Introduction
Cholestatic liver disease (CLD) manifests as a multitude of etiological
heterogeneous hepatobiliary disorders, mainly including primary biliary
cholangitis (PBC) and primary sclerosing cholangitis (PSC), in adults.
CLD, which is also called cholestasis, describes a range of conditions
caused by the accumulation of bile acids in the liver, resulting in
hepatocellular necrosis, apoptosis, progressive fibrosis, and even
end-stage liver
disease[1].
Accumulated evidence has proved that ursodeoxycholic acid (UDCA) can
decrease the progression of
PBC[2]; however,
approximately one-third of patients do not respond to UDCA
treatment[3]. In
2016, obeticholic acid (OCA), a farnesoid X receptor (FXR) agonist, was
found could decrease serum alkaline phosphatase (AP) level in CLD
patient, was approved by the FDA for UDCA
nonresponders[4,
5]. In spite of this, long-term
follow-up is needed to confirm the safety and effectiveness of this
novel treatment. At present, there is no clinical evidence that there is
any medical therapy can alter the course of PSC. Current treatment
regimens focus on symptom management and treatment of
cholangitis[6].
Effective drugs for PBC and PSC are still limited and new treatment
strategies are urgently needed.
Increasing evidence shows that human cholestasis is closely related to
the disorder of microbiome
composition[7,
8], increased intestinal permeability,
enhanced translocation of pathogenic bacteria and bacterial toxins, such
as lipopolysaccharide (LPS) into the
liver[9,
10]. Inflammasomes and
proinflammatory cytokines are then activated due to these
microbe-derived products, which are recognized by the innate immune
system via pathogen recognition receptors (TLRs and
NLRs)[11].When persistent liver inflammation is unresolved, the proinflammatory
milieu can play a detrimental role in parenchymal and nonparenchymal
liver cells resulting in fibrosis and ultimately loss of
function[12].
Macrophages, which are composed of resident tissue macrophages and
monocyte-derived recruited cells, can differentiate into either
classically activated macrophages (a pro-inflammatory phenotype, also
called M1 polarity) or alternatively activated macrophages (M2 polarity)
which express anti-inflammatory
cytokines[13].
Recent studies have demonstrated that the intestinal microbiome leads to
the process of cholestasis-mediated cell death and inflammation by
activating the mechanisms of the inflammasome in
macrophages[14].
Additionally, clinical studies have suggested that in cholestasis
patients, the recruitment of monocytes and macrophages in diseased liver
is significantly
increased[15]. The
expression of various monocyte chemotactic proteins, such as monocyte
chemoattractant protein (MCP)-1 is significantly increased in the livers
of patients with
cholestasis[16]. In
parallel to these findings, C–C chemokine receptor type-2 (CCR-2)
expressed by liver macrophages is accompanied by increasing macrophage
numbers in the livers of cholestasis
patients[17].
Importantly, macrophages in CLD patients are more susceptible to
stimulation signals such as endotoxin (for example, LPS), which have
also been shown to be increased in cholestasis
patients[18]. In
line with the above findings, the expression and activity of TLR4 (the
primary receptor of LPS) are upregulated on the monocytes of CLD
patients, leading to LPS hyperreactivity and increased production of
proinflammatory cytokines [i.e., interleukin (IL)-1β, IL-6, and
IL-8][19]. It is
reported that macrophage-derived IL-1β is a key cytokine activating
hepatocyte nuclear factor κB (NFκb), which is the master inflammation
regulator. Activation of NFκb can also interfere with FXR and liver X
receptor (LXR) signaling, which results in transcriptional suppression
of bile and sterol transporters, finally culminating in
cholestasis[20].
The above evidence indicates that hepatic macrophages, namely Kupffer
cells (KCs), exert a pivotal role during the development of CLD.
Traditional Chinese medicines (TCMs) have been demonstrated to be an
important source for potential drug
discovery[21].
Tectorigenin (TEC), a plant isoflavone, has attracted much attention due
to its multiple activities such as antiproliferation, antiinflammatory
and antioxidant effects. We and others have demonstrated that TEC can
inhibit macrophage activation (M1 polarity) in vivo and in
vitro[22,
23]. However, the underlying molecular
mechanisms still require further investigation. It is noteworthy that
TEC has been widely reported to provide protective functions in the
liver[24-26]. This
evidence prompted us to hypothesize that TEC could alleviate CLD by
suppressing hepatic macrophage recruitment and activation. Thus, in the
present study, we investigated whether TEC intervention could improve
the development of CLD in the ANIT-induced and DDC-induced mouse models.
We also investigated the molecular mechanism by which TEC regulates the
polarization of bone marrow-derived macrophages (BMDMs) and primary
mouse KCs.