Yes, I mean it—to you, not for you. That’s just too easy. We all know that without oxygen, we die. Neurons die in minutes (stroke); heart muscle dies in a few hours (heart attack or myocardial infarction in medical-speak). Metabolically, oxygen is absolutely essential for oxidizing glucose and providing us with energy in the form of ATP. Without ATP, everything grinds to a halt.
But wait a minute, you might ask. Didn’t you wax scientifically poetic about the amazing turtle that can live for months on end with little or no oxygen? Indeed, I did. How did this clumsy, slow, and lovable animal do it? In two complementary ways: (1) by switching its glucose metabolism from the highly efficient, oxygen-dependent, oxidative phosphorylation (38 ATP per glucose molecule) to the inefficient, oxygen-independent, glycolysis (6 ATP per glucose molecule) and (2) reducing its rate of metabolism, thereby reducing its demand for energy (or ATP). This is why they are so clumsy and slow, but what counts is that ,in the long run, they win the race.
A less lovable example
In the first paragraph, I stated that “metabolically, oxygen is absolutely essential for oxidizing glucose…” and then went on to contradict myself in describing the turtle’s adaptation to anaerobic (without oxygen) oxidation of glucose. But as clumsy and lovable we humans may be, we are not as metabolically adaptable as the turtle. So how do we handle hypoxic stress?
There are two situations in which our tissues suffer hypoxia: a wound and cancer. In fact, more than 20 years ago, Dr. Dvořak, a professor at Harvard (not the Czech composer), characterized cancer as “a wound that never heals”. This was a profound insight, and here is why.
Wounds and solid tumors are hypoxic
One of the things we associate with injury is bleeding. If we looked at the bleeding wound through the microscope, we’d make an obvious observation: The blood is coming out of disrupted blood vessels. Obvious, but not trivial.
The consequence of disruption of blood supply is that the tissue that had received oxygen from these vessels became hypoxic (subnormal oxygen concentration), or even anoxic (no oxygen at all). This state of affairs triggers a cascade of urgent responses: Injured and dying cells release their contents, among which are a variety of peptides that attract inflammatory cells to the area of devastation.
Within minutes to hours, these cells, called phagocytes, literally meaning “cell-eaters”, engulf and remove the tissue debris. This, in turn, sets the stage for the healing process. The phagocytes secrete peptides that attract cells that will lay down scar tissue as well as blood vessels needed to provide oxygen and nourishment to the newly created tissue.
[An interesting side comment: How could the inflammatory cells themselves survive in the hypoxic environment of a fresh wound? The answer is by using anaerobic glycolysis, just like our friend the turtle.]
Solid tumors (as distinguished from blood cancers) have a very interesting life history. A solitary cell undergoes certain mutations. Endow it with immortality, and this transforms it into a cancer cell. The newly minted cancer cell divides into two daughter cells; those, in turn, divide into two daughter cells each, and so on and so on. Assuming that each cell divides every 12-24 hours, after the first day we’d have 2 cells, the next day 4, the day after 8, then 16, 32, and so on. You can see how these numbers escalate rapidly so that within a very short time, we’d get a million cells; the next day, 2 million; and not much longer, a billion cells. And before long, we get into the trillions. A trillion here, a trillion there—and pretty soon there will be more tumor cells than normal cells. A nightmare scenario—except that it doesn’t happen quite this way. When the number of cells reaches about 1 billion, the tumor stops growing. Its size is about 1 cubic millimeter, barely visible even with our most sophisticated imaging techniques. What stopped the tumor in its tracks?
Hypoxia and its consequences
This tiny speck of tumor cells gets into a state of semi-dormancy because it does not have an adequate blood supply to provide oxygen. So, the cells have to resort to using anaerobic glycolysis to generate their energy supply. Like the turtle, these oxygen-starved cancer cells “dial down” their metabolism to drastically reduce their energy requirements.
If this state of affairs persisted, we could live happily ever after with our dormant, harmless tiny tumors. In fact, a study of male cadavers showed that a significant percentage of males have tiny, harmless prostate cancer tumors when they die. With improved imaging techniques and heightened awareness of breast cancer, many women are facing the vexing dilemma of what to do about tiny, barely malignant tumors discovered in their breasts. Do they justify total mastectomy? Partial mastectomy? Lumpectomy? Chemotherapy? We just don’t have evidence-based answers yet.
But something dramatic happens to these tiny 1 mm³ tumors when new blood vessels start coursing through the adjacent tissue heading toward the tumor mass. It is as though the blood vessels are responding to some cryptic message broadcast from the beacon of tumor mass. Indeed, that’s exactly what is happening.
After a period of dormancy that can last days or years, depending on the tumor type, some of the cells in the tumor mass begin secreting into their environment certain peptides that attract blood vessels to grow in their direction. This process, called chemo-attraction, very quickly provides the tumor with a new blood supply, ample oxygen, and plenty of nutrients. A new burst of cell division ensues and the tumor grows to dimensions that are easily detected by imaging techniques and even simple palpation.
And then the cycle repeats: In the center of the tumor which is not well vascularized, cells switch to anaerobic glycolysis or just die (this is called central necrosis). In the process, they release more peptides that attract additional blood vessels and the vicious cycle (quite literally) continues.
Once the tumor attains a certain size there is another effect on the body. As we mentioned before, glycolysis is terribly inefficient with only 6 ATPs being produced from one glucose molecule. Unlike what happens in turtles where metabolism and, thus, glucose requirements are dialed down in an anerobic state, the tumor cells do the opposite. They increase by many folds the uptake of glucose into the cell. This then provides enough fuel for the inefficient anaerobic metabolism of glucose to generate an undiminished amount of energy in the form of ATP. And so, the larger the tumor, the larger the proportion of nutrients consumed by the tumor as opposed to the body’s normal cells. This is part of the reason for the weight loss characteristic of cancer.
A note of hope
Does this all sound way too complicated? Well, it is. But complexity is not a reason for despair. Au contraire, the more complexity, the more opportunity there is to find vulnerable points of attack.
Indeed, a few years ago, a biotechnology company (Genentech) received approval for an engineered protein (an antibody) that binds to the peptide that attracts the blood vessel growth toward the tumor. The peptide is called vascular endothelial growth factor, or VEGF. When the antibody binds to VEGF, it inhibits it from binding to the blood vessel cells and thereby prevents them from growing toward the tumor. This drug, called Avastin, is highly effective in the treatment of colon cancer, breast cancer, and is being studied in other cancers.
What is the take-home lesson?
To me, the most basic lesson, and most fascinating, is how biology in all its diversity is interconnected. How could one guess that the lessons learned from turtles, wounds, tumors, and many other seemingly irrelevant phenomena would have such an incredible impact on our understanding of human disease? One couldn’t. But the implications for public policy (i.e., funding for basic science research) are obvious and quite literally of existential importance.