ArticlePDF AvailableLiterature Review

The role of hepatic microenvironment in hepatic fibrosis development

Taylor & Francis
Annals of Medicine
Authors:
  • 北京理工大学

Abstract and Figures

Aim Fibrosis is a common pathological feature of most types of chronic liver injuries. There is no specific treatment for liver fibrosis at present. The liver microenvironment, which fosters the survival and activity of liver cells, plays an important role in maintaining the normal structure and physiological function of the liver. The aim of this review is to deeply understand the role of the liver microenvironment in the dynamic and complicated development of liver fibrosis. Methods After searching in Elsevier ScienceDirect, PubMed and Web of Science databases using ‘liver fibrosis’ and ‘microenvironment’ as keywords, studies related to microenvironment in liver fibrosis was compiled and examined. Results The homeostasis of the liver microenvironment is disrupted during the development of liver fibrosis, affecting liver cell function, causing various types of cell reactions, and changing the cell-cell and cell-matrix interactions, eventually affecting fibrosis formation. Conclusion Liver microenvironment may be important for identifying potential therapeutic targets, and restoring microenvironment homeostasis may be an important strategy for promoting the reversal of liver fibrosis. KEY MESSAGES The homeostasis of the liver microenvironment is disrupted in liver fibrosis; A pro-fibrotic microenvironment is formed during the development of liver fibrosis; Restoring microenvironment homeostasis may be an important strategy for promoting the reversal of liver fibrosis.
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REVIEW ARTICLE
The role of hepatic microenvironment in hepatic fibrosis development
Ying Meng
a
, Tong Zhao
b
, Zhengyi Zhang
a
and Dekui Zhang
c
a
Department of General Medicine, Lanzhou University Second Hospital, Lanzhou, Gansu, China;
b
Department of Orthopedics,
Lanzhou University First Hospital, Lanzhou, Gansu, China;
c
Department of Gastroenterology, Lanzhou University Second Hospital,
Lanzhou, Gansu, China
ABSTRACT
Aim: Fibrosis is a common pathological feature of most types of chronic liver injuries. There is
no specific treatment for liver fibrosis at present. The liver microenvironment, which fosters the
survival and activity of liver cells, plays an important role in maintaining the normal structure
and physiological function of the liver. The aim of this review is to deeply understand the role
of the liver microenvironment in the dynamic and complicated development of liver fibrosis.
Methods: After searching in Elsevier ScienceDirect, PubMed and Web of Science databases
using liver fibrosisand microenvironmentas keywords, studies related to microenvironment in
liver fibrosis was compiled and examined.
Results: The homeostasis of the liver microenvironment is disrupted during the development of
liver fibrosis, affecting liver cell function, causing various types of cell reactions, and changing
the cell-cell and cell-matrix interactions, eventually affecting fibrosis formation.
Conclusion: Liver microenvironment may be important for identifying potential therapeutic tar-
gets, and restoring microenvironment homeostasis may be an important strategy for promoting
the reversal of liver fibrosis.
KEY MESSAGES
The homeostasis of the liver microenvironment is disrupted in liver fibrosis;
A pro-fibrotic microenvironment is formed during the development of liver fibrosis;
Restoring microenvironment homeostasis may be an important strategy for promoting the
reversal of liver fibrosis.
ARTICLE HISTORY
Received 18 July 2022
Revised 22 September 2022
Accepted 30 September 2022
KEYWORDS
Hepatic fibrosis;
microenvironment; therapy
1. Background
Chronic liver disease is a global public health problem. It
is estimated that currently, 844 million people suffer
from chronic liver disease worldwide, with an annual
death rate of about two million [1]. This is mainly
because most chronic liver injuries, such as toxic liver
disease, alcoholic liver disease, non-alcoholic fatty liver
disease, chronic viral hepatitis, and cholestatic liver dis-
ease, can develop into liver fibrosis [2,3]. This is a patho-
logical change resulting in increased extracellular matrix
(ECM) and decreased parenchymal cells in the liver.
Although mild fibrosis is mostly asymptomatic, it eventu-
ally progresses to cirrhosis and is often accompanied by
serious structural disorders and vascular distortion, which
is the leading cause of liver-related morbidity and mor-
tality [4]. Since most patients had already developed
obvious liver fibrosis or cirrhosis when they were first
clinically identified, anti-fibrosis drugs that can prevent
the progression of liver fibrosis or induce cirrhosis
regression are urgently needed [5].
2. Liver microenvironment and liver fibrosis
A significant number of literature reviews on the
mechanism of liver fibrosis have demonstrated that
the development of liver fibrosis is a complex and
dynamic process involving a variety of cells and mole-
cules, and the interaction between them is crucial to
the development direction of the disease. This com-
plex regulatory process makes the liver microenviron-
ment the focus of research. Multiple components,
including not only hepatocytes that account for the
largest proportion, but a variety of interstitial liver
cells, such as liver sinusoidal endothelial cells (LSEC),
liver macrophages, hepatic stellate cells (HSC), and
CONTACT Dekui Zhang zhangdk8616@126.com Department of Gastroenterology, Lanzhou University Second Hospital, Lanzhou University,
Lanzhou, Gansu 730030, China
ß2022 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (http://creativecommons.org/licenses/by-nc/4.0/),
which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
ANNALS OF MEDICINE
2022, VOL. 54, NO. 1, 28292843
https://doi.org/10.1080/07853890.2022.2132418
intrahepatic lymphocytes are included in the liver
microenvironment. These cells and their secreted cyto-
kines and ECM form a complex interaction network
essential for maintaining the livers normal physio-
logical functions.
