THE STABILITY OF LIVER MESSENGER RNA

Messenger ribonucleic acid of cerebral nuclei

1. RNA was isolated from crude nuclear preparations and from ribosomes derived from rat brain and liver. Nuclear RNA was obtained by lysis of the nuclei with sodium dodecyl sulphate, followed by denaturation and removal of DNA and protein with hot phenol. 2. Base composition analyses indicated that the cerebral nuclear RNA preparation contained a higher proportion of non-ribosomal RNA than the analogous hepatic preparation. 3. Sucrose-density-gradient analyses revealed a heterogeneous profile for each nuclear RNA preparation, with two major peaks possessing the sedimentation properties of ribosomal RNA (18s and 28s). 4. Template activities of both preparations were widely distributed through the sucrose density gradients. 5. The cerebral nuclear RNA preparation was more active than the hepatic nuclear RNA preparation in promoting amino acid incorporation in cell-free systems from Escherichia coli and rat brain. 6. Cerebral nuclear RNA stimulated amino acid incorporation in a cerebral ribosomal system even in the presence of an excess of purified E. coli transfer RNA. 7. It is concluded that a significant proportion of cerebral nuclear RNA has the characteristics of messenger RNA.

Synthesis and degradation of phosphoenolpyruvate carboxylase in rat liver and adipose tissue. Changes during a starvation-re-feeding cycle

A specific antibody against liver cytosol phosphoenolpyruvate carboxylase (EC 4.1.1.32) was used to isolate the enzyme from liver and adipose tissue. With this technique we have shown that phosphoenolpyruvate carboxylase synthesis in starved rats accounts for 3% of the total synthesis of cytosol protein in each tissue. Re-feeding starved animals decreases this relative rate of phosphoenolpyruvate carboxylase synthesis to 0.2% and 1% respectively in liver and adipose tissue, and the activity of the enzyme in each tissue is decreased to 25% of the starvation value. An additional starvation period is accompanied by an increased rate of enzyme synthesis, but the response to starvation is considerably slower than that caused by re-feeding. The degradation rate of phosphoenolpyruvate carboxylase is also subject to regulation. Thus re-feeding starved animals decreases the half-life of the enzyme in liver from 13h to 5.2h, but the rapid rate of degradation is maintained at least during the first 20h of subsequent starvation. Only slight changes in the degradation rate of phosphoenolpyruvate carboxylase are found in adipose tissue. We conclude that the large alterations in the rate of enzyme synthesis during a starvation-re-feeding cycle are the major cause of fluctuations in activity.

LONG-LIVED MESSENGER RNA: EVIDENCE FROM COTTON SEED GERMINATION

In germinating cotton embryos the partial inhibition of RNA synthesis by actinomycin D does not inhibit the incorporation of leucine-(14)C into soluble protein nor cause a loss of polyribosomes during the first 16 hours of germination. This suggests that the protein synthesis observed during this period is directed by messenger RNA which exists in the mature seed and which is not rapidly degraded and resynthesized.

Physical change in cytoplasmic messenger ribonucleoproteins in cells treated with inhibitors of mRNA transcription

Exposure of intact cells to UV light brings about cross-linking of polyadenylated mRNA to a set of cytoplasmic proteins which are in direct contact with the mRNA in vivo. Substantial amounts of an additional protein of molecular weight 38,000 (38K) become cross-linked to the mRNA when cells are treated with inhibitors of mRNA synthesis (actinomycin D, camptothecin, and 5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole) or after infection with vesicular stomatitis virus. Cordycepin, which inhibits polyadenylation but not mRNA synthesis, has no such effect. Inhibitors of protein synthesis and of rRNA synthesis are also without effect on 38K cross-linking to mRNA. The onset of the effect of inhibitors of mRNA synthesis on the UV cross-linkable interaction between mRNA and 38K is rapid and reaches a maximal level in less than 60 min, and it is completely and rapidly reversible. In cells treated with actinomycin D, the amount of 38K which becomes cross-linked to mRNA is proportional to the extent of inhibition of mRNA synthesis. The association of 38K with mRNA during transcriptional arrest does not require protein synthesis because simultaneous treatment with the protein synthesis inhibitor emetine does not interfere with it. The effectors which promote the interaction of 38K with mRNA do not affect the proteins which are in contact with polyadenylated heterogeneous nuclear RNA and do not markedly affect protein synthesis in the cell. The 38K protein can be isolated with the polyribosomal polyadenylated fraction from which it was purified, and monoclonal antibodies against it were prepared. Immunofluorescence microscopy shows mostly cytoplasmic and some nuclear staining. These observations demonstrate that commonly used inhibitors of transcription affect the physical state of messenger ribonucleoproteins in vivo.

