Assessment of Immunogenicity and Efficacy of CV0501 mRNA-Based Omicron COVID-19 Vaccination in Small Animal Models

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) Omicron and its subvariants (BA.2, BA.4, BA.5) represented the most commonly circulating variants of concern (VOC) in the coronavirus disease 2019 (COVID-19) pandemic in 2022. Despite high vaccination rates with approved SARS-CoV-2 vaccines encoding the ancestral spike (S) protein, these Omicron subvariants have collectively resulted in increased viral transmission and disease incidence. This necessitates the development and characterization of vaccines incorporating later emerging S proteins to enhance protection against VOC. In this context, bivalent vaccine formulations may induce broad protection against VOC and potential future SARS-CoV-2 variants. Here, we report preclinical data for a lipid nanoparticle (LNP)-formulated RNActive^® N1-methylpseudouridine (N1mΨ) modified mRNA vaccine (CV0501) based on our second-generation SARS-CoV-2 vaccine CV2CoV, encoding the S protein of Omicron BA.1. The immunogenicity of CV0501, alone or in combination with a corresponding vaccine encoding the ancestral S protein (ancestral N1mΨ), was first measured in dose-response and booster immunization studies performed in Wistar rats. Both monovalent CV0501 and bivalent CV0501/ancestral N1mΨ immunization induced robust neutralizing antibody titers against the BA.1, BA.2 and BA.5 Omicron subvariants, in addition to other SARS-CoV-2 variants in a booster immunization study. The protective efficacy of monovalent CV0501 against live SARS-CoV-2 BA.2 infection was then assessed in hamsters. Monovalent CV0501 significantly reduced SARS-CoV-2 BA.2 viral loads in the airways, demonstrating protection induced by CV0501 vaccination. CV0501 has now advanced into human Phase 1 clinical trials (ClinicalTrials.gov Identifier: NCT05477186).

A third dose of the unmodified COVID-19 mRNA vaccine CVnCoV enhances quality and quantity of immune responses

A third vaccine dose is often required to achieve potent, long-lasting immune responses. We investigated the impact of three 8 μg doses of CVnCoV, CureVac's SARS-CoV-2 vaccine candidate containing sequence-optimized unmodified mRNA encoding spike (S) glycoprotein, administered at 0, 4 and 28 weeks on immune responses in rhesus macaques. Following the third dose S-specific binding and neutralizing antibodies increased 50-fold compared with post-dose 2 levels, with increased responses also evident in the lower airways and against the SARS-CoV-2 B.1.1.7 (Alpha), B.1.351 (Beta), P.1 (Gamma) and B.1.617.2 (Delta) variants. Enhanced binding affinity of serum antibodies after the third dose correlated with higher somatic hypermutation in S-specific B cells, corresponding with improved binding properties of monoclonal antibodies expressed from isolated B cells. Administration of low dose mRNA led to fewer cells expressing antigen in vivo at the injection site and in the draining lymph nodes compared with a tenfold higher dose, possibly reducing the engagement of precursor cells with the antigen and resulting in the suboptimal response observed following two-dose vaccination schedules in phase IIb/III clinical trials of CVnCoV. However, when immune memory is established, a third dose efficiently boosts the immunological responses as well as improves antibody affinity and breadth.

mRNA vaccines induce rapid antibody responses in mice

mRNA vaccines can be developed and produced quickly, making them prime candidates for immediate outbreak responses. Furthermore, clinical trials have demonstrated rapid protection following mRNA vaccination. Thus, we sought to investigate how quickly mRNA vaccines elicit antibody responses compared to other vaccine modalities. We first compared the immune kinetics of mRNA and DNA vaccines expressing SARS-CoV-2 spike in mice. We observed rapid induction of antigen-specific binding and neutralizing antibodies by day 5 following mRNA (4 µg/mouse), but not DNA (50 µg/mouse), immunization. Comparing innate responses hours post immunization, the mRNA vaccine induced increased levels of IL-5, IL-6, and MCP-1 cytokines which maybe promoting humoral responses downstream. We then evaluated the immune kinetics of an HIV-1 mRNA vaccine in comparison to DNA, protein, and rhesus adenovirus 52 (RhAd52) vaccines of the same HIV-1 envelope antigen in mice. Again, induction of envelope-specific antibodies was observed by day 5 following mRNA vaccination, whereas antibodies were detected by day 7–14 following DNA, protein, and RhAd52 vaccination. Thus, eliciting rapid humoral immunity may be a unique and advantageous property of mRNA vaccines for controlling infectious disease outbreaks.

Optimization of non-coding regions for a non-modified mRNA COVID-19 vaccine

The CVnCoV (CureVac) mRNA vaccine for severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) was recently evaluated in a phase 2b/3 efficacy trial in humans1. CV2CoV is a second-generation mRNA vaccine containing non-modified nucleosides but with optimized non-coding regions and enhanced antigen expression. Here we report the results of a head-to-head comparison of the immunogenicity and protective efficacy of CVnCoV and CV2CoV in non-human primates. We immunized 18 cynomolgus macaques with two doses of 12 μg lipid nanoparticle-formulated CVnCoV or CV2CoV or with sham (n = 6 per group). Compared with CVnCoV, CV2CoV induced substantially higher titres of binding and neutralizing antibodies, memory B cell responses and T cell responses as well as more potent neutralizing antibody responses against SARS-CoV-2 variants, including the Delta variant. Moreover, CV2CoV was found to be comparably immunogenic to the BNT162b2 (Pfizer) vaccine in macaques. Although CVnCoV provided partial protection against SARS-CoV-2 challenge, CV2CoV afforded more robust protection with markedly lower viral loads in the upper and lower respiratory tracts. Binding and neutralizing antibody titres were correlated with protective efficacy. These data demonstrate that optimization of non-coding regions can greatly improve the immunogenicity and protective efficacy of a non-modified mRNA SARS-CoV-2 vaccine in non-human primates.

mRNA Vaccines Induce Rapid Antibody Responses in Mice

mRNA vaccines can be developed and produced quickly, making them attractive for immediate outbreak responses. Furthermore, clinical trials have demonstrated rapid protection following mRNA vaccination. We sought to investigate how quickly mRNA vaccines elicit antibody responses compared to other vaccine modalities. We first examined immune kinetics of mRNA and DNA vaccines expressing SARS-CoV-2 spike in mice. We observed rapid induction of antigen-specific binding and neutralizing antibodies by day 5 following mRNA, but not DNA, immunization. The mRNA vaccine also induced increased levels of IL-5, IL-6 and MCP-1. We then evaluated immune kinetics of an HIV-1 mRNA vaccine in comparison to DNA, protein, and rhesus adenovirus 52 (RhAd52) vaccines with the same HIV-1 envelope antigen in mice. Induction of envelope-specific antibodies was observed by day 5 following mRNA vaccination, whereas antibodies were detected by day 7-14 following DNA, protein, and RhAd52 vaccination. Eliciting rapid humoral immunity may be an advantageous property of mRNA vaccines for controlling infectious disease outbreaks.

