b MD PhD Combined Degree Program
b MD-PhD Combined Degree Program, University of Texas Medical Branch, Galveston, TX, 77555, USA
c Institute for Cancer Genetics and Department of Pathology and Cell Biology, College of Physicians and Surgeons, Columbia University, New York, 10032, USA
One carbon metabolism
Isotope traing mass spectrometry
A major hallmark of cancer is a perturbed metabolism resulting in high demand for various metabolites, glucose being the most well studied. While glucose can be converted into pyruvate for ATP production, the serine synthesis pathway (SSP) can divert glucose to generate serine, glycine, and methionine. In the process, the carbon unit from serine is incorporated into the one-carbon pool which makes methionine and maintains S-adenosylmethionine levels, which are needed to maintain the epigenetic landscape and ultimately controlling what genes are available for transcription. Alternatively, the carbon unit can be used for purine and thymidylate synthesis. We present here an approach to follow the flux through this pathway in cultured human cells using stable isotope enriched glucose and gas chromatography mass spectrometry analysis of serine, glycine, and methionine. We demonstrate that in three different cell lines this pathway contributes only 1–2% of total in-tracellular methionine. This suggests under high extracellular methionine conditions, the predominance of carbon units from this pathway are used to synthesize nucleic acids.
Human tumors have a distinct metabolism, most notably a pre-dilection for glucose. However, the utility and fate of those carbon atoms remains incompletely understood. Normally in the absence of oxygen glucose is converted to pyruvate and ultimately lactate. This generates only a fraction of the ATP possible by aerobic cellular re-spiration, which requires oxygen as the final electron donor. While Peroxynitrite glycolysis can be accomplished by most cells, cancer cells have the unusual property of converting much of their glucose to lac-tate, even in the presence of oxygen. The process is termed aerobic glycolysis. Aerobic glycolysis underlies the Warburg effect, long re-cognized as a defining property of many cancers . Much of the glucose can of course be used as a source of ATP, albeit less efficiently than oxidative phosphorylation which yield 2 ATP and 36 ATP per equivalent of glucose, respectively. But, there may be some benefits despite the trade-off, such as a higher rate of ATP generation on de-mand [1,2].
While it may seem paradoxical to use a less efficient system to
* Corresponding author. E-mail address: [email protected] (K. Zhang).
generate ATP, glycolytic intermediates feed into many other important pathways. For example, the pentose phosphate pathway is used to generate ribose sugars and NADPH, both of which are important in DNA/RNA synthesis and anabolic processes. Others have argued gly-colysis allows a tumor a certain plasticity in order to rapidly respond to a changing microenvironment . Additionally the glycolytic inter-mediate, 3-phosphoglycerate, can be diverted to generate serine which can be utilized to synthesize nucleic acids that are essential for cell proliferation .
In this study we have investigated the incorporation of carbon from glucose into the amino acids glycine, serine and methionine (Fig. 1). Phosphoglycerate dehydrogenase (PHDGH) is the rate-limiting enzyme in the conversion of 3-phosphoglycerate into serine . Serine can donate a carbon atom to tetrahydrofolate by way of serine hydro-xymethyl transferase (SHMT), which can then be used for de novo purine synthesis or thymidylate synthesis [6,7]. Alternatively, the carbon unit can be transferred from serine to homocysteine to form methionine.