Receptor recognition and specificity of interleukin-8 is determined by residues that cluster near a surface-accessible hydrophobic pocket

Conformational plasticity and dynamic interactions of the N-terminal domain of the chemokine receptor CXCR1

The dynamic interactions between G protein-coupled receptors (GPCRs) and their cognate protein partners are central to several cell signaling pathways. For example, the association of CXC chemokine receptor 1 (CXCR1) with its cognate chemokine, interleukin-8 (IL8 or CXCL8) initiates pathways leading to neutrophil-mediated immune responses. The N-terminal domain of chemokine receptors confers ligand selectivity, but unfortunately the conformational dynamics of this intrinsically disordered region remains unresolved. In this work, we have explored the interaction of CXCR1 with IL8 by microsecond time scale coarse-grain simulations, complemented by atomistic models and NMR chemical shift predictions. We show that the conformational plasticity of the apo-receptor N-terminal domain is restricted upon ligand binding, driving it to an open C-shaped conformation. Importantly, we corroborated the dynamic complex sampled in our simulations against chemical shift perturbations reported by previous NMR studies and show that the trends are similar. Our results indicate that chemical shift perturbation is often not a reporter of residue contacts in such dynamic associations. We believe our results represent a step forward in devising a strategy to understand intrinsically disordered regions in GPCRs and how they acquire functionally important conformational ensembles in dynamic protein-protein interfaces.

Porcine CXCR1/2 antagonist CXCL8(3-72)G31P inhibits lung inflammation in LPS-challenged mice

Swine pneumonia is a great threat for pig industry around the world, which is usually accompanied with neutrophils infiltration in the airway. Although interleukin-8 (CXCL8) and its receptors, CXC chemokine receptor 1 and 2 (CXCR1/2) in human have been well documented, the expression and function of CXCR1/2 is still unknown in swine. To explore the feasibility to develop new veterinary anti-inflammatory drugs targeting porcine CXCR1/2, we detected CXCR1/2 expression in swine pneumonia through Real-Time PCR and immunohistochemistry for the first time. Two porcine CXCR1/2 antagonists, CXCL8(3-72)N11R/G31P (pN11R) and CXCL8(3-72)G31P (pG31P) were prepared and their anti-inflammatory effects were evaluated using cell chemotaxis assays and animal experiments. Our data showed that CXCR1/2 expression, which was closely related to neutrophil infiltration in the lung, was significantly up-regulated in swine pneumonia. The pN11R and pG31P could effectively inhibit the directional migration of neutrophils in vitro. In vivo data also indicated that both pN11R and pG31P significantly relieved LPS-induced pneumonia in mice through decreasing the expression of TNF-α, CXCL8, and IL-1β, and inhibiting neutrophil influx into the lung. pG31P was more efficient. Our study suggested that it is possible to develop new veterinary anti-inflammatory drugs targeting porcine CXCR1/2, and pG31P is a promising candidate.

Epitope Identification and Affinity Determination of an Inhibiting Human Antibody to Interleukin IL8 (CXCL8) by SPR- Biosensor-Mass Spectrometry Combination

The polypeptide chemokine Interleukin-8 (IL8) plays a crucial role in inflammatory processes in humans. IL8 is involved in chronic inflammatory lung diseases, rheumatoid arthritis, and cancer. Previous studies have shown that the interaction of IL8 with its natural receptors CXCR1 and CXCR2 is critical in these diseases. Antibodies have been used to study the receptor interaction of IL8; however, the binding epitopes were hitherto unknown. Identification of the antibody epitope(s) could lead to a molecular understanding of the inhibiting mechanism and development of improved inhibitors. Here, we report the epitope identification and the affinity characterization of IL8 to a monoclonal anti-human IL8 antibody inhibiting the receptor binding by a combination of surface plasmon resonance (SPR) biosensor analysis and MALDI-mass spectrometry. SPR determination of IL8 with the immobilized antibody revealed high affinity (K_D, 82.2 nM). Epitope identification of IL-8 was obtained by proteolytic epitope-extraction mass spectrometry of the peptide fragments upon high pressure trypsin digestion, using an affinity microcolumn with immobilized anti-IL-8 antibody. MALDI-MS of the affinity-bound peptide elution fraction revealed an assembled (discontinuous) epitope comprising two specific peptides, IL8 [12-20] and IL8 [55-60]. Identical epitope peptides were identified by direct MALDI-MS of the eluted epitope fraction from the immobilized anti-IL8 antibody on the SPR chip. SPR determination of the synthetic epitope peptides provided high affinities confirming their binding specificity. The previously reported finding that the anti-Il8 antibody is inhibiting the IL8-CXCR1 interaction is well consistent with the overlapping region of epitope interactions identified in the present study.

How do chemokines navigate neutrophils to the target site: Dissecting the structural mechanisms and signaling pathways

Chemokines play crucial roles in combating microbial infection and initiating tissue repair by recruiting neutrophils in a timely and coordinated manner. In humans, no less than seven chemokines (CXCL1, CXCL2, CXCL3, CXCL5, CXCL6, CXCL7, and CXCL8) and two receptors (CXCR1 and CXCR2) mediate neutrophil functions but in a context dependent manner. Neutrophil-activating chemokines reversibly exist as monomers and dimers, and their receptor binding triggers conformational changes that are coupled to G-protein and β-arrestin signaling pathways. G-protein signaling activates a variety of effectors including Ca2+ channels and phospholipase C. β-arrestin serves as a multifunctional adaptor and is coupled to several signaling hubs including MAP kinase and tyrosine kinase pathways. Both G-protein and β-arrestin signaling pathways play important non-overlapping roles in neutrophil trafficking and activation. Functional studies have established many similarities but distinct differences for a given chemokine and between chemokines at the level of monomer vs. dimer, CXCR1 vs. CXCR2 activation, and G-protein vs. β-arrestin pathways. We propose that two forms of the ligand binding two receptors and activating two signaling pathways enables fine-tuned neutrophil function compared to a single form, a single receptor, or a single pathway. We summarize the current knowledge on the molecular mechanisms by which chemokine monomers/dimers activate CXCR1/CXCR2 and how these interactions trigger G-protein/β-arrestin-coupled signaling pathways. We also discuss current challenges and knowledge gaps, and likely advances in the near future that will lead to a better understanding of the relationship between the chemokine-CXCR1/CXCR2-G-protein/β-arrestin axis and neutrophil function.

Structure of monomeric Interleukin-8 and its interactions with the N-terminal Binding Site-I of CXCR1 by solution NMR spectroscopy

The structure of monomeric human chemokine IL-8 (residues 1-66) was determined in aqueous solution by NMR spectroscopy. The structure of the monomer is similar to that of each subunit in the dimeric full-length protein (residues 1-72), with the main differences being the location of the N-loop (residues 10-22) relative to the C-terminal α-helix and the position of the side chain of phenylalanine 65 near the truncated dimerization interface (residues 67-72). NMR was used to analyze the interactions of monomeric IL-8 (1-66) with ND-CXCR1 (residues 1-38), a soluble polypeptide corresponding to the N-terminal portion of the ligand binding site (Binding Site-I) of the chemokine receptor CXCR1 in aqueous solution, and with 1TM-CXCR1 (residues 1-72), a membrane-associated polypeptide that includes the same N-terminal portion of the binding site, the first trans-membrane helix, and the first intracellular loop of the receptor in nanodiscs. The presence of neither the first transmembrane helix of the receptor nor the lipid bilayer significantly affected the interactions of IL-8 with Binding Site-I of CXCR1.