IMPORTANCE

mRNA vaccines can be developed and produced in record time. Here we demonstrate induction of rapid antibody responses by mRNA vaccines encoding two different viral antigens by day 5 following immunization in mice. The rapid immune kinetics of mRNA vaccines can be an advantageous property that makes them well suited for rapid control of infectious disease outbreaks.

Safety and immunogenicity of an mRNA-lipid nanoparticle vaccine candidate against SARS-CoV-2 : A phase 1 randomized clinical trial

BACKGROUND

We used the RNActive® technology platform (CureVac N.V., Tübingen, Germany) to prepare CVnCoV, a COVID-19 vaccine containing sequence-optimized mRNA coding for a stabilized form of SARS-CoV‑2 spike (S) protein encapsulated in lipid nanoparticles (LNP).

METHODS

This is an interim analysis of a dosage escalation phase 1 study in healthy 18-60-year-old volunteers in Hannover, Munich and Tübingen, Germany, and Ghent, Belgium. After giving 2 intramuscular doses of CVnCoV or placebo 28 days apart we assessed solicited local and systemic adverse events (AE) for 7 days and unsolicited AEs for 28 days after each vaccination. Immunogenicity was measured as enzyme-linked immunosorbent assay (ELISA) IgG antibodies to SARS-CoV‑2 S‑protein and receptor binding domain (RBD), and SARS-CoV‑2 neutralizing titers (MN50).

RESULTS

In 245 volunteers who received 2 CVnCoV vaccinations (2 μg, n = 47, 4 μg, n = 48, 6 μg, n = 46, 8 μg, n = 44, 12 μg, n = 28) or placebo (n = 32) there were no vaccine-related serious AEs. Dosage-dependent increases in frequency and severity of solicited systemic AEs, and to a lesser extent local AEs, were mainly mild or moderate and transient in duration. Dosage-dependent increases in IgG antibodies to S‑protein and RBD and MN50 were evident in all groups 2 weeks after the second dose when 100% (23/23) seroconverted to S‑protein or RBD, and 83% (19/23) seroconverted for MN50 in the 12 μg group. Responses to 12 μg were comparable to those observed in convalescent sera from known COVID-19 patients.

CONCLUSION

In this study 2 CVnCoV doses were safe, with acceptable reactogenicity and 12 μg dosages elicited levels of immune responses that overlapped those observed in convalescent sera.

IABS/CEPI platform technology webinar: Is it possible to reduce the vaccine development time?

The International Alliance for Biological Standardization and the Coalition for Epidemic Preparedness Innovations organized a joint webinar on the use of platform technologies for vaccine development. To tackle new emerging infectious diseases, including SARS-CoV-2, rapid response platforms, using the same basic components as a backbone, yet adaptable for use against different pathogens by inserting new genetic or protein sequences, are essential. Furthermore, it is evident that development of platform technologies needs to continue, due to the emerging variants of SARS-CoV-2. The objective of the meeting was to discuss techniques for platform manufacturing that have been used for COVID-19 vaccine development, with input from regulatory authorities on their experiences with, and expectations of, the platforms.

Industry and regulators have been very successful in cooperating, having completed the whole process from development to licensing at an unprecedented speed. However, we should learn from the experiences, to be able to be even faster when a next pandemic of disease X occurs.

Drug biokinetic and toxicity assessments in rat and human primary hepatocytes and HepaRG cells within the EU-funded Predict-IV project

The overall aim of Predict-IV (EU-funded collaborative project #202222) was to develop improved testing strategies for drug safety in the late discovery phase. One major focus was the prediction of hepatotoxicity as liver remains one of the major organ leading to failure in drug development, drug withdrawal and has a poor predictivity from animal experiments. In this overview we describe the use and applicability of the three cell models employed, i.e., primary rat hepatocytes, primary human hepatocytes and the human HepaRG cell line, using four model compounds, chlorpromazine, ibuprofen, cyclosporine A and amiodarone. This overview described the data generated on mode of action of liver toxicity after long-term repeat-dosing. Moreover we have quantified parent compound and its distribution in various in vitro compartments, which allowed us to develop biokinetic models where we could derive real exposure concentrations in vitro. In conclusion, the complex data set enables quantitative measurements that proved the concept that we can define human relevant free and toxic exposure levels in vitro. Further compounds have to be analyzed in a broader concentration range to fully exploit these promising results for improved prediction of hepatotoxicity and hazard assessment for humans.

Comprehensive summary--Predict-IV: A systems toxicology approach to improve pharmaceutical drug safety testing

This special issue of Toxicology in Vitro is dedicated to disseminating the results of the EU-funded collaborative project "Profiling the toxicity of new drugs: a non animal-based approach integrating toxicodynamics and biokinetics" (Predict-IV; Grant 202222). The project's overall aim was to develop strategies to improve the assessment of drug safety in the early stage of development and late discovery phase, by an intelligent combination of non animal-based test systems, cell biology, mechanistic toxicology and in silico modeling, in a rapid and cost effective manner. This overview introduces the scope and overall achievements of Predict-IV.

Transcriptomic hepatotoxicity signature of chlorpromazine after short- and long-term exposure in primary human sandwich cultures

Drug-induced liver injury is the most frequent reason for market withdrawal of approved drugs, and is difficult to predict in animal models. Here, we analyzed transcriptomic data derived from short- and long-term cultured primary human hepatocytes (PHH) exposed to the well known human hepatotoxin chlorpromazine (CPZ). Samples were collected from five PHH cultures after short-term (1 and 3 days) and long-term (14 days) repeat daily treatment with 0.1 or 0.2 µM CPZ, corresponding to C(max). Two PHH cultures were additionally treated with 1 µM CPZ, and the three others with 0.02 µM CPZ. Differences in the total number of gene changes were seen between donors and throughout treatment. Specific transcriptomic hepatotoxicity signatures were created for CPZ and consisted of inflammation/hepatitis, cholestasis, and liver proliferation in all five donors, as well as fibrosis and steatosis, which were observed in four of five donors. Necrosis was present in three of five donors, and an indicative signature of cirrhosis was observed after long-term 14-day repeat treatment, also in three of five donors. The inter-donor variability in the inflammatory response to CPZ treatment was associated with variability in the strength of the response of the transcriptomic hepatotoxicity signatures, suggesting that features of inflammation could be related to the idiosyncratic hepatotoxic effects of CPZ in humans.

