International Journal of
Molecular Sciences
Review
Phytochemicals against TNFα-Mediated
Neuroinflammatory Diseases
Lalita Subedi 1,† , Si Eun Lee 1,† , Syeda Madiha 2 , Bhakta Prasad Gaire 1 , Mirim Jin 3 ,
Silvia Yumnam 1, * and Sun Yeou Kim 1, *
1
2
3
*
†
College of Pharmacy, Gachon University, #191, Hambakmoero, Yeonsu-gu, Incheon 21936, Korea;
subedilali@gmail.com (L.S.); dltldms90@nate.com (S.E.L.); samarpanbp@gmail.com (B.P.G.)
Neurochemistry and Biochemical Neuropharmacology Research Unit, Department of Biochemistry,
University of Karachi, Karachi-75270, Pakistan; syedamadiha2010@live.com
College of Medicine and Department of Health Science and Technology, GAIHST, Gachon University #155,
Gaebeol-ro, Yeonsu-gu, Incheon 21999, Korea; mirimj@gachon.ac.kr
Correspondence: silviayumnam@gmail.com (S.Y.); sunnykim@gachon.ac.kr (S.Y.K.);
Tel.: +82-32-820-4931 (S.Y. & S.Y.K.); Fax: +82-32-820-4932 (S.Y. & S.Y.K.)
These authors contributed equally to this study.
Received: 5 December 2019; Accepted: 21 January 2020; Published: 24 January 2020
Abstract: Tumor necrosis factor-alpha (TNF-α) is a well-known pro-inflammatory cytokine responsible
for the modulation of the immune system. TNF-α plays a critical role in almost every type of
inflammatory disorder, including central nervous system (CNS) diseases. Although TNF-α is a
well-studied component of inflammatory responses, its functioning in diverse cell types is still unclear.
TNF-α functions through its two main receptors: tumor necrosis factor receptor 1 and 2 (TNFR1,
TNFR2), also known as p55 and p75, respectively. Normally, the functions of soluble TNF-α-induced
TNFR1 activation are reported to be pro-inflammatory and apoptotic. While TNF-α mediated
TNFR2 activation has a dual role. Several synthetic drugs used as inhibitors of TNF-α for diverse
inflammatory diseases possess serious adverse effects, which make patients and researchers turn their
focus toward natural medicines, phytochemicals in particular. Phytochemicals targeting TNF-α can
significantly improve disease conditions involving TNF-α with fewer side effects. Here, we reviewed
known TNF-α inhibitors, as well as lately studied phytochemicals, with a role in inhibiting TNF-α
itself, and TNF-α-mediated signaling in inflammatory diseases focusing mainly on CNS disorders.
Keywords: TNF-α; TNFR1; TNFR2; neuroinflammation; neurodegeneration; phytochemicals
1. Introduction
Cytokines are involved in autocrine, paracrine, and endocrine functions, and are either
pro-inflammatory or anti-inflammatory in nature. Pro-inflammatory cytokines are involved in
both the induction and progression of inflammatory reactions or pathologies, including inflammation,
pain, and cancer [1]. Among such inflammatory cytokines, tumor necrosis factor-alpha (TNF-α) is a
well-known pro-inflammatory cytokine responsible for the modulation of the immune system. TNF-α
is synthesized as a type II transmembrane protein made up of 233 amino acids, which then undergo
proteolytic cleavage through TNF-α converting enzyme (TACE) to form active TNF-α [2]. Various
human cells are capable of producing TNF-α, especially macrophages and monocytes. Enhanced
TNF-α production can alleviate cellular signaling that can cause cells to undergo necrosis or apoptosis.
Despite roles in defense mechanism against inflammatory conditions, TNF-α is well characterized
as a pathogenic mediator in diverse inflammatory diseases, including Alzheimer’s disease (AD),
Parkinson’s disease (PD), stroke, psoriasis, arthritis, septic shock, and pulmonary disorders [3–5].
Int. J. Mol. Sci. 2020, 21, 764; doi:10.3390/ijms21030764
www.mdpi.com/journal/ijms
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The biological functions of TNF-α are mediated through its two main receptors: tumor
necrosis factor receptor 1 (TNFR1) (p55) and tumor necrosis factor receptor 2 (TNFR2) (p75) [6].
Activation of TNFR1 is known to initiate inflammatory, apoptotic, and degenerative cascades,
whereas TNF-α signaling through TNFR2 is anti-inflammatory and cytoprotective, resulting in
the induction of proliferation, differentiation, angiogenesis, and tissue repair [7,8]. Soluble TNF-α and
transmembrane TNF-α are the two main forms of TNF-α, and each has a distinct role and signaling
pattern. Soluble TNF-α is usually considered to have inflammatory effects through binding to its
TNFR1 receptor, whereas transmembrane TNF-α preferentially binds to TNFR2 receptors and exerts
anti-inflammatory effects. Binding of TNF-α to TNFR1 can initiate cell apoptosis through activation
of mitogen activated protein kinase (MAPK), caspases, and transcription through NF-κB (Nuclear
factor kappa-light-chain-enhancer of activated B cells) signaling, which is responsible for cell death
and pro-inflammatory conditions [9]. These receptors, as well as TNF-α, are well expressed/present
in brain tissues [10]. TNFR1 mediated signaling induces the activation of PI3K (Phosphoinositide
3-kinase) signaling, which further activates caspase 8/3 and BH3 interacting-domain death agonist
(BID). This is followed by the induction of oxidative stress, necrosis, and apoptosis [11,12], which
are the main causes of neurodegeneration. TNFR2 activation by TNF-α is reported to increase cell
survival as well as re-myelination of degenerated neurons in multiple sclerosis (MS) lesions, supporting
the hypothesis that TNFR2 plays an opposite and beneficial role to that of TNFR1 in animal and
human physiology [13]. When TNF-α binds to TNFR2, it can activate CXC motif chemokine 12
(CXCL12)/CXC chemokine receptor type 4 (CXCR4), responsible for the proliferation, differentiation,
and re-myelination of the demyelinated neurons in MS lesions [14]. TNFR2-mediated activation of
PI3K/AKT (Protein kinase B) and vascular endothelial growth factor receptor 2 (VEGFR2) is responsible
for angiogenesis [15], and NADPH oxidase 4 (Nox4)/reactive oxygen species (ROS)-mediated heme
oxygenase 2 (HO-2)/CO production, which is controlled by TNFR2, can induce cell survival during
various injuries or insults in organs [16]. TNF-α/TNFR2 is also able to activate PI3K/VEGFR2 signaling,
and this pathway causes angiogenesis, as Nox4/ROS-mediated HO-2/CO activation is responsible
for cell survival and can inhibit neurodegeneration. Signaling via this pathway can also produce
anti-inflammatory effects [15,17].
