Amplitude and Frequency Modulation of Metabolic Signals in Leukocytes: Synergistic Role of IFN-γ in IL-6- and IL-2-Mediated Cell Activation

Abstract

AbstractMany stimuli cause intracellular concentration oscillations of second messengers or metabolites, which, in turn, may encode information in their amplitudes and frequencies. We now test the hypothesis that synergistic cellular responses to dual cytokine exposure correlate with cross-talk between metabolic signaling pathways of leukocytes. Polarized RAW264.7 macrophages and human neutrophils and monocytes exhibited NAD(P)H autofluorescence oscillation periods of ≅20 s. IFN-γ tripled the NAD(P)H oscillatory amplitude for these cells. Although IL-6 had no effect, incubation of cells with IFN-γ and IL-6 increased both oscillatory amplitude and frequency. Parallel changes were noted after treatment with IFN-γ and IL-2. However, IL-1β and TNF-α did not display frequency doubling with or without IFN-γ exposure. To determine whether frequency doubling required complete IFN-γ signaling or simply metabolic amplitude modulation, an electric field was applied to cells at NAD(P)H troughs, which has been shown to enhance NAD(P)H amplitudes. Electric field application led to frequency doubling in the presence of IL-6 or IL-2 alone, suggesting that amplitude modulation is crucial to synergism. Because NADPH participates in electron trafficking to NO, we tested NO production during cytokine exposure. Although IL-6 and IL-2 alone had no effect, IFN-γ plus IL-6 and IFN-γ plus IL-2 enhanced NO release in comparison to IFN-γ treatment alone. When NO production was examined for single cells, it incrementally increased with the same phase and period as NAD(P)H. We suggest that amplitude and frequency modulation of cellular metabolic oscillations contribute to intracellular signaling synergy and entrain NO production.