Physical change in cytoplasmic messenger ribonucleoproteins in cells treated with inhibitors of mRNA transcription

Exposure of intact cells to UV light brings about cross-linking of polyadenylated mRNA to a set of cytoplasmic proteins which are in direct contact with the mRNA in vivo. Substantial amounts of an additional protein of molecular weight 38,000 (38K) become cross-linked to the mRNA when cells are treated with inhibitors of mRNA synthesis (actinomycin D, camptothecin, and 5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole) or after infection with vesicular stomatitis virus. Cordycepin, which inhibits polyadenylation but not mRNA synthesis, has no such effect. Inhibitors of protein synthesis and of rRNA synthesis are also without effect on 38K cross-linking to mRNA. The onset of the effect of inhibitors of mRNA synthesis on the UV cross-linkable interaction between mRNA and 38K is rapid and reaches a maximal level in less than 60 min, and it is completely and rapidly reversible. In cells treated with actinomycin D, the amount of 38K which becomes cross-linked to mRNA is proportional to the extent of inhibition of mRNA synthesis. The association of 38K with mRNA during transcriptional arrest does not require protein synthesis because simultaneous treatment with the protein synthesis inhibitor emetine does not interfere with it. The effectors which promote the interaction of 38K with mRNA do not affect the proteins which are in contact with polyadenylated heterogeneous nuclear RNA and do not markedly affect protein synthesis in the cell. The 38K protein can be isolated with the polyribosomal polyadenylated fraction from which it was purified, and monoclonal antibodies against it were prepared. Immunofluorescence microscopy shows mostly cytoplasmic and some nuclear staining. These observations demonstrate that commonly used inhibitors of transcription affect the physical state of messenger ribonucleoproteins in vivo.

Changes in rat pancreatic protein synthesis after a single feeding with diets containing raw or heated soybeans

The patterns of protein synthesis following the administration of a single meal containing defatted ground soybeans (RSD) or heated defatted ground soybeans (HSD) were compared. A double label method was used so that the determination of the relative rate of synthesis of an enzyme could not be obscured by a possible alteration of the activity or quantity of the enzyme. Rats were given one meal of RSD or HSD and were subsequently killed at different times after the meal. Eight hours after the meal, the relative rate of synthesis of one of the three trypsinogens was substantially increased with RSD feeding, whereas that of the amylases and one chymotrypsinogen were somewhat lower. The synthetic rate of lipase, ribonclease, proelastase, another chymotrypsinogen and of two trypsinogens was unaffected when feeding RSD is compared to feeding HSD. The relative rate of synthesis of one of the trypsinogens was unaffected 8 hours after RSD feeding, but was increased 16 hours after RSD feeding. Actinomycin D could suppress the effects of RSD feeding on the protein synthetic rate of some, but not of all, secretory proteins.

Stimulation of in vitro translation of messenger RNA by actinomycin D and cordycepin

Actinomycin D and cordycepin were tested for their effect on translation in the wheat germ embryo extract and reticulocyte lysate assays for in vitro protein synthesis. Both drugs were found to stimulate the incorporation of 35S-labeled methionine into protein up to threefold as compared to control assays. This was true for synthesis directed by murine myeloma polyadenylate-containing RNA, tobacco mosaic virus RNA, and endogenous reticulocyte messenger RNA.

Nuclear and cytoplasmic RNase-activity in regenerating mouse liver

Nuclear and cytoplasmic RNase activities at pH 5.0 and 7.6 were analyzed in regenerating mouse liver at 6, 12, 24, 48, and 72 h after partial hepatectomy. Two different nucleus-isolation methods were used, one in a EDTA-spermidine medium free from divalent cations, and one in a sucrose medium containing these ions. During regeneration, the cytoplasmic alkaline RNase activity in the sucrose medium was unchanged, but in the spermidine medium showed an increase toward the end of the period. Also the cytoplasmic acid RNase activity was unchanged in sucrose medium, whereas in the spermidine it slightly increased during regeneration. The nuclear alkaline RNase activity showed a notable peak 6 h after the operation and later decreased. Also the nuclear acid RNase activity displayed a similar marked peak 6 h after operation, then decreased, but remained high throughout the period. The nuclear RNase activities were about 1% of the corresponding cytoplasmic RNase activities. The absolute activities varied greatly according to the nucleus-isolation methods. In the controls, the absolute activity of nuclear alkaline RNase was slightly above (1.2 times) that of the corresponding acid activity after the spermidine method. After the sucrose method the nuclear alkaline activity was 2.7 times that of the acid activity. The absoluted activity of cytoplasmic alkaline RNase was slightly above (1.2 times) the acid activity after the spermidine method but after the sucrose method it was only 0.25 times that of the acid activity. In sham-operated animals, cytoplasmic acid and alkaline RNase activities generally were fairly similar to the normal value, but corresponding nuclear activities showed marked variations indicating an influence by anesthesia.

Ribonucleic acid synthesis in porphyric rat liver induced by 3,5-dicarbethoxy-1,4-dihydrocollidine

1. Administration of 3,5-dicarbethoxy-1,4-dihydrocollidine to rats caused a marked increase in the activity of delta-aminolaevulinic acid synthetase as well as a slight net increase in RNA in the livers. 2. 3,5-Dicarbethoxy-1,4-dihydrocollidine primarily stimulated the synthesis of RNA in the nucleus of the liver cell. 3. The decrease in RNA synthesis after administration of this drug resulted in a rapid decrease in the activity of this induced enzyme. This was also confirmed by treatment with actinomycin D. 4. From kinetic experiments on synthesis of RNA and delta-aminolaevulinic acid synthetase in vivo the induction mechanism of this enzyme was discussed.

Stability of cytoplasmic messenger RNA in HeLa cels

The metabolic stability of pulse-labeled or long-term labeled messenger RNA (mRNA) from cytoplasmic free polysomes was measured in HeLa cells, with chase conditions that do not involve inhibitors of RNA synthesis, and chromatography on benzoylated-DEAE-cellulose or poly(T)-cellulose for isolation of mRNA. For these studies, a new chase technique was developed that allows analysis of the stability of mRNA labeled during a short [(3)H]uridine pulse. Pulse-labeled and long-term labeled mRNA were found to decay with an estimated average half-life of about 2 and 3 days, respectively, much longer than hitherto assumed.

Characterization of the rapidly labeled hybridizable RNA synthesized in L5178Y mouse leukemic cells. Hybridization lifetime studies

An attempt is made to characterize the rapidly labeled hybridizable RNA of L5178Y mouse leukemic cells which has been shown to have similar base sequences when synthesized in two different stages of the cell cycle. The size of rapidly labeled RNA molecules was heterogeneous. For labeling times of 20 min or less, the per cent of hybridization was maximal. With longer labeling times, the per cent of hybridization decreased as radioactivity appeared in long-lived species of low hybridization efficiency; the radioactivity profile resembled the optical density profile in sucrose gradients. The lifetime of newly synthesized hybridizable RNA was studied by pulse labeling exponentially growing cells and then "chasing" with nonradioactive uridine. The per cent of hybridization was studied as a function of chase time. Three RNA groups, which comprised different proportions of rapidly labeled hybridizable RNA, were distinguished. The short-lived group had a half-life of 10 min, much less than the values reported in the literature for messenger RNA of mammalian cells. The half-life of 1-1(1/2) hr observed for a medium-lived group more closely corresponds to that of messenger RNA. A long-lived group had a half-life of approximately 20 hr. Specific activity measurements during chase indicate the presence of a "pool" of labeled uridine derivatives. The uridine of this pool appears to be nonexchangeable with but dilutable by exogenous uridine. A nontoxic concentration of actinomycin D was added to the chase media in an attempt to block the "pool effect". A rapidly degradable RNA was demonstrable both by specific activity and per cent of hybridization measurements.

The effect of whole-body x-irradiation of guinea pigs on liver ribonucleic acid synthesis

1. Liver RNA synthesis was studied within 24h after whole-body X-irradiation of guinea pigs that had been starved for 22-24h. 2. Microsomal RNA was labelled in vivo for 3h with [(14)C]orotic acid and the isolated labelled RNA was fractionated by sucrose-density-gradient centrifugation. Incorporation was 50-100% higher between 3 and 12h after 2000rd X-irradiation and at 22h was not elevated any further. Whole nuclear RNA was labelled with [(14)C]orotic acid for 15min. At 5h after irradiation the incorporation showed a 50-100% increase. Incorporation increased in all types of RNA studied. 3. The RNA phosphorus/DNA phosphorus ratio of whole liver gradually increased after X-irradiation. Maximal increase was found between 24 and 36h, which corresponds to a value about 40% above that of the starved control. The RNA phosphorus content of isolated ribonucleoproteins obtained from various cell fractions of the liver was similarly increased after X-irradiation. 4. Liver microsomes were obtained from X-irradiated and control animals. Microsomes were incubated in vitro with [(14)C]phenylalanine in the presence and absence of polyuridylic acid. After the incubation the microsomes were fractionated by sucrose-density-gradient centrifugation. The polyuridylic acid enhancement was twice as great in the microsomes of the control preparation as in the irradiated one. The experiment demonstrated a higher saturation of microsomes by endogenous messenger after X-irradiation. 5. RNA polymerase activity of the purified nuclear preparation was assayed. The activity of the Mg(2+)-dependent RNA polymerase activity was 50 and 200% respectively above the control values at 6 and 9h after X-irradiation. 6. Animals were treated with actinomycin D shortly before X-irradiation. This treatment abolished the radiation-induced enrichment of polyribosomes and the increase of protein-synthesizing activity. The effect of X-irradiation on the transcription of the genetic code of the liver is discussed.

Further studies on phosphorylation and the thiol-disulphide ratio of histones in growth and development

1. The proportion of thiol groups in the total thiol+disulphide of histone extracts from fertilized eggs from Echinus and Psammechinus was increased during periods of structural alterations in the nucleus. 2. The probable start of DNA synthesis in the fertilized eggs coincided with periods of maximum thiol content. 3. Histone extracts from rat liver and regenerating liver were predominantly in the thiol form and no significant variations could be detected during the first 30hr. after partial hepatectomy. 4. An assay system was developed to follow the phosphorylation believed to be associated with the arginine-rich histone F3. Phosphorylation increased by 50% at 1 and 2hr. after partial hepatectomy. The phosphate content also increased during the period of DNA synthesis. 5. The increased phosphorylation found 1hr. after partial hepatectomy was not prevented by actinomycin or prior irradiation. 6. The phosphate content of histone F1 was very high in livers from foetal rats and declined in neonatal rats similarly to the decline in DNA synthesis.

Metabolic control mechanisms in mammalian systems. Hormonal regulation of phosphofructokinase in the rat prostate and seminal vesicles