Regulation of Chemokine Function: The Roles of GAG-Binding and Post-Translational Nitration

The primary function of chemokines is to direct the migration of leukocytes to the site of injury during inflammation. The effects of chemokines are modulated by several means, including binding to G-protein coupled receptors (GPCRs), binding to glycosaminoglycans (GAGs), and through post-translational modifications (PTMs). GAGs, present on cell surfaces, bind chemokines released in response to injury. Chemokines bind leukocytes via their GPCRs, which directs migration and contributes to local inflammation. Studies have shown that GAGs or GAG-binding peptides can be used to interfere with chemokine binding and reduce leukocyte recruitment. Post-translational modifications of chemokines, such as nitration, which occurs due to the production of reactive species during oxidative stress, can also alter their biological activity. This review describes the regulation of chemokine function by GAG-binding ability and by post-translational nitration. These are both aspects of chemokine biology that could be targeted if the therapeutic potential of chemokines, like CXCL8, to modulate inflammation is to be realised.

Effects of K11R and G31P Mutations on the Structure and Biological Activities of CXCL8: Solution Structure of Human CXCL8(3-72)K11R/G31P

The ELR-CXC chemokines are important to neutrophil inflammation in many acute and chronic diseases. Among them, CXCL8 (interleukin-8, IL-8), the expression levels of which are elevated in many inflammatory diseases, binds to both the CXCR1 and CXCR2 receptors with high affinity. Recently, an analogue of human CXCL8, CXCL8_(3–72)K11R/G31P (hG31P) has been developed. It has been demonstrated that hG31P is a high affinity antagonist for both the CXCR1 and CXCR2. Herein, we have determined the solution structure and the CXCR1 N-terminal peptide binding sites of hG31P by NMR spectroscopy. We have found that the displacement within the tertiary structure of the 30 s loop and the N-terminal region and more specifically change of the loop conformation (especially H33), of hG31P may affect its binding to the CXCR1 receptor and thereby inhibit human neutrophil chemotactic responses induced by ELR-CXC chemokines. Our results provide a structural basis for future clinical investigations of this CXCR1/CXCR2 receptor antagonist and for the further development of CXCL8 based antagonists.

CXCR2-Mediated Granulocytic Myeloid-Derived Suppressor Cells' Functional Characterization and Their Role in Maternal Fetal Interface

To investigate CXC chemokine receptor 2 (CXCR2)-mediated granulocytic myeloid-derived suppressor cells' (MDSCs) (G-MDSCs) functional characterization and their role in maternal-fetal interface. Proportions of CXCR2(+) MDSCs and CXCR2 protein levels in total MDSCs were lower in abortion-prone CBA/J×DBA/2 mice than in CBA/J×BALB/c mice with normal pregnancy. Treatment with CXCR2 neutralizing antibody in vivo at early stage of pregnancy significantly increased the embryo resorption rates and reduced MDSCs abundance in mice from CBA/J×BALB/c matings. Adoptive transfer of MDSCs improved pregnancy outcomes in anti-CXCR2-pretreated CBA/J mice in CBA/J×BALB/C matings. CXCR2 was capable of enhancing the migration of G-MDSCs efficiently instead of monocytic MDSCs (M-MDSCs). In addition to preferential G-MDSC accumulation, arginase I expression as well as arginase I activity of G-MDSCs were regulated by CXCR2. CXCL1, as one of CXCR2 ligands, correlated well with CXCR2-mediated G-MDSCs migration and arginase I activity. CXCR2/CXCL1 axis promotes G-MDSC recruitment and facilitates arginase I expression and activity of these cells at maternal-fetal interface. These findings provide comprehensive insights into how G-MDSCs are recruited to decidual tissues and how local G-MDSCs maintain pregnancy tolerance.

A fluorescence polarization assay for the experimental validation of an in silico model of the chemokine CXCL8 binding to receptor-derived peptides

Peptide based inhibitors of protein-protein interactions are of great interest in proteomics, structural biology and medicinal chemistry. Optimized inhibitors can be designed by experimental approaches or by computational prediction. Ideally, computational models are adjusted to the peptide-protein complex of interest according to experimental data obtained in specific binding experiments. The chemokine CXCL8 (interleukin-8) is an interesting target for drug discovery due to its role in inflammatory diseases. Given the available structural data and information on its receptor interactions it constitutes a basis for the rational design of inhibitor peptides. Starting from the reported structure of CXCL8 in complex with a peptide derived from its receptor CXCR1 we developed a computational docking procedure to estimate the changes in binding energy as a function of individual amino acid exchanges. This indicates whether the respective amino acid residue must be preserved or can be substituted to maintain or improve affinity, respectively. To validate and improve the assumptions made in this docking simulation we established a fluorescence polarization assay for receptor-derived peptides binding to CXCL8. A peptide library was tested comprising selected mutants characterized by docking simulations. A number of predictions regarding electrostatic interactions were confirmed by these experiments and it was revealed that the model needed to be corrected for backbone flexibility. Therefore, the assay presented here is a promising tool to systematically improve the computational model by iterative cycles of modeling, experimental validation and refinement of the algorithm, leading to a more reliable model and peptides with improved affinity.

Role of Spinal CXCL1 (GROα) in Opioid Tolerance

Background:

The pivotal role of glial activation and up-regulated inflammatory mediators in the opioid tolerance has been confirmed in rodents but not yet in humans. Here, the authors investigated the intraspinal cytokine and chemokine profiles of opioid-tolerant cancer patients; and to determine if up-regulated chemokines could modify opioid tolerance in rats.

Methods:

Cerebrospinal fluid samples from opioid-tolerant cancer patients and opioid-naive subjects were compared. The cerebrospinal fluid levels of tumor necrosis factor-alpha, CXCL1, CXCL10, CCL2, and CX3CL1 were assayed. The rat tail flick test was utilized to assess the effects of intrathecal CXCL1 on morphine-induced acute antinociception and analgesic tolerance.

Results:

CXCL1 level in cerebrospinal fluid was significantly up-regulated in the opioid-tolerant group (n = 30, 18.8 pg/ml vs. 13.2 pg/ml, P = 0.02) and was positively correlated (r2 = 0.49, P < 0.01) with opioid dosage. In rat experiment, after induction of tolerance by morphine infusion, the spinal cord CXCL1 messenger RNA was up-regulated to 32.5 ± 11.9-fold. Although CXCL1 infusion alone did not affect baseline tail-flick latency, the analgesic efficacy of a single intraperitoneal injection of morphine dropped significantly on day 1 to day 3 after intrathecal infusion of CXCL1. After establishing tolerance by intrathecal continuous infusion of morphine, its development was accelerated by coadministration of CXCL1 and attenuated by coadministration of CXCL1-neutralizing antibody or CXCR2 antagonist.

Conclusions:

CXCL1 is up-regulated in both opioid-tolerant patients and rodents. The onset and extent of opioid tolerance was affected by antagonizing intrathecal CXCL1/CXCR2 signaling. Therefore, the CXCL1/CXCR2 signal pathway may be a novel target for the treatment of opioid tolerance.

Rational design of a peptide capture agent for CXCL8 based on a model of the CXCL8:CXCR1 complex

A CXCL8-binding peptide designed from the interaction sites of CXCR1 with CXCL8 serves as a capture agent and inhibits neutrophil migration.

