Nuclear translocation of the estradiol receptor: partial inhibition by ethidium bromide

Novel DiOC3 96-well real-time efflux assay for discovery of NorA efflux pump inhibitors in Staphylococcus aureus

The NorA efflux pump is one of the most studied efflux systems in Staphylococcus aureus and confers multidrug resistance to a variety of dyes and antimicrobial compounds. Hence, inhibition of the NorA efflux pump might be a viable option for restoring susceptibility to antibiotics like fluoroquinolones. Fluorescent real-time efflux assays are important tools to identify putative efflux pump inhibitors. Nevertheless, the number of available compounds for usage in Staphylococcus aureus is limited. Previously, a 3-dipropyloxacarbocyanine iodide (DiOC₃) efflux assay was published that circumvented problems associated with the usage of ethidium bromide, namely slow efflux and suggested mutagenicity. However, the DiOC₃ assay protocol was cuvette - based and therefore needs to be adapted to the 96-well plate format. Hence, we optimized this assay for usage with 96-well plates. The new assay allows for rapid high-throughput efflux pump inhibitor screening.

Stereochemical complementary of DNA and steroid agonists and antagonists

Modelling studies in our laboratories over the past decade have demonstrated that a variety of natural products exhibit stereochemical complementarity with nucleic acids. In the case of steroid hormones, the basic cyclopentanophenanthrene skeleton fits between base pairs in partially unwound double stranded DNA; heteroatoms on the steroids form stereospecific donor/acceptor linkages to hydrogen bonding heteroatoms on the DNA. Each of the hormones appears to fit best in the site, i.e. 5'-dTdG-3'.5'dCdA-3'; the pattern of donor/acceptor linkages is unique for each type of hormonal activity. Steroid hormone agonists fit into the same site as the parent hormone; degree of fit correlates with degree of hormonal activity. Steroid hormone antagonists (e.g. RU 486; tamoxifen; anandron) fit into the same site as the agonists but possess different donor/acceptor linkages than the parent hormone; these linkages occur within the site between the base pairs or along the outside surface of the DNA helix in the major or minor grooves. A chronological review of the underlying concepts and observations leading to these discoveries is presented. Work in progress and some potential implications of the emerging technology are also discussed.

The stereochemical complementarity of DNA and reproductive steroid hormones correlates with biological activity

Modeling studies revealed that progesterone, testosterone, and estradiol are stereochemically complementary to the cavity formed between base pairs in the DNA sequence, 5'-dTdG-3' X 5'-dCdA-3'. Each steroid aligned precisely with the topography of the cavity and formed 2 stereospecific hydrogen bonds linking phosphate oxygens on adjacent DNA strands. Hydrogen bonding donor-acceptor relationships were different for each hormone. The remarkable stereochemical specificity of the hormone-DNA complexes was demonstrated by the lack of complementarity of steroid enantiomers and steroid analogs having alternate ring systems and/or changes in the position of functional groups. Fit of molecules into DNA in the manner of the parent hormone correlated with biological activity. Antagonists also fit into the cavity but differed from agonists in their hydrogen bonding linkages to DNA and/or extended out of the cavity beyond the helix. Unlike flat intercalating agents which form stable complexes with DNA, wedge shaped steroids may thus be capable of forming reversible sequence-specific complexes with DNA. We conclude that the stereochemistry of DNA can be used to predict hormonal activity.

Apparent stereochemical complementarity of estrogens and helical cavities between DNA base pairs: implications for the mechanism of action of steroids

The shape of the space occupied by a model of the estrogenic steroid hormone estradiol-17 beta conforms closely to a helical cavity between neighboring base pairs in partially coiled B-DNA. The orientation of estradiol-17 beta when fitted into DNA allows stereochemically complementary hydrogen bonding of both the 3- and 17 beta-hydroxyl groups to phosphate oxygens of the deoxyribose-phosphate backbone on adjacent strands. Changes in the chirality (handedness) of the steroid skeleton or in the absolute stereochemistry of hydrogen bonding groups prevent formation of complementary fits in the DNA. Synthetic estrogens can also adopt conformations which are stereochemically complementary to the cavities between base pairs. The complementary relationships between active estrogens and nucleic acids may be related to constraints on the evolution of the structure and the biological function of steroids.

Enhanced hormonal responsiveness in mammary glands from parous mice: molecular mechanisms

Mammary glands from parous mice required lower concentrations of hormones in vitro than those from virgins to effect differentiation, as measured by lactose synthetase activity. This phenomenon could not be explained by changes in receptor levels, since both mammary gland insulin and prolactin binding, although elevated at midpregnancy, returned to baseline in tissue from parous mice. Ethidium bromide, an intercalating dye, was a potent inhibitor of lactose synthetase induction in explants from virgins but much less potent in tissue from pregnant mice; explants from parous animals displayed intermediate sensitivity, suggesting that DNA structure was permanently altered. However, casein synthesis in glands from parous mice required hormone concentrations as high as in virgins and are just as susceptible to ethidium bromide as in virgins. Similarly, the vulnerability of the casein gene to DNAase I digestion is low in mammary epithelial cells from virgins, high in cells from pregnant mice, and low again in cells from parous animals. These data suggest that during the first pregnancy of mice, there are changes in the chromatin configuration that may facilitate the transcription of milk-related mRNA. Furthermore, after mammary gland involution these changes in the casein gene undergo reversion, while those involved with lactose synthetase activity persist; this may explain the disparate hormonal responsiveness seen in these animals with respect to casein and lactose synthesis.

Early effect of estrogen on chromatin ultrastructure in endometrial nuclei

The effect of estradiol on chromatin ultrastructure in interphase nuclei was studied in immature rat and lamb endometrium. Physiological doses of estradiol within the first hour transformed the condensed chromatin into dispersed chromatin both in vivo and in vitro. These ultrastructural modifications were specifically induced by hormones translocating the estrogen receptor to the nucleus of estrogen-responsive tissues. Conversely, the antiestrogen tamoxifen gave a hypercondensation of chromatin. The addition of actinomycin D, cordycepin or alpha-amanitin, but not of cycloheximide, prevented the effect of estradiol both on the ultrastructural change and on [3H]uridine incorporation, suggesting that chromatin decondensation was closely related to transcriptional activity. These results indicate that in endometrium, estrogen rapidly provokes a large and extended modification of chromatin ultrastructure, which suggest a general effect on chromatin function rather than a selective activation of a limited number of genes.

Characterization of the avian progesterone receptor through the use of inhibitors

Several chemical agents have been identified which block interaction of the avian progesterone receptor with isolated nuclei, ATP-Sepharose, DNA-cellulose or phosphocellulose. Four of these inhibitors, rifamycin AF/103, o-phenanthroline, aurintricarboxylic acid and pyridoxal 5-phosphate appear to block directly binding of the activated receptor complex to the above "acceptors." Another inhibitor, sodium molybdate, only blocks receptor interactions when added before receptor activation and therefore appears to interfere with the activation process. When nuclear receptor complexes were formed in vivo and labeled by nuclear exchange with [3H]progesterone in vitro, these complexes could not be disrupted by incubation of the nuclei with inhibitors. Therefore, the receptor complex bound in nuclei appears to be modified or masked in a way which resists the action of these chemical agents. These results indicate the value of inhibitors as chemical probes for the analysis of steroid receptors.