Immunohistochemical localization of FAP-1, an inhibitor of Fas-mediated apoptosis, in normal and neoplastic human tissues

Fas, a death receptor, is widely expressed in human tissue, but its expression, although a prerequisite for the induction of apoptosis, does not predict its biological function. To understand the mechanisms of Fas resistance in human tissues in vivo, we performed immunohistochemistry using an antibody against Fas-associated phosphatase-1 (FAP-1), which interacts with the cytosolic domain of Fas and inhibits Fas-mediated apoptosis. In normal human tissues, FAP-1 immunostaining was easily detected, for example, in renal tubules, skeletal muscle, myocardiocytes, pituitary gland, parathyroid gland, pancreatic islets, hepatocytes, testicular germ cells, prostatic glands, neurons, epithelium of fallopian tube, endometrial glands, trophoblasts, bronchial epithelial cells, and some types of gastrointestinal epithelial cells. In 123 (78%) of 158 cancers of various origins, including breast carcinomas, stomach carcinomas, colon carcinomas, lung carcinomas and several types of sarcomas, variable intensities of FAP-1 expression were evident. Taken together, these findings demonstrated that FAP-1 is widely expressed in normal human tissues and partly overlapped with Fas expression described in earlier reports, suggesting that FAP-1 may have an important role in the regulation of apoptosis in vivo. In addition, FAP-1 expression in cancers suggests that many cancers may be resistant to Fas-mediated apoptosis through the action of FAP-1 in vivo.