Autoregulation of Actin Synthesis in Hepatocytes by Transcriptional and Posttranscriptional Mechanisms

Abstract

Treatment of rat hepatocytes with the filamentous-actin-stabilizing toxin phalloidin decreased the amount of globular actin by 77% in the cytosol and by 80% in the nucleus within 12 h. Simultaneously, actin mRNA was specifically increased by 230%. The de-novo synthesis of actin mRNA, as measured by nuclear run-on transcription, was enhanced by 250%. Treatment of cells with actinomycin D blocked the increase of actin mRNA. The apparent half-life of actin mRNA was not significantly altered during treatment with phalloidin.In contrast, the globular-actin-stabilizing botulinum C2 toxin increased the amount of cytosolic globular actin by 50% within 12 h. Simultaneously, the actin mRNA level was decreased by 62%. However, de-novo synthesis of actin mRNA was not impaired. The apparent half-life of actin mRNA was decreased by approximately 60 % during treatment with C2 toxin.The data strongly suggest an autoregulatory control of actin synthesis on the basis of the globular/ filamentous actin ratio in rat hepatocytes at the transcriptional as well as at the posttranscriptional levels.

Microinjection of ADP-ribosylated actin inhibits actin synthesis in hepatocyte-hepatoma hybrid cells

“…Autoregulation of actin synthesis by the level of monomeric Gactin or polymeric F-actin has been demonstrated recently in primary cultures of rat hepatocytes [13,14]. To prove whether Gactin is responsible for this regulation, the cellular level of monomeric actin was increased by microinjection into isolated cells.…”

“…It was recently demonstrated that autoregulation of actin synthesis in cultured hepatocytes is due to transcriptional as well as post-transcriptional mechanisms depending on the increase or decrease in the G-actin\F-actin equilibrium effected by C2 toxin and phalloidin [14]. However, it could not be excluded that direct effects of the toxins might influence actin synthesis.…”

“…However, it could not be excluded that direct effects of the toxins might influence actin synthesis. Moreover it was not clear whether the actual cellular concentrations of G-actin or of F-actin, or even the G-actin\F-actin ratio, is responsible for the regulation of actin synthesis in hepatocytes [14].…”

“…An increase in the G-actin\F-actin ratio caused by C2 toxin was followed by a decrease in actin mRNA, whereas a decrease in G-actin\F-actin ratio caused by phalloidin was followed by an increase in actin mRNA [13]. Recently it has been demonstrated that the decrease in the G-actin\F-actin ratio in response to phalloidin treatment results in a transcriptional enhancement of actin synthesis, whereas the increase in the G-actin\F-actin ratio in response to C2 toxin treatment is accompanied by a decreased stability of actin mRNA [14]. The latter effect was confirmed in 3T3 cells and HeLa cells, which showed a post-transcriptional decrease in actin synthesis after treatment with C2 toxin and latrunculin A, a metabolite from the Red Sea sponge that binds G-actin and prevents its assembly [15].…”

Transcriptional activation of the haem oxygenase-1 gene by cGMP via a cAMP response element/activator protein-1 element in primary cultures of rat hepatocytes

“…Hepatocytes (1i10() were used for isolation of nuclei for the nuclear run-on assay. The isolation procedure was performed as described previously [27]. The nuclear run-on reaction was performed with 2i10' nuclei in a vol.…”