C/EBPdelta (
CCAAT/enhancer binding protein delta) has been implicated as a regulator of
acute-phase response (APR) genes in hepatocytes. Its expression increases dramatically in liver during the APR and can be induced in hepatic cell lines by
interleukin-6 (IL-6), an acute-phase mediator that activates transcription of many APR genes. Here we have investigated the mechanism by which C/EBPdelta expression is regulated by
IL-6 in
hepatoma cells. C/EBPdelta promoter sequences to -125 bp are sufficient for
IL-6 inducibility of a reporter gene and include an APR
element (APRE) that is essential for
IL-6 responsiveness.
DNA binding experiments and transactivation assays demonstrate that Stat3, but not Stat1, interacts with this APRE. Two Sp1 sites, one of which is adjacent to the APRE, are required for
IL-6 induction and transactivation by Stat3. Thus, Stat3 and Sp1 function cooperatively to activate the C/EBPdelta promoter. Replacement of the APRE with Stat binding elements (SBEs) from the
ICAM-1 or
C/EBPbeta promoter, both of which recognize both Stat1 and Stat3, confers responsiveness to
gamma interferon, a
cytokine that selectively activates Stat1. Sequence comparisons suggest that the distinct Stat binding specificities of the C/EBPdelta and
C/EBPbeta SBEs are determined primarily by a single base pair difference. Our findings indicate that the
cytokine specificity of C/EBPdelta gene expression is governed by the APRE sequence.