Isolation of RNA for dot hybridization by heparin-DNase I treatment of whole cell lysate

Analysis of heat shock transcription factor for suppression of polyglutamine toxicity

Individually over-expressed chaperones can interfere with cytotoxicity and aggregation of polyglutamine proteins in disease models. As chaperones cooperate, the analysis of suppression or reversal of polyglutamine pathology may require ways to up-regulate multiple chaperone coding genes. This condition might be achieved by exogenous expression of de-repressed forms of heat shock transcription factor 1 (HSF1), which mediates induction of several genes coding cytosolic and nuclear chaperones. Here we present the rationale behind this possible approach and the caveats, and employ a non-neuronal cell system to test whether Ataxin-1 aggregation can be modulated by de-repressed HSF1 mutants through augmented expression of chaperone coding genes. In our experiments, HSF1 mutants have induced heat shock protein 70 and Human DnaJ (HDJ)-1 to intermediate levels. Cells expressing such mutants also showed partial reduction of Ataxin-1 [31Q] aggregation. A consolidated positive outcome of these tests in cellular models would encourage experiments in transgenic mice and prospects for pharmacological modulation of HSF1 activity or delivery.

The role of the LDL receptor in apolipoprotein B secretion

Familial hypercholesterolemia is caused by mutations in the LDL receptor gene (Ldlr). Elevated plasma LDL levels result from slower LDL catabolism and a paradoxical lipoprotein overproduction. We explored the relationship between the presence of the LDL receptor and lipoprotein secretion in hepatocytes from both wild-type and LDL receptor-deficient mice. Ldlr(-/-) hepatocytes secreted apoB100 at a 3.5-fold higher rate than did wild-type hepatocytes. ApoB mRNA abundance, initial apoB synthetic rate, and abundance of the microsomal triglyceride transfer protein 97-kDa subunit did not differ between wild-type and Ldlr(-/-) cells. Pulse-chase analysis and multicompartmental modeling revealed that in wild-type hepatocytes, approximately 55% of newly synthesized apoB100 was degraded. However, in Ldlr(-/-) cells, less than 20% of apoB was degraded. In wild-type hepatocytes, approximately equal amounts of LDL receptor-dependent apoB100 degradation occured via reuptake and presecretory mechanisms. Adenovirus-mediated overexpression of the LDL receptor in Ldlr(-/-) cells resulted in degradation of approximately 90% of newly synthesized apoB100. These studies show that the LDL receptor alters the proportion of apoB that escapes co- or post-translational presecretory degradation and mediates the reuptake of newly secreted apoB-containing lipoprotein particles.

Glutamine synthetase expression in perinatal spiny mouse liver

The pronounced increase in the protein/mRNA ratio of ammonia-metabolising enzymes in rat liver in the last prenatal week represents a clear example of a post-transcriptional level of control of gene expression. Both the underlying mechanism, namely an increase in translational efficiency of the mRNA and/or enhanced stability of the protein, and its importance for perinatal adaptation are unknown. We investigated this process in spiny mouse liver, because the comparison of rat and spiny mouse can discriminate adaptively from developmentally regulated processes in the perinatal period. We focused on glutamine synthetase (GS) because of the conveniently small size of its mRNA. Prenatally, GS enzyme activity slowly accumulated to approximately 1.3 U x g-1 liver at birth and postnatally more rapidly to 5.5 U x g-1 at 2 weeks. Both phases of enzyme accumulation obeyed exponential functions. Western-blot analysis showed that changes in GS activity reflected changes in GS protein content. GS mRNA content of the liver was 45 fmol x g-1 at 2 weeks before birth and slowly declined to approximately 25 fmol x g-1 at 2 weeks after birth. The GS protein/mRNA ratio increased 2.5-fold prenatally and sixfold postnatally. Analysis of prenatal and postnatal polysome profiles revealed no evidence of GS mRNA-containing ribonucleoprotein particles. Instead, GS mRNAs were (fully) occupied by 12 ribosomes, indicating regulation at the level of elongation. The kinetics of GS protein accumulation, in conjunction with GS mRNA content, are consistent with an approximately sixfold increase in its rate of synthesis at birth as the result of a corresponding stimulation of the rate of elongation.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.

Multiple functions of Drosophila heat shock transcription factor in vivo

Heat shock transcription factor (HSF) is a transcriptional activator of heat shock protein (hsp) genes in eukaryotes. In order to elucidate the physiological functions of HSF in Drosophila, we have isolated lethal mutations in the hsf gene. Using a conditional allele, we show that HSF has an essential role in the ability of the organism to survive extreme heat stress. In contrast to previous results obtained with yeast HSF, the Drosophila protein is dispensable for general cell growth or viability. However, it is required under normal growth conditions for oogenesis and early larval development. These two developmental functions of Drosophila HSF are genetically separable and appear not to be mediated through the induction of HSPs, implicating a novel action of HSF that may be unrelated to its characteristic function as a stress-responsive transcriptional activator.

Displacement of sequence-specific transcription factors from mitotic chromatin

The general inhibition in transcriptional activity during mitosis abolishes the stress-inducible expression of the human hsp70 gene. Among the four transcription factors that bind to the human hsp70 promoter, the DNA-binding activities of three (C/EBP, GBP, and HSF1) were normal, while Sp1 showed reduced binding activity in mitotic cell extracts. In vivo footprinting and immunocytochemical analyses revealed that all of the sequence-specific transcription factors were displaced from promoter sequences as well as from bulk chromatin during mitosis. The correlation of transcription factor displacement with chromatin condensation suggests an involvement of chromatin structure in mitotic repression. However, retention of DNase I hypersensitivity suggests that the hsp70 promoter was not organized in a canonical nucleosome structure in mitotic chromatin. Displacement of transcription factors from mitotic chromosomes could present another window in the cell cycle for resetting transcriptional programs.

