Researchers believe they may have discovered a method for treating and preventing radiation-induced gastrointestinal toxicity.
The investigators found that inhibiting prolyl hydroxylase domain (PHD) proteins in mice could help protect them from radiation-induced toxicity and prolong their life spans.
“We were very surprised by the amount of protection the animals received,” said Amato Giaccia, PhD, of the Stanford University School of Medicine in California.
“The important thing to note is that we didn’t change the amount of damage the intestinal cells sustained as a result of the radiation. We simply changed the physiology of that tissue and how it responded to that damage.”
Dr Giaccia and his colleagues described this research in Science Translational Medicine.
The study began with an interest in hypoxia-inducible factor (HIF) proteins, which are known to help cells survive stressful conditions.
“Previous studies from our group and others have suggested that the HIF proteins are important in protecting cells from many types of stress,” Dr Giaccia said. “So we wondered whether stabilizing HIF proteins, and therefore increasing their levels within the cells, could also protect the intestine from the effects of radiation.”
The researchers inhibited the degradation of HIF proteins in 2 ways. In the first experiment, they engineered mice that were unable to express PHD isoforms, a group of 3 proteins that tag HIF proteins for destruction.
In another experiment, the investigators treated unmodified mice with a small molecule called dimethyloxyallyl glycine (DMOG), which also inhibits the activity of PHD proteins.
In both cases, the levels of HIF1 and HIF2 proteins increased significantly in the manipulated mice, as compared to controls.
In addition, 70% of the genetically modified mice lived for at least 30 days after receiving a normally lethal dose of abdominal radiation, and 27% survived at least 30 days after a normally lethal dose of whole-body radiation.
Sixty-seven percent of DMOG-treated mice survived for at least 60 days after receiving a normally lethal dose of abdominal radiation, and 40% lived for at least 30 days after a normally lethal dose of whole-body radiation.
The control mice in both experiments did not survive longer than 10 days after either type of radiation exposure.
Elucidating the mechanism
Further experiments showed that HIF2, rather than HIF1, is responsible for the radioprotection the researchers observed.
To determine the cause of the treated animals’ prolonged survival, the investigators looked directly at the epithelial cells lining the intestines.
Treated mice exhibited lower levels of cell death in response to abdominal radiation exposure and improved survival of crypts, which host the rapidly dividing stem cells necessary to accommodate the intestines’ need for repeated cell turnover.
The treated animals also experienced less diarrhea and fewer imbalances in fluid and electrolyte levels than untreated animals exposed to the same dose of radiation. And they quickly gained back the weight they had lost as a result of the exposure.
Treatment after radiation exposure
“The animals that survived the abdominal radiation have a life span that is similar to unexposed animals, which was very exciting to us,” Dr Giaccia said. “However, we realized it would be impossible to pretreat humans unexpectedly exposed to large amounts of radiation like at Chernobyl or Fukushima because those exposures are, by nature, unpredictable.”
So Dr Giaccia and his colleagues experimented with treating the mice with DMOG after abdominal radiation exposure. They found that, although the protective qualities of the molecule were diminished, it did help.
When DMOG was given 4 hours after radiation exposure, 45% of the treated mice, but no untreated mice, survived at least 10 days.
After 24 hours, the effect was more subtle. DMOG treatment showed little benefit at higher doses of radiation. But at a lower dose, 75% of the treated animals lived for at least 30 days, compared to 18.2% of the untreated animals.
“We found we were still able to rescue a significant proportion of the animals,” Dr Giaccia said.
Finally, the researchers tested the effect of DMOG treatment 24 hours after total-body irradiation.
They found that 37.5% of the treated mice survived for at least 30 days, but only if the mice were also given a bone marrow transplant to restore blood and immune stem cells killed by the radiation. None of the untreated mice lived beyond 10 days.
The investigators pointed out that, although this study suggests a possible way to mitigate the effects of therapeutic radiation exposure, more work remains. But the next steps are clear.
“There are a number of drug molecules that act in a manner similar to DMOG that are already in clinical trials for unrelated conditions,” Dr Giaccia said. “Our next step will be to test some of these molecules to see if they also offer radioprotection.”
Stanford University has filed a patent application, “Use of Prolyl Hydroxylase Inhibitors as a Radioprotective Drug for the Lower Gastrointestinal Tract” (international application No. PCT/US2012/052232), based on the results of this study.
