Tumor-associated fibroblasts are functionally and phenotypically distinct from normal fibroblasts that are not in the tumor microenvironment. Fibroblast activation
protein is a 95 kDa
cell surface glycoprotein expressed by
tumor stromal fibroblasts, and has been shown to have
dipeptidyl peptidase and
collagenase activity. Site-directed mutagenesis at the catalytic site of fibroblast activation
protein, Ser624 --> Ala624, resulted in an approximately 100,000-fold loss of fibroblast activation
protein dipeptidyl peptidase (DPP) activity. HEK293 cells transfected with wild-type fibroblast activation
protein, enzymatic mutant (S624A) fibroblast activation
protein, or vector alone, were inoculated subcutaneously into immunodeficient mouse to assess the contribution of fibroblast activation
protein enzymatic activity to
tumor growth. Overexpression of wild-type fibroblast activation
protein showed growth potentiation and enhanced tumorigenicity compared with both fibroblast activation
protein S624A and vector-transfected HEK293 xenografts. HEK293 cells transfected with fibroblast activation
protein S624A showed
tumor growth rates and tumorigenicity potential similar only to vector-transfected HEK293. In vivo assessment of fibroblast activation
protein DPP activity of these
tumors showed enhanced enzymatic activity of wild-type fibroblast activation
protein, with only baseline levels of fibroblast activation
protein DPP activity in either fibroblast activation
protein S624A or vector-only xenografts. These results indicate that the enzymatic activity of fibroblast activation
protein is necessary for fibroblast activation
protein-driven
tumor growth in the HEK293 xenograft model system. This establishes the proof-of-principle that the enzymatic activity of fibroblast activation
protein plays an important role in the promotion of
tumor growth, and provides an attractive target for
therapeutics designed to alter fibroblast activation
protein-induced
tumor growth by targeting its enzymatic activity.