Basic fibroblast growth factor exhibits dual and rapid regulation of cyclin D1 and p27 to stimulate proliferation of rat cerebral cortical precursors.
While extracellular signals play a major role in brain neurogenesis, little is known about the cell cycle machinery underlying mitogen stimulation of precursor proliferation. Current models suggest that the D cyclins function as primary sensors of extracellular mitogens. Here we define the mechanisms by which basic fibroblast growth factor (bFGF) stimulates cortical precursors, with particular attention to the responses of cell cycle promitogenic and antimitogenic regulators. bFGF produced a 4-fold increase in DNA synthesis and a 3-fold rise in bromodeoxyuridine labeling, suggesting that the factor promotes the G1/S transition. There was also a 3-fold increase in cyclin-dependent kinase 2 (CDK2) kinase activity, which is critical for S phase entry. CDK2 activation was apparently cyclin E dependent, since only its protein and mRNA levels were elevated at 24 h, whereas CDK2, p27KIP1 and p57KIP2 levels were unaltered. Late G1 phase CDK2/cyclin E activity depends on early G1 D cyclin function. Indeed, cyclin D1, but not cyclin D3, was upregulated selectively at 8 h after bFGF treatment, a time when cyclin E was unchanged. The sequential activation of cyclin D1 and cyclin E supports the idea that cyclin E gene transcription is regulated by cyclin-D/CDK4/6-mediated pRb phosphorylation and subsequent E2F transcription factor release. However, in addition to increased D1 cyclin, we unexpectedly detected a 75% reduction in p27KIP1 protein at 8 h, suggesting that both pro- and antimitogenic regulators are targets of extracellular mitogens during brain development.
Li
Baogang
B
Department of Neuroscience and Cell Biology, University of Medicine and Dentistry of New Jersey-Robert Wood Johnson Medical School, Piscataway, NJ 08854, USA. liba@umdnj.edu
DiCicco-Bloom
Emanuel
E
D000818:Animals; D001973:Bromodeoxyuridine; D042846:CDC2-CDC28 Kinases / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000378:metabolism* / Q000494:pharmacology; D002454:Cell Differentiation / Q000187:drug effects / Q000502:physiology; D049109:Cell Proliferation / Q000187:drug effects; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000166:cytology / Q000196:embryology / Q000378:metabolism*; D019938:Cyclin D1 / Q000187:drug effects / Q000378:metabolism*; D019927:Cyclin E / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D051357:Cyclin-Dependent Kinase 2; D050760:Cyclin-Dependent Kinase Inhibitor p27; D004247:DNA / Q000096:biosynthesis / Q000187:drug effects; D015536:Down-Regulation / Q000187:drug effects / Q000235:genetics; D016222:Fibroblast Growth Factor 2 / Q000378:metabolism* / Q000494:pharmacology; D009474:Neurons / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D012333:RNA, Messenger / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013234:Stem Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D025521:Tumor Suppressor Proteins / Q000378:metabolism*; D015854:Up-Regulation / Q000187:drug effects / Q000502:physiology
C493962:Cdkn1b protein, rat; D018797:Cell Cycle Proteins; D019927:Cyclin E; D012333:RNA, Messenger; D025521:Tumor Suppressor Proteins; D016222:Fibroblast Growth Factor 2; D019938:Cyclin D1; D050760:Cyclin-Dependent Kinase Inhibitor p27; D004247:DNA; D042846:CDC2-CDC28 Kinases; C495892:Cdk2 protein, rat; D051357:Cyclin-Dependent Kinase 2; D001973:Bromodeoxyuridine