Abstract C210: Atypical chemokine receptor 3/CXCR7 and EGFR interact to control breast cancer growth.
Authors: Salazar Nicole , Muñoz Daniel , Lokeshwar Bal L. 
Published: August 2014

Abstract
Introduction: Breast cancer (BrCa) is a major burden for women in the US, ranking second in both incidence and cancer related deaths. Many studies comparing cancer vs. normal gene expression have identified CXCR7 as being preferentially expressed in several different cancer types, including breast cancer. CXCR7 is the latest CXC-chemokine receptor implicated in breast cancer growth. The chemokine receptor CXCR7 is an atypical G-protein coupled receptor, in that it does not bind G-proteins as GPCRs do. It is so atypical, that convention by HUGO has recently changed the name of CXCR7 to ACKR3, atypical chemokine receptor 3.
Methods: We used the database Oncomine to confirm the correlation of CXCR7 with breast cancer at both mRNA and tissue level. We also show functional expression of CXCR7 on relevant human breast cancer cell lines and tissue. To identify the mechanism of function of CXCR7 in BrCa we studied its interaction with EGFR and beta-arrestin2 (beta-ARR2), which are relevant partners important in the mitogenic growth cascade. We used a proximity ligation assay (PLA) to visualize CXCR7-EGFR co-localization both in cell culture and in human normal and breast cancer tissue. To visualize interaction with beta-ARR2, we used live cell confocal microscopy.
Results: CXCR7 was over-expressed in ER-α positive BrCa cells, compared to normal breast epithelial cells. ER-negative cells also expressed CXCR7, however, at lower levels. Depletion of CXCR7 in MCF-7 BrCa cells by RNAi decreased cell proliferation and caused cell cycle arrest. Since CXCR7 is an atypical G-protein coupled receptor where ligand activation does not elicit conventional intracellular signaling, we hypothesized that CXCR7 regulates BrCa proliferation through interaction with EGFR or activation of other potential targets of the MAP-kinase pathway. CXCR7 depletion reduced site-specific phosphorylation of EGFR at Tyrosine 1110 after EGF-stimulation. CXCR7 depletion also reduced phosphorylation of ERK-1/2, indicating a potentially direct interaction with mitogenic signaling in MCF-7 cells. We found EGFR and CXCR7 co-localize as demonstrated by confocal microscopy, with increased co-localization after EGF stimulation. To clarify whether CXCR7-EGFR interaction is direct or indirect, we analyzed the role beta-ARR2 plays in this transactivation. SDF1 addition to MCF7 cells over expressing beta-ARR2-EGFP induced minimal movement of beta-ARR2-EGFP over a 30 min window. EGF addition induced an obvious shift of beta-ARR2-EGFP localization. When CXCR7 was down-regulated by siRNA, there was decreased rearrangement of beta-ARR2-EGFP after EGF stimulation, suggesting this CXCR7-EGFR interaction is beta-ARR2 mediated.
Conclusion: These data demonstrate significant molecular interactions between two cell surface receptors that have high clinical significance. The impact of this work may potentially lead to better treatment strategies based on cell surface targets.
Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C210.
Citation Format: Nicole Salazar, Daniel Muñoz, Bal L. Lokeshwar. Atypical chemokine receptor 3/CXCR7 and EGFR interact to control breast cancer growth. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C210.