1. Background
Pulmonary fibrosis (PF) is a type of interstitial lung disease that arises due
to various causative factors, both inside and outside the lungs. It is
characterized by the infiltration of inflammatory cells such as macrophages and
lymphocytes, the proliferation of myofibroblasts, and the accumulation of
extracellular matrix (ECM) within the interstitial space. Most interstitial lung
diseases ultimately result in PF, which seriously affects patient health and can
even lead to death. The etiology of idiopathic PF (IPF) is unknown [1]. IPF often
exhibits chronic progression, massive fibroblast proliferation, ECM deposition,
restrictive ventilatory dysfunction in the lungs, and eventually death because of
respiratory failure [2]. The prevalence rate of IPF ranges from 0.5 to 18 cases
per 100,000 individuals, while the average lifespan after diagnosis is typically
only 2–3 years. At present, there are few effective approaches for the
prevention and treatment of IPF [1, 2]. Hence, there is an urgent need to
investigate the development of PF, and especially the molecular mechanism
underlying IPF. This should lead to the discovery of more effective drugs for the
prevention and treatment of PF.
Abnormal myofibroblast proliferation and massive ECM secretion are prominent
features of PF [3, 4]. The classic pathogenesis of fibrotic diseases defines
resident fibroblasts as the main source of myofibroblasts. However, recent
studies suggest that pericytes are one of the essential sources of myofibroblasts
[5, 6, 7, 8, 9, 10]. Studies on liver and kidney injuries have confirmed the role of pericytes
in fibrosis [6, 7]. Moreover, studies on spinal cord injuries suggest that
pericytes are direct effector cells in fibrosis [8, 9]. Kuppe et al. [10]
employed single cell RNA sequencing (RNA-seq) to comprehensively map the entire human kidney. They
profiled the transcriptomes of both proximal and non-proximal tubule cells in
healthy and fibrotic kidneys. This approach identified all matrix-producing cells
with high precision, revealing distinct subpopulations of pericytes and
fibroblasts as the primary cell types responsible for generating scar-forming
myofibroblasts during fibrosis in the human kidney. Studies conducted on
different organs and employing cell-fate mapping have provided support for
pericyte-myofibroblast transition (PMT) in PF. Pericytes may therefore be a novel
target in the treatment of fibrotic diseases [5, 6, 7]. Wang et al. [11]
found that regulation of the Lysine-specific demethylase 5B
(KDM5B)/platelet-derived growth factor receptor alpha or beta
(PDGFR/) signaling pathway through overexpression of growth
arrest-specific transcript 5 inhibited transforming growth factor-1
(TGF-1)-induced pericytomyoblast differentiation, thereby reducing
bleomycin-induced PF in mice. Hannan et al. [12] found that pulmonary
pericytes were myofibroblast progenitors in a bleomycin-induced model of lung
injury. Sun et al. [13] reported that inhibition of fucosyltransferase 8
expression effectively impeded the transdifferentiation of pericytes into
myofibroblasts. Finally, pericytes have also been shown to participate in airway
remodeling and to transform into myofibroblasts in a mouse model of chronic
allergic asthma [14]. In light of the above findings, we herein describe the
characteristics of pericytes and their possible mechanism of involvement in PF.
We also provide a theoretical basis for the use of pericytes to treat PF.
2. The Epidemiology of Pulmonary Fibrosis
Interstitial lung disease refers to a group of diseases in which cell
proliferation, interstitial inflammation and fibrosis occur after alveolar wall
damage due to various causes. Idiopathic interstitial pneumonia, specifically
IPF, is widely recognized as the most prevalent and severe form within this
category [1, 2]. IPF is a fatal, irreversible, progressive, and chronic fibrotic
respiratory disease of unknown etiology. This non-cancer-related lung disease has
poor prognosis and a high prevalence. Because of similar risk factors, the 2019
coronavirus disease pandemic has further increased the burden of IPF [15]. The
prevalence of IPF has been increasing over the years, with estimates ranging from
2.8 to 18 cases per year per 100,000 individuals in Europe and North America.
Although data on global variation is limited, the occurrence of IPF is thought to
be comparatively lower in Asia and South America, with estimates ranging from 0.5
to 4.2 cases per 100,000 individuals annually. IPF tends to affect males more
frequently and is uncommon in individuals aged 50 years, with the typical age
at diagnosis being about 65 years [2]. This disease exhibits a range of outcomes
and is difficult to predict, with an average survival period of 2–3 years
following diagnosis. Respiratory failure typically ensues as a result of the
illness. IPF has an unfavorable prognosis and does not respond well to
conventional medications used in fibrosis treatment. Individuals with IPF may
encounter sudden episodes of the condition due to unknown causes, leading to high
mortality rates [16]. Hence, in order to promptly diagnose IPF it is important to
understand the underlying cellular and molecular mechanisms, thereby also
facilitating appropriate drug discovery.
