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Mol. Cell. Biol., Sep 1997, 5588-5597, Vol 17, No. 9
A Sewing, B Wiseman, AC Lloyd and H Land
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.
Copyright © 1997, American Society for Microbiology
High-intensity Raf signal causes cell cycle arrest mediated by p21Cip1
Imperial Cancer Research Fund, London, United Kingdom.
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