cells
Review
Roles of NF-κB in Cancer and Inflammatory Diseases
and Their Therapeutic Approaches
Mi Hee Park and Jin Tae Hong *
College of Pharmacy and Medical Research Center, Chungbuk National University, 194-31,
Osongsaengmyeong 1-ro, Osong-eup, Cheongwon-gun, Chungbuk 28160, Korea; pmh5205@hanmail.net
* Correspondence: jinthong@chungbuk.ac.kr; Tel.: +82-43-261-2813; Fax: +82-43-268-2732
Academic Editor: Ruaidhri Carmody
Received: 22 February 2016; Accepted: 24 March 2016; Published: 29 March 2016
Abstract:
Nuclear factor-
κ
B (NF-
κ
B) is a transcription factor that plays a crucial role in various
biological processes, including immune response, inflammation, cell growth and survival, and
development. NF-
κ
B is critical for human health, and aberrant NF-
κ
B activation contributes
to development of various autoimmune, inflammatory and malignant disorders including
rheumatoid arthritis, atherosclerosis, inflammatory bowel diseases, multiple sclerosis and malignant
tumors. Thus, inhibiting NF-
κ
B signaling has potential therapeutic applications in cancer and
inflammatory diseases.
Keywords:
NF-
κ
B; canonical pathway; non-canonical pathway; cancer; inflammatory disease;
therapeutic approaches
1. Introduction
The nuclear factor-
κ
B (NF-
κ
B) family of transcription factors control the expression of genes
involved in many critical physiological responses such as inflammatory responses, proliferation,
differentiation, cell adhesion and apoptosis [
1
]. NF-
κ
B transcription complexes have a variety of
homo- and heterodimers consisting of the subunits p50, p52, c-Rel, RelA (p65) and RelB [
2
]. NF-
κ
B
signaling pathways can be divided into canonical and noncanonical pathways. In the canonical
pathway, I kappa B kinase (IKK) phosphorylates I
κ
B
α
at two N-terminal serines, triggering its
ubiquitination and proteasomal degradation; this leads to the nuclear translocation of NF-
κ
B
complexes, predominantly p50/RelA and p50/c-Rel dimers [
3
]. The noncanonical NF-
κ
B pathway
involves different signaling molecules and leads to the activation of the p52/RelB dimer [4].
NF-
κ
B is able to induce several of these cellular alterations and has been shown to be constitutively
activated in some types of cancer cells. Constitutively activated NF-
κ
B transcription factors have
been associated with several aspects of tumorigenesis, including promoting cancer-cell proliferation,
preventing apoptosis, and increasing a tumor
'
s angiogenic and metastatic potential. Activation of the
NF-
κ
B/Rel by nuclear translocation plays a role in inflammation through induction of transcription
of several proinflammatory genes [
5
]. Recent data indicate that activation of IKK-
β
, rather than
IKK-
α
, participates in the primary pathway of proinflammatory genes [
6
]. IKK-
β
is expressed
in fibroblast-like synoviocytes and plays a central role in TNF-
α
–mediated NF-
κ
B activation and
expression of proinflammatory genes [
7
]. IKK-
β
also activates NF-
κ
B and inflammatory gene
transcription in monocytes and CD4+ T lymphocytes [
7
]. Many natural products and drugs that have
been involved in anti-cancer and anti-inflammatory activity have also been shown to inhibit NF-κB.
This review provides the signaling mechanisms and biological functions of the NF-
κ
B pathway,
and the role of NF-
κ
B in cancer and inflammatory diseases, and the multitude of NF-
κ
B inhibitors that
have been reported.
Cells 2016, 5, 15; doi:10.3390/cells5020015 www.mdpi.com/journal/cells
Cells 2016, 5, 15 2 of 13
2. NF-κB
Nuclear factor-
κ
B (NF-
κ
B) is a transcription factor that plays a crucial role in various biological
processes, including immune response, inflammation, cell growth and survival, and development [
1
,
8
].