Development of stably transfected human and rat hepatoma cell lines for the species-specific assessment of xenobiotic response enhancer module (XREM)-dependent induction of drug metabolism

Based on our current knowledge, PXR holds a key position in the induction of a selective battery of enzymes and transporters of drug metabolism. In order to prevent serious adverse drug effects or unpredicted drug-drug interactions (DDI), it is compulsory to investigate the possible inducing potency of drugs under development. Furthermore, analysis of the inducing potency of environmental pollutants and new or manufactured chemicals is part of toxicological risk assessment. In non-transfected human HepG2 and rat H4IIE hepatoma cells, we examined the characteristics of expression of 45 genes involved in drug metabolism. A few gene products such as CYP2B6 or CYP3A4 mRNA were prominent in HepG2 cells while their major rat counterparts were, e.g., CYP2B3 or CYP3A1/3A3. Furthermore, a number of xenobiotic receptors including PXR were expressed in both cell lines. A number of genes were regulated in a cell type and species-specific manner after incubation with the prototypical PXR agonists rifampicin or dexamethasone, respectively. Then, we established cell-based reporter gene assays for screening for PXR-dependent induction of drug metabolism. HepG2 and H4IIE cells were stably transfected with a reporter gene containing PXR responsive elements (XREMs) which mediate the induction of PXR target genes such as CYP3A enzymes. With both stable cell lines the CYP inducers clotrimazole, dexamethasone, omeprazole, phenobarbital, rifampicin, as well as the drug candidate EMD 392949 and the brominated flame retardants hexabromocylododecane (HBCD) and a pentabromodiphenyl ether (pentaBDE) mixture were screened. In the human HepG2-XREM3 and rat H4IIE-XREM3 cells, clotrimazole and HBCD were found as common activators of the human and rat PXR whereas pentaBDE was more effective with the human cell system. Omeprazole and phenobarbital did not induce the rat PXR-dependent reporter gene expression in H4IIE-XREM3 cells, while a moderate increase was found in HepG2-XREM3 cells. EMD 392949 also acted as inducer in human but not in rat cells confirming in vivo observations. In summary, the established PXR-dependent in vitro system allows the simultaneous, fast, and species-specific screening of chemicals, environmental contaminants, food ingredients and drugs for CYP3A induction in cells of human and rat origin.

Activation of P53 in HepG2 cells as surrogate to detect mutagens and promutagens in vitro

The current genotoxicity tests of the standard in vitro battery, especially those using mammalian cells, are limited by their low specificity and highlight the importance of new in vitro tools. This study aimed to evaluate the suitability of HepG2 cells for assaying mutagens and promutagens. We determined P53 activity as surrogate genotoxicity endpoint in HepG2 cells. Our results revealed a significant P53-induction by actinomycin D, methyl methanesulfonate and etoposide. Prior to the investigation of promutagens we characterized HepG2 cells by analyzing the expression of 45 genes involved in xenobiotic metabolism and measuring the activity of selected Cytochrome-P450 (CYP) enzymes. We determined a limited metabolic capacity prompting us to employ a co-treatment with rat liver S9 as metabolic activation system (MAS) for promutagens. While cyclophosphamide showed an elevation of activated P53 in the presence of S9, 7,12-dimethylbenz[a]anthracene and aflatoxin B(1) responded without the MAS. Inhibition of cellular CYP3A4 or CYP1A/1B suppressed the aflatoxin B(1)- and dimethylbenz[a]anthracene-mediated P53 response, respectively, indicating that HepG2 cells are capable of metabolizing these compounds in a CYP1A/B/3A4-dependent manner. In summary, our results indicate that P53 activation in HepG2 cells combined with a MAS can be used for the identification of new (pro)genotoxicants.

Extracellular matrix modulates sensitivity of hepatocytes to fibroblastoid dedifferentiation and transforming growth factor beta-induced apoptosis

Hepatocytes in culture are a valuable tool to investigate mechanisms involved in the response of the liver to cytokines. However, it is well established that hepatocytes cultured on monolayers of dried stiff collagen dedifferentiate, losing specialized liver functions. In this study, we show that hepatocyte dedifferentiation is a reversible consequence of a specific signaling network constellation triggered by the extracellular matrix. A dried stiff collagen activates focal adhesion kinase (FAK) via Src, leading to activation of the Akt and extracellular signal-regulated kinase (ERK) 1/2 pathways. Akt causes resistance to transforming growth factor beta (TGF-beta)-induced apoptosis by antagonizing p38, whereas ERK1/2 signaling opens the route to epithelial-mesenchymal transition (EMT). Apoptosis resistance is reversible by inhibiting Akt or Src, and EMT can be abrogated by blocking the ERK1/2 pathway. In contrast to stiff collagen, a softer collagen gel does not activate FAK, keeping the hepatocytes in a state where they remain sensitive to TGF-beta-induced apoptosis and do not undergo EMT. In this culture system, inhibition of p38 as well as overexpression of constitutively active Akt causes apoptosis resistance, whereas constitutively active Ras induces EMT. Finally, we show that matrix-induced EMT is reversible by replating cells from dried stiff to soft gel collagen. Our results demonstrate that hepatocyte dedifferentiation in vitro is an active process driven by FAK-mediated Akt and ERK1/2 signaling. This leads to similar functional and morphological alterations as observed for regenerating hepatocytes in vivo and is reversible when Akt and/or ERK1/2 signaling pathways are antagonized.

CONCLUSION

Hepatocytes can exist in a differentiated and a dedifferentiated state that are reversible and can be switched by manipulating the responsible key factors of the signaling network.

