Treatment of hepatocyte-
hepatoma hybrid cells with
Clostridium botulinum C2 toxin led to a 167% increase in monomeric globular actin (
G-actin) and to a 57% decrease in filamentous actin (
F-actin) within 2 h. Simultaneously, the level of actin
mRNA was specifically decreased to 49% and actin synthesis was significantly diminished. In contrast, treatment 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. Microinjection of C2 toxin or of
phalloidin into hepatocyte-
hepatoma hybrid cells had the same effects on actin synthesis as incubation with either toxin in the culture medium. Microinjection of nonpolymerizable
ADP-ribosylated
G-actin into hepatocyte-
hepatoma hybrid cells specifically decreased the incorporation of [35S]
methionine into newly synthesized actin within 1 h. This decrease continued for at least 19 h. Microinjection of
ADP-ribosylated actin led to rounding of cells and obvious disaggregation 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 actin also resulted in decreased actin synthesis, the concentration of monomeric
G-actin seems to be responsible for the regulation of actin synthesis in hepatocyte-
hepatoma hybrid cells, which can be regarded as immortalized hepatocytes.