Size, complexity and abundance of a specific poly(A)-containing RNA of liver from male Xenopus induced to vitellogenin synthesis by estrogen

Roles of Estrogens in the Healthy and Diseased Oviparous Vertebrate Liver

The liver is a vital organ that sustains multiple functions beneficial for the whole organism. It is sexually dimorphic, presenting sex-biased gene expression with implications for the phenotypic differences between males and females. Estrogens are involved in this sex dimorphism and their actions in the liver of several reptiles, fishes, amphibians, and birds are discussed. The liver participates in reproduction by producing vitellogenins (yolk proteins) and eggshell proteins under the control of estrogens that act via two types of receptors active either mainly in the cell nucleus (ESR) or the cell membrane (GPER1). Estrogens also control hepatic lipid and lipoprotein metabolisms, with a triglyceride carrier role for VLDL from the liver to the ovaries during oogenesis. Moreover, the activation of the vitellogenin genes is used as a robust biomarker for exposure to xenoestrogens. In the context of liver diseases, high plasma estrogen levels are observed in fatty liver hemorrhagic syndrome (FLHS) in chicken implicating estrogens in the disease progression. Fishes are also used to investigate liver diseases, including models generated by mutation and transgenesis. In conclusion, studies on the roles of estrogens in the non-mammalian oviparous vertebrate liver have contributed enormously to unveil hormone-dependent physiological and physiopathological processes.

Cellular distribution of Mr 25,000 protein, a protein partially overlapping phosvitin and lipovitellin 2 in vitellogenin B1, and yolk proteins in Xenopus laevis oocytes and embryos

A phosphorylated protein with molecular mass of 25,000 (pp25) can be derived from Xenopus laevis vitellogenin B1. In order to clarify the distribution of pp25, the changes in the concentration and localization of this protein in oocytes and embryos were examined by immunoblotting and immunohistochemistry using anti-pp25 antibodies, and compared with those of yolk proteins. In oocytes, pp25 was shown to localize characteristically at the surface just below the plasma membrane by immunohistochemical analysis. Interestingly, during embryogenesis, immunocytochemical staining revealed a transition of the pp25 distribution from beneath the outer surface of each germ layers to endoderm during tailbudding. In contrast, yolk proteins were localized in endoderm constantly throughout the developmental stages. However, the level of pp25 in the cytoplasm gradually decreased following the growth of embryos at the tailbud stage and disappeared at the tadpole stage, as shown by immunoblot analysis. These results suggest that pp25 could play different roles from those of yolk proteins such as lipovitellin and phosvitin in X. laevis oocytes and developing embryos.

Mr 25 000 protein, a substrate for protein serine/threonine kinases, is identified as a part of Xenopus laevis vitellogenin B1

A phosphorylated protein with a molecular mass of 25 000 (pp25) previously purified from the cytosolic fraction of Xenopus laevis oocytes is an effective phosphate acceptor for casein kinases and protein kinase C. In this study, based on the partial amino acid sequence of pp25, a cDNA was isolated that encodes a new yolk precursor protein, Xenopus vitellogenin B1, which contained the sequence encoding pp25. Both mRNA and protein of vitellogenin B1 were expressed in all of the female organs examined. In agreement with a previous report, the amount of vitellogenin B1 protein in the liver increased after stimulation with estrogen. These results suggest that pp25 is a cytosolic non-crystallized yolk protein nutrient source, but it might also play a role in rapid development.

Precursor-product relationship between vitellogenin and the yolk proteins as derived from the complete sequence of a Xenopus vitellogenin gene

In Xenopus laevis four estrogen-responsive genes are expressed simultaneously to produce vitellogenin, the precursor of the yolk proteins. One of these four genes, the gene A2, was sequenced completely, as well as cDNAs representing 75% of the coding region of the gene. From this data the exon-intron structure of the gene was established, revealing 35 exons that give a transcript of 5,619 bp without the poly A-tail. This A2 transcript encodes a vitellogenin of 1,807 amino acids, whose structure is discussed with respect to its function. At the nucleic acid as well as at the protein level no extensive homologies with any sequences other than vitellogenin were observed. Comparison of the amino acid sequence of the vitellogenin A2 molecule with biochemical data obtained from the different yolk proteins allowed us to localize the cleavage products on the vitellogenin precursor as follows: NH2 - lipovitellin I - phosvitin (or phosvette II - phosvette I) - lipovitellin II - COOH.