Oestrogen receptors pathways to oestrogen responsive elements: the transactivation function-1 acts as the keystone of oestrogen receptor (ER)beta-mediated transcriptional repression of ERalpha

Oestrogen receptors (ER)alpha and beta modify the expression of genes involved in cell growth, proliferation and differentiation through binding to oestrogen response elements (EREs) located in a number of gene promoters. Transient transfection of different luciferase reporter vectors 3xEREs-Vit, 2xEREs-tk and ERE-C3 showed that the transactivation capacity of both ER subtypes was influenced by 1) the nature of the inducer (oestradiol (E2), phyto- and anti-oestrogen (AE)), 2) the structure of the promoter (nucleotidic sequence, number of ERE, length of the promoter sequence) and 3) the cell line (containing endogenous ER (MCF-7) or in which ER was stably expressed (MDA-MB-231-HE-5 (ERalpha+) or MDA-MB-231-HERB (ERbeta+)). ER subtype did not display the same efficacy on the different constructions in the presence of E2 and of AE according to the cell (e.g. in MCF-7 cells: tk>>Vit>>C3 approximately 0 while in MDA-MB-231 cells: Vit>>tk approximately C3). E2 response was higher in MCF-7 cells, probably due to higher ER expression level (maximal at 10(-10)M instead of 10(-8)M for E2 in HE-5 cells). Finally, the same ligand could exert opposite activities on the same promoter according to the ER isoform expressed: in the MDA-MB-231 cells, AE acted as inducers of the C3 promoter via ERbeta whereas ERalpha/AE complexes down-regulated this promoter. Approximately 70% of breast tumours express ER and most tumour cells coexpress both ER isotypes. Thus, different types of ER dimers can be formed in such tumours (ERbeta or ERalpha homodimers or ERalpha/ERbeta heterodimers). We therefore studied the influence of the coexistence of the two ERs on the ligand-induced transcriptional process following transient transfection of ERalpha in ERbeta+ cells, and inversely ERbeta in ERalpha+ cells. ERbeta-transfection inhibited the E2- and genistein-induced ERalpha-dependent transcription on all promoters in all cell lines except C3 in MCF-7; this inhibitory effect was lost following transfection of ERbeta deleted of its AF-1 (ERbeta-AF-2). These results suggest that the dominant negative properties of ERbeta are mainly due to its AF-1 function. Interestingly, transfection of an ERbeta-AF-2 construct into MCF-7 cells potentiated the transcription inhibitory capacity of 4-OH-tamoxifen (OHT) on the Vit and tk promoters. Thus, (1) OHT exerts an agonistic activity through the AF-1 function of ER and (2) expression of ERbeta in breast cancer cells seems to favour the AE treatment. Contrary to ERbeta, ERalpha-transfection had little effect on ERbeta transactivation capacity in HERB cells. Finally, the ratio ERalpha/ERbeta constitutes one decisive parameters to orientate the transcriptional mechanism of a target gene in the presence of agonist as well as of antagonist ligands.

Alternatives in Pharmaceutical Toxicology: Global and focussed approaches – Two case studies

Safety testing of potential drugs has been and will continue to be a challenging task for the toxicologist in the pharmaceutical industry. We present two examples for the use of target-specific cell models to detect and assess species-specific toxicity. In the first example, adrenal models based on primary cells as well as a permanent human adrenal cell line were used. Both cell systems enabled a good prediction of adrenal effects in rodents, non-rodents and humans. The second example made use of primary hepatocytes. In this project, a potential drug candidate showed unexpected toxicity in vitro as well as species-specific cytochrome P450 (CYP) induction in vivo. We therefore analysed CYP induction and gene expression signatures in rat and human hepatocytes as well as in samples from in vivo animal toxicity studies. By this approach, the rat hepatocyte model correctly predicted the effects observed in rats and the in vitro/in vivo comparison enabled a solid extrapolation of consequences in humans. These examples demonstrate that an intelligent testing strategy, using alternative methods, can enable a meaningful safety assessment for humans by adding a ''tailor-made'' range of technologies to ''classic'' toxicological methods.

Human reporter gene assays: Transcriptional activity of the androgen receptor is modulated by the cellular environment and promoter context

The androgen receptor (AR) is a member of the nuclear receptor superfamily and mediates the physiological effects of androgens. Androgens are essential for male development and disruption of androgen signaling may cause androgen-dependent developmental defects and/or tumors. Here we present a comparative analysis of various model systems for the investigation of endocrine active compounds in human cell lines. We generated reporter plasmids containing androgen response elements derived from the human secretory component or the rat probasin genes as well as the glucocorticoid consensus response element and compared their activities to that of the mouse mammary tumor virus promotor. Additionally, we generated an expression plasmid containing the AR cDNA derived from LNCaP cells. In 22Rv1 cells transiently transfected with human AR, all reporters displayed a dose-dependent, high activity when treated with androgens. Interestingly, the potency of testosterone and its metabolite dihydrotestosterone was very low in HepG2 but not in 22Rv1 cells, independent of the reporter used. The efficacies of the androgens tested were comparable in both cell lines but highly dependent on the reporter used. Based on these results, 22Rv1 cells provide a highly sensitive in vitro test system to analyze endocrine activities of xenobiotics. Furthermore, this study highlights the need to investigate the (anti-) androgenic activity of compounds in dependence of the cellular and promoter context.

Effects of cell culture conditions on primary rat hepatocytes—Cell morphology and differential gene expression

We incubated primary rat hepatocytes on collagen monolayer as well as in collagen sandwich cultures with serum-containing or serum-free medium formulations. Morphological monitoring of hepatocytes revealed that hepatocytes cultured on collagen monolayer adopted their polygonal shape and started to create aggregates earlier than sandwich-cultured cells. Bile canaliculi-like structures were observed in every cell culture system but were more prominent in serum-free cultures. Hepatocytes in collagen-sandwich configuration and serum-free medium were the most viable after 72 h of culture, still displaying polygonal shape, clear cytoplasm and stable canaliculi-like structures. Differential gene expression patterns were determined for each cell culture condition using quantitative TaqMan Low Density Arrays (LDA). Gene expression analysis revealed distinct profiles in monolayer versus sandwich cultures and in particular in serum-free versus serum-containing culture medium. The hepatocytes cultured in the collagen-sandwich with serum-free medium showed the least variation in expression values over time. Importantly, stress markers were not induced in the serum-free sandwich culture, in contrast to the monolayer and the serum-containing sandwich cultures. Additionally, expression of the investigated cytochrome P450 genes was maintained in the serum-free monolayer and the sandwich cultures. In conclusion, culturing primary rat hepatocytes in a sandwich between two layers of gelled collagen and in a serum-free medium formulation, appears to be most suitable for long-term in vitro hepatotoxicity screening.

