Nf-kappa B, chemokine gene transcription and tumour growth

Abstract

The constitutive expression of angiogenic and tumorigenic chemokines by tumour cells facilitates the growth of tumours. The transcription of these angiogenic and tumorigenic chemokine genes is modulated, in part, by the nuclear factor-kappa B (NF-kappa B) family of transcription factors. In some tumours, there is constitutive activation of the kinases that modulate the activity of inhibitor of NF-kappa B (I kappa B) kinase (IKK), which leads to the constitutive activation of members of the NF-kappa B family. This activation of NF-kappa B is associated with the dysregulation of transcription of genes that encode cytokines, chemokines, adhesion factors and inhibitors of apoptosis. In this review, I discuss the factors that lie upstream of the NF-kappa B cascade that are activated during tumorigenesis and the role of the putative NF-kappa B enhanceosome in constitutive chemokine gene transcription during tumorigenesis.

Figure 1. NF-κB is activated by plasma-membrane receptors that transduce signals to kinases such as PI3K

Phosphatidylinositol 3-kinase (PI3K) can activate AKT, which potentially affects the phosphorylation of both inhibitor of NF-κB (IκB) kinase (IKK; a complex of IKKα, IKKβ and NEMO) and p65 (RELA). In addition, linker proteins, such as tumour-necrosis factor (TNF)-receptor-associated factors (TRAFs), participate in the activation of NF-κB-inducing kinase (NIK), as well as transforming-growth-factor-β-activated kinase 1 (TAK1). NIK-mediated enhancement of NF-κB activity might be facilitated by its activation of mitogen-activated protein (MAP) kinases, such as extracellular-signal-regulated kinase 1 (ERK1) and ERK2. The activation of IKK leads to enhanced phosphorylation of IκB, followed by its ubiquitylation then degradation by the 26S proteasome. This frees the NF-κB p50 and p65 subunits to be transported to the nucleus. Nuclear NF-κB p50–p65 heterodimers that are not phosphorylated on the p65 subunit can only partially activate gene transcription, whereas if p65 is phosphorylated, the enhancement of transcription is greatly magnified. LTβ, lymphotoxin-β; LTβR, lymphotoxin-β receptor; NEMO, NF-κB essential modulator; PtdInsP₃, phosphatidylinositol-3,4,5-trisphosphate; PTEN, phosphatase and tensin homologue.

Figure 2. Similarities between the proposed enhanceosomes for IL-6, CXCL8 and CXCL1

Similar key regulatory elements participate in the modulation of transcription of CXCL8, interleukin-6 (IL-6) and CXCL1. The transcription of CXCL8 is largely modulated through a nuclear factor-κB (NF-κB) element, which works in concert with adjacent activating protein 1 (AP1) and/or C/EBP elements to modulate the expression of this chemokine. The transcription of CXCL8 is also modulated by an NF-κB-repressing factor (NRF). This negative regulatory element has a dual role; in the absence of stimulation, NRF inhibits the transcription of CXCL8, but after an IL-1 signal, the NRF is required for the full induction of CXCL8 transcription. An enhanceosome model of cytokine gene expression, analogous to the CXCL8 paradigm, has been proposed for the regulation of the IL-6 promoter. Both CXCL8 and IL-6 promoters have binding sequences for NF-κB, C/EBP and the TATA BOX. There is competition between NF-κB and an inhibitory factor, RBP-Jκ for the NF-κB promotor element. Promoter activation relies on the p65 NF-κB subunit, which recruits CREB-binding protein (CBP/p300) to the site. The binding of CBP/p300, which has histone acetylase (HAT) activity, will stabilize transcription from these promoters. In an independent study, CCAAT displacement protein (CDP) has been shown to interact physically with CBP/p300 and is a target for acetylation at specific residues near the homeodomain. So, CDP and CBP seem to have antagonistic roles in the regulation of IL-6 and CXCL8 transcription. The transcriptional repression of CDP is probably associated with its ability to recruit the histone-deacetylase activity of HDAC1. HDAC1 deacetylates the histones and, as a result, the chromatin structure is closed and unavailable for transcription, with the end result being the consequent silencing of the chromatin. We propose that similar interactions between NF-κB, CBP and CDP might be involved in the transcription of CXCL1. c/EBP, CCAAT, enhancer-binding protein; CRE, cyclic AMP response element; IRE, interferon response element; IUR, immediate upstream region; PARP, poly-ADP ribose polymerase; RBP-Jκ, recombination signal binding protein-Jκ.

Figure 3. Model of potential components involved in the constitutive activation of NF-κB and enhanced expression of CXCL1 and CXCL8 in melanoma

We postulate that the constitutive activation of nuclear factor-κB (NF-κB) is modulated by receptor-mediated signals, such as those mediated through chemokine receptors and the LTβ receptor, which result in the persistent activation of NF-κB-inducing kinase (NIK), AKT and, potentially, mitogen-activated protein (MAP) kinase kinase kinase 1 (MEKK1). This activation of the inhibitor of NF-κB (IκB) kinase (IKK; a complex of IKKα, IKKβ and NEMO) leads to the phosphorylation and degradation of IκB and the phosphorylation of RELA/p65, which can, in turn, lead to constitutive expression of chemokines. We postulate that chemokine expression can be blocked by treatment with non-steroidal anti-inflammatory drugs (NSAIDs), NEMO-binding peptide and PS-341, which would target tumour cells for apoptosis. CBP, CREB-binding protein; CDP, CCAAT displacement protein; HAT, histone acetyltransferase; HDAC1, histone deacetylase 1; IUR, immediate upstream region; LTβ, lymphotoxin-β; LTβR, lymphotoxin-β receptor; NEMO, NF-κB essential modulator; PARP, poly-ADP ribose polymerase; PI3K, phosphatidylinositol 3-kinase; PtdInsP₃, phosphatidylinositol-3,4,5-trisphosphate; PTEN, phosphatase and tensin homologue; TRAF, tumour-necrosis-factor-receptor-associated factor.