Neuroinflammation is inflammation of the neurons where cells of the central nervous system
(CNS), such as neurons, macroglia, and microglia are involved. Microglia are the resident macrophage
of the CNS and determine the fate of other neural cells. Upon encountering an endogenous or exogenous
stimuli microglia are activated and initiates neuroinflammation by secreting various pro-inflammatory
cytokines, such as TNF-α, interleukin-1β (IL-1β), IL-6, IL-18, reactive oxygen, and nitrogen
species [18,19]. Neuroinflammation is an important feature of most of the neurodegenerative diseases,
including Alzheimer’s disease (AD), Parkinson’s disease (PD), multiple sclerosis (MS), Huntington’s
disease (HD), and amyotrophic lateral sclerosis (ALS). The pro-inflammatory cytokines, TNF-α, are an
important component of neuroinflammation, and play both homeostatic and pathophysiological roles
in CNS. In healthy CNS, TNF-α regulates many physiological processes, including sleep, learning
and memory, synaptic plasticity, and astrocyte-induced synaptic strengthening. [20,21]. While in
pathological conditions, microglia and astrocytes release large amounts of TNF-α, leading to the
release of neuroinflammatory cascades that are associated with several neurodegenerative disorders.
Miscontrol of inflammatory signaling has been involved in the pathogenesis of several neurological
disorders, including AD and PD. Increase of TNF-α, a central mediator of neuroinflammation, occurs
with the onset of early neurological diseases, which develop typical pathologies in age-related
neurological diseases [22]. Additionally, TNF-α and TNFR1 in neurodegenerative disorders also
contribute to amyloidogenesis [23]. Therefore, TNF-α is a promising candidate for future TNF-α
-based neuroinflammation therapy. This review summarizes the role of TNF-α in neuroinflammation
and discusses various phytochemicals that inhibit TNF-α and its neuroprotective mechanism against
neurodegenerative diseases.
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2. TNF-α Signaling in Neuroinflammation
In neuroinflammatory disorders, secretion of large amounts of TNF-α from microglia is mostly
responsible for conditions such as neuroinflammation and excitotoxicity [24]. Astrocytes and neurons
can also express TNF-α receptors and secrete TNF-α, which can trigger inflammatory cascades following
neurological disorders. However, microglia cells are a major source of TNF-α in the brain compared
to other cells [21]. Moreover, neuroinflammatory disease as well as neurodegenerative disorders are
characterized by extensively elevated levels of pro-inflammatory cytokines, including TNF-α, IL-1β,
IL-6, and IL-18. These overall cascades begin by TNF-α binding and are summarized in Figure 1.
Moreover, increased levels of TNF-α, IL-1β, and nerve growth factor (NGF) were detected in the
inflamed paws of mice. Administration of an anti-TNF-α antibody significantly reduced the levels of
TNF-α and IL-1β, followed by a reduction in paw inflammation [25]. TNF-α binding with its cell surface
receptor upregulate mitogen activated protein kinase (MAPK) signaling. MAPK signaling includes
p38, extracellular-signal-regulated kinases (ERKs), and cJun NH2-terminal kinases (JNKs). MAPK
signaling activation leads upregulation of the production of pro-inflammatory cytokines, such as IL6,
IL-1β, and TNF-α as a secondary response. TNF-α increased in this way is responsible for the biological
activity [26]. TNF-α-mediated stress stimuli induce activation of ERK, p38, and JNK, non-specifically.
Stress-activated MAP kinase signaling, such as p38 and JNK are dramatically upregulated following
TNF-α treatment pathways [27]. This mechanism of JNK and p38 MAPK pathway activation has
been associated with sustained TNFα signaling during the cell death response [26]. On the other
hand, activation of the JNK through any stress stimuli can actively participate in the macrophage
activation towards the inflammatory M1 phenotype via increased TNF-α. Hence, JNK activation
is believed to be involved in the secretion of the TNF-α and the neuroinflammatory cascades [28].
In addition, TNF-α-mediated cytokine production, followed by intracellular adhesion molecule 1
(ICAM-1), vascular cell adhesion protein 1(VCAM-1), E-selectin, and P-selectin expression, can cause
cell infiltration and inflammation [29]. Increased levels of TNF-α in the blood activates matrix
metalloproteinase 9 (MMP-9), which causes blood-brain barrier (BBB) disruptions and induces related
neurological disorders [30].
Figure 1. Receptor-mediated endogenous signaling pathways of tumor necrosis factor-alpha (TNF-α).
