Interaction between CCAAT/enhancer binding protein and cyclic AMP responseelement binding protein 1 regulates human immunodeficiency virus type 1 transcription in cells of the monocyte/macrophage lineage

Recent observations have shown two CCAAT/enhancer binding protein (C/EBP) b inding sites to be critically important for efficient human immunodeficienc y virus type 1 (HIV-1) replication within cells of the monocyte/ macrophage lineage, a cell type likely involved in transport of the virus to the brai n. Additionally, sequence variation at C/EBP site I, which lies immediately upstream of the distal nuclear factor kappa B site and immediately downstr eam of a binding site for activating transcription factor (ATF)/cyclic AMP response element binding protein (CREB), has been shown to affect HIV-1 lon g terminal repeat (LTR) activity. Given that C/EBP proteins have been shown to interact with many other transcription factors including members of the ATF/ CREB family, we proceeded to determine whether an adjacent ATF/CREB b inding site could affect C/EBP protein binding to C/EBP site I. Electrophor etic mobility shift analyses indicated that selected ATF/CREB site variants assisted in the recruitment of C/EBP proteins to an adjacent, naturally oc curring, low-sanity C/EBP site. This biophysical interaction appears to occ ur via at least two mechanisms. First, low amounts of CREB-1 and C/EBP appe ar to heterodimerize and bind to a site consisting of a half site from both the ATF/CREB and C/EBP binding sites. In addition, CREB-1 homodimers bind to the ATF/CREB site and recruit C/EBP dimers to their cognate weak binding sites. This interaction is reciprocal, since C/EBP dimer binding to a stro ng C/EBP site leads to enhanced CREB-1 recruitment to ATF/CREB sites that a re weakly bound by CREB. Sequence variation at both C/EBP and ATF/CREB site s affects the molecular interactions involved in mediating both of these me chanisms. Most importantly, sequence variation at the ATF/CREB binding site affected basal LTR activity as well as LTR function following interleukin- 6 stimulation, a treatment that leads to increases in C/EBP activation. Thu s, HIV-1 LTR ATF/CREB binding site sequence variation may modulate cellular signaling at the viral promoter through the C/EBP pathway.