Estrogen receptor alpha and beta subtype expression and transactivation capacity are differentially affected by receptor-, hsp90- and immunophilin-ligands in human breast cancer cells

In MCF-7 (estrogen receptor (ER)+) and in MDA-MB-231 (ER-) cells stably transfected with either estrogen receptor alpha (ERalpha) or beta (ERbeta) subtype (MDA-MB-231 stably transfected with the mouse ERalpha cDNA (MERA) and MDA-MB-231 stably transfected with the human ERbeta cDNA (HERB), respectively) N-term heat shock protein of 90kDa (hsp90) ligands (geldanamycin and radicicol) and C-term hsp90 ligands (novobiocin) decrease the basal and estradiol (E(2))-induced transcription activity of ER on an estrogen responsive element (ERE)-LUC reporter construct concomitantly with or 1h after E(2) treatment. All hsp90 ligands induced an E(2)- and MG132-inhibited decrease of both ER cell content. However, the kinetics of these degradations are slower than those induced by the selective estrogen receptor down-regulator RU 58668 (RU). This suggests that inhibition of the hsp90 ATPase activity targets both ERs to the 26S proteasome and that hsp90 interacts with both ER subtypes. Rapamycin (Rapa) and cyclosporin A (CsA), ligands of immunophilins FK506 binding protein (FKBP52) and cyclophilin of 40kDa (CYP40) interacting in separate ER-hsp90 complexes, both induced a proteasomal-mediated degradation of ERs but not of their cognate immunophilin. Moreover, they also decrease the E(2)-induced luciferase transcription but weaker than RU and hsp90 ligands. Fluorescence activated cell sorter (FACS) analysis revealed a blockade of cell progression by RU and 4-hydroxy-tamoxifen at the G(1) phase of the cell cycle and an induction of apoptosis in MCF-7 cells. Rapa and mainly CsA (but not FK506) and hsp90 ligands promote by their own apoptosis in MCF-7, in MERA, and in HERB cells and in MDA-MB-231 ER-null cells. These data suggest that (1) hsp90, as for all steroid receptors, acts as a molecular chaperone for ERbeta; (2) ER-ligands (except tamoxifen), hsp90- and immunophilin-ligands (except FK506) target the two ER subtypes to a proteasome-mediated proteolysis via different signalling pathways; (3) hsp90- and immunophilin-ligands Rapa and CsA, alone or in association with anti-estrogens such as RU, may constitute a potential therapeutic strategy for breast cancer treatment.

Endogenous estrogen receptor beta is transcriptionally active in primary ovarian cells from estrogen receptor knockout mice

The estrogen receptor (ER) alpha is a hormone-inducible transcription factor that has a pivotal physiological role. Intriguingly, a clear and undisputed physiological function of the recently described ERbeta remains elusive, with the exception of the ovary where a cooperative role of ERalpha and ERbeta has been demonstrated. We have, therefore, investigated whether endogenous ERs, in particular ERbeta, act as ligand-inducible transcription factors in primary ovarian cells derived from wild-type, ERalpha or ERbeta knockout mice. Granulosa-enriched cell fractions naturally expressing ERbeta and thecal cell fractions that express ERalpha were analyzed in transactivation assays using the vitellogenin A2 consensus estrogen response element and potent ER agonists diethylstilbestrol and S-indenestrol A. We studied also the potency-selective ERbeta agonist R-indenestrol A, the pure ERalpha agonist and ERbeta antagonist R,R-diethyl-tetrahydrochrysene and the pure ERalpha agonist propylpyrazole-triol. Using ER subtype-specific physiological cell models and these ER subtype-specific structural probes, we analyzed trans-activation of ERalpha and ERbeta. This analysis revealed that endogenously expressed ERbeta is indeed functional as a transcription factor, that it responds to estrogens appropriately, and that the ligands used are true ER subtype-specific probes in primary ovarian cells. In conclusion, this study demonstrates that endogenously expressed ERbeta is capable of regulating gene transcription independent of ERalpha.

Phytoestrogens and their human metabolites show distinct agonistic and antagonistic properties on estrogen receptor alpha (ERalpha) and ERbeta in human cells

Phytoestrogens exert pleiotropic effects on cellular signaling and show some beneficial effects on estrogen-dependent diseases. However, due to activation/inhibition of the estrogen receptors ERalpha or ERbeta, these compounds may induce or inhibit estrogen action and, therefore, have the potential to disrupt estrogen signaling. We performed a comprehensive analysis and potency comparison of phytoestrogens and their human metabolites for ER binding, induction/suppression of ERalpha and ERbeta transactivation, and coactivator recruitment in human cells. The soy-derived genistein, coumestrol, and equol displayed a preference for transactivation of ERbeta compared to ERalpha and were 10- to 100-fold less potent than diethylstilbestrol. In contrast, zearalenone was the most potent phytoestrogen tested and activated preferentially ERalpha. All other phytoestrogens tested, including resveratrol and human metabolites of daidzein and enterolactone, were weak ER agonists. Interestingly, the daidzein metabolites 3',4',7-isoflavone and 4',6,7-isoflavone were superagonists on ERalpha and ERbeta. All phytoestrogens tested showed reduced potencies to activate ERalpha and ERbeta compared to diethylstilbestrol on the estrogen-responsive C3 promoter compared to a consensus estrogen response element indicating a degree of promoter dependency. Zearalenone and resveratrol were antagonistic on both ERalpha and ERbeta at high doses. The phytoestrogens enhanced preferentially recruitment of GRIP1 to ERalpha similar to 17beta-estradiol. In contrast, for ERbeta no distinct preference for one coactivator (GRIP1 or SRC-1) was apparent and the overall coactivator association was less pronounced than for ERalpha. Due to their abundance and (anti)-estrogenic potencies, the soy-derived isoflavones, coumestrol, resveratrol, and zearalenone would appear to have the potential for effectively functioning as endocrine disruptors.

