Fibroblast activation protein, a dual specificity serine protease expressed in reactive human tumor stromal fibroblasts

Proteolytic degradation of extracellular matrix (ECM) components during tis sue remodeling plays a pivotal role in normal and pathological processes in cluding wound healing, inflammation, tumor invasion, and metastasis. Proteo lytic enzymes in tumors may activate or release growth factors from the ECM or act directly on the ECM itself thereby facilitating angiogenesis or tum or cell migration. Fibroblast activation protein (FAP) is a cell surface an tigen of reactive tumor stromal fibroblasts found in epithelial cancers and in granulation tissue during wound healing. It is absent from most normal adult human tissues. FAP is conserved throughout chordate evolution, with h omologues in mouse and Xenopus laevis, whose expression correlates with tis sue remodeling events. Using recombinant and purified natural FAP, we show that FAP has both dipeptidyl peptidase activity and a collagenolytic activi ty capable of degrading gelatin and type I collagen; by sequence, FAP belon gs to the serine protease family rather than the matrix metalloprotease fam ily. Mutation of the putative catalytic serine residue of FAP to alanine ab olishes both enzymatic activities. Consistent with its in vivo expression p attern determined by immunohistochemistry, FAP enzyme activity was detected by an immunocapture assay in human cancerous tissues but not in matched no rmal tissues. This study demonstrates that FAP is present as an active cell surface-bound collagenase in epithelial tumor stroma and opens up investig ation into physiological substrates of its novel, tumor-associated dipeptid yl peptidase activity.