Suitability of enhanced green fluorescent protein as a reporter component for bioassays

Two-color fluorescent proteins reporting survivin regulation in breast cancer cells for high throughput drug screening

Reporter gene assay is widely used for high throughput drug screening and drug action mechanism evaluation. In this study, we developed a robust dual-fluorescent reporter assay to detect drugs repressing the transcription of survivin, a cancer biomarker from the inhibitor of apoptosis family, in breast cancer cells cultured in three-dimensional (3D) microbioreactors. Survivin is overexpressed in numerous malignancies but almost silent in normal tissue cells and is considered a lead target for cancer therapy. Breast cancer MCF-7 cells were engineered to express enhanced green fluorescent protein driven by a survivin promoter and red fluorescent protein driven by a cytomegalovirus promoter as internal control to detect changes in survivin expression in cells as affected by drugs. This 3D dual-fluorescent reporter assay was validated with YM155 and doxorubicin, which were known to downregulate survivin in cancer cells, and further evaluated with two widely used anticancer compounds, cisplatin, and epigallocatechin gallate, to evaluate their effects on survivin expression. The results showed that the 3D dual-fluorescent reporter assay was robust for high throughput screening of drugs targeting survivin in breast cancer cells.

An in-vitro approach for water quality determination: activation of NF-κB as marker for cancer-related stress responses induced by anthropogenic pollutants of drinking water

Epidemiological studies show that there is a link between urban water pollution and increase in human morbidity and mortality. With the increase in number of new substances arising from the chemical, pharmaceutical, and agricultural industries, there is an urgent need to develop biological test systems for fast evaluation of potential risks to humans and the environmental ecosystems. Here, a combined cellular reporter assay based on the cellular survival and the stress-induced activation of the survival-promoting factor nuclear factor κB (NF-κB) and its use for the detection of cytotoxicity and cancer-related stress responses is presented. A total of 14 chemicals that may be found in trace-amounts in ground water levels are applied and tested with the presented assay. The project is embedded within the joint research project TOX-BOX which aims to develop a harmonized testing strategy for risk management of anthropogenic trace substances in potable water. The assay identified carbendazim as a NF-κB-activating agent in mammalian cells.

Space experiment "Cellular Responses to Radiation in Space (CellRad)": Hardware and biological system tests

One factor contributing to the high uncertainty in radiation risk assessment for long-term space missions is the insufficient knowledge about possible interactions of radiation with other spaceflight environmental factors. Such factors, e.g. microgravity, have to be considered as possibly additive or even synergistic factors in cancerogenesis. Regarding the effects of microgravity on signal transduction, it cannot be excluded that microgravity alters the cellular response to cosmic radiation, which comprises a complex network of signaling pathways. The purpose of the experiment "Cellular Responses to Radiation in Space" (CellRad, formerly CERASP) is to study the effects of combined exposure to microgravity, radiation and general space flight conditions on mammalian cells, in particular Human Embryonic Kidney (HEK) cells that are stably transfected with different plasmids allowing monitoring of proliferation and the Nuclear Factor κB (NF-κB) pathway by means of fluorescent proteins. The cells will be seeded on ground in multiwell plate units (MPUs), transported to the ISS, and irradiated by an artificial radiation source after an adaptation period at 0 × g and 1 × g. After different incubation periods, the cells will be fixed by pumping a formaldehyde solution into the MPUs. Ground control samples will be treated in the same way. For implementation of CellRad in the Biolab on the International Space Station (ISS), tests of the hardware and the biological systems were performed. The sequence of different steps in MPU fabrication (cutting, drilling, cleaning, growth surface coating, and sterilization) was optimized in order to reach full biocompatibility. Different coatings of the foil used as growth surface revealed that coating with 0.1 mg/ml poly-D-lysine supports cell attachment better than collagen type I. The tests of prototype hardware (Science Model) proved its full functionality for automated medium change, irradiation and fixation of cells. Exposure of HEK cells to the β-rays emitted by the radiation source dose-dependently decreased cell growth and increased NF-κB activation. The signal of the fluorescent proteins after formaldehyde fixation was stable for at least six months after fixation, allowing storage of the MPUs after fixation for several months before the transport back to Earth and evaluation of the fluorescence intensity. In conclusion, these tests show the feasibility of CellRad on the ISS with the currently available transport mechanisms.

