Abstract 3336: The heterotypic interaction between CXCR7 and EGFR is an alternative proliferation mechanism in breast cancer
Authors: Salazar Nicole , Muñoz Daniel , Singh Rajendra K. , Lokeshwar Bal L. 
Published: May 2015

Abstract
Introduction: CXCR7 is selectively over expressed in Breast Cancer (BrCa), suggesting its potential importance in BrCa development and function. CXCR7 is the latest CXC-chemokine receptor implicated in BrCa growth. Although it is endogenously over expressed in BrCa, the CXCR7 mechanism of tumor growth enhancement is still vaguely described. BrCa is a heterogeneous disease in several ways. An investigation of the interaction of CXCR7 with EGFR will help determine the relative contribution and consequence of such interaction in BrCa cell proliferation and progression.
Results: CXCR7 is expressed predominantly in Estrogen Receptor positive (ER+) BrCa cells, MCF7 and BT474, and was absent in common ER- BrCa cells. CXCR7 was expressed unevenly in normal breast and ER+ cancer tissues. Depletion of CXCR7 in BrCa cells by RNAi decreased cell proliferation and caused cell cycle arrest, with significant changes in cell cycle-associated proteins. Addition, depletion, and rescue of CXCR7-ligand, SDF-1a, had no significant effect on cell proliferation or phosphorylation of ERK1/2. Depletion of CXCR7 led to inhibition of EGF-mediated phosphorylation of EGFR at Tyrosine 1110 and ERK1/2 phosphorylation. We examined whether CXCR7 regulates BrCa proliferation through interaction with EGFR or by activating other potential targets of MAP-kinase activation and found that CXCR7 complexes with EGFR, as demonstrated by co-immunoprecipitation followed by immunoblotting and with proximity ligation assays, using the Duo-link fluorescence technique. Investigation of whether CXCR7-EGFR interaction is direct or indirect, led to test potential translocation of fluorescent β-Arrestin-2 from the plasma membrane to the cytoplasm. SDF-1a addition to MCF7 cells, over expressing β-Arrestin-2-EGFP, induced minimal movement of β-Arrestin-2-EGFP over a 30 min window, whereas EGF addition induced re-arrangement of β-Arrestin-2-EGFP localization. When CXCR7 was down-regulated by siRNA, the beta-ARR2-EGFP rearrangement was significantly reduced, suggesting this CXCR7-EGFR interaction is facilitated by β-Arrestin-2.
Conclusion: Our results demonstrate a heterotypic interaction between CXCR7 and EGFR in breast cancer independent of SDF-1a. Recent studies remind us that seventy percent of breast cancers are ER+ even at the metastatic stage, making the expression of CXCR7 clinically relevant. The innate heterogeneity of BrCa makes it clear that proliferation mechanisms change and may be altered during cancer progression, so that CXCR7 and other receptors may step in to support proliferation. These results demonstrate the important role of CXCR7 in breast cancer growth and progression.
Citation Format: Nicole Salazar, Daniel Muñoz, Rajendra K. Singh, Bal L. Lokeshwar. The heterotypic interaction between CXCR7 and EGFR is an alternative proliferation mechanism in breast cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3336. doi:10.1158/1538-7445.AM2014-3336