Effect of actinomycin D and cycloheximide on lipoproteins in blood serum and liver cytosol of rats under oxidative stress

The influence of protein synthesis inhibitors on rat lipoprotein content and composition under oxidative stress caused by cobalt chloride injection has been investigated in the present work. It has been concluded that apoprotein synthesis is very important process influencing on adaptive reactions under free-radical oxidation activation conditions. Co-administration of cobalt chloride and actinomycin D or cycloheximide (the inhibitors of the protein synthesis) has realy prevented hyperlipoproteinemia in many cases but hasn't influenced on lipoprotein oxidation. Pre-beta- and beta-lipoproteins were discussed to have mRNA pool in hepatocytes.     

Effect of vitamin E an actinomycin D on protein and RNA synthesis in rat liver

In vitro experiments showed that RNA synthesis intensity in the rat liver with E-hypovitaminosis decreases considerably while the level of the labelled precursors incorporation into protein does not differ from the norm. Under conditions of E-hypovitaminosis the inhibitory effect of actinomycin D on the RNA synthesis is pronounced more clearly as compared to the norm. In the case of the E-hypovitaminous rate liver chyme preincubation with alpha-tocopherol there is no inhibitory effect of actinomycin D on the protein biosynthesis.     

Stimulation of liver cholesterol synthesis by actinomycin D

1. An eightfold increase in the incorporation of [2-(14)C]acetate into liver cholesterol in vivo was observed 24hr. after starved rats had been given actinomycin D (0.5mg./kg. of body wt.). Liver cholesterol radioactivity declined faster in the treated animals, suggesting a greater rate of cholesterol turnover. 2. Liver slices from treated animals showed a tenfold increase in the incorporation of [2-(14)C]acetate into cholesterol; conversion into CO(2) and into fatty acids was less markedly increased, and conversion into ketone bodies was not significantly affected. 3. The patterns of conversion into liver cholesterol in vivo of the lactone and the sodium salt of mevalonic acid differed markedly. The former was converted at a faster rate and to a greater extent than the latter. Treatment with actinomycin D increased the conversion of both forms of mevalonic acid into liver cholesterol, but only to a small extent. 4. Stimulation of the incorporation of acetate into cholesterol occurred at 4hr. after the administration of actinomycin D but not at 2hr. The response was abolished by the simultaneous administration of dl-ethionine or puromycin. 5. Pre-feeding with a cholesterol-rich diet greatly diminished the stimulation of conversion of acetate into cholesterol caused by actinomycin D, though it did not completely suppress it. Adrenalectomized animals responded to the drug, but much less markedly. 6. It is concluded that actinomycin D stimulates the synthesis of cholesterol in the liver at a stage in the pathway before mevalonic acid, by a mechanism that probably requires protein synthesis. A likely site would be the beta-hydroxy-beta-methylglutaryl-CoA reductase, the rate-limiting enzyme in cholesterol biosynthesis. Some possible mechanisms by which the drug may lead to increased activity of this enzyme are considered.     

Effect of cycloheximide on protein biosynthesis in rat liver

1. The liver ribosomes of rats given cycloheximide by intraperitoneal injection incorporate less amino acid into protein than ribosomes from control rat liver when they are incubated in vitro with excess of Sephadex-treated cell sap. The effect is rapid, marked and persistent. 2. Cell sap from liver of cycloheximide-treated animals is inhibitory but the inhibition can be relieved almost entirely by treating the cell sap with Sephadex. No damage has been done to the cell-sap factors: it is suggested that the dissolved cycloheximide in the cell sap causes the inhibition. 3. Cycloheximide added in vitro inhibits amino acid incorporation into protein in the presence or absence of polyuridylic acid. The inhibition is lessened by addition of excess of cell sap but is not abolished. 4. The differences between these results and those obtained with mouse liver (Trakatellis, Montjar & Axelrod, 1965) might arise because of species differences in sensitivity to the drug.     

Effect of actinomycin D on DNA replication and c-myc expression during rat liver regeneration

In an attempt to elucidate the mechanisms that control the hepatic DNA replication, effect of low doses of actinomycin D on DNA synthesis and c-myc expression during the early stage of liver regeneration was investigated. Small amounts of actinomycin D, in amounts that had no effect on the rate of RNA synthesis in normal rats, were multiple injected at 0, 2 and 4 hours after partial hepatectomy. DNA synthesis and c-myc expression in these rats were compared with those in untreated rats. Hepatic DNA synthesis in the treated rats was delayed about 4 to 6 hours in comparison with control rats. In contrast, time course of c-myc expression in inhibitor treated rats was very similar to that in control rats.