In human immunodeficiency virus type 1 (HIV-1) latently infected cells, NF-kappaB (NF-kappaB) plays a critical role in the transcriptional induction of the HIV-1 promoter. The trans-activating ability of NF-kappaB can be modified by another nuclear factor C/EBPbeta that can physically bind to NF-kappaB and regulate its activity. Because the HIV-1 promoter also contains a C/EBPbeta site adjacent to the NF-kappaB site, the present study examined cooperative functional in vivo interaction of the p65 subunit of NF-kappaB and C/EBPbeta, 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/EBPbeta and Tat leads to a decrease in p65 binding, which allows C/EBPbeta to bind more efficiently to the LTR. Inhibition of p65 expression by siRNA significantly decreases C/EBPbeta-binding and LTR expression. Using ChIP assay, we confirmed the existence of an interchange between p65 and C/EBPbeta and their abilities to bind to the LTR in vivo. These observations demonstrate that a delicate balance of interaction between p65, C/EBPbeta, and Tat can dictate the level of HIV-1 LTR transcription.
c 2006 Wiley-Liss, Inc.