Regenerative changes in C/EBP alpha and C/EBP beta expression modulate binding to the C/EBP site in the c-fos promoter.
CCAAT/enhancer binding proteins are a family of basic zipper DNA binding proteins that regulate transcription of several liver-specific genes and certain growth-related genes. Growth-related variations in the nuclear expression of one or more of the CCAAT/enhancer binding proteins may regulate the transition from the nonproliferative, differentiated phenotype of adult liver to the proliferative phenotype of regenerating liver. To evaluate this possibility, we used Northern- and Western-blot analyses to profile the expression of selected CCAAT/enhancer binding proteins in regenerating liver. Variations in CCAAT/enhancer binding protein expression were then correlated with changes in binding to the CCAAT/enhancer binding protein site of the c-fos promoter. Expression of both CCAAT/enhancer binding protein alpha and CCAAT/enhancer binding protein beta increases after partial hepatectomy. Steady-state levels of CCAAT/enhancer binding protein alpha mRNA increase 30% within an hour of partial hepatectomy (p < 0.05). This is followed by a transient increase in nuclear levels of CCAAT/enhancer binding protein alpha protein at 3 hr after partial hepatectomy (p = 0.08). In contrast, increases in CCAAT/enhancer binding protein beta mRNA and protein are more sustained. Levels of CCAAT/enhancer binding protein beta mRNA increase 400% to 500% within an hour of partial hepatectomy and remain increased throughout most of the prereplicative period (p < 0.01). Nuclear levels of CCAAT/enhancer binding protein beta protein are 200% to 300% greater than prehepatectomy levels at 3 hr (p < 0.001) to 6 hr (p < 0.05) and do not approach basal levels until 24 hr after partial hepatectomy. Gel mobility shift assays of nuclear extracts from regenerating livers indicate that these increases in nuclear protein expression are associated with increased DNA binding of CCAAT/enhancer binding protein alpha-beta heterodimers and beta-beta homodimers. These results demonstrate growth-related variations in the expression and DNA binding of both CCAAT/enhancer binding protein alpha and beta during liver regeneration and support the theory that altered CCAAT/enhancer binding protein DNA binding may contribute to regeneration-associated changes in liver cell phenotype.
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