HSP47: Family Members
Hsp47 is a member of the SERPIN superfamily of proteinase inhibitors. The SERPIN superfamily comprises the most common proteinase inhibitors and can be found in all five kingdoms of life 20. serpin family members are encoded by a multigene family comprising 35 putatively functional protein-coding genes and six pseudogenes in humans, with many of the genes found in two clusters on chromosomes 14 (most clade A serpins) and 18 (clade B serpins), reflecting the role of gene duplication and differentiation in their familial expansion (Table 2). Serpins are classified into clades due to their sequence similarity and phylogenetic relationship. 16 Clades have been defined (termed A–P), with 9 clades (A–I) covering human serpins (Table 2). The two largest clades of serpins identified so far are the extracellular ‘clade A’ members and the intracellular ‘clade B’ serpins.
Clade A comprises three pseudogenes (SERPINA7P1, SERPINA13P, SERPINA15P) as well as 12 functional genes, encoding extracellular serpins that are characterized as being anti-trypsin-like. Serpin A1, also termed a1-antitrypsin or a1-protease inhibitor (a1-PI), is an inhibitory molecule crucially involved in the blockage of neutrophil elastase 39. SERPINA2 was originally described as a pseudogene, but new lines of evidence indicate it is a functional gene coding for ER-resident Serpin A2 with putative serine proteinase activity 40. Serpin A3, also termed a1-antichymotrypsin, represents an inflammatory response molecule with inhibitory function. Although its physiological function remains unsolved, Serpin A3 is capable of blocking neutrophil cathepsin G and mast cell chymase, both of which are able to convert inactive angiotensin-1 to the active angiotensin-2 41. Serpin A4 and Serpin A5 are further inhibitory serpins. While Serpin A4 acts as a kallikrein inhibitor 42, Serpin A5 functions as a pro-coagulant and pro-inflammatory agent by blocking the activation of the anti-coagulant, protein C factor 43, 44. Serpin A5 also functions as an anti-coagulant by blocking blood coagulation factors and fibrinolytic enzymes including urokinase (uPA) 45, reflecting the ambivalent nature of this serpin. Clade A serpins also encompass the non-inhibitory hormone-transport molecules Serpin A6 (corticosteroid-binding globulin, CNP) and Serpin A7 (thyroxine-binding globulin). While Serpin A6 has been shown to bind to hormones (e.g. glucocorticoids) 46, Serpin A7 represents the main thyroid hormone transport protein in serum 47. Serpin A8 (angiotensinogen, AGT) is an indispensable component of the renin-angiotensin system (RAS), and functions as a strong regulator of blood pressure, body fluid and electrolyte homeostasis 48, 49. Serpin A9 (centerin) represents a serine proteinase inhibitor highly expressed in normal germinal center (GC) B-cells and GC B-cell-derived malignancies, playing a pivotal role in naïve B cell maintenance 50. Serpin A10 is expressed in the liver and secreted into the plasma. Serpin A10 triggers blood coagulation as it inhibits the coagulation protease factor Xa in the presence of protein Z and factor XIa in the absence of co-factors 51. Serpin A10 has an N-terminal extension which is relevant for adjuvant functions and responsible for a fundamental increase of its molecular mass 52. Serpin A11 was originally characterized as being a pseudogene. Nowadays it has become apparent that Serpin A11 acts as a serpin proteinase inhibitor highly expressed in liver 53. Serpin A12 (vaspin) is expressed in visceral adipose tissues and functions as an adipokine which modulates insulin sensitivity by blocking its target protease, kallikrein-7 54.
