Chemical composition of an oestrogen-induced calcium-binding glycolipophosphoprotein in Xenopus laevis

A vitellogenin polyserine cleavage site: highly disordered conformation protected from proteolysis by phosphorylation

Vitellogenin (Vg) is an egg-yolk precursor protein in most oviparous species. In honeybee (Apis mellifera), the protein (AmVg) also affects social behavior and life-span plasticity. Despite its manifold functions, the AmVg molecule remains poorly understood. The subject of our structure-oriented AmVg study is its polyserine tract - a little-investigated repetitive protein segment mostly found in insects. We previously reported that AmVg is tissue specifically cleaved in the vicinity of this tract. Here, we show that, despite its potential for an open, disordered structure, AmVg is unexpectedly resistant to trypsin/chymotrypsin digestion at the tract. Our findings suggest that multiple phosphorylation plays a role in this resilience. Sequence variation is highly pronounced at the polyserine region in insect Vgs. We demonstrate that sequence differences in this region can lead to structural variation, as NMR and circular dichroism (CD) evidence assign different conformational propensities to polyserine peptides from the honeybee and the jewel wasp Nasonia vitripennis; the former is extended and disordered and the latter more compact and helical. CD analysis of the polyserine region of bumblebee Bombus ignitus and wasp Pimpla nipponica supports a random coil structure in these species. The spectroscopic results strengthen our model of the AmVg polyserine tract as a flexible domain linker shielded by phosphorylation.

A PTH/PTHrP receptor antagonist blocks the hypercalcemic response to estradiol-17β

Estradiol (E2) increases circulating calcium and phosphate levels in fish, thus acting as a hypercalcemic and hyperphosphatemic factor during periods of high calcium requirements, such as during vitellogenesis. Since parathyroid hormone (PTH)-related protein (PTHrP) has been shown to be calciotropic in fish, we hypothesized that the two hormones could be mediating the same process. Sea bream ( Sparus auratus) juveniles receiving a single intraperitoneal injection of piscine PTHrP(1-34) showed an elevation in calcium plasma levels within 24 h. In contrast, injections of the PTH/PTHrP receptor antagonist PTHrP(7-34) decreased circulating levels of calcium in the same period. Intraperitoneal implants of estradiol-17β (E2; 10 μg/g) evoked significant increases of circulating plasma levels of calcium and phosphorus and a sustained increases of circulating plasma levels of PTHrP. However, a combined treatment of E2 and PTHrP(7-34) evoked a markedly lower calcium response compared with E2 alone. We conclude that PTHrP or a related peptide that binds the PTH/PTHrP receptor mediates, at least in part, the hypercalcemic effect of E2 in calcium and phosphate balance in fish.

Tilapia (Oreochromis niloticus) vitellogenins: development of homologous and heterologous ELISAs and analysis of vitellogenin pathway through the ovarian follicle

Vitellogenin (VTG) of Oreochromis niloticus was again purified, due to the conflicting results found in the literature. Three purification processes have been used: electrophoresis and electro-elution, double chromatography (gel filtration and ion-exchange chromatography) and single ion-exchange chromatography. Using SDS-PAGE we confirmed in all cases the presence of two polypeptidic forms of plasma VTG of 130 kDa (VTG1) and 170 kDa (VTG2). We raised polyclonal antibodies against each VTG form and we demonstrated the complete cross-reactivity of each antibody with both forms of VTG by Enzyme Immuno-Assay (EIA) and Western blots. The homologous ELISAs developed exhibited a detection limit of 6 ng x ml(-1), equivalent to 60 ng x ml(-1) of plasma VTG and allowed us to quantify the total plasma VTG of O. niloticus with high specificity and sensitivity. Using photonic and electron immunomicroscopy, we followed the pathway of VTG into the ovarian follicle (OF) demonstrating that VTG enters the oocyte at stage 3 of OF development, at the same time as cortical alveoli and lipid globules appear. Heterologous ELISAs performed on other cichlid species allowed us to quantify plasma VTG in Oreochromis aureus and Sarotherodon melanotheron and to detect it in Hemichromis fasciatus, Hemichromis bimaculatus and Tilapia zillii, constituting a reliable tool for monitoring the presence of xeno-estrogens in the environment of these fish species.

