Transcriptional and post-transcriptional inhibition of albumin gene expression by estrogen in Xenopus liver

Overexpression of Lin28a delays Xenopus metamorphosis and down-regulates albumin independently of its translational regulation domain

BACKGROUND

Lin28 regulates stem cell biology and developmental timing. At the molecular level Lin28 inhibits the biogenesis of the micro RNA let-7 and directly controls the transcription and translation of several genes. In Xenopus, Lin28 overexpression delays metamorphosis and affects the expression of genes of the thyroid hormone (TH) axis. The TH carrier albumin, synthesized by the liver, is down-regulated in limbs and tail after Lin28 overexpression. The molecular mechanisms underlying the interaction between Lin28, let-7, and the hypothalamus-pituitary-thyroid gland (HPT) axis are unknown.

RESULTS

We found that precursor and mature forms of let-7 increase during Xenopus metamorphosis. In the liver, lin28b is down-regulated and albumin is up-regulated during metamorphosis. Overexpression of a truncated form of Lin28a (Lin28aΔC), which has been shown not to interact with RNA helicase A to regulate translation, delays metamorphosis, indicating that the translational regulation domain is not required to inhibit the HPT axis. Importantly, both full length Lin28a and Lin28aΔC block the increase of albumin mRNA in the liver independently of changes in TH signaling.

CONCLUSIONS

These results suggest that Lin28 delays metamorphosis through regulation of let-7 and that the decrease of the TH carrier albumin is one of the early changes after Lin28 overexpression.

Mechanisms of endonuclease-mediated mRNA decay

Endonuclease cleavage was one of the first identified mechanisms of mRNA decay but until recently it was thought to play a minor role to the better-known processes of deadenylation, decapping, and exonuclease-catalyzed decay. Most of the early examples of endonuclease decay came from studies of a particular mRNA whose turnover changed in response to hormone, cytokine, developmental, or nutritional stimuli. Only a few of these examples of endonuclease-mediated mRNA decay progressed to the point where the enzyme responsible for the initiating event was identified and studied in detail. The discovery of microRNAs and RISC-catalyzed endonuclease cleavage followed by the identification of PIN (pilT N-terminal) domains that impart endonuclease activity to a number of the proteins involved in mRNA decay has led to a resurgence of interest in endonuclease-mediated mRNA decay. PIN domains show no substrate selectivity and their involvement in a number of decay pathways highlights a recurring theme that the context in which an endonuclease function is a primary factor in determining whether any given mRNA will be targeted for decay by this or the default exonuclease-mediated decay processes.

Regulation of pathways of mRNA destabilization and stabilization

The level of an mRNA in the cytoplasm represents a balance between the rate at which the mRNA precursor is synthesized in the nucleus and the rates of nuclear RNA processing and export and cytoplasmic mRNA degradation. Although most studies of gene expression have focused on gene transcription and in the area of eukaryotic mRNA degradation, but to provide a short general discussion of the importance of mRNA degradation and its regulation and a brief overview of recent findings and present knowledge. The overview is followed by a more in-depth discussion of one of the several pathways for mRNA degradation. We concentrate on the pathway for regulated mRNA degradation mediated by mRNA-binding proteins and endonucleases that cleave within the body of mRNAs. As a potential example of this type of control, we focus on the regulated degradation of the egg yolk precursor protein vitellogenin on the mRNA-binding protein vigilin and the mRNA endonuclease polysomal ribonuclease 1 (PMR-1).

Effects of 17beta-estradiol and testosterone on hepatic mRNA/protein levels and catalytic activities of CYP2M1, CYP2K1, and CYP3A27 in rainbow trout (Oncorhynchus mykiss)