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Furthermore, induction of insulin resistance is an unwanted condition, both in terms of neurological
and diabetic complications. TNF-α is capable of inducing such conditions via JNK/protein kinase
C-related kinase (PKR)/IKKα (IkappaB kinase alpha) signaling-mediated disturbances in eukaryotic
initiation factor 2-alpha (eIF2α)/insulin receptor substrate 1 (IRS-1) insulin signaling, and causing
impaired synapses, as well as impaired behavioral control in the brain [31–33]. Activation of c-Fos
and c-Myc through TNF-α-activated phospho (p)-IκB-α/NF-κB is responsible for carcinogenicity [34].
Previous studies revealed that TNF-α mediates signaling via various pathways such as JNK/IKKB
(Inhibitor of nuclear factor kappa-B kinase subunit beta), which is responsible for AD pathology [35],
NF-κB/activator protein 1 (AP-1) for PD, TACE/SOD1 (Superoxide dismutase 1) for ALS, and caspases
for HD [36].
3. TNF-α in Neurodegenerative Disorders
TNF-α-mediated elevation in amyloid beta (Aβ) plaque burden and β-secretase 1 (BACE1)
expression is responsible for abnormal Aβ processing. This abnormal processing induces synaptic
loss followed by neuronal loss and neuronal cell death, ultimately causing dementia-characterizing
AD lesions [37]. Additionally, TNF-α-mediated transcytosis allows TNF-α to permeate easily through
the BBB, causing further BBB disruption and AD pathology [38]. Phase I and IIa clinical trials on
TNF-α inhibitors have revealed their role in controlling cognitive and memory decline in AD patients.
The supporting evidence clearly suggests the role of TNF-α in the pathology of AD, and inhibition of
TNF-α production or expression cannot only obstruct the disease pathology, but also prevent further
damage and severity [22].
PD is characterized by the loss of dopaminergic neurons, neuroinflammation, and toxicity-induced
neuronal death. Over-activated glial cells, such as microglia and astrocytes, are responsible for the
production of a number of inflammatory mediators or pro-inflammatory cytokines, including TNF-α,
which can induce neuronal death and conditions, under which continuous degeneration or death of
the dopaminergic neurons can occur [39]. TNF-α-mediated MAPK or NF-κB/activator protein 1 (AP-1)
signaling target the induction of apoptosis through various apoptotic pathways. In MD, activation of
JNK by increased TNF-α is responsible for apoptosis and myelin degeneration [40]. In addition, the
activation of TNFR2 by TNF-α may play a role in increasing angiogenesis, proliferation, differentiation,
and re-myelination of degenerative or demyelinated axons of the neurons through CXCL12/CXCR4 in
the CNS, especially in astrocytes [14].
TNF-α is considered to be one of the key pathogenic factors that participate in the initiation and
progression of the ALS pathogenesis. High levels of TNF-α and its soluble receptors TNFR1/TNFR2
were observed in the blood, cerebrospinal fluid, and nerve tissues of ALS patients and animal
models [41]. Thalidomide and lenalidomide, which are TNF-α inhibitors, had been tested for ALS;
however, they could not pass clinical trials [42]. Pathologically, mutation of SOD1 in ALS raises interest
to find its interconnection with ALS as a first target [43]. It was revealed that extracellular SOD1 is
not directly involved in the pathogenesis of ALS; however, it utilizes CD14 (Cluster of differentiation
14)-TLR2 pathway, which is triggered through the activated microglia-mediated release of TNF-α [44]
that subsequently aggravate motor neuron degeneration, a characteristic feature in ALS. Further,
activation of the ionotropic purinergic receptor P2X7 in SOD1G93A microglia trigger the production
of high levels of TNFα, which exert neurotoxic effects on the motor neuron [45], contributing to the
pathogenesis of ALS. In addition, high levels of TNFRs are observed in the spinal cords of mutant
SOD1 mice, which are associated with the activation of diverse signaling involving MKK3-6, MKK4,
ASK1 (Apoptosis signaling-regulating kinase 1), and phosphorylated p38 MAPK (p-p38) in motor
neurons and glial cells, which ultimately lead to the degeneration of motor neurons [41]. Altogether,
these previous reports indicate that SOD/TNFRs/ASK1/p38 MAPK signaling have actively participated
in spinal motor neuron degeneration associated with ALS. In addition, accumulation of SOD1-induced
iron in the ventral motor neuron influenced the enzymatic activity of TACE, resulting in excessive
TNF-α production [46]. TNF-α production-mediate glutamate excitotoxicity is also the prime cause for
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the motor neuron toxicity [41]. Collectively, TNF-α has a critical role in motor neuron degeneration,
the major pathogenesis in ALS. The overall roles of TNF-α in various neurological disorders are
summarized in Figure 2.
Figure 2. Involvement of TNF-α in various neurological disorders via action on different targets.
TNF-α also plays a dominant role during the inflammation in non-neuronal disorders. In case of
diseases, such as rheumatoid arthritis (RA), Crohn’s disease (CD), psoriasis, retinitis, multiple myeloma,
diabetes, obesity, human immunodeficiency virus (HIV-AIDS)-mediated inflammations, and cognitive
impairment, anti-TNF agents are the first drug of choice to lower the inflammation [47,48]. This suggests
that phytochemicals, having the potential to lower the secretion or inflammatory cascades of TNF-α,
could be a great alternative for the treatment of immune mediated inflammatory conditions. Targeting
TNF-α by natural lead compounds can be beneficial, not only for lowering neuroinflammation, but
non-neuronal inflammation as well.
4. Commercially Available TNF-α Inhibitors and Their Side Effects
Many commercialized TNF-α inhibitors are available, such as adalimumab, apratastat,
certolizumab, golimumab, infliximab, minocycline, thalidomide, GW333, BMS-561393, and etanercept
(listed in Table 1). These drugs have roles in treating many disease conditions, including neurological
disorders. However, the severe side effects of these inhibitors are the key reason for the withdrawal of
such medicines. Such drugs can cause significant and even life-threatening adverse effects and are
accordingly blacklisted. The three main negative effects of TNF-α inhibitors are as follows: (1) serious
infections such as erysipelas, abscess, and candidiasis, (2) neoplasms, including squamous cell
carcinoma and basal cell carcinoma, and (3) a very rare type of hepatosplenic T-cell lymphoma [49,50].
