Inhibition of ligand induced promoter occupancy in vivo by a dominant negative RXR

BACKGROUND

Retinoid X receptors (RXRs) heterodimerize with other nuclear hormone receptors and control ligand mediated transcription. To address how RXRs function as heterodimers, we investigated activities of truncated RXR alpha and RXR beta that lack approximately 20 conserved C-terminal amino acids.

RESULTS

The truncated RXRs formed heterodimers and bound to respective DNA elements in vitro. By transient reporter assays we found that these RXRs act as dominant negative receptors and inhibit ligand dependent transcription by the retinoic acid receptor (RAR) and vitamin D receptor. P19 embryonal carcinoma cells stably expressing the truncated RXR beta (termed delta C2) were deficient in activating the endogenous RAR beta gene and an RA responsive reporter. To study the dominant negative activity of delta C2 further, genomic footprinting analysis was performed for the RAR beta2 promoter. In control P19 clones, the RA responsive element (RARE) and other elements in the promoter were protected after RA treatment. However, in delta C2 clones RA-induced protection was markedly inhibited at all elements.

CONCLUSIONS

These results indicate that the C-terminal region of RXR is required for full RARE occupancy in vivo, a RA dependent process that leads to the recruitment of other factors to the promoter and the subsequent transcriptional activation. Thus, RXRs play an integral role in ligand dependent transcription.

A water-vapour giga-maser in the active galaxy TXFS2226-184

Active galactic nuclei are thought to be powered by gas falling into a massive black hole; the different types of active galaxy may arise because we view them through a thick torus of molecular gas at varying angles of inclination. One way to determine whether the black hole is surrounded by a torus, which would obscure the accretion disk around the black hole along certain lines of sight, is to search for water masers, as these exist only in regions with plentiful molecular gas. Since the first detection of an extra-galactic water maser in 1979, they have come to be associated primarily with active galaxies, and have even been used to probe the mass of the central engine. Here we report the detection of a water giga-maser in the radio galaxy TXFS2226-184. The strength of the emission supports a recently proposed theory of maser pumping that allows for even more powerful masers, which might be detectable at cosmological distances. Water masers may accordingly provide a way to determine distances to galaxies outside the usual distance ladder, providing an independent calibration of the Hubble constant.

Displacement of sequence-specific transcription factors from mitotic chromatin

The general inhibition in transcriptional activity during mitosis abolishes the stress-inducible expression of the human hsp70 gene. Among the four transcription factors that bind to the human hsp70 promoter, the DNA-binding activities of three (C/EBP, GBP, and HSF1) were normal, while Sp1 showed reduced binding activity in mitotic cell extracts. In vivo footprinting and immunocytochemical analyses revealed that all of the sequence-specific transcription factors were displaced from promoter sequences as well as from bulk chromatin during mitosis. The correlation of transcription factor displacement with chromatin condensation suggests an involvement of chromatin structure in mitotic repression. However, retention of DNase I hypersensitivity suggests that the hsp70 promoter was not organized in a canonical nucleosome structure in mitotic chromatin. Displacement of transcription factors from mitotic chromosomes could present another window in the cell cycle for resetting transcriptional programs.

The efficacy of podiatric orthoses as an adjunct to the treatment of plantar ulceration in leprosy

This study examines the outcome of a management approach to plantar ulceration secondary to leprotic neuropathy. Locally-available resources were used to produce podiatric orthoses which were supplied to an experimental group. The effects on healing time and quality of healing were compared with a control group. Both groups were ambulant (the programme sought not to interfere with socioeconomic independence). Using standard nonparametric methods of analysis, it was demonstrated that the effects on healing rate, attributed to the experimental intervention, were highly significant. Over a 7-month period, 57% of the experimental group ulcers healed, while only 12.5% of the control group ulcers healed. The experimental intervention also demonstrated a positive effect on the quality of ulcer healing. This effect was not reflected in the control group.

Patterns of cocaine use among individuals in substance abuse treatment

Recognition of the heterogeneity among substance abusers has led to increased attention to the issue of client-treatment matching. One approach involves identifying different patterns of use within samples of substance abusers. The present preliminary report employed a cluster analysis of various aspects of cocaine use to examine subgroups of cocaine users within a sample of individuals in treatment for substance abuse. Three clusters emerged, differentiated primarily by quantity/frequency of use and route of administration. The resulting subgroups were compared on measures of external validity. Results suggest that subgroups of cocaine use patterns do exist within a sample of substance abusers. In addition, these subgroups differ in age of cocaine use onset and the severity of negative consequences. Future research is needed to establish the clinical utility of these clusters.

