Chemokines are a family of low molecular weight proteins (8-13 kDa) that are classified into four distinct groups depending on the positioning of the cysteine motif at the amino terminus. The family members comprise CXC, CC, XC, and CX3C chemokines of which CXC and CC are the largest and most characterized. The CXC chemokines include interleukin-8 (IL-8), neutrophil-activating protein-2 (NAP-2), growth-related oncogenes GRO-α, GRO-β, GRO-γ, epithelial cell-derived neutrophil activating factor-78 (ENA-78), granulocyte chemoattractant protein-2 (GCP-2), γ-interferon-inducible protein-10 (γIP-10), interferon-inducible T cell α-chemoattractant (I-TAC), monokine induced by γ-interferon (Mig) and platelet factor-4 (PF-4). CC chemokines include RANTES (regulated on activation normal T cell expressed and secreted), macrophage inflammatory proteins MIP-1α, MIP-1β, monocyte chemoattractant proteins MCP-1, MCP-2, MCP-3 and eotaxin. The XC family comprises two members, lymphotactin-α and lymphotactin-β, and the CX3C family consists only of a single chemokine named fractalkine (Murphy et al., Pharmacol. Rev. 52: 145-176, 2000).
Chemokines mediate their biological effects by binding to cell surface molecules, which belong to the superfamily of seven-transmembrane spanning receptors that signal through coupling to heterotrimeric G proteins. Although most chemokine receptors recognize more than one chemokine, they are almost always restricted to a single subclass. Chemokine receptor binding initiates a cascade of intracellular events of which the first step is the binding of the receptor by its high-affinity ligand. This induces a conformational change leading to a dissociation of the receptor-associated heterotrimeric G proteins into α and βγ subunits. These G protein subunits are able to activate various effector proteins, including phospholipases leading to generation of inositol trisphosphate, an increase in cytosolic calcium, and activation of protein kinases. This cascade of intracellular events mediates a wide range of functions in different leukocytes such as chemotaxis, degranulation, oxidative burst, phagocytosis, and lipid mediator synthesis.
Interleukin-8 (IL-8) is a key mediator of immunological reactions in inflammatory disorders such as atherosclerosis, ischemia/reperfusion injury, rheumatoid arthritis, chronic obstructive pulmonary disease, respiratory distress syndrome, asthma, cystic fibrosis, and psoriasis (Bizarri et al., Curr. Med. Chem. 2: 67-79, 2003). IL-8 is the most characterized member of the CXC subfamily of chemokines. Leukocyte responses to IL-8 are mediated via specific cell surface receptors, CXCR1 and CXCR2. Whereas CXCR1 is selectively activated by IL-8, CXCR2 responds to several additional chemokines including growth-related oncogenes GRO-α, GRO-β, GRO-γ, neutrophil-activating protein-2 (NAP-2), epithelial cell-derived neutrophil activating factor-78 (ENA-78), and granulocyte chemoattractant protein-2 (GCP-2). The common denominator shared by all chemokines that activate CXCR2 is a Glu-Leu-Arg (ELR) sequence in the amino terminus, which appears to serve as a recognition sequence for receptor binding and activation (Herbert et al., J. Biol. Chem. 266: 18989-18994, 1991).
Early investigations concentrated on the effect of IL-8 on neutrophils, which respond to IL-8 with calcium mobilization, actin polymerization, enzyme release, chemotaxis, and the respiratory burst. Despite similar affinities for IL-8 and similar receptor numbers of CXCR1 and CXCR2 on neutrophils, both receptors are functionally different. Responses such as calcium mobilization and the release of granule enzymes are mediated through both receptors, whereas the respiratory burst and the activation of phospholipase D depend exclusively on stimulation of CXCR1 (Jones et al., Proc. Natl. Acad. Sci. USA 93: 6682-6686, 1996). Due to their prominent role in neutrophil recruitment, CXCR1 and CXCR2 are thought to be important in several acute neutrophil-mediated diseases such as acute respiratory distress syndrome and ischemia/reperfusion injuries, as well as in chronic diseases such as asthma, psoriasis, dermatitis, and arthritis.
It has been shown that CXCR2 is also expressed by monocytes. Despite IL-8's inactivity in monocyte chemotaxis assay, this factor induces calcium flux and respiratory burst in monocytes and enhances adhesion of monocytes in static assays. Similarly, GRO-α enhances adhesion of monocytes to stimulated endothelial cells. Moreover, IL-8 is able to induce firm arrest of monocytes on endothelial cells under conditions of physiological flow (Gerszten et al., Nature 398: 718-723,1999). Since CXCR2 is strongly expressed on monocytes and macrophages in atherosclerotic lesions where it is suggested to play a key role in chemoattraction, retension, expansion, and activation of monocytes and macrophages, this strongly suggests that CXCR2 and one or more of its ligands (IL-8, GRO-α ) play a pathophysiological role in atherosclerosis (Huo et al., J. Clin. Invest. 108: 1307-1314, 2001).
Apart from neutrophils and monocytes, numerous cell types have been shown to express IL-8 receptors. These cell types include neurons, various cancer cells, keratinocytes, and endothelial cells. Several lines of evidence indicate that IL-8 plays a direct role in angiogenesis via stimulation of CXCR2 expressed on endothelial cells. IL-8 has been shown to bind specifically to endothelial cells and induce chemotaxis. IL-8 is able to induce neovascularization in the absence of inflammatory responses (Koche et al., Science 258: 1798-1801, 1992). Moreover, there is accumulating evidence that IL-8 could play a key role in melanoma progression and metastasis as patients with melanoma metastases have elevated serum levels of IL-8. IL-8 is supposed to act as an autocrine growth and metastatic factor for melanoma cells (Schadendorf et al., J. Immunol: 151-157, 1993).
Due to the wide range of actions of IL-8, such as attraction and activation of neutrophils and monocytes/macrophages as well as promotion of endothelial cell proliferation and cancer cell growth, the inhibition of chemokine receptors CXCR1 and CXCR2 is expected to be beneficial in the prevention and treatment of numerous diseases. Besides acute and chronic inflammatory diseases such as atherosclerosis, ischemia/reperfusion injuries, chronic obstructive pulmonary disease, asthma, and rheumatoid arthritis, chemokine (such as, but not limited to IL-8, GRO-α, GRO-β, GRO-γ, NAP-2, ENA-78, or GCP-2) mediated diseases include adult respiratory distress syndrome, inflammatory bowel disease, ulcerative colitis, Crohn's disease, atopic dermatitis, cystic fibrosis, psoriasis, multiple sclerosis, angiogenesis, restenosis, osteoarthritis, septic shock, endotoxic shock, gram negative sepsis, toxic shock syndrome, stroke, glomerulonephritis, thrombosis, graft vs. host reaction, allograft rejections, alzheimers disease, malaria, viral infections, traumatic brain injury, pulmonary fibrosis, and cancer.
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