The Epstein-Barr virus protein BRLF1 activates S phase entry through E2F1 induction

The Epstein-Barr Virus (EBV) immediate-early protein BRLF1 is one of two tr ansactivators which mediate the switch from latent to lytic replication in EBV-infected cells. DNA viruses often modulate the function of critical cel l cycle proteins to maximize the efficiency of virus replication. Here we h ave examined the effect of BRLF1 on cell cycle progression. A replication-d eficient adenovirus expressing BRLF1 (AdBRLF1) was used to infect normal hu man fibroblasts and various epithelial cell lines. BRLF1 expression induced S phase entry in contact-inhibited fibroblasts and in the human osteosarco ma cell line U-2 OS. AdBRLF1 infection produced a dramatic increase in the level of E2F1 but not E2F4, In contrast, the levels of Rb, p107, and p130 w ere decreased in AdBRLF1-infected cells. Electrophoretic mobility shift ass ays confirmed an increased level of free E2F1 in the AdBRLF1-inFected human fibroblasts. Consistent with the previously described effect of E2F1, AdBR LF1-infected fibroblasts had increased levels of p53 and p21 and died by ap optosis. BRLF1-induced activation of E2F1 may be required for efficient EBV lytic replication, since at least one critical viral replication gene (the viral DNA polymerase) is activated by E2F (Ct Liu, N. D. Sista, and J. S. Pagano, J. Virol. 70:2545-2555, 1996).