If you think back to your high school biology or chemistry classes, at one point you were probably forced to memorize different metabolic pathways. Does the “Krebs cycle” or “glycolysis” ring any bells? If you were anything like me in high school, while you were learning this stuff you were probably thinking to yourself, “What have I done to deserve this?”, or equally baffling, “How the heck did someone figure this all out?”
Metabolic pathway maps are complicated at first glance, and following them can be like trying to read a map of the New York City subway system. What makes them even more confusing is the fact that they differ depending on the organ or cell type you’re looking at. For example, we know that cancer cells rewire their metabolism and increase aerobic glycolysis, as opposed to the more efficient oxidative phosphorylation, giving them a competitive growth advantage. This phenomenon is known as the Warburg Effect, and the exact mechanisms that underline how and why cancer cells do this continues to elude researchers nearly a century after the phenomenon was discovered. A breakthrough in this area would undoubtedly lead to the development of better cancer therapies and improve overall patient outcomes; and as we’re starting to understand, it’s merely one of the hundreds of different dysregulated metabolic pathways cancers use to outcompete the other cells in our body.
So how do researchers know that these highly complicated pathways exist, let alone study how they are altered throughout disease? After all, we are talking about teeny tiny chemicals that are inside a single cell that make up an individual organ, all inside a person’s body. Today, we use a technique called “stable isotope tracing”. Sounds complicated, I know, but stick with me, the principles are actually quite simple.
What is an isotope?
Think of isotopes like identical twins: they look the exact same, but in fact are two individuals with different personalities. Isotopes are atoms of the same chemical element (ex. carbon (“C”)) with the same number of protons and electrons, but a different number of neutrons (i.e. C12, C13, C14). Most importantly (at least with respect to studying metabolism), some of these isotopes are stable (i.e., not radioactive) and have different molecular weights, enabling us to detect their presence in individual chemicals and metabolites. So, while our cells can’t tell the difference between metabolites composed of C12 or C13 carbons, researchers can distinguish between them by using a mass spectrometer.
So what?
Let’s go back to the example of the Warburg Effect. We know that healthy cells and cancer cells metabolize glucose differently, but we’re still trying to figure out why. Much like a tourist navigating the streets of New York City, if you don’t have a map, you’re going to get lost. Stable isotopes give us that map, and here’s how.
Glucose is made up of six carbons (see the figure below), and while isotopes are naturally occurring, they make up a very small percentage of the carbons in the glucose pool, where C12 is 98.9% abundant, and C13 makes up a negligible amount (around 1.1%). Therefore, almost all the glucose we get from our food will be entirely C12-glucose. However, chemists have figured out a way to produce glucose that is made up entirely of isotopic C13. Therefore, when we feed our cells C13-glucose, we can trace the breakdown of that molecule as it’s metabolized through the pathway. This allows us to see what the cells are doing with the glucose, and where it’s going. Think of it like the Google Maps car driving through the city and mapping out all the roads so you can find your way to your destination.
Now, researchers can choose different isotopic formations using various other elements including nitrogen [15N] and hydrogen [2H], enabling nearly every metabolic pathway to be mapped and studied using this technique. This has become a vital tool in the study of how metabolism contributes to diseases such as cancer, Alzheimer’s, tuberculosis and many more. As this technology continues to improve and isotope tracing is adopted by research labs across the world, the hope is that we will soon have our own little Google Maps system to show us how every aspect of metabolism behaves throughout numerous diseases and cell types. Who knows, we might even answer the question Warburg posed so many years ago.
Photo by José Martín Ramírez Carrasco on Unsplash
