Serpin A1 is a prototype member of the Serpin superfamily of the serine protease inhibitors. As one of the most abundant proteinase inhibitors in the circulation, it is synthesized in hepatocytes, and to a lesser extent, in macrophages as well as intestinal epithelial cell lines and secreted as the abundant proteinase inhibitor in the circulation whose targets include elastase, plasmin, thrombin, trypsin, chymotrypsin, and plasminogen activator. Point mutations in the native SerpinA1 variants result in Serpin A1 deficiency, and consequently lead to several clinical complications such as pulmonary emphysema, juvenile hepatitis, cirrhosis, and hepatocellular carcinoma. For example, the Z variants (Glu342 to Lys) forms intracellular inclusion bodies, is not secreted, and leads to a severe SerpinA1 deficiency. Accordingly, Serpin A1 deficiency in circulation is associated with emphysema or liver disease.
SerpinA1, also known as Alpha-1 antitrypsin (AAT), is a prototype member of the Serpin superfamily of the serine protease inhibitors. This serine protease inhibitor blocks the protease, neutrophil elastase. Alpha-1 antitrypsin is mainly produced in the liver and acts as an antiprotease. Its principal function is to inactivate neutrophil elastase, preventing tissue damage. SerpinA1, an acute phase protein and the classical neutrophil elastase inhibitor, is localized within lipid rafts in primary human monocytes in vitro. It association with monocytes is inhibited by cholesterol depleting/efflux-stimulating agents and oxidized low-density lipoprotein (oxLDL) and conversely, enhanced by free cholesterol. Furthermore, SerpinA1/monocyte association per se depletes lipid raft cholesterol as characterized by the activation of extracellular signal-regulated kinase 2, formation of cytosolic lipid droplets, and a complete inhibition of oxLDL uptake by monocytes. Alpha-1 antitrypsin deficiency is a recently identified genetic disease that occurs almost as frequently as cystic fibrosis. It is caused by various mutations in the SerpinA1 gene, and has numerous clinical implications. Alpha-1 antitrypsin deficiency is an inherited disease affecting the lung and liver. In the liver, alpha-1 antitrypsin deficiency may manifest as benign neonatal hepatitis syndrome; a small percentage of adults develop liver fibrosis, with progression to cirrhosis and hepatocellular carcinoma.
SerpinA3, also known as Alpha 1-antichymotrypsin (AACT), is a plasma alpha globulin glycoprotein, and is a member of serpin superfamily of the serine protease inhibitors consisting of at least 35 members. SerpinA3 has been demonstrated to inhibit the activity of certain serine proteases, such as cathepsin G found in neutrophils, and chymases present in mast cells, by inducing a major conformational rearrangement, and thus protects some tissues from damage caused by proteolytic enzymes. This enzyme is produced primarily in the liver, and is identified as an acute-phase inflammatory protein. SerpinA3 deficiency has been associated with liver disease, and mutations of this gene have been observed in patients with Parkinson disease and chronic obstructive pulmonary disease. In addition, ACT gene polymorphism has been implicated with Alzheimer’s disease (AD), cerebral amyloid angiopathy (CAA), as well as stroke, since SerpinA3 is a major constituent of the plaques in AD and an inhibitor of amyloid beta peptide degradation.
Serpin A3 belongs to the Serpin superfamily of serine protease inhibitors. Serpin A3 has been shown to inhibit some serine proteases, such as neutrophil cathepsin G and mast cell chymase. Serpin A3 is synthesized initially in the liver and secreted in plasma. Serpin A3 has been found in the amyloid plaques from the hippocampus of Alzheimer disease brains. In addition to, Serpin A3 is associated with liver disease and Parkinson disease and chronic obstructive pulmonary disease.
Serpin Peptidase Inhibitor, Clade A (α-1 Antiproteinase, Antitrypsin), Member 4 (Serpin A4) is a member of the Serpin family. Serpin A4 exists as a monomer and some homodimers. Serpin A4 is expressed by the liver and secreted in plasma. Serpin A4 is a regulator of vascular homeostasis capable of controlling a wide spectrum of biological actions in the cardiovascular and renal systems. It can inhibit intracellular reactive oxygen species formation in cultured cardiac and renal cells. In addition, Serpin A4 has anti-inflammatory effect. Heparin blocks kallistatin's complex formation with tissue kallikrein and abolishes its inhibitory effect on tissue kallikrein's activity.
Serpin A5 is a member of the human Serpin superfamily consists of at least 35 members. It is synthesized in the liver and has been detected in saliva, cerebral spinal fluid, amniotic fluid, tears and semen. As a potent inhibitor of the protein C anticoagulant pathway at the levels of both zymogen activation and enzyme inhibition, Serpin A5 additionally inhibits a variety of serine protease including thrombin, factor Xa, several kallekreins and acrosin. It plays a critical role in the processes of blood of blood coagulation and fertilization. Serpin A5 also inhibits urinary plasminogen activator (uPA), a mediator of tumor cell invasion, and regulates tumor growth and metastasis by inhibiting angiogenesis. Furthermore, recent studies have identified PCI as a potent and direct inhibitor of activated HGFA (hepatocyte growth factor activator), suggesting a novel function in the regulation of tissue repair and regeneration. Similar to Serpins C1 and D1, the thrombin inhibitory activity of serpinA5 is enhanced by heparin.
Corticosteroid-binding globulin (CBG), also known as SerpinA6, is a non-inhibitory member of the serine proteinase inhibitor (serpin) superfamily. It is the high-affinity transport protein for glucocorticoids in vertebrate blood. CBG is specifically cleaved by this protease at a precise site close to its carboxy-terminus. This induces a conformation change and disrupts the binding between glucocorticoids and CBG, and promotes a significant and local release of glucocorticoids (over 90% of them are bound to CBG in human plasma). In this context, CBG directs glucocorticoids to sites of inflammation, and plays in consequence a crucial role in efficient glucocorticoid action in physiology. The SerpinA6 protein is mainly secreted by the liver. This negative acute phase protein regulates free cortisol levels in the blood and distributes cortisol to its target tissues. SerpinA6 deficiency is an extremely rare hereditary disorder characterized by reduced corticosteroid-binding capacity with normal or low plasma corticosteroid-binding globulin concentration, and normal or low basal cortisol levels associated with hypo-/hypertension and muscle fatigue. There are three heritable, human CBG gene mutations that can reduce CBG-cortisol binding affinity and/or reduce circulating CBG levels.
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