The Tax Oncoprotein of Human T-cell Leukemia Virus Type 1 Associates with and Persistently Activates IκB Kinases Containing IKKα and IKKβ

Abstract

The Tax oncoprotein of human T-cell leukemia virus type 1 (HTLV1) chronically activates transcription factor NF-B by a mechanism involving degradation of IB␣, an NF-B-associated cytoplasmic inhibitor. Taxinduced breakdown of IB␣ requires phosphorylation of the inhibitor at Ser-32 and Ser-36, which is also a prerequisite for the transient activation of NF-B in cytokine-treated T lymphocytes. However, it remained unclear how Tax interfaces with the cellular NF-B/IB signaling machinery to generate a chronic rather than a transient NF-B response. We now demonstrate that Tax associates with cytokine-inducible IB kinase (IKK) complexes containing catalytic subunits IKK␣ and IKK␤, which mediate phosphorylation of IB␣ at Ser-32 and Ser-36. Unlike their transiently activated counterparts in cytokine-treated cells, Tax-associated forms of IKK are constitutively active in either Tax transfectants or HTLV1-infected T lymphocytes. Moreover, point mutations in Tax that ablate its IKK-binding function also prevent Tax-mediated activation of IKK and NF-B. Together, these findings suggest that the persistent activation of NF-B in HTLV1-infected T-cells is mediated by a direct Tax/IKK coupling mechanism.The NF-B/Rel family of transcription factors plays an important regulatory role in T-cell homeostasis and antigendriven proliferation (1-4). In quiescent T lymphocytes, NF-B is trapped in the cytoplasm by various inhibitory proteins, including IB␣ (5). In response to mitogenic signals, IB␣ is targeted for destruction by the ubiquitin-proteasome pathway and then NF-B translocates to the nucleus (6). Proteolytic inactivation of IB␣ requires signal-dependent phosphorylation of the inhibitor at Ser-32 and Ser-36 (7-10). Recent studies have identified two cytokine-inducible IB kinases (IKKs) 1 , termed IKK␣ and IKK␤, which appear to form heterodimers and catalyze these site-specific modifications in the context of a multisubunit enzyme complex (11)(12)(13)(14)(15).Human T-cell leukemia virus type 1 (HTLV1) is the etiologic agent of an aggressive malignancy of activated CD4 ϩ T lymphocytes (16). The HTLV1 provirus encodes a 40-kDa oncoprotein, termed Tax, which potently induces the constitutive nuclear expression of . Studies with Tax-transgenic mice suggest that this viral/host interaction is required to maintain the transformed phenotype of HTLV1-infected cells (20). We demonstrated recently that Tax converts IB␣ into a labile proteasome substrate by a mechanism involving phosphorylation of the inhibitor at 21,22). These findings suggested that Tax induces a chronic NF-B response by acting upstream of one or more IKKs (7). However, the precise mechanism by which Tax accesses the host NF-B signaling pathway remained unknown.We demonstrate here that cytokine-inducible IB kinases containing IKK␣ and IKK␤, which normally function in a transient manner (12-14), are expressed as constitutively active signal transducers in HTLV1-infected T lymphocytes. These activated forms of IKK associate stably with Tax when the oncoprotein is expressed i...

Transcriptional activation of survivin through the NF‐κB pathway by human T‐cell leukemia virus type I tax

“…Tax M22 fails to associate with IKK complexes known to mediate phosphorylation of IkBs. 33 Tax 703 is defective for p300/CREB binding protein-associated factor, which contains a coactivator transcription function on the viral LTR. 34 Whereas WT Tax and the 703 mutant increased the activity of survivin-driven reporter gene, no significant activation of this reporter was observed with the M22 mutant (Fig.…”

Activation of NF-κB by HTLV-I and implications for cell transformation

“…More direct evidence for the involvement of a cellular kinase in Tax-mediated NF-kB activation came from the finding that Tax induces IkBa phosphorylation at two regulatory serines, serine-32 and serine-36 (Brockman et al, 1995), which also serve as the sites of IkBa phosphorylation induced by cellular stimuli (Brown et al, 1995;Traenckner et al, 1995). Indeed, soon after the identification of IKK, it became clear that Tax is an intracellular stimulator of this pivotal cellular signaling molecule (Chu et al, 1998;Geleziunas et al, 1998;Uhlik et al, 1998;Yin et al, 1998). Constitutive IKK activity was detected in both HTLV-I-infected and Tax-transfected cells.…”

“…Nevertheless, Tax physically associates with IKK, which likely contributes to the sustained IKK activation (Sun and Ballard, 1999) (Figure 1). In HTLV-I-transformed T cells, Tax is stably assembled into the canonical IKK complex, in which IKK remains chronically phosphorylated and activated (Chu et al, 1998;Carter et al, 2001). The Tax/ IKK complex formation relies on physical interaction between Tax and the IKK regulatory subunit, IKKg (Chu et al, 1999;Jin et al, 1999).…”

Human T-cell Lymphotropic Virus Type 1 Tax Induction of Biologically Active NF-κB Requires IκB Kinase-1-mediated Phosphorylation of RelA/p65

“…Thus, Tax appears to regulate NF-B in both the proximal and distal phases of the activation pathway. Unlike the transient release of cytokines, Tax is continually expressed in HTLV-1-infected cells, leading to constitutive NF-B action (37). The ability of Tax to promote increased transcriptional activity of NF-B through the phosphorylation of the C-terminal transactivation domain in RelA/p65 likely plays a role in sustaining the NF-B transcription response and contributes to leukemogenesis induced by HTLV-1 infection.…”