title: Differential regulation of cyclin D1 and D2 in protecting against cardiomyocyte proliferation.
abstract: Cardiomyocytes withdraw from cell cycle after terminal differentiation due in part to impaired nuclear import of cyclin D1. Thus, we have previously shown that expression of nuclear localization signal-tagged cyclin D1 (D1NLS) and cyclin-dependent kinase 4 promotes cardiomyocyte proliferation both in vitro and in vivo. Here we show that cyclin D2 fails to stimulate cell cycle in cardiomyocytes through a mechanism distinct from that of cyclin D1. We demonstrate that cyclin D2 can express in the nucleus much more efficiently than cyclin D1. Cyclin D2, however, was much less effective in activating CDK2 and cell proliferation than cyclin D1 when expressed transiently in the nucleus of cardiomyocytes using nuclear localization signals. Consistent with such an observation, CDK inhibitors p21(cip1) and p27(kip1) remained bound to CDK2 in cells expressing cyclin D2, whereas p21 and p27 were sequestered to cyclin D1 in cells expressing D1NLS. These data suggest that cyclin D2 has weaker affinities to the CDK inhibitors and therefore is less efficient in activating cell cycle than cyclin D1. According to such a notion, double knockdown of p21 and p27 in cells expressing D2NLS induced activation of CDK2/CDC2 and BrdU incorporation to levels similar to those in cells expressing D1NLS. Taken together, our data suggest that distinct mechanisms keep cyclin D1 and cyclin D2 from activating cell cycle in terminally differentiated cardiomyocytes.
authors.affiliation: Department of Biochemical Genetics, Medical Research Institute, Bunkyo-ku, Tokyo, Japan. mtam.bgen@mri.tmd.ac.jp
authors.forename: Mimi
authors.initials: M
authors.lastname: Tamamori-Adachi
authors.forename: Ikuko
authors.initials: I
authors.lastname: Goto
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Yamada
authors.forename: Shigetaka
authors.initials: S
authors.lastname: Kitajima
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D002453:Cell Cycle; D049109:Cell Proliferation; D019925:Cyclin A / Q000378:metabolism; D019938:Cyclin D1 / Q000737:chemistry / Q000378:metabolism*; D056742:Cyclin D2; D019927:Cyclin E / Q000378:metabolism; D051357:Cyclin-Dependent Kinase 2 / Q000378:metabolism; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000378:metabolism; D050760:Cyclin-Dependent Kinase Inhibitor p27 / Q000378:metabolism; D016213:Cyclins / Q000737:chemistry / Q000378:metabolism*; D019610:Cytoprotection; D008954:Models, Biological; D032383:Myocytes, Cardiac / Q000166:cytology* / Q000201:enzymology / Q000378:metabolism*; D019913:Nuclear Localization Signals / Q000378:metabolism; D011485:Protein Binding; D034741:RNA, Small Interfering / Q000378:metabolism; D051381:Rats; D044967:Serum
chemical_list: C530105:Ccnd1 protein, mouse; C486196:Ccnd2 protein, mouse; D019925:Cyclin A; D056742:Cyclin D2; D019927:Cyclin E; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D019913:Nuclear Localization Signals; D034741:RNA, Small Interfering; D019938:Cyclin D1; D050760:Cyclin-Dependent Kinase Inhibitor p27; D051357:Cyclin-Dependent Kinase 2