Multihormonal control of vitellogenesis in lower vertebrates

The comparative approach on how and when vitellogenesis occurs in the diverse reproductive strategies displayed by aquatic and terrestrial lower vertebrates is presented in this chapter; moreover, attention has been paid to the multihormonal control of hepatic vitellogenin synthesis as it is related to seasonal changes and to vitellogenin use by growing oocytes. The hormonal mechanisms regulating vitellogenin synthesis are also considered, and the effects of environmental estrogens on the feminization process in wildlife and humans have been reported. It is then considered how fundamental nonmammalian models appear to be, for vitellogenesis research, addressed to clarifying the yolkless egg and the evolution of eutherian viviparity.

Carbohydrate components of lipovitellin of the sand crab Emerita asiatica

Lipovitellin II (Lv II), the major yolk protein of the anomuran crab Emerita asiatica, was purified using heparin-sepharose affinity column chromatography. The purified Lv II was a glycoprotein as it was stainable with periodic acid-Schiff's reagent. Quantitative analysis of sugars showed the presence of fucose, mannose, galactosamine, N-linked oligosaccharides, as well as O-linked oligosaccharides containing N-acetyl hexosamine as the terminal residue. The amount of N-linked oligosaccharides is higher than that of the O-linked oligosaccharides. Biogel P-4 column chromatographic separation of the radiolabeled oligosaccharides of Lv II showed the presence of five different O-linked oligosaccharides and four different N-linked oligosaccharide species. HPTLC separation of the neoglycolipids prepared from the O-linked oligosaccharides also showed the presence of five different O-linked oligosaccharide species. N-linked oligosaccharides contain significant quantities of mannose. Unisil column chromatographic purification in conjunction with HPTLC separation revealed three neutral glycolipid species such as monoglycosylceramide, diglycosylceramide, and triglycosylceramide in the Lv II. The functional significance of these carbohydrate components of the major yolk protein during embryogenesis of the sand crab is discussed.

Susceptibility to oxidation of copper-induced plasma lipoproteins from Japanese eel: protective effect of vitellogenin on the oxidation of very low density lipoprotein

The susceptibility to oxidation of copper-induced plasma lipoproteins from Japanese eel Anguilla japonica was examined with the guidance of thiobarbituric acid-reactive substances (TBARS). The TBARS values of copper-induced plasma lipoproteins increased with increasing the lipid-to-apolipoprotein ratios and very low density lipoprotein (VLDL) exhibited the highest TBARS value. On the other hand, vitellogenin, estrogen-induced precursor of egg yolk proteins, was resistant to copper-induced oxidation and seemed to chelate low concentrations of copper ion. Vitellogenin also protected the copper-induced oxidation of VLDL because of its antioxidant function. Vitellogenin seemed to serve as transition metals-binding lipoprotein by which free-radical reactions in the oocytes were extensively depressed.

Purification, partial characterization and peptide sequences of vitellogenin from a reptile, the tuatara (Sphenodon punctatus)

Vitellogenin (Vg), a major precursor to egg yolk proteins, was purified from plasma of an estradiol-treated female tuatara (Sphenodon punctatus) by MgCl2-EDTA precipitation and DEAE-cellulose chromatography. The amino acid composition of tuatara Vg is similar to that of other vertebtate Vgs and contains a large proportion of serine (13.7 mol/100 mol of total amino acid). The amino acid sequences of the N-terminus of mature Vg (33 residues) and of several trypsin- and CNBr-generated peptides were determined. Six peptide sequences obtained from tuatara Vg could be aligned with Vg sequences from other vertebrates. Reduced and non-reduced forms of tuatara Vg have the same apparent molecular mass (approximately 218 kDa) when resolved by SDS-PAGE, indicating that inter-chain disulfide bonds are not a feature of the molecule in this species. Western blot analysis with anti-tuatara Vg antiserum indicated that at least some epitopes are shared among Vgs of turtle, alligator and tuatara.

