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Biomedical subjects

H Ponta

Publications and source records attributed to H Ponta.

At least 91 records · Page 5Linked to original sources
PubMed1990

Antitumor promotion and antiinflammation: down-modulation of AP-1 (Fos/Jun) activity by glucocorticoid hormone.

Glucocorticoid hormones counteract inflammation and phorbol ester tumor promotion and drastically decrease the expression of several extracellular proteases, including collagenase I. Glucocorticoid hormone inhibits basal and induced transcription of collagenase by interfering with AP-1, the major enhancer factor of the collagenase promoter. The mechanism of interference is novel in that it does not require protein synthesis, it depends on the hormone receptor but not its binding to DNA, it occurs at hormone doses one order of magnitude below those required for gene activation, and it involves down-modulation of the trans-activating function of preexisting unbound and DNA-bound AP-1. Coprecipitation experiments suggest direct AP-1-hormone receptor interaction, which also possibly explains the reverse experiment: overexpression of Fos or Jun inhibits the expression of hormone-dependent genes.

Animals↗
PubMed1990

The long terminal repeat region of the mouse mammary tumour virus contains multiple regulatory elements.

Mouse mammary tumour virus (MMTV) is the major aetiologic agent of mouse mammary tumour formation. The expression of this virus is regulated by steroid hormones and cell type specific factors. The nucleotide sequence that controls the steroid hormone response has already been localized between -202 and -59 upstream of the start of transcription in the long terminal repeat (LTR) region of the proviral DNA. Through transfection experiments in three different cultured mouse cell lines (NIH3T3, NMuMG and GR), we have investigated which sequences in the MMTV LTR play a role in the cell type specific expression at the proviral promoter. We have identified two elements on the MMTV LTR from -631 to -560 and from -428 to -364 that have the potential to influence expression at the MMTV LTR promoter. The -631 to -560 element mediated a negative response in all the cell types we studied whereas the -428 to -364 element had negative effects in the mouse fibroblast NIH3T3 and the normal mouse mammary gland cell NMuMG but not in the mouse mammary tumour epithelial GR cells. The -428 to -364 element therefore contributes to the cell type specific expression at MMTV LTR promoter. We have also identified another regulatory element between -1094 and -739 that had a slight positive regulatory effect at the MMTV LTR promoter but greatly enhanced expression at a foreign promoter when present at this promoter in an orientation that is the reverse of its own orientation in the MMTV LTR. This orientation-dependent effect was only observed in the mouse mammary epithelial cells NMuMG and GR but not in mouse fibroblastic cell line NIH3T3. This element may be important in regulating the expression of neighbouring genes in a cell type specific manner. These results show that the MMTV LTR contains multiple regulatory elements necessary for the control of expression at its own promoter and the expression of neighbouring genes.

Animals↗
PubMed1990

Radiation-induced activation of transcription factors in mammalian cells.

In mammalian cells radiation induces the enhanced transcription of several genes. The cis acting elements in the control region of inducible genes have been delimited by site directed mutagenesis. Several different elements have been found in different genes. They do not only activate gene transcription in response to radiation but also in response to growth factors and to tumor promoter phorbol esters. The transcription factors binding to these elements are present also in non-irradiated cells, but their DNA binding activity and their transactivating capability is increased upon irradiation. The signal chain linking the primary radiation-induced signal (damaged DNA) to the activation of transcription factors involves the action of (a) protein kinase(s).

Animals↗
PubMed1990

Transforming growth factor-beta represses transcription of the mouse mammary tumour virus DNA in cultured mouse mammary cells.

Increased expression of mouse mammary tumour virus (MMTV) is associated with hyperplastic alveolar growth and subsequent development of mammary cancers in the mouse. The expression of this virus is repressed when mammary tumour cells undergo sarcomatous transformation. We have demonstrated that a spontaneous progression of mouse mammary adenocarcinoma cells into highly malignant cells with the transformed phenotype is accompanied by an increased expression of transforming growth factors alpha and beta (TGF alpha and TGF beta), as well as a decreased expression of MMTV. Mouse mammary adenocarcinoma cells transformed with activated ras oncogene also expressed high levels of the transforming growth factors and a low level of MMTV. Thus a reverse correlation exists between the increased expression of the transforming growth factors and a low level of expression of MMTV. Mouse mammary cells that express high levels of MMTV when treated with exogenous TGF alpha and TGF beta 1 showed a down regulation of MMTV expression in response to TGF beta 1 but not to TGF alpha. These results demonstrate that the repression of MMTV expression in mouse mammary tumour progression may be due in part to an increased expression of TGF beta.

