Persistent activation of NF-kappaB by the tax transforming protein of HTLV-1: hijacking cellular IkappaB kinases

Activation of IKKalpha and IKKbeta through their fusion with HTLV-I tax protein

Human T-cell leukemia virus type I (HTLV-I) Tax protein persistently stimulates the activity of IkappaB kinase (IKK), resulting in constitutive activation of the transcription factor NF-kappaB. Tax activation of IKK requires physical interaction of this viral protein with the IKK regulatory subunit, IKKgamma. The Tax/IKKgamma interaction allows Tax to engage the IKK catalytic subunits, IKKalpha and IKKbeta, although it remains unclear whether this linker function of IKKgamma is sufficient for supporting the Tax-specific IKK activation. To address this question, we have examined the sequences of IKKgamma required for modulating the Tax/IKK signaling. We demonstrate that when fused to Tax, a small N-terminal fragment of IKKgamma, containing its minimal IKKalpha/beta-binding domain, is sufficient for bringing Tax to and activating the IKK catalytic subunits. Disruption of the IKKalpha/beta-binding activity of this domain abolishes its function in modulating the Tax/IKK signaling. We further demonstrate that direct fusion of Tax to IKKalpha and IKKbeta leads to activation of these kinases. These findings suggest that the IKKgamma-directed Tax/IKK association serves as a molecular trigger for IKK activation.

The NF-kappa B signaling pathway is not required for Fas ligand gene induction but mediates protection from activation-induced cell death

Stimulation of T cells by antigens or mitogens triggers multiple signaling pathways leading to activation of genes encoding interleukin-2 and other growth-regulatory cytokines. The same stimuli also activate the gene encoding an apoptosis-inducing molecule, Fas ligand (FasL), which contributes to activation-induced cell death. It has been proposed that the signaling pathways involved in cytokine gene induction also contribute to activation-induced FasL expression; however, genetic evidence for this proposal is lacking. In the present study, the role of the NF-kappaB signaling pathway in FasL gene expression was examined using a mutant T cell line deficient in an essential NF-kappaB signaling component, IkappaB kinase gamma. These mutant cells have a blockade in signal-induced activation of NF-kappaB but remained normal in the activation of NF-AT and AP-1 transcription factors. Interestingly, the NF-kappaB signaling defect has no effect on mitogen-stimulated FasL gene expression, although it completely blocks the interleukin-2 gene induction. We further demonstrate that NF-kappaB activation is required for protecting T cells from apoptosis induction by mitogens and an agonistic anti-Fas antibody. These genetic results suggest that the NF-kappaB signaling pathway is not required for activation-induced FasL expression but rather mediates cell growth and protection from activation-induced cell death.

Negative regulation of the nuclear factor kappa B-inducing kinase by a cis-acting domain

Nuclear factor kappaB (NF-kappaB)-inducing kinase (NIK) participates in the activation of NF-kappaB, a family of eukaryotic transcription factors that mediate cell growth and transformation. NIK activates the IkappaB kinase both in vivo and in vitro, although how the activity of NIK is regulated has remained unclear. Here we show that the N-terminal region of NIK contains a negative-regulatory domain (NRD), which is composed of a basic motif and a proline-rich repeat motif. Deletion of these motifs leads to a marked enhancement of NIK function. We further demonstrate that the N-terminal NRD interacts with the C-terminal region of NIK, thereby inhibiting the binding of NIK to its substrate IkappaB kinase. Consistently, when expressed alone, the NRD potently inhibits NIK-mediated NF-kappaB signaling. These results provide a new insight into the mechanism of NIK regulation.