NF-
κ
B is activated by various inflammatory stimuli such as growth factors and infectious microbes.
NF-
κ
B controls expression of a number of genes that regulate immune responses, cell growth and
proliferation, survival and apoptosis, stress responses and embryogenesis and development of a
variety of stimuli [
7
,
9
]. NF-
κ
B is critical for human health, and aberrant NF-
κ
B activation contributes
to development of various autoimmune, inflammatory and malignant disorders including rheumatoid
arthritis, atherosclerosis, inflammatory bowel diseases, multiple sclerosis and malignant tumors [
10
,
11
].
2.1. NF-κB Subunits
The mammalian NF-
κ
B family is composed of five members, including RelA (p65), RelB, c-Rel,
NF-
κ
B1 p50, and NF-
κ
B2 p52, which form various dimeric complexes that transactivate numerous
target genes via binding to the
κ
B enhancer [
2
]. The NF-
κ
B proteins are normally sequestered in the
cytoplasm by a family of inhibitors, including I
κ
B
α
and other ankyrin repeat-containing proteins [
6
,
12
].
Proteasome-mediated processing of p105 and p100 produces the mature NF-
κ
B1 and NF-
κ
B2 proteins
(p50 and p52) and results in disruption of the IκB-like function of these precursor proteins [13].
The NF-
κ
B transcription factor family in mammals consists of five proteins including p65
(RelA), RelB, c-Rel, p105/p50 (NF-
κ
B1), and p100/52 (NF-
κ
B2) that associate with each other to form
distinct transcriptionally active homo- and heterodimeric complexes [
13
,
14
]. Through combinatorial
associations, the Rel protein family members can form up to 15 different dimers. Among them,
the p50/65 heterodimer clearly represents the most abundant of Rel dimers, being found in almost
all cell types [
15
]. In addition, dimeric complexes of p65/p65, p65/c-Rel, p65/p52, c-Rel/c-Rel,
p52/c-Rel, p50/c-Rel, p50/p50, RelB/p50, and RelB/p52 have been described, some of them only in
limited subsets of cells [
10
–
12
]. NF-
κ
B family shares a Rel homology domain in their N-terminus. A
subfamily of NF-
κ
B proteins, including RelA (p65), RelB and c-Rel has a transactivation domain in
their C-termini [
16
,
17
]. After processing of p105 and p100 by the ubiquitin/proteasome pathway that
involves selective degradation of their C-terminal region containing ankyrin repeats, mature NF-
κ
B
subunits such as p50 and p52 are generated [
16
,
17
]. Actually, the p50 and p52 proteins have no intrinsic
ability to activate transcription and act as transcriptional repressors when binding
κ
B elements as
homodimers [18].
2.2. NF-κB Signaling Pathway
The NF-
κ
B dimers are sequestered in the cytoplasm by a family of inhibitors, called I
κ
Bs (Inhibitor
of
κ
B), which are proteins that contain multiple copies of a sequence called ankyrin repeats, in
unstimulated cells [
5
,
14
]. The I
κ
B proteins mask the nuclear localization signals (NLS) of NF-
κ
B
proteins and keep them sequestered in an inactive state in the cytoplasm by virtue of their ankyrin
repeat domains [
11
,
16
]. Because the presence of ankyrin repeats in their C-terminal halves, p105
and p100 also function as I
κ
B proteins. The C-terminal half of p100, that is often referred to as I
κ
B
δ
,
also functions as an inhibitor [
19
]. I
κ
B
δ
degradation in response to developmental stimuli, such as
those transduced through LT
β
R, potentiate NF-
κ
B dimer activation in a NIK dependent non-canonical
pathway [20].