The Rev protein of human immunodeficiency virus type 1 counteracts the effect of an AU-rich negative element in the human papillomavirus type 1 late 3' untranslated region

We have identified a sequence in the late 3' untranslated region of human papillomavirus type 1 mRNAs that acts posttranscriptionally to repress gene expression. Deletion analysis localized the inhibitory element to an AU-rich sequence between nucleotides 6958 and 6984 on the human papillomavirus type 1 genome. This sequence inhibits gene expression in an orientation-dependent manner. Upon transfection of eucaryotic cells with plasmids containing this sequence, approximately 4-fold-lower cytoplasmic mRNA levels and 64- to 128-fold-lower protein levels were produced compared with those produced by plasmids lacking the inhibitory sequence. Interestingly, providing the constitutive transport element of simian retrovirus type 1 in sense orientation counteracted inhibition exerted by the human papillomavirus type 1 sequence. Inhibition could also be overcome by the presence of human immunodeficiency virus type 1 Rev protein in trans and its target sequence, the Rev-responsive element, in cis. Rev is a nuclear protein and acts by promoting nuclear export of human immunodeficiency virus type 1 mRNAs encoding structural proteins. Our results are consistent with a model for human papillomavirus type 1 late-gene expression in which mRNAs containing human papillomavirus type 1 inhibitory sequences enter a nonproductive route in the nucleus, resulting in inefficient mRNA utilization. Rev directs mRNA containing inhibitory sequences to a productive route by interacting with the Rev-responsive element.

Suppression of sterol 27-hydroxylase mRNA and transcriptional activity by bile acids in cultured rat hepatocytes

In previous work we have demonstrated suppression of cholesterol 7 alpha-hydroxylase by bile acids at the level of mRNA and transcription, resulting in a similar decline in bile acid synthesis in cultured rat hepatocytes [Twisk, Lehmann and Princen (1993) Biochem. J. 290, 685-691]. In view of the substantial contribution of the 'alternative' or '27-hydroxylase' route to total bile acid synthesis, as demonstrated in cultured rat hepatocytes and in vivo in humans, we here evaluate the effects of various bile acids commonly found in bile of rats on the regulation of sterol 27-hydroxylase in cultured rat hepatocytes. Addition of taurocholic acid, the predominant bile acid in rat bile, to the culture medium of rat hepatocytes resulted in a 72% inhibition of sterol 27-hydroxylase activity. The effect was exerted at the level of sterol 27-hydroxylase mRNA, showing a time- and dose-dependent decline with a maximal suppression (-75%) at 50 microM taurocholic acid after 24 h of culture. The decline in mRNA followed first-order kinetics with an apparent half-life of 13 h. Under these conditions cholesterol 7 alpha-hydroxylase mRNA (-91%) and bile acid synthesis (i.e. chenodeoxycholic and beta-muricholic acid, -81%) were also maximally suppressed. In contrast, no change was found in the level of lithocholic acid 6 beta-hydroxylase mRNA. Assessment of the transcriptional activity of a number of genes involved in routing of cholesterol towards bile acids showed similar suppressive effects of taurocholate on expression of the sterol 27-hydroxylase and cholesterol 7 alpha-hydroxylase genes (-43% and -42% respectively), whereas expression of the lithocholic 6 beta-hydroxylase gene was not affected. Taurocholic acid and unconjugated cholic acid were equally as effective in suppressing sterol 27-hydroxylase mRNA. The more hydrophobic bile acids, chenodeoxycholic acid and deoxycholic acid, also produced a strong inhibition of 57% and 76% respectively, whereas the hydrophilic beta-muricholic acid was not active. We conclude that (1) a number of bile acids, at physiological concentrations, suppress sterol 27-hydroxylase by down-regulation of sterol 27-hydroxylase mRNA and transcriptional activity and (2) co-ordinated suppression of both sterol 27-hydroxylase and cholesterol 7 alpha-hydroxylase results in inhibition of bile acid synthesis in cultured rat hepatocytes.

Rev of human immunodeficiency virus and Rex of the human T-cell leukemia virus type I can counteract an mRNA downregulatory element of the transferrin receptor mRNA

Expression of the structural proteins of the human immunodeficiency virus type 1 (HIV-1), the human T-cell leukemia virus type I (HTLV-I), and of the transferrin receptor (TfR) mRNA depends on posttranscriptional regulatory mechanisms involving both positive and negative elements. In these systems the presence of elements decreasing mRNA expression have been demonstrated. The regulatory proteins (Rev, Rex or iron response element binding protein IRE-BP) antagonize the effects of the downregulatory elements by interacting directly with specific mRNA sites (Rev responsive element, RRE, Rex responsive element, RXRE, or iron responsive elements, IREs) resulting in stabilization and efficient expression of the corresponding mRNAs. To investigate whether this strategy involves common pathways of mRNA utilization, we have studied expression from hybrid mRNAs that contained these previously identified HIV-1 or TfR instability determinants and the binding sites of the regulatory proteins Rev, Rex and/or IRE-BP. Our results demonstrate that only low levels of these hybrid mRNAs accumulate in the absence of the positive regulatory factors Rev, Rex or IRE-BP. The presence of these factors counteracts the effect of heterologous downregulatory elements resulting in increased accumulation of the hybrid mRNAs. However, while Rev or Rex regulation also resulted in efficient protein expression, the IRE-BP only affected mRNA levels without significantly affecting protein expression, suggesting that the pathways of mRNA stabilization/expression are different in these systems.

