Introduction: Breast cancer (BrCa) ranks second in both incidence and cancer deaths for women in the USA. Recent advances have revealed a significant contribution of chemokines and their receptors in tumor growth, survival after chemotherapy, and organ-specific metastasis. CXCR7 is the latest CXC-chemokine receptor implicated in BrCa growth. It is endogenously over expressed in BrCa, but its mechanism of tumor growth enhancement is still not clearly defined. CXCR7 heterodimerizes with CXCR4 and binds to SDF-1 (CXCL12) and CXCL11. No direct ligand mediated physiological action has been implicated for CXCR7 in BrCa other than facilitating CXCR4-mediated cell motility, however, studies in other cancers have implicated CXCR7 in cell proliferation, anti-apoptotic activity and cell-cell adhesion. The expression pattern of CXCR7 in BrCa tissues and its potential contribution in BrCa cell proliferation and survival were investigated in representative BrCa cell lines using knock down and knock-in experiments.
Results: CXCR7 was only expressed in ER-α positive BrCa cells (e.g., MCF-7) and was not found in BrCa cells with known metastatic potential (MDA-MB231 and SKBr3). Depletion of CXCR7 in MCF-7 BrCa cells by both siRNA and shRNA 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 activating other potential targets of MAP-kinase activation. 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 tested for direct interaction of CXCR7 with EGFR by co-immunoprecipitation and immunofluorescence studies. We found EGFR and CXCR7 co-immunoprecipitate and co-localize as demonstrated by confocal microscopy. Interestingly, our results also demonstrate CXCR7 and EGFR are co-localized before EGF stimulation. Further corroboration of CXCR7-EGFR interaction was established using the proximity ligation assay (PLA) technique that allowed us to visualize CXCR7-EGFR co-localization both in cell culture and in human normal and breast cancer tissue.
Conclusion: This data demonstrates an important role for CXCR7 in BrCa growth and explores the mechanism of action of direct coupling of EGFR with CXCR7.
Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P5-07-01.