By David N. Leff
Former President Bush put broccoli on the map. He hated it.
Perhaps he didn't realize the green vegetable is so wholesome. It turns out that broccoli is particularly rich in antioxidant molecules, which defang the harmful free-oxygen-radical oxidants thought to cause a host of afflictions, from Alzheimer's and Parkinson's to cardiovascular disease and cancer.
"Atherosclerosis," said cardiology fellow Kaikobad Irani, at Johns Hopkins University, "is believed to be at least partially mediated by oxidants, which may be generated by the endothelial and smooth-muscle cells within the blood vessels themselves."
Irani is lead author of an article in the current issue of Science, dated March 14, 1997, titled: "Mitogenic signaling mediated by oxidants in ras-transformed fibroblasts."
"We have been interested in oxidant injury and oxidant signaling for a long time," Irani told BioWorld Today. "Because it is difficult to develop cell lines from cardiovascular tissue, we expressed the ras oncogene in mouse fibroblasts, which we had rendered cancerous."
His study shows that the activated ras protein transmits its oncogenic, cancer-promoting, message via production of free-radical oxidants. In macrophages, which scavenge and kill bacteria, ras-like proteins produce the toxic superoxide — a free radical with an extra electron tacked on the oxygen — involved in the killing.
"We wondered," Irani said, "if the same superoxide was produced by the activated, mutated form of ras in these fibroblasts. Indeed it was. But superoxide," he pointed out, "is a very labile molecule. Other cellular enzymes, superoxide dismutase and catalase, very rapidly convert it to hydrogen peroxide, and other reactive oxygen species."
Peroxide, he noted, is used as a germ-killing antiseptic.
He and his co-authors made their mouse fibroblasts cancerous by treating them with ras oncogene. "Since ras is involved in proliferation," he recalled, "lo and behold, those cells multiplied much more rapidly than non-cancerous controls, even in the absence of growth factors."
Then they added a chemical antioxidant, N-acetyl-L-cysteine, which blocked these oxidative species inside the cells.
Antioxidant Stops Fast-Growing Cancer Cells
"We observed," Irani said, "that the runaway growth of these ras-transformed cells was suppressed up to 50 percent by this chemical antioxidant." He went on to emphasize "that this is not universally true for all cancer cells, but is true for cells transformed by ras.
"Another feature of our Science article," Irani observed, "was that we also looked at a different cancer cell line, transformed by a different oncogene. N-acetyl-L-cysteine had practically no effect on their growth.
"This tells you," he pointed out, "that superoxide and other reactive oxygen species are playing a significant role in transmitting the growth message of ras into those cancerous cells. That fits in very neatly with the epidemiological observation that people who eat foods rich in antioxidants have a lower incidence of developing many different forms of cancer."
That dietary phenomenon jibes with the molecular concept that "oxidants mutate and damage DNA, transform ras genes from protooncogene to oncogenic, and therefore make cells cancerous." Irani added, "This gives further insight into why antioxidants in the diet may be beneficial toward curbing the progression of certain forms of cancer."
While disclaiming any specific knowledge of broccoli in this context, Irani did observe: " I'm sure there are studies done that show particular forms of antioxidants are present in leafy, green vegetables such as broccoli, brussel sprouts, and vitamins such as C and E."
N-acetyl-L-cysteine proved too toxic to treat asthma, for which it was originally developed. It is still used medicinally to rescue livers threatened with destruction by Tylenol overdose. "Tylenol causes a lot of free-radical generation in the liver," Irani noted.
Insights Now; Therapeutic Possibilities Later
Although his Science study's main virtue is "adding insight into the mechanism of cancer generation and transformed cell propagation," Irani allowed that "it could have potential therapeutic implications, in that if one would direct antioxidants into ras-transformed cells, one might inhibit their growth, without affecting the growth of other cells within the body."
He also suggested that gene therapy might express genes for superoxide dismutase or catalase, both oxidant-scavenging enzymes, in ras-transformed cells, and block their cancerous growth.
Meanwhile, he and his colleagues are pursuing a different lead: "There is evidence," Irani concluded, "that cancerous cells bypass the body's normal immune surveillance, through a mechanism yet to be defined. We are looking into the hypothesis that oxidants produced in ras-transformed cells protect these cells from the normal immune defenses of the body."
An editorial accompanying Irani's paper commented that, if its antioxidant results pan out, "maybe even former president George Bush will think twice about refusing to eat broccoli." *