Clade B encompasses intracellular serpins that are crucially involved in inflammatory processes 55, mucous production 56, and regulating immune responses 57-59. Ubiquitously expressed Serpin B1 (leukocyte elastase inhibitor, LEI) is a regulator of neutrophil proteases including cathepsin G, chymase, chymotrypsin, elastase, proteinase-3, and kallikrein-3 and acts as a potent granzyme H inhibitor 60. Serpin B2, plasminogen activator inhibitor (PAI-2), represents a urokinase (uPA) inhibitor which plays a prominent role in fibrinolysis 61. The squamous cell-specific Serpin B3 and Serpin B4 are closely related proteinase inhibitors that modulate the host’s immune response against tumor cells 62, 63. The corresponding genes are regularly co-expressed in the same tissues, including bladder, blood, esophagus, lung, heart, prostate, testis, thymus, and trachea, and uterus, with the exception of SERPINB4 which could not be identified in bladder and thymus 64. The non-inhibitory Serpin B5 (maspin) functions as a tumor suppressor in normal mammary epithelial cells. Interestingly, Serpin B5 does not undergo the characteristic serpin-like conformational change found in active serpins 65. The ubiquitously expressed Serpin B6 represents a cathepsin G inhibitor known to block inappropriate activity of cytotoxic apoptotic proteases 66. Serpin B6 has been proposed to protect the inner ear against leakage of lysosomal content during stress. Failure of this protection leads to cell death and sensorineural hearing loss 67. Serpin B7 (megsin) is preferentially expressed in mesangial cells where it regulates the cell proliferation and secretion of collagen type IV 68. Serpin B9, also known as peptidase inhibitor 9 (PI-9), acts as a specific inhibitor of granzyme B and plays a crucial role in mediating immune responses 69, 70. Inhibitory Serpin B10 (bomapin) holds a key role in regulating protease activities during TNF-mediated hematopoiesis and apoptosis 71, 72. Serpin B10 is expressed specifically in myeloid cells and the bone marrow 72. Serpin B11 does not harbor any protease inhibitory activity, possibly due to single nucleotide polymorphisms encoding amino acid variants in the serpin scaffold that impede conformational re-arrangements 73. It is obviously expressed in a limited number of tissues only, including lung, placenta, prostate, and tonsil 73. Serpin B12 (yukopin) acts as a trypsin and plasmin inhibitor, but not as an inhibitor of thrombin 74. The serpin was detected in several tissues, including brain, bone marrow, heart, intestine, lymph node, lung, liver, pancreas, testis, and ovary 74. Finally, Serpin B13 (headpin, hurpin) represents a cysteine endopeptidase inhibitor which exerts regulatory activities in keratinocyte development 75.
Clade C and Clade D contain a single serpin only. Serpin C1 (also named antithrombin, antithrombin-III) represents a crucial serine proteinase inhibitor in plasma which regulates the blood coagulation cascade. Serpin C1 was found to block thrombin, matriptase-3/TMPRSS7 as well as the coagulation factors IXa, Xa and XIa 76. Serpin D1 (heparin cofactor II) can be found in the plasma and predominantly in liver where it acts as a prominent thrombin and chymotrypsin inhibitor 77. Serpin D1 has been demonstrated to utilize co-factor recruitment and conformational transition to achieve striking inhibitory activity 78. Serpin C1 and Serpin D1 bear sizable N-terminal extensions supposed to mediate adjuvant functions of the molecules 52.
Clade E encompasses three extracellular serpins. Serpin E1 (plasminogen activator inhibitor 1, PAI-1) is the primary inhibitor of tissue- (tPA) and urokinase-type plasminogen activators (uPA), enzymes that convert plasminogen to plasmin, leading to fibrin degradation. Serpin E1, one of the main anti-fibrinolytic proteins, has been found to block thrombin and matriptase-3/TMPRSS7 76, 79. It is predominantly expressed in endothelial cells and can be found not only in plasma, but also in platelets as well as in hepatoma and fibrosarcoma cells 80, 81. Serpin E2, also named Glia-derived nexin (GDN), is an extracellular serine proteinase inhibitor known to inhibit several proteases including thrombin 82. GDN has been shown to affect the kinetics of re-innervation and the recovery of sensory and motor functions after peripheral nerve injury 83. It is ubiquitiously expressed with highest expression levels found in decidua. The poorly characterized Serpin E3 (nexin-related serine proteinase inhibitor) represents a putative extracellular serine proteinase inhibitor which is highly expressed in liver. Two isoforms have been identified for this molecule as the result of alternative splicing yielding proteins 424 and 404 amino acid residues in length, respectively.
Clade F contains two extracellular serpins. Serpin F1 (pigment epithelium-derived factor, PEDF) has been noted to act as a neurotrophic factor which induces extensive neuronal differentiation 84. As Serpin F1 does not exhibit the well characterized conformational change of active serpins, it lacks any serine proteinase inhibitory activity 85. Moreover, this serpin serves as a potent inhibitor of angiogenesis 86. Serpin F1 is ubiquitiously expressed with high expression levels found in the pigmented layer of retina. Serpin F2, also termed a2-antiplasmin (a2-AP), represents a strong inhibitor of fibrinolysis as it targets plasmin and trypsin, but it also blocks matriptase-3/TMPRSS7 and chymotrypsin 76. Serpin F2 is predominantly expressed in the liver and secreted into the plasma. Two isoforms have been identified for this molecule as the result of alternative splicing yielding proteins 491 and 427 amino acid residues in length, respectively.