Characterization of the species-specificity of peroxisome proliferators in rat and human hepatocytes

Peroxisome proliferation is a well-defined pleiotropic effect that is mediated by the ligand inducible transcription factor peroxisome proliferator-activated receptor (PPAR) alpha. Because marked peroxisome proliferation occurs in rodents but not in humans, we aimed to elucidate the molecular and cellular determinants of this species-specificity in hepatocytes. Analysis of peroxisomal marker enzyme activities confirmed that peroxisome proliferators induced acyl-CoA oxidase (ACOX) and to a lesser extent catalase in rat hepatocytes, but not in human hepatoma HepG2 cells. Transient transfection assays revealed that ciprofibrate and Wy 14,643 induced rat but not human PPARalpha-mediated reporter gene activity in rat FAO and primary hepatocytes on rat but not on human PPARalpha response elements (PPREs). In contrast, in human HepG2 and primary human hepatocytes, peroxisome proliferators did not induce either human or rat PPARalpha activity regardless of rat or human PPRE sequences. In addition, no induction of ACOX gene expression was observed in human hepatocytes independent of the expression level of human PPARalpha. Remarkably, no distinct peroxisome proliferation related responses were observed in human hepatocytes when rat PPARalpha was transfected, although human hepatocytes were responsive to PPARalpha-mediated induction of carnitine palmitoyl transferase-1A and 3-hydroxy-3-methylglutaryl-CoA synthase. These results confirmed that PPARalpha and PPREs are important determinants for the species-specificity of peroxisome proliferation. Nevertheless, our results showed that human hepatocytes limit the extent of peroxisome proliferation regardless of PPARalpha expression.

Xenoestrogens: mechanisms of action and detection methods

Estrogenic compounds exert pleiotropic effects in wildlife and humans, and endogenous estrogens, like 17 beta-estradiol, regulate growth and development of their target tissues. Environmental, industrial, or naturally occurring chemicals that possess estrogenic and/or antiestrogenic activities are termed xenoestrogens and may interfere with endocrine systems. These xenoestrogens are therefore defined as endocrine-active or endocrine-disrupting compounds. The estrogen receptor (ER) is the major regulatory unit within the estrogen-signaling pathway and the molecular mechanisms of estrogen and ER actions are described briefly. Based on the mechanism of ER action, in vitro test systems are described that can be employed for screening but also for the elucidation of mechanisms of action of (anti)estrogenic compounds. How screening assays and mechanistic studies can aid in human risk assessment for potential endocrine-active compounds is discussed also.

Increased cell proliferation is associated with genomic instability: elevated micronuclei frequencies in estradiol-treated human ovarian cancer cells

Estrogen-related cancers are often associated with the hormone's tumor promoting activity. Recently, estradiol has also been demonstrated to induce gene mutations in the physiological concentration range. Mitotic disturbances are found at higher concentrations. In the present study we demonstrate data suggesting an additional mechanism for the induction of genetic damage, i.e. chromosomal breakage. Estrogen receptor-positive (BG-1) and -negative (UCI) human ovarian cancer cell lines were investigated for micronucleus formation after treatment with estradiol. BG-1 cells but not UCI cells showed an increase in micronucleus formation which correlated with the estradiol-induced cell proliferation. The specific estradiol receptor antagonist hydroxytamoxifen suppressed the formation of micronuclei in BG-1 cells. Increased micronucleus frequencies were also seen after normalization of the data to the number of cell divisions. Kinetochore analysis revealed a difference between micronuclei induced by picomolar concentrations of estradiol (kinetochore-negative) and micromolar concentrations (predominantly kinetochore-positive) leading to mitotic disturbances. In accordance with this finding, analysis of the cell cycle revealed decreased cell numbers in G(2)/M phase after treatment with picomolar concentrations, usually not found after mitotic disturbances. We hypothesize that hormone-specific forcing of responsive cells through the cell cycle leads to an override of checkpoints operating under homeostatic control of the cell cycle, resulting in genomic instability.

Activation of estrogen receptor alpha and ERbeta by 4-methylbenzylidene-camphor in human and rat cells: comparison with phyto- and xenoestrogens

4-Methylbenzylidene-camphor (4-MBC) is an organic sunscreen that protects against UV radiation and may therefore help in the prevention of skin cancer. Recent results on the estrogenicity of 4-MBC have raised concerns about a potential of 4-MBC to act as an endocrine disruptor. Here, we investigated the direct interaction of 4-MBC with estrogen receptor (ER) alpha and ERbeta in a series of studies including receptor binding, ER transactivation and functional tests in human and rat cells. 4-MBC induced alkaline phosphatase activity, a surrogate marker for estrogenic activity, in human endometrial Ishikawa cells. Interestingly, 4-MBC induced weakly ERalpha and with a higher potency ERbeta mediated transactivation in Ishikawa cells at doses more than 1 microM, but showed no distinct binding affinity to ERalpha or ERbeta. In addition, 4-MBC was an effective antagonist for ERalpha and ERbeta. In an attempt to put 4-MBC's estrogenic activity into perspective we compared binding affinity and potency to activate ER with phyto- and xenoestrogens. 4-MBC showed lower estrogenic potency than genistein, coumestrol, resveratrol, bisphenol A and also camphor. Analysis of a potential metabolic activation of 4-MBC that could account for 4-MBC's more distinct estrogenic effects observed in vivo revealed that no estrogenic metabolites of 4-MBC are formed in primary rat or human hepatocytes. In conclusion, we were able to show that 4-MBC is able to induce ERalpha and ERbeta activity. However, for a hazard assessment of 4-MBC's estrogenic effects, the very high doses of 4-MBC required to elicit the reported effects, its anti-estrogenic properties as well as its low estrogenic potency compared to phytoestrogens and camphor has to be taken into account.

Molecular determinants of the stereoselectivity of agonist activity of estrogen receptors (ER) alpha and beta

The two known estrogen receptors, ERalpha and ERbeta, are hormone-inducible transcription factors that have distinct roles in regulating cell proliferation and differentiation. Previously, our laboratory demonstrated that ERalpha exhibits stereoselective ligand binding and transactivation for several structural derivatives and metabolites of the synthetic estrogen diethylstilbestrol. We have previously described the properties of indenestrol A (IA) enantiomers on ERalpha. In the study presented here, the estrogenic properties of the S and R enantiomers of IA, IA-S and IA-R, respectively, were expanded to examine the activity in different cell and promoter contexts using ERalpha and ERbeta. Using human cell lines stably expressing either ERalpha or ERbeta, we found that IA-S was a more potent activator of transcription than IA-R through ERalpha in human endometrial Ishikawa and breast MDA-MB 231 (MDA) cells. Interestingly, IA-R was more potent on ERbeta when compared with ERalpha in MDA, but not in Ishikawa cells, and IA-R exhibited equally low binding affinities to ERalpha and ERbeta in vitro in contrast to its cell line-dependent preferential activation of ERbeta. Alignment of the protein structures of the ligand-binding domains of ERalpha and ERbeta revealed one mismatched residue, Leu-384 in ERalpha and Met-283 in ERbeta, which may be responsible for making contact with the methyl substituent at the chiral carbon of IA-S and IA-R. Mutagenesis and exchange of this one residue showed that the binding of IA-R and IA-S was not affected by this mutation in ERalpha and ERbeta. However, in transactivation studies, IA-R showed higher potency in activating L384M-mutated ERalpha and wild-type ERbeta compared with wild-type ERalpha and M283L-mutated ERbeta in all cell and promoter contexts examined. Furthermore, IA-R-bound ERalpha L384M and wild-type ERbeta displayed enhanced interactions with the nuclear receptor interaction domains of the coactivators SRC-1 and GRIP1. These data demonstrate that a single residue in the ligand-binding domain determines the stereoselectivity of ERalpha and ERbeta for indenestrol ligands and that IA-R shows cell type selectivity through ERbeta.

