Functions of the aryl hydrocarbon receptor (AHR) beyond the canonical AHR/ARNT signaling pathway

The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor regulating adaptive and maladaptive responses toward exogenous and endogenous signals. Research from various biomedical disciplines has provided compelling evidence that the AHR is critically involved in the pathogenesis of a variety of diseases and disorders, including autoimmunity, inflammatory diseases, endocrine disruption, premature aging and cancer. Accordingly, AHR is considered an attractive target for the development of novel preventive and therapeutic measures. However, the ligand-based targeting of AHR is considerably complicated by the fact that the receptor does not always follow the beaten track, i.e. the canonical AHR/ARNT signaling pathway. Instead, AHR might team up with other transcription factors and signaling molecules to shape gene expression patterns and associated physiological or pathophysiological functions in a ligand-, cell- and micromilieu-dependent manner. Herein, we provide an overview about some of the most important non-canonical functions of AHR, including crosstalk with major signaling pathways involved in controlling cell fate and function, immune responses, adaptation to low oxygen levels and oxidative stress, ubiquitination and proteasomal degradation. Further research on these diverse and exciting yet often ambivalent facets of AHR biology is urgently needed in order to exploit the full potential of AHR modulation for disease prevention and treatment.

Unraveling the differential impact of PAHs and dioxin-like compounds on AKR1C3 reveals the EGFR extracellular domain as a critical determinant of the AHR response

Polycyclic aromatic hydrocarbons (PAHs), dioxin-like compounds (DLCs) and structurally-related environmental pollutants may contribute to the pathogenesis of various diseases and disorders, primarily by activating the aryl hydrocarbon receptor (AHR) and modulating downstream cellular responses. Accordingly, AHR is considered an attractive molecular target for preventive and therapeutic measures. However, toxicological risk assessment of AHR-modulating compounds as well as drug development is complicated by the fact that different ligands elicit remarkably different AHR responses. By elucidating the differential effects of PAHs and DLCs on aldo-keto reductase 1C3 expression and associated prostaglandin D₂ metabolism, we here provide evidence that the epidermal growth factor receptor (EGFR) substantially shapes AHR ligand-induced responses in human epithelial cells, i.e. primary and immortalized keratinocytes and breast cancer cells. Exposure to benzo[a]pyrene (B[a]P) and dioxin-like polychlorinated biphenyl (PCB) 126 resulted in a rapid c-Src-mediated phosphorylation of EGFR. Moreover, both AHR agonists stimulated protein kinase C activity and enhanced the ectodomain shedding of cell surface-bound EGFR ligands. However, only upon B[a]P treatment, this process resulted in an auto-/paracrine activation of EGFR and a subsequent induction of aldo-keto reductase 1C3 and 11-ketoreduction of prostaglandin D₂. Receptor binding and internalization assays, docking analyses and mutational amino acid exchange confirmed that DLCs, but not B[a]P, bind to the EGFR extracellular domain, thereby blocking EGFR activation by growth factors. Finally, nanopore long-read RNA-seq revealed hundreds of genes, whose expression is regulated by B[a]P, but not by PCB126, and sensitive towards pharmacological EGFR inhibition. Our data provide novel mechanistic insights into the ligand response of AHR signaling and identify EGFR as an effector of environmental chemicals.

Investigations on the use of C-21-steroids as precursors for placental oestrogen synthesis in the cow

Cotyledonary homogenates from 220 and 270 day pregnant and term cows were incubated (NADPH- and NAD(+)-regenerating system) with 3H-pregnenolone and 3H-progesterone, respectively. Identification of metabolites was based on separation on HPLC and the respective retention times. On days 220/270 the major metabolite formed after incubation with 3H-pregnenolone was progesterone, followed by 17 alpha-hydroxyprogesterone and dehydroepiandrosterone/17 alpha-hydroxypregnenolone; the formation of estrone was low (up to 6%), while it was the major metabolite after incubation of pregnenolone with term placenta. At all stages of pregnancy investigated, the only metabolite found after incubation with 3H-progesterone was 17 alpha-hydroxyprogesterone. These data provide evidence that 17 alpha-hydroxyprogesterone is the endpoint of steroid biosynthesis in the bovine placenta along the delta 4-route and that oestrogen synthesis follows the delta 5-pathway. Based on the high activity of 3 beta-hydroxysteroiddehydrogenase/delta 5/4-isomerase also found on days 220/270, the key enzyme allowing for placental oestrogen synthesis in the cow seems to be cytochrome P450c17 alpha. Thus the situation in the cow is similar to that in the sheep and the increased turnover of pregnenolone into estrone may explain the decrease of placental progesterone production in the cow towards the end of gestation.

