Control of synthesis and wastage of ribosomal RNA in lymphocytes

Enhanced insulin-regulated phagocytic activities support extreme health span and longevity in multiple populations

The immune system plays a central role in many processes of age-related disorders and it remains unclear if the innate immune system may play roles in shaping extreme longevity. By an integrated analysis with multiple bulk and single cell transcriptomic, so as DNA methylomic datasets of white blood cells, a previously unappreciated yet commonly activated status of the innate monocyte phagocytic activities is identified. Detailed analyses revealed that the life cycle of these monocytes is enhanced and primed to a M2-like macrophage phenotype. Functional characterization unexpectedly revealed an insulin-driven immunometabolic network which supports multiple aspects of phagocytosis. Such reprogramming is associated to a skewed trend of DNA demethylation at the promoter regions of multiple phagocytic genes, so as a direct transcriptional effect induced by nuclear-localized insulin receptor. Together, these highlighted that preservation of insulin sensitivity is a key to healthy lifespan and extended longevity, via boosting the function of innate immune system in advanced ages.

Suboptimal T-cell receptor signaling compromises protein translation, ribosome biogenesis, and proliferation of mouse CD8 T cells

Global transcriptomic and proteomic analyses of T cells have been rich sources of unbiased data for understanding T-cell activation. Lack of full concordance of these datasets has illustrated that important facets of T-cell activation are controlled at the level of translation. We undertook translatome analysis of CD8 T-cell activation, combining polysome profiling and microarray analysis. We revealed that altering T-cell receptor stimulation influenced recruitment of mRNAs to heavy polysomes and translation of subsets of genes. A major pathway that was compromised, when TCR signaling was suboptimal, was linked to ribosome biogenesis, a rate-limiting factor in both cell growth and proliferation. Defective TCR signaling affected transcription and processing of ribosomal RNA precursors, as well as the translation of specific ribosomal proteins and translation factors. Mechanistically, IL-2 production was compromised in weakly stimulated T cells, affecting the abundance of Myc protein, a known regulator of ribosome biogenesis. Consequently, weakly activated T cells showed impaired production of ribosomes and a failure to maintain proliferative capacity after stimulation. We demonstrate that primary T cells respond to various environmental cues by regulating ribosome biogenesis and mRNA translation at multiple levels to sustain proliferation and differentiation.

Calcium, cyclic AMP and protein kinase C--partners in mitogenesis

Evidence is steadily mounting that the proto-oncogenes, whose products organize and start the programs that drive normal eukaryotic cells through their chromosome replication/mitosis cycles, are transiently stimulated by sequential signals from a multi-purpose, receptor-operated mechanism (consisting of internal surges of Ca2+ and bursts of protein kinase C activity resulting from phosphatidylinositol 4,5-bisphosphate breakdown and the opening of membrane Ca2+ channels induced by receptor-associated tyrosine-protein kinase activity) and bursts of cyclic AMP-dependent kinase activity. The bypassing or subversion of the receptor-operated Ca2+/phospholipid breakdown/protein kinase C signalling mechanism is probably the basis of the freeing of cell proliferation from external controls that characterizes all neoplastic transformations.

Cellular and molecular aspects of immune system aging

We begin with a brief discussion of the importance and advantages of immune studies to the problem of aging. This is followed by a short over-view of immune system aging at the systemic level. The major portion of the article is a review of observation, both at the cellular and molecular level, of changes in aging immune cells, with sections on intercellular communication, membrane phenomena, cyclic nucleotides, and molecular genetic changes.

