Retroviral oncoprotein Tax induces processing of NF-kappaB2/p100 in T cells: evidence for the involvement of IKKalpha

Cooperation of NF-kappaB2/p100 activation and the PDZ domain binding motif signal in human T-cell leukemia virus type 1 (HTLV-1) Tax1 but not HTLV-2 Tax2 is crucial for interleukin-2-independent growth transformation of a T-cell line

Human T-cell leukemia virus type 1 (HTLV-1) but not HTLV-2 is associated with adult T-cell leukemia, and the distinct pathogenicity of these two closely related viruses is thought to stem from the distinct biological functions of the respective transforming proteins, HTLV-1 Tax1 and HTLV-2 Tax2. In this study, we demonstrate that Tax1 but not Tax2 interacts with NF-kappaB2/p100 and activates it by inducing the cleavage of p100 into the active transcription factor p52. Using RNA interference methods, we further show that NF-kappaB2/p100 is required for the transformation induced by Tax1, as determined by the ability to convert a T-cell line (CTLL-2) from interleukin-2 (IL-2)-dependent to -independent growth. While Tax2 shows a reduced transforming activity relative to Tax1, Tax2 fused with a PDZ domain binding motif (PBM) present only in Tax1 shows transforming activity equivalent to that of Tax1 in CTLL-2 cells expressing an inducer of p52 processing. These results reveal that the activation of NF-kappaB2/p100 plays a crucial role in the Tax1-mediated transformation of T cells and that NF-kappaB2/p100 activation and PBM function are both responsible for the augmented transforming activity of Tax1 relative to Tax2, thus suggesting that these Tax1-specific functions play crucial roles in HTLV-1 leukemogenesis.

Negative regulation of TCR signaling by NF-kappaB2/p100

The positive regulation of the NF-kappaB-signaling pathway in response to TCR stimulation has been well-studied. However, little is known about the negative regulation of this pathway in T cells. This negative regulation is crucial in controlling the duration of TCR signaling and preventing abnormal lymphocyte activation and proliferation. Therefore, understanding the negative regulation of TCR-mediated NF-kappaB signaling is essential in understanding the mechanisms involved in T cell function and homeostasis. TCR stimulation of human CD4+ T cells resulted in an increase in NF-kappaB2/p100 expression with no appreciable increase in p52, its cleavage product. Due to the presence of inhibitory ankyrin repeats in the unprocessed p100, this observation suggests that p100 may function as a negative regulator of the NF-kappaB pathway. Consistent with this hypothesis, ectopic expression of p100 inhibited TCR-mediated NF-kappaB activity and IL-2 production in Jurkat T cells. Conversely, knockdown of p100 expression enhanced NF-kappaB transcriptional activity and IL-2 production upon TCR activation. p100 inhibited the pathway by binding and sequestering Rel transcription factors in the cytoplasm without affecting the activity of the upstream IkappaB kinase. The kinetics and IkappaB kinase gamma/NF-kappaB essential modulator dependency of p100 induction suggest that NF-kappaB2/p100 acts as a late-acting negative-feedback signaling molecule in the TCR-mediated NF-kappaB pathway.

Diversity and regulation in the NF-kappaB system

The nuclear factor (NF)-kappaB family of transcription factors is a key participant in multiple biological processes, most notably in the immune and inflammatory response. Five proteins make up the NF-kappaB family, and these proteins can hetero- and homo-dimerize, giving rise to diversity. Recently, it has been shown that certain members can also interact directly with other transcription factors such as signal transducers of activated transcription, interferon regulatory factor family members and p53, providing further diversity. We propose that this promiscuity might help explain the many of roles of NF-kappaB in specialized cell function and fate. Furthermore, the state of a cell and its cellular background in addition to overall promoter structure and variations in the kappaB target sequence will all define the composition and activity of multimeric NF-kappaB complexes.

Autophagy and NF-kappaB: fight for fate

Autophagy/macroautophagy is known for its role in cellular homeostasis, development, cell survival, aging, immunity, cancer and neurodegeneration. However, until recently, the mechanisms by which autophagy contributes to these important processes were largely unknown. The demonstration of a direct cross-talk between autophagy and NF-kappaB opens up new frontiers for deciphering the role of autophagy in diverse biological processes. Here, we review our current understanding of autophagy, with a focus on its role in tumor suppression, NF-kappaB inactivation and selective protein degradation in mammals. We also list some most intriguing and outstanding questions that are likely to engage researchers in the near future.

Aberrant NF-kappaB signaling in lymphoma: mechanisms, consequences, and therapeutic implications

The transcription factor NF-kappaB is a tightly regulated positive mediator of T- and B-cell development, proliferation, and survival. The controlled activity of NF-kappaB is required for the coordination of physiologic immune responses. However, constitutive NF-kappaB activation can promote continuous lymphocyte proliferation and survival and has recently been recognized as a critical pathogenetic factor in lymphoma. Various molecular events lead to deregulation of NF-kappaB signaling in Hodgkin disease and a variety of T- and B-cell non-Hodgkin lymphomas either up-stream or downstream of the central IkappaB kinase. These alterations are prerequisites for lymphoma cell cycling and blockage of apoptosis. This review provides an overview of the NF-kappaB pathway and discusses the mechanisms of NF-kappaB deregulation in distinct lymphoma entities with defined aberrant pathways: Hodgkin lymphoma (HL), diffuse large B-cell lymphoma (DLBCL), mucosa-associated lymphoid tissue (MALT) lymphoma, primary effusion lymphoma (PEL), and adult T-cell lymphoma/leukemia (ATL). In addition, we summarize recent data that validates the NF-kappaB signaling pathway as an attractive therapeutic target in T- and B-cell malignancies.

HTLV-1 Tax mutants that do not induce G1 arrest are disabled in activating the anaphase promoting complex

HTLV-1 Tax is a potent activator of viral transcription and NF-κB. Recent data indicate that Tax activates the anaphase promoting complex/cyclosome (APC/C) ahead of schedule, causing premature degradation of cyclin A, cyclin B1, securin, and Skp2. Premature loss of these mitotic regulators is accompanied by mitotic aberrations and leads to rapid senescence and cell cycle arrest in HeLa and S. cerevisiae cells. Tax-induced rapid senescence (tax-IRS) of HeLa cells is mediated primarily by a dramatic stabilization of p27^KIP and is also accompanied by a great surge in the level of p21^CIP1mRNA and protein. Deficiencies in p27^KIP prevent Tax-IRS. A collection of tax point mutants that permit normal growth of S. cerevisiae have been isolated. Like wild-type tax, many of them (C23W, A108T, L159F, and L235F) transactivate both the HTLV-LTR and the NF-κB reporters. One of them, V19M, preferentially activates NF-κB, but is attenuated for LTR activation. None of the mutants significantly elevated the levels of p21^CIP1and p27^KIP1, indicating that the dramatic surge in p21^CIP1/WAF1and p27^{KIP 1}induced by Tax is brought about by a mechanism distinct from NF-κB or LTR activation. Importantly, the ability of these mutants to activate APC/C is attenuated or abrogated. These data indicate that Tax-induced rapid senescence is causally associated with APC/C activation.