Effects of antiestrogens on the induction of vitellogenin and its mRNA in Xenopus laevis

The egg yolk protein precursor vitellogenin is induced by estrogen in the liver of male Xenopus laevis. The large rise in serum vitellogenin is accompanied by a corresponding increase in intracellular levels of vitellogenin and its mRNA. In the present study this model system was used to examine the subcellular sites of action of triphenylethylene antiestrogens (e.g. tamoxifen). Tamoxifen was extensively metabolized to 4-hydroxytamoxifen in Xenopus and both of these antiestrogens were used in this study. Pre-injection with tamoxifen or 4-hydroxytamoxifen suppressed the estrogen-dependent induction of vitellogenin in serum. 4-Hydroxytamoxifen also inhibited the induction of intracellular vitellogenin and its mRNA by estrogen suggesting that this metabolite of tamoxifen is able to inhibit estrogen-induced transcription of the vitellogenin genes. Neither tamoxifen nor 4-hydroxytamoxifen stimulated the production of serum vitellogenin as assayed by a sensitive dot immunoblot assay. However either compound alone induced low amounts of vitellogenin mRNA and stimulated the production of intracellular vitellogenin to levels 10-40% of those produced by similar doses of estradiol. Since 10-40% of the serum levels of vitellogenin produced by estradiol would have been detected by the dot immunoblot assay, these data suggest that antiestrogens may have effects on post-translational processing or secretion of vitellogenin in addition to their effects on vitellogenin transcription.

Differential stability of Drosophila embryonic mRNAs during subsequent larval development

The relative stabilities of specific embryonic mRNAs that persist in Drosophila melanogaster larvae were determined using an approach that combined RNA density labeling with cell-free translation. Unlike the other methods commonly used to measure the decay of individual mRNAs, the density labeling approach does not depend on the use of transcriptional inhibitors or on the measurement of precursor pool specific activities. Using this approach, we have determined that different embryonic mRNA species persist for varying periods during subsequent development, with half-lives ranging from approximately 2 to approximately 30 h. The embryonic histone mRNAs are relatively unstable; they are no longer detectable by 9 h of larval development. By 41 h of larval development, 90% of the nonhistone mRNAs assayed have decayed considerably; computerized scanning densitometry of translation products indicates that these transcripts are not decaying as members of discrete half-life classes. The persisting mRNAs that remain are very long-lived; their in vitro translation products can still be detected after 91 h of larval development. We have tentatively identified the mRNAs that encode actin, tropomyosin, and tubulin as members of this stable mRNA population. Although embryonic mRNAs do fall into these three broad classes of stability, they appear to decay with a continuum of half-lives. Because the range of half-lives is so great, mRNA stability is probably an important factor controlling mRNA abundance during Drosophila development.

Injection of partially purified estrogen receptor protein from Xenopus liver nuclei into oocytes activates the silent vitellogenin locus

Injection of extracts from Xenopus liver nuclei that are enriched 2000 times in estradiol receptor into Xenopus oocytes induces transcription of the silent vitellogenin locus, which is activated in liver by estradiol, but not of the albumin locus, which is active in liver but suppressed by high levels of estradiol. Transcription initiates within the 5'-end region of the gene we have studied and probably continues into the 3' third. The activation seems to be very efficient, but most of the primary transcripts are probably rapidly and inaccurately processed. New proteins are also made and secreted by the oocytes.

Effect of estrogen on Xenopus laevis albumin mRNA levels

Accurate quantitation of concentrations of albumin mRNA by hybridization to albumin cDNA allows analysis of the estrogen effect on the Xenopus laevis albumin mRNA levels in the liver cytoplasm during the primary stimulation. Administration of estradiol-17 beta to male Xenopus laevis increases the amount of both total cellular RNA and poly(A)-RNA but has no effect on the albumin mRNA level and the amount of albumin mRNA per cell is constant.

