Estrogen-induced destabilization of yolk precursor protein mRNAs in avian liver

Expression of Thyroid Hormone Responsive SPOT 14 Gene Is Regulated by Estrogen in Chicken (Gallus gallus)

Thyroid hormone responsive spot 14 (THRSP) is a small nuclear protein that responds rapidly to thyroid hormone. It has been shown that THRSP is abundant in lipogenic tissues such as liver, fat and the mammary gland in mammals. The THRSP gene acts as a key lipogenic activator and can be activated by thyroid hormone triiodothyronine (T3), glucose, carbohydrate and insulin. Here we report that chicken THRSP is also abundant in lipogenic tissues including the liver and the abdominal fat, and its expression levels increased with sex maturation and reached the highest level at the peak of egg production. Structure analysis of the THRSP gene indicates that there is a conscious estrogen response element (ERE) located in the −2390 – −2402 range of the gene promoter region. Further studies by ChIP-qPCR proved that the ERα interacts with the putative ERE site. In addition, THRSP was significantly upregulated (P < 0.05) when chickens or chicken primary hepatocytes were treated with 17β-estradiol in both the in vivo and in vitro conditions. We therefore conclude that THRSP is directly regulated by estrogen and is involved in the estrogen regulation network in chicken.

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.

Sensitivity of expression of perivitelline membrane glycoprotein ZP1 mRNA in the liver of Japanese quail (Coturnix japonica) to estrogenic compounds

Avian perivitelline membrane protein, ZP1, is synthesized and secreted by the liver with the stimulation of estrogens. In the present study, we measured the expression of ZP1 gene in the liver of immature male quail treated with various estrogenic compounds and in the liver of male quail embryos that were developed in the fertilized eggs laid by mother quail injected with various estrogenic compounds during vitellogenesis. Total RNA extracted from the liver was reverse-transcribed and cDNA was subjected to real-time PCR. Both diethylstilbestrol and ethinyl estradiol caused significant effect on the increase in mRNA in immature male quail. In contrast, diethylstilbestrol administered via the route of maternal injection was not effective for induction of embryonic mRNA, although the effect of ethinyl estradiol administered via the same route was prominent. These results showed that direct administration of estrogenic compounds, diethylstilbestrol and ethinyl estradiol, stimulates the induction of ZP1 gene, but the rate of accumulation of these compounds in the yolk is different during vitellogenesis. The present studies suggest that although ZP1 gene is a sensitive biomarker to evaluate the effects of endocrine disruptors, the route of administration is an important factor to compare the effectiveness.

Effect of melengestrol acetate as an alternative to induce molting in hens on the expression of yolk proteins and turnover of oviductal epithelium

Inducing hens to molt increases egg quality, egg production and extends the productive life of hens. It has been previously demonstrated that melengestrol acetate (MGA), an orally active progestin, decreased gonadotropic support for the ovary, which decreased the steroidogenic support for the oviduct and resulted in the cessation of lay. Estradiol produced by the theca cells of small follicles stimulates the production of the yolk proteins vitellogenin II and apolipoprotein II by the liver and supports the oviductal epithelial cells. The objective of the present experiment was to determine gene expression for yolk proteins and oviductal epithelial cell turn-over in response to a MGA-induced molt. Hy-Line W-36 laying hens were fed either 0 or 8mg MGA per day for 28 days in a balanced diet and then returned to a standard layer ration until day 36. Four birds per treatment on days 1, 8, 16, 28 and 36 were euthanized and the liver was removed and snap frozen in liquid nitrogen until RNA was extracted. Expression of vitellogenin II and apolipoprotein II genes was determined using real-time RT-PCR. A portion of the magnum was removed to determine proliferation and programmed cell death for secretory and ciliated luminal epithelium. Vitellogenin II and apolipoprotein II gene expression was reduced in hens fed 8mg MGA compared to those fed 0mg MGA. There was no effect of day on gene expression of either yolk protein. Cell proliferation was increased in the ciliated epithelial cells of the oviduct in hens receiving 8mg MGA compared to those receiving 0mg. However, programmed cell death of the ciliated epithelial cells was not different between controls and MGA treatment. Programmed cell death and proliferation increased in the secretory epithelial cells in hens receiving 8mg MGA compared to controls. Therefore, utilizing MGA as an alternative method to induce molt results in some, but not all, of the physiological changes previously described for hens molted by feed withdrawal. These findings lead us to suggest that some of the observed physiological changes resulting from feed withdrawal are required to increase egg quality and egg production following molt and other changes are not necessary, but are just a result of nutrient deprivation.

