Abstract 535: Paving the path to metastasis/invasion through GIV: Identification of a polarized ex(o)citing machinery for cancer invasion
Authors: Rohena Cristina C. , Sun Nina N. , Aznar Nicolas , Ghosh Pradipta 
Published: July 2017

Abstract
Polarized exocytosis is a fundamental process involving the delivery of membrane and cargo proteins to target sites the plasma membrane (PM) and it is essential for a wide range of biological functions such as cell growth, morphogenesis, and polarized cell migration. Polarized exocytosis requires precise spatial control of vesicle tethering to target sites at the PM. This is mediated by the octameric exocyst complex that is conserved from yeast to man. How the exocyst complex imparts polarity during exocytosis remained a mystery until recent insights emerged from studies on budding yeast. A functional interaction between exocyst component Exo70p and the yeast polarity determinant scaffold protein Bem1p was reported, and it was shown that Bem1p regulates the targeting of Exo70p to polarized exocytic sites. Despite these insights, the counterpart of Bem1p in higher species remained elusive. Here we demonstrate that a novel polarity-determinant protein called GIV (Gα-interacting vesicle associated protein/Girdin) fulfills the key criteria and functions of its yeast counterpart Bem1p. GIV provides an evolutionary upgrade to the process of polarized exocytosis by making it responsive to growth factors and other external cues and by modulating it through G protein signaling that is unique to mammalian cells. GIV was originally identified in a yeast-2 hybrid screen as a binding partner of EXOC1, and subsequently validated by us as a PI4P-binding protein that is a direct binding partner of mammalian Exo70. Biochemical assays have confirmed that the mechanism of GIV:Exo70 interaction bears close semblance to the Bem1p:Exo70p interaction. Sequence alignment studies led to the identification of single point mutants in Bem1p and GIV that are selectively defective in binding to Exo70. Using genetic manipulation of cancer cells we have observed that GIV is essential for polarized tethering of Exo70-coated vesicles at the cell periphery. Consistent with the previously described prometastatic role of GIV, we found that GIV is required for EGF-stimulated exocytosis of matrix metalloproteases that aid in cancer invasion in MDA-MB-231 breast cancer cells and that specific point mutants of GIV that interrupt the interaction with Exo70 cause a reduction in the ability of these breast cancer cells to invade and degrade components of the extracellular matrix. Furthermore, GIV interacts specifically with TC-10•GTP[active], a monomeric GTPase that regulates fusion of exocytic vesicles at the plasma membrane. Insights gained continue to provide an in-depth understanding of how GIV affects a fundamental process of exocytosis, and what impact that may have on cancer invasion and metastasis.
Citation Format: Cristina C. Rohena, Nina N. Sun, Nicolas Aznar, Pradipta Ghosh. Paving the path to metastasis/invasion through GIV: Identification of a polarized ex(o)citing machinery for cancer invasion [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 535. doi:10.1158/1538-7445.AM2017-535