Plasminogen activator inhibitor-2 explained

Plasminogen activator inhibitor-2 (placental PAI, SerpinB2, PAI-2), a serine protease inhibitor of the serpin superfamily, is a coagulation factor that inactivates tissue plasminogen activator and urokinase. It is present in most cells, especially monocytes/macrophages. PAI-2 exists in two forms, a 60-kDa extracellular glycosylated form and a 43-kDa intracellular form.

It is present only at detectable quantities in blood during pregnancy, as it is produced by the placenta, and may explain partially the increased rate of thrombosis during pregnancy. The majority of expressed PAI-2 remains unsecreted due to the presence of an inefficient internal signal peptide.

Interactions

PAI-2 has been reported to bind a series of intracellular and extracellular proteins. Whether PAI-2's physiological function is inhibition of the extracellular protease urokinase and/or whether PAI-2 has intracellular activities remains controversial. At least one of PAI-2's physiological functions may involve regulation of adaptive immunity.^[1]

Structure and polymerization

Like other serpins, PAI-2 has three beta sheets (A, B, C) and nine alpha helices (hA-hI).^{[2] [3]} The structure of PAI-2 mutants have been solved, in which the 33-amino acid loop connecting helices C and D is deleted. This CD-loop is particularly flexible and difficult to stabilize, as the loop is known to translocate up to 54 Å during the formation of intramolecular disulfide bonds.^[4] In addition to the CD-loop, notable motifs include the reactive center loop (RCL) spanning amino acids 379-383 and an N-terminal hydrophobic signal sequence.

Despite their similar inhibitory targets, PAI-2 is phylogenetically distant from its counterpart plasminogen activator inhibitor-1 (PAI-1). As a member of the ovalbumin-related serpin family, PAI-2 is genetically similar to chicken ovalbumin (Gallus gallus), and is a close mammalian homolog.^[5] Both ovalbumin and PAI-2 undergo secretion via uncleaved secretory signal peptides, although PAI-2 secretion is relatively much less efficient.^[6]

PAI-2 exists in three polymeric states: monomeric, polymerigenic, and polymer (inactive state). Polymerization occurs by a so-called "loop-sheet" mechanism, in which the RCL of one molecule sequentially inserts into the A-beta-sheet of the next molecule. This process occurs preferentially when PAI-2 is in its polymerigenic form, which is stabilized by a disulfide bond between Cys-79 (located in the CD-loop) and Cys-161.^[7] When PAI-2 is in its monomeric form, the CD-loop is vastly out-of-position for this disulfide linkage, and it must translocate a distance of 54 Å to become sufficiently close to Cys-161. Nevertheless, since the CD-loop is quite flexible, the monomeric and polymerigenic forms are fully interconvertible, and one state can be favored over the other by altering the redox environment of the protein. Polymerization of PAI-2 occurs spontaneously under physiological conditions, for instance in the cytosol of placental cells.^[8] Cytosolic PAI-2 tends to be monomeric, while PAI-2 in secretory organelles (which tend to be more oxidizing than the cytosol) is more prone to polymerization. For these combined reasons, it is thought that PAI-2 may sense and respond to environmental redox potential.

Mechanism

PAI-2 uses a suicide inhibition mechanism (a common mechanism for serpins) to irreversibly inactivate tissue plasminogen activator and urokinase. First, the target serine protease docks to PAI-2 and catalyzes cleavage of the RCL, between residues Arg-380 and Thr-381. At this point, two outcomes are possible: the protease escapes, leaving an inactive PAI-2; or the protease forms a permanent, covalently-bonded complex with PAI-2, in which the protease is significantly distorted.

Biological Functions

Although extracellular (glycosylated) PAI-2 functions to regulate fibrinolysis, it remains unclear whether this inhibitory role is the main function of PAI-2. PAI-2 is predominantly intracellular. The secretory signal peptide of PAI-2 is relatively inefficient, perhaps by evolutionary design, as various mutations to the signal sequence can significantly enhance secretion efficiency. PAI-2 is undetectable in adult plasma, and is typically only detectable during pregnancy, in myelomonocytic leukemias, or in gingival crevicular fluid; moreover, PAI-2 is a slower inhibitor than its counterpart PAI-1 by orders of magnitude (based on second order rate constants).^[9] On the other hand, detailed intracellular roles for PAI-2 have not yet been conclusively established.

