HOME

TheInfoList



OR:

Protease-activated receptors (PAR) are a subfamily of related
G protein-coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily related ...
s that are activated by cleavage of part of their extracellular domain. They are highly expressed in
platelet Platelets or thrombocytes () are a part of blood whose function (along with the coagulation#Coagulation factors, coagulation factors) is to react to bleeding from blood vessel injury by clumping to form a thrombus, blood clot. Platelets have no ...
s, and also on
endothelial cells The endothelium (: endothelia) is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the res ...
, fibroblasts, immune cells,
myocyte A muscle cell, also known as a myocyte, is a mature contractile Cell (biology), cell in the muscle of an animal. In humans and other vertebrates there are three types: skeletal muscle, skeletal, smooth muscle, smooth, and Cardiac muscle, cardiac ...
s,
neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...
s, and tissues that line the gastrointestinal tract. Protease-activated receptors, or PARs, are not to be mistaken with PAR proteins, a group of regulators of cellular polarity named after their associated partitioning
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...
.


Classification

There are four mammalian members of the protease-activated receptor (PAR) family: PAR1 – encoded by the gene ''F2R'', PAR2 – ''F2RL1'', PAR3 – ''F2RL2'' and PAR4 – '' F2RL3'', each of these genes has its locus on
chromosome 5 Chromosome 5 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 5 spans about 182 million base pairs (the building blocks of DNA) and represents almost 6% of the total DNA in cells. Ch ...
except that of PAR4, which is on
chromosome 19 Chromosome 19 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 19 spans more than 61.7 million base pairs, the building material of DNA. It is considered the most Gene density, gene-ri ...
. The protease specificities, expression patterns, and functions of each PAR vary across a range of tissues and cell types. They are also members of the seven-transmembrane
G-protein-coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large protein family, group of evoluti ...
superfamily, and are expressed throughout the body.


History

When scientists were researching the process of blood clotting in the late 1980s, they made the discovery of protease-activated receptors (PARs). A novel protein that was activated by thrombin, a crucial part of the clotting cascade, was discovered by a research team at the University of California, San Francisco in 1991. The team was directed by Shaun Coughlin. This protein, which was eventually given the designation protease-activated receptor 1 (PAR1), was the first to be recognized as a member of the PAR family. A second thrombin-activated protein, later known as PAR3, was identified in 1994. Later research revealed that PAR3 works as a cofactor for PAR4 but lacks a useful intracellular domain. A third member of the PAR family, known as PAR2, was discovered as a protein triggered by in 1996.