HIV-1 matrix protein p17 binds to the IL-8 receptor CXCR1 and shows IL-8–like chemokine activity on monocytes through Rho/ROCK activation

Exogenous HIV-1 matrix protein p17 was found to deregulate biologic activities of many different immune cells that are directly or indirectly involved in AIDS pathogenesis after binding to unknown cellular receptor(s). In particular, p17 was found to induce a functional program in monocytes related to activation and inflammation. In the present study, we demonstrate that CXCR1 is the receptor molecule responsible for p17 chemokine–like activity on monocytes. After CXCR1 binding, p17 was capable of triggering rapid adhesion and chemotaxis of monocytes through a pathway that involved Rho/ROCK. Moreover, CXCR1-silenced primary monocytes lost responsiveness to p17 chemoattraction, whereas CXCR1-transfected Jurkat cells acquired responsiveness. Surface plasmon resonance studies confirmed the capacity of p17 to bind CXCR1 and showed that the p17/CXCR1 interaction occurred with a low affinity compared with that measured for IL-8, the physiologic CXCR1 ligand. In all of its activities, p17 mimicked IL-8, the natural high-affinity ligand of CXCR1. Recent studies have highlighted the role of IL-8 and CXCR1 in HIV-1 replication and AIDS pathogenesis. Our findings herein call for an exploration of the therapeutic potential of blocking the p17/IL-8/CXCR1 axis in HIV-1 infection.

hCXCR1 and hCXCR2 antagonists derived from combinatorial peptide libraries

The human CXCL8 plays important roles in inflammation by activation of neutrophils through the hCXCR1 and hCXCR2 receptors. The role of hCXCR1 and hCXCR2 in the pathogenesis of inflammatory responses has encouraged the development of antagonists of these receptors. In this study, we used phage display peptide libraries to identify peptides antagonists that block the interactions between hCXCL8 and hCXCR1/2. Two linear hexapeptides (MSRAKE and CAKELR) and two disulfide-constrained hexapeptides (CLRSGRFC and CLPWKENC) were recovered by panning phage libraries on hCXCR1- and hCXCR2-transfected murine pre-B cells after specific elution with hCXCL8. Sequence analysis revealed homology between the linear hexapeptides and the N-terminal domain (1-SAKELR-6), whereas the constrained peptides are composed of non-contiguous amino acids mimicking spatial structure on the surface of folded C-terminal portion of hCXCL8 (50-CLDPKENWVQRVVEKFLKRAENS-72). The synthetic linear and structurally constrained peptides competed for (125)I-hCXCL8 binding to hCXCR1 and hCXCR2 (IC(50) comprised between 10 and 100μM). Furthermore, they inhibited the intracellular calcium flux and the migration of hCXCR1/hCXCR2 transfectants; and desensitized hCXCR1 and hCXCR2 receptors on neutrophils, reducing their chemotactic responses induced by ELR-CXC chemokines (hCXCL8, hCXCL1, hCXCL2, hCXCL3, and hCXCL5). To better characterize the residues required for hCXCL8 binding, we identified three linear peptides MLRQTR, HASILP and KKEPWI specific to hCXCL8. These peptides similarly displaced the binding of (125)I-hCXCL8 to hCXCR1 (IC(50) ranging from 8.5 to 10μM) in a dose-dependent manner, inhibited hCXCL8 induced increases in the intracellular calcium, and migration of hCXCR1- and hCXCR2-transfected cells. The identified peptides could be used as antagonists of hCXCL8-induced activities related to its interaction with hCXCR1 and hCXCR2 receptors and may help in the design of new anti-inflammatory therapeutic molecules.

The role of CXC chemokines in the transition of chronic inflammation to esophageal and gastric cancer

Chronic inflammation may increase the risk to develop cancer, for instance esophagitis or gastritis may lead to development of esophageal or gastric cancer, respectively. The key molecules attracting leukocytes to local inflammatory sites are chemokines. We here provide a systematic review on the impact of CXC chemokines (binding the receptors CXCR1, CXCR2, CXCR3 and CXCR4) on the transition of chronic inflammation in the upper gastrointestinal tract to neoplasia. CXCR2 ligands, including GRO-α,β,γ/CXCL1,2,3, ENA-78/CXCL5 and IL-8/CXCL8 chemoattract pro-tumoral neutrophils. In addition, angiogenic CXCR2 ligands stimulate the formation of new blood vessels, facilitating tumor progression. The CXCR4 ligand SDF-1/CXCL12 also promotes tumor development by stimulating angiogenesis and by favoring metastasis of CXCR4-positive tumor cells to distant organs producing SDF-1/CXCL12. Furthermore, these angiogenic chemokines also directly enhance tumor cell survival and proliferation. In contrast, the CXCR3 ligands Mig/CXCL9, IP-10/CXCL10 and I-TAC/CXCL11 are angiostatic and attract anti-tumoral T lymphocytes and may therefore mediate tumor growth retardation and regression. Thus, chemokines exert diverging, sometimes dual roles in tumor biology as described for esophageal and gastric cancer. Therefore extensive research is needed to completely unravel the complex chemokine code in specific cancers. Possibly, chemokine-targeted cancer therapy will have to be adapted to the individual's chemokine profile.

A CXCL8 receptor antagonist based on the structure of N-acetyl-proline-glycine-proline

A role for the collagen-derived tripeptide, N-acetyl proline-glycine-proline (NAc-PGP), in neutrophil recruitment in chronic airway inflammatory diseases, including COPD and cystic fibrosis, has recently been delineated. Due to structural similarity to an important motif for interleukin-8 (CXCL8) binding to its receptor, NAc-PGP binds to CXCR1/2 receptors, leading to neutrophil activation and chemotaxis. In an effort to develop novel CXCL8 antagonists, we describe the synthesis of four chiral isomers of NAc-PGP (NAc-L-Pro-Gly-L-Pro (LL-NAc-PGP), NAc-L-Pro-Gly-D-Pro (LD-NAc-PGP), NAc-D-Pro-Gly-L-Pro (DL-NAc-PGP), and NAc-D-Pro-Gly-D-Pro (DD-NAc-PGP)), characterize them by circular dichroism and NMR spectroscopy, compare their structures to the equivalent region of CXCL8, and test them as potential antagonists of ll-NAc-PGP and CXCL8. We find that LL-NAc-PGP superimposes onto the CXCR1/2 contacting E(29)S(30)G(31)P(32) region of CXCL8 (0.59A rmsd for heavy atoms). In contrast, DD-NAc-PGP has an opposing orientation of key functional groups as compared to the G(31)P(32) region of CXCL8. As a consequence, DD-NAc-PGP binds CXCR1/2, as demonstrated by competition with radiolabeled CXCL8 binding in a radioreceptor assay, yet acts as a receptor antagonist as evidenced by inhibition of CXCL8 and LL-NAc-PGP mediated neutrophil chemotaxis. The ability of DD-NAc-PGP to prevent the activation of CXC receptors indicates that DD-NAc-PGP may serve as a lead compound for the development of CXCR1/2 inhibitors. In addition, this study further proves that using a different technical approach, namely preincubation of NAc-PGP instead of simultaneous addition of NAc-PGP with radiolabeled CXCL8, the direct binding of NAc-PGP to the CXCL8 receptor is evident.

MIF-chemokine receptor interactions in atherogenesis are dependent on an N-loop-based 2-site binding mechanism

Macrophage migration inhibitory factor (MIF) is a cytokine that mediates inflammatory diseases. MIF promotes atherogenic leukocyte recruitment through a promiscuous, yet highly affine, interaction with CXCR2 and CXCR4. Binding to CXCR2 is dependent on a pseudo-(E)LR motif in MIF, but a second interaction site has been elusive. Here we identified an N-like loop in MIF, suggesting that MIF binding to CXCR2 follows the 2-site binding mode of bona fide chemokines. For MIF, the model predicts interactions between the N-like loop and the CXCR2 N domain (site 1) and pseudo-(E)LR and extracellular loops (ELs) of CXCR2 (site 2). Applying biophysical and peptide array analysis, we demonstrated an interaction between MIF and the CXCR2 N domain, which was pseudo-(E)LR independent. Peptide array analysis also indicated that the pseudo-(E)LR motif is responsible for MIF binding to EL2 and 3. Notably, peptides MIF-(40-49) and MIF-(47-56), representing N-like-loop-derived peptides, but not a scrambled control peptide, significantly blocked MIF/CXCR2 binding, MIF-mediated monocyte arrest under flow on aortic endothelial cells in vitro (IC(50): 1.24×10(-6) M), and MIF-dependent monocyte adhesion to atherosclerotic mouse carotid arteries in vivo. Thus, the N-like loop in MIF is critical for MIF's noncognate interaction with CXCR2 and proatherogenic functions. The 2-site binding model that explains chemokine receptor activation also applies to MIF.

