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

Effect of carbohydrate and corticosteroids on activity of -glucosidases in intestine of the infant rat

The activities of intestinal sucrase and isomaltase are not detectable in rats before 15-16 days of age, but administration of corticosteroids precociously induces the activities of these two alpha-glucosidases. 9-day old rats were removed from their mothers, warmed in an incubator, and fed by constant infusion through gastrostomies. The basic diet was a soya preparation to which various sugars were added. When the diet contained 2% sucrose, diarrhea ensued for 48 hr, but subsided when intestinal sucrase and isomaltase appeared precociously. In animals fed sucrose, the activities of sucrase and isomaltase were markedly increased as compared to animals on carbohydrate-free diets (sucrase 2.41+/-0.23 vs. 0.63+/-0.13 U, isomaltase 3.43+/-0.42 vs. 0.78+/-0.18 U). Maltase activity was doubled, while lactase was not altered significantly. The mitotic index of crypt cells, the depth of crypts, and incorporation of thymidine-(3)H into DNA were increased. In adrenalectomized rats, activities of sucrase and isomaltase were not detected nor induced by sucrose. Steroids given to adrenalectomized rats caused appearance of the enzymes; but if cortisone and sucrose were given together, there was synergism evidenced by a marked increase in activities (sucrase 7.2+/-1.1 vs. 0.68+/-0.12 U). In contrast to observations in adult animals, the effect of sucrose on alpha-glucosidases in developing animals demands the participation of the adrenal gland.

The metabolism of testosterone and dihydrotestosterone in an androgen-dependent tumour. A possible correlation between dihydrotestosterone and tumour growth in vivo

The effects of dihydrotestosterone (17beta-hydroxy-5alpha-androstan-3-one) and testosterone on the growth of the androgen-dependent Shionogi SC-115 tumour in mice have been compared and the metabolites in the tumour arising from each steroid have been identified. After the transfer of SC-115 tumour cells to castrated male mice, treatment of the recipients with dihydrotestosterone produced a striking proliferative response that enabled earlier tumour detection and led to a higher tumour incidence than obtained with testosterone. At short intervals after the intravenous injection of 200muCi of [1,2-(3)H]testosterone the amounts of radioactivity in tumour, muscle and seminal vesicles were almost equal. The metabolism of [1,2-(3)H]testosterone in tumour and muscle was slight in comparison with the extensive metabolism in seminal vesciles. Whereas up to 7% of the total neutral steroid recovered from whole tumour tissue and isolated nuclei was in the form of [1,2-(3)H]dihydrotestosterone, the amount of this compound in the corresponding preparations from seminal vesciles was several times greater. When the metabolism of [1,2-(3)H]dihydrotestosterone in tumour tissue was studied, it was found that more than 60% of the total neutral steroid in both cytoplasm and nuclei consisted of [1,2-(3)H]dihydrotestosterone. Thus much higher intracellular concentrations of dihydrotestosterone occurred with the administration of this steroid than with testosterone. Tumour cell proliferation was suppressed by oestradiol and the amount of androgen in nuclei was significantly decreased by high doses of this hormone.

Expression of differentiated functions in hepatoma cell hybrids: reappearance of tyrosine aminotransferase inducibility after the loss of chromosomes

HYBRIDS FROM A CROSS OF RAT HEPATOMA CELLS WITH DIPLOID EPITHELIAL CELLS FROM RAT LIVER HAVE BEEN STUDIED WITH RESPECT TO KARYOTYPE AND EXPRESSION OF TWO FUNCTIONS LIMITED TO THE HEPATOMA PARENT: high level of the enzyme tyrosine aminotransferase (EC 2.6.1.5; L-tyrosine:2-oxoglutarate aminotransferase) and its inducibility with steroid hormones. The hybrids that contain the complete chromosomal complements from both parents show low enzyme activity and no inducibility. One hybrid clone, and all of its derivatives, which have lost 30-40% of the chromosomes initially present, show enzyme inducibility. Induction of tyrosine aminotransferase in the hepatoma and hybrid cells responds similarly to inhibition by cycloheximide and actinomycin D, and to steroid concentration. The enzymes from induced and noninduced hepatoma cells and from induced hybrid cells are similar in heat sensitivity and intracellular distribution; those from noninduced hybrid and diploid rat epithelial cells are different.

Regulation of protein synthesis in HeLa cells. 3. Inhibition during poliovirus infection

The mechanism of inhibition of host cell protein synthesis by poliovirus has been studied by examining the behavior of host polyribosomes and polyribosome-associated messenger ribonucleic acid (mRNA). Virus infection appears to result in a specific inhibition of the initiation of translation of host cell mRNA. The site of inhibition does not appear to be mRNA itself. Human cells respond to virus infection by producing a factor antagonistic to the virus inhibition which promotes the initiation of host mRNA translation. The production of the host factor is sensitive to actinomycin; however, the behavior of the host cell factor and that of host mRNA appear distinctly different.

Expression of tyrosine aminotransferase activity in somatic-cell heterokaryons: evidence for negative control of enzyme expression

Cells of the tyrosine aminotransferase (EC 2.6.1.5)-inducible line, HTC, were fused with those of the BRL-62 line, which contain little or no enzyme and are unresponsive to the usual steroid inducers. Histochemical examination of the resulting heterokaryons within 24 hr of the fusion showed them to have little or no enzyme and to be unresponsive to steroids. These experiments disclose that the observed negative dominance is universal in the fused cells and that such dominance occurs promptly after fusion, when loss of chromosomes has not occurred.

Actinomycin-D enhancement of glutamine synthetase activity in chick embryo retinas cultured in the presence of cortisol

Cortisol can prematurely induce glutamine synthetase activity in the chick embryo retina. Under appropriate conditions, this effect can be enhanced by addition of low levels of actinomycin D; this enhancement is reversibly inhibited by cycloheximide. The magnitude of the effect is a function of time of exposure to hormone as well as antibiotic and is also a function of the age of the embryo; within the limits of the present study it did not appear to be a function of actinomycin-D concentration. The data are discussed in terms of current ideas of possible control mechanisms in animal cells.

On the differential cytotoxicity of actinomycin D

Actinomycin D (AMD) at concentrations that inhibit cellular RNA synthesis by 85% or more causes an acute phase of lethal cell degeneration in HeLa cultures beginning as early as 3 hr after drug exposure, resulting in the nearly complete loss of viable cells by 12 hr. The loss of cells during this acute phase of lethality is closely dose dependent. Vero, WI38, or L cells are not susceptible to this early acute cyto-intoxication by AMD, and may begin to die only after 1-2 days. Differential susceptibility to acute cyto-intoxication by AMD, or other inhibitors of RNA synthesis (daunomycin or nogalamycin), among different types of cultured cells is analogous to that observed in vivo in certain tissues and tumors, and cannot be accounted for by differences in the effect of AMD on RNA, DNA, or protein syntheses, or by the over-all loss of preformed RNA. Actinomycin D in a dose that inhibits RNA synthesis causes an equivalent loss of the prelabeled RNA in all the cell types studied. Inhibition of protein synthesis with streptovitacin A or of DNA synthesis with hydroxyurea does not cause acute lethal injury in HeLa cells as does inhibition of RNA synthesis. Furthermore, since Vero or L cells divide at about the same rate as HeLa cells, no correlation can be drawn between the rate of cell proliferation and susceptibility to the cytotoxicity of AMD. Susceptibile cells are most vulnerable to intoxication by AMD in the G(1)-S interphase or early S phase. Inhibition of protein synthesis (which protects cells against damage by other agents affecting DNA) does not protect against AMD-induced injury. Although HeLa cells bind more AMD at a given dose than Vero or L cells, the latter cell types, given higher doses, can be made to bind proportionally more AMD without succumbing to acute cyto-intoxication. It is suggested that the differential susceptibility of these cell types to acute poisoning by AMD may reflect differences among various cells in the function or stability of certain RNA species not directly involved in translation whose presence is vital to cells. In HeLa cells, these critical species of RNA are presumed to have a short half-life.