In clade G and clade H, only a single serpin each is present. Serpin G1 (plasma protease C1 inhibitor) serves as a serine proteinase inhibitor which is active against complement C1s and C1r, kallikrein as well as factor XII 87. It is broadly expressed with highest levels found in the liver. Serpin G1 bears an N-terminal extension of approximately 100 amino acid resues which is highly glycosylated, thereby increasing the molecular mass of the circulating protein to ~104 kDa 88. The non-inhibitory Serpin H1, also termed Hsp47, is an ER-resident unique molecular chaperone crucially involved in the correct folding of procollagen 2, 3. Serpin H1, whose expression is induced by cell stress including heat shock 15, can be found in a broad spectrum of tissues with highest expression levels found in smooth muscle tissue. An autosomal-recessive missense mutation in the SERPINH1 gene coding for Hsp47 has been identified to cause a severe phenotype of osteogenesis imperfecta 89.
Clade I comprises two extracellular serpins. Serpin I1, also known as neuroserpin, acts as a serine proteinase inhibitor which blocks tissue- and urokinase-type plasminogen activators (tPA, uPA) as well as plasmin, but not thrombin 90. Serpin I1 is widely expressed with highest levels found in the brain 91 and has been proposed to play a pivotal role in the organization of synaptic connections as well as for synaptic plasticity in the adult nervous system 92. Noteworthy, Serpin I1 containing an S49P mutation has been characterized to cause a new form of dementia, familial encephalopathy with neuroserpin inclusion bodies (FENIB) 93. A significant correlation was drawn between Serpin I1 expression and adult glioma by Rajaraman and colleagues 94. Serpin I2 (pancpin), which is predominantly expressed in pancreas and adipose tissues 95, exhibits serine proteinase inhibitory activity against pancreatic elastase and chymotrypsin 96. Serpin I2 is down-regulated in pancreatic and breast cancer, and associated with acinar cell apoptosis and pancreatic insufficiency 95, 96.
Table 2a: The clade A of the human Serpin superfamily of proteinase inhibitors
|Protein||UniProt ID||Aliases||Length (aa)||Chromosome||Gene ID|
|Serpin A1||P01009||a1-antitrypsin, a1-protease inhibitor (a1-PI)||418, 359, 306||14q32.13||5265|
|Serpin A2||P20848||putative a1-antitrypsin-related protein||420||14q32.13||390502|
|Serpin A3||P01011||a1-antichymotrypsin, cell growth-inhibiting gene 24/25 protein||423, 216, 95||14q32.13||12|
|Serpin A4||P29622||Kallistatin, kallikrein inhibitor, peptidase inhibitor 4||427||14q32.13||5267|
|Serpin A5||P05154||Plasma serine protease inhibitor, plasminogen activator inhibitor 3 (PAI-3), protein C inhibitor (PCI)||406||14q32.13||5104|
|Serpin A6||P08185||Corticosteroid-binding globulin (CBP), transcortin||405||14q32.13||866|
|Serpin A7||P05543||Thyroxin-binding globulin, T4-binding globulin||415||Xq22.3||6906|
|Serpin A8||P01019||Angiotensinogen (AGT)||485||1q42.2||183|
|Serpin A9||Q86WD7||Centerin, germinal center B-cell-expressed transcript 1 protein (GCET1), serine proteinase inhibitor A11||417, 335, 201, 334, 286, 337, 435||14q32.13||327657|
|Serpin A10||Q9UK55||Protein Z-dependent protease inhibitor (PZI)||444||14q32.13||51156|
|Serpin A11||Q86U17||Antiproteinase-like 2||422||14q32.13||256394|
|Serpin A12||Q8IW75||OL-64, visceral adipose tissue-derived serine protease inhibitor (vaspin)||414||14q32.13||145264|
Table 2b: The clade B of the human Serpin superfamily of proteinase inhibhitors