Structural basis for differential binding of the interleukin-8 monomer and dimer to the CXCR1 N-domain: role of coupled interactions and dynamics

Interleukin-8 (IL-8 or CXCL8) plays a critical role in orchestrating the immune response by binding and activating the receptor CXCR1 that belongs to the GPCR class. IL-8 exists as both monomers and dimers, and both bind CXCR1 but with differential affinities. It is well established that the monomer is the high-affinity ligand and that the interactions between the ligand N-loop and receptor N-domain play a critical role in determining binding affinity. In order to characterize the structural basis of differential binding of the IL-8 monomer and dimer to the CXCR1 N-domain, we analyzed binding-induced NMR chemical shift and peak intensity changes and show that they are exquisitely sensitive and can provide detailed insights into the binding process. We used three IL-8 variants, a designed monomer, a trapped disulfide-linked dimer, and WT at dimeric concentrations. NMR data for the monomer show that nonsequential residues that span the entire N-loop are involved in the binding process and that the binding is mediated by a network of extensive direct and indirect coupled interactions. Interestingly, in the case of WT, binding induces dissociation of the dimer-receptor complex to the monomer-receptor complex, and in the case of the trapped dimer, binding results in increased global conformational flexibility. Increased dynamics is evidence of unfavorable interactions, indicating that binding of the WT dimer triggers conformational changes that disrupt dimer-interface interactions, resulting in its dissociation. These results together provide evidence that binding is not a localized event but results in extensive coupled interactions within the monomer and across the dimer interface and that these interactions play a fundamental role in determining binding affinity.

Molecular requirements for sorting of the chemokine interleukin-8/CXCL8 to endothelial Weibel-Palade bodies

Sorting of proteins to Weibel-Palade bodies (WPB) of endothelial cells allows rapid regulated secretion of leukocyte-recruiting P-selectin and chemokines as well as procoagulant von Willebrand factor (VWF). Here we show by domain swap studies that the exposed aspartic acid in loop 2 (Ser(44)-Asp(45)-Gly(46)) of the CXC chemokine interleukin (IL)-8 is crucial for targeting to WPB. Loop 2 also governs sorting of chemokines to alpha-granules of platelets, but the fingerprint of the loop 2 of these chemokines differs from that of IL-8. On the other hand, loop 2 of IL-8 closely resembles a surface-exposed sequence of the VWF propeptide, the region of VWF that directs sorting of the protein to WPB. We conclude that loop 2 of IL-8 constitutes a critical signal for sorting to WPB and propose a general role for this loop in the sorting of chemokines to compartments of regulated secretion.

Identification of GRO1 as a critical determinant for mutant p53 gain of function

Mutant p53 gain of function contributes to cancer progression, increased invasion and metastasis potentials, and resistance to anticancer therapy. The ability of mutant p53 to acquire its gain of function is shown to correlate with increased expression of progrowth genes, such as c-MYC, MDR1, and NF-kappaB2. However, most of the published studies to identify mutant p53 target genes were performed in a cell system that artificially overexpresses mutant p53. Thus, it remains unclear whether such mutant p53 targets can be regulated by endogenous physiological levels of mutant p53. Here, we utilized SW480 and MIA-PaCa-2 cells, in which endogenous mutant p53 can be inducibly knocked down, to identify mutant p53 target genes that potentially mediate mutant p53 gain of function. We found that knockdown of mutant p53 inhibits GRO1 expression, whereas ectopic expression of mutant R175H in p53-null HCT116 cells increases GRO1 expression. In addition, we found that endogenous mutant p53 is capable of binding to and activating the GRO1 promoter. Interestingly, ectopic expression of GRO1 can rescue the proliferative defect in SW480 and MIA-PaCa-2 cells induced by knockdown of mutant p53. Conversely, knockdown of endogenous GRO1 inhibits cell proliferation and thus abrogates mutant p53 gain of function in SW480 cells. Taken together, our findings define a novel mechanism by which mutant p53 acquires its gain of function via transactivating the GRO1 gene in cancer cells. Thus, targeting GRO1 for cancer therapy would be applicable to a large portion of human tumors with mutant p53, but the exploration of GRO1 as a potential target should take the mutation status of p53 into consideration.

Melanoma and innate immunity--aActive inflammation or just erroneous attraction? Melanoma as the source of leukocyte-attracting chemokines

Unwanted growth breeds response--in the garden as well as in the tumor microenvironment. Innate immune cells mediate the earliest responses against melanoma or its precursors. However, the actual benefit by those cellular efforts is questionable. Why can early melanoma lesions actually develop in the face of rapid innate responses, and why is neutrophil- and macrophage-attracting chemokine secretion observed in melanoma? A surprisingly similar choice of chemokine receptors and chemokines are present in both innate immune cells and melanoma. Here we focus on analogies and differences between the two. Melanoma cell clusters show active chemokine signalling, with mostly tumor growth-enhancing and leukocyte-attracting effects. However, infiltrating leukocytes have only weak tumoricidal effects. Therefore, the observed leukocyte infiltration in melanoma might be at least in part an epiphenomenon of neoplastic self-stimulation rather than a full-fledged innate anti-tumor immune response.

Characterization of interleukin 8 in woodchucks with chronic hepatitis B and hepatocellular carcinoma

The eastern woodchuck, Marmota monax, represents a useful animal model to study hepatitis B virus infection in humans. However, immunological studies in this model have been impeded by a lack of basic information about the components of the immune system such as cytokines and chemokines. To clarify the role(s) of interleukin 8 (IL-8) in chronic hepatitis B and hepatocellular carcinoma (HCC) in the woodchuck model, we cloned and characterized the woodchuck IL-8 cDNA and genomic DNA. Sequence analysis revealed that the organization of the wk-IL-8 gene is similar to that of the human IL-8 gene and consists of four exons and three introns. Woodchuck IL-8 protein exhibits the conserved ELRCXC motif of IL-8 and shows 87, 82, 82 and 79% similarity with rabbit, ovine, bovine and human IL-8 proteins, respectively. The biological activity of wk-IL-8 was demonstrated using neutrophil chemotaxis assays. Wk-IL-8 could be readily detected in both tumor and non-tumor tissues with higher expression in the non-tumor tissues in most cases. The results from this study will facilitate the investigation of IL-8 in the immunopathogenesis of hepadnavirus-related diseases by the woodchuck model.

Artificial chemokines: combining chemistry and molecular biology for the elucidation of interleukin-8 functionality

How can we understand the contribution of individual parts or segments to complex structures? A typical strategy to answer this question is simulation of a segmental replacement followed by realization and investigation of the resulting effect in structure-activity studies. For proteins, this problem is commonly addressed by site-directed mutagenesis. A more general approach represents the exchange of whole secondary structure elements by rationally designed segments. For a demonstration of this possibility we identified the alpha-helix at the C-terminus of human interleukin-8 (hIL-8). Since this chemokine possesses four conserved cysteine residues, it can easily be altered by ligation strategies. A set of different segments, which are able to form amphiphilic helices, was synthesized to mimic the C-terminal alpha-helix. Beside sequences of alpha-amino acids, oligomers of non-natural beta(3)-amino acids with the side chains of canonical amino acids were introduced. Such beta-peptides form helices, which differ from the alpha-helix in handedness and dipole orientation. Variants of the semisynthetic hIL-8 proteins demonstrated clearly that the exact side chain orientation is of more importance than helix handedness and dipole orientation. The activity of a chimeric protein with a beta-peptide helix that mimics the side chain orientation of the native alpha-helix most perfectly is comparable to that of the native hIL-8. Concepts like this could be a first step toward the synthesis of proteins consisting of large artificial secondary structure elements.

