A serpin was identified in normal mammary gland by differential cDNA sequencing. In situ hybridization has detected this serpin exclusively in the myoepithelial cells on the normal and noninvasive mammary epithelial side of the basement membrane and thus was named myoepithelium-derived serine proteinase inhibitor (MEPI). No MEPI expression was detected in the malignant breast carcinomas. MEPI encodes a 405-aa precursor, including an 18-residue secretion signal with a calculated molecular mass of 46 kDa. The predicted sequence of the new protein shares 33% sequence identity and 58% sequence similarity to plasminogen activator inhibitor (PAI)-1 and PAI-2. To determine whether MEPI can modulate the in vivo growth and progression of human breast cancers, we transfected a full-length MEPI cDNA into human breast cancer cells and studied the orthotopic growth of MEPI-transfected vs. control clones in the mammary fat pad of athymic nude mice. Overexpression of MEPI inhibited the invasion of the cells in the in vitro invasion assay. When injected orthotopically into nude mice, the primary tumor volumes, axillary lymph node metastasis, and lung metastasis were significantly inhibited in MEPI-transfected clones as compared with controls. The expression of MEPI in myoepithelial cells may prevent breast cancer malignant progression leading to metastasis.The serine protease plasminogen/plasmin activator(PA) system contributes significantly to extracellular proteolysis in a wide variety of physiological processes of normal development and pathological processes in the etiology of diseases such as tumor invasion and metastasis (1-2). Compelling experimental evidence has suggested an important and apparently causal role for the tumor-associated urokinase type PA (uPA) and the receptor uPAR in cancer invasion and metastasis (1-3). Consistent with its role in cancer metastasis, overexpression and unrestrained activity of u-PA has been shown clinically to be a prognostic marker in many different types of human cancer (4-10). The down-regulation of uPA may occur at the level of transcriptional regulation of the genes and through interaction with specific endogenous inhibitor serpin such as plasminogen activator inhibitor (PAI). The best characterized serpins are PAI-1 and PAI-2. Both PAI-1 and PAI-2 have been shown to inhibit extracellular matrix degradation in vitro (11-12). These results suggest that the inhibitory activity of PAIs may be important in inhibiting tumor metastatic progression leading to metastasis. In fact, administration of a recombinant PAI-2 to mice decreases tumor growth (13), whereas overexpression of either PAI-1 or PAI-2 inhibits tumor metastasis (14-15).It has been demonstrated extensively that high tumor levels of uPA and PAI-1 are statistically independent and poor prognostic factors for disease-free and overall survival in breast cancer (6,(16)(17)(18)(19). In contrast to PAI-1, high levels of PAI-2 expression may be a favorable prognostic marker in breast cancer (2-3). In breast carcinomas with h...