Abstract
Queuine is a eukaryotic micronutrient, derived exclusively from eubacteria. It is incorporated into both cytosolic and mitochondrial transfer RNA to generate a queuosine nucleotide at position 34 of the anticodon loop. The transfer RNA of primary tumors has been shown to be hypomodified with respect to queuosine, with decreased levels correlating with disease progression and poor patient survival. Here, we assess the impact of queuine deficiency on mitochondrial bioenergetics and substrate metabolism in HeLa cells. Queuine depletion is shown to promote a Warburg type metabolism, characterized by increased aerobic glycolysis and glutaminolysis, concomitant with increased ammonia and lactate production and elevated levels of lactate dehydrogenase activity but in the absence of significant changes to proliferation. In intact cells, queuine deficiency caused an increased rate of mitochondrial proton leak and a decreased rate of ATP synthesis, correlating with an observed reduction in cellular ATP levels. Data from permeabilized cells demonstrated that the activity of individual complexes of the mitochondrial electron transport chain were not affected by the micronutrient. Notably, in queuine free cells that had been adapted to grow in galactose medium, the re-introduction of glucose permitted the mitochondrial F1FO-ATP synthase to operate in the reverse direction, acting to hyperpolarize the mitochondrial membrane potential; a commonly observed but poorly understood cancer trait. Together, our data suggest that queuosine hypomodification is a deliberate and advantageous adaptation of cancer cells to facilitate the metabolic switch between oxidative phosphorylation and aerobic glycolysis.
Highlights
Cancer metabolism is characteristically skewed towards a high rate of glycolysis even in the presence of sufficient oxygen, commonly referred to as aerobic glycolysis or the Warburg effect.The near universality of this phenomenon is evidenced by the clinical success of the glucoseNutrients 2020, 12, 871; doi:10.3390/nu12030871 www.mdpi.com/journal/nutrientsNutrients 2020, 12, 871 analogue [18 F]-fluorodeoxyglucose to positively diagnose and stage most primary and metastatic lesions [1]
The effect of queuine on cell proliferation was examined using horse serum or dialyzed fetal bovine serum, which have low to undetectable amounts of the micronutrient [16,18,32]
There was no statistically significant difference in the proliferation rate of cells grown in the absence or presence of queuine (Figure 1C)
Summary
Cancer metabolism is characteristically skewed towards a high rate of glycolysis even in the presence of sufficient oxygen, commonly referred to as aerobic glycolysis or the Warburg effect. The addition of queuine to the culture medium was found to decrease HeLaS3 cell proliferation and to increase growth under aerobic conditions. Acetyl-CoA from glucose and oxaloacetate (OAA) from required for cancer cell growth and division, with the concomitant secretion of lactate, alanine, and glutamine converge to produce citrate, which is exported to the cytoplasm and converted to fatty ammonia. (F–J) Cells were grown in serum free medium without further additions (white bar) or supplemented daily (gray bar) with 1 μM queuine. Mean (± s.d.) for triplicate samples. n ≥ 2. ***p < 0.001, t-test
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