Glycosaminoglycan-Mediated Downstream Signaling of CXCL8 Binding to Endothelial Cells.

The recruitment of leukocytes, mediated by endothelium bound chemokine gradients, is a vital process in inflammation. The highly negatively charged, unbranched polysaccharide family of glycosaminoglycans (GAGs), such as heparan sulfate and chondroitin sulfate mediate chemokine immobilization. Specifically the binding of CXCL8 (interleukin 8) to GAGs on endothelial cell surfaces is known to regulate neutrophil recruitment. Currently, it is not clear if binding of CXCL8 to GAGs leads to endothelial downstream signaling in addition to the typical CXCR1/CXCR2 (C-X-C motif chemokine receptor 1 and 2)-mediated signaling which activates neutrophils. Here we have investigated the changes in protein expression of human microvascular endothelial cells induced by CXCL8. Tumor necrosis factor alpha (TNFα) stimulation was used to mimic an inflammatory state which allowed us to identify syndecan-4 (SDC4) as the potential proteoglycan co-receptor of CXCL8 by gene array, real-time PCR and flow cytometry experiments. Enzymatic GAG depolymerization via heparinase III and chondroitinase ABC was used to emulate the effect of glycocalyx remodeling on CXCL8-induced endothelial downstream signaling. Proteomic analyses showed changes in the expression pattern of a number of endothelial proteins such as Zyxin and Caldesmon involved in cytoskeletal organization, cell adhesion and cell mobility. These results demonstrate for the first time a potential role of GAG-mediated endothelial downstream signaling in addition to the well-known CXCL8-CXCR1/CXCR2 signaling pathways in neutrophils.

Glycosaminoglycans are important mediators of neutrophilic inflammation in vivo.

The pro-inflammatory chemokine interleukin-8 (CXCL8) exerts its function by establishing a chemotactic gradient in infected or damaged tissues to guide neutrophil granulocytes to the site of inflammation via its G protein-coupled receptors (GPCRs) CXCR1 and CXCR2 located on neutrophils. Endothelial glycosaminoglycans (GAGs) have been proposed to support the chemotactic gradient formation and thus the inflammatory response by presenting the chemokine to approaching leukocytes. In this study, we show that neutrophil transmigration in vitro can be reduced by adding soluble GAGs and that this process is specific with respect to the nature of the glycan. To further investigate the GAG influence on neutrophil migration, we have used an engineered CXCL8 mutant protein (termed PA401) which exhibits a much higher affinity towards GAGs and an impaired GPCR activity. This dominant-negative mutant chemokine showed anti-inflammatory activity in various animal models of neutrophil-driven inflammation, i.e. in urinary tract infection, bleomycin-induced lung fibrosis, and experimental autoimmune uveitis. In all cases, treatment with PA401 resulted in a strong reduction of transmigrated inflammatory cells which became evident from histology sections and bronchoalveolar lavage. Since our CXCL8-based decoy targets GAGs and not GPCRs, our results show for the first time the crucial involvement of this glycan class in CXCL8/neutrophil-mediated inflammation and will thus pave the way to novel approaches of anti-inflammatory treatment.

Targeting of CCL2-CCR2-Glycosaminoglycan Axis Using a CCL2 Decoy Protein Attenuates Metastasis through Inhibition of Tumor Cell Seeding.

The CCL2-CCR2 chemokine axis has an important role in cancer progression where it contributes to metastatic dissemination of several cancer types (e.g., colon, breast, prostate). Tumor cell-derived CCL2 was shown to promote the recruitment of CCR2(+)/Ly6C(hi) monocytes and to induce vascular permeability of CCR2(+) endothelial cells in the lungs. Here we describe a novel decoy protein consisting of a CCL2 mutant protein fused to human serum albumin (dnCCL2-HSA chimera) with enhanced binding affinity to glycosaminoglycans that was tested in vivo. The monocyte-mediated tumor cell transendothelial migration was strongly reduced upon unfused dnCCL2 mutant treatment in vitro. dnCCL2-HSA chimera had an extended serum half-life and thus a prolonged exposure in vivo compared with the dnCCL2 mutant. dnCCL2-HSA chimera bound to the lung vasculature but caused minimal alterations in the leukocyte recruitment to the lungs. However, dnCCL2-HSA chimera treatment strongly reduced both lung vascular permeability and tumor cell seeding. Metastasis of MC-38GFP, 3LL, and LLC1 cells was significantly attenuated upon dnCCL2-HSA chimera treatment. Tumor cell seeding to the lungs resulted in enhanced expression of a proteoglycan syndecan-4 by endothelial cells that correlated with accumulation of the dnCCL2-HSA chimera in the vicinity of tumor cells. These findings demonstrate that the CCL2-based decoy protein effectively binds to the activated endothelium in lungs and blocks tumor cell extravasation through inhibition of vascular permeability.