3. Growing Focus on the Involvement of Pericytes in the Development of
Pulmonary Fibrosis
PF is a consequence of various genetic and environmental risk factors, including
but not limited to patient exposure to tobacco smoke or asbestos, autoimmune
disorders, and inflammatory conditions [17]. Infection, trauma, cellular stress,
and inflammation serve as triggers for wound healing, which tries to restore the
normal functions of damaged tissues before eventually ceasing. Prolonged or
recurrent injuries lead to excessive production of angiogenic factors alongside
the inflammatory and fibrotic cytokines. This dysregulated and unresolved
wound-healing response results in fibrosis, which is characterized by the
replacement of normal tissues with scar tissue, and leads to a pathological
decline in organ function [18, 19, 20]. After sustaining an injury, lung epithelial
cells release a variety of potent factors that can stimulate fibroblasts,
including TGF-, connective tissue growth factor, prostaglandin E2, sonic
hedgehog, Wnt1-inducible signaling pathway protein 1, and
interleukin-1 (IL-1) [18, 21]. Myofibroblasts
play a crucial role in fibrotic diseases as they are primarily responsible for
constructing and modifying the ECM. The expression of alpha-smooth muscle actin
(-SMA) by myofibroblasts distinguishes them from other cell types. It
is well established that fibroblasts produce collagens and numerous other
proteins that form the ECM of fibrous connective tissues. Vimentin, collagen
triple helix repeat containing 1, and fibroblast-specific protein 11 have served
as markers to identify fibroblasts by immunohistochemical techniques [22, 23]. Due
to their abundance in active scar lesions and their rarity in healthy organs,
myofibroblasts have emerged as a significant therapeutic target for fibrotic
diseases. Consequently, extensive research has been conducted to investigate the
origin of myofibroblasts. Various cell types have been proposed as potential
sources for their generation, including pericytes, interstitial
-SMA-fibroblasts, epithelial cells, endothelial cells, and
hematopoietic fibroblasts [17]. Additionally, ECM increases pulmonary ischemia
and hypoxia, thus further promoting PF [24].
Using a bleomycin-induced mouse model of fibrosis, previous studies have shown
that most pulmonary myofibroblasts are derived from pericytes. Pericytes can be
distinguished from fibrotic cells in many organs. Recent analysis has revealed
that pericyte-like cells are important myofibroblast progenitors [24]. Other
findings also support the notion that pericytes are an important source of
myofibroblasts [25]. Researchers have recently applied genetic fate mapping
technology to investigate whether pericytes are an important source of pulmonary
interstitial myofibroblasts [17]. The mechanism underlying pericyte activation
and the subsequent molecular biological effects could provide new intervention
strategies for the treatment of PF. Research suggests that several cell types
release cytokines such as TGF- and platelet-derived growth factor (PDGF) including alveolar epithelial
cells, vascular endothelial cells, and macrophages. These cytokines then activate
signaling pathways in pericytes. Consequently, downstream proteins are
phosphorylated, ultimately leading to pericyte activation [26, 27, 28, 29]. Zhao
et al. [29] reported that myofibroblasts cause scar formation during
healing. TGF- is a fibrogenic cytokine, and TGF-beta-induced
phosphorylation of Smad2 and Smad3 in the nucleus mediate pro-fibrotic gene
expression. Siedlecki et al. [28] found that PDGF signal transduction
was crucial for pericellular cells. The recruitment of these cells induced
vascular maturation, which was mainly dependent on PDGF. Pericellular cells
migrate to new blood vessels along the PDGF gradient, proliferate, and then bind
to the inner duct to achieve vascular stability via physical contact. The above
studies show that excessive activation of multiple signaling pathways can cause
pericyte activation. The potential treatment of PF could thus involve targeting
pericytes to effectively inhibit the progression of this condition.
4. Biological Characteristics of Pericytes
The Definition and Origin of Pericytes
The lung plays a vital role in various biological processes, including the
exchange of gases, immune surveillance, and the maintenance of a protective
barrier. This highly vascularized organ receives blood supply from both the
pulmonary and bronchial arteries. Therefore, pericytes are likely to have
significant involvement in lung physiology due to their strategic positioning
within the perivascular niche [30]. Pulmonary pericytes play a crucial role in
the development of blood vessels and ensure the integrity of endothelial cells,
thereby contributing to the maintenance of organ homeostasis [31]. Clark
et al. [32] first reported the existence of pericytes in 1925 when they
observed that connective tissue components of tadpole larvae formed pericytes on
capillaries. Subsequent investigations employing chicken-quail chimeras and
specific markers for individual cells demonstrated that pericytes originate from
the neural crest and are predominantly found in the head region and central
nervous system [33, 34]. Fate-mapping analysis in mice utilized gene reporters to
demonstrate that pericytes in the gut, liver, heart, and lung originate from
cells derived from the mesothelium through processes such as
epithelial-mesenchymal transition (EMT), stratification, and migration [35].
Pericytes are vascular wall cells within the basement membrane and are connected
to endothelial cells. They are a heterogeneous cell population that vary greatly
depending on the tissue type [36]. As pericytes come into contact with the
microvascular endothelium, they play a crucial role in promoting vascular
maturation and ensuring its stability. This includes capillaries, arterioles
before capillaries, venules after capillaries, and collecting venules [30].
Pericytes function in the regulation of blood vessels, and they are key cells in
cardiovascular diseases.