Bone marrow stromal cells prevent apoptosis of lymphoma cells by upregulation of anti-apoptotic proteins associated with activation of NF-kappaB (RelB/p52) in non-Hodgkin's lymphoma cells

Stromal cells are an essential component of the bone marrow microenvironment that regulate or supports tumor survival. In this study we therefore studied the role of stromal cells in lymphoma cell survival. We demonstrated that adhesion of the B-cell lymphoma cell lines SUDH-4 and 10 to bone marrow stroma inhibited mitoxantrone-induced apoptosis. This adhesion-dependent inhibition of mitoxantrone-induced apoptosis correlated with decreased activation of caspases-8 and 9, and cleavage of caspase 3 and PARP. Electrophoretic mobility shift assays (EMSA) analysis demonstrated significantly increased NF-kappaB binding activity in lymphoma cells adhered to stroma cells compared to lymphoma cells in suspension. This DNA binding activity could be attributed to cell adhesion-mediated proteolysis of the NF-kappaB precursor, p100 (NF-kappaB2). This resulted in the generation of active p52, which translocated to the nucleus in complex with p65 and RelB. Coculture with stromal cells also induced expression of the NF-kappaB-regulated anti-apoptotic molecules, XIAP, cIAP(1) and cIAP(2). Inhibition of NF-kappaB significantly suppressed HS-5-induced protection against apoptosis in lymphoma cell lines as well as in primary lymphoma cells. Thus, bone marrow stroma protects B-cell lymphoma cells against apoptosis, at least in part through activation of NF-kappaB dependent mechanism involving up-regulation of NF-kappaB regulated antiapoptotic proteins. Consequently, this study suggests a new approach to decrease the resistance of lymphoma to chemotherapy.

Human T-cell leukaemia virus type 1 (HTLV-1) infectivity and cellular transformation

It has been 30 years since a 'new' leukaemia termed adult T-cell leukaemia (ATL) was described in Japan, and more than 25 years since the isolation of the retrovirus, human T-cell leukaemia virus type 1 (HTLV-1), that causes this disease. We discuss HTLV-1 infectivity and how the HTLV-1 Tax oncoprotein initiates transformation by creating a cellular environment favouring aneuploidy and clastogenic DNA damage. We also explore the contribution of a newly discovered protein and RNA on the HTLV-1 minus strand, HTLV-1 basic leucine zipper factor (HBZ), to the maintenance of virus-induced leukaemia.

Deubiquitinating enzyme CYLD regulates the peripheral development and naive phenotype maintenance of B cells

Deubiquitinating enzymes (DUB) form a family of cysteine proteases that digests ubiquitin chains and reverses the process of protein ubiquitination. Despite the identification of a large number of DUBs, their physiological functions remain poorly defined. Here we provide genetic evidence that CYLD, a recently identified DUB, plays a crucial role in regulating the peripheral development and activation of B cells. Disruption of the CYLD gene in mice results in B cell hyperplasia and lymphoid organ enlargement. The CYLD-deficient B cells display surface markers indicative of spontaneous activation and are hyperproliferative upon in vitro stimulation. When challenged with antigens, the CYLD(-/-) mice develop exacerbated lymphoid organ abnormalities and abnormal B cell responses. Although the loss of CYLD has only a minor effect on B cell development in bone marrow, this genetic deficiency disrupts the balance of peripheral B cell populations with a significant increase in marginal zone B cells. In keeping with these functional abnormalities, the CYLD(-/-) B cells exhibit constitutive activation of the transcription factor NF-kappaB due to spontaneous activation of IkappaB kinase beta and degradation of the NF-kappaB inhibitor IkappaBalpha. These findings demonstrate a critical role for CYLD in regulating the basal activity of NF-kappaB and maintaining the naive phenotype and proper activation of B cells.

Endoproteolytic processing of C-terminally truncated NF-kappaB2 precursors at kappaB-containing promoters

The C-terminal, partially truncated forms of the NF-kappaB2/p52 precursor p100, p100DeltaCs, manifest constitutive processing and oncogenic ability, although the responsible mechanisms remain unknown. Here, we report that p100DeltaCs are specifically processed in association with binding to promoter DNA-containing kappaB sites. In the nucleus, p100DeltaCs bind to the kappaB promoter DNA and subsequently recruit the proteasome to form a stable proteasome/p100DeltaC/DNA complex, which mediates the processing of p100DeltaCs. Notably, the processing at the kappaB promoter is initiated by a proteasome-mediated endoproteolytic cleavage at amino acid D(415) of p100DeltaCs, and the processed p52, but not the precursors themselves, is oncogenic by up-regulating a subset of target genes. Our studies demonstrate a different mechanism of p100 processing and also present evidence showing that the proteasome modulates the action of transcription factors at promoter regions through endoproteolysis.

Tgat, a Rho-specific guanine nucleotide exchange factor, activates NF-kappaB via physical association with IkappaB kinase complexes

Constitutive activity of NF-kappaB is associated with various human cancers including adult T-cell leukemia (ATL). In this study, we have found Tgat that activates NF-kappaB by screening a cDNA expression library derived from ATL cells. We previously identified Tgat as the oncogene, which consists of the Rho-guanine nucleotide exchange factor (Rho-GEF) domain and the unique C-terminal region, as a consequence of alternative splicing of the Trio transcript. Tgat activated the IKK activity by binding with the IkappaB kinase (IKK) complex. The Tgat mutants lacking the C-terminal region failed to associate with the IKK complex suggesting an essential role of the unique sequence. The mutation causing the loss of GEF activity also abolished the NF-kappaB activation. Moreover, co-expressed p100 was efficiently processed into p52 in the Tgat-expressing cells, suggesting the co-involvement of non-canonical pathway.

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

Human T cell leukemia virus type 1 (HTLV-1) is the etiological agent of adult T cell leukemia. HTLV-1 encodes a trans-activating protein, Tax, which is largely responsible for the oncogenic properties of the virus. Tax promotes T cell transformation by deregulating the activity of various cellular factors, including the transcription factor NF-kappaB. Tax activates the IkappaB kinase (IKK) via physical interaction with the regulatory subunit, IKKgamma, although it is unknown precisely how Tax activates the IKK complex. Here we show that Tax modulates the cellular localization of the IKK complex. The IKKs relocalize from a broad distribution in the cytoplasm to concentrated perinuclear "hot spots" in both HTLV-1-transformed lines and in Tax-expressing Jurkat cells. Relocalization of IKK is not observed with Tax mutants unable to activate NF-kappaB, suggesting that only activated forms of IKK are relocalized. However, relocalization of IKK is strictly dependent on Tax expression because it does not occur in ATL cell lines that lack Tax expression or in Jurkat cells treated with phorbol 12-myristate 13-acetate and ionomycin. Furthermore, IKKgamma is required for redistribution because cells lacking IKKgamma were unable to relocalize IKKalpha upon expression of Tax. We also find that Tax ubiquitination likely regulates IKK relocalization because mutation of three critical lysine residues in Tax renders it unable to relocalize IKK and activate the canonical and noncanonical NF-kappaB pathways. Finally, we have observed that the perinuclear IKK in Tax-expressing cells colocalizes with the Golgi, and disruption of Golgi with either nocodazole or brefeldin A leads to a redistribution of IKK to the cytoplasm. Together, these results demonstrate that Tax induces relocalization of the IKK complex in a ubiquitin-dependent manner, and dynamic changes in the subcellular localization of the IKK complex may be critical for Tax function.

IkappaB kinase complexes: gateways to NF-kappaB activation and transcription

Transcription factors of the NF-kappaB family regulate hundreds of genes in the context of multiple important physiological and pathological processes. NF-kappaB activation depends on phosphorylation-induced proteolysis of inhibitory IkappaB molecules and NF-kappaB precursors by the ubiquitin-proteasome system. Most of the diverse signaling pathways that activate NF-kappaB converge on IkappaB kinases (IKK), which are essential for signal transmission. Many important details of the composition, regulation and biological function of IKK have been revealed in the last years. This review summarizes current aspects of structure and function of the regular stoichiometric components, the regulatory transient protein interactions of IKK and the mechanisms that contribute to its activation, deactivation and homeostasis. Both phosphorylation and ubiquitinatin (destructive as well as non-destructive) are crucial post-translational events in these processes. In addition to controlling induced IkappaB degradation in the cytoplasm and processing of the NF-kappaB precursor p100, nuclear IKK components have been found to act directly at the chromatin level of induced genes and to mediate responses to DNA damage. Finally, IKK is engaged in cross talk with other pathways and confers functions independently of NF-kappaB.

