Biomedical subjects

R Ge

Publications and source records attributed to R Ge.

28 records · Page 2Linked to original sources

PubMed1998

Effect of dichloroacetic acid and trichloroacetic acid on DNA methylation in liver and tumors of female B6C3F1 mice.

Dichloroacetic acid (DCA) and trichloroacetic acid (TCA) are found in drinking water and are metabolites of trichloroethylene. They are carcinogenic and promote liver tumors in B6C3F1 mice. Hypomethylation of DNA is a proposed nongenotoxic mechanism involved in carcinogenesis and tumor promotion. We determined the effect of DCA and TCA on the level of DNA methylation in mouse liver and tumors. Female B6C3F1 mice 15 days of age were administered 25 mg/kg N-methyl-N-nitrosourea and at 6 weeks started to receive 25 mmol/liter of either DCA or TCA in their drinking water until euthanized 44 weeks later. Other animals not administered MNU were euthanized after 11 days of exposure to either DCA or TCA. DNA was isolated from liver and tumors, and after hydrolysis 5-methylcytosine (5MeC) and the four bases were separated and quantitated by HPLC. In animals exposed to either DCA or TCA for 11 days but not 44 weeks, the level of 5MeC in DNA was decreased in the liver. 5MeC was also decreased in liver tumors from animals exposed to either chloroacetic acid. The level of 5MeC in TCA-promoted carcinomas appeared to be less than in adenomas. Termination of exposure to DCA, but not to TCA, resulted in an increase in the level of 5MeC in adenomas to the level found in noninvolved liver. Thus, hypomethylated DNA was found in DCA and TCA promoted liver tumors and the difference in the response of DNA methylation to termination of exposure appeared to support the hypothesis of different mechanisms for their carcinogenic activity.

5-Methylcytosine↗

PubMed1996

Evidence for the involvement of a nuclear NF-kappa B inhibitor in global down-regulation of the major histocompatibility complex class I enhancer in adenovirus type 12-transformed cells.

Diminished expression of major histocompatibility complex class I antigens on the surface of adenovirus type 12 (Ad12)-transformed cells contributes to their high tumorigenic potential by enabling them to escape immune recognition by cytotoxic T lymphocytes. This low class I antigen expression is due to a block in class I transcription, which is mediated by Ad12 E1A. Genetic analysis has shown that the class I enhancer is the target for transcriptional down-regulation. In this study, we show that the ability of the R1 element of the class I enhancer to stimulate transcription is greatly reduced in Ad12-transformed cells. The loss of functional activity by the R1 element was attributed to loss of binding by the NF-kappa B p50-p65 heterodimer. NF-kappa B binding appears to be blocked within the nucleus rather than at the level of nuclear translocation. Significantly, NF-kappa B binding activity could be recovered from the nuclear extracts of Ad12-transformed cells following detergent treatment, suggesting that the block is mediated through a nuclear inhibitor present in the Ad12-transformed cells. These results, taken together with the fact that the R2 element of the class I enhancer exhibits strong binding to the transcriptional repressor COUP-TF, suggest that the class I enhancer is globally down-regulated in Ad12-transformed cells.

Adenoviruses, Human↗

PubMed1994

Retinoid X receptor homodimers function as transcriptional activators in yeast.

The possibility that different retinoids activate transcription from a specific retinoic acid (RA)-responsive element known as site A via different homo and heterodimeric versions of RA receptors cannot be evaluated in mammalian cells because they contain endogenous RA receptors (RAR). However, this limitation can be overcome by using yeast cells, which do not contain endogenous RAR, to study retinoid signaling pathways. Here, we describe heterologous expression of the human retinoid X receptor (RXR alpha) in yeast and hormone-dependent activation of a reporter construct containing site A upstream from a yeast promoter fused to the lacZ gene of Escherichia coli. Western blot analysis of yeast extracts containing RXR alpha revealed a distinct immunoreactive polypeptide co-migrating with the mammalian-produced RXR alpha. Electrophoretic mobility shift assays demonstrated that RXR alpha produced in yeast binds efficiently to site A in the absence of 9-cis-RA. However, transcription activation experiments showed that RXR alpha transactivates a yeast basal promoter linked to site A only in the presence of 9-cis-RA. We conclude that RXR alpha homodimers bind to site A in the absence of 9-cis-RA, but function as ligand-dependent transactivators in yeast cells. This retinoid-responsive transcription unit created in yeast cells provides a powerful genetic tool for the systemic unraveling of the synergistic interactions between RXR alpha and its heterodimeric partners.

Base Sequence↗

PubMed1994

Transcriptional repression of apolipoprotein AI gene expression by orphan receptor ARP-1.