When the liver suffers a persistent injury, microenvir-
onmental homeostasis is disrupted. The imbalances in
the microenvironment significantly affect cell function,
leading to various cellular responses and changing cell-
cell and cell-matrix interactions. Simultaneously, the dif-
ferent components of the liver microenvironment
change in this process, resulting in significant changes in
the physical and chemical properties as well as the struc-
ture of the liver microenvironment. The unique micro-
environment created by the above changes will
significantly influence the establishment and develop-
ment of fibrosis. Understanding the above-mentioned
alterations in the liver microenvironment may lead to a
better understanding of the regulatory effects of the
liver microenvironment, which is critical for finding
potential therapeutic targets.
2.1. Changes in the liver microenvironment in
liver fibrosis
2.1.1. Metabolic changes in the liver
microenvironment
The liver is the main metabolic organ of the body and
can regulate the metabolism of various nutrients [6].
Liver cells undergo metabolic reprogramming during
liver fibrosis development to adapt to the environ-
mental changes [7], which can affect a variety of meta-
bolic processes, including glucose, lipid, and amino
acid metabolism [8], and can also lead to mitochon-
drial dysfunction, the centre of cellular energy metab-
olism [9].
2.1.1.1. Alteration of glucose metabolism. Glucose
metabolism alters after chronic liver injury [10]. The
energy generation process from glucose will mostly
shift to glycolysis [11]. This enhanced glycolysis in an
aerobic state is called the Warburg effect [12,13], and
this kind of metabolic shift is strongly associated with
liver fibrosis development. In liver fibrosis, HSC is the
main source of myofibroblasts, and its activation is
accompanied by increased glycolysis [14]. It is possible
that glycolysis can better meet the activation energy
requirements of HSC since it produces ATPs more
quickly. Glycolysis inhibition could have toxic effects
on activated HSC, demonstrated as reduced cell prolif-
eration [15], and inhibited cell contraction [16]. The
above studies suggest glycolysis is necessary for
transdifferentiation from HSC to myofibroblasts. In
addition to HSC, studies have demonstrated that the
mode of energy production of hepatocytes changes
into glycolysis in the early stage of hepatic fibrosis.
However, this method of glucose metabolism cannot
generate enough energy, eventually leading to hep-
atocyte dysfunction, aggravating hepatocyte damage,
and fibrosis development [17]. In addition to the direct
influence of glycolysis energy supply on liver fibrosis,
the metabolism products in this process will also par-
ticipate in the fibrosis process. Glycolysis intermediates
have been reported to support cell anabolism and
promote cell proliferation [18,19]. Recent studies also
demonstrated that glycolysis metabolism products
could promote collagen synthesis in myofibroblast,
and inhibiting the formation of glycolytic intermedi-
ates in liver fibrosis mice model can reverse fibrosis
[20]. Furthermore, pyruvic acid, a product of glycolysis,
can be converted into lactic acid, leading to high lactic
acid levels [21,22], thus promoting the formation of an
acidic microenvironment, which can promote TGFb
activation and myofibroblast formation [23,24].
Several regulatory factors related to this glucose
metabolism pattern have been identified in liver fibro-
sis. Lactate dehydrogenase-A (LDH-A) is thought to
play an important role in this regulation of glycolysis.
Its high expression promotes glycolysis, which meets
the energy requirements for rapid cell proliferation. A
natural compound Oroxylin A has been found to block
aerobic glycolysis by inhibiting LDH-A, which inhibited
HSC contraction and alleviated liver fibrosis [16]. M2
type pyruvate kinase (PKM2) is also an important regu-
lator of aerobic glycolysis. Studies have found that
PKM2 expression is significantly upregulated in fibrotic
livers of mice and humans, of which its dimer struc-
ture induces HSC activation, while changing it into a
tetrameric structure will reduce aerobic glycolysis and
inhibit HSC activation [25]. Further studies have dem-
onstrated that induction of this glucose metabolism
process is significantly correlated with the Hedgehog
signalling pathway and the activity of hypoxia inducer
1a(HIF1a)[15]. Some enzymes in other metabolic
pathways also may influence sugar metabolic reprog-
ramming. Geranylgeranyl diphosphate synthase
(GGPPS) is a key enzyme in the mevalonate pathway,
and it is considered to be the risk factor for the pro-
gression of non-alcoholic fatty liver disease (NAFLD) to
fibrosis. The study found that the liver specificity lack-
ing GGPPS can enhance aerobic glycolysis process by
undermining the mitochondrial function, which can
induce liver inflammation and aggravate the progres-
sion of fibrosis [26].
2830 Y. MENG ET AL.
2.1.1.2. Alteration of lipid metabolism. Lipid metab-
olism disorder may occur during liver fibrosis develop-
ment [27,28]. Hepatocyte damage caused by various
harmful factors can lead to fatty acid oxidation dis-
order, causing an accumulation of free fatty acids, free
cholesterol, and other lipids in the liver. Excessive lipid
accumulation can promote liver fibrosis through differ-
ent mechanisms, such as disrupting energy metabol-
ism by promoting mitochondrial and hepatocyte
damage, causing hepatocytes to produce several pro-
fibrotic mediators, and promoting HSC activation [29].
A study has demonstrated that HSC activation is
accompanied by the gradual loss of retinol-containing
lipid droplets stored in HSC [30]. It has also been dem-
onstrated that the loss of lipid droplets is related to
autophagy activation, which promotes the decompos-
ition of lipids into fatty acids for b-oxidation, and the
resultant energy is essential for HSC activation and is
closely related to the ECM secretion and contraction
function of activated HSC [31].
Studies on the mechanisms of lipid metabolism dis-
orders in liver fibrosis suggest some possible targets
for intervention. Dual specificity phosphatase 9
(Dusp9) is a member of the DUSP protein family.
