In vivo equivalence of a cell-free system from rat liver for ribosomal RNA processing and transport

Immunofluorescent analysis of creatine kinase in cultured astrocytes by conventional and confocal microscopy: a nuclear localization

The subcellular localization of creatine kinase (CK) was examined in primary cultures of astrocytes with immunofluorescent labeling methods and detection by both standard fluorescence microscopy and confocal laser-scanning microscopy. With conventional microscopy, the pattern of CK staining was uniform throughout the cell cytoplasm and appeared to stain the nuclear region intensely. Staining of CK in the nuclear region co-localized with the DNA-specific Hoechst nuclear stain. CK produced a diffuse cytoplasmic staining pattern that was different from the staining pattern produced by the cytoskeletal proteins glial fibrillary acidic protein and tubulin, both of which showed a filamentous cytoskeletal network that excluded the nucleus. To examine the structural details of CK in the nuclear region, serial optical sections were taken through the cell monolayer with a confocal microscope. The cells were immunostained for CK, and the CK-staining pattern was compared with the staining pattern produced by propidium iodide, which is specific for DNA in RNase-treated samples and stains total nucleic acid in untreated samples. CK staining was present within the nucleus in each section taken through the monolayer. The nucleolus did not stain for CK. The pattern of CK staining in the nucleus (and cytoplasm) was distinctly different from the staining pattern of either DNA or total nucleic acid. Nuclear CK appeared to have a granular, particulate pattern, which is suggestive of a nucleoplasmic distribution.

Induction of an oncofetal marker protein upon X-radiation of rat mammary glands

Fifty-day-old female Sprague--Dawley rats were given carcinogenic and sub-carcinogenic doses of X-radiation to the mammary glands to evaluate the induction of a 60-kDa oncofetal protein (OFP-60). This protein has previously been shown to be produced in the target organ and released to circulation during chemical carcinogenesis and tumorigenesis. In a time course experiment, the mammary glands of the rats were irradiated with a single dose of 1.85 gray (Gy). The OFP-60 marker protein was detected in peripheral blood at 21 days post-irradiation. Irradiation of the mammary gland with single X-ray doses ranging from 0.22 to 1.85 Gy produced a linear relationship between X-ray dose and plasma concentration of OFP-60 determined at 21 days post-treatment. This initiation-related parameter correlates with the known linear relationship between dose of X-radiation and potential tumor incidence.

Hybridization analysis of RNA transported from rat liver nuclei in response to 35 kDa normal and 60 kDa messenger RNA transport factors

The transport of messenger RNA (mRNA) in response to normal adult (35 kDa) and oncofetal (60 kDa) transport factors has been studied in a reconstituted cell-free system. Poly(A)+ mRNA sequences transported by the 35 kDa and 60 kDa transport factors were compared by cDNA:RNA hybridization kinetics. Heterologous hybridization reactions indicated that a proportion of messengers transported in response to the 35 kDa factor were absent or at a markedly reduced abundance in the mRNA released by the 60 kDa factor. Recombinant DNA probes containing cDNA inserts were used to quantitate transport of rat-liver-specific alpha 2 mu-globulin and albumin mRNA from isolated nuclei in presence of the normal and tumor-specific transport factors. More alpha 2 mu-globulin and albumin messenger sequences were transported in response to the 35 kDa transport factor as compared to the 60 kDa factor. These results indicate that the 35 kDa transport protein isolated from rat liver cytosol and the 60 kDa transport protein isolated from hepatoma cytosol, differ significantly in specificity for the classes of RNA sequences released from nuclei. Monoclonal antibodies against the 60 kDa factor do not cross-react with the 35 kDa factor or other proteins as determined by the immunobioassay and by the Western blot technique.

Inflammation-associated events in liver nuclei during acute-phase reaction

Isolated liver nucleoli from rats undergoing turpentine-induced inflammation (acute-phase reaction) synthesize rRNA at a rate significantly higher than normal. This increase is associated with, and possibly preceded by, an enhanced methylation of RNA, which further increases when rRNA synthesis has reached a plateau level. Five hours after turpentine treatment, before clear activation of RNA synthesis and methylations, the nucleocytoplasmic transport of rRNA (largely 40S and 60S subunits) and the related ATPase activity of isolated nuclei are significantly increased. Apparently, posttranscriptional control is affected before transcription of rRNA during the onset of the acute-phase reaction: both kinds of events eventually contribute to the expansion of the ribosome population which occurs in the liver cells from rats undergoing an inflammatory process. All these processes are activated before the liver starts the synthesis of acute-phase proteins.

