Control of specific gene expression in higher organisms. Expression of mammalian genes may be controlled by repressors acting on the translation of messenger RNA

Messenger RNA synthesis in synchronized Chinese hamster ovary cells

Chinese hamster ovary cells were synchronized without inhibitors by mitotic selection and labelled in G1, S or G2 phase by incubation for 90 min with [3H]- OR [14C]uridine. Purified polyribosomes were extracted with phenol and the polyadenylated mRNA prepared by poly(U)-Sepharose chromatography. Poly-adenylated [3H]uridine-labelled mRNA from the G1 phase of the cell cycle was compared by exponential polyacrylamide gel electrophoresis in formamide with [14C] uridine-labelled polyadenylated nRNA from the S or G2 phase. The electrophoretic patterns obtained correspond to the size range expected for mRNA (7-28 S). No prominent differences were detected between mRNAs synthesized in different phases of the cell cycle. From these data we conclude that the major size classes of polyribosomal poly(A)-containing mRNA are synthesized in equal ratios throughout the cell cycle.

Cellular and molecular toxicology of lead. II. Effect of lead on delta-aminolevulinic acid synthetase of cultured cells

The effect of lead nitrate on delta-aminolevulinic acid synthetase (ALA synthetase), the first and rate-limiting enzyme of the heme biosynthetic pathway, was studied by using an established rat liver cell line (RLC-GAI). Lead was shown to produce a time-dependent increase in ALA synthetase activity, with a maximum after incubation of the cells for 24 h with 10(-5) M lead nitrate. The effect of lead was not liver-specific in that increases in enzyme activity were produced in other tissue-derived cell lines. Cycloheximide but not actinomycin D, cordycepin, or hydroxyurea, at concentrations that inhibit the synthesis of protein, RNA, and DNA, prevented the lead-associated increase in ALA synthetase activity. Heme, added to the cells as hemin, also prevented the effect of lead. These results indicate that lead induced the synthesis of ALA synthetase secondary to an inhibition of the synthesis of heme.

Synthesis and secretion of transferrin by cultured mouse hepatoma cells

The mouse hepatoma cell (Hepa-1) in tissue culture has been shown to synthesize and secrete three electrophoretically distinct transferrins. Each of these forms of transferrin has a molecular weight of 77,000, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The concentration of each form is indicated by its staining intensity, which is highest in the form with the fastest mobility and lowest in the form with the slowest mobility. The relative rate of transferrin synthesis has been determined in log-phase and stationary-phase cells; the data indicate that the relative rate of synthesis increases twofold in stationary-phase cells. When the incorporation of [3H]leucine into transferrin reaches steady state, the rate of secretion is equal to the rate of synthesis; the rate of secretion also increases twofold in stationary-phase cells. Our studies also show that transferrin synthesis accounts for 0.98% of the total protein synthesis in log-phase cells and for 1.8% in stationary-phase cells. This is the level of synthesis that has been determined by in vivo studies. We conclude that after continuous culture for several years these hepatoma cells have maintained one of the characteristics of the differentiated liver cell, namely, the ability to synthesize and secrete transferrin.

Cycloheximide causes increased accumulation of translatable mRNA for tyrosine aminotransferase and tryptophan oxygenase in livers of cortisol-treated rats

Messenger RNA activities for two cortisol-inducible enzymes, tyrosine aminotransferase and tryptophan oxygenase, have been determined by translation in a wheat germ system. The effects of cycloheximide on the two mRNA activities have been evaluated. Cortisol leads to an increase of the translatable mRNAs for tyrosine aminotransferase and tryptophan oxygenase with a maximum at approximately 6 h. Cycloheximide was administered 4 h after treatment with cortisol; 2 h later, the activities of tyrosine aminotransferase and tryptophan oxygenase mRNA had increased five-fold and two-fold, respectively, compared to the activities reached with cortisol alone. Thereafter the amount of the two translatable mRNAs declined, though 14 h after cortisol administration the mRNA activities were still several fold higher than in control animals. Application of alpha-amanitin together with cycloheximide did not prevent an increased accumulation of specific translatable mRNAs. The increase in tyrosine aminotransferase and tryptophan oxygenase activity by cortisol was immediately blocked by cycloheximide. Whereas tryptophan oxygenase activity rapidly declined after cycloheximide application, tyrosine aminotransferase activity remained at the same level. Approximately 4 h thereafter, both enzyme activities increased again.

