Cancer cells avoid aerobics
You probably remember from biology class that your cells can process glucose to release energy two main ways: with oxygen, (a process called aerobic respiration), and without oxygen (using glycolysis, which literally means “breaking glucose”).
Aerobic respiration uses the mitochondria to completely break down cellular fuel into carbon dioxide and water, and creates far more energy-packed ATP than glycolysis. When you want to extract as much energy as possible, and oxygen is plentiful, aerobic respiration is the way to go.
Most cancer cells, though, usually don’t opt for the high-energy pathway. As a result, they end up using a lot more glucose to fuel their incredible growth. How does this happen? It helps if you understand a bit about how cells metabolize glucose. When glucose enters the cell, a series of enzymes take it through the glycolysis process to release energy, and produce pyruvate. This pyruvate is then imported into the mitochondria, where the aerobic respiration process takes place:
At the very last step of glycolysis, the enzyme that makes pyruvate has a mutation that tends to shunt the pyruvate into an alternate pathway that makes lactic acid, keeping it away from the mitochondria. The reasons why aren’t entirely clear, but there are some theories. First of all, if the glycolysis pathway gets backed up or slowed down, many of the interim products are siphoned off to other pathways that manufacture fatty acids and amino acids. A less-efficient pathway could increase the amounts of lipid and amino acid byproducts, which enables the cancer cells to divide and grow more quickly.
It is also possible that the cancer cells end up shutting down the mitochondria, because they are a key part of apoptosis, or programmed cell death. Apoptosis usually kicks in when cells realize something is wrong with the DNA. Since cancer involves serious DNA disruptions, anything that holds off apoptosis helps the cancer cells survive.
The fact that cancer cells use a lot more glucose to get the job done is great news for oncologists trying to track down cancer sites in a patient. They inject radioactively tagged glucose into the bloodstream, and then take radiographs. The cancer cells take up a most of the tagged glucose very quickly, and show as bright spots in the images, like this PET image below showing colorectal cancer that has metastasized to the liver:
The Biology of Cancer, Robert Weinberg
Wikipedia article on Warburg Effect
Image credit:Wikimedia user RegisFrey, Creative Commons By-SA 3.0
Edited to add a link to this paper on the Warburg Effect that goes into a lot more detail and is worth reading if you want a deep dive into the subject.
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