The H2A.Z and NuRD associated protein HMG20A controls early head and heart developmental transcription programs.

Specialized chromatin-binding proteins are required for DNA-based processes during development. We recently established PWWP2A as a direct histone variant H2A.Z interactor involved in mitosis and craniofacial development. Here, we identify the H2A.Z/PWWP2A-associated protein HMG20A as part of several chromatin-modifying complexes, including NuRD, and show that it localizes to distinct genomic regulatory regions. Hmg20a depletion causes severe head and heart developmental defects in Xenopus laevis. Our data indicate that craniofacial malformations are caused by defects in neural crest cell (NCC) migration and cartilage formation. These developmental failures are phenocopied in Hmg20a-depleted mESCs, which show inefficient differentiation into NCCs and cardiomyocytes (CM). Consequently, loss of HMG20A, which marks open promoters and enhancers, results in chromatin accessibility changes and a striking deregulation of transcription programs involved in epithelial-mesenchymal transition (EMT) and differentiation processes. Collectively, our findings implicate HMG20A as part of the H2A.Z/PWWP2A/NuRD-axis and reveal it as a key modulator of intricate developmental transcription programs that guide the differentiation of NCCs and CMs.

Glucocorticoids regulate the human occludin gene through a single imperfect palindromic glucocorticoid response element.

The 65kDa protein occludin is an essential element of the blood-brain barrier. This integral membrane protein represents an important part of the tight junctions, which seal and protect the blood brain barrier against paracellular diffusion of solutes to the brain parenchyme and are therefore responsible for the high resistance and low permeability between cerebral capillary endothelial cells. However, the molecular basis for the regulation of occludin gene expression is only incompletely understood. In former projects we showed that treatment of a brain microvascular cell line, cEND, with glucocorticoids resulted in increased occludin expression in cell-cell-contacts [Förster, C., Silwedel, C., Golenhofen, N., Burek, M., Kietz, S., Mankertz, J., Drenckhahn, D., 2005. Occludin as direct target for glucocorticoid-induced improvement of blood-brain barrier properties in a murine in vitro system. J. Physiol. 565, Pt 2, 475-486]. Induction of occludin expression by glucocorticoids was shown to be dependent on the glucocorticoid receptor. This study aims to identify the underlying molecular mechanism of gene expression and to identify potential glucocorticoid receptor binding sites within the occludin promoter, the glucocorticoid response elements. We identified one candidate glucocorticoid response element within the distal part of the occludin promoter that differs from the consensus glucocorticoid response element by the presence of a 4-basepair instead of a 3-basepair spacer between two highly degenerate halfsites (5'-ACATGTGTTTACAAAT-3'). Chromatin immunoprecipitation assay and site-directed mutagenesis confirmed binding of the glucocorticoid receptor to this site. The need for glucocorticoid receptor dimerization to induce gene expression was further confirmed by transfection studies using wild type and glucocorticoid receptor dimerization-deficient expression vectors, indicating that transactivation of occludin occurs through the glucocorticoid response element (GRE).

p66alpha and p66beta of the Mi-2/NuRD complex mediate MBD2 and histone interaction.

The Mi-2/NuRD complex is a multi-subunit protein complex with enzymatic activities involving chromatin remodeling and histone deacetylation. Targeting of Mi-2/NuRD to methylated CpG sequences mediates gene repression. The function of p66alpha and of p66beta within the multiple subunits has not been addressed. Here, we analyzed the in vivo function and binding of both p66-paralogs. Both factors function in synergy, since knocking-down p66alpha affects the repressive function of p66beta and vice versa. Both proteins interact with MBD2 functionally and biochemically. Mutation of a single amino acid of p66alpha abolishes in vivo binding to MBD2 and interferes with MBD2-mediated repression. This loss of binding results in a diffuse nuclear localization in contrast to wild-type p66alpha that shows a speckled nuclear distribution. Furthermore, wild-type subnuclear distribution of p66alpha and p66beta depends on the presence of MBD2. Both proteins interact with the tails of all octamer histones in vitro, and acetylation of histone tails interferes with p66 binding. The conserved region 2 of p66alpha is required for histone tail interaction as well as for wild-type subnuclear distribution. These results suggest a two-interaction forward feedback binding mode, with a stable chromatin association only after deacetylation of the histones has occurred.

Glucocorticoid effects on mouse microvascular endothelial barrier permeability are brain specific.

