Tissue-type plasminogen
activator (tPA)

PLAT (gene locus)

Gene: PLAT maps on chromosome 8p12-q11.2.
mRNA: size: ? kb.
Protein: a serine protease that activates the proenzyme plasminogen to plasmin, which in turn is responsible for fibrinolytic activity. tPA is synthesized as a single polypeptide chain. Its proteolytic cleavage at a centrally located arginine-isoleucine bond by plasmin gives rise to a 2-chain disulfide-linked form, composed of the N-terminally derived heavy chain and the C-terminal light chain.

Cell lines:
- tPA activity of MCF-7 BCC treated with estradiol (E₂, 10 nM) was 3-fold higher, that of cells cultured on laminin and treated with E₂ was 15-fold higher, than that of control. Northern-blot analysis showed that tPA mRNA levels were up-regulated by E₂ and laminin, whereas PAI-1 mRNA levels were down-regulated by laminin and not affected by E₂. Concomitant treatment with laminin and E₂ decreased PAI-1 mRNA and increased tPA mRNA levels, accounting for the synergistic increase in tPA activity (Sonohara S. et al., 1998).

- The maspin protein has tumor suppressor activity in breast and prostate cancers. It inhibits cell motility and invasion in vitro and tumor growth and metastasis in nude mice. It was found that single-chain tissue plasminogen activator (sctPA) specifically interacts with the maspin reactive site loop peptide and forms a stable complex with recombinant maspin [rMaspin(i)]. Major effects of rMaspin(i) were observed on plasminogen activation by sctPA. First, rMaspin(i) activated free sctPA. Second, it inhibited sctPA preactivated by poly-D-lysine. Third, rMaspin(i) exerted a biphasic effect on the activity of sctPA preactivated by fibrinogen/gelatin, acting as a competitive inhibitor at low concentrations (< 0.5 microM) and as a stimulator at higher concentrations. Fourth, 38-kDa C-terminal truncated rMaspin(i) further stimulated fibrinogen/gelatin-associated sctPA. rMaspin(i) did not inhibit urokinase-type plasminogen activator, plasmin, chymotrypsin, trypsin, or elastase. The kinetic data were 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 maspin and sctPA in vitro suggest a mechanism by which maspin regulates plasminogen activation by sctPA bound to the epithelial cell surface (Sheng S. et al., 1998).

- Cytokeratin 8 (CK8) is an intermediate filament protein that penetrates to the external surfaces of breast cancer cells and is released from cells in the form of soluble heteropolymers. CK8 binds plasminogen and tissue-type plasminogen activator (t-PA) and accelerates plasminogen activation on cancer cell surfaces. The plasminogen-binding site is located at the C-terminus of CK8. It was demonstrated that CK8 may promote plasminogen activation by t-PA only when present in an oligomerized state. CK18 may participate in the oligomer, together with CK8, based on its ability to bind t-PA (Kralovich K.R. et al., 1998).

Tumors:
- In a prospective study of 130 patients with node-negative invasive breast cancer who underwent radical operation (the median follow-up was 52.6 months), patients with high u-PA, high PAI-1, or low t-PA had significantly higher relapse rates than did those with low u-PA, low PAI-1, or high t-PA, respectively, by the Kaplan-Meier method (P = 0.006, 0.032, and 0.028, respectively). Analyses of the combinations of both u-PA and PAI-1 or both u-PA and t-PA showed that the differences in relapse rate between the high- and low-risk groups were statistically very significant. In the univariate analysis, u-PA, PAI-1, t-PA, progesterone receptor, and tumor size (T3 versus T1) were significantly correlated with relapse. However, the multivariate analysis revealed that only u-PA (P = 0.023) was an independent prognostic factor (Kim S.J. et al., 1998).

Bell L.D. et al. (1988) Chemical synthesis, cloning and expression in mammalian cells of a gene coding for human tissue-type plasminogen activator. Gene 63, 155-163.
Kim S.J. et al. (1998) Prognostic impact of urokinase-type plasminogen activator (PA), PA inhibitor type-1, and tissue-type PA antigen levels in node-negative breast cancer: a prospective study on multicenter basis. Clin. Cancer Res. 4, 177-182.
Kralovich K.R. et al. (1998) Characterization of the binding sites for plasminogen and tissue-type plasminogen activator in cytokeratin 8 and cytokeratin 18. J. Protein Chem. 17, 845-854.
Sheng S. et al. (1998) Tissue-type plasminogen activator is a target of the tumor suppressor gene maspin. Proc. Natl. Acad. Sci. USA 95, 499-504.
Sonohara S. et al. (1998) Laminin and estradiol regulation of the plasminogen-activator system in MCF-7 breast-carcinoma cells. Int. J. Cancer 76, 77-85.
Yang-Feng T.L. et al. (1985) Mapping of the human tissue-type plasminogen activator (PLAT) gene to chromosome 8 (8p12-q11.2). (Abstract) Cytogenet. Cell Genet. 40, 784.

Plasminogen activator inhibitor, type 1 (PAI-1), Plasminogen activator inhibitor, type 2 (PAI-2), urokinase-type plasminogen activator (uPA)