Effects of the chemokine CXCL12 and combined internalization of its receptors CXCR4 and CXCR7 in human MCF-7 breast cancer cells

The chemokine CXCL12 (stromal cell-derived factor-1, SDF-1) and its receptor CXCR4 play a major role in tumor initiation, promotion, progression and metastasis, especially for breast cancer cells. Recently, CXCR7 has been identified as a second receptor for CXCL12; nevertheless, it also binds CXCL11 (interferon-inducible T cell α chemoattractant, I-TAC). However, little is known about the co-expression of the two receptors and their interactions. Quantitative reverse transcription plus the polymerase chain reaction has demonstrated that both receptors are frequently co-expressed in breast cancer cell lines, whereas other tumor cell lines often express only one of them. For interaction studies, we chose MCF-7 breast cancer cells, since they highly express CXCR4 and CXCR7 at the protein level but not CXCR3 (another target for CXCL11). Immunofluorescence and gold–labeling by light and electron microscopy, respectively, revealed that both receptors were localized at the cell surface in non-stimulated cells. After exposure to CXCL12 or CXCL11, the receptors were rapidly internalized alone or in close proximity. Stimulation with the CXCR4- or CXCR7-selective non-peptide antagonists AMD3100 and CCX733 resulted not only in single internalization but partly also in co-internalization of the two receptors. Furthermore, both chemokine ligands reduced staurosporine-induced apoptosis and caspase-3/7 activation; however, the selective inhibitors merely had partial inhibitory effects on these biological responses. Our findings suggest that CXCR4 and CXCR7 closely interact in breast cancer cells. Both are co-internalized, transduce signals and induce further biological effects partly independently of a selective stimulus or antagonist. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00441-014-1823-y) contains supplementary material, which is available to authorized users.