1. The hormonal regulation of phosphofructokinase was investigated in the accessory reproductive organs of the orchidectomized rat. 2. Phosphofructokinase activities declined to 51% and 47% in the prostate and 9% and 6% of the normal values in seminal vesicles 4 and 8 weeks after castration respectively. Administration of testosterone (100mug./100g. body wt.) for 3 days reversed substantially the effects of orchidectomy, and phosphofructokinase activity increased to 173% in the prostate and 536% in seminal vesicles as compared with the values of castrated controls. 3. Time-course studies demonstrated that after a single injection of testosterone (5mg./100g. body wt.) phosphofructokinase activity was maximally elevated to 236% in the prostate and 342% in seminal vesicles at 24hr. 4. Dose-response studies revealed that 2.5mg. of testosterone propionate/100g. body wt. was the minimal amount necessary to induce significant increases in enzyme activity in both accessory sex organs; maximal increases were obtained with a dose of 5mg./100g. body wt. 5. The observed enzyme increases induced by testosterone were inhibited by the simultaneous administration of oestradiol-17beta, and phosphofructokinase activity in this group of rats remained at 97% in the prostate and 137% of the control values in seminal vesicles. Oestradiol-17beta by itself failed to produce any significant effect on enzyme activity in either of these secondary sexual tissues. 6. The nature of the testosterone-induced increases in phosphofructokinase activity was studied by using a variety of inhibitors of RNA and protein synthesis. Cycloheximide, 5-fluorouracil and ethionine largely blocked the androgen-stimulated rise in enzyme activity observed 24hr. after steroid injection. The inhibitory effect of ethionine was completely reversed by the simultaneous administration of methionine. 7. Actinomycin, which is known to inhibit the synthesis of messenger RNA as well as the synthesis of other cellular RNA fractions, when given simultaneously with the hormone, also inhibited the testosterone-induced increases in prostatic and seminal-vesicular phosphofructokinase. However, when the antibiotic was given 6 or 12hr. after injection of the steroid, practically no inhibition of phosphofructokinase induction was obtained. This indicates that, once the enzyme-forming machinery is turned on and allowed to operate for a few hours, actinomycin is incapable of reversing the hormone-induced enzyme responses. 8. The results presented suggest that new RNA and protein synthesis may be involved in the observed androgen-induced increases in phosphofructokinase activity in the prostate and seminal vesicles of the orchidectomized rat.

Hybridizable ribonucleic acid of rat brain

1. Cerebral RNA of adult and newborn rats was labelled in vivo by intracervical injection of [5-(3)H]uridine or [(32)P]phosphate. Hepatic RNA of similar animals was labelled by intraperitoneal administration of [6-(14)C]orotic acid. Nuclear and cytoplasmic fractions were isolated and purified by procedures involving extraction with phenol and repeated precipitation with ethanol. 2. The fraction of pulse-labelled RNA from cerebral nuclei that hybridized to homologous DNA exhibited a wide range of turnover values and was heterogeneous in sucrose density gradients. 3. Base composition of the hybridizable RNA was similar to that of the total pulse-labelled material; both were DNA-like. 4. Pulse-labelled cerebral nuclear RNA hybridized to a greater extent than cytoplasmic RNA for at least a week after administration of labelled precursor. This finding suggested that cerebral nuclei contained a hybridizable component that was not transferred to cytoplasm. 5. The rates of decay of the hybridizable fractions of cerebral nuclei and cytoplasm were faster in the newborn animal than in the adult. Presumably a larger proportion of labile messenger RNA molecules was present in the immature brain. 6. Cerebral nuclear and cytoplasmic RNA fractions from newborn or adult rats, labelled either in vivo for periods varying from 4min. to 7 days or in vitro by exposure to [(3)H]-dimethyl sulphate, uniformly hybridized more effectively than the corresponding hepatic preparation. These data suggested that a larger proportion of RNA synthesis was oriented towards messenger RNA formation in brain than in liver.

Alterations in the structure and function of the rough-surfaced endoplasmic reticulum during necrosis in vitro

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The effect of actinomycin D on the synthesis of ribonucleic acid and protein in rat liver parenchymal cells in suspension and liver slices

