TISSUE-TYPE PLASMINOGEN-ACTIVATOR IS A TARGET OF THE TUMOR-SUPPRESSORGENE MASPIN

The maspin protein has tumor suppressor activity in breast and prostat e cancers. It inhibits cell motility and invasion in vitro and tumor g rowth and metastasis in nude mice, Maspin is structurally a member of the serpin (serine protease inhibitors) superfamily but deviates somew hat from classical serpins, We find that single-chain tissue plasminog en activator (sctPA) specifically interacts with the maspin reactive s ite loop peptide and forms a stable complex with recombinant maspin [r Maspin(i)]. Major effects of rMaspin(i) are observed on plasminogen ac tivation by sctPA, First, rMaspin(i) activates free sctPA, Second, it inhibits sctPA preactivated by poly-D-lysine. Third, rMaspin(i) exerts a biphasic effect on the activity of sctPA preactivated by fibrinogen /gelatin, acting as a competitive inhibitor at low concentrations (<0. 5 mu M) and as a stimulator at higher concentrations, Fourth, 38-kDa C -terminal truncated rMaspin(i) further stimulates fibrinogen/gelatin-a ssociated sctPA, rMaspin(i) acts specifically; it does not inhibit uro kinase-type plasminogen activator, plasmin, chymotrypsin, trypsin, or elastase. Our kinetic data are quantitatively consistent with a model in which two segregated domains of maspin interact with the catalytic and activating domains of sctPA. These complex interactions between ma spin and sctPA in vitro suggest a mechanism by which maspin regulates plasminogen activation by sctPA bound to the epithelial cell surface.