Nonmonotonic Classical Magnetoconductivity of a Two-Dimensional Electron Gas in a Disordered Array of Obstacles.

Magnetotransport measurements in combination with molecular dynamics simulations on two-dimensional disordered Lorentz gases in the classical regime are reported. In quantitative agreement between experiment and simulation, the magnetoconductivity displays a pronounced peak as a function of the perpendicular magnetic field B which cannot be explained by existing kinetic theories. This peak is linked to the onset of a directed motion of the electrons along the contour of the disordered obstacle matrix when the cyclotron radius becomes smaller than the size of the obstacles. This directed motion leads to transient superdiffusive motion and strong scaling corrections in the vicinity of the insulator-to-conductor transitions of the Lorentz gas.

Antagonism between granulocytic maturation and deacetylase inhibitor-induced apoptosis in acute promyelocytic leukaemia cells.

BACKGROUND: Transcriptional repression is a key mechanism driving leukaemogenesis. In acute promyelocytic leukaemia (APL), the fusion protein promyelocytic leukaemia-retinoic acid receptor-α fusion (PML-RARα) recruits transcriptional repressors to myeloid differentiation genes. All-trans-retinoic acid (ATRA) induces the proteasomal degradation of PML-RARα and granulocytic differentiation. Histone deacetylases (HDACs) fall into four classes (I-IV) and contribute to the transcription block caused by PML-RARα. METHODS: Immunoblot, flow cytometry, and May-Grünwald-Giemsa staining were used to analyze differentiation and induction of apoptosis. RESULTS: A PML-RARα- and ATRA-dependent differentiation programme induces granulocytic maturation associated with an accumulation of the myeloid transcription factor CCAAT/enhancer binding protein (C/EBP)ɛ and of the surface protein CD11b. While this process protects APL cells from inhibitors of class I HDAC activity, inhibition of all Zinc-dependent HDACs (classes I, II, and IV) with the pan-HDACi (histone deacetylase inhibitor(s)) LBH589 induces apoptosis of immature and differentiated APL cells. LBH589 can eliminate C/EBPɛ and the mitochondrial apoptosis regulator B-cell lymphoma (BCL)-xL in immature and differentiated NB4 cells. Thus, BCL-xL and C/EBPɛ are newly identified molecular markers for the efficacy of HDACi against APL cells. CONCLUSIONS: Our results could explain the therapeutic limitations occurring with ATRA and class I HDACi combinations. Pro-apoptotic effects caused by pan-HDAC inhibition are not blunted by ATRA-induced differentiation and may provide a clinically interesting alternative.

SIAH ubiquitin ligases target the nonreceptor tyrosine kinase ACK1 for ubiquitinylation and proteasomal degradation.

Activated Cdc42-associated kinase 1 (ACK1) is a nonreceptor tyrosine kinase linked to cellular transformation. The aberrant regulation of ACK1 promotes tumor progression and metastasis. Therefore, ACK1 is regarded as a valid target in cancer therapy. Seven in absentia homolog (SIAH) ubiquitin ligases facilitate substrate ubiquitinylation that targets proteins to the proteasomal degradation pathway. Here we report that ACK1 and SIAH1 from Homo sapiens interact in a yeast two-hybrid screen. Protein-protein interaction studies and protein degradation analyses using deletion and point mutants of ACK1 verify that SIAH1 and the related SIAH2 interact with ACK1. The association between SIAHs and ACK1 depends on the integrity of a highly conserved SIAH-binding motif located in the far C-terminus of ACK1. Furthermore, we demonstrate that the interaction of ACK1 with SIAH1 and the induction of proteasomal degradation of ACK1 by SIAH1 are independent of ACK1's kinase activity. Chemical inhibitors blocking proteasomal activity corroborate that SIAH1 and SIAH2 destabilize the ACK1 protein by inducing its proteasomal turnover. This mechanism apparently differs from the lysosomal pathway targeting ACK1 after stimulation with the epidermal growth factor. Our data also show that ACK1, but not ACK1 mutants lacking the SIAH binding motif, has a discernable negative effect on SIAH levels. Additionally, knockdown approaches targeting the SIAH2 mRNA uncover specifically that the induction of SIAH2 expression, by hormonally-induced estrogen receptor (ER) activation, decreases the levels of ACK1 in luminal human breast cancer cells. Collectively, our data provide novel insights into the molecular mechanisms modulating ACK1 and they position SIAH ubiquitin ligases as negative regulators of ACK1 in transformed cells.