Overview of in vitro tools to assess the estrogenic and antiestrogenic activity of phytoestrogens

There is an intense discussion in the scientific and even more so in the public community as well as regulatory agencies about the potential benefits or detrimental effects of plant-derived compounds that may affect the endocrine system, especially estrogen signaling pathways. These so-called phytoestrogens are found in the normal western diet and predominantly in an eastern or soy-based diet and the potency of the isolated compounds to interact with the known receptors for estrogen varies tremendously. The estrogen receptors, ER alpha and ER beta, mediate the effects of endogenous estrogens, i.e. regulation of reproductive function, tissue development, cell proliferation and differentiation. In this review, in vitro test systems available to date for the screening of estrogenic and antiestrogenic activity including mechanism-based assays are described. The potency of phytoestrogens determined using these in vitro assays are compared with the potency of endogenous estrogens and results obtained in vitro are compared with effects in vivo. Finally, the impact of in vitro assays to determine estrogenicity on human hazard assessment is discussed as well as other non ER-mediated mechanisms that may contribute to potential beneficial or adverse effects of phytoestrogens in man.

Mammary gland development in adult mice requires epithelial and stromal estrogen receptor alpha

Complete mammary gland development takes place following puberty and depends on the estrogen receptor (ER)alpha and the progesterone receptor (PR) and is tightly regulated by the interaction of the mammary epithelium with the stromal compartment. Studies using mammary tissues of immature mice have indicated that stromal but not epithelial ER alpha is required for mammary gland growth. This study investigates whether these same tissue growth requirements of neonate tissue are necessary for mammary development and response in adult mice. Mammary epithelial cells were isolated from adult mice with a targeted disruption of the ER alpha gene (alpha ERKO) or from wild-type counterparts and injected into epithelial-free mammary fat pads of 3-wk-old female alpha ERKO or wild-type mice. Ten weeks after cell injection, analysis of mammary gland whole mounts showed that both stromal and epithelial ER alpha were required for complete mammary gland development in adult mice. However, when the mice were treated with high doses of estradiol (E2) and progesterone, stromal ER alpha was sufficient to generate full mammary gland growth. Surprisingly, ER alpha-deficient epithelial cells were able to proliferate and develop into a rudimentary mammary ductal structure in an ER alpha-negative stroma, indicating that neither stromal nor epithelial ER alpha are required for the mammary rudiment to form in the adult mouse, as confirmed by the phenotype of the alpha ERKO mammary gland. Use of this in vivo model system has demonstrated that neonatal and adult mammary tissues use a different tissue-specific role for ER alpha in mammary response. Immunostaining for ER alpha and PR in the mammary outgrowths supported the view that both stromal and epithelial ER alpha, in cooperation with epithelial PR, govern mammary gland development in adult mice.

Estrogen receptors and endocrine diseases: lessons from estrogen receptor knockout mice

The estrogen receptors ERalpha and ERbeta are the main mediators of estrogen action and estrogens play an important role in a variety of aspects of physiology besides their well acknowledged function in reproduction. In vivo and in vitro studies indicate that the estrogen receptors are mechanistically implicated in endocrine-related diseases. Recent studies with estrogen receptor knockout mice have helped to unravel the role of the estrogen receptors in brain degeneration, osteoporosis, cardiovascular diseases and obesity.

Immortalized testis cell lines from estrogen receptor (ER) alpha knock-out and wild-type mice expressing functional ERalpha or ERbeta

The surprising findings that male mice lacking the estrogen receptor (ER) alpha (alphaERKO) have atrophic testis and are infertile proved that ERalpha is involved in normal testicular function. To obtain compatible in vitro model systems for alphaERKO male mice, we immortalized different cell lines from the testis of alphaERKO and wild-type (C57BL/6) mice with the human papilloma virus E6/E7 genes. The established cell lines were characterized for Sertoli, Leydig, and peritubular cell markers by means of messenger RNA expression and functional assays. One wild-type-derived cell line showed Leydig cell-specific marker gene expression and produced testosterone after stimulation with cyclic adenosine monophosphate. Most wild-type cell lines expressed androgen receptor and a functional ERalpha as shown by high estrogenic activity in a luciferase-based transactivation assay. Most notably, the wild-type-derived WL3, and the ES4 cell line derived from alphaERKO mice expressed ERbeta and showed ER-mediated transcriptional activity, but no ERalpha protein expression. These cell lines with and without functional ERalpha or ERbeta enable the analyses of ER subtype-specific responses and their function in testicular cell signaling, morphogenesis, and neoplasia.

Supra-additive genotoxicity of a combination of gamma-irradiation and ethyl methanesulfonate in mouse lymphoma L5178Y cells

While testing for genotoxicity is usually performed on single chemicals, exposure of humans always comprises a number of genotoxic agents. The investigation of potentially synergistic effects of combinations therefore is an important issue in toxicology. Combinations of 511 keV gamma-radiation with the chemical alkylating agent ethyl methane-sulfonate were investigated in the in vitro micronucleus test in mouse lymphoma L5178Y cells. With combinations in the low dose linear effect range for the individual agents (0. 25-2 Gy and 0.8-3.2 mM, respectively), supra-additivity by 34-86% was seen. The synergism was more pronounced at the higher dose levels. Supra-additivity was confirmed in experiments using cytochalasin B and analyzing binucleate cells only, to control for putative effects on the cell cycle. Statistical significance was shown by a 2-factor analysis of variance with interaction. The results indicate that damage to DNA by gamma-radiation and alkylation could affect different rate limiting steps in the formation of micronuclei. Further investigations will have to show whether the observations are of general validity, in particular, whether other end-points of genotoxicity produce the same results and whether the degree of supra-additivity is always dose dependent. The latter would have a strong impact on risk assessment for mixtures at low doses.