Studies have found that Dusp9 expression in liver tis-
sue is significantly decreased during the development
of liver fibrosis induced by a high-fat diet; however,
Dusp9 overexpression can prevent lipid accumulation
and inhibit the development of liver fibrosis by block-
ing apoptosis signal-regulated kinase 1 (ASK1) related
signalling pathway [32]. Fang et al. showed that cathe-
psin B (CatB) played a role in hepatic lipid accumula-
tion, and inhibition of CatB expression could inhibit
lipid production and improve fibrosis by blocking
CD36 and PPARcexpression [33]. Adenosine mono-
phosphate-activated protein kinase (AMPK) is also
involved in metabolic regulation in the development
of chronic liver disease. Its activation can reduce de
novo adipogenesis, increase fatty acid oxidation, and
reduce steatosis; meanwhile, AMPK activator has been
shown to reduce liver fat content and inhibit fibrogen-
esis in rodents [34]. Milk fat globule-epidermal growth
factor 8 (MFG-E8) is a multifunctional glycoprotein,
which has an anti-fibrosis effect, and Hu et al. further
found that it plays a vital role in the process of non-
alcoholic steatohepatitis (NASH). When MFG-E8 is
knocked down, liver steatosis and lipid accumulation
are aggravated, and fibrosis is promoted, confirming
that this is related to the activation of the TLR4/NF-jB
signalling pathway after the knockdown of MFG-E8
[35]. Sodium-glucose cotransporter 2 (SGLT2) inhibitors
can improve hepatic steatosis and fibrosis in patients
with chronic liver disease. Recent studies have shown
that the mechanism of the hepatic protective effect of
SGLT2 inhibitors is independent of glycemic control
but promotes liver phospholipid balance by regulating
lipid composition and thus forms the protective lipid
structure to play a beneficial role in the liver [36].
Genetic factors may also be involved in the progres-
sion of such diseases through lipid metabolism regula-
tion. Two genes, PNPLA3 and TM6SF2, were found to
be associated with NAFLD in genome-wide
Association Studies (GWAS), and they are considered
to be markers of steatosis and more severe fibrosis.
Detection of PNPLA3 and TM6SF2 gene variants may
enhance the risk assessment of such diseases [37].
Different drugs targeting lipid metabolic disorders in
liver fibrosis are being screened. A study has found
that Jiaqi Ganxian granules may inhibit collagen depos-
ition in the liver by regulating lipid metabolism [38].
Bile acid receptor (FXR) can inhibit hepatic bile acid
synthesis and promote hepatic bile acid transport
in vivo. Studies have demonstrated that the FXR agonist
GW4064 can significantly reduce liver injury in model
animals with cholestatic liver disease [39]. GW4064 may
also inhibit the development of liver fibrosis by inhibit-
ing HSC activation [40]. Peroxisome proliferator-acti-
vated receptor (PPAR) regulates fatty acid metabolism
in the liver, and recent studies have demonstrated that
PPARaantagonist GW6471 can improve NASH [41].
Other studies have demonstrated that lipid-regulating
drugs fenofibrate and ciprofibrate can reduce pulmon-
ary fibrosis, collagen production, and myofibroblast dif-
ferentiation in mice [42,43]. Edible insects as a possible
dietary supplement may play a role in antifibrosis
through liver lipid metabolism regulation. A study
found that oral allomyrina dichotoma Larva extract
(ADLE) can improve the high-fat diet-induced liver
fibrosis in mice, which is closely related to the regula-
tion of lipid metabolism. They further found that ADLE
not only inhibits the expression of genes related to adi-
pogenesis but also attenuates hepatic steatosis by
increasing AMPK phosphorylation [44].
2.1.1.3. Alteration of amino acids metabolism.
Proteins are the building blocks of life. As the basic
unit of proteins, amino acids are essential for various
life activities of cells. The liver is the main organ
responsible for amino acid metabolism. In liver fibrosis,
there is an imbalance in the proportion of amino acids,
manifested by a decrease in branched amino acids and
an increase in aromatic amino acids [45]. Previous stud-
ies have demonstrated that a decrease in the ratio of
branched amino acids to tyrosine (BTR) is closely
ANNALS OF MEDICINE 2831
related to liver fibrosis progression [46]. Gaggini et al.
believed that glutamate was an amino acid more
closely related to the severity of liver fibrosis. It was
found that the concentrations of glycine and serine in
plasma of patients with non-alcoholic fatty liver disease
decreased while the concentration of glutamate
increased, and combined with liver histological analysis,
they proposed that GSG index (glutamate/[serine þgly-
cine]) may be a useful index for evaluating the severity
of liver fibrosis [47]. Furthermore, glutamine metabolism
was found to be associated with liver fibrosis.
Glutamine metabolism can provide cells with energy
and raw materials for macromolecular synthesis.
Glutamine is metabolised into a ketone glutaric acid
that can be used as amino acid and lipid precursor and
can also provide energy support to the Krebs cycle.
Synthetic metabolism is also thought to be associated
with liver fibrosis. Du et al. found that glutaminase
expression in the livers of mice and humans rises sig-
nificantly in fibrosis, prompting an increase in glutam-
ine metabolism. They further depicted that glutaminase
promotes glutamine decomposition in myofibroblasts,
which plays an essential role in maintaining cell pheno-
type and can adjust the proliferation and migration of
myofibroblasts [48]. Li et al. also noticed that the
expression of genes involved in glutamine metabolism,
including glutaminase (GLS), aspartate aminotransferase
(GOT1), and glutamate dehydrogenase (GLUD1), were
significantly upregulated in the human fibrotic liver,
suggesting an increase in glutamine catabolism.
Furthermore, in vitro studies have displayed that glu-
tamine breakdown plays an important role in HSC acti-
vation and proliferation [49].
Liver metabolic function damage is the main cause
of amino acid imbalance in the development of liver
fibrosis after liver injury. However, Huang et al. recently
reported that the ECM microstructure could regulate
amino acid metabolism through the mechanism associ-
ated with integrin b1. It has a regulatory effect on tryp-
tophan, branched-chain amino acid and methionine in
the hepatocyte, which will provide new ideas for further
study of amino acid metabolism in the fibrotic liver [50].
Some drugs targeting changes in amino acid
metabolism in liver fibrosis may play a beneficial role.