In addition, pericytes may serve as progenitor cells for tissue regeneration
[37, 38]. According to Birbrair et al. [39], two distinct pericellular
subtypes exist in the interstitium of skeletal muscle: type 1
(Nestin/NG2) and type 2 (Nestin/NG2). Their findings
indicate that type 2 pericytes are involved in muscle regeneration following
injury, without the generation of fat tissue. Conversely, type 1 pericytes do not
contribute to muscle formation, but instead play a role in the accumulation of
adipose tissue. This implies that both type 1 and type 2 pericytes have the
ability to regenerate, and that their differences may arise from the equilibrium
between myogenic and non-myogenic processes. However, further investigation is
required to determine whether the transplantation of type 2 pericytes enhances
physiological performance and the repair and regeneration of skeletal muscle
[39]. Another study showed that pericytes exist in the small blood vessels of
skeletal muscle and promote the growth and regeneration of skeletal muscle after
birth [40]. Type 1 rather than type 2 pericytes exhibit fibrogenic properties
both in vitro and in vivo, and are involved in the accumulation
of fibrous tissue within various organs such as skeletal muscle and lungs [41].
In vivo pericyte lineage tracking studies showed that endogenous
pericytes can differentiate into osteoblasts and osteoblasts, thereby serving as
a source of osteoblasts for the promotion of fracture healing [42]. This suggests
that pericytes are potential candidates for the treatment of bone diseases. In
addition to their involvement in skeletal muscle, recent research suggests that
CD146 pericytes derived from microtia tissue could be a valuable cell
source for the regeneration of ear cartilage [43]. Type 1 and type 2 pericytes
have distinct roles in neurogenesis. While type 1 pericytes generate
-SMA pericytes, they do not give rise to nerve cells. Conversely, under
optimized culture conditions, type 2 pericytes can produce neural progenitors
that resemble brain neuron-glial antigen2 (NG2) glial cells
[44]. Teichert et al. [45] demonstrated the significance of
pericyte-expressed Tie2 receptor in regulating angiogenesis and vascular
maturation. In contrast to type 1 pericytes, type 2 pericytes play a significant
role in angiogenesis in tumors including lung cancer and glioblastoma [46, 47].
Together, these studies suggest that pericytes have unique regenerative
capabilities, which could lead to the development of new therapeutic strategies.
5. Physiological Functions of Pericytes
5.1 Pericyte biomarkers
Pericyte characteristics include pericyte/endothelial cell coverage, shape, and
several markers that validate pericyte heterogeneity [31]. These cells are
commonly identified using markers such as -SMA, NG2, alkaline
phosphatase, and PDGFR. Additional markers linked to pericytes include
desmin, vimentin, aminopeptidase N, CD13, CD133, and CD146 [37, 48, 49, 50]. Novel
pericyte identifiers include the regulator of G protein signaling 5, endosialin,
and delta-like homolog 1. Expression of the key pericyte markers is dynamic and
varies according to developmental stage, disease, and specific tissue [35].
Marker levels also vary depending on the stage and location of the pericyte. For
example, pericytes expressing -SMA are frequently observed in the
retina and retinopathy. The expression of NG2 and PDGFR- is elevated in
pericytes associated with cardiac fibrosis and PF [51]. Furthermore, increased
levels of G protein signaling 5 were consistently observed in angiogenic
pericytes, indicating the extent of vascular remodeling [52]. However, due to the
nature of pericytes as potential progenitors, none of the markers examined was
specific. Moreover, pericytes share common markers with neighboring cells. For
example, PDGFR- is a well-known pericyte marker, but is also expressed
in smooth muscle cells, myofibroblasts, stem cells, and neuronal progenitor cells
[53]. NG2 is also a well-known pericyte marker, but has been found in adult skin
stem cells, adipocytes and oligodendrocyte progenitors [54, 55]. -SMA
serves as a common marker for pericytes with contractile properties, smooth
muscle cells within blood vessels, and myofibroblasts [56]. Currently, there are
no specific markers for identifying pericytes, and use of the above markers is
not a reliable way to track pericytes. However, a seminal report by Yamaguchi
et al. [57] showed that simultaneous positivity for PDGFRB and
chondroitin sulfate proteoglycan 4 detected only pericytes in the mouse brain. It
should therefore be emphasized that the markers described above are expressed not
only in pericytes but also in other cell types. Therefore, it is often necessary
to combine multiple markers in order to identify and verify the presence and
function of pericytes. Further study of pericytes should lead to the discovery of
new markers, or refinement in the use of existing markers.
5.2 Interaction between Pericytes and Endothelial Cells
Reciprocal regulation between pericytes and endothelial cells occurs throughout
embryonic development, adult homeostasis, and injury responses. PDGF, notch,
epidermal growth factor, hedgehog, ephrin, sphingosine-1-phosphate receptor, and
stromal-derived factor-1 are thought to establish connections between pericytes
and endothelial cells through specific pathways. These interactions play a
crucial role in modulating the proliferation, differentiation, localization, and
stability of the vasculature during development. Pericytes and endothelial cells
work together to deposit and organize the vascular basement membrane.
Additionally, pericytes can detach from the endothelium and transform into
myofibroblasts, thereby disrupting vascular homeostasis [17].