Elements distinct from human immunodeficiency virus type 1 splice sites are responsible for the Rev dependence of env mRNA

In the absence of the viral regulatory protein Rev, the human immunodeficiency virus type 1 gag/pol and env mRNAs are inefficiently expressed, since nucleocytoplasmic transport, stability, and polysomal loading are impaired. It has been suggested that splicing is necessary for Rev function and that the low expression of the unspliced and intermediate spliced mRNAs in the absence of Rev is associated with specific splice sites. Previous studies identified distinct RNA elements within the gag/pol region responsible for low expression that are not associated with splice sites. Here we study the determinants for Rev dependence of the authentic env mRNA. We demonstrate that upon removal of all the utilized splice sites, the env mRNA is still Rev dependent and Rev responsive for expression in human cells. We have identified several regions within the env mRNA that inhibit expression of a gag-env hybrid mRNA. Elimination of one of these elements, located within the Rev-responsive element, did not result in virus expression, supporting our model that several independently acting elements are responsible for the downregulatory effect. By analogy to the RNA elements within the gag/pol region, we propose that elements unrelated to utilized splice sites are responsible for the posttranscriptional regulation of env mRNA.

Differential feedback regulation of cholesterol 7 alpha-hydroxylase mRNA and transcriptional activity by rat bile acids in primary monolayer cultures of rat hepatocytes

We have used primary monolayer cultures of rat hepatocytes to study the effects of physiological concentrations of various bile acids, commonly found in bile of normal rats, on the mechanism of regulation of cholesterol 7 alpha-hydroxylase and bile acid synthesis. Addition of taurocholic acid, the most predominant bile acid in rat bile, to the culture medium suppressed cholesterol 7 alpha-hydroxylase activity and mRNA time- and dose-dependently. The decrease in enzyme activity paralleled the changes in mRNA. Maximal suppression of cholesterol 7 alpha-hydroxylase mRNA (-91%) and enzyme activity (-89%) was observed after a 16 h incubation period with 50 microM taurocholic acid. The declines in mRNA and enzyme caused by taurocholic acid were tightly coupled and followed first-order kinetics with a half-life of 4 h. Transcriptional activity, as assessed with nuclear run-on assays, was decreased by 44% at 50 microM taurocholic acid. Mass production of bile acids (chenodeoxycholic acid and beta-muricholic acid) was inhibited to a similar extent as the cholesterol 7 alpha-hydroxylase when different concentrations of taurocholic acid were used, giving maximal inhibition (-81%) at 50 microM taurocholic acid. Glycocholic acid and unconjugated cholic acid were equally effective as taurocholic acid in suppressing cholesterol 7 alpha-hydroxylase mRNA. The more hydrophobic bile acids (chenodeoxycholic acid and deoxycholic acid) showed profound suppression of the cholesterol 7 alpha-hydroxylase mRNA by 85% and 75% respectively, whereas the other trihydroxy bile acids in rat bile, alpha- and beta-muricholic acid, were not or only marginally active. We conclude that rat bile acids, in particular the more hydrophobic ones, in concentrations commonly observed in portal blood, exert negative feedback control at the level of cholesterol 7 alpha-hydroxylase mRNA in cultured rat hepatocytes through a direct effect on the hepatocytes, and that down-regulation of transcription is only one of the mechanisms involved in this regulation.

The Rev protein of human immunodeficiency virus type 1 promotes polysomal association and translation of gag/pol and vpu/env mRNAs

Biochemical examination of the Rev-dependent expression of gag mRNAs produced from gag-Rev-responsive element (RRE) expression plasmids showed a large discrepancy between the level of cytoplasmic gag mRNA and the produced Gag protein. Significant levels of the mRNA produced in the absence of Rev were localized in the cytoplasm, while very low levels of Gag protein were produced. In the presence of Rev, the levels of mRNA increased by 4- to 16-fold, while the Gag protein production increased by 800-fold. These findings indicated that in addition to promoting nucleus-to-cytoplasm transport, Rev increased the utilization of cytoplasmic viral mRNA. Poly(A) selection and in vitro translation of cytoplasmic gag mRNA verified that the mRNA produced in the absence of Rev was functional. To analyze the translational defect in the absence of Rev, we examined the association of the cytoplasmic gag mRNA with ribosomes. gag mRNA produced in the absence of Rev was excluded from polysomes, while gag mRNA produced in the presence of Rev was associated with polysomes and produced Gag protein. These observations showed that the presence of Rev was required for efficient loading of gag mRNA onto polysomes. This effect required the presence of the RRE on the mRNA. Analysis of mRNAs produced from a rev-minus proviral clone confirmed that the presence of Rev promoted polysomal loading of both gag/pol and vpu/env mRNAs. The localization of gag mRNA was also examined by in situ hybridization. This analysis showed that in the presence of Rev, most of the gag mRNA was found in the cytoplasm, while in the absence of Rev, most of the gag mRNA was found in the nucleus and in the region surrounding the nucleus. These results suggest that a substantial fraction of the gag mRNA is retained in distinct cytoplasmic compartments in the absence and presence of Rev. These findings indicate that the presence of Rev is required along the entire mRNA transport and utilization pathway for the stabilization, correct localization, and efficient translation of RRE-containing mRNAs.