Plasma vitellogenin of grouper (Epinephelus malabaricus): isolation and properties

Radiolabelled vitellogenin produced by juvenile grouper following injection of 3H-leucine, 32P-orthophosphate and estradiol-stimulation was purified by anion-exchange and gel filtration chromatography and characterized by electrophoresis and chemical analysis. The radiolabelled product was found to exist in two heterogeneous molecular weight forms by electrophoresis under nondenaturing conditions on polyacrylamide gel with M(r), of 260,000 and 525,000. It showed two protein monomers (M(r) 113,000 and 140,000) on electrophoresis under denaturing condition on a sodium dodecyl sulphate-polyacrylamide gel. The purified material contained 17.2% total lipid with 1.3% cholesterol and 2.1% triglycerides, 8-21 micrograms/mg protein total carbohydrate and 6.8 micrograms phosphate/mg protein; amino acid analysis showed a profile comparable to that found for vitellogenins isolated from the grouper, medaka, goldfish and rainbow trout. The study showed grouper vitellogenin to be a glycophospholipoprotein similar in composition to vitellogenins from other teleosts and demonstrated that vitellogenins can be induced in juveniles by injection with estradiol-17 beta.

Protease activity appeared after trypsin treatment of the purified vitellogenin from eel Anguilla japonica

Density gradient ultracentrifugation and anion-exchange chromatography combination were effective for the purification of the eel vitellogenin from the plasma of estradiol-treated eels. The vitellogenin was very high density glycolipoprotein (P = 1.27 g/ml) and its apolipoprotein was M(r) 196 k in both reduced and non-reduced conditions by SDS-PAGE. The major lipid component was phospholipid. The N-terminal amino-acid sequence of the vitellogenin was as follows: (Ac)Thr-Pro-Ala-Leu/Ala-Asp-Tyr. Amino-acid composition of the eel vitellogenin was similar to those of other teleosts. The protease activity appeared in the trypsinized vitellogenin, but was not detected in the purified vitellogenin. The protease was separated from the used trypsin and the other cleaved vitellogenin by a dextran sulfate cellulose column. The molecular weight of the protease was determined by zymogram using SDS-polyacrylamide gel containing casein and was 50 k. It was concluded that the eel vitellogenin possesses the protease activity as a latent form.

Identification of vitellogenin in the little skate (Raja erinacea)

1. Vitellogenin was isolated from mature female skates by selective precipitation with MgCl2/EDTA followed by chromatography on DEAE-cellulose columns. 2. A single monomer of approximately 205 kDa was identified on 6.0% SDS-PAGE gels. 3. In addition, isolation of yolk proteins with ammonium sulfate yielded proteins of 94 and 38 kDa (putative phosvitins) and putative lipovitellins of ca 105, 91 and 67 kDa. 4. In vivo phosphate incorporation in female and male skates implanted with estradiol indicated that vitellogenin was phosphorylated. 5. Total protein phosphate incorporation was significantly higher in females than male skates. 6. In male skates treated with estradiol, phosphate incorporation increased from 2 days after implantation to a maximum at approximately 11 days after implantation. 7. Determination of the rate of disappearance of 32P-labeled protein suggests a half-life of ca 200 hr in normal female skate plasma.

Human estrogen receptor introduced into the Xenopus oocyte represses expression from an artificial frog estrogen response element

Although the estrogen responsiveness and estrogen receptors of Xenopus hepatocytes have been well described, oocytes of this species have not previously been shown to contain estrogen receptors (ER). Recombinant human ER (HER) was expressed in oocytes in a dose dependent fashion as measured by [35S]methionine incorporation into newly synthesized proteins. Chloramphenicol acetyl transferase (CAT) reporter plasmids, driven by a herpes simplex thymidine kinase promotor with or without a 17 base pair estrogen response element (ERE) from the vitellogenin A2 gene, were also injected into oocytes. When injected without the accompanying HER sequences, the construct containing the ERE expressed 10-fold more CAT activity, and this response was saturable as demonstrated by injecting increasing amounts of reporter plasmid. These results suggest either the activity of small amounts of a Xenopus ER (measured here by LH-20 assay), or the presence of some endogenous oocyte protein other than the ER that can interact with this ERE. When HER was co-expressed with ERECAT, CAT expression was suppressed over a wide range of HER concentrations. This unexpected repression may be due to displacement of an estrogen receptor or other endogenous oocyte regulatory protein on the ERE. HER's positive regulatory activity may require transcription factors that are lacking or insufficient in the oocyte. Alternatively the simple 17 base pair ERE may not provide DNA binding sites for such transcription factors.

