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The DNA Binding Activity of C/EBP Transcription Factors Is Regulated in the G Phase of the Hepatocyte Cell Cycle
We have isolated the promoter of the rat C/EBPα gene and find a high degree of homology with the mouse gene, particularly in putative regulatory domains. Transactivation of this promoter by ectopic expression of rat C/EBPβ occurs through a C/EBP regulatory domain at position â170 to â195. An o...
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Published in: | The Journal of biological chemistry 1995-07, Vol.270 (30), p.18123 |
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Main Authors: | , , , , |
Format: | Article |
Language: | English |
Online Access: | Get full text |
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Summary: | We have isolated the promoter of the rat C/EBPα gene and find a high degree of homology with the mouse gene, particularly
in putative regulatory domains. Transactivation of this promoter by ectopic expression of rat C/EBPβ occurs through a C/EBP
regulatory domain at position â170 to â195. An oligonucleotide corresponding to this domain binds to complexes expressed in
rat liver that comprise C/EBPα-C/EBPβ heterodimers (αβ) as well as C/EBPβ complexed with itself and/or other unidentified
nuclear factors (β1, β2, and β3). The DNA binding activity of these complexes changes both qualitatively and quantitatively
following partial hepatectomy. Within 2-5 h postsurgery, the binding activity of the αβ complexes drops severalfold, reaching
a nadir by 20 h. During the ensuing 3-8 days, as regeneration nears completion, this activity slowly returns to normal quiescent
liver levels. Western blot analysis shows 3 major C/EBPα polypeptide species (42, 40, and 30 kDa), whose abundance in general
parallels the decrease and recovery in DNA binding activity. In contrast to C/EBPα behavior, the DNA binding activity of the
β complexes is transiently induced severalfold during the early G 1 period between 2 and 6 h posthepatectomy. The major C/EBPβ polypeptide is the 32-kDa LAP protein, whereas the LIP protein
(21 kDa) is weakly expressed. Both remain essentially constant throughout the course of regeneration, suggesting that changes
in DNA binding activity may reflect changes in the complexed proteins rather than the C/EBPβ polypeptides themselves. In primary
hepatocyte cultures, under growth supporting conditions, in the absence of growth factors proliferation is negligible; C/EBPα
is abundantly expressed at the outset, but is then extensively down-regulated. Epidermal growth factor causes further decay
of C/EBPα polypeptides and DNA binding activity, and down-regulates C/EBPβ DNA binding activity as well. Addition of transforming
growth factor β completely antagonizes the effects of epidermal growth factor on C/EBPβ activity, and partially overcomes
the effect on C/EBPα. These results demonstrate that the DNA binding activity of C/EBPα and C/EBPβ complexes is regulated
in the regenerating liver, and in hepatocyte cultures responding to growth factors that regulate their proliferation. |
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ISSN: | 0021-9258 1083-351X |
DOI: | 10.1074/jbc.270.30.18123 |