Post‐translational and cell type‐specific regulation of CXCR4 expression by cytokines

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Abstract
We have investigated the regulation and function of the chemokine receptor CXCR4 on neutrophils. CXCR4 is hardly detectable on neutrophils in the peripheral blood. However, overnight culture strongly up‐regulates CXCR4 expression on the cell surface. The functional activity of CXCR4 on cultured neutrophils was confirmed by stromal cell‐derived factor (SDF)‐induced migration and up‐regulation of the integrins CD11b and CD11c. CXCR4 surface expression on neutrophils but not on lymphocytes and monocytes is rapidly down‐regulated after stimulation with TNF‐α and IFN‐γ, resulting in significantly decreased SDF‐induced functional responses of neutrophils. In contrast to surface expression, CXCR4 mRNA expression was several‐fold increased in cytokine‐stimulated neutrophils, suggesting a post‐translational regulation. By confocal microscopy we demonstrate that CXCR4 is internalized after stimulation with TNF‐α and IFN‐γ. The down‐modulation of CXCR4 surface expression in response to TNF‐α and IFN‐γ was fully reversible after cytokine removal. Further, CXCR4 down‐modulation could be completely blocked by hypertonic sucrose and significantly reduced by chlorpromazine indicating the involvement of clathrin‐coated pits. Internalization of CXCR4 by cytokines in a cell type‐specific manner is a novel and functionally important mechanism of chemokine receptor regulation.