Targeting the energy metabolism of melanoma cells: FX-11 acts as a mitochondrial uncoupler

In a compound screen on melanoma cells, which included different microenvironmental contexts aiming at better representing the environmental and growth characteristics of tumours, we identified several drugs efficiently suppressing cell viability. Here we focus on results obtained with FX-11, reported to be an inhibitor of lactate dehydrogenase (LDH). FX-11 dose-dependently inhibited growth of 624Mel and Wm3248 melanoma cells grown in a modular physiologic medium (MPM). Importantly, FX-11 was able to reduce the growth of the corresponding drug-resistant melanoma sublines equipotently. When testing the on-target activity of FX-11, the results were unexpected: in contrast to other LDH inhibitors (used as positive controls), FX-11 did not decrease the NAD+/NADH ratio, the glucose uptake, or the lactate secretion of melanoma cells. However, in seahorse assays, FX-11 increased the oxygen consumption rate as well as the extracellular acidification rate of cells, behaving like the mitochondrial uncouplers FCCP and Bam15. Treatment with FX-11, FCCP, or Bam15 induced an increase in acetyl-CoA carboxylase phosphorylation and a decrease in serine 65 phosphorylation of the eukaryotic initiation factor 4E-binding protein 1, indicative of AMPK activation by a decreased ATP/ADP ratio. Importantly, FX-11 and Bam15 drastically decreased the mitochondrial membrane potential in contrast to the LDH inhibitors LDH-IN-I and GNE-140. Taken together, we provide evidence that FX-11 effectively inhibits the growth of melanoma cells, including drug-resistant ones. FX-11 profoundly affects their energy metabolism, although it does not seem to act as other LDH inhibitors, but as a mitochondrial uncoupler.