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Role of the NLRP3 Inflammasome in Preeclampsia


Abstract and Figures

Reproduction involves tightly regulated series of events and the immune system is involved in an array of reproductive processes. Disruption of well-controlled immune functions leads to infertility, placental inflammation, and numerous pregnancy complications, including preeclampsia (PE). Inflammasomes are involved in the process of pathogen clearance and sterile inflammation. They are large multi-protein complexes that are located in the cytosol and play key roles in the production of the pivotal inflammatory cytokines, interleukin (IL)-1β and IL-18, and pyroptosis. The nucleotide-binding oligomerization domain, leucine-rich repeat-, and pyrin domain-containing 3 (NLRP3) inflammasome is a key mediator of sterile inflammation induced by various types of damage-associated molecular patterns (DAMPs). Recent evidence indicates that the NLRP3 inflammasome is involved in pregnancy dysfunction, including PE. Many DAMPs (uric acid, palmitic acid, high-mobility group box 1, advanced glycation end products, extracellular vesicles, cell-free DNA, and free fatty acids) are increased and associated with pregnancy complications, especially PE. This review focuses on the role of the NLRP3 inflammasome in the pathophysiology of PE.
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published: 25 February 2020
doi: 10.3389/fendo.2020.00080
Frontiers in Endocrinology | 1February 2020 | Volume 11 | Article 80
Edited by:
John Even Schjenken,
University of Adelaide, Australia
Reviewed by:
Xian-Hui He,
Jinan University, China
Udo Jeschke,
München, Germany
Koumei Shirasuna
Specialty section:
This article was submitted to
a section of the journal
Frontiers in Endocrinology
Received: 10 October 2019
Accepted: 07 February 2020
Published: 25 February 2020
Shirasuna K, Karasawa T and
Takahashi M (2020) Role of the NLRP3
Inflammasome in Preeclampsia.
Front. Endocrinol. 11:80.
doi: 10.3389/fendo.2020.00080
Role of the NLRP3 Inflammasome in
Koumei Shirasuna 1
*, Tadayoshi Karasawa 2and Masafumi Takahashi 2
1Department of Animal Science, Tokyo University of Agriculture, Atsugi, Japan, 2Division of Inflammation Research, Center
for Molecular Medicine, Jichi Medical University, Shimotsuke, Japan
Reproduction involves tightly regulated series of events and the immune system
is involved in an array of reproductive processes. Disruption of well-controlled
immune functions leads to infertility, placental inflammation, and numerous pregnancy
complications, including preeclampsia (PE). Inflammasomes are involved in the
process of pathogen clearance and sterile inflammation. They are large multi-protein
complexes that are located in the cytosol and play key roles in the production of
the pivotal inflammatory cytokines, interleukin (IL)-1βand IL-18, and pyroptosis. The
nucleotide-binding oligomerization domain, leucine-rich repeat-, and pyrin domain-
containing 3 (NLRP3) inflammasome is a key mediator of sterile inflammation induced
by various types of damage-associated molecular patterns (DAMPs). Recent evidence
indicates that the NLRP3 inflammasome is involved in pregnancy dysfunction, including
PE. Many DAMPs (uric acid, palmitic acid, high-mobility group box 1, advanced glycation
end products, extracellular vesicles, cell-free DNA, and free fatty acids) are increased and
associated with pregnancy complications, especially PE. This review focuses on the role
of the NLRP3 inflammasome in the pathophysiology of PE.
Keywords: NLRP3 inflammasome, pregnancy, preeclampsia, interleukin-1β, inflammation
Reproduction, including development of oocyte and sperm, ovulation, corpus luteum function,
fertilization, implantation, placentation, maintenance of pregnancy, and parturition, is essential
for species maintenance, and reproductive events for next generation are tightly regulated (1).
Pregnancy has been studied extensively over the years (2). From the perspective of the maternal
immune system, a conceptus is a semi-allogeneic tissue that must be rejected; however, that does
not generally happen. It was quickly ruled out that the fetus is shielded from the maternal immune
system via the placenta acting as a physical barrier because the fetal extravillous trophoblast cells
deeply penetrate the uterine mucosa and directly communicate with various maternal immune cells
to avoid rejection (3).
Inflammation is basically a complex protective immune response to harmful stimuli such as
pathogens, damaged or dead cells, and irritants (4). This response is tightly regulated by the
host, enabling survival after infection or injury and maintaining tissue homeostasis. However,
excessive inflammation may cause chronic or systemic inflammatory diseases. On the other hand,
the immune system also contributes to the regulation of reproductive function and pregnancy
(5). Immune-mediated processes such as tissue growth, remodeling, and differentiation are
crucial to maintain pregnancy (1,5). Disruption of well-controlled immune functions leads to
infertility, placental inflammation, and numerous pregnancy complications, such as preeclampsia
Shirasuna et al. NLRP3 Inflammasome in Preeclampsia
(PE), obesity during pregnancy, gestational diabetes mellitus
(GDM), spontaneous abortion, and recurrent pregnancy
loss (68).
There is an increasing body of evidence to suggest that
inflammation and immune cells are involved in both physiology
and pathophysiology of pregnancy. Since infection is not
involved in the majority of the phenomena related to pregnancy
physiology and pathology, it remains unclear why inflammation
is involved. Recently, there have been numerous reports of
inflammasome mechanisms that control sterile inflammation
involved in pregnancy pathologies. Inflammasomes are large
multi-protein complexes found in the cytosol that play key
roles in the production of the pivotal inflammatory cytokines,
interleukin (IL)-1βand IL-18, and pyroptosis (inflammatory
cell death) [(911); Figure 1]. In particular, nucleotide-binding
oligomerization domain, leucine-rich repeat-, and pyrin domain-
containing 3 (NLRP3) inflammasome is a key mediator of sterile
inflammation. Excessive activation of the NLRP3 inflammasome
contributes to the pathogenesis of a wide variety of diseases,
such as diabetes, atherosclerosis, and obesity-induced insulin
resistance (1217). The present review focuses on the role of the
NLRP3 inflammasome in placental inflammation and pregnancy
complications, especially PE.
The most important immune cells that induce pregnancy
immune tolerance is CD4+regulatory T cells (Tregs) (18).
The transcription factor, forkhead boxP3 (Foxp3), is a master
regulator of the development and function of Tregs (19).
The frequency of Foxp3+Tregs increases during normal
pregnancy in the decidua and peripheral blood in humans
and mice (2022). Shima et al. (23) used an animal model
to demonstrate that CD4+CD25+Foxp3+Tregs play a critical
role in regulating immune tolerance at the implantation site to
support implantation and successful pregnancy. The frequency
of Tregs is lower in human pregnancy complications such
as PE or miscarriage (24). In addition, seminal fluid induces
and accumulates paternal-specific Tregs that are involved in
the preimplantation uterus, and insufficient expansion of Tregs
against paternal antigens may trigger spontaneous abortion (25).
Natural killer (NK) cells, particularly decidual NK cells, are
also essential immune cells involved in establishing pregnancy;
they are the most abundant leukocyte population during the
first trimester of human pregnancy (1,26). Decidual NK cells
directly communicate with extravillous trophoblast cells and
other immune cells in the fetal-maternal boundary area, and
promote fetal tolerance and pregnancy progression (26).
Monocytes also accumulate in the decidua, in a process
that involves communication with trophoblast cells (1,27).
They can differentiate into dendritic cells (DCs) in the decidua
during murine and human pregnancy (28,29). DCs regulate
immune tolerance by inducing effector T cell apoptosis and
expansion of Tregs due to reduced antigen presentation, reduced
expression of co-stimulatory molecules, or enhanced production
of anti-inflammatory IL-10 (1,30). Monocytes also differentiate
into macrophages depending on the tissue, and polarization of
macrophages is well-understood (inflammatory M1 and anti-
inflammatory M2 type macrophages). It has been suggested
that dysfunction of decidual macrophages and dysregulation
of M1/M2 balance are critical events in the pathogenesis
of PE. Moreover, activation of NLRP3 inflammasome in the
reproductive organs including placenta is known to occur by
these macrophages.
