Heterophilic interactions between cell adhesion molecule L1 and α_vβ₃-integrin induce HUVEC process extension in vitro and angiogenesis in vivo

Cell adhesion molecule L1 was implicated in angiogenic processes, tumor formation and metastasis. Here, we provide evidence that the sixth Ig-like domain of L1 (L1Ig6) interacts with α_vβ₃ to induce process extension of human umbilical vein endothelial cells (HUVECs) in vitro and angiogenesis in vivo. HUVECs formed network-like structures on full-length L1 or L1Ig6 substrates comparable to structures found on matrigel. In the presence of mab α_vβ₃ or cyclic RGD, apoptosis was induced. In fibrin matrices where L1Ig6 was covalently incorporated, HUVECs formed multicellular and hollow processes through interactions between cell-surface α_vβ₃ and RGD-sites of matrix-immobilized L1Ig6. No such processes were induced by L1Ig6 having non-functional RDG-sites, or in the presence of mab α _vβ₃ or cyclic RGD. In those matrices, increased apoptosis was found. Co-immunoprecipitation of L1 or L1Ig6 with α_vβ₃ suggests close interactions. Furthermore, L1Ig6 stimulated HUVECs showed increased tyrosine phosphorylation of α_vβ₃ and phosphorylation of MAP kinases (ERK1 and ERK2) but not AKT indicating specific activation of α _vβ₃ followed by activation of downstream kinases. Application of L1Ig6-modified fibrin matrices on CAMs induced 50-60% increased α_v and α_vβ₃ protein expression and in vivo angiogenesis indicated by ∼50% increased mean vascular length density. The results demonstrate angiogenic potential of L1Ig6 involving ligation and activation of α_vβ₃.