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D J Shapiro

Publications and source records attributed to D J Shapiro.

At least 55 records · Page 3Linked to original sources
PubMed1990

An NF1-related vitellogenin activator element mediates transcription from the estrogen-regulated Xenopus laevis vitellogenin promoter.

We have used a homologous transfection system employing cell lines derived from Xenopus hepatocytes and fibroblasts and cloned Xenopus estrogen receptor to identify a DNA sequence essential for efficient transcription of the powerful estrogen-regulated Xenopus laevis vitellogenin B1 promoter. Although deletion of the CAAT box, the conserved nematode box, and several other sequences had little effect on vitellogenin promoter activity, deletion or mutation of a short sequence related to the NF1 transcription activator reduced estrogen-dependent transcription 10-20-fold in both cell lines. The activity of the vitellogenin activator (VA) element, which is located at positions -48 to -41, is strongly position dependent. Insertion of even five copies of the VA element at -445 does not restore activity to a VA deletion, while insertion of a single VA element at -62 largely restores promoter activity. The VA element is not liver-specific, since VA deletions, mutations, and insertions elicited similar effects on vitellogenin promoter activity in the cell lines derived from hepatocytes and fibroblasts. Gel mobility shift assays using nuclear extracts from the Xenopus hepatocyte cell line demonstrate the presence of a protein which binds with high affinity and specificity to the VA sequence. The NF1 consensus sequence, but not a functional AP1 consensus sequence, which actually exhibits higher homology to the VA element than the NF1 sequence, competes for binding to the VA element. In transfections using simplified promoters in which single copies of the VA region or NF1 sequence are linked to a TATA box, the VA region, which does not contain a palindrome, is 2.4-fold more active than the consensus NF1 palindrome. The VA-binding protein therefore appears to be related to, but not identical to, the NF1 transcription activator.

Animals↗
PubMed1990

Nucleotide sequence and estrogen induction of Xenopus laevis 3-hydroxy-3-methylglutaryl-coenzyme A reductase.

We have isolated and sequenced overlapping cDNA clones encompassing the entire protein coding region of Xenopus laevis 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase). This 3-kilobase cDNA codes for an 883-amino acid protein, which exhibits extensive amino acid sequence homology to mammalian HMG-CoA reductases. The highly conserved 341-amino acid membrane spanning domain exhibits 89% amino acid sequence identity with hamster HMG-CoA reductase. In contrast to the high degree of homology in the coding regions of the mRNAs, the nucleotide sequences of the 5'- and 3'-untranslated regions of Xenopus and mammalian HMG-CoA reductase mRNAs exhibit negligible homology. Primer extension data suggests that Xenopus HMG-CoA reductase mRNA is transcribed from multiple initiation sites. The Xenopus HMG-CoA reductase cDNA clones were used as hybridization probes to examine estrogen regulation of hepatic HMG-CoA reductase mRNA levels. Northern blot analysis demonstrates that estradiol-17 beta induced HMG-CoA reductase mRNA in livers of male X. laevis 10-fold within 24 h. Reductase mRNA levels reach a plateau 23-fold higher than basal levels between 5 and 8 days after initial estrogen administration. In contrast to the vitellogenin and retinol binding protein genes whose transcription is induced by estrogen in Xenopus liver, the runoff transcription rate of HMG-CoA reductase does not increase significantly on administration of estradiol-17 beta. These data suggest that the estrogen induction of HMG-CoA reductase mRNA may be achieved through posttranscriptional mechanisms.

Amino Acid Sequence↗
PubMed1990

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.

Animals↗
PubMed1989

Ribosome loading, but not protein synthesis, is required for estrogen stabilization of Xenopus laevis vitellogenin mRNA.

We have examined the effect of protein synthesis and of ribosome loading on the estrogen-mediated stabilization of hepatic Xenopus laevis vitellogenin mRNA. Removal of estradiol-17 beta from the culture medium, which destabilizes vitellogenin mRNA, does not alter the density of ribosomes on polysomal vitellogenin mRNA, or change the proportion of vitellogenin mRNA associated with the endoplasmic reticulum. Cycloheximide, which inhibits elongation, without changing the density of ribosomes on vitellogenin mRNA, does not block estrogen-mediated stabilization. In contrast, 2-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide, (MDMP), which inhibits initiation, greatly reduces the density of ribosomes on vitellogenin mRNA, and completely blocks estrogen-mediated stabilization. Vitellogenin mRNA in MDMP treated cells is degraded at a rate similar to that seen when untreated cells are transferred from medium containing estrogen to estrogen-free medium. This suggests that a ribosome-associated degradative system may not be responsible for vitellogenin mRNA degradation. The failure of estrogen to stabilize vitellogenin mRNA in MDMP-treated cells is not due to the release of vitellogenin mRNA from the endoplasmic reticulum. Vitellogenin mRNA in MDMP-treated cells remains associated with the endoplasmic reticulum in small polysomes containing 3-5 ribosomes. These data demonstrate that maintaining a high density of ribosomes on vitellogenin mRNA, but not continuing protein synthesis, is necessary for estrogen-mediated stabilization of vitellogenin mRNA.

