Physical exercise has been found to mobilize the immune system of cancerous mice, resulting in a dramatic reduction in tumor size. Though this has yet to be observed in humans, the researchers behind the discovery have outlined a direct pathway by which this effect is brought about in the rodents, providing a key starting point for future research into the benefits of exercise in human cancer treatment.
Building on previous studies that have shown that physical exertion stimulates an increase in the number of immune cells called natural killer cells (NK cells), the new study suggests that this increase is driven by the surge in epinephrine – also known as adrenaline – which normally accompanies exercise.
These cells are then guided towards tumors by an immune signaling molecule called IL-6, which is released by muscles as they contract.
Publishing their paper in the journal Cell Metabolism, the researchers explain how mice were split into two groups, one of which was conditioned to exercise regularly while the other was not. After being injected with various types of tumors, the mice that exercised regularly were found to have recovered considerably six weeks later.
For instance, compared to the control group, tumor growth was reduced by 61 percent in exercising mice with lung cancer. Furthermore, 75 percent of non-exercising mice that had been injected with a cancer-inducing compound called diethylnitrosamine went on to develop liver tumors, compared with just 31 percent of exercising mice.
Concentrations of NK cells were found to be upregulated in the affected organs of active mice, while they were barely detectable in the control group, suggesting that these cells play a major role in combating cancer and are mobilized by exercise.
Next, the researchers injected the mice with propranolol, which blocks the effects of epinephrine. They report that this “completely blunted the effect of running on tumor volume and abolished the increased NK cell infiltration in tumors from running mice,” indicating that these cells are activated by epinephrine.
To confirm this, the study authors injected the non-running mice with epinephrine, and found that this produced an increase in NK cell numbers and a reduction in tumor size. However, this effect was not as pronounced as it was in the exercising mice, since a lower proportion of these NK cells found their way to the tumors, therefore hinting that adrenaline does not work alone in mobilizing these cells.
Suspecting that IL-6 may provide the missing piece of the puzzle, the researchers injected the running mice with antibodies that blocked this molecule. This was found to diminish the inhibiting effect of exercise on tumor growth, without preventing the production of NK cells.
Taking all of this data into account, the authors conclude that exercise helps to combat tumor growth in mice by increasing adrenaline levels, which in turn generates an increase in NK cell numbers. Simultaneously, the release of IL-6 by contracting muscles acts as a kind of guide that helps these NK cells infiltrate tumors in order to destroy them.
While more research is needed in order to determine whether physical activity has the same effect on tumor growth in humans, previous studies have indicated that cancer survivors who exercise regularly are less likely to suffer a recurrence of their illness than those who do not.