SummaryWe previously reported that LPS activates the transcription of the IL-10 gene through the Sp1 and C/EBP binding sites and indicated that Sp1, C/EBPb and C/EBPd can coactivate the IL-10 gene expression in mouse macrophage cells [Liu Y.-W., Tseng H.-P., Chen L.-C., Chen B.-K., Chang W.-C. J. Immunol. 171: 821-828, 2003]. In the present report, we demonstrated the direct physical interaction between C/EBPd and Sp1, and also mapped the interaction domains of these two proteins. C/EBPd binds to Sp1 via its basic region leucine zipper domain. The C-terminus of Sp1 was also the major region interacting with C/EBPd. However, both glutamine-and serine/threonine-rich homologus regions of Sp1 also interacted with C/ EBPd. The binding of Sp1 and C/EBPd as a complex to the Sp1 binding site on the promoter of IL-10 was further confirmed by using the DNA affinity precipitation assay. By using Sp1-deficient SL2 cells, we also found that the overexpressions of C/EBPd and Sp1 synergically activate the transcriptional activity of IL-10 gene. Taken together, our present results revealed a novel mechanism of a superactivation of Sp1 by C/ EBPd via a direct interaction between these two transcription factors leading to the activation of the IL-10 gene in mouse macrophage cells.
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