The purification and partial characterization of carp, Cyprinus carpio, vitellogenin

A procedure is described for the isolation of intact vitellogenin (c-VTG) from the carp, Cyprinus carpio. VTG was induced in juvenile females using oestradiol-17β and purified from the plasma using a combination of gel-filtration chromatography on Sepharose 6B and ion exchange chromatography on DEAE-cellulose. Purification procedures were conducted at low temperatures (below 9°C) in the presence of the proteolytic enzyme inhibitor aprotinin to prevent degradation. Intact c-VTG had an apparent molecular mass of 390,000 Daltons, but when extracted from plasma in the absence of aprotinin it underwent proteolysis into at least 2 protein fragments (apparent molecular masses of 230,000 and 96,000 Daltons), showing an instability of the native dimer. An amino acid analysis of c-VTG showed that its composition was almost identical to goldfish VTG, a species closely allied to the true carps and also similar to other oviparous vertebrate VTGs. Collectively, these data indicate that using these purification procedures VTG from carp, and probably other teleost species, can be isolated in an intact, highly purified form.

Cholesterol metabolism in frog (Rana esculenta) liver: seasonal and sex-related variations

Many aspects of lipid metabolism have been studied in amphibians, but seasonal lipid modulation in male and female frogs has not been investigated. We describe here the yearlong patterns of hepatic lipid content and enzyme activities related to cholesterol homeostasis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity and acyl coenzyme A:cholesterol acyltransferase (ACAT) activity in liver of the male and female frog, Rana esculenta. Lipid storage follows distinct seasonal patterns, with an increase in June that is more pronounced in the female than in the male frog. Cholesterol content and cholesterol storage as cholesteryl ester in male liver are consistent with the activity of HMG-CoA reductase and of ACAT enzymes. HMG-CoA reductase activity of the female frog shows an extra peak in fall unrelated to cholesterol storage and probably related to the production of essential compound for oogenesis.

Vitellogenin induction by estradiol in channel catfish, Ictalurus punctatus

In oviparous vertebrates estrogens induce hepatic synthesis of vitellogenin (VG), a blood protein sequestered in vitellogenic oocytes and from which lipovitellin (LV) and phosvitin are derived. Our objective was to identify VG in the channel catfish, Ictalurus punctatus. An intraperitoneal injection of estradiol-17 beta into adult male fish induced a dose-dependent accumulation of a 150 kDa protein (EP) in the plasma. EP was detectable in Coomassie blue-stained polyacrylamide gels within 24 hr after injection of 2 mg hormone/100 g body weight. During the next 4 days, EP increased from 5 to about 25% of the total plasma protein. Electrophoretic mobility, peptide mapping, and immunological crossreactivity showed EP to be indistinguishable from a plasma protein in adult females with vitellogenic ovaries. Two major yolk polypeptides, YP1 (120 kDa) and YP2 (29.6 kDa), were precipitated by (NH4)2SO4 from a yolk protein extract. YP1 but not YP2 reacted with an anti-EP polyclonal antiserum in Western blots. Peptide mapping after proteolysis with trypsin showed YPs 1 and 2 to be unique and revealed structural homologies between YP1 and EP. Liver but not pancreatic explants from an estradiol-treated male synthesized and secreted a [35S]methionine-labeled, 150 kDa protein beginning about 2 hr after initial exposure to the label. We tentatively conclude that EP and YP1 represent VG and LV, respectively. YP2 remains unidentified.

Relationship between estradiol-17 beta seasonal profile and annual vitellogenin content of liver, fat body, plasma, and ovary in the frog (Rana esculenta)

The seasonal plasma estradiol-17 beta (E2-17 beta) profile and annual vitellogenin content of liver, fat body, plasma, and ovary were investigated in Rana esculenta. Concomitant with the increase in E2-17 beta, vitellogenin peaked in liver, plasma, and ovary during autumn and winter, while it remained at a relatively high concentration in fat body during spring. In vitro experiments showed that E2-17 beta (10(-9) M) is ineffective in inducing vitellogenin production in fat body, but is effective in inducing vitellogenin production in liver. As fat bodies do not produce the vitellogenin they contain, we suggest that fat bodies are involved in the transfer of vitellogenin to the ovary.