Metabolism of polyadenylated mRNA in growing human lymphocytes

The kinetics of degradation of newly synthesized, cytoplasmic polyadenylated RNA have been examined in normal human lymphocytes stimulated to grow with phytohemagglutinin. A single class of poly(A)-bearing RNA was identified with a half-life of approximately 50 h. In the presence of actinomycin D, the half-life was 5 to 6 h, and virtually no decay of pulse-labeled material was detectable after 6 h of chase incubation with cordycepin. These findings contrast sharply with data obtained from other growing human cells used as controls: polyadenylated mRNA in MOLT-4 cells, a cultured line of T lymphocytes, had a half-life of 2 h in the presence of actinomycin D. The stability of poly(A)-containing RNA in stimulated lymphocytes from normal donors is therefore not simply a manifestation of cell proliferation. In normal resting lymphocytes, Berger and Copper [(1975) Proc. Natl. Acad. Sci. U.S. 72, 3873--3877] reported the existence of 2 classes of polyadenylated mRNA with half-lives of under an hour and greater than 20 h, respectively. Since short-lived poly(A)-bearing mRNA is absent from mitogen-stimulated lymphocytes, the data suggest that stabilization of previously labile poly(A)-bearing RNA is one of many carefully regulated processes accompanying growth induction in normal lymphoid cells.

RNA synthesis in growing and stationary cells of a culture of Scarlet Rose. Disproportionate synthesis of ribosomal subunits in the stationary state

RNA synthesis has been investigated in resting and growing cells of a culture of Scarlet rose. The rates of messenger RNA (mRNA) and ribosomal RNA (rRNA) synthesis are five- and ten-fold higher, respectively, in the growing culture. In stationary phase cultures, newly synthesized 26S and 18S rRNA do not appear in the cytoplasm in equimolar amounts. Rather, the 26S/18S ratio of [3H]-uridine labeled rRNA of stationary cells ranged from 0.9 to 1.3 while the ratio of the corresponding fraction from growing cells was 1.6 to 2.0. A similar result was obtained when cells were labeled with [3H-CH3] methionine. Pulse chase experiments demonstrated that the nascent pre-rRNA in resting cells could be chased into polysomes. These data are interpreted to indicate that a major part of the regulation of rRNA synthesis in stationary cells is at the level of the processing of the rRNA transcript.

Effect of extreme amino acid starvation on the protein synthetic machinery of CHO cells

When CHO cells are incubated under conditions of extreme amino acid starvation, effected by withdrawal of an amino acid from the medium together with genetic or chemical interference with the activity of the corresponding aminoacyl-tRNA synthetase, there is a rapid and profound decline in the functional capacity of the protein synthetic machinery. The effect was observed for all amino acids tested including leucine, asparagine, histidine, methionine and glutamine. This decline in protein synthetic potential appears to be due to a progressive permanent inactivation of the specific aminoacyl-tRNA synthetase concerned, as shown by a decline in the amount of cellular, specific aminoacyl-tRNA and a decline in the cell-free enzyme activity, measured after reversal of the starvation conditions. When cells are left for more than several hours under these starvation conditions, they shrink in size, lose viability and eventually disintegrate, with anomalous rapidity. We suggest that the progressive loss of protein synthetic capacity of the cells is the prime cause of these subsequent events. If the starvation conditions are reversed before cell death, regeneration of the protein synthetic potential occurs rapidly but requires protein synthesis itself, implying the existence of strong control mechanisms for cellular aminoacyl-tRNA synthetase activities.

Biochemical characterization of RNA and protein synthesis in erythrocyte development

