DUAL MECHANISM OF LAMININ MODULATION OF ECTO-5'-NUCLEOTIDASE ACTIVITY

The myoblast cell surface activity of ecto-5'-nucleotidase was stimula ted by a laminin substrate, whereas fibronectin and gelatin did not in crease 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 o f an intracellular pool of enzyme. Furthermore, laminin and its fragme nts E1' and E8 modified the AMPase activity of the ecto-5'-nucleotidas e purified from chicken striated muscle and reconstituted in liposomes . Over the range of concentrations used, intact laminin and its fragme nt E8, consisting of the distal half of the long arm, stimulated the A MPase activity of ecto-5'-nucleotidase. By contrast, the large fragmen t derived from the short arms, designated E1', inhibited the AMPase ac tivity. 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 ef fect of fragment E1'. In conclusion, laminin stimulates the AMPase act ivity of ecto-5'-nucleotidase by two mechanisms: inducing the expressi on of ecto-5'-nucleotidase to the cell surface and direct modulation o f the enzymatic activity. (C) 1993 Wiley-Liss, Inc.