PAI-2 is upregulated during both pregnancy and immune responses. During pregnancy, PAI-2 is particularly present in the decidua and amniotic fluid, where it may protect membranes from digestion and aid in remodeling fetal and uterine tissues.^[10] PAI-2 assists PAI-1 in regulating fibrinolysis and may help prevent overexpression of PAI-1, which increases risk of thrombosis.^[11] Over the course of a pregnancy, PAI-2 plasma concentration rises from nearly-undetectable levels to 250 ng/mL (mostly in glycosylated form).

Among immune cells, macrophages are the main producers of PAI-2, as both B-cells and T-cells do not produce significant amounts.^[12] PAI-2 plays a role in inflammatory responses and infections, potentially in downregulating T cells that secrete IgG2c and interferon type II.

Due to its position on chromosome 18 close to the bcl-2 protooncogene and several other serpins, PAI-2's role in apoptosis has been investigated, but current evidence remains inconclusive.^[13] A recent study suggests PAI-2 may be a direct downstream target and activator of p53, and may directly stabilize p21; in addition, PAI-2 expression is increased in senescent fibroblasts and may arrest growth of young fibroplasts.^[14]

Potential roles in cancer

The role of PAI-2 in cancer growth and metastasis is complex, as PAI-2 may have tumor-promoting and tumor-inhibiting effects. Notably, it is high expression of PAI-2 by tumor cells, not the host organism, which influences cancer growth.^[15] Cancer cells may facilitate export of PAI-2 via microparticles.

PAI-2 provides protection for cancer cells against plasmin-induced cell death, which can exert a lethal effect on tumors. This protection is particularly salient in brain metastases, which tend to express high levels of PAI-2 and neuroserpin, and whose growth may be partially inhibited by knockout of PAI-2.^[16] Due to its high expression in tumor cells, PAI-2 has been used to track and study the spread of angiotropic melanoma cells.^[17]

Although PAI-2 expression can promote metastasis to the brain, in other cases high PAI-2 expression significantly decreases metastasis to the lungs and other organs.^[18] The particular effects of PAI-2 on metastasis may depend on cancer type and location in the body.

