Application of beta-galactosidase enzyme complementation technology as a high throughput screening format for antagonists of the epidermal growth factor receptor

Evaluation of the InteraX™ System Technology in a High-Throughput Screening Environment

The authors have developed a cell-based high-throughput screening (HTS)-compatible assay tomeasure EGFRdimerization using the InteraX TMenzyme complementation technology of Applied Biosystems. The cells contain 2 chimeric proteins with complementing deletionmutants of the beta galactosidase enzyme, each fused to the extracellular and transmembrane part of EGFR. On binding of EGF, EGF receptor dimerizes and an active beta galactosidase is built. The authors used this homogeneous 384-well assay to screen about 20,000 diverse compounds. From 2 independent primary screen runs 239 hits were identified. For run 1, amean S/Bratio of 4.26 and ameanZβ factor of 0.74were obtained, for run 2 amean S/Bratio of 3.88 and amean Zβ factor of 0.71 were obtained. After hit confirmation, repeated 4 times, 112 hits remainedwith a confirmation rate of 48.9%. Thirty of the 112 could be identified as cytotoxic. Fifty-one of the remaining 82 compounds could be shown to be inhibitors of the beta galactosidase enzymeitself. In summary, 31 compounds remained as potential EGFRdimerization or EGF stimulation inhibitors. The authors conclude that the InteraX TMsystemtechnology is HTS capable and can detect smallmolecule inhibitors capable of inhibiting protein-protein interactions.

The use of mammalian two-hybrid technologies for high-throughput drug screening

Developing agents that target protein-protein interactions (PPIs) is an emerging field in drug discovery. Although this target class has hitherto remained underexplored, it holds exceptional promise related to the large amount of potential PPI targets compared to single protein targets and it offers important opportunities to increase the specificity of therapeutic molecules. While several PPI modulating therapeutics have recently been reported and a number of these are in clinical trial, progress in the field has been hampered by the lack of efficient screening systems. Recently, a number of cellular approaches have been developed that complement classical in vitro screening methods and which exhibit a number of important assets related to the physiological context they provide. Here we discuss the utility of two-hybrid technologies towards high-throughput screening for PPI inhibitors, in particular those that operate in a mammalian cellular background. We review a number of cases where mammalian two-hybrids have been successfully applied to identify small molecule disruptors of PPIs and zoom in further on the MAPPIT (Mammalian Protein-Protein Interaction Trap) technology platform. The value of this approach for drug discovery is illustrated by recent data from MAPPIT-based screening projects.

Overview of high-throughput screening

High-throughput screening (HTS) is a key process used in drug discovery to identify hits from compound libraries that may become leads for medicinal chemistry optimization. This updated overview discusses the utilization of compound libraries, compounds derived from combinatorial and parallel synthesis campaigns and natural product sources; creation of mother and daughter plates; and compound storage, handling, and bar coding in HTS. The unit also presents an overview of established and emerging assay technologies (i.e., time-resolved fluorescence, fluorescence polarization, fluorescence-correlation spectroscopy, functional whole cell assays, and high-content assays) and their integration in automation hardware and IT systems. This revised unit provides updated descriptions of state-of-the-art instrumentation and technologies in this rapidly changing environment. The section on assay methodologies now also covers enzyme complementation assays and methods for high-throughput screening of ion channel activities. Finally, a section on criteria for assay robustness is included discussing the Z'-factor, which is now a widely accepted criterion for evaluation and validation of high throughput screening assays.

Seeking Ligand Bias: Assessing GPCR Coupling to Beta-Arrestins for Drug Discovery

G protein-coupled receptors (GPCR) are the major site of action for endogenous hormones and neurotransmitters. Early drug discovery efforts focused on determining whether ligands could engage G protein coupling and subsequently activate or inhibit cognate "second messengers." Gone are those simple days as we now realize that receptors can also couple βarrestins. As we delve into the complexity of ligand-directed signaling and receptosome scaffolds, we are faced with what may seem like endless possibilities triggered by receptor-ligand mediated events.