Apratastat is a well-known TACE and MMP inhibitor that is mostly prescribed for inflammatory joint
disorders, but its approval has been terminated after clinical trials, because of its lack of efficacy [51].
Certolizumab shows potency against CNS infection and CD, where it inhibits TNF-α binding and
hence inhibits TNF-α-mediated toxicity and inflammatory cascade activation. There are known side
effects, these include moderate pain, abdominal disorders, injection site reaction, skin rashes, and
urinary tract disorders, while serious infections, malignancies, and heart failure are severe side effects
that have also been reported [52,53]. Similarly, golimumab and infliximab are also very important
TNF-α inhibitors, which bind to soluble and transmembrane forms of TNF-α and block it from binding
to its receptors [54,55]. These effects are useful for the treatment of conditions such as RA, psoriasis,
psoriatic arthritis, ulcerative colitis, CD, and Wegener’s granulomatosis [54,55]. Besides such benefits,
treatment of many disorders with compounds such as adalimumab, certolizumab, golimumab, and
infliximab produces many side effects, including serious infections, such as a lower respiratory tract
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infection, skin infection, and tuberculosis [54], as shown in Table 1. Similarly, minocycline (a TNF-α
synthesis inhibitor) and thalidomide (a TNF-α degradation inducer) also show side effects. Minocycline
is normally prescribed for the treatment of ALS, MS, AD, stroke, traumatic brain injury (TBI), and
spinal cord injury, while thalidomide is used to treat multiple myeloma, CD, human immunodeficiency
virus (HIV), lupus, and leprosy. Their adverse effects are also well known in many treatment therapies.
Minocycline mediates vestibular side effects, as well as leukopenia and weight loss, which are more
specifically associated with females [56]. The adverse effects associated with thalidomide are more
serious: thalidomide toxicity includes deep vein thrombosis, teratogenicity, constipation, pyrexia,
fatigue, osteonecrosis of the jaw, pain, peripheral neuropathy (PN), and somnolence [57]. Recently,
morphea was observed as a new side effect of TNF-α inhibitors [58]. In addition, the drug interactions
of TNF-α inhibitors with methotrexate, anakinra, and abatacept [54,59] are another shortcoming of
the known TNF-α inhibitors [60]. The immunogenicity, autoimmune disorders, and congestive heart
failure caused by TNF-α inhibitors are also some negative effects associated with these drugs. Five
main TNF blockers (etanercept, infliximab, adalimumab, golimumab, and certolizumab) are approved
for clinical use, but the immune suppression and demyelination of the central and peripheral nervous
system by these candidates, while using not only against neuronal disorders, but also against systemic
inflammation, raise a big question for their safety profile and usability in the future, suggesting the
importance of phytochemicals having potential, similar to that of TNF-α inhibitors/blockers with lesser
side effects and better usability[61].
Table 1. Commercially available TNF-α inhibitors prescribed for several inflammatory disorders.
TNF-α
Inhibitor
Disease(s)
Toxicity/Side Effect
Mechanism of Action
Reference(s)
Adalimumab
CD, retinitis pigmentosa,
psoriatic arthritis
Increased risk of
infections, lymphoma
Blocks the effects of
TNF-α, reduces
oxidative stress
[62,63]
Apratastat
RA
Less effective, dropped
from clinical trial
TACE and MMP
inhibitor
[64]
Certolizumab
RA, CD
Meningococcal
meningoencephalitis,
palmoplantar pustulosis
Inhibits soluble TNF-α
binding
[52,53]
Golimumab
RA, ankylosing
spondylitis, psoriatic
arthritis
Bacterial and viral
infection, fungal
infection, tuberculosis
Prevents TNF-α binding
with TNFR1 and TNFR2
[54]
Infliximab
CD, psoriasis, cognitive
improvements, AD
Parkinsonism
Binds with high affinity
to soluble and
transmembrane forms of
TNF-α, progressive MS
[54,55]
Minocycline
ALS, MS, AD, stroke,
TBI, spinal cord injury
Dizziness, vertigo,
lightheadedness
TNF-α synthesis
inhibition
[56,65]
Thalidomide
Multiple myeloma, CD,
Behcet’s disease, HIV,
lupus, leprosy
Congenital
abnormalities, birth
defects, sensorimotor
peripheral neuropathy,
somnolence, rash,
fatigue, constipation
Increases TNF-α
degradation
[66,67]
GW3333
RA, other inflammation
NA
TACE and MMP
inhibitor
[64]
BMS-561392
RA, other inflammation
NA
Specific TACE inhibitor
[64]
Etanercept
Acute and chronic stroke,
post-stroke cognitive
impairment, chronic
brain injury
NA
Inhibits natural TNF-α,
TNF blockade
[68,69]
Abbreviations: CD, Chorn’s disease; RA, rheumatoid arthritis; AD, Alzheimer’s disease ALS, amytrophic lateral
sclerosis; MS, multiple sclerosis; NA, not applicable.
