Deubiquitinases: A Backdoor into Undruggable Targets?

Molecular model of the yeast proteasome.
Molecular model of the yeast proteasome.

Ubiquitin modification of a protein directs events such as targeting for proteasomal degradation. Targeting a protein for degradation through ubiquitin modification is one way to regulate the amount of time a signaling protein, such as a kinase or other enzyme, is available to participate in cell signaling events. Deubiquitinases (DUBs) are enzymes that cleave the ubiquitin tags from proteins, and they have been implicated in several diseases, including cancer.

With their roles in the stabilization of proteins involved in cell cycle progression and other critical processes, DUBs are promising targets for small molecule inhibitors, particularly since they may provide a “back door” for targeting otherwise intractable, undruggable proteins by modulating their half lives. However, finding small molecule inhibitors of the ubiquitin proteases to date has not been trivial. Here we highlight two papers describing the identification and characterization of small molecule inhibitors against the DUB USP7. Continue reading “Deubiquitinases: A Backdoor into Undruggable Targets?”

Review: Validation of a high throughput cell-based assay to screen compounds for mitochondrial toxicity

mitotox_covertipDrug-induced mitochondrial toxicity is a concern for pharmaceuticals that was, until recently, limited by the availability of a cell-based assay that is amenable to rapid high-throughput screening. Incorporating high-throughput assay chemistry that can detect mitochondrial dysfunction early in drug discovery programs provides the opportunity to identify potential mitotoxicants before they reach clinical trials or the market population.

In the paper reviewed here (1) Swiss and colleagues have validated the use of a commercially available Mitochondrial ToxGlo™ Asssay (2) to replace traditional methods that not only require expensive reagents and/or equipment but also do not lend themselves easily to high-throughput screening formats. Continue reading “Review: Validation of a high throughput cell-based assay to screen compounds for mitochondrial toxicity”

Shining a Bright Light on Deep Questions in Biology with Bioluminescence

artists view inside a cellA quick search of the PubMed database for “dual luciferase” quickly returns over 1,000 papers. The Dual-Luciferase® Reporter Assay is a powerful tool that allows researchers to ask a multitude of questions about gene control and expression in a system that itself could be normalized and internally controlled. For more than 15 years, firefly and Renilla luciferases have been valuable tools for researchers asking many different kinds of questions in the life sciences. In a recent webinar, Biologically Relevant Assays for Oncology: Harnessing the Power of Bioluminescence, Neal Cosby discussed how bioluminescent chemistries have formed the basis of a range of powerful assays and research tools for scientists who are asking questions about the deep and complex genetic and cellular story associated with cancer. Here we talk a bit of about bioluminescent chemistries, some of the newest bioluminescent tools available, and how some of these tools can be used to probe the deeper questions of cell biology, including cancer biology. Continue reading “Shining a Bright Light on Deep Questions in Biology with Bioluminescence”

Screening for Inhibitors of CD73 (5´-ectonucleotidase) Using a Metabolite Assay

CD73CD73 also known as 5´-Ectonucleotidase (NT5E) is a membrane-anchored protein that acts at the outer surface of the cell to convert AMP to adenosine and free phosphate. CD73 activity is associated with immunosuppression and prometastatic effects, including angiogenesis. CD73 is highly expressed on the surfaces of many types of cancer cells and other immunosuppressive cells (1). A recent study by Quezada and colleagues showed that the high concentration of adenosine produced by the CD73-catalyzed reaction on glioblastoma multiforme cells, which are characterized by extreme chemoresistance, triggered adenosine signaling and in turn, the multi-drug resistance (MDR) phenotype of these cells (2).

Because of the roles of adenosine in immunosuppression, angiogenesis and MDR phenotypes, CD73 (NT5E) is an attractive therapeutic target. However, the current methods of assaying for the ectonucleotidase activity, HPLC and a malachite green assay, are cumbersome and not suited to high-throughput screening. The HPLC assay is expensive and difficult to automate and miniaturize (3). The malachite green assay is sensitive to phosphate found in media, buffers and other solutions used in the compound-screening environment.

