**1.1 Basic nuclear factor-κB (NF-κB) family and signaling pathways**

So what is NF-κB? In mammals, NF-κB is a collective term for a small family of dimeric transcription factors [comprising p65 (RelA) and RelB, c-Rel, p50/p105 (NF-κB1), and p52/p100 (NF-κB2)]. All NF-κB proteins share a Rel homology domain (RHD), which is responsible for DNA binding and dimerization. Only p65, RelB, and c-Rel contain potent transactivation domains within sequences from C-terminal to the RHD. Therefore, p50 and p52 cannot act as transcriptional activators by themselves. Dimers of these two proteins have been reported to repress NF-κB-dependent transcription *in vivo*, most likely by competing with other transcriptionally active dimers. These proteins form homo- and heterodimers, and their activity is regulated by the canonical or alternative pathways described as following [1]. A simple diagram of canonical NF-κB signaling, which will be the focus of this chapter, is shown in **Figure 1**. The canonical pathway is activated by multiple stimuli, including proinflammatory cytokines (e.g., tumor necrosis factor, TNF; interleukin 1, IL-1), and the components of the bacterial wall (Lipopolysaccharide, LPS). Exterior signals lead to the phosphorylation and degradation of the inhibitory complex IκB, which is modulated by the IκB kinase (IKK), and its degradation allows for the release of the typical NF-κB

heterodimer, p65/p50, to translocate into the nucleus. NF-κB binds to its cognate DNA elements and can transcriptionally activate different target genes among which 200–500 genes have been implicated in cell survival/apoptosis, cell growth, immune response, and inflammation [2].

The alternative or noncanonical pathway is activated by the members of the TNF cytokine family, such as B-cell activating factor (BAFF), cluster of differentiation 40 ligand (CD40L), receptor activator of nuclear factor-κB ligand (RANKL), and lymphotoxin-β2 (LTβ2), and requires recruitment of the p52/RelB dimers to activate transcription. Firstly, activation of NIK (NF-κB-inducing kinase) leads to

#### **Figure 1.**

*Pathway of canonical and non-canonical NF-κB signaling. Under the canonical pathway of NF-κB signaling, activation of the NF-κB is initiated by a stimulus resulting in phosphorylation and subsequent proteasomal degradation of IκBα. This allows the release of the p65/p50 heterodimer into the nucleus, where they can bind to their cognate DNA elements and promote NF-κB target gene expression [1]. On the other hand, under noncanonical activation of NF-κB, NIK (NF-κB-inducing kinase) leads to activation of IKKα in this pathway. Subsequent phosphorylation of the NF-κB1 precursor molecule, p100, triggers partial proteolysis giving rise to p52, which preferentially dimerizes with RelB. This allows translocation of p52/RelB to the nucleus where they can bind to cognate DNA elements and promote gene transcription [1]. Figure adapted and simplified from Hoesel et al. [103].*

**49**

*Phosphorylation of NF-κB in Cancer*

*DOI: http://dx.doi.org/10.5772/intechopen.83650*

promote gene transcription (**Figure 1**).

**1.2 Important role of NF-κB in cancer**

in affected patients [2].

both solid tumors and hematological malignancies.

activation of IKKα in this pathway. This event leads to the subsequent phosphorylation of the NF-κB1 precursor molecule, p100, and triggers partial proteolysis to give rise to p52, which preferentially dimerizes with RelB [1]. The p52/RelB heterodimer then translocates to the nucleus where they can bind to cognate DNA elements and

NF-κB was first discovered by Dr. Ranjan Sen in 1986 [3]. This family of transcription factors plays important roles in the regulation of apoptosis, proliferation, inflammation, and immune response in both normal and cancer cells. Generally, in normal cells, the central transcription factor NF-κB is transiently activated in response to certain stimuli. However, cancer cells usually exhibit sustained activation of NF-κB [4, 5] which significantly contributes to their survival. Moreover, NF-κB activity plays critical roles in many of the well-known "hallmarks" of cancer, via its regulation of target genes involved in tumor cell proliferation, suppression of apoptosis, activation of angiogenesis as well as induction of the epithelial-to-mesenchymal transition (EMT) phenotype, a critical step in metastasis [6]. Constitutively active NF-κB has been found in many types of cancer. For instance, in thyroid cancer, oncogenic proteins including "rearranged during transfection" (RET), "Rat Sarcoma" (RAS), and "v-Raf murine sarcoma viral oncogene homolog B" (BRAF) were shown to induce NF-κB activation, which in turn activated proliferative and antiapoptotic signaling pathways [7]. Moreover, in renal cell carcinoma (RCC), NF-κB is constitutively activated. The phosphorylated p65, a major subunit of NF-κB, exhibited a significant increase in the RCC samples compared with corresponding normal tissues [8]. Furthermore, Nogueira et al. showed that in glioblastoma (GBM), deletion of IκB showed a phenotype similar to that of epidermal growth factor receptor (EGFR) amplification in the pathogenesis of GBM. This was also correlated with low survival rates