Inflammasomes recognize various inflammation-inducing
stimuli, such as endogenous danger/damage-associated
molecular patterns (DAMPs) and exogenous pathogen-
associated molecular patterns (PAMPs). They tightly regulate
the production of proinflammatory cytokines such as IL-1βand
IL-18 (9,13,31). The NLRP3 inflammasome is the most widely
studied and is activated in response to a wide array of stimuli,
including exogenous and endogenous danger signals [(9,11);
Figure 1]. The NLRP3 inflammasome is typically composed
of NLRP3, apoptosis-associated speck-like protein containing
a caspase recruitment domain (ASC), and caspase-1 as an
IL-1β-converting enzyme (32). Activation of NLRP3 in response
to danger signals leads to nucleation of ASC into prion-like
filaments via pyrin domain (PYD)–PYD interactions (33).
ASC is then linearly ubiquitinated for NLRP3 inflammasome
assembly, followed by procaspase-1 interaction with ASC using
caspase recruitment domain (CARD)-CARD interactions,
forming its own prion-like filaments (34). Activated caspase-1
(a cysteine protease) cleaves the precursor cytokines, pro-IL-1β
and pro-IL-18, generating the biologically active cytokines, IL-1β
and IL-18, respectively (911). Moreover, active caspase-1 is
able to induce pyroptosis as an inflammatory form of cell death
due to cleaved gasdermin D (GSDMD) (35,36). Caspase-1
proteolytically cleaves GSDMD into a N-terminal domain and
C-terminal domain. Cleaved N-terminal domain of GSDMD
binds to phosphatidylinositol phosphates and phosphatidylserine
in the cell membrane, forming a 10–20 nm pore and induces
a lytic form of cell death, pyroptosis (36). Another feature of
gasdermin D-dependent pyroptosis is the release of IL-1βand
IL-18 via GSDMD-forming cell membrane pore.
The production and secretion of mature IL-1βare regulated
via two steps, including the transcription of pro-IL-1β
and proteolytic processing into a mature form IL-1βby
inflammasomes (911). Prior to its activation, NLRP3 must be
primed in most cell types. Nuclear factor κB (NF-κB)-activating
stimuli, such as lipopolysaccharide (LPS), upregulate mRNA
expression of NLRP3 and IL-1β, resulting in elevated expression
of NLRP3 and pro-IL-1βprotein (911). On the other hand,
another priming step facilitates the rapid induction of the NLRP3
inflammasome via deubiquitination of NLRP3 (37,38).
The upstream mechanisms of NLRP3 activation have
been elucidated by many studies, and include the release of
cathepsins into the cytosol after lysosomal destabilization,
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FIGURE 1 | Schematic mechanisms of NLRP3 inflammasome activation. NLRP3 is activated by various endogenous DAMPs: uric acid crystals (MSU), cholesterol
crustal, cell-free DNA (cfDNA), high-mobility group box 1 (HMGB1), extracellular debris, extracellular vesicles (EVs), advanced glycation end-products (AGEs), and free
fatty acid. Various events such as intracellular ATP release, NLRP3 deubiqutination, relocalization, reactive oxygen species (ROS) generation, mitochondrial
dysfunction, lysosome rapture, and cathepsin release occur depending on the effects of damage-associated molecular patterns (DAMPs). Then, inflammasome
components, including NLRP3, ASC, and procaspase-1, form the NLRP3 inflammasome complexes. Finally, activated caspase-1 induces the inflammatory form of
cell death known as pyroptosis and cleaves the precursor cytokines pro-IL-1βand pro-IL-18, generating the biologically active cytokines IL-1βand IL-18.
potassium efflux, generation of mitochondrial reactive oxygen
species (ROS), and release of mitochondrial DNA (39,40).
Cytosolic leakage of cathepsin B via lysosomal rupture is
essential for NLRP3 inflammasome activation, especially by
endogenous DAMPs (41). Leakage of cathepsin B also leads to
potassium efflux and mitochondrial damage. Potassium efflux
and reduced potassium concentration within cells result in
NLRP3 inflammasome activation (10). In response to potassium
efflux, NEK7 (a member of the family of mammalian NIMA-
related kinases) directly interacts with NLRP3 inflammasome
(42,43). Cellular and mitochondrial ROS production also act as
NLRP3 inflammasome activators (44,45). Furthermore, recent
studies have demonstrated that the NLRP3 inflammasome
is tightly regulated by multiple mechanisms, including
ubiquitination, phosphorylation, nitrosylation, microRNAs, and
endogenous regulators (e.g., pyrin-only proteins and CARD-only
proteins) (9,4648).
Following NLRP3 activation through the above mentioned
regulatory mechanisms, NLRP3 relocates from endoplasmic
reticulum to the mitochondria, where it forms complexes with
ASC (49). IL-1βand IL-18 secretion is regulated by caspase-1
activation by many NLRP3 inflammasome activators, including
monosodium urate (MSU) crystals, silica crystals, asbestos, and
cholesterol crystals (12,13,31,50). Additionally to the canonical
pathway of the NLRP3 inflammasome, the inflammasome
activation can also be indirectly triggered by caspase-11 in mice
(or the homologs caspase-4 and caspase-5 in humans), which
has been termed the non-canonical inflammasome pathway (51).
In this non-canonical pathway, caspase-11 directly recognized
and binds to intracellular LPS, resulting in its oligomerization
and activation by autoproteolytic cleavage (35). Then, caspase-
11 can directly induce the cleavage of GSDMD to induce
pyroptosis (35,36). Details of the structure and activation
mechanism of the NLRP3 inflammasome are refer to following
great reviews (10,17,39,40,52).
PE is a pregnancy-specific hypertensive syndrome that
complicates around 5–10% of all pregnancies worldwide
(53), and is a leading cause of maternal and fetal morbidity and
mortality. It is characterized by the onset of hypertension and
proteinuria in the third trimester of pregnancy, and is associated
with 12% of infants with fetal growth restriction (FGR) and
approximately 20% of preterm deliveries (54). The clinical
manifestations of PE reflect widespread systemic inflammation
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and endothelial dysfunction, resulting in vasoconstriction,
end-organ ischemia and increased vascular permeability (55).
The placenta has been shown to play a central role in the
pathogenesis of PE due to the rapid disappearance of disease
symptoms after delivery. Thus, placenta-derived circulating
factor(s) may induce excessive inflammation and endothelial
defects, leading to PE (56).
During normal pregnancy, trophoblast cells invade, and
remodeling of maternal spiral arteries and the fetoplacental unit
produce angiogenic factors, such as vascular endothelial growth
factor (VEGF) and placental growth factor (PlGF), to support the
developing placenta (57,58). Inadequate trophoblast remodeling
of spiral arteries, which is a key feature of PE, is believed to result
of dysregulation in placental angiogenesis and maternal immune
response (55). Following that, various inflammatory factors are
produced by the diseased and hypoxic placenta, which activates
systemic inflammatory responses (27,59). It is widely recognized
that antiangiogenic factors, including soluble endoglin (sEng; a
coreceptor for transforming growth factor β) and soluble fms-
like tyrosine kinase (sFlt-1; a receptor for VEGF), induce PE-
like phenomena (57,60). Indeed, overexpression of sEng and
sFlt-1 in pregnant rats leads to severe PE symptoms including
hypertension, proteinuria, renal and endothelial dysfunction,
hemolysis, elevated liver enzymes, and FGR (60).
Pathophysiological changes of PE include inflammation and
immune cell activation (6163). The main pathological features
of PE include a general inflammatory response by cytokines,
such as IL-1β, IL-6, IL-8, and tumor necrosis factor-α(TNFα)
(7,64,65). Siljee et al. (66) reported that IL-1βhas a potential to
improve prediction of PE during the first trimester. A decreased
frequency of peripheral Tregs is characteristic immune cell
dynamics seen in PE patients (6). On the other hand, M2-like
immunomodulatory macrophages are abundantly present in the
decidua in healthy pregnant women and participate in spiral
artery remodeling via the angiogenic factors, VEGF and PlGF
(27). Increased numbers of M1-like inflammatory macrophages
are observed in PE patients and may be associated with increase
in inflammatory cytokines, decreased spiral artery remodeling,
and increased production of sFlt-1 and sEng (27).