There is growing concern that exposure to chemicals in the environment can disrupt the endocrine systems of wildlife and humans, causing reproductive problems or other adverse effects. The expression of many cytochrome P450s (CYPs) is under hormonal control, hence, levels of these enzymes can be affected by exposure to endocrine-disrupting chemicals. Previous research has reported that treatment of fish and other animals with the estrogenic and androgenic hormones 17beta-estradiol (E2) and testosterone (T) alters the P450 content or enzyme activities in the treated animals. However, the results of many of these studies are either incomplete or in disagreement and in most cases the effect on specific P450 forms has not been determined. Therefore, to better understand the effects of gonadal hormones on the expression of P450s and their associated enzyme activities, it was of interest to undertake a comprehensive investigation of the transcriptional and translational expression of three constitutive hepatic P450s in the rainbow trout (Oncorhynchus mykiss) following hormone exposure. Accordingly, juvenile trout were injected intraperitoneally with propylene glycol vehicle and the most active estrogenic and androgenic hormones E2 (3 mg/kg) or T (3 mg/kg) on days 1, 4, 7, 13, and 15 and euthanized on day 19. After treatment with E2, hepatic microsomes showed significantly lower levels (percentage of control) in total P450 contents (52%), lauric acid hydroxylase (32%), and 6beta-progesterone hydroxylase activities (27%), [(3)H]aflatoxin-DNA binding (31%), and the protein levels of individual cytochrome P450s (CYPs) LMC1 (CYP2M1), LMC2, (CYP2K1), and LMC5 (CYP3A27) (average for three isoforms a reduction to 29% of control values) with only minor differences between sexes. Treatment with T had either no effect or resulted in small increases in total P450 in males (42%), in lauric acid hydroxylase in females (24%), and in 6beta-progesterone hydroxylase activity in males (21%). Biological variabilities among fish were high and a polymorphic or new LMC2-like form was detected at about 52 kDa in some liver microsomal samples after exposure of fish to either hormone. Female liver RNAs were analyzed through Northern blots and an average decrease of 94% in CYP2 M1, CYP2K1, and CYP3A27 mRNA levels occurred in the E2-treated trout. In livers from T-treated trout, the changes of mRNA levels of CYP2M1 and CYP3A27 were negligible, but CYP2K1 mRNA level decreased by about 60%. Additional CYP2K1 cDNA hybridizable mRNAs were seen in some fish as faint bands at about 2.8 kb for both hormone treatments. Results of this study, therefore, indicated that E2 down-regulated while T produced small but variable effects on the hepatic mRNA/protein levels of CYP2K1, CYP2M1, and CYP3A27 in juvenile rainbow trout. This study, therefore, suggests that exposure of fish and other wildlife to environmental endocrine disruptors, especially estrogen mimics, can adversely affect a number of physiological processes through mechanisms involving altered levels of expression of specific P450 isozymes.

Regulated nuclear polyadenylation of Xenopus albumin pre-mRNA

Cytoplasmic regulation of the length of poly(A) on mRNA is a well-characterized process involved in translational control during development. In contrast, there is no direct in vivo evidence for regulation of the length of poly(A) added during nuclear pre-mRNA processing in somatic cells. We previously reported that Xenopus serum albumin [Schoenberg et al. (1989) Mol. Endocrinol. 3, 805-815] and transferrin [Pastori et al. (1992) J. Steroid Biochem. Mol. Biol. 42, 649-657], mRNA have exceptionally short poly(A) tails ranging from 12 to 17 residues, whereas vitellogenin mRNA has long poly(A). An RT-PCR protocol was adapted to determine the length of poly(A) added onto pre-mRNA, defined here as that species bearing the terminal intron. Using this assay we show that vitellogenin pre-mRNA has the same long poly(A) tail as mature vitellogenin mRNA. In contrast, albumin pre-mRNA has the same short poly(A) as found on fully-processed albumin mRNA. These results indicate that the short poly(A) tail on albumin mRNA results from regulation of poly(A) addition during nuclear 3' processing.