Newts (Triturus cristatus) made anemic with acetylphenylhydrazine (APH) fail to regenerate erythrocytes (RBC's) immediately and exhibit a latent period of 1.5-2 wk during which animals lack RBC's and are aplastic. With the establishment of erythroid regeneration at 10-14 days, relatively homogeneous populations of successive erythropoietic stages occur in the blood. This feature makes possible biochemical analyses of events in early, intermediate, and late developmental stages, respectively, each of which can be obtained in vivo with minimal contamination by other stages. Previous studies have described a primitive cell population referred to as "erythroid precursor cells" (EPC's) which precedes the appearance of definitive erythroid elements. The present studies show that EPC's and early erythroid cells are engaged mainly in ribosomal production, including synthesis of rRNA and ribosomal proteins. Moreover, EPC's and early erythroid cells also synthesize tRNA and a presumed Hb-mRNA which has been identified by its sedimentation rate at 9-12 s and its content of polyadenylic acid. In intermediate stages, there occurs a fourfold decrease in the level of RNA synthesis and, while rRNA continues to be formed, there is a disproportionate accumulation of the two major cytoplasmic rRNA species in favor of the large ribosomal subunit RNA. In late developmental stages, the level of RNA synthesis is markedly diminished with little or no evidence of formation of defined RNA classes. Correlated radioautographic and biochemical studies with radioactive delta-aminolevulinic acid and leucine indicate that EPC's and other early erythroid elements synthesize not only hemoglobin but also ferritin and ribosomal proteins. It is concluded that: (a) erythroid RNA synthesis is most pronounced in the early developmental stages, being manifested predominantly by rRNA production but including tRNA and Hb-mRNA; (b) intermediate developmental stages show both "ribosomal wastage" and decreased growth rate, marking a pivotal point between the transcriptional activities of early stages and translational activities of late stages; (c) EPC's represent a cell population already committed to RBC formation and are excluded from a role as the pluripotential stem cell.

RNA synthesis and processing as a measure of phenotypic variability in cytodifferentiation and neoplasia

Neoplastic cell lines exhibit RNA synthesis and process patterns which are related to phenotypic attributes more complex than merely the rate of proliferation. Mouse neuroblastoma cells of the same genotype but different differentiated states have different ribosomal RNA precursor processing patterns, while plasmacytoma cells of different genotypes but the same differentiated state have the same pre-ribosomal RNA processing pattern. In addition, our observations indicate that chromatin-associated RNA is involved in cytodifferentiation and is closely related to phenotypic variability. When neuroblastoma cells are induced to differentiate, there is a 2- to 3-fold increase in the labeling of chromatin-associated RNA. Both of the differentiated cell lines, human myeloma and mouse neuroblastoma, have slow-labeling, stable chromatin-associated RNA while this same fraction from HeLa cells is labeled rapidly and is unstable.

Effect of puromycin on synthesis, processing, and nucleocytoplasmic translocation of rRNA in Tetrahymena pyriformis

1. Treatment of Tetrahymena pyriformis with various concentrations of puromycin results in a more pronounced inhibition of [3H]uridine accumulation in stable RNA than of protein synthesis. 2. At a concentration of 500 micrograms/ml, which is almost completely inhibitory to [3H]uridine incorporation in vivo, puromycin has no influence on the incorporation of [3H]UTP into RNA in isolated macronuclei. Pretreatment of the cells with the antibiotic, however, reduces the activity of RNA polymerases in isolated nuclei to less than 30%. 3. In puromycin-treated cells a small amount of pre-rRNA is synthesized but not processed into cytoplasmic rRNAs. 4. Puromycin reduces the nucleocytoplasmic translocation of pre-existing RNA to about 25% of the control rate within 5 min, resulting in an accumulation of relatively stable rRNA precursor molecules in the macronucleus.

Molecular basis of a control mechanism of DNA synthesis in mammalian cells

Personal observations made on the model of isoproterenol-stimulated DNA synthesis have pointed out the following: 1) cell hypertrophy precedes constantly the onset of DNA synthesis; 2) the length of the G1 phase is mass-dependent; 3) accumulation of ribosomes is needed for cell progress through G1; 4) ribosomal protein synthesis is involved in cell growth activation. These results together with a consideration of the pertinent literature allow us to formulate a hypothesis on the control of cell division in mammalian cells. DNA synthesis might be the terminal event in a chain of metabolic processes whereby a cell adjusts itself to increased functional demands (Increased Functional Demand Hypothesis). The main points of this model are the following: the interaction of the extracellular effector on the target cells first activates the pre-existent protein-synthesizing apparatus of the cell, which in turn brings about the activation of the "translation-transcription connecting mechanism" whereby the cells adjusts itself to an increased need for protein synthesis. Such a mechanism is characterized by cytoplasmic signals arising from the protein-synthesizing apparatus of the cell which reach the nucleus and call forth a messenger RNA for ribosomal proteins. The latter, once synthesized, protect the nascent ribosomal RNA from nuclease attack, resulting in an accumulation of ribosomes in the cytoplasm. Once the ribosomes have reached a "critical amount", the cell is triggered to enter DNA synthesis. As a link between the enhanced ribosomal RNA synthesis and DNA synthesis a reduction in the capacity of the ribonucleotide pool as source of DNA precursors has been suggested.