Somatic mutagenesis studies of NF-kappa B signaling in human T cells: evidence for an essential role of IKK gamma in NF-kappa B activation by T-cell costimulatory signals and HTLV-I Tax protein

NF-kappa B plays a pivotal role in normal T-cell activation and may also mediate human T-cell leukemia virus (HTLV)-induced T-cell transformation. Activation of NF-kappa B by both T-cell costimulatory signals and the HTLV Tax protein involves stimulation of I kappa B kinase (IKK). As a genetic approach to dissect the intermediate steps involved in NF-kappa B activation in human T cells, we performed somatic cell mutagenesis to isolate signaling-defective mutant Jurkat T-cell lines. One of the mutant cell lines was shown to have a specific blockade in the IKK signaling pathway but remained competent in the c-Jun N-terminal kinase and MAP kinase pathways. Interestingly, this mutant cell line lacks expression of IKK gamma, a non-catalytic component of the IKK complex. Expression of exogenous IKK gamma in the mutant cells restored NF-kappa B activation by both the T-cell costimulation agents and Tax. These findings provide genetic evidence for the requirement of IKK gamma in NF-kappa B signaling triggered by both T-cell costimulatory signals and HTLV-I Tax protein.

How NF-kappaB is activated: the role of the IkappaB kinase (IKK) complex

Rel/NF-kappaB transcription factors are primarily regulated by association with inhibitor IkappaB proteins. Thus, in most cells NF-kappaB exists in the cytoplasm in an inactive complex bound to IkappaB. Most agents that activate NF-kappaB do so through a common pathway based on phosphorylation-induced, proteasome-mediated degradation of IkappaB. The key regulatory step in this pathway involves activation of a high molecular weight IkappaB kinase (IKK) complex, whose catalysis is generally carried out by a heterodimeric kinase consisting of IKKalpha and IKKbeta subunits. This review describes the identification of proteins in the IKK complex, and the regulation and physiological functions of IKK.

Epstein-barr virus transformation: involvement of latent membrane protein 1-mediated activation of NF-kappaB

Epstein-Barr virus (EBV) transforms resting primary human B lymphocytes into indefinitely proliferating lymphoblastoid cell lines in vitro and is associated with several human malignancies in vivo. Recombinant EBV genetic analyses combined with in vitro B lymphocyte transformation assays demonstrate that latent infection membrane protein 1 (LMP1) is essential for EBV-mediated lymphocyte transformation. LMP1 has no intrinsic enzymatic activity but instead aggregates cellular proteins of the tumor necrosis factor receptor signaling pathway to activate transcription factor NF-kappaB. Mutants rendering LMP1 defective in these protein interactions are impaired in their abilities to activate NF-kappaB in reporter gene assays. Concordantly, EBV recombinants with LMP1 mutations that are compromised for NF-kappaB activation are impaired for growth transformation. Thus, EBV-mediated growth transformation is genetically and biochemically linked to LMP1-mediated activation of NF-kappaB.

Aberrant rel/nfkb genes and activity in human cancer

Rel/NF-kappaB transcription factors are key regulators of immune, inflammatory and acute phase responses and are also implicated in the control of cell proliferation and apoptosis. Remarkable progress has been made in understanding the signal transduction pathways that lead to the activation of Rel/NF-kappaB factors and the consequent induction of gene expression. Evidence linking deregulated Rel/NF-kappaB activity to oncogenesis in mammalian systems has emerged in recent years, consistent with the acute oncogenicity of the viral oncoprotein v-Rel in animal models. Chromosomal amplification, overexpression and rearrangement of genes coding for Rel/NF-kappaB factors have been noted in many human hematopoietic and solid tumors. Persistent nuclear NF-kappaB activity was also described in several human cancer cell types, as a result of constitutive activation of upstream signaling kinases or mutations inactivating inhibitory IkappaB subunits. Studies point to a correlation between the activation of cellular gene expression by Rel/NF-kappaB factors and their participation in the malignant process. Experiments implicating NF-kappaB in the control of the apoptotic response also support a role in oncogenesis and in the resistance of tumor cells to chemotherapy. This review focuses on the status of the rel, nfkb and ikb genes and their activity in human tumors and their association with the onset or progression of malignancies.