Regulation of Leukocyte Integrins in Inflammatory Cell Recruitment

Division Of Basic Sciences - Nci

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Abstract

(a) Inside-out signaling regulation of beta2-integrins. Several pathways have been identified to mediate the inside out-signaling activation of integrin activity. Such pathways include the Rap-1-RapL pathway, the talin-pathway, cytohesin-1 as well as 14-3-3 proteins. These pathways may be regulated by ubiquitin ligases. We found that activation of the leukocyte beta2-integrin LFA-1 in cells of the innate immunity as well as inflammatory cell recruitment in vitro and in vivo can be regulated by the crosstalk between 14-3-3 proteins and the ubiquitin ligase cbl-b. Specifically, we found that cbl-b deficiency induces LFA-1 activation. Neutrophil and macrophage recruitment was enhanced in cbl-b-/- mice. Additionally, cbl-b-/- macrophages showed increased spreading and LFA-1-dependent adhesion in vitro. Whereas cbl-b-/- cells had no increased Rap1 activity, we found that upon cbl-b-deficiency the interaction between 14-3-3 proteins with the beta2 integrin chain was increased, resulting in higher LFA-1 activation. This was due to increased phosphorylation of the cytoplasmic tail of the beta2-chain of LFA-1 at position T758 due to cbl-b deficiency. (b) The role of the receptor of advanced glycation endproducts (RAGE) and its ligand HMGB1 in the regulation of leukocyte integrin activity. RAGE was characterized as a counter-receptor of the beta2-integrin Mac-1. One of the RAGE ligands, high mobility group B1 (HMGB1) is a nuclear protein, that can be actively secreted by macrophages and dendritic cells upon inflammatory stimuli and is also passively released by necrotic but not apoptotic cells. Extracellular HMGB1 evokes a very strong inflammatory response, thus HMGB1 represents a warning system for tissue injury. Our efforts have defined that HMGB1 has a direct chemotactic activity for inflammatory cells in vitro and in vivo, which is mediated by the leukocyte beta2-integrin Mac-1. HMGB1 activates the formation of a functional complex on the neutrophil surface between Mac-1 and RAGE, thereby stimulating Mac-1-dependent inflammatory cell adhesion and migration in a RAGE-dependent manner. (c)Developmentally regulated endothelial locus-1 (Del-1) is produced and secreted by endothelial cells. We found expression of Del-1 in the mouse lung and brain. We identified a novel interaction between Del-1 and leukocyte beta2-integrins. By this interaction Del-1 secreted from endothelial cells, could regulate inflammatory cell recruitment.