Weak doses of radiation prolong life of female flies, scientists find

A group of scientists from the Laboratory of the Genetics of Aging and Longevity at MIPT, the Engelhardt Institute of Molecular Biology at the Russian Academy of Sciences, the Komi Research Center and Syktyvkar University under the leadership of Alexey Moskalev is studying the mechanism of radiation hormesis, the effect by which moderate stress has a stimulating effect on an organism and enables the prolongation of life. This effect was first observed at the end of the XIX century by the German pharmacologist Hugo Shulz who discovered that small doses of poison speed up the growth of yeast cells. The effect was later found in many other organisms, in particular in peppermint and roundworms.

"Small doses of poison or the moderate influence of other stress factors affect the organism in such a way that the stimulation effect overcomes the harm. As a consequence, this can lead to an increase in life expectancy," explains Svetlana Zhikrivetskaya, lead author of the article.

In recent years scientists have been actively studying the influence of radiation hormesis, observing beneficial effects of weak doses of ionizing radiation. It is a commonly accepted view that there can be no safe doses of radiation, as any radiation will damage the molecules of DNA. An acceptable background is considered to be that at which the risk of cancer is negligibly small.

However, a number of experiments have demonstrated an improvement "under radiation" of indicators of life expectancy in mice and cell cultures. Indirect confirmation of radiation hormesis can be seen in cases of accidental irradiation of large groups of people over extended periods of time.

In particular, in 1982 in Taiwan, during smelting, 20 tonnes of steel were accidentally contaminated with cobalt-60. This steel was then used as construction material and, for about 20 years, about 10 thousand people were subjected to the effects of irradiation at a level of approximately one thousand times that of the natural background. Scientists who have investigated these people have drawn the conclusion that the number of cases of cancer in this group of unwilling experimental subjects is lower than normal. This conclusion has, however, been criticized and the very idea of radiation hormesis has remained debatable.

Moskalev's group has researched the effect of low radiation doses. The scientists had previously established that the radiation has a statistically significant influence on the larvae of drosophila flies: their life expectancy grows. Moreover, they become more resilient to stress, for example to overheating. In their new research the group has shown that gamma radiation effects adult drosophila and, importantly, the way in which the activity of the various genes changes.

"It is suggested that hormesis is universal, that its mechanism of operation, the stress genes and stress paths, are very similar to those of wounded animals. We are looking for these common paths and are compiling a list of the relevant genes. It is possible to do this more quickly, simply and cheaply using flies rather than mice and other animals," says Zhikrivetskaya.

The experiment has researched about two thousand drosophila flies (one thousand males, one thousand females) and there was a control group of two thousand flies. The experimental group was divided into four sub-groups, each of which received their dose of radiation from a radium 266 source -- from 0.05 to 0.4 grays (Gy). The amount of radiation energy absorbed by the various parts (for example the skin of the organism) was measured in grays. A dose of 6-8 grays received by a person will cause acute radiation sickness within a few days, a dose of 3 grays will act within a few weeks and a dose of 0.05 grays is the accepted dose limit for people whose work is connected with radiation.

Adult flies in contrast to people bear radiation well, even a gigantic dose of 1000 grays does not lead to instantaneous death but only to a certain shortening of life. Therefore, doses of tens and hundreds of grays, which could cause radiation sickness in people, do not seriously affect flies.

"The results of our experiment have demonstrated that life expectancy of the drosophila grows. Furthermore, the effect is significantly stronger in the female: for females life expectancy was extended by 7.6%, whilst for males it grew by just 3.4%. On average a drosophila lives for about two months, that is, there is a significant increase of about a week," Svetlana Zhikrivetskaya adds.

But the scientists are considerably more interested in which genes are active in this process and how. "If we can understand the genetic mechanisms at work here, in the future we will be able to activate them without stress, that is, for example, we can activate the mechanism that leads to beneficial effects from small radiation doses without the radiation itself," the researcher explains.

The scientists in their experiment with drosophila investigated the changes in the activity of 29 genes. And these changes were measured many times: immediately after radiation, after 6 hours, after 24 hours and then after 48 and 72 hours.

"We have discovered that several genes which are involved in the repair (restoration) of DNA are superstressed for a period of 48-77 hours after exposure to radiation. In a number of cases, the sex of the fly affected whether genes demonstrated superstress," says Zhikrivetskaya.

In her words, the scientists are planning future research into the phenomenon of radiation hormesis in other living organisms, mice in particular. "In the final analysis, this will allow us to find methods of extending human life expectancy," concludes the researcher.