Basic fibroblast growth factor exhibits dual and rapid regulation of cyclin D1 and p27KIP1 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 G(1)/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, p27(KIP1) and p57(KIP2) levels were unaltered. Late G(1) phase CDK2/cyclin E activity depends on early G(1)D cyclin function. Indeed, cyclin D-1, but not cyclin D-3, was upregulated selectively at 8 h after bFGF treatment, a time when cyclin E was unchanged. The sequential activation of cyclin D-1 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 D-1 cyclin, we unexpectedly detected a 75% reduction in p27(KIP1) protein at 8 h, suggesting that both pro- and antimitogenic regulators are targets of extracellular mitogens during brain development. Copyright (C) 2004 S. Karger AG, Basel.