A new protocol for high-yield purification of recombinant human CXCL8((3-72))K11R/G31P expressed in Escherichia coli

The ELR-CXC chemokines are important to neutrophil inflammation in many acute and chronic diseases. Among them, CXCL8 (interleukin-8, IL-8), binds to both the CXCR1 and CXCR2 receptors with high affinity and the expression levels of CXCL8 are elevated in many inflammatory diseases. Recently, an analogue of human CXCL8, CXCL8((3-72))K11R/G31P (hG31P) has been developed. It has been demonstrated that hG31P is a high affinity antagonist for both CXCR1 and CXCR2. To obtain large quantities of hG31P, we have successfully constructed and expressed hG31P in Escherichia coli. Moreover, we have developed a new protocol for high-yield purification of hG31P and for the removal of lipopolysaccharide (LPS, endotoxin) associated with hG31P due to the expression in E. coli. The purity of hG31P is more than 95% and the final yield is 9.7mg hG31P per gram of cell paste. The purified hG31P was tested by various biological assays. In addition, the structural properties of hG31P were studied by circular dichroism (CD), ultracentrifuge, isothermal titration calorimetry (ITC), and nuclear magnetic resonance (NMR) spectroscopy. Our results indicate that this purification protocol is very simple and easy to amplify at a large scale. The results of this study will provide an effective route to produce enough hG31P for future clinical studies.

Relationships between glycosaminoglycan and receptor binding sites in chemokines-the CXCL12 example

Chemokines are small proteins, promoting directional migration and activation of different cells through binding to specific receptors. Most chemokines also bind to heparan sulfate (HS), a family of complex and highly sulfated glycosaminoglycan (GAG) found at the cell surface and in the extracellular matrix. This class of molecules has recently emerged as critical regulators of many events involving cell response to the external environment. Binding to HS is thought to be functionally important. Current models suggested that HS ensures the correct positioning of chemokines within tissues and maintains haptotactic gradients of the proteins along cell surfaces, thus providing directional cues for migrating cells. On the chemokine surface, the GAG binding epitopes can be displayed on different areas, some of which overlap the receptor binding domain, while others are clearly separated. We review here some structural aspects of the interaction between GAGs or receptors and chemokines. In particular, we will address the case of CXCL12, a chemokine whose receptor binding site is distinct from the GAG binding site and whose different isoforms display different GAG binding abilities. This chemokine system thus offers an unprecedented opportunity to ascertain the importance of chemokine/GAG interaction in the regulation of cell migration.

Structural basis of chemokine receptor function--a model for binding affinity and ligand selectivity

Chemokine receptors play fundamental roles in human physiology from embryogenesis to inflammatory response. The receptors belong to the G-protein coupled receptor class, and are activated by chemokine ligands with a range of specificities and affinities that result in a complicated network of interactions. The molecular basis for function is largely a black box, and can be directly attributed to the lack of structural information on the receptors. Studies to date indicate that function can be best described by a two-site model, that involves interactions between the receptor N-domain and ligand N-terminal loop residues (site-I), and between receptor extracellular loop and the ligand N-terminal residues (site-II). In this review, we describe how the two-site model could modulate binding affinity and ligand selectivity, and also highlight some of the unique chemokine receptor features, and their role in function.

CXCL1 induced by prostaglandin E2 promotes angiogenesis in colorectal cancer

Chronic inflammation is a well-known risk factor for cancer. Proinflammatory mediators such as prostaglandin E2 (PGE2) promote colorectal tumor growth by stimulating angiogenesis, cell invasion, and cell growth, and inhibiting apoptosis. Molecules that regulate tumor-associated angiogenesis provide promising therapeutic targets for treatment of colorectal cancer (CRC) as indicated by the recent development of the novel anti-angiogenic agent bevacizumab (Avastin). However, use of this drug only prolongs survival by several months, highlighting the importance of finding more effective treatment regimens. We report here that PGE2 induces expression of CXCL1 (growth-regulated oncogene alpha), a pro-angiogenic chemokine, in human CRC cells. More importantly, CXCL1 released from carcinoma cells induces microvascular endothelial cell migration and tube formation in vitro. Furthermore, PGE2 promotes tumor growth in vivo by induction of CXCL1 expression, which results in increased tumor microvessel formation. These results have potential clinical significance because we found that CXCL1 expression correlates with PGE2 levels in human CRCs. Collectively, our findings show for the first time that CXCL1 is regulated by PGE2 and indicate that CXCL1 inhibitors should be evaluated further as potential anti-angiogenic agents for treatment of CRC.

Characterization of Nef-CXCR4 interactions important for apoptosis induction

The HIV-1 Nef protein was analyzed for apoptotic structural motifs that interact with the CXCR4 receptor and induce apoptosis in CD4(+) lymphocytes. Two apoptotic motifs were identified. One centered on Nef amino acids (aa) 50 to 60, with the overlapping 20-mer peptides retaining about 82% of the activity of the full Nef protein. The second centered on aa 170 to 180, with the overlapping 20-mer peptides retaining about 30% of the activity of the full protein. Significant apoptotic abilities were observed for 11-mer motif peptides spanning aa 50 to 60 and aa 170 to 180, with a scrambled version of the 11-mer motif peptide corresponding to aa 50 to 60 showing no apoptotic ability. Hallmarks of apoptosis, such as the formation of DNA ladders and caspase activation, that were observed with the full-length protein were equally evident upon exposure of cells to these motif peptides. A CXCR4 antibody and the endogenous ligand SDF-1alpha were effective in blocking Nef peptide-induced apoptosis as well as the physical binding of a fluorescently tagged Nef protein, while CCR5 antibodies were ineffective. The CXCR4-negative cell line MDA-MB-468 was resistant to the apoptotic peptides and became sensitive to the apoptotic peptides upon transfection with a CXCR4-expressing vector. A fluorescently tagged motif peptide and Nef protein displayed physical binding to CXCR4-transfected MDA-MB-468 cells, but not to CCR5-transfected cells. The removal of the apoptotic motif sequences from the full-length protein completely eliminated the ability of Nef to induce apoptosis. However, these modified Nef proteins still retained the ability to enhance viral infectivity. Thus, specific sequences in the Nef protein appear to be necessary for Nef protein-induced apoptosis as well as for physical interaction with CXCR4 receptors.

Ligand Selectivity and Affinity of Chemokine Receptor CXCR1

Glu-Leu-Arg ("ELR") CXC chemokines interleukin-8 (IL-8) and melanoma growth stimulatory activity (MGSA) recruit neutrophils by binding and activating two receptors, CXCR1 and CXCR2. CXCR1 is specific, binding only IL-8 with nanomolar affinity, whereas CXCR2 is promiscuous, binding all ELRCXC chemokines with high affinity. Receptor signaling consists of two events: interactions between the ligand N-terminal loop (N-loop) and receptor N-terminal domain (N-domain) residues (site I), and between the ligand N-terminal ELR and the receptor juxtamembrane domain (J-domain) residues (site II). It is not known how these interactions mediate ligand affinity and selectivity, and whether binding at one site influences binding and function at the other. Sequence analysis and structure-function studies have suggested that the receptor N-domain plays an important role in ligand selectivity. Here, we report ligand-binding properties and structural characteristics of the CXCR1 N-domain in solution and in detergent micelles that mimic the native membrane environment. We find that IL-8 binds the N-domain with significantly higher affinity in micelles than in solution (approximately 1 microM versus approximately 20 microM) and that MGSA does not bind the N-domain in solution but does in micelles with appreciable affinity (approximately 3 microM). We find that the N-domain is structured in micelles and that the entire N-domain interacts with the micelle in an extended fashion. We conclude that the micellar environment constrains the N-domain, and this conformational restraint influences its ligand-binding properties. Most importantly, our data suggest that for both ligands, site I interaction provides similar affinity and that differential coupling between site I and II interactions is responsible for the observed differences in affinity.