Animals↗
PubMed1989

A highly sensitive, mixed-phase assay for chloramphenicol acetyltransferase activity in transfected cells.

We describe a simple, rapid yet extremely sensitive assay for chloramphenicol acetyltransferase (CAT) activity in extracts from transfected eukaryotic cells. Using our modified reaction conditions and the mixed-phase assay, less than 0.000010 unit of CAT activity in transfected cells can be reliably detected. The mixed-phase assay is based on the inability of the polar [3H]-acetyl-Coenzyme A (CoA) substrate to partition out of a urea containing aqueous phase into the nonpolar scintillation fluor, while the [3H]chloramphenicol reaction products partition into the toluene scintillation fluor and are quantitated by scintillation counting. The increased sensitivity of this assay is due to the optimization of the acetyl-CoA concentration, to a urea-containing aqueous phase which lowers the assay background, and to the use of extract blanks. The mixed-phase assay is simpler, is quantitative, uses less costly substrates, and is far more sensitive than the most widely used CAT assays, which require solvent extraction followed by thin-layer chromatography to separate the unreacted substrate from product.

Animals↗
PubMed1988

Persistent estrogen induction of hepatic Xenopus laevis serum retinol binding protein mRNA.

Administration of estradiol-17 beta to male Xenopus laevis induces the hepatic mRNA coding for the serum retinol binding protein (RBP) approximately 10-fold, both in vivo and in primary liver cultures. Estrogen induction of RBP mRNA is completely blocked by the anti-estrogen, hydroxytamoxifen. Testosterone administration reduces the elevated level of RBP mRNA observed in livers of female X. laevis to the constitutive level seen in livers of control male animals, and partially blocks the estrogen induction of RBP mRNA. Intracellular RBP mRNA levels therefore represent a balance between the opposing effects of estradiol-17 beta and testosterone. In marked contrast to the estrogen induction of vitellogenin mRNA, which requires the continuous presence of exogenous estrogen, induction of RBP mRNA persists for at least 4 months after a single injection of estrogen. Runoff transcription measurements demonstrate that persistent induction of RBP mRNA is due to an increased rate of RBP gene transcription. Administration of hydroxytamoxifen abolishes persistent induction of RBP mRNA, suggesting that residual hormone receptor complex plays a role in the persistent induction of RBP gene transcription. The persistent estrogen induction of RBP mRNA provides the first demonstration of long-term activation of the transcription of a hormone-responsive gene in response to a transient dose of a steroid hormone.

Animals↗
PubMed1988

Transient administration of estradiol-17 beta establishes an autoregulatory loop permanently inducing estrogen receptor mRNA.

A single transient dose of estradiol-17 beta is sufficient to elicit the permanent induction of hepatic estrogen receptor mRNA, which is induced 18-fold (from 0.13 to 2.4 molecules per cell) and then remains fully induced for at least 125 days. In primary liver cultures, extremely low concentrations of estradiol-17 beta, which are below the Kd of the Xenopus laevis estrogen receptor, maintain persistent induction of estrogen receptor mRNA but not of estrogen-inducible vitellogenin mRNA. These data and the ability of the antiestrogen, hydroxytamoxifen, to reverse persistent induction of estrogen receptor mRNA, support a model in which transient doses of estradiol-17 beta induce the estrogen receptor and thereby establish an autoregulatory loop. The low levels of estradiol-17 beta normally circulating in male X. laevis and the elevated level of receptor provide sufficient hormone-receptor complex to permanently maintain the induced level of expression of the estrogen receptor gene. The permanent induction of the estrogen receptor may be the regulatory switch that results in the persistent expression of a recently identified class of proteins that exhibit long-term responses to estrogen.

Animals↗
PubMed1987

Estrogen induces transcription of the Xenopus laevis serum retinol-binding protein gene.

We have isolated and sequenced a full-length cDNA cloned tentatively identified as encoding Xenopus laevis serum retinol-binding protein (RBP) mRNA. The derived amino acid sequence of the Xenopus protein is 63% homologous to the sequence of the human RBP. 17 of 19 amino acids identified as critical in the retinol-binding pocket of human RBP are identical or conservative replacements in the Xenopus protein. The RBP cDNA clone has been used as a hybridization probe to demonstrate that administration of estradiol-17 beta to male X. laevis induces hepatic RBP mRNA 10-fold from its constitutive in vivo level of 1,800 molecules/cell to approximately 18,000 molecules/cell. Using a simplified method for determining relative rates of gene transcription, we demonstrate an estrogen-mediated increase in the rate of RBP gene transcription. These quantitative data provide the first demonstration that a steroid hormone regulates the levels of vertebrate retinol-binding protein mRNA.

Amino Acid Sequence↗
PubMed1987

The Xenopus laevis estrogen receptor: sequence homology with human and avian receptors and identification of multiple estrogen receptor messenger ribonucleic acids.

