Constitutive and chemokine-dependent internalization and recycling of CXCR7 in breast cancer cells to degrade chemokine ligands.
CXCR7 is a receptor for chemokines including CXCL12 (stromal-derived factor-1), a molecule that promotes tumor growth and metastasis in breast cancer and other malignancies. Building on the recent observation that CXCR7 sequesters CXCL12, we investigated mechanisms for CXCR7-dependent uptake of chemokines. Breast cancer cells expressing CXCR7 accumulated chemokines CXCL12 and CXC11 present at concentrations <1 ng/ml, unlike cells expressing CXCR4. CXCR7-dependent accumulation of chemokines was reduced by inhibitors of clathrin-mediated endocytosis. After CXCR7-mediated internalization, CXCL12 trafficked to lysosomes and was degraded, although levels of CXCR7 remained stable. CXCR7 reduced CXCL12 in the extracellular space, limiting the amounts of chemokine available to acutely stimulate signaling through CXCR4. CXCR7 constitutively internalized and recycled to the cell membrane even in the absence of ligand, and addition of chemokines did not significantly enhance receptor internalization. Chemokines at concentrations less than the Kd values for ligand-receptor binding did not alter levels of CXCR7 at the cell surface. Higher concentrations of chemokine ligands reduced the total cell surface expression of CXCR7 without affecting receptor internalization, indicating that receptor recycling was inhibited. CXCR7-dependent uptake of chemokines and receptor trafficking were regulated by beta-arrestin 2. These studies establish mechanisms through which CXCR7 regulates the availability of chemokine ligands in the extracellular space.
Department of Radiology, Center for Molecular Imaging, University of Michigan Medical School, 109 Zina Pitcher Place, A526 BSRB, Ann Arbor, MI 48109-2200, USA.
K E
KE
Luker
J M
JM
Steele
L A
LA
Mihalko
P
P
Ray
G D
GD
Luker
D019390:Arrestins / Q000378:metabolism; D001943:Breast Neoplasms / Q000235:genetics / Q000378:metabolism* / Q000473:pathology*; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000378:metabolism; D054371:Chemokine CXCL11 / Q000378:metabolism; D054377:Chemokine CXCL12 / Q000378:metabolism; D018925:Chemokines / Q000378:metabolism*; D002966:Clathrin / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D004705:Endocytosis; D005110:Extracellular Space / Q000378:metabolism; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D008024:Ligands; D008247:Lysosomes / Q000378:metabolism; D021381:Protein Transport; D054387:Receptors, CXCR / Q000378:metabolism*; D019718:Receptors, CXCR4 / Q000378:metabolism; D000071560:beta-Arrestin 2; D000071557:beta-Arrestins
C000605254:ARRB2 protein, human; D019390:Arrestins; D054371:Chemokine CXCL11; D054377:Chemokine CXCL12; D018925:Chemokines; D002966:Clathrin; D008024:Ligands; D054387:Receptors, CXCR; D019718:Receptors, CXCR4; D000071560:beta-Arrestin 2; D000071557:beta-Arrestins