2.2.1. Canonical Pathway
Canonical NF-
κ
B pathway of NF-
κ
B is activated after degradation of I
κ
B
α
, which results in nuclear
translocation of various NF-
κ
B complexes, predominantly the p50/p65 dimer [
3
] (Figure 1). The
degradation of I
κ
B
α
is mediated by phosphorylation through the I
κ
B kinase (IKK), a trimeric complex
composed of two catalytic subunits, IKK
α
and IKK
β
, and a regulatory subunit, IKK
γ
(also termed
NF-
κ
B essential modulator or NEMO) [
21
]. When activated by signals, the I
κ
B kinase phosphorylates
Cells 2016, 5, 15 3 of 13
two serine residues located in an I
κ
B regulatory domain [
19
,
20
]. When phosphorylated on these serines
(e.g., serines 32 and 36 in human I
κ
B
α
), the I
κ
B inhibitor molecules are processed by ubiquitination,
which then leads them to be degraded by a cell structure called the proteasome [
22
,
23
] . With the
degradation of I
κ
B, the NF-
κ
B complex then enters into the nucleus where it can
'
turn on
'
the expression
of several genes that have DNA-binding sites for NF-
κ
B [
22
,
23
]. The activation of these genes by
NF-
κ
B then leads to the given physiological response, for example, an inflammatory or immune
response, a cell survival response, or cellular proliferation [
24
]. NF-
κ
B turns on expression of its own
repressor, I
κ
B
α
. The newly synthesized I
κ
B
α
then re-inhibits NF-
κ
B and forms an auto feedback loop,
which results in oscillating levels of NF-
κ
B activity [
22
,
23
]. Genetic evidence suggests that this NF-
κ
B
pathway regulates important biological functions, such as lymphoid organogenesis, B-cell survival
and maturation, dendritic cell activation, and bone metabolism.
κ
κ α κ
κ κ
κ
κ
κ
κ α κ α κ
κ
κ
κ
β
β κ κ
κ κ
α
κ
κ
κ κ
κ α
κ
κ
Figure 1.
The canonical NF-
κ
B pathway (
left
) induced by signals including antigens, TLR ligands
and cytokines such as TNF uses a wide variety of signaling adaptors to engage and activate the
IKK
β
subunit of the IKK complex. IKK
β
phosphorylation of I
κ
B proteins bound to NF-
κ
B dimers
results in ubiquitination (Ub) of I
κ
B and proteasome-induced degradation. This allows NF-
κ
B to enter
the nucleus and be involved in controlling the transcription of gene encoding functions as diverse as
inflammation, cell survival and cell division. The noncanonical pathway (
right
) engaged in by members
of the TNF-like family of cytokines requires NIK to activate IKK
α
, which then phosphorylates p100
(NF-
κ
B2), triggering its proteosomal processing needed for the activation of p52-RelB dimers. Among its
functions, this specific NF-
κ
B heterodimer controls gene expression crucial for lymphoid organogenesis.
2.2.2. Non-Canonical Pathway
The non-canonical NF-
κ
B pathway activates the RelB/p52 NF-
κ
B complex using a mechanism that
relies on the inducible processing of p100 instead of degradation of I
κ
B
α
(Figure 1). The processing of
p100 serves to both generate p52 and induce the nuclear translocation of the RelB/p52 heterodimer [
4
].
The discovery of non-canonical NF-
κ
B signaling pathway came from the study of p100 processing [
25
].
In contrast to the constitutive and co-translational processing of p105, the processing of p100 is tightly
regulated. In most cell types, p100 is the predominant product of NF-
κ
B2 [
26
,
27
]. Overexpressed
p100 is barely converted to p52 in mammalian cells, as opposed to the constitutive production of p50
from p105 [
25
]. However, p52 is actively generated in specific cell types, such as B cells, leading to
Cells 2016, 5, 15 4 of 13
the idea that p100 processing might be a signal-regulated event [
25
,
28
]. Indeed, the NF-
κ
B-inducing
kinase (NIK) induces p100 processing and is required for p100 processing in splenocytes. Moreover,
endogenous p100 processing can be activated by various receptor signals in an NIK-dependent
manner [29,30].
