Regulation and Function of NOTCH2 in B-CLL Cells.

B-cell chronic lymphocytic leukemia (B-CLL) represents a clonal expansion of self-reactive CD5+ B-lymphocytes which seems to be resistant to apoptosis in vivo. One of the characteristics of B-CLL lymphocytes is the high expression of the B-cell differentiation/activation marker CD23 which we recently identified as a target gene for NOTCH2 signaling. NOTCH2 is implicated in the development/homeostasis of murine CD5+ (B-1a) B-cells, suggesting a function for human NOTCH2 in B-CLL leukemogenesis. Here we show that peripheral B-CLL lymphocytes overexpress a transcriptionally active form of NOTCH2 (N2^IC) irrespective of their prognostic marker profile (ie. IgVH mutational status, CD38 expression, and cytogenetics). Although the majority of unstimulated B-CLL samples downregulate their N2^IC activity in vitro, DNA-bound N2^IC complexes could be maintained by the protein kinase C (PKC) activator TPA (12-O-tetradecanoylphorbol 13-acetate) accompanied by an upregulation of the NOTCH2 target gene CD23 and increased B-CLL cell viability. These effects are sensitive to the PKC-δ selective inhibitor Rottlerin. In 80% of B-CLL cases, NOTCH2 signaling was found to be resistant to the γ-secretase inhibitors (GSI’s) Dapt and compound E, indicative for the expression of truncated forms of NOTCH2 which do not require γ-secretase for processing and function. Inhibition of NOTCH2 either by Dapt in GSI sensitive B-CLL cases or, more specifically, by RNA interference downregulates CD23 expression on the mRNA and protein level and sensitizes B-CLL cells for apoptosis. Since self-reactive B-cells are normally eliminated either by chronic (anergy) or apoptotic (negative selection) B-cell receptor (BCR) signaling, we asked whether NOTCH2 modulates B-cell fate decisions triggered by the BCR. For this reason, we stably transduced the human B-cell line BL41 with constitutive active forms of NOTCH2 and found that NOTCH2 inhibits BCR mediated apoptosis induced by surface-IgM cross-linking. In summary, the data demonstrate that NOTCH2 signaling is deregulated in B-CLL cells and might be critically involved in the PKC-dependent maintenance of their malignant phenotype.