Dual mechanism of laminin modulation of ecto-5'-nucleotidase activity.

The myoblast cell surface activity of ecto-5'-nucleotidase was stimulated by a laminin substrate, whereas fibronectin and gelatin did not increase the AMPase activity of ecto-5'-nucleotidase. This increase was related to a higher expression of ecto-5'-nucleotidase on the surface of cells seeded on a laminin substrate, but without the mobilization of an intracellular pool of enzyme. Furthermore, laminin and its fragments E'1 and E8 modified the AMPase activity of the ecto-5'-nucleotidase purified from chicken striated muscle and reconstituted in liposomes. Over the range of concentrations used, intact laminin and its fragment E8, consisting of the distal half of the long arm, stimulated the AMPase activity of ecto-5'-nucleotidase. By contrast, the large fragment derived from the short arms, designated E'1, inhibited the AMPase activity. Furthermore, the monoclonal anti-ecto-5'-nucleotidase antibody, CG37, abolished the stimulatory effect of fragment E8 on the AMPase activity of ecto-5'-nucleotidase but did not reverse the inhibitory effect of fragment E'1. In conclusion, laminin stimulates the AMPase activity of ecto-5'-nucleotidase by two mechanisms: inducing the expression of ecto-5'-nucleotidase to the cell surface and direct modulation of the enzymatic activity.