In this pathway, activation of the NF-
κ
B inducing kinase (NIK) led to the phosphorylation and
subsequent proteasomal processing of the NF-
κ
B2 precursor protein p100 into mature p52 subunits
in an IKK1/IKKa dependent manner [
31
]. Then, p52 dimerizes with RelB to appear as a nuclear
RelB/p52 DNA binding activity and regulate a distinct class of genes [
32
]. In contrast to the canonical
signaling that relies upon NF-kB essential modulator (NEMO)-IKK2 mediated degradation of I
κ
B
α
, -
β
,
-
ε
, the non-canonical signaling critically depends on NIK mediated processing of p100 into p52 [
30
,
31
].
Recent studies showed that synthesis of the constituents of the non-canonical pathway, RelB and
p52, is controlled by the canonical IKK2-I
κ
B-RelA: p50 signaling [
30
,
31
]. These studies suggest that
an integrated NF-
κ
B system network underlies activation of both RelA and RelB containing dimer
and that a malfunctioning canonical pathway will lead to an aberrant cellular response also through
the non-canonical pathway [
30
,
31
]. Deregulated non-canonical NF-
κ
B signaling is associated with
lymphoid malignancies [
28
,
33
]. Cell-differentiating or developmental stimuli such as B-Cell activation
factor (BAFF), receptor activator of nuclear factor kappa-B ligand (RANKL) or lymphotoxin-
α
, activate
the non-canonical NF-κB pathway [34].
3. Role of NF-κB in Diseases
NF-
κ
B activation affects hallmarks of cancer and inflammatory diseases through the transcription
of genes involved in cell proliferation, survival, angiogenesis, inflammation and tumor promotion and
metastasis as shown in Figure 2.
κ
κ
κ
κ α
β ε
κ
κ
κ
α κ
κ
κ
κ
Figure 2.
NF-
κ
B activation affects hallmarks of cancer and inflammatory diseases through the
transcription of genes involved in cell proliferation, survival, angiogenesis, inflammation and tumor
promotion and metastasis. BCL2, B-cell lymphoma protein 2; BCL-XL, also known as BCL2-like 1; BFL1,
also known as BCL2A1; CDK2, cyclin-dependent kinase 2; COX2, cyclooxygenase 2; CXCL, chemokine
(C-X-C motif) ligand; DR, death receptor; ELAM1, endothelial adhesion molecule 1; FLIP, also
known as CASP8; GADD45beta, growth arrest and DNA-damage-inducible protein beta; HIF1alpha,
hypoxia-inducible factor 1alpha; ICAM1, intracellular adhesion molecule 1; IEX-1L, radiation-inducible
immediate early gene (also known as IER3); IL, interleukin; iNOS, inducible nitric oxide synthase;
KAL1, Kallmann syndrome 1 sequence; MCP1, monocyte chemoattractant protein 1 (also known
as CCL2); MIP2, macrophage inflammatory protein 2; MMP, matrix metalloproteinase; MnSOD,
manganese superoxide dismutase (also known as SOD2); TNF, tumour necrosis factor; TRAF, TNF
receptor-associated factor; uPA, urokinase plasminogen activator; VCAM1, vascular cell adhesion
molecule 1; VEGF, vascular endothelial growth factor; XIAP, X-linked inhibitor of apoptosis protein.
Cells 2016, 5, 15 5 of 13
3.1. NF-κB and Cancer
NF-
κ
B regulates the genes that control cell proliferation and cell survival [
35
]. Many different
types of human tumors have misregulated NF-
κ
B; that is, NF-
κ
B is constitutively active. Active
NF-
κ
B turns on the expression of genes that keeps the cell proliferating and protects the cell from
conditions that would otherwise cause it to die via apoptosis [
36
]. Cancer-associated chromosomal
translocations, deletions and mutations might also disrupt genes that encode NF-
κ
B and I
κ
B proteins,
uncoupling NF-
κ
B factors from their regulators and causing constitutive NF-
κ
B activation [
37
].
