Characterization of the G protein-coupled receptor kinase 6 promoter reveals a functional CREB binding site

Background G protein-coupled receptor kinase 6 (GRK6) is part of the G protein-coupled receptor kinase family, whose members act as key regulators of seven-transmembrane receptor signalling. GRK6 seems to play a role in regulation of inflammatory processes, but mechanisms of transcriptional regulation of GRK6 expression in inflammatory cell lines have not been characterized. Protein kinase C (PKC) signalling is also involved in inflammatory regulation and an impact of PKC activation on GRK6 protein expression was described previously. Thus, the aim of this study was to 1) characterize the GRK6 promoter, and 2) investigate a potential influence of PKC on GRK6 expression. Methods Five deletion constructs of the GRK6 promoter were cloned. After transient transfection into a human T cell line, promoter activity was assessed using luciferase reporter gene assays. Putative transcription factor binding sites were identified, mutated, and binding was investigated using electrophoretic mobility shift assays (EMSA). Following stimulation with a PKC activator, GRK6 expression on mRNA and protein levels was assessed by reverse transcriptase qPCR and Western blots. Results Investigation of the GRK6 promoter revealed a putative cAMP responsive element (CRE), whose mutation led to decreased promoter activity (p = 0.0006). Functionality of the CRE binding protein (CREB) binding site was verified in EMSA blots. Stimulation with a PKC activator resulted in decreased GRK6 promoter activity (p = 0.0027), mRNA (p = 0.04) and protein expression. Conclusion We characterized the human GRK6 promoter and identified promoter activity to be influenced by a CREB binding site. PKC might be one determinant contributing to altered GRK6 expression.

Interaction between CCAAT/Enhancer Binding Protein and Cyclic AMP Response Element Binding Protein 1 Regulates Human Immunodeficiency Virus Type 1 Transcription in Cells of the Monocyte/Macrophage Lineage

ABSTRACT Recent observations have shown two CCAAT/enhancer binding protein (C/EBP) binding sites to be critically important for efficient human immunodeficiency 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 brain. Additionally, sequence variation at C/EBP site I, which lies immediately upstream of the distal nuclear factor kappa B site and immediately downstream of a binding site for activating transcription factor (ATF)/cyclic AMP response element binding protein (CREB), has been shown to affect HIV-1 long 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 binding site could affect C/EBP protein binding to C/EBP site I. Electrophoretic mobility shift analyses indicated that selected ATF/CREB site variants assisted in the recruitment of C/EBP proteins to an adjacent, naturally occurring, low-affinity C/EBP site. This biophysical interaction appears to occur via at least two mechanisms. First, low amounts of CREB-1 and C/EBP appear 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 strong C/EBP site leads to enhanced CREB-1 recruitment to ATF/CREB sites that are weakly bound by CREB. Sequence variation at both C/EBP and ATF/CREB sites affects the molecular interactions involved in mediating both of these mechanisms. 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. Thus, HIV-1 LTR ATF/CREB binding site sequence variation may modulate cellular signaling at the viral promoter through the C/EBP pathway.