Stimulation of the high-affinity IgE receptor results in the tyrosine phosphorylation of a 60 kD protein which is associated with the protein-tyrosine kinase, Csk

The inositol 5-phosphatase SHIP-1 and adaptors Dok-1 and 2 play central roles in CD4-mediated inhibitory signaling

CD4 functions to enhance the sensitivity of T cells to antigenic peptide/MHC class II. However, if aggregated in isolation, e.g. in the absence of T cell receptor (TCR), CD4 can transduce yet undefined signals that lead to T cell unresponsiveness to antigen and apoptosis. In Human Immunodeficiency Virus-1 (HIV-1) disease, CD4(+) T cell loss can result from gp120-induced CD4 signaling in uninfected cells. We show here that CD4 aggregation leads to Lck-dependent phosphorylation of the RasGAP adaptors Downstream of kinase-1/2 (Dok-1/2) and the inositol 5-phosphatase-1 (SHIP-1) and association of the two molecules. Studies using SHIP-1 shRNA, knockout mice and decoy inhibitors further indicate that CD4-mediated inhibition of TCR-mediated T cell activation is SHIP-1 and Dok-1/2 dependent, and involves SHIP-1 hydrolysis of Phosphatidylinositol 3,4,5-trisphosophate (PI(3,4,5)P3) needed for TCR signaling. Our studies provide evidence for a novel mechanism by which ill-timed CD4-mediated signals activated by ligands such as HIV-1 gp120 lead to disarmament of the immune system.

Adaptor proteins in lymphocyte activation

Adaptor proteins are unique, as they contain modular domains and lack intrinsic enzymatic activity. These proteins are scaffolds for the organization of macromolecular complexes and they recruit other proteins for correct localization during molecular signal transduction. Numerous recent advances have been made through the elucidation of new adaptor proteins and the recognition of novel functions for previously identified molecules. In addition, the roles of adaptors in both the positive and negative regulation of lymphocyte activation have been further clarified.

Adaptors as central mediators of signal transduction in immune cells

Adaptors are molecular scaffolds that recruit effectors, which are critical for immune cell activation. Recent work has underscored the requirement for adaptors in propagating stimulatory signals as well as their ability to inhibit immune cell function. The mechanisms by which adaptors function rely not only on the intermolecular interactions they mediate, but also on where they are localized within the cell. The use of sophisticated genetic, biochemical, cellular and imaging approaches has provided important new insights into the biology of adaptor protein function. Here we focus on T lymphocytes as a model to illustrate the critical roles adaptors play as regulators of cellular activation.

Inhibition of the motility and growth of B16F10 mouse melanoma cells by dominant negative mutants of Dok-1

Dok-1 (p62(Dok)) is a multiple-site docking protein that acts downstream of receptor and nonreceptor tyrosine kinases. Although it has been proposed to contribute to the control of cell growth and migration through association with the Ras GTPase-activating protein and the adapter protein Nck, the role of Dok-1 remains largely unknown. The functions of Dok-1 have now been investigated by the generation of two different COOH-terminal truncation mutants of this protein: one (DokPH+PTB) containing the pleckstrin homology and phosphotyrosine-binding domains, and the other (DokPH) composed only of the pleckstrin homology domain. Both of these mutant proteins were shown to act in a dominant negative manner. Overexpression of each of the mutants in highly metastatic B16F10 mouse melanoma cells thus both inhibited the tyrosine phosphorylation of endogenous Dok-1 induced by cell adhesion as well as reduced the association of the endogenous protein with cellular membranes and the cytoskeleton. Overexpression of DokPH+PTB in these cells also markedly reduced both the rates of cell spreading, migration, and growth as well as the extent of Ras activation. The effects of DokPH on these processes were less pronounced than were those of DokPH+PTB, indicating the importance of the phosphotyrosine-binding domain. These results suggest that at least in B16F10 cells, Dok-1 positively regulates not only cell spreading and migration but also cell growth and Ras activity.