Roquin-2 promotes ubiquitin-mediated degradation of ASK1 to regulate stress responses

Apoptosis signal-regulating kinase 1 (ASK1, also known as MAP3K5) mediates reactive oxygen species (ROS)-induced cell death. When activated by ROS, ASK1 ultimately becomes ubiquitinated and degraded by the proteasome, a process that is antagonized by the ubiquitin-specific protease USP9X. Using a functional siRNA (small interfering RNA) screen in HeLa cells, we identified Roquin-2 (also called RC3H2) as an E3 ubiquitin ligase required for ROS-induced ubiquitination and degradation of ASK1. In cells treated with H2O2, knockdown of Roquin-2 promoted sustained activation of ASK1 and the downstream stress-responsive kinases JNK (c-Jun amino-terminal kinase) and p38 MAPK (mitogen-activated protein kinase), and led to cell death. The nematode Caenorhabditis elegans produces ROS as a defense mechanism in response to bacterial infection. In C. elegans, mutation of the gene encoding the Roquin-2 ortholog RLE-1 promoted accumulation of the activated form of the ASK1 ortholog NSY-1 and conferred resistance to infection by the bacteria Pseudomonas aeruginosa. Thus, these data suggest that degradation of ASK1 mediated by Roquin-2 is an evolutionarily conserved mechanism required for the appropriate regulation of stress responses, including pathogen resistance and cell death.

Abstract SY02-04: Regulation of invadopodia formation and cancer cell invasion

Cell invasion plays a central role in a wide variety of biological phenomena, and is the cause of tumor growth and metastasis. Understanding the biochemical mechanisms that control cell invasion is one of the major goals of our laboratory. Podosomes and invadopodia are specialized cellular structures present in cells with physiological or pathological invasive behaviors. These transient structures are localized at the ventral cell surface, contain an array of different proteins, and facilitate cell-substrate adhesion as well as the local proteolytic activity necessary for extracellular matrix remodeling and subsequent cellular invasion. We have previously shown that the adaptor protein and Src substrate Tks5 is required for both podosome and invadopodia formation, and for tumor growth in vivo. More recently we also defined a role for the Tks5-mediated generation of reactive oxygen species (ROS). We have developed a high content, cell-based high throughput screening assay that allows us to identify inhibitors and activators of invadopodia formation. As a proof of principle, we tested the LOPAC1280 collection of small molecules and found 7 inhibitory compounds and 2 activators. Of the 7 inhibitors, 4 are cyclin-dependent kinase (Cdk) inhibitors. Loss-of-function experiments demonstrated that Cdk5 is the responsible kinase. On the other hand, paclitaxel was one of the two compounds that increased invadopodia formation. We have also used our high content assay to screen a large compound collection as well as an siRNA library targeting the entire kinome. The mechanisms by which Cdk5 and tubulin regulate invadopodia formation, and the nature of the other regulators, are the subjects of our current investigations.

Citation Format: Barbara Blouw, Matt Buschman, Pilar Cejudo-Martin, Begoña Diaz, Christine Gould, Danielle Murphy, Manuela Quintavalle, Susanne Heynen, Behrad Azimi, Jeff Price, Sara A. Courtneidge. Regulation of invadopodia formation and cancer cell invasion [abstract]. In: Proceedings of the AACR 101st Annual Meeting 2010; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr SY02-04

Statistics of Assay Validation in High Throughput Cell Imaging of Nuclear Factor κB Nuclear Translocation

This report describes statistical validation methods implemented on assay data for inhibition of subcellular redistribution of nuclear factor kappaB (NF kappaB) in HeLa cells. We quantified cellular inhibition of cytoplasmic-nuclear translocation of NF kappaB in response to a range of concentrations of interleukin-1 (IL-1) receptor antagonist in the presence of IL-1alpha using eight replicate rows in each four 96-well plates scanned five times on each of 2 days. Translocation was measured as the fractional localized intensity of the nucleus (FLIN), an implementation of our more general fractional localized intensity of the compartments (FLIC), which analyzes whole compartments in the context of the entire cell. The NF kappaB antagonist assay (inhibition of IL-1- induced NF kappaB translocation) data were collected on a Q3DM (San Diego, CA) EIDAQtrade mark 100 high throughput microscopy system. [In 2003, Q3DM was purchased by Beckman Coulter Inc. (Fullerton, CA), which released the IC 100 successor to the EIDAQ 100.] The generalized FLIC method is described along with two-point (minimum-maximum) and multiple point titration statistical methods. As a ratio of compartment intensities that tend to change proportionally, FLIN was resistant to photobleaching errors. Two-point minimum-maximum statistical analyses yielded the following: a Z' of 0.174 with the data as n = 320 independent well samples; Z' by row data in a range of 0.393-0.933, with a mean of 0.766; by-plate Z' data of 0.310, 0.443, 0.545, and 0.794; and by-plate means of columns Z' data of 0.879, 0.927, 0.945, and 0.963. The mean 50% inhibitory concentration (IC50) for IL-1 receptor antagonist over all experiments was 213 ng/ml. The combined IC50 coefficients of variation (CVs) were 0.74%, 0.85%, 2.09%, and 2.52% for the four plates. Repeatability IC50 CVs were as follows: day to day 3.0%, row to row 8.0%, plate to plate 2.8%, and day to day 0.6%. The number of cells required for statistically resolvable differences in dose concentrations, plotted in a family of FLIN sigma/deltamicro (SD/range) curves and tabulated, demonstrated cell-by-cell assay precision with our combined sigma/deltamicro = 0.32 that required approximately 10-fold fewer cells than in a previously reported NF kappaB assay with sigma/deltamicro = 1.52. To better understand the relationship between cell-by-cell measurements and IC50 precision, 500 Monte Carlo simulations with varying cell-measurement SDs were used to explore three-, five-, seven-, and 11-point model titrations. The reductions in deltaIC50 90% confidence intervals from 11- to three-point titrations were 10-fold with the previously reported sigma/deltamicro = 1.52 and twofold with our sigma/deltamicro = 0.32. With these normalized parameters, this report provides a common statistical foundation, independent of the assay details, for evaluating the performance of imaging data on any instrument.

Advances in molecular labeling, high throughput imaging and machine intelligence portend powerful functional cellular biochemistry tools

Cellular behavior is complex. Successfully understanding systems at ever-increasing complexity is fundamental to advances in modern science and unraveling the functional details of cellular behavior is no exception. We present a collection of prospectives to provide a glimpse of the techniques that will aid in collecting, managing and utilizing information on complex cellular processes via molecular imaging tools. These include: 1) visualizing intracellular protein activity with fluorescent markers, 2) high throughput (and automated) imaging of multilabeled cells in statistically significant numbers, and 3) machine intelligence to analyze subcellular image localization and pattern. Although not addressed here, the importance of combining cell-image-based information with detailed molecular structure and ligand-receptor binding models cannot be overlooked. Advanced molecular imaging techniques have the potential to impact cellular diagnostics for cancer screening, clinical correlations of tissue molecular patterns for cancer biology, and cellular molecular interactions for accelerating drug discovery. The goal of finally understanding all cellular components and behaviors will be achieved by advances in both instrumentation engineering (software and hardware) and molecular biochemistry.