Nanoluciferase Reporter Gene System Directed by Tandemly Repeated Pseudo-Palindromic NFAT-Response Elements Facilitates Analysis of Biological Endpoint Effects of Cellular Ca(2+) Mobilization

NFAT is a cytoplasm-localized hyper-phosphorylated transcription factor that is activated through dephosphorylation by calcineurin, a Ca(2+)/calmodulin-dependent phosphatase. A non-palindromic NFAT-response element (RE) found in the IL2 promoter region has been commonly used for a Ca(2+)-response reporter gene system, but requirement of concomitant activation of AP-1 (Fos/Jun) often complicates the interpretation of obtained results. A new nanoluciferase (NanoLuc) reporter gene containing nine-tandem repeats of a pseudo-palindromic NFAT-RE located upstream of the IL8 promoter was designed to monitor Ca(2+)-induced transactivation activity of NFAT in human embryonic kidney (HEK) 293 cells by measuring luciferase activities of NanoLuc and co-expressed firefly luciferase for normalization. Ionomycin treatment enhanced the relative luciferase activity (RLA), which was suppressed by calcineurin inhibitors. HEK293 cells that stably express human STIM1 and Orai1, components of the store-operated calcium entry (SOCE) machinery, gave a much higher RLA by stimulation with thapsigargin, an inhibitor of sarcoplasmic/endoplamic reticulum Ca(2+)-ATPase (SERCA). HEK293 cells deficient in a penta-EF-hand Ca(2+)-binding protein ALG-2 showed a higher RLA value than the parental cells by stimulation with an acetylcholine receptor agonist carbachol. The novel reporter gene system is found to be useful for applications to cell signaling research to monitor biological endpoint effects of cellular Ca(2+) mobilization.