See also

• Plasminogen activator inhibitor-1
• Serpin

Further reading

• Rasmussen HH, van Damme J, Puype M, Gesser B, Celis JE, Vandekerckhove J . Microsequences of 145 proteins recorded in the two-dimensional gel protein database of normal human epidermal keratinocytes . Electrophoresis . 13 . 12 . 960–9 . December 1992 . 1286667 . 10.1002/elps.11501301199 . 41855774 .
• Ellis V, Wun TC, Behrendt N, Rønne E, Danø K . Inhibition of receptor-bound urokinase by plasminogen-activator inhibitors . The Journal of Biological Chemistry . 265 . 17 . 9904–8 . June 1990 . 10.1016/S0021-9258(19)38757-5 . 2161846 . free .
• Estreicher A, Mühlhauser J, Carpentier JL, Orci L, Vassalli JD . The receptor for urokinase type plasminogen activator polarizes expression of the protease to the leading edge of migrating monocytes and promotes degradation of enzyme inhibitor complexes . The Journal of Cell Biology . 111 . 2 . 783–92 . August 1990 . 2166055 . 2116194 . 10.1083/jcb.111.2.783 .
• Samia JA, Alexander SJ, Horton KW, Auron PE, Byers MG, Shows TB, Webb AC . Chromosomal organization and localization of the human urokinase inhibitor gene: perfect structural conservation with ovalbumin . Genomics . 6 . 1 . 159–67 . January 1990 . 2303256 . 10.1016/0888-7543(90)90461-3 .
• Schwartz BS, Monroe MC, Bradshaw JD . Endotoxin-induced production of plasminogen activator inhibitor by human monocytes is autonomous and can be inhibited by lipid X . Blood . 73 . 8 . 2188–95 . June 1989 . 2471561 . 10.1182/blood.V73.8.2188.2188. free .
• Ye RD, Ahern SM, Le Beau MM, Lebo RV, Sadler JE . Structure of the gene for human plasminogen activator inhibitor-2. The nearest mammalian homologue of chicken ovalbumin . The Journal of Biological Chemistry . 264 . 10 . 5495–502 . April 1989 . 10.1016/S0021-9258(18)83572-4 . 2494165 . free .
• Laug WE, Aebersold R, Jong A, Rideout W, Bergman BL, Baker J . Isolation of multiple types of plasminogen activator inhibitors from vascular smooth muscle cells . Thrombosis and Haemostasis . 61 . 3 . 517–21 . June 1989 . 2799763 . 10.1055/s-0038-1646626. 2349338 .
• Kruithof EK, Cousin E . Plasminogen activator inhibitor 2. Isolation and characterization of the promoter region of the gene . Biochemical and Biophysical Research Communications . 156 . 1 . 383–8 . October 1988 . 2845977 . 10.1016/S0006-291X(88)80852-0 .
• Ye RD, Wun TC, Sadler JE . cDNA cloning and expression in Escherichia coli of a plasminogen activator inhibitor from human placenta . The Journal of Biological Chemistry . 262 . 8 . 3718–25 . March 1987 . 10.1016/S0021-9258(18)61414-0 . 3029122 . free .
• Antalis TM, Clark MA, Barnes T, Lehrbach PR, Devine PL, Schevzov G, Goss NH, Stephens RW, Tolstoshev P . Cloning and expression of a cDNA coding for a human monocyte-derived plasminogen activator inhibitor . Proceedings of the National Academy of Sciences of the United States of America . 85 . 4 . 985–9 . February 1988 . 3257578 . 279685 . 10.1073/pnas.85.4.985 . 1988PNAS...85..985A . free .