In recent years, there has been a rapid increase in reports
that the NLRP3 inflammasome is involved in the pathogenesis of
PE (Figure 2). Higher expression of components of the NLRP3
inflammasome has been reported in peripheral blood mono-
nuclear cells and placental tissue from PE patients compared with
that of healthy normal pregnant women (6769). In addition to
immune cells, human trophoblast cells express NLRP3, ASC and
caspase-1 that are components of the NLRP3 inflammasome (70
72). IL-1βsecretion is induced in response to nigericin or nano-
silica crystals, typical activators of the NLRP3 inflammasome, in
human trophoblast cells (71,72).
Maternal hypertension is a characteristic of PE and the renin–
angiotensin system has been implicated in its pathogenesis of
PE (73,74) generated a mouse model of PE-like symptoms by
mating females expressing human angiotensinogen with males
expressing human renin, resulting in mice exhibiting maternal
hypertension, proteinuria, and FGR. Angiotensin II (AngII) is
a strong vasoconstrictor that contributes to hypertension and
stimulates sFlt-1 production and secretion from the placenta
in mice (75). Infusion of AngII in pregnant mice can lead to
high maternal blood pressure, proteinuria, and FGR (75,76).
Deficiency of NLRP3 inflammasome components attenuates the
development of AngII-induced hypertension, but does not affect
FGR, proteinuria, or sFlt1 levels (76).
Furthermore, during non-pregnant conditions, infusion
of AngII induces hypertension with activation of NLRP3
inflammasome in the aorta, and NLRP3 deficiency attenuated
AngII-induced hypertension via inhibition of NLRP3
inflammasome activation in mice (77). A murine experimental
hypertension model (uninephrectomy and treatment with
deoxycorticosterone acetate and 0.9% NaCl in the drinking
water) induced activation of the NLRP3 inflammasome in kidney
and specific NLRP3 inhibitor, MCC950, inhibited the NLRP3
inflammasome and inflammation, resulting in improvement of
hypertension in mice (78). In rats, salt-induced hypertension
occurs partly due to the role of NLRP3 inflammasome activation
in the hypothalamic paraventricular nucleus, while blockade
of brain NLRP3 attenuates the hypertensive response (79).
An absence of ASC also reduces pulmonary hypertension
induced by hypoxia (80). These findings suggest that the NLRP3
inflammasome contributes to the development of hypertension
in both pregnant and non-pregnant situations. On the other
hand, NLRP3 inflammasome has been shown to contribute
to a wide range of acute and chronic kidney diseases (81);
the importance of NLRP3 inflammasome in renal pathologic
abnormalities in PE pathology is not well-understood.
Release of DAMPs from various cells during stress has
been implicated in pregnancy complications. In PE patients,
many DAMPs, such as, cholesterol, uric acid crystals (MSU),
extracellular DNA, high-mobility group box 1 (HMGB1),
extracellular cell debris, advanced glycation end-products
(AGEs), and free fatty acids, have been detected in higher levels
in the peripheral blood and placenta (Figure 2) and act as NLRP3
inflammasome activators.
Cholesterol crystals activate inflammatory responses and
promote inflammatory cell infiltration, resulting in progression
of atherosclerosis and development of cardiovascular disease
(16,82). Cholesterol crystals also cause lysosome rupture,
resulting in the release of cathepsin B to the cytosol, and are a
candidate activator of the NLRP3 inflammasome (82,83).
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FIGURE 2 | Concept of association of NLRP3 inflammasome in pathogenesis of pregnancy complications. Maternal risk such as hypertension and obesity are
associated with the elevation of damage-associated molecular patterns (DAMPs). DAMPs activate NLRP3 inflammasome, accumulate immune cells, and induce
inflammatory cytokine production and vascular dysfunction in placenta. These events result in placental inflammation and dysfunction, leading to pregnancy
complications, such as preeclampsia, spontaneous abortion, recurrent pregnancy loss, and fetal growth restriction.
Maternal cholesterol serum levels are elevated in PE and
cholesterol accumulates in placenta of PE patients, along with
increased levels of NLRP3 and IL-1βexpression (84,85). In
an in vitro human placental explant experiment, treatment
with cholesterol crystals significantly increased the release
of IL-1β, and cholesterol crystal-induced IL-1βsecretion
was suppressed by treatment with MCC950, as a specific
inhibitor of the NLRP3 inflammasome (84). Cholesterol
crystals also strongly activated the NLRP3 inflammasome
in macrophages and induced IL-1βsecretion, dependent
on activation of the NLRP3 inflammasome (82,83,86). In
addition to macrophages, cholesterol crystals markedly increase
the formation and activation of NLRP3 inflammasome in
endothelial cells, as demonstrated by increased colocalization
of NLRP3 with ASC or caspase-1, enhanced caspase-
1 activity, and elevated IL-1βsecretion in mice (87).
These findings indicate that cholesterol induces placental
inflammation via the NLRP3 inflammasome pathway in human
placenta, suggesting the contribution of enhanced NLRP3
inflammasome activation to harmful placental inflammation
in PE.
Saturation of uric acid in body fluids results in the formation of
MSU crystals. These are identified as danger signals from dying
cells, resulting in an acute and/or chronic inflammatory response
known as gout, which is associated with the deposition of MSU
crystals (41,88) demonstrated that MSU crystals activate the
NLRP3 inflammasome, resulting in the production of active IL-
1βand neutrophil accumulation in mice, suggesting a pivotal
role for inflammasomes in inflammatory diseases. In terms of the
mechanisms of NLRP3 inflammasome activation, MSU crystals
are taken up by phagocytosis and lysosomal damage is induced,
resulting in the release of cathepsin B and stimulation of ROS
production from mitochondria (89).
Elevated levels of MSU in the maternal circulation have
been shown in many pregnancy complications, especially PE
(69,84,90,91). In human first trimester trophoblast cell lines,
IL-1βwas produced in response to MSU crystals via the NLRP3
inflammasome (91). Brien et al. (91) reported that MSU crystals
induce a proinflammatory profile with predominant secretion of
IL-1βand IL-6 in human placental explants, and these effects
were IL-1-dependent, as confirmed using a caspase-1 inhibitor
and IL-1 receptor antagonist. In addition, administration of
MSU crystals to pregnant rats induced placental inflammation
(increase IL-1β, IL-6, and TNFαproduction, and macrophage
accumulation) and FGR. Indeed, MSU crystals elicit an increase
in the recruitment of macrophages and neutrophils with IL-
1βsecretion in the NLRP3 inflammasome-dependent manner
(41,92). These findings suggest that higher levels of MSU
in PE patients trigger placental inflammation via NLRP3
inflammasome activation, resulting in the pathogenesis of PE.
Extracellular released cell-free DNA (cfDNA) circulating in
the blood, which is considered a product of apoptosis and/or
necrosis, acts as a DAMP and is related to many types of
inflammatory diseases (93,94). Toll-like receptor 9 (TLR9),
originally identified as a sensor of exogenous DNA fragments,
contributes to cfDNA-mediated inflammatory processes (95).
It is activated by bacterial DNA rich in unmethylated CpG
motifs, and can also be activated by DNA from mammalian
cells such as nucleic and mitochondrial DNA. Therefore, TLR9
signaling is of interest as a candidate molecule responsible for
the first signal in sterile inflammation (96). It was previously
reported that cfDNA released from apoptotic hepatocytes
activates TLR9 systems, which in turn triggers a signaling cascade
that increases transcription of the genes encoding pro-IL-1β
and pro-IL-18. Furthermore, mice lacking components of the
NLRP3 inflammasome showed reduced amounts of cfDNA and
improved liver injury (96). Pan et al., reported that mitochondrial
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DNA is directly recognized and binds with NLRP3, resulting
in the formation of NLRP3 inflammasome complex and its
activation (97).
During pregnancy, the amount of total cfDNA and cf-fetal
DNA (cffDNA) is significantly increased in the maternal blood
depending on the stage of pregnancy (98). There are also
significant associations between elevated cfDNA and cffDNA
with pregnancy complications such as PE and FGR (98104).