Importantly, our laboratory also found that in colon cancer, NF-κB can be activated by the Y-box protein 1 (YBX1), a critical event correlated with increased colon cancer cell proliferation and anchorage-independent growth [9, 10]. Additionally, in pancreatic cancer, a mutant oncogenic KrasG12D (glycine to aspartic acid) mutation induced positive feedback loops of interleukin 1α (IL-1α) and p62 expression to sustain constitutive IKKβ (inhibitor of NF-κB kinase subunit β)/NF-κB activation [11]. In breast cancer, moderately elevated NF-κB led to chronic inflammatory conditions that result in some cells escaping immune surveillance [12]. Additionally, NF-κB activation was shown to upregulate the expression of cyclin D1, cyclin-dependent kinase 2 (CDK2), and c-Myc, which drives cell cycle progression and causes uncontrolled cell proliferation [13]. Moreover, in breast cancer, NF-κB has been shown to induce and stabilize the expression of EMT markers (Snail and twist-related protein1) [14], a pivotal process in tumor metastasis. In addition to the role of NF-κB in solid tumors as described thus far, Gasparini et al. has thoroughly reviewed the important tumorgenic role of NF-κB in hematological malignancies, including acute lymphocytic lymphoma (ALL), acute myeloid leukemia (AML), chronic lymphocytic leukemia (CLL), chromic myeloid leukemia (CML), B lymphomas, diffuse large B-cell lymphomas (DLBCLs), Hodgkin's lymphoma, adult T-cell lymphomas (ATLL), anaplastic large-cell lymphomas (ALCL), and multiple myeloma, which will not be further discussed in this chapter [15]. Overall, these studies highlight a prominent role of dysregulated NF-κB in multiple aspects of cancer progression in

### *Phosphorylation of NF-κB in Cancer DOI: http://dx.doi.org/10.5772/intechopen.83650*

*Adenosine Triphosphate in Health and Disease*

immune response, and inflammation [2].

heterodimer, p65/p50, to translocate into the nucleus. NF-κB binds to its cognate DNA elements and can transcriptionally activate different target genes among which 200–500 genes have been implicated in cell survival/apoptosis, cell growth,

The alternative or noncanonical pathway is activated by the members of the TNF cytokine family, such as B-cell activating factor (BAFF), cluster of differentiation 40 ligand (CD40L), receptor activator of nuclear factor-κB ligand (RANKL), and lymphotoxin-β2 (LTβ2), and requires recruitment of the p52/RelB dimers to activate transcription. Firstly, activation of NIK (NF-κB-inducing kinase) leads to

*Pathway of canonical and non-canonical NF-κB signaling. Under the canonical pathway of NF-κB signaling, activation of the NF-κB is initiated by a stimulus resulting in phosphorylation and subsequent proteasomal degradation of IκBα. This allows the release of the p65/p50 heterodimer into the nucleus, where they can bind to their cognate DNA elements and promote NF-κB target gene expression [1]. On the other hand, under noncanonical activation of NF-κB, NIK (NF-κB-inducing kinase) leads to activation of IKKα in this pathway. Subsequent phosphorylation of the NF-κB1 precursor molecule, p100, triggers partial proteolysis giving rise to p52, which preferentially dimerizes with RelB. This allows translocation of p52/RelB to the nucleus where they can bind to cognate DNA elements and promote gene transcription [1]. Figure adapted and simplified from* 

**48**

**Figure 1.**

*Hoesel et al. [103].*

activation of IKKα in this pathway. This event leads to the subsequent phosphorylation of the NF-κB1 precursor molecule, p100, and triggers partial proteolysis to give rise to p52, which preferentially dimerizes with RelB [1]. The p52/RelB heterodimer then translocates to the nucleus where they can bind to cognate DNA elements and promote gene transcription (**Figure 1**).
