Human neural cell adhesion molecule L1 and rat homologue NILE are ligands for integrin alpha(v)beta(3)

Integrin alpha(v) beta(3) is distinct in its capacity to recognize the sequence Arg-Gly-Asp (RGD) in many extra-cellular matrix (ECM) components. Here, we demonstrate that in addition to the recognition of ECM components, alpha(v) beta(3) can interact with the neural cell adhesion molecule L1-CAM; a member of the immunoglobulin superfamily (IgSF). M21 melanoma cells displayed significant Ca++-dependent adhesion and spreading on immunopurified rat L1 (NILE). This adhesion was found to be dependent on the expression of the alpha(v)-integrin subunit and could be significantly inhibited by an antibody to the alpha(v) beta(3) heterodimer. M21 cells also displayed some alpha(v) beta(3)-dependent adhesion and spreading on immunopurified human L1. Ligation between this ligand and alpha(v) beta(3) was also observed to promote significant haptotactic cell migration. To map the site of alpha(v) beta(3) ligation we used recombinant L1 fragments comprising the entire extracellular domain of human L1. Significant alpha(v) beta(3)-dependent adhesion and spreading was evident on a L1 fragment containing Ig-like domains 4, 5, and 6. Importantly, mutation of an RGD sequence present in the sixth Ig-like domain of L1 abrogated M21 cell adhesion. We conclude that alpha(v) beta(3)-dependent recognition of human L1 is dependent on ligation of this RGD site. Despite high levels of L1 expression the M21 melanoma cells did not display significant adhesion via a homophilic L1-L1 interaction. These data suggest that M21 melanoma cells recognize and adhere to L1 through a mechanism that is primarily heterophilic and integrin dependent. Finally, we present evidence that melanoma cells can shed and deposit L1 in occluding ECM. In this regard, alpha(v) beta(3) may recognize L1 in a cell-cell or cell-substrate interaction.