Center for Molecular Imaging, Department of Radiology, University of Michigan, Ann Arbor, MI 48109, U.S.A.
Biochem J. 2012 Mar 1;442(2):433-42. doi: 10.1042/BJ20111341.
Chemokine CXCL12 (CXC chemokine ligand 12) signalling through CXCR (CXC chemokine receptor) 4 and CXCR7 has essential functions in development and underlies diseases including cancer, atherosclerosis and autoimmunity. Chemokines may form homodimers that regulate receptor binding and signalling, but previous studies with synthetic CXCL12 have produced conflicting evidence for homodimerization. We used bioluminescence imaging with GL (Gaussia luciferase) fusions to investigate dimerization of CXCL12 secreted from mammalian cells. Using column chromatography and GL complementation, we established that CXCL12 was secreted from mammalian cells as both monomers and dimers. Secreted CXCL12 also formed homodimers in the extracellular space. Monomeric CXCL12 preferentially activated CXCR4 signalling through Gαi and Akt, whereas dimeric CXCL12 more effectively promoted recruitment of β-arrestin 2 to CXCR4 and chemotaxis of CXCR4-expressing breast cancer cells. We also showed that CXCR7 preferentially sequestered monomeric CXCL12 from the extracellular space and had minimal effects on dimeric CXCL12 in cell-based assays and an orthotopic tumour xenograft model of human breast cancer. These studies establish that CXCL12 secreted from mammalian cells forms homodimers under physiological conditions. Since monomeric and dimeric CXCL12 have distinct effects on cell signalling and function, our results have important implications for ongoing efforts to target CXCL12 pathways for therapy.
趋化因子 CXCL12（CXC 趋化因子配体 12）通过 CXCR（CXC 趋化因子受体）4 和 CXCR7 的信号转导在发育过程中具有重要功能，并导致包括癌症、动脉粥样硬化和自身免疫在内的多种疾病。趋化因子可能形成同源二聚体，调节受体结合和信号转导，但以前使用合成 CXCL12 的研究对同源二聚化产生了相互矛盾的证据。我们使用带有 GL（Gaussia 荧光素酶）融合的生物发光成像来研究从哺乳动物细胞分泌的 CXCL12 的二聚化。通过柱层析和 GL 互补，我们确定 CXCL12 从哺乳动物细胞分泌为单体和二聚体。分泌的 CXCL12 也在细胞外空间形成同源二聚体。单体 CXCL12 优先通过 Gαi 和 Akt 激活 CXCR4 信号转导，而二聚体 CXCL12 更有效地促进β-arrestin 2 募集到 CXCR4 并趋化表达 CXCR4 的乳腺癌细胞。我们还表明，CXCR7 优先从细胞外空间隔离单体 CXCL12，并且在细胞测定和人乳腺癌的原位肿瘤异种移植模型中对二聚体 CXCL12 的影响最小。这些研究表明，在生理条件下，从哺乳动物细胞分泌的 CXCL12 形成同源二聚体。由于单体和二聚体 CXCL12 对细胞信号转导和功能有不同的影响，因此我们的研究结果对于为治疗靶向 CXCL12 途径的持续努力具有重要意义。