Neurotrophic protein regulates muscle acetylcholinesterase in culture

Skeletal muscles lose acetylcholinesterase in culture as a result of denervation. A protein fraction isolated from peripheral nerves maintained the level of acetylcholinesterase in cultures of aneural embryonic muscle or denervated adult chicken muscle. These results indicate that trophic regulation of muscle acetylcholinesterase might be mediated by a protein produced by nerves.

Regulation of cardiac protein balance by hydrocortisone: interaction with insulin

In fetal mouse hearts in organ culture the rate of protein synthesis was substantially reduced and the rate of protein degradation slightly increased by hydrocortisone in the absence of insulin, but in the presence of insulin the steroid caused a small increase in protein synthesis and a significant reduction in protein degradation. Hydrocortisone promoted the net uptake (or reduced the net release) of branched-chain amino acids independent of insulin and independent of simultaneous changes in protein balance. The specific activities of the lysosomal enzymes cathepsin D and glucosaminidase were reduced by hydrocortisone in all media, whereas the specific activity of creatine kinase increased when the medium contained insulin but decreased in the absence of insulin. It is concluded that hydrocortisone regulates cardiac protein balance via alterations both in synthesis and in degradation. Some of the hormone's myocardial effects are influenced by insulin so that hydrocortisone is anabolic in its presence but catabolic in its absence.

Steroids, aspirin, and inflammation

The ability of adrenal corticosteroids to both both suppress inflammation and compromise host defenses has been well documented. Recently, a series of in vitro and in vivo experiments, based on our new knowledge of the cell biology of inflammation and the biochemistry of the phagocytic cell itself, has provided new insights into the mechanism of steroid action in the inflammatory process. Evidence is presented that pharmacologic doses of steroids are capable of inhibiting each of the steps in phagocytic-micro-organism interaction: chemotaxis, recognition and opsonization, phagocytosis, membrane fusion, and degranulation. In addition, steroid alteration of the postphagocytic superoxide production, hydrogen peroxide generation, and prostaglandin and thromboxane synthesis is described. The antiinflammatory effects of aspirin and indomethacin can be explained almost entirely by virtue of their ability to inhibit cyclooxygenase, this preventing the transformation of arachidonic acid to both prostaglandins and thromboxanes. The cortisol-induced inhibition of endoperoxides, prostaglandins, and thromboxanes (at a site proximal to the release of arachidonic acid) may well explain those antiinflammatory actions that cortisone shares with aspirin. However, patients treated with nonsteroidal antiinflammatory agents effectively combat infections. In contrast, corticosteroids have more profound effects, as can be seen by the inhibition of superoxide production, with the subsequent decrease in hydrogen peroxide generation and the diminution in release of the antibacterial lysosomal hydrolases within the phagocytic vacuole. Thus, corticosteroids interfere with the killing of microorganisms. This new understanding of the pharmacologic action of cortisol on phagocytic cells explains, we believe, how glucocorticoids alleviate inflammation while, at the same time, they permit multiplication of the offending microorganism within the phagocyte.

The stability of tyrosine aminotransferase and other proteins in enucleated rat hepatoma tissue culture cells

The role of the nucleus in bringing about the induction of tyrosine aminotransferase (TAT) by glucocorticosteroid hormone and its deinduction upon steroid removal has been studied in enucleated rat hepatoma tissue culture cells (FU5-5). Both processes require the presence of the nucleus. However, cytoplasts from preinduced cells show an initial rapid decline in enzyme activity immediately after enucleation followed by maintenance of a constant level of activity. This initial decline in enzyme activity can be partially prevented by trypan blue, an inhibitor of lysosomal activity. This suggests that the early fall in enzyme activity could be due to an increase in the level of lysosomal activity immediately after enucleation. The subsequent constant level of activity seems due to maintenance rather than synthesis and degradation since it is not affected by cycloheximide. The absence of degradation applies to other kinds of proteins in enucleated FU5-5 cells and enucleated mouse fibroblast L cells. These experiments suggest that some kind of labile RNA or protein dependent on the presence of the nucleus is required for the degradation of all classes of proteins in different kinds of cells.

