Abstract
CXCL4 is the most abundant chemokine secreted from human platelet and it is critically involved in several biological processes that drive inflammation, chemotaxis, atherosclerosis, heparin-induced thrombocytopenia, inhibition of HIV-1 infection and angiogenesis. CXCL4 has anti-angiogenic properties thought to be mediated by different mechanisms, including CXCR3 receptor activation. Chemokines have distinct oligomerization states that are correlated with their biological functions. CXCL4 exists as a stable tetramer in physiological conditions. It is unclear whether the oligomerization state impacts CXCL4-receptor interactions and how CXCL4 executes different functions in correlation with the receptor. We noticed that the CXCL4 tetramer was sensitive to pH and salt concentration. Furthermore, it was determined that residue Glu-28 was important for tetramer formation, and the first β-strand and the C-terminal helix were critical for dimerization. The CXCL4 monomer acts as the active unit for activating CXCR3A, and N-terminal tyrosine sulfations of the receptor are involved in binding. Noticeably, CXCL4L1, a CXCL4 variant with three-residue difference in the C-terminal helix, could activate CXCR3A. CXCL4L1 but not native CXCL4 showed a higher tendency to directly dissociate into monomers. This indicates that monomeric CXCL4 behaves like CXCL4L1. Thus, in this chemokine family, being in the monomeric state seems critical for CXCR3A binding and activation. Our study clearly indicated chemokines can convert their biological functions by interfering with their oligomerization state. Due to the wide cellular distribution of CXCR3 receptor, this study is relevant in many physiological and pathological situations, such as immunity, angiogenesis, tumor development or metastatic spread.