Beta 1-integrin-mediated cell signaling in T lymphocytes.

beta1-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 beta1-integrins. Our previous study had shown that PLC-gamma, pp125FAK (focal adhesion kinase), pp105, paxillin, p59fyn, p56lck and ERK1/2 are phosphorylated in their tyrosine residues upon engagement of beta1-integrins. We identified pp105 as Cas (Crk-associated substrate)-related protein and successfully cloned its cDNA. pp105 is a Cas homologue predominantly expressed in the cells of lymphoid lineage, which led us to designate it as Cas-L. Like p130Cas, Cas-L contains a single SH3 domain and multiple SH2 binding sites (YXXP 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 and phosphorylates its tyrosine residues upon beta1-integrin stimulation. Since Cas-L is preferentially expressed in lymphocytes, it is conceivable that Cas-L plays an important role in lymphocyte-specific signals. We have shown that Cas-L is involved in the T-cell receptor (TCR)/CD3 signaling pathway as well as the beta1-integrin signaling pathway. Cas-L is transiently phosphorylated following CD3 cross-linking, and tyrosine-phosphorylated Cas-L binds to Crk and C3G. Furthermore, a Cas-L mutant (Cas-LDeltaSH3), which lacks the binding site for FAK, is still tyrosine-phosphorylated upon CD3 cross-linking, but not upon beta1-integrin cross-linking, suggesting that FAK is not involved in CD3-dependent Cas-L phosphorylation. Finally, we have identified a crucial role of Cas-L in beta1-integrin-mediated T-cell co-stimulation. beta1-integrins have known to provide a co-stimulus for TCR/CD3-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 cells compared with peripheral T-cells. The transfection of Cas-L cDNA into Jurkat cells restored the beta1-integrin-mediated co-stimulation, while the transfection of Cas-LDeltaSH3 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 beta1-integrin-mediated T-cell co-stimulation. Taken together, Cas-L might be the bi-modal docking protein that assembles the signals through beta1-integrins and TCR/CD3, and participates in a variety of T-cell functions.