p22 is a novel plasminogen fragment with antiangiogenic activity

Tumor or tumor-associated cells cleave circulating plasminogen into three o r four kringle-containing antiangiogenic fragments, collectively referred t o as angiostatin. Angiostatin blocks tumor growth and metastasis by prevent ing the growth of endothelial cells that are critical for tumor vasculariza tion. Here, we show that cancer and normal cells convert plasminogen into a novel 22 kDa fragment (p22). Production of this plasminogen fragment in a cell-free system has allowed characterization of the structure and activity of the protein. p22 consists of amino acid residues 78-180 of plasminogen and therefore embodies the first plasminogen kringle (residues 84-162) as w ell as additional N- and C-terminal residues. Circular dichroism and intrin sic fluorescence spectrum analysis have defined structural differences betw een p22 and recombinant plasminogen kringle 1 (rK1), therefore suggesting a unique conformation for kringle 1 within p22. Proliferation of capillary e ndothelial cells but not cells of other lineages was selectively inhibited by p22 in vitro. In addition, p22 prevented vascular growth of chick chorio allantoic membranes (CAMs) in vivo. Furthermore, administration of p22 at l ow dose suppressed the growth of murine Lewis lung carcinoma (LLC) metastat ic foci in vivo. This is the first identification of a single kringle-conta ining antiangiogenic plasminogen fragment produced under physiological cond itions.