No abstract

The integral membrane adaptor protein, linker for activation of T cells (LAT), 1 is a substrate of the protein tyrosine kinases activated by engagement of the T cell antigen receptor (TCR) (1, 2). Upon LAT tyrosine phosphorylation, multiple adaptor molecules and enzymes critical to T cell activation bind to LAT directly by virtue of the SH2 domains that these proteins contain. These proteins can also bring other associated molecules to these LAT-nucleated complexes. Co-immunoprecipitation experiments have demonstrated that following TCR binding, phosphorylated LAT associates with Grb2, Gads, SOS, PLC-␥1, Vav, SLP-76, Cbl, and the p85 subunit of phosphatidylinositol 3-kinase. The enzymes recruited by LAT can themselves be activated in these complexes by tyrosine phosphorylation and find higher concentrations of their substrates in the plasma membrane. In particular, phospholipase-C␥ is activated by tyrosine phosphorylation to cleave phosphoinositides into diacylglycerol, an activator of protein kinase C, and inositol-1,4,5-triphosphate (IP 3 ), which induces intracellular calcium elevation (3). Tyrosine phosphorylation of Vav, the guanine nucleotide exchange factor for the small G proteins Rac, Rho, and cdc42, also activates its enzymatic activity (4). Phosphatidylinositol 3-kinase and SOS, the guanine nucleotide exchange factor for Ras, are not activated by tyrosine phosphorylation in T cells but are recruited to the membrane following receptor engagement via interaction with adaptor proteins (5, 6).The central role of LAT in T cell activation has also been demonstrated in several other experiments. The LAT gene has been disrupted by gene targeting experiments, and intrathymic development in the resulting mice is completely blocked at an early stage (7). Thus receptor-mediated events in immature T cells are dependent on the presence of LAT, and presumably, the molecules LAT recruits in these cells. In Jurkat T cells, overexpression of a mutant form of LAT lacking two tyrosine residues predicted to bind Grb2 had a dominant negative effect on T cell signaling (1). Following TCR activation of cells expressing this mutant, a number of signaling molecules failed to bind to the mutant LAT molecule, and TCR-coupled transcriptional events, AP-1 and NF-AT activation, were partially disrupted. The functional significance of the LAT molecule in signaling events coupled to the TCR was clearly demonstrated in the study of two independently derived Jurkat T cell variants (8,9). Both of these lines were shown to be deficient in LAT expression. TCR engagement in these two lines failed to induce tyrosine phosphorylation of PLC-␥1, calcium elevation, Ras and Erk activation, or transcriptional activation of AP-1 and NF-AT. These defects were corrected by expression of wild-type LAT.The importance of this molecule in regulating both intrathymic T cell development and activation of mature T cells, as modeled by the Jurkat cell line, led us to investigate the specificity of binding to LAT. Can the interaction of multiple signaling mo...

Linker for activation of B cells (LAB, also called NTAL; a product of wbscr5 gene) is a newly identified transmembrane adaptor protein that is expressed in B cells, NK cells, and mast cells. Upon BCR activation, LAB is phosphorylated and interacts with Grb2. LAB is capable of rescuing thymocyte development in LAT-deficient mice. To study the in vivo function of LAB, LAB-deficient mice were generated. Although disruption of the Lab gene did not affect lymphocyte development, it caused mast cells to be hyperresponsive to stimulation via the FcɛRI, evidenced by enhanced Erk activation, calcium mobilization, degranulation, and cytokine production. These data suggested that LAB negatively regulates mast cell function. However, mast cells that lacked both linker for activation of T cells (LAT) and LAB proteins had a more severe block in FcɛRI-mediated signaling than LAT−/− mast cells, demonstrating that LAB also shares a redundant function with LAT to play a positive role in FcɛRI-mediated signaling.