The role of AtMUS81 in DNA repair and its genetic interaction with the helicase AtRecQ4A

The endonuclease MUS81 has been shown in a variety of organisms to be involved in DNA repair in mitotic and meiotic cells. Homologues of the MUS81 gene exist in the genomes of all eukaryotes, pointing to a conserved role of the protein. However, the biological role of MUS81 varies between different eukaryotes. For example, while loss of the gene results in strongly impaired fertility in Saccharomyces cerevisiae and nearly complete sterility in Schizosaccharomyces pombe, it is not essential for meiosis in mammals. We identified a functional homologue (AtMUS81/At4g30870) in the genome of Arabidopsis thaliana and isolated a full-length cDNA of this gene. Analysing two independent T-DNA insertion lines of AtMUS81, we found that they are sensitive to the mutagens MMS and MMC. Both mutants have a deficiency in homologous recombination in somatic cells but only after induction by genotoxic stress. In contrast to yeast, no meiotic defect of AtMUS81 mutants was detectable and the mutants are viable. Crosses with a hyperrecombinogenic mutant of the AtRecQ4A helicase resulted in synthetic lethality in the double mutant. Thus, the nuclease AtMUS81 and the helicase AtRecQ4A seem to be involved in two alternative pathways of resolution of replicative DNA structures in somatic cells.

Comparison of the contact surface pattern between virtual and milled Cerec 3D full-ceramic crowns

PURPOSE

The objective of this study was to compare resulting contact point patterns in the context of crown production with the Cerec 3D system. The contact surface patterns of full-ceramic crowns milled with the Cerec 3D system were examined for agreement with the corresponding virtually produced crowns.

MATERIALS AND METHODS

The agreement was determined with regard to the parameters number, size, position, and shape. Twenty-eight models were mounted centrally in a mechanical articulator. In these model cases, at least one second premolar and one first molar in the maxilla and mandible had to be prepared for crown production (preparation). A total of 50 teeth was prepared. Crowns were then designed with the aid of the Cerec 3D system. The crown designs were milled and integrated in the model. Silicone bite registrations, the contact point patterns of which were compared with the virtually designed crowns, were produced in static occlusion in the region of the crowns. Different evaluation methods were used for the comparison.

RESULTS

There was an agreement of 78% with regard to number, 76% in terms of localization, 65% regarding size, and 65% in terms of shape.

CONCLUSION

It was thus confirmed that very good, highly reproducible results can be achieved with the Cerec 3D system.

Molecular characterization of homologues of both subunits A (SPO11) and B of the archaebacterial topoisomerase 6 in plants

The Spo11 protein is an eukaryotic homologue of the topoisomerase 6 subunit A from archaebacteria. In yeast Spo11p has been found to bind covalently to double-strand breaks (DSBs) during meiosis. Single homologues of the SPO11 gene exist in various eukaryotes, except plants. Previously, we found in the Arabidopsis thaliana genome two ancient paralogs, AtSPO11-1 and 2. Here we report on the molecular characterization of a third one, AtSPO11-3. This puzzling finding might be explained by the fact that we detected additionally--for the first time outside of the archaebacterial kingdom--a homologue of the subunit B of topoisomerase 6, AtTOP6B. Both AtSPO11-3 and AtTOP6B are abundantly expressed in Arabidopsis and EST comparisons indicate the presence of both genes in various plant species. Via two hybrid studies we could demonstrate that full length AtTop6B is able to interact with AtSpo11-2 and 3 but not with AtSpo11-1. Our data suggest that plants possess in contrast to other eukaryotes an additional archaebacterial kind of topoisomerase.

