Measurement of cellular ATP levels

Intracellular ATP concentration was determined by the commercially available CellTiter-Glo® Luminescent Cell Viability Assay (Promega, Madison, WI, USA) (13). Cells were lysed in 100 μL Cell Titer-Glo reagent according to the manufacturer's recommendations, and the luminescent signal was recorded for 1s on a high-sensitivity luminometer (Synergy 2; Biotek, Winooski, VT, USA). The assay is based on ATP requiring luciferin-oxyluciferin conversion mediated by a thermostable luciferase that generates a stable “glow-type” luminescent signal.