1. Rat liver parenchymal cells in suspension are shown to require a higher concentration of actinomycin D than liver slices for equivalent inhibition of the incorporation of [(14)C]adenine, [(14)C]uracil and [(32)P]phosphate into RNA, and of (14)C-labelled amino acids into protein; protein synthesis is much less susceptible to actinomycin D inhibition than RNA synthesis in both the tissue preparations. Possible causes for these differences are discussed. 2. The uptake of [(3)H]actinomycin D in the first few minutes was much greater in the cell suspensions than in the tissue slices; that in the next 1-4hr. was about the same in both the cases. The uptake by both the tissue preparations was at all times proportional to the concentration of the drug within the range 0.5-2.0mug./ml. 3. In the slices actinomycin D taken up initially was concentrated almost exclusively in the nuclei; with time the concentration of the drug in the mitochondria and the supernatant increased more rapidly than in the nuclei though at no stage did it exceed that in the nuclei. In the cell suspension the largest concentration of the drug taken up initially was found in the supernatant; most of the drug taken up subsequently also stayed in the supernatant. 4. When the drug concentration in the incubation medium was 1mug./ml., its concentration within the parenchymal cells in suspension and the parenchymal cells in the slices reached 2.2 and 1.6mug./cm.(3) of cellular volume respectively. On average, 7% of the drug was removed from the medium by the cells in suspension and 23% by the cells in the slices; the average ratio of intracellular to extracellular concentration was 2.4 in the former and 2.1 in the latter case.

Molecular hybridization used to characterize the RNA synthesized by isolated bovine thymus nuclei

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Synthesis of ribonucleic acid in normal bone in vitro

1. The incorporation of [2-(14)C]uridine into nucleic acids of bone cells was studied in rat and pig trabecular-bone fragments surviving in vitro. 2. The rapid uptake of uridine into trichloroacetic acid-soluble material, and its subsequent incorporation into a crude nucleic acid fraction of bone or purified RNA extracted from isolated bone cells, was proportional to uridine concentration in the incubation medium over a range 0.5-20.0mum. 3. During continued exposure to radioactive uridine, bulk RNA became labelled in a curvilinear fashion. Radioactivity rapidly entered nuclear RNA, which approached its maximum specific activity by 2hr. of incubation; cytoplasmic RNA, and particularly microsomal RNA, was more slowly labelled. The kinetics of labelling and rapid decline of the nuclear/microsomal specific activity ratio were consistent with a precursor-product relationship. 4. Bulk RNA preparations were resolved by zonal centrifugation in sucrose density gradients into components with approximate sedimentation coefficients 28s, 18s and 4s. 5. Rapidly labelled RNA, predominantly nuclear in location, demonstrated a polydisperse sedimentation pattern that did not conform to the major types of stable cellular RNA. Material of highest specific activity, sedimenting in the 4-18s region and insoluble in 10% (w/v) sodium chloride, rapidly achieved its maximum activity during continued exposure to radioactive precursor and decayed equally rapidly during ;chase' incubation, exhibiting an average half-life of 4.3hr. 6. Ribosomal 28s and 18s RNA were of lower specific activity, which increased linearly for at least 6hr. in the continued presence of radioactive uridine. There was persistent but variable incorporation into ribosomal RNA during ;chase' incubation despite rapid decline in total radioactivity of the acid-soluble pool containing RNA precursors.

Protein synthetic activity of heart microsomes and robosomes during left ventricular hypertrophy in rabbits

Certain aspects of cytoplasmic protein synthesis have been studied during the early stages (0 to 5 days) of left ventricular hypertrophy produced by supravalvular constriction of the ascending aorta in adult rabbits. At 5 days after constriction, cell-free protein synthesis by microsomal preparations from hypertrophied heart muscle was increased, both from free phenylalanine or leucine (50 to 130%), and from phenylalanyl-soluble RNA, both with (70 to 250%) or without (70 to 150%) polyuridylic acid as artificial messenger. Such increased activity was observed as early as 24 hr, and persisted to 5 days (the limit of the study). Sham preparations also displayed increased activity at 24 hr, which subsided by 5 days. The increased activity could be localized to the microsomal fraction: no differences were found when soluble fractions from normal, sham, and hypertrophied hearts, or from liver, were interchanged. Ribosomes from hypertrophied hearts were no more active than normal ribosomes. Increased yields of microsomal, ribosomal, and soluble RNA were found. It is suggested that the increased protein synthesis occurring in hypertrophy is localized in the microsomes, that soluble factors are not of major significance, and that an increased number of ribosomes (and microsomes) play a major role.

Nucleolar pathology produced by acridine orange and proflavine

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