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5. Phytochemicals Inhibiting TNF-α for Lowering the Neuroinflammatory and
Neurodegenerative Disorders
TNF-α, being a key target that can modulate the pathology of multiple neurological disorders,
has gathered enormous attention in the recent past. However, the serious side effects of commercial
TNF-α inhibitors bring question to their efficiency, and there is increasing effort to screen TNF-α
inhibitors/blockers from natural products, phytochemicals, and nutraceuticals. Novel TNF-α
inhibitors/blockers may prove to be an alternative way of treating disorders in which TNF-α
is involved as a key player. Many phytochemicals, such as curcumin, shogaol, paradol, and
equol are known to have crucial roles in inhibiting TNF-α with lesser side effects [70–73]. Recent
findings have suggested that phytochemicals, such as allyl isothiocyanate (AITC), quercetin, and
kaempferol show potential to control neuronal disorders by inhibiting TNF-α production, as shown
in Table 2. In addition, 6-shogaol, gingerol, and their derivatives from Zingiber officinale, bear
great potential for limiting TNF-α activity, either by inhibiting binding or inducing degradation.
Apigenin, naringenin, and myricetin also inhibit inflammatory cascades in diverse inflammatory
disorders through significant inhibition of TNF-α expression. Moreover, we have identified many
phytochemicals that show anti-inflammatory activity either via the inhibition of TNF-α binding and
activity, or by direct inhibition. Most of these phytochemicals inhibit the production of TNF-α via
inhibition of the NF-κB mediated transcription regulated by MAPK or PI3K signaling. Butein and
hesperetin inhibit TNF-α secretion. Wogonin, morin, chrysin, eudesmin, mandolin and honokiol
inhibit TNF-α through regulation of JAK/STAT3 (Janus kinase/signal transducer and activators
of transcription) pathway or PI3K/Akt/MAPKs-NF-κB pathways, suggesting them to be possible
candidates against AD pathology. Ginsenoside compounds from Panax ginseng also inhibit TNF-α
significantly with subsequent inhibition of the inflammatory cascades, suggesting the phytochemicals
as potent anti-inflammatory candidates. On a similar note, phytochemicals, such as nicotine, berberine,
capsaicin, and kavalactone inhibit the TNF-α with concomitant amelioration of inflammatory and
oxidative stress, resulting in anti-inflammatory effects, making them good candidates for inhibition
of inflammation during AD and PD pathologies. Diallyl sulfide, present in Allium sativum, is also
reported to have a strong anti-inflammatory effect via downregulating production of pro-inflammatory
cytokines, such as TNF-α [74]. Similarly, curcumin, a major spice used in Asian countries, such
as India, Nepal, and Pakistan, is a well-reported anti-inflammatory agent, having the potential to
lower most pro-inflammatory mediators, including TNF-α. Many of the in vitro and in vivo studies
have been performed regarding this potent compound; however, the solubility issues kept this in
the shade [70,75,76]. Designing the better formulation could enhance the utilization of curcumin
against several inflammatory conditions, including neuroinflammation and neurodegeneration, in
which TNF-α could be the major pathogenic target. Overall, phytochemicals isolated from natural
resources with the ability to inhibit TNF-α binding to TNFR1 and/or inhibit TNF-α activity could be
vital candidates for the treatment of a number of neuroinflammatory disorders in humans. Table 2
summarizes the key phytochemicals that are reported to have a TNF-α inhibitory effect in diverse
experimental models, indicating their potential as plausible anti-TNF-α therapy.
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Table 2. TNF-α secretion-inhibiting phytochemicals that play beneficial roles in controlling the pathogenesis of several neuronal disorders.
Medicinal Plant
Active Compound
Compound Full Name
Mechanism of Action
Disease/
Pathogenesis
Experiment
Model
Reference(s)
Wasabia japonica
AITC
(isothiocyanate)
3-isothiocyanatoprop-1-ene
Inhibits JNK/NF-κB signaling
and inhibit TNF-α secretion
Neuroinflammation
In vitro
[77]
Zingiber officinale
6-Shogaol
(phenols)
(E)-1-(4-hydroxy-3-methoxyphenyl)dec4-en-3-one
Attenuates LPS-induced
TNF-alpha secretion, protects
dopaminergic neurons
Neuroinflammation,
PD
In vitro, In vivo
[78,79]
Glycine max
Genistein
(isoflavone)
5,7-dihydroxy-3-(4hydroxyphenyl)chromen-4-one
Inhibits ERK activation and
NF-κB regulation by blocking
the cleavage of IκB-α
Inflammation,
muscular dystrophy
In vitro
[80–82]
Brassica oleracea
Quercetin (aglycon)
2-(3,4-dihydroxyphenyl)-3,5,7trihydroxychromen-4-one
Inhibits nuclear translocation
of NF-κB and phosphorylated
Akt
MPTP-induced
neurotoxicity
In vitro, in vivo,
human subject
[83,84]
Allium fistulosum
Kaempferol
(flavonoid)
3,5,7-trihydroxy-2-(4hydroxyphenyl)chromen-4-one
Inhibits TLR4 and
corresponding downstream
activation of NF-κB, JNK, p38
MAPK, and Akt
Neuroinflammation
In vitro
[83]
Psidium guajava
Apigenin
(flavone)
5,7-dihydroxy-2-(4hydroxyphenyl)chromen-4-one
Attenuates the upregulation of
NF-κB gene
PD
In vitro, In vivo
[85]
Citrus paradisi
Naringenin
(flavanone)
(2S)-5,7-dihydroxy-2-(4-hydroxyphenyl)-2,3dihydrochromen-4-one
Inhibits iNOS/NO, decreases
α-synuclein expression and
neuroinflammation in PD
Neuroinflammatory
injury
In vitro
[86]
Brassica oleracea
Myricetin
(flavonoid)
3,5,7-trihydroxy-2-(3,4,5trihydroxyphenyl)chromen-4-one
Attenuates the activation of the
MAPK and NF-κB signaling
pathways
AD, PD
In vitro
[87]
Rhus verniciflua
Butein
(polyphenol)
(E)-1-(2,4-dihydroxyphenyl)-3-(3,4dihydroxyphenyl)prop-2-en-1-one
Inhibits the production of
IL-1β, IL-6, and TNF-α
Neuroinflammation,
neurotoxicity
In vitro
[88,89]
Citrus sinensis
Hesperetin
(flavanone)
(2S)-5,7-dihydroxy-2-(3-hydroxy-4methoxyphenyl)-2,3-dihydrochromen-4-one
Inhibits iNOS expression and
TNF-α production
Neuroinflammatory
injury
In vitro
[90,91]
Scutellaria baicalensis
Wogoni
n(flavone)
5,7-dihydroxy-8-methoxy-2-phenylchromen4-one
Alteration of JAK/STAT
pathways
AD, PD
In vitro
[92]
Maclura pomifera
Morin
(flavonol)
2-(2,4-dihydroxyphenyl)-3,5,7trihydroxychromen-4-one
Inhibits NF-κB- and
AP-1-mediated transcription
and phosphorylation of
MAPKs and Akt
Neuroinflammation,
AD
In vivo
[93,94]
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Table 2. Cont.