Endothelial-pericyte communication is generally mediated via specialized
intercellular junctions. For example, the peg-socket structure binds to
endothelial cells and pericytes, and connects to the endothelial cells and cell
matrix [37, 48]. The degree of pericyte vascular coverage varies between different
organs and anatomical structures, with a relatively high occurrence observed in
the lungs. In pulmonary tissue, pericytes are found within the capillary basement
membrane and are able to enhance peg-socket interactions with endothelial cells,
eventually leading to the formation of adhesions, gaps, and tight junctions
between one or more endothelial cells and a pericyte [58, 59, 60]. The
pericyte-endothelial ratio varies across tissues, and the number and size of
pericyte-endothelial contacts also varies widely between different tissues and
vessels. The ratio of endothelial cells to pericytes in normal tissues can range
from 1:1 to 10:1. The ratio is highest (1:1) in neural tissues and especially in
the retina, probably due to their high metabolic activity and the need to control
blood flow. The opposite is observed in skeletal muscle tissues, where the ratio
of endothelial cells to pericytes is 10:1 [37, 61]. Differences in the dispersion
and arrangement of pericytes suggest a wide range of microvascular variation and
adaptability at the cellular level, probably due to the array of pericyte
functions in governing various physiological systems. Close connections exist
between endothelial cells and pericytes, and their mutual communication is
essential for neovascularization and the maintenance of vascular stability.
Endothelial cell-pericyte interactions play a crucial role in
the regulation of vascular remodeling and stability, together with various
factors such as TGF-, angiopoietin 1/angiopoietin 2/Tie-2 receptor,
PDGF-B/PDGFR-, vascular endothelial growth factor (VEGF), and
sphingosine-1-phosphate receptor [35]. A previous study found that
recombinant fucosyltransferase 8, PDGFR and TGFR are
key proteins involved in pericyte activation [25]. Interactions between pericytes
and endothelial cells trigger activation of the latent TGF- signaling
pathway, thereby facilitating the differentiation and proliferation of both cell
types by engaging activin receptor-like kinase 1, activin receptor-like kinase 5,
and endorphins as receptors. Additionally, pericytes are stimulated to secrete
extracellular collagen via Smad2 signaling [62]. Angiopoietin/iron (Ang/Tie)
signals also control the association between endothelial cells and pericytes.
Tie2 expressed by endothelial cells is activated by Ang1 expressed on pericytes,
thus helping to maintain the static phenotype of endothelial cells. Ang1-driven
Tie2 phosphorylation activates downstream pathways that mediate cell survival,
proliferation, migration and anti-inflammatory signals [45]. PDGF
secreted by angiogenic endothelial cells is the most characteristic growth factor
and can recruit pericytes that express PDGFR. Pericytes migrate and
proliferate to neovascularization along the PDGF gradient and bind to the
endothelial tube. Vascular stability is achieved through physical contact, VEGF
and the Ang1/Tie2 system [28]. The sphingosine-1-phosphate (S1P)/S1P1 receptor
signaling axis also plays a crucial role in pericyte coverage by regulating
N-cadherin, with pericyte coverage being an important sign of vascular maturation
[62].
5.3 Pericyte Secretion
Pericytes release various cytokines, immunomodulatory factors and ECM, which
regulate tissue repair and regeneration [63, 64]. These cells also contain various
pro-inflammatory factors, including IL-6, IL-8, TNF-alpha, and interferon
gamma-inducible protein 10 (IP-10) that play a role in triggering inflammation
responses and influencing T-cell activity [65, 66, 67]. Pericytes also release
substances such as leukemia inhibitory factor (LIF), cyclooxygenase-2 (COX-2),
and heme oxygenase-1 (HMOX-1) that suppress the inflammatory response during
periods of inflammation [68, 69]. Furthermore, pericytes prevent cell failure by
releasing bone morphogenetic protein-4, 6, 7 (Bmp-4, 6, 7) and preserving the
regenerative capacity of stem cells [70, 71]. VEGF stimulates the differentiation
and proliferation of vascular endothelial cells and pericytes, thereby
contributing to angiogenesis and vascular stabilization [72]. Moreover, ECM
secreted by pericytes is crucial for tissue repair and regeneration [73, 74]. In
addition, pericytes contribute to the regulation of ECM formation and fibrosis by
producing secreted protein acidic and cysteine-rich (SPARC)
proteins [75]. Overall, pericytes secrete numerous cytokines and growth factors,
making them pivotal for the maintenance of tissue and organ functions (Table 1,
Ref. [65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75]).
Table 1.Pericyte secretion products and functions.
Pericyte secretion products |
Functions |
|
IL-6, 8, TNF-, IP-10 |
Induce inflammation and contribute to T cell activity |
[65, 66, 67] |
COX-2, HMOX-1, LIF |
Inhibit the inflammatory response during inflammation |
[68, 69] |
Bmp-4, 6, 7 |
Prevent cell exhaustion and preserve stem cell regeneration |
[70, 71] |
VEGF |
Stimulates the differentiation and proliferation of vascular endothelial cells and pericytes, and contributes to the formation and stability of blood vessels |
[72] |
ECM |
Promotes tissue repair and regeneration processes |
[73, 74] |
SPARC |
Irritable fibrosis |
[75] |
IL-6, interleukin-6; TNF-, tumor necrosis factor-alpha; IP-10,
interferon gamma-inducible protein 10; COX-2, cyclooxygenase-2; HMOX-1, heme
oxygenase-1; LIF, leukemia inhibitory factor; Bmp-4, 6, 7, bone morphogenetic
protein-4, 6, 7; VEGF, vascular endothelial growth factor; ECM, extracellular
matrix; SPARC, secreted protein acidic and cysteine-rich.
