| Research Abstract |
Productive infection/replication of herpesviruses usually occurs in growth arrested cells but there has been no direct evidence in case of Epstein-Barr virus (EBV) since an efficient lytic replication system without external stimuli does not exist for the virus. Expression of EBV lytic switch transactivator BZLF1 protein in EBV-negative epithelial tumor cell lines, however, is known to arrest cell cycle in G0/G1 by induction of the tumor suppressor protein, p53 and the cyclin dependent kinase inhibitors, p21WAF-1/CIP-1 and p27KIP-1 followed by the accumulation of hypophosphorylated form of Rb protein. In order to determine the effect of onset of the lytic viral replication on cellular events in EBV-latently infected B lymphoblastoid cell lines, a tightly controlled induction system of the EBV lytic replication program by inducible BZLF1 protein expression was established in B95-8 cells. The induction of the lytic replication completely arrested cell cycle progression and cellular DNA replication. Surprisingly, the levels of p53, p21WAF-1/CIP-1, and p27KIP-1 were constant before and after the lytic program induction, indicating that the cell cycle arrest induced by the lytic program is not mediated through p53 and the CDK inhibitors. Furthermore, although cellular DNA replication was blocked, elevation of cyclin E/A expression and accumulation of hyperphosphorylated forms of Rb protein were observed, being a post-G1/S phase characteristics of cells. Thus, while the EBV lytic program promoted specific cell cycle associated activities involved in the progression from G1 to S phase, it inhibited cellular DNA synthesis. Such cellular condition appears to most favors viral lytic replication.
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