Persistence, methylation and expression of vitellogenin gene derivatives after injection into fertilized eggs of Xenopus laevis

We report the fate of different derivatives of the vitellogenin genes after injection into fertilized eggs of Xenopus. We injected a constructed minigene as well as a 5' fragment of the A2 vitellogenin gene. The minigene survives in embryogenesis much better than the 5' A2 fragment and is retained more frequently and at a higher level in frog tissues. The mosaic distribution of the foreign DNA in different frog tissues indicates that no integration occurred before the first cleavage stage. The persisting DNA may be partially integrated but is mostly found in an episome-like form. This unintegrated form is not supercoiled and is rearranged. Methylation of the Hpa II sites prior to injection has no influence on the survival of the injected sequences and the Hpa II sites of the surviving DNA are unmethylated irrespective whether the injected DNA was methylated or not. Whereas the derivatives are transcribed in embryos, they cannot be activated by estrogen in the liver of young frogs.

Structure and evolution of the Xenopus laevis albumin genes

The 68K and 74K albumin genes of Xenopus laevis arose by duplication approximately 30 million years ago. Electron microscopic analysis showed that both genes contain 15 coding sequences. The lengths of corresponding coding sequences are almost identical and are extremely similar to those of mammalian albumin genes. A block of four coding sequences, which in mammals codes for one protein domain, is repeated three times. The corresponding introns are usually different in length and have therefore diverged as a result of insertion/deletion events. The extensive homology between these gene sequences is neither confined to nor most extensive in the coding sequences and similar amounts of homologous sequences are found in the flanking DNAs as in the gene regions. Various structures were formed in the 5'-flanking DNA by mutually exclusive pairing of different homology regions. Analysis of the two 74K albumin gene sequences isolated suggests that the X. laevis genome may contain one 68K albumin gene and two very closely related 74K albumin genes.

The role of estrogen receptor in Xenopus laevis vitellogenin gene expression

Administration of estradiol 17 beta [estra-1,3,5(10)-triene-3,17-beta-diol] to male Xenopus laevis induces the massive synthesis by the liver of the egg yolk precursor phospholipoglycoprotein, vitellogenin, and its cognate mRNAs. Restimulation of male X. laevis that have been previously induced to synthesize vitellogenin mRNA but are inactive in vitellogenin mRNA synthesis at the time of restimulation with estrogen results in more rapid accumulation of vitellogenin mRNA and more efficient transcription of the vitellogenin genes than occurs following primary estrogen stimulation. The estrogen receptor system that mediates estrogen action in this organism exhibits several unusual properties. The cytoplasm of unstimulated liver cells contains high levels of a middle-affinity estrogen-specific binding protein and little if any estrogen receptor. The properties of the estrogen binding protein are consistent with a role in protecting estradiol 17 beta against metabolism, as a fraction of cytoplasmic estradiol 17 beta is not subject to rapid metabolism. In addition, similar binding activities are found in all Xenopus tissues surveyed that respond to steroid hormones. The induction of nuclear estrogen receptor is coincident with the onset of vitellogenin mRNA accumulation. However, an increased level of estrogen receptor is not responsible for the elevated rate of vitellogenin gene transcription observed following restimulation with estrogen.

Comparative study of the physiology of vitellogenesis in Japanese quail

1. During a study of vitellogenesis in the Japanese quail, male or female quail were given, intramuscularly, one to three doses of estradiol-17 beta (16 mumol/100 g body weight). 2. In treated males, the plasma concentrations of protein-bound P (PBP, a measure of the phosphate groups in vitellogenin), Ca, and total protein fluctuated in unison. 3. Packed cell volume (PCV) values varied approx, inversely as the PBP concentrations. 4. Maxima of approx. 0.85 mg PBP/ml occurred six, five, and four days after the primary, secondary, and tertiary injections. Following each maximum, the PBP concentrations declined rapidly to the 0.004 mg/ml level found in the plasma of untreated male quail. 5. During periods of elevated PBP levels, Ca and PBP were present in equimolar quantities. 6. When laying female quail were given a single injection of estradiol, the PBP concentration rose from 0.12 to 0.97 mg/ml. 7. PCV values for both male and female estrogen-treated quail decreased from approx. 0.50 to 0.30. The decreases in PCV values were attributed to increases in plasma volume as measured using 125I-labelled serum albumin. No marked change in total erythrocyte mass was measured using 51CrO2-(4) as the cell label. 8. Perhaps the decrease in plasma serum albumin concentrations reported by some investigators for estrogen-treated animals is due to dilution rather than to repression of serum albumin synthesis.

