Interleukin-8 (IL-8), also known as CXCL8, was discovered over 30 years ago as a small molecule that could promote chemotaxis and induce the production of both superoxide and hydrogen peroxide (H202) (Schroder et al, 1987; Yoshimura et al, 1987). IL-8 is now recognised as a chemokine, which are small chemoattractant molecules that are secreted by cells of the immune system. Chemokines induce integrin expression and primarily attract leukocytes to the site of infection, although their roles have been discovered to be much more diverse (Moser et al, 2013; Constantin et al, 2000). 44 chemokines and 23 chemokine receptors have been identified to date (Nomiyama et al, 2013). Structurally, IL-8 contains two N-terminal cysteine motifs, which classify it as a CXCL family member (Oppenheim et al, 1991). These cysteine motifs are separated by an amino acid which is essential for the specific binding of IL-8 to its membranous receptors CXCR1 and CXCR2 (Herbert et al, 1991). Classification of CXCR1 and CXCR2 determined their expression on several cell types, including endothelial and epithelial cells, as well as fibroblasts and neutrophils (Sturm et al, 2005; Schonbeck et al, 1995). IL-8 is primarily produced by macrophages, epithelial cells, and endothelial cells, and has an important role in cell migration, targeting neutrophils (Klezovitch et al, 2001; Schuerer-Maly, 1994, Bickel, 1993).
IL-8 can binds to the cell surface receptors CXCR1 and CXCR2, with a greater affinity of IL-8 for CXCR1, with the ligation of IL-8 to CXCR1 induces chemotactic signalling downstream (Hammond et al, 1995). The binding of IL-8 to CXCR1 or CXCR2 triggers a conformational change and leads to the dissociation of cytoplasmic G-coupled protein subunits, Ga and Gbg, facilitating the activation of a variety of signalling pathways, including the mitogen-associated protein kinase (MAPK), phosphatidyl-inositol 3’ kinase/Akt (PI3K/Akt), phospholipase C/protein kinase C (PLC/PKC) pathways (Wu et al, 1993). MAPK signalling leads to the transcription of multiple genes that promote cell proliferation and survival, in addition to pro-inflammatory genes. IL-8 – induced activation of both MAPK and PI3K facilitates the induction of adhesion molecules, such as Mac-1 and integrins, which are critical molecules for mediating chemotaxis (Takami et al, 2002). Furthermore, the production of the second messenger molecule 3,4,5-inositol triphosphate (IP3) leads to the release of intracellular calcium from the endoplasmic reticulum stores, which culminates in the degranulation of neutrophils, a process mediating the release of antimicrobial, cytotoxic molecules (Faurschou and Borregaard, 2003). Overall, IL-8 drives chemotaxis by recruiting neutrophils via a series of complex signalling processes and the secretion of adhesion molecules. Transcription of IL-8 is induced by stimulation with TNFaLPS, IL-1, or by viral infection (Leonard and Toshimura, 1990).
IL-8 in Pathogenesis
It has been long established that elevated levels of IL-8 contribute to the pathogeneses of multiple inflammatory diseases, such as inflammatory bowel disease (Daig et al, 1996; Harada et al, 1996). During chronic inflammatory pathogenesis, increased infiltration of neutrophils concurrent with increased levels of IL-8 is noted. Targeting IL-8 production via the inhibition of NF-kB activation leads to reduce IL-8 transcription in intestinal epithelial cells lead to reduced inflammation in the gastrointestinal tract (Jobin et al, 1998). Moreover, IL-8 induces malignancies, angiogenesis, and cellular invasion. In vitro experiments shave shown that over expression of IL-8 in colon cancer cell lines promotes cell proliferation, angiogenesis and migration (Ning et al, 2011). Recent studies have demonstrated that IL-8 and IL-6 signal synergistically to enhance cellular migration and promotes metastasis, with dual inhibition of both IL-8 and IL-6 leading to a reduced metastatic phenotype in breast cancer (Jayatilaka et al, 2016). High levels of IL-8 has also been associated with poor response to chemotherapy, and downregulating IL-8 levels reduced chemoresistance in hepatocellular carcinoma (Park et al, 2014). Interestingly, CXCR1 and CXCR2 have been identified on immune cells in the CNS; astrocytes and microglia, as well as on neurons (Goczalik et al, 2008; Flynn et al, 2003). Recently, the activation of microglia and astrocytes facilitates the chemokine-induced infiltration of neutrophils into the CNS in response to b-amyloid pathogeneses has been implicated in Alzheimer’s disease pathology (Liu et al, 2014).
Targeting IL-8 as a Therapeutic Strategy
IL-8 inhibition has been explored as a therapeutic target yielding promising results (Schinke et al, 2015). The use of monoclonal antibody to neutralize IL-8 has been demonstrated to reduce the IL-8 – induced detrimental effects in inflammatory pathogeneses (Skov et al, 2008). Targeting the IL-8 receptors is another approach, and notably, it is required that both CXCR1 and CXCR2 are inhibited in order to eliminate the harmful effects of IL-8. Several small molecule inhibitors of CXCR1/CXCR2 have been developed, including repertaxin (Dompé, Italy), SCH479833 (Merck), and SCH527123 (Merck), which have all shown positive anti-tumour results in cases of melanoma, breast cancer and colon cancer (Ning et al, 2012; Genestier et al, 2010; Singh et al, 2009). Due to the pleiotropic nature of many cytokines, including IL-8, it is important to be cautious when developing signalling inhibitors. As IL-8 – induced neutrophil invasion is a homeostatic part of immunosurveillance, the effects of IL-8 inhibitors must be carefully monitored and balanced when developing therapeutics.
Figure 1: IL-8 signalling. IL-8 binds extracellularly to either CXCR1 or CXCR2 at the membrane. This induces a conformational change in the G protein subunits, Gα and Gβγ, which are intracellularly coupled to the CXCR1 and CXCR2 receptors. The conformational change leads to the dissociation of these G protein subunits from the receptor complex activating downstream signalling. The Gα subunit generates cyclic AMP (cAMP) and cAMP which subsequently activate protein kinase A (PKA). The Gβγ subunit activates phospholipids to produce inositol 3,4,5-triphosphate (IP3), which mediates the release of calcium from the endoplasmic reticulum and degranulation, and diacylglycerol (DAG), which signals through PKC to activate MAPK activation.
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