FAP is predominantly expressed in disease states, including liver and lung fibrosis, solid tumours, arthritis and atherosclerosis. Substrates of this protease include α-2-antiplasmin, collagen I and Neuropeptide Y (NPY).
We measured degradation kinetics of NPY, PYY and substance P in plasma to show that NPY is a physiological substrate of FAP in mouse and human, and that DPP4 and FAP account for all the cleavage of the N-terminal dipeptide of NPY. These neuropeptides were immune-localised in liver.
In a diet-induced obesity model, we have found that FAP gene knockout (gko) mice have improved glucose tolerance and liver histopathology, and less insulin resistance and fatty liver, compared to wild type mice. FAPgko mice resist liver fibrosis.
Serum levels of FAP enzyme activity co-segregated with liver stiffness as a measure of fibrosis in patients with type 2 diabetes or morbid obesity and fatty liver (NAFLD). A serum FAP level below 730 pmol AMC/min/mL had a negative predictive value for significant fibrosis of 95%. FAP level was added to the NAFLD fibrosis score (NFS) to correctly reclassify 49% of patients as low risk of severe fibrosis who by NFS had been classified as intermediate risk.
Cell lines derived from FAP gko mice were engineered to express functional human FAP enzyme (FAPe+) vs inactive FAP (FAPe-). In vitro ‘wound healing’ found that cells with FAP activity exhibited greater cell migration but comparable proliferation and apoptosis.
Conclusions: (1) NPY is a physiological substrate of FAP. (2) FAP has an important role in glucose and lipid metabolism and in fibrosis progression. (2) Adding a FAP serum measurement to the existing clinical NFS algorithm may correctly diagnose as non-fibrotic about half of the patients who would otherwise receive an uncertain diagnosis and require further testing. (3) FAP enzyme activity causes increased cell migration.