Designing a mutant CCL2-HSA chimera with high glycosaminoglycan-binding affinity and selectivity.

Chemokines like CCL2 mediate leukocyte migration to inflammatory sites by binding to G-protein coupled receptors on the target cell as well as to glycosaminoglycans (GAGs) on the endothelium of the inflamed tissue. We have recently shown that the dominant-negative Met-CCL2 mutant Y13A/S21K/Q23R with improved GAG binding affinity is highly bio-active in several animal models of inflammatory diseases. For chronic indications, we have performed here a fusion to human serum albumin (HSA) in order to extend the serum half-life of the chemokine mutant. To compensate a potential drop in GAG-binding affinity due to steric hindrance by HSA, a series of novel CCL2 mutants was generated with additional basic amino acids which were genetically introduced at sites oriented towards the GAG ligand. From this set of mutants, the Met-CCL2 variant Y13A/N17K/S21K/Q23K/S34K exhibited high GAG-binding affinity and a similar selectivity as wild type (wt) CCL2. From a set of different HSA-chemokine chimeric constructs, the linked HSA(C34A)(Gly)4Ser-Met-CCL2(Y13A/N17K/S21K/Q23K/S34K) fusion protein was found to show the best overall GAG-binding characteristics. Molecular modeling demonstrated an energetically beneficial fold of this novel protein chimera. This was experimentally supported by GdmCl-induced unfolding studies, in which the fusion construct exhibited a well-defined secondary structure and a transition point significantly higher than both the wt and the unfused CCL2 mutant protein. Unlike the wt chemokine, the quaternary structure of the HSA-fusion protein is monomeric according to size-exclusion chromatography experiments. In competition experiments, the HSA-fusion construct displaced only two of seven unrelated chemokines from heparan sulfate, whereas the unfused CCL2 mutant protein displaced five other chemokines. The most effective concentration of the HSA-fusion protein in inhibiting CCL2-mediated monocyte attachment to endothelial cells, as detected in the flow chamber, was 8.6 µg/ml. This novel HSA-fusion protein exhibits not only high affinity but also selective displacement of chemokines from GAGs binding. HSA is therefore proposed to be a highly promising scaffold candidate for therapeutic, GAG-targeting chemokine mutants.

Targeting glycosaminoglycans in the lung by an engineered CXCL8 as a novel therapeutic approach to lung inflammation.

It is broadly recognized that chemokine-activated neutrophils play a crucial role in the inflammation and disruption of lung tissue observed in several acute and chronic lung diseases. Since glycosaminoglycan side chains of proteoglycans act as chemokine co-receptors in inflammation, we have used a CXCL8-based dominant-negative mutant, dnCXCL8, to displace neutrophil-related chemokines in murine lungs using models of lung inflammation. Treatment with dnCXCL8 resulted in a dose-dependent reduction of neutrophil counts in bronchoalveolar lavage (BAL) of mice exposed to lipopolysaccharide after intravenous, subcutaneous and intratracheal administration. A strong and significant therapeutic effect was achieved already at a dose of 40 µg/kg of dnCXCL8. A similar dose response, but showing a broader spectrum of reduced inflammatory cells and soluble inflammatory markers, was observed in a murine model of tobacco smoke (TS)-induced lung inflammation. The broad spectrum of reduced inflammatory cells and markers can be due to the strong inhibition of neutrophil extravasation into the lung parenchyma, and/or to a relatively broad protein displacement profile of dnCXCL8 which may compete not only with wtCXCL8 for glycosaminoglycan-binding but possibly also with other related glycosaminoglycan-binding pro-inflammatory chemokines. Overall our results demonstrate that antagonizing CXCL8/glycosaminoglycan binding reduces lung inflammation as well as associated lung tissue damage due to LPS and TS and may therefore be a new therapeutic approach for lung pathologies characterized by a neutrophilic inflammatory phenotype.