AXA1665 is a novel amino acid component, and
research suggests that it can improve amino acid
imbalance and has good safety and tolerability when
given orally to cirrhotic subjects [51]. As the concen-
tration of branched-chain amino acids (BCAA)
decreases in the development of liver fibrosis, it is rec-
ommended to use BCAA to improve protein balance.
However, clinical trials do not show the significant
benefits of BCAA supplements. Researchers have
speculated that supplementing the branched chain
ketones acid may correct the amino acid imbalance,
which still needs further study to evaluate its role in
hepatic fibrosis [52].
2.1.1.4. Mitochondrial dysfunction. Mitochondria pro-
duce a small number of reactive oxygen species under
physiological conditions. When mitochondria are dys-
functional, significant electron leakage occurs, resulting
in numerous reactive oxygen species, and the subse-
quent oxidative stress reaction can cause cell damage
[53]. The production of a large number of reactive oxy-
gen species due to mitochondrial dysfunction is consid-
ered a key driver of liver fibrosis, and it has been found
that liver mitochondrial function is significantly
impaired in liver fibrosis development [5456]. Protein
spectrum analysis suggested that oxidative stress and
lipid peroxidation-related proteins were upregulated in
the rat liver fibrosis model [57]. Proteomic analysis of
human fibrotic liver tissue also depicted that oxidative
stress, and liver mitochondrial dysfunction was signifi-
cantly correlated with fibrosis development [58]. In add-
ition to causing mitochondrial dysfunction, excessive
reactive oxygen species generated by mitochondria can
activate inflammatory pathways and directly promote
the expression of pro-fibrosis factor transforming
growth factor-beta (TGFb)[59].
There have been some studies on the mechanism of
mitochondrial dysfunction in liver fibrosis. Mitochondrial
membrane lipid composition is essential to maintain the
structure and function of mitochondria. Studies have
found that cardiolipin and phosphatidylethanolamine in
mitochondrial lipids gradually decrease in the progres-
sion of chronic liver disease, which is associated with
increased oxidative stress in mitochondria. The research-
ers also believe these changes in mitochondrial lipids
are the early events of mitochondrial dysfunction and
the progression of chronic liver disease [60]. Fernandez-
tussy et al. found that glycine-N-methyltransferase
(GNMT) is an important regulator of Complex II activity
in the mitochondrial electron transport chain. GNMT
expression is controlled by Mir-873-5p in hepatocytes of
NASH patients, and up-regulation of Mir-873-5p leads to
the down-regulation of GNMT, thereby inhibiting mito-
chondrial function. While inhibition of Mir-873-5p in the
corresponding mouse model can improve mitochondrial
function and inhibit fibrosis progression [61]. Krishnan
et al. found that mitochondrial dysfunction in liver fibro-
sis is associated with the liver pyruvate kinase (L-PK)
expression, which can alter mitochondrial pyruvate flux
and its binding to citrate. Silencing L-PK in a mouse
2832 Y. MENG ET AL.
fibrosis model improves mitochondrial function and
reduces fibrosis [62]. Moreover, Pusec et al. found that
hexokinase domain protein 1 (HKDC1), a member of the
hexokinase family, was highly expressed in the fibrotic
liver of mice, localized to mitochondria of hepatocytes
and caused mitochondrial dysfunction, manifested by
decreased cellular respiration and decreased mitochon-
drial membrane potential. Furthermore, this was mainly
related to increased mitochondrial dynamic-related pro-
tein 1 (DRP1) [63].
A significant amount of data indicates that metabolic
changes in the liver microenvironment are strongly
associated with the development of liver fibrosis. It is
not only characteristic of liver fibrosis but also plays an
important role in disease development. The metabolites
produced due to the unbalanced metabolic environ-
ment can directly damage parenchymal cells and further
induce inflammation to aggravate liver injury. More
importantly, the metabolic reprogramming process cre-
ates an environment conducive to the information of
myofibroblast. The process provides the necessary
energy for myofibroblast formation, and the metabolites
provide the biomolecular raw materials for maintaining
phenotype and promoting myofibroblast proliferation.
All these changes promote the development of liver
fibrosis (Figure 1). The in-depth study of the above
metabolic abnormalities will not only further clarify the
complex mechanism of liver fibrosis but may also pro-
vide effective new targets for treating liver fibrosis.
2.1.2. The immune microenvironment in liver
fibrosis
Persistent chronic inflammation is the basis of hepatic
fibrosis and continues throughout all stages of fibrosis
development. Under various liver injury conditions,
various liver innate immune cells, including liver mac-
rophages and lymphocytes, are activated to initiate
inflammatory responses [64,65]. Kupffer cells can pro-
duce active oxygen species, TGFb, platelet-derived
growth factor (PDGF), and other pro-inflammatory and
pro-fibrosis factors, as well as secrete many chemo-
kines to recruit circulating immune cells (monocytes,
lymphocytes) to the site of injury [6670]. Natural killer
(NK) cells and natural killer T (NKT) cells are abundant
in the liver. NK cells express effector molecules to kill
or promote activated HSC apoptosis through the
degranulation effect after activation [71]. However, its
anti-fibrosis activity is limited to the early stages of
liver fibrosis and inhibited in the later stage [72,73].
Activated NKT cells can promote fibrosis by producing
pro-fibrotic cytokines such as IL-4, IL-13, hedgehog lig-
and, and osteopontin [74]. Furthermore, damaged
LSECs can secrete pro-inflammatory cytokines, such as
TGF-b1, interleukin, and tumour necrosis factor a
(TNFa), to promote the recruitment of circulating
inflammatory cells [75]. The damaged hepatocytes
secrete various pro-inflammatory and pro-fibrotic fac-
tors to maintain their survival, thereby directly or indir-
ectly activating HSC [76]. Immune cells recruited to
the damaged liver can also continue to synthesize and
secrete a variety of pro-inflammatory and pro-fibrotic
factors: mononuclear cells will differentiate into macro-
phages and produce a variety of cytokines, including
TGFb, platelet-derived growth factor (PDGF), TNFa,
tumour necrosis factor b(TNFb) and interleukin-1b(IL-
1b), further promoting HSC activation [77]. T helper
cell 2 (Th2) stimulates TGFbsynthesis by secreting IL-
13 and macrophage differentiation by secreting IL-4
and IL-13 [78]. T helper cell 17 (Th17) can secrete IL-7,
which can either directly induce HSC activation or
Figure 1. Metabolic changes in the liver microenvironment.