Expression of the apolipoprotein AI (apoAI) gene in the liver is controlled by a liver-specific enhancer. The function of this enhancer depends on synergistic interactions between transcription factors bound to at least three sites (designated A, B, and C) located within this enhancer. We have previously shown that an apoAI gene reporter construct containing the entire enhancer is expressed efficiently in a hepatoma cell line and that its activity is repressed by the orphan receptor ARP-1. Moreover, repression by ARP-1 is overcome by the retinoid X receptor RXR alpha in the presence of retinoic acid. In this study, we show that ARP-1 represses the apoAI promoter by binding to site A of the apoAI liver-specific enhancer, the repression being a promoter context-specific event. Mapping analysis of ARP-1 indicated that its DNA binding domain is essential but not sufficient for repression. Two separate repression domains located at the amino- and carboxyl-terminal halves of ARP-1 were found to individually complement the DNA binding domain for efficient repression. We also demonstrate the reversibility of ARP-1 repression by transcription factors C/EBP and Egr-1, which might also be involved in apoAI gene expression. Significantly, repression by ARP-1 was found to be a prerequisite for C/EBP-mediated transactivation. We interpret our results in terms of a model in which ARP-1 repression via its interaction with site A is an obligatory intermediate step in switching from one activated state of the apoAI gene to another.

Animals↗

PubMed1994

E1A oncogene of adenovirus-12 mediates trans-repression of MHC class I transcription in Ad5/Ad12 somatic hybrid transformed cells.

Reduction of the Major Histocompatibility Complex (MHC) class I antigens on the surface of adenovirus type 12 (Ad12) transformed cells is thought to contribute to their tumorigenic potential. The E1A gene of Ad12 mediates this effect by repressing the MHC class I transcriptional enhancer. By contrast, Ad5-transformed cells are not blocked in class I gene expression and are nontumorigenic. Because E1A proteins modulate transcription by interacting with several different cellular factors, we inquired whether the Ad5 E1A proteins could interfere with the ability of Ad12 E1A proteins to repress class I transcription. Somatic cell hybrids, produced by fusing Ad5- and Ad12-transformed cells, expressed E1A proteins of both virus serotypes. The level of class I expression and transcription in the 5/12 hybrid cell lines was reduced to the same degree as in the original Ad12-transformed cells. Also observed in the 5/12 hybrid cell lines was strong binding activity to the R2 subelement of the H-2 class I enhancer which, in Ad12-transformed cells, correlates with repression of class I transcription. These results demonstrate that Ad12 E1A proteins mediate repression of the class I enhancer through a mechanism that is unaffected by Ad5 E1A proteins.

Adenoviridae↗

PubMed1994

Negative regulation by the R2 element of the MHC class I enhancer in adenovirus-12 transformed cells correlates with high levels of COUP-TF binding.

The transcriptional down-regulation of the major histocompatibility complex (MHC) class I antigens in adenovirus type 12 (Ad12) transformed cells gives them the potential to escape immunosurveillance and to form tumors. The enhancer of the class I promoter is the target of transcriptional repression which is mediated by the E1A gene of Ad12. The R2 region within the class I enhancer acts as a negative element in Ad12-transformed cells and exhibits a stronger binding activity than is observed in nontumorigenic Ad5-transformed cells, which are not reduced in class I expression. The R2 element contains a nuclear hormone receptor half-site consensus sequence, AGGTCA, which is required for both the binding activity and the ability of R2 to act as a negative element in Ad12-transformed cells. In this study, we show that an orphan hormone receptor protein, COUP-TF, contributes to the differential R2 binding activity observed between Ad12- and Ad5-transformed cells. Additionally, COUP-TF was shown to bind as a dimer to the R2 element and to use the consensus AGGTCA as one half-site and its 3' flanking sequence as a probable second degenerate half-site. Since COUP-TF can act as a transcriptional repressor, we suggest that the higher COUP-TF binding activity to the R2 element in Ad12-transformed cells contributes to down-regulation of class I transcription and, consequently, tumorigenesis.

Adenoviruses, Human↗

PubMed1994

Down-regulation of MHC class I antigen in insulinoma cells controlled by the R1 element of the H-2 enhancer.