6. Possible Mechanism Underlying the Transformation of Pericytes to
Myofibroblasts in Pulmonary Fibrosis
6.1 The Connection between Pericytes and Myofibroblasts in Pulmonary
Fibrosis
Myofibroblast proliferation and massive ECM secretion are prominent features of
PF [3, 4]. Resident fibroblasts are considered to be the main source of
myofibroblasts. However, studies on systemic sclerosis and on fibrosis in a
variety of organs have shown that pericytes act as precursors of myofibroblasts
[76, 77, 78]. Following tissue injury, pericytes readily detach from the vasculature
and differentiate into myofibroblasts. Subsequently, these cells deposit ECM
which leads to fibrosis [35, 79]. The mechanisms underlying the production of ECM
by pericytes remain unclear [80]. Hung et al. [4] reported that
pericytes expressing myofibroblast markers were present in mouse pulmonary
fibrotic lesions. These authors performed functional enrichment analysis of
differentially expressed genes identified by microarray experiments and found
that Foxd1-derived pericytes expressed the typical markers PDGFR, NG2,
and CD146 during cell culture. Following treatment with TGF-,
-SMA expression was upregulated and collagen type 1 was expressed,
indicating that peri-pulmonary cells can express these two myofibroblast markers.
After bleomycin-induced lung injury in mice, Foxd1 ancestral pericytes account
for a large proportion of -SMA myofibroblasts, which proliferate
locally and produce collagen in fibroblast foci [4]. Subsequently, Yamaguchi
et al. [57] reported the presence of pericytes in the fibrotic lesions
of IPF patients. These authors also performed cell experiments to show that
pericytes could transform into myofibroblasts after TGF- treatment. The
aforementioned results indicate the importance of PMT in the onset and
progression of PF.
6.2 Fibrosis is Characterized by an Increased Number of
Fibrotic Cells and a Build-up of Extracellular Matrix
Fibrotic cells encompass fibroblasts and myofibroblasts. The development of
fibrosis is closely associated with pericytes due to the proliferation and
transdifferentiation of fibroblasts into myofibroblasts [81]. Numerous cytokines
secreted by pericytes have the ability to stimulate the growth and activation of
fibrotic cells. For example, TGF- derived from pericytes plays a
critical role in the progression of fibrosis as it enhances ECM secretion and
promotes the activation and proliferation of fibrotic cells [82].
In addition to their direct interaction with fibrotic cells, pericytes
contribute to fibrosis by assuming a similar role to fibrotic cells. ECM is the
primary component of fibrosis and consists of collagen, elastic fibers, and
proteoglycans [83]. ECM production is primarily attributed to myofibroblasts,
although a recent investigation found that pericytes can also secrete collagen
and exhibit comparable functionalities to myofibroblasts [39]. Moreover,
pericytes secrete SPARC protein, which is responsible for regulating the ECM
[75]. -SMA, PDGFR-, and endosialin (CD248) are common markers
of pericytes and fibrotic cells. Pericytes possess myofibroblast properties in
the fibrotic lung microenvironment [37] and are a critical factor in the fibrosis
process due to their similarities with fibrotic cells.
7. Signaling Pathways Associated with the Transformation of Pericytes
to Myofibroblasts
7.1 TGF- Pathway
TGF- serves as a regulator that promotes fibrosis. Injured epithelial
cells generate TGF- and other signals known as damage-associated
molecular patterns (DAMPs) that can be detected by immune receptors on both
immune and non-immune cells [84]. Several studies have investigated the effect of
TGF- on PMT and the mechanism underlying the pathogenesis of fibrosis.
TGF- ligand was reported bind and phosphorylate TGF-R, thus
promoting ECM formation. Various downstream regulators are
activated during this process, including the Smad2/3 signaling pathway, non-Smad
pathways such as the mitogen-activated protein kinase (MAPK) pathway, the
rho-like GTPase signaling pathway, and the phosphatidylinositol 3-Kinase
(PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway. Smad2 and Smad3 play
crucial roles by stimulating pericyte proliferation, differentiation, and
migration via TGF-. The regulation of mTOR activity in pericytes may be
influenced to some extent by Akt. These findings suggest that pericyte
proliferation and PMT induced by TGF- are mediated through activation of
the Smad2/3 and Akt/mTOR signaling pathways, while TGF- attenuates
laser-induced fibrosis by inhibiting the Akt/mTOR pathway [36].
Castellano et al. [85] reported that PMT occurs in a
model of lipopolysaccharide (LPS)-induced acute renal fibrosis. Renal pericytes
are activated in response to acute stimuli, such as LPS-induced acute kidney
injury. In both in vivo and in vitro settings, PMT can be
observed following 9 hours of LPS stimulation. In addition, the regulation of PMT
involves interaction between toll-like receptor 4 (TLR-4) and TGF-
signaling. Enhanced fibroblast responsiveness to TGF- stimulation is
observed upon activation of TLR4 signaling. Moreover, LPS increases the
phosphorylation of SMAD2/3 and extracellular regulated protein kinase 1 (ERK1),
whereas TGF- directly induces the phosphorylation of SMAD2/3. In
summary, LPS-induced pericytes express high levels of TLR-4 and TGF-,
promote the phosphorylation of SMAD2/3 and ERK1, and undergo PMT, thus leading to
acute renal fibrosis [85].
