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 mit ogen-activated protein (MAP) kinases of the extracellular-signal-regulated kinase (ERK) family [1,2], The relationship between ERK and cell-cycle cont rol is, however, complex, as ERK activation can also lead to terminal diffe rentiation [3] or a senescence-like growth arrest [4], Here, we report that reversible cell-cycle exit induced by serum withdrawal in primary avian fi broblasts 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 MAP kinase 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 ac tivatory phosphorylation mediated by MEK is thought to play an important ro le in promoting nuclear translocation [5,6]. These results indicate that tr ansient deactivation of ERK signalling can be sufficient for stable cell-cy cle exit, and that MEK-mediated phosphorylation is not sufficient for nucle ar translocation of active ERK in G0. Cytoplasmic sequestration may prevent active ERK from accessing critical nuclear cell-cycle targets, thus allowi ng quiescent or post-mitotic cells to retain ERK activity for other physiol ogical functions.