Nuclear disintegration in chicken peritoneal macrophages exposed to fumonisin B1 from Indian maize

The effects of fumonisin B1 (FB1) on the morphology of chicken peritoneal macrophages were studied. It was found that FB1-exposure caused nuclear disintegrations in peritoneal macrophages. The number of macrophages experiencing nuclear disintegration increased with the rise in the dose of FB1 from 6 to 18 micrograms ml-1. The finding implies that dietary intake of FB1 may damage the macrophages in consumers rendering the latter susceptible to infection.

Ligand-dependent occupancy of the retinoic acid receptor beta 2 promoter in vivo

Retinoic acid (RA) activates transcription of the RA receptor beta 2 (RAR beta 2) gene in embryonal carcinoma (EC) cells. This activation involves binding of the RAR/retinoid X receptor (RAR/RXR) heterodimer to the RA-responsive element (beta RARE). Dimethyl sulfate-based genomic footprinting was performed to examine occupancy of this promoter in P19 EC cells. No footprint was detected at the beta RARE prior to RA treatment, but a footprint was detected within the first hour of RA treatment. Concomitantly, other elements in the promoter, the cyclic AMP-responsive element and tetradecanoyl phorbol acetate-like-responsive element became footprinted. Footprints at these elements were induced by RA without requiring new protein synthesis and remained for the entire duration of RA treatment but rapidly reversed upon withdrawal of RA. A delayed protection observed at the initiator site was also reversed upon RA withdrawal. The RA-inducible footprint was not due to induction of factors that bind to these element, since in vitro assays showed that these factors are present in P19 cell extracts before RA treatment. Significantly, no RA-induced footprint was observed at any of these elements in P19 cells expressing a dominant negative RXR beta, in which RXR heterodimers are unable to bind to the beta RARE. Results indicate that binding of a liganded heterodimer receptor to the beta RARE is the initial event that allows other elements to gain access to the factors. In accordance, reporter analyses showed that a mutation in the beta RARE, but not those in other elements, abrogates RA activation of the promoter. It is likely that the RAR beta 2 promoter opens in a hierarchically ordered manner, signalled by the occupancy of liganded heterodimers.

Dominant negative retinoid X receptor beta inhibits retinoic acid-responsive gene regulation in embryonal carcinoma cells

Retinoid X receptors (RXRs) heterodimerize with multiple nuclear hormone receptors and are thought to exert pleiotropic functions. To address the role of RXRs in retinoic acid- (RA) mediated gene regulation, we designed a dominant negative RXR beta. This mutated receptor, termed DBD-, lacked the DNA binding domain but retained the ability to dimerize with partner receptors, resulting in formation of nonfunctional dimers. DBD- was transfected into P19 murine embryonal carcinoma (EC) cells, in which reporters containing the RA-responsive elements (RAREs) were activated by RA through the activity of endogenous RXR-RA receptor (RAR) heterodimers. We found that DBD- had a dominant negative activity on the RARE reporter activity in these cells. P19 clones stably expressing DBD- were established; these clones also failed to activate RARE-driven reporters in response to RA. Further, these cells were defective in RA-induced mRNA expression of Hox-1.3 and RAR beta, as well as in RA-induced down-regulation of Oct3 mRNA. Gel mobility shift assays demonstrated that RA treatment of control P19 cells induces RARE-binding activity, of which RXR beta is a major component. However, the RA-induced binding activity was greatly reduced in cells expressing DBD-. By genomic footprinting, we show that RA treatment induces in vivo occupancy of the RARE in the endogenous RAR beta gene in control P19 cells but that this occupancy is not observed with the DBD- cells. These data provide evidence that the dominant negative activity of DBD- is caused by the lack of receptor binding to target DNA. Finally, we show that in F9 EC cells expression of DBD- leads to inhibition of the growth arrest that accompanies RA-induced differentiation. Taken together, these results demonstrate that RXR beta and partner receptors play a central role in RA-mediated gene regulation and in the control of growth and differentiation in EC cells.

