A histone deacetylase inhibitor potentiates estrogen receptor activation of a stably integrated vitellogenin promoter in HepG2 cells

Integrated Decision-tree Testing Strategies for Environmental Toxicity with Respect to the Requirements of the EU REACH Legislation

Liverpool John Moores University and FRAME recently conducted a research project sponsored by Defra on the status of alternatives to animal testing with regard to the European Union REACH (Registration, Evaluation and Authorisation of Chemicals) system for safety testing and risk assessment of chemicals. The project covered all the main toxicity endpoints associated with the REACH system. This paper focuses on the prospects for using alternative methods (both in vitro and in silico) for environmental (aquatic) toxicity testing. The manuscript reviews tests based on fish cells and cell lines, fish embryos, lower organisms, and the many expert systems and QSARs for aquatic toxicity testing. Ways in which reduction and refinement measures can be used are also discussed, including the Upper Threshold Concentration -- Step Down (UTC) approach, which has recently been retrospectively validated by ECVAM and subsequently endorsed by the ECVAM Scientific Advisory Committee (ESAC). It is hoped that the application of this approach could reduce the number of fish used in acute toxicity studies by around 65-70%. Decision-tree style integrated testing strategies are also proposed for acute aquatic toxicity and chronic toxicity (including bioaccumulation), followed by a number of recommendations for the future facilitation of aquatic toxicity testing with respect to environmental risk assessment.

Integrated decision-tree testing strategies for environmental toxicity with respect to the requirements of the EU REACH legislation

Liverpool John Moores University and FRAME recently conducted a research project sponsored by Defra on the status of alternatives to animal testing with regard to the European Union REACH (Registration, Evaluation and Authorisation of Chemicals) system for safety testing and risk assessment of chemicals. The project covered all the main toxicity endpoints associated with the REACH system. This paper focuses on the prospects for using alternative methods (both in vitro and in silico) for environmental (aquatic) toxicity testing. The manuscript reviews tests based on fish cells and cell lines, fish embryos, lower organisms, and the many expert systems and QSARs for aquatic toxicity testing. Ways in which reduction and refinement measures can be used are also discussed, including the Upper Threshold Concentration--Step Down (UTC) approach, which has recently been retrospectively validated by ECVAM and subsequently endorsed by the ECVAM Scientific Advisory Committee (ESAC). It is hoped that the application of this approach could reduce the number of fish used in acute toxicity studies by around 65-70%. Decision-tree style integrated testing strategies are also proposed for acute aquatic toxicity and chronic toxicity (including bioaccumulation), followed by a number of recommendations for the future facilitation of aquatic toxicity testing with respect to environmental risk assessment.

Regulation of ER alpha signaling pathway in neuronal HN10 cells: role of protein acetylation and Hsp90

Estrogen has a variety of neuroprotective effects but the molecular basis of its function is still mainly unclear. Estrogen receptor (ER) signaling is highly dependent on posttranslational modifications and the assembly of coactivator and corepressor complexes. Several proteins involved in ER alpha signaling have recently been found to be acetylated, including ER alpha itself and Hsp90, a key chaperone in the functional regulation of ER alpha. ER alpha complexes also contain histone deacetylases (HDAC) which repress transactivation. Our purpose was to clarify the role of protein acetylation and Hsp90 function in the ERE-mediated ER alpha signaling in neuronal HN10 cells. We observed that increasing protein/histone acetylation status with trichostatin A, a potent HDAC inhibitor, increased the 17beta-estradiol (E2)-induced transactivation of ERE-driven luciferase in non-transfected cells, and even more extensively in pER alpha-transfected cells. E2-induced ERE-driven transactivation was blocked by ICI 182.780. Several ER antagonists, such as raloxifene and tamoxifen, were unresponsive. Valproate, an antiepileptic drug which is recently characterized as a HDAC inhibitor, was also able to potentiate the E2-induced ERE-transactivation. Inhibition of the function of Hsp90 chaperone with geldanamycin strongly inhibited the E2-induced ERE-transactivation. Overexpression of SIRT2 protein deacetylase did not inhibit the acetylation-potentiated ERE-driven transactivation indicating that SIRT2 deacetylase is not involved in ER alpha signaling. Our results reveal that ER alpha signaling is dependent on protein acetylation and epigenetic regulation.

Different Modulation of ER-Mediated Transactivation by Xenobiotic Nuclear Receptors Depending on the Estrogen Response Elements and Estrogen Target Cell Types

Recent studies demonstrated that constitutive androstane receptor (CAR) inhibits ER-mediated transactivation of both endogenous and synthetic estrogen responsive promotor in Hep G2. Whereas steroid and xenobiotic receptor (SXR) but not peroxisome proliferator-activated receptor-gamma (PPAR-gamma) was also reported to repress estrogen receptor (ER) transactivation of the synthetic 4ERE in Hep G2, the effects of these xenobiotic nuclear receptors (XNRs) on the endogenous estrogen responsive promotor remain to be determined. Effects of CAR, SXR, and PPAR-gamma on ER transactivation were also examined in three different kinds of breast cancer cell lines. However, except in MCF-7, studies were limited either in single dose response (MDA-MB-231) or with CAR only (MCF-7-K3). And there is presently no report on the effects of CAR, SXR, and PPAR-gamma on ER-mediated transactivation in ovarian-derived CHO-S cells. Accordingly, this article further examined the effects of the endogenous vitellogenin B1 estrogen responsive promotor on the SXR- and PPAR-gamma-modulated ER transactivation in Hep G2, and either dose-dependent or single dose effects of SXR, PPAR-gamma, and CAR in two different breast cancer cell lines and the ovarian-derived cell line respectively, on the ER-mediated transactivation of the synthetic (4ERE)-tk-luciferase reporter. Consistent with the previous report, CAR significantly repressed ER-mediated transactivation of the endogenous vitellogenin B1 promotor in Hep G2 cells. However, contrary to the effects on the synthetic promotor, PPAR-gamma potentiated whereas SXR did not have any effects on the ER transactivation of the vitellogenin promotor in Hep G2. In the breast cancer cell line of MDA-MB-231 in which endogenous ER is known not to be expressed, CAR modestly stimulated ER transactivation of the synthetic 4ERE in a low dose whereas both SXR and PPAR-gamma did not have any effects in all doses examined (20-500 ng). And in both CHO-S and estrogen-independent breast cancer cell line, MCF-7-K3, none of the three xenobiotic receptors significantly influenced the ER-mediated 4ERE transactivation in all doses examined. XNRs modulate ER-mediated transactivation depending on the estrogen response elements (EREs) and estrogen target cell types.

