Notch1 antiapoptotic activity is abrogated by caspase cleavage in dying T lymphocytes.

Excessive signaling via the Notch1 receptor inhibits apoptosis in T lymphocytes. Since several antiapoptotic proteins are cleaved by caspases during cell death, we investigated whether Notch1 was a caspase substrate. Results demonstrate that the intracellular domain of Notch1 (NICD) is cleaved into six fragments during apoptosis in Jurkat cells or peripheral T lymphocytes. Notch1 cleavage is prevented by the caspase inhibitors DEVD-fmk and VEID-fmk or by Bcl-2 expression. Caspase-3 and caspase-6 cleave the NICD into six fragments using sites located within the NF-kappaB binding domain, the ankyrin repeats and the transactivation domain. Notch1 cleavage correlates with the loss of HES-1 expression in apoptotic T cells. Notch1 fragments cannot inhibit activation-induced cell death in a T-cell hybridoma, confirming the abrogation of Notch1 antiapoptotic activity by caspases. The ability of the NICD but not the fragments to antagonize Nur77 activity supports a role for this factor in Notch1 antiapoptotic function.