Lactate preconditioning promotes a HIF-1α-mediated metabolic shift from OXPHOS to glycolysis in normal human diploid fibroblasts

Abstract

Recent evidence has emerged that cancer cells can use various metabolites as fuel sources. Restricting cultured cancer cells to sole metabolite fuel sources can promote metabolic changes leading to enhanced glycolysis or mitochondrial OXPHOS. However, the effect of metabolite-restriction on non-transformed cells remains largely unexplored. Here we examined the effect of restricting media fuel sources, including glucose, pyruvate or lactate, on the metabolic state of cultured human dermal fibroblasts. Fibroblasts cultured in lactate-only medium exhibited reduced PDH phosphorylation, indicative of OXPHOS, and a concurrent elevation of ROS. Lactate exposure primed fibroblasts to switch to glycolysis by increasing transcript abundance of genes encoding glycolytic enzymes and, upon exposure to glucose, increasing glycolytic enzyme levels. furthermore, lactate treatment stabilized HIF-1α, a master regulator of glycolysis, in a manner attenuated by antioxidant exposure. our findings indicate that lactate preconditioning primes fibroblasts to switch from OXPHOS to glycolysis metabolism, in part, through ROS-mediated HIF-1α stabilization. interestingly, we found that lactate preconditioning results in increased transcript abundance of MYC and SNAI1, key facilitators of early somatic cell reprogramming. Defined metabolite treatment may represent a novel approach to increasing somatic cell reprogramming efficiency by amplifying a critical metabolic switch that occurs during ipSc generation. The preferential use of glycolysis even in the presence of oxygen is known as aerobic glycolysis or the Warburg effect, a unique form of metabolism originally identified in cancer cells, but also found in many non-transformed cells 1,2. By shuttling glucose primarily through glycolysis, cancer cells fuel their proliferation by accumulating glycolytic intermediates required for fatty acid and nucleic acid synthesis. As lactate is a by-product of glycolysis, cancer cells exist in an acidic microenvironment which serves to facilitate angiogenesis and tumour invasion 3. Research examining the relationship between cancer cells and their microenvironment has revealed a novel phenomenon known as the reverse Warburg effect 4. The reverse Warburg effect is based on the theory that cells within a tumour can switch between glycolysis and oxidative phosphorylation (OXPHOS) 5-7. The reverse Warburg effect postulates that oxidative cancer cells secrete reactive oxygen species (ROS) which induce oxidative stress in surrounding stromal cells such as cancer-associated fibroblast (CAF) cells 8,9. To respond to this stress, CAFs adopt aerobic glycolysis as their primary form of metabolism 8,9. Glycolytic CAFs secrete lactate and precursors for nucleic acid and fatty acid synthesis, which are then taken up by adjacent cancer cells to fuel OXPHOS and support proliferation, thereby continuing the cycle 8,9. Long considered merely a by-product of glycolysis, lactate is emerging as an important signalling molecule and energy source 10-15. In addition to promoti...

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