We recently showed that expression levels of TLR9 and the
amount of cffDNA from the placenta were higher in PE patients
compared with that in women with normal placenta (NP), and
PE-derived cffDNA stimulated levels of inflammatory cytokine,
including IL-1βand sEng secretion depend on TLR9 signaling,
compared with NP-derived cffDNA (105). Moreover, a synthetic
TLR9 ligand activated inflammatory responses including IL-
6 secretion together with stimulation of sFlt1 secretion, while
inhibition of TLR9 reduced sFlt1 secretion in human trophoblast
cells (106). In mice, administration of a TLR9 ligand induces
PE-like symptoms, such as hypertension, proteinuria, placental
inflammation, and FGR. Moreover, injection of human fetal
DNA, but not adult DNA, induces placental inflammation, fetal
resorption, and preterm birth in pregnant mice, and notably,
these adverse effects are improved in TLR9-knockout mice (107).
These findings suggest that excessive extracellular DNA acts as
a DAMP and causes pregnancy complications, especially PE, via
TLR9 signaling.
In trophoblast cells, cfDNA is also capable of detecting
danger signals via the intracellular DNA sensor, interferon-
inducible protein 16 (IFI16). Indeed, IFI16 agonist poly(dA:dT)
stimulates sFlt-1 and sEng production in human trophoblast
cells in an IFI16-dependent manner (108). Extracellular DNA
plays an essential role in the induction of inflammatory
responses; however, further research is required to clarify the
role of extracellular DNA in NLRP3 inflammasome activation in
pregnancy complications.
HMGB1 is an important DAMP that acts as an architectural
chromatin-binding factor and is generally located in the nucleus
of most cell types under physiological conditions (109). When
cells are exposed to stress, HMGB1 is translocated into the
extracellular milieu and elicits inflammatory responses via the
production of proinflammatory mediators and accumulation
of inflammatory cells. HMGB1 interacts with TLR2, TLR4,
and receptor for AGE (RAGE), resulting in elevated levels of
HMGB1 in tissues and serum associated with the development of
inflammation during pathological conditions (110). It is reported
that HMGB1 induces the formation of the NLRP3 inflammasome
(111). HMGB1 also activates the NLRP3 inflammasome since
that stimulation with HMGB1 induces the release of IL-1β
with increase in NLRP3 inflammasome component, these effects
can be attenuated by inhibition of the NLRP3 inflammasome
(112). In addition, Deng et al. (113) demonstrated that
HMGB1 directly binds LPS and targets its internalization into
the lysosomes of cells via the RAGE, resulting activation of
caspase-11-dependent non-canonical inflammasome signaling.
On the contrary, NLRP3 inflammasome activation accelerates
atherosclerosis induced by HMGB1 secretion, indicating that
HMGB1 is a key downstream signaling molecule of NLRP3
inflammasome activation (114). Therefore, the vicious cycle
of HMGB1 and the NLRP3 inflammasome may exacerbate
inflammation and pathological conditions.
In peripheral blood, HMGB1 concentrations are significantly
elevated in PE patients compared with those of healthy pregnant
and non-pregnant women (115,116). Compared with healthy
placenta, protein and mRNA expression of HMGB1 and its
receptor RAGE, are increased in severe PE placentas (116).
In human trophoblast cells, HMGB1 stimulates inflammatory
cytokine production dependent on NF-κB activation and ROS
signaling via TLR4 (117). In human placenta, treatment with PE
serum increased the expression and release of HMGB1, which
induced endothelial cell activation (118). In addition, HMGB1
treatment increased NLRP3 protein expression and activation
of caspase-1, resulting in an increase of mature IL-1βsecretion
in human chorioamniotic membranes (119). These findings
indicated that HMGB1 contributes to placental inflammation
and NLRP3 inflammasome activation as endogenous DAMPs,
leading to PE. Interestingly, the expression levels of HMGB1 in
the uterus are lowest during the expected time of implantation,
and exogenous administration of HMGB1 leads to pregnancy
failure accompanied by induction of inflammatory responses in
rats, indicating a role of excessive extracellular HMGB1 in PE as
well as infertility (120).
The outer layer of the placenta is covered by a single
syncytiotrophoblast that forms the maternal-fetal interface
(1). When portions of the syncytiotrophoblast become
damaged, cellular debris is extruded into the maternal blood
in membrane-enclosed vesicles (121). During normal healthy
pregnancy, trophoblastic debris is produced by programmed
cell death/apoptosis in the placenta. This extracellular debris is
believed to induce a tolerogenic response in maternal endothelial
and immune cells (122). On the other hand, extracellular debris
from PE placenta mainly originates from necrotic cell death, and
exposing endothelial cells to necrotic trophoblastic debris leads
to their activation (123). The amount of trophoblastic debris
shed into the maternal blood is greatly increased in PE patients
compared with that in healthy pregnant women (108).
It is likely that trophoblastic debris includes various types
of danger signals, such as DNA, RNA, adenosine, HMGB1,
and MSU (118,124). The degree of trophoblastic debris from
human placenta is increased by treatment with PE serum
and antiphospholipid antibodies, resulting in the activation of
endothelial cell activation and induction of immune cell adhesion
(118). Interestingly, necrotic, but not apoptotic, trophoblastic
debris contains IL-1βprotein, whereas much of the trophoblastic
debris is dead cell corpses that might not be able to produce new
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Shirasuna et al. NLRP3 Inflammasome in Preeclampsia
proteins (124). On the other hand, adenosine in trophoblastic
debris and cell surface adenosine receptor A2B signaling also
contributes to the pathogenesis of PE (125). Iriyama et al. (125)
demonstrated that chronically elevated placental adenosine leads
to the hallmark features of PE (hypertension, proteinuria, and
FGR) in a mouse model. Moreover, elevated adenosine in PE
patients is correlated with Th1/Th2 imbalance, and adenosine
directly induces sFlt-1 production from placenta (126). Baron
et al. (127) showed that extracellular adenosine activates the
NLRP3 inflammasome and IL-1βsecretion by interaction with
adenosine receptors and through adenosine cellular uptake using
nucleotide transporters. These findings suggest that adenosine
signaling in debris activates NLRP3 inflammasome in placenta,
resulting in PE.
Extracellular vesicles (EVs) are also produced and released
by living cells and can be detected in all biological fluids,
including blood. EVs are nanosized particles that are traditionally
classified into subtypes, such as exosomes, microvesicles, and
apoptotic/necrotic bodies (debris). EV cargo includes bioactive
molecules such as protein, lipids, and nucleic acid (DNA,
mRNA, microRNA, and non-coding RNA) (128). Significantly
higher levels of syncytiotrophoblast-derived EVs are found in
the peripheral blood of women with PE compared with women
with normal pregnancies (129). EVs isolated from PE patients
differ phenotypically and functionally from those isolated from
healthy pregnant women (130). Indeed, syncytiotrophoblast-
derived EVs (including exosomes) from patients with PE contain
higher levels of sFlt-1, sEng, and neprilysin, and treatment
with EVs from PE patients impairs angiogenesis of endothelial
cells and changes the characteristics of monocytes in vitro
(131,132). In addition, exosomes from PE patients cause
vascular dysfunction and directly result in adverse PE-like
birth outcomes in mice (131). Kohli et al. (133) demonstrated
that administration of EVs led to accumulation of activated
platelets and induced activation of NLRP3 inflammasome within
the placenta, resulting in a PE-like phenotype in pregnant
mice. Intriguingly, genetic deletion of NLRP3 inflammasome or
pharmacological inhibition of inflammasome abolished this PE-
like phenotype, indicating the pathogenesis of PE by EVs was
dependent the NLRP3 inflammasome.
Obesity is a major risk factor for PE and FGR (134,135). Obesity
represents low-grade chronic systemic inflammation (136), and
maternal obesity increases the risk of the offspring developing
obesity and insulin resistance in the later stages of life (137
141). The NLRP3 inflammasome is involved in the pathogenesis
of obesity-related inflammatory diseases, including metabolic
syndrome, type 2 diabetes, and cardiovascular diseases (12,13,
31,50). There are many common mechanisms between PE and
obesity-related pregnancy complications, and obesity accelerates
the systemic features of PE.
