Search

International audience; Like many integral membrane glycoproteins, the extracellular domain of L-selectin undergoes rapid shedding, which occurs on both resting and activated host leucocytes. Incubating normal or transformed leukocytes with phorbol esters can also artificially induce shedding of L-selectin, providing multiple possibilities for the source of soluble forms of L-selectin found in the serum of patients with hematological malignancies. Here, using genetically engineered L-selectin-deficient mouse models, we have measured the release of soluble circulating forms of L-selectin in the serum of lymphoma-bearing mice. We found that L-selectin-deficient lymphoma cells could not induce an elevation of circulating soluble forms of L-selectin in normal mice, as compared to lymphoma cells expressing L-selectin. Moreover, soluble forms of L-selectin were detected in the serum in mice bearing lymphoma induced by injection of T lymphoma cells expressing L-selectins. Interestingly, we also found that lymphoma cells that are unable to shed L-selectin in vitro following exposure to phorbol ester can generate soluble forms of serum L-selectin in vivo. Taken together, these results indicate that lymphoma cells are the major contributors to levels of soluble forms of L-selectins in lymphoma-bearing mice.

p120 GTPase-activating protein (p120GAP) is a major negative regulator of p21ras activity in several cell types including T cells. Catalytic activity of this enzyme is regulated in part by its interaction with several associated tyrosine-phosphorylated proteins. Sam68 was initially described as associated with p120GAP. It has been further established that Sam68 is a substrate of src kinases in mitosis and that it is not associated with p120GAP in transformed fibroblasts. We describe herein that Sam68 associates with p120GAP and PLCγ1 in human mature T cells and in a T cell line expressing the CD4 molecule HUT78 CD4+. This association is present in nonactivated cells and increases after anti-CD3 activation. It is dependent on CD4 expression and, in part, on the association of CD4 with p56lck, as shown by the strongly decreased association of Sam68 with p120GAP in the CD4− mutants, HUT78 CD4−, and by the reduced association of Sam68 with both p120GAP and p56lck in the HUT78 T cell line expressing a CD4 mutant unable to interact with p56lck, HUT78 C420/22. We propose that recruitment of Sam68, via CD4/p56lck, to the inner face of the plasma membrane may permit, via its docking properties, the correct association of key signaling molecules including PLCγ1 and p120GAP. This formation of transduction modules will enable the activation of different signaling cascades including the p21ras pathway and an array of downstream events, ultimately leading to T cell activation.