IkappaB kinase subunits alpha and gamma are required for activation of NF-kappaB and induction of apoptosis by mammalian reovirus

Reoviruses induce apoptosis both in cultured cells and in vivo. Apoptosis plays a major role in the pathogenesis of reovirus encephalitis and myocarditis in infected mice. Reovirus-induced apoptosis is dependent on the activation of transcription factor NF-kappaB and downstream cellular genes. To better understand the mechanism of NF-kappaB activation by reovirus, NF-kappaB signaling intermediates under reovirus control were investigated at the level of Rel, IkappaB, and IkappaB kinase (IKK) proteins. We found that reovirus infection leads initially to nuclear translocation of p50 and RelA, followed by delayed mobilization of c-Rel and p52. This biphasic pattern of Rel protein activation is associated with the degradation of the NF-kappaB inhibitor IkappaBalpha but not the structurally related inhibitors IkappaBbeta or IkappaBepsilon. Using IKK subunit-specific small interfering RNAs and cells deficient in individual IKK subunits, we demonstrate that IKKalpha but not IKKbeta is required for reovirus-induced NF-kappaB activation and apoptosis. Despite the preferential usage of IKKalpha, both NF-kappaB activation and apoptosis were attenuated in cells lacking IKKgamma/Nemo, an essential regulatory subunit of IKKbeta. Moreover, deletion of the gene encoding NF-kappaB-inducing kinase, which is known to modulate IKKalpha function, had no inhibitory effect on either response in reovirus-infected cells. Collectively, these findings indicate a novel pathway of NF-kappaB/Rel activation involving IKKalpha and IKKgamma/Nemo, which together mediate the expression of downstream proapoptotic genes in reovirus-infected cells.

Hsp90 inhibition results in autophagy-mediated proteasome-independent degradation of IκB kinase (IKK)

Autophagic and proteasomal proteolysis are two major pathways for degradation of cellular constituents. Current models suggest that autophagy is responsible for the nonselective bulk degradation of long-lived proteins and organelles while the proteasome specifically degrades short-lived proteins including misfolded proteins caused by the absence of Hsp90 function. Here, we show that the IkappaB kinase (IKK), an essential activator of NF-kappaB, is selectively degraded by autophagy when Hsp90 is inhibited by geldanamycin (GA), a specific Hsp90 inhibitor showing highly effective anti-tumor activity. We find that in this case inactivation of ubiquitination or proteasome fails to block IKK degradation. However, inhibition of autophagy by an autophagy inhibitor or knockout of Atg5, a key component of the autophagy pathway, significantly rescues IKK from GA-induced degradation. These findings provide the first evidence that an Hsp90 client may be degraded by a mechanism different from the proteasome pathway and establish a molecular link among Hsp90, NF-kappaB and autophagy.

Kaposi sarcoma-associated herpesvirus and other viruses in human lymphomagenesis

Kaposi sarcoma-associated herpesvirus (KSHV), also called human herpesvirus 8 (HHV-8), is associated with a specific subset of lymphoproliferative disorders. These include two main categories. The first is primary effusion lymphomas and related solid variants. The second is multicentric Castleman disease, from which KSHV-positive plasmablastic lymphomas can arise. KSHV contributes to lymphomagenesis by subverting the host cell molecular signaling machinery to deregulate cell growth and survival. KSHV expresses a selected set of genes in the lymphoma cells, encoding viral proteins that play important roles in KSHV lymphomagenesis. Deregulation of the NF-kappaB pathway is an important strategy used by KSHV to promote lymphoma cell survival, and the viral protein vFLIP is essential for this process. Two other viruses that are well documented to be causally associated with lymphoid neoplasia in humans are Epstein-Barr virus (EBV/HHV-4) and human T-cell lymphotropic virus (HTLV-1). Both of these are similar to KSHV in their use of viral proteins to promote cell survival by deregulating the NF-kappaB pathway. Here we review the basic information and recent developments that have contributed to our knowledge of lymphomas caused by KSHV and other viruses. The understanding of the mechanisms of viral lymphomagenesis should lead to the identification of novel therapeutic targets and to the development of rationally designed therapies.

Activation of non-canonical NF-kappaB pathway mediated by STP-A11, an oncoprotein of Herpesvirus saimiri

Although Saimiri Transforming Protein (STP)-A11, an oncoprotein of Herpesvirus saimiri, has been known to activate NF-kappaB signaling pathway, the detailed mechanism has not been reported yet. We herein report that STP-A11 activates non-canonical NF-kappaB pathway, resulting in p100 processing to p52. In addition, translocation of p52 protein (NF-kappaB2) into the nucleus is observed by the expression of STP-A11. STP-A11-mediated processing of p100 to p52 protein requires proteosome-mediated proteolysis because MG132 treatment clearly blocked p52 production in spite of the expression of STP-A11. Analysis of STP-A11 mutants to activate NF-kappaB2 pathway discloses the requirement of TRAF6-binding site not Src-binding site for STP-A11-mediated NF-kappaB2 pathway. Blockage of STP-A11-mediated p52 production using siRNA against p52 enhanced a chemotherapeutic drug-mediated cell death, suggesting that p52 production induced by the expression of STP-A11 would contribute to cellular transformation, which results from a resistance to cell death.

IkappaB kinase epsilon interacts with p52 and promotes transactivation via p65

The members of the NF-kappaB transcription factor family are key regulators of gene expression in the immune response. Different combinations of NF-kappaB subunits not only diverge in timing to induce transcription but also recognize varying sequences of the NF-kappaB-binding site of their target genes. The p52 subunit is generated as a result of processing of NF-kappaB2 p100. Here, we demonstrate that the non-canonical IkappaB kinase epsilon (IKKepsilon) directly interacts with p100. In a transactivation assay, IKKepsilon promoted the ability of p52 to transactivate gene expression. This effect was indirect, requiring p65, which was shown to be part of the IKKepsilon-p52 complex and to be phosphorylated by IKKepsilon. These novel interactions reveal a hitherto unknown function of IKKepsilon in the regulation of the alternative NF-kappaB activation pathway involving p52 and p65.

The alternative NF-kappaB pathway from biochemistry to biology: pitfalls and promises for future drug development

The past two decades have led to a tremendous work on the transcription factor NF-kappaB and its molecular mechanisms of activation. The nuclear translocation of NF-kappaB is controlled by two main pathways: the classical and the alternative NF-kappaB pathways. The classical NF-kappaB pathway activates the IKK complex that controls the inducible degradation of most IkappaB family members that are IkappaBalpha, IkappaBbeta, IkappaBvarepsilon and p105. The alternative NF-kappaB pathway induces p100 processing and p52 generation through the activation of at least two kinases, which are NIK and IKKalpha. Genetic studies have shown that IKKgamma is dispensable for the alternative pathway, which suggests the existence of an alternative IKKalpha-containing complex. It is noteworthy that activation of particular p52 heterodimers like p52/RelB requires solely the alternative pathway while activation of p52/p65 or p52/c-Rel involves a "hybrid pathway". Among others, LTbetaR, BAFF-R, CD40 and RANK have the ability to induce the alternative pathway. The latter plays some roles in biological functions controlled by these receptors, which are the development of secondary lymphoid organs, the proliferation, survival and maturation of B cell, and the osteoclastogenesis. Exacerbated activation of the alternative pathway is potentially associated to a wide range of disorders like rheumatoid arthritis, ulcerative colitis or B cell lymphomas. Therefore, inhibitors of the alternative pathway could be valuable tools for the treatment of inflammatory disorders and cancers.

