Cell cycle-dependent morphological changes in the actin cytoskeleton induced by agents which elevate cyclic AMP

Agents which increase the intracellular concentration of cyclic adenosine-5'-monophosphate, induce a highly arborised morphology in a proportion of sub-confluent Swiss 3T3 fibroblasts. During this process the organisation of actin filaments progressively changes from a characteristic stress fibre pattern to leave a network of actin filaments within each and every arborisation. Despite this massive reorganisation of the actin cytoskeleton no changes are observed in the extent of polymerisation of actin during arborisation. The proportion of cells in asynchronous cultures undergoing arborisation at maximal concentrations of agents reaches a maximum of 30%; suggesting that the effect might be mediated only in cells during a restricted period of the cell cycle. More than 80% of serum-starved cells responded to these agents between 1 and 8 hours after readdition of serum, but not at other times, suggesting that the arborisation response can occur only in the G1 phase of the cell cycle.