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Mol. Cell. Biol., 09 1997, 5598-5611, Vol 17, No. 9
D Woods, D Parry, H Cherwinski, E Bosch, E Lees and M McMahon
The Raf family of protein kinases display differences in their abilities to
promote the entry of quiescent NIH 3T3 cells into the S phase of the cell
cycle. Although conditional activation of deltaA- Raf:ER promoted cell
cycle progression, activation of deltaRaf-1:ER and deltaB-Raf:ER elicited a
G1 arrest that was not overcome by exogenously added growth factors.
Activation of all three deltaRaf:ER kinases led to elevated expression of
cyclin D1 and cyclin E and reduced expression of p27Kip1. However,
activation of deltaB-Raf:ER and deltaRaf-1:ER induced the expression of
p21Cip1, whereas activation of deltaA-Raf:ER did not. A catalytically
potentiated form of deltaA-Raf:ER, generated by point mutation, strongly
induced p21Cip1 expression and elicited cell cycle arrest similarly to
deltaB-Raf:ER and deltaRaf-1:ER. These data suggested that the strength and
duration of signaling by Raf kinases might influence the biological outcome
of activation of this pathway. By titration of deltaB-Raf:ER activity we
demonstrated that low levels of Raf activity led to activation of cyclin
D1-cdk4 and cyclin E-cdk2 complexes and to cell cycle progression whereas
higher Raf activity elicited cell cycle arrest correlating with p21Cip1
induction and inhibition of cyclin-cdk activity. Using green fluorescent
protein-tagged forms of deltaRaf-1:ER in primary mouse embryo fibroblasts
(MEFs) we demonstrated that p21Cip1 was induced by Raf in a p53-independent
manner, leading to cell cycle arrest. By contrast, activation of Raf in
p21Cip1(-/-) MEFs led to a robust mitogenic response that was similar to
that observed in response to platelet-derived growth factor. These data
indicate that, depending on the level of kinase activity, Raf can elicit
either cell cycle progression or cell cycle arrest in mouse fibroblasts.
The ability of Raf to elicit cell cycle arrest is strongly associated with
its ability to induce the expression of the cyclin-dependent kinase
inhibitor p21Cip1 in a manner that bears analogy to alpha-factor arrest in
Saccharomyces cerevisiae. These data are consistent with a role for Raf
kinases in both proliferation and differentiation of mammalian cells.
Copyright © 1997, American Society for Microbiology
Raf-induced proliferation or cell cycle arrest is determined by the level of Raf activity with arrest mediated by p21Cip1
Department of Cell Signaling, DNAX Research Institute, Palo Alto, California 94304, USA.
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