Hormonal control of hyaluronic acid production in fibroblasts and its relation to nucleic acid and protein synthesis

Whole serum and elevated pH previously had been found to stimulate both cell multiplication and hyaluronic acid production by chick embryo fibroblasts in culture. In a study to determine whether cell multiplication and hyaluronic acid production both respond to a single well-defined substance, insulin was found to stimulate, and cortisol to inhibit both processes coordinately. It appears, therefore, that multiplication and differentiated function in fibroblasts respond to a common underlying regulatory signal. Inhibition of ribosomal RNA synthesis by actinomycin D does not prevent serum stimulation of hyaluronic acid production, but inhibition of total RNA synthesis does. If total RNA synthesis is inhibited only after the hyaluronic acid production has reached a new high level, it continues at that level for the next five hours. The stimulatory treatment causes an increase in the activity of the enzyme hyaluronate synthetase. Inhibition of protein synthesis prevents any increase in hyaluronic acid production, and reduces the basal level of production. Reduction of the availability of Mg2+ in the medium coordinately inhibits DNA synthesis and hyaluronic acid production. The results are discussed in the light of a model for coordinate control growth and metabolism based on the availability of Mg2+.

Discussion paper: induction by progesterone and a "maturation-promoting factor" of soluble proteins in Xenopus laevis oocytes in vitro

In vitro incubation of isolated X. laevis oocytes with progesterone induces the specific labeling of proteins during maturation. They are detected as several discrete peaks with a double-labeling technique that involves the injection of [3H] leucine into hormone-treated and [14C] leucine into control cells. The peaks are separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Identical proteins are obtained with two other steroids that provoke maturation and with p-chloromercuribenzoate. The same progesterone-induced proteins are observed even if oocytes have been treated with actinomycin D or ethidium bromide or have been enucleated. Cycloheximide suppresses the labeling of proteins that are induced by progesterone, if administered either at the time of radioactive amino acid exposure or after prelabeling the oocytes to determine whether the hormone-induced peaks are due to compartmental changes of the proteins. A cytoplasmic fraction, obtained from maturing oocytes before GVBD, promotes maturation when injected into resting oocytes and therefore contains an MPF. After this injection, GVBD occurs earlier than during incubation with progesterone and is accompanied by the formation of the same proteins and of MPF, both suppressed by cycloheximide. The possibility that MPF is an induced protein is then discussed. It has been also found that there is a decrease in membrane permeability, as evidenced by decreased leucine uptake into proteins of oocytes incubated in radioactive amino acids and exposed to progesterone. The same decrease occurs after injection of MPF-containing cytoplasm into oocytes.

Evidence for nuclear and DNA binding forms of the receptor-glucocorticoid complex from hepatoma tissue culture cells

Binding to DNA associated with cellulose has been used to investigate the receptor-glucocorticoid complex isolated from a line of rat hepatoma tissue culture cells. The amount of activated complex that bound to DNA was approximately half that which bound to nuclei. Additional results suggest the existence of two forms of the activated glucocorticoid receptor-steroid complex in about equal amounts: one form binds only to nuclei and the other binds to DNA and nuclei. The two forms also differ in their stability, with the DNA/nuclei binging form being relatively labile. The binding of either form to the appropriate acceptor is reduced by cytosol inhibitors by the same mechanism.

Early events in the induction of glutamine synthetase activity by hydrocortisone in embryonic chick neural retina

Primary cultures of 10-day embryonic chick neural retinas were used to investigate early aspects of the mechanism of hydrocortisone action on glutamine synthetase activity. As little as 2 hr of hydrocortisone exposure served to initiate significant increases in the glutamine synthetase activity levels assayed after 24 hr culture. Time course studies indicated that the increase in glutamine synthetase activity observed after 24 hr in culture resulted from a two-phase rise in activity and that cycloheximide was effective in suppressing the second-phase rise. Additional inhibition studies demonstrated that the second-phase increase in enzyme activity required continuous protein synthesis during the initial 6 hr. The evidence suggests a mechanism of hydrocortisone action involving the production of a protein which is important for the induction of glutamine synthetase activity by hydrocortisone.

A model for cytoplasm-governed gene regulation

A model of cytoplasm-governed transcription is presented. The nuclear membrane has a selective permeability towards nuclear pre-mRNA molecules which are provided with group-specific non-translated "passwords". RNA transcription on the chromatin proceeds under a dual control. One of them is gene regulation according to the Britten-Davidson and Georgiev models. The other is cytoplasm-governed regulation mediated through the selective transport of mRNA from nucleus to cytoplasm. Pre-mRNA molecules which are not "in immediate demand" by the cytoplasm and therefore accumulating the nucleus repress their own synthesis by end-product inhibition. The interrelationship of the two types of regulation in the course of cell development is discussed.

Translation of mRNA from rat-liver polysomes into tyrosine aminotransferase and tryptophan oxygenase in a protein-synthesizing system from wheat germ. Effects of cortisol on the translatable levels of mRNA for these two enzymes

Messenger RNA was isolated from rat liver polysomes by phenol/chloroform extraction and subsequent oligo(dT)-cellulose chromatography. The mRNA was translated in a protein-synthesizing system in vitro derived from wheat germ. The system was optimized in respect to Mg2+ and K+. The presence of spermidine or spermine is necessary for the synthesis of polypeptides having molecular weights of over 20 000. In the absence of the bases only small molecular weight products are formed. The amount of protein synthesized is linearly dependent on the amount of mRNA added up to concentrations of 80 mug mRNA/ml. The synthesis of tyrosine aminotransferase and tryptophan oxygenase in the system in vitro has been demonstrated by specific immunoprecipitation and sodium-dodecylsulfate polyacrylamide gel electrophoresis of the precipitate with enzyme proteins as marker. The amount of specific product formed is linearly dependent on the amount of mRNA present. The amount of translatable tyrosine aminotransferase mRNA and tryptophan oxygenase mRNA increases after administration of hydrocortisone to adrenalectomized rats. At low doses of hormone (2 mg/100 g body weight) maximal values are observed at 4 h, control levels being reached at 6-8 h after hormone application. With higher doses of hydrocortisone (20 mg/100 g body weight) maximal values are attained at 6 h, tending to control levels 14 h after treatment. The enzyme activity curves are parallel to the mRNA curves, the peak of enzyme activity occurring 2 h after the peak of mRNA activity.

The qualitative dynamics of a class of biochemical control circuits

The dynamical behavior of a class of biochemical control circuits that regulate enzyme or protein synthesis by end-product feedback is analyzed. Both inducible and repressible systems are studied and it is proven that in the former unique steady states are globally asymptotically stable. This precludes periodic solutions in these systems. A similar result holds for repressible systems under certain constraints on kinetic parameters and binding contants. However, when the reaction sequence is sufficiently long, or when a large enough number of effector molecules bind to each repressor molecule, repressible systems can show zero-amplitude ("soft") bifurcations: these are predicted by Hopf's bifurcation theorem.

The regulation of protein synthesis in animal cells by serum factors

We have investigated the regulation of protein synthesis in animal cells by serum factors. Withdrawal of serum from the medium of actively dividing Vero cells resulted in an immediate decline in the rate of peptide chain elongation (Hassell and Engelhardt, 1973). Assay of elongation factor I (EFI) activity in the post-ribosomal supernatant as well as that associated with the ribosomes revealed that serum deprivation resulted also in reduction in the activity of this factor. The decline in the activity of EFI after serum deprivation occurred to the same extent and at the same time as the decline in the in vivo rate of protein synthesis and the in vitro peptide synthetic capacity of cell-free extracts. A temporal correlation therefore exists among the in vivo rate of protein synthesis, the peptide synthetic activity of cell-free extracts, and the activity of EFI. The activity of peptidyl transferase was not altered by serum deprivation. The loss of extract peptide synthetic activity resulting from serum deprivation was reversible since serum addition to previously serum-starved cultures resulted in full restoration of activity for polyphenylalanine (polyPhe) synthesis within 3 h. Moreover, RNA synthesis was not required for this turn-on of polyPhe synthesis. Vased on these data we conclude that a translational control mechanism is operative in Vero cells deprived of serum.

Regulation of nitrate reductase at the transcriptional and translational levels in Escherichia coli

Nitrate reductase from Escherichia coli is induced by nitrate and derepressed by oxygen removal after a lag phase. Elimination of inducer, shift to aerobic conditions and addition of actinomycin D causes the decline in the rate of its synthesis, which eventually may stop. Kinetic analysis of the sensitivity of the biosynthetic process to oxygen, chloramphenicol, actinomycin D and rifampicin gave results which we interprete as evidence that oxygen (and possibly nitrate) affect simultaneously both the transcriptional and translational processes.