Activated Notch1 can transiently substitute for EBNA2 in the maintenance of proliferation of LMP1-expressing immortalized B cells.

Epstein-Barr virus (EBV) nuclear antigen 2 (EBNA2) and latent membrane protein 1 (LMP1) are essential for immortalization of human B cells by EBV. EBNA2 and activated Notch transactivate genes by interacting with the cellular transcription factor RBP-Jkappa/CBF1. Therefore, EBNA2 can be regarded as a functional homologue of activated Notch. We have shown previously that the intracellular domain of Notch1 (Notch1-IC) is able to transactivate EBNA2-regulated viral promoters and to induce phenotypic changes in B cells similar to those caused by EBNA2. Here we investigated whether Notch1-IC can substitute for EBNA2 in the maintenance of B-cell proliferation. Using an EBV-immortalized lymphoblastoid cell line in which EBNA2 function can be regulated by estrogen, we demonstrate that murine Notch1-IC, in the absence of functional EBNA2, is unable to maintain LMP1 expression and to maintain cell proliferation. However, in a lymphoblastoid cell line expressing LMP1 independently of EBNA2, murine Notch1-IC can transiently maintain proliferation after EBNA2 inactivation. After 4 days, cell numbers do not increase further, and cells in the G2 phase of the cell cycle start to die. In contrast to EBNA2, murine Notch1-IC is unable to upregulate the expression of the c-myc gene in these cells.