Medicinal Plant
Active Compound
Compound Full Name
Mechanism of Action
Disease/
Pathogenesis
Experiment
Model
Reference(s)
Honey, propolis
Chrysin
(hydroxyflavone)
5,7-dihydroxy-2-phenylchromen-4-one
Inhibits iNOS, COX2, NO
Neuroinflammation
In vitro
[95]
Zanthoxylum armatum
Eudesmin
(lignan)
3,6-bis(3,4-dimethoxyphenyl)-1,3,3a,4,6,6ahexahydrofuro[3,4-c]furan
Suppression of NF-κB
Inflammation
In vitro
[96]
Magnolia
fargesii
Magnolin
(lignan)
(3S,3aR,6S,6aR)-3-(3,4-dimethoxyphenyl)-6(3,4,5-trimethoxyphenyl)-1,3,3a,4,6,6ahexahydrofuro[3,4-c]furan
Suppression of NF-κB, NO,
PGE2
Inflammation
In vitro
[97,98]
Magnolia
officinalis
Honokiol
(lignan)
2-(4-hydroxy-3-prop-2-enylphenyl)-4-prop2-enylphenol
Inhibits the phosphorylation of
PI3K/Akt/MAP kinases, NF-κB,
and CB2 receptor
Neurodegenerative
diseases
(e. g. AD)
In vitro
[99,100]
D-galactose-induced
aging
(related to AD)
In vito, in vivo
[101]
Panax ginseng
Ginsenoside Rg1
(triterpene glycosides)
(2R,3R,4S,5S,6R)-2-[[(3S,5R,6S,8R,9R,10R,12R,13R,14R,17S)-3,12dihydroxy-4,4,8,10,14-pentamethyl-17-[(2S)6-methyl-2-[(2S,3R,4S,5S,6R)-3,4,5trihydroxy-6-(hydroxymethyl)oxan-2Reduces the levels of IL-1β,
yl]oxyhept-5-en-2-yl]IL-6, and TNF-α
2,3,5,6,7,9,11,12,13,15,16,17-dodecahydro1H-cyclopenta[a]phenanthren-6-yl]oxy]-6(hydroxymethyl)oxane-3,4,5-triol
Panax ginseng
Ginsenoside Rb2
(triterpene glycoside)
(2S,3R,4S,5S,6R)-2-[(2R,3R,4S,5S,6R)-4,5dihydroxy-6-(hydroxymethyl)-2[[(3S,5R,8R,9R,10R,12R,13R,14R,17S)-12hydroxy-4,4,8,10,14-pentamethyl-17-[(2S)-6methyl-2-[(2S,3R,4S,5S,6R)-3,4,5-trihydroxy6-[[(2S,3R,4S,5S)-3,4,5-trihydroxyoxan-2yl]oxymethyl]oxan-2-yl]oxyhept-5-en-2-yl]2,3,5,6,7,9,11,12,13,15,16,17-dodecahydro1H-cyclopenta[a]phenanthren-3yl]oxy]oxan-3-yl]oxy-6(hydroxymethyl)oxane-3,4,5-triol
Nicotiana tabacum
Nicotine
(alkaloid)
3-[(2S)-1-methylpyrrolidin-2-yl]pyridine
Immune modulation, alteration
of MYD88/NF-κB downstream
pathway
AD, PD, MS
In vitro, In vivo
and in patients
[103–106]
Berberis vulgaris
Berberine
(alkaloid)
16,17-dimethoxy-5,7-dioxa-13azoniapentacyclo[11.8.0.02,10.04,8.015,20]
henicosa-1(13),2,4(8),9,14,16,18,20-octaene
Downregulates
acetylcholinesterase and
inhibits the activation of the
NF-κB signaling pathway
AD
In vitro and
In vivo
[107–109]
Suppresses TNF-α production
via NF-κB inhibition
AD, PD, MS
In vivo
[102]
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Table 2. Cont.
