Induction of apoptosis and changes in nuclear G-actin are mediated by different pathways: The effect of inhibitors of protein and RNA synthesis in isolated rat hepatocytes

Stressor-induced changes in the cytoskeleton, of which actin is a major com ponent, may lead to apoptosis. The role of drug-induced changes in nuclear G-actin and apoptosis was studied in freshly isolated hepatocytes. Several protein synthesis inhibitors, cycloheximide, puromycin, and emetine, induce d 10 to 15% apoptosis in hepatocytes after 4 h, as was determined by change s in nuclear morphology and flow cytometric analysis of Annexin V-positive cells. Apoptosis induced by protein synthesis inhibition could be prevented by the caspase inhibitors Z-Val-Ala-DL-Asp fluormethylketone (zVAD-fmk) an d Ac-Asp-Glu-Val-Asp-aldehyde (DEVD-cho). Several (chemical) stressors cause a rapid increase in nuclear G-actin stai ning in hepatocytes or cell lines (Meijerman et al., Biochem. Biophys. Res. Commun. 240, 697-700, 1997). The protein synthesis inhibitors also increas ed G-actin staining in nuclei after 2 h; this could not be inhibited by zVA D-fmk or DEVD-cho. Changes in the cytosolic F-actin pattern did not occur u ntil nuclear G-actin staining had already increased. The mRNA synthesis inh ibitor actinomycin D, also increased nuclear G-actin staining, but did not induce apoptosis within the studied time frame. The results suggest that th e induction of apoptosis and the increased nuclear staining of G-actin by p rotein synthesis inhibition are differently controlled. (C) 1999 Academic P ress.