Endothelial cells (ECs) from different vascular beds display certain common qualities, but each subtype is uniquely adapted to meet the demands of the underlying tissues. The structural peculiarities of intercellular junctions are, for instance, considered to account for the differences in permeability displayed by various vascular beds: strong occludin expression is unique to cerebral ECs and considered to account for the high electrical resistance and low paracellular permeability of brain microvessels which constitute the blood-brain barrier (BBB). The integrity of the BBB is compromised in many disorders of the human CNS; therapeutic strategies include treatment with glucocorticoids (GCs), which improve barrier properties of the BBB. In contrast, positive effects of GCs on peripheral vascular permeability could not be demonstrated clearly, while side-effects of prolonged GC treatment are considerable. In an effort to elucidate this difference, we analysed the expression of occludin and the glucocorticoid receptor (GR) in BBB and non-BBB (myocardium) endothelial cells. Our results demonstrate complete GR downregulation by GCs in murine non-BBB endothelial cells in vivo, whereas GC administration led to nuclear concentration of GRs in BBB endothelium. In correlation with these in vivo data, the use of cerebral and myocardial endothelial cell lines proved GR downregulation in non-BBB cells in vitro in response to GC treatment. Divergent transactivating activity of GRs in the BBB and non-BBB endothelial cellular context could be demonstrated after transfection of endothelial cells with a model GC-responsive test promoter plasmid in the presence and absence of dexamethasone. Our results thus suggest differential signalling mechanisms involved in endothelial barrier regulation, arguing for the development of tissue-specific drugs for therapeutic applications.

Estrogen receptor-alpha regulates SOCS-3 expression in human breast cancer cells.

The suppressor of cytokine signalling (SOCS) protein family negatively regulates cytokine action. In this study, we investigated the effects of estrogen (E2) on SOCS-3 expression in T47D and MCF-7 human breast cancer cells. Real-time PCR analysis of E2-treated T47D cells revealed a ligand and time-dependent increase in of SOCS-3 mRNA levels. Cloning of a 1.7 kb fragment of the human SOCS-3 5' flanking sequence, and subsequent analysis of potential transcription factor-binding sites identified an incomplete ERE motif located -1493 to -1489 upstream of the start site. Transient transfection of the cloned fragment in MCF-7 cells showed that both E2 and genistein treatment caused an increase in reporter gene activity, which was inhibited by co-treatment with ICI 182,780. Chromatin immunoprecipitation analysis revealed an E2 and time-dependent recruitment of ERalpha to the E2 responsive region of the human SOCS-3 promoter. In summary, this study shows that ERalpha directly regulates human SOCS-3 promoter activity in human breast cancer cells, thus modulating cytokine activity.

Activation of PPAR gamma in colon tumor cell lines by oxidized metabolites of linoleic acid, endogenous ligands for PPAR gamma.

The nuclear hormone receptor peroxisome proliferator-activated receptor (PPAR) gamma plays an important role in the differentiation of intestinal cells and other tissues. Real-time PCR examination of PPAR mRNA for gamma1, gamma2 and gamma3, in Caco-2 and HCT-116 colon cell lines showed that gamma3 is the most abundant message in both lines. Treatment of Caco-2 cells with sodium butyrate, which induces cell differentiation, also leads to an increase in all three PPAR mRNAs. In contrast, treatment of HCT-116 cells with sodium butyrate, which does not lead to differentiation of these cells, causes a decrease in the amount of all three PPAR mRNAs. Furthermore, the amount of PPAR mRNA is greater in Caco-2 cells than in HCT-116 cells at all times examined. As several oxidative metabolites of linoleic acid, including 13-hydroxyoctadecadienoic acid (13-HODE) and 13-oxooctadecadienoic acid (13-OXO) have been shown to bind PPAR, and there is a strong positive correlation between enzymes for metabolism of linoleate oxidation products, intestinal cell differentiation and the distribution of PPAR, we also performed a detailed investigation of the activation of PPAR gamma by 13-HODE and 13-OXO. For these experiments, Caco-2 and HCT-116 cells were transfected with constructs containing PPAR gamma1 or gamma2 then a PPRE-luc reporter construct. Exposure of transfected cells to micromolar concentrations of 13-HODE or 13-OXO produced concentration-dependent increases in luciferase activity. In addition, the two linoleate metabolites activate endogenous PPAR in these cell lines transfected with only PPRE-luc. The data substantiate the contention that oxidation products of linoleic acid are metabolically produced endogenous ligands for PPAR gamma and that PPAR gamma plays an important role in the differentiation of intestinal cells.