Tripathi Abhishek, Davis Jeffrey D, Staren Daniel M, Volkman Brian F, Majetschak Matthias
Department of Surgery, Loyola University Chicago, Stritch School of Medicine, United States.
Department of Biochemistry, Medical College of Wisconsin, United States.
Cytokine. 2014 Feb;65(2):121-5. doi: 10.1016/j.cyto.2013.12.008. Epub 2013 Dec 24.
Recently, we reported that extracellular ubiquitin functions as another agonist of CXC chemokine receptor (CXCR)4. Whereas the cognate CXCR4 ligand, stromal cell-derived factor (SDF)-1α, is also a CXCR7 agonist, ubiquitin does not bind to CXCR7. Because both ligands are present in the extracellular environment, co-activation of CXCR4 appears to be physiologically relevant. CXCR4 mediated effects of ubiquitin, however, are not well understood and consequences of co-activation of CXCR4 with both ligands are unknown. Utilizing proximity ligation assays and flow cytometry, we detected CXCR4, but not CXCR7, on the cell surface of THP-1 cells, which suggests that confounding effects of CXCR7 are unlikely. Time course and magnitude of reduction of cell surface CXCR4 expression were comparable after stimulation of THP-1 cells with both ligands. SDF-1α was more efficacious than ubiquitin to mobilize Ca(2+). Co-stimulation of THP-1 cells with both ligands resulted in synergistic effects on Ca(2+) fluxes at suboptimal ligand concentrations. Homologous desensitization of Ca(2+) fluxes was detectable with both ligands. SDF-1α pre-stimulation desensitized ubiquitin induced Ca(2+) fluxes, but not vice versa. Effects of SDF-1α and ubiquitin on cAMP levels, Akt and ERK1/2 phosphorylation and chemotactic responses were additive. The chemotactic activities of ubiquitin and SDF-1α were sensitive to AMD3100, pertussis toxin, U73122, LY94002 and U0126. These data suggest that CXCR4 activation with SDF-1α and ubiquitin results in partially synergistic effects on cellular signaling events and in differential effects on receptor desensitization. The ligand ratio that is present in the extracellular environment may contribute to the regulation of CXCR4 mediated functions.
最近，我们报道细胞外泛素作为CXC趋化因子受体（CXCR）4的另一种激动剂发挥作用。而同源CXCR4配体，基质细胞衍生因子（SDF）-1α，也是CXCR7激动剂，泛素并不与CXCR7结合。由于两种配体都存在于细胞外环境中，CXCR4的共激活似乎具有生理相关性。然而，泛素介导的CXCR4效应尚未得到充分理解，且两种配体共同激活CXCR4的后果尚不清楚。利用邻近连接分析和流式细胞术，我们在THP-1细胞的细胞表面检测到了CXCR4，而非CXCR7，这表明CXCR7的混杂效应不太可能存在。在用两种配体刺激THP-1细胞后，细胞表面CXCR4表达降低的时间进程和幅度相当。SDF-1α在动员Ca(2+)方面比泛素更有效。在次优配体浓度下，用两种配体共同刺激THP-1细胞对Ca(2+)通量产生协同效应。两种配体均可检测到Ca(2+)通量的同源脱敏。SDF-1α预刺激可使泛素诱导的Ca(2+)通量脱敏，但反之则不然。SDF-1α和泛素对cAMP水平、Akt和ERK1/2磷酸化以及趋化反应的影响是相加的。泛素和SDF-1α的趋化活性对AMD3100、百日咳毒素、U73122、LY94002和U0126敏感。这些数据表明，用SDF-1α和泛素激活CXCR4会对细胞信号事件产生部分协同效应，并对受体脱敏产生不同影响。细胞外环境中存在的配体比例可能有助于调节CXCR4介导的功能。
