l-Selectin activates the Ras pathway via the tyrosine kinase p56lck

Selectins mediate rolling, the initial step of leukocyte adhesion to endothelial cells [Springer, T. A. (1995) Annu. Rev. Physiol . 57, 827–872 and Butcher, E. C. (1991) Cell 67, 1033–1036]. In this study we show that l -selectin triggering of Jurkat cells using different antibodies or glycomimetics resulted in activation of the src-tyrosine kinase p56 lck ; tyrosine phosphorylation of intracellular proteins, in particular mitogen-activating protein kinase and l -selectin; and association of Grb2/Sos with l -selectin. This association correlated with an activation of p21Ras, mitogen-activating protein kinase, Rac2, and a transient increase of O 2 − synthesis. Stimulation of the Ras pathway by l -selectin requires functional p56 lck , since p56 lck -deficient Jurkat cells (JCaM1.6) do not show tyrosine phosphorylation, association of l -selectin with Grb2/Sos, and activation of Ras upon l -selectin triggering. Transfection of JCaM1.6 cells with p56 lck reconstitutes the observed signaling events. Genetic inhibition of Ras or Rac2 prevented Rac2 stimulation and O 2 − synthesis, respectively. The specificity and the physiological significance of the observed signaling cascade is indicated by stimulation of l -selectin-transfected P815, l -selectin-positive CEM or peripheral blood lymphocytes resulting in the same activation events as in Jurkat cells. Our results point to a signaling cascade from l -selectin via p56 lck , Grb2/Sos, Ras, and Rac2 to O 2 − . signal transduction adhesion proteins G-proteins lymphocytes