Medicinal Plant
Active Compound
Compound Full Name
Mechanism of Action
Disease/
Pathogenesis
Experiment
Model
Reference(s)
Capsicum annuum
Capsaicin
(alkaloid)
(E)-N-[(4-hydroxy-3methoxyphenyl)methyl]-8-methylnon6-enamide
Inhibits glial
activation-mediated oxidative
stress and neuroinflammation
PD
In vitro and
In vivo
[110–112]
Piper methysticum
Kavalactones
(polyketide)
Reduces intracellular oxidative
stress
AD, stroke
In vitro and
In vivo
[113–115]
Vitis vinifera
Resveratrol
(Polyphenol)
5-[(E)-2-(4-hydroxyphenyl)ethenyl]benzene1,3-diol
Upregulates the expression of
the suppressor of SOCS-1
PD
In vitro and
In vivo
[116–119]
Lindera neesiana
Koaburaside
(ether)
(2S,3R,4S,5S,6R)-2-(4-hydroxy-3,5dimethoxyphenoxy)-6(hydroxymethyl)oxane-3,4,5-triol
Inhibits inflammatory
mediators, pro-inflammatory
cytokines in LPS-activated
microglia, prevents neuronal
death
AD, PD, MS lesions
In vitro
[120]
Patrinia saniculaefolia
Nardostachin
(iridoid)
[(1S,4aS,6S,7R,7aS)-6-hydroxy-7-methyl-1-(3methylbutanoyloxy)-1,4a,5,6,7,7ahexahydrocyclopenta[c]pyran-4-yl]methyl
3-methylbutanoate
Reduces cytokines, COX-2, and
PGE2
Inflammatory
disorders such as
neuroinflammation
In vitro
[121]
Perilla frutescens
Magnosalin,
Andamanicin
(neolignans)
1-[(1R,2R,3R,4R)-2,3-dimethyl-4-(2,4,5trimethoxyphenyl)cyclobutyl]-2,4,5trimethoxybenzene
Inhibits neuroinflammation
and cell death
Degenerative disease
In vitro
[122]
Petrosaspongiolid M
[(1S,3R,4aR,4bS,6aS,10aS,10bS,12aS)-3-(2hydroxy-5-oxo-2H-furan-3-yl)-4b,7,7,10atetramethyl1,3,4,4a,5,6,6a,8,9,10,10b,11,12,12atetradecahydronaphtho[2,1-f]isochromen1-yl] acetate
Reduces PGE2, NO, and TNF-α
levels
Acute and Chronic
Inflammation
In vivo
[123]
Neuropathic pain,
Neuroinflammation
In vitro and
in vivo
[124,125]
AD, activated
microglia-mediated
brain disorders
In vitro
[126]
Petrosaspongianigra
Salvia miltiorrhiza
Tanshinone
(diterpene)
1,6-dimethylnaphtho[1,2-g][1]benzofuran10,11-dione
Selectively suppresses
pro-inflammatory gene
expression and partially
decreased anti-inflammatory
genes expression
Vitamin A
Retinoic acid
(terpenes)
(2E,4E,6E,8E)-3,7-dimethyl-9-(2,6,6trimethylcyclohexen-1-yl)nona-2,4,6,8tetraenoic acid
Inhibits TNF-α and iNOS in
(Aβ) or LPS-induced
microglia-mediated
neuroinflammation
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Table 2. Cont.
Medicinal Plant
Active Compound
Compound Full Name
Mechanism of Action
Disease/
Pathogenesis
Experiment
Model
Reference(s)
Inflammation,
Allergy
In vitro
[74]
Inflammation,
In vitro, in vivo
and in human
[70,75]
Allium sativum
Diallyl sulfide
(sulfide)
3-prop-2-enylsulfanylprop-1-ene
Suppress pro-inflammatory
cytokines by decreased ROS
production through-induced
PI3K/Akt and reduced NF-κB
and AP-1
Curcuma longa
Curcumin
(polyphenol)
(1E,6E)-1,7-bis(4-hydroxy-3methoxyphenyl)hepta-1,6-diene-3,5-dione
Reduction of NF-κB mediated
transcription
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TNF-α mediated inflammatory cascades can be lowered by four main mechanisms, such as (1) by
inhibiting the binding of TNF-α to TNFR1, (2) by lowering the activation of TACE that lowered the
TNF-α production, (3) lowering the activation signaling that are responsible for TNF-α production,
and (4) TNF-α degradation. Above mentioned medicinal plants, and their active chemicals target
in one/more of these pathways and hence lowers the TNF-α and its related inflammatory pathways.
Though the exact mechanism of those phytochemicals to inhibit the TNF-α is not known yet, most of
them significantly lowered the TNF-α secretion either in vitro in the activated microglia/macrophage
cells or in-vivo in the mice model of inflammation, respectively. These phytochemicals are in need of
more research to explore their exact role to lower the TNF-α secretion and its inflammatory activity
in cells and animal models. Only then could we reach closer to the TNF-α inhibiting drug discovery
from natural products that could be effective, from the experimental table to the patient’s bed, in the
near future.
Natural products are still considered to be a major source of lead compounds with desired
pharmacological effects, those could be possible candidates for the drug discovery against several
biological ailments covering inflammation to cancer [127]. However, research on natural products
are mostly limited in the in vitro and in vivo studies, and very few have reached clinical trials, and
even failed there [128]. These difficulties of drug discovery from natural resources have been because
of various limitations of natural products, including complex molecular structure, solubility issues,
the selectivity of the compounds to certain targets, and the specificity of the compounds towards
experimental species [128,129]. The solubility issue of the phytochemicals rendering their activities
can be achieved by designing better formulation through nanoparticle formulation, drug micelle
formation, prodrug formulation, designing semisynthetic derivatives with the addition of functional
groups that help to improve the bioavailability, and BBB permeability. Besides, proper structure
activity relationship (SAR), bioactivity guided isolation of the natural product and experiments from
complete in vitro/vivo models to drug discovery, is the prime necessity of this era, in order to get rid of
the unwanted side effects of synthetic drugs that are readily available. Through SAR, the relationship
between the chemical structure of compounds and its biological activity can be assessed, and easily
determine the particular chemical group inducing a particular biological effect. In Chinese dragon’s
blood chalcone, homoisoflavanone and flavone were found to suppress the neuroinflammatory
process in neurodegenerative diseases [130]. By SAR analysis, it was observed that the active moiety
(1-(2,4-dimethoxy-phenyl)-propenone), substituent position in the phenyl ring was the key component
in imparting the anti-inflammatory effect of pyrazole chalcones [131]. SAR can be very helpful in
confirming the lead potential compounds for future drug discovery.