Induction of cell death in adult T-cell leukemia cells by a novel IkappaB kinase inhibitor

NF-kappaB is constitutively activated in adult T-cell leukemia (ATL) and is considered responsible for cell growth and prevention of cell death. In this study, we demonstrate that NF-kappaB is constitutively activated in various HTLV-1-infected T-cell lines and ATL-derived cell lines irrespectively of Tax expression as evidenced by the phosphorylation of IkappaBalpha and p65 subunit of NF-kappaB, activation of NF-kappaB DNA binding, and upregulation of various target genes including bcl-xL, bcl-2, XIAP, c-IAP1, survivin, cyclinD1, ICAM-1 and VCAM-1. The effects of a novel IkappaB kinase (IKK) inhibitor, 2-amino-6-[2-(cyclopropylmethoxy)-6-hydroxyphenyl]-4-piperidin-4-yl nicotinonitrile (ACHP), were examined on cell growth of these cell lines and fresh ATL leukemic cells. We found that ACHP could inhibit the phosphorylation of IkappaBalpha and p65, as well as NF-kappaB DNA-binding, associated with downregulation of the NF-kappaB target genes and induce cell growth arrest and apoptosis in these cells. When Tax-active and Tax-inactive cell lines were compared, ACHP could preferentially inhibit cell growth of Tax-active cells. Moreover, ACHP exhibited strong apoptosis-inducing activity in fresh ATL cells. These findings indicate that ACHP and its derivatives are effective in inducing ATL cell death and thus feasible candidates for the treatment of ATL.

Versatile reporter systems show that transactivation by human T-cell leukemia virus type 1 Tax occurs independently of chromatin remodeling factor BRG1

Potent activation of human T-cell leukemia virus type 1 (HTLV-1) gene expression is mediated by the virus-encoded transactivator protein Tax and three imperfect 21-bp repeats in the viral long terminal repeats. Each 21-bp repeat contains a cAMP-responsive-element core flanked by 5' G-rich and 3' C-rich sequences. Tax alone does not bind DNA. Rather, it interacts with basic domain-leucine zipper transcription factors CREB and ATF-1 to form ternary complexes with the 21-bp repeats. In the context of the ternary complexes, Tax contacts the G/C-rich sequences and recruits transcriptional coactivators CREB-binding protein (CBP)/p300 to effect potent transcriptional activation. Using an easily transduced and chromosomally integrated reporter system derived from a self-inactivating lentivirus vector, we showed in a BRG1- and BRM1-deficient adrenal carcinoma cell line, SW-13, that Tax- and 21-bp repeat-mediated transactivation does not require BRG1 or BRM1 and is not enhanced by BRG1. With a similar reporter system, we further demonstrated that Tax- and tumor necrosis factor alpha-induced NF-kappaB activation occurs readily in SW-13 cells in the absence of BRG1 and BRM1. These results suggest that the assembly of stable multiprotein complexes containing Tax, CREB/ATF-1, and CBP/p300 on the 21-bp repeats is the principal mechanism employed by Tax to preclude nucleosome formation at the HTLV-1 enhancer/promoter. This most likely bypasses the need for BRG1-containing chromatin-remodeling complexes. Likewise, recruitment of CBP/p300 by NF-kappaB may be sufficient to disrupt histone-DNA interaction for the initiation of transcription.

beta-TrCP binding and processing of NF-kappaB2/p100 involve its phosphorylation at serines 866 and 870

Processing of the NF-kappaB2 precursor protein p100 is a major step in noncanonical NF-kappaB signaling. This signaling step requires the NF-kappaB inducing kinase (NIK) and its downstream kinase, IkappaB kinase alpha (IKKalpha). We show here that p100 undergoes phosphorylation at serines 866, 870, and possibly 872, in cells stimulated with noncanonical NF-kappaB stimuli or transfected with NIK and IKKalpha. Phosphorylation of this serine cluster creates a binding site for beta-TrCP, the receptor subunit of the beta-TrCP(SCF) ubiquitin ligase. Mutation of either serine 866 or serine 870 abolishes the beta-TrCP recruitment and ubiquitination of p100. The functional significance of p100 phosphorylation is further supported by the finding that this molecular event occurs in a NIK- and IKKalpha-dependent manner. Additionally, induction of p100 phosphorylation can be blocked by a protein synthesis inhibitor, suggesting the requirement of de novo protein synthesis. These data suggest that p100 processing involves its phosphorylation at specific terminal serines, which form a binding site for beta-TrCP thereby regulating p100 ubiquitination.

Extending the nuclear roles of IkappaB kinase subunits

The transcription factor NF-kappaB plays a key role in a wide variety of cellular processes such as innate and adaptive immunity, cellular proliferation, apoptosis and development. In unstimulated cells, NF-kappaB is sequestered in the cytoplasm through its tight association with inhibitory proteins called IkappaBs, comprising notably IkappaBalpha. A key step in NF-kappaB activation is the phosphorylation of IkappaBalpha by the so-called IkappaB kinase (IKK) complex, which targets the inhibitory protein for proteasomal degradation and allows the freed NF-kappaB to enter the nucleus where it can transactivate its target genes. The IKK complex is composed of two catalytic subunits called IKKalpha and IKKbeta, and a regulatory subunit called NEMO/IKKgamma. Despite their key role in mediating IkappaBalpha phosphorylation in the cytoplasm, recent works have provided evidence that IKK subunits also translocate into the nucleus to regulate NF-kappaB-dependent and -independent gene expression, paving the way of a novel and exciting field of research. In this review, we will describe the current knowledge in that research area.

Deregulated NF-kappaB activity in haematological malignancies

The NF-kappaB family of transcription factors plays key roles in the control of cell proliferation and apoptosis. Constitutive NF-kappaB activation is a common feature for most haematological malignancies and is therefore believed to be a crucial event for enhanced proliferation and survival of these malignant cells. In this review, we will describe the molecular mechanisms underlying NF-kappaB deregulation in haematological malignancies and will highlight what is still unclear in this field, 20 years after the discovery of this transcription factor.

NF-kappaB activation by reactive oxygen species: fifteen years later

The transcription factor NF-kappaB plays a major role in coordinating innate and adaptative immunity, cellular proliferation, apoptosis and development. Since the discovery in 1991 that NF-kappaB may be activated by H(2)O(2), several laboratories have put a considerable effort into dissecting the molecular mechanisms underlying this activation. Whereas early studies revealed an atypical mechanism of activation, leading to IkappaBalpha Y42 phosphorylation independently of IkappaB kinase (IKK), recent findings suggest that H(2)O(2) activates NF-kappaB mainly through the classical IKK-dependent pathway. The molecular mechanisms leading to IKK activation are, however, cell-type specific and will be presented here. In this review, we also describe the effect of other ROS (HOCl and (1)O(2)) and reactive nitrogen species on NF-kappaB activation. Finally, we critically review the recent data highlighting the role of ROS in NF-kappaB activation by proinflammatory cytokines (TNF-alpha and IL-1beta) and lipopolysaccharide (LPS), two major components of innate immunity.

