Abstract
In human immunodeficiency virus type 1 (HIV‐1) latently infected cells, NF‐kappaB (NF‐κB) plays a critical role in the transcriptional induction of the HIV‐1 promoter. The trans‐activating ability of NF‐κB can be modified by another nuclear factor C/EBPβ that can physically bind to NF‐κB and regulate its activity. Because the HIV‐1 promoter also contains a C/EBPβ site adjacent to the NF‐κB site, the present study examined cooperative functional in vivo interaction of the p65 subunit of NF‐κB and C/EBPβ, and the impact of Tat in this event. We demonstrated that ectopic expression of p65 along with Tat increases p65 binding to HIV‐1 LTR, and that this increase correlates with enhanced HIV‐1 promoter activity. Further, co‐expression of C/EBPβ and Tat leads to a decrease in p65 binding, which allows C/EBPβ to bind more efficiently to the LTR. Inhibition of p65 expression by siRNA significantly decreases C/EBPβ‐binding and LTR expression. Using ChIP assay, we confirmed the existence of an interchange between p65 and C/EBPβ and their abilities to bind to the LTR in vivo. These observations demonstrate that a delicate balance of interaction between p65, C/EBPβ, and Tat can dictate the level of HIV‐1 LTR transcription. J. Cell. Biochem. 100: 1210–1216, 2007. © 2006 Wiley‐Liss, Inc.