The adaptor molecule, linker for activation of T cells (LAT), is essential in T cell activation and development; a similar molecule in B cells has not yet been identified. Here, we report the identification of a new adaptor protein, linker for activation of B cells (LAB). Like LAT, LAB was localized to lipid rafts. Upon activation via the B cell receptor (BCR), LAB was phosphorylated and interacted with the adaptor protein Grb2. Decreased LAB expression led to a reduction in BCR-mediated calcium flux and Erk activation. LAB rescued thymocyte development but not normal T cell activation in LAT(-/-) mice. Our data suggest that LAB links BCR engagement to downstream signaling pathways.

The alpha-fetoprotein (AFP) gene is highly activated in fetal liver but is dramatically repressed shortly after birth. The mechanisms that underlie AFP transcriptional repression in postpartum liver are not well understood. AFP enhancer, repressor region, and promoter are implicated to be involved in AFP postnatal repression, but the major transcriptional repressor remains undefined. We previously identified a zinc finger protein gene ZBTB20. To determine its physiological functions in vivo, we have generated hepatocyte-specific ZBTB20 knockout mice by the Cre/loxP approach and demonstrated here that ZBTB20 ablation in liver led to dramatic derepression of the AFP gene in entire liver throughout adult life, although the hepatocytes were normally under nonproliferating status. Furthermore, we found that ZBTB20 was a transcriptional repressor capable of specifically inhibiting AFP promoter-driven transcriptional activity. Liver chromatin immunoprecipitation and mobility shift assays showed that ZBTB20 bound to AFP promoter directly. ZBTB20 was developmentally activated in liver after birth and inversely correlated with its AFP gene expression, suggesting that activated ZBTB20 expression in liver mediated AFP gene repression. Our data point to ZBTB20 as a key regulator governing AFP gene transcription and postulate a new model for the postnatal gene repression of AFP in liver.gene regulation ͉ transcription factor ͉ transcriptional repressor

Linker for activation of T cells (LAT) is a membrane-associated adaptor protein that is phosphorylated on multiple tyrosines upon TCR cross-linking. Previous studies show that LAT is essential for TCR-mediated signaling and thymocyte development. In this study, we expressed a series of LAT Tyr to Phe mutants in LAT-deficient J.CaM2.5 cells and examined their tyrosine phosphorylation; association with Grb2, Gads, and phospholipase C (PLC)-gamma1; and function in T cell activation. Our results showed that the five membrane-distal tyrosines were phosphorylated upon T cell activation. Grb2, Gads, and PLC-gamma1 associated with LAT preferentially via different sets of tyrosine residues; however, they failed to interact with LAT mutants containing only one tyrosine. We also determined the minimal requirement of LAT tyrosine residues in T cell activation and thymocyte development. Our results showed that a minimum of three tyrosines is required for LAT to function in T cell activation and thymocyte development. LAT mutants that were capable of binding Grb2 and PLC-gamma1 could reconstitute T cell activation in LAT-deficient cells and thymocyte development in LAT-deficient mice.

To estimate the distribution of lymphoid neoplasms in China, we conducted a comprehensive analysis, based on subtype, age, sex, and lesion, of primary and resected biopsy specimens of 4,638 lymphoid neoplasms diagnosed from 2004 to 2008 at 5 large hospitals. Of the 4,638 patients, mature B-cell neoplasms accounted for 64.3% of all lymphoid neoplasms, mature T/NK-cell neoplasms for 23.3%, and Hodgkin lymphoma for 8.6%. The most common subtype was diffuse large B-cell lymphoma (36.2%), followed by extranodal NK/T-cell lymphoma, nasal type (11.0%), classic Hodgkin lymphoma (8.4%), extranodal marginal zone B-cell lymphoma (7.7%), plasmacytic neoplasm (5.0%), and peripheral T-cell lymphoma, not otherwise specified (3.9%). For most lymphoid neoplasm subtypes, male subjects showed higher rates than female subjects. In summary, our study showed that the epidemiologic features of lymphoid neoplasms in China are distinct from those in Western countries and similar in many ways to those in other countries of the Far East.

FAP is highly expressed in carcinoma cells and fibroblasts in PDAC tissues, and its expression is associated with desmoplasia and worse prognosis.