The Rev protein of human immunodeficiency virus type 1 promotes polysomal association and translation of gag/pol and vpu/env mRNAs

Biochemical examination of the Rev-dependent expression of gag mRNAs produced from gag-Rev-responsive element (RRE) expression plasmids showed a large discrepancy between the level of cytoplasmic gag mRNA and the produced Gag protein. Significant levels of the mRNA produced in the absence of Rev were localized in the cytoplasm, while very low levels of Gag protein were produced. In the presence of Rev, the levels of mRNA increased by 4- to 16-fold, while the Gag protein production increased by 800-fold. These findings indicated that in addition to promoting nucleus-to-cytoplasm transport, Rev increased the utilization of cytoplasmic viral mRNA. Poly(A) selection and in vitro translation of cytoplasmic gag mRNA verified that the mRNA produced in the absence of Rev was functional. To analyze the translational defect in the absence of Rev, we examined the association of the cytoplasmic gag mRNA with ribosomes. gag mRNA produced in the absence of Rev was excluded from polysomes, while gag mRNA produced in the presence of Rev was associated with polysomes and produced Gag protein. These observations showed that the presence of Rev was required for efficient loading of gag mRNA onto polysomes. This effect required the presence of the RRE on the mRNA. Analysis of mRNAs produced from a rev-minus proviral clone confirmed that the presence of Rev promoted polysomal loading of both gag/pol and vpu/env mRNAs. The localization of gag mRNA was also examined by in situ hybridization. This analysis showed that in the presence of Rev, most of the gag mRNA was found in the cytoplasm, while in the absence of Rev, most of the gag mRNA was found in the nucleus and in the region surrounding the nucleus. These results suggest that a substantial fraction of the gag mRNA is retained in distinct cytoplasmic compartments in the absence and presence of Rev. These findings indicate that the presence of Rev is required along the entire mRNA transport and utilization pathway for the stabilization, correct localization, and efficient translation of RRE-containing mRNAs.

Mechanism of translation of monocistronic and multicistronic human immunodeficiency virus type 1 mRNAs

We have used a panel of cDNA clones expressing wild-type and mutant human immunodeficiency virus type 1 (HIV-1) mRNAs to study translation of these mRNAs in eucaryotic cells. The tat open reading frame (ORF) has a strong signal for translation initiation, while rev and vpu ORFs have weaker signals. The expression of downstream ORFs is inhibited in mRNAs that contain the tat ORF as the first ORF. In contrast, downstream ORFs are expressed efficiently from mRNAs that have rev or vpu as the first ORF. All env mRNAs contain the upstream vpu ORF. Expression of HIV-1 Env protein requires a weak vpu AUG, which allows leaky scanning to occur, thereby allowing ribosomes access to the downstream env ORF. We concluded that HIV-1 mRNAs are translated by the scanning mechanism and that expression of more than one protein from each mRNA was caused by leaky scanning at the first AUG of the mRNA.

Mechanism of translation of monocistronic and multicistronic human immunodeficiency virus type 1 mRNAs

We have used a panel of cDNA clones expressing wild-type and mutant human immunodeficiency virus type 1 (HIV-1) mRNAs to study translation of these mRNAs in eucaryotic cells. The tat open reading frame (ORF) has a strong signal for translation initiation, while rev and vpu ORFs have weaker signals. The expression of downstream ORFs is inhibited in mRNAs that contain the tat ORF as the first ORF. In contrast, downstream ORFs are expressed efficiently from mRNAs that have rev or vpu as the first ORF. All env mRNAs contain the upstream vpu ORF. Expression of HIV-1 Env protein requires a weak vpu AUG, which allows leaky scanning to occur, thereby allowing ribosomes access to the downstream env ORF. We concluded that HIV-1 mRNAs are translated by the scanning mechanism and that expression of more than one protein from each mRNA was caused by leaky scanning at the first AUG of the mRNA.

Distinct RNA sequences in the gag region of human immunodeficiency virus type 1 decrease RNA stability and inhibit expression in the absence of Rev protein

The expression of Gag, Pol, Vif, Vpr, Vpu, and Env proteins from unspliced and partially spliced human immunodeficiency virus type 1 (HIV-1) mRNAs depends on the viral protein Rev, while the production of Tat, Rev, and Nef from multiply spliced mRNAs does not require Rev. To investigate the difference between gag and tat mRNAs, we generated plasmids expressing tat-gag hybrid mRNAs. Insertion of the gag gene downstream of the tat open reading frame in the tat cDNA resulted in the inhibition of Tat production. This inhibition was caused, at least in part, by a decrease in the stability of the produced mRNA. Deletions in gag defined a 218-nucleotide inhibitory sequence named INS-1 and located at the 5' end of the gag gene. Further experiments indicated the presence of more than one inhibitory sequence in the gag-protease gene region of the viral genome. The inhibitory effect of INS-1 was counteracted by the positive effect mediated by the Rev-Rev-responsive element interaction, indicating that this sequence is important for Rev-regulated gag expression. The INS-1 sequence did not contain any known HIV-1 splice sites and acted independently of splicing. It was found to have an unusually high AU content (61.5% AU), a common feature among cellular mRNAs with short half-lives. These results suggest that HIV-1 and possibly other lentiviruses have evolved to express unstable mRNAs which require additional regulatory factors for their expression. This strategy may offer the virus several advantages, including the ability to enter a state of low or latent expression in the host.

Stage-specific cellular regulation of inhibin alpha-subunit mRNA expression in the rat seminiferous epithelium

To find out the local regulation of inhibin production and its possible paracrine role in the seminiferous epithelium, inhibin alpha mRNA levels were measured in sequential 1 mm segments of rat seminiferous tubules accurately staged by transillumination technique. Highest levels were found at stages XIV-I-IV of the cycle, and lowest at stages VI-VIIb of the cycle. When dividing spermatogonia were selectively destroyed by 3 Gy of high-energy X-irradiation, stage-specific inhibin alpha mRNA levels remained unchanged until 26 and 38 days after irradiation when stages VII and VIII of the cycle showed 6- and 4-fold increases during a selective reduction of pachytene spermatocyte and round spermatid numbers, respectively. The results suggest that these cells at a strictly stage-specific fashion have a paracrine inhibitory effect on Sertoli cell inhibin alpha gene expression. Inhibin alpha mRNA level also correlates closely to the follicle-stimulating hormone-stimulated cAMP production during the cycle of the seminiferous epithelium, but does not seem to have a correlation to spermatogonial DNA synthesis.

