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Departments of ¹ Medical Oncology and ² Biostatistics, ³ Institute for Cancer Research, Fox Chase Cancer Center, Philadelphia, Pennsylvania; and ⁴ Department of Biochemistry, Tufts University School of Medicine, Boston, Massachusetts

Requests for reprints: Jonathan D. Cheng, Department of Medical Oncology, Fox Chase Cancer Center, 333 Cottman Avenue, Philadelphia, PA 19111-2497. Phone: 215-728-2450; Fax: 215-728-3639. E-mail: j_cheng{at}fccc.edu

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 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.

Key Words: Fibroblast activation protein • tumor stromal fibroblasts • serine protease • tumor growth • animal model

Grant support: Supported by grants CA090468, CA103991-01, CA09035-28, the Frank Strick Foundation, the Bernard A. and Rebecca S. Bernard Foundation, and an appropriation from the Commonwealth of Pennsylvania and the American Cancer Society.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

⁵ http://www.cgl.ucsf.edu/chimera

Received 10/ 4/04; revised 1/ 6/05; accepted 1/14/05.