Quantized magnetic confinement in quantum wires.

Ballistic quantum wires are exposed to longitudinal profiles of perpendicular magnetic fields composed of a spike and a homogeneous part. An asymmetric magnetoconductance peak as a function of the homogeneous magnetic field is found, comprising quantized conductance steps in the interval where the homogeneous magnetic field and the magnetic barrier have identical polarities, and a characteristic shoulder with several resonances in the interval of opposite polarities. The observations are interpreted in terms of inhomogeneous diamagnetic shifts of the quantum wire modes leading to magnetic confinement.

Ubiquitin conjugase UBCH8 targets active FMS-like tyrosine kinase 3 for proteasomal degradation.

The class III receptor tyrosine kinase FMS-like tyrosine kinase 3 (FLT3) regulates normal hematopoiesis and immunological functions. Nonetheless, constitutively active mutant FLT3 (FLT3-ITD) causally contributes to transformation and is associated with poor prognosis of acute myeloid leukemia (AML) patients. Histone deacetylase inhibitors (HDACi) can counteract deregulated gene expression profiles and decrease oncoprotein stability, which renders them candidate drugs for AML treatment. However, these drugs have pleiotropic effects and it is often unclear how they correct oncogenic transcriptomes and proteomes. We report here that treatment of AML cells with the HDACi LBH589 induces the ubiquitin-conjugating enzyme UBCH8 and degradation of FLT3-ITD. Gain- and loss-of-function approaches show that UBCH8 and the ubiquitin-ligase SIAH1 physically interact with and target FLT3-ITD for proteasomal degradation. These ubiquitinylating enzymes though have a significantly lesser effect on wild-type FLT3. Furthermore, physiological and pharmacological stimulation of FLT3 phosphorylation, inhibition of FLT3-ITD autophosphorylation and analysis of kinase-inactive FLT3-ITD revealed that tyrosine phosphorylation determines degradation of FLT3 and FLT3-ITD by the proteasome. These results provide novel insights into antileukemic activities of HDACi and position UBCH8, which have been implicated primarily in processes in the nucleus, as a previously unrecognized important modulator of FLT3-ITD stability and leukemic cell survival.

Cross talk between stimulated NF-kappaB and the tumor suppressor p53.

Nuclear factor-kappaB (NF-kappaB) and p53 critically determine cancer development and progression. Defining the cross talk between these transcription factors can expand our knowledge on molecular mechanisms of tumorigenesis. Here, we show that induction of replicational stress activates NF-kappaB p65 and triggers its interaction with p53 in the nucleus. Experiments with knockout cells show that p65 and p53 are both required for enhanced NF-kappaB activity during S-phase checkpoint activation involving ataxia-telangiectasia mutated and checkpoint kinase-1. Accordingly, the pro-inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) also triggers formation of a transcriptionally active complex containing nuclear p65 and p53 on kappaB response elements. Gene expression analyses revealed that, independent of NF-kappaB activation in the cytosol, TNF-induced NF-kappaB-directed gene expression relies on p53. Hence, p53 is unexpectedly necessary for NF-kappaB-mediated gene expression induced by atypical and classical stimuli. Remarkably, data from gain- and loss-of function approaches argue that anti-apoptotic NF-kappaB p65 activity is constitutively evoked by a p53 hot-spot mutant frequently found in tumors. Our observations suggest explanations for the outstanding question why p53 mutations rather than p53 deletions arise in tumors of various origins.

Influence of the polymerization potential on the transport properties of polypyrrole films.