Akt regulates basal and induced processing of NF-kappaB2 (p100) to p52

NF-kappaB is a family of transcription factors important for innate and adaptive immunity. NF-kappaB is restricted to the cytoplasm by inhibitory proteins that are degraded when specifically phosphorylated, permitting NF-kappaB to enter the nucleus and activate target genes. Phosphorylation of the inhibitory proteins is mediated by an IkappaB kinase (IKK) complex, which can be composed of two subunits with enzymatic activity, IKKalpha and IKKbeta. The preferred substrate for IKKbeta is IkappaBalpha, degradation of which liberates p65 (RelA) to enter the nucleus where it induces genes important to innate immunity. IKKalpha activates a non-canonical NF-kappaB pathway in which p100 (NF-kappaB2) is processed to p52. Once produced, p52 can enter the nucleus and induce genes important to adaptive immunity. This study shows that Akt binds to and increases the activity of IKKalpha and thereby increases p52 production in cells. Constitutively active Akt augments non-canonical NF-kappaB activity, whereas kinase dead Akt or inhibition of phosphatidylinositol 3-kinase have the opposite effect. Basal and ligand-induced p52 production is reduced in mouse embryo fibroblasts deficient in Akt1 and Akt2 compared with parental cells. These observations show that Akt plays a role in activation of basal and induced non-canonical NF-kappaB activity.

Melatonin suppresses macrophage cyclooxygenase-2 and inducible nitric oxide synthase expression by inhibiting p52 acetylation and binding

Melatonin has been shown to be produced by nonpineal cells and possess anti-inflammatory actions in animal models. In the present study, we tested the hypothesis that melatonin suppresses the expression of proinflammatory genes such as cyclooxygenase-2 (COX2) and inducible nitric oxide synthase (INOS) by a common transcriptional mechanism. Melatonin but not tryptophan or serotonin inhibited lipopolysaccharide (LPS)-induced COX-2 and iNOS protein levels and promoter activities in RAW 264.7 cells in a time- and concentration-dependent manner. LPS or LPS plus interferon-gamma (IFNgamma) increased binding of all 5 isoforms of NF-kappaB to COX-2 and iNOS promoters. Melatonin selectively inhibited p52 binding without affecting p100 expression, p52 generation from p100, or p52 nuclear translocation. p52 acetylation was enhanced by LPS, which was abrogated by melatonin. Melatonin inhibited p300 histone acetyltransferase (HAT) activity and abrogated p300-augmented COX-2 and iNOS expression. HAT inhibitors suppressed LPS-induced p52 binding and acetylation to an extent similar to melatonin, and melatonin did not potentiate the effect of HAT inhibitors. These results suggest that melatonin inhibits COX-2 and iNOS transcriptional activation by inhibiting p300 HAT activity, thereby suppressing p52 acetylation, binding, and transactivation.

Down-regulating constitutive activation of the NF-κB canonical pathway overcomes the resistance of cutaneous T-cell lymphoma to apoptosis

Constitutive activation of the nuclear factor-kappaB (NF-κB) pathway has been shown to be involved in the resistance of tumor cells to apoptosis in several human malignancies of the hematopoietic lineage. By using electrophoretic mobility shift assay (EMSA) and confocal microscopic analysis, we demonstrate that NF-κB is constitutively activated in cutaneous T-cell lymphoma (CTCL) cell lines HuT-78, MyLa, and SeAx and in peripheral blood lymphocytes (PBLs) from patients with Sézary syndrome (SS) presenting a high ratio of tumor cells, with evidence of p50 and RelA/p65 in DNA-linked complexes. Transfection of SeAx line with a κB/luciferase reporter plasmid showed that translocated NF-κB complexes were functional. Selective inhibition of NF-κB, by transfecting CTCL cell lines with a super-repressor form of IκBα, led to apoptosis. We evidenced down-regulation of NF-κB activation and induction of CTCL cell apoptosis in the presence of proteasome 26S inhibitors ALLN, MG132, and bortezomib. Bortezomib at nanomolar concentrations inhibited constitutive activation of NF-κB and induced apoptosis of CTCL cells, with evidence of an upregulation of Bax expression. These results demonstrate the key role played by NF-κB in the resistance of CTCL to apoptosis and suggest that bortezomib might be useful for the treatment of patients with advanced stages of CTCL refractory to standard antineoplastic chemotherapy.

Regulation of T cell development by the deubiquitinating enzyme CYLD

T cell receptor signaling is essential for the generation and maturation of T lymphocyte precursors. Here we identify the deubiquitinating enzyme CYLD as a positive regulator of proximal T cell receptor signaling in thymocytes. CYLD physically interacted with active Lck and promoted recruitment of active Lck to its substrate, Zap70. CYLD also removed both Lys 48- and Lys 63-linked polyubiquitin chains from Lck. Because of a cell-autonomous defect in T cell development, CYLD-deficient mice had substantially fewer mature CD4(+) and CD8(+) single-positive thymocytes and peripheral T cells.

Activation of the NF-kappaB pathway by the leukemogenic TEL-Jak2 and TEL-Abl fusion proteins leads to the accumulation of antiapoptotic IAP proteins and involves IKKalpha

Abnormal activation of tyrosine kinases and of signaling pathways they control plays a critical role in the neoplastic process of human hematopoietic malignancy. The nuclear factor-kappaB (NF-kappaB) pathway is one of the signalings activated by the TEL-Jak2 and TEL-Abl oncoproteins and required for their antiapoptotic activity. To define the signal relay responsible for this activation, we used mouse embryonic fibroblast (MEF) cells and observed that TEL-Jak2- and TEL-Abl-mediated NF-kappaB induction was abolished in cells lacking the IkappaB kinase (IKK)alpha but not in IKKbeta(-/-) cells. Similar observations were performed with oncogenic forms of the FMS-like tyrosine kinase 3 (Flt-3) involved in the pathogenesis of one-third of acute myeloid leukemias. Rescue of TEL-Jak2-mediated NF-kappaB activation was obtained with a kinase-proficient form of IKKalpha in IKKalpha(-/-) MEF. Hematopoietic cells transformed by TEL-Jak2 and TEL-Abl showed sustained IKKalpha activity without promotion of NF-kappaB2/p100 processing, generally associated to IKKalpha functions. Furthermore, IAP1, IAP2 and XIAP, which are central regulators of the NF-kappaB-mediated survival pathway, were highly expressed in cells transformed by these oncoproteins. Our results indicate that these oncogenic tyrosine kinases preferentially use an IKKalpha-dependent mechanism to induce a persistent NF-kappaB activity and allow the production of antiapoptotic effectors that participate to their leukemogenic properties.

Stabilization of Basally Translated NF-κB-inducing Kinase (NIK) Protein Functions as a Molecular Switch of Processing of NF-κB2 p100

The non-canonical pathway based on processing of NF-kappaB2 precursor protein p100 to generate p52 plays a critical role in controlling B cell function and lymphoid organogenesis. Activation of this unique pathway by extracellular stimuli requires NF-kappaB-inducing kinase (NIK) and de novo protein synthesis. However, how NIK is regulated is largely unknown. Here, we systematically analyzed NIK expression at different levels in the presence or absence of different NF-kappaB stimuli. We found that NIK mRNA is relatively abundant and undergoes constitutive protein synthesis in resting B cells. However, NIK protein is undetectable. Interestingly, protein expression of NIK is steadily induced by B cell-activating factor or CD40 ligand, two major physiological inducers of p100 processing, but not by mitogen phorbol 12-myristate 13-acetate/ionomycin or cytokine tumor necrosis factor alpha, two well known inducers of the canonical NF-kappaB signaling. Remarkably, both B cell-activating factor and CD40 ligand do not significantly induce expression of NIK at translational or transcriptional level but rather rescue the basally translated NIK protein from undergoing degradation. Furthermore, overexpressed or purified NIK protein triggers p100 processing in the presence of protein synthesis inhibitor. Taken together, these studies define one important mechanism of NIK regulation and the central role of NIK stabilization in the induction of p100 processing. These studies also provide the first evidence explaining why activation of the non-canonical NF-kappaB signaling is delayed and can be inhibited by protein synthesis inhibitor as well as why most classical NF-kappaB stimuli, including mitogens and tumor necrosis factor alpha, fail to induce p100 processing.

