Role of beta-arrestins in chemokine receptor signaling

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PROJECT SUMMARY The heterotrimeric G protein-coupled receptor (GPCR) C-X-C motif receptor 4 (CXCR4) and its cognate ligand CXCL12 play important roles in health and disease. A large body of evidence indicates that CXCR4 signaling is linked to cancer progression. CXCR4 expression and signaling in cancer correlates with poor prognosis2-5, mainly because cancer cells expressing CXCR4 colonize distant anatomical sites where CXCL12 is located, resulting in metastatic disease, the cause of most cancer related deaths. CXCR4 signaling regulates several aspects of cell physiology linked to cancer progression. This includes directed cell migration and cell survival, which occur via several discrete signaling pathways. Yet the mechanisms remain poorly understood. The focus of this proposal is on the signal transduction mechanisms that regulate CXCR4-mediated chemotaxis towards CXCL12. We recently reported that CXCR4-mediated chemotaxis occurs via a novel mechanism involving a complex formed between endocytic adaptor proteins b-arrestin1 (barr1) and STAM1 (barr1:STAM1). The barr1:STAM1 complex does not act on Akt or ERK-1/2 signaling pathways, but instead is necessary for activating focal adhesion kinase (FAK), which is also necessary for CXCL12 driven chemotaxis. FAK is typically linked to integrin signaling and focal adhesion dynamics, but these aspects of FAK function are not regulated by the barr1:STAM1 complex. Despite our contribution, how barr1:STAM1 activates FAK downstream of CXCR4 to promote chemotaxis remains poorly understood. The overall objective of this proposal is to fill in knowledge gaps. Based on our published and preliminary studies we hypothesize that G protein-dependent barr1:STAM1 signaling spatially and temporally controls FAK activity required for CXCR4- dependent chemotaxis. To test this hypothesis we will pursue the following specific aims: Aim 1. To elucidate the role of CXCR4 site-specific phosphorylation on FAK activation; Aim 2. To identify the structural and biophysical properties of the barr1 interaction with STAM1; Aim 3. To elucidate the functional role of the barr1:STAM1 complex in chemotaxis. Because of the mechanistic focus of our proposal we will use cell culture models and other in vitro approaches spanning techniques in cell and molecular biology, genetics, biochemistry and biophysics plus advanced live cell imaging strategies and mass spectrometry approaches. At the conclusion of this project we will have learned novel signal transduction mechanisms by which barr1:STAM1 collaborate to activate FAK to promote chemotaxis. This is significant because it will reveal novel aspects of CXCR4 signaling that could be targeted therapeutically.