FOS Promoter is Overactive Outside of Genome Context and Weakly Regulated by Changes in the Na+i/K+i Ratio

Changes in the Na⁺ and K⁺ intracellular concentrations affect expression of the FOS gene. Here, we obtained a genetic construct coding for the TurboGFP-dest1 protein under control of the human FOS promoter (−549 to +155) and studied its expression in HEK293T cells exposed to monovalent metal cations. Amplification of the FOS promoter sequence from genomic DNA was efficient only in the presence of Li⁺ ions. Incubation of cells with ouabain or in a medium containing Li⁺ ions instead of Na⁺ ions caused intracellular accumulation of Na⁺ and Li⁺ ions, respectively. In addition, both stimuli increased the levels of endogenous FOS mRNA and the average fluorescence intensity of TurboGFP-dest1 in transfected cells. The mRNA levels of TurboGFP-dest1 were significantly higher than the FOS mRNA levels and were little affected by the stimuli.