Polypyrrole films have been prepared by potentiostatic electrochemical polymerization at low temperatures. The cyclic voltammograms and the electronic transport properties of the films are investigated as a function of the polymerization potential. As the potential increases from 520 mV to 1.2 V, the oxidation peak moves to larger voltages, while above 1.2 V, the peak voltage drops again. The film conductivity drops monotonously as the polymerization potential is increased. However, the localization length of the current-carrying states, which characterizes the temperature dependence of the conductivity, correlates with the oxidation peak and shows a minimum for films polymerized at 1.2 V. Furthermore, we show that, with an independent doping step after polymerization, the conductivity of the films can be increased by up to 50%. A maximum conductivity of 1360 S/cm has been observed.

Histone deacetylase inhibitors and hydroxyurea modulate the cell cycle and cooperatively induce apoptosis.

Therapy resistance represents a major problem for disease management in oncology. Histone deacetylase inhibitors (HDACi) have been shown to modulate the cell cycle, to induce apoptosis and to sensitize cancer cells for other chemotherapeutics. Our study shows that the HDACi valproic acid (VPA) and the ribonucleotide reductase inhibitor hydroxyurea (HU) potentiate the pro-apoptotic effects of each other towards several cancer cell lines. This correlates with the HU-induced degradation of the cyclin-dependent kinase inhibitors (CDKI) p21 and p27, mediated by the proteasome or caspase-3. Moreover, we found that caspase-3 activation is required for VPA-induced apoptosis. Remarkably, p21 and p27 can confer resistance against VPA and HU. Both CDKI interact with caspase-3 and compete with other caspase-3 substrates. Hence, p21 and p27 may contribute to chemotherapy resistance as apoptosis inhibitors. Since the biological effects of VPA and HU could be achieved at concentrations used in current treatment protocols, the combined application of these compounds might be considered as a potential strategy for cancer treatment.

Valproic acid (VPA) in patients with refractory advanced cancer: a dose escalating phase I clinical trial.

Altered histone deacetylase (HDAC) activity has been identified in several types of cancer. This study was designed to determine the safety and maximum tolerated dose (MTD) of valproic acid (VPA) as an HDAC inhibitor in cancer patients. Twenty-six pre-treated patients with progressing solid tumours were enrolled in dose-escalating three-patient cohorts, starting at a dose of VPA 30 mg kg(-1) day(-1). VPA was administered as an 1-h infusion daily for 5 consecutive days in a 21-day cycle. Neurocognitive impairment dominated the toxicity profile, with grade 3 or 4 neurological side effects occurring in 8 out of 26 patients. No grade 3 or 4 haematological toxicity was observed. The MTD of infusional VPA was 60 mg kg(-1) day(-1). Biomonitoring of peripheral blood lymphocytes demonstrated the induction of histone hyperacetylation in the majority of patients and downmodulation of HDAC2. Pharmacokinetic studies showed increased mean and maximum serum VPA concentrations >120 and >250 mg l(-1), respectively, in the 90 and 120 mg kg(-1) cohorts, correlating well with the incidence of dose-limiting toxicity (DLT). Neurotoxicity was the main DLT of infusional VPA, doses up to 60 mg kg(-1) day(-1) for 5 consecutive days are well tolerated and show detectable biological activity. Further investigations are warranted to evaluate the effectivity of VPA alone and in combination with other cytotoxic drugs.

Negative glucocorticoid receptor response elements and their role in glucocorticoid action.

