EXPRESSION OF THE GENE ENCODING ALPHA(1)-ACID GLYCOPROTEIN IN RABBIT LIVER UNDER ACUTE-PHASE CONDITIONS INVOLVES INDUCTION AND ACTIVATION OF BETA-CCAAT AND DELTA-CCAAT ENHANCER BINDING-PROTEINS

Transcription of the gene encoding alpha(1)-acid glycoprotein is highly induced during acute inflammation which has been previously shown to be mediated by some inducible members of the CCAAT-enhancer-binding (C/EBP) transcription-factor family. In this study, we demonstrate that the involved inducible C/EBP isoforms are C/EBP-beta and C/EBP-delta, and together they control the high-level induction of the alpha(1)-acid glycoprotein gene in response to inflammatory signals. We observed that dephosphorylation severely inhibits the DNA-binding ability of C/EBP-delta and its transactivating potential increases in the presence of cellular phosphatase inhibitors, such as okadaic acid and sodium orthovanadate. These results suggest that C/EBP-delta is regulated by phosphorylation. Transient transfections using expression vectors of C/EBP-alpha, C/EBP-beta and C/EBP-delta have shown that while individually all three isoforms can transactivate the alpha(1)-acid glycoprotein-chloramphenicol-acetyltransferase gene transcription, co-expression of C/EBP-alpha and C/EBP-beta isoforms results in lower levels of reporter gene expression than the levels predicted from their additive transactivation level. In vitro DNA-binding studies have shown that C/EBP-alpha and C/EBP-beta isoforms both interact and form complexes with the alpha(1)-acid glycoprotein gene C/EBP-binding element under normal noninduced conditions during which alpha(1)-acid glycoprotein is expressed at a very low level. Higher than additive levels of reporter gene expression are observed when combinations of C/EBP-delta and C/EBP-beta or C/EBP-delta and C/EBP-alpha are used. Together, these data demonstrate that C/EBP-beta and C/EBP-delta are the major proteins responsible for the acute-phase induction of alpha(1)-acid-glycoprotein gene expression and they require phosphorylation for transactivation potential.