Suppressor of Cytokine Signaling 3 (SOCS3) Regulates CXCL12-Induced Adhesion to VCAM1 during B Cell Development.

CXCL12 (SDF-1) induces prolonged FAK phosphorylation in bone marrow (BM) progenitor B cells, but only induces transient FAK phosphorylation in peripheral B cells. In addition, FAK signaling is tightly correlated with CXCL12-induced adhesion to VCAM-1. Given that the CXCR4 receptor expression is similar at all B cell stages, we hypothesized that progenitor and mature B cells possess different mechanisms to control FAK phosphorylation. Here we demonstrate that SOCS3 regulates CXCL12-induced FAK signaling through the ubiquitin-proteosome pathway. First we show that CXCL12 triggers increased FAK ubiquitination in human peripheral B cells and mouse spleen B cells, but not in human and mouse BM B cells. In accordance with this finding, SOCS3 expression is relatively low in progenitor BM B cells and high in peripheral blood and splenic B cells. Through over-expression of SOCS3 in the progenitor B cell line, REH, we demonstrate that SOCS3 co-associates with FAK following CXCL12 stimulation. Moreover, over-expression of SOCS3 results in decreased FAK phosphorylation (intensity and duration), and impaired CXCL12- induced chemotaxis and adhesion to VCAM1. Next, we isolated splenic B cells from MMTV-Cre SOCS3 ^fl/fl mice and found that SOCS3 deficient mature B cells have increased and prolonged FAK phosphorylation and also show enhanced chemotaxis and sustained adhesion to VCAM-1, similar to progenitor B cells. Flow cytometric analysis revealed that MMTV-Cre SOCS3 ^fl/fl mice have an increased number of IgM+, Ig D-, mature B cells in the BM compared to wild type mice. Taken together, our results suggest that regulation of FAK signaling through SOCS3 may be important for B cell maturation in the BM and for the egress of mature B cells into the circulation.