Expression, secretion, and lipid-binding characterization of the N-terminal 17% of apolipoprotein B

The N-terminal 17% of human apolipoprotein B (apoB-17) was expressed in murine C127 cells following transfection with a bovine papilloma virus-based expression vector. A permanent cell line overexpressing the expected 89-kDa protein was selected and characterized. Pulse-chase experiments showed that the depletion of intracellular apoB-17 follows an apparent first-order kinetics with t1/2 = 51 min. Under conditions of continuous labeling, greater than 60% of the total synthesized apoB-17 was secreted in a soluble form, approximately 98% lipid-poor and approximately 2% lipid-bound. Inclusion of 1.2 mM oleate resulted in 5- and 2.5-fold increases in the amount of labeled apoB-17 in the p less than 1.063 g/ml and 1.063 less than p less than 1.21 g/ml fractions, respectively, which was coordinated with increased secretion of radiolabeled core lipids, triacylglycerols, and cholesteryl esters. Thus under conditions in which lipid pools are enriched a greater fraction of apoB-17 may be secreted on lipoprotein-like particles. The lipid-poor apoB-17 present in p greater than 1.21 g/ml readily associates with exogenously added dimyristoylphosphatidylcholine (DMPC) multilamellar vesicles to form discoidal particles. Discs formed with DMPC/apoB-17, 7:1 (wt/wt), are 239 +/- 43 A in diameter and 61 +/- 4 A thick and contain approximately 2 molecules of apoB-17 and 2250 molecules of DMPC per disc. Based on volume calculations we conclude that apoB-17 forms an annulus about one bilayer high and 10 A thick surrounding the DMPC disc. Circular dichroic spectra of apoB-17 on DMPC discs showed apoB-17 to contain 39% alpha-helix, 36% beta-sheet, 9% beta-turn, and 16% random coil. To be consistent with this model greater than 70% of apoB-17 on DMPC discs must bind to lipid. These data suggest that the N-terminal 17% of apoB-100 can bind lipid and may contribute to some extent to the stabilization of triglyceride-rich lipoproteins.

Expression of human immunodeficiency virus type 1 vif and vpr mRNAs is Rev-dependent and regulated by splicing

We have analyzed the structure and expression of the HIV-1 vif and vpr mRNAs. The results revealed that the predominant vif and vpr mRNAs belong to the intermediate size class of HIV-1 mRNAs and that their expression is dependent on the presence of Rev protein. In addition, low levels of a small multiply spliced vpr mRNA were produced by HIV-1. cDNA cloning and expression of vpr cDNAs in eucaryotic cells revealed that high levels of Vpr were produced only from the intermediate-size mRNA in the presence of Rev. Thus, as demonstrated for the viral structural proteins, expression of Vif and Vpr is regulated by Rev. The arrangement of the splice sites and the Rev-RRE interaction are responsible for the regulation of viral expression, and especially for the switching from an early stage, producing only or primarily Tat, Rev, and Nef from multiply spliced mRNAs, to a late stage, leading to the production of Gag, Pol, Env, Vpu, Vif, and Vpr from unspliced and partially spliced mRNAs.

Expression and characterization of recombinant mouse beta 2-microglobulin type a in insect cells infected with recombinant baculoviruses

The murine beta 2-microglobulina cDNA was cloned into pAc373 and pVL941 transfer vectors and introduced via homologous recombination into the genome of Autographa californica nuclear polyhedrosis virus downstream of the polyhedrin promoter. Both types of recombinant baculoviruses were isolated and used to infect Spodoptera frugiperda (Sf9) lepidopteran cells. beta 2m was synthesized at a substantially higher rate in cells infected with the pVL941-derived virus than when the pAc373-based virus was used. beta 2m was secreted into the culture medium where it accumulated and, under the best conditions, reached an approximate level of 10 micrograms/10(6) cells. Pulse-chase experiments after metabolic labelling with 35S-methionine followed by immunoprecipitation showed that beta 2m was stable, but that the secretion process in infected cells was relatively slow. Recombinant beta 2m was endowed with biological activity and was indistinguishable from that produced by mouse cells in 2D gel analysis. beta 2m was purified to near homogeneity from serum-free culture medium conditioned by recombinant baculovirus-infected cells by using an immunoaffinity column. The use of the insect cell/baculovirus expression system should constitute a suitable source of mouse beta 2m and should aid experiments aimed at unraveling its interactions with mouse class I histocompatibility molecules.

