Beta 1-integrin-mediated cell signaling in T lymphocytes

beta 1-integrins play crucial roles in a variety of cell processes such as adhesion, migration, proliferation, and differentiation of lymphocytes. For understanding the molecular mechanisms of these various biological effects , it may be particularly important to analyze cell signaling through the be ta 1-integrins. Our previous study had shown that PLC-gamma, pp125FAK (foca l adhesion kinase), pp105, paxillin, p59fyn, p56lck and ERK1/2 are phosphor ylated in their tyrosine residues upon engagement of pl-integrins. We ident ified pp105 as Cas (Crk-associated substrate)-related protein and successfu lly cloned its cDNA. pp105 is a Cas homologue predominantly expressed in th e cells of lymphoid lineage, which led us to designate it as Cas-L, Like p1 30Cas, Cas-L contains a single SH3 domain and multiple SH2 binding sites (Y XXP motif), which is suggested to bind SH2 domains of Crk, Nck, and SHPTP2. Subsequent studies revealed that pp125FAK binds Cas-L on its SH3 domain an d phosphorylates its tyrosine residues upon beta 1-integrin stimulation. Si nce Cas-L is preferentially expressed in lymphocytes, it is conceivable tha t Cas-L plays an important role in lymphocyte-specific signals. We have sho wn that Cas-L is involved in the T-cell receptor (TCR)/CD3 signaling pathwa y as well as the beta 1-integrin signaling pathway. Cas-L is transiently ph osphorylated following CD3 cross-linking, and tyrosine-phosphorylated Cas-L binds to Crk and C3G. Furthermore, a Cas-L mutant (Cas-L Delta SH3), which lacks the binding site for FAK, is still tyrosine-phosphorylated upon CD3 cross-linking, but not upon beta 1-integrin cross-linking, suggesting that FAK is not involved in CD3-dependent Cas-L phosphorylation. Finally, we hav e identified a crucial role of Cas-L in beta 1-integrin-mediated T-cell co- stimulation. beta 1-integrins have known to provide a co-stimulus for TCR/C D3-driven interleukin-2 production and proliferation of peripheral T-cells. We have found that this co-stimulatory pathway is impaired in the Jurkat T -cell line, and that the expression level of Cas-L is reduced in Jurkat cel ls compared with peripheral T-cells. The transfection of Cas-L cDNA into Ju rkat cells restored the beta 1-integrin-mediated co-stimulation, while the transfection of Cas-L Delta SH3 mutant failed to do so, showing a contrast to the case with CD3-mediated signaling. These results indicate that Cas-L plays a key role through the association and phosphorylation by FAK in the beta 1-integrin-mediated T-cell co-stimulation. Taken together, Cas-L might be the bi-modal docking protein that assembles the signals through beta 1- integrins and TCR/CD3. and participates in a variety of T-cell functions. ( C) 2000 Elsevier Science Ireland Ltd. All rights reserved.