Plasminogen activator inhibitor-2

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.[5]

Like other serpins, PAI-2 has three beta sheets (A, B, C) and nine alpha helices (hA-hI).[6][7] 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.[8] 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.

Reactive center loop (RCL) of plasminogen activator inhibitor-2. PyMol rendering of PDB 2ARR.

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.[9] Both ovalbumin and PAI-2 undergo secretion via uncleaved secretory signal peptides, although PAI-2 secretion is relatively much less efficient.[10]

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.[11] 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.[8] Polymerization of PAI-2 occurs spontaneously under physiological conditions, for instance in the cytosol of placental cells.[12] 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.[11] For these combined reasons, it is thought that PAI-2 may sense and respond to environmental redox potential.[8]

PAI-2 uses a suicide inhibition mechanism (a common mechanism for serpins) to irreversibly inactivate tissue plasminogen activator and urokinase.[6] 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.

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.[10] 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).[13] 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.[14] PAI-2 assists PAI-1 in regulating fibrinolysis and may help prevent overexpression of PAI-1, which increases risk of thrombosis.[14][15] Over the course of a pregnancy, PAI-2 plasma concentration rises from nearly-undetectable levels to 250 ng/mL (mostly in glycosylated form).[13]

Among immune cells, macrophages are the main producers of PAI-2, as both B-cells and T-cells do not produce significant amounts.[16] PAI-2 plays a role in inflammatory responses and infections, potentially in downregulating T cells that secrete IgG2c and interferon type II.[16]

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][17] 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.[18]

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.[19] Cancer cells may facilitate export of PAI-2 via microparticles.[19]

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.[20] Due to its high expression in tumor cells, PAI-2 has been used to track and study the spread of angiotropic melanoma cells.[21]

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.[19][22] The particular effects of PAI-2 on metastasis may depend on cancer type and location in the body.

• Plasminogen activator inhibitor-1
• Serpin

• The MEROPS online database for peptidases and their inhibitors: I04.007
• Plasminogen+Activator+Inhibitor+2 at the U.S. National Library of Medicine Medical Subject Headings (MeSH)