Estradiol-induced down-regulation of estrogen receptor. Effect of various modulators of protein synthesis and expression

Incubation of MCF-7 cells with estradiol (E2) down-regulates estrogen receptor (ER) resulting in a progressive reduction of the capacity of cells to concentrate selectively [3H]E2. Scatchard plot analysis failed to detect any transformation of residual receptors into peptides of lower binding affinity. [3H]Estrone gave an identical ER disappearance pattern with an ER half-life comprised between 2 and 3 h. A similar value was established by incubating the cells with [3H]tamoxifenaziridine ([3H]TAZ) for 1 h before the addition of excessive unlabeled E2 which induced ER-down regulation and impeded any further labeling of the residual receptors. Submission of the [3H]TAZ labeled cell extracts to SDS-PAGE revealed no progressive emergence of low molecular weight cleavage products of the receptor (< 67 kDa). Two inhibitors of protein kinases, H-7 at 40 microM and H-89 at 20 microM, failed to block the E2-induced ER down-regulation. On the contrary, the protein phosphatases 1 and 2A inhibitor, okadaic acid, was effective with concentrations higher than 0.1 microM indicating that a dephosphorylation mechanism was involved in this phenomenon. Cycloheximide (CHX) also significantly reduced the receptor decrease at concentrations higher than 1 microM. G-C specific intercalating agents [actinomycin D (AMD) and chromomycin A3 at 1 microM] also prevented ER disappearance; ethidium bromide (EB) and quinacrine were ineffective. AMD and CHX operated immediately after their addition to the medium indicating an inhibitory action on the synthesis of an RNA and/or a peptide with high turnover rate involved in ER decline. Moreover, AMD produced its suppressive effects under conditions impeding any labeling of newly synthetized receptors (i.e. [3H]TAZ with an excess of unlabeled E2) rejecting the possibility of an increasing ER production which may partially hamper its disappearance. Finally, E2-induced ER mRNA down-regulation was similarly abolished by AMD while EB and CHX were devoid of effect.

Differential regulation and polyadenylation of transferrin mRNA in Xenopus liver and oviduct

Estrogen destabilizes transferrin mRNA in male Xenopus liver in the same manner as observed for albumin and gamma-fibrinogen. The present study examined estrogen regulation of transferrin gene expression in female Xenopus liver and oviduct. In female Xenopus liver estrogen causes the same enhanced degradation of transferrin mRNA from the cytoplasm as seen in males. In contrast, transferrin is induced 3- to 4-fold in both oviduct nuclear and cytoplasmic RNA. The similar increase in transferrin RNA in both preparations suggests a transcriptional mechanism is responsible for this stimulation. Therefore, transferrin expression is differentially regulated in these tissues by the same hormone. Previous experiments showed that Xenopus serum albumin mRNA has a very short (17 residue) poly(A) tail that may play a role in its hormone-regulated instability. Transferrin mRNA has a similarly short poly(A) tail in liver of both male and female Xenopus. Estrogen has no effect on transferrin polyadenylation in liver. Similarly short poly(A) is found on transferrin mRNA from estrogen-deprived oviducts in explant culture. However, addition of estradiol to the medium results in the appearance of a 50-200 nucleotide poly(A) concurrent with induction. Therefore, transferrin mRNA is differentially polyadenylated in Xenopus liver and oviduct. In the latter tissue polyadenylation is under hormonal control.

Estrogen-induced ribonuclease activity in Xenopus liver

Estrogen administration to male Xenopus causes the cytoplasmic destabilization of the hepatic serum protein coding mRNAs, most notably, albumin, yet has little effect on mRNAs encoding intracellular proteins such as ferritin. This report describes an estrogen-inducible ribonuclease activity found in liver polysomes that degrades albumin mRNA 4 times faster in vitro than it degrades ferritin mRNA. This differential rate of degradation was observed upon incubation of polysome extract with free liver RNA, isolated liver mRNPs, or transcripts from plasmid vectors. A cleavage fragment consisting of a doublet of approximately 194 nucleotides in length was consistently observed upon digestion of transcripts for the full length or 5' half of albumin mRNA. The generation of this cleavage fragment was used as an assay to study properties of the polysome nuclease activity. The 194 doublet is produced by the action of a Mg(2+)-independent endonuclease. This distinguishes the Xenopus liver enzyme from the enzymes that degrade histone or c-myc mRNA in vitro. It is inactivated by 400 mM NaCl or heating at 90 degrees C, but not by placental ribonuclease inhibitor or N-ethylmaleimide. Finally, the polysomal nuclease activity does not degrade double-stranded RNA. We believe the estrogen-induced nuclease activity contains an enzyme(s) that may mediate hormone-regulated changes in mRNA stability in this tissue.

