By Dean A. Haycock
Special to BioWorld Today
Free radical molecules running wild in the body are about as welcome these days as free radical protestors running wild outside a political party convention. Superoxide radicals are so highly reactive, for example, that they ravage more stable biomolecules in their search for chemical stability.
Although they have productive uses in the body, free radicals in excess can kill cells by inducing them to self-destruct. That, of course, is one of the reasons antioxidants have become popular food supplements. It may also, one day, become the reason certain estrogen derivatives are developed into anticancer drugs.
It may be too early to think of estrogen derivatives as a radical new line of research for developing novel anticancer drugs. Nevertheless, there definitely is something radical about the way 2-methoxyoestradiol (2-ME) - an estrogen derivative that does not bind to estrogen receptors - kills human leukemia cells while sparing normal cells.
Researchers from the University of Texas M.D. Anderson Cancer Center in Houston report in the Sept. 21, 2000, issue of Nature that 2-ME kills certain cancer cells by inhibiting enzymes that inactivate or neutralize superoxide radicals. These enzymes, designated superoxide dismutases (SODs), play a crucial role in protecting cells from the effects of free radicals, which are byproducts of normal and, in the case of cancer cells, abnormal cellular metabolism.
Increase Of P53 Was Initial Goal
The authors began their 2-ME research using the estrogen derivative to increase levels of the p53 protein, a growth inhibitor or suppressor, as they tried to boost the effects of other anticancer drugs. Their control experiments, in which 2-ME was used by itself on human leukemia cells, produced a surprising result: 2-ME killed cancer cells all by itself. Other research teams had ascribed anticancer properties to 2-ME but no one had figured out definitively how it worked. The authors used complementary DNA microarray and in vitro biochemical techniques to show that 2-ME and a derivative target two SOD enzymes, CuZnSOD and MnSOD. Furthermore, they identified a crucial site in the 2-ME molecule necessary for SOD inhibition and the induction of cell death.
First author Peng Huang explained to BioWorld Today the hypothesis he and his colleagues are pursuing. Some cancer cells have significantly higher rates of metabolism than normal cells. Consequently, they generate more reactive oxygen species than normal cells. As Huang and his colleagues explain in their article, "Superoxide dismutase as a target for the selective killing of cancer cells," this apparently is a key factor behind 2-ME's anticancer properties. Since certain cancers cells produce lots of free radicals yet contain comparatively small amounts of the protective SOD enzymes, they have enough SOD to survive and flourish but not enough to make them invulnerable to SOD inhibition. Normal cells, on the other hand, at least in these preclinical studies, appear to be able to handle 2-ME SOD inhibition and survive unharmed.
"We have quite a few preliminary studies with other solid tumor cells," Huang reported.
"We find that 2-ME also is effective in solid tumor cell lines such as ovarian cancer, lung cancer and, to some extent, pancreatic cancer cells. But I have to say, all those are laboratory studies in cell culture."
Huang cautioned that in certain situations some normal cells may have high levels of free radicals. "In these situations, the normal cell may be subject to the toxicity of SOD inhibitors," he said. "On the other hand, if certain types of tumors are not active in free radical metabolism, they may not be sensitive to SOD inhibition," Huang said. This led Huang and his co-authors to write, "The role of [superoxide] in causing apoptosis may vary among different cell types, and thus inhibition of SOD should not be considered a general approach to cancer therapy."
For this reason, the Texas researchers believe it might be worthwhile to investigate use of estrogen derivatives such as 2-ME in combination with other anticancer therapies including therapeutic agents (anthracyclines and bleomycin, for example), or ionizing radiation. "We know that some of the therapeutic agents now in clinical use kill cancer cells by generating free radicals," Huang noted. Therefore, he reasoned, use of 2-ME with these therapies might have a cancer-fighting synergistic effect.
John Cleveland and Michael Kastan of the St. Jude Children's Research Hospital in Memphis, Tenn., writing in a News and Views column in the same issue of Nature, agreed. "The results raise hope that inhibiting SODs at the same time as treating cells with agents that increase the levels of reactive oxygen species is a promising way of treating at least some cancers," they wrote. "At the very least, even if SODs turn out not to be ideal drug targets, we can now be confident that it is reasonable to try to exploit the differences in the ways that normal and tumor cells control their redox status." The commentators describe the results as "provocative."
Meanwhile, the group in Texas has begun studying details of the molecular mechanism, the signaling transduction pathways, behind the induction of cell suicide produced by free radicals. "We hopefully are trying to identify another set of targets which, for example, would be synergistically used in combination with 2-ME," Huang said. They are also designing new protocols to test different combinations of drugs.
After several years of researching the SOD inhibitors, the M.D. Anderson scientists have recently begun to collaborate with scientists from EntreMed Inc. in Rockville, Md. "They were not involved in the initial study of SOD inhibition but they are aware of these results and they are also quite excited and interested in doing some collaborations," Huang said. "Recently we have had some discussion about moving forward with some development testing."
Huang cautioned against speculating about how these compounds would work in a clinical setting. "Sometimes the laboratory result may not really spell out what would happen in a real situation but we are really excited about it and are planning to move forward in the study and are planning for clinical trials."