Cytosolic targeting of hen egg lysozyme gives rise to a short-lived protein presented by class I but not class II major histocompatibility complex molecules

A way to study the role of intracellular trafficking of an antigen in its presentation to T cells is to target the antigen to various cell compartments of the antigen-presenting cells (APC) and compare the nature of the complexes associating major histocompatibility complex (MHC) molecules and antigenic peptides, expressed on the cell surface. MHC class I+ and MHC class II+ mouse L fibroblasts secreting hen egg lysozyme (HELs cells) or expressing HEL in their cytosol (HELc cells) were obtained after transfection with HEL cDNA and signal sequence-deleted HEL cDNA, respectively. HEL was evidenced in both HELs- and HELc-transfected cells and the former type of transfectant secreted a large amount of HEL. However, HEL produced in the cytosol exhibited a short half-life of less than 5 min. HEL-derived peptides could not be shown biochemically either in HELc- nor in HELs-transfected cells. We then studied the capacity of these cells to present HEL to HEL-specific class I- and class II-restricted T cells. Both cell types could be recognized by the HEL-specific MHC class I-restricted CTL clones. In contrast, MHC class II-HEL peptide complexes, recognized by HEL-specific helper T cell hybridomas, could be detected on MHC class II+ HELs- but not HELc-transfected cells. In vivo experiments showed, however, that HELc-transfected cells could provide host APC with HELc-derived peptides able to associate with MHC class II molecules. This was inferred from the capacity of MHC class II-HELc-transfected cells, unable by themselves to elicit any anti-HEL antibody response, to prime syngeneic and allogeneic mice against HEL. The priming was revealed by the induction of an antibody response after a boost with an amount of HEL unable itself to elicit an antibody response.

Modulation of IFN-mediated Ly-6E antigen induction by cAMP in a T cell lymphoma: opposite effects on the responses to IFN-gamma and IFN-alpha/beta

This study was initiated to examine the role of cyclic nucleotides in the regulation of the expression of the Ly-6E cell surface Ag by IFN. As a model system, we used the YAC T cell lymphoma where this Ag is constitutively absent but is highly inducible by both IFN-gamma and IFN-alpha/beta. Treatment with cAMP or cGMP analogs did not cause Ly-6E expression in the absence of IFN. However, treatment with cAMP analogs, but not with cGMP analogs, markedly altered Ly-6E expression triggered by IFN, both at the mRNA and at the cell surface protein levels. Interestingly, these effects depended on whether Ly-6E induction was mediated by IFN-gamma or IFN-alpha/beta. Thus, the response to IFN-gamma was enhanced up to tenfold, whereas the response to IFN-alpha/beta was suppressed by 90-95%. Similar differential modulations of Ly-6E induction were also exerted by forskolin and cholera toxin, which are known to elevate intracellular cAMP concentration through distinct mechanisms. A YAC cell variant (C10) was isolated, where cAMP analogs or cAMP inducers could not modify Ly-6E induction. Although resistant to the biological effect of cAMP, the C10 mutant exhibited normal IFN-mediated Ly-6E responses. Moreover, in this mutant, Ly-6E induction was still affected by the PKC activator PMA, as in wild-type YAC cells. Altogether, our data demonstrate that cAMP (and cGMP) is not directly involved as second messenger in Ly-6E induction mediated by IFNs. However, a rise of cAMP modulates in an opposite fashion the Ly-6E-inducing actions of IFN-gamma and IFN-alpha/beta, which suggests that the two types of IFN utilize separate biochemical pathways to regulate Ly-6E expression.

Stage- and cell-specific expression of the ornithine decarboxylase gene during rat and mouse spermatogenesis

Ornithine decarboxylase (ODC) is an enzyme that has been shown to be induced in the growth, differentiation and proliferation of cells. We have used a cDNA probe to determine ODC mRNA levels in different stages of the cycle of rat and mouse seminiferous epithelium. For Northern and slot-blot hybridizations, RNA was isolated from microdissected staged seminiferous tubules. Cell-specific localization of ODC mRNA was studied by in situ hybridization. In the rat, in situ hybridization showed increasing mRNA levels during prophase of meiosis with the highest mRNA levels seen in late pachytene spermatocytes and step 3-5 spermatids. In the mouse, the mRNA levels increased in a similar fashion and the highest mRNA levels were found in step 1-8 spermatids. In the rat, Northern blot hybridizations revealed three molecular sizes of ODC mRNA: 2.2, 2.7 and 1.6 kb. The levels of all molecular sizes were highest in stages VII-VIII, and the lowest mRNA levels were seen in stage I of the seminiferous epithelial cycle. The level of the 2.2 kb transcript was low during stages XIII-I. In the mouse, the Northern blot hybridizations also showed three molecular sizes of ODC mRNA: 2.2 and 2.7 kb and very low levels of 1.6 kb transcript. The levels of the transcripts were steady throughout the cycle. In the mouse, the 2.2 kb transcript was more abundant than the 2.7 kb transcript indicating a species difference between rat and mouse in the usage of the two polyadenylation signals within the ODC gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Human fibroblasts show expression of the leukotriene-A4-hydrolase gene, which is increased after simian-virus-40 transformation

Human fibroblasts in cell culture converted the epoxide intermediate leukotriene A4 into the potent chemotaxin leukotriene B4. The identity of leukotriene B4 was ascertained by its mobility in reverse-phase high performance liquid chromatography, ultraviolet spectroscopy and gas chromatography/mass spectrometry. The presence of the enzyme responsible for the conversion (i.e. leukotriene A4 hydrolase), as well as the corresponding mRNA, were demonstrated by Western and Northern blot analyses. Leukotriene-A4-hydrolase enzyme activity, protein and mRNA were all enhanced (approximately threefold) in human fibroblasts that had been transformed by simian virus 40.

