Metabolic changes in living human lymphoma cells intervening NAD+ metabolism as revealed by NAD(P)H-fluorescence lifetime imaging and para-hydrogen induced polarization NMR.

Proliferating cells have a sustained high demand for regeneration of electron acceptors as NAD(P)+/NAD(P)H is involved in a number of critical redox-reactions within cells. However, their analysis in living cells is still challenging. We propose that combining label-free NAD(P)H fluorescence lifetime imaging (NAD(P)H-FLIM) and signal-enhanced magnetic resonance (MR) spectroscopy allows new, deeper insights into changes of specific metabolic pathways in living cells. For proof of principle NAD+-metabolism was perturbed by specific inhibiton of the rate-limiting enzyme of the NAD+ „Salvage pathway“ Nicotinamide phosphoribosyltransferase (NAMPT) by FK866 in RAMOS human lymphoma cells. FK866 treatment leads to NAD(H) reduction followed by reduced RAMOS cell proliferation. The NAD(P)H-FLIM analysis revealed increased general NAD(P)H-dependent metabolic activity indicated by increased ratios of enzyme-bound to total NAD(P)H concentration upon NAMPT-inhibition. More importantly, a marked reduced lactate dehydrogenase (LDH) activity accompanied by NADPH oxidase activity increase is observed. Using signal-enhanced MR spectroscopy a reduced flux of pyruvate to lactate catalyzed by LDH is detectable in real time in living cells. This strongly supports NAD(P)-FLIM analysis and demonstrates that intervening into the NAD+ „Salvage pathway“ can have specific and global consequences for cells. Our principle study shows how spatially-resolved metabolic imaging techniques, i.e. NAD(P)H-FLIM, are complemented by real-time MR, paving the way towards a comprehensive spatio-temporal understanding of metabolic pathways in living cells.