It has long been postulated that certain primary tumors or host macrophages produce a specific inhibitor of metastasis. Angiostatin (AS), which functions as a local inhibitor of angiogenesis and therefore metastatic tumor growth, was recently isolated from Lewis lung carcinoma cell (LLCC) tumor bearing mice (O'Reiley, 1994). AS is a 38 kDa degradation product of plasminogen (PSM) that is identical to kringle regions 1-4 of PSM. The specific proteinase operative in vivo has heretofore been undescribed. We have found that MMP are capable of cleaving PSM to AS by N-terminal sequencing. In order of efficiency of cleavage, these include macrophage elastase, Gelatinase B, stromelysin and matrilysin. The MME degradation product of PSM inhibited endothelial cell proliferation by 20-40%. Interestingly, in vitro endothelial cell differentiation was also inhibited. In order to investigate the role of MME in tumor metastasis in vivo, LLCC tumors were produced in MME-deficient mice (MME -/-) and wild-type controls (MME +/+). 5 weeks following subcutaneous injection of 1 10 6 tumor cells, there were no discernible differences in the development of primary tumors, however, pulmonary metastasis differed significantly between the two kindreds. MME (+/+) mice developed small nests of perivascular tumor cells surrounded by MME-producing macrophages, without gross metastasis. In contrast, MME (-/-) mice developed > 8 gross pulmonary metastasis per mouse which were highly vascularized tumors. In sum, we have found that certain MMPs, specifically MME, are capable of degrading PSM to AS. The MME/PSM product inhibits endothelial cell proliferation, but, more importantly, also inhibits endothelial cell differentiation in vitro. Finally, the development of significant pulmonary metastasis in mice with subcutaneous LLCC tumors is MME-dependent. Studies are ongoing to identify the specific function of MME in the development of pulmonary metastasis and its potential role in the production of AS in this model.
