Abbreviations
PARs- Protease activated receptors
I/R- Ischemia reperfusion
AKI- Acute kidney injury
PASD- Periodic acid- Schiff diastase
WT- Wild type
KIM-1- Kidney injury molecule-1
MCP-1- Monocyte chemoattractant protein-1
UUO- Unilateral ureteral obstruction
TEM- Transmission electron microscopy
LC3-II- Light chain 3-II
HE- Hematoxylin and eosin staining
PAN- Puromycin aminonucleoside
NS- Nephrotic syndrome
DIN-Drug induced nephropathy
ESRD-End stage renal disease
TF-Tissue factor
PLA- Proximity ligation assays
ERK- Extracellular signal-regulated kinase
RT-PCR- Real-Time polymerase chain reaction
IHC- Immunohistochemistry
eNOS- Endothelial NO synthase
uPA- Urokinase plasminogen activator
uPAR- Urokinase plasminogen activator receptor
PTC-Proximal tubule cells
Introduction
PARs are cell surface receptors, of the family G-protein- coupled
receptors (GPCRs) with extracellular amino terminus domain, and
consisting of four identified subtypes namely; PAR1, PAR2, PAR3, and
PAR4 [1-3]. Although the pathological role of PAR1 and PAR2 is well
described in certain experimental kidney injury models, less information
is available on PAR3 and PAR4. Notably, some endogenous proteases such
as trypsin, tryptase, cathepsin, urokinase, and kallikreins are
activators of PARs that regulate renal homeostasis, inflammation, and
tissue remodeling as both are interlinked with the pathogenesis of acute
kidney injury (AKI) [4-8]. Similarly, during disease conditions such
as sepsis-induced glomerulonephritis or septic shock, PARs can also be
activated by clotting factors namely thrombin, FVIIa, and FXa. These
pathological conditions might involve hyper-coagulability, and
inflammation which can lead to clotting factor-dependent up-regulation
of PAR receptor expressions and this further induce(s) glomerular fibrin
deposition and macrophage infiltration [9-10]. Moreover, during the
diseased conditions, activation of PARs by proteolytic cleavage at the
extracellular amino terminus domain results in the formation of tethered
ligand. Later, this newly formed tethered ligand binds to the receptor
body and can activate the transmembrane signaling molecules involving
MAPKs, NF-kB and PI3K/AKT/mTOR, which are responsible for producing
various pathophysiological responses such as mesangial cell
proliferation, extracellular matrix production, renal fibrin deposition,
podocyte apoptosis and also necrosis in kidney disease [1-3, 11-14].
Both PAR1 and PAR2 subtypes are expressed in the kidney and known to
promote fibro-proliferative disorders and exacerbate diabetic
nephropathy (DN) [6, 15-18]. Additionally, PAR1 and PAR2 are also
responsible for initiating podocyte cell injury, tubular epithelial cell
inflammation and mesangial cell expansion in the kidney [5, 19, 20].
Moreover, few studies have reported that the PAR2 receptor subtype
contributes to the progression of kidney damage in cisplatin and also
IgA-induced nephropathy, which suggests the disease’s specific
involvement of particular subtypes [21-23]. This review summarizes
the various findings which have tested the role of PAR’s in the
pathophysiology of kidney diseases, provides in-depth insights into the
mechanisms and discusses the standing challenges in therapeutic testing
of these receptor subtypes in kidney diseases.
A PubMed based literature survey was performed using the following
phrases in all possible combinations mainly: “protease activating
receptor subtypes in kidney diseases” (43 results), “role of PAR-1 in
renal injuries” (09 results), “role of PAR-2 in kidney disease” (12
results), “role of PAR-3 in renal disease” (5 results), “PAR in
glomerular and tubular injury” (10 results), “PAR in drug-induced
nephrotoxicities” (27 results), “PAR in kidneys inflammation and
fibrosis” (27 results), “effects of PAR in diabetic kidney disease”
(25 results), “role of PAR 1 and PAR2 in ischemia injury in rats” (4
results) This survey resulted into a total of 162 published
articles and after scrutinization only 13 original research
evidence focused on the involvement of PARs in renal diseases, published
between 1995-2021. The selected studies tested the role of different PAR
subtypes in diverse renal injuries including ischemia-reperfusion
injury, type I/II diabetic nephropathy, sepsis-induced acute kidney
injury, nephrotic syndrome, biologicals based renal injury, obstructive
injury, drug-induced nephrotoxicities, podocytopenia, in-vitro and
ex-vivo findings, glomerular as well as tubular diseases. Evidence
pertaining to the impact of PARs subtypes in co-morbidities, other organ
injuries, and clinical manifestations were excluded from the present
review.
