noNormal metabolic activity generates reactive oxygen species (ROS), which can damage cellular components if not removed. As a result, ROS from overactive mitochondria are frequently cited as a cause of DNA damage. Despite this long-held belief, few studies have clearly demonstrated this link.
Tobias DansenThe redox biologist at the University Medical Center Utrecht admits that even during graduate school he believed that ROS produced in mitochondria could damage DNA. This view began to change as he attended more conferences and met biochemists studying redox biology. “I started to realize that you actually have to pass a lot of material through to get from the mitochondria to the nucleus and damage DNA,” he says, adding that because of ROS’s reactivity, they’re probably captured along the way.
Tobias Danssen (left) and Daan van Soest (right) challenged the idea that mitochondrial ROS cause DNA damage, showing that these products do not efficiently reach the nucleus under normal conditions.
Verle Hove
Dansen and his graduate student co-author on the study. Daan van Soestwe tested this assumption. Nature Communications, The team proved that hydrogen peroxideROS generated by mitochondria do not diffuse into the nucleus.1 Considering the presence of DNA bases mutated by hydroxyl radicals in tumors, this finding supports a nuclear source of ROS that has yet to be identified.
To specifically study the location-dependent effects of hydrogen peroxide release, the researchers D-amino acid oxidase (DAAO) produces this ROS when D-alanine is added.2 The team fused DAAO to either nucleosomal proteins or mitochondrial membrane proteins and studied the effects of hydrogen peroxide produced at these specific locations.
Hydrogen peroxide has poor permeability across the nuclear membrane, so the team assessed whether increasing its concentration could improve this transport. Hyper 7 (Variations of the original probe, HyperNamed after hydrogen peroxide, this substance becomes active in the presence of hydrogen peroxide.3,4 The research team demonstrated that hydrogen peroxide produced by DAAO in the nucleus activates nuclear-localized HyPer7, but mitochondrial-bound DAAO does not significantly activate nuclear HyPer7 in the absence of toxic levels of hydrogen peroxide.
The research team hypothesized that hydrogen peroxide levels below the detection level of HyPer7 could still cause DNA damage. Nuclear DAAO activation promoted the activation of DNA damage repair proteins and led to DNA strand breaks, whereas DAAO bound to mitochondria did not cause either of these effects. Furthermore, hydrogen peroxide produced by DAAO at the mitochondrial membrane did not cause cell cycle arrest, whereas hydrogen peroxide produced by DAAO in the nucleus did pause the cell cycle.
Low levels of hydrogen peroxide produced by DAAO in mitochondrial membranes do not damage DNA or pause the cell cycle, but higher concentrations reduce cell viability. The researchers confirmed that increased production of hydrogen peroxide does not interfere with mitochondrial function. They investigated a form of apoptosis caused by hydroxyl radicals formed in the presence of iron and found that although these compounds contribute to cell death, cells also died in the presence of increased hydrogen peroxide, suggesting an alternative mechanism.
“It will be interesting to see how quickly or easily this idea is adopted. [hydrogen peroxide] “It has not penetrated the core,” he said. Ryan Burns“It’s important to understand how cells function in a way that is consistent with how they function,” said Dr. Stanley, a cell biologist at the University of Kansas who was not involved in the study.
ROS generated by mitochondria do not translocate efficiently from the cytoplasm (left image) to the nucleus (right image), as shown by the color map.
Daan van Soest
Burns found the methodology rigorous and the answer to the overarching question that mitochondrial ROS damage DNA compelling, but he said future experiments could explore the possibility that the evolution of cancers that expose cells to long-term hypoxia might weaken antioxidant defenses, altering hydrogen peroxide’s ability to diffuse elsewhere.
Dansen says one thing he and his team are still puzzled by is the observation that, like many other studies, treating cells directly with hydrogen peroxide triggers a DNA damage response and growth arrest but does not kill the cells. But in their study, hydrogen peroxide produced in high concentrations in mitochondria induced cell death before it reached the nucleus. “This is something I don’t understand,” Dansen says. “It’s one of the reasons why this dogma was pretty strong.” His group is now investigating the discrepancy.
“This proves once again that we need to be careful of dogmas that may exist in the biological community,” van Soest said, “It shows that we must always challenge old ideas with new tools that have been developed over the years, because sometimes old ideas and old findings don’t really explain things very well.”
