EVIDENCE FOR POSTTRANSCRIPTIONAL REGULATION OF C EBP-ALPHA AND C/EBP-BETA ISOFORM EXPRESSION DURING THE LIPOPOLYSACCHARIDE-MEDIATED ACUTE-PHASE RESPONSE/

The mRNAs of the CCAAT/enhancer-binding trans-activator proteins (C/EB P alpha and C/EBP beta) serve as templates for the differential transl ation of several isoforms which have specific transcriptional regulato ry functions. By using an oligonucleotide corresponding to the C/EBP b inding site of the mouse alpha(1)-acid glycoprotein promoter, we detec ted multiple forms of C/EBP alpha and C/EBP beta proteins in the mouse liver that have DNA-binding activity, By using specific antisera, we detected C/EBP alpha s 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 signific antly after lipopolysaccharide (LPS) treatment. The binding activity a nd protein levels of the 20-kDa isoform are low in controls and increa se dramatically after LPS treatment. C/EBP beta isoforms with molecula r masses of 35, 20, and 16 kDa were also detected. The 35-kDa pool lev el did not change whereas the 20-kDa isoform was strongly induced in r esponse to LPS. Western (immunoblot) and Southwestern (DNA-protein) an alyses show that p42(C/EBP alpha) forms specific complexes with the al pha(1)-acid glycoprotein oligonucleotide in control nuclear extract an d that p20(C/EBP beta) forms complexes in UPS-treated liver, Our studi es suggest that synthesis of specific C/EBP alpha and C/EBP beta isofo rms occurred in the normal liver in vivo and that LPS mediated a diffe rential initiation and inhibition of translation at specific AUG sites within each mRNA, The qualitative and quantitative changes in C/EBP a lpha and C/EBP beta isoform pool levels suggest that LPS or an UPS-sti mulated factor can regulate the selection of AUG start sites for both activation and repression of translation. This regulation appears to i nvolve an UPS-mediated down-regulation of initiation at the first AUG codon of the 42-kDa C/EBP alpha and dramatic translational up-regulati on at the fifth AUG codon of the 20-kDa C/EBP alpha and the third AUG codon of the 20-kDa C/EBP beta, These regulatory events suggest the ex istence of proteins that may act as translational trans-acting factors .