SOLUBLE LATENT MEMBRANE-TYPE 1 MATRIX METALLOPROTEASE SECRETED BY HUMAN MESANGIAL CELLS IS ACTIVATED BY UROKINASE

Background. Matrix metalloprotease 2 (MMP2) is secreted in a latent in active form (pro-MMP2) that is activated on the cell surface by a memb rane-type 1 MMP (MT1-MMP) in the presence of the tissue inhibitor of M MP (TIMP2). In spite of evidence for the synthesis of MT1-MMP shown by immunoblotting, immunocytochemistry and RT-PCR, and of TIMP2, MMP2 wa s found exclusively in a latent form in human mesangial cells (HMC) se rum-free culture medium. Methods and Results. On purified membranes of HMC, MT1-MMP was found in a 63 kD latent form and as a faint band of 55 kD. The 55 kD band was also present in the ultracentrifuged conditi oned medium and likely represented MT1-MMP cleaved from its transmembr ane domain, since Northern blot analysis showed only one transcription product. The addition of urokinase plasminogen activator (uPA, 100 nM ) to HMC membranes induced the activation of pro-MMP2 via the activati on of latent membrane-associated MT1-MMP as reflected by the cleavage of the 63 and 55 kD forms. In addition, when the conditioned medium wa s successively incubated with uPA and alpha(2)-macroglobulin and analy zed by immunoblotting, MT1-MMP decreased, indicating that the soluble MT1-MMP was in a latent form and was activated by uPA. Conclusion. Our results provide the first evidence, to our knowledge, of the existenc e of a soluble latent form of MT1-MMP secreted by primary human cells in culture, confirming that MT1-MMP is an ectoenzyme, and show that uP A can regulate MT1-MMP activity in a soluble phase.