Identification of iNOS inhibitors using InteraX

Inducible nitric oxide synthase (iNOS) is active as a homodimer. A cell-based assay suitable for high-throughput screening (HTS) was generated to identify inhibitors of iNOS dimerization using the InteraX enzyme complementation technology of Applied Biosystems. The cells contain 2 chimeric proteins of complementing deletion mutants of beta-galactosidase, each fused to the oxygenase domain of human iNOS. The assay was characterized using known iNOS dimerization inhibitors, which gave a decrease in beta-galactosidase activity. Surprisingly, the assay was also able to identify compounds that have the same profile as known inhibitors of fully formed dimeric iNOS by causing an increase in beta-galactosidase activity. The iNOS InteraX assay was used to screen approximately 800,000 compounds in a 384-well format. After hit confirmation, 3359 compounds were taken forward for full IC50 determination in InteraX and cytotoxicity assays. Of these compounds 40.5% were confirmed as greater than 10-fold more active in InteraX compared to a cytotoxicity assay and were classified as potential iNOS dimerization inhibitors as they did not inhibit beta-galactosidase alone. In the same primary screen, 901 compounds gave a significant increase in beta-galactosidase activity. Many of these were known inhibitors of iNOS. After IC50 determination in InteraX and cytotoxicity assays, 182 novel compounds remained as potential arginine-competitive inhibitors of dimeric iNOS.

Beta galactosidase complementation: a cell-based luminescent assay platform for drug discovery

Many cell-based assays interrogating cell pathway activation employ protocols that require microscopic imaging techniques. However, such assays are not in general widely adopted for primary screening. Protein complementation, particularly of enzymes, provides an alternative approach for cell pathway analysis, with a principal advantage that is amenable to high throughput screening using microtiter plate protocols. Notably, alpha complementation of the enzyme beta-galactosidase has been exploited as a technology in this regard, using substrates that generates luminescent signals. This review describes the various uses of this flexible technology to cell-based assay development.

Firefly luciferase enzyme fragment complementation for imaging in cells and living animals

We identified different fragments of the firefly luciferase gene based on the crystal structure of firefly luciferase. These split reporter genes which encode for protein fragments, unlike the fragments currently used for studying protein-protein interactions, can self-complement and provide luciferase enzyme activity in different cell lines in culture and in living mice. The comparison of the fragment complementation associated recovery of firefly luciferase enzyme activity with intact firefly luciferase was estimated for different fragment combinations and ranged from 0.01 to 4% of the full firefly luciferase activity. Using a cooled optical charge-coupled device camera, the analysis of firefly luciferase fragment complementation in transiently transfected subcutaneous 293T cell implants in living mice showed significant detectable enzyme activity upon injecting d-luciferin, especially from the combinations of fragments identified (Nfluc and Cfluc are the N and C fragments of the firefly luciferase gene, respectively): Nfluc (1-475)/Cfluc (245-550), Nfluc (1-475)/Cfluc (265-550), and Nfluc (1-475)/Cfluc (300-550). The Cfluc (265-550) fragment, upon expression with the nuclear localization signal (NLS) peptide of SV40, shows reduced enzyme activity when the cells are cotransfected with the Nfluc (1-475) fragment expressed without NLS. We also proved in this study that the complementing fragments could be efficiently used for screening macromolecule delivery vehicles by delivering TAT-Cfluc (265-550) to cells stably expressing Nfluc (1-475) and recovering signal. These complementing fragments should be useful for many reporter-based assays including intracellular localization of proteins, studying cellular macromolecule delivery vehicles, studying cell-cell fusions, and also developing intracellular phosphorylation sensors based on fragment complementation.

Novel curcumin- and emodin-related compounds identified by in silico 2D/3D conformer screening induce apoptosis in tumor cells

Background

Inhibition of the COP9 signalosome (CSN) associated kinases CK2 and PKD by curcumin causes stabilization of the tumor suppressor p53. It has been shown that curcumin induces tumor cell death and apoptosis. Curcumin and emodin block the CSN-directed c-Jun signaling pathway, which results in diminished c-Jun steady state levels in HeLa cells. The aim of this work was to search for new CSN kinase inhibitors analogue to curcumin and emodin by means of an in silico screening method.

