Regulation of G(1) cyclin-dependent kinases in the liver: role of nuclear localization and p27 sequestration.
Recent studies suggest that cyclin D1 mediates progression of hepatocytes through G(1) phase of the cell cycle. The present study further examines the regulation of cyclin D1-dependent kinase activity and the interplay between cyclin D1 and other G(1) phase regulatory proteins during liver regeneration. After 70% partial hepatectomy in rats, there was upregulation of kinase activity associated with cyclins (A, D1, D3, and E), cyclin-dependent kinases (Cdk2 and Cdk4), and Cdk-inhibitory proteins (p27, p107, and p130). Although cyclin D1/Cdk4 complexes were more abundant in the cytoplasmic fraction after partial hepatectomy, kinase activity was detected primarily in the nuclear fraction. Cytoplasmic cyclin D1/Cdk4 complexes were activated by recombinant cyclin H/Cdk7. Because endogenous Cdk7 activity was found in the nucleus, this suggests that activation of cyclin D1/Cdk4 requires nuclear importation and subsequent phosphorylation by cyclin H/Cdk7. Recombinant cyclin E/Cdk2 was inhibited by extracts from quiescent liver, and cyclin D1 could titrate out this inhibitory activity. Induction of cyclin D1 was accompanied by increased abundance of cyclin D1/p27 complexes, and most p27 was sequestered by cyclin D1 after partial hepatectomy. Thus cyclin D1 appears to play two roles during G(1) phase progression in the regenerating liver: it forms a nuclear kinase complex, and it promotes activation of Cdk2 by sequestering inhibitory proteins such as p27. These experiments underscore the complexity of cyclin/Cdk regulatory networks in the regenerating liver.
Albrecht
J H
JH
Department of Medicine, Hennepin County Medical Center, Minneapolis 55415, Minnesota. albre010@tc.umn.edu
Rieland
B M
BM
Nelsen
C J
CJ
Ahonen
C L
CL
D000818:Animals; D042846:CDC2-CDC28 Kinases; D018797:Cell Cycle Proteins; D002467:Cell Nucleus / Q000201:enzymology; D019938:Cyclin D1 / Q000378:metabolism; D019927:Cyclin E / Q000378:metabolism; D056746:Cyclin G; D056766:Cyclin G1; D051357:Cyclin-Dependent Kinase 2; D051358:Cyclin-Dependent Kinase 4; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D016213:Cyclins / Q000378:metabolism*; D003593:Cytoplasm / Q000201:enzymology; D004791:Enzyme Inhibitors / Q000378:metabolism; D008099:Liver / Q000201:enzymology*; D008115:Liver Regeneration / Q000502:physiology*; D008297:Male; D008869:Microtubule-Associated Proteins / Q000378:metabolism*; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins; D051381:Rats; D017207:Rats, Sprague-Dawley; D016196:S Phase / Q000502:physiology; D025521:Tumor Suppressor Proteins
C534704:Ccng1 protein, rat; C493962:Cdkn1b protein, rat; D018797:Cell Cycle Proteins; D019927:Cyclin E; D056746:Cyclin G; D056766:Cyclin G1; D016213:Cyclins; D004791:Enzyme Inhibitors; D008869:Microtubule-Associated Proteins; D011518:Proto-Oncogene Proteins; D025521:Tumor Suppressor Proteins; D019938:Cyclin D1; D050760:Cyclin-Dependent Kinase Inhibitor p27; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495892:Cdk2 protein, rat; C495903:Cdk4 protein, rat; D051357:Cyclin-Dependent Kinase 2; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases