Effect of diabetes and insulin treatment on protein synthetic activity of rat liver ribosomes

Effects of vitamin C and E on toxic action of alcohol on partial hepatectomy-induced liver regeneration in rats

The purpose of this study was to investigate the influence of vitamins C and E on the toxic action of alcohol in rat liver regeneration. Male Sprague-Dawley rats subjected to 70% partial hepatectomy were divided into five groups (Groups 1 to 5). Rats in Groups 2 to 5 were only provided alcohol for drinking. Additionally, vitamin C, vitamin E, and vitamin C in combination with vitamin E were administered to Groups 3, 4, and 5, respectively. Alcohol inhibits liver regeneration, resulting in an increase in free radicals produced by alcohol metabolism and thus causing cellular damage and altering liver function. During liver regeneration, vitamins C and E significantly ameliorated liver injury from alcohol administration by reducing hepatic lipid peroxidation. Vitamins C and E protect against liver injury and dysfunction, attenuate lipid peroxidation, and thus may be more effective in combination than either vitamin alone against alcohol-mediated toxic effects during liver regeneration.

[Diabetic glomerulosclerosis: current status of its morphology and pathogenesis (author's transl)]

The diabetic glomerulosclerosis as a consequence of the abnormal metabolic state is characterized by an uniform thickening of the glomerular basement membrane (GBM) and an augmentation of the mesangial matrix. Both alterations begin already few years after the onset of the diabetes and are observed in all glomeruli to the same extent (=diffuse type). Later on, nodular deposits of glycoproteins are additionally found in the mesangium (=nodular type). Only these nodules are morphologically specific for the diabetic disorder. In association with both the diffuse and the nodular glomerulosclerosis, insudation of plasma can be seen in the afferent and efferent vessels and the glomerular capillary loops. The mechanism of the increase in the amount of GBM-material is not known, since contradictory data have been reported with regards to both the chemical composition and metabolism of the GBM in human and experimental diabetes. Some postulate that the abnormal deposition of GBM-mateiral is due to an excessive synthesis (anabolic disorder), others argue that a further decrease in the normally slow breakdown and disposal (catabolic disorder) might be responsible. This review presents the different pathogenetic concepts of the glomerulosclerosis and attemps to explain the possible causes for the current discrepancies.

Acute effect of ethanol on membranes of the endoplasmic reticulum and on protein synthesis in rat liver

The acute effect of ethanol on the membranes of hepatic endoplasmic reticulum, on the in vitro protein-synthetic activities of hepatic free and membrane-bound polyribosomes and on the plasma proteins of rats fasted overnight was investigated. Ethanol (0.75 g/100 g body weight) was tube-fed as a 50% (v/v) solution in saline 3 hr before sacrifice. Hepatic endoplasmic reticulum membranes from control and ethanol-treated rats were compared using the following techniques: (1) lactoperoxidase-catalyzed radioiodination of proteins and sodium dodecylsulfate-polyacrylamide gel electrophoresis, and (2) measurement of 14C-choline incorporation into membranes. Hepatic microsomal membranes from ethanol-treated rats incorporated in vitro les 125I into total proteins (as well as into the 55,000 molecular weight proteins) and incorporated in vivo less 14C-choline into microsomal membranes than membranes of control rats. Ethanol administration inhibited in vivo incorporation of 14C-leucine or 14C-phenylalanine into liver protein and plasma albumin and globulin. The data also indicate that an acute dose of ethanol reduced the in vitro protein-synthetic activity of hepatic membrane-bound polyribosomes, while free polyribosomes were relatively unaffected.

Phenobarbitone-induced stimulation of protein synthesis in liver of diabetic rat

The effect of phenobarbitone on liver weight, on the rate of protein synthesis and on the sedimentation profiles of polyribosomes from livers was studied in diabetic rats. The rate of protein synthesis by isolated postmitochondrial supernatants from diabetic rats is lower than that from normal animals. The analysis of polyribosome profiles and the effect of Sephadex chromatography on protein synthesis demonstrated that the reduction was dependent in part on polyribosomal disaggregation and in part on the presence in the cytosol of low molecular weight inhibitor(s). Phenobarbitone administration had the same effect in either diabetic or normal rats in that it increased, (a) the degree of polyribosomal aggregation, (b) the rate of protein synthesis by the isolated postmitochondrial supernatants, (c) liver weight and (d) the activity of the inducible enzyme, NADPH-cytochrome c reductase. Both polyribosomal and soluble factors appear to be involved in the phenobarbitone effect. As the diabetic rats do not secret insulin the results suggest that insulin is not involved in the control of protein synthesis by phenobarbitone. It is suggested that the intracellular redox state has a major influence on the rate of protein synthesis.

