Cancer and Reactive Oxygen Species

Cruciferous Vegetables

As many cancer patients can attest, adverse side effects are often par for the course when undergoing certain treatments – and it is known that these side effects are largely the result of healthy cells being killed along with cancerous ones.

Clearly, targeting cancer cells while leaving normal cells unharmed is the name of the game – and the scientific community is striving to develop treatments that do just that. A recently published review* looks at the role of something called reactive oxygen species (ROS) in killing cancerous cells.

ROS: What they are, how they work

ROS are normal molecular byproducts of oxygen metabolism – one of the compounds our bodies produce when we use oxygen. Generated constantly, ROS exist in every human cell, their numbers determined by production level and biochemical antioxidants. Ironically, these natural byproducts can actually cause healthy cells to become cancerous if ROS are over-generated.

“ROS attack our proteins, lipids and DNA, which results in genetic damage that can lead to cancer development,” says co-author Xinjiang Wu, MD, PhD, of the University of Pennsylvania’s Abramson Family Cancer Research Institute. “Previous studies have shown that ROS can transform normal cells into cancerous ones.”

“All cells are governed by biological and physiological processes that depend upon signaling pathways, and ROS is essential for those pathways to function,” he says. “Therefore, it is not accurate to say that ROS are a bad thing overall.”

Since the basal level of ROS in cancerous cells is higher than that in the normal cells, Wu says, this difference may render cancer cells more vulnerable than normal cells to further ROS increase.

In fact, the studies reviewed by Wu and Hua indicate that targeting ROS has the potential to be a valuable tool in the efforts to prevent and treat cancer – if the appropriate level is introduced to cancer cells at the right point.

Because ROS fuels the rapid replication characteristic of cancer cells, these cells maintain a higher basal level of ROS than healthy cells do. They are therefore very sensitive to increases in ROS, which can be lethal to them, Wu explains.

The opposite is true for healthy cells. They have a lower basal level of ROS than cancer cells do – and, as previously mentioned, require a small amount of ROS to function properly. But too many ROS in normal cells hasten their proliferation (and even induce cell deaths at high level), which can cause the side effects typical of many cancer treatments, Wu says.

The goal is to understand the ROS threshold that triggers death in particular cells, and to manipulate ROS within a sensitive cellular framework so that it kills only cancer cells and prevents healthy cells from becoming cancerous.

“Because side effects are a big problem with cancer treatments like radiotherapy and chemotherapy, this strategy is very attractive for a lot of reasons,” Wu says.

What do cruciferous vegetables have to do with this?

Cruciferous vegetables – which include broccoli, watercress, Brussels sprouts, cauliflower, and many kinds of greens and cabbage – are high in phenylethyl isothiocyanate (PEITC), a family of naturally occurring compounds that contain sulfur and promote ROS production.

(Onions and garlic are also rich in some sulfur-containing compounds which have been shown to kill cancer cells associated with ROS increase.)

Evidence suggests that cancer patients, therefore, might benefit from a diet high in foods that contain a lot of PEITC or other sulfur-containing compounds.
“Studies indicate that this type of diet may slow the progress of cancer via multiple mechanisms. These findings suggest that killing cancer cells by targeting ROS with sulfur-containing vegetables will be beneficial for cancer patients,” Wu says. “And because ROS exists in every human cell, this strategy eventually could be applicable to many types of cancers.”

But, he continues, the medical community “must be careful in increasing ROS intake in human clinical applications. On one hand, we kill cancer cells; on the other hand, there’s a possibility of increasing the growth of normal cells, as well.”

The upshot is that additional research is needed, Wu says. Most ROS studies to date have been conducted in laboratories, and little human epidemiological data exists. In the meantime, those who wish to learn more about the cancer-related effects of a diet high in cruciferous vegetables or other high-sulfur foods should speak with their physicians.

* Wu XJ and Hua X, Targeting ROS: Selective Killing of Cancer Cells by a Cruciferous Vegetable Derived Pro-Oxidant Compound. Cancer Biol Ther. 2007 May 1; 6(5)

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