Concanavalin A reactivity of vitellogenin and yolk proteins of the threespined stickleback Gasterosteus aculeatus (Teleostei)

1. Concanavalin A (con A) reactive proteins have been detected in the plasma and ovaries of the oestradiol treated Gasterosteus aculeatus. 2. Concanavalin A-horseradish peroxidase (HRP) technique applied on nitrocellulose membranes reveals that vitellogenin (Vg) is the only mannose and glucose rich glycoprotein present in the plasma of oestradiol treated sticklebacks. Stickleback Vg can be purified by con A-Sepharose chromatography. 3. Con A reactivity in the ovary changes in the course of development of the oocytes. First, the yolk vesicles, which are synthesized by the oocyte itself, become con A positive. Later, the yolk granules, which contain vitellogenin synthesized in the liver and taken up from the plasma, show a clear affinity for con A. Con A staining disappears when mannopyranoside is added. 4. No con A staining is found in the periodic acid/Schiff staining chorion.

Vitellogenesis in the lizard Lacerta vivipara jacquin. I. Purification and partial characterization of plasma vitellogenin

The polypeptide moiety of lizard plasma vitellogenin was reproducibly dissociated and separated by SDS-PAGE into two subunits: Vg alpha 2-2.2 X 10(5) Da and Vg beta 1-1.1 X 10(5) Da. In estrogenized females, active incorporation of inorganic 32P into both vitellogenin chains present in the plasma, and in the culture medium of the liver, was identified following autoradiography of SDS-polyacrylamide gels. Lacerta vivipara vitellogenin was isolated from pooled plasma collected from heavily estrogenized males by the two-step precipitation procedure of H. S. Wiley, L. Opresko, and R. A. Wallace, (1980, Anal. Biochem. 97, 145-152), followed by chromatography on DEAE-cellulose. This preparation of L. vivipara vitellogenin was of sufficient purity to generate in rabbits an immune serum which cross-reacted very slightly with plasma free of vitellogenin (rocket immunoelectrophoresis). Using the double immunodiffusion procedure it was shown that the anti-vitellogenin serum recognized identical antigenic determinants in plasma from a vitellogenic female or from estrogenized lizards, and in crude vitellus. The immunodetection of L. vivipara native vitellogenin consistently allowed two circulating forms to be distinguished. After autoradiography of a rocket immunoelectrophoresis plate it was demonstrated that both native forms had incorporated inorganic 32P into the polypeptide moiety and/or the phospholipid moiety.

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.

Identification of the vitellogenin proteins in the newt Pleurodeles waltl (urodele amphibian)

Vitellogenin derived from the blood of estrogen-treated Pleurodeles waltl was identified by immunochemical and electrophoretic analyses, using an antiserum against plasma vitellogenin isolated by dimethylformamide precipitation. Pleurodeles vitellogenin migrates as four bands on native PAGE, designated alpha-, beta-, gamma- and delta- VTG, with apparent mol. wts of 250,000, 270,000, 280,000 and 520,000 respectively. In the plasma, from estrogen-treated males like from ovariectomized estrogen-treated females, an additional band (mu-VTG) was found by native PAGE, never observed in estrogen-treated female plasma. It has a mol. wt of about 380,000 and shows complete immunological cross-reactivity with the vitellogenin antiserum. At least two polypeptides, termed VTG-I and VTG-II (mol. wt = 180,000 and 210,000) were identified by SDS-PAGE. Rocket immunoelectrophoresis displays three distinct precipitate lines indicating major immunological differences between the plasma vitellogenins.

Specific and non-specific immunosuppressive factors in salamander pregnancy serum

In vitro experiments employing pregnant Salamandra salamandra have previously shown a maternal specific cytotoxic cellular reaction against larval cells and an inhibition of this reaction by maternal serum. Two serum fractions, 19S and 13S, have been implicated in this inhibition. In the present work, the site of action and the identity of the factors responsible for this activity have been determined. Pre-incubation experiments indicate that the 19S fraction from pregnancy serum specifically protects embryonic epithelial cells derived from the same pregnant female. The 13S fraction non-specifically inhibits cytotoxic activity of maternal cells. Both factors implicated in these reactions are respectively an IgM and an alpha 2-macroglobulin. The question of whether the alpha 2M is responsible for immunosuppression is discussed, with particular reference to its vitellogenin analogue and also in view of the fact that the presence of this alpha 2M is not always linked with the immunosuppressive properties (except during pregnancy).