Axonal modulation of myelin gene expression in the peripheral nerve

Myelin gene expression (P0, MBP, P2, and MAG) was investigated during Wallerian degeneration and in the presence or absence of subsequent axonal regeneration and remyelination. The steady state levels of mRNA and protein were assessed in the crushed or permanently transected rat sciatic nerve at 0, 1, 4, 7, 10, 12, 14, 21, and 35 days after injury. The mRNA and protein steady state levels of the myelin specific genes, P0 and the MBPs, decreased to low yet detectable levels during Wallerian degeneration and returned to normal levels with subsequent axonal regeneration. The steady state level of P2 protein also followed a similar pattern of expression. The steady state level of MAG mRNA decreased to undetectable levels by 4 days of injury in the permanently transected nerve. After crush injury, re-expression of MAG to levels comparable to those of normal nerves preceded that of P2 by 2 days and that of P0 and the MBPs by 3 weeks during axonal regeneration and remyelination. These results support the proposed roles for MAG in the formation of initial Schwann cell-axonal contact required for myelin assembly, for P2 in fatty acid transport during myelination, and for P0 and the MBPs in the maintenance of the integrity and compactness of the myelin sheath. In addition, these results indicate that the expression of the myelin specific genes, P0 and MBP, is constitutive and that the level of myelin specific mRNAs is modulated by axonal contact and myelin assembly.

Regulation of apolipoprotein E gene expression after injury of the rat sciatic nerve

The expression of apolipoprotein E (apo E) is dramatically increased following peripheral nerve injury. This increased expression has been postulated to be negatively influenced by unknown mechanisms during subsequent axonal regeneration (Muller et al.: Science 228:499-501, 1985). The present study investigates the role of the regenerating axon in regulating apo E gene expression in two experimental paradigms which permit or prevent axonal regeneration in the adult rat sciatic nerve--the crush or permanent transection injuries. The nerves in these two models undergo axonal degeneration, demyelination, and Schwann cell proliferation; however, subsequent axonal regeneration and remyelination occur only in the distal segment of the crush-injured and not in the permanently transected nerve. The steady-state levels of apo E mRNA in both models increase sharply between 1 and 4 days and reach a maximum level at 12-14 days, which did not change significantly between 14 and 35 days after injury. No significant difference is observed in the steady-state levels of apo E mRNA between the crushed and permanently transected nerves as a function of time after injury. The steady-state protein level of apo E in the endoneurial segments initially increases, peaks at 14-21 days, and then decreases between 35 and 60 days after injury in both models. In contrast, the rate of newly translated and secreted apo E significantly increases by fourfold (P less than 0.005) between 35 and 60 days after permanent transection whereas it does not significantly differ at these times after crush injury. The increased rate of translation and secretion of apo E after transection compared to the constant rate observed after crush injury, together with the comparable steady-state levels of apo E mRNA and protein in both models, suggests translational or post-translational control, but not transcriptional and/or posttranscriptional control, by the regenerating axons. Furthermore, the increasing rate of biosynthesis and secretion of apo E after permanent transection concomitant with the decreasing steady-state levels of the protein suggests that apo E is either removed from the endoneurium or subsequently utilized or degraded by mechanisms that are independent from nerve regeneration.

Activation of the glucose-regulated gene (grp78) in regenerating rat liver is nonspecific and is related to acute phase response

The expression pattern of the hsp70 gene family during regeneration or rat liver has been investigated. Northern blots were prepared from total RNA isolated from livers at 0 h (control), 12 h (end of prereplication phase), 24 h (maximum of DNA synthesis) and 36 h (postmitotic phase) after partial hepatectomy. Blots were hybridized with probes specific for the hsp70 (heat-inducible), hsc70 (constitutively expressed), hst70 (testis-specific) and grp78 (glucose-regulated) gene. No hsp70 and hst70 gene transcripts have been detected at any time point investigated, and only a low increase of the hsc70 mRNA level has been observed 24 h after surgery. In contrast, a significant accumulation of the transcript coded by the grp78 gene has been detected in liver remnant 12 and 24 h after partial hepatectomy. However, we observed a comparable activation of this gene in livers of sham-operated rats or in rats injected with turpentine to cause sterile inflammation. Our results indicate that the activation of the grp78 gene in liver of wounded rats (partial hepatectomy or sham operation) is presumably a part of acute-phase response.

Isolation and analysis of ribonucleic acids from skeletal tissues

We report a method for the isolation of total cellular RNA from mineralized or cartilaginous tissues. The procedure accommodates the large amount of hydroxyapatite and high buoyant density proteoglycans present in skeletal tissue samples, as well as the low cell density characteristic of these tissues. The procedure can be reliably used for processing a large number of small (100-800 mg) tissue samples. Tissues are homogenized in guanidine hydrochloride solution, then centrifuged at low speed, and filtered to remove the nonsolubilized extracellular matrix proteins. Subsequent high speed density gradient centrifugation produces a high yield of RNA (0.2-0.6 micrograms RNA/mg tissue) which is precipitated in a low pH sodium acetate solution. RNA extracted by this method has been analyzed for the expression of various genes by Northern blotting. In addition to mRNAs of bone- and cartilage-specific proteins, messenger RNA for growth factors, proto-oncogenes, and heat shock proteins can be detected.

Arrest of epidermal growth factor-dependent growth in fetal hepatocytes after ethanol exposure

Exposure of the fetal rat hepatocyte to ethanol in vitro blocks epidermal growth factor (EGF)-dependent cell replication. To define possible mechanisms for this growth arrest, we determined the effects of ethanol on EGF binding and EGF receptor (EGF-R) levels. During a 24-h exposure to ethanol (1.7 mg/ml, 31 mM), cell replication was completely blocked while EGF binding per cell doubled. This effect was no specific for EGF, with variable degrees of increased binding noted for insulin, transferrin, and glucagon. Significantly increased EGF binding was seen after 6 h of ethanol exposure, and both growth arrest and enhanced EGF binding were reversed within 12 h of ethanol withdrawal. Increases in both "high" and "low" affinity sites were seen, with no changes in the apparent Kd's. Total RNA, beta-actin mRNA, and EGF-R mRNA were increased 50-70% in ethanol exposed cells. However, direct measurements of EGF-R synthesis rates by [35S]methionine incorporation revealed no differences between control and ethanol exposed cells. Internalization of EGF-R was significantly altered by ethanol exposure. A 2-h incubation resulted in the internalization of 57% of the ligand in control cells, while only 31% of bound EGF was internalized in the ethanol exposed cells. Thus, the enhanced EGF binding may be due to decreased efficiency of internalization.