Vitellogenesis and the vitellogenin gene family

Vitellogenin is synthesized under estrogen control in the liver, extensively modified, transported to the ovary, and there processed to the yolk proteins lipovitellin and phosvitin. In the frog Xenopus laevis there are at least four distinct but related vitellogenin genes. The two genes A1 and A2 have a 95 percent sequence homology in their messenger RNA coding regions, and contain 33 introns that interrupt the coding region (exons) at homologous positions. Sequences and lengths of analogous introns differ, and many introns contain repetitive DNA elements. The introns in these two genes that have apparently arisen by duplication have diverged extensively by events that include deletions, insertions, and probably duplications. Rapid evolutionary change involving rearrangements and the presence of repeated DNA suggests that the bulk of the sequences within introns may not have any specific function.

Catfish vitellogenin and its messenger RNA are smaller than their chicken and Xenopus counterparts

Injection of male bullheads (Ameiurus nebulosus) with estradiol induced the production of a major serum phosphoprotein of molecular weight 145,000. This protein was immunoprecipitable by antisera raised against lipovitellin from bullhead eggs and was absent from the serum of control males. Production of this serum protein coincided with changes in the liver mRNA population, which suggested that estradiol had induced the synthesis of additional mRNA sequences in the high-frequency class. Agarose gel electrophoresis in the presence of methyl mercury hydroxide showed that this mRNA population contained at least one species which was not present in the liver of uninjected males. This new RNA was the major polyadenylated species present in the total cellular RNA and its size relative to ribosomal RNAs and locust vitellogenin mRNA was estimated as 5800 nucleotides. When the liver total RNAs were translated in the mRNA-dependent rabbit reticulocyte lysate system the major translation product from the induced fish had the same molecular weight (145,000) as the serum phosphoprotein and was immunoprecipitable by antilipovitellin antisera. This translation product was not coded for by RNA from control fish. These observations are consistent with the induction of vitellogenesis by estradiol as reported in other egg-laying vertebrates and they show that bullhead vitellogenin and its mRNA are significantly smaller than their avian and amphibian counterparts.

Four different vitellogenin proteins of Xenopus identified by translation in vitro

Kinetic analysis of vitellogenin mRNA translation in a cell-free reticulocyte lysate translation system revealed that a serine-rich sequence, most probably containing the phosvitin molecule, is located toward the end of the translational product and therefore resides near to the carboxy terminus of the vitellogenin molecule. Translation of the four different vitellogenin mRNAs in vitro and cleavage of the translational products with cyanogen bromide revealed that vitellogenin consists of four different polypeptides, each containing a serine-rich sequence toward its carboxy terminus.

Processing of ribosomal precursor RNAs in Physarum polycephalum

The processing of intermediates of Physarum rRNA has been investigated using two cloned rDNA restriction fragments as sequence specific probes. The size of the largest stable pre-rRNA detected is 11.8 Kb, as determined by denaturing gel electrophoresis. R-loop analysis revealed that this pre-rRNA fraction mainly consists of transcripts which do not contain the two insertion sequences present in the 26 S gene. However, a small number of molecules are found which still contain one or the other, or both, of these introns. These observations suggest that the two introns are transcribed but spliced out in a random order before RNA synthesis has reached the 3-end of the primary transcript. Transcription starts at about 17.7 Kb and stops at 4.4 Kb as measured from the ends of the linear palindromic rDNA molecule. Besides the 11.8 Kb transcript several further ribosomal RNA precursors have been identified, ranging in size from 2.5 to 8.8 Kb. In addition, the genes for 5.8 S RNA have been located by R-loop mapping.