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

T-cell transformation by the human T-cell leukemia virus type I (HTLV-I) involves deregulation of cellular transcription factors, including members of the NF-kappaB family. In normal T cells, NF-kappaB activation occurs transiently in response to immune stimuli, which is required for antigen-stimulated T-cell proliferation and survival. However, HTLV-I induces persistent activation of NF-kappaB, causing deregulated expression of a large array of cellular genes, which in turn contributes to the induction of T-cell transformation. The HTLV-I transforming protein Tax functions as an intracellular stimulator of IkappaB kinase (IKK), a cellular kinase mediating NF-kappaB activation by diverse stimuli. Tax physically interacts with IKK and renders this inducible kinase constitutively active. By assembling different Tax/IKK complexes, Tax targets the persistent activation of both canonical and noncanonical NF-kappaB signaling pathways. Whereas Tax plays a primary role in HTLV-I-mediated NF-kappaB activation, recent studies reveal that the IKK/NF-kappaB signaling pathway is also activated in freshly isolated adult T-cell leukemia (ATL) cells that often lack detectable Tax expression. The mechanism underlying this Tax-independent pathway of NF-kappaB activation remains poorly understood. Clarifying the precise nature and consequences of the constitutive NF-kappaB activation in ATL cells is important for developing rational therapeutic strategies for this T-cell malignancy.

The NF-κB signalling pathway: a therapeutic target in lymphoid malignancies?

Nuclear factor-kappaB/reticuloendotheliosis (NF-kappaB/Rel) designates a family of transcription factors that influence the activation of a multitude of genes critically involved in immune and inflammatory responses. Recently, genetic and biochemical evidence has accumulated, suggesting that constitutive activation of NF-kappaB/Rel proteins plays an important role in the development/progression of B and T cell lymphoid malignancies. In particular, genetic and molecular alterations of NF-kappaB family members and their transcriptional target genes have been implicated in the development of diffuse large B cell lymphoma and mucosa-associated lymphoid tissue lymphoma. Although NF-kappaB/Rel proteins represent an integrating point of several pathways, potentially contributing to several diseases, their unique activation depends on cell type and stimulus. Considering the NF-kappaB specificity in lymphoid cells, molecules that finely modulate the activity of these NF-kappaB components and dampen the inappropriate proliferation of lymphocytes may represent a novel pharmacological intervention to several lymphoid malignancies.

Mechanisms of persistent NF-kappaB activation by HTLV-I tax

Human T cell leukemia virus type I (HTLV-I) is the causative agent of a fatal malignancy known as adult T cell leukemia (ATL). The HTLV-I Tax protein is thought to play a significant role in the initiation and pathogenesis of HTLV-I-mediated disease. Tax is a potent oncogene that deregulates cellular gene expression by persistently activating signaling pathways such as NF-kappaB. Tax activation of NF-kappaB is critical for the immortalization and survival of HTLV-I-infected T cells. In this review, we describe recent insights into the mechanisms employed by Tax to activate the canonical and noncanonical NF-kappaB signaling pathways. The adaptor function of Tax appears to be a common and important mechanism for the pathological activation of both NF-kappaB pathways.

Histone deacetylase inhibition down-regulates cyclin D1 transcription by inhibiting nuclear factor-kappaB/p65 DNA binding

Histone deacetylase (HDAC) inhibitors are emerging as a promising new class of cancer therapeutic agents. HDAC inhibitors relieve the deacetylation of histone proteins. However, little is known about the nonhistone targets of HDAC inhibitors and their roles in gene regulation. In this study, we addressed the molecular basis of the down-regulation of the nuclear factor-kappaB (NF-kappaB)-responsive gene cyclin D1 by the HDAC inhibitor trichostatin A in mouse JB6 cells. Cyclin D1 plays a critical role in cell proliferation and tumor progression. Trichostatin A inhibits cyclin D1 expression in a NF-kappaB-dependent manner in JB6 cells. Electrophoretic mobility shift assay studies showed that trichostatin A treatment prevents p65 dimer binding to NF-kappaB sites on DNA. Moreover, a chromatin immunoprecipitation assay shows that trichostatin A treatment inhibits endogenous cyclin D1 gene transcription by preventing p65 binding to the cyclin D1 promoter. However, acetylation of p65 is not affected by trichostatin A treatment. Instead, trichostatin A enhances p52 acetylation and increases p52 protein level by enhancing p100 processing. This is the first report that trichostatin A, a HDAC inhibitor, activates p100 processing and relieves the repression of p52 acetylation. The enhanced acetylation of p52 in the nuclei may operate to cause nuclear retention of p65 by increasing the p52/p65 interaction and preventing IkappaBalpha-p65 binding. The enhanced p52 acetylation coincides with decreased p65 DNA binding, suggesting a potential role of p52 acetylation in NF-kappaB regulation. Together, the results provide the first demonstration that HDAC inhibitor trichostatin A inhibits cyclin D1 gene transcription through targeting transcription factor NF-kappaB/p65 DNA binding. NF-kappaB is therefore identified as a transcription factor target of trichostatin A treatment.

Distinct IκB kinase regulation in adult T cell leukemia and HTLV-I-transformed cells

We have recently shown constitutive IkappaB kinase (IKK) activation and aberrant p52 expression in adult T cell leukemia (ATL) cells that do not express human T cell leukemia virus type I (HTLV-I) Tax, but the mechanism of IKK activation in these cells has remained unknown. Here, we demonstrate distinct regulation of IKK activity in ATL and HTLV-I-transformed T cells in response to protein synthesis inhibition or arsenite treatment. Protein synthesis inhibition for 4 h by cycloheximide (CHX) barely affects IKK activity in Tax-positive HTLV-I-transformed cells, while it diminishes IKK activity in Tax-negative ATL cells. Treatment of ATL cells with a proteasome inhibitor MG132 prior to protein synthesis inhibition reverses the inhibitory effect of CHX, and MG132 alone greatly enhances IKK activity. In addition, treatment of HTLV-I-transformed cells with arsenite for 1 h results in down-regulation of IKK activity without affecting Tax expression, while 8 h of arsenite treatment does not impair IKK activity in ATL cells. These results indicate that a labile protein sensitive to proteasome-dependent degradation governs IKK activation in ATL cells, and suggest a molecular mechanism of IKK activation in ATL cells distinct from that in HTLV-I-transformed T cells.

Stimulation of CD30 in anaplastic large cell lymphoma leads to production of nuclear factor-kappaB p52, which is associated with hyperphosphorylated Bcl-3

Anaplastic large cell lymphoma (ALCL) and Hodgkin lymphoma (HL) express CD30 at high levels, but stimulation of this molecule has been reported to induce contradictory effects. To elucidate the molecular mechanism of CD30-mediated apoptosis of ALCL, we compared the gene expression profiles of t(2;5)(p23;q35)-positive ALCL with those of HL altered by CD30 agonistic stimulation. The results showed that BCL3, the high-level expression of which in ALCL was previously reported, was further upregulated in response to CD30 stimulation, along with several pro-apoptotic genes. Bcl-3 protein was present as an intermediate phospho-form in the resting-state ALCL, becoming hyperphosphorylated (Bcl-3P) upon stimulation. We next found that the stimulation promoted de novo synthesis of the nuclear factor (NF)-kappaB2/p100 precursor as well as processing to p52, and a series of immunoprecipitation and western blotting analyses consistently showed association of Bcl-3P with p52 in CD30-stimulated ALCL. An electrophoretic mobility shift assay revealed the induction of kappaB binding activity of the p52 homodimer, and nuclear colocalization of Bcl-3 and p52 was demonstrated in anaplastic lymphoma kinase-positive ALCL tumor tissues by immunohistochemistry. As Bcl-3 can act as an anti-repressor or transactivator or both, we propose that the (p52)2/Bcl-3P ternary complex, which is specifically induced in CD30-stimulated ALCL, can modulate expression of apoptosis-related genes regulated by NF-kappaB, thereby accounting for CD30-mediated apoptosis of ALCL.