Animals↗
PubMed1989

Autoregulation of fos: the dyad symmetry element as the major target of repression.

Fos and Jun co-operatively repress the fos promoter. Removal of all putative Fos/Jun binding sites from the fos promoter neither obliterates the repression by Fos/Jun in transient cotransfection experiments in NIH3T3 cells nor the turn-off kinetics of serum-induced fos expression in stably transfected NIH3T3 cells. The dyad symmetry element (DSE) suffices to subject a promoter to this type of repression. However, one of the putative Fos/Jun binding sites (-292 to -299 and thus located immediately adjacent to the DSE), determines the very low level of basal expression.

Animals↗
PubMed1989

The regulation of expression of mouse mammary tumor virus DNA by steroid hormones and growth factors.

Mouse mammary tumor virus (MMTV) expression is associated with hyperplastic alveolar growth and subsequent development of mammary cancers in the mouse. The expression of this virus is also controlled by factors involved in the normal proliferation and differentiation of the mammary epithelium. During pregnancy when the mammary gland undergoes massive proliferation, MMTV expression is increased. Steroid hormones and growth factors that play an important role in the proliferation of mammary gland cells are responsible for the increased MMTV expression. In sarcomatous transformation of mouse mammary epithelial cells, MMTV expression is repressed. This repression is due to negative control of MMTV expression by transforming growth factor-beta (TGF beta). This growth factor is produced in high amounts when mammary epithelial cells progress into the transformed state. The expression of MMTV is therefore under multiple control by steroid hormones and growth factors.

Animals↗
PubMed1989

'Nuclear' oncogenes convert extracellular stimuli into changes in the genetic program.

Recent discoveries have advanced our understanding of oncogene action: transcription factors are encoded by cellular (nuclear) oncogenes and the activity of transcription factors is modulated in response to extracellular stimuli. Moreover, the nuclear oncoproteins appear to represent the gene-proximal ends of a network of cellular oncoproteins that link extracellular stimuli to the transcriptional machinery and thus to the program of genes a cell will express.

Gene Expression Regulation↗
PubMed1989

UV-induced transcription from the human immunodeficiency virus type 1 (HIV-1) long terminal repeat and UV-induced secretion of an extracellular factor that induces HIV-1 transcription in nonirradiated cells.

UV irradiation, but not visible sunlight, induces the transcription of human immunodeficiency virus type 1 (HIV-1). Chimeric constructs carrying all or parts of the HIV-1 long terminal repeat linked to an indicator gene were transfected into HeLa cells or murine and human T-cell lines, and their response to irradiation was tested. The cis-acting element conferring UV responsiveness is identical to the sequence binding transcription factor NF kappa B. UV irradiation enhances NF kappa B binding activity as assayed by gel retardation experiments. Interestingly, the requirement for UV irradiation can be replaced by cocultivation of transfected cells with UV-irradiated nontransfected (HIV-1-negative) cells. A UV-induced extracellular protein factor is detected in the culture medium conditioned by UV-treated cells. The factor is produced upon UV irradiation by several murine and human cell lines, including HeLa, Molt-4, and Jurkat, and acts on several cells. These data suggest that the UV response of keratinocytes in human skin can be magnified and spread to deeper layers that are more shielded, including the Langerhans cells, and that this indirect UV response may contribute to the activation of HIV-1 in humans.

Cell Line↗
PubMed1989

Different regions of the estrogen receptor are required for synergistic action with the glucocorticoid and progesterone receptors.

