Selective Homogeneous Assay for Circulating Endopeptidase Fibroblast Activation Protein (FAP)
Travis W Bainbridge, Diana Ronai Dunshee, Noelyn M Kljavin, Nicholas J Skelton, Junichiro Sonoda, James A Ernst, Travis W Bainbridge, Diana Ronai Dunshee, Noelyn M Kljavin, Nicholas J Skelton, Junichiro Sonoda, James A Ernst
Abstract
Fibroblast Activation Protein (FAP) is a membrane-bound serine protease whose expression is often elevated in activated fibroblasts associated with tissue remodeling in various common diseases such as cancer, arthritis and fibrosis. Like the closely related dipeptidyl peptidase DPPIV, the extracellular domain of FAP can be released into circulation as a functional enzyme, and limited studies suggest that the circulating level of FAP correlates with the degree of tissue fibrosis. Here we describe a novel homogeneous fluorescence intensity assay for circulating FAP activity based on a recently identified natural substrate, FGF21. This assay is unique in that it can effectively distinguish endopeptidase activity of FAP from that of other related enzymes such as prolyl endopeptidase (PREP) and was validated using Fap-deficient mice. Structural modeling was used to elucidate the mechanistic basis for the observed specificity in substrate recognition by FAP, but not by DPPIV or PREP. Finally, the assay was used to detect elevated FAP activity in human patients diagnosed with liver cirrhosis and to determine the effectiveness of a chemical inhibitor for FAP in mice. We propose that the assay presented here could thus be utilized for diagnosis of FAP-related pathologies and for the therapeutic development of FAP inhibitors.
Conflict of interest statement
This work was funded by Genentech, Inc. All the authors are present or former paid employees of Genentech/Roche. Genentech has filed patent applications related to this work.
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References
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