Transient deactivation of ERK signalling is sufficient for stable entry into G0 in primary avian fibroblasts.
Re-entry into the cell cycle from quiescence requires the activation of mitogen-activated protein (MAP) kinases of the extracellular-signal-regulated kinase (ERK) family [1,2]. The relationship between ERK and cell-cycle control is, however, complex, as ERK activation can also lead to terminal differentiation [3] or a senescence-like growth arrest [4]. Here, we report that reversible cell-cycle exit induced by serum withdrawal in primary avian fibroblasts is associated with rapid deactivation of ERK, but ERK activity is subsequently regenerated and sustained at high levels in fully quiescent (G0) cells. As in proliferating cells, ERK activation during G0 required the MAPkinase kinase MEK and was partially dependent on cell adhesion. Active, phosphorylated ERK was concentrated in the nucleus in cycling cells, but was largely confined to the cytoplasm during G0. This was unexpected, as activatory phosphorylation mediated by MEK is thought to play an important role in promoting nuclear translocation [5,6]. These results indicate that transient deactivation of ERK signalling can be sufficient for stable cell-cycle exit, and that MEK-mediated phosphorylation is not sufficient for nuclear translocation of active ERK in G0. Cytoplasmic sequestration may prevent active ERK from accessing critical nuclear cell-cycle targets, thus allowing quiescent or post-mitotic cells to retain ERK activity for other physiological functions.
Black
E J
EJ
Beatson Institute for Cancer Research, Cancer Research Campaign Beatson Laboratories, Garscube Estate, Switchback Road, Bearsden, G61 1BD, Glasgow, UK.
Clark
W
W
Gillespie
D A
DA
D016475:3T3 Cells; D000818:Animals; D002448:Cell Adhesion; D002453:Cell Cycle; D002467:Cell Nucleus / Q000201:enzymology; D002478:Cells, Cultured; D002642:Chick Embryo; D003470:Culture Media; D003593:Cytoplasm / Q000201:enzymology; D005347:Fibroblasts; D005434:Flow Cytometry; D006133:Growth Substances / Q000378:metabolism; D048728:MAP Kinase Kinase Kinase 1; D020935:MAP Kinase Signaling System; D051379:Mice; D018613:Microscopy, Confocal; D020928:Mitogen-Activated Protein Kinases / Q000276:immunology / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D051381:Rats; D016192:Resting Phase, Cell Cycle
D003470:Culture Media; D006133:Growth Substances; D017346:Protein Serine-Threonine Kinases; D020928:Mitogen-Activated Protein Kinases; D048728:MAP Kinase Kinase Kinase 1; C482578:Map3k1 protein, mouse