• Schleuning WD, Medcalf RL, Hession C, Rothenbühler R, Shaw A, Kruithof EK . Plasminogen activator inhibitor 2: regulation of gene transcription during phorbol ester-mediated differentiation of U-937 human histiocytic lymphoma cells . Molecular and Cellular Biology . 7 . 12 . 4564–7 . December 1987 . 3325828 . 368144 . 10.1128/mcb.7.12.4564 .
• Webb AC, Collins KL, Snyder SE, Alexander SJ, Rosenwasser LJ, Eddy RL, Shows TB, Auron PE . Human monocyte Arg-Serpin cDNA. Sequence, chromosomal assignment, and homology to plasminogen activator-inhibitor . The Journal of Experimental Medicine . 166 . 1 . 77–94 . July 1987 . 3496414 . 2188630 . 10.1084/jem.166.1.77 .
• Dickinson JL, Bates EJ, Ferrante A, Antalis TM . Plasminogen activator inhibitor type 2 inhibits tumor necrosis factor alpha-induced apoptosis. Evidence for an alternate biological function . The Journal of Biological Chemistry . 270 . 46 . 27894–904 . November 1995 . 7499264 . 10.1074/jbc.270.46.27894 . free .
• Mikus P, Urano T, Liljeström P, Ny T . Plasminogen-activator inhibitor type 2 (PAI-2) is a spontaneously polymerising SERPIN. Biochemical characterisation of the recombinant intracellular and extracellular forms . European Journal of Biochemistry . 218 . 3 . 1071–82 . December 1993 . 7506655 . 10.1111/j.1432-1033.1993.tb18467.x .
• Jensen PJ, Wu Q, Janowitz P, Ando Y, Schechter NM . Plasminogen activator inhibitor type 2: an intracellular keratinocyte differentiation product that is incorporated into the cornified envelope . Experimental Cell Research . 217 . 1 . 65–71 . March 1995 . 7867722 . 10.1006/excr.1995.1064 . free .
• Akiyama H, Ikeda K, Kondo H, Kato M, McGeer PL . Microglia express the type 2 plasminogen activator inhibitor in the brain of control subjects and patients with Alzheimer's disease . Neuroscience Letters . 164 . 1–2 . 233–5 . December 1993 . 8152607 . 10.1016/0304-3940(93)90899-V . 40620114 .
• Ragno P, Montuori N, Vassalli JD, Rossi G . Processing of complex between urokinase and its type-2 inhibitor on the cell surface. A possible regulatory mechanism of urokinase activity . FEBS Letters . 323 . 3 . 279–84 . June 1993 . 8388810 . 10.1016/0014-5793(93)81357-6 . 1822666 . free . 1993FEBSL.323..279R .
• Bartuski AJ, Kamachi Y, Schick C, Overhauser J, Silverman GA . Cytoplasmic antiproteinase 2 (PI8) and bomapin (PI10) map to the serpin cluster at 18q21.3 . Genomics . 43 . 3 . 321–8 . August 1997 . 9268635 . 10.1006/geno.1997.4827 .
• Mahony D, Stringer BW, Dickinson JL, Antalis TM . DNase I hypersensitive sites in the 5' flanking region of the human plasminogen activator inhibitor type 2 (PAI-2) gene are associated with basal and tumor necrosis factor-alpha-induced transcription in monocytes . European Journal of Biochemistry . 256 . 3 . 550–9 . September 1998 . 9780231 . 10.1046/j.1432-1327.1998.2560550.x . free .
• Nishida Y, Hayashi Y, Imai Y, Itoh H . Expression and localization of the urokinase-type plasminogen activator receptor (uPAR) in the human placenta . The Kobe Journal of Medical Sciences . 44 . 1 . 31–43 . February 1998 . 9846056 .

