Elevated expression of the serine protease inhibitor alpha-1- antitrypsin (alpha-1-AT) by MCF-7 breast cancer cells is associated with decreased release of transforming growth factor-alpha (TGFalpha) and reduced colony formation in soft agar. Consistent with this an 84 KDa, multi-domain, type II membrane serine protease, which we have called epithin, because of its homology to a recently described protein from mouse thymocytes, was cloned from MCF-7 cells. Northern blotting showed highest expression of epithin in mouse and human and epithelial cells particularly those from breast, kidney, trophoblast and uterine glands. Southern analysis indicated the presence of sequences homologous to human epithin in baboon, mouse, and rabbit but not in chicken or drosophila DNA. The domain structure and properties of epithin appear to be analogous to those of the ADAMs family of matrix metalloproteases. Immunohistochemistry of normal human breast tissue showed flat the epithin protein was restricted to the surface of ductal epithelial cells where it co- localized with the EGFr the receptor for TGFalpha. In addition, epithin expression appeared to be elevated in DCIS and in invasive breast carcinoma. A role for epithin in invasion and tumorigenesis is highly likely as epithin antibodies block TGFalpha release and invasion of BT-20 human breast cancer cells. Under the same conditions, invasion was found to be significantly inhibited by alpha-1-AT. Our results suggest that serine proteases and serine protease inhibitors acting in concert may regulate the effects of E2 on breast cancer cell tumorigenesis. / NOTE:Final rept. 15 Aug 1996-14 Aug 2000.