Respiratory syncytial virus influences NF-kappaB-dependent gene expression through a novel pathway involving MAP3K14/NIK expression and nuclear complex formation with NF-kappaB2

A member of the Paramyxoviridae family of RNA viruses, respiratory syncytial virus (RSV), is a leading cause of epidemic respiratory tract infection in children. In children, RSV primarily replicates in the airway mucosa, a process that alters epithelial cell chemokine expression, thereby inducing airway inflammation. We investigated the role of the mitogen-activated protein kinase kinase kinase 14/NF-kappaB-inducing kinase (NIK) in the activation of NF-kappaB-dependent genes in alveolus-like A549 cells. RSV infection induces a time dependent increase of NIK mRNA and protein expression that peaks 12 to 24 h after viral exposure. Immunoprecipitation kinase assays indicate that NIK kinase activity is activated even more rapidly (within 6 h of RSV adsorption) associated with an endogenous approximately 50-kDa NF-kappaB2 substrate. Because NIK associates with IKKalpha to mediate processing of the 100-kDa NF-kappaB2 precursor into its 52-kDa DNA binding isoform ("p52"), the effects of RSV on NIK complex formation with IKKalpha and NF-kappaB2 were determined by coimmunoprecipitation assay. We find that NIK, IKKalpha, and both 100 kDa- and 52-kDa NF-kappaB2 isoforms strongly complex 15 h after exposure to RSV at times subsequent to NIK kinase activation. Western immunoblot and microaffinity DNA pull-down assays showed a parallel increase in nuclear translocation and DNA binding of the NF-kappaB2-Rel B complex. Interestingly, we make the novel observations that NIK also transiently translocates into the nucleus complexed with 52-kDa NF-kappaB2. Small interfering RNA-mediated NIK "knock-down" blocked RSV-inducible 52-kDa NF-kappaB2 processing and interfered with the early activation of a subset of NF-kappaB-dependent genes, indicating the importance of this activation pathway in the genomic NF-kappaB response to RSV. Together, these data indicate that RSV infection rapidly activates the noncanonical NF-kappaB activation pathway prior to the more potent canonical pathway activation. This appears to be through a novel mechanism involving induction of NIK kinase activity, expression, and nuclear translocation of a ternary complex with IKKalpha and processed NF-kappaB2.

Aberrant NF-kappaB2/p52 expression in Hodgkin/Reed-Sternberg cells and CD30-transformed rat fibroblasts

Overexpression of CD30 and constitutive nuclear factor-kappaB (NF-kappaB) activation are hallmarks of the malignant Hodgkin Reed-Sternberg (H-RS) cells. Previous investigations have demonstrated that both proliferation and survival of H-RS cells require constitutive NF-kappaB activity, which is comprised of the p50 and RelA subunits. We report here enhanced expression of NF-kappaB2/p52 and RelB-containing NF-kappaB DNA-binding activity in Epstein-Barr virus-negative H-RS cells. Kinetic studies revealed that a proteasome inhibitor MG132 induced p100 accumulation with reduced p52 expression in H-RS cells, suggesting proteasome-dependent processing of p100. In addition, treatment with a protein synthesis inhibitor cycloheximide rapidly downregulated inhibitor of NF-kappaB (IkappaB) kinase activity in H-RS cells. We also demonstrate that overexpression of CD30 in rat fibroblasts at levels comparable to those in H-RS cells results in constitutive IkappaB kinase activation, proteasome-dependent p100 processing, and NF-kappaB-dependent cell transformation. Our results thus indicate that CD30 triggers the noncanonical NF-kappaB activation pathway, and suggest that deregulated CD30 signaling contributes to the neoplastic features of H-RS cells.

Regulation of the deubiquitinating enzyme CYLD by IkappaB kinase gamma-dependent phosphorylation

Tumor suppressor CYLD is a deubiquitinating enzyme (DUB) that inhibits the ubiquitination of key signaling molecules, including tumor necrosis factor (TNF) receptor-associated factor 2 (TRAF2). However, how the function of CYLD is regulated remains unknown. Here we provide evidence that inducible phosphorylation of CYLD is an important mechanism of its regulation. Under normal conditions, CYLD dominantly suppresses the ubiquitination of TRAF2. In response to cellular stimuli, CYLD undergoes rapid and transient phosphorylation, which is required for signal-induced TRAF2 ubiquitination and activation of downstream signaling events. Interestingly, the CYLD phosphorylation requires IkappaB kinase gamma (IKKgamma) and can be induced by IKK catalytic subunits. These findings suggest that CYLD serves as a novel target of IKK and that the site-specific phosphorylation of CYLD regulates its signaling function.

Elevated expression of CD30 in adult T-cell leukemia cell lines: possible role in constitutive NF-kappaB activation

Background

Human T-cell leukemia virus type 1 (HTLV-1) is associated with the development of adult T-cell leukemia (ATL). HTLV-1 encoded Tax1 oncoprotein activates the transcription of genes involved in cell growth and anti-apoptosis through the NF-κB pathway, and is thought to play a critical role in the pathogenesis of ATL. While Tax1 expression is usually lost or minimal in ATL cells, these cells still show high constitutive NF-κB activity, indicating that genetic or epigenetic changes in ATL cells induce activation independent of Tax1. The aim of this study was to identify the molecules responsible for the constitutive activation of NF-κB in ATL cells using a retroviral functional cloning strategy.

Results

Using enhanced green fluorescent protein (EGFP) expression and blasticidin-resistance as selection markers, several retroviral cDNA clones exhibiting constitutive NF-κB activity in Rat-1 cells, including full-length CD30, were obtained from an ATL cell line. Exogenous stable expression of CD30 in Rat-1 cells constitutively activated NF-κB. Elevated expression of CD30 was identified in all ATL lines examined, and primary ATL cells from a small number of patients (8 out of 66 cases).

Conclusion

Elevated CD30 expression is considered one of the causes of constitutive NF-κB activation in ATL cells, and may be involved in ATL development.

p21WAF1 modulates NF-kappaB signaling and induces anti-apoptotic protein Bcl-2 in Tax-expressing rat fibroblast

Of the cell cycle-associated genes regulated by human T-cell leukemia virus type-1 (HTLV-1) Tax, cyclin-dependent kinase (CDK) inhibitor p21WAF1 is upregulated in HTLV-1-infected cells. Previously, we reported that p21WAF1 stimulated Tax-dependent NF-kappaB activation which influences a variety of cellular processes, including proliferation, differentiation, and apoptosis. In HTLV-1-infected cells, Tax is primarily involved in the constitutive activation of NF-kappaB signaling. Here, we demonstrate that p21WAF1 affects Tax-dependent NF-kappaB signaling by inducing p100/52, an NF-kappaB-related protein. W4, a Tax-transformed rat fibroblast cell line, exhibits the constitutive activation of NF-kappaB signaling, potentially mediated by overexpression of RelB. Ectopic expression of p21WAF1 in W4 cells, which lack endogenous expression due to methylation of the p21WAF1 promoter, induces the expression of p100/52. Bcl-2 expression was also upregulated by ectopic p21WAF1 in this cell line, suggesting that p21WAF1 plays an important role in the regulation of apoptosis by modulating NF-kappaB signaling in Tax-expressing rat fibroblasts. We also address the expression of NF-kappaB-related proteins in HTLV-1-infected cells.