Comparative analysis of the structural organization of two closely related vitellogenin genes in X. laevis

The structural organization of the two closely related vitellogenin genes A1 and A2 has been determined and compared by electron microscopy. In both genes the mRNA-coding sequence of 6 kb is interrupted 33 times, leading to a total gene length of 21 kb for gene A1 and 16 kb for gene A2. Thus both genes have a mean exon length of 0.175 kb, while the mean intron length is 0.45 kb in gene A1 and 0.31 kb in gene A2. Because the introns interrupt the structural sequence at homologous positions in genes A1 and A2, we suggest that these two genes are the products of a duplication of an ancestral gene which has an intron-exon arrangement similar to that of the extant genes. Since the duplication event, the sequence and length of the analogous introns have changed rapidly, whereas homologous exons have diverged to an extent of only 5% of their sequences. The results suggest different mechanisms of evolution for exons and introns. While the exons evolved primarily by point mutations, such mutations, as well as deletion, insertion and duplication events, were important in the evolution of the introns.

Isolation of two closely related vitellogenin genes, including their flanking regions, from a Xenopus laevis gene library

A gene library of Xenopus laevis was constructed from embryonic DNA partially digested with restriction endonucleases Hae III and Alu I and joined to the phage lambda Charon 4 cloning vector with EcoRI linkers. Nucleotide sequences from three of the four related vitellogenin genes have been isolated. Two of the genes (called A1 and A2) were isolated in their entirety together with long stretches of flanking sequences. These two closely related vitellogenin genes have lengths of about 21 and 16 kilobases, but both produce a vitellogenin mRNA of 6.3 kilobases.

Isolation and translation in vitro of four related vitellogenin mRNAs of estrogen-stimulated Xenopus laevis

Cloning of vitellogenin cDNA of Xenopus laevis revealed that vitellogenin is encoded in a small family of genes representing two distantly related main groups A and B, each comprising two more closely related subgroups A1, A2, and B1, B2 respectively. To characterize the proteins derived from these genes we have isolated the corresponding mRNAs by hybridizing, under stringent conditions, cytoplasmic poly(a)-containing RNA from the liver of estrogen-stimulated Xenopus to filter-bound cDNA clones containing sequences specific for all four vitellogenin genes. Hybridization of the isolated mRNAs with nick-translated cDNA clones revealed that contamination of the mRNAs by those of the other main group was less than 0.1%. Melting curves of the hybrids prepared with the isolated mRNAs and cDNA clones specific for the four vitellogenin genes showed that the isolated vitellogenin mRNAs are also specific for the four subgroups. Analysis of R loops formed between isolated mRNAs and cDNA clones representing the corresponding subgroup further indicated about 10% cross-contamination between the more closely related mRNAs. In a reticulocyte lysate each of the four mRNAs coded for a 200 000-Mr protein immunoprecipitable by monospecific vitellogenin antibody. From these results we conclude that the four different mRNAs A1, A2, B1 and B2, which all can be isolated efficiently, code for vitellogenin and are expressed simultaneously in response to estrogen stimulation.

Identification, organization and processing intermediates of the putative precursors of Xenopus vitellogenin messenger RNA

To understand the mechanism of estrogen-induced activation of the vitellogenin genes in the liver of Xenopus, it is essential to characterize the transcriptional products of these genes. In this paper we describe large nuclear RNAs containing vitellogenin mRNA sequences as revealed by hybridization of cloned vitellogenin cDNAs to nuclear RNA separated on agarose gels. Putative vitellogenin mRNA precursors, which are recovered as poly(A)-containing RNA, have been identified for the four known vitellogenin mRNAs. From electron microscopic analysis of R loops, prepared between enriched mRNA precursors and cDNA specific for the A1 vitellogenin mRNA, we conclude that the precursor molecules contain sequences complementary to vitellogenin mRNA which are interrupted by additional RNA segments probably representing transcribed introns. Within the 3.7 kb of the 3' end of the A1 vitellogenin mRNA we have discovered seven large and at least five small transcribed introns. Some of the R loops have been found to contain only a few transcribed introns, and we assume that they represent processing intermediates. Comparison of these putative intermediates suggests that the splicing order of different introns does not follow a single pathway.

