The aim of this proposal is to elucidate the mechanism by which latent TGF-beta1 (LTGF-beta1) or a molecule very similar to LTGF-beta1 is activated in cocultures of endothelial cells (ECs) and smooth muscle cells (SMCs). We have established that endogenous LTGF-beta1 is activated by plasmin in specific heterotypic cocultures of bovine cells. We hypothesize that in these heterotypic cocultures, cell-cell contact induces changes in the PA-plasmin system which leads to the effective conversion of LTGF-beta1 into active TGF-beta1. This results in an inhibition of EC movement, mitosis, and perhaps, invasion. We hypothesize that upon heterotypic cell-cell contact, changes in the PA-plasmin system occur which lead to the availability of plasmin in a form which converts LTGF-beta1 to TGF-beta1. To test this hypothesis we shall (1) explore other combinations of cells to assess the generality of heterotypic cell-cell activation, (2) characterize changes in the PA-plasmin system which might lead to activation, and (3) characterize the role of gap junctions in this process.