A Critical Role for the Bcl-2 Pathway in Notch-Mediated B Cell Apoptosis, Where Growth Arrest Is Independent of the E2A, IL-3 and p21Waf1/p27Kip1 Pathways.

The Notch receptor pathway regulates critical cell fate decisions in multiple developmental systems, including hematopoiesis. We have previously demonstrated that Notch signaling induces growth arrest and apoptosis in a wide range of human B cell malignancies and has potential as a B cell-specific therapeutic approach. In order to identify the mechanisms of growth arrest and apoptosis we analyzed an immortalized murine progenitor B cell line derived from Bax/Bak double knockout mice. These cells are unable to undergo apoptosis since they lack the pro-apoptotic effectors of the Bcl-2 pathway, and have been shown to be resistant to multiple apoptotic stimuli.

Here we report that induction of Notch signaling through expression of several family members (Notch1, Notch4, Hes1) leads to rapid growth arrest, but not apoptosis, within 48 hours in these Bax-/Bak- progenitor B cells. These findings provide the first evidence for a critical role of the Bcl-2 pathway in Notch-mediated B cell apoptosis, and establish a mitochondrial-dependent mechanism for this effect. Importantly, the kinetics of growth arrest are accelerated with the expression of the Notch downstream target Hes1 as compared to the Notch receptors 1 and 4. These results extend our observation that Hes1 is sufficient to reproduce Notch-mediated B cell death, by demonstrating that Hes1 is more proximal to the critical growth inhibiting events, and may therefore provide a therapeutic target.

In this model system we can isolate growth arrest from the effects on the apoptotic cascade. This provides a unique opportunity to explore the mechanism of Notch-mediated growth arrest. Prior studies have suggested that Notch signaling may induce growth arrest through inhibition of the E2A pathway, or through upregulation of the cell cycle regulators p21Waf1 and p27Kip1. In this model system, inhibition of the E2A pathway is not sufficient to induce growth arrest. Similarly, Hes1 does not upregulate either p21Waf1 or p27Kip1, suggesting that this is not the mechanism of growth arrest. To explore whether Notch/Hes1 induce growth arrest through inhibition of the IL-3 pathway, we compared phenotypic and functional aspects of Hes1 expression and IL-3 withdrawal. Although the timing and phenotypic effects (cell size, cell cycle and metabolic studies) were quite similar, Hes1 growth arrested cells lose their ability to migrate in response to the pan-B chemo-attractant SDF1a compared to IL-3 withdrawn cells.

In summary, these results demonstrate that Notch/Hes1-mediated B cell apoptosis relies critically on pro-apoptotic members of the Bcl-2 pathway, Bax/Bak. Furthermore, growth arrest when isolated from apoptosis does not rely on inhibition of the E2A or IL-3 pathways, nor upregulation of p21Waf1/ p27Kip1. These findings provide the first insight into the mechanisms of Notch/Hes1-mediated B cell growth arrest and apoptosis and will help guide the development of Notch/Hes1 signaling as a cell-type specific therapeutic approach for B cell malignancies.