Role of mRNA transcript stability in modulation of expression of the gene encoding thrombin activable fibrinolysis inhibitor

Regulation of mRNA stability has emerged as a major control point in eukaryotic gene expression. The abundance of a particular mRNA can be rapidly regulated in response to a stimulus by altering the stability of existing translatable transcripts rather than by altering the rate of transcription initiation. Alternative polyadenylation of transcripts during mRNA processing can be important in determining transcript abundance if the different forms of mRNA possess different stabilities or translatability. The mRNA transcript encoding thrombin activable fibrinolysis inhibitor (TAFI) is an attractive candidate for regulation of mRNA stability because of the relatively long length of its 3'-untranslated region and because the transcript can be polyadenylated at three different sites. As well, we have previously reported that treatment of HepG2 cells with interleukins (IL) - 1beta and - 6 destabilizes the endogenous TAFI mRNA expressed in this cell line. In the current study, we report that the TAFI 3'-untranslated region contains cis-acting instability element(s) and that these elements in fact determine the intrinsic stability of the TAFI transcript. Moreover, we found that the three different polyadenylated mRNA forms have different intrinsic stabilities, with the mRNA half-life increasing from the longest to the shortest transcript. Interestingly, treatment with IL-1beta plus IL-6 not only resulted in a 2-fold decrease in stability of the transcript produced using the 3'-most polyadenylation site but also resulted in profound shifts in the relative abundances of the respective polyadenylated forms through changes in the frequency of utilization of the three polyadenylation sites. As such, in the presence of IL-1beta and IL-6, the longest transcript is over a thousand times more abundant than the two shorter transcripts whereas in the absence of the stimulus it comprises only 1% of the total TAFI transcripts.

Identification of two tamoxifen target proteins by photolabeling with 4-(2-morpholinoethoxy)benzophenone

Our quest to identify target proteins involved in the activity of tamoxifen led to the design of photoaffinity ligand analogues of tamoxifen able to cross-link such proteins. A new tritiated photoprobe, 4-(2-morpholinoethoxy)benzophenone (MBoPE), was synthesized and used to identify proteins involved in tamoxifen binding in rat liver. MBoPE, which has structural features in common with the potential antagonist of the intracellular histamine receptor (N,N-diethyl-2-[(4-phenylmethyl)phenoxy]ethanamine HCl: DPPE) is unable to bind the estrogen receptor although it does compete with tamoxifen for an antiestrogen binding site (AEBS). This tritiated benzophenone derivative was obtained by metal-catalyzed halogen-tritium replacement reaction. Because of its high specific activity, four target proteins could be photolabeled, three of which were identified with M(r) of 60,000, 49,500, and 14,000, while the fourth at 27,500 was in too low an amount and could not be sequenced. The 49.5 kDa protein corresponded by mass spectrometry to the microsomal epoxide hydrolase already identified with an aryl azide photoprobe [Mesange, F., et al. (1998) Biochem. J. 334, 107-112]. The 60 and 14 kDa proteins were identified as the carboxylesterase (ES10) and the liver fatty acid binding protein (L-FABP), respectively. The inhibitory effect of tamoxifen on carboxylesterase activity and the competitive efficacy of oleic acid on [(3)H]tamoxifen binding suggest that both proteins are AEBS subunits. Moreover, treatment of hepatocytes with antisense mRNA directed against ES10 or L-FABP abolished both tamoxifen and MBoPE binding. On the basis of previous pharmacological arguments, the 27.5 kDa protein might correspond to the sigma I receptor. Altogether, these results confirm that the microsomal epoxide hydrolase is a target for tamoxifen and provide evidence of two new target proteins implicated in cell lipid metabolism.

