By Devorah Saffern
A study done at the Rabinowitz lab in the Chemistry and Integrative Genomics Carl Icahn Laboratory at Princeton University was published in this month’s edition of Nature. The work, which analyzed the role of glucose in the nutrient-metabolizing tricarboxylic acid cycle (TCA cycle), is particularly relevant now because November is pancreatic cancer awareness month. The study analyzes the role of different metabolites in the TCA cycle, such as lactate and glucose, that are involved in the breakdown of food to give energy to our tissues. The Rabinowitz lab’s discovery is directly applicable to tumor proliferation in the pancreas as well as to different types of cancer research, shedding some light on the pathways and importance of lactate as an intermediary metabolite. This paper elucidates some of the questions about the particular mechanisms of glucose in our metabolism.
Glucose is an essential nutrient found in many of our foods, and is broken down into CO2 through the TCA cycle to obtain energy or is turned into lactate via glycolysis. Lactate is a metabolic intermediate, exchanged between cells and tissues during these processes. The circulatory flux of a metabolite is the rate that tissues consume the metabolite, which circulates through the bloodstream. In the experiment, researchers analyzed these fluxes of various nutrients and their pathways, including glucose and lactate, in a mouse model. Previously, glucose was assumed to be the primary source of circulating carbon in the TCA cycle, but data had not yet been published about the measured amounts of carbon from lactate. Researchers in this experiment explored this unknown area by using 13C isotopic labeling in fasting mice to determine glucose and lactate concentrations. They gave intravenous infusions of these nutrients labeled with 13C, and after measuring the 13C concentration, they found that lactate had the greatest flux of the metabolites in fasting mice, even surpassing that of glucose. Lactate showed 2.5-fold higher circulatory turnover flux on a molar basis than glucose did, showing that lactate is the primary source of carbon in the TCA cycle and not glucose as previously thought.
The researchers also measured the consumption rates of other metabolites, and they found that while pyruvate is consumed just as rapidly as lactate, it exists in low concentrations and therefore does not have a considerable flux. Scientists had previously assumed that lactate abundance over glucose was due to the constant exchanges between lactate and pyruvate that occur in the body in response to depleted oxygen or excess lactate. This conversion process, however, would not change 13C labels once infused during the exchanges between lactate and pyruvate and therefore would not contribute to this large measured flux. This showed that the data is indeed an indicator that glucose hardly directly contributes to flux; instead, the majority enters as lactate.
The researchers then repeated the experiment in fed mice, and found that the circulatory flux of glucose increased significantly (3.1 fold), and the circulatory flux of lactate increased slightly. Glucose still contributed to the TCA cycle through lactate in all tissues except the brain and muscles (in muscles about half is directly fed to the TCA cycle as glucose). In fed mice, 40% of glucose in bodily tissues contributes to the TCA cycle via circulating lactate. In addition, the researchers found that the difference between lactate labeling in arteries and veins, not previously understood, depends on the circulatory flux rather than metabolite flow. They derived a differential equation to model this difference and showed that it confirms the high circulatory turnover flux of lactate that they had found.
In tumor TCA cycles, the study showed that lactate contribution still exceeded glucose contribution by two-fold. In most tissues and GEMM (genetically engineered mouse model) tumors, glucose contributes to the TCA cycle most commonly in the form of lactate. Lactate is the largest TCA contributor in lung cancer (whereas in pancreatic cancer, the largest TCA contributor is glutamine). Further research can be done to understand this process in other tumor types and in more case studies, to better link the role of lactate flux to tumor proliferation.
The researchers’ novel findings that lactate can be the primary source of carbon for the TCA cycle shows that glycolysis and the TCA cycle are uncoupled at the lactate level in all tissues except the brain. In addition, the study outlined this important role of lactate, and revealed that glucose does not significantly contribute in a direct manner to the TCA cycle. Rather, glucose is turned into lactate in a process called gluconeogenesis in order to feed the TCA cycle. The group also tested the direct TCA substrate usage from each tissue, and found direct glucose flux to TCA to be near zero, confirming their results that glucose only fees the TCA cycle as lactate.
Future directions include follow-up experiments to assess lactate and glucose levels in mice at states other than fed and fasting, such as during exercise. The way in which glucose changes to lactate within the tissues can also be further explored. The findings indicate that the role of glucose and lactate is not as simple as previously thought, and the study demonstrates useful ways to measure circulatory flux that can be helpful for future experiments. These initial results, however, show significant progress toward greater understandings of processes relevant to tumor growth and metabolic processes at large.
The views and opinions expressed in this article are mine alone and do not represent the views of the Princeton Public Health Review.
Image found on http://animalia-life.club/other/human-cells-under-a-microscope.html