To address the problem of developing a reliable high-throughput screening assay for CD73, Sachsenmeier and colleagues (3) looked to a luminescent ATP-detection reagent. Continue reading “Screening for Inhibitors of CD73 (5´-ectonucleotidase) Using a Metabolite Assay”

The Ideal Kinase Assay

Kinome_FullIf you could design the ideal kinase assay system what would it look like?

  • Would it be able to match, point for point, the results of the tried-and-true isotopic assay methods but not have any of the associated safety and waste disposal issues?
  • Would it avoid the use of specific antibodies?
  • Would it minimize false hits associated with many of the fluorescence-based assays?
  • Would it be affordable technology, adaptable to any laboratory’s throughput from 96-well to 1,536-well automated screening?
  • Would it be universal—able to assess the function of any kind of kinase (protein, lipid or sugar) that uses any kind of substrate?
  • Would it be able to detect low conversion rates (low-activity enzymes) with a high signal-to-background ratio?
  • Would you be able to use it with substrates that are multiphosphorylated?

If you answered “yes” to any of the above questions, you might want to take a look at the Promega Webinar “Enabling Kinase Research with a Luminescent ADP Detection Platform and Complete Kinase Enzyme Systems”, presented by Hicham Zegzouti, PhD, research scientist at Promega. Here he describes the ADP-Glo™ Assay platform, which meets these and several other criteria.

The precise molecular lesion that occurs with the Philadelphia Chromosome translocation—a rearrangement that creates a bcr-abl fusion in which the abl tyrosine kinase is constitutively active leading to the development of chronic myeloid leukemia is the first description of dysregulation of a kinase leading to a particular disease state. However, the human genome contains 518 protein kinases and many other atypical kinases, and one-third of all human proteins are phosphorylated. It is now estimated that over 400 human diseases are caused by dysregulation or mutation of kinases, making kinases a major target for drug discovery efforts. Continue reading “The Ideal Kinase Assay”

Screening for Drug-Drug Interactions with PXR and CYP3A4 Activation

Cytochrome P450 3A4 Enzyme

Numerous pharmaceutical companies have adopted assays for detecting activation of pregnane X receptor (PXR), a nuclear receptor known to regulate expression of cytochrome P450 (CYP450) drug-metabolizing enzymes (1). PXR is a transcription factor that has been designated the “master xenosensor” due to its ability to upregulate cellular levels of a variety of drug-metabolizing enzymes in response to drugs and foreign chemicals. Elevated levels of CYP450 enzymes can elicit alterations in the pharmacokinetics of co-administered drugs, which can result in adverse drug-drug interactions (DDI) or diminished bioavailability. By assessing PXR activation and CYP450 enzyme induction early in the drug development process, many companies hope to reduce late-stage clinical failures and minimize the high costs associated with bringing a new drug to market.

Proportion of drugs metabolized by different CYPs

A recent paper by Shukla et al. (2) examined over 2,800 clinically used drugs for their ability to activate human PXR (hPXR) and rat PXR (rPXR), induce human cytochrome P450 3A4 enzyme (CYP3A4) at the cellular level, and bind hPXR at the protein level. Several studies have identified PXR as playing a key role in regulating the expression of CYP3A4, an enzyme involved in the metabolism of more than 50% of all drugs prescribed in humans. Continue reading “Screening for Drug-Drug Interactions with PXR and CYP3A4 Activation”

Describing Life and Death in the Cell

4621CALife is complicated. So is death. And when the cells in your multiwell plate die after compound treatment, it’s not enough to know that they died. You need to know how they died: apoptosis or necrosis? Or, have you really just reduced viability, rather than induced death? Is the cytotoxicity you see dose-dependent? If you look earlier during drug treatment of your cells, do you see markers of apoptosis? If you wait longer, do you observe necrosis? If you reduce the dosage of your test compound, is it still cytotoxic? Continue reading “Describing Life and Death in the Cell”