CCAAT/enhancer binding protein alpha regulates p21 protein and hepatocyte proliferation in newborn mice.
CCAAT/enhancer binding protein alpha (C/EBP alpha) is expressed at high levels in quiescent hepatocytes and in differentiated adipocytes. In cultured cells, C/EBP alpha inhibits cell proliferation in part via stabilization of the p21 protein. The role of C/EBP alpha in regulating hepatocyte proliferation in vivo is presented herein. In C/EBP alpha knockout newborn mice, p21 protein levels are reduced in the liver, and the fraction of hepatocytes synthesizing DNA is increased. Greater than 30% of the hepatocytes in C/EBP alpha knockout animals continue to proliferate at day 17 of postnatal life when cell division in wild-type littermates is low (3%). p21 protein levels are relatively high in wild-type neonates but undetectable in C/EBP alpha knockout mice. The reduction of p21 protein in the highly proliferating livers that lack C/EBP alpha suggests that p21 is responsible for C/EBP alpha-mediated control of liver proliferation in newborn mice. During rat liver regeneration, the amounts of both C/EBP alpha and p21 proteins are decreased before DNA synthesis (6 to 12 h) and then return to presurgery levels at 48 h. Although C/EBP alpha controls p21 protein levels, p21 mRNA is not influenced by C/EBP alpha in liver. Using coimmunoprecipitation and a mammalian two-hybrid assay system, we have shown the interaction of C/EBP alpha and p21 proteins. Study of p21 stability in liver nuclear extracts showed that C/EBP alpha blocks proteolytic degradation of p21. Our data demonstrate that C/EBP alpha regulates hepatocyte proliferation in newborn mice and that in liver, the level of p21 protein is under posttranscriptional control, consistent with the hypothesis that protein-protein interaction with C/EBP alpha determines p21 levels.
Timchenko
N A
NA
Department of Pathology, Baylor College of Medicine, Houston, Texas 77030, USA.
Harris
T E
TE
Wilde
M
M
Bilyeu
T A
TA
Burgess-Beusse
B L
BL
Finegold
M J
MJ
Darlington
G J
GJ
D000818:Animals; D000831:Animals, Newborn; D022762:CCAAT-Enhancer-Binding Proteins; D002455:Cell Division / Q000235:genetics / Q000502:physiology; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins / Q000235:genetics / Q000378:metabolism*; D004247:DNA / Q000096:biosynthesis; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D066298:In Vitro Techniques; D008099:Liver / Q000166:cytology* / Q000378:metabolism*; D008115:Liver Regeneration / Q000235:genetics / Q000502:physiology; D051379:Mice; D018345:Mice, Knockout; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D051381:Rats
D022762:CCAAT-Enhancer-Binding Proteins; C493957:Cdkn1a protein, mouse; C493958:Cdkn1a protein, rat; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D012333:RNA, Messenger; D004247:DNA