The estrogen-regulated destabilization of Xenopus albumin mRNA is independent of translation

Protein synthesis inhibitors have been shown to increase the stability of a number of labile mRNAs. In Xenopus laevis serum albumin mRNA is destabilized in the liver cell cytoplasm following estrogen administration. The present study examined the effect of translation inhibitors on this process. The initiation inhibitor 2-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide causes accumulation of albumin mRNA in 20-80S mRNP particles whereas the elongation inhibitor cycloheximide causes albumin mRNA to accumulate in polysomes. Neither inhibitor blocked the disappearance of albumin mRNA from liver cell cytoplasm when added with estradiol to the medium of liver explant cultures. We conclude that unlike a number of labile mRNAs the instability of Xenopus albumin mRNA following estradiol is independent of translation.

Estradiol and estrogen receptor-dependent stabilization of a minivitellogenin mRNA lacking 5,100 nucleotides of coding sequence

We have developed a transfection assay to investigate the estrogen-mediated stabilization of cytoplasmic vitellogenin mRNA. A minivitellogenin (MV5) gene containing the 5' and 3' untranslated and coding regions but lacking 5,075 nucleotides of internal coding sequence was constructed. Cotransfection of the MV5 plasmid and a Xenopus estrogen receptor expression plasmid into Xenopus liver tissue culture cells yielded a 529-nucleotide MV5 mRNA, which was specifically stabilized by estrogen. MV5 mRNA exhibited the increased stability indicative of positive regulation when the estradiol-estrogen receptor complex was present and was not destabilized by unliganded estrogen receptor. Transfected estrogen receptor, estradiol, and 529 nucleotides of the 5,604-nucleotide vitellogenin B1 mRNA were sufficient for stabilization.

Estradiol and estrogen receptor-dependent stabilization of a minivitellogenin mRNA lacking 5,100 nucleotides of coding sequence

We have developed a transfection assay to investigate the estrogen-mediated stabilization of cytoplasmic vitellogenin mRNA. A minivitellogenin (MV5) gene containing the 5' and 3' untranslated and coding regions but lacking 5,075 nucleotides of internal coding sequence was constructed. Cotransfection of the MV5 plasmid and a Xenopus estrogen receptor expression plasmid into Xenopus liver tissue culture cells yielded a 529-nucleotide MV5 mRNA, which was specifically stabilized by estrogen. MV5 mRNA exhibited the increased stability indicative of positive regulation when the estradiol-estrogen receptor complex was present and was not destabilized by unliganded estrogen receptor. Transfected estrogen receptor, estradiol, and 529 nucleotides of the 5,604-nucleotide vitellogenin B1 mRNA were sufficient for stabilization.

Molecular aspects of estrogen receptor activation factor (E-RAF) function

The estrogen receptor activation factor (E-RAF)-mediated binding of the receptor-estrogen complex to uterine nuclei was found to involve at least two classes of nuclear macromolecules: (1) the DNA, and (2) a proteinacious component. Evidences are presented to show that at least a portion of (2) is represented by the nuclear RNA polymerases. The receptor-estrogen complex associated in vivo with the nuclear RNA polymerases existed in two distinct forms which sedimented at 3.8 S and 4.8 S on sucrose density gradients. Almost 2/3 of the total radioactivity was associated with the 3.8 S species. Saturation kinetics of the two forms showed that while the 4.8 S form displayed characteristics similar to the classical type I nuclear binding site, the features displayed by the 3.8 S form were closely similar to those of the nuclear type II site. The 4.8 S species is a DNA binding form while the 3.8 S form is non-DNA binding. Anti-E-RAF IIA IgG cross-reacted with both the binding components. Goat uterine E-RAF I, IIA and IIB were purified to homogeneity as described earlier. While E-RAF IIA and IIB destabilized the native DNA structure and induced separation of the DNA strands, E-RAF I performed the opposite function. The reactions required the presence of ATP; all three of them displayed DNA-dependent ATPase activity. In an in vitro transcription system which contained purified RNA polymerase B (rat liver enzyme) and goat uterine DNA, E-RAF IIA and IIB enhanced transcription 7-fold over the control while E-RAF I totally suppressed the transcription process, irrespective of whether it was stimulated earlier by the other two E-RAF forms or not. This E-RAF property remained unchanged even after its association with the 4 S receptor-estrogen complex forming 5 S complex.

