NF-IL6 (C/EBPβ) vigorously activates il1b gene expression via a Spi-1 (PU.1) protein-protein tether

Two classes of transcription factors, ETS and bZIP, stand out as key mediators of monocyte commitment and differentiation. The ETS domain factor Spi-1 (also called PU.1) and the bZIP factor NF-IL6 (also called C/EBP beta) have been shown to be involved in the transcriptional regulation of interleukin-1 beta gene (il1b) and other monocyte-specific genes. We now show that these two factors strongly cooperate on the il1b core promoter (-59/+12) in the absence of direct NF-IL6 binding to DNA. Transient transfection assays, using mutated il1b core promoters, showed that the Spi-1, but not the NF-IL6, binding site is absolutely required for functional cooperativity, Furthermore, the NF-IL6 transactivation domain (TAD) is functionally indispensable and more critical than that of Spi-1. Additionally, TAD-deficient NF-IL6 functions as a dominant negative for Spi-1-mediated activation, suggesting the involvement of the bZIP DNA binding domain. This is supported by the demonstration of in vitro interaction between the NF-IL6 bZIP and Spi-1 winged helix-turn-helix (wHTH) DNA binding domains, arguing that NF-IL6 vigorously activates the il1b core promoter via protein-tethered transactivation mediated by Spi-1.