rev protein of human immunodeficiency virus type 1 affects the stability and transport of the viral mRNA

rev (trs/art) is an essential human immunodeficiency virus type 1 (HIV-1) regulatory protein. rev increases the levels of the gag- and env-producing mRNAs via a cis-acting element in the env region of HIV-1, named rev-responsive element. Our results show that rev increases the stability of the unspliced viral mRNA, while it does not affect the stability of the multiply spliced viral mRNAs that do not contain the rev-responsive element. The study of mutated proviral constructs producing mRNA that cannot be spliced revealed that the effect of rev on stability is independent of splicing. Our experiments also indicate that rev promotes the transport of the viral mRNA containing the rev-responsive element from the nucleus to the cytoplasm. The proposed functions of rev are consistent with its nuclear localization as shown by immunofluorescence. The selective effects of rev on the levels of the viral mRNA suggest a model for feedback regulation by rev leading to a steady state of viral expression.

Effect of age and busulphan treatment on the hsp70 gene-related transcript level in rat testes

Northern blot analysis was used to determine the expression pattern of a testis-specific, heat shock gene (hsp70)-related transcript (hst70 RNA) in rats during sexual maturation, following administration of busulphan and in aged rats with progressive tubular degeneration. The level of the hst70 RNA in testes of rats aged 4, 7, 12, 19, 21 or 24 months decreased gradually with age. In the atrophic testes of old rats with tubules depleted of germ cells, the hst70 transcript was undetectable. Transient arrest of spermatogenesis in rats aged 2 months following injection of a single dose of busulphan (10 mg/kg) resulted in a parallel but transient decrease in the transcript level to undetectable values, followed by its reappearance after resumption of spermatogenesis. The transcript was absent from testes containing tubules depleted of spermatocytes and spermatids and reappeared when a new generation of round spermatids repopulated the seminiferous tubules. Possible expression of the hst70 gene prior to the formation of early spermatids is discussed.

Expression of a testis-specific hsp70 gene-related RNA in defined stages of rat seminiferous epithelium

Changes in the level of a testis-specific hsp70 gene-related transcript (hst70 RNA) and its cellular localization during the cycle of rat seminiferous epithelium have been investigated. Segments of seminiferous tubules at defined stages of the cycle were isolated in living condition by transillumination-assisted microdissection and the exact stages identified by phase-contrast microscopy of live cell squashes. The levels of the hst70 RNA were determined by Northern and slot blotting of whole cell lysates. High levels were found in stages XII-XIV and I to early VII of the cycle, and low levels were found in other stages, i.e., late VII (VIId) through VIII-XI of the cycle. The in situ hybridization revealed that the hst70 gene was activated in late pachytene primary spermatocytes during stage XII of the cycle, and that mRNA was then present in cells during differentiation through diakinesis, meiotic divisions, and early spermiogenesis (steps 1 through early 7). The activation of the gene coding for hst70 RNA shortly before meiotic divisions may indicate that the gene product is needed either during differentiation of late spermatocytes into spermatids or later during spermiogenesis, and that the mRNA may be stored in early spermatids.

Posttranscriptional regulation of hsp70 expression in human cells: effects of heat shock, inhibition of protein synthesis, and adenovirus infection on translation and mRNA stability

We have examined the posttranscriptional regulation of hsp70 gene expression in two human cell lines, HeLa and 293 cells, which constitutively express high levels of HSP70. HSP70 mRNA translates with high efficiency in both control and heat-shocked cells. Therefore, heat shock is not required for the efficient translation of HSP70 mRNA. Rather, the main effect of heat shock on translation is to suppress the translatability of non-heat shock mRNAs. Heat shock, however, has a marked effect on the stability of HSP70 mRNA; in non-heat-shocked cells the half-life of HSP70 mRNA is approximately 50 min, and its stability increases at least 10-fold upon heat shock. Moreover, HSP70 mRNA is more stable in cells treated with protein synthesis inhibitors, suggesting that a heat shock-sensitive labile protein regulates its turnover. An additional effect on posttranscriptional regulation of hsp70 expression can be found in adenovirus-infected cells, in which HSP70 mRNA levels decline precipititously late during infection although hsp70 transcription continues unabated.

Posttranscriptional regulation of hsp70 expression in human cells: effects of heat shock, inhibition of protein synthesis, and adenovirus infection on translation and mRNA stability

We have examined the posttranscriptional regulation of hsp70 gene expression in two human cell lines, HeLa and 293 cells, which constitutively express high levels of HSP70. HSP70 mRNA translates with high efficiency in both control and heat-shocked cells. Therefore, heat shock is not required for the efficient translation of HSP70 mRNA. Rather, the main effect of heat shock on translation is to suppress the translatability of non-heat shock mRNAs. Heat shock, however, has a marked effect on the stability of HSP70 mRNA; in non-heat-shocked cells the half-life of HSP70 mRNA is approximately 50 min, and its stability increases at least 10-fold upon heat shock. Moreover, HSP70 mRNA is more stable in cells treated with protein synthesis inhibitors, suggesting that a heat shock-sensitive labile protein regulates its turnover. An additional effect on posttranscriptional regulation of hsp70 expression can be found in adenovirus-infected cells, in which HSP70 mRNA levels decline precipititously late during infection although hsp70 transcription continues unabated.