Enrichment and characterization of the DNA coding for vitellogenin in Xenopus laevis

Purified vitellogenin mRNA of Xenopus laevis was incubated with mechanically sheared DNA in high concentrations of formamide and the resulting R-loops (i.e. RNA . DNA hybrid fragments) separated from the bulk DNA by caesium chloride buoyant density centrifugation. Hybridization with 125I-labeled vitellogenin mRNA revealed a 15--30-fold enrichment of the DNA coding for vitellogenin. Restriction analysis of the R-loop-enriched DNA demonstrated that all known endonuclease HindIII fragments coding for vitellogenin of unfractionated Xenopus DNA were also present in the enriched material, including the specific fragments for the oligo(A)-containing segment of the RNA. Comparison of these restriction data with the structure found in cloned vitellogenin cDNA, indicates the presence of at least one intervening sequence in the genomic DNA coding for vitellogenin.

Vitellogenin in Xenopus laevis is encoded in a small family of genes

Vitellogenin, the yolk protein precursor, is produced in X. laevis liver from a 6.3 kilobase (kb) mRNA. Sequences of this mRNA have been transcribed into cDNA and cloned in E. coli. Some properties of 21 of these cloned DNAs, ranging in size from 1 to 3.7 kb, have been reported by Wahli et al. (1978b). This paper reports restriction endonuclease mapping, cross hybridization, heteroduplex mapping in the electron microscope and heteroduplex melting experiments with these DNAs. We conclude that the cloned DNAs fall into two main groups of sequences which differ from each other in approximately 20% of their nucleotides. Each main group contains two subgroups which differ from each other by about 5% sequence divergence. By hybridizing cloned DNAs with restricted genomic DNA, we showed that sequences corresponding to all four sequence groups are present in a single animal. Furthermore, we have obtained tentative evidence for the presence of large intervening sequences in genomic vitellogenin DNA. Analysis of R loop molecules demonstrated that all four sequences are present in the vitellogenin mRNA population purified from individual animals. While some alternate explanations are not entirely excluded, we suggest that vitellogenin is encoded by a small family of related genes in Xenopus.

Sub-cellular distribution of the cytoplasmic myosin heavy chain mRNA during myogenesis

In the light of earlier work [1] which demonstrated the presence of a large number of myosin heavy chain (MHC) transcripts in chick myoblasts prior to cell fusion and the burst of MHC synthesis it was of great interest to determine the subcellular localization of the still inactive transcripts. It has been determined in differentiating muscle cells in culture. Two populations of cells were examined -- monucleated myoblasts just prior to cell fusion and myotubes where at least 80% of the cells were fused. Utilizing a myosin complementary DNA (cDNA) probe [2] it is observed that just prior to cell fusion, when the "burst" of myosin synthesis has not yet occurred, the vast majority of cytoplasmic myosin mRNA transcripts are found in a stored messenger RNA protein complex with a minimal amount found in the heavy polysome fraction. In differentiated myotube cultures, when myosin synthesis is progressing at a high rate, the reverse is found, i.e, the amount of stored myosin messenger RNA (mRNA) is minimal while the largest amount of myosin mRNA transcripts are localized in the polysome fraction. The number of total cytoplasmic myosin transcripts is found to decrease after cell fusion at a time when myosin synthesis is maximal suggesting that the efficiency of translation of myosin mRNA increases during terminal differentiation.

Cell specific effect of ecdysone on RNA synthesis in the differentiated salivary gland of Acricotopus lucidus

The influence of the steroid hormone ecdysone on the RNA metabolism of 2 cell types (main and anterior lobe) of the Acricotopus salivary gland has been investigated. Ecdysone has been found to increase the synthesis of 4S RNA and poly(A)+ RNA in the main lobe, while the anterior lobe remained unaffected by the same treatement. The stimulated poly(A)+ RNA is characterized by its relatively low cytoplasmic turnover. No evidence has been found for the stimulation of ribosomal RNA synthesis in both cell types. Analysis of the metabolic stability of cytoplasmic poly(A)+ RNA after ecdysone administration revealed a different response of the main and anterior lobe cells. Only in the anterior lobe ecdysone causes a shortening of the lifetime of that part of poly(A)+ RNA which is peculiar in its early appearance and relatively low stability in the cytoplasm.

Synthesis of vitellogenin, an attractive model for investigating hormone-induced gene activation

The estrogen-induced synthesis of vitellogenin in the frog Xenopus and the chicken is an attractive system for investigating the molecular events leading to the activation of a specific gene. In this review article the events occurring at the level of the protein and mRNA in the cytoplasm are discussed. The available data show that the induction of vitellogenin synthesis is due to the accumulation of the corresponding vitellogenin mRNA in the cytoplasm. This suggests that transcriptional or posttranscriptional events in the nucleus are activated by the hormone. The few experiments investigating the processes in the nuclear compartment are reviewed.