EFFECTS OF CYCLOHEXIMIDE AND ACTINOMYCIN D ON PRODUCTION OF ACIDIC NUCLEAR PROTEINS IN HUMAN LYMPHOCYTES ACTIVATED BY PHYTOHEMAGGLUTININ

Previous studies have implicated a role for acidic nuclear proteins in the PHA stimulated activation of human lymphocytes. This study examines the mechanism of synthesis of these proteins. Cycloheximide inhibited their production, while Actinomycin D did not. It is suggested that synthesis proceeds by translation of dormant messenger RNA rather than of newly synthesized messenger RNA.

Ribonucleic acid synthesis in normal and immune macrophages after antigenic stimulus

Macrophage ribonucleic acid (RNA) synthesis is an important metabolic process intimately related to the function of these cells. Mouse peritoneal macrophage RNA was extracted with phenol in the presence of bentonite and electrophoresed on composite agarose-polyacrylamide gels. The pulse-chase technique was used to follow the precursor relationships in macrophage ribosomal RNA (rRNA) maturation. The rRNA species at 18S and 28S appeared at 15 and 45 min, respectively, after RNA synthesis was halted. Their appearance corresponded closely to decreases in the rRNA precursors at 45S, 36S, and 34S. Studies of RNA methylation aided in confirming the identity of these ribosomal species. Unmethylated RNA species appeared as messenger RNA between 5S and 15S, and at about 55S probably represented heterodisperse nuclear RNA. When normal macrophages were incubated with heat-killed Salmonella enteritidis, an acceleration in the maturation of RNA was observed. The accelerated maturation was indicated by the earlier appearance of 28S rRNA and the more rapid development of an equilibrium state, where further labeling did not change the RNA profile. In macrophage RNA from mice immunized with S. enteritidis, rRNA species appeared rapidly but did not accumulate to the same extent as observed for normal macrophages. Precursor rRNA and other RNA species developed as usual, suggesting specific degradation of mature rRNA. Such rRNA wastage could indicate a mechanism controlling ribosome assembly in the non-proliferating activated macrophage. The pattern of RNA synthesis in immune macrophages was essentially unchanged by the presence of heat-killed S. enteritidis in vitro.

Wasting of 18 S ribosomal RNA by human myeloma cells cultured in adenosine

When human myeloma cells are pulsed for one hour with 3H-uridine and chased for six hours in fresh medium containing unlabeled uridine, the processing of 45 S rRNA precursor into the stable 28 S and 18 S rRNA components can be followed. However, when the cells are chased in exogenous adenosine instead of uridine, the accumulation of 18 S rRNA is selectively inhibited. Cells pulsed with 3H-adenosine and chased in the absence of exogenous nucleosides exhibit normal rRNA precursor processing, while cells pulsed simultaneously with 3H-uridine and 3H-adenosine and chased with uridine and adenosine are deficient in labeled 18 S rRNA. Consequently, the inhibition of 18 S rRNA accumulation by adenosine is not an artifact of labeling nor is it relieved by an equal molar concentration of uridine. The wasting of 18 S rRNA in human myeloma cells is similar to that reported to occur in normal lymphocytes during the quiescent state.

