TRIB2 regulates normal and stress-induced thymocyte proliferation.

TRIB2, a serine/threonine pseudokinase identified as an oncogene, is expressed at high levels in the T-cell compartment of hematopoiesis. The proliferation of developing thymocytes is tightly controlled to prevent leukemic transformation of T cells. Here we examine Trib2 loss in murine hematopoiesis under steady state and proliferative stress conditions, including genotoxic and oncogenic stress. Trib2 (-/-) developing thymocytes show increased proliferation, and Trib2 (-/-) mice have significantly higher thymic cellularity at steady state. During stress hematopoiesis, Trib2 (-/-) developing thymocytes undergo accelerated proliferation and demonstrate hypersensitivity to 5-fluorouracil (5-FU)-induced cell death. Despite the increased cell death post 5-FU-induced proliferative stress, Trib2 (-/-) mice exhibit accelerated thymopoietic recovery post treatment due to increased cell division kinetics of developing thymocytes. The increased proliferation in Trib2 (-/-) thymocytes was exacerbated under oncogenic stress. In an experimental murine T-cell acute lymphoblastic leukemia (T-ALL) model, Trib2 (-/-) mice had reduced latency in vivo, which associated with impaired MAP kinase (MAPK) activation. High and low expression levels of Trib2 correlate with immature and mature subtypes of human T-ALL, respectively, and associate with MAPK. Thus, TRIB2 emerges as a novel regulator of thymocyte cellular proliferation, important for the thymopoietic response to genotoxic and oncogenic stress, and possessing tumor suppressor function.

Use of the hollow fiber assay for the evaluation of DNA damaging agents.

INTRODUCTION: The preclinical development and clinical progression of potential anticancer agents are highly time and resource-intensive. Traditionally, promising compounds in vitro undergo further screening in xenograft models, a long process that uses large numbers of animals. In order to hasten compound progression, the hollow fiber assay (HFA) was developed by the US National Cancer Institute as an additional filtering step in drug development, bridging the gap between in vitro and xenograft compound screening. The HFA demonstrates a good correlation in terms of clinical predictivity, and has significant reduction and refinement benefits for animal usage. In addition, the assay enables the study of basic pharmacological properties of compounds under investigation. The HFA has been mainly used as a rapid in vivo cytotoxicity screen, but has also been shown to be amenable to study drug/target interactions in vivo. One of the challenges of the HFA is the small sample sizes obtained, which can limit sensitivity. METHODS: Here we specifically focus on the detection of DNA double-strand breaks, monitoring the effects of standard and novel anti-cancer agents on human lung, colon and breast cancer cell lines using immunoblotting and flow cytometry techniques for γ-H2A.X. This presented a further challenge due to the low abundance of the target event. RESULTS: We found a good correlation between techniques in terms of rate of detection and sensitivity confirming the ability to use the HFA for detection of these specific drug-target interactions. DISCUSSION: The results demonstrate good sensitivity and predictability for drug behavior in an assay where cell number is limited. In contrast to conventional xenograft studies, this short-term assay also enables analysis of pharmacodynamic endpoints in tumor cells in vivo. Importantly, there is a significant impact on reduction and refinement of the use of animals in incorporating this assay into the drug development process.

Leukemia-stimulated bone marrow endothelium promotes leukemia cell survival.

Extensive endothelial cell proliferation and marked neovascularization are the most pronounced microenvironmental changes consistently observed in the bone marrow (BM) of patients with acute lymphoblastic leukemia (ALL). It is not known whether ALL cells induce this phenotype and whether they receive critical signals from the tumor-associated BM endothelium. Here, we show that leukemia cells actively stimulate BM endothelium, promote de novo angiogenesis, and induce neovascularization in the leukemic BM. Soluble factors, present in the leukemic BM microenvironment, promote the proliferation, migration, and morphogenesis of BM endothelial cells, which are critical processes in tumor angiogenesis. We also show in vitro that leukemia cells display directional motion towards assembled BM endothelium and following adherence exhibit cell polarization, pseudopodia, and ultrastructural features that suggest the existence of leukemia-endothelium cross-talk. Finally, we show that BM endothelium promotes leukemia cell survival through a mechanism mediated through the anti-apoptotic molecule bcl-2. These studies indicate that ALL cells actively recruit BM endothelium and mediate the leukemia-associated neovascularization observed in ALL. Therefore, disruption of interactions between leukemia cells and BM endothelium may constitute a valid therapeutic strategy.

IL-7-dependent human leukemia T-cell line as a valuable tool for drug discovery in T-ALL.

The specific targeting of critical signaling molecules may provide efficient therapies for T-cell acute lymphoblastic leukemia (T-ALL). However, target identification and drug development are limited by insufficient numbers of primary T-ALL cells and by their high rate of spontaneous apoptosis. We established a human interleukin-7 (IL-7)-dependent T-ALL cell line, TAIL7, that maintains several biologic and signaling properties of its parental leukemia cells. TAIL7 cells are pre-T-ALL cells that proliferate in response to IL-7 and IL-4. IL-7 stimulation of TAIL7 cells prevents spontaneous in vitro apoptosis and induces cell activation and cell cycle progression. The signaling events triggered by IL-7 include down-regulation of p27(kip1) and hyperphosphorylation of retinoblastoma protein (Rb). Stimulation of TAIL7 cells by IL-7 leads to phosphorylation of Janus kinase 3 (JAK3), signal transducer and activator of transcription 5 (STAT5), Akt/PKB (protein kinase B), and extracellular-regulated kinase 1 and 2 (Erk1/2). Importantly, specific blockade of JAK3 by its inhibitor WHI-P131 abrogates the IL-7-mediated proliferation and survival of TAIL7 cells, suggesting that activation of JAK3 is critical for IL-7 responsiveness by these cells. Because TAIL7 cells seem to be a biologic surrogate for primary leukemia T cells, this cell line constitutes a valuable tool for the study of the signaling pathways implicated in T-ALL. Exploitation of this cell line should allow the identification of molecular targets and promote the rational design and validation of antileukemia signaling inhibitors.