Role of the Matrix Metalloproteinase and Plasminogen Activator-Plasmin Systems in Angiogenesis

The phenomenon of inhibition of tumor growth by tumor mass has been repeatedly studied, but without elucidation of a satisfactory mechanism. In our animal model, a primary tumor inhibits its remote metastases. After tumor removal, metastases neovascularize and grow. When the primary tumor is present, metastatic growth is suppressed by a circulating angiogenesis inhibitor. Serum and urine from tumor-bearing mice, but not from controls, specifically inhibit endothelial cell proliferation. The activity copurifies with a 38 kDa plasminogen fragment that we have sequenced and named angiostatin. A corresponding fragment of human plasminogen has similar activity. Systemic administration of angiostatin, but not intact plasminogen, potently blocks neovascularization and growth of metastases. We here show that the inhibition of metastases by a primary mouse tumor is mediated, at least in part, by angiostatin.

MT1-MMP is a membrane-bound matrix metalloproteinase (MT-MMP) capable of mediating pericellular proteolysis of extracellular matrix components. MT1-MMP is therefore thought to be an important molecular tool for cellular remodeling of the surrounding matrix. To establish the biological role of this membrane proteinase we generated MT1-MMP-deficient mice by gene targeting. MT1-MMP deficiency causes craniofacial dysmorphism, arthritis, osteopenia, dwarfism, and fibrosis of soft tissues due to ablation of a collagenolytic activity that is essential for modeling of skeletal and extraskeletal connective tissues. Our findings demonstrate the pivotal function of MT1-MMP in connective tissue metabolism, and illustrate that modeling of the soft connective tissue matrix by resident cells is essential for the development and maintenance of the hard tissues of the skeleton.