Regulated transport of messenger ribonucleic acid from isolated liver nuclei by nucleic acid binding proteins

Rat liver nucleocytosolic messenger ribonucleic acid (mRNA) transport is shown to be regulated by proteins with a high affinity for nucleic acids. In the cell-free system described, the energy-dependent transport of all RNA classes [transfer RNA (tRNA), mRNA, and ribosomal RNA (rRNA)] exhibited a dependence upon the availability of discrete minor sets of cytosol proteins. In addition to having a different level of saturation, only the mRNA "transport protein" activities are increased by adenosine cyclic 3',5'-phosphate (cAMP), an effect most likely mediated by a cAMP-dependent protein kinase. The mRNA transport proteins were isolated from cytosol by precipitation with streptomycin sulfate followed by deoxyribonucleic acid (DNA)-cellulose affinity chromatography, or from oligo-(thymidylate)-cellulose bound cytoplasmic messenger ribonucleoprotein (mRNP) particles by high-salt extraction. Either method yielded a protein fraction which exhibited a 1000-fold increase in mRNA transport activity as compared to cytosol. Over one-half of the mRNA transport activity is associated with the mRNP of the cell. A partial homology between the cytosol and mRNP-derived proteins was demonstrated by polyacrylamide gel electrophoresis. One major (20 000 daltons) and several minor proteins (23 000, 52 000, 54 000, and 72 000 daltons) were in common. Nuclear 4-5S exited from in vitro incubated nuclei in three phases, according to their differential in vivo rates of labeling and intranuclear pool sizes. The amount of nuclear RNA transported in vitro as mRNA (about 1.0%) agrees wtih the in vivo estimates. Additional evidence for in vivo equivalence was provided by the physicochemical characterization and bioassay of the RNA. The transported mRNA sedimented in urea-sucrose gradients as an 8-18S heterodisperse product. This RNA initiated cell-free translation with the synthesis of precursor peptides as diverse in size as those for albumin and alpha 2U-globulin. The relative abundancies of various transported mRNAs were different than the corresponding abundancies of liver cytoplasmic mRNAs.     

Regulation of tyrosine aminotransferase messenger ribonucleic acid in rat liver. effect of cycloheximide on messenger ribonucleic acid turnover

Tyrosine aminotransferase messenger ribonucleic acid (mRNA) activity in rat liver was rapidly increased 3-6-fold following in vivo administration of hydrocortisone acetate, dibutyryladenosine cyclic 3',5'-phosphate, or the protein synthesis inhibitor cycloheximide. Treatment with the steroid hormone or cyclic nucleotide in combination with cycloheximide resulted in levels of tyrosine aminotransferase mRNA 10-20-fold greater than control values. These changes in mRNA activity were not accompanied by changes in albumin mRNA or total liver template activity. The rapid decline in tyrosine aminotransferase mRNA activity following cordycepin inhibition of de novo RNA synthesis was prevented by cycloheximide treatment. This protection was not observed when pactamycin was substituted for cycloheximide, demonstrating that the inhibition of protein synthesis per se was not responsible for the stabilization of tyrosine aminotransferase mRNA. Based upon the effects of cycloheximide and pactamycin on rat liver polysome structure, it is concluded that the cycloheximide-mediated increase in tyrosine aminotransferase mRNA activity is the result of stabilization of the mRNA molecule which renders the message less susceptible to inactivation and degradation in the cytoplasm. The action of cycloheximide is very specific for tyrosine aminotransferase, phosphoenolpyruvate carboxykinase, and probably several other mRNAs that code for minor liver proteins that turn over rapidly in response to hormonal or metabolic stimuli.     

Optical studies on the structure of hemoglobin messenger ribonucleic acid

The secondary structure of hemoglobin mRNA (HbmRNA has been investigated by optical rotatory dispersion (ORD), circular dichroism (CD) and ultraviolet absorbance spectrophotometry. The dependence on temperature or reaction with formaldehyde of the CD and absorbance are characteristic of a structure with substantial base pairings at 20°C. The presence of Cotton effects in one of the major ultraviolet absorption bands indicates a highly ordered secondary structure. The UV hyperchromism on thermal denaturation is consistent with a value of 58% double helical content.     

Estrogen effects on histone messenger ribonucleic acid levels in the rat uterus

RNA was isolated from uteri of immature rats before and after estrogen treatment. The concentration of histone mRNA was analyzed by Northern hybridization and compared with messenger RNA concentration of alpha-actin, beta-actin, and beta-tubulin. Steady state levels of common histone mRNAs did not change up to 9 h after hormone administration. After that time the histone mRNA levels increased significantly and reached a maximum at 18 h, several hours later than the time of maximal histone protein biosynthesis induced by estrogen. The concentration of control mRNAs (alpha- and beta-actin and beta-tubulins) increased shortly after estradiol injection and reached a peak at 9 h. These results show that the pattern of histone gene expression induced by estrogen has some features similar to those observed during embryogenesis.     