The upstream stem-loop domain of the 3' untranslated region of apolipoprotein II mRNA binds the estrogen-regulated mRNA stabilizing factor

Apolipoprotein II (apoII), a component of the very low density lipoprotein (VLDL) particle, is a yolk protein expressed in the liver in response to estrogen. Its expression is modulated by estrogen-mediated stimulation of transcription as well as stabilization of its mRNA. This stabilization is due to the estrogen-regulated mRNA stabilizing factor (E-RmRNASF) [Cell. Mol. Biol. Res. 41 (1995) 583). E-RmRNASF protects apoII mRNA from targeted endonucleolytic degradation. The expression of E-RmRNASF itself is under estrogenic control. The hepatic expression of E-RmRNASF is also modulated by certain estrogenic and anti-estrogenic non-steroidal environmental xenobiotics [Biochem. Pharmacol. 53 (1997) 1425]. Studies involving RNA affinity-based depletion of mRNA stabilization activity indicated that E-RmRNASF binds to apoII mRNA. E-RmRNASF binds apoII mRNA in a region-specific manner. The region of binding has been narrowed down to the upstream domain of stem-loop secondary structure spanning nucleotides (nt) 402-558 in the 3' untranslated region (3'UTR). A RNA affinity chromatography procedure using this portion of apoII mRNA was utilized for the purification of E-RmRNASF. A gel filtration (GF) chromatography step preceding the RNA affinity chromatography was required for additional enrichment of E-RmRNASF. A functional assay involving the in vitro stabilization of apoII mRNA from degradation was utilized to detect E-RmRNASF during chromatography. E-RmRNASF appears to be a protein of apparent molecular weight of 20-25kDa visualized by SDS polyacrylamide gel electrophoresis.

Estrogen modulates HNF-3beta mRNA levels in the developing chick oviduct

Steroid hormones are involved in many physiological processes, including tissue-specific gene expression, homeostasis, and development. The chick oviduct represents an excellent system in which to study many of these events, as it is highly steroid responsive. Here, we report the cloning of chick HNF-3beta from an oviduct cDNA library and its expression pattern in adult tissues and in the developing oviduct in response to estrogen treatment. Overall, cHNF-3beta was expressed at high levels in the immature chick oviduct and lung and, to a lesser extent, in the liver, kidney, and muscle. This expression pattern is divergent from that of mammalian HNF-3beta, which is not expressed in kidney or muscle. Furthermore, several lengths of cHNF-3beta mRNA transcripts were detected that were expressed tissue specifically. Interestingly, cHNF-3beta mRNA levels were differentially influenced by estrogen as a result of a post-transcriptional effect on the cHNF-3beta message in some tissues. Finally, a role for cHNF-3beta is proposed in the estrogen-stimulated differentiation and development of the oviduct, as cHNF-3beta mRNA expression is induced in the early stages of oviduct development and declines as the animal becomes sexually mature.

Developmental regulation of the 3-methylcholanthrene- and dioxin-inducible CYP1A5 gene in chick embryo liver in vivo

The cDNA sequences for two dioxin-inducible cytochrome P450s in chicken, CYP1A4 and CYP1A5, have recently been reported which correspond to two dioxin-inducible forms of P450 previously designated as TCDDAHH and TCDDAA, respectively. The developmental expression of CYP1A4-associated aryl hydrocarbon (benzo[a]pyrene) hydroxylase (AHH) activity and its association with expression of the Ah receptor had previously been characterized in chick embryo liver. The purpose of this study was to examine the developmental regulation of the second dioxin-inducible P450 gene, CYP1A5, in chick embryo liver. A partial gene sequence for CYP1A5 indicated that the intron/exon organization of this gene was identical to that of the CYP1A1 and CYP1A2 mammalian genes and was present in a single copy in the genome. CYP1A5 mRNA was expressed basally in chick embryo liver and was highly inducible by the Ah receptor ligands, 3-methylcholanthrene, beta-naphthoflavone, and 3,4,3', 4'-tetrachlorobiphenyl (TCB), but not by the phenobarbital analog, glutethimide. CYP1A5 mRNA levels were increased 40- to 50-fold within 5 h after a single TCB treatment, corresponding to a 30- to 40-fold increase in the transcription rate of the CYP1A5 gene at this time point. In contrast to a previous report that CYP1A5 mRNA expression was inducible by estradiol, we observed no effects of estradiol or dexamethasone on CYP1A5 mRNA expression, either alone or in combination with TCB. Basal and TCB-inducible CYP1A5 mRNA expression was maximal in liver at 8 days of development and remained high throughout the remainder of embryonic development. Thus, CYP1A5 appears to be regulated in a very similar manner to CYP1A4 in chick embryo liver.

