Environmental contaminants affect a wide variety of biological events in many species. Dioxins are typical environmental contaminants that exert adverse oestrogen-related effects. Although their anti-oestrogenic actions are well described, dioxins can also induce endometriosis and oestrogen-dependent tumours, implying possible oestrogenic effects. However, the molecular mechanism underlying oestrogen-related actions of dioxins remains largely unknown. A heterodimer of the dioxin receptor (AhR) and Arnt, which are basic helix-loop-helix/PAS-family transcription factors, mediates most of the toxic effects of dioxins. Here we show that the agonist-activated AhR/Arnt heterodimer directly associates with oestrogen receptors ER-alpha and ER-beta. This association results in the recruitment of unliganded ER and the co-activator p300 to oestrogen-responsive gene promoters, leading to activation of transcription and oestrogenic effects. The function of liganded ER is attenuated. Oestrogenic actions of AhR agonists were detected in wild-type ovariectomized mouse uteri, but were absent in AhR-/- or ER-alpha-/- ovariectomized mice. Our findings suggest a novel mechanism by which ER-mediated oestrogen signalling is modulated by a co-regulatory-like function of activated AhR/Arnt, giving rise to adverse oestrogen-related actions of dioxin-type environmental contaminants.

MicroRNAs (miRNAs) control cell proliferation, differentiation and fate through modulation of gene expression by partially base-pairing with target mRNA sequences. Drosha is an RNase III enzyme that is the catalytic subunit of a large complex that cleaves pri-miRNAs with distinct structures into pre-miRNAs. Here, we show that both the p68 and p72 DEAD-box RNA helicase subunits in the mouse Drosha complex are indispensable for survival in mice, and both are required for primary miRNA and rRNA processing. Gene disruption of either p68 or p72 in mice resulted in early lethality, and in both p68(-/-) and p72(-/-) embryos, expression levels of a set of, but not all, miRNAs and 5.8S rRNA were significantly lowered. In p72(-/-) MEF cells, expression of p72, but not a mutant lacking ATPase activity, restored the impaired expression of miRNAs and 5.8S rRNA. Furthermore, we purified the large complex of mouse Drosha and showed it could generate pre-miRNA and 5.8S rRNA in vitro. Thus, we suggest that DEAD-box RNA helicase subunits are required for recognition of a subset of primary miRNAs in mDrosha-mediated processing.

Histone modifications induced by activated signalling cascades are crucial to cell-lineage decisions. Osteoblast and adipocyte differentiation from common mesenchymal stem cells is under transcriptional control by numerous factors. Although PPAR-gamma (peroxisome proliferator activated receptor-gamma) has been established as a prime inducer of adipogenesis, cellular signalling factors that determine cell lineage in bone marrow remain generally unknown. Here, we show that the non-canonical Wnt pathway through CaMKII-TAK1-TAB2-NLK transcriptionally represses PPAR-gamma transactivation and induces Runx2 expression, promoting osteoblastogenesis in preference to adipogenesis in bone marrow mesenchymal progenitors. Wnt-5a activates NLK (Nemo-like kinase), which in turn phosphorylates a histone methyltransferase, SETDB1 (SET domain bifurcated 1), leading to the formation of a co-repressor complex that inactivates PPAR-gamma function through histone H3-K9 methylation. These findings suggest that the non-canonical Wnt signalling pathway suppresses PPAR-gamma function through chromatin inactivation triggered by recruitment of a repressing histone methyltransferase, thus leading to an osteoblastic cell lineage from mesenchymal stem cells.

Chromatin reorganization is governed by multiple post-translational modifications of chromosomal proteins and DNA. These histone modifications are reversible, dynamic events that can regulate DNA-driven cellular processes. However, the molecular mechanisms that coordinate histone modification patterns remain largely unknown. In metazoans, reversible protein modification by O-linked N-acetylglucosamine (GlcNAc) is catalysed by two enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). However, the significance of GlcNAcylation in chromatin reorganization remains elusive. Here we report that histone H2B is GlcNAcylated at residue S112 by OGT in vitro and in living cells. Histone GlcNAcylation fluctuated in response to extracellular glucose through the hexosamine biosynthesis pathway (HBP). H2B S112 GlcNAcylation promotes K120 monoubiquitination, in which the GlcNAc moiety can serve as an anchor for a histone H2B ubiquitin ligase. H2B S112 GlcNAc was localized to euchromatic areas on fly polytene chromosomes. In a genome-wide analysis, H2B S112 GlcNAcylation sites were observed widely distributed over chromosomes including transcribed gene loci, with some sites co-localizing with H2B K120 monoubiquitination. These findings suggest that H2B S112 GlcNAcylation is a histone modification that facilitates H2BK120 monoubiquitination, presumably for transcriptional activation.

One class of the nuclear receptor AF-2 coactivator complexes contains the SRC-1/TIF2 family, CBP/p300 and an RNA coactivator, SRA. We identi®ed a subfamily of RNA-binding DEAD-box proteins (p72/p68) as a human estrogen receptor a (hERa) coactivator in the complex containing these factors. p72/p68 interacted with both the AD2 of any SRC-1/TIF2 family protein and the hERa A/B domain, but not with any other nuclear receptor tested. p72/p68, TIF2 (SRC-1) and SRA were co-immunoprecipitated with estrogenbound hERa in MCF7 cells and in partially puri®ed complexes associated with hERa from HeLa nuclear extracts. Estrogen induced co-localization of p72 with hERa and TIF2 in the nucleus. The presence of p72/ p68 potentiated the estrogen-induced expression of the endogenous pS2 gene in MCF7 cells. In a transient expression assay, a combination of p72/p68 with SRA and one TIF2 brought an ultimate synergism to the estrogen-induced transactivation of hERa. These ®nd-ings indicate that p72/p68 acts as an ER subtypeselective coactivator through ERa AF-1 by associating with the coactivator complex to bind its AF-2 through direct binding with SRA and the SRC-1/TIF2 family proteins.