Regulation of transcription of genes of ribosomal rna during amphibian oogenesis. A biochemical and morphological study

Natural changes in the transcription of rRNA genes were studied in nucleoli from three oogenic stages of the newt Triturus alpestris with electron microscope, auto-radiographic, and biochemical techniques. From determinations of the uridine triphosphate pool sizes and [3H]uridine uptake, phosphorylation, and incorporation into 28S and 18S rRNAs in vivo it was estimated that the rate of rRNA synthesis was about 0.01% in previtellogenic oocytes and 13% in mature oocytes when compared to midvitellogenesis. Spread preparations of nucleoli showed significant morphological changes in the transcriptional complexes. The total number of lateral fibrils, i.e., ribonucleoproteins containing the nascent rRNA precursor, were drastically decreased in stages of reduced synthetic activity. This indicates that rRNA synthesis is regulated primarily at the level of transcription. The resulting patterns of fibril coverage of the nucleolar chromatin axes revealed a marked heterogeneity. On the same nucleolar axis occurred matrix units that were completely devoid of lateral fibrils, matrix units that were almost fully covered with lateral fibrils, and various forms of matrix units with a range of lateral fibril densities intermediate between the two extremes. Granular particles that were tentatively identified as RNA polymerase molecules were not restricted to the transciptional complexes. They were observed, although less regularly and separated by greater distances, in untranscribed spacer regions as well as in untranscribed gene intercepts. The results show that the pattern of transcriptional control of rRNA genes differs widely in different genes, even in the same genetic unit.

Ribonucleic acid labelling and nucleotide pools during compensatory renal hypertrophy

During the first 48h of compensatory renal hypertrophy induced by unilateral nephrectomy, RNA content per cell increased by 20-40%. During this period, rates of RNA synthesis derived from the rates of labelling of UTP and RNA after a single injection of [5-(3)H]uridine showed no change in the rate of RNA synthesis (3.1nmol of UTP incorporated into RNA/min per mg of RNA). ATP and ADP pools were not changed. The rate of RNA synthesis was considerably in excess of the increment of total RNA appearing in the kidneys. With [5-(3)H]uridine as label, only continuous infusion for 24h could produce an increase (60%) in the specific radioactivity of renal rRNA in mice with contralateral nephrectomies. With a single injection of [methyl-(3)H]methionine used to identify methyl groups inserted into newly synthesized rRNA, the specific radioactivity of this rRNA was unchanged 5h after contralateral nephrectomy, increased by 60% at 9-48h, and returned to normal values at 120h. Most RNA synthesized in both nephrectomized and sham-nephrectomized mice has a short half-life. Since total cellular RNA content increases in compensatory hypertrophy despite unchanged rates of rRNA synthesis, the accretion of RNA might involve conservation of ribosomal precursor RNA or a change in rate of degradation of mature rRNA.

Early estrogen action: stimulation of the metabolism of high molecular weight and ribosomal RNAs (estradiol-17 -RNA isolation-uterus-ribosomal RNA precursors-actinomycin D)

Samples of RNA, isolated from uteri of ovariectomized adult rats treated with estrogen, have been analyzed on sucrose gradients. Treatment with estrogen either for 20 min or 2 hr increased the specific activity of all classes of uterine RNA, but produced no significant alteration in the distribution of radioactivity in the gradients, when animals received [(3)H]uridine intraperitoneally 15 min before they were killed. After labeling periods of 30 min, 1 hr, or 2 hr, however, the RNAs isolated from animals treated with estrogen had a smaller percentage of rapidly sedimenting (faster than 28S) species of RNA than did RNA from animals not treated with the hormone. The decreased percentage of high molecular weight RNA correlated with increases in both the specific activity of 28S and 18S RNA and the concentration of RNA in the whole organ. The labeled RNA of high molecular weight was also demonstrated, by the use of actinomycin D in vivo, to have a more rapid turnover rate in the estrogen-stimulated uterus. Our results indicate that estrogen increases not only the rate of synthesis of ribosomal RNA in the uterus of the ovariectomized adult rat, but also the rate or efficiency of processing of precursor RNA species of high molecular weight.