Effect of the CCAAT/enhancer binding protein on expression of the gene for chicken malic enzyme.

The gene for malic enzyme is expressed at a high level in chick embryo-hepatocytes (CEH) treated with triiodothyronine (T3) and at a low level in the absence of T3. In chick-embryo fibroblasts (CEF), expression of the malic enzyme gene is low and not regulated by T3. Specific nuclear proteins from both CEH and CEF bound to a consensus CCAAT/enhancer binding protein (C/EBP) site at -335 to -327 bp of the malic enzyme gene. The level of binding was much higher in extracts from CEH than in extracts of CEF, and the complexes formed had different mobilities. C/EBPalpha was present in the complex that bound to the C/EBP site in nuclear extracts from CEH but not in those from CEF. The C/EBP element was necessary and sufficient to bestow full T3 responsiveness to 5800 bp of 5'-flanking DNA of the malic enzyme gene in CEH. C/EBPalpha was not detectable in wild-type CEF, and deletion of the C/EBP binding site had no effect on expression of transgenes containing 5800 bp of 5'-flanking DNA of the malic enzyme gene. In CEF, overexpression of C/EBPalpha stimulated promoter activity of constructs that contained the C/EBP site linked to the malic enzyme promoter or a heterologous reporter. The results suggest that C/EBPalpha or a closely related isoform is involved in the tissue-specific expression of the malic enzyme gene.