Evidence for a posttranscriptional effect of retinoic acid on connexin43 gene expression via the 3'-untranslated region

All-trans retinoic acid (10(-7) M) induces cell-cell communication and the expression of the gap junction protein connexin43 in mouse F9 teratocarcinoma cells. Previous experiments revealed an increase of mRNA but no change in the transcription of connexin43, suggesting a posttranscriptional mechanism responsible for the regulation of connexin43 gene expression. In transient transfection experiments using an expression vector containing the 3'-untranslated region of the connexin43 gene downstream of the luciferase coding sequence driven by the connexin43 promoter we show here that retinoic acid enhances luciferase activity via the connexin43 3'-untranslated region due to altered stability of the mRNA. Thus, retinoic acid is able to influence connexin43 gene expression at the level of mRNA stability via elements located in the 3'-untranslated region.

Stabilization of myelin mRNAs as measured in a brain slice system

The stabilization and destabilization of myelin mRNA is increasingly recognized as a major control point in regulating myelin gene expression. A brain slice system was developed and characterized to study mRNA stability in actively myelinating oligodendrocytes. The mRNA half-life of a major CNS myelin protein, proteolipid protein (PLP), was measured to be 5 hr. The half-life of another CNS myelin protein mRNA, myelin basic protein (MBP), was measured to be greater than 12 hr. A long half-life for MBP mRNA is consistent with MBP mRNA being stable long enough to be translocated to the myelin internode where it is then translated. Using semi-quantitative reverse transcriptase-PCR, it was determined that there was no differential stabilization between the two major PLP mRNA isoforms, PLP and DM20. It was also determined that protein synthesis was required for the specific stabilization of PLP/DM20 mRNAs. Inasmuch as PLP is a major structural protein of the CNS myelin, the PLP/DM20 mRNAs have relatively short half-lives. However, the PLP/DM20 mRNAs half-lives may be increased by the action of trans-acting factors that are themselves very labile.

Modulation of the hepatic expression of the estrogen-regulated mRNA stabilizing factor by estrogenic and antiestrogenic nonsteroidal xenobiotics

Estrogen-mediated accumulation of apolipoprotein II (apoII) mRNA in the avian liver is due, in part, to its stabilization. This stabilization appears to be due to the estrogen-regulated mRNA stabilizing factor (E-RmRNASF) that is expressed in response to estrogen. The E-RmRNASF protects the mRNA from targeted endonucleolytic degradation (Ratnasabapathy, Cell Mol Biol Res 41: 583-594, 1995). To determine whether certain environmental xenobiotics altered the expression of the gene encoding E-RmRNASF by mimicking estrogen, roosters were given estrogen, tamoxifen, clomiphene, hexachlorophene, lindane, rotenone, chlordecone, dichlorodiphenyltrichloroethane (DDT); Araclor, methoxychlor, dieldrin, toxaphene, or bisphenol-A parenterally. Uniformly radiolabeled, capped, and polyadenylated apoII mRNA, incubated in vitro in the presence of liver cytosolic extracts from birds that received estrogen, tamoxifen, hexachlorophene, chlordecone, or Araclor, remained stable, indicating that these agents were estrogenic and stimulated the expression of E-RmRNASF. However, the same mRNA was degraded in similar extracts from control roosters and those treated with clomiphene, DDT, methoxychlor, dieldrin, rotenone, toxaphene, lindane, or bisphenol-A. To determine whether the latter agents were antiestrogenic, roosters were given a 1:5 molar combination of estrogen and each of the xenobiotics. ApoII mRNA showed degradation in liver extracts from roosters that received clomiphene, toxaphene, or bisphenol-A, indicating that these agents prevented estrogenic stimulation of expression of the E-RmRNASF and were antiestrogenic. However, the rest of the xenobiotics failed to antagonize estrogenic stimulation of E-RmRNASF gene expression. These results set a precedent in showing the estrogenic and antiestrogenic effects in vivo of environmental xenobiotics on the expression of a regulatory protein involved in estrogen-mediated mRNA stabilization.

