The CXCL12/CXCR4 axis is involved in many cellular responses for host homeostasis, and malfunction of this signaling pathway is associated with a variety of diseases. It is now known that CXCL12 also binds to another newly identified chemokine receptor, CXCR7, which does not couple with a G-protein. CXCR7 can form homodimers, or heterodimers with CXCR4, and is believed to sequester the chemokine CXCL12, although the CXCL12/CXCR7 axis activates MAP kinases through β-arrestin. Therefore, it has not been well defined how CXCR7 activation affects CXCL12-induced cellular events. To elucidate the function of CXCR7, we prepared CXCR7 agonist Compound 1. Compound 1 is a selective and potent CXCR7 agonist that clearly has the activity to recruit β-arrestin toward CXCR7. It also activates MAP kinases Akt and ERK. Using this compound, we confirmed that the CXCR7 agonist, but not an antagonistic antibody, did inhibit CXCL12 induced HUVEC tube formation, suggesting that activation of CXCR7 ameliorates CXCL12 induced cellular events, probably by affecting on CXCR4 function. We show that β-arrestin recruitment to CXCR4 is reduced by over-expression of CXCR7 and activation of CXCR7 by agonist treatment reduces the protein level of CXCR4. Based on our results, together with reported information, we propose that CXCR7, when up-regulated upon inflammation, can act as a negative regulator of CXCR4 by heterodimerizing with CXCR4, inducing its internalization and degradation. This mechanism suggests that CXCR7 agonists can have a therapeutic effect on CXCL12 causing diseases by countering the effects of CXCL12.
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