Hierarchy of ADAM12 binding to integrins in tumor cells

ADAMs (a disintegrin and metalloprotease) comprise a family of cell surface proteins with protease and cell-binding activities. Usingdifferent forms and fragments of ADAM 12 as substrates in cell adhesion and spreading assays, we demonstrated that alpha 9 beta 1 integrin is the main receptor for ADAM12. However, when alpha 9 beta 1 integrin is not expressed-as in many carcinoma cells-other members of the beta 1 integrin family can replace its ligand binding activity. In attachment assays, the recombinant disintegrin domain of ADAM12 only supported alpha 9 integrin-dependent tumor cell attachment, whereas full-length ADAM12 supported attachment via alpha 9 integrin and other integrin receptors. Cells that attached to full-length ADAM12 in an alpha 9 integrin-dependent manner also attached to ADAM12 in which the putative alpha 9 beta 1 integrin-binding motif in the disintegrin domain had been mutated. This attachment was mediated through use of an alternate beta 1 integrin. We also found that cell spreading in response to ADAM 12 is dependent on the apparent level of integrin activation. Binding of cells to ADAM 12 via the alpha 9 beta 1 integrin was Mn2+ -independent and resulted in attachment of cells with a rounded morphology; attachment of cells with a spread morphology required further activation of the alpha 9 beta 1 integrin. We demonstrated that phosphoinositide-3-kinase appears to be central in regulating alpha 9 beta 1 integrin cell spreading activity in response to ADAM12. (c) 2005 Elsevier Inc. All rights reserved.