MICROINJECTION OF ADP-RIBOSYLATED ACTIN INHIBITS ACTIN SYNTHESIS IN HEPATOCYTE-HEPATOMA HYBRID-CELLS

Treatment of hepatocyte-hepatoma hybrid cells with Clostridium botulin um C2 toxin led to a 167% increase in monomeric globular act in (G-act in) and to a 57% decrease in filamentous actin (F-actin) within 2 h. S imultaneously, the level of actin mRNA was specifically decreased to 4 9% and actin synthesis was significantly diminished. In contrast, trea tment of hybrid cells with phalloidin led to a decrease in G-actin to 55% and to a reciprocal increase in actin mRNA to 244% and an increase in actin synthesis. These alterations of actin synthesis depending on the G-actin/F-actin ratio corresponded to the autoregulation of actin synthesis observed in primary cultures of rat hepatocytes, Microinjec tion of C2 toxin or of phalloidin into hepatocyte-hepatoma hybrid cell s had the same effects on actin synthesis as incubation with either to xin in the culture medium. Microinjection of nonpolymerizable ADP-ribo sylated G-actin into hepatocyte-hepatoma hybrid cells specifically dec reased the incorporation of [S-35]methionine into newly synthesized ac tin within 1 h. This decrease continued for at least 19 h. Microinject ion of ADP-ribosylated actin led to rounding of cells and obvious disa ggregation of actin filaments, which might be due to capping of actin filaments by the ADP-ribosylated actin. Because stabilization of actin filaments by phalloidin before microinjection of ADP-ribosylated acti n also resulted in decreased actin synthesis, the concentration of mon omeric G-actin seems to be responsible for the regulation of actin syn thesis in hepatocyte-hepatoma hybrid cells, which can be regarded as i mmortalized hepatocytes.