Fluorescent labeling for clonal selection of Marc 145 cells secreting high levels of recombinant protein PBD-1

Despite the powerful impact gene expression markers like the green fluorescent protein (GFP) or enhanced GFP (EGFP) exert on linking the expression of recombinant protein for selection of high producers in recent years, there is still a strong incentive to develop more economical and efficient methods for isolating mammalian cell clones secreting high levels of recombinant proteins. Here we present a new method based on the co-expression of EGFP that allows clonal selection in standard 96-well cell culture plates. The genes encoding the EGFP protein and the related protein are linked by an internal ribosome entry site and thus are transcribed into the same mRNA in an independent translation process. Since both proteins arise from a common mRNA, the EGFP expression level correlates with the expression level of the therapeutic protein in each clone. By expressing recombinant porcine β-defensin 1 in Marc 145 cells, we demonstrate the robustness and performance of this technique. The method can be served as an alternative to identify high-producer clones with various cell sorting methods.

Cytotoxicity and genotoxicity reporter systems based on the use of mammalian cells

With the dramatic increase in the number of new agents arising from the chemical, pharmaceutical, and agricultural industries, there is an urgent need to develop assays for rapid evaluation of potential risks to man and environment. The panel of conventional tests used for cytotoxicity and genotoxicity and the strategies to progress from small scale assays to high content screening in toxicology are discussed. The properties of components necessary as sensors and reporters for new reporter assays, and the application of genetic strategies to design assays are reviewed. The concept of cellular reporters is based on the use of promoters of chemical stress-regulated genes ligated to a suitable luminescent or fluorescent reporter gene. Current reporter assays designed from constructs transferred into suitable cell lines are presented.

Detection of UV-induced activation of NF-kappaB in a recombinant human cell line by means of Enhanced Green Fluorescent Protein (EGFP)

The cellular protection reaction known as ultraviolet (UV) response leads to increased transcription of several genes. Parts of this transcriptional response are transmitted via activation of the Nuclear factor kappaB (NF-kappaB). The contribution of different UV radiation qualities to this process is not yet known. In a previous work, a stably transfected human cell line was developed which indicates activation of the NF-kappaB pathway by fluorescence of the reporters Enhanced Green Fluorescent Protein (EGFP) and its destabilized variant (d2EGFP) thereby allowing a fast and reliable monitoring of UV effects on the NF-kappaB pathway. Cells were exposed to a mercury low-pressure lamp or to simulated sunlight of different wavelength ranges and subjected to flow cytometric analysis after different post-irradiation periods. Growth capacity of cells after UV irradiation was quantified using a luminance measurement of crystal violet stained cell layers. In contrast to UVC and UVB, UVA radiation induced d2EGFP expression and NF-kappaB activation in a non-cytotoxic dose range. These results show that NF-kappaB plays a role in the UVA-induced gene activation in a non-cytotoxic dose range in a human epithelial cell line.