Estrogen and progesterone or estrogen and glucocorticoid receptors functionally cooperate in gene activation if their cognate binding sites are close to one another. These interactions have been described as synergism of action of the steroid receptors. The mechanism by which synergism is achieved is not clear, although protein-protein interaction of the receptors is one of the favorite models. In transfection experiments with receptor expression vectors and a reporter gene containing estrogen and progesterone-glucocorticoid receptor binding sites, we have examined the effects that different portions of the various receptors have on synergism. N-terminal domains of the chicken progesterone and human glucocorticoid receptors, when deleted, abolished the synergistic action of these receptors with the estrogen receptor. Deletion of the carboxy-terminal amino acids 341 to 595 of the estrogen receptor produced a mutant receptor that could not trans-activate on its own. This mutant receptor did not affect the action of the glucocorticoid receptor but functioned synergistically with the progesterone receptor. We therefore conclude that the synergistic action of the receptors for estrogen and progesterone is mechanistically different from the synergistic action of the receptors for estrogen and glucocorticoid.

Animals↗
PubMed1989

The Fos and Jun/AP-1 proteins are involved in the downregulation of Fos transcription.

The low basal expression of Fos and the rapid and effective turn-off of serum induced Fos transcription is due to autoregulation. Fos and Jun/AP-1 protein cooperate in the repression mechanism. Overexpressions of Fos and Jun decrease basal and induced transcription from Fos-CAT constructs and from the endogenous gene in NIH3T3 cells. The introduction into cells of either antisense Fos or antisense Jun sequences leads to elevated basal Fos promoter activity. Gel retardation experiments with synthetic oligonucleotides define two target sequences in the Fos promoter which bind Fos-Jun/AP-1 (centering at about -296 and -60). In vivo competition with these oligonucleotides relieves repression.

Chloramphenicol O-Acetyltransferase↗
PubMed1988

Requirement for fos gene expression in the transcriptional activation of collagenase by other oncogenes and phorbol esters.

Transcription from the c-fos promoter and from minimal promoter constructs carrying the phorbol ester-responsive element [12-O-tetradecanoylphorbol-13-acetate (TPA) responsive element (TRE)] corresponding to the sequence in the human collagenase gene is activated by elevated levels of the oncogene products v-src, c-Ha-ras, activated c-Ha-ras, and v-mos, as well as by phorbol ester. Elevated c- or v-fos expression stimulates TRE-dependent transcription but represses the c-fos promoter. Antisense fos sequences abolish basal and induced transcription from TRE constructs and derepress the c-fos promoter. These results establish a key role for fos in signal transduction and implicate the fos protein as a trans-activating and -repressing molecule.

Enzyme Induction↗
PubMed1988

DNA sequences outside the receptor-binding sites differently modulate the responsiveness of the mouse mammary tumour virus promoter to various steroid hormones.

Glucocorticoids, progestins and androgens all induce the transcription of the mouse mammary tumour virus (MMTV) DNA upon binding of their respective receptors to the hormone response element (HRE). This element is located between -202 and -59 5' upstream of the start of transcription on the MMTV long terminal repeat (LTR) region. The HRE contains four repeats of the hexanucleotide 5'-TGTTCT-3' to which the steroid hormone receptors are thought to bind. To investigate the contribution of the individual receptor-binding sites and neighbouring sequences to the steroid hormone action at the MMTV LTR promoter, we mutated various regions of the HRE and studied their response in transfection experiments. Each of the four receptor-binding sites was found to contribute substantially to the overall induction of transcription by all the various steroid hormones tested. This indicates that each individual receptor-binding site on the HRE is important for maximum hormone response. Additionally, we identified four separate sequences outside the receptor binding sites that differentially modulated the response of the MMTV LTR promoter to various steroids. One of these sequences binds the cellular factor, NFI. Thus the interaction of trans-acting factors with sequences outside the hormone receptor-binding sites controls the hormone response of the MMTV LTR promoter.

Acetyltransferases↗
PubMed1988

Estrogen and progesterone receptor-binding sites on the chicken vitellogenin II gene: synergism of steroid hormone action.

