Nrf2 and its cellular dance partner keap1 play a dual role in the body's response to oxidative stress. Keap1 usually inhibits the transcription factor nrf2. Oxidative stressors release nrf2 from inhibition and allow what's known as the "electrophile counterattack" to proceed, which simultaneously gets rid of the offending oxidizers and any cellular components they have damaged. Simultaneously, in a feedback loop, nrf2 activates the transcription of keap1, which ultimately shuts it back off.
In the January 2008 issue of Developmental Cell, researchers from the University of Rochester debuted a new model system for studying the pathway: a drosophila engineered to express green fluorescent protein when the nrf2/keap homologous pathway is activated.
Typically, biomedical research moves up the evolutionary ladder, with promising research in flies making the jump to vertebrates. In the case of keap and nrf2, there already is an active research effort under way in vertebrates. Indeed, keap agonists have a number of clinical trials under their belt - though by and large, not very successful ones - as chemoprevention agents for people at high risk of developing cancer.
As senior author Dirk Bohmann succinctly summarized, this leads rather directly to the question "why bother with flies?"
His answer is that the green fly might be useful as a drug discovery model. "We are not saying that we will replace vertebrate models," Bohmann told BioWorld Today. But the fruit fly is a simple and "genetically accessible model" that allows researchers to study the interaction between different pathways.
In this case, its short lifespan also is what allowed Bohmann and first author Gerasimos Sykiotis to make an unexpected finding in a different area. Males - though not females - missing one copy of the keap gene lived 8 percent to 10 percent longer than their wild-type cousins.
In their Developmental Cell paper, Sykiotis and Bohmann first identified the most likely drosophila homolog of nrf2: the protein cncC. CncC is expressed in the gut, giving it a good location for first-line defense against free radicals. By binding to an antioxidative response element, cncC regulates the oxidative stress response gene gstD1. It also has the same reciprocal relationship with the drosophila version of keap1 that is found in mammals.
The researchers then made a fly that has green fluorescent protein linked, as a reporter gene, to gstD1. They found that under baseline conditions, the flies expressed green fluorescent protein mainly in the gut. However, when they subjected flies to a variety of oxidative stimuli - ranging from paraquat, a toxin that generates free radicals, to hydrogen peroxide - gstD1 became more widely expressed, and the flies became greener. Flies overexpressing cncC were more likely to survive an equivalent dose of the oxidant paraquat than wild types, while flies whose cncC expression was knocked down were more likely to die from it.
In a final set of experiments, the scientists tested Aventis SA's investigational drug oltipraz on their flies. Originally developed as an antiparasitic drug, oltipraz also is in clinical trials for the prevention of cancer in high-risk individuals. Oltipraz activated the fluorescent reporter, providing further evidence that cncC is a nrf2 analogue and, in the words of the authors, that "Drosophila could be used as a platform for the discovery or preclinical testing of novel Nrf2 activators."
Given the relationship between aging and cancer, perhaps the effects of the nrf2/keap pathway are not completely surprising. Nevertheless, "the relationship between oxidative stress and lifespan has been known for decades," Bohmann said. "But that this particular pathway might have an effect had not been recognized." Why the effect was only apparent in males is not clear, but Bohmann said that lifespan effects, for reasons as yet unknown, are "commonly different in males than in females."
Whether the results have any relevance to humans, either male or female, is yet anther question. Bohmann cautioned that drosophila's very simplicity also means that it's hard to extrapolate to humans directly. But, he said, the results nevertheless suggested that "targeting the pathway might be effective against certain aspects of age-associated decline."