Recruitment of histone deacetylase 4 to the N-terminal region of estrogen receptor alpha

Transcriptional activation of estrogen receptor alpha (ERalpha) is regulated by the ligand-dependent activation function 2 and the constitutively active N-terminal activation function 1. To identify ERalpha N-terminal-specific coregulators, we screened a breast cDNA library by T7 phage display and isolated histone deacetylase 4 (HDAC4). HDAC4 interacts with the ERalpha N terminus both in vitro and in vivo. Presence of the ERalpha DNA binding domain and hinge region reduce HDAC4 recruitment whereas full-length ERalpha enhances recruitment. HDAC4 interaction is selective for the ERalpha and not ERbeta N terminus and occurs in the nucleus. We demonstrate in vivo that HDAC4 is recruited by the N terminus to the promoter of an endogenous estrogen responsive gene. HDAC4 suppresses transcriptional activation of ERalpha by estrogen and selective ER modulators (SERMs) such as tamoxifen in a cell type-dependent manner. Consistently, silencing of HDAC4 promotes the agonist effect of SERMs (tamoxifen and raloxifene) in a cell type-specific manner. These findings indicate a role for HDAC4 in regulating ERalpha activity as a novel N-terminal coregulator and uncover a mechanism by which certain cell types regulate SERM behavior.

Estrogen Receptor-dependent and Estrogen Receptor-independent Pathways for Tamoxifen and 4-Hydroxytamoxifen-induced Programmed Cell Death

The therapeutic efficacy of tamoxifen (TAM) in cancer therapy is thought to arise primarily from its ability to compete with estrogens for binding to the estrogen receptor (ER). We show that TAM and its active metabolite, 4-hydroxytamoxifen (OHT), can actively induce programmed cell death through distinct ER-dependent and ER-independent pathways. The ER-independent pathway is activated by 10-20 microm TAM and OHT and by 10-20 microm 17beta-estradiol and raloxifene, and occurs in ER-negative cells. The ER dependence of a second pathway, caused by submicromolar concentrations of TAM and OHT, was demonstrated by the ability of the ER ligands 17beta-estradiol, raloxifene, and ICI 182,780 to effectively block the cell death-inducing effects of TAM and OHT. Because the p38-specific inhibitor SB203580 blocks OHT.ER-induced cell death, stress kinase pathways are likely involved. ER-independent cell death triggers classic caspase-dependent apoptosis. However, although OHT.ER triggers some hallmarks of apoptosis, including Bax translocation and cytochrome c release, the absence of poly(ADP-ribose) polymerase cleavage or DNA laddering indicates that the death pathway involved is caspase-independent. The OHT.ER-dependent cell death pathway appears to diverge from classical apoptosis at the level of caspase 9 activation. The ability to promote ER-dependent programmed cell death represents a novel activity of TAM and OHT.

Inhibitory cross-talk between estrogen receptor (ER) and constitutively activated androstane receptor (CAR). CAR inhibits ER-mediated signaling pathway by squelching p160 coactivators

Estrogen receptor (ER) activity can be modulated by the action of other nuclear receptors. To study whether ER activity is altered by orphan nuclear receptors that mediate the cellular response to xenobiotics, cross-talk between ER and constitutive androstane receptor (CAR), steroid and xenobiotic receptor, or peroxisome proliferator-activated receptor gamma was examined in HepG2 cells. Of these receptors, CAR substantially inhibited ER-mediated transcriptional activity of the vitellogenin B1 promoter as well as a synthetic estrogen responsive element (ERE)-containing promoter. Treatment with an agonist of CAR, 1,4-bis-(2-(3,5-dichloropyridoxyl))benzene, potentiated CAR-mediated transcriptional repression. In contrast, an antagonist of CAR, androstenol, alleviated the repression effect. Although CAR interacted with the ER in solution, CAR did not interact with the ER bound to the ERE. CAR/retinoid X receptor bound to the ERE but with much lower affinity than ER. Incremental amounts of CAR elicited a progressive reduction of the ER activity induced by the p160 coactivator glucocorticoid receptor interacting protein 1 (GRIP-1). In turn, increasing amounts of GRIP-1 progressively reversed the depression of ER activity by CAR. An agonist or antagonist of CAR potentiated or alleviated, respectively, the CAR-mediated repression of the GRIP-1-enhanced ER activity, which is consistent with the ability of theses ligands to increase or decrease, respectively, the interaction of CAR with GRIP-1. A CAR mutant that did not interact with GRIP-1 did not inhibit ER-mediated transactivation. Our data demonstrate that xenobiotic nuclear receptor CAR antagonizes ER-mediated transcriptional activity by squelching limiting amounts of p160 coactivator and imply that xenobiotics may influence ER function of female reproductive physiology, cell differentiation, tumorigenesis, and lipid metabolism.