Previous studies have also shown that pericytes occur in the abundant
microvasculature of the human lung. The production of TGF- by
fibroblasts and resident macrophages in the interstitial space of the lung leads
to accumulation of -SMA pericytes in fibrotic lesions. Activation
of the Smad signaling pathway is triggered by the interaction between
TGF- and TGF-R located on the pericyte cell membrane. P-Smad2/3
accumulation in the nucleus increases binding to Smad-binding elements, thereby
increasing the transcription of fibrotic genes. Thus, increased secretion of
matrix proteins (including type I collagen) leads to increased ECM stiffness. In
a stiff environment, focal adhesion kinase increases the translocation of
megakaryoblastic leukemia 1, yes-associated protein (YAP), and transcriptional
coactivators with PDZ-binding motif (TAZ), thus increasing the expression of
-SMA and myofibroblast transformation [5].
7.2 The PDGFR Pathway
PDGF plays a crucial role in promoting fibrosis in various organs. Inhibition of
PDGF could potentially serve as an effective therapeutic approach for numerous
fibrotic diseases. Fibrosis encompasses several pathways, with PDGF signaling
being a key mediator. The proliferation, migration, and production of ECM are key
processes in fibrosis and are driven by the activation of stromal-mesenchymal
cells that express PDGFR- and -. The bleomycin model is widely
recognized as an effective in vivo model for the study of PF. A
significant increase in PDGF expression was reported following the induction of
PF by bleomycin [3]. PDGF has the ability to promote cell proliferation,
movement, and EMT. The various isoforms of PDGFR have been implicated in
several pathological conditions of mesenchymal cells that contribute to the
development of diseases such as PF. Additionally, the expression of PDGF-A and
PDGF-C has been observed in other cell types present in injured or fibrotic lungs
[86, 87]. Moreover, PDGFR- expression is often increased in glomerular
disease [88]. Further study of lung fibrosis and of signaling associated with
PDGFR is therefore crucial for future intervention strategies [80].
PF is characterized by the conversion of pericytes into myofibroblasts through
activation of the TGF-/Smad2/3 and PDGF-/ERK signaling
pathways. Crosstalk between TGF- and PDGF signaling is commonly found in
endothelial cells and pericytes. TGF- signaling induces phosphorylation
of Smad2/3 proteins, thereby regulating the expression of fiber genes, while PDGF
signaling activates ERK1/2 in pericytes. The progression of PF involves the
transformation of pericytes into myofibroblasts via stimulation of the
TGF-/Smad2/3 and PDGF-/ERK signaling cascades. Additionally,
Sun et al. [25] reported that glycosylation can affect the binding of
receptors (TGFR and PDGFR) to related ligands. These authors
showed that inhibition of the glycosylation modification of TGFR and
PDGFR can block pericyte activation, thereby alleviating PF. Similarly,
Wang et al. [11] reported that IPF progression can be delayed by
blocking activation of the PDGFR/ signaling pathway. In IPF,
lncRNA growth arrest Specific 5 hinders PMT by suppressing
PDGFR/ expression via demethylation of KDM5B-mediated
H3K4me2/3.
7.3 The Receptorless-associated Integration Site
(Wnt)/–catenin Pathway
Wnt signaling is a highly conserved signal transduction pathway that facilitates
intercellular communication to regulate various cellular processes. These include
the determination of cell fate, establishment of polarity, promotion of
differentiation, and the facilitation of migration. This complex signaling
cascade plays a critical role during organ and embryo development. Wnt signaling
has also been implicated in the wound healing process. In this particular
context, canonical signaling stabilizes the -catenin complex within the
cytoplasm before it is transported to the nucleus. This transportation
subsequently triggers gene transcription events that are vital for tissue repair
and regeneration. Certain target genes controlled by -catenin play
significant roles in tissue repair and regeneration processes, including
cyclinD1, CD44, and VEGF [89]. Wnt/-catenin as well as atypical Wnt
signaling are activated during human renal fibrosis, and this could play a
dominant role in the persistence of fibrotic cells [90]. FoxM1 has been
recognized as a crucial regulator of the Wnt/-catenin signaling pathway
by retaining -catenin within the nucleus and enhancing its transcription
factor activity through direct binding to the -catenin promoter region.
FoxM1 interacts directly with Smad3 proteins to facilitate their nuclear
localization and also binds to specific sequences in the -catenin
promoter, thereby promoting fibrogenesis. Exosomes derived from pulmonary
vascular endothelial cells and characterized by low levels of let-7d drive
fibrosis in lung pericytes through activation of the
TGFRI/FoxM1/Smad/-catenin signaling cascade [91].
7.4 Notch Pathway
The communication mechanism known as Notch signaling is a highly conserved
process that plays a significant role in maintaining lung function, responding to
injury, and facilitating tissue repair [92]. The Notch signaling pathway has been
implicated in pericyte differentiation and contributes to the development and
progression of IPF. Rat models of lung fibrosis show increased levels of the
Notch1 receptor and its ligands. Additionally, Notch1 promotes angiogenesis by
stimulating the expression of PDGFR in pericytes [16]. Notch1 also
regulates the activity of PDGFR, which in turn controls various
downstream signaling molecules such as Ras, PI3K, and phospholipase C. Moreover,
PDGFR plays a role in the migration of vascular smooth muscle cells by
modulating Rho-related protein kinase 1 (ROCK1). Given that ROCK1 is also
implicated in the development of PF, the PDGFR–ROCK1 interaction has
been speculated to contribute to IPF. In summary, Notch1 induces the
proliferation of pericytes and their transformation into myofibroblasts through
involvement of the PDGFR/ROCK1 pathway [92].