Differential processing of stromal-derived factor-1alpha and stromal-derived factor-1beta explains functional diversity

The chemokine stromal-derived factor-1 (SDF-1), which is constitutively expressed in most tissues as SDF-1alpha and SDF-1beta resulting from alternative gene splicing, regulates hematopoiesis, lymphocyte homing, B-lineage cell growth, and angiogenesis. Because SDF-1alpha and SDF-1beta are constitutively and ubiquitously expressed, their degradation must serve an important regulatory role. Here we show that SDF-1alpha and SDF-1beta are secreted as full-length molecules. When exposed to human serum, full-length SDF-1alpha (1-68) undergoes processing first at the COOH terminus to produce SDF-1alpha 1-67 and then at the NH2 terminus to produce SDF-1alpha 3-67. By contrast, full-length SDF-1beta (1-72) is processed only at the NH2 terminus to produce SDF-1beta 3-72. CD26/dipeptidyl peptidase is responsible for serum cleavage of SDF-1alpha and SDF-1beta at the NH2 terminus. Serum processing of SDF-1alpha at the COOH terminus, which has not been previously reported, reduces the ability of the polypeptide to bind to heparin and to cells and to stimulate B-cell proliferation and chemotaxis. The additional processing at the NH2 terminus renders both forms of SDF-1 unable to bind to heparin and to activate cells. The differential processing of SDF-1alpha and SDF-1beta provides biologic significance to the existence of 2 splice forms of the chemokine and adds a tool to precisely regulate SDF-1's biologic activity by changes in specific activity.

Binding of ALX40-4C to APJ, a CNS-based receptor, inhibits its utilization as a co-receptor by HIV-1

APJ, a G protein-coupled seven-transmembrane receptor, has been shown to serve as a co-receptor for the entry of human immunodeficiency virus type 1 (HIV-1), and it is dramatically expressed in central nervous system (CNS)-based cells. ALX40-4C was identified as a small-molecule antagonist of the chemokine receptor CXCR4, which can specifically inhibit HIV-1 entry via this co-receptor. In this study, we demonstrated that ALX40-4C inhibited both APJ- and CXCR4/APJ-mediated cell membrane fusion in a dose-dependent manner. In competitive binding assays, (125)I-Apelin13 was replaced by ALX40-4C with an IC(50) of 2.9 microM, as compared with an IC(50) of 0.2 nM for Apelin13. Furthermore, ALX40-4C could block ligand-induced APJ internalization and signaling. ALX40-4C, as an antagonist to APJ, directly binds to and prevents use of APJ as a HIV-1 co-receptor. Thus, ALX-4C has potential utility for further elucidation of HIV-1 neuropathogenesis and therapy of HIV-1-induced encephalopathy.

Cell surface heparan sulfate participates in CXCL1-induced signaling

The CXC subfamily of chemokines plays an important role in diverse processes, including inflammation, wound healing, growth regulation, angiogenesis, and tumorigenesis. The ELR-CXC chemokine, CXCL1 or MGSA/GROalpha, is traditionally considered to attract neutrophils to sites of inflammation. The non-ELR-CXC chemokine, CXCL10 or IP-10, is chemotactic for monocytes, B cells, and activated T lymphocytes. In addition to its role in leukocyte migration, CXCL10 inhibits the angiogenic functions of the ELR-CXC chemokines as well as bFGF and VEGF. Heparan sulfate proteoglycans (HSPGs) are required for the interaction of bFGF and vEGF ligands and their receptors. However, the role of HSPGs in regulating the ELR-chemokines signaling and biological functions is poorly understood. We show here that the CXCL1 maximal binding to CXCR2 expressed on HEK293 and CHO-K1 cells is dependent on the presence of cell surface HSPGs. The cell surface HSPGs on cells are required for CXCL1-induced PAK1 activation. Moreover, CXCL10 can inhibit CXCL1-induced PAK1 and ERK activation as well as the CXCL1-induced chemotaxis through decreasing CXCL1 binding to cell surface heparan sulfate. These data indicate that HSPGs are involved in modulating CXCL1-induced PAK1 activation and chemotaxis through regulating CXCL1 binding activity to CXCR2 receptor. CXCL10 inhibits CXCL1-induced PAK1 activation and chemotaxis by interfering with appropriate binding of CXCL1 to CXCR2 receptor.

The role of chemokines in melanoma tumor growth and metastasis

Chemokines represent a large family of polypeptide signaling molecules that are notable for their role in chemotaxis, leukocyte homing, directional migration, and G protein coupled receptor activation. Chemo kines have recently been implicated in tumor progression and metastasis. The demonstration of chemokine expression and receptor activation in melanoma tumor cells themselves, and the tumor infiltrating leukocytes, may have important implications in terms of tumor progression and tumor cell homing to metastatic sites. In addition to their chemotactic and cell homing properties, chemokines and their receptors also play a part in other biologic functions relevant to oncogenesis, including cell proliferation, protease induction, tumor growth, and angiogenesis. Melanomas, and the cells derived from them, have been found to express a number of chemokines, including CXCL8 (interleukin-8), CXCL1-3 (MGSA-GROalpha-gamma), CCL5 (RANTES), and CCL2 (monocyte chemotactic protein-1), which have been implicated in tumor growth and progression. Furthermore, recent studies have demonstrated organ-specific patterns of melanoma metastasis that correlate with their expression of specific chemokine receptors, including CXCR4, CCR7, and CCR10. This review will focus on the current biology of chemokines and chemokine receptors in the context of understanding their potential roles in melanoma progression and metastasis, and is not meant to be a comprehensive review of chemokine biology. Continued understanding and progress in the determination of the role of chemokines and their receptors in tumorigenesis and metastasis, including melanoma, may lead to novel approaches in the treatment and management of this disease.

Structure, function, and inhibition of chemokines

Chemokines are the largest family of cytokines in human immunophysiology. These proteins are defined by four invariant cysteines and are categorized based on the sequence around the first two cysteines, which leads to two major and two minor subfamilies. Chemokines function by activating specific G protein-coupled receptors, which results in, among other functions, the migration of inflammatory and noninflammatory cells to the appropriate tissues or compartments within tissues. Some of these proteins and receptors have been implicated or shown to be involved in inflammation, autoimmune diseases, and infection by HIV-1. The three-dimensional structure of each monomer is virtually identical, but the quaternary structure of chemokines is different for each subfamily. Structure-function studies reveal several regions of chemokines to be involved in function, with the N-terminal region playing a dominant role. A number of proteins and small-molecule antagonists have been identified that inhibit chemokine activities. In this review, we discuss aspects of the structure, function, and inhibition of chemokines.