ANNALS OF MEDICINE 2833
indirectly promote fibrosis by upregulating TGFb
expression to promote neutrophil recruitment to the
liver [78,79].
Although the inflammation induced by liver injury
at the initial stage promotes tissue repair, the influ-
ence of persistent injury factors on liver homeostasis
disrupts the livers inflammatory balance, leading to
excessive inflammation, effectively changing the inter-
action between liver cells, and driving the liver micro-
environment towards promoting fibrosis. The final
result is that HSC, the main source of myofibroblasts
in liver fibrosis, transforms into an activated state and
enters the permanent stage under the stimulation of
various cell products [80]. Furthermore, other intrahe-
patic cells, including fibroblasts, hepatocytes, and
extrahepatic bone marrow-derived cells recruited into
liver tissue, may also be transformed into myofibro-
blast phenotype under the above factors [8185].
Myofibroblasts are the core components of fibrotic
lesions; their increased concentration leads to exces-
sive ECM synthesis, thereby promoting fibrosis
progression.
The changes of immune microenvironment in the
development of fibrosis are caused by innate immune
cell activation in the liver, the recruitment of systemic
immune cells to the liver tissue, and their interaction
with intrahepatic cells, all of which promote the
phenotypic transformation of liver cells. The trans-
formed cells can secrete pro-inflammatory factors and
continue to receive inflammatory stimulation, which
maintains the activated state of myofibroblasts.
Chemokines are also secreted to recruit more immune
cells to the damaged liver, thereby aggravating the
inflammatory damage [86,87]. All these factors lead to
an uncontrollable vicious cycle and aggravate the
damage to the intrahepatic microenvironment homeo-
stasis (Figure 2).
However, studies have also found that liver cells
may play different roles in the inflammatory response.
Regulating cellular interactions by promoting the sta-
bility of the liver microenvironment may help restore
inflammatory balance and even promote fibrosis
regression. Macrophages will form different pheno-
types under the influence of the tissues microenviron-
ment, and their specific signals can promote or inhibit
the fibrosis process. For example, hepatic macro-
phages can promote fibrosis by activating HSC in liver
injury. However, studies have also found that in the
CCL
4
-induced liver injury model, when macrophages
develop a recovery phenotype, they can inhibit the
fibrosis process by inhibiting inflammation and reduc-
ing the formation of myofibroblasts [88] and may pro-
mote the regression of fibrosis by secreting a variety
of matrix metalloproteinases [89,90]. NKT cells are also
plastic, and their differentiation into subtypes depends
on the microenvironment in which they are located.
Its different subtypes regulate immune processes
through different cytokine expression profiles, and
studies suggest that such different regulatory effects
on inflammation may inhibit the process of fibrosis
[91]. Studies have found that NKT cells can relieve liver
fibrosis by releasing IFN-cto kill activated HSC [92].
Other studies have demonstrated that IL-30 can
improve liver fibrosis by killing activated HSC through
Figure 2. The changes of immune microenvironment in liver fibrosis.
2834 Y. MENG ET AL.
NKT cells [93]. T lymphocytes can also differentiate
into different cell phenotypes in response to different
microenvironments such when they are differentiated
into TH1 cells, interferon-creleased by them can
antagonize the activity of TGFband thus alleviate liver
fibrosis [79].
In summary, the outcome of the inflammatory
response is influenced by the local microenvironment,
and it is essential to understand how it varies under
different disease conditions. Inflammation may act as
a double-edged sword in the development of liver
fibrosis; uncontrolled inflammation resulting from liver
homeostasis imbalance can aggravate the hepatic
injury and promote the production of myofibroblasts.
However, it may also restore inflammatory equilibrium
by changing the liver microenvironment, which may
produce an integrated regulatory effect on liver cells,
and promote the inflammatory reaction in the direc-
tion of tissue repair and fibrogenolysis.
2.1.3. Vascular changes in the liver
microenvironment
The development of liver fibrosis is accompanied by
changes in the vascular system, primarily manifested
as vascular remodelling and angiogenesis (Figure 3).
Vascular remodelling is characterized by hepatic sinus-
oid capillarization, mainly caused by structural dys-
function of LSEC, which constitutes the wall of the
hepatic sinusoid [9497]. Normal LSEC lacks a base-
ment membrane and has the characteristic structure
of fenestration, which plays an important role in regu-
lating blood flow and material exchange in the hep-
atic sinusoid [98,99]. Studies have depicted that under
various liver injury factors, LSEC will lose the character-
istic fenestration, resulting in the formation of a
continuous basement membrane of hepatic sinusoid
[100,101]. This change is also known as the capillariza-
tion of LSEC. Capillarized LSECs are also closely related
to fibrosis development, and research indicates that
capillarized LSECs will lose their ability to inhibit HSC
activation [102]. Meanwhile, structural dysfunction of
LSEC is considered the leading cause of increased por-
tal vein resistance in patients with liver dis-
ease [103,104].
The progression of liver fibrosis is also accompanied
by angiogenesis. Due to the massive ECM deposition,
the liver structure is disorganized, resulting in the
obstruction of hepatic sinusoidal blood flow and even-
tual hypoxia. The dysfunction of LSEC structure and
function will affect oxygen exchange, further aggravat-
ing insufficient oxygen supply to the tissues. Hypoxic
conditions can lead to the upregulation of angiogenic
factors, thus inducing new angiogenesis [66,105].
