NF-κB Serves as a Cellular Sensor of Kaposi's Sarcoma-Associated Herpesvirus Latency and Negatively Regulates K-Rta by Antagonizing the RBP-Jκ Coactivator

Successful viral replication is dependent on a conducive cellular environment; thus, viruses must be sensitive to the state of their host cells. We examined the idea that an interplay between viral and cellular regulatory factors determines the switch from Kaposi's sarcoma-associated herpesvirus (KSHV) latency to lytic replication. The immediate-early gene product K-Rta is the first viral protein expressed and an essential factor in reactivation; accordingly, this viral protein is in a key position to serve as a viral sensor of cellular physiology. Our approach aimed to define a host transcription factor, i.e., host sensor, which modulates K-Rta activity on viral promoters. To this end, we developed a panel of reporter plasmids containing all 83 putative viral promoters for a comprehensive survey of the response to both K-Rta and cellular transcription factors. Interestingly, members of the NF-kappa B family were shown to be strong negative regulators of K-Rta transactivation for all but two viral promoters (Ori-RNA and K12). Recruitment of K-Rta to the ORF57 and K-bZIP promoters, but not the K12 promoter, was significantly impaired when NF-kappa B expression was induced. Many K-Rta-responsive promoters modulated by NF-kappa B contain the sequence of the RBP-J kappa binding site, a major coactivator which anchors K-Rta to target promoters via consensus motifs which overlap with that of NF-kappa B. Gel shift assays demonstrated that NF-kappa B inhibits the binding of RBP-J kappa and forms a complex with RBP-J kappa. Our results support a model in which a balance between K-Rta/RBP-J kappa and NF-kappa B activities determines KSHV reactivation. An important feature of this model is that the interplay between RBP-J kappa and NF-kappa B on viral promoters controls viral gene expression mediated by K-Rta.