6. Plant Extract or Phytochemical for Neurological Complications
Several medicinal plants are well-reported to have efficacy against a variety of neurological
disorders; however, very few have reached clinical trials. Curcumin, a well-studied medicinal
spice, is reported to have anti-inflammatory and anti-neurodegenerative effects in in vitro and in vivo
experimental models, and also in patients with dementia (AD pathologies in particular) [132–135].
Similarly, resveratrol, a naturally occurring phenolic compound widely present in grapes and berries,
such as blueberries, mulberries and raspberries, is reported to have protective effects against several
neurological complications in cellular and physiological systems, and in clinical trials [136–138].
Ginger (Zingiber officinale) and its constituents, 6-gingerol, 6-shogaol, 6-paradol, zingerone, Ginkgo
biloba extract, quercetin, berberine, and apigenin are effective for AD in cell cultures and in animal
models [139–145]. Spicatoside A, which is one of the active compounds of Liriope platyphylla, also
possess anti-inflammatory and memory repairment in animal models of dementia [146]. Scutellaria
baicalensis Georgi and Lindera neesiana extract have potential anti-inflammation and neuroprotective
properties, and can be used during AD progression [120,147,148]. Broccoli extract enriched in
sulforaphane attenuates neuroinflammation, and because of its neuroprotective role, it can be used
for combating neurodegenerative diseases, such as AD, PD, MS, and ischemia [149]. Fatty acids
Int. J. Mol. Sci. 2020, 21, 764
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isolated from stem bark of Sorbus commixta have been found to have neuroprotective activity [150].
Bacopa monnieri attenuated symptoms of anxiety and epileptic disorders in animal models [142]. The
neuroprotective efficacy of these phytochemicals clearly indicate that they could be strong candidates
for the management of neurological complications.
Neurological disorders have a very complex pathology, in which a huge number of genes
or proteins are involved. Inflammation is the major cause of such conditions, and induction of
TNF-α secretion and its secondary toxicity leads to chronic neuroinflammation. In TNF-α-mediated
neuroinflammation, several secondary targets are also involved. Preliminary screening of the extracts
from the dietary substances or edible natural products controlling those sub-targets of TNF-α can help
design the multi-targeted phytochemical or extract formulation, which could be an alternative strategy
for the downregulation of neuro-inflammatory cascades. Dietary products with traditional uses against
neuroinflammation could provide greater safety and affordability to patients. From the preliminary
screening of the dietary extracts for their anti-inflammatory effects, preparation of optimal mixture of
extracts can help design the functional food as well. Determination of these effects could help for in vivo
and clinical studies. Utilization of such medicinal foods not only promote the use of nutraceuticals
medically, either to prevent or to ameliorate the negative effect of inflammation, but also help to
financially discover a new drug that could have an appealing strategy for neurological complications.
7. Conclusions
TNF-α is a key cytokine involved in many neurological disorders, ranging from simple
inflammation to dementia, depression, and infectious disorders. For decades, inhibitors or neutralizing
antibodies for TNF-α have been an important alternative for the treatment of such conditions. However,
their effect is still not entirely satisfactory, and therefore, screening for natural phytochemicals that
can control TNF-α activity is essential for the treatment of various types of inflammatory disorders in
human organs, especially in the brain.
As many of the commercially available anti-inflammatory drugs, including TNF-α inhibitor, have
severe side effects, the use of natural products are considered to be the most desired candidates for
drug discovery against various biological ailments. From the studies conducted so far, candidate
phytochemicals could inhibit TNF-α signaling and activity through TNFR1 or could block TACE
and thereby inhibit inflammation, apoptosis, degeneration, cancer, and immune disorders in the
brain. In addition, phytochemicals that activate TNF-α/TNFR2 activity could improve conditions by
inhibiting inflammation and related disorders. This review summarizes phytochemicals against TNF-α
and their neuroprotective properties in various neurodegenerative diseases. Even though the use of
phytochemicals as anti-inflammatory agents has been widely studied in in vitro and in vivo studies,
clinical trials using these natural phytochemicals are still limited due to their complex structures
and solubility issues. Future studies can focus on overcoming this solubility issue by designing
better formulation through nanoparticle formulation, drug micelle formation, prodrug formulation,
and designing semisynthetic derivatives with the addition of functional groups that help improve
bioavailability and BBB permeability. Hence, developing natural phytochemical drugs against TNF-α
may be a potential therapeutic intervention in neurodegenerative diseases. Conclusively, we strongly
suggest that neuroinflammation relating to TNF-α and TNFR1-mediated signal transduction may
be potential therapeutic targets against various neurodegenerative and age-related diseases. TNF-α
inhibitors from natural products might ameliorate neuroinflammation and cognitive dysfunction
in neurodegenerative disease patients. Therefore, phytochemicals may ameliorate TNF-α-induced
neurological impairments through anti-inflammatory effects, and it could be an effective dietary
supplement and nutraceuticals against brain aging and neurodegenerative diseases. In addition,
these investigations may provide a novel therapeutic candidate in the treatment and prevention of
AD and other neuroinflammatory diseases. In the present review, we summarized the evidence
supporting the beneficial role of anti-TNF-α phytochemicals to prevent or slow the progression of
various neurodegenerative diseases to modulate TNF-α induced neuroinflammation.
Int. J. Mol. Sci. 2020, 21, 764
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Author Contributions: All authors have read and agreed to the published version of the manuscript. Literature
review – L.S., S.E.L., S.M.; writing – original and draft preperation: L.S.; S.E.L.; B.P.G.; writing – review and editing
– M.J.; S.Y., supervision: S.Y.K.
Acknowledgments: This work was supported by the Gachon University Research Fund of 2019 (GCU-2019-0351).
Conflicts of Interest: The authors declare no conflict of interest.
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