Deletion of the plexin-D1 ectodomain leads to anoikis by suppressing integrin inside-out signaling

Plexin-D1, mainly expressed in endothelial and cancer cells, regulates diverse effects, suppresses endothelial cell growth, and induces cancer cell migration and proliferation. Here, we demonstrated that plexin-D1 was cleaved by proteinase on cancer cells. To examine the role of cleaved plexin-D1 in cells, Madin–Darby canine kidney (MDCK) cells overexpressing truncated plexin-D1 were cultured in Matrigel. MDCK cells expressing plexin-D1 lacking the ectodomain (plexin-D1 ΔEC) underwent apoptosis. An adhesion assay for extracellular matrix (ECM) molecules showed that plexin-D1 ΔEC–expressing MDCK cells lost their affinity for the ECM. These results suggest that plexin-D1 ΔEC blocks integrin inside-out signaling, leading to detachment from the ECM and apoptosis, so-called anoikis. By contrast, MDCK cells expressing full-length plexin-D1 or plexin-D1 lacking the cytoplasmic domain (plexin-D1 ΔIC) developed multicellular branching tubular structures in Matrigel. This morphological change was blocked in plexin-D1–expressing MDCK cells by the hepatocyte growth factor receptor (Met) loss of function or by Met inhibitors. These results suggest that plexin-D1 associates with Met through the plexin-D1 extracellular domain, and this activates Met cytoplasmic kinase activity. We therefore conclude that plexin-D1 contains distinct domains that determine the fate of cancer cells.