Abstract
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 materialThe online version of this article (doi:10.1007/s00441-014-1823-y) contains supplementary material, which is available to authorized users.
Highlights
Chemokines were initially discovered as small, 8- to 10-kDa chemotactic cytokines in the immune system; they attract leukocytes by interacting with their G-protein-coupled seven-transmembrane domain receptors
We investigated the expression of the chemokine receptors CXCR7 and CXCR4 in various
The CXCR4/7 ligand CXCL12 is expressed at a low level, whereas the CXCR3/7 ligand CXCL11 is just above the detection limit. c Western blots of chemokine receptors CXCR4 and CXCR7 in membrane fractions of various types of human tumor cells clearly reveal a dual production of CXCR4 and CXCR7 in MCF-7 breast cancer cells, a single expression of CXCR7 in U343 glioma cells and an absence of both receptors in LOX melanoma cells, as found by qRT-polymerase chain reaction (PCR)
Summary
Chemokines were initially discovered as small, 8- to 10-kDa chemotactic cytokines in the immune system; they attract leukocytes by interacting with their G-protein-coupled seven-transmembrane domain receptors. Later, chemokines and their receptors were shown to exhibit a much broader function in tissue development and homeostasis and in many pathological conditions (for a review, see Mentlein et al 2013). Chemokines and their receptors were shown to exhibit a much broader function in tissue development and homeostasis and in many pathological conditions (for a review, see Mentlein et al 2013) They play decisive roles in tumor initiation, promotion, progression and metastasis (for a review, see Zlotnik et al 2011). In addition to CXCL12, CXCR7 binds with ten-fold lower affinity to CXCL11/I-TAC (interferon-inducible T cell α chemoattractant), which is a ligand for CXCR3 (which is targeted by CXCL9/Mig and CXCL10/IP-10)
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