Study links TiO2 nanoparticles with potential for brain-cell damage
Preliminary results suggest that titanium dioxide nanoparticles, which are widely used in consumer products, could damage brain cells.
In a new study posted to ES&T’s Research ASAP website (DOI: 10.1021/es060589n), researchers report that titanium dioxide (TiO2) nanoparticles can trigger rapid and long-lasting defensive responses in mouse microglia, specialized cells that protect the brain from harmful external stimuli. The study—according to corresponding author Bellina Veronesi, a researcher at the U.S. EPA’s National Health and Environmental Effects Research Laboratory—is the first to examine the potential neurotoxicity of nanoscale TiO2, which is widely used in consumer products like sunscreen and cosmetics.
The authors followed a protocol for nanotoxicity testing that they hope other researchers will adopt in order to facilitate comparison across studies. “You carefully characterize the particles,” Veronesi explains, “and you begin with a very simple in vitro model. Then you move up the ladder” to more complicated systems and in vivo experiments.
Evidence is mounting that relatively nontoxic materials, like TiO2, become increasingly harmful at smaller sizes. Although they have yet to study the phenomenon in humans, toxicologists have begun to explore how nanomaterials spur the generation of biologically active molecules, known as reactive oxygen