Free fatty acids levels are elevated in the plasma of obese
humans (142), and it has been proposed that they act to
promote inflammatory responses by directly engaging TLRs
and inducing the NF-κB-dependent production of inflammatory
cytokines (143,144). In particular, one of the major saturated
fatty acids, palmitic acid (PA), causes intracellular crystallization,
which in turn activates the NLRP3 inflammasome via lysosomal
dysfunction in macrophages (145). PA also induces NLRP3
inflammasome activation by generating ROS and inducing
autophagy dysfunction, resulting in secretion of mature IL-
1β(144,146,147). Similar to other crystalline molecules,
intraperitoneal administration of PA crystal induces neutrophil
recruitment in an IL-1β-dependent manner (145).
Serum PA levels are increased in women with PE and FGR
(148150). Treatment with free fatty acid solution to mimic
the plasma of PE patients induces lipid droplet accumulation,
mitochondrial dysfunction, and apoptosis in human umbilical
vein endothelial cells (149). In addition, PA induces activation
of the NLRP3 inflammasome, resulting in the secretion of
mature IL-1βby human trophoblast cells (147). NF-κB activation
and IL-6 production are associated with higher levels of lipid
accumulation in the placenta of obese women compared with
those of lean women (151). These findings suggest that saturated
fatty acids directly induce placental inflammation, resulting
in PE.
AGEs are heterogeneous, reactive, and irreversibly crosslinked
molecules formed from the non-enzymatic glycation of proteins,
lipids, and nucleic acids (152,153). They interact with RAGE
and/or TLR4 to induce inflammatory responses (154,155).
AGE-RAGE interactions may increase and perpetuate the
inflammatory condition, leading to obesity, diabetes mellitus,
and cardiovascular and kidney diseases. Both in vivo and
in vitro experiments have demonstrated that AGEs stimulate
NLRP3 inflammasome activation and IL-1βsecretion in human
umbilical vein endothelial cells, kidney, and pancreatic islets
(117,156,157). Ablation of the NLRP3 inflammasome improved
AGE-induced abnormal insulin sensitivity, pancreatic islet
damage, and inflammatory responses (158). These findings
suggest that consumption of AGEs increases obesity-related
dysfunction via NLRP3 inflammasome activation.
Increasing evidence indicates that AGEs and IL-1βare
associated with PE and obesity in pregnant women (134,135,
159161). In human placenta, AGEs increase in vitro release
of IL-1β, IL-6, IL-8, and TNFαdepend on NF-κB activation
(162). We also demonstrated that in human placental tissues,
AGEs directly increase both the transcription and secretion of
IL-1β(117). In addition, AGEs stimulate pro-IL-1βproduction,
resulting in the acceleration of mature IL-1βsecretion by NLRP3
inflammasome activation in human trophoblast cells. AGEs also
induce sFlt-1 production through RAGE signaling, suggesting a
Frontiers in Endocrinology | 7February 2020 | Volume 11 | Article 80
Shirasuna et al. NLRP3 Inflammasome in Preeclampsia
direct link with the pathology of PE (163). Antoniotti et al. (164)
reported that AGEs led to activated inflammatory responses
in endometrial cells, impaired decidualization, compromised
implantation of blastocyst, and suppressed trophoblast invasion.
Therefore, AGEs adversely may impact not only PE but also
endometrial function and embryo implantation.
GDM is also classed as an obesity-related pregnancy
complication. In GDM, high levels of serum glucose are
associated with increased inflammation in blood as well
as placenta (165). Excess glucose induces IL-1βsecretion
from human trophoblast cells depending on the NLRP3
inflammasome (166). In addition to the placenta, caspase-
1 activation and mature IL-1βsecretion are higher in the
adipose tissue of pregnant patients with GDM compared
with healthy pregnant women (167), and treatment with
caspase-1 inhibitor suppresses IL-1βsecretion, suggesting
the contribution of NLRP3 inflammasome activation
in GDM.
Inflammation of the maternal-fetal interface such as
intra-amniotic inflammation or chorioamnionitis, which
can be induced by intra-amniotic infection or DAMPs, is a
causal link to spontaneous preterm birth, which is a leading
cause of perinatal mortality and morbidity (168). In a non-
primate rhesus macaques chorioamnionitis model induced
by intra-amniotic injection of LPS, the amnion upregulated
neutrophil accumulation via the chemoattractant IL-8 in an IL-
1-dependent manner (169). In a mouse model of intra-amniotic
inflammation-induced preterm birth, the NLRP3 inflammasome
was activated following IL-1βsecretion in the fetal membranes
and decidua basalis (170). In addition, IL-1βblockade decreased
inflammation-induced preterm labor in mice (171). These
findings suggest that the NLRP3 inflammasome plays a pivotal
role in inflammation of the maternal-fetal interface associated
with preterm birth, and IL-1 is a potential therapeutic target for
these conditions.
To understand the role of the NLRP3 inflammasome in
normal pregnancy and pregnancy complications, please refer the
essential review (172).
Accumulating evidence suggests that the NLRP3 inflammasome
plays an essential role in the pathogenesis of pregnancy
inflammatory complications. Various types of DAMPs act
as danger signals to activate the NLRP3 inflammasome in
reproductive organs, resulting in pregnancy inflammatory
complications (Figure 2). Once activated, the NLRP3
inflammasome drives the robust release of mature IL-1β,
initiating a positive feedback loop that results in the accumulation
of other immune cells (neutrophils and macrophages) and an
increase in the “danger” cytokines and chemokines. Considering
the potential for excessive NLRP3 inflammasome and IL-1β
production, it is not unexpected that several negative regulatory
mechanisms exist in nature to control inflammasome function.
Understanding how the NLRP3 inflammasome regulates
pregnancy complications and how to control excessive NLRP3
inflammasome activation is essential for the identification of new
targets for the treatment of reproductive dysfunction.
KS and TK wrote the manuscript. MT critically revised the
manuscript. All authors read and approved the final manuscript.
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Conflict of Interest: The authors declare that the research was conducted in the
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Frontiers in Endocrinology | 13 February 2020 | Volume 11 | Article 80
... This implies that placental infection-induced inflammation is associated with lower placental weight. As mentioned above, the increase in pro-inflammatory cytokines, and activation of the components of the inflammasomes cascade, is consistently reported in pregnancies associated with placental inflammation [17,37,[47][48][49][50][51][52][53][54][55][56][57][58][59]. This study demonstrates that the reduction of placental weight and birth weight in FGR may be associated with sterile inflammation, specifically when there are no signs of infection associated with the FGR placentae included in this study. ...
... NLRP3 is active in immune cells, such as macrophages, during the innate immune response through the activation of CASP1 and IL-1β [52]. However, the activation of NLRP3 is not limited to bacterial exposure, but also to various types of damage-associated molecular patterns (DAMPs), including uric acid, cholesterol crystals, palmitic acid, ROS, and HMGB1 [12,50,55,60,61]. In the placenta, previous studies show the upregulation of NLRP3 in decidual stromal cells and trophoblasts following LPS stimulation, indicating that NLRP3 may contribute to the placental innate immune response [62]. ...
... In the placenta, previous studies show the upregulation of NLRP3 in decidual stromal cells and trophoblasts following LPS stimulation, indicating that NLRP3 may contribute to the placental innate immune response [62]. Furthermore, DAMP-induced NLRP3 levels have been extensively investigated in placental inflammation and dysfunction through the accumulation of cytokines (e.g., IL-1β), all of which were implicated in various pregnancy complications including preeclampsia [55] and spontaneous pre-term labour [63]. In the pathogenesis of preeclampsia, the activation of NLRP3 by DAMPs leads to an increased blood pressure through sympathetic nervous system activation, activation of the renin-angiotensin-aldosterone system (RAAS), tubulointerstitial inflammation, and placental abruption [56]. ...
