|Ph.D Student||Kigel Boaz Avraham|
|Subject||The Role of Class-3 Semaphorin and their Receptors,|
Type-A Plexins, in Angiogenesis and Tumor
|Department||Department of Medicine||Supervisor||Professor Emeritus Gera Neufeld|
|Full Thesis text|
Class-3 semaphorins include seven family members. Six of bind to neuropilin-1, neuropilin-2 or both, while the seventh, sema3E, binds to the plexin-D1 receptor. Sema3B and sema3F were previously characterized as tumor suppressors and as inhibitors of tumor angiogenesis. To determine if additional class-3 semaphorins possess anti-angiogenic and anti-tumorigenic properties, we expressed the recombinant semaphorins in four different tumorigenic cell lines expressing different combinations of its receptors. We demonstrate that sema3A, sema3D, sema3E and sema3G can function as potent anti-tumorigenic agents. All the semaphorins we examined were also able to reduce the concentration of tumor associated blood vessels although the potencies of the anti-angiogenic effects varied depending on the tumor cell type. There was little correlation between the ability of a semaphorin to inhibit tumor angiogenesis and their anti-tumorigenic activity, but its ability to inhibit tumor progression highly correlated with the tumor cell expression of specific signal transducing receptors for particular semaphorins. To better understand if class-3 semaphorins could indeed be used as therapeutic agents, we purified large amount of sema3E and injected it systemically into mice bearing 4T1 breast cancer cell line. Indeed, sema3E was able to inhibit tumor progression and even decreased the lung metastatic index, as compared to the control mice. This result suggests that that class-3 semaphorins might be used as therapeutic agents and that combination of them may be more effective than single semaphorins in cases in which tumor cells express more than one type of semaphorin receptor.
Most of the semaphorins are known to tranduce inhibitory signals. A few were characterized as stimulatory ligands. In order to understand the apparent dual roles of type-A plexins in endothelial cells (EC) and in tumor cells we silenced their expression using specific shRNAs. In EC, the inhibition caused plexin-specific morphological changes while the morphology of tumor cells remained unaffected. The silencing also inhibited bFGF-induced EC proliferation. Using various angiogenic assays we found that silencing plexin-A4 expression also results in the inhibition of angiogenesis in in-vitro assays. Silencing of plexin-A4 or plexin-A1 expression in tumor cells from different origins results in inhibition of their proliferation. Furthermore, tumor development from U87MG glioblastoma cells in which the expression of plexin-A4 was silenced was strongly inhibited, suggesting that plexin-A4 may represent a novel target for the development of cancer therapeutics. We also discover that the inhibitory effects seen while silencing plexin-A4 might be as a result of breaking a sema6B\plexin-A4 autocrine signaling.