Lewis‐Tuffin, Laura J.

Abstract The RhoA GTPase is thought to play a crucial role in cancer development. Its activation is tightly controlled by numerous guanine nucleotide-exchange factors (GEFs). We previously showed that Syx, a RhoA-GEF, promotes junctional integrity in endothelial cells and governs cell polarity and directed migration in glioblastoma (GBM) cells. GBM is a grade IV glioma with a median survival of approximately 14 months. Importantly, the gene locus of Syx (Plekhg5) is located within chromosome 1p36, a region deleted in oligodendroglioma (a type II/III glioma) and associated with better prognosis, suggesting a role of Syx in GBM aggressiveness. Here we show that Syx is a potential target for the treatment of GBM. Knockdown of Syx resulted in altered expression of cell cycle regulators, including cyclins A, B, and E, cyclin-dependent kinase inhibitors (CDIs), namely p21 and p27, and mitosis modulators CDC20 and survivin. These defects resulted in G2/M arrest and dysfunction of cell division, leading to improper chromosome segregation and DNA double-strand breaks. However, DNA damage response could not be activated effectively for DNA repair, leading to cellular apoptosis. Consistent with these findings, we used an orthotopic patient-derived xenograft GBM model to show that Syx silencing results in decreased tumor growth accompanied by increased animal survival. Agents that cause DNA damage and/or disruption of DNA repair pathways are often used for cancer therapy. We thus tested the possibility of targeting Syx for GBM treatment, and showed that Syx knockdown in combination with temozolomide, a first-line drug for GBM, synergistically inhibit cell growth. In summary, this is the first study showing the role of Syx in GBM cell growth and that targeting this signaling pathway can be beneficial for GBM therapy. Citation Format: Wan-Hsin Lin, Ryan Feathers, Laura Lewis-Tuffin, Panos Anastasiadis. Targeting the RhoA guanine nucleotide-exchange factor Syx for the treatment of glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4365.

Vascular endothelial growth factor (VEGF) and Ang1 (Angiopoietin-1) have opposing effects on vascular permeability, but the molecular basis of these effects is not fully known. We report in this paper that VEGF and Ang1 regulate endothelial cell (EC) junctions by determining the localization of the RhoA-specific guanine nucleotide exchange factor Syx. Syx was recruited to junctions by members of the Crumbs polarity complex and promoted junction integrity by activating Diaphanous. VEGF caused translocation of Syx from cell junctions, promoting junction disassembly, whereas Ang1 maintained Syx at the junctions, inducing junction stabilization. The VEGF-induced translocation of Syx from EC junctions was caused by PKD1 (protein kinase D1)-mediated phosphorylation of Syx at Ser806, which reduced Syx association to its junctional anchors. In support of the pivotal role of Syx in regulating EC junctions, syx−/− mice had defective junctions, resulting in vascular leakiness, edema, and impaired heart function.