NF-kB in development and progression of human cancer

The nuclear factor kB (NF-kB) comprises a family of transcription factors involved in the regulation of a wide variety of biological responses. NF-kB plays a well-known function in the regulation of immune responses and inflammation, but growing evidences support a major role in oncogenesis. NF-kB regulates the expression of genes involved in many processes that play a key role in the development and progression of cancer such as proliferation, migration and apoptosis. Aberrant or constitutive NF-kB activation has been detected in many human malignancies. In recent years, numerous studies have focused on elucidating the functional consequences of NF-kB activation as well as its signaling mechanisms. NF-kB has turned out to be an interesting therapeutic target for treatment of cancer.

Phosphorylation of NF-kappaB and IkappaB proteins: implications in cancer and inflammation

Nuclear factor-kappaB (NF-kappaB) is a transcription factor that has crucial roles in inflammation, immunity, cell proliferation and apoptosis. Activation of NF-kappaB mainly occurs via IkappaB kinase (IKK)-mediated phosphorylation of inhibitory molecules, including IkappaBalpha. Optimal induction of NF-kappaB target genes also requires phosphorylation of NF-kappaB proteins, such as p65, within their transactivation domain by a variety of kinases in response to distinct stimuli. Whether, and how, phosphorylation modulates the function of other NF-kappaB and IkappaB proteins, such as B-cell lymphoma 3, remains unclear. The identification and characterization of all the kinases known to phosphorylate NF-kappaB and IkappaB proteins are described here. Because deregulation of NF-kappaB and IkappaB phosphorylations is a hallmark of chronic inflammatory diseases and cancer, newly designed drugs targeting these constitutively activated signalling pathways represent promising therapeutic tools.

A Novel NF-κB Pathway Involving IKKβ and p65/RelA Ser-536 Phosphorylation Results in p53 Inhibition in the Absence of NF-κB Transcriptional Activity

Nuclear factor kappaB (NF-kappaB) plays an important role in regulating cellular transformation and apoptosis. The human T-cell lymphotropic virus type I protein, Tax, which is critical for viral transformation, modulates the transcription of several cellular genes through activation of NF-kappaB. We have demonstrated previously that Tax inhibits p53 activity through the p65/RelA subunit of NF-kappaB. We now present evidence that suggests that the upstream kinase IKKbeta plays an important role in Tax-induced p53 inhibition through phosphorylation of p65/RelA at Ser-536. First, mouse embryo fibroblast (MEF) IKKbeta-/-cells did not support Tax-mediated p53 inhibition, whereas MEFs lacking IKKalpha allowed Tax inhibition of p53. Second, transfection of IKKbeta wild type (WT), but not a kinase-dead mutant, into IKKbeta-/-cells rescued p53 inhibition by Tax. Third, the IKKbeta-specific inhibitor SC-514 decreased the ability of Tax to inhibit p53. Fourth, we show that phosphorylation of p65/RelA at Ser-536 is important for Tax inhibition of p53 using MEF p65/RelA-/-cells transfected with p65/RelA WT or mutant plasmids. Moreover, Tax induced p65/RelA Ser-536 phosphorylation in WT or IKKalpha-/- cells but failed to induce the phosphorylation of p65/RelA Ser-536 in IKKbeta-/-cells, suggesting a link between IKKbeta and p65/RelA phosphorylation. Consistent with this observation, blocking IKKbeta kinase activity by SC-514 decreases the phosphorylation of p65/RelA at Ser-536 in the presence of Tax in human T-cell lymphotropic virus type I-transformed cells. Finally, the ability of Tax to inhibit p53 is distinguished from the NF-kappaB transcription activation pathway. Our work, therefore, describes a novel Tax-NF-kappaB p65/RelA pathway that functions to inhibit p53 but does not require NF-kappaB transcription activity.

Herpes virus proteins ICP0 and BICP0 can activate NF-κB by catalyzing IκBα ubiquitination

The immediate early proteins ICP0 and BICP0 from Herpes virus are promiscuous activators of both viral and cellular genes and play a critical role in virus life cycle. Here we report that ICP0 and BICP0 could induce NF-kappaB translocation from cytoplasm into nucleus and strongly activate NF-kappaB responsive genes specifically. This process was dependent on the RING domain of both proteins. In addition, ICP0 interacted specifically with IkappaBalpha and its activating effect was attenuated by Ubch5A(C85A) and MG132, but not by IkappaBalpha(S32A/S36A). Remarkably, IkappaBalpha was poly-ubiquitinated by both ICP0 and BICP0, in vitro and in vivo. These data indicate that ICP0 and BICP0, functioning as ubiquitin ligases, are bona fide activators of NF-kappaB signaling pathway. Our study identifies a new way ICP0 and BICP0 explore to regulate gene expression.

Essential role of IkappaB kinase alpha in the constitutive processing of NF-kappaB2 p100

Processing of NF-kappaB2 precursor protein p100 to generate p52 is tightly controlled, which is important for proper function of NF-kappaB. Accordingly, constitutive processing of p100, caused by the loss of its C-terminal processing inhibitory domain due to nfkappab2 gene rearrangements, is associated with the development of various lymphomas and leukemia. In contrast to the physiological processing of p100 triggered by NF-kappaB-inducing kinase (NIK) and its downstream kinase, IkappaB kinase alpha (IKKalpha), which requires the E3 ligase, beta-transducin repeat-containing protein (beta-TrCP), and occurs only in the cytoplasm, the constitutive processing of p100 is independent of beta-TrCP but rather is regulated by the nuclear shuttling of p100. Here, we show that constitutive processing of p100 also requires IKKalpha, but not IKKbeta (IkappaB kinase beta) or IKKgamma (IkappaB kinase gamma). It seems that NIK is also dispensable for this pathogenic processing of p100. These results demonstrate a general role of IKKalpha in p100 processing under both physiological and pathogenic conditions. Additionally, we find that IKKalpha is not required for the nuclear translocation of p100. Thus, these results also indicate that p100 nuclear translocation is not sufficient for the constitutive processing of p100.

An atypical tumor necrosis factor (TNF) receptor-associated factor-binding motif of B cell-activating factor belonging to the TNF family (BAFF) receptor mediates induction of the noncanonical NF-kappaB signaling pathway

BAFF receptor (BAFFR) is a member of the TNF receptor (TNFR) superfamily that regulates the survival and maturation of B cells. BAFFR exerts its signaling function by inducing activation of NF-kappaB, although the underlying mechanism has not been well defined. By using a chimeric BAFFR, we show that BAFFR preferentially induces the noncanonical NF-kappaB signaling pathway. This specific function of BAFFR is mediated by a sequence motif, PVPAT, which is homologous to the TRAF-binding site (PVQET) present in CD40, a TNFR known to induce both the canonical and noncanonical NF-kappaB pathways. Mutation of this putative TRAF-binding motif within BAFFR abolishes its interaction with TRAF3 as well as its ability to induce noncanonical NF-kappaB. Interestingly, modification of the PVPAT sequence to the typical TRAF-binding sequence, PVQET, is sufficient to render the BAFFR capable of inducing strong canonical NF-kappaB signaling. Further, this functional acquisition of the modified BAFFR is associated with its stronger and more rapid association with TRAF3. These findings suggest that the PVPAT sequence of BAFFR not only functions as a key signaling motif of BAFFR but also determines its signaling specificity in the induction of the noncanonical NF-kappaB pathway.