18 The Warburg Revival

NOBEL LAUREATE OTTO Heinrich Warburg (1883–1970) was born in Freiburg, in southwestern Germany. He was the son of Emil Warburg, a prominent physics professor at the University of Freiburg, and grew up around contemporaries like Albert Einstein and Max Planck, both of whom would also become legendary scientists.

Otto Warburg’s specific research interest was cellular energetics, where he applied the rigorous methods of the physical sciences (chemistry and physics) to biology. How much energy do cells require? How do they generate that energy? This obsession would lead him eventually to his life’s research: what he called the “cancer problem.” How did cancer cells differ in their energy metabolism from normal cells?

Normally, cells may generate energy in the form of adenosine triphosphate (ATP) in two different ways: oxidative phosphorylation (OxPhos), also called respiration; and glycolysis, also called fermentation. OxPhos, which occurs in the mitochondrion, burns each glucose molecule with oxygen to generate thirty-six ATP. Without oxygen, normal cells must resort to glycolysis, which generates only two ATP and two lactic acid molecules per glucose molecule. For example, during intense exercise, muscles require so much energy so quickly that the blood flow cannot keep up with the oxygen demand. The muscles switch to glycolysis, which does not require oxygen and generates far less energy per glucose molecule. Eventually, lactic acid builds up, causing muscle fatigue, which is why you may feel like you can’t go any further in the midst of a particularly challenging workout. Normal cells do not function well in an acidic environment. As your body rests, oxygen demand slows, and your muscle cells resume OxPhos once oxygen supply catches up.

Cells generating a lot of energy using OxPhos naturally require more oxygen. Warburg noted this phenomenon when he observed fertilized sea urchin eggs growing rapidly. He speculated that rapidly growing cancer cells would also consume oxygen prodigiously. But he was wrong. In 1923, Warburg noticed with some amazement that fast-growing rat tumor cells used no more oxygen than regular cells.

The cancer cells were instead using ten times more glucose and producing lactic acid at seventy times the rate of normal tissues (see Figure 18.1).¹ Warburg calculated that tumor cells converted an astounding 66 percent of the glucose they took up to lactate.²

Figure 18.1

Despite the ready availability of oxygen, cancer cells were generating energy using the less efficient glycolysis pathway. This surprising process is now referred to as the Warburg effect.

Because glycolysis generates much less ATP per glucose, cancer cells must drink up glucose like a camel drinks water after a long desert trek. Today we use the Warburg effect for the common cancer imaging test, the PET scan. As we discussed in chapter 3, PET scans measure how much glucose is being consumed by cells. Active cancer cells guzzle the glucose much faster than normal surrounding cells, and the PET scan picks up these hot spots.

Aerobic glycolysis (glycolysis in the presence of lots of oxygen) is unique to cancer. Normal cells almost always choose OxPhos if oxygen is plentiful. Even in situations where cells grow quickly and require large amounts of energy, such as during wound healing, the Warburg effect is not found. But why? It seems very strange.

Think about it. We know that cancer can be distinguished by four hallmarks:

It grows.
It is immortal.
It moves around.
It uses the Warburg effect: it deliberately uses a less efficient method of energy extraction.

One of these does not fit with the others. Immortal cancer cells are super busy, constantly growing, and moving all over the body. This requires lots and lots of energy. Why on earth would cancer choose a less efficient manner of energy extraction?

Suppose you build a fast sports car, sleek and low to the ground and with a spoiler on the back to reduce drag. Then you take out its 600-horsepower motor and put in a 9-horsepower lawnmower engine. Huh? That’s bizarre. Cancer effectively does the same thing by deliberately choosing a less efficient energy-generating method. Yet it cannot simply be a coincidence, as about 80 percent of all known cancers use the Warburg effect. Whatever the reason, it is critical to cancer’s genesis and not merely a metabolic mistake. Cancer has not persisted for millennia, in animals ranging from hydra to dogs to cats to humans, by making mistakes.

In a famous 1956 research paper titled “On the Origin of Cancer,” Warburg hypothesized that the anomalous switch to aerobic glycolysis is so bizarre that it must be cancer’s inciting event. To review, the two main requirements for OxPhos are oxygen and functional mitochondria, the cellular structures where OxPhos takes place. Because oxygen is plentiful, Warburg deduced that it must be the mitochondria that are dysfunctional, forcing the cancer cell to revert to the less efficient glycolysis pathway.³ Warburg hypothesized that cancer was caused primarily by mitochondrial damage.

While the Warburg effect is a well-established fact, many observations argue against Warburg’s hypothesis.⁴ Mitochondria from cancer cells often function normally with preserved respiration.⁵ Most cancer cells have normal mitochondrial function, meaning they aren’t exclusively reliant on glycolysis for energy production—they could switch back to OxPhos if necessary.⁶ Cancer was not being forced to use glycolysis, it was choosing it. But why?