7.5 Other Signaling Pathways
Gui et al. [93] reported that mTORC49 and YAP/Taz were activated in
TGF-induced renal fibrosis. The Hippo pathway is a highly conserved
kinase cascade that plays a crucial role in the regulation of cell proliferation
and tissue regeneration. Key constituents of this pathway include Mst1/2, Sav1,
Lats1/2, YAP, and the coactivator Taz. In response to specific stimuli, YAP and
Taz become activated due to a reduction in phosphorylation levels mediated by the
canonical Hippo pathway. Recent studies have shown that YAP/Taz can induce
fibroblast activation and contribute to renal fibrosis [93]. The mTOR, a member
of the phosphatidylinositol-3-OH-kinase-related family, is highly conserved
throughout evolution and plays crucial roles in regulating cell growth,
proliferation, and survival. Previous research found that mTORC2 signaling
promotes fibroblast activation and contributes to renal fibrosis. Moreover, the
inhibition of mTORC2 effectively delays fibroblast/pericyte activation and
subsequent renal fibrosis by suppressing YAP/Taz activation [93].
The Gasdermin (GSDM) family of proteins is involved in regulating innate immune
responses and cell death. GSDMD can independently induce the release of
inflammatory mediators, such as IL-1. Previous research has shown that
macrophages utilize the GSDMD/IL-1 pathway to secrete IL-1.
Additionally, IL-1 is involved in promoting the conversion of pericytes
into fibroblasts. The expression of GSDMD is upregulated in peritoneal fibrosis,
whereas knockdown of GSDMD decreases the level of IL-1 in pericytes and
peritoneal fibrosis. GSDMD knockdown also inhibits fibrosis and VEGF/PI3K
pathways in pericytes. These findings suggest that modulation of the VEGF/PI3K
pathway through the GSDMD/IL-1 axis by macrophages can influence the
transition of pericytes to peritoneal fibrosis [94].
The release of alarmins or DAMPs by damaged or dying cells can be detected by
immune receptors present on both immune and non-immune cells. Recognition of
DAMPs by pericytes involves the participation of TLR2 and TLR4 as important
pericyte receptors. Extensive studies conducted in vitro and in
vivo have shown that MyD88 present within pericytes is a significant regulator
of the injury response. MyD88 can facilitate signaling pathways related to TLR
and IL-1. During the development of fibrotic disease, it is crucial that
pericytes engage in signaling mediated by MyD88 and IL-1 receptor-associated
kinase 4. In summary, DAMPs are released when cells are damaged and bind to
TLR2/4 receptors on pericytes. This induces changes in MyD88 and IL-1
receptor-associated kinase 4 signaling, leading to the development of PF [84].
Sirtuin 3 (SIRT3) is the principal regulator of mitochondrial protein
deacetylation. It regulates various physiological and pathological processes,
including metabolic homeostasis, oxidative stress, apoptosis, and aging. Previous
research has shown that SIRT3 plays a protective role in reducing
tubulointerstitial fibrosis in hypertensive kidneys, which contain elevated
levels of TGF- and reactive oxygen species (ROS). Decreased expression
of SIRT3 was observed during angiotensin II (Ang-II)-induced cardiac fibrosis and
PMT. Depletion of SIRT3 enhanced the induction of TGF- expression and
the generation of ROS in response to Ang-II [93, 94]. In addition, Feng et
al. [95] demonstrated that fibrosis induced by Ang-II could be attributed to a
reduction in SIRT3 levels and upregulation of the ROS-TGF-1 pathway,
thus facilitating PMT.
Valproic acid (VPA) is a short-chain fatty acid used as a first-line drug for
the treatment of epilepsy and depression [1]. A recent study showed that VPA
inhibits histone deacetylase (HDAC), thereby increasing the levels of histone
acetylation [2]. Other studies have shown that HDAC mitigates cardiac fibrosis in
rats, and that differentiation of pericardial cells and myofibroblasts is
HDAC4-dependent. HDAC4 can induce phosphorylase, which then dephosphorylates ERK
[96]. The MAPK pathway is critical for cell proliferation, programmed cell death,
and cellular differentiation. Among the three major MAPKs, ERK1/2 is the primary
kinase responsible for growth signaling. The phosphorylation of ERK is tightly
regulated by both kinases and protein phosphatases. A significant increase in the
level of phosphorylated ERK1/2 protein was observed in a rat model of cardiac
fibrosis [96]. Studies have also shown that knockdown of VPA and HDAC4 lead to
decreased ERK phosphorylation and inhibition of PMT, thereby reducing organ
fibrosis [96].
8. Therapeutic Potential of Pericytes
Although the cause of PF remains unclear, there is growing recognition of the
significant contribution made by pericytes to the onset and progression of PF.
This cell type is therefore a potential target for the treatment of PF. Several
treatment methods involving pericytes have been proposed in recent years, such as
TGF-R and PDGFR antagonists. Through the use of a PDGFR inhibitor,
Johnson et al. [14] have suggested that pericytes could serve as a
reservoir for airway resident mesenchymal cells, thereby playing a role in the
airway remodeling process observed in individuals with chronic asthma. The dual
effects of TGF-R and PDGF receptor antagonists on pericellular cells
means that further studies are required to clarify the effects of these
antagonists.
