Antiplasmin-cleaving enzyme is a soluble form of fibroblast activation protein (Lee et al, 2006)

[SNT Gatchaman]

Antiplasmin-cleaving enzyme is a soluble form of fibroblast activation protein
Kyung N. Lee, Kenneth W. Jackson, Victoria J. Christiansen, Chung S. Lee, Jin-Geun Chun, Patrick A. McKee

Circulating antiplasmin-cleaving enzyme (APCE) has a role in fibrinolysis and appears structurally similar to fibroblast activation protein (FAP), a cell-surface proteinase that promotes invasiveness of certain epithelial cancers. To explore this potential relationship, we performed comparative structure/function analyses of the 2 enzymes.

APCE from human plasma and recombinant FAP (rFAP) exhibited identical pH optima of 7.5, extinction coefficients (in(280 nm)(1%)) of 20.2 and 20.5, common sequences of tryptic peptides, and cross-reactivity with FAP antibody. APCE and rFAP are homodimers with monomeric subunits of 97 and 93 kDa. Only homodimers appear to have enzymatic activity, with essentially identical kinetics toward Met-alpha2-antiplasmin (Met-alpha2AP) and peptide substrates. APCE and rFAP cleave both Pro3-Leu4 and Pro12-Asn13 bonds of Met-alpha2AP, but relative kcat/Km values for Pro12-Asn13 are about 16-fold higher than for Pro3-Leu4. APCE and rFAP demonstrate higher kcat/Km values toward a peptide modeled on P4-P4' sequence surrounding the Pro12-Asn13 primary cleavage site than for Z-Gly-Pro-AMC and Ala-Pro-AFC substrates.

These data support APCE as a soluble derivative of FAP and Met-alpha2AP as its physiologic substrate. Conversion of Met-alpha2AP by membrane or soluble FAP to the more easily fibrin-incorporable form, Asn-alpha2AP, may increase plasmin inhibition within fibrin surrounding certain neoplasms and have an impact on growth and therapeutic susceptibility.

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[Pyrrhus]

Before anyone makes the mistake of trying to Google "antiplasmin-cleaving enzyme (APCE)", the authors defined it themselves:

We purified the plasma proteinase that converts Met-Alpha2AP to Asn-Alpha2AP and termed it antiplasmin-cleaving enzyme (APCE). Its N-terminal amino acid sequence corresponded to residues 24-38 of fibroblast activation protein (FAP), also known as seprase, a type II integral membrane glycoprotein, which is predicted to have its first 6 N-terminal residues within the fibroblast cytoplasm, followed by a 20-residue transmembrane domain, and then a 734-residue extracellular C-terminal catalytic domain.

It appears that the authors are suggesting that "APCE" is similar to the extracellular (catalytic) portion of the FAP protein, but with 4000 Daltons of something added to it. (FAP is normally membrane-bound.) Has no one found the gene for this "APCE"?

 

[SNT Gatchaman]

I think this 2006 paper called it "APCE" but when it was later established that it was soluble FAP, that name has been superseded. I thought it was good to include this older paper, both for their initial thinking and also to relate FAP to antiplasmin in words.

From the 2014 paper Understanding fibroblast activation protein (FAP): Substrates, activities, expression and targeting for cancer therapy (my emphasis) —

Although FAP is involved in numerous (patho-) physiological processes, its substrate repertoire remains mostly unknown.

...two physiological endopeptidase substrates have so far been discovered for FAP; denatured type I collagen (CN-I) and a2-antiplasmin (a2-AP)

During tissue repair, fibrin is deposited to form a fibrin clot. Fibrinolysis is the natural process in which a fibrin clot is dissolved by plasmin leading to scar resolution. a2-AP is an inhibitor of plasmin and therefore reduces the rate of lysis of the fibrin clot. However, cleavage of a2-AP by FAP converts a2-AP into a more potent inhibitor of plasmin. We have therefore hypothesised that this activity of FAP leads to a reduction in fibrinolysis and promotion of scar formation in tissues.

As well as being a cell surface protease, FAP also exists in a truncated, soluble form in human plasma, lacking the transmembrane domain. Soluble FAP cleaves a2-AP at prolyl bonds Pro3-leucine4 and Pro12-Asn13.

So FAP is a common factor in both metabolic and coagulation function/dysfunction. Also interesting to note its relationship to collagen (bearing in mind EDS).

 

[Pyrrhus]

Ah, I think I see. So:

1. FAP is a membrane-bound protein that can inactivate the metabolic hormone Fibroblast growth factor 21 (FGF21).
2. There is also a form of FAP that circulates in the blood which makes Alpha 2-antiplasmin (a2-AP) more potent.
3. a2-AP stops clots from dissolving, by inhibiting plasmin.