Efficiently generating energy (ATP) is an advantage only under conditions of scarcity. If there is a lot of glucose around, then why does it matter if each glucose produces only two ATP instead of thirty-six? Glycolysis produces ATP less efficiently but more quickly. In the time that normal cells metabolize one glucose to thirty-six ATP, cancer cells metabolize eleven glucose molecules to twenty-two ATP and twenty-two lactic acid. Because lactic acid can be converted to ATP one to one, this gives cancer a potential total of forty-four ATP. Cancer cells produce energy quicker, although it requires ten times more glucose to do so.⁷

Imagine two people. One burns 2,000 calories per day while the other, being more energy efficient, burns only 1,000 calories. The increased energy efficiency is not an advantage if you are eating 2,500 calories per day. OxPhos is advantageous only when glucose is scarce, but given the recent obesity and type 2 diabetes epidemics, glucose levels tend to run high, not low. So, the OxPhos “advantage” of energy efficiency is largely illusory in this current environment.

The fact that almost every known cancer uses this pathway suggests that it is neither coincidence nor a mistake, but integral to cancer development. It must confer some selective advantage. But what?

Cells need more than just energy to grow. They also need basic building blocks. Because we are carbon-based life-forms, our cell growth is reliant on carbon to build basic molecules. During OxPhos, most of the carbons in glucose are metabolized for energy, leaving behind carbon dioxide, which is exhaled. During glycolysis, only a small percentage of carbons are completely burned for energy. The leftover carbons can be metabolized into carbon building blocks to make new amino acids and fatty acids.

Consider this analogy: Building a house requires both energy (the hard work of the builders) and materials (bricks). Having builders but no bricks is useless. Similarly, rapidly growing cells require both energy (ATP) and materials (carbons). OxPhos generates pure energy alone, which does not maximize growth. Glycolysis better supports rapid growth because it provides both energy and materials, while OxPhos generates only pure energy.⁸ This may explain the advantage of the Warburg effect for cancer growth.

By the 1970s, Warburg’s focus on cancer cell metabolism was looking increasingly shaky. The genetic revolution was well under way, and cancer researchers were drawn to the somatic mutation theory like iron filings to a magnet. The question of how cancer fueled its growth and the anomalous, curious predilection for glycolysis was a conveniently ignored mystery. Entire years would pass without any scientific publications on the Warburg effect. These two scientific fields of inquiry, cancer’s growth and cancer’s metabolism, were complete strangers to each other. Then, unexpectedly in the late 1990s, they united in a shotgun marriage.

THE WARBURG REVIVAL

The pathways that govern a cell’s growth and its metabolism had always been considered distinct. But Lew Cantley’s groundbreaking research linked the well-known metabolic hormone insulin directly to growth pathways through PI3K. Cancer cell growth and metabolism were inextricably linked by the exact same genes and hormones.⁹ For example, the oncogene myc controls not only growth, but also a metabolic enzyme that turns on the Warburg effect. Cantley found a direct link between nutrient sensors, metabolism, the Warburg effect, and cellular proliferation.¹⁰ Genes controlling growth also controlled metabolism.

All these newly discovered oncogenes and tumor suppressor genes also influenced metabolic pathways. Many oncogenes control enzymes called tyrosine kinases, which regulate both cell growth and glucose metabolism. The ubiquitous p53 tumor suppressor gene influences growth, and it also regulates cell metabolism by affecting mitochondrial respiration and glycolysis.

Cancer cells can’t stop growing, but they also can’t stop eating. Does cancer grow because it can’t stop eating, or does it eat because it can’t stop growing? Most likely, both. Diseases of growth were diseases of metabolism—and this applied to more than just glucose metabolism.

Cancer cells love eating glucose, but not exclusively. The metabolic pathways of the amino acid glutamine are also disrupted in cancer.¹¹ Amino acids are the building blocks of proteins, and glutamine is the most abundant amino acid in the blood. Some cancer cells consumed more than ten times the normal amount of glutamine.¹² Some cancers, such as neuroblastoma, lymphoma, and kidney and pancreatic cancer, appeared so “addicted” to glutamine that they simply couldn’t survive without it.¹³

Warburg believed that cancer was solely dependent on glucose for energy, but this was not entirely true. Cancer can also metabolize glutamine, and more recent studies show that cancer may also metabolize fatty acids and other amino acids.¹⁴ Cancer competes with other cells for fuel in a crowded environment, so having the flexibility to use a variety of fuels is advantageous for growth. While Warburg’s original hypothesis may not have panned out, his hunch that cancer’s metabolism was vitally important was spot on. The Warburg effect did have a purpose. It provided cancer cells with a strategic advantage in their struggle for survival. The large volume of lactic acid produced during the Warburg effect was not a waste product, as previously assumed, but a major benefit, providing the cancer cell with a significant survival advantage.