Activation of Nuclear Factor κB by Different Agents

The transcription factor nuclear factor kappaB (NF-kappaB) or other components of this pathway have been identified as possible therapeutic targets in inflammatory processes, cancer, and autoimmune diseases. In order to clarify the role of NF-kappaB in epithelial cells in response to different stresses, a cell-based screening assay for activation of NF-kappaB-dependent gene transcription in human embryonic kidney cells (HEK/293) was developed. This assay allows detection of NF-kappaB activation by measurement of the fluorescence of the reporter protein destabilized enhanced green fluorescent protein (d2EGFP). For characterization of the cell-based assay, activation of the pathway by several agents, for example, tumor necrosis factor alpha (TNF-alpha), interleukin-1beta (IL-1beta), lipopolysaccharide (LPS), camptothecin and phorbol ester (PMA), and the influence of the culture conditions on NF-kappaB activation by TNF-alpha were examined. NF-kappaB was activated by TNF-alpha, IL-1beta, PMA, and camptothecin in a dose-dependent manner, but not by LPS. TNF-alpha results in the strongest induction of NF-kappaB-dependent gene expression. However, this response fluctuated from 30 to 90% of the cell population showing d2EGFP expression. This variation can be explained by differences in growth duration and cell density at the time of treatment. With increasing confluence of the cells, the activation potential decreased. In a confluent cell layer, only 20-35% of the cell population showed d2EGFP expression. The underlying mechanism of this phenomenon can be the production of soluble factors by the cells inhibiting the NF-kappaB activation or direct communication via gap junctions in the cell layer diminishing the TNF-alpha response.

Novel reporter gene in a fluorescent-based whole cell sensing system

A common problem encountered when using fluorescence detection in real samples analysis is that the matrix may contain compounds that autofluorescence or that can be excited at the wavelengths of commonly employed fluorescent reporter molecules. This causes an increase in background fluorescence, which in turn tends to compromise the detection limits of the system. To address this issue, we investigated the use of a reporter enzyme that produces fluorescent compounds, which can be excited at wavelengths that are not commonly encountered in compounds present in real samples. For that, a whole cell-based sensing system for arsenite that employs cobA as the reporter gene was developed. The system utilizes genetically engineered bacteria that incorporate the specificity of the ars operon with the sensitivity of the cobA gene. The cobA gene codes for uroporphyrinogen III methyltransferase that converts the substrate uroporphyrinogen (urogen) III into two fluorescent compounds sirohydrochlorin and trimethylpyrrocorphin. Urogen III is ubiquitous within the cell, however, because the cells use it for vitamin B12 and siroheme biosynthesis, this sensing system is limited by substrate availability. By supplementing the media with ALA, a precursor of urogen III, a more stable and reproducible response was obtained. We observed three excitation maxima at 357, 378, and 498 nm, with a single emission maximum at 605 nm. Excitation at 498 nm was selected because it results in less background interference as most endogenous substances are not active at this wavelength. Advantages and limitations of using the cobA gene in whole-cell sensing applications are presented.

Generation of stably transfected Mammalian cell lines as fluorescent screening assay for NF-kappaB activation-dependent gene expression

Cellular stress protection responses lead to increased transcription of several genes via modulation of transcription factors. Activation of the Nuclear Factor kappaB (NF-kappaB) pathway as a possible antiapoptotic route represents one important cellular stress response. To identify conditions that are capable of modifying this pathway, a screening assay for detection of NF-kappaB-dependent gene activation using the reporter protein Enhanced Green Fluorescent Protein (EGFP) and its destabilized variant (d2EGFP) was developed. Human Embryonic Kidney (HEK/293) cells were stably transfected with a vector carrying EGFP or d2EGFP under control of a synthetic promoter containing 4 copies of the NF-kappaB response element. Treatment with tumor necrosis factor alpha (TNF-alpha) gave rise to substantial EGFP/d2EGFP expression in up to 90% of the cells and was therefore used to screen different stably transfected clones for induction of NF-kappaB-dependent gene expression. The time course of NF-kappaB activation leading to d2EGFP expression was measured in an oligonucleotide-based NF-kappaB-ELISA. NF-kappaB binding in-creased after 15-min incubation with TNF-alpha. In parallel, d2EGFP increased after 3 h and reached its maximum at 24 h. These results show (1) the time lag between NF-kappaB activation and d2EGFP transcription, translation, and protein folding and (2) the increased reporter gene expression after treatment with TNF-alpha to be caused by the activation of NF-kappaB. The detection of d2EGFP expression required FACS analysis or fluorescence microscopy, while EGFP could also be measured in the microplate reader, rendering the assay useful for high-throughput screening.