CELL CYCLE-DEPENDENT MORPHOLOGICAL-CHANGES IN THE ACTIN CYTOSKELETON INDUCED BY AGENTS WHICH ELEVATE CYCLIC-AMP

Agents which increase the intracellular concentration of cyclic adenos ine-5'-monophosphate, induce a highly arborised morphology in a propor tion of sub-confluent Swiss 3T3 fibroblasts. During this process the o rganisation of actin filaments progessively changes from a characteris tic stress fibre pattern to leave a network of actin filaments within each and every arborisation. Despite this massive reorganisation of th e actin cytoskeleton no changes are observed in the extent of polymeri sation of actin during arborisation. The proportion of cells in asynch ronous cultures undergoing arborisation at maximal concentrations of a gents reaches a maximum of 30%; suggesting that the effect might be me diated only in cells during a restricted period of the cell cycle. Mor e than 80% of serum-starved cells responded to these agents between 1 and 8 hours after readdition of serum, but not at other times, suggest ing that the arborisation response can occur only in the G1 phase of t he cell cycle.