Use of the protein A-gold immunocytochemical and enzyme-gold cytochemical techniques in studies of vitellogenesis

Vitellogenesis in the frog hepatocyte was investigated by applying the protein A-gold immunocytochemical and RNase-gold cytochemical techniques in conjunction with morphometric and biochemical analyses. The morphometric studies demonstrated that the surface density of rough endoplasmic reticulum (RER) and nucleolar size increased more than fourfold and 1.25-fold, respectively, while the nuclear size and the mitochondrial compartment size remained constant following estrogen treatment. Concurrently, liver RNA concentration increased 2.5-fold while protein and DNA concentrations did not change. In addition, total plasma protein more than doubled, with vitellogenin accounting for 40% of the final volume. The secretory proteins vitellogenin and protein-RcX (a nonvitellogenin, estrogen-induced plasma protein of unknown function, found in the plasma of Rana catesbeiana) were detected immunocytochemically in the RER, Golgi apparatus, and secretory granules in hepatocytes only of estrogen-treated frogs. Lysosomes also were labeled. These observations established that protein-RcX was synthesized and secreted by the hepatocyte in parallel with vitellogenin and that both of these export proteins were confined to the secretory pathway and lysosomes. Quantitation of labeling density indicated that the concentration of vitellogenin increased as it progressed along the secretory vector. Albumin was detected immunocytochemically also within these same hepatocyte entities from both untreated and treated animals. In the untreated animals, albumin concentration also increased progressively along the secretory vector. A marked alteration of albumin processing was observed following estrogen treatment. While albumin concentration in the RER was unchanged, its concentrations within the Golgi apparatus and secretory granules were lower than those observed in the RER or in counterpart compartments under control conditions. RNase-gold cytochemistry for total RNA demonstrated a 1.5-fold increase in labeling density over the nucleolus but no change in RER labeling following estrogen treatment. These labeling data, in combination with the morphometric data, suggest an increase of approximately 80% in the total amount of RNA in the nucleolus and 430% in the RER in response to estrogen. This review thus illustrates the significant contributions which can be made by gold-probe techniques, alone or in combination with morphometric and biochemical techniques, to investigations of the intracellular processing of secretory proteins.

Inhibition of gastrin gene expression by somatostatin

Previous studies performed in this laboratory have demonstrated somatostatin-containing cells in close proximity to gastrin cells in antral mucosa and have shown that somatostatin exerts a local regulatory effect on gastrin release. The present studies were directed to determine whether the effects of somatostatin on the antral gastrin cell involve pretranslational events. The effects of somatostatin on gastrin mRNA were determined by dot blot hybridization using a gastrin antisense RNA probe derived from human gastrin cDNA. Inclusion of somatostatin in the incubation medium caused a dose-dependent inhibition of steady-state gastrin mRNA. Conversely, when antral somatostatin was neutralized by the addition of specific somatostatin antibodies to the incubation medium, gastrin mRNA levels increased by 116 +/- 31% over control values (P less than 0.01). Northern blot hybridization of total antral RNA demonstrated a single major band with a molecular size of approximately 620 nucleotides, closely matching the predicted size of gastrin mRNA. The effect of somatostatin on the rate of gastrin gene transcription was examined using nuclear run-off transcription assays. Inclusion of antibodies to somatostatin in the incubation medium resulted in a 33.8 +/- 3.3% increase in gastrin gene transcriptional activity (P less than 0.01). These studies indicate that, in addition to its established effect on peptide release, somatostatin exerts inhibitory effects on antral gastrin cells at the pretranslational level. Although this inhibition appears to occur in part at the gene transcriptional level, the results also indicate that somatostatin may affect posttranscriptional processing of gastrin mRNA.

