Cell Motility in Chronic Lymphocytic Leukemia: Defective Rap1 and ALB2 Activation by Chemokine

Chemokine-induced activation of A4B1 and ALB2 integrins (by conformational change and clustering) is required for lymphocyte transendothelial migration (TEM) and entry into lymph nodes. We have previously reported that chemokineinduced TEM is defective in chronic lymphocytic leukemia (CLL) and that this defect is a result of failure of the chemokine to induce polar clustering of ALB2; engagement of A4B1 and autocrine vascular endothelial growth factor (VEGF) restore clustering and TEM. The aim of the present study was to characterize the nature of this defect in ALB2 activation and determine how it is corrected. We show here that the ALB2 of CLL cells is already in variably activated conformations, which are not further altered by chemokine treatment. Importantly, such treatment usually does not cause an increase in the GTP-loading of Rap1, a GTPase central to chemokine-induced activation of integrins. Furthermore, we show that this defect in Rap1 GTP-loading is at the level of the GTPase and is corrected in CLL cells cultured in the absence of exogenous stimuli, suggesting that the defect is the result of in vivo stimulation. Finally, we show that, because Rap1induced activation of both A4B1 and ALB2 is defective, autocrine VEGF and chemokine are necessary to activate A4B1 for ligand binding. Subsequently, this binding and both VEGF and chemokine stimulation are all needed for ALB2 activation for motility and TEM. The present study not only clarifies the nature of the ALB2 defect of CLL cells but is the first to implicate activation of Rap1 in the pathophysiology of CLL. [Cancer Res 2008;68(20):8429–36]