Furthermore, studies have demonstrated that chronic
liver inflammation stimulates epithelial cells, HSC, and
LSEC to express a variety of angiogenic factors, such
as fibroblast growth factor (FGF), vascular endothelial
growth factor (VEGF), platelet-derived growth factor
(PDGF), and angiopoietin (Ang) [96,106]. Other studies
have demonstrated that VN 1-rich exosomes secreted
by lipotoxic hepatocytes can also promote angiogen-
esis [107]. Different studies have demonstrated that
angiogenesis under such pathological conditions is
also associated with liver fibrosis progression, which
may disrupt normal tissue repair and maintain inflam-
matory processes to promote fibrosis [108]. Other
studies have revealed that, in addition to promoting
angiogenesis, angiogenic factors can promote liver
fibrosis by increasing ECM production and portal vein
pressure [109].
Figure 3. Vascular changes in the liver microenvironment.
ANNALS OF MEDICINE 2835
The role of vascular remodelling and angiogenesis
in promoting liver fibrosis is closely related to their
interaction with the liver microenvironment. LSEC, an
important component of liver microcirculation, can
regulate the balance between liver regeneration and
fibrosis by producing angiocrine factors. However, the
angiocrine factors produced by LSEC become pro-
fibrotic in the microenvironment of liver fibrosis [110].
Although angiogenesis can provide nutrition for tis-
sues, they become the source of inflammatory factors
in the process of fibrosis and promote the vicious
cycle by maintaining local chronic inflammation, thus
promoting disease progression [111,112].
Due to the association of vascular remodelling and
angiogenesis with fibrosis, vascular alterations in liver
fibrosis may also be a therapeutic target. Several stud-
ies have shown that inhibiting LSEC capillarization can
stabilize the HSC phenotype, and inhibiting angiogen-
esis in liver fibrosis can reduce portal pressure and
liver inflammation, both of which can inhibit fibrosis
development [106]. Taura et al. reported that blocking
angiogenic signalling of HSC in BALB/C mice with liver
fibrosis induced by carbon tetrachloride (CCl
4
) or bile
duct ligation (BDL) could inhibit fibrosis [113].
Therefore, targeting hepatic vascular regulation may
also be an option for maintaining organ homeostasis
in liver fibrosis. However, the timing of vascular inter-
vention must be thoroughly evaluated, as physio-
logical angiogenesis is also necessary for injury repair.
2.1.4. Changes in ECM in the liver
microenvironment
ECM consists of collagen, fibronectin, hyaluronic acid,
proteoglycan, and other molecules that undergo highly
dynamic changes during synthesis and breakdown. ECM
is essential for maintaining the structural integrity of tis-
sues under physiological conditions. During the devel-
opment of liver fibrosis, persistent inflammation and
changes in metabolic and vascular components in the
microenvironment lead to the production of a large
number of myofibroblasts, the immediate cause of
excessive ECM. The expression of tissue inhibitors of
matrix metalloproteinases (TIMP), especially TIMP-1,
increases in the damaged liver tissue, and it inactivates
MMPs by binding to them, resulting in reduced ECM
degradation [114]. Furthermore, increased collagen
cross-linking hinders collagen degradation, primarily
mediated by lysyl oxidase (LOX) and transglutaminase
(TGase). Different studies have shown that the activity
and expression of LOX and TGase are significantly
increased in fibrotic liver, which can increase liver hard-
ness and inhibit the breakdown effect of MMP on ECM
by mediating collagen cross-linking to increase its stabil-
ity [115120]. Furthermore, as liver fibrosis develops, not
only does the amount of ECM increase, but also the
composition and properties of ECM change significantly.
The main components change from type IV and VI colla-
gen to type I and III collagen, which becomes resistant
to degradation [121,122]. Under the influence of the
above factors, ECM remodelling is excessive and uncon-
trollable, leading to a large amount of ECM deposition.
ECM deposition is not only the result of the devel-
opment of liver fibrosis but also an essential compo-
nent of the hepatic fibrosis microenvironment.
Excessive ECM deposition can significantly alter liver
microenvironmental homeostasis and affect various
liver cells. Different studies have demonstrated that
ECM composition and configuration changes can be
used as extracellular signals to activate intracellular
signal transduction, influencing cell phenotype and
function [123]. The direct interaction between ECM
and cells is mainly mediated by cell surface receptor
integrin and discoidin domain receptor (DDR). A study
showed that the integrin aVb6 expression was signifi-
cantly upregulated in both rodent models of liver
fibrosis and patients with chronic liver disease [124].
Integrin has been found to play a critical role in latent
TGFbactivation, a key regulator of fibrosis [125].
Increased DDR1 expression has also been noticed in
liver fibrosis, and research indicates that the inter-
action between DDR1 and collagen promotes white
blood cell migration in liver tissues [126]. Since ECM
also serves as a reservoir of cytokines in tissues, its
components can regulate cell function by capturing or
releasing cytokines [127]. There is evidence that the
biological activity of cytokines can be regulated by
binding to ECM molecules [128]. Activation of ECM-
stored cytokines changes the behaviour of LSEC [129].
Mechanical stimulation caused by changes in ECM
configuration and density also impacts the fibrosis
process, significantly promoting the differentiation of
HSC and portal fibroblasts into myofibroblasts
[130,131]. Liu et al. reported that mechanical signals
generated by collagen fibres promoted intracellular
signal transduction through DDR2 receptors on HSC
and were also closely related to HSC activation [132].
Moreover, sclerotic ECM has been revealed to promote
the progression of liver fibrosis by activating Rho-asso-
ciated protein kinase (ROCK) and the YAP/TAZ signal-
ling pathway in myofibroblasts [120].
Changes in the liver microenvironment promote
ECM synthesis over ECM degradation, producing
pathological ECM in large quantities. However, ECM
overproduction is not the end point of fibrosis
2836 Y. MENG ET AL.
development but rather a component of the micro-
environment to influence the behaviour of liver cells
through abnormal physical and chemical signals. This
can establish a vicious cycle and lead to persistent
fibrosis progression even after the damage factors are
removed (Figure 4).