The glucocorticoid receptor (GR) belongs to the steroid hormone receptor subclass of nuclear receptors and controls physiological processes through activation and repression of specific target genes. The ligand-activated receptor dimer activates gene expression by binding to specific DNA sequences (glucocorticoid response element, GRE) in the promoter regions of glucocorticoid-regulated genes. In contrast to the regulation of these classical GREs, the repression of negatively regulated target genes is mediated by negative GREs (nGRE), composite GREs or by transrepression. Due to their broad therapeutic spectrum and superior therapeutic effects glucocorticoids (GCs) are the most effective drugs used for the treatment of acute and chronic inflammatory diseases. Unfortunately, long term systemic therapy with GCs is restricted due to their metabolic side effects. It is assumed that transrepression of transcription factors such as AP-1 and NF-kappa B is the main mechanism by which glucocorticoids mediate their anti-inflammatory activity, whereas the side effects of GCs are mainly mediated by GR-DNA-interaction either by activation or by negative regulation of gene expression. While trans-repression has been characterized in detail, the molecular mechanisms of DNA-dependent cis-repression remain unclear. In this review, we focus on current knowledge about nGRE-mediated target gene repression and the relevance and function of these genes for glucocorticoid action. Negative GREs contribute to the regulation of the hypothalamic-pituitary-adrenal (HPA) axis (POMC and CRH), bone (osteocalcin) and skin (keratins) function, inflammation (IL-1beta), angiogenesis (proliferin) and lactation (prolactin). The discovery of the underlying mechanisms, especially the comparison to positive GREs and trans-repression may help in the future to discover and analyze novel selective GR agonists.

Valproic acid defines a novel class of HDAC inhibitors inducing differentiation of transformed cells.

Histone deacetylases (HDACs) play important roles in transcriptional regulation and pathogenesis of cancer. Thus, HDAC inhibitors are candidate drugs for differentiation therapy of cancer. Here, we show that the well-tolerated antiepileptic drug valproic acid is a powerful HDAC inhibitor. Valproic acid relieves HDAC-dependent transcriptional repression and causes hyperacetylation of histones in cultured cells and in vivo. Valproic acid inhibits HDAC activity in vitro, most probably by binding to the catalytic center of HDACs. Most importantly, valproic acid induces differentiation of carcinoma cells, transformed hematopoietic progenitor cells and leukemic blasts from acute myeloid leukemia patients. More over, tumor growth and metastasis formation are significantly reduced in animal experiments. Therefore, valproic acid might serve as an effective drug for cancer therapy.

Energy spectra of quantum rings.

Quantum mechanical experiments in ring geometries have long fascinated physicists. Open rings connected to leads, for example, allow the observation of the Aharonov-Bohm effect, one of the best examples of quantum mechanical phase coherence. The phase coherence of electrons travelling through a quantum dot embedded in one arm of an open ring has also been demonstrated. The energy spectra of closed rings have only recently been studied by optical spectroscopy. The prediction that they allow persistent current has been explored in various experiments. Here we report magnetotransport experiments on closed rings in the Coulomb blockade regime. Our experiments show that a microscopic understanding of energy levels, so far limited to few-electron quantum dots, can be extended to a many-electron system. A semiclassical interpretation of our results indicates that electron motion in the rings is governed by regular rather than chaotic motion, an unexplored regime in many-electron quantum dots. This opens a way to experiments where even more complex structures can be investigated at a quantum mechanical level.

Histone deacetylase as a therapeutic target.

The maintenance of health depends on the coordinated and tightly regulated expression of genetic information. Certain forms of leukemia have become paradigms for the pathogenic role of aberrant repression of differentiation genes. In these acute leukemias, fusion proteins generated by chromosomal translocations no longer function as transcriptional activators, but instead repress target genes by recruiting histone deacetylases (HDACs). The potential benefit of HDAC inhibition has been established by the use of enzyme inhibitors in vitro and in a single reported case of experimental therapy. Because recently identified HDAC inhibitors appear to overcome many drawbacks of early inhibitory compounds in clinical use, the stage is set to test the therapeutic value of HDAC inhibition in leukemias and in other diseases, including solid tumors and aberrant hormonal signaling. This review summarizes the range of diseases expected to respond to HDAC inhibition.

Shot noise by quantum scattering in chaotic cavities.

We have experimentally studied shot noise of chaotic cavities defined by two quantum point contacts in series. The cavity noise is determined as (1/4)2e/I/ in agreement with theory and can be well distinguished from other contributions to noise generated at the contacts. Subsequently, we have found that cavity noise decreases if one of the contacts is further opened and reaches nearly zero for a highly asymmetric cavity. Heating inside the cavity due to electron-electron interaction can slightly enhance the noise of large cavities and is also discussed quantitatively.

Signatures of spin pairing in chaotic quantum dots.

