Lef-1: NOTCHed up in T-cell lymphomas

A potential interaction between Notch and Wnt, 2 signaling pathways essential for T-cell development, opens a new perspective for unraveling the molecular basis of leukemia. Activation of Notch signaling by Delta-like 1 (DLL) ligands is required for commitment of early thymic progenitors to the T-cell lineage. Notch signaling is also required for the double negative 3a (DN3a) to DN3b thymocyte transition.¹  On the other hand, Wnt signaling, mainly through TCF-1, is required for the development of early thymic progenitors as well as the progression through β-selection up until the immature single positive (ISP) stage.^2,3 

Deregulated activation of Notch is etiologically linked to T-ALL both in humans and in animal models. Over 50% of all examined human T-ALL cases spanning the entire spectrum of the identified T-ALL subtypes bear Notch-activating mutations. Moreover, transgenic expression of activated Notch1 or Notch3 results in thymocyte transformation involving modulation of pre–T-cell receptor (pre-TCR) signaling, inhibition of the E2A pathway, and up-regulation of c-Myc as well as E2A-PBX. Conversely, lymphomas in E2A- or p53-deficient mice or in mice expressing activated Tal1/SCL require or select for activation of Notch.^4,5  Although activation of Notch1 is a frequent event in T-cell leukemia, the etiology of the cases that do not depend on it remains elusive.⁶ 

We have recently provided evidence for a Notch-independent path to T-cell leukemia involving the deregulated activation of β-catenin, which binds to Lef-1/Tcf-1 and activates transcription in response to Wnt signals. Conditional activation of this protein leads to CD4⁺CD8⁺ double-positive (DP) lymphomas requiring Rag activity, and additional secondary genetic events, including the up-regulation of c-Myc, whose activity is required for transformation. Unlike other T-cell leukemia models, the β-catenin–induced lymphomas do not select for Notch-activating mutations or up-regulation of Notch target genes.⁷  This finding leads us to speculate that the Wnt/β-catenin pathway may be etiologically linked with leukemias that do not show Notch-activating mutations.

The report by Spaulding and colleagues raises the possibility that the 2 pathways may actually cooperate, with Notch transcriptionally regulating Lef-1, a downstream effector of the Wnt signaling pathway. In this elegant study, these authors used a number of in vivo and in vitro approaches to show that expression of Lef-1, which is elevated in T-cell lymphomas, is directly regulated by Notch and that Notch is present at the Lef-1 promoter. Interestingly, Notch-dependent regulation of Lef-1 was detected in lymphomas expressing activated forms of Notch, but not in primary T-lineage cells. This probably reflects a high threshold level of Notch activity necessary to overcome the low affinity of CSL for the promoter of Lef-1. Lef-1 activity is functionally significant for the maintenance of lymphoma cell lines because suppression of Lef-1 by virally expressed shRNAs results in increased levels of apoptosis. These findings are in line with a recent report showing elevated levels of Lef-1 in human acute lymphocytic leukemias.⁸ 

Crosstalk between Notch and Wnt signaling has been proposed for multiple developmental checkpoints and organisms. It has also been reported that Lef-1 may induce the expression of DLL. Spaulding and colleagues provide evidence for a direct mechanism of crosstalk between these pathways that may not involve extracellular Wnt signals but does result in transcriptional up-regulation of Lef-1, a downstream component of the Wnt cascade. Lef-1 exerts its function in at least 3 ways: first, by bending DNA and allowing distant regulatory proteins on a gene to come into proximity; second, as a transcriptional activator in complex with β-catenin; and third, as a transcriptional repressor with Groucho and in the absence of β-catenin. The current study raises the question of whether the role of Lef-1 in leukemia involves its interaction with β-catenin. On the one hand, we have recently reported that stabilization of β-catenin alone leads to Notch-independent leukemia. On the other hand, Spaulding and colleagues now show that Lef-1 is directly regulated by Notch. This invites the provocative speculation that β-catenin may play a role in the etiology of Notch-dependent leukemia as well. Bearing in mind that all these observations need to be confirmed in human leukemia, they open new perspectives for unraveling the crosstalk of oncogenic pathways and designing new, targeted therapies for T-cell leukemia.