Modeling key metabolic pathways of cancer cells using label-free fluorescence lifetime imaging

Glycolysis, glutaminolysis, and oxidative phosphorylation (OXPHOS) are the main cellular metabolic pathways used to generate energy. Reduced nicotinamide adenine dinucleotide (NADH) and oxidized flavin adenine dinucleotide (FAD) are coenzymes in these metabolism pathways, and detection of their endogenous fluorescence lifetime offers a label-free and quantitative method to study redox state and model cellular metabolism. Many cancer cells depend on glycolysis instead of oxidative phosphorylation to produce energy even in an aerobic environment, which is known as the Warburg effect. Here, autofluorescence lifetime images of NADH and FAD were obtained from MCF-7 breast cancer cells using multiphoton fluorescence lifetime microscopy. Cells were cultured in the Dulbecco's Modified Eagle's Medium (DMEM), and glycolysis, glutaminolysis, and OXPHOS were inhibited using 2-DG (2-Dexoy-D-glucose), sodium cyanide, and BPTES (Bis-2-(5-phenylacetamido-1,3,4-thiadiazol-2-yl) ethyl sulfide) respectively. The fraction of free NADH decreases when glycolysis is inhibited and increases when OXPHOS is inhibited. The mean NADH lifetime is increased when glycolysis is inhibited and is reduced when OXPHOS is inhibited. NADH and FAD fluorescence lifetime features vary when inhibiting glutaminolysis. Altogether, this investigation offers a non-invasive method to image key metabolic pathways at a cellular level, which improves the identification of different metabolic states and drug responses of cancer cells.