The chicken vitellogenin II gene is transcriptionally activated by estrogens. In transient transfection experiments in human T47D cells that contain receptors for various steroids, we showed estradiol, progestin, and androgen responses of a chimeric chicken vitellogenin II construct. This construct consists of DNA sequences from -626 to -590 upstream of the start of transcription of the chicken vitellogenin gene linked to the herpes simplex virus thymidine kinase promoter driving the transcription of the bacterial chloramphenicol acetyltransferase gene. Treatment of the transfected T47D cells with a combination of estradiol and the progestin R5020 led to a superinduction of chloramphenicol acetyltransferase activity, showing a synergistic action of these two steroids. This synergism was not observed upon treatment of the transfected cells with estradiol and the androgen dihydrotestosterone. Using point mutations in the vitellogenin gene fragment, we showed in functional and in in vitro DNase I footprinting assays with a purified progesterone receptor that, for the synergistic action of estradiol and R5020 to occur, the progesterone receptor must be bound to the vitellogenin gene fragment. The progesterone receptor-binding site was localized at -610 to -590, close to the consensus sequence (-626 to -613) for estrogen receptor binding and function. We therefore demonstrate here that two different steroid hormones can be functionally synergistic through the interaction of their corresponding receptors with two different binding sites adjacent to one another.

Animals↗
PubMed1987

The hormone response element of the mouse mammary tumour virus DNA mediates the progestin and androgen induction of transcription in the proviral long terminal repeat region.

Mouse mammary tumour virus (MMTV) gene expression has been shown to be regulated by glucocorticoids. A hormone response element (HRE) located between -202 and -59 upstream of the start of transcription in the long terminal repeat (LTR) region of the proviral DNA is required for this induction. We have investigated the role played by the HRE in the induction of MMTV LTR transcription by other classes of steroid hormones. Chimaeric constructs containing the HRE and the authentic LTR promoter linked to an indicator gene or the HRE linked to an otherwise hormone insensitive promoter directing the transcription of an indicator gene, were transfected into the human mammary tumour cell line T47D. Transcription at the MMTV LTR promoter or at the previously hormone-insensitive promoter was induced by progestins and androgens but not by oestradiol in transfected cells that contained functional receptors for these hormones. These results identify the HRE as the cis-acting element that mediates the progestin and androgen induction of MMTV LTR transcription. The HRE is therefore a DNA element that is required not just for glucocorticoid but also for progesterone and androgen induction of MMTV LTR transcription.

Acetyltransferases↗
PubMed1986

The endogenous mouse mammary tumour virus locus Mtv-8 contains a defective envelope gene.

The Mtv-8 associated provirus, GR40, is not expressed in vivo. However, upon transfection into rat XC cells, the two MMTV specific mRNAs of 35S and 24S are transcribed. Further, the level of these transcripts is augmented when the transfected cells are grown in the presence of dexamethasone (Ponta et al., 1983). No virus can be detected in the medium of the transfected cells. Intracellular protein analysis of these transfected cells shows that although apparently authentic gag proteins are synthesized, a novel protein of 68 kDa is the only env related protein detectable. The sequence of the env gene of GR40 was determined and the predicted amino acid sequence of the env protein obtained. A premature termination codon is present 68 amino acids before the COOH-terminus of the known MMTV env precursor Pr73env. This would result in an env protein of about 68 kDa, in agreement with the size of the env protein found in GR40 transfected cells. The lack of the carboxy terminus may be responsible for the non-processing of the aberrant env precursor protein, p68.

Amino Acid Sequence↗
PubMed1985

Hormonal regulation of cell surface expression of the major histocompatibility antigen H-2Ld in transfected cells.

The murine major histocompatibility antigens are cell surface glycoproteins which play an important role in the recognition of foreign antigens by cytotoxic T lymphocytes. Modulation of the level of expression of histocompatibility antigens could therefore be useful for the study of the interaction between the antigen presenting cells and T lymphocytes. The glucocorticoid hormone-inducible promoter, located in the long terminal repeat of mouse mammary tumor virus, was used to replace the promoter region of a cloned H-2Ld class I gene. The chimeric gene was introduced into cultured cells. Glucocorticoid induction of MMTV LTR H-2Ld mRNA could be shown by blot analysis. An S1 nuclease protection assay indicated that the transfected cells accurately initiate the chimeric mRNA. Immunoprecipitation of H-2Ld protein with a specific monoclonal antibody showed inducibility also at the cellular protein level. Fluorescence-activated cell sorter analysis monitored a 3-fold increase of H-2Ld on the cell surface when the transfected cells were grown in the presence of dexamethasone. This increase of H-2Ld expression was accompanied by a corresponding decrease on the cell surface of the endogenous H-2Kk.

Animals↗