|Protein||UniProt ID||Aliases||Length (aa)||Chromosome||Gene ID|
|Serpin B1||P30740||leukocyte elastase inhibitor (LEI), monocyte/neutrophil elastase inhibitor (EI), peptidase inhibitor 2 (PI-2), epididymis luminal protein 57, epididymis secretory protein Li 27||379, 228||6p25.2||1992|
|Serpin B2||P05120||Plasminogen activator inhibitor (PAI-2), urokinase inhibitor, placental plasminogen activator inhibitor||415||18q21.33-q22.1||5055|
|Serpin B3||P29508||Protein T4-A, squamous cell carcinoma antigen 1 (SCCA-1)||390, 338||18q21.33||6317|
|Serpin B4||P48594||Leupin, peptidase inhibitor 11 (PI-11), squamous cell carcinoma antigen 2 (SCCA-2)||390||18q21.33||6318|
|Serpin B5||P36952||Maspin, peptidase inhibitor 5 (PI-5)||375, 231||18q21.33||5268|
|Serpin B6||P35237||Cytoplasmic antiproteinase (CAP), peptidase inhibitor 6 (PI-6), placental thrombin inhibitor||376||6p25.2||5269|
|Serpin B7||O75635||Megsin, TP55||380, 363||18q21.33||8710|
|Serpin B9||P50453||Cytoplasmic antiproteinase 3 (CAP-3), peptidase inhibitor 9 (PI-9), testicular tissue protein Li 180||376||6p25.2||5272|
|Serpin B10||P48595||Bomapin, peptidase inhibitor 10 (PI-10)||397||18q22.1||5273|
|Serpin B11||Q96P15||Serine (or cysteine) proteinase inhibitor, clade B (ovalbumin), member 11||392, 305, 190||18q21.33||89778|
|Serpin B12||Q96P63||Yukopin, serine (or cysteine) proteinase inhibitor, clade B (ovalbumin), member 12||405, 425||18q21.33||89777|
|Serpin B13||Q9UIV8||Headpin, HaCaT UV-repressible serpin (hurpin), UV-B repressed sequence (HUR7), peptidase inhibitor 13 (PI-13)||391, 339||18q21.33||5275|
Table 2c: The clades C – I of the human Serpin superfamily of proteinase inhibhitors
|Protein||UniProt ID||Aliases||Length (aa)||Chromosome||Gene ID|
|Serpin C1||P01008||Antithrombin-III (ATIII), serpin peptidase inhibitor clade C member 1, antithrombin||464||1q25.1||462|
|Serpin D1||P05546||Heparin cofactor 2, HC-II, protease inhibitor leuserpin-2 (HLS-2)||499||22q11.21
|Serpin E1||P05121||Plasminogen activator inhibitor 1 (PAI-1), endothelial plasminogen activator inhibitor||402, 387||7q22.1||5054|
|Serpin E2||P07093||Glia-derived nexin (GDN), peptidase inhibitor 7 (PI-7), protease nexin 1 (PN-1)||398, 397, 409||2q36.1||5270|
|Serpin E3||A8MV23||Nexin-related serine protease inhibitor; serpin peptidase inhibitor, clade E (nexin, plasminogen activator inhibitor type 1), member 3||424, 404||13q14.3||647174|
|Serpin F1||P36955||Pigment epithelium-derived factor (PEDF), EPC-1, cell proliferation-inducing gene 35 protein, testis tissue sperm-binding protein Li 70n||418||17p13.3||5176|
|Serpin F2||P08697||a2-antiplasmin (a2-AP), a2-plasmin inhibitor (a2-PI)||491, 427||17p13.3||5345|
|Serpin G1||P05155||Plasma protease C1 inhibitor (C1Inh), C1 esterase inhibitor, C1-inhibiting factor, complement component 1 inhibitor, epididymis secretory sperm binding protein||500, 448, 505||11q12.1||710|
|Serpin H1||P50454||47 kDa heat shock protein (Hsp47), arsenic-transactivated protein 3 (AsTP3), cell proliferation-inducing gene 14 protein, collagen-binding protein 1/-2 (CBP-1/-2), colligin-1/-2, rheumatoid arthritis-related antigen RA-A47, gp46, J6||418||11q13.5||871|
|Serpin I1||Q99574||Neuroserpin, peptidase inhibitor 12 (PI-12), serpin peptidase inhibitor clade I member 1||410||3q26.1||5274|
|Serpin I2||O75830||Pancpin, myoepithelium-derived serine protease inhibitor, pancreas-specific protein TSA2004, peptidase inhibitor 14 (PI-14), serpin peptidase inhibitor clade I member 2||405||3q26.1||5276|