The Role of Pyruvate Dehydrogenase in Cell Growth
Rajagopalan, Kartik N.
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Otto Warburg's observation that tumor cells have increased rates of glucose uptake and lactate secretion in comparison to normal cells spawned his notion that tumors have dysfunctional mitochondria. However, in addition to metabolizing glucose to lactate, tumors in vivo exhibit mitochondrial glucose oxidation, indicating activity of pyruvate dehydrognase (PDH), which gates entry of glucose derived carbon into the tricarboxylic acid (TCA) cycle. To test whether cells require glucose oxidation for proliferation, the work in this thesis establishes a model wherein PDH activity is suppressed using RNA interference. Small hairpin RNAs against the transcript encoding the PDHE1α protein were cloned into a retroviral vector which allowed doxycycline-inducible control of expression. Metabolism of cancer cells was studied in vitro using a combination of metabolomics and metabolic flux analysis. Growth in monolayer culture was performed in medium containing lipid-replete serum as well as serum in which lipids had been depleted. As expected, suppression of PDH activity reduced flow of carbon from glucose to the TCA cycle and to de novo fatty acid synthesis. Surprisingly, H460 lung cancer cells could tolerate a 60% reduction of PDH flux without any significant effect on proliferation rate, as long as lipids were present in the medium. Further examination of the effects of PDH silencing on the overall network of central carbon metabolism revealed enhanced channeling of carbon from glutamine to fatty acids and an increase in scavenging free fatty acids. Lipid depletion caused a reduction in growth rate of PDH deficient cells, and this defect was completely rescued by supplying free fatty acids to the medium. Together the data indicate that proliferating cells exhibit PDH activity that allows transfer of glucose carbon to citrate and the TCA cycle as well as ultimately into fatty acids. Importantly, suppression of PDH activity limits growth in conditions in which cancer cells do not have access to extracellular lipids. This work illustrates that compensatory pathways that sustain cell proliferation are activated during suppression of mitochondrial oxidation of glucose.