Notch signalling via RBP-J promotes myeloid differentiation.

The expression of Notch receptors on hematopoietic cells and of cognate ligands on bone marrow stromal cells suggests a possible role for Notch signalling in the regulation of hematopoiesis. In order to assess the involvement of Notch1 signalling in myelopoiesis, 32D myeloid progenitor cell lines were engineered to permit the conditional induction of the constitutively active intracellular domain of murine Notch1 (mN1(IC)) by the 4-hydroxytamoxifen-inducible system. The induction of mN1(IC) resulted in accelerated and increased granulocytic differentiation. These effects were observed under growth conditions that support differentiation and, to a lesser degree, under conditions that normally promote self-renewal. Transient transfection of mN1(IC) deletion mutants showed that the differentiation promoting activity correlated with RBP-J transactivation. Furthermore, expression of a transcriptionally active derivative of RBP-J (RBP-J-VP16) increased myeloid differentiation. To test further the role of Notch signalling in a physiological context, 32D cells expressing mNotch1 were cultured on fibroblast layers that either expressed or did not express the Notch ligand Jagged1. Similar to the induction of mN1(IC), Jagged1 accelerated granulocytic differentiation of 32D cells. Taken together, our data suggest that activation of mNotch1 promotes myeloid differentiation via RBP-J transactivation.