Evidence for induction by cortisol in vitro of a protein inhibitor of transport and phosphorylation processes in rat thymocytes

Studies of intact rat thymocytes incubated in vitro with cortisol, actinomycin D, puromycin, and cycloheximide indicate that distinct inhibitory effects of cortisol on transport and phosphorylation are due to an action on mRNA synthesis with consequent induction of synthesis of protein(s) with inhibitory influence. Incubation of thymocytes with cortisol results in inhibition of the rate of labeled orthophosphate incorporation into ATP and the entry of rubidium ion and hexoses into the cells. Continuing protein synthesis is required for the progressive and persistent manifestation of the inhibitory effects of the steroid. RNA synthesis is also required during the initial phase of incubation of cells with cortisol, but significant inhibitory effects of cortisol, once initiated, are evident for at least 60-120 min after addition of actinomycin D. In contrast, addition of cycloheximide some time after cortisol results in prevention or reversal of the effects of the steroid. In the absence of cortisol, the antibiotics exert relatively little effect on orthophosphate incorporation and on the transport processes studied. It is suggested that the sequence of events leading to dissolution of thymocytes exposed to cortisol is initiated by the synthesis of mRNA coding for inhibitory protein(s) with more rapid turnover rates than that of the mRNA, and that these events are modulated by the relative sensitivity of different cellular processes to the protein inhibitor(s).

Partial purification by affinity chromatography of tyrosine aminotransferase-synthesizing ribosomes from hepatoma tissue culture cells

This report describes a cell-free protein-synthesizing system derived from hepatoma tissue culture cells in which the synthesis of tyrosine aminotransferase is enhanced in relation to total protein synthesis. The enhancement was achieved by using affinity chromatography to select a specific fraction of ribosomes in which the proportion of tyrosine aminotransferase synthesis is higher than it is in unfractionated ribosomes.

Specific cytoplasmic glucocorticoid hormone receptors in hepatoma tissue culture cells

Kinetic and equilibrium studies are presented for the reversible binding of [(3)H]dexamethasone by "specific" macromolecular receptors in the cytoplasmic fraction of cultured rat hepatoma cells. As in the case of the nuclear receptors in the same cells, the binding affinities of various steroids for the cytoplasmic receptors are closely correlated with the activities of these compounds as inducers of both tyrosine aminotransferase (EC 2.6.1.5) and cell adhesiveness. This suggests that the binding reaction is important for the biological effects of the hormones. Steroid-binding activity is inhibited by various proteases, mercurials, and 1 M KCl, but not by DNase or RNase. The receptors sediment in sucrose gradients in 0.5 M KCl near 4S, and at lower ionic strength near 7S; some of their physical properties are altered upon binding steroid. Bound dexamethasone can be recovered from the receptors as the unaltered steroid.

Post-transcriptional control of interferon synthesis

Low to moderate doses of cycloheximide had a stimulatory effect on interferon production in rabbit kidney cell cultures treated with double-stranded polyinosinate-polycytidylate (poly I:poly C). A very marked stimulation occurred in the presence of a dose of cycloheximide inhibiting amino acid incorporation into total cellular protein by about 75%. Higher doses of cycloheximide caused a shift in interferon release towards later intervals and a gradual decrease in the overall degree of stimulation. An even greater increase in the amount of interferon produced was observed if cells were treated with cycloheximide for only 3 to 4 hr immediately after their exposure to poly I:poly C. Under the latter conditions, a rapid burst of interferon production occurred after the reversal of cycloheximide action. Treatment with a high dose of actinomycin D before the reversal of cycloheximide action caused a further increase and a marked prolongation of interferon production. It is postulated that inhibitors of protein synthesis suppress the accumulation of a cellular regulatory protein (repressor) which interacts with the interferon messenger ribonucleic acid mRNA and thereby prevents its translation. Therefore, active interferon mRNA can apparently accumulate in rabbit kidney cells which, after exposure to poly I:poly C, are kept in the presence of an inhibitor of protein synthesis. Some of this accumulated interferon mRNA can be translated during a partial block of cellular protein synthesis, but its most efficient translation occurs after the reversal of the action of the protein synthesis inhibitor.

Regulation of sugar transport in Neurospora crassa

Sugar uptake systems in Neurospora crassa are catabolically repressed by glucose. Synthesis of a low K(m) glucose uptake system (system II) in Neurospora is derepressed during starvation for an externally supplied source of carbon and energy. Fasting also results in the derepression of uptake systems for fructose, galactose, and lactose. In contrast to the repression observed when cells were grown on glucose, sucrose, or fructose, system II was not repressed by growth on tryptone and casein hydrolysate. System II was inactivated in the presence of 0.1 m glucose and glucose plus cycloheximide but not by cycloheximide alone. Inactivation followed first-order kinetics with a half-time of 40 min. The addition of glycerol to the uptake medium had no significant effect on the kinetics of 3-0-methyl glucose uptake, suggesting that the system was not feedback inhibitable by catabolites of glycerol metabolism.