Lipid rafts play an important role in signal integration and cellular activation of a number of cytokine and growth factor receptors. Flotillin proteins have recently been shown to be recruited to lipid raft microdomains upon cellular activation and have been implicated in neural cell regeneration, receptor signaling and lymphocyte activation. However, little is known about the relevance of the flotillin proteins in T cell responses to chemoattractant stimulation. To this end, cytoplasmic and lipid raft fractions from human T cells were analyzed for flotillin protein redistribution prior to and after CXCL12 stimulation. Flotillin-1 but not flotillin-2 redistributes to lipid rafts upon CXCR4 ligation. Moreover, in CXCL12-treated T cells, flotillin-1 also associates with several raft proteins including LAT, Lck, CD48 and CD11a. In addition, an increase in CXCR4 association with flotillin-1 in lipid rafts was observed after chemokine treatment. RNAi technology was also utilized to inhibit the expression of flotillin-1 resulting in an inhibition of CXCL12-mediated signaling, function and CXCR4 recruitment into lipid rafts. Together, these data suggest that the association of flotillin-1 with lipid raft during chemokine exposure may play an important role in chemokine receptor recruitment to and signaling in lipid rafts and possibly in leading edge formation. Overall, we believe that a greater understanding of the various signaling and cell surface proteins associated with lipid rafts may provide insight into age-related alterations in cell signaling and trafficking.? ? Additional studies have also revealed that dexamethasone (DM)-treated T cells demonstrate enhanced migration in response to the chemokine CXCL12, possibly through altering the cell membranes and rafts or through direct interaction with the chemokine and T-cell receptor signaling pathways. DM is a synthetic member of the glucocorticoid (GC) class of hormones that possesses anti-inflammatory and immunosuppressant activity and is commonly utilized to treat chronic inflammatory disorders, severe allergies and other disease states. While glucocorticoids are known to mediate well-defined transcriptional effects via GC receptors, there is increasing evidence that GCs also initiate rapid non-genomic signaling events in a variety of cell types. Here, we report that dexamethasone appears to induce the phosphorylation of Lck and the activation of other down stream mediators including p59Fyn, Zap70, Rac1 and Vav in resting but not activated human T cells. DM treatment also appears to augment CXCL12-mediated signaling in resting T cells through its cell surface receptor, CXCR4 resulting in the enhanced actin polymerization and cell migration upon ligand exposure. Lck was found to be a critical intermediate in these DM-induced signaling activities. Moreover, DM-mediated Lck phosphorylation in T cells was dependent on the presence of both the glucocorticoid receptor and the CD45 molecule. Overall, these results elucidate additional non-genomic effects of DM on resting human T cells, inducing Lck activation and augmenting chemokine signaling and function.
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