Involvement of PKC-δ in the Regulation of Notch2 in B-CLL.

We have recently shown that NOTCH2 signaling is involved in the overexpression of CD23 in B-cell chronic lymphocytic leukemia (B-CLL) cells (Hubmann et al., BLOOD 2002). There is an increasing evidence that NOTCH2 plays a determining role in the development/homeostasis of self-reactive CD5+ B-cells, suggesting a potential function of NOTCH2 in B-cell leukemogenesis. Here we study the regulation of NOTCH2 signaling in B-CLL cells and its possible function in B-lymphocytes using NOTCH2 transduced BL41 cells as a model system. Cultured B-CLL samples (n=30) lose their nuclear NOTCH2 (N2IC) activity within one day as demonstrated by electrophoretic mobility shift assays (EMSA). However, DNA-bound NOTCH2 complexes could be maintained in culture by exposure to the phorbolester TPA and is accompained by increased cell viability. The effect of TPA is prevented by the PKC-δ inhibitor Rottlerin indicating that PKC-δ is involved in the regulation of NOTCH2 signaling in B-CLL cells. The activity of N2IC in the leukemic cells appeared to be resistant to the γ-secretase inhibitors DAPT and compound E, two substances known to block ligand mediated release of the NOTCH2 intracellular domain (N2IC). Since B-CLL cells are locked in an anergic state, we next asked whether NOTCH2 modulates B-cell receptor (BCR) signaling and found that retrovirally transduced N2IC rescues the B-cell line BL41 from surface immunoglobulin M (sIgM) mediated apoptosis, a mechanism proposed to prevent the uncontrolled expansion of self-reactive CD5+ B-cells. In summary, our data suggest that B-CLL cells express an activated form of NOTCH2 which might be involved in the protection of the malignant clone from peripheral negative selection.