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

Roles of Estrogens in the Healthy and Diseased Oviparous Vertebrate Liver

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

Activation of the silent progesterone receptor gene by ectopic expression of estrogen receptors in a rat fibroblast cell line

We describe the construction and characterization of a novel estrogen (E2)-responsive cell line, Rat1+ER, which ectopically expresses estrogen receptor (ER). Human ER cDNA was introduced by retrovirus-mediated gene transfer into the Rat1 cell line, which does not express functional ER endogenously. Rat1+ER cells express functional ER based on radioreceptor assays, immunoblotting, and transient transfection experiments using E2-responsive reporter plasmids. The effects of this ectopic ER expression were studied on three endogenous E2-responsive genes, prolactin (PRL), progesterone receptor (PR), and epidermal growth factor receptor (EGFR). PRL, usually expressed in the lactotrophs of the pituitary, is not expressed at all in Rat1+ER cells, with or without E2 addition, and appears to require other factors for expression. In contrast, although PR is not expressed in Rat1 cells, it is induced in Rat1+ER cells upon the addition of E2. This induction appears to occur at the transcriptional level and is insensitive to cycloheximide treatment. This is one of the few examples where the expression of one gene activates an otherwise silent gene. Another contrasting observation is that, although EGFR is basally expressed in Rat1+ER cells, the addition of E2 has no effect. Our studies paint a complicated picture of E2 regulation of endogenous genes: the activation of the PR gene may only require the presence of E2 and ER, whereas EGFR and PRL genes require factors in addition to ER for basal as well as E2-regulated expression.

Xenopus transcription factors: key molecules in the developmental regulation of differential gene expression

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Activation of chromosomal vitellogenin genes in Xenopus oocytes by pure estrogen receptor and independent activation of albumin genes

We generate pure estrogen receptor protein in Xenopus oocytes by injecting them with estrogen receptor mRNA synthesized in vitro. A chromosomal vitellogenin gene, which normally responds to estrogen only in liver cells, is activated. Primer extension shows that initiation is accurate, and ribonuclease mapping shows that the first exon is correctly spliced out of the initial transcript. Long transcripts are produced, one being equal in length to poly(A)- vitellogenin mRNA. Immunochemical estimates of receptor levels in the oocyte nuclei suggest that pure receptor, acting alone, cannot activate oocyte vitellogenin genes unless unusually large amounts are present. However, when a receptor-free extract from liver cells is also injected, the amount of receptor required is reduced. Such an extract, but not pure receptor, can also activate albumin genes in oocytes.

Activation of chromosomal vitellogenin genes in Xenopus oocytes by pure estrogen receptor and independent activation of albumin genes

We generate pure estrogen receptor protein in Xenopus oocytes by injecting them with estrogen receptor mRNA synthesized in vitro. A chromosomal vitellogenin gene, which normally responds to estrogen only in liver cells, is activated. Primer extension shows that initiation is accurate, and ribonuclease mapping shows that the first exon is correctly spliced out of the initial transcript. Long transcripts are produced, one being equal in length to poly(A)- vitellogenin mRNA. Immunochemical estimates of receptor levels in the oocyte nuclei suggest that pure receptor, acting alone, cannot activate oocyte vitellogenin genes unless unusually large amounts are present. However, when a receptor-free extract from liver cells is also injected, the amount of receptor required is reduced. Such an extract, but not pure receptor, can also activate albumin genes in oocytes.

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.

A novel nuclear transcription system which responds correctly to cloned estrogen receptor

We describe here a novel but simple nuclear transcription system in which nuclei make RNA in an isotonic buffer for a long time and respond to a cloned external factor by accurately initiating new transcription.

The human estrogen receptor can regulate exogenous but not endogenous vitellogenin gene promoters in a Xenopus cell line

Transfection of a human estrogen receptor cDNA expression vector (HEO) into cultured Xenopus kidney cells confers estrogen responsiveness to the recipient cells as demonstrated by the hormone dependent expression of co-transfected Xenopus vitellogenin-CAT chimeric genes. The estrogen stimulation of these vit-CAT genes is dependent upon the presence of the vitellogenin estrogen responsive element (ERE) in their 5' flanking region. Thus, functional human estrogen receptor (hER) can be synthesized in heterologous lower vertebrate cells and can act as a trans-acting regulatory factor that is necessary, together with estradiol, for the induction of the vit-CAT constructs in these cells. In addition, vitellogenin minigenes co-transfected with the HEO expression vector also respond to hormonal stimulation. Their induction is not higher than that of the vit-CAT chimeric genes. It suggests that in the Xenopus kidney cell line B 3.2, the structural parts of the vitellogenin minigenes do not play a role in the induction process. Furthermore, no stabilizing effect of estrogen on vitellogenin mRNA is observed in these cells. In contrast to the transfected genes, the endogenous chromosomal vitellogenin genes remain silent, demonstrating that in spite of the presence of the hER and the hormone, the conditions necessary for their activation are not fulfilled.

Trans-activation of transcription, from promoters containing immunoglobulin gene octamer sequences, by myeloma cell mRNA in Xenopus oocytes

To study factors required for immunoglobulin gene transcription hybrid promoters were made by linking octamer elements to a Xenopus albumin gene construct containing only 50bp of the albumin gene promoter. When injected into oocytes these hybrid promoters directed transcription far less efficiently than the unmodified 50bp albumin gene promoter fragment. Activity of the hybrid promoter, but not the unmodified albumin promoter, could be stimulated by preinjection of poly(A)+ RNA from NS1 myeloma cells. This stimulation may be caused by translation of the NS1 poly(A)+ RNA into transcription factors that act on the octamer. Both the reduction in transcription caused by octamer insertion and the extent of the inducibility by NS1 RNA are greater when two, rather than one, octamers are inserted.

The anti-oestrogen drug tamoxifen is an elongation inhibitor of eukaryotic protein biosynthesis

The drug tamoxifen is widely used in the chemotherapy of breast cancer but its action is not explained completely by its anti-oestrogen properties. We now present evidence indicating that it is also a potent inhibitor of eukaryotic protein synthesis as demonstrated in Xenopus oocytes, intact reticulocytes and reticulocyte lysates. The inhibition affects general protein synthesis, is transient in oocytes and not reversed by oestrogen. The drug appears to act by inhibiting polypeptide chain elongation. This action of tamoxifen is independent of oestrogen receptors and may explain its therapeutic effectiveness in oestrogen-independent tumours.