9. Summary and Perspectives
Pericytes play a crucial role in the pathogenesis of PF and are involved in
diverse physiological processes including inflammation, ECM remodeling, vascular
homeostasis, and EMT (Fig. 1). The primary function of these cells is to secrete
multiple types of inflammatory mediators such as TGF-, IL-6,
IL-1, together with cytokines including VEGF, PDGF, and Notch. These
molecules can effectively induce myofibroblast proliferation and collagen
deposition, thereby advancing the progression of PF. To date, TGF- and
PDGF have been extensively investigated as key targets in the progression of PF,
providing valuable avenues for the development of novel therapeutics. However,
research on related pathways such as the Wnt/-catenin signaling pathway,
VEGF, TLR, MAPK, and the PI3K-AKT pathway is still relatively scarce.
Fig. 1.
The mechanism underlying pericyte transformation into
myofibroblasts. (a) transforming growth factor- (TGF-)
production was increased (damaged epithelial cells produce TGF-;
toll-like receptor 4 (TLR-4) and TGF- were highly expressed in pericytes
upon lipopolysaccharide (LPS) induction; and Sirtuin 3 (SIRT3) highly expresses
TGF- by increasing reactive oxygen species (ROS) levels), and then
TGF- with receptor phosphorylation, multiple downstream adjustment
factor is activated (e.g., Smad2/3 signaling, and non-Smad pathways).
Phosphorylated TGF- promotes the phosphorylation of Smad2/3 and
extracellular regulated protein kinase 1 (ERK1), leading to
pericyte-myofibroblast transition (PMT). Phosphorylated Smad2/3 (P-Smad2/3)
translocated to the nucleus, and the accumulation of P-Smad2/3 in the nucleus
increases the binding to Smad-binding elements (SBE), leading to increased
transcription of fibrotic genes, which in turn increases matrix proteins
(including collagen type I) leading to increased extracellular matrix (ECM)
deposition. Binding of FAK to the ECM leads to increased translocation of MKL-1,
yes-associated protein (YAP), and transcriptional coactivators with PDZ-binding
motifs (TAZ), which in turn lead to increased -SMA expression and
myofibroblast transition, leading to idiopathic pulmonary fibrosis (IPF). (b)
Notch1 and exogenous bleomycin can regulate platelet-derived growth factor
receptor (PDGFR) activity, and PDGFR activation can regulate a variety of
downstream signaling molecules, such as Ras, phosphatidylinositol 3-Kinase (PI3K)
and phospholipase C (PLC). In addition, PDGFR also regulates Rho-related protein
kinase 1 (ROCK1), which is involved in the pathogenesis of pulmonary fibrosis,
while PDGF signaling can stimulate ERK1/2 activation in pericytes. (c)
Macrophages can modulate the vascular endothelial growth factor (VEGF) /PI3K
pathway through the gasdermin D (GSDMD)/ interleukin-1 (IL-1)
axis, thereby altering the transition of pericytes to fibrosis. (d)
damage-associated molecular patterns (DAMPs) occur when cells are damaged, bind
to receptors TLR2/4 on pericytes, induce changes in MyD88 and interleukin-1
receptor associated kinase 4 (RAK4) signaling and develop pulmonary fibrosis. (e)
The mTOR is a member of the phosphatidylinositol-3-OH-kinase-related family,
TGF- binds to its receptor and activates Akt/mTOR
signaling pathway to mediate the proliferation of ECM and PMT.
Activation of the Wnt/-catenin signaling pathway in PF leads to
enhanced stability and accumulation of -catenin within the cell nucleus.
This subsequently triggers the activation of transcription factors that promote
fibroblast proliferation and collagen synthesis, thereby driving disease
progression. -catenin also plays a role in regulating pathophysiological
functions such as EMT and the modulation of inflammation. However, there is a
paucity of research on downstream pathways associated with -catenin.
Therefore, further investigation is warranted into the role of -catenin
in the pathogenesis and progression of PF. Despite some encouraging progress in
this field, a comprehensive understanding of the intricate mechanisms and
regulatory networks that govern the involvement of pericytes in PF requires
further research. Future investigations should clarify the precise contribution
of pericytes during lung fibrogenesis and seek to obtain novel insights into
their potential therapeutic application.
Author Contributions
XH and YF contributed to the conceptualization ideas and
writing of the original draft. YL, YY consulted the relevant
literature and summarized the literature. YW contributed to the formal analysis,
reviewing, and editing of the manuscript. XT put forward insightful opinions on
the project and revised the manuscript. All authors have
participated sufficiently in the work to take public responsibility for
appropriate portions of the content and agreed to be accountable for all aspects
of the work in ensuring that questions related to its accuracy or integrity. All
authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript.
Ethics Approval and Consent to Participate
Not applicable.
Acknowledgment
We thank State Key Laboratory of Respiratory Disease and Guangzhou Science and
Technology Burea for the fnancial support.
Funding
This work was supported by the Open Project of the State Key
Laboratory of Respiratory Disease (No. SKLRD-OP-202320) and
the Key Research and Development Project of
Guangzhou Science and Technology Burea
(No.2023B01J1002).
Conflict of Interest
The authors declare no conflict of interest.