Constitutively activated NF-
κ
B transcription factors have been associated with several aspects of
tumorigenesis, including promoting cancer-cell proliferation, preventing apoptosis, and increasing a
tumor
'
s angiogenic and metastatic potential [
37
,
38
]. In tumor cells, NF-
κ
B is consequently activated
because mutations in genes encoding the NF-
κ
B transcription factors themselves or in genes that
control NF-
κ
B activity. In addition, some tumor cells secrete factors that cause NF-
κ
B to become
active [
39
,
40
]. Blocking NF-
κ
B can cause tumor cells to stop proliferating, to die, or to become more
sensitive to the action of anti-tumor agents [
41
]. NF-
κ
B stimulates the transcription of genes that
encode G1 cyclins [
1
,
40
]. A
κ
B site is present within the cyclin D1 promoter and there is strong
evidence that NF-
κ
B dependent cyclin D1 induction drives the proliferation of mammary epithelial
cells during pregnancy [
42
,
43
]. NF-
κ
B is also an inhibitor of programmed cell death [
44
,
45
]. This factor
activates the transcription of several target genes that block the induction of apoptosis by TNF-
α
and
other pro-apoptotic members of this family [
22
]. The anti-apoptotic factors that are induced by NF-
κ
B
include cellular inhibitors of apoptosis (cIAPs), caspase-8/FADD (FAS-associated death domain)-like
IL-1beta-converting enzyme (FLICE) inhibitory protein (c-FLIP) and members of the BCL2 family
(such as A1/BFL1 and BCL-XL) [
22
]. Cells with elevated NF-
κ
B activity deregulate production of
chemokines, which increases migratory activity [
1
]. At least one NF-
κ
B-regulated chemokine, IL-8, has
been shown to promote angiogenesis [
46
]. In addition,
κ
B sites were identified in the promoters of
genes that encode several matrix metalloproteinases (MMPs) that are proteolytic enzymes involved
in promoting tumor invasion of surrounding tissue [
47
]. NF-
κ
B contributes to extracellular matrix
destruction by cancer cells [
48
,
49
]. NF-
κ
B has also been shown to be involved in the development of
carcinomas—cancers of epithelial origin, such as breast cancer [
50
]. Several studies have documented
elevated or constitutive NF-
κ
B DNA-binding activity both in mammary carcinoma cell lines and
primary breast cancer cells [51,52].
In inflammatory cells, continuous NF-
κ
B activity could promote the production of reactive oxygen
species, thereby damaging DNA of surrounding epithelial cells [
53
]. Some of the best circumstantial
evidence that supports such a role for NF-
κ
B comes from various gastrointestinal cancers [
54
]. NF-
κ
B
activation is also associated with colorectal cancer. Colon cancer cell lines, human tumor samples, and
stromal macrophages in sporadic adenomatous polyps also have increased NF-
κ
B activity [
55
]. It has
been shown that canonical NF-
κ
B is a Fas transcription activator and the alternative NF-
κ
B is a Fas
transcription repressor [56].
3.2. NF-κB and Inflammatory Disease
NF-
κ
B is a major transcription factor that regulates genes responsible for both the innate and
adaptive immune response [
57
]. After activation of T- or B-cell receptors, NF-
κ
B is activated through
distinct signaling [
58
]. Upon ligation of the T-cell receptor, protein kinase Lck is recruited and
phosphorylates the immunoreceptor tyrosine-based activation motifs (ITAMs) of the CD3 cytoplasmic
tail [
59
]. ZAP70 is then recruited to the phosphorylated ITAMs and helps recruit Linker-for-activation
of T cells (LAT) and Phospholipase C (PLC)-
γ
, which causes activation of Protein kinase C (PKC) [
60
].
Through a cascade of phosphorylation, the kinase complex is activated and NF-
κ
B enter the nucleus
to upregulate genes involved in T-cell proliferation, maturation and development [
61
]. NF-
κ
B
is chronically activated in many inflammatory diseases, including inflammatory bowel disease,
arthritis, sepsis, gastritis, asthma, atherosclerosis and others [
62
]. It is important to note, though,
that elevation of some NF-κB activators, such as osteoprotegerin (OPG), are associated with elevated