LACTIC ACID

As a tumor grows, new cancer cells crop up farther and farther away from the main blood supply that provides oxygen and clears waste products. Cells closer to the blood vessels are well supplied and thrive. Cells farther away do not get enough oxygen to survive. In between these two regions is the area known as the hypoxic zone, where cells receiving barely enough oxygen to survive activate an enzyme called hypoxia-inducible factor (HIF1). The struggle for survival in this hypoxic zone acts as a potent selective evolutionary pressure.

First, HIF stimulates the release of vascular endothelial growth factor (VEGF), which promotes the growth of new blood vessels. New blood reserves deliver more oxygen and allow the tumor to grow larger. “Inducing angiogenesis” is one of the key hallmarks of cancer described by Weinberg and Hanahan.

Second, HIF makes it easier for the normally stationary cells to become more mobile. The adhesion molecules that anchor cells to their proper position are disrupted, and the basement membranes that limit cells to certain areas are degraded.¹⁵ This makes it easier for cells to “activate invasion and metastasis,” another key hallmark of cancer.

Third, because oxygen is scarce, HIF reprograms the cell’s metabolism from OxPhos and toward glycolysis. Because more glucose is needed for energy production, HIF increases the expression of cellular glucose receptors. At the same time, HIF decreases the production of new mitochondria, which are essential for OxPhos.¹⁶ In essence, HIF is responsible for the phenomenon known as the Warburg effect, yet another key hallmark.¹⁷

This package of changes induced by HIF improves survival in a low-oxygen environment. Cells deprived of oxygen attempt to build new blood vessels, move away from the hypoxic region, and use less oxygen. Not coincidently, these are also behaviors typical of cancer cells, and this is precisely the environment that provides a unicellular organism with an advantage over that of its multicellular cousin. The Warburg effect is not simply a metabolic “mistake.” It provides cancer cells with a unique survival advantage when competing against other cells.

Cancer cells produce lactic acid during glycolysis, and they dump that acid into their surrounding environment in the same way a chemical plant might dump toxic waste in its surroundings. This is no accident, and the lactic acid is not merely a waste by-product. Tumors are using precious energy to deliberately manufacture and pump more acid into their immediate surroundings, which are already acidic.¹⁸ Compared to normal cells, which live in an environment with a pH of 7.2 to 7.4, tumors generate a surrounding microenvironment of pH 6.5 to 6.9.¹⁹ Why do cancer cells put so much effort into acidifying their surroundings?²⁰ Because the acidity give the cells a huge survival advantage. Normal cells are injured in an acidic environment and undergo apoptosis, while cancer cells tolerate the acidity fairly well.

There are two ways to win: get better or make your competitor worse. Both work. Cancer is playing a cellular Game of Thrones. You win or you die. While normal cells play nice and cooperate, unicellular organisms compete by sabotaging opponents. Cancer cells secrete the noxious lactic acid to impede nearby cells. Killing your neighbors is a time-tested survival strategy, and is common in the unicellular universe.

In 1928, Sir Alexander Fleming discovered that the fungus Penicillium notatum secreted a noxious chemical into its surroundings that killed competing bacteria. That chemical eventually became the breakthrough antibiotic penicillin. On Easter Island, rapamycin was discovered from a bacterium that secreted a noxious chemical into its surroundings to kill competing fungi.

The caustic acidic environment degrades the extracellular matrix, the normal supporting structure of the cell. This allows for the cancer cell to invade through the basement membrane more easily, an important prerequisite for metastasis. The damage caused by the lactic acid also provokes inflammation. This attracts immune cells that secrete growth factors, which would be useful in wound healing, but ultimately benefit the cancer cell.

Cancer has been called “the wound that never heals” because of its similarity to the hypergrowth seen in wound healing. In normal wound healing, new blood vessels replace the torn old ones, cellular debris is cleared, and the wound heals. The main difference is that the wound-healing program eventually comes to an end, whereas cancer’s growth program does not.

Even when oxygen is freely available, cancer continues to use glycolysis because it offers the singular survival advantage of pumping out lactic acid (the Warburg effect). The inflammation induced by lactic acid also inhibits those immune cells that normally target and kill cancer cells.²¹ Thus, the increased lactic acid from the Warburg effect:

Suppresses normal cell function;
Degrades the extracellular matrix, facilitating invasion;
Provokes inflammatory response and growth factor secretion;
Reduces immune response; and
Increases angiogenesis.

Cancer didn’t choose glycolysis over OxPhos (the Warburg effect) by accident. It’s not a mistake. It’s a logical choice because of the survival advantage offered by lactic acid. The tradeoff is the requirement for much more glucose as feedstock. During conditions of ample glucose, the balance is tipped toward cancer growth. It is the Warburg effect that sets the stage for the next step in the development of cancer by making it easier for cells to invade tissues and move around. This stage is largely responsible for cancer’s lethality.