Evolutionary background of PAR subtypes
The major breakthrough came in the 1990s when it was first discovered
that in a wide variety of tissues some membrane-spanning proteins do
exist on the cell surfaces and are known as protease-activated receptors
(PAR) that belong to the G protein-coupled receptor family. In the year
1991, the first member of PAR receptors was identified and cloned known
as protease-activated receptor-1 (PAR-1) [24]. To date, four members
of PARs have been identified, of which PAR1, PAR3, and PAR4 are known as
thrombin receptors, and on the other hand PAR2 is known as serine
receptor [25]. Notably, a study in PAR1 knockout mice lead to the
discovery of PAR3 and PAR4 receptors which showed a similar response to
thrombin receptor on platelet as that of PAR1 [26]. In the later
year, PAR2 was discovered serendipitously and was found to be activated
only by a serine receptor protein called trypsin [25]. Presently
Vorapaxar and atopaxar are available PAR-1 antagonists that have
undergone extensive clinical development and are approved as
antiplatelet agents [27].
Expression of PARs in the kidneys
In the kidneys, the expression of PAR1 and PAR2 is most abundant in
contrast to that of other receptor subtypes [2, 28-30]. Previous in
vitro studies using the RT-PCR technique have detected the transcripts
of PAR1, PAR2, and PAR4 in both isolated endothelial and epithelial
cells of humans and rodent kidneys. However, mRNA for PAR3 was found to
be absent in two of these studies [2, 29]. The PAR
receptors are endogenously activated by serine proteases which are also
part of the clotting system, where thrombin is responsible for PAR1,
PAR3, PAR4 activation, and trypsin activates PAR2. Moreover, some
urinary proteases including urokinase, kallikrein can also activate PAR1
and PAR2 and have been summarized in table 1 [1, 2, 28]. The
Urokinase enzyme is also called a urokinase plasminogen activator (uPA).
It is synthesized in the kidneys by proximal and distal tubular
epithelial cells and is secreted in the urine. Other sources which also
synthesize urokinase are monocytes (macrophages), fibroblast, and
myofibroblasts [31,32]. Moreover, it is believed that urokinase is
released during inflammation or injury as confirmed by experimental
studies in mice [33]. This study showed that mRNA expression of
urokinase plasminogen activator receptor (uPAR) was absent in normal
kidneys whereas, it was up-regulated in the kidneys of UUO induced
chronic kidney injury in mice. Another urinary protease, kallikrein is
also released in the kidneys from the distal convoluted tubular cells
and secreted into the urine. It is also reported that renal kallikrein
plays an important role in the regulation of renal hemodynamics and in
glomerular filtration [34, 35]. Moreover, the up-regulation of
tissue factor upon injury or inflammatory state leads to activation of
PAR1 and PAR2 by involving FXa [36, 37], which subsequently,
activates various signaling pathways including PKC, MAPK, NF-kB, thus
making PARs responsible for triggering several inflammation processes
[4, 12, 13]. Eventually, the activation of these signaling molecules
increases the release of pro-inflammatory markers such as TNF-α, IL-1β,
IL-6/8; which alter the renal hemodynamics including GFR and also fluid
reabsorption [4, 13, 29, 37]. Apart from endogenous proteases and
FXa, there are some synthetic peptide activators of PAR1 (TRAP-6,
TFLLR-NH2) and PAR2 (SLIGKV-NH2, SLIGL-NH2) [2, 38], and an
endogenous protein called activated protein C (APC) which also acts as
an agonist for both PAR1 and PAR2 [4, 36, 39].
Table.1: A summary of PAR activators