Analysis of homodimeric protein interfaces by graph-spectral methods

The quaternary structures impart structural and functional credibility to proteins. In a multi-subunit protein, it is important to understand the factors that drive the association or dissociation of the subunits. It is a well known fact that both hydrophobic and charged interactions contribute to the stability of the protein interface. The interface residues are also known to be highly conserved. Though they are buried in the oligomer, these residues are either exposed or partially exposed in the monomer. It is felt that a systematic and objective method of identifying interface clusters and their analysis can significantly contribute to the identification of a residue or a collection of residues important for oligomerization. Recently, we have applied the techniques of graph-spectral methods to a variety of problems related to protein structure and folding. A major advantage of this methodology is that the problem is viewed from a global protein topology point of view rather than localized regions of the protein structure. In the present investigation, we have applied the methods of graph-spectral analysis to identify side chain clusters at the interface and the centers of these clusters in a set of homodimeric proteins. These clusters are analyzed in terms of properties such as amino acid composition, accessibility to solvent and conservation of residues. Interesting results such as participation of charged and aromatic residues like arginine, glutamic acid, histidine, phenylalanine and tyrosine, consistent with earlier investigations, have emerged from these analyses. Important additional information is that the residues involved are a part of a cluster(s) and that they are sequentially distant residues which have come closer to each other in the three-dimensional structure of the protein. These residues can easily be detected using our graph-spectral algorithm. This method has also been used to identify important residues ('hot spots') in dimerization and also to detect dimerization sites on the monomer. The residues predicted using the present algorithm have correlated well with the experiments indicating the efficacy of this method in predicting residues involved in dimer stability.

Molecular determinants of receptor binding and signaling by the CX3C chemokine fractalkine

Fractalkine/CX3CL1 is a membrane-tethered chemokine that functions as a chemoattractant and adhesion protein by interacting with the receptor CX3CR1. To understand the molecular basis for the interaction, an extensive mutagenesis study of fractalkine's chemokine domain was undertaken. The results reveal a cluster of basic residues (Lys-8, Lys-15, Lys-37, Arg-45, and Arg-48) and one aromatic (Phe-50) that are critical for binding and/or signaling. The mutant R48A could bind but not induce chemotaxis, demonstrating that Arg-48 is a signaling trigger. This result also shows that signaling residues are not confined to chemokine N termini, as generally thought. F50A showed no detectable binding, underscoring its importance to the stability of the complex. K15A displayed unique signaling characteristics, eliciting a wild-type calcium flux but minimal chemotaxis, suggesting that this mutant can activate some, but not all, pathways required for migration. Fractalkine also binds the human cytomegalovirus receptor US28, and analysis of the mutants indicates that US28 recognizes many of the same epitopes of fractalkine as CX3CR1. Comparison of the binding surfaces of fractalkine and the CC chemokine MCP-1 reveals structural details that may account for their dual recognition by US28 and their selective recognition by host receptors.

Nuclear magnetic resonance solution structure of truncated human GRObeta [5-73] and its structural comparison with CXC chemokine family members GROalpha and IL-8

The three-dimensional structure of a novel four amino acid truncated form of the CXC chemokine GRObeta [5-73] isolated from bone marrow stromal cells with potent hematopoietic and anti-infective activities has been determined by two-dimensional (1)H nuclear magnetic resonance (NMR) spectroscopy in solution. On the basis of 1878 upper distance constraints derived from nuclear Overhauser effects (NOE) and 314 dihedral angle constraints, a group of 20 conformers representing the solution structure of the human GRObeta [5-73] was computed with the program DYANA. At the concentrations used for NMR study, GRObeta [5-73] forms a dimer in solution that is architectured by a six-stranded antiparallel beta-sheet (residues 25 to 29, 39 to 44, 49 to 52) and a pair of helices (residues 58 to 68) with 2-fold symmetry, while the C terminus of the protein is disordered. The average of the pairwise root-mean-square deviations of individual NMR conformers relative to the mean coordinates for the backbone atoms N, C(alpha) and C' of residues 5 to 68 is 0.47 A. Overall, the global fold of GRObeta [5-73] is similar to that of the previously reported NMR structure of GROalpha and the NMR and X-ray structures of interleukin-8. Among these three CXC chemokines, GRObeta [5-73] is most similar in structure to GROalpha. Significant differences between GRObeta [5-73], GROalpha and interleukin-8 are in the N-terminal loop comprising residues 12 to 19. The N-terminal arm containing the conserved ELR motif and the loop of residues 30 to 38 containing the GPH motif are different among these three CXC chemokines. The structural differences in these two regions may be responsible for the specificity of the receptor binding and biological activity of different chemokines.

Identification of residues in the monocyte chemotactic protein-1 that contact the MCP-1 receptor, CCR2

The CC chemokine, MCP-1, has been identified as a major chemoattractant for T cells and monocytes, and plays a significant role in the pathology of inflammatory diseases. To identify the regions of MCP-1 that contact its receptor, CCR2, we substituted all surface-exposed residues with alanine. Some residues were also mutated to other amino acids to identify the importance of charge, hydrophobicity, or aromaticity at specific positions. The binding affinity of each mutant for CCR2 was assayed with THP-1 and CCR2-transfected CHL cells. The majority of point mutations had no effect. Residues at the N-terminus of the protein, known to be crucial for signaling, contribute less than a factor of 10 to the binding affinity. However, two clusters of primarily basic residues (R24, K35, K38, K49, and Y13), separated by a 35 A hydrophobic groove, reduced the level of binding by 15-100-fold. A peptide fragment encompassing residues 13-35 recapitulated some of the mutational data derived from the intact protein. It exhibited modest binding as a linear peptide and dramatically improved affinity when the region which adopts a single turn of a 3(10)-helix in the protein, which includes R24, was constrained by a disulfide bond. Additional constraints at the ends of the peptide, corresponding to the disulfide between the first and third cysteines in MCP-1, yielded further improvements in affinity. Together, these data suggest a model in which a large surface area of MCP-1 contacts the receptor, and the accumulation of a number of weak interactions results in the 35 pM affinity observed for the wild-type (WT) protein. The receptor binding site of MCP-1 also is significantly different from the binding sites of RANTES and IL-8, providing insight into the issue of receptor specificity. It was previously shown that the N-terminus of CCR2 is critical for binding MCP-1 [Monteclaro, F. S., and Charo, I. F. (1996) J. Biol. Chem. 271, 19084-92; Monteclaro, F. S., and Charo, I. F. (1997) J. Biol. Chem. 272, 23186-90]. Point mutations of six acidic residues in this region of the receptor were made to test their role in ligand binding. This identified D25 and D27 of the DYDY motif as being important. On the basis of our data, we propose a model in which the receptor N-terminus lies along the hydrophobic groove in an extended fashion, placing the DYDY motif near the basic cluster involving R24 and K49 of MCP-1. This in turn orients the signaling residues (Y13 and the N-terminus) for productive interaction with the receptor.

Differential mechanisms of recognition and activation of interleukin-8 receptor subtypes

We have probed an epitope sequence (His18-Pro19-Lys20-Phe21) in interleukin-8 (IL-8) by site-directed mutagenesis. This work shows that single and double Ala substitutions of His18 and Phe21 in IL-8 reduced up to 77-fold the binding affinity to IL-8 receptor subtypes A (CXCR1) and B (CXCR2) and to the Duffy antigen. These Ala mutants triggered neutrophil degranulation and induced calcium responses mediated by CXCR1 and CXCR2. Single Asp or Ser substitutions, H18D, F21D, F21S, and double substitutions, H18A/F21D, H18A/F21S, and H18D/F21D, reduced up to 431-fold the binding affinity to CXCR1, CXCR2, and the Duffy antigen. Interestingly, double mutants with charged residue substitutions failed to trigger degranulation or to induce wild-type calcium responses mediated by CXCR1. Except for the H18A and F21A mutants, all other IL-8 mutants failed to induce superoxide production in neutrophils. This study demonstrates that IL-8 recognizes and activates CXCR1, CXCR2, and the Duffy antigen by distinct mechanisms.

