Transcription factors of the bHLH and LIM families: synergistic mediators of T cell acute leukemia?

Aberrant signaling in T-cell acute lymphoblastic leukemia: biological and therapeutic implications

T-cell acute lymphoblastic leukemia (T-ALL) is a biologically heterogeneous disease with respect to phenotype, gene expression profile and activation of particular intracellular signaling pathways. Despite very significant improvements, current therapeutic regimens still fail to cure a portion of the patients and frequently implicate the use of aggressive protocols with long-term side effects. In this review, we focused on how deregulation of critical signaling pathways, in particular Notch, PI3K/Akt, MAPK, Jak/STAT and TGF-beta, may contribute to T-ALL. Identifying the alterations that affect intracellular pathways that regulate cell cycle and apoptosis is essential to understanding the biology of this malignancy, to define more effective markers for the correct stratification of patients into appropriate therapeutic regimens and to identify novel targets for the development of specific, less detrimental therapies for T-ALL.

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 p27kip1 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.

Combined expression of pTalpha and Notch3 in T cell leukemia identifies the requirement of preTCR for leukemogenesis

Notch receptors are conserved regulators of cell fate and have been implicated in the regulation of T cell differentiation and lymphomagenesis. However, neither the generality of Notch involvement in leukemia, nor the molecules with which Notch may interact have been clarified. Recently, we showed that transgenic mice expressing the constitutively active intracellular domain of Notch3 in thymocytes and T cells developed early and aggressive T cell neoplasias. Although primarily splenic, the tumors sustained features of immature thymocytes, including expression of pTalpha, a defining component of the pre T cell receptor, known to be a potent signaling complex provoking thymocyte survival, proliferation, and activation. Thus, enforced expression of Notch3, which is ordinarily down-regulated as thymocytes mature, may sustain pre T cell receptor expression, causing dysregulated hyperplasia. This hypothesis has been successfully tested in this article by the observation that deletion of pTalpha in Notch3 transgenic mice abrogates tumor development, indicating a crucial role for pTalpha in T cell leukemogenesis. Parallel observations were made in humans, in that all T cell acute lymphoblastic leukemias examined showed expression of Notch3 and of the Notch target gene HES-1, as well as of pTalpha a and b transcripts, whereas the expression of all these genes was dramatically reduced or absent in remission. Together, these results suggest that the combined expression of Notch3 and pTalpha sustains T cell leukemogenesis and may represent pathognomonic molecular features of human T-ALL.