Primary induction of vitellogenin synthesis in monolayer cultures of amphibian hepatocytes

Direct induction of vitellogenin production in cultured male amphibian hepatocytes by estradiol-17 beta has been accomplished. Liver cells were isolated from adult male bullfrogs by collagenase perfusion and maintained as monolayers in serum-free medium containing insulin and estradiol. Vitellogenin production was measured by direct immunoprecipitation from radioactively labeled secreted protein with a specific antiserum against vitellogenin. Significant quantities of vitellogenin were detected in the exported protein on the second day of hormone treatment. Vitellogenin production increased with duration of culture in the presence of estradiol until by the eighth day approximately 90% of secreted protein was vitellogenin. This response is largely comparable to that obtainable in vivo. Indirect immunofluorescence microscopy was used to identify cells synthesizing vitellogenin in response to estradiol. An increase in cytoplasmic fluorescence could be seen in cells throughout the cultures, with increasing time in the presence of estradiol. By the sixth day of treatment, the majority of cells showed significant fluorescence labeling. The results suggest that studies on the mechanisms underlying the primary activation of the vitellogenin gene may now be conducted under well defined conditions in a monolayer liver cell culture system.

Characterisation of bacterial clones containing DNA sequences derived from Xenopus laevis vitellogenin mRNA

A 1700 nucleotide DNA sequence derived from Xenopus vitellogenin mRNA has been cloned in the bacterial plasmid pBR322. The identity of the cloned sequence was verified in two ways. Firstly, the plasmid DNA was shown to hybridise to an RNA of the correct size (6,700 nucleotides). This was shown by in situ hybridisation to electrophoretically separated RNA and also by the formation of "R-loops" with purified vitellogenin mRNA. Then, using a novel procedure in which plasmid DNA covalently bound to diazotised paper is used to select complementary mRNA sequences, the cloned sequence was shown to hybridise to an mRNA which directed the synthesis of vitellogenin when translated in a reticulocyte lysate cell-free system.

Oestrogen-induced cholesterol and fatty acid biosynthesis in Xenopus laevis liver during vitellogenic response

1. Oestradiol-17beta induces livers of Xenopus laevis (South African clawed toad) to synthesize and secrete into the serum large quantities of the egg-yolk-protein precursor, vitellogenin. The peak of this response occurs 9-16 days after hormone treatment [Dolphin, Ansari, Lazier, Munday & Akhtar (1971) Biochem. J.124, 751-758]. It is now shown that 6 days after hormone treatment a 120-160-fold stimulation of the synthesis of cholesterol and fatty acid compared with control values occurred. 2. A cell-free system, derived from Xenopus liver, which synthesizes squalene and fatty acid is described. By using this system, several hundredfold stimulation of incorporation of [(14)C]acetate into squalene was recorded 6 days after the administration of oestradiol-17beta, compared with a 3-4-fold stimulation of incorporation of [(3)H]mevalonate compared with control values. It is argued that oestradiol-17beta must affect enzyme(s) catalysing step(s) between acetate and mevalonate in the biosynthetic pathway to cholesterol. 3. In incubation of liver slices in vitro, most of the lipid and cholesterol synthesized in response to the steroid hormone was associated with those subcellular fractions that contained membranes. Moreover, pulse-labelling experiments in vivo showed that 70% of this lipid and cholesterol was retained in the liver. The remainder appeared in the serum, where it was equally distributed between vitellogenin and vitellogenin-free serum. 4. G.l.c. analyses of the cholesterol content of liver microsomal fractions of Xenopus laevis indicated that the cholesterol content was at least 50% higher in microsomal fractions obtained from livers that had been exposed to oestradiol-17beta. Meanwhile, g.l.c. analysis of the lipid moiety of secreted vitellogenin showed that up to 35% of its lipid was cholesterol.

Ontogeny of the vitellogenic response to oestradiol and of the soluble nuclear oestrogen receptor in embryonic-chick liver

A specific high-affinity oestradiol-binding protein was characterized in salt extracts of liver nuclei of the developing chick embryo. It is present in very small amounts at day 10 of development and is marginally stimulated by oestradiol injection into the yolk sac on day 8. Injection of oestradiol on day 10 evokes a substantial increase in the nuclear oestradiol-binding activity measured on day 12 of development. This oestradiol-binding protein has properties of sedimentation, hormone specificity and high-affinity binding very similar to those of the soluble nuclear receptor in hatched chicks. Livers from the 12-day embryos injected 48 h earlier with oestradiol do not synthesize vitellogenin, as judged by a specific immunochemical and electrophoretic assay for this oestrogen-induced protein. Traces of vitellogenin synthesis can be induced in 13-day-embryo liver, and a substantial response, equivalent to that in hatched chicks, is seen in liver from 15-day embryos injected on day 13. The development of the ability of oestradiol to increase the concentration of the soluble nuclear receptor appears to be one, but not the only, critical factor involved in the development of the ability of chick liver to synthesize vitellogenin.

