Notch signaling is a highly conserved mechanism of intercellular communication that controls the developmental fate in all animal species studied to date. Specific transmembrane ligands activate Notch receptors on neighboring cells, thereby inducing proteolytic cleavage and nuclear translocation of the Notch intracellular domain (Notch^IC). Notch^IC associates with the transcriptional repressor RBP-J (recombination recognition sequence binding protein at the Jĸ site), also known as CSL [CBF1/Su(H)/Lag-1], and converts it to an activator. In conjunction with chromatin remodeling enzymes, components of the transcriptional machinery and the activity of other cofactors, Notch^IC induces transcription of downstream target genes, including genes of the Hes (hairy and enhancer of split) and Hey (also called Hes-related repressor Herp, Hesr, Hrt, CHF, gridlock) family. Recent evidence has shown that the Notch pathway is involved in multiple aspects of hematopoietic development. In this review, we summarize the current knowledge of the components and mechanisms of the Notch signaling pathway and discuss the role of Notch in embryonic and adult myelopoiesis. Finally, we will focus on mediators of Notch signaling in the hematopoietic system. We propose that besides suppression of differentiation mediated by the Hes/Hey family, Notch/ RBP-J signaling mediates lineage decisions by direct activation of transcription factors such as PU.1, that are critically involved in directing cells along certain cell lineages, and further influences maturation by activation of functional genes, for example β-globin.

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Signaling mediated by activation of the transmembrane receptor Notch influences cell-fate decisions, differentiation, proliferation, and cell survival. Activated Notch reduces proliferation by altering cell-cycle kinetics and promotes differentiation in hematopoietic progenitor cells. Here, we investigated if the G(1) arrest and differentiation induced by activated mNotch1 are dependent on tumor suppressor p53, a critical mediator of cellular growth arrest. Multipotent wild-type p53-expressing (p53(wt)) and p53-deficient (p53(null)) hematopoietic progenitor cell lines (FDCP-mix) carrying an inducible mNotch1 system were used to investigate the effects of proliferation and differentiation upon mNotch1 signaling. While activated Notch reduced proliferation of p53(wt)-cells, no change was observed in p53(null)-cells. Activated Notch upregulated the p53 target p21(cip/waf) in p53(wt)-cells, but not in p53(null)-cells. Induction of the p21(cip/waf) gene by activated Notch was mediated by increased binding of p53 to p53-binding sites in the p21(cip/waf) promoter and was independent of the canonical RBP-J binding site. Re-expression of p53(wt) in p53(null) cells restored the inhibition of proliferation by activated Notch. Thus, activated Notch inhibits proliferation of multipotent hematopoietic progenitor cells via a p53-dependent pathway. In contrast, myeloid and erythroid differentiation was similarly induced in p53(wt) and p53(null) cells. These data suggest that Notch signaling triggers two distinct pathways, a p53-dependent one leading to a block in proliferation and a p53-independent one promoting differentiation.