An investigation into the role of CD45 isoforms in T cell antigen receptor signal transduction was carried out by transfecting CD45-negative CD4؉ CD8 ؉ HPB-ALL T cells with the CD45R0, CD45RBC, and CD45RABC isoforms. Fluorescence resonance energy transfer analysis showed that the CD45R0 isoform, but not the CD45RBC or CD45RABC isoforms, was found as homodimers and also preferentially associated with CD4 and CD8 at the cell-surface. A comparison was therefore made of T cell antigen receptor signaling between sub-clones expressing either CD45R0 or CD45RBC. Under basal conditions CD4-associated p56 lck tyrosine kinase activity and cellular protein tyrosine phosphorylation levels were higher in the CD45R0 ؉ than in the CD45RBC ؉ sub-clones. The CD45 tyrosine phosphatase regulates the threshold of T cell antigen receptor (TCR) 1 signaling by modulating the actions of the p56 lck and p59 fyn tyrosine kinases (1-4). The p56 lck kinase associates with the CD4 and CD8 co-receptors and initiates TCR signaling cascades by phosphorylation of immunoreceptor tyrosine-based activation motifs located in the TCRand CD3-⑀ chains (5, 6). In CD45 Ϫ/Ϫ mice T cell development is severely affected (7,8), and elevated TCR signaling thresholds are marked by dysfunctional p56 lck , together with defective TCR-chain phosphorylation and ZAP-70 recruitment (9). CD45 exerts a positive effect on p56 lck actions by de-phosphorylating the inhibitory C-terminal pTyr-505 (10 -13), but can also negatively regulate the kinase by dephosphorylating the pTyr-394 autophosphorylation site (14, 15). The actions of CD45 on p56 lck in CD4 ϩ T cells are directed selectively at the CD4-associated pool of the kinase (16). The dominant effect of CD45 in the T-lineage appears to be at pTyr-505, since T cell development in CD45 Ϫ/Ϫ mice can be largely restored by backcrossing to mice expressing the mutant lck Y505F -active transgene (17,18).Alternative splicing generates up to eight different CD45 isoforms (19,20) of which five are expressed at significant levels in T cells (21). All T cells express more than one CD45 isoform, and differential isoform expression is tightly controlled during thymic development and the activation of mature T cells (reviewed in Ref.1). However, the molecular consequences of differential CD45 isoform expression for TCR signaling, if any, are not yet well understood. Reconstitution of a murine CD45 Ϫ cell line with CD45 isoforms suggested that the CD45R0 isoform (lacking the A, B, and C exon-encoded segments of the ectodomain) promoted greater interleukin-2 secretion upon engagement of the TCR with the cognate major histocompatibility complex-peptide as compared with other isoforms (22). Co-capping experiments in these cells revealed preferential CD4-CD45R0 association (23). However, more detailed capping and co-immunoprecipitation studies indicated a basal association of CD45R0 with the TCR independent of CD4 expression and suggested that co-capping of CD4 with CD45R0 was mediated by this prior CD45R0-TCR association (24). Neverthel...

The CD45 tyrosine phosphatase lowers T-cell antigen receptor signalling thresholds by its positive actions on p56(lck) tyrosine kinase function. We now show that mice expressing active lck(F505) at non-oncogenic levels develop aggressive thymic lymphomas on a CD45(-/-) background. CD45 suppresses the tumorigenic potential of the kinase by dephosphorylation of the Tyr394 autophosphorylation site. In CD45(-/-) thymocytes the kinase is switched to a hyperactive oncogenic state, resulting in increased resistance to apoptosis. Transformation occurs in early CD4(-)CD8(-) thymocytes during the process of TCR-beta chain rearrangement by a recombinase-independent mechanism. Our findings represent the first example in which a tyrosine phosphatase in situ prevents the oncogenic actions of a SRC: family tyrosine kinase.

In order to develop new cancer therapeutics, rapid, reliable, and relevant biological models are required to screen and validate drug candidates for both efficacy and safety. In recent years, the zebrafish (Danio rerio) has emerged as an excellent model organism suited for these goals. Larval fish or immunocompromised adult fish are used to engraft human cancer cells and serve as a platform for screening potential drug candidates. With zebrafish sharing ~80% of disease-related orthologous genes with humans, they provide a low cost, high-throughput alternative to mouse xenografts that is relevant to human biology. In this review, we provide background on the methods and utility of zebrafish xenograft models in cancer research.

Resistance to chemotherapy is a major cause of treatment failure and poor overall survival in patients with lung cancer. Identification of molecular targets present in resistant cancer cells is essential for addressing therapeutic resistance and prolonging lung cancer patient survival. Members of the B-cell lymphoma 2 (Bcl-2) family of proteins are associated with chemotherapeutic resistance. In this study, we found that pro-survival protein Bcl-2 is upregulated in paclitaxel resistant cells, potentially contributing to chemotherapy resistance. To exploit the increase in Bcl-2 expression for targeting therapy resistance, we investigated the effects of a peptide derived from the nuclear receptor Nur77 that converts Bcl-2 from an anti-apoptotic protein to a pro-apoptotic protein. The Nur77 derived peptide preferentially induced apoptosis in paclitaxel-resistant cancer cells with high expression of Bcl-2. This peptide also induced apoptosis of multidrug resistant H69AR lung cancer cells that express Bcl-2 and inhibited their growth in 3D spheroids. The Nur77 peptide strongly suppressed the growth of paclitaxel-resistant lung cancer cells in a zebrafish xenograft tumor model. Taken together, our data supports a new strategy for treating lung cancers that acquire resistance to chemotherapy through overexpression of Bcl-2.

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