Analysis of nuclear RNA processing and transport by temperature perturbation of a cell-free system from mammalian cells

The similarity of the Arrhenius plots relating temperature to messenger RNA (mRNA) transport from intact and membrane-denuded rat liver nuclei demonstrates that the ATP and cytosol-dependent transport is independent of the lipid phase of the nuclear membrane. This temperature dependence of RNA release was confirmed for alpha 2u-globulin mRNA by use of a recombinant DNA probe. Ribosomal RNA (rRNA) release showed a similar temperature dependence, suggesting that both mRNA and rRNA share a common temperature-sensitive step. The kinetics of RNA release at different temperatures suggest that RNA transport from mammalian cell nuclei is a rate-controlled rather than a graded unlocking phenomenon. The processing of mRNA precursors also exhibits a temperature dependence as shown by the linear increase in the ratio of total alpha 2u-globulin RNA to alpha 2u-globulin precursor as a function of time at 30 degrees C but not at 14 degrees C in spite of residual transport at the lower temperature. This temperature dependence of mRNA processing was confirmed by Northern blot analysis of the nuclear RNA following a 45 min incubation. Thus, both the processing and transport of RNA show temperature-sensitive steps when analyzed in cell-free systems derived from mammalian cells.

Nucleocytoplasmic RNA transport

A number of closely related post-transcriptional facets of RNA metabolism show nuclear compartmentation, including capping, methylation, splicing reactions, and packaging in ribonucleoprotein particles (RNP). These nuclear 'processing' events are followed by the translocation of the finished product across the nuclear envelope. Due to the inherent complexity of these interrelated events, in vitro systems have been designed to examine the processes separately, particularly so with regard to translocation. A few studies have utilized nuclear transplantation/microinjection techniques and specialized systems to show that RNA transport occurs as a regulated phenomenon. While isolated nuclei swell in aqueous media and dramatic loss of nuclear protein is associated with this swelling, loss of RNA is not substantial, and most studies on RNA translocation have employed isolated nuclei. The quantity of RNA transported from isolated nuclei is related to hydrolysis of high-energy phosphate bonds in nucleotide additives. The RNA is released predominantly in RNP: messenger-like RNA is released in RNP which have buoyant density and polypeptide composition similar to cytoplasmic messenger RNP, but which have distinctly different composition from those in heterogeneous nuclear RNP. Mature 18 and 28S ribosomal RNA is released in 40 and 60S RNP which represent mature ribosomal subunits. RNA transport proceeds with characteristics of an energy-requiring process, and proceeds independently of the presence or state of fluidity of nuclear membranes. The energy for transport appears to be utilized by a nucleoside triphosphatase (NTPase) which is distributed mainly within heterochromatin at the peripheral lamina. Photoaffinity labeling has identified the pertinent NTPase as a 46 kD polypeptide which is associated with nuclear envelope and matrix preparations. The NTPase does not appear to be modulated via direct phosphorylation or to reflect kinase-phosphatase activities. A large number of additives (including RNA and insulin) produce parallel effects upon RNA transport and nuclear envelope NTPase, strengthening the correlative relationship between these activities. Of particular interest has been the finding that carcinogens induce specific, long-lasting increases in nuclear envelope (and matrix) NTPase; this derangement may underlie the alterations in RNA transport associated with cancer and carcinogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Release of rRNA from liver nuclei during the early stages of the acute-phase reaction

Liver nuclei isolated from rats 5 h after turpentine injection show an increased release of rRNA, of the transport-related nucleoside-triphosphatase activity and of the amount of nuclear RNA; RNA methylation is also likely to undergo some activation. These changes occur when RNA synthesis is still normal.

Thermal down-regulation of exportable rRNA in nuclei

The export of rRNP particles from nuclei isolated from Tetrahymena was investigated after preincubating the nuclei at different temperatures under nonpermissive export-conditions. We observed a new phenomenon: Temperature elevation from the sublethal cells' growth temperature, 8 degrees C, to the optimal temperature, 28 degrees C, lead to a gradual down-regulation in the maximal proportion of rRNP particles subsequently exported from nuclei at 28 degrees C. This thermal down-regulation is apparently not due to qualitative changes in the exported rRNP particles, a derangement in the gross nuclear organization, a degradation and/or nicking of the nuclear rRNA, a gross decomposition of the major nuclear proteins, a random cross-linking of nuclear components by disulfide bonds, or an elution of nuclear factors possibly required for rRNP export. Moreover, there is a corresponding thermal down-regulation in nuclear envelope-free nuclei. Our data indicate that nuclei possess a mechanism that regulates the number of potentially exportable rRNP particles at a level preceding the rRNP passage through the nuclear envelope.

