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/EBP alpha) (p42) has been implicated in the transcriptional activation of adipocyte genes including the 422(aP2) and C/EBP alpha genes during differentiation of 3T3-L1 preadipocytes. We have identified a 30-kDa isoform (p30) of C/EBP alpha that is expressed by 3T3-L1 adipocytes, mouse adipose tissue, and rat liver. In vitro translation of wild-type C/EBP alpha mRNA or transient transfection with a wild-type C/EBP alpha 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/EBP alpha message. p30C/EBP alpha binds to the C/EBP sites within and activates reporter gene expression driven by the 422(aP2) and C/EBP alpha gene promoters. Although transfection of 3T3-L1 preadipocytes with a strong p30C/EBP alpha expression vector is insufficient to induce differentiation, this vector advances the differentiation program. Unlike p42C/EBP alpha, which inhibits cell proliferation, p30C/EBP alpha is not antimitotic. Thus, the N-terminal 12-kDa segment of full-length C/EBP alpha contains an amino acid sequence necessary for antimitotic activity. During differentiation of 3T3-L1 preadipocytes and during hepatocyte development, the cellular p42C/EBP alpha/p30C/EBP alpha ratio changes, raising the possibility of a regulatory role.
Lin
F T
FT
Department of Biological Chemistry, Johns Hopkins University School of Medicine, Baltimore, MD 21205.
MacDougald
O A
OA
Diehl
A M
AM
Lane
M D
MD
D016475:3T3 Cells; D017667:Adipocytes; D017398:Alternative Splicing; D000818:Animals; D022762:CCAAT-Enhancer-Binding Proteins; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000235:genetics*; D006131:Growth Inhibitors; D066298:In Vitro Techniques; D008099:Liver / Q000502:physiology; D051379:Mice; D008938:Mitosis; D008970:Molecular Weight; D009687:Nuclear Proteins / Q000737:chemistry / Q000235:genetics*; D011401:Promoter Regions, Genetic; D012045:Regulatory Sequences, Nucleic Acid; D014157:Transcription Factors / Q000235:genetics*; D014158:Transcription, Genetic; D015533:Transcriptional Activation
D022762:CCAAT-Enhancer-Binding Proteins; D004268:DNA-Binding Proteins; D006131:Growth Inhibitors; D009687:Nuclear Proteins; D014157:Transcription Factors