Molecular characterisation of RecQ homologues in Arabidopsis thaliana

Members of the RecQ family of DNA helicases are involved in processes linked to DNA replication, DNA recombination and gene silencing. RecQ homologues of various animals have been described recently. Here, for the first time for plants, we characterised cDNAs of all in all six different RecQ-like proteins that are expressed to different extents in Arabidopsis thaliana. Surprisingly, three of these proteins are small in size [AtRecQl1, AtRecQl2, AtRecQl3-606, 705 and 713 amino acids (aa), respectively], whereas the two bigger proteins result from a duplication event during plant evolution [AtRecQl4A and AtRecQl4B-1150 and 1182 aa, respectively]. Another homologue (AtRecQsim, 858 aa) most probably arose by insertion of an unrelated sequence within its helicase domain. The presence of these homologues demonstrates the conservation of RecQ family functions in higher eukaryotes. We also detected a small gene (AtWRNexo) encoding 285 aa which, being devoid of any RecQ-like helicase domain, reveals a striking homology to the exonuclease domain of human Werner protein, a prominent RecQ helicase of larger size. By means of the two-hybrid assay we were able to detect an interaction between AtWRNexo and AtRecQl2, indicating that activities that reside in a single protein chain in mammals might in plants be complemented in trans.

Molecular characterisation of two paralogous SPO11 homologues in Arabidopsis thaliana

The Spo11 protein of yeast has been found to be covalently bound to double-strand breaks in meiosis, demonstrating a unique role of the protein in the formation of these breaks. Homologues of the SPO11 gene have been found in various eukaryotes, indicating that the machinery involved in meiotic recombination is conserved in eukaryotes. Here we report on SPO11 homologues in plants. In contrast to what is known from other eukaryotes, Arabidopsis thaliana carries in its genome at least two SPO11 homologues, AtSPO11-1 and AtSPO11-2. Both genes are not more closely related to each other than to other eukaryotic SPO11 homologues, indicating that they did not arise via a recent duplication event during higher plant evolution. For both genes three different poly-adenylation sites were found. AtSPO11-1 is expressed not only in generative but also to a lesser extent in somatic tissues. We were able to detect in different organs various AtSPO11-1 cDNAs in which introns were differently spliced-a surprising phenomenon also reported for SPO11 homologues in mammals. In the case of AtSPO11-2 we found that the 3' end of the mRNA is overlapping with a mRNA produced by a gene located in inverse orientation next to it. This points to a possible antisense regulation mechanism. Our findings hint to the intriguing possibility that, at least for plants, Spo11-like proteins might have more and possibly other biological functions than originally anticipated for yeast.

Analysis of the hyperpolarizing effects of forskolin in guinea-pig atrial heart muscle

The effects of forskolin on action potential configuration and on both uptake and efflux of 86Rb+ were studied in guinea-pig left atria. The action potential was prolonged by forskolin in the plateau range but shortened at the end of repolarization; maximal upstroke velocity and amplitude of slow response potentials were enhanced. In partially depolarized preparations, the resting potential was increased by forskolin; this effect was not prevented by atropine 1 mumol/l. Forskolin augmented the rate constant of 86Rb+ efflux in beating and in resting preparations. The uptake of 86Rb+ was enhanced by forskolin in resting preparations. It is concluded that forskolin stimulates the Na+,K+-pump and activates a background potassium conductance. Both effects may account for the shortening effect of the drug on the action potential and the increase in resting potential seen in partially depolarized preparations.

[Ambulatory therapy of trichomoniasis vaginalis. Clinical trial of a new Trichomonas-active azide substance]

Report on the results of the clinical trial of a new trichomonacide in ambulatory patients in a gynaecological practice. A report is given on a new drug for the treatment of vaginal trichomoniasis, which has been tried in 87 patients of a gynaecological practice under outpatient conditions. The rate of failures was 5.7% (5 of 87 cases). The advantages over alternative preparations is to be seen in the short duration of treatment (24 hours at the most), and in the favourable tolerance in spite of high doses.