The shortest method for regression of excessive ECM
in liver fibrosis is to promote ECM degradation.
Regulating the release and activity of collagenase may
be advantageous. Research has shown that collagenase
encapsulated with nanoparticles can reverse fibrosis after
targeted delivery to the fibrotic liver and release it in an
active form [133]. Furthermore, research on cellular ther-
apy for liver fibrosis has shown that bone-marrow-
derived macrophages differentiated under the condition
of CSF-1 stimulation can inhibit fibrosis when injected
into mice with liver fibrosis. This is believed to be related
to increased MMP expression, promoting collagen deg-
radation [134]. Other studies have suggested that LOXL2
inhibition plays an important role in preventing the
development of pathological microenvironment in
fibrotic diseases. The inhibitory monoclonal antibody
AB0023 against LOXL2 can significantly reduce fibroblast
activation and the production of related growth factors
and cytokines in fibrotic diseases [135]. Ikenaga et al.
also reported that inhibiting LOXL2 in liver fibrosis can
effectively inhibit liver fibrosis progression [119]. The
regulation of mechanical signals in ECM may also inhibit
fibrosis development. Yes-associated protein (YAP) is an
essential transcriptional coactivator for mechanical signal
transduction. Inhibiting YAP expression or pharmaco-
logical inhibition of YAP can prevent HSC activation
in vitro, whereas pharmacological inhibition of YAP can
inhibit CCL
4
-induced liver fibrosis in mice [136]. ROCK
activity is essential for cellular mechanical perception,
and selective ROCK inhibitor Y27632 has been shown to
prevent CCL
4
-induced liver fibrosis, reduce collagen con-
tent, and HSC activation in rats [137].
2.2. Therapeutic strategies based on liver fibrosis
microenvironment
Liver fibrosis is a complicated heterogeneous disease
characterized by forming a pro-fibrotic microenviron-
ment in the liver due to interactions between different
types of cells, thereby promoting disease progression.
Figure 4. changes in ECM in the liver microenvironment.
ANNALS OF MEDICINE 2837
Liver fibrosis was initially thought to be irreversible.
However, cholestatic and viral liver disease studies
demonstrate that liver fibrosis regression is possible
even in advanced stages [138,139]. The strong regen-
erative ability of the liver further stimulates the
research into the reversal of liver fibrosis.
There are no specific drugs against liver fibrosis,
and the best treatment for liver fibrosis is still the
removal of damage factors [140,141]. However, this is
insufficient to improve the prognosis of all patients
with liver fibrosis. Based on microenvironmental
changes, the pathogenesis of liver fibrosis indicates
that its development is a complex dynamic process,
and targeting a single pathway in the anti-fibrosis
strategy is unlikely to prevent or reverse fibrosis.
Similar clinical studies have confirmed that single-tar-
get selective drugs cannot successfully treat fibrosis in
the human body [142,143]. Since various types of liver
cells and molecules are involved in liver fibrosis devel-
opment, therapeutic strategies to promote microenvir-
onmental homeostasis may have a global regulatory
action on the functional conditions of various liver
cells, inhibiting cell transition to a pro-fibrotic state,
thereby forming a homeostasis feedback loop, a key
to reverse fibrosis. It has been demonstrated that hep-
atocytes from cirrhotic tissues regained their metabolic
activity and ability to secrete liver-specific proteins
after exposure to a healthy liver microenvironment
[144]. Other studies have demonstrated that myofibro-
blasts could revert to an inactive phenotype as fibrosis
regresses and pro-fibrosis signals diminish [145,146].
These studies suggest that regulating the microenvir-
onment in response to the characteristic changes of
the fibrotic microenvironment may play an important
role in preventing or reversing fibrosis.
Currently, most of the studies on liver fibrosis focus
on a specific factor or cell necessary for the occurrence
and development of liver fibrosis but is insufficient to
explain the multi-step process of continuous progres-
sion of liver fibrosis. Changes in the microenvironment
during liver fibrosis development suggest that it is a
complex multicellular disease. Although the massive
production of myofibroblasts is the key factor in liver
fibrosis development, it is not the only intervention tar-
get. Changes in the microenvironment during this pro-
cess need to be identified. It is necessary to restore
microenvironmental homeostasis to fundamentally
inhibit the formation and sustained activation of myofi-
broblasts in liver fibrosis, thus inhibiting the dynamic
progression of the disease. Based on the characteristics
of the liver fibrotic microenvironment, a holistic thera-
peutic strategy for reshaping microenvironmental
homeostasis may improve or even reverse fibrosis since
it can restore and maintain normal liver cell function.
This requires multi-target combination therapy.
However, further evaluation of possible interactions
between multiple drug combinations is required to
improve efficacy.
3. Conclusion
Liver fibrosis is the result of multicellular interaction in
a pro-fibrotic microenvironment. The involvement of
liver microenvironment changes in its progression sug-
gests that the therapeutic target for liver fibrosis
should be identified in the liver microenvironment.
Restoration of microenvironmental homeostasis from a
holistic perspective can enable all kinds of liver cells
to maintain a more stable and long-lasting condition
and inhibit the transformation of liver cells into a pro-
fibrotic state, which may be an important strategy to
promote the reversal of liver fibrosis.
Author contributions
YM and TZ conceived and designed the paper, and reviewed
and summarized the related literature; YM wrote the first
draft of the manuscript; YM, TZ, ZZ and DZ contributed to
interpreting data, revising the manuscript, and approved the
published version. All authors agree to be accountable for
all aspects of the work.
Disclosure statement
No potential conflict of interest was reported by
the author(s).
Funding
The study was supported by the Cuiying Scientific and
Technological Innovation Program of Lanzhou University
Second Hospital [No.2020QN-11] and the Natural Science
Foundation of Gansu [21JR7RA397].
