title: High-intensity Raf signal causes cell cycle arrest mediated by p21Cip1.
abstract: Activated Raf has been linked to such opposing cellular responses as the induction of DNA synthesis and the inhibition of proliferation. However, it remains unclear how such a switch in signal specificity is regulated. We have addressed this question with a regulatable Raf-androgen receptor fusion protein in murine fibroblasts. We show that Raf can cause a G1-specific cell cycle arrest through induction of p21Cip1. This in turn leads to inhibition of cyclin D- and cyclin E-dependent kinases and an accumulation of hypophosphorylated Rb. Importantly, this behavior can be observed only in response to a strong Raf signal. In contrast, moderate Raf activity induces DNA synthesis and is sufficient to induce cyclin D expression. Therefore, Raf signal specificity can be determined by modulation of signal strength presumably through the induction of distinct protein expression patterns. Similar to induction of Raf, a strong induction of activated Ras via a tetracycline-dependent promoter also causes inhibition of proliferation and p21Cip1 induction at high expression levels. Thus, p21Cip1 plays a key role in determining cellular responses to Ras and Raf signalling. As predicted by this finding we show that Ras and loss of p21 cooperate to confer a proliferative advantage to mouse embryo fibroblasts.
authors.affiliation: Imperial Cancer Research Fund, London, United Kingdom.
authors.forename: A
authors.initials: A
authors.lastname: Sewing
authors.forename: B
authors.initials: B
authors.lastname: Wiseman
authors.forename: A C
authors.initials: AC
authors.lastname: Lloyd
authors.forename: H
authors.initials: H
authors.lastname: Land
mesh_terms: D016475:3T3 Cells; D000818:Animals; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D002453:Cell Cycle; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000378:metabolism*; D005347:Fibroblasts / Q000166:cytology / Q000378:metabolism; D016193:G1 Phase; D051379:Mice; D027384:Oncogene Proteins v-raf; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D015682:Retroviridae Proteins, Oncogenic / Q000378:metabolism*; D015398:Signal Transduction; D018631:ras Proteins / Q000378:metabolism
chemical_list: C493957:Cdkn1a protein, mouse; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D004791:Enzyme Inhibitors; D015682:Retroviridae Proteins, Oncogenic; D011505:Protein-Tyrosine Kinases; D027384:Oncogene Proteins v-raf; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D018631:ras Proteins