Estrogen increases proenkephalin messenger ribonucleic acid levels in the ventromedial hypothalamus of the rat

The effects of estrogen on proenkephalin (PE) gene expression were measured in neurons of the ventromedial hypothalamus. Slot blot hybridization analysis indicates that the levels of PE mRNA in the ventromedial hypothalamus of ovariectomized rats increase 3.1-fold after 2 weeks of estrogen replacement. In situ hybridization reveals that the estrogen-inducible enkephalinergic neurons are located in the ventrolateral aspect of the ventromedial nucleus, a subnucleus known to contain many estrogen-concentrating neurons. The increase in PE mRNA levels is due to both a 63% increase in the number of detectable PE mRNA-containing neurons and a 2.0-fold increase in the levels of PE mRNA per enkephalinergic neuron (1.63 x 2.0 = 3.3-fold overall induction). This estrogen-regulated enkephalinergic cell group may represent part of the neural network mediating estrogen's effects on reproductive behavior and/or other neuroendocrine processes.     

Ribonucleic acid synthesis in isolated rat liver nuclei under conditions of ribonucleic acid processing and transport.

A cell-free system is described which permits a significant and prolonged synthesis of RNA in isolated rat liver nuclei, under conditions previously demonstrated to support normal nuclear processing and transport of both rRNA and mRNA. The system contains cytosol but not (NH4)2SO4 or other non-physiological components. Evidence is presented for cytosol factors which stimulate ribosomal, and to a lesser degree, non-ribosomal RNA synthesis.     

Effects of dexamethasone on the levels of plasma corticosteroid binding globulin in rats and monkeys.

Rat marrow cells were preincubated for 45 hours with 5.5 × 10^?4M sodium hexachloroiridate. This treatment abolished DNA synthesis whilst improving cell survival over that of controls. The synthesis of RNA, protein and glycoprotein continued and could be further increased by the addition of erythropoietin for up to 44 more hours. Heme synthesis also continued in the absence of DNA synthesis but could not be stimulated by erythropoietin.     

An amino acid substitution in biobreeding rat corticosteroid binding globulin results in reduced steroid binding affinity

BioBreeding (BB) rats are derived from an outbred colony of Wistar rats and are used as a model of autoimmune diabetes mellitus. A corticosteroid binding globulin (CBG) variant with reduced affinity for glucocorticoids has now been found in the blood of these animals. The dissociation rate constants of BB CBG for cortisol (4.42 nM) and corticosterone (1.43 nM) are both about 50% higher than those associated with Wistar CBG, but no obvious difference in the steroid binding specificity of BB and Wistar CBGs was detected. Purified BB and Wistar CBGs exhibit the same size heterogeneity when examined by polyacrylamide gel electrophoresis under denaturing conditions, and the sizes of their respective hepatic mRNAs are identical. The genetic basis for this abnormality was therefore determined by comparing the cDNA sequences for BB and Wistar CBG, and this revealed a point mutation that results in a single amino acid substitution at residue 276 (Ile in BB CBG and Met in Wistar CBG). To confirm that this mutation is responsible for the reduced steroid binding affinity associated with BB CBG, the cDNAs for rat CBG-Ile276 and CBG-Met276 were expressed in Chinese hamster ovary cells. The steroid binding affinities of the CBGs secreted by these cells were essentially identical with those observed in the corresponding serum samples from these two rat strains. The amino acid substitution identified in BB rat CBG therefore clearly accounts for the reduction in its steroid binding affinity, and further analysis of this and other natural CBG variants may reveal important information about the CBG steroid binding site. It is also possible that this mutation may contribute to the etiology of pathological abnormalities that are characteristic of the BB rat.     

A variant estrogen receptor messenger ribonucleic acid is associated with reduced levels of estrogen binding in human mammary tumors

An analysis of the human estrogen receptor (ER) mRNA was performed on 71 human breast tumors using an RNase protection assay. Complementary DNA clones to the human estrogen receptor (lambda R8 and lambda R3) were used to generate small antisense 32P-labeled RNA molecules that were hybridized to the tumor RNA. We determined the relative amounts of ER mRNA in each tumor by measuring the amount of RNases A and T1 resistant hybrids. Moreover, because RNase A has the ability to cleave single-base mismatches within RNA/RNA duplexes, we were able to use the assay to screen for possible mutations or deletions in the ER mRNA. A significant correlation was found between the ER mRNA levels and the estrogen binding concentrations determined by a dextran-coated charcoal assay (r = 0.68; P less than 0.0001; n = 58). We also identified a subpopulation of tumors in which a mismatch in the ER mRNA was detected. This message modification, in the B region of the message, significantly correlated with low levels of estrogen binding. This result suggests that the observed B variant might lead to the production of receptors with altered properties.