Nuclear factor of κB (NF-κB) is a sequence-specific transcription factor that is known to be involved in the inflammatory and innate immune responses. Although the importance of NF-κB in immunity is undisputed, recent evidence indicates that NF-κB and the signalling pathways that are involved in its activation are also important for tumour development. NF-κB should therefore receive as much attention from cancer researchers as it has already from immunologists.

NF-κB in cancer: from innocent bystander to major culprit

The abilities of NF-κB to promote cell proliferation, suppress apoptosis, promote cell migration, and suppress differentiation apparently have been co-opted by cellular and viral oncoproteins to promote oncogenesis (Figure ​(Figure2).2). Direct evidence, using both in vitro and in vivo models, indicates that NF-κB is required for oncogenesis, probably at multiple levels. NF-κB likely plays an important role in the early events of oncogenesis, possibly functioning primarily in protecting against transformation-associated apoptosis. In most late-stage tumor cells, classic NF-κB (the p50-p65 heterodimer) is clearly not the only survival factor, because its inhibition does not induce apoptosis in many of these tumor cells. This observation suggests that other events have occurred to upregulate NF-κB–independent cell survival pathways. However, clearly some cancer cells depend on NF-κB for their survival. NF-κB also can contribute to cell progression by transcriptionally upregulating cyclin D1 with corresponding hyperphosphorylation of the tumor suppressor protein Rb. The induction of NF-κB–controlled proliferation may correlate with loss of differentiation in certain settings (47), which may promote oncogenesis. NF-κB is known to regulate certain genes associated with metastasis, such as matrix metalloproteinase 9, tissue plasminogen activator, and ICAM-1. Thus, a more relevant role for NF-κB in later-stage oncogenesis may be to promote metastasis and angiogenesis. Although many tumor cells display some level of constitutive nuclear NF-κB, higher levels of NF-κB and the transcriptional potential of NF-κB can be further enhanced in response to certain types of chemotherapy. Consistent with this, inhibition of NF-κB in parallel with certain (but apparently not all) chemotherapy treatments strongly enhances the apoptotic potential of the chemotherapy. This observation indicates that NF-κB plays an important role in inducible chemoresistance and establishes NF-κB inhibition as a new adjuvant approach in chemotherapy.

https://dm5migu4zj3pb.cloudfront.net/manuscripts/11000/11991/JCI0111991.pdf

Control of oncogenesis and cancer therapy resistance by the transcription factor NF-κB

Complementation Cloning of NEMO, a Component of the IκB Kinase Complex Essential for NF-κB Activation

Gene expression profiling has revealed that diffuse large B cell lymphoma (DLBCL) consists of at least two distinct diseases Patients with one DLBCL subtype, termed activated B cell–like (ABC) DLBCL, have a distinctly inferior prognosis An untapped potential of gene expression profiling is its ability to identify pathogenic signaling pathways in cancer that are amenable to therapeutic attack The gene expression profiles of ABC DLBCLs were notable for the high expression of target genes of the nuclear factor (NF)-κB transcription factors, raising the possibility that constitutive activity of the NF-κB pathway may contribute to the poor prognosis of these patients Two cell line models of ABC DLBCL had high nuclear NF-κB DNA binding activity, constitutive IκB kinase (IKK) activity, and rapid IκBα degradation that was not seen in cell lines representing the other DLBCL subtype, germinal center B-like (GCB) DLBCL Retroviral transduction of a super-repressor form of IκBα or dominant negative forms of IKKβ was toxic to ABC DLBCL cells but not GCB DLBCL cells DNA content analysis showed that NF-κB inhibition caused both cell death and G1-phase growth arrest These findings establish the NF-κB pathway as a new molecular target for drug development in the most clinically intractable subtype of DLBCL and demonstrate that the two DLBCL subtypes defined by gene expression profiling utilize distinct pathogenetic mechanisms

/pdf/constitutive-nuclear-factor-kb-activity-is-required-for-1t3dhq0ksw.pdf

Constitutive Nuclear Factor κB Activity Is Required for Survival of Activated B Cell–like Diffuse Large B Cell Lymphoma Cells

