Cytoskeletal rearrangement and Src and PI-3K-dependent Akt activation

The γ-amino butyric acid (GABA) type B receptors (GABA B R) function as chemoattractant receptors in response to GABA B R agonists in human neutrophils. The goal of this study was to define signaling mechanisms regulating GABA B R-mediated chemotaxis and cytoskeletal rearrangement. In a proteomic study we identified serine/threonine kinase Akt, tyrosine kinases Src and Pyk2, microtubule regulator kinesin and microtubule affinity-regulating kinase (MARK) co-immunoprecipitating with GABA B R. To define the contributions of these candidate signaling events in GABA B R-mediated chemotaxis, we used rat basophilic leukemic cells (RBL-2H3 cells) stably transfected with human GABA B1b and GABA B2 receptors. The GABA B R agonist baclofen induced Akt phosphorylation and chemotaxis by binding to its specific GABA B R since pretreatment of cells with CGP52432, a GABA B R antagonist, blocked such effects. Moreover, baclofen induced Akt phosphorylation was shown to be dependent upon PI-3K and Src kinases. Baclofen failed to stimulate actin polymerization in suspended RBL cells unless exposed to a baclofen gradient. However, baclofen stimulated both actin and tubulin polymerization in adherent RBL-GABA B R cells. Blockade of actin and tubulin polymerization by treatment of cells with cytochalasin D or nocodazole respectively, abolished baclofen-mediated chemotaxis. Furthermore, baclofen stimulated Pyk2 and STAT3 phosphorylation, both known regulators of cell migration. In conclusion, GABA B R stimulation promotes chemotaxis in RBL cells which is dependent on signaling via PI3-K/Akt, Src kinases and on rearrangement of both microtubules and actin cytoskeleton. These data define mechanisms of GABA B R-mediated chemotaxis which may potentially be used to therapeutically regulate cellular response to injury and disease.