Effects of thyroid hormone on the androgenic expression of KAP gene in mouse kidney

The kidney androgen-regulated protein (KAP) gene exhibits a cell-specific hormonal regulation of its expression in the epithelial cells of proximal tubules of mouse kidney, where T3 is required for constitutive expression in the straight segments and androgens for expression in the convoluted ones. By using different models of hypothyroidism, we demonstrate that maximal androgen-mediated induction of the gene depends on thyroid hormone as well. This constitutes a specific event, since vitamin D3 cannot mimic the effects of T3, albeit their remarkable functional relationship. It is also shown that while congenital hypothyroid hyt/hyt male mice, exposed to maternal T3 in the gestational period, exhibit diminished but existent androgen-dependent cortical responses, mice exposed to goitrogens during gestation and postnatally are unable to express the gene even at postnatal day ninety. Impairment of KAP cortical expression in hypothyroid animals does not correlate with lower levels of androgens or androgen receptor expression.

Estrogen induces the assembly of a multiprotein messenger ribonucleoprotein complex on the 3'-untranslated region of chicken apolipoprotein II mRNA

UV cross-linking was used to identify estrogen-induced hepatocyte proteins that bind to apoII mRNA. Probes spanning the entire message revealed the presence of eight estrogen-induced proteins cross-linked to the 3'-untranslated region (UTR), but not to the coding region or the 5'-UTR. Two estrogen-induced proteins of 132 and 50 kDa were either absent or barely detectable in control animals, whereas six additional proteins of 93, 83, 74, 65, 58, and 45 kDa were clearly present in control animals and increased 2-5-fold by estrogen. A similar profile of estrogen-induced proteins was seen with the 3'-UTRs of the estrogen-regulated mRNAs for apoB and vitellogenin II, but not with the 3'-UTRs of the non-estrogen-regulated mRNAs for apoA-I and glyceraldehyde-phosphate dehydrogenase. These findings indicate that the estrogen-induced proteins discriminate among mRNAs and suggest that they interact selectively with the family of estrogen-regulated mRNAs. The estrogen-induced proteins are found in the cytoplasmic fraction of liver extracts, and a subset of them are also found in adrenal glands, testes, heart, brain, and kidneys, but they are estrogen-induced only in the liver. Deletion analysis defined a 150-nucleotide region of the apoII 3'-UTR that is necessary for maximal binding of the estrogen-induced proteins. An internal deletion of endonucleolytic cleavage sites previously identified within the apoII 3'-UTR selectively reduced the binding of the 58-kDa protein. These findings reveal remarkable complexity in estrogen-stimulated protein-RNA interactions within the 3'-UTRs of estrogen-regulated mRNAs. These proteins may participate in the mRNA degradation process or in other aspects of cytoplasmic mRNA metabolism that accompany estrogen-stimulated vitellogenesis.

Chicken lecithin-cholesterol acyltransferase. Molecular characterization reveals unusual structure and expression pattern

Rapidly growing oocytes in the laying hen are, in addition to the liver, targets of the so-called "reverse cholesterol transport" (RCT) (Vieira, P.M., Vieira, A.V., Sanders, E.J., Steyrer, E., Nimpf, J., and Schneider, W.J. (1995) J. Lipid Res. 36, 601-610), pointing to the importance of this process in nonplacental reproduction. We have begun to delineate the details of this unique transport pathway branch by molecular characterization of the first nonmammalian lecithin-cholesterol acyltransferase (LCAT), the enzyme that catalyzes an early step in RCT. The biological significance of the enzyme is underscored by the high degree of protein sequence identity (73%) maintained from chicken to man. Interestingly, the conservation extends much less to the cysteine residues; in fact, two of the cysteines thought to be important in mammalian enzymes (residues 31 and 184 in man) are absent from the chicken enzyme, providing proof of their dispensability for enzymatic activity. Antibodies prepared against a chicken LCAT fusion protein cross-react with human LCAT and identify a 64-kDa protein present in enzymatically active fractions obtained by hydrophobic chromatography of chicken serum. The developmental and tissue distribution pattern of LCAT in females is striking; during embryogenesis and adolescence, LCAT expression is extremely high in liver but undetectable in brain. Upon onset of laying, however, brain LCAT mRNA increases suddenly and is maintained at levels 5 times higher than in liver, in stark contrast to most mammals. In adult roosters, the levels of LCAT transcripts in brain are lower than in liver. Together with the molecular characterization of chicken LCAT, these newly discovered developmental changes and gender differences in its expression establish the avian oocyte/liver system as a powerful model to delineate in vivo regulatory elements of RCT.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.

