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 prot ein 1 (LMP1) are essential for immortalization of human B cells by EBV. EBN A2 and activated Notch transactivate genes by interacting with the cellular transcription factor RBP-J kappa /CBF1. Therefore, EBNA2 can be regarded a s a functional homologue of activated Notch. We have shown previously that the intracellular domain of Notch1 (Notch1-IC) is able to transactivate EBN A2-regulated viral promoters and to induce phenotypic changes in B cells si milar to those caused by EBNA2. Here we investigated whether Notch1-IC can substitute for EBNA2 in the maintenance of B-cell proliferation. Using an E BV-immortalized lymphoblastoid cell line in which EBNA2 function can be reg ulated by estrogen, we demonstrate that murine Notch1-IC, in the absence of functional EBNA2, is unable to maintain LMP1 expression and to maintain ce ll proliferation. However, in a lymphoblastoid cell line expressing LMP1 in dependently of EBNA2, murine Notch1-IC can transiently maintain proliferati on after EBNA2 inactivation. After 4 days, cell numbers do not increase fur ther, and cells in the G(2) phase of the cell cycle start to die. In contra st to EBNA2, murine Notch1-IC: is unable to upregulate the expression of th e c-myc gene in these cells.