Evidence for posttranscriptional regulation of C/EBPα and C/EBPβ isoform expression during the lipopolysaccharide-mediated acute-phase response

The mRNAs of the CCAAT/enhancer-binding trans-activator proteins (C/EBPα and C/EBPβ) serve as templates for the differential translation of several isoforms which have specific transcriptional regulatory functions. By using an oligonucleotide corresponding to the C/EBP binding site of the mouse α₁- acid glycoprotein promoter, we detected multiple forms of C/EBPα and C/EBPβ proteins in the mouse liver that have DNA-binding activity. By using specific antisera, we detected C/EBPαs with molecular masses of 42, 38, 30, and 20 kDa that have DNA-binding activity. The pool levels of the 42- and 30-kDa isoforms were high in control nuclear extracts and decreased significantly after lipopolysaccharide (LPS) treatment. The binding activity and protein levels of the 20-kDa isoform are low in controls and increase dramatically after LPS treatment. C/EBPβ isoforms with molecular masses of 35, 20, and 16 kDa were also detected. The 35-kDa pool level did not change whereas the 20- kDa isoform was strongly induced in response to LPS. Western (immunoblot) and Southwestern (DNA-protein) analyses show that p42(C/EBPα) forms specific complexes with the α₁-acid glycoprotein oligonucleotide in control nuclear extract and that p20(C/EBPβ) forms complexes in LPS-treated liver. Our studies suggest that synthesis of specific C/EBpα and C/EBPβ isoforms occurred in the normal liver in vivo and that LPS mediated a differential initiation and inhibition of translation at specific AUG sites within each mRNA. The qualitative and quantitative changes in C/EBPα and C/EBPβ isoform pool levels suggest that LPS or an LPS-stimulated factor can regulate the selection of AUG start sites for both activation and repression of translation. This regulation appears to involve an LPS-mediated down- regulation of initiation at the first AUG codon of the 42-kDa C/EBPα and dramatic translational up-regulation at the fifth AUG codon of the 20-kDa C/EBPα and the third AUG codon of the 20-kDa C/EBPβ. These regulatory events suggest the existence of proteins that may act as translational trans- acting factors.