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Fetal growth restriction (FGR) is commonly associated with placental insufficiency and inflammation. Nonetheless, the role played by inflammasomes in the pathogenesis of FGR is poorly understood. We hypothesised that placental inflammasomes are differentially expressed and contribute to the aberrant trophoblast function. Inflammasome gene expression profiles were characterised by real-time PCR on human placental tissues collected from third trimester FGR and gestation-matched control pregnancies (n = 25/group). The functional significance of a candidate inflammasome was then investigated using lipopolysaccharide (LPS)-induced models of inflammation in human trophoblast organoids, BeWo cells in vitro, and a murine model of FGR in vivo. Placental mRNA expression of NLRP3, caspases 1, 3, and 8, and interleukin 6 increased (>2-fold), while that of the anti-inflammatory cytokine, IL-10, decreased (<2-fold) in FGR compared with control pregnancies. LPS treatment increased NLRP3 and caspase-1 expression (>2-fold) in trophoblast organoids and BeWo cell cultures in vitro, and in the spongiotrophoblast and labyrinth in the murine model of FGR. However, the LPS-induced rise in NLRP3 was attenuated by its siRNA-induced down-regulation in BeWo cell cultures, which correlated with reduced activity of the apoptotic markers, caspase-3 and 8, compared to the control siRNA-treated cells. Our findings support the role of the NLRP3 inflammasome in the inflammation-induced aberrant trophoblast function, which may contribute to FGR.
... In addition, Nrf2 was shown to inhibit pyroptosis in vascular endothelial cells (Hu et al. 2018) and rat cortical neurons . Only a few studies have focused on the possible roles of pyroptosis in preeclampsia (Shirasuna et al. 2020;Cheng et al. 2019), and further investigations are required. Previous studies showed that microRNA-155 (miR-155) could bind to and inhibit Nrf2 signaling pathway in lung cancer (Gu et al. 2017) and liver injury . ...
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Background This study aimed to investigate the effects of LINC00240/miR-155/Nrf2 axis on trophoblast function and macrophage polarization in the pathogenesis of preeclampsia. Methods Bindings between LINC00240, miR-155 and Nrf2 were validated by dual luciferase reporter assay or RNA-immunoprecipitation. Cell proliferation, migration, invasion, and pyroptosis were detected by CCK-8, clone formation, wound healing, Transwell system, and flow cytometry, respectively. Macrophage polarization was tested by flow cytometry. The expression levels of LINC00240, miR-155, Nrf2, and oxidative stress and pyroptosis-related markers in in vitro and in vivo preeclampsia models were analyzed by qPCR, western blot, or ELISA assays. Blood pressure, urine protein levels, liver and kidney damages, and trophoblast markers in placenta tissues were further studied in vivo. Results Placenta tissues from preeclampsia patients and animals showed decreased LINC00240 and Nrf2 and increased miR-155 expression levels, and the decreased M2 macrophage polarization. LINC00240 directly bound and inhibited expression of miR-155, which then inhibited oxidative stress-induced pyroptosis, promoting proliferation, migration and invasion abilities of trophoblasts, and M2 macrophage polarization. Inhibition of miR-155 led to increased Nrf2 expression and similar changes as LINC00240 overexpression in trophoblast function and macrophage polarization. Overexpression of LINC00240 in in vivo preeclampsia model decreased blood pressure, urine protein, liver and kidney damages, increased fetal weight and length, and induced trophoblast function and M2 macrophage polarization. Conclusion LINC00240 inhibited symptoms of preeclampsia through regulation on miR-155/Nrf2 axis, which suppressed oxidative stress-induced pyroptosis to improve trophoblast function and M2 macrophage polarization. LINC00240 could be a potential therapeutic target for preeclampsia.
... The first study investigating the correlation between NLRP3 and hypertension was in the exploration of relieving high blood pressure symptom in preeclampsia (86). In previous study, sterile inflammation was supposed to involve the pathogenesis, but whether NLRP3 inflammasome participates in the process remained undefined. ...
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Cardiovascular diseases (CVDs) are the prevalent cause of mortality around the world. Activation of inflammasome contributes to the pathological progression of cardiovascular diseases, including atherosclerosis, abdominal aortic aneurysm, myocardial infarction, dilated cardiomyopathy, diabetic cardiomyopathy, heart failure, and calcific aortic valve disease. The nucleotide oligomerization domain-, leucine-rich repeat-, and pyrin domain-containing protein 3 (NLRP3) inflammasome plays a critical role in the innate immune response, requiring priming and activation signals to provoke the inflammation. Evidence shows that NLRP3 inflammasome not only boosts the cleavage and release of IL-1 family cytokines, but also leads to a distinct cell programmed death: pyroptosis. The significance of NLRP3 inflammasome in the CVDs-related inflammation has been extensively explored. In this review, we summarized current understandings of the function of NLRP3 inflammasome in CVDs and discussed possible therapeutic options targeting the NLRP3 inflammasome.
... Elevated of UA levels can also inhibit placental amino acid uptake, trophoblast invasion and the incorporation of trophoblast into endothelial monolayers, leading to placental hypoperfusion [26][27][28]. Additionally, during late gestation, UA crystals activate the nod-like receptor protein_3 (NLRP3) inflammatory pathwayvia an IL-1-dependent pathway, causing placental interface inflammation and affecting fetal development [29,30]. Numerous population-level studies have also investigated the association between UA and adverse maternal and infant outcomes, although their conclusions have been inconsistent. ...
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Background: In recent years, results on the association between serum uric acid (UA) and pregnancy outcomes have been inconsistent, and the association between urea nitrogen (UN) and adverse pregnancy outcomes in normal pregnant women has not been reported. Thus, we examined the association of UA and UN levels during gestation with the risk of adverse pregnancy outcomes in a relatively large population. Methods: A total of 1602 singleton mothers from Union Shenzhen Hospital of Huazhong University of Science and Technology at January 2015 to December 2018 were included. Both UA and UN levels were collected and measured during the second (16-18th week) and third (28-30th week) trimesters of gestation respectively. Statistical analysis was performed using multivariate logistic regression. Results: After adjustment, the highest quartile of UA in the third trimester increased the risk of premature rupture of membranes (PROM) and small for gestational age infants (SGA) by 48% (odds ratio [OR]: 1.48, 95% confidence interval [CI]: 1.04-2.10) and 99% (95% CI: 1.01-3.89) compared to those in the lowest quartile. The adjusted OR (95% CI) in the highest quartile of UN for the risk of SGA was 2.18 (95% CI: 1.16-4.13) and 2.29 (95% CI: 1.20-4.36) in the second and third trimester, respectively. In the second trimester, when UA and UN levels were both in the highest quartile, the adjusted OR (95% CI) for the risk of SGA was 2.51 (95% CI: 1.23-5.10). In the third trimester, when the group 1 (both indicators are in the first quartile) was compared, the adjusted ORs (95% CI) for the risk of SGA were 1.98 (95% CI: 1.22-3.23) and 2.31 (95% CI: 1.16-4.61) for group 2 (UA or UN is in the second or third quartile) and group 3 (both indicators are in the fourth quartile), respectively. Conclusions: Higher UA and UN levels increased the risk of maternal and fetal outcomes. The simultaneous elevation of UA and UN levels was a high-risk factors for the development of SGA, regardless of whether they were in the second or third trimester.
The cold-inducible proteins (CIPs) are essential for post-transcriptional gene regulation playing diverse tissue-specific roles in maintaining normal cellular function and morphogenesis. The potential implications of CIPs in reproductive events raise questions about their role in the physiology of the bovine reproductive tract. However, the expression changes of CIPs during the bovine estrous cycle have not been studied so far. Here, we hypothesized that the bovine estrous cycle could affect the mRNA expression of the CIPs and other candidate transcripts in the reproductive tract. This study aimed to examine estrous cycle-dependent mRNA expression patterns in the bovine endometrium and ampulla of three of the major described CIPs (CIRBP, RBM3, SRSF5), a set of inflammatory cytokines (IL-10, IL-18, IL-1β), and other candidate genes (IL-10RA, IL-10RB, BCL2, NLRP3, STAT1, STAT3, STAT5A, STAT6). Endometrial and ampullar tissues were assessed by RT-qPCR. Additionally, the mRNA expression levels were correlated among them and with follicular progesterone and estradiol concentrations. The transcript levels of CIPs increased in the endometrium during stage III (Days 11–17) compared to stage I (Days 1–4) and IV (Days 18–20). In the ampulla, the mRNA expression of CIRBP increased during the late luteal phase (stage III), but no differences in the expression of other CIPs were observed. This study expands the current knowledge regarding mRNA expression in the endometrium and oviductal ampulla of cycling heifers, focusing mainly on the CIPs. A better understanding of the mechanisms within the uterus and oviduct during the estrous cycle is crucial to improving the fertility rate.
