Modulation of human cytomegalovirus immediate-early gene enhancer by mitogen-activated protein kinase kinase kinase-1.

The immediate-early (IE) promoter of human cytomegalovirus (HCMV) constitutes a primary genetic switch, which determines the progression of viral infection. Earlier reports by others have shown mitogen-activated protein kinase kinase kinase-1 (MEKK1) to be able to up-regulate HCMV-IE promoter through downstream mitogen-activated protein kinase (MAPK) pathways. However, we noticed that the activation of the HCMV-IE promoter by constitutively active MEKK1 (MEKK1-TRU) might not be through the MAPK pathways. Using a HCMV-IE enhancer/promoter (- 522 to + 72) driving a luciferase reporter, we demonstrated that the downstream MAPK activation actually repressed the up-regulation of the promoter by MEKK1 in CHO-K1 and human 293 cells. We further found that the up-regulation of HCMV-IE promoter by MEKK1 could be in great extent suppressed by over-expression of IkappaBalpha. Deletion of the NFkappaB/rel sites in the HCMV-IE enhancer region by mutagenesis proportionally reduced the transcriptional activation by MEKK1-TRU, whereas deletion of the ATF/CREB binding sites or cyclic AMP response elements (CRE) had no effects. Furthermore, the NFkappaB/rel deletion mutant also showed repression on the basic transcription activity of the HCMV-IE promoter. Our results indicate that the NFkappaB/rel sites are not only responsible for the modulation of HCMV-IE enhancer activity by MEKK1 but also control the basic transcription activity of the HCMV-IE promoter. On the other hand, the four consensus CRE sites were found to have no function in the activation of the promoter by MEKK1.