Inhibition of the proliferation of Daudi cells by exposure to human lymphoblastoid interferons is associated with an early and marked decrease in the incorporation into DNA of exogenous [3H]thymidine when cells are incubated with trace amounts of this precursor. In contrast, incorporation of exogenous deoxyadenosine into DNA is unchanged under the same conditions. Interferon treatment results in a lowering of thymidine kinase activity, an effect which may be largely responsible for the inhibition of incorporation of labelled thymidine into DNA. At higher concentrations of exogenous thymidine, which minimize the contribution of intracellular sources to the dTTP pool, the inhibition of thymidine incorporation is abolished. Under conditions in which exogenous thymidine is rigorously excluded from the medium or, conversely, in which cells are entirely dependent on exogenous thymidine for growth, the magnitude of the inhibition of cell proliferation by interferons is the same as under normal culture conditions. We conclude that, even though cell growth is impaired, the rate of DNA synthesis is not grossly inhibited up to 48 h after commencement of interferon treatment. Furthermore, changes in neither the utilization of exogenous thymidine nor the synthesis of nucleotides de novo are responsible for the effect on cell proliferation.
In rats fed on a protein-deficient diet, albumin synthesis as a percentage of total liver protein synthesis falls from the normal value of approx. 15% to about 8%. We have extracted total cytoplasmic RNA from individual rat livers and measured the concentration of active albumin mRNA by translation in a reticulocyte lysate system from which the endogenous mRNA had been removed [Pelham & Jackson (1976) Eur. J. Biochem. 67, 247-256]. In this messenger-dependent system it is possible to measure the synthesis of albumin as a proportion of the overall protein synthesis promoted by the addition of the hepatic RNA. The results show that the concentration of translatable albumin mRNA in samples of total cytoplasmic RNA from livers of protein-deficient rats is decreased markedly. These findings suggest that dietary protein supply affects selectively the synthesis and/or functional stability of albumin mRNA in rat liver.
Treatment of the Daudi line of human lymphoblastoid cells with concentrations of human interferons within the physiological range progressively inhibits cell proliferation over 1-4 days. Rigorous measurement of the overall rate of protein synthesis during this period, using a concentration of [3H]phenylalanine sufficient to equalize the specific radioactivity of intracellular and extracellular precursor pools, shows that protein synthesis becomes progressively inhibited as the growth inhibition develops. There is a strong correlation between inhibition of amino acid incorporation and inhibition of cell proliferation. In contrast, we find no evidence for any increase in protein degradation rate under these conditions. These results suggest that interferon treatment of susceptible cells can inhibit protein synthesis even in the absence of virus infection and that this inhibition is of a sufficient magnitude to account for the anti-proliferative effect.
INTRODUCTIONThe ability of interferons to inhibit viral replication in infected cells through effects on [4][5][6]. On the other hand,-inhibition of incorporation of labelled amino acids has been observed in some systems in which cell proliferation is inhibited [7][8][9]. The human lymphoblastoid Daudi cell line is extremely sensitive in exhibiting a marked suppression of growth when human interferons are added in culture [10][11][12]. We have investigated, using this system, the extent to which the changes in growth rate can be correlated with changes in protein synthesis. In order to do this it is necessary to quantify both processes rigorously. Such an approach also allows indirect estimates to be made of the rate of protein breakdown, and of the relative contributions of changes in synthesis versus breakdown to the inhibition of cell growth caused by interferon treatment.The rate of incorporation of a radioactive amino acid into protein reflects both the actual rate at which protein is synthesized and the specific radioactivity of the precursor which is being incorporated. If the values of the latter are different between two populations of cells then spurious conclusions can be drawn. In an attempt to overcome any such problems we have adapted procedures developed for measurement of protein synthesis rates in perfused organs [13] and in tissues of
To determine the subcellular distribution of initiation-factor eIF-2 activity, Ehrlich ascites-cell homogenates were fractionated to give (a) a rapidly sedimenting fraction, (b) a microsomal fraction and (c) post-microsomal supernatant. The first two fractions were washed in 0.5 M-KCI to render the associated protein-synthesis factor soluble. As much as 60% of the total recoverable eIF-2 was obtained from the rapidly sedimenting material.
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