Methods

Here we present a novel method to identify efficient inhibitors of CSN-associated kinases. Using curcumin and emodin as lead structures an in silico screening with our in-house database containing more than 10⁶ structures was carried out. Thirty-five compounds were identified and further evaluated by the Lipinski's rule-of-five. Two groups of compounds can be clearly discriminated according to their structures: the curcumin-group and the emodin-group. The compounds were evaluated in in vitro kinase assays and in cell culture experiments.

Results

The data revealed 3 compounds of the curcumin-group (e.g. piceatannol) and 4 of the emodin-group (e.g. anthrachinone) as potent inhibitors of CSN-associated kinases. Identified agents increased p53 levels and induced apoptosis in tumor cells as determined by annexin V-FITC binding, DNA fragmentation and caspase activity assays.

Conclusion

Our data demonstrate that the new in silico screening method is highly efficient for identifying potential anti-tumor drugs.

Engineered protein function by selective amino acid diversification

Almost all protein engineering methods rely upon making changes to naturally occurring proteins that already possess some of the desired properties. This will probably remain the case as long as we lack a complete understanding of the way that an amino acid sequence gives rise to a protein with a precisely defined biological function. Common to all methods for altering an existing protein is the selection of a subset of amino acids in the protein for variation and a choice of which substitutions to make at each position. Variants are then tested empirically and further variants are created based upon their performance. Differences between protein engineering methods are the ways in which amino acids are chosen for variation, the protocols followed for creating the variants, and how information regarding variant properties is used in creating subsequent variants. In this article, we describe these differences and provide examples of how the experimental parameters of specific projects determine which method is most suitable.

Cell-based high-throughput screening assay system for monitoring G protein-coupled receptor activation using beta-galactosidase enzyme complementation technology

A novel cell-based functional assay to directly monitor G protein-coupled receptor (GPCR) activation in a high-throughput format, based on a common GPCR regulation mechanism, the interaction between beta-arrestin and ligand-activated GPCR, is described. A protein-protein interaction technology, the InteraX trade mark system, uses a pair of inactive beta-galactosidase (beta-gal) deletion mutants as fusion partners to the protein targets of interest. To monitor GPCR activation, stable cell lines expressing both GPCR- and beta-arrestin-beta-gal fusion proteins are generated. Following ligand stimulation, beta-arrestin binds to the activated GPCR, and this interaction drives functional complementation of the beta-gal mutant fragments. GPCR activation is measured directly by quantitating restored beta-gal activity. The authors have validated this assay system with two functionally divergent GPCRs: the beta2-adrenergic amine receptor and the CXCR2 chemokine-binding receptor. Both receptors are activated or blocked with known agonists and antagonists in a dose-dependent manner. The beta2-adrenergic receptor cell line was screened with the LOPAC trade mark compound library to identify both agonists and antagonists, validating this system for high-throughput screening performance in a 96-well microplate format. Hit specificity was confirmed by quantitating the level of cAMP. This assay system has also been performed in a high-density (384-well) microplate format. This system provides a specific, sensitive, and robust methodology for studying and screening GPCR-mediated signaling pathways.

Novel detection strategies for drug discovery

The Human Genome Project is expected to increase the number of potential drug targets from the current figure of 500 to approximately 3,000-4,000. This increased number of targets, and increasing knowledge of signaling-pathway networks and their complexities, sets new demands for efficiency on HTS assay technologies. Assessment of the total efficacy of a given drug candidate requires not only the classical assays, but also a wide variety of assays related to signaling cascades and cellular functions. Discrete functional assays traditionally involved Ca(2+) flux, kinases and cAMP, but today extend to the whole signaling network, from ligand binding to expression. This review discusses emerging novel non-radioisotopic assays, such as ligand-stimulated GTP-binding, the inositol triphosphate assay, cellular receptor trafficking, and protein-protein interactions.