Recombinant Mouse LIX Protein

Mouse LIX [lipopolysaccharide (LPS)-induced CXC chemokine], also previously called GCP-2, is a member of the ELR+ group of 7-9 kDa neutrophil and monocyte chemotactic proteins in the CXCL cytokine family (1). This group contains the ELR/GluLeuArg amino acid (aa) motif immediately N-terminal to the CXC motif. Other ELR+ chemokines also include mouse CXCL1 thru 3 and 15 (or KC, MIP-2, DCIP-1 and lungkine, respectively), plus human CXCL1 thru 3 and 5 thru 8 (or GRO alpha, GRO beta, GRO gamma, ENA-78, GCP-2, NAP-2 and IL-8, respectively) (1-4). Mouse LIX cDNA encodes 132 aa's, including a 40 aa signal peptide and up to a 92 aa mature protein that has a longer C-terminus than other ELR+ cytokines (5). The first 78 aa of mature LIX shares 63% and 55% aa identity with human CXCL6/GCP-2 and CXCL5/ENA-78, respectively. Its activity is similar but not identical to these human chemokines (1, 5). In the mouse, CXCL1/KC and CXCL2/MIP-2 expression and function may overlap with that of LIX (1, 6-10).

LIX can be produced by a variety of cells including fibroblasts, epithelial cells (including alveolar type II epithelia), endothelial cells, platelets, cardiac myocytes, preosteoblasts, oligodendrocytes, and adipose-resident macrophages (1, 4-8, 11-16). It is mainly produced when induced by LPS, IL-17 and/or TNF-alpha (1, 5-7, 11, 15). LIX can also be stored within specialized granules, such as platelet alpha-granules and endothelial cytoplasmic granules, but not Weibel-Palade bodies (6, 13). Endotoxemia increases LIX expression, especially in the heart, but also in the lung, spleen, and liver (8). LIX is downregulated by glucocorticoids and is considered a glucocorticoid-attenuated response gene or GARG (5, 8). It can also be downregulated by IL-10 and viral proteins (12, 17). Natural mouse LIX includes short forms that may be N-terminally cleaved by MMP1, 2, 8, 9, 12 or 13, and/or C-terminally cleaved by MMP1, 8, 9 or 12 (1-3, 18, 19).

Unlike other ELR+ chemokines, it is not cleaved within the ELR motif, and short forms show enhanced activity as compared to the full-length form (1-3, 18, 19). LIX activities are mainly mediated by its receptor CXCR2, which is expressed on neutrophils, mast cells and macrophages (9, 16, 17). Unlike other ELR+ chemokines, mouse LIX and human IL-8 can also signal through CXCR1 (2, 17). LIX is the most potent mouse neutrophil chemoattractant and activator (1-4, 7-9, 11, 13, 14). CXCR2-expressing cells treated with LIX show activation of NF kappaB signaling pathways, resulting in increased production of inflammatory cytokines such as IL-1 beta and TNF-alpha (9, 17). TNF-alpha produced as a result of endotoxemia or ischemia-reperfusion can, in turn, induce cardiomyocyte production of LIX, influx of neutrophils, and impedance of cardiac contractile activity (7, 14). LIX participates in the induction of LPS-induced acute lung injury and in lung ischemia-induced angiogenesis (10, 11). LIX protects neurons from apoptosis, and its downregulation by viral proteins is thought to allow neuronal apoptotic death (12). It is thought to play a protective role in inflammatory bone loss (15). High expression of LIX in obese white adipose tissue is thought to contribute to insulin resistance (16). LIX also binds the erythrocyte receptor, DARC (Duffy Antigen Receptor for Chemokines) and is reported to impair chemokine scavenging by DARC (13).