A regulatory element in the beta 2-microglobulin promoter identified by in vivo footprinting

Expression of the beta 2-microglobulin (beta 2-m) and major histocompatibility complex (MHC) class I genes is coordinately regulated. By ligation-mediated polymerase chain reaction, we have analyzed in vivo factor binding to the promoter region of the murine beta 2-m gene. In adult spleen, in which beta 2-m is expressed, strong protection was found in three elements. Two of these elements, the beta 2-m NF-kappa B binding site and the interferon consensus sequence, are homologous to the regulatory elements of the MHC class I genes and were also found to be protected in spleen. A third protected element, PAM, identified in this work, is unique to the beta 2-m gene. None of the elements showed protection in brain tissue, in which neither the beta 2-m nor the MHC class I gene is expressed. In vivo footprinting was also performed with F9 embryonal carcinoma cells, in which expression of the beta 2-m and MHC class I genes is induced at a low level only upon stimulation with retinoic acid (RA). No in vivo protection was detected before and after RA treatment of F9 cells, indicating that RA induction of beta 2-m (and MHC class I) expression occurs without detectable in vivo factor occupancy, whereas EL4 T lymphocytes expressing beta 2-m at a high level exhibited strong protection similar to that in spleen. Despite the lack of in vivo occupancy, the nuclear factors specific for each of the three elements were present in brain tissue and F9 cells as well as in spleen tissue and EL4 cells. We show that PAM, an element identified by its in vivo protection, binds nuclear factors ranging from 40 to 50 kDa in size and is capable of enhancing transcription of a reporter in F9 and other cells. Taken together, these results indicate that in vivo factor occupancy for the beta 2-m and MHC class I promoters is coordinated and occurs through a mechanism other than mere expression of relevant factors.

Occupancy of upstream regulatory sites in vivo coincides with major histocompatibility complex class I gene expression in mouse tissues

The major histocompatibility complex (MHC) class I HLA-B7 transgene carrying a 660-bp upstream sequence is expressed in the mouse with tissue specificity that parallels that of the expression of endogenous mouse MHC class I (H-2) genes. We have performed in vivo genomic footprinting for the HLA-B7 transgene and the endogenous H-2Kb gene. We show that the upstream region of both the transgene and the endogenous gene was extensively occupied in spleen tissue, where these genes are expressed at high levels. In contrast, no occupancy was detected in brain tissue, where expression of these genes is virtually absent. Sites exhibiting in vivo protection correspond to cis elements previously shown to bind to nuclear factors in vitro, including the constitutive enhancer region I and the interferon response element. The strongest tissue-specific protection was detected at site alpha, located downstream from the interferon response element. Site alpha bound a constitutively expressed nuclear factor(s) in vitro that exhibited an overlapping specificity which may involve a nuclear hormone receptor, RXR, and an AP-1-related factor. Site alpha was functional in vivo, as it enhanced MHC class I transcription in lymphocytes. These results show that the tissue-specific occupancy of the MHC class I regulatory sequences in vivo correlates with their expression and suggest that in vivo occupancy is controlled by a mechanism other than the mere presence of factors capable of binding to these sites. Our results suggest that a sequence present in the 660-bp upstream region in a human leukocyte antigen gene directs tissue-specific occupancy of MHC class I genes in vivo, independently of their position and copy number, illustrating a potential advantage of using a transgene for delimitation of the sequence requirement for in vivo occupancy.

The AP-1 site and the cAMP- and serum response elements of the c-fos gene are constitutively occupied in vivo

The c-fos proto-oncogene is inducible by cAMP, phorbol esters, serum, and growth factors. The induction by cAMP is mediated by the conserved cAMP response element (CRE), while induction by phorbol esters, serum, and growth factors requires a distal element called the serum response element (SRE). In addition to these elements, a consensus AP-1 transcription factor binding site is located next to SRE. Upstream regions of the mouse and human c-fos genes were footprinted in vivo by the ligation-mediated polymerase chain (PCR). Our results show that all three elements are constitutively protected in mouse liver and lung and in cultured human A431 cells. No major change in the protection profile was detected in A431 cells following stimulation with epidermal growth factor or in mice at birth, when c-fos is known to be induced. These results suggest that the inducible cis elements of the c-fos gene are poised, ready to respond immediately to external signals.