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... MCP-1 is one of the key cytokines that regulates the migration and infiltration of monocytes/macrophages, which initiates further inflammatory cellular responses (Tedgui and Mallat, 2006). MCP-1 is induced and secreted in endothelial cells stimulated by TNF-α (Meng et al., 2022). Therefore, we tested whether AT or ATF prevented TNF-α-induced MCP-1. ...
... Recently, we reported that AT significantly blocked cholestasis-induced liver damage via the inhibition of inflammation and fibrosis (Bae et al., 2022). In hepatic diseases such as hepatitis and cirrhosis, elevated systemic and hepatic TNF-α levels have been linked to vascular dysfunction (Meng et al., 2022). Endothelial cells in a vessel are a major component cell type involved in vascular function since they act as the first barrier against injury (Kitada et al., 2016). ...
... NF-κB is a key transcriptional factor that regulates inflammatory genes (Liu et al., 2017). The NF-κB motif in the promoters of ICAM-1, VCAM-1, and MCP-1 is particularly important for the induction of these genes (Rothgiesser et al., 2010;Meng et al., 2022). AT and ATF inhibited the TNF-α-induced nuclear translocation and transcriptional activity of NF-κB. ...
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Acer tegmentosum Maxim (AT) is a medicinal plant used to treat hepatic, neurological diseases, and cancer. However, the beneficial effects of AT on endothelial dysfunction have not been reported yet. In this study, we evaluated the effects of AT and the main compounds against TNF-α-mediated inflammatory responses and their possible mechanism of action. The anti-inflammatory effect and its molecular mechanism were analyzed by adhesion assay, immunoblotting, promoter-luciferase assay, ELISA, RT-PCR, immunocytochemistry, immunoprecipitation, siRNA gene knockdown, docking, and molecular dynamics simulation. AT and its compounds salidroside and tyrosol reduced TNF-α-induced adhesion between monocytes and endothelial cells. Fermentation of AT with Bacillus subtilis converted salidroside to tyrosol, which is salidroside’s aglycone. The fermented AT product (ATF) potently inhibited TNF-α-mediated monocyte adhesion with higher potency than AT. AT or ATF abrogated TNF-α-induced expression of adhesion molecules (VCAM-1 and ICAM-1) and production of MCP-1 with the inhibition of phosphorylated MAP kinases. TNF-α-mediated NF-κB transactivation and RelA/p65 acetylation were suppressed by AT and ATF through the interaction of NF-κB with sirtuin-1 (SIRT1), an NAD⁺-dependent histone deacetylase. Sirt1 gene knockdown diminished the protective effects of AT and ATF against TNF-α-mediated signaling and inflammatory response. Interestingly, SIRT1 protein expression was significantly increased by ATF and tyrosol rather than by AT and salidroside, respectively. Molecular docking showed that the tyrosol moiety is critical for the interaction with Glu²³⁰ of SIRT1 (PDB ID: 4ZZH and 4ZZJ) for the deacetylase activity. Molecular dynamics revealed that tyrosol can induce the movement of the N-terminal domain toward the catalytic domain of SIRT1. This study demonstrates the potential of AT and ATF to prevent endothelial inflammation and vascular dysfunction of the retina by the MAPK/NF-κB/SIRT1 signaling pathways and targeting of the tyrosol moiety to Glu²³⁰ in SIRT1.
... The liver microenvironment plays an important role in the formation of liver fibrosis. Factors such as the presence of inflammatory cells, particularly macrophages, and the ECM's composition can also significantly influence HSC activation and proliferation [21]. During liver injury, activated hepatic stellate cells (HSCs), macrophages, endothelial cells, and hepatocytes produce the platelet-derived growth factor (PDGF), which plays a key role in liver fibrosis. ...
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... Moreover, TGF-β1-induced increased levels of ROS and 8-OHdG, a DNA damage marker, were inhibited by circABHD3 knockdown (Fig 2D and 2E). In addition, hepatic fibrosis is associated with mitochondrial dysfunction, which can lead to hepatocyte damage, immune cell activation, inflammation, and trans-differentiation of hepatic stellate cells [30][31][32]. We found that TGF-β1-induced increased JC-1 monomer and decreased JC-1 aggregate in TGF-β1-treated THLE-2 and AML12 cells were reduced by circABHD3 knockdown, suggesting that circABHD3 knockdown suppressed TGF-β1-induced impaired mitochondrial membrane potential (Fig 2F). ...
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The EMPA-REG OUTCOME (Empagliflozin, Cardiovascular Outcome Event Trial in patients with Type 2 Diabetes Mellitus (T2DM)) trial evidenced the potential of sodium-glucose cotransporter 2 (SGLT2) inhibitors for the treatment of patients with diabetes and cardiovascular disease. Recent evidences have shown the benefits of the SGLT2 inhibitor empagliflozin on improving liver steatosis and fibrosis in patients with T2DM. Metabolomic studies have been shown to be very useful to improve the understanding of liver pathophysiology during the development and progression of metabolic hepatic diseases, and because the effects of empagliflozin and of other SGLT2 inhibitors on the complete metabolic profile of the liver has never been analysed before, we decided to study the impact on the liver of male Zucker diabetic fatty (ZDF) rats of a treatment for 6 weeks with empagliflozin using an untargeted metabolomics approach, with the purpose to help to clarify the benefits of the use of empagliflozin at hepatic level. We found that empagliflozin is able to change the hepatic lipidome towards a protective profile, through an increase of monounsaturated and polyunsaturated glycerides, phosphatidylcholines, phosphatidylethanolamines, lysophosphatidylinositols and lysophosphatidylcholines. Empagliflozin also induces a decrease in the levels of the markers of inflammation IL-6, chemerin and chemerin receptor in the liver. Our results provide new evidences regarding the molecular pathways through which empagliflozin could exert hepatoprotector beneficial effects in T2DM.
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