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.

Proteins associated with the messenger ribonucleoprotein particle for the estrogen-regulated apolipoprotein II mRNA

The stability of the mRNA for apolipoprotein (apo) II is regulated by estrogen [Gordon et al. (1988) J. Biol. Chem. 263, 2625-2631]. On the hypothesis tha estrogen regulation of apoII mRNA stability is mediated through mRNA-protein interaction, we have examined the messenger ribonucleoprotein particle (mRNP) for apoII mRNA following release from chicken liver polyribosomes. Polyribosomes containing undegraded apoII mRNA were obtained when tissue was homogenized without detergent, and polyribosomes were isolated following simultaneous addition of detergent and magnesium to a 20000g supernatant. ApoII mRNP released by EDTA sedimented at 12-18 S in sucrose gradients, and banded at rho = 1.4 g/mL in CsCl isopycnic centrifugation, indicative of a 3:1 ratio of protein to mRNA. A fraction in which apoII mRNP was enriched to 40-50% of total mRNP was prepared by successive size fractionation steps on sucrose gradients. Proteins associated with sucrose gradient enriched apoII mRNP were examined by iodination of UV-cross-linked proteins followed by SDS-polyacrylamide gel electrophoresis. Comparisons of proteins in highly enriched apoII mRNP to proteins in mRNP from non-estrogen-treated rooster liver did not reveal any differences. This result suggests that the major proteins associated with apoII mRNA are mRNP proteins also associated with the bulk of liver mRNAs.

Estradiol-induced vitellogenin gene expression in a teleost fish, Oreochromis aureus

Vitellogenesis presents a versatile model for the study of hormone-induced gene expression. We report here the effects of estradiol-17 beta-propionate on vitellogenin gene expression in male Oreochromis aureus, a teleost fish. Vitellogenin mRNA of 6500 nucleotides has been elucidated from the livers of female and estradiol-treated male O. aureus. By hybridization with a specific O. aureus cDNA probe, the vitellogenin mRNA transcript was detected as early as 1 hr following primary and secondary estradiol-stimulations, although for the latter, the rate of accumulation of vitellogenin-specific mRNA was 20-fold higher. The vitellogenin mRNA peaked at 72 and 48 hr, respectively, for primary and secondary stimulations. At the translational level, the increase in plasma vitellogenin was further enhanced during the secondary stimulation. There was a distinct shift in the peak of plasma vitellogenin from Day 14 in the primary induction to Day 3 in the secondary stimulation. The plasma vitellogenin presented in two forms, 300 and 500 kDa, both of which were immunologically confirmed by Western blot analysis to be vitellogenin proteins.

ApoVLDLII gene transcription in immature cockerels without estradiol stimulation

1. Previous studies have led to the conclusion that the gene encoding the apo very low density lipoprotein II (ApoVLDLII) is under full estrogen control. However, by using a sensitive hybridization technique we found a weak expression of this gene in some males in the absence of hormone. 2. As the vitellogenin II gene, which is also estrogen dependent, remains inactive in these animals it is likely that they carry a deregulated allele of the ApoVLDLII gene. 3. The highest level of ApoVLDLII mRNA is found in genetically fat males suggesting a possible involvement of the ApoVLDLII gene in fatness determinism.