Differentiation and oncogenesis: phenotypically distinct lens tumors in transgenic mice

We report on two lines of transgenic mice that express a murine alpha A-crystallin/SV40 tumor antigen fusion gene in the eye lens. The alpha T1 line develops fast growing, poorly differentiated lens tumors, whereas the alpha T2 line produces lens tumors that are slow growing and well differentiated. There is a striking difference between these two lines in the temporal and spatial patterns of tumor antigen expression during initial lens development. In the alpha T1 line, the transgene is expressed very early in development in most lens cells, and no primary fiber differentiation takes place. In the alpha T2 line, transgene expression occurs after primary fiber formation has been initiated, and is restricted to differentiating fiber cells. The anterior epithelium from both alpha T lines undergoes normal development and remains morphologically normal until after birth, although in alpha T1 mice, these anterior cells produce considerable amounts of SV40 tumor antigens. This suggests that the state of differentiation of the lens cell plays an important role in its response to oncogene products.

Cell-specific induction of mouse Cyp1a1 mRNA during development

The dioxin-inducible cytochrome P(1)450 (Cyp1a1 gene) and P(3)450 (Cyp1a2 gene) enzymes have been implicated in the metabolism of numerous polycyclic hydrocarbons and arylamines, respectively. The prototypic inducer 3-methylcholanthrene was given to the pregnant mouse, and the intrauterine response was examined with the use of in situ hybridization. During the early postimplantation stage, inducible Cyp1a1 mRNA is detected in specific cell types in the extraembryonic tissues only. This selective expression along with the lack of detectable constitutive Cyp1a1 and constitutive or inducible Cyp1a2 gene transcripts between 5.3 and 14.5 days of gestation suggest that (i) these two genes appear to play no endogenous role during differentiation and (ii) the metabolic activity of the inducible Cyp1a1 enzyme may be important to the embryo and fetus from the standpoint of protection against toxic foodstuff and other environmental chemicals.

Effect of high tibial osteotomy on upper tibial venous drainage: study by intraosseous phlebography in primary osteoarthritis of knee joint

The effect of high tibial osteotomy on the upper tibial venous pattern in primary painful osteoarthritis of the knee joint was studied by preoperative and three to six months postoperative intraosseous phlebograms. The normal phlebographic pattern was established by phlebograms in five patients with normal knees. The preoperative engorgement, tortuosity of the medullary sinusoids, and slow dye clearance showed a remarkable conversion to a near normal appearance after the operation. Rest pain disappeared in all patients after the osteotomy, suggesting that venous congestion is the cause of 'rest pain'.

Affinity binding of non-histone chromatin proteins to the X chromosome of Drosophila by in situ chromatin reconstitution and its significance

Cytophotometric analysis of the in situ binding affinity of non-histone chromosomal protein (NHCP) to the polytenic X chromosome and autosome of Drosophila melanogaster has been carried out using Feulgen-Napthol Yellow S staining technique. The results reveal that the mean transformed absorbance ratio (male:female) with a 547 nm interference band filter for the two specific segments of the X chromosome is close to 0.5, while for a specific segment of an autosome it is close to 1.0, in the two sets of control; namely, the positive control (no treatment) and the negative control (treated with 1 M-urea+2M-NaCl) as well as in the reconstituted chromosomal preparations, which received 1 M-urea+2M-NaCl and the NHCP isolated from D. melanogaster. In contrast, the transformed absorbance ratios (male:female) with a 433 nm interference band filter yielded an interestingly different result. The ratios with a 433 nm filter for the X chromosome segments are significantly greater than 0.5 in all three sets of experiments. This finding by itself suggests that the NHCP binding affinity is dissimilar for the X chromosomes of male and female. When the 433 to 547 nm absorbance ratios were compared among the three sets, the data clearly revealed that in both positive control and NHCP reconstituted samples, the absorbance ratios (i.e. 433:547 nm) are significantly different between X chromosomes from males and those from females, while they are different between autosomes from males and females. The ratios are also not significantly different between male and female, either for the X chromosome or for the autosome in the negative control. These findings, therefore, suggest that there is a stronger binding affinity of NHCP for the male X chromosome of Drosophila, and reinstate the view that the X-chromosomal hyperactivity in male Drosophila is the consequence of a regulated organizational change in the DNA template.