Molecular cloning of fibroblast activation protein α, a member of the serine protease family selectively expressed in stromal fibroblasts of epithelial cancers

Abstract

The human fibroblast activation protein α (FAPα) is a M(r) 95,000 cell surface antigen selectively expressed in reactive stromal fibroblasts of epithelial cancers, granulation tissue of healing wounds, and malignant cells of bone and soft tissue sarcomas. Normal adult tissues are generally FAPα- , but some fetal mesenchymal tissues transiently express the molecule. Because of its restricted normal tissue distribution and abundant expression in the stroma of over 90% of breast, colorectal, and lung carcinomas, FAPα is under clinical evaluation as a target for immunodetection and immunotherapy of epithelial cancers. In the present study, we have isolated a full-length cDNA for FAPα through expression cloning in COS-1 cells. The FAPα cDNA codes for a type II integral membrane protein with a large extracellular domain, transmembrane segment, and short cytoplasmic tail. FAPα shows 48% amino acid sequence identity to the T-cell activation antigen CD26, a membrane-bound protein with dipeptidyl peptidase IV (DPPIV) activity; however, unlike FAPα, CD26 is widely expressed in normal tissues. Three catalytic domains shared by DPPIV homologues in different species and by other serine proteases are conserved in FAPα. Immunochemical analysis of COS-1 cells coexpressing FAPα and CD26 revealed that the two molecules form heteromeric cell surface complexes, suggesting that a previously identified FAPα-associated M(r) 105,000 protein of cultured fibroblasts and growth factor-stimulated melanocytes, FAPβ, is identical to CD26. In vivo coexpression of FAPα and CD26 is found in reactive fibroblasts of healing wounds but not in tumor stromal fibroblasts or sarcomas (FAPα+/CD26-). The putative serine protease activity of FAPα and its in vivo induction pattern may indicate a role for this molecule in the control of fibroblast growth or epithelial-mesenchymal interactions during development, tissue repair, and epithelial carcinogenesis.