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A new tuberculosis diagnostic cartridge assay, which detects a 3-gene tuberculosis signature in whole blood, was not diagnostic in women with maternal tuberculosis disease in India (AUC=0.72). In a cohort of pregnant women, we identified a novel gene set for TB diagnosis (AUC=0.97) and one for TB progression (AUC=0.96).
Atherosclerosis (AS) is chronic pathological process based on the inflammatory reaction associated with factors including vascular endothelial dysfunction, inflammation, and autoimmunity. Inflammasomes are known to be at the core of the inflammatory response. As a pattern recognition receptor of innate immunity, the NLRP3 inflammasome mediates the secretion of inflammatory factors by activating the Caspase-1, which is important for maintaining the immune system and regulating the gut microbiome, and participates in the occurrence and development of AS. The intestinal microecology is composed of a large number of complex structures of gut microbiota and its metabolites, which play an important role in AS. The gut microbiota and its metabolites regulate the activation of the NLRP3 inflammasome. Targeting the NLRP3 inflammasome and regulating intestinal microecology represent a new direction for the treatment of AS. This paper systematically reviews the interaction between the NLRP3 inflammasome and gut microbiome in AS, strategies for targeting the NLRP3 inflammasome and gut microbiome for the treatment of AS, and provides new ideas for the research and development of drugs for the treatment of AS.
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Cigarette smoke (CS) alters cutaneous biological processes such as redox homeostasis and inflammation response that might be involved in promoting skin inflammatory conditions. Exposure to CS has also been linked to a destabilization of the NLRP3 inflammasome in pollution target tissues such as the lung epithelium, resulting in a more vulnerable immunological response to several exogenous and endogenous stimuli related to oxidative stress. Thus, CS has an adverse effect on host defense, increasing the susceptibility to develop lung infections and pathologies. In the skin, another direct target of pollution, inflammasome disorders have been linked to an increasing number of diseases such as melanoma, psoriasis, vitiligo, atopic dermatitis, and acne, all conditions that have been connected directly or indirectly to pollution exposure. The inflammasome machinery is an important innate immune sensor in human keratinocytes. However, the role of CS in the NLRP1 and NLRP3 inflammasome in the cutaneous barrier has still not been investigated. In the present study, we were able to determine in keratinocytes exposed to CS an increased oxidative damage evaluated by 4-HNE protein adduct and carbonyl formation. Of note is that, while CS inhibited NLRP3 activation, it was able to activate NLRP1, leading to an increased secretion of the proinflammatory cytokines IL-1β and IL-18. This study highlights the importance of the inflammasome machinery in CS that more in general, in pollution, affects cutaneous tissues and the important cross-talk between different members of the NLRP inflammasome family.
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Inflammasomes are cytoplasmic multiprotein complexes that coordinate inflammatory responses, including those that take place during pregnancy. Inflammasomes and their downstream mediators caspase-1 and IL-1β are expressed by gestational tissues (e.g., the placenta and chorioamniotic membranes) during normal pregnancy. Yet, only the activation of the NLRP3 inflammasome in the chorioamniotic membranes has been partially implicated in the sterile inflammatory process of term parturition. In vivo and ex vivo studies have consistently shown that the activation of the NLRP3 inflammasome is a mechanism whereby preterm labor and birth occur in the context of microbial- or alarmin-induced inflammation. In the placenta, the activation of the NLRP3 inflammasome is involved in the pathogenesis of preeclampsia and other pregnancy syndromes associated with placental inflammation. This evidence suggests that inhibition of the NLRP3 inflammasome or its downstream mediators may foster the development of novel anti-inflammatory therapies for the prevention or treatment of pregnancy complications.
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The NLRP3 inflammasome can be activated by stimuli that include nigericin, uric acid crystals, amyloid-β fibrils and extracellular ATP. The mitotic kinase NEK7 licenses the assembly and activation of the NLRP3 inflammasome in interphase. Here we report a cryo-electron microscopy structure of inactive human NLRP3 in complex with NEK7, at a resolution of 3.8 Å. The earring-shaped NLRP3 consists of curved leucine-rich-repeat and globular NACHT domains, and the C-terminal lobe of NEK7 nestles against both NLRP3 domains. Structural recognition between NLRP3 and NEK7 is confirmed by mutagenesis both in vitro and in cells. Modelling of an active NLRP3–NEK7 conformation based on the NLRC4 inflammasome predicts an additional contact between an NLRP3-bound NEK7 and a neighbouring NLRP3. Mutations to this interface abolish the ability of NEK7 or NLRP3 to rescue NLRP3 activation in NEK7-knockout or NLRP3-knockout cells. These data suggest that NEK7 bridges adjacent NLRP3 subunits with bipartite interactions to mediate the activation of the NLRP3 inflammasome.
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NEP (neprilysin) is a widely expressed membrane-bound metalloprotease, which binds and cleaves a variety of peptides including vasodilators, natriuretics, and diuretics. Higher levels of NEP result in hypertension-a cardinal feature of the placental disease preeclampsia. Syncytiotrophoblast-derived extracellular vesicles (EVs), comprising microvesicles and exosomes, are released into the peripheral circulation in pregnancy and are postulated as a key mechanism coupling placental dysfunction and maternal phenotype in preeclampsia. We aimed to determine whether higher levels of active NEP are found in syncytiotrophoblast-derived EVs in preeclampsia compared with normal pregnancy. Using immunostaining and Western blotting, we first demonstrated that NEP levels are greater not only in preeclampsia placental tissue but also in syncytiotrophoblast-derived microvesicles and exosomes isolated from preeclampsia placentas ( P<0.05, n=5). We confirmed placental origin using antibody-coated magnetic beads to isolate NEP-bound vesicles, finding that they stain for placental alkaline phosphatase. NEP on syncytiotrophoblast-derived EVs is active and inhibited by thiorphan ( P<0.01, n=3; specific inhibitor). Syncytiotrophoblast-derived microvesicles, isolated from peripheral plasma, demonstrated higher NEP expression in preeclampsia using flow cytometry ( P<0.05, n=8). We isolated plasma exosomes using size-exclusion chromatography and showed greater NEP activity in preeclampsia ( P<0.05, n=8). These findings show that the placenta releases active NEP into the maternal circulation on syncytiotrophoblast-derived EVs, at significantly greater levels in preeclampsia. NEP has pathological roles in hypertension, heart failure, and amyloid deposition, all of which are features of preeclampsia. Circulating syncytiotrophoblast-derived EV-bound NEP thus may contribute to the pathogenesis of this disease.
The advanced glycation end products (AGEs) are the products of non‐enzymatic glycation reaction of proteins and sugars, which can induce the generation of free radicals and the expression of inflammatory factors, thereby playing an important role in vascular dysfunction in diabetes. To investigate the effects of caffeic acid (CA) on glycation formed by glucose and protein, various spectroscopic techniques and molecular docking methods were carried out. Furthermore, the protective effects of CA on human umbilical vein endothelial cells (HUVECs) damaged by AGEs were explored. The results indicated that CA inhibited AGEs formation in vitro, decreased the expression of IL‐1β, IL‐18, ICAM‐1, VCAM‐1, NLRP3, Caspase‐1 and CRP and reduced the ROS in HUVECs exposed to AGEs. Our findings suggested that the supplementation with dietary CA could prevent and delay the AGEs‐induced vascular dysfunction in diabetes.
