Berlin · New York Autoregulation of Actin Synthesis by Physiological Alterations of the G-Actin Level in Hepatocytes

Summary: Hypotonie treatment of cultured rat hepatocytes significantly decreased the monomeric G-actin level by 18 % after 120 min while the level of filamentous F-actin remained essentially unchanged. Simultaneously the level of cellular actin mRNA was increased by 53%. Incubation of hepatocytes for 120 min with the F-actin stabilizing toxin phalloidin from Amanita phalloides led to a decrease of G-actin by 70 % and an increase of F-actin by 55%. Although the toxin dependent decrease of G-actin was much more pronounced than the decrease after hypotonic treatment, the increase of actin mRNA was similar under both conditions. Simultaneous treatment with hypotonic medium did not result in a further decrease of the G-actin level. On the other hand, the G-actin elevating C2 toxin from Clostridium botulinum completely blocked the effects of osmotic stress on G-actin and actin-mRNA content. The results demonstrate that already an essentially physiological decrease of G-actin without alterations of F-actin results in a substancial enhancement of the actin mRNA level, indicating the physiological significance of this autoregulation. Introduction the equilibrium between G- and F-actin can be shifted A... ~. ι ι ι r. to the monomeric or to the polymeric form by biologicalActm is a main component of the cytoskeletal frame-... * · · r i. / ™- ι, „ „..., ,. toxins, which arrest actin in one of the two forms. Phal