Avian adipose lipoprotein lipase: cDNA sequence and reciprocal regulation of mRNA levels in adipose and heart

cDNA clones for chicken adipose lipoprotein lipase were isolated from an expression library in lambda gt11 by antibody screening and characterized by hybridization selection and nucleotide sequencing. Based on the cDNA sequence and on N-terminal sequence analysis of the purified enzyme, chicken adipose lipoprotein lipase is a mature protein of 465 amino acids with a signal peptide of 19 or 25 amino acids, depending on which of two methionine residues is used for translation initiation. The predicted amino-acid sequence was found to be 73-77% identical to the four known mammalian adipose lipoprotein lipase sequences, with conservation of position of cysteine residues and putative functional domains, and number of potential N-glycosylation sites. Chicken lipoprotein lipase differs from mammalian lipoprotein lipases with respect to the position of one N-glycosylation site and the presence of an additional 15-17 C-terminal amino acids. 32P-labeled cDNA clones hybridized to mRNA species of 3.7 and 4.0 kb in Northern blots of heart and adipose, but not of liver RNA. In chickens that were fasted for 48 h and then refed, lipoprotein lipase mRNA levels in adipose increased to a maximal level of 350% that of controls at 10 h, whereas heart lipoprotein lipase mRNA levels fell to 40% of controls at 14 h. Concomitantly, no changes in total RNA were observed. Thus, avian lipoprotein lipase is subject to reciprocal pretranslational regulation in adipose and heart.

Physiological regulation at the level of mRNA: analysis of steady-state levels of specific mRNAs by DNA-excess solution hybridization

Recent appreciation for how systems may be regulated at the protein and mRNA levels has dictated that physiological questions be addressed at both. The application of novel and sensitive biochemical and molecular biological techniques may be necessary to answer questions fundamental to our understanding of physiological regulation. For measurement of low-abundance proteins, such as receptors for hormones and growth factors, and for measurement of their corresponding mRNAs, the need for highly sensitive assays is critical. The present work focuses both on recent studies of the regulation of steady-state levels of mRNAs and on how mRNAs are quantified by DNA-excess solution hybridization assays using highly specific single-stranded probes. Examples of physiological regulation are provided in which analysis of mRNA levels has been a key facet to our understanding of the mode of regulation. The construction and characterization of probes for solution hybridization assays are described using the hamster beta 2-adrenergic receptor as a model. Studies on the regulation of beta-adrenergic receptors by agonists and by permissive hormones as well as investigations on the regulation of apolipoproteins by estrogen and by cholesterol in vivo are used to illustrate the many advantages of analyzing the steady-state levels of specific mRNAs by DNA-excess solution hybridization.

Estrogens and the skeleton: cellular and molecular mechanisms

Postmenopausal women lose bone mineral density and this loss can be prevented by estrogen administration. Although the skeletal effects of estrogens have been regarded previously as indirect, estrogen receptors have been discovered in cultured human osteoblasts and related cell lines. The UMR106 cell line derived from a rat osteogenic osteosarcoma is such an osteoblast model. We have shown direct effects of estradiol (E) on these cells in vitro, inhibiting growth and stimulating alkaline phosphatase activity (AP) corrected for cell number. This response was maximal at E conc. of 10(-10) M in serum and Phenol Red free medium, was metabolite specific and cell cycle-dependent. These cells contain high affinity binding sites with a Kd of 0.5 nM. Estrogen receptors were detected by the monoclonal antibody H-222 on Western blot after initial immunoprecipitation to concentrate the proteins. E treatment increased several enzymes including creatine kinase and LDH isoenzymes along with increments in intracellular transferrin. Transforming growth factor-beta is secreted by these cells. Secretion of this peptide was stimulated by E. TGF-beta mediated the transient growth inhibition associated with E treatment. Insulin like growth factors (IGF) are also secreted by these cells with IGF-II concentrations in the culture medium being eight times higher than IGF-I levels. E treatment increased the concentrations of both IGFs in the culture medium after a 3 day incubation. Exposure of E treated cells manifested a mitogenic response and reduced AP, indicating that E induced receptors for IGFs. These findings establish direct effects of E on osteoblastic cells in vitro and demonstrate responses to E at many levels. These osteoblast responses in vitro suggest an important role for sex steroids in the development and function of the osteoblast lineage.