Mesenchymal and Neuroectodermal Origin Activation Protein in Normal and Transformed Cells of Regulation and Heteromeric Structure of the Fibroblast

The human fibroblast activation protein (FAP), defined by monoclonal antibody F19, is expressed in vivo in reactive stromal fibroblasts of epi thelial cancers, subsets of bone and soft tissue sarcomas, and granulation tissue of healing wounds. FAP is generally absent from the stroma of benign epithelial tumors and normal adult tissues. In vitro FAP induction is observed in proliferating cultured fibroblasts and in melanocytes grown with fibroblast growth factor and phorbol ester. In the present study, we show that fibroblast and melanocyte FAP is a cell surface protein com prising noncovalently linked M, 95,000 (p95) and M,. 105,000 (plOS) subunits. In contrast, cultured sarcoma and melanoma cell lines express only p95 or are FAP negative. Immunoblot experiments show that p95, but not plOS, carries the epitope defined by monoclonal antibody F19. Further more, peptide maps of purified p95 and plOS differ, suggesting that they may be distinct gene products. Loss of FAP or a change from p95/pl05 to p95 expression accompanies the acquisition of growth factor independence and tumorigenicity in several in vitro test systems, including simian virus 40 transformation of normal fibroblasts, ila-ras transformation of normal melanocytes, supertransformation of osteosarcoma cells, and enhanced N-MYC expression in variant neuroblastoma cells, whereas serum-starved normal fibroblasts continue to express p95/pl05. Thus, FAP expression appears to be linked to the growth factor-dependent proliferativi' capacity of normal cells and is not merely a secondary event in proliferating cells; furthermore, FAP expression is inversely correlated with growth factor independence and tumorigenicity in transformed cell lines. This distribu tion pattern is consistent with a role for p95/pl05 in mediating extrinsic, growth regulatory signals in normal cells, possibly as a heteromeric cell surface receptor. Such a physiological function may be obviated when oncogenes with cytoplasmic or nuclear sites of action are activated, reduc ing extrinsic growth factor dependence and permitting down-regulation of FAP in certain transformed cells.