The 9ORF1 gene encodes an adenovirus E4 region oncoprotein that requires a C-terminal region for transforming activity. Screening a λgt11 cDNA expression library with a 9ORF1 protein probe yielded a novel cellular PDZ domain-containing protein, 9BP-1, which binds to wild-type, but not a transformation-defective, C-terminal, mutant 9ORF1 protein. The fact that PDZ domains complex with specific sequences at the free C-terminal end of some proteins led to the recognition that the 9ORF1 C-terminal region contained such a consensus-binding motif. This discovery prompted investigations into whether the 9ORF1 protein associates with additional cellular proteins having PDZ domains. It was found that the 9ORF1 protein interacts directly, in vitro and in vivo, with the PDZ domain-containing protein hDlg/SAP97 (DLG), which is a mammalian homolog of the Drosophila discs large tumor suppressor protein and which also binds the adenomatous polyposis coli tumor suppressor protein. Of interest, in forming complexes, the 9ORF1 protein preferentially associated with the second PDZ domain of DLG, similar to adenomatous polyposis coli protein. Human T cell leukemia virus type 1 Tax and most oncogenic human papillomavirus E6 oncoproteins also possessed PDZ domain-binding motifs at their C termini and, significantly, human T cell leukemia virus type 1 Tax and human papillomavirus 18 E6 proteins bound DLG in vitro. Considering the requirement of the 9ORF1 C-terminal region in transformation, these findings suggest that interactions with the cellular factor DLG may contribute to the tumorigenic potentials of several different human virus oncoproteins.

https://www.pnas.org/content/pnas/94/13/6670.full.pdf

Binding of human virus oncoproteins to hDlg/SAP97, a mammalian homolog of the Drosophila discs large tumor suppressor protein

Human T-cell leukemia virus type I (HTLV-I) encodes a 40 kDa trans-acting protein, Tax, that regulates transcription of both the proviral and cellular genes, and can transform rat fibroblasts. To determine the functional importance of its trans-acting capacities in cell transformation, we have examined two representative pathways of transcriptional activation--HTLV-I long terminal repeat (LTR) mediated and NF-kappa B dependent--by mutational analysis of Tax. In contrast to a previous report, mutants lacking the ability to activate an NF-kappaB-dependent promoter failed to transform rat fibroblasts, whereas a mutation which abolishes the HTLV-I LTR-mediated trans-activation demonstrated a wild-type capacity for cell transformation. Stable expression of Tax competent for transformation caused enhanced DNA binding of NF-kappa B in rat fibroblasts. We also demonstrate that stable co-expression of the NFKB2 precursor, known as a member of the I kappa B proteins, with wild-type Tax blocked transformation as well as eliminated aberrant NF-kappaB activation by Tax without interference with the HTLV-I LTR-mediated trans-activation. Our results indicate that constitutive activation of NF-kappa B is essential for Tax-mediated transformation of rat fibroblasts.

Constitutive activation of NF-kappa B is essential for transformation of rat fibroblasts by the human T-cell leukemia virus type I Tax protein.

Cytotoxic Activity of rev Protein of Human Immunodeficiency Virus Type 1 by Nucleolar Dysfunction

The transcription initiation signals in retroviruses lie within the long terminal repeat (LTR) sequences. We have found a new transcriptional promoter in a central portion of the genome of human T-cell leukaemia virus type I (HTLV-I). The transcription start site is located just upstream to the ATG codon of the transcriptional trans-activator molecule, tax protein (Tax). The internal promoter may provide a new insight into gene expression of HTLV-I. The mechanism of leukaemogenesis by the defective HTLV-I is also discussed. Furthermore, we have identified two repeats of a novel enhancer sequence AGTTCT, which are located around the initiation site. We call the sequence HIRE (HTLV-I Internal Regulatory Element).

Novel internal promoter/enhancer of HTLV-I for Tax expression.

The transcriptional trans-activator molecule of HTLV-I, Tax, is known to transform rodent fibroblasts. A revertant clone expressing Tax was obtained by treating transformed Rat-I cells harboring a single copy of the tax gene with a mutagen. Sequence analysis of the tax gene of the revertant clone revealed that it had one point mutation at codon 12(CTT----TTT), resulting in a change from Leu to Phe. The colony-forming efficiencies of the cells transfected with the mutant tax gene (mu71 tax) were significantly lower than those transfected with the wild-type by the soft-agar method. This difference was shown to be due to the instability of mu71 Tax.

Mamoru Sakurai

Papers

Constitutive activation of NF-kappa B is essential for transformation of rat fibroblasts by the human T-cell leukemia virus type I Tax protein.

Cytotoxic Activity of rev Protein of Human Immunodeficiency Virus Type 1 by Nucleolar Dysfunction

Novel internal promoter/enhancer of HTLV-I for Tax expression.

Transforming activity and the level of tax protein: Effect of one point mutation in HTLV‐I tax gene