Helicobacter pylori induces DNA damage in vitro

A close association between Helicobacter pylori infection and the development of gastric adenocarcinoma in humans has been demonstrated. Therefore, the direct induction of DNA damage by H. pylori was investigated here using the in vitro micronucleus assay. After 5 days of incubation with bacterial lysate a dose-dependent formation of micronuclei was found, which was not limited to cytotoxic protein concentrations and was not observed after treatment with Escherichia coli lysate (control). This induction of DNA damage may be a link between chronic H. pylori infection and development of adenocarcinoma of the stomach.

Characterization of the genotoxicity of anthraquinones in mammalian cells

Naturally occurring 1,8-dihydroxyanthraquinones are under consideration as possible carcinogens. Here we wanted to elucidate a possible mechanism of their genotoxicity. All three tested anthraquinones, emodin, aloe-emodin, and danthron, showed capabilities to inhibit the non-covalent binding of bisbenzimide Hoechst 33342 to isolated DNA and in mouse lymphoma L5178Y cells comparable to the topoisomerase II inhibitor and intercalator m-amsacrine. In a cell-free decatenation assay, emodin exerted a stronger, danthron a similar and aloe-emodin a weaker inhibition of topoisomerase II activity than m-amsacrine. Analysis of the chromosomal extent of DNA damage induced by these anthraquinones was performed in mouse lymphoma L5178Y cells. Anthraquinone-induced mutant cell clones showed similar chromosomal lesions when compared to the topoisomerase II inhibitors etoposide and m-amsacrine, but were different from mutants induced by the DNA alkylator ethyl methanesulfonate. These data support the idea that inhibition of the catalytic activity of topoisomerase II contributes to anthraquinone-induced genotoxicity and mutagenicity.

Occurrence of emodin, chrysophanol and physcion in vegetables, herbs and liquors. Genotoxicity and anti-genotoxicity of the anthraquinones and of the whole plants

1,8-Dihydroxyanthraquinones, present in laxatives, fungi imperfecti, Chinese herbs and possibly vegetables, are in debate as human carcinogens. We screened a variety of vegetables (cabbage lettuce, beans, peas), some herbs and herbal-flavoured liquors for their content of the 'free' anthraquinones emodin, chrysophanol and physcion. For qualitative and quantitative analysis, reversed-phase HPLC (RP-LC), gas chromatography-mass spectrometry (GC-MS) and RP-LC-MS were used. The vegetables showed a large batch-to-batch variability, from 0.04 to 3.6, 5.9 and 36 mg total anthraquinone per kg fresh weight in peas, cabbage lettuce, and beans, respectively. Physcion predominated in all vegetables. In the herbs grape vine leaves, couch grass root and plantain herb, anthraquinones were above the limit of detection. Contents ranged below 1 mg/kg (dry weight). All three anthraquinones were also found in seven of 11 herbal-flavoured liquors, in a range of 0.05 mg/kg to 7.6 mg/kg. The genotoxicity of the analysed anthraquinones was investigated in the comet assay, the micronucleus test and the mutation assay in mouse lymphoma L5178Y tk+/- cells. Emodin was genotoxic, whereas chrysophanol and physcion showed no effects. Complete vegetable extract on its own did not show any effect in the micronucleus test. A lettuce extract completely abolished the induction of micronuclei by the genotoxic anthraquinone danthron. Taking into consideration the measured concentrations of anthraquinones, estimated daily intakes, the genotoxic potency, as well as protective effects of the food matrix, the analysed constituents do not represent a high priority genotoxic risk in a balanced human diet.

Biotransformation of the anthraquinones emodin and chrysophanol by cytochrome P450 enzymes. Bioactivation to genotoxic metabolites

The studies presented here were designed to elucidate the enzymes involved in the biotransformation of naturally occurring 1, 8-dihydroxyanthraquinones and to investigate whether biotransformation of 1,8-dihydroxyanthraquinones may represent a bioactivation pathway. We first studied the metabolism of emodin (1, 3,8-trihydroxy-6-methylanthraquinone), a compound present in pharmaceutical preparations. With rat liver microsomes, the formation of two emodin metabolites, omega-hydroxyemodin and 2-hydroxyemodin, was observed. The rates of formation of omega-hydroxyemodin were not different with microsomes from rats that had been pretreated with inducers for different cytochrome P450 enzymes. Thus, the formation of omega-hydroxyemodin seems to be catalyzed by several cytochrome P450 enzymes at low rates. The formation of 2-hydroxyemodin was increased in liver microsomes from 3-methylcholanthrene-pretreated rats and was inhibited by alpha-naphthoflavone, by an anti-rat cytochrome P450 1A1/2 antibody, and, to a lesser degree, by an anti-rat cytochrome P450 1A1 antibody. These data suggest the involvement of cytochrome P450 1A2 in the formation of this metabolite. However, other cytochrome P450 enzymes also seem to catalyze this reaction. The anthraquinone chrysophanol (1,8-dihydroxy-3-methylanthraquinone) is transformed, in a cytochrome P450-dependent oxidation, to aloe-emodin (1, 8-dihydroxy-3-hydroxymethylanthraquinone) as the major product formed. The mutagenicity of the parent dihydroxyanthraquinones and their metabolites was compared in the in vitro micronucleus test in mouse lymphoma L5178Y cells. 2-Hydroxyemodin induced much higher micronucleus frequencies, compared with emodin. omega-Hydroxyemodin induced lower micronucleus frequencies, compared with emodin. Aloe-emodin induced significantly higher micronucleus frequencies than did chrysophanol. These data indicate that the cytochrome P450-dependent biotransformation of emodin and chrysophanol may represent bioactivation pathways for these compounds.

Structural analysis of the interaction of the tRNA modifying enzymes Tgt and QueA with a substrate tRNA

The enzymes tRNA guanine-transglycosylase (Tgt) and S-adenosylmethionine :tRNA ribosyltransferase-isomerase (QueA) participate in the biosynthesis of the hypermodified tRNA nucleoside queuosine (Q) in Escherichia coli. Here we show by HPLC analysis and gel retardation that both enzymes interact with an in vitro transcribed tRNA(ASP) from yeast, specifically modified with a Q precursor molecule. RNase I footprinting experiments showed strong protein tRNA contacts in the anticodon stem-loop and a minor interaction with the dihydrouridine loop. This suggests that all identity elements for the recognition of Q-specific tRNAs are clustered in the anticodon region and explains earlier results that both enzymes accept a RNA microhelix with the sequence of an anticodon stem-loop as substrate.