CD31, also known as platelet endothelial cell adhesion molecule-1 (PECAM-1), is a 130 kDa heavily glycosylated transmembrane protein belonging to the immunoglobulin (Ig) superfamily of cell adhesion molecules. CD31 is highly expressed on endothelial cells and at a lower level on platelets, granulocytes, macrophages, dendritic cells, T and B cells, and natural killer (NK) cells. It is involved in cell adhesion and is required for transepithelial migration of leukocytes (TEM). CD31 is composed of an extracellular domain (ECD) of 574 amino acids containing six Ig-like domains, a transmembrane domain, and a 118 aa cytoplasmic domain. The latter undergoes alternative splicing which generates multiple isoforms showing altered adhesive properties compared to full length CD31. The human CD31 ECD shares 63% and 61% aa sequence identity with mouse and rat CD31, respectively. CD31 acts as a homophilic receptor through its extracellular domain and is involved in downstream signaling via its cytoplasmic domain. This domain contains highly conserved ITIM motifs which, once tyrosine phosphorylated, recruit and activate the signaling molecules Src and SHP-2. The resulting inhibition of TCR signaling increases the activation threshold of T cells, thus reinforcing peripheral tolerance and preventing development of autoimmunity. CD31 additionally regulates immune responses by acting as a key inhibitory receptor in dendritic cell development. Besides its role in TEM, CD31 appears to regulate T cell trafficking through a complex coordination of endothelial cell junctions and T cell extravasation.
Pentraxin-related protein PTX3, also known as Tumor necrosis factor alpha-induced protein 5, Tumor necrosis factor-inducible gene 14 protein, TSG-14, PTX3 and TNFAIP5, is a secreted protein that contains one pentaxin domain. PTX3 plays a role in the regulation of innate resistance to pathogens, inflammatory reactions, possibly clearance of self-components and female fertility. Pentraxins are a family of evolutionarily conserved multifunctional pattern-recognition proteins characterized by a cyclic multimeric structure. Based on the primary structure of the subunit, the pentraxins are divided into two groups: short pentraxins and long pentraxins. C-reactive protein (CRP) and serum amyloid P-component (SAP) are the two short pentraxins. The prototype protein of the long pentraxin group is pentraxin 3 (PTX3). CRP and SAP are produced primarily in the liver in response to IL-6, while PTX3 is produced by a variety of tissues and cells and in particular by innate immunity cells in response to proinflammatory signals and Toll-like receptor (TLR) engagement. PTX3 is essential in female fertility by acting as a nodal point for the assembly of the cumulus oophorus hyaluronan-rich extracellular matrix. PTX3 interacts with several ligands, including growth factors, extracellular matrix components and selected pathogens, playing a role in complement activation and facilitating pathogen recognition by phagocytes, acting as a predecessor of antibodies. PTX3 may also contribute to the pathogenesis of atherosclerosis.
Pepsinogen C, also known as PGC, is an aspartic proteinase that belongs to the peptidase family A1. Pepsinogen C is synthesized in the gastric mucosa as inactive precursors, known as zymogens. Pepsinogen C contains a prosegment that serves to stabilize the inactive form and prevent entry of the substrate to the active site. At low PH conditions, Pepsinogen C undergoes conversion into active enzyme. Pepsinogen C has been found expressed in all regions of the stomach mucosa and also in the proximal duodenal mucosa. In stomach cancer tissues and cancer cell lines, the expressions of the pepsinogen genes were decreased or lost, in good accordance with their pepsinogen productions. No gross structural changes of the pepsinogen genes were observed in these cancers, but the methylation patterns of the pepsinogen genes were found to be altered in different ways in different cancers. Serum levels of Pepsinogen C are used as a biomarker for certain gastric diseases including Helicobacter pylori related gastritis.
PEPD belongs to the peptidase M24B family of Eukaryotic-type prolidase subfamily. PEPD is a cytosolic dipeptidase that hydrolyzes dipeptides with proline or hydroxyproline at the carboxy terminus. It is important in collagen metabolism because of the high levels of imino acids. Defects in PEPD are a cause of prolidase deficiency which is an autosomal recessive disorder associated with iminodipeptiduria.
Osteoblast-Specific Factor OSF-2 (POSTN, Periostin, OSF-2) is a secreted, homodimeric protein that belongs to the periostin family of the FAS1 superfamily of molecules. Periostin is a disulfide linked 90kDa bone adhesion protein secreted by osteoblasts and osteoblast-like cell lines and the protein is an attachment agent for osteoblasts. It is a TGF-beta inducible molecule that serves as both an adhesion molecule and tumor suppressor. It is synthesized by smooth muscle cells, fibroblasts and osteoblasts, as well as in the periosteum and periodontal ligament. Periostin functions as a ligand for alpha-V/beta-3 and alpha-V/beta-5 integrins to support adhesion and migration of epithelial cells.
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