BioWorld International Correspondent
LONDON - Researchers working in the UK believe they might have discovered a new class of agent to prevent cancer: small interfering RNA, targeted against a protein that helps determine whether a battery of genes involved in protecting cells against the effects of harmful chemicals is switched on or off.
John Hayes, professor of molecular carcinogenesis at the University of Dundee in Scotland, told BioWorld International: "This is terribly good news. It means that the protein we have down-regulated is an excellent drug target for those wanting to develop agents to prevent cancer."
Hayes, together with Lesley McLellan, lecturer at the University of Dundee, and colleagues, reported their findings in the May 2, 2005, issue of Proceedings of the National Academy of Sciences in a paper titled "Utility of siRNA against Keap1 as a strategy to stimulate a cancer chemopreventive phenotype."
Over the past five years, numerous studies in mice have established that the transcription factor known as Nrf2 regulates about 200 genes, many of them involved in protecting the cell against oxidative stress and harmful environmental agents. Groups in Japan and the U.S. also have developed knockout mice lacking functional copies of the gene encoding Nrf2. When experimentally exposed to chemical carcinogens, those mice develop cancer at a rate two to four times higher than that of wild-type mice, and many of them also develop autoimmune diseases associated with old age in humans.
The finding links with others made by groups investigating cancer. Many of those cancer chemopreventive agents activate the Nrf2 transcription factor and thus switch on the battery of defense genes that Nrf2 controls.
However, if those cancer chemopreventive agents are given to the Nrf2 knockout mice, they have no effect on the rate at which the animals develop cancer.
The studies all suggest that Nrf2 is central to mammalian defenses against cancer and degenerative diseases.
Hayes, McLellan and their colleagues set out to study how Nrf2 is controlled in human cells. Hayes said: "Like many transcription factors, Nrf2 is not very long-lived. We believe that it is made at a constant rate and that, under normal conditions when the cell is not under threat, it is rapidly destroyed."
Research had shown that a protein called Keap1 continuously binds to Nrf2 and delivers it to the proteosome. If a cancer chemopreventive agent is given to the cell, or if oxidative stress is present, Keap1 no longer is able to carry out that role, with the result that Nrf2 accumulates and the battery of defense genes is switched on.
"This is a controversial area, and we don't know all the details," Hayes said. "Nevertheless, it is clear that Keap1 does control the half-life of Nrf2. We wanted to know if we could reduce levels of Keap1 and would this alone be enough to allow Nrf2 to accumulate?"
To get the answer, the team used a type of liposome to deliver siRNA to cultured human keratinocytes. The siRNA used was identical in sequence to the messenger RNA produced by the gene encoding Keap1. When double-stranded RNA of that kind is detected in mammalian cells, the cells destroy any messenger RNA of the same sequence. As a result, the gene cannot make its protein and effectively is silenced.
The experiments by Hayes, McLellan and their team showed that simply reducing the levels of Keap1 in the cells was enough to activate the protective gene battery.
It might be possible, Hayes speculated, to develop the siRNA approach into a cancer chemopreventive agent. Some agents, such as sulforaphane-enriched broccoli sprout extracts, already are in clinical trials, he said, "but it is possible that these agents, in isolation, could have some undesirable side effects. By contrast, as far as we are aware, any side effects caused by siRNA should be much less severe."
The group now wants to define the battery of genes in human cells that are regulated by the Nrf2/Keap1 pathway. Hayes said: "This is important because preliminary results suggest the genes in humans that are controlled by Nrf2 differ from those in rats and mice that are controlled by this factor. Furthermore, it will allow the types of cancer-causing chemicals that human cells can be protected against by this pathway to be identified."