We have isolated and sequenced a cDNA clone encompassing the entire protein coding region of the Xenopus laevis estrogen receptor (xER). The Xenopus ER, the first steroid hormone receptor to be sequenced from a cold-blooded organism, exhibits two regions of striking amino acid homology with the human and avian ERs. In the putative DNA binding region, the amino acid sequence of the xER differs from those of the human and avian ERs at only one of 83 amino acids. The putative hormone binding region contains 44 and 46 amino acid blocks in which the sequence is identical in the Xenopus and human ERs. Blot hybridizations of Xenopus liver RNA suggest that the xER is encoded by four mRNAs with lengths of approximately 9, 6.5, 2.8, and 2.5 kilobases. In contrast, hybridization of human RNA to a human ER cDNA clone reveals only a single major ER RNA, approximately 6.7 kilobases in length.

Amino Acid Sequence↗
PubMed1986

Dexamethasone and estrogen regulate Xenopus laevis albumin mRNA levels.

The regulation of albumin mRNA levels by steroid hormones was examined in a primary Xenopus liver culture system. In the absence of exogenous steroid hormone, albumin mRNA levels declined rapidly in culture. Dexamethasone is required for preservation of albumin mRNA levels in culture and effects a greater than 10 fold induction of albumin mRNA in cultures maintained in steroid hormone-free medium. Estrogen can override the effect of dexamethasone and elicits a decline of greater than 80% in albumin mRNA levels. Our cDNA clones of the mRNAs encoding the two 74,000 dalton and one 72,000 dalton cellular albumins allowed us to show that all three albumin mRNAs were down regulated by estrogen.

Animals↗
PubMed1985

Regulation of rat liver cell 3-hydroxy-3-methylglutaryl coenzyme A reductase by methoxypolyoxyethylated cholesterol.

A water-soluble derivative of cholesterol, methoxypolyoxyethylated (MPOE) cholesterol, has been synthesized and used to study the regulation of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the key regulatory enzyme in cholesterol biosynthesis. MPOE cholesterol causes a specific, rapid and linear decline in HMG-CoA reductase in cultured rat liver cells. MPOE cholesterol is not a direct allosteric inhibitor of HMG-CoA reductase, does not appear to regulate HMG-CoA reductase through changes in membrane environment, and does not change the phosphorylation state and level of activation of rat liver cell HMG-CoA reductase. In order to confirm our data, which were consistent with a model in which MPOE cholesterol regulates the amount of HMG-CoA reductase and not its activity, we made direct measurements of reductase mRNA levels. The decline in HMG-CoA reductase in MPOE cholesterol-treated rat liver cells is preceded by the rapid disappearance of HMG-CoA reductase mRNA. As a water-soluble cholesterol derivative, MPOE cholesterol represents a useful model compound for studies on the regulation of the level of HMG-CoA reductase and its cognate mRNA.

Animals↗
PubMed1985

Coordinate estrogen induction of vitellogenin and a small serum protein mRNA in Xenopus laevis liver.

We have used plus-minus hybridization to identify Xenopus liver cDNA clones of mRNAs whose levels are regulated by estrogen. One clone identified in this way was shown to be a nearly full-length cDNA clone of the mRNA coding for a small 22 000 dalton estrogen-inducible serum protein (EISP). Quantitation of EISP mRNA levels by in vitro translation and by hybridization to the cloned DNA demonstrated a 7-12-fold estrogen induction of EISP mRNA, both in vivo and in primary Xenopus liver cultures. The kinetics of induction of EISP mRNA closely parallel those of the mRNA coding for the abundant estrogen-inducible serum protein, vitellogenin. In contrast, the massive, and toxic, estrogen-mediated accumulation of vitellogenin in serum of male Xenopus laevis is accompanied by a sharp decline in the levels of albumin mRNA and in the levels of the mRNAs coding for several other serum proteins.

Animals↗
PubMed1985

3-Hydroxy-3-methylglutaryl-coenzyme A reductase is present in peroxisomes in normal rat liver cells.

The location inside rat liver parenchymal cells of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase; EC 1.1.1.34), the key regulatory enzyme in cholesterol biosynthesis, has been examined by immunoelectron microscopy and by subcellular fractionation. Although HMG-CoA reductase is generally thought to be exclusively a microsomal enzyme, we find that a substantial portion of cellular HMG-CoA reductase is localized in peroxisomes. Immunoelectron microscopic labeling of ultrathin frozen sections of normal rat liver, using two monoclonal antibodies to purified HMG-CoA reductase, showed that the enzyme is present in the peroxisomes at a higher concentration than at any other site inside the hepatocytes. Subcellular fractionation studies using Percoll and metrizamide gradients demonstrated a close correspondence of peaks of HMG-CoA reductase activity and of catalase activity, again revealing the presence of the reductase enzyme in peroxisomes. HMG-CoA reductase is therefore localized in peroxisomes in addition to being in the microsomal fraction.

Animals↗