Fibroblast activation
protein (FAP), a membrane prolyl-specific
proteinase with both
dipeptidase and
endopeptidase activities, is overexpressed by reactive stromal fibroblasts during epithelial-derived
cancer growth. FAP digests extracellular matrix as tissue is remodeled during
cancer expansion and may also promote an immunotolerant tumor microenvironment. Recent studies suggest that nonspecific FAP inhibitors suppress human
cancer xenografts in mouse models.
Prolyl oligopeptidase (POP), another prolyl-specific
serine proteinase, is also elevated in many
cancers and may have a regulatory role in angiogenesis promotion. FAP and POP cell-associated activities may be targets for diagnosis and treatment of various
cancers, but their accessibilities to highly effective specific inhibitors have not been shown for cells important to
cancer growth. Despite their frequent simultaneous expression in many
cancers and their overlapping activities toward commonly used substrates, precise, separate measurement of FAP or POP activity has largely been ignored. To distinguish each of the two activities, we synthesized highly specific substrates and inhibitors for FAP or POP based on amino acid sequences surrounding the scissile bonds of their respective putative substrates. We found varying amounts of FAP and POP
protein and activities on activated fibroblasts, mesenchymal cells, normal breast cells, and one
breast cancer cell line, with some cells exhibiting more POP than FAP activity. Replicating endothelial cells (ECs) expressed POP but not FAP until tubulogenesis began. Targeting FAP-positive cells, especially mesenchymal stem cells and cancer-associated fibroblasts for inactivation or destruction, and inhibiting POP-producing EC may abrogate stromal invasion and angiogenesis simultaneously and thereby diminish
cancer growth.