Objectives: Preeclampsia, a pregnancy-specific syndrome, is associated with maternal systemic and placental inflammatory responses. Cell-free DNA (cfDNA) and cf-foetal DNA (cffDNA) in the blood are elevated in patients with preeclampsia and act as danger signals. Placenta-derived foetal DNA induces inflammatory responses and pregnancy complications in mice. However, whether extracellular DNA from the placenta really causes inflammatory responses remains unclear. Therefore, we investigated the effect of serum cfDNA and placental cffDNA on inflammatory responses using normal pregnant women and preeclampsia patients. Methods: Sera were taken from normal pregnant women and preeclampsia patients, and human trophoblast cell line Sw.71 cells were treated with serum with or without toll-like receptor 9 (TLR9; a sensor of exogenous DNA) inhibitor and genome elimination reagent. For cffDNA collection, placental tissue from the participants was cultured, and the released cffDNA was administrated to Sw.71 cells. Results: The amount of serum cfDNA was higher in preeclampsia patients than in normal pregnant women. Treatment of preeclampsia serum stimulated inflammatory cytokine secretion, which was inhibited by a genome elimination reagent. Expression levels of TLR9 and amount of cffDNA from the placenta were higher in preeclampsia patients than of normal pregnant women. Preeclampsia-derived cffDNA increased inflammatory cytokine levels compared with normal pregnant derived cffDNA. Conclusion: In human trophoblast cells, preeclampsia patient-derived cfDNA increased inflammatory cytokine levels via TLR9. Preeclampsia placenta released more cffDNA, which stimulated inflammatory cytokine. We suggest that elevated circulating cfDNA and cffDNA induces placental inflammatory responses, resulting in accelerated pathological features of preeclampsia.
Inflammasomes are multiprotein innate immune complexes that regulate caspase-dependent inflammation and cell death. Pattern recognition receptors, such as nucleotide-binding oligomerization domain (NOD)-like receptors and absent in melanoma 2 (AIM2)-like receptors, sense danger signals or cellular events to activate canonical inflammasomes, resulting in caspase 1 activation, pyroptosis and the secretion of IL-1β and IL-18. Non-canonical inflammasomes can be activated by intracellular lipopolysaccharides, toxins and some cell signalling pathways. These inflammasomes regulate the activation of alternative caspases (caspase 4, caspase 5, caspase 11 and caspase 8) that lead to pyroptosis, apoptosis and the regulation of other cellular pathways. Many inflammasome-related genes and proteins have been implicated in animal models of kidney disease. In particular, the NLRP3 (NOD-, LRR- and pyrin domain-containing 3) inflammasome has been shown to contribute to a wide range of acute and chronic microbial and non-microbial kidney diseases via canonical and non-canonical mechanisms that regulate inflammation, pyroptosis, apoptosis and fibrosis. In patients with chronic kidney disease, immunomodulation therapies targeting IL-1β such as canakinumab have been shown to prevent cardiovascular events. Moreover, findings in experimental models of kidney disease suggest that small-molecule inhibitors targeting NLRP3 and other inflammasome components are promising therapeutic agents.
NLRP3 (NOD-, LRR- and pyrin domain-containing protein 3) is an intracellular sensor that detects a broad range of microbial motifs, endogenous danger signals and environmental irritants, resulting in the formation and activation of the NLRP3 inflammasome. Assembly of the NLRP3 inflammasome leads to caspase 1-dependent release of the pro-inflammatory cytokines IL-1β and IL-18, as well as to gasdermin D-mediated pyroptotic cell death. Recent studies have revealed new regulators of the NLRP3 inflammasome, including new interacting or regulatory proteins, metabolic pathways and a regulatory mitochondrial hub. In this Review, we present the molecular, cell biological and biochemical bases of NLRP3 activation and regulation and describe how this mechanistic understanding is leading to potential therapeutics that target the NLRP3 inflammasome. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.
Problem To investigate whether metabolic syndrome (MetS) is associated with exacerbation of inflammatory responses in preeclamptic (PE) patients, the dynamic changes of Th17 and Treg cells, monocytes, cytokines and transcription pattern of inflammasome related genes were analyzed in 35 women with PE suffering from MetS in comparison to 38 PE women without MetS and healthy pregnant women. Method of study Expression of inflammasome related genes, cytokines and also TLR4 were measured using Real‐Time PCR. Serum and medium supernatant cytokines levels of PBMCs and serum levels of HMGB1 and Caspase‐1 were also evaluated by ELISA. Monocytes, Th17 and Treg cells frequency were also determined by flow cytometry. Result PE women with MetS exhibited increased percentage of non‐classical and intermediate monocytes and Th17 cells (p=0.025). Furthermore, decreased Treg cells frequency was also observed in PE women with MetS compared to PE women (p=0.019). The mRNA expression of inflammasome‐related genes (Caspase‐1, NLRP3, HMGB1), TLR4, IL‐1β, IL‐6, IL‐17, IL‐18, and TNF‐α were significantly higher in PE patients with MetS than that of the healthy pregnant individuals (p<0.0001) and PE patients (p<0.0001). Serum levels of TGF‐β and TNF‐α in PE patients with MetS were increased compared to other two groups, while IL‐10 levels significantly was reduced. A significant sFlt (p=0.016), Caspase‐1 (p=0.012), HMGB1 (p=0.016) upregulation and VEGF (p=0.023) downregulation were also observed in the serum of PE women having MetS compared to PE women. Conclusion MetS is closely related to the exacerbation of inflammatory reactions in PE. This study indicates that, in order to diminish the systemic features of PE, prior to conceive and start a pregnancy, Mets should be severely considered and managed. This article is protected by copyright. All rights reserved.
High mobility group box-1 protein (HMGB-1) is one of the most important DAMPs and has been previously shown to promote the formation of the NOD-like receptor with pyrin domain containing-3 (NLRP3) inflammasome in microglia. Interleukin 4 (IL4) is a Th2-derived cytokine that plays a significant role in the function of various immune cells. However, the underlying molecular mechanism by which IL4 signaling antagonizes NLRP3 inflammasome is poorly characterized. In particular, whether IL4 could modulate NLRP3 inflammasome in astrocytes remains unknown. In the present study, we elucidated this phenomenon and the mechanism by which IL4 inhibits HMGB1-mediated NLRP3 inflammasome formation in astrocytes. For this purpose, we cultured and extracted primary astrocytes, setup different concentrations of HMGB1, and used immunofluorescence and western blotting to detect NLRP3 inflammasome formation, including NLRP3, ASC and caspase-1, and signaling changes in the nuclear factor κB (NF-κB). Meanwhile, BAY 11–7082 and IL4 were added with HMGB1 to observe the NLRP3 inflammasome and changes in NF-κB expression. Our data showed that HMGB1 could effectively promote NLRP3 inflammasome formation by activating NF-κB in astrocytes. This effect can be inhibited by BAY 11–7082, a NF-κB inhibitor. Meanwhile, IL4 could activate PPARγ via the STAT6 singling pathway and inhibit NF-κB activation, significantly decreasing formation of the NLRP3 inflammasome complex. Our study demonstrated that the NLRP3 inflammasome complex is also expressed in astrocytes, and IL4 could inhibit HMGB1-mediated NLRP3 inflammasome formation, through negative regulation of NF-κB activity and promotion of PPARγ activation.
Intra-amniotic inflammation/infection is strongly associated with spontaneous preterm labor and birth, the leading cause of perinatal mortality and morbidity worldwide. Previous studies have suggested a role for the NLRP3 (NLR family pyrin domain-containing protein 3) inflammasome in the mechanisms that lead to preterm labor and birth. However, a causal link between the NLRP3 inflammasome and preterm labor/birth induced by intra-amniotic inflammation has not been established. Herein, using an animal model of lipopolysaccharide-induced intra-amniotic inflammation (IAI), we demonstrated that there was priming of the NLRP3 inflammasome 1) at the transcriptional level, indicated by enhanced mRNA expression of inflammasome-related genes (Nlrp3, Casp1, Il1b); and 2) at the protein level, indicated by greater protein concentrations of NLRP3, in both the fetal membranes and decidua basalis prior to preterm birth. Additionally, we showed that there was canonical activation of the NLRP3 inflammasome in the fetal membranes, but not in the decidua basalis, prior to IAI-induced preterm birth as evidenced by increased protein levels of active caspase-1. Protein concentrations of released IL1β were also increased in both the fetal membranes and decidua basalis, as well as in the amniotic fluid, prior to IAI-induced preterm birth. Finally, using the specific NLRP3 inhibitor, MCC950, we showed that in vivo inhibition of the NLRP3 inflammasome reduced IAI-induced preterm birth and neonatal mortality. Collectively, these results provide a causal link between NLRP3 inflammasome activation and spontaneous preterm labor and birth in the context of intra-amniotic inflammation. We also showed that, by targeting the NLRP3 inflammasome, adverse pregnancy and neonatal outcomes can be significantly reduced.