Expression of the gene encoding α 1 ‐acid glycoprotein in rabbit liver under acute‐phase conditions involves induction and activation of β and δ CCAAT‐enhancer‐binding proteins

Transcription of the gene encoding α 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‐β and C/EBP‐δ, and together they control the high‐level induction of the α 1 ‐acid glycoprotein gene in response to inflammatory signals. We observed that dephosphorylation severely inhibits the DNA‐binding ability of C/EBP‐δ 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‐δ is regulated by phosphorylation. Transient transfections using expression vectors of C/EBP‐α, C/EBP‐β and C/EBP‐δ have shown that while individually all three isoforms can transactivate the α 1 ‐acid glycoprotein–chloramphenicol‐acetyltransferase gene transcription, co‐expression of C/EBP‐α and C/EBP‐β 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‐α and C/EBP‐β isoforms both interact and form complexes with the α 1 ‐acid glycoprotein gene C/EBP‐binding element under normal non‐induced conditions during which α 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‐δ and C/EBP‐β or C/EBP‐δ and C/EBP‐α are used. Together, these data demonstrate that C/EBP‐β and C/EBP‐δ are the major proteins responsible for the acute‐phase induction of α 1 ‐acid‐glycoprotein gene expression and they require phosphorylation for transactivation potential.