A 30-KDA ALTERNATIVE TRANSLATION PRODUCT OF THE CCAAT ENHANCER-BINDING PROTEIN-ALPHA MESSAGE - TRANSCRIPTIONAL ACTIVATOR LACKING ANTIMITOTIC ACTIVITY

Full-length (42 kDa) CCAAT/enhancer binding protein alpha (C/EBPalpha) (p42) has been implicated in the transcriptional activation of adipocyte genes including the 422(aP2) and C/EBPalpha genes during differentiation of 3T3-L1 preadipocytes. We have identified a 30-kDa isoform (p30) of C/EBPalpha that is expressed by 3T3-L1 adipocytes, mouse adipose tissue, and rat liver. In vitro translation of wild-type C/EBPalpha mRNA or transient transfection with a wild-type C/EBPalpha vector gave rise to similar levels of p42 and p30. Mutational analysis revealed that p30 is an alternative translation product initiated at the third in-frame methionine codon of the C/EBPalpha message. p30C/EBPalpha binds to the C/EBP sites within and activates reporter gene expression driven by the 422(aP2) and C/EBPalpha gene promoters. Although transfection of 3T3-L1 preadipocytes with a strong p30C/EBPalpha expression vector is insufficient to induce differentiation, this vector advances the differentiation program. Unlike p42C/EBPalpha, which inhibits cell proliferation, p30C/EBPalpha is not antimitotic. Thus, the N-terminal 12-kDa segment of full-length C/EBPalpha contains an amino acid sequence necessary for antimitotic activity. During differentiation of 3T3-L1 preadipocytes and during hepatocyte development, the cellular p42C/EBPalpha/p30C/EBPalpha ratio changes, raising the possibility of a regulatory role.