Continuous ERK activation downregulates antiproliferative genes throughout G1 phase to allow cell-cycle progression.
BACKGROUND
The ERK family of MAP kinase plays a critical role in growth factor-stimulated cell-cycle progression from G0/G1 to S phase. It has been suggested that sustained activation, but not transient activation, of ERK is necessary for inducing S phase entry. Although the essential role of ERK MAP kinase in growth factor-stimulated gene expression, especially expression of immediate-early genes, is well established, it has remained unclear how ERK activity duration affects the promotion of G1 phase progression to S phase.


RESULTS
We have found that inhibition of ERK activation by the MEK inhibitor or dominant-negative MEK1 even immediately before the onset of S phase leads to the cessation of S phase entry. Our analyses reveal that there are ERK-dependent downregulated genes, whose expression levels return to their original levels rapidly after ERK inactivation, and that their downregulation mostly requires AP-1 activity. Remarkably, microinjection experiments demonstrate that many of the downregulated genes act as antiproliferative genes during G1 phase and that their forced expression to the levels before growth factor stimulation even in late G1 phase blocks S phase entry.


CONCLUSIONS
Thus, continuous ERK activation downregulates antiproliferative genes until the onset of S phase to allow successful G1 phase progression. This mechanism may also work as a fail-safe mechanism, which prevents inappropriate stimuli that induce transient ERK activation from causing S phase entry.
Yamamoto
Takuya
T
Department of Cell and Developmental Biology, Graduate School of Biostudies, Kyoto University, Kyoto 606-8502, Japan.
Ebisuya
Miki
M
Ashida
Fumito
F
Okamoto
Kazuo
K
Yonehara
Shin
S
Nishida
Eisuke
E
D000818:Animals; D049109:Cell Proliferation; D015536:Down-Regulation; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism* / Q000502:physiology; D016193:G1 Phase / Q000235:genetics*; D020869:Gene Expression Profiling; D018816:Genes, cdc; D051379:Mice; D041681:NIH 3T3 Cells; D016196:S Phase / Q000235:genetics; D018808:Transcription Factor AP-1 / Q000235:genetics / Q000378:metabolism
D018808:Transcription Factor AP-1; D048049:Extracellular Signal-Regulated MAP Kinases