Cyclin D3 maintains growth-inhibitory activity of C/EBPalpha by stabilizing C/EBPalpha-cdk2 and C/EBPalpha-Brm complexes.
C/EBPalpha arrests proliferation of young livers by inhibition of cdk2. In old mice, C/EBPalpha inhibits growth by repression of E2F-dependent promoters through the C/EBPalpha-Brm complex. In this paper, we show that cyclin D3-cdk4/cdk6 supports the ability of C/EBPalpha to inhibit liver proliferation in both age groups. Although cyclin D3-cdk4/cdk6 kinases are involved in the promotion of growth, they are expressed in terminally differentiated cells, suggesting that they have additional functions in these settings. We demonstrate that C/EBPalpha represents a target for phosphorylation by cyclin D3-cdk4/cdk6 complexes in differentiated liver cells and in differentiated adipocytes. Cyclin D3-cdk4/cdk6 specifically phosphorylate C/EBPalpha at Ser193 in vitro and in the liver and support growth-inhibitory C/EBPalpha-cdk2 and C/EBPalpha-Brm complexes. We found that cyclin D3 is increased in old livers and activates cdk4/cdk6, resulting in stabilization of the C/EBPalpha-Brm complex. Old livers fail to reduce the activity of cyclin D3-cdk4/cdk6 after partial hepatectomy, leading to high levels of C/EBPalpha-Brm complexes after partial hepatectomy, which correlate with weak proliferation. We examined the role of cyclin D3 in the stabilization of C/EBPalpha-cdk2 and C/EBPalpha-Brm by using 3T3-L1 differentiated cells. In these cells, cyclin D3 is increased during differentiation and phosphorylates C/EBPalpha at Ser193, leading to the formation of growth-inhibitory C/EBPalpha-cdk2 and C/EBPalpha-Brm complexes. The inhibition of cyclin D3 blocks the formation of these complexes. Thus, these studies provide a new function of cyclin D3, which is to support the growth-inhibitory activity of C/EBPalpha.
Huffington Center on Aging, Baylor College of Medicine, One Baylor Plaza, Houston, Texas 77030, USA.
Guo-Li
GL
Wang
Xiurong
X
Shi
Elizabeth
E
Salisbury
Yuxiang
Y
Sun
Jeffrey H
JH
Albrecht
Roy G
RG
Smith
Nikolai A
NA
Timchenko
D041721:3T3-L1 Cells; D000375:Aging; D000818:Animals; D022763:CCAAT-Enhancer-Binding Protein-alpha / Q000378:metabolism*; D002454:Cell Differentiation; D056743:Cyclin D3; D051357:Cyclin-Dependent Kinase 2 / Q000378:metabolism*; D051358:Cyclin-Dependent Kinase 4 / Q000378:metabolism; D051361:Cyclin-Dependent Kinase 6 / Q000378:metabolism; D016213:Cyclins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006131:Growth Inhibitors / Q000378:metabolism*; D006498:Hepatectomy; D008099:Liver / Q000378:metabolism; D051379:Mice; D008970:Molecular Weight; D010766:Phosphorylation; D011485:Protein Binding; D020033:Protein Isoforms; D012694:Serine / Q000378:metabolism; D014157:Transcription Factors / Q000378:metabolism*
D022763:CCAAT-Enhancer-Binding Protein-alpha; C534697:Ccnd3 protein, mouse; D056743:Cyclin D3; D016213:Cyclins; D006131:Growth Inhibitors; D020033:Protein Isoforms; C499503:Smarca2 protein, mouse; D014157:Transcription Factors; D012694:Serine; D051357:Cyclin-Dependent Kinase 2; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6