Comparative studies on immunochemical properties of female-specific serum protein and egg yolk proteins in rainbow trout (Salmo gairdneri)

1. Female-specific serum protein (FS) and egg yolk proteins of mature female rainbow trout (Salmo gairdneri) were studied comparatively with reference to their immunological and some physicochemical properties. 2. FS was considered to be composed of two egg proteins, E1 and E2, which were isolated from the egg yolk by gel filtration. 3. The molecular weights of FS, E1 and E2 were estimated by gel filtration to be approx 600,000, 300,000 and 35,000, respectively. 4. The analysis with sodium dodecyl sulphate electrophoresis indicated that E1 consisted of two subunits with molecular weights of 90,000 and 15,000, and E2 was estimated to be a dimer of a molecule of 15,000. 5. Male and immature female trout were injected with estradiol-17 beta, and the production of proteins with the same antigenicity with FS was demonstrated.

Regulation by estrogen of the vitellogenin gene

The vitellogenin gene is inactive in the liver of male Xenopus laevis, unless exogenous estrogen is administered. We have previously shown that conventional doses of estradiol-17beta result in the appearance of new hepatic messenger RNAs, some of which are encoded for vitellogenin. We now report that much higher doses of the hormone (2 mg/frog per day for 4 days) are required to elicit maximal responses. The relative levels of membrane-bound polysomes and vitellogenin mRNA were determined as a function of time and dose of hormone. Translation of total polysomal RNA in a cell-free system derived from wheat germ was used to estimate the relative levels of vitellogenin messenger RNA. Faithful translation of this messenger RNA was indicated by two lines of evidence: labeled cell-free products were immunoprecipitated with antivitellogenin antibody, and the migration of the major labeled product in sodium dodecyl sulfate/acrylamide gels was identical to that of native vitellogenin. Our results establish conditions for maximal estrogen-induced responses in this system, and are compatible with the hypothesis that a major regulatory mechanism of steroid hormones in the control of protein synthesis is that of gene activation and regulation of messenger RNA levels.

Studies on the biosynthesis, assembly and secretion of vitellogenin, an oestrogen-induced multicomponent protein

1. The process by which the egg-yolk protein precursor vitellogenin is biosynthesized, assembled and secreted by Xenopus laevis (South African clawed toad) liver was studied. It was previously shown in other laboratories that vitellogenin contains the two egg-yolk proteins lipovitellin (mol.wt. 140 000) and phosvitin (mol.wt. 35 000). 2. Evidence is presented which shows that Xenopus liver microsomal fractions synthesize precursors of vitellogenin. These precursors were solubilized from the membranes with detergent and analysed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. This analysis indicated that there is only one precursor polypeptide, and this has mol.wt. approx. 200 000 +/- 20 000. This demonstrates that the egg-yolk proteins are translated as part of this larger polypeptide. 3. Experiments also demonstrate the existence of a microsomal proteinase which is able to cleave the precursor into smaller fragments. The nature of these fragments provided some indirect evidence that phosvitin and lipovitellin light chains are situated together within the precursor molecule. 4. These precursor data fit in well with structural studies on serum vitellogenin, since it has been shown that the latter protein consists of two identical subunits each with a mobility on sodium dodecyl sulphate/polyacrylamide gels identical with that shown by the microsomal precursor. This indicates that both the intracellular precursor and subunit of vitellogenin have similar (but not necessarily identical) molecular weights. 5. It was also shown that trypsin or chymotrypsin can cleave the serum vitellogenin into leucine- and serine-rich fragments which resemble lipovitellin and phosvitin respectively. Attention is, however, drawn to the fact that the serine-rich fragment is not identical with phosvitin, since it contains eight times more leucine than that expected for the authentic phosvitin molecule [Penning (1976) Ph.D. Thesis, University of Southampton].