Cooperative binding and synergistic activation by RelA and C/EBPβ on the intercellular adhesion molecule-1 promoter

Intercellular adhesion molecule-1 (ICAM-1) is up-regulated on numerous cell types in response to inflammatory cytokines. Tumor necrosis factor-α (TNF-α) activates the ICAM-1 promoter through a variant nuclear factor-κB (NF-κB) site at -187/-178 bp upstream of the transcription start site. In this investigation, we provide biochemical and functional evidence that an adjacent CCAAT/enhancer binding protein (C/EBP) site and this variant NF-κB site define a composite element for activation of the ICAM-1 promoter in certain cell lines. We detected an endogenous TNF-α-inducible DNA-protein complex in nuclear extracts from A549, HeLa, and EVC304 cells that contained both RelA and C/EBPβ but not other family members. Complex formation required intact C/EBP and NF-κB sites and was absolutely dependent on translocation of RelA into the nucleus. Complex formation and cooperative binding were also demonstrated using recombinant proteins, and as above, both binding sites were necessary. Interestingly, the RelA/C/EBPβ complex was not detected in either Jurkat or Raji cells, indicating cell type specificity. Functional studies with various reporter gene constructs revealed that both binding sites were required for maximal activation of the ICAM-1 promoter in response to TNF-α and for synergistic activation by RelA and C/EBPβ. This is the first detailed analysis of how RelA and C/EBPβ function to regulate ICAM-1 expression, and this study has important implications for how this gene is activated in specific cell types.