Activity of a cloned Xenopus albumin gene promoter in the homologous frog oocyte system

The activity of the liver-specific promoter from the Xenopus laevis 68 kDa albumin gene in homologous oocytes has been analysed by microinjection. We find that the albumin promoter functions relatively efficiently in oocytes, directing the synthesis of correctly initiated transcripts, and deletion analysis reveals that only a small amount of upstream sequence is required for full activity.

A correlated morphometric and cytochemical study on hepatocyte nucleolar size and RNA distribution during vitellogenesis

The enzyme-gold cytochemical technique was used to label RNA in the nucleolus and rough endoplasmic reticulum (RER) of the hepatocytes of normal, male American bullfrogs (Rana catesbeiana) and in bullfrogs eight days following treatment with estradiol-17 beta. Concurrently, stereology was applied to quantitate: (1) the density of RNA labelling, and (2) changes in the size of the nucleus and nucleolus in response to estrogen treatment. In the hepatocytes from untreated frogs, specific labelling for RNA was present over the fibrillar and, to a greater extent, the granular portions of the nucleolus, and, to the greatest extent, over the RER. Following estrogen treatment, the density of RNA labelling increased over both parts of the nucleolus but was unchanged over the RER. The size of the nucleolus enlarged in response to estrogen: in combination with the increase in its RNA labelling, this suggested an increase of about 80% in the total amount of RNA in the nucleolus. Previous data on enlargement of the RER compartment, along with the present data on RNA labelling of RER, suggested that the total amount of this nucleic acid increased about 430% in this entity, in response to estrogen. However, the density of RNA labelling over the RER appears to be constant in spite of changes in the amount of RER.

5'-flanking and 5'-proximal exon regions of the two Xenopus albumin genes. Deletion analysis of constitutive promoter function

The 5'-flanking regions and the first two exons of the 68kd and 74kd albumin genes of Xenopus laevis reveal extensive sequence homology between the two in the exon part, in the 5'-flanking region up to position -400 as well as in the first intron. Sequence comparisons of the Xenopus genes with either the albumin genes of the chicken and mammals or the mammalian alpha-fetoprotein genes reveals no homology in the 5'-flanking region but some conserved features in the first exon. The analysis of the chromatin structure demonstrates a DNase I hypersensitive region in the promoter of the 68kd albumin gene specific for hepatocytes that express the albumin gene. Deletion analysis of albumin-CAT fusion genes indicates that a 69 base-pair fragment extending from -50 to +19 of the 68 kd albumin gene is sufficient for constitutive transcription in microinjected Xenopus oocytes. The addition of 5'-flanking sequences did not change the transcriptional activity. This is consistent with the sequence data that revealed no other promoter element in this region other than the TATA box. The absence of a CCAAT box distinguishes the Xenopus albumin genes from the mammalian albumin genes but is in agreement with the promoter structure of the alpha-fetoprotein genes.

Estrogen induction of a 45 kDa secreted protein coordinately with vitellogenin in Xenopus liver

In the frog Xenopus laevis, vitellogenin is the major estrogen-induced protein in the liver. We have characterized an additional secreted protein, of 45,000 Da and designated Ep45, which is also responsive to estrogen treatment. Like vitellogenin, Ep45 is not normally found in the plasma nor synthesized by the liver of the male frog. Its synthesis increases 6-fold between days 2 and 8 following a single 2 mg injection of estradiol-17 beta. For comparison, we have also studied a third estrogen-regulated protein, Ep20, with a molecular weight of approximately 20,000. This protein exhibits a different set of characteristics with regard to hormone responsiveness. Ep20 is synthesized in the liver of normal males and therefore is not absolutely hormone-dependent. Its level increases only about 4-fold following estrogen stimulation. The messenger RNAs for both Ep45 and Ep20 have been identified and purified, using a high-resolution RNA fractionation technique. By this procedure, it was possible to demonstrate that following high doses of estrogen the predominant mRNAs in the liver are those coding for vitellogenin, Ep45 and Ep20. Thus estrogen suppression of virtually all other liver proteins appears to act at the messenger RNA level for intracellular as well as secreted proteins.