A one-arm homologous recombination approach for developing nuclear receptor assays in somatic cells.

Drug discovery is in need of technologies that enable investigators to develop cell-based assays that accurately reflect the functional consequence of small molecule intervention on biological processes. Here, we describe a strategy that uses both one-arm homologous recombination and the beta-lactamase (BLA) reporter system, a sensitive and robust transcriptional reporter for gene activation. We demonstrate that this powerful approach can be utilized for developing cell-based assays for the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) in HEK293 somatic cells. Specifically, one-arm homologous recombination was used to introduce the GAL4 DNA-binding domain (GAL4DBD) in the GR and MR genomic loci such that a chimeric GAL4DBD-GR (ligand-binding domain) [GAL4DBD-GR(LBD)] and GAL4DBD-MR(LBD) transcript is produced from the strong CMV promoter in HEK293 cells previously stably transfected with the UAS(GAL4)-BLA reporter construct. Dexamethasone- and aldosterone-responding BLA-positive cells were isolated by fluorescence-activated cell sorting, and then further expanded into separate cell lines. The sensitivity and robustness of the resulting GR and MR assays are demonstrated by the fact that the addition of dexamethasone and aldosterone to the two transgenic clonal cell lines for 16 h results in high Z' values (>0.8) and EC(50) values of 1 and 0.3 nM, respectively. These assays illustrate the flexibility of this technology to generate high-performance cellular assays for nuclear receptor targets without the need for target-specific cDNA.

Integration of G-protein coupled receptor signaling pathways for activation of a transcription factor (EGR-3).

We recently reported the use of a gene-trapping approach to isolate cell clones in which a reporter gene had integrated into genes modulated by T-cell activation. We have now tested a panel of clones from that report and identified the one that responds to a variety of G-protein coupled receptors (GPCR). The beta-lactamase tagged EGR-3 Jurkat cell was used to dissect specific GPCR signaling in vivo. Three GPCRs were studied, including the chemokine receptor CXCR4 (Gi-coupled) that was endogenously expressed, the platelet activation factor (PAF) receptor (Gq-coupled), and beta2 adrenergic receptor (Gs-coupled) that was both stably transfected. Agonists for each receptor activated transcription of the beta-lactamase tagged EGR-3 gene. Induction of EGR-3 through CXCR4 was blocked by pertussis toxin and PD58059, a specific inhibitor of MEK (MAPK/ERK kinase). Neither of these inhibitors blocked isoproterenol or PAF-mediated activation of EGR-3. Conversely, beta2- and PAF-mediated EGR-3 activation was blocked by the p38, specific inhibitor SB580. In addition, both beta2- and PAF-mediated EGR-3 activation could be synergistically activated by CXCR4 activation. This combined result indicates that EGR-3 can be activated through distinct signal transduction pathways by different GPCRs and that signals can be integrated and amplified to efficiently tune the level of activation.