Activation of the IKK/NF-κB signaling pathway is a hallmark of human cytomegalovirus (HCMV) infection and its importance in the viral replication has recently been the focus of several studies. However, its role in regulating major immediate-early promoter (MIEP)-dependent transcription and HCMV replication remains controversial. In this study, we used a combination of genetic approaches to investigate the effects of cell culture conditions on the importance of virus-induced NF-κB activation during the infection of endothelial cells or fibroblasts. Adenoviral-mediated expression of a dominant negative mutant of IKK2 kinase (dnIKK2) in human umbilical vein endothelial cells (HUVECs) resulted in a strong reduction of IκBα degradation and NF-κB activation following infection with an HCMV clinical isolate. Viral replication was impaired in dnIKK2-expressing cells that were growth-arrested before infection, but not in replicating cells. The inhibitory effect of dnIKK2 was independent from that of the virus strain and the cell type, since replication of the laboratory AD169 strain was impaired as well in dnIKK2-expressing quiescent fibroblasts. Moreover, progressive disruption of NF-κB response elements within the MIEP in recombinant HCMV viruses derived from the clinical isolate prevented their replication in quiescent cells but not in actively growing cells. These results indicate an essential role of virus-induced IKK/NF-κB activity to trigger both viral IE gene expression and productive replication in quiescent cells.
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