Thermal diminution and augmentation of the retention of transportable rRNA in nuclear envelope-free nuclei

We have examined the effect of temperature on the rRNA transport from nuclei isolated from Tetrahymena after removal of both nuclear membranes and pore complexes by 1% Triton X-100. These nuclei export rRNA as precursor ribosomal ribonucleoprotein particles at both 28 degrees C and 8 degrees C which are qualitatively the same in terms of rRNA pattern, sedimentation coefficients and buoyant densities. At 8 degrees C, however, significantly fewer ribosomal ribonucleoprotein particles can be maximally exported than at 28 degrees C, though nuclei contain enough potentially transportable particles. These are increasingly released with increasing temperatures. Under conditions non-permissive for export, temperature elevation decreases the number of the potentially transportable ribosomal ribonucleoprotein particles in nuclei. Our data show: transportable ribosomal ribonucleoprotein particles inside nuclei are not 'free', but rather are subject to a complex temperature-sensitive retention: this retention is gradually diminished under export conditions and augmented under non-permissive export conditions with increasing temperatures. These retention mechanisms operate at an intranuclear level preceding the ribosomal ribonucleoprotein passage through the nuclear envelope pore complexes, i.e., the nuclear envelope regulates neither the number of potentially transportable ribosomal ribonucleoprotein particles in nuclei nor the number of those particles which can be maximally exported from nuclei at a given temperature. We suggest that these retention mechanisms involve temperature-sensitive domains of the nuclear matrix.

Effect of ethanol ingestion on the nucleocytoplasmic transport of hepatic RNA

Influence of prolonged ethanol ingestion on the nucleocytoplasmic transport of RNA has been examined in a cell-free system. The nucleocytoplasmic transport of RNA was energy- and temperature-dependent. Ethanol treatment of rats for 10 weeks led to a significant increase in the release of isotopically labelled nuclear RNA from the nucleus, suggesting a partial loss of nuclear restrictive control. Results from crossover experiments led to the conclusion that the observed effect of ethanol ingestion was mediated through factors present in the cytosol as well as in the nuclear fraction.

Effect of physiological concentrations of insulin and antidiabetic drugs on RNA release from isolated liver nuclei

The addition of 10(-11) M insulin to a cell-free system from rat liver promotes the release of messengerlike RNA from isolated prelabeled nuclei. The stimulation was similar whether the nuclei were preincubated with insulin, or if insulin was added directly to the cell-free system with or without a protease inhibitor. Dot blot hybridization using cloned cDNA for alpha 2u-globulin mRNA showed that this was one of the messages whose release was enhanced by insulin. Nuclei isolated from rats treated with either of the antidiabetics tolbutamide or tolazamide showed no increase in RNA release in the presence of insulin over the concentration range 10(-5) - 10(-14) M. Furthermore, these nuclei did not release detectable levels of alpha 2u-globulin mRNA.

Transport of functional messenger RNA from liver nuclei in a reconstituted cell-free system

The reliability of a reconstituted cell-free system for messenger RNA processing and transport, consisting of isolated nuclei in fortified cytosol, has been evaluated in terms of the functionality and regulated release of the transported product. The poly(A) messenger RNA transport in vitro formed appropriate initiation complexes with ribosomes in an optimized translation system and had template activity comparable to that transported in vivo. The intra-nuclear origin of this messenger RNA is supported by pulse-labeling studies, its transport from detergent-treated nuclei and the absence of the release under non-transport conditions. Serum albumin was identified by immunoprecipitation and electrophoresis as one of the products synthesized when the transported RNA was translated in vitro. The transport of messenger RNA in the cell-free system was dependent on specific cytosol (soluble cytoplasmic) proteins. These proteins, which constitutes less than 0.1% of the total cytosol proteins, are precipitated wtih streptomycin with high specificity.

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.

In vitro ribosomal ribonucleoprotein transport upon nuclear expansion

The interdependence of nuclear rRNA release and nuclear size is investigated in macronuclei isolated from Tetrahymena. Nuclei are induced to contract and to expand, without any structural disintegration of the nuclear envelope, by final Ca2+/Mg2+ (3:2) concentrations of 5 and 1.5 mM, respectively. Upon expansion, the average volume of nuclei increases from 600 +/- 42 to 811 +/- 76 micron3. Concomitantly, nuclei begin to release RNA following saturation kinetics. This RNA release stops immediately upon nuclear contraction. Similar to the in vivo situation, only advanced rRNA processing products are released in the form of ribosomal precursor particles, as identified in detail by polyacrylamide gel electrophoresis and rate zonal and isopycnic density gradient centrifugation. Three particle ty9es are released having average buoyant densities of 1.495, 1.470, and 1.532 g/cm3, exhibiting average sedimentation coefficients of 62, 62, and 35 S, and containing the immediate precursor to the 25S rRNA, 26S rRNA, and 17S rRNA, respectively. Tje rRNP release if ATP independent and noncoincident with the release of endogenous nuclear Pi, though it is Be2+ sensitive. Our data are compatible with the views that nuclear expansion is the prerequisite rather than the cause for the rRNP release and that nuclear pore complex associated ATPases play only, if at all, a minor role in nucleocytoplasmic exchange of rRNP.