Tumorigenesis in mice of the rat insulin promoter [RIP]-simian virus 40 tumor antigen [SV40 Tag] transgenic lineages, RIP1-Tag2 and RIP1-Tag4, is a process initiated by expression of SV40 Tag in pancreatic beta cells, evolution of islet cell hyperplasia and insulinoma appearance. Analysis of major histocompatibility complex [MHC] class I gene expression during this process revealed a normal level of MHC class I molecules at the surface of pancreatic islet cells of RIP1-Tag4 mice, while hyperplastic islets from the same mice contained cells expressing a normal level and cells expressing a low level of MHC class I antigen. Insulinomas themselves expressed very low levels or no MHC class I gene product. Thus, down-regulation of MHC class I gene appears to accompany tumor progression of SV40 Tag-transformed beta islet cells. MHC class I antigen expression in a series of clonally derived cell lines of beta cell origin from different SV40 Tag-induced insulinomas ranged from quite low to undetectable, although expression was inducible by interferon-gamma. Nuclear run-on and transient transfection analyses indicated that expression of the MHC class I gene in these cells in controlled at the transcriptional level, and that the decreased expression is paralleled by reduced binding of transcription factors to the R1 element of the H-2 enhancer.

Animals↗

PubMed1993

[Histochemical studies on the glycoconjugates of experimental alkali burned cornea in rabbit].

The model of experimental alkali burn of cornea in rabbit was obtained using 1 mol/L NaOH solution. After being burned for 1, 3, 7 and 14 days, the histochemical changes of these models were studied by using labelled concanavalin A (ConA-FITC) and wheat germ agglutinin (WGA-FITC). The results showed that after being burned, the amount of glycoconjugates interacted with these lectins decreased, and increased gradually after 7 days, then reached the level higher than that in the normal cornea. These changes were related to the recovery of alkali burned cornea.

Animals↗

PubMed1992

Down-regulation of the major histocompatibility complex class I enhancer in adenovirus type 12-transformed cells is accompanied by an increase in factor binding.

In transformed cells, the E1A gene of adenovirus type 12 (Ad12) represses transcription of class I genes of the major histocompatibility complex. The tumorigenic potential of Ad12-transformed cells correlates with this diminished class I expression. In contrast, the E1A gene of the nontumorigenic Ad5 does not affect class I expression. We show here that a transfected reporter chloramphenicol acetyltransferase plasmid driven by an H-2K promoter (-1049 bp) was expressed at much lower levels in Ad12- than in Ad5-transformed mouse cells. Analysis of mutant constructs revealed that only 83 bp of H-2 DNA, consisting of the enhancer juxtaposed to the basal promoter, was sufficient for this differential expression. Whereas the H-2 basal promoter alone was somewhat less active in Ad12-transformed cells, the H-2 TATA box itself did not appear to be important. The H-2 enhancer proved to be the principal element in Ad12 E1A-mediated repression, since (i) substitution of the H-2 enhancer by simian virus 40 enhancers overcame the repression, and (ii) when juxtaposed to either its native or heterologous basal promoters, the H-2 enhancer was functional in Ad5- but not Ad12-transformed cells. Mobility shift assays showed that there is a DNA-binding activity to the 5' site (R2 element) of the enhancer that is significantly higher in Ad12- than in Ad5-transformed cells. These results suggest that decreased class I enhancer activity in Ad12-transformed cells may, at least in part, be due to the higher levels of an enhancer-specific factor, possibly acting as a repressor.

Adenovirus E1A Proteins↗

PubMed1992

Negative regulation of the major histocompatibility complex class I enhancer in adenovirus type 12-transformed cells via a retinoic acid response element.

In cells transformed by the highly oncogenic adenovirus type 12 (Ad12), the viral E1A proteins mediate transcriptional repression of the major histocompatibility class I genes. In contrast, class I transcription is not reduced in cells transformed by the nononcogenic Ad5. The decreased rate of class I transcription is, at least in part, the result of a reduced major histocompatibility complex class I enhancer activity in Ad12-transformed cells and correlates with an increase in the levels of a DNA-binding activity to the R2 element of the enhancer (R. Ge, A. Kralli, R. Weinmann, and R. P. Ricciardi, J. Virol. 66:6969-6978, 1992). Employing transient transfection assays, we now provide direct evidence that the R2 element can confer repression in Ad12- but not Ad5-transformed cells. Repression by R2 was observed only in the presence of the positive enhancer element R1 and was dependent on (i) the number of the R2 elements and (ii) the relative arrangement of R2 and R1 elements. The putative R2-binding repressor protein, R2BF, was similar in molecular weight and binding specificity to members of the thyroid hormone/retinoic acid (RA) receptor family. RA treatment abrogated the R2-mediated repression in Ad12-transformed cells and had no effect on the activity of R2/R1-containing promoters in Ad5-transformed cells. These results are consistent with the presence of an R2-binding repressor in Ad12-transformed cells. In the absence of RA, the repressor compromises enhancer activity by interfering with the activity of the positive cis element R1. RA treatment of Ad12-transformed cells may render the repressor inactive.

Adenovirus E1A Proteins↗