Summary The Merlin/NF2 tumor suppressor restrains cell growth and tumorigenesis by controlling contact-dependent inhibition of proliferation. We have identified a tight-junction-associated protein complex comprising Merlin, Angiomotin, Patj, and Pals1. We demonstrate that Angiomotin functions downstream of Merlin and upstream of Rich1, a small GTPase Activating Protein, as a positive regulator of Rac1. Merlin, through competitive binding to Angiomotin, releases Rich1 from the Angiomotin-inhibitory complex, allowing Rich1 to inactivate Rac1, ultimately leading to attenuation of Rac1 and Ras-MAPK pathways. Patient-derived Merlin mutants show diminished binding capacities to Angiomotin and are unable to dissociate Rich1 from Angiomotin or inhibit MAPK signaling. Depletion of Angiomotin in Nf2−/− Schwann cells attenuates the Ras-MAPK signaling pathway, impedes cellular proliferation in vitro and tumorigenesis in vivo.

Controlled regulation of Rho GTPase activity is an essential component mediating growth factor-stimulated migration. We have previously shown that angiomotin (Amot), a membrane-associated scaffold protein, plays a critical role during vascular patterning and endothelial migration during embryogenesis. However, the signaling pathways by which Amot controls directional migration are not known.

Tumor endothelial cells have long been regarded as genomically stable and therefore less likely to develop resistance to antiangiogenic therapies. However, recent findings have challenged this notion. We have shown that DNA can be transferred between cells through phagocytosis of apoptotic bodies by adjacent viable cells. Propagation of the ingested DNA is prevented by the activation of the p53-p21 pathway. In this study, we examined whether concomitant transfer of tumor DNA with genes that inactivate the p53 pathway could overcome the barrier to tumor DNA propagation. Our results demonstrate that fibroblasts and endothelial cells are capable of acquiring and replicating tumor DNA when the apoptotic tumor cells contain the SV40 large T antigen. Analysis of the tumor stroma of xenotransplanted tumors in severe combined immunodeficient mice revealed that a sub-population of the endothelial cells contained tumor DNA. These cells maintained the ability to form functional vessels in an in vivo assay and concurrently express tumor-encoded and endothelial-specific genes. Malignant and non-malignant cells are dependent on a functional blood circulation for the supply of oxygen and nutrients.1 Evidence from transgenic mouse models demonstrate that tumors initially lack the ability to induce blood vessel formation but subsequently acquire the capacity to recruit vessels from the adjoining stroma.2 This 'angiogenic switch' coincides with increase in tumor mass and invasiveness. Tumor cells secrete a plethora of angiogenic factors that induce sprouting angiogenesis (i.e. the formation of new capillaries from pre-existing capillaries) and the differentiation of hematopoietic stem cells into endothelial cells, which then contribute to tumor vessel formation. 3A potential advantage in targeting the endothelial cells of the tumor vasculature is that these cells are regarded as diploid and genetically stable. However, recent data have challenged this notion. Uveal melanomas have been reported to form their own circulatory network by the formation of vascular channels lined by tumor cells. 4 These vasculogenic channels apparently link directly to normal vessels without evidence of angiogenesis. Streubel et al.5 analyzed the cellular origin of the microvascular endothelial cells of B-cell lymphomas in patients. Chromosomal translocations specific for the tumor cells were detected in the tumor endothelium, suggesting a close relationship between the two cell types. Furthermore, analyses of human tumors transplanted in severe combined immunodeficient (SCID) mice demonstrate that the endothelial cells of solid human tumors grown in mice are genetically unstable and exhibit aneuploidy with multiple chromosomes and centrosomes. 6 Cumulatively, these studies potentially imply that tumor endothelial cells exhibit genetic instability albeit the mechanisms by which genetic alterations are induced are presently unclear.Our previous studies and those of others have demonstrated that viral and chromosomal DNA can be efficiently transferred be...