Telfer, Janice C.

Abstract Notch signaling plays a role in normal lymphocyte development and function. Activating Notch1-mutations, leading to aberrant downstream signaling, have been identified in human T-cell acute lymphoblastic leukemia (T-ALL). While this highlights the contribution of Notch signaling to T-ALL pathogenesis, the mechanisms by which Notch regulates proliferation and survival in normal and leukemic T cells are not fully understood. Our findings identify a role for Notch signaling in G1-S progression of cell cycle in T cells. Here we show that expression of the G1 proteins, cyclin D3, CDK4, and CDK6, is Notch-dependent both in vitro and in vivo, and we outline a possible mechanism for the regulated expression of cyclin D3 in activated T cells via CSL (CBF-1, mammals; suppressor of hairless, Drosophila melanogaster; Lag-1, Caenorhabditis elegans), as well as a noncanonical Notch signaling pathway. While cyclin D3 expression contributes to cell-cycle progression in Notch-dependent human T-ALL cell lines, ectopic expression of CDK4 or CDK6 together with cyclin D3 shows partial rescue from γ-secretase inhibitor (GSI)-induced G1 arrest in these cell lines. Importantly, cyclin D3 and CDK4 are highly overexpressed in Notch-dependent T-cell lymphomas, justifying the combined use of cell-cycle inhibitors and GSI in treating human T-cell malignancies.

Abstract Notch and its ligands have been implicated in the regulation and differentiation of various CD4+ T-helper cells. Regulatory T cells (Tregs), which express the transcription factor Foxp3, suppress aberrant immune responses that are typically associated with autoimmunity or excessive inflammation. Previous studies have shown that transforming growth factor beta (TGFβ1) induces Foxp3 expression and a regulatory phenotype in peripheral T cells. Here, we show that pharmacologic inhibition of Notch signaling using γ-secretase inhibitor (GSI) treatment blocks (1) TGFβ1-induced Foxp3 expression, (2) the up-regulation of Foxp3-target genes, and (3) the ability to suppress naive T-cell proliferation. In addition, the binding of Notch1, CSL, and Smad to conserved binding sites in the foxp3 promoter can be inhibited by treatment with GSI. Finally, in vivo administration of GSI results in reduced Foxp3 expression and development of symptoms consistent with autoimmune hepatitis, a disease previously found to result from dysregulation of TGFβ signaling and regulatory T cells. Together, these findings indicate that the Notch and TGFβ signaling pathways cooperatively regulate Foxp3 expression and regulatory T-cell maintenance both in vitro and in vivo.