Activation

Protease activated receptors are integral
membrane protein Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several broad categories depending on their location. Integral membrane proteins are a permanent part of a cell membrane ...
s that are coupled to G-proteins and are activated by proteolytic cleavage of the amino terminal sequence that exposes a new N-terminal sequence functions as a tethered ligand, which bind a conserved region on extracellular loop 2 (ECL2). Such binding causes activation of intracellular pathways and the specific change in conformation of the PAR. The most effective activator of PAR4 is thrombin. However, trypsin and factor Xa activate PAR4 by cleaving the receptor at various places in its N-terminal domain. Four types of PAR receptors have been identified by molecular cloning, and classified according to the main enzyme that is able to activate it. It has been determined that a large group of proteases cleave and activate PARs receptors, including various endogenous proteases from: a) the
coagulation cascade Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The process of coagulat ...
, b) inflammatory cells, and c) the
digestive tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the Digestion, digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascula ...
. On the other hand, PARs can be specifically cleaved and irreversibly activated even by exogenous proteases originated from insects, bacteria or plants and fungi. The wide distribution of PARs in a variety of cells supports the idea that they are involved in many process related with the gastrointestinal physiology and cardiovascular physiology. Although the proteolysis is the main mechanism for PAR activation, it is well known that a synthetic peptide (SLIGKV) that mimics the new N-terminal sequence produced after the cleavage, activates PAR-2 receptors without its proteolytic processing. In this sense, here we report that TFF3 isolated from human breast milk activates PAR-2 receptors of intestinal epithelial cells HT-29. These findings suggest that TFF3 activates intestinal epithelial cells through G-protein-coupled PAR-2, and could actively participate in the immune system of breastfed babies inducing the production of peptides related to innate defense, such as
defensin Defensins are small cysteine-rich cationic proteins across cellular life, including vertebrate and invertebrate animals, plants, and fungi. They are host defense peptides, with members displaying either direct Antimicrobial, antimicrobial activit ...
s and
cytokine Cytokines () are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are produced by a broad range of cells, including immune cells like macrophages, B cell, B lymphocytes, T cell, T lymphocytes ...
s. PARs are activated by the action of
serine protease Serine proteases (or serine endopeptidases) are enzymes that cleave peptide bonds in proteins. Serine serves as the nucleophilic amino acid at the (enzyme's) active site. They are found ubiquitously in both eukaryotes and prokaryotes. Serin ...
s such as
thrombin Prothrombin (coagulation factor II) is encoded in the human by the F2-gene. It is proteolytically cleaved during the clotting process by the prothrombinase enzyme complex to form thrombin. Thrombin (Factor IIa) (, fibrose, thrombase, throm ...
(acts on PARs 1, 3 and 4) and
trypsin Trypsin is an enzyme in the first section of the small intestine that starts the digestion of protein molecules by cutting long chains of amino acids into smaller pieces. It is a serine protease from the PA clan superfamily, found in the dig ...
(PAR 2). These
enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...
cleave the N-terminus of the receptor, which in turn acts as a tethered ligand. In the cleaved state, part of the receptor itself acts as the
agonist An agonist is a chemical that activates a Receptor (biochemistry), receptor to produce a biological response. Receptors are Cell (biology), cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an R ...
, causing a physiological response. Most of the PAR family act through the actions of G-proteins i (
cAMP Camp may refer to: Areas of confinement, imprisonment, or for execution * Concentration camp, an internment camp for political prisoners or politically targeted demographics, such as members of national or minority ethnic groups * Extermination ...
inhibitory), 12/13 (Rho and Ras activation) and q (calcium signalling) to cause cellular actions.


Function

PARs play a role in a multitude of physiological processes such as hemostasis, thrombosis, inflammation, and pain sensation. The cellular effects of thrombin are mediated by protease-activated receptors (PARs). Endothelial PARs participate in the regulation of vascular tone and permeability while in vascular smooth muscle they mediate contraction, proliferation, and
hypertrophy Hypertrophy is the increase in the volume of an organ or tissue due to the enlargement of its component cells. It is distinguished from hyperplasia, in which the cells remain approximately the same size but increase in number. Although hypertro ...
. In endothelial cells PARs play a key role in promotion vascular barrier function as they provide a positive signals for endothelial adhesion molecules ( vascular cell adhesion molecule-1 or VCAM-1,
intercellular adhesion molecule-1 Cell adhesion molecules (CAMs) are a subset of cell surface proteins that are involved in the binding of cells with other cells or with the extracellular matrix (ECM), in a process called cell adhesion. In essence, CAMs help cells stick to each ...
or ICAM-1, and
E-selectin E-selectin, also known as CD62 antigen-like family member E (CD62E), endothelial-leukocyte adhesion molecule 1 (ELAM-1), or leukocyte-endothelial cell adhesion molecule 2 (LECAM2), is a selectin cell adhesion molecule expressed only on endotheli ...
). PARs contribute to the pro-inflammatory response. For example PAR4 induces leukocyte migration and PAR2 helps
macrophage Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...
s to produce cytokines such as
interleukin-8 Interleukin 8 (IL-8 or chemokine (C-X-C motif) ligand 8, CXCL8) is a chemokine produced by macrophages and other cell types such as epithelial cells, airway smooth muscle cells and endothelial cells. Endothelial cells store IL-8 in their storage ...
(IL-8). Recent research has also implicated these novel receptors in muscle growth and bone cell differentiation and proliferation.


See also

* Protease-activated receptor 1 * Protease-activated receptor 2 * Protease-activated receptor 3


References


Further reading

* * * *


External links

* * {{G protein-coupled receptors G protein-coupled receptors