,45,46). It also determines the phosphorylation of the RelA subunit of NF-B, a prerequisite for activation of RelA and p50 heterodimers in the nucleus (30). However, Tax also colocalizes in discrete nuclear bodies (NB) with the two subunits of NF-B, p50 and RelA, in addition to RNA polymerase II, and the assembly of these nuclear structures correlates with Tax transcriptional activity (4-6, 37). Thus, Tax-mediated activation o...

Current insights into the regulation of programmed cell death by NF-κB

“…Recent studies have shown that Tax undergoes ubiquitination and sumoylation, and that these two modifications act coordinately to regulate Tax localization to nuclear bodies and recruitment of RelA and NEMO to activate NF-kBdependent gene expression (Lamsoul et al, 2005). Together with many prior reports, these studies reinforce the notion that many viruses utilize the NF-kB signaling pathway to manipulate the host apoptotic response to favor viral transformation and pathogenicity.…”

Ubiquitylated Tax targets and binds the IKK signalosome at the centrosome

“…We also showed that Tax ubiquitylation on C-terminal lysines is critical for both Tax binding to IKK and for the nuclear translocation of RelA (Nasr et al, 2006). In addition, we and others showed that Tax is post-translationally modified by small ubiquitin-related modifier (SUMO) in the nucleus and that sumoylation is critical for the formation of RelA-enriched Tax nuclear bodies (Lamsoul et al, 2005;Nasr et al, 2006).…”

Activation of NF-κB by the Human T Cell Leukemia Virus Type I Tax Oncoprotein Is Associated with Ubiquitin-dependent Relocalization of IκB Kinase

“…Ubiquitinated Tax resides in the cytoplasm and activates NF-B, whereas sumoylated Tax is located in nuclear bodies together with RelA (17,18). Ubiquitination and sumoylation of Tax occur on overlapping lysine residues of which lysines 263, 280, and 284 are critical for NF-B activation (17,18). In this report, we have monitored the subcellular localization of the individual endogenous IKK subunits in Tax-expressing T cells as well as HTLV-1-transformed cells.…”

“…However, it is unclear exactly how Tax binding to IKK␥ triggers the catalytic activity of the IKKs. Several recent studies have reported that ubiquitination and sumoylation are important post-transcriptional regulators of Tax-mediated NF-B activation (17,18). Ubiquitinated Tax resides in the cytoplasm and activates NF-B, whereas sumoylated Tax is located in nuclear bodies together with RelA (17,18).…”

“…Several recent studies have reported that ubiquitination and sumoylation are important post-transcriptional regulators of Tax-mediated NF-B activation (17,18). Ubiquitinated Tax resides in the cytoplasm and activates NF-B, whereas sumoylated Tax is located in nuclear bodies together with RelA (17,18). Ubiquitination and sumoylation of Tax occur on overlapping lysine residues of which lysines 263, 280, and 284 are critical for NF-B activation (17,18).…”