External links

• The MEROPS online database for peptidases and their inhibitors: I04.007

Notes and References

1. Schroder WA, Major L, Suhrbier A . The role of SerpinB2 in immunity . . 31 . 1 . 15–30 . 2011 . 21395508 . 10.1615/critrevimmunol.v31.i1.20 .
2. Law RH, Zhang Q, McGowan S, Buckle AM, Silverman GA, Wong W, Rosado CJ, Langendorf CG, Pike RN, Bird PI, Whisstock JC . An overview of the serpin superfamily . Genome Biology . 7 . 5 . 216 . 2006 . 16737556 . 1779521 . 10.1186/gb-2006-7-5-216 . free .
3. Di Giusto DA, Sutherland AP, Jankova L, Harrop SJ, Curmi PM, King GC . Plasminogen activator inhibitor-2 is highly tolerant to P8 residue substitution--implications for serpin mechanistic model and prediction of nsSNP activities . Journal of Molecular Biology . 353 . 5 . 1069–80 . November 2005 . 16214170 . 10.1016/j.jmb.2005.09.008 .
4. Lobov S, Wilczynska M, Bergström F, Johansson LB, Ny T . Structural bases of the redox-dependent conformational switch in the serpin PAI-2 . Journal of Molecular Biology . 344 . 5 . 1359–68 . December 2004 . 15561148 . 10.1016/j.jmb.2004.10.010 .
5. J. Evan Sadler . Ye RD, Ahern SM, Le Beau MM, Lebo RV, Sadler JE . Structure of the gene for human plasminogen activator inhibitor-2. The nearest mammalian homologue of chicken ovalbumin . The Journal of Biological Chemistry . 264 . 10 . 5495–502 . April 1989 . 10.1016/S0021-9258(18)83572-4 . 2494165 . free .
6. Belin D, Guzman LM, Bost S, Konakova M, Silva F, Beckwith J . Functional activity of eukaryotic signal sequences in Escherichia coli: the ovalbumin family of serine protease inhibitors . Journal of Molecular Biology . 335 . 2 . 437–53 . January 2004 . 14672654 . 10.1016/j.jmb.2003.10.076 .
7. Wilczynska M, Lobov S, Ohlsson PI, Ny T . A redox-sensitive loop regulates plasminogen activator inhibitor type 2 (PAI-2) polymerization . The EMBO Journal . 22 . 8 . 1753–61 . April 2003 . 12682008 . 154470 . 10.1093/emboj/cdg178 .
8. Mikus P, Ny T . Intracellular polymerization of the serpin plasminogen activator inhibitor type 2 . The Journal of Biological Chemistry . 271 . 17 . 10048–53 . April 1996 . 8626560 . 10.1074/jbc.271.17.10048 . free .
9. Kruithof EK, Baker MS, Bunn CL . Biological and clinical aspects of plasminogen activator inhibitor type 2 . Blood . 86 . 11 . 4007–24 . December 1995 . 7492756 . 10.1182/blood.v86.11.4007.bloodjournal86114007 . free .
10. Astedt B, Lindoff C, Lecander I . Significance of the plasminogen activator inhibitor of placental type (PAI-2) in pregnancy . Seminars in Thrombosis and Hemostasis . 24 . 5 . 431–5 . 1998 . 9834009 . 10.1055/s-2007-996035 . 39347062 .
11. Thompson PN, Cho E, Blumenstock FA, Shah DM, Saba TM . Rebound elevation of fibronectin after tissue injury and ischemia: role of fibronectin synthesis . The American Journal of Physiology . 263 . 4 Pt 1 . G437–45 . October 1992 . 1415704 . 10.1152/ajpgi.1992.263.4.G437 .
12. Schroder WA, Le TT, Major L, Street S, Gardner J, Lambley E, Markey K, MacDonald KP, Fish RJ, Thomas R, Suhrbier A . A physiological function of inflammation-associated SerpinB2 is regulation of adaptive immunity . Journal of Immunology . 184 . 5 . 2663–70 . March 2010 . 20130210 . 10.4049/jimmunol.0902187 . free .
13. Lee JA, Cochran BJ, Lobov S, Ranson M . Forty years later and the role of plasminogen activator inhibitor type 2/SERPINB2 is still an enigma . Seminars in Thrombosis and Hemostasis . 37 . 4 . 395–407 . June 2011 . 21805446 . 10.1055/s-0031-1276589 . 260316614 .
14. Hsieh HH, Chen YC, Jhan JR, Lin JJ . The serine protease inhibitor serpinB2 binds and stabilizes p21 in senescent cells . Journal of Cell Science . 130 . 19 . 3272–3281 . October 2017 . 28794016 . 10.1242/jcs.204974 . free .
15. Schroder WA, Major LD, Le TT, Gardner J, Sweet MJ, Janciauskiene S, Suhrbier A . Tumor cell-expressed SerpinB2 is present on microparticles and inhibits metastasis . Cancer Medicine . 3 . 3 . 500–13 . June 2014 . 24644264 . 4101741 . 10.1002/cam4.229 .
16. Valiente M, Obenauf AC, Jin X, Chen Q, Zhang XH, Lee DJ, Chaft JE, Kris MG, Huse JT, Brogi E, Massagué J . Serpins promote cancer cell survival and vascular co-option in brain metastasis . Cell . 156 . 5 . 1002–16 . February 2014 . 24581498 . 3988473 . 10.1016/j.cell.2014.01.040 .
17. Bentolila LA, Prakash R, Mihic-Probst D, Wadehra M, Kleinman HK, Carmichael TS, Péault B, Barnhill RL, Lugassy C . Imaging of Angiotropism/Vascular Co-Option in a Murine Model of Brain Melanoma: Implications for Melanoma Progression along Extravascular Pathways . Scientific Reports . 6 . 23834 . April 2016 . 27048955 . 4822155 . 10.1038/srep23834 . 2016NatSR...623834B .
18. Mueller BM, Yu YB, Laug WE . Overexpression of plasminogen activator inhibitor 2 in human melanoma cells inhibits spontaneous metastasis in scid/scid mice . Proceedings of the National Academy of Sciences of the United States of America . 92 . 1 . 205–9 . January 1995 . 7816818 . 42846 . 10.1073/pnas.92.1.205 . 1995PNAS...92..205M . free .