Estradiol-induced accumulation of vitellogenin mRNA and secretion of vitellogenin in liver cultures of Xenopus

Explants of male Xenopus liver maintained in a serum-free culture medium respond to stimulation by 2 X 10(-8) M 17beta-estradiol with an increasing rate of accumulation of vitellogenin mRNA, as revealed by hybridization of cDNA to the total cytoplasmic RNA extracted from the cultures. A similar response is observed for secretion of 32PO4-labeled vitellogenin into the culture medium. The in vitro response is improved in liver tissue of prestimulated animals, and by adaptation of liver explants to the culture medium prior to hormone treatment, but attains only about 10% of the in vivo response. Since essential features of the in vivo response are maintained in liver explants, organ culture appears suitable for investigating initial events of estradiol action leading to enhanced synthesis of vitellogenin.

Characterization of uterine heterogeneous nuclear ribonucleic acid and the effect of oestradiol-17 beta on its synthesis

An early response to the administration of oestradiol-17 beta to immature rats is a dramatic stimulation in the synthesis of uterine hnRNA (heterogenous nuclear RNA). High-molecular-weight fractions of the hnRNA were purified and subfractionated on poly(U)-Sepharose into fractions that differed in their poly(A) content and their size profile on polyacrylamide gels. Oestrogen treatment of the rats stimulated the synthesis of all three fractions of high-molecular-weight hnRNA, but the kinetics of synthesis, degree of stimulation and size distribution of the newly synthesize RNA differed in each fraction.

Electron-microscopic demonstration of terminal and internal initiation sites for cDNA synthesis on vitellogenin mRNA

cDNA synthesized on purified vitellogenin mRNA from Xenopus liver was hybridized to the template in formamide/urea at 22 degrees C to avoid degradation of the RNA. The hybrids formed were visualized by spreading for electron microscopy. Contour length measurements proved that most of the RNA molecules in the hybrids were still intact showing the expected molecular weight of 2.3 x 10(6). The hybridized cDNA corresponded on the average to 12% of the RNA length. In about 80% of the molecules the cDNA was located at one end. Since cDNA synthesis was primed by oligo(dT), the terminal duplex region marks the 3' end of the vitellogenin mRNA molecule. Internal duplex regions were mainly located at a specific position starting about 2800 nucleotides from the 3' end. Since the cDNA hybridizing at the internal position could specifically be synthesized on a vitellogenin RNA fragment isolated on poly(U)-Sepharose as an oligo(A)-containing RNA, we conclude that cDNA synthesis is not only initiated by the poly(A) of the 3' end, but also by a specific internal sequence.

Hormonal control of mRNA synthesis studied by nucleic acid hybridization

The regulation of protein synthesis by steroids is thought to be due to hormonal effects primarily on mRNA concentration. Experimental evidence to support this conclusion has come largely from the use of DNA probes complementary (cDNA) to mRNA molecules or by translation of the mRNA in vitro. In this review the experimental procedures involved and the application to hormone action of cDNA hybridization will be reviewed. (1) mRNA concentrations can be assayed in tissue RNA samples by hybridization with radiolabelled complementary DNA probes (cDNA). From the rate of hybridization of an mRNA preparation to a cDNA probe it is possible to estimate specific mRNA concentrations and thereby study their hormonal regulation within tissues of subcellular fractions (2) Rates of synthesis of a specific RNA can be measured by hybridization of pulse-labelled RNA with excess cold cDNA as illustrated in studies of the glucocorticoid induction of MMTV RNA. (3) Hormore-induced alterations of mRNA populations as a whole can be investigated. From the kinetics of hybridization of mRNA with its complementary DNA it is possible to estimate the number of different RNA sequences in tissues and to approximate the number of copies of each sequence per cell. Consequently, by comparing mRNA samples isolated from tissues of different hormonal status it is possible to demonstrate specific hormone-inducible mRNA species and, in some cases, identify their translation products.