Cell Motility in Chronic Lymphocytic Leukemia: Defective Rap1 and αLβ2 Activation by Chemokine

Chemokine-induced activation of alpha 4 beta 1 and alpha L beta 2 integrins (by conformational change and clustering) is required for lymphocyte transendothelial migration (TEM) and entry into lymph nodes. We have previously reported that chemokine-induced 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 alpha L beta 2; engagement of alpha 4 beta 1 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 alpha L beta 2 activation and determine how it is corrected. We show here that the alpha L beta 2 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 Rapt 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 Rap1-induced activation of both alpha 4 beta 1 and alpha L beta 2 is defective, autocrine VEGF and chemokine are necessary to activate alpha 4 beta 1 for ligand binding. Subsequently, this binding and both VEGF and chemokine stimulation are all needed for alpha L beta 2 activation for motility and TEM. The present study not only clarifies the nature of the alpha L beta 2 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]