Muscle protein biosynthesis in the tumour-bearing rat. A defect in a post-initiation stage of translation

1. The decreased ability of polysomes isolated from the gastrocnemius of rats bearing the Walker 256 carcinoma to incorporate L-[14C]leucine into protein persisted in the presence of 5-10-5 M aurin tricarboxylic acid and 1-10-2 M NaF. 2. Poly(U)-directed phenylalanine incorporation by such polysome preparations was less than that of similar preparations from normal rats. 3. The ability of gastrocnemius polysomes from tumour-bearing rats to react with puromycin was markedly decreased. Cycloheximide inhibited peptidyl puromycin formation by polysome preparations from both normal and tumour-bearing rats. 4. The ability of recombined 60 S and 40 S subunits prepared from polysomes of tumour-bearing rats to carry out poly(U)-directed polyphenylalanine synthesis was much reduced when assayed over a wide range of magnesium concentrations. Cross-over experiments with subunits from normal and tumour-bearing animals suggested that this was due to a defect in the smaller ribosomal subunit.

The effect of phenobarbitone on protein synthesis by liver polyribosomes in fed and starved rats

1. The effect of phenobarbitone on the rate of protein synthesis and on the sedimentation patterns of various liver subcellular fractions containing ribosomes was studied in rats. 2. Phenobarbitone treatment increased the incorporation of [114C]leucine into protein by all preparations, provided they had not been subjected to preliminary treatment with Sephadex G-25. The phenobarbitone-induced effect on incorporation was associated with a gain in liver weight and a higher degree of polyribosomal aggregation. 3. Preparations that were treated with Sephadex G-25 incorporated more radioactivity into protein, but did not show the response to phenobarbitone treatment. 4. When the influence of starvation and phenobarbitone was studied separately on membrane-bound and membrane-free polyribosomes, it was shown that whereas both classes of polyribosomes were affected by starvation, apparently only the former class was susceptible to phenobarbitone stimulation of protein synthesis. 5. The decreased capacity for protein synthesis of polyribosomes from starved rats was independent of their association with the membranes of the endoplasmic reticulum, but resulted from polyribosomal disaggregation, from an intrinsic defect of the polyribosomes themselves and from changes in composition of the cell cap. 6. The results are discussed in relation to the problem of the control of protein biosynthesis and of the functional separation of membrane-bound and membrane-free polyribosomes.

Characteristics of membrane-bound and free hepatic ribosomes from insulin-deficient rats. I. Acute experimental diabetes mellitus

Membrane-bound and free ribosomes were prepared by discontinuous density gradient centrifugation from livers of rats 2-3 days after receiving alloxan (75 mg/kg) or streptozotocin (100 mg/kg). Hepatocytes from these animals were also examined by electron microscopy and subjected to quantitative morphometric analysis. The results indicated that the two populations of hepatic ribosomes respond differently to acute insulin deficiency. There was an overall reduction (P < 0.001) in total number of bound ribosomes per volume cytoplasm: the remaining bound ribosomes underwent a shift to smaller-sized ribosomal messenger RNA (mRNA) aggregates (P < 0.02); and the proteinsynthetic activity of these bound ribosomes was less than normal (P < 0.02) when protein synthesis was directed by endogenous mRNA. However, there was no difference between bound ribosomes from livers of normal and diabetic rats when protein synthesis was directed by polyuridylic acid. In contrast, free ribosomes were unchanged in number and degree of ribosomal mRNA aggregation, but displayed a significantly increased rate of in vitro protein synthesis (P < 0.01) as compared to normal controls. This increased protein-synthetic activity occurred when amino acid incorporation was directed by endogenous mRNA or polyuridylic acid. These changes in structure and function of bound and free hepatic ribosomes were prevented by the concomitant administration of insulin. The decrease in protein-synthetic activity of bound hepatic ribosomes from acutely diabetic rats seems to be secondary to marked disruption and disaggregation of the rough endoplasmic reticulum (RER) with production of smaller ribosomal mRNA aggregates which incorporate less amino acids into protein. Increased protein synthetic activity of free ribosome appears to be related to the ability of these ribosomes to copy mRNA more efficiently.