Solution structure and dynamics of the CX3C chemokine domain of fractalkine and its interaction with an N-terminal fragment of CX3CR1

Fractalkine, a novel CX3C chemokine, is unusual because of both its membrane-associated structure and its direct role in cell adhesion. We have solved the solution structure of the chemokine domain of fractalkine (residues 1-76) by heteronuclear NMR methods. The 20 lowest energy structures in the ensemble have an average backbone rmsd of 0.43 A, excluding the termini. In contrast to many other chemokines which form homodimers, fractalkine's chemokine module is monomeric. Comparison of the structure to CC and CXC chemokines reveals interesting differences which are likely to be relevant to receptor binding. These include a bulge formed by the CX3C motif, the relative orientation of the N-terminus and 30's loop (residues 30-38), and the conformation of the N-loop (residues 9-19). 15N backbone relaxation experiments indicate that these same regions of the protein are dynamic. We also titrated 15N-labeled protein with a peptide from the N-terminus of the receptor CX3CR1 and confirmed that this region of the receptor contacts the fractalkine chemokine domain. Interestingly, the binding site maps roughly to the regions of greatest flexibility and structural variability. Together, these data provide a first glimpse of how fractalkine interacts with its receptor and should help guide mutagenesis studies to further elucidate the molecular details of binding and signaling through CX3CR1.

Crystal structure of chemically synthesized [N33A] stromal cell-derived factor 1alpha, a potent ligand for the HIV-1 "fusin" coreceptor

Stromal cell-derived factor-1alpha (SDF-1alpha ) is a member of the chemokine superfamily and functions as a growth factor and chemoattractant through activation of CXCR4/LESTR/Fusin, a G protein-coupled receptor. This receptor also functions as a coreceptor for T-tropic syncytium-inducing strains of HIV-1. SDF-1alpha antagonizes infectivity of these strains by competing with gp120 for binding to the receptor. The crystal structure of a variant SDF-1alpha ([N33A]SDF-1alpha ) prepared by total chemical synthesis has been refined to 2.2-A resolution. Although SDF-1alpha adopts a typical chemokine beta-beta-beta-alpha topology, the packing of the alpha-helix against the beta-sheet is strikingly different. Comparison of SDF-1alpha with other chemokine structures confirms the hypothesis that SDF-1alpha may be either an ancestral protein from which all other chemokines evolved or the chemokine that is the least divergent from a primordial chemokine. The structure of SDF-1alpha reveals a positively charged surface ideal for binding to the negatively charged extracellular loops of the CXCR4 HIV-1 coreceptor. This ionic complementarity is likely to promote the interaction of the mobile N-terminal segment of SDF-1alpha with interhelical sites of the receptor, resulting in a biological response.

Chemokines and chemotaxis of leukocytes in infectious meningitis

Chemokines constitute a constantly growing family of small inflammatory cytokines. They have been implied in many different diseases of the CNS including trauma, stroke and inflammation, e.g., multiple sclerosis. In this review we focus on the role of chemokines in infectious meningitis of bacterial or viral origin. In experimental bacterial meningitis induced by Listeria monocytogeneses both CXC and CC chemokines namely MIP-1alpha, MIP-1beta and MIP-2 are produced intrathecally by meningeal macrophages and leukocytes which infiltrate into the CNS. In patients with bacterial meningitis, IL-8, GROalpha, MCP-1, MIP-1alpha and MIP-1beta are detectable in the CSF. These chemokines contribute to CSF mediated chemotaxis on neutrophils and PBMC in vitro. In viral meningitis IL-8, IP-10 and MCP-1 are identified in the CSF to be responsible for chemotactic activity on neutrophils, PBMC and activated T cells. Taken collectively these data indicate that the recruitment of leukocytes in infectious meningitis involves the intrathecal production of chemokines.

Inhibition of adenylyl cyclase by alpha chemokines IL-8 and GRO-alpha in Chinese hamster ovary cells expressing R1 and R2 receptors

Interleukin-8 (IL-8) and growth-related oncogene protein-alpha (GRO-alpha) belong to a family of alpha chemokines. The biologic effects of IL-8 are realized by binding to two seven-transmembrane receptors R1 and R2 and that of GRO-alpha by binding to receptor R2. Chinese hamster ovary (CHO) cells stably expressing R1 and R2 have been used to demonstrate that the ligand-dependent signaling by both receptors is via the inhibition of adenylyl cyclase. This inhibition is pertussis toxin sensitive and could be mediated by G(alpha)i2, which is present in CHO cells. GRO-alpha inhibits adenylyl cyclase exclusively in CHO-R2 cells, and IL-8 inhibits in both CHO-R1 and CHO-R2 cells. The cAMP status in cells is an easy, reliable, quantifiable signal that is amenable to high throughput screening for small molecule analogs.

New strategies for chemokine inhibition and modulation: you take the high road and I'll take the low road

Chemokines are low molecular weight cytokines that induce extravasation, chemotaxis, and activation of a wide variety of leukocytes. Members of the different chemokine families are defined by the orientation of specific critical cysteine residues, and are designated as C-X-C (e.g. interleukin-8), C-C (e.g. regulated upon activation normally T cell expressed and secreted, RANTES), or C (lymphotactin). All chemokines bind to members of a G-protein coupled serpentine receptor superfamily that span the leukocyte cell surface membrane seven times and mediate the biological activities of the individual ligands. Most chemokines possess two major binding surfaces: a high affinity site responsible for specific ligand/receptor interactions and a lower affinity site, also called the heparin-binding or glycosaminoglycan-binding domain, believed to be responsible for the establishment and presentation of chemokine gradients on the surface of endothelial cells and within the extracellular matrix. Although chemokines are clearly beneficial in wound healing, hemopoiesis, and the clearance of infectious organisms, the continued expression of chemokines is associated with chronic inflammation. Therefore, this class of cytokines are attractive targets for the creation of antagonists that abrogate one or more chemokine functions. It is envisioned that such antagonists could serve as a new class of anti-inflammatory drugs. In this commentary, we will discuss two different but related strategies for antagonizing chemokine-induced functions, namely, disruption of the low and high affinity binding sites.

Enhanced tumor-forming capacity for immortalized melanocytes expressing melanoma growth stimulatory activity/growth-regulated cytokine beta and gamma proteins

Three human MGSA/GRO genes encode 3 highly related chemokines, MGSA/GRO alpha, -beta and -gamma. All 3 MGSA/GRO proteins bind to the same receptors, but with differing affinities, and stimulate a number of biological responses including chemotaxis, angiogenesis, and growth regulation. We have previously demonstrated that MGSA/GRO alpha can be isolated from culture medium conditioned by malignant melanoma cells and that continuous secretion of MGSA/GRO alpha contributes to the transformation of immortalized murine melanocytes. The present study was designed to determine whether MGSA/GRO beta or -gamma have similar effects on melanocyte tumorigenicity. Stable Melan-a clones expressing either human MGSA/GRO beta or -gamma exhibited enhanced ability to form large colonies in soft agar and tumors in nude mice. The clones expressing the MGSA/GRO beta or -gamma transgene formed tumors within 2 months after injection; the tumors were highly pigmented and expressed immunoreactive MGSA/GRO beta or -gamma protein. Furthermore, when conditioned medium from Melan-a clones expressing MGSA/GRO alpha, -beta or -gamma transgenes were examined for the ability to induce angiogenesis in the rat cornea, strong angiogenic responses were observed. This angiogenic response was blocked by antibodies to the respective MGSA/GRO protein, but not by normal rabbit serum. By contrast, angiogenic responses were observed in only 2 of 12 corneal implants (17%) containing medium conditioned by Melan-a clones expressing the neomycin resistance marker alone.