Coulomb blockade resonances are measured in a GaAs quantum dot in which both shape deformations and interactions are small. The parametric evolution of the Coulomb blockade peaks shows a pronounced pair correlation in both position and amplitude, which is interpreted as spin pairing. As a consequence, the nearest-neighbor distribution of peak spacings can be well approximated by a modified bimodal Wigner surmise, in which interactions are taken into account beyond the constant interaction model.

Multiple regions of ETO cooperate in transcriptional repression.

In acute myeloid leukemias (AMLs) with t(8;21), the transcription factor AML1 is juxtaposed to the zinc finger nuclear protein ETO (Eight-Twenty-One), resulting in transcriptional repression of AML1 target genes. ETO has been shown to interact with corepressors, such as N-CoR and mSin3A to form complexes containing histone deacetylases. To define regions of ETO required for maximal repressor activity, we analyzed amino-terminal deletions in a transcriptional repression assay. We found that ETO mutants lacking the first 236 amino acids were not affected in their repressor activity, whereas a further deletion of 85 amino acids drastically reduced repressor function and high molecular weight complex formation. This latter mutant can still homodimerize and bind to N-CoR but shows only weak binding to mSin3A. Furthermore, we could show that a "core repressor domain" comprising nervy homology region 2 and its amino- and carboxyl-terminal flanking sequences recruits mSin3A and induces transcriptional repression. These results suggest that mSin3A and N-CoR bind to ETO independently and that both binding sites cooperate to maximize ETO-mediated transcriptional repression. Thus, ETO has a modular structure, and the interaction between the individual elements is essential for the formation of a stable repressor complex and efficient transcriptional repression.

Analysis of the metallic phase of two-dimensional holes in SiGe in terms of temperature dependent screening

We find that temperature dependent screening can quantitatively explain the metallic behavior of the resistivity on the metallic side of the so-called metal-insulator transition in p-SiGe. Interference and interaction effects exhibit the usual insulating behavior which is expected to overpower the metallic background at sufficiently low temperatures. We find empirically that the concept of a Fermi liquid describes our system with its large interaction parameter r(s) approximately 8.

VDR-Alien: a novel, DNA-selective vitamin D(3) receptor-corepressor partnership.

The vitamin D receptor (VDR) is a transcription factor that transmits incoming 1,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) signaling via combined contact with coactivator proteins and specific DNA binding sites (VDREs), which ultimately results in activation of transcription. In contrast, the mechanisms of transcriptional repression via the VDR are less well understood. This study documents VDR-dependent transcriptional repression largely via histone deacetylase (HDAC) activity. Direct, ligand-sensitive protein-protein interaction of the VDR with the nuclear receptor corepressor (NCoR) and a novel corepressor, called Alien, was demonstrated to be comparable but independent of the VDR AF-2 trans-activation domain. Functional assays indicated that Alien, but not NCoR, displays selectivity for different VDRE structures for transferring these repressive effects into gene regulatory activities. Moreover, superrepression via Alien was found to be affected only in part by HDAC inhibitors such as trichostatin A. Finally, for a dissociation of VDR-Alien complexes in vitro and in vivo, higher ligand concentrations were needed than for a dissociation of VDR-NCoR complexes. This suggests that Alien and NCoR are using different interfaces for interaction with the VDR and different pathways for mediating superrepression, which in turn characterizes Alien as a representative of a new class of corepressors. Taken together, association of the VDR with corepressor proteins provides a further level of transcriptional regulation, which is emerging as a complex network of protein-protein interaction-mediated control.

The fermionic hanbury brown and twiss experiment

A Hanbury Brown and Twiss experiment for a beam of electrons has been realized in a two-dimensional electron gas in the quantum Hall regime. A metallic split gate serves as a tunable beam splitter to partition the incident beam into transmitted and reflected partial beams. In the nonequilibrium case the fluctuations in the partial beams are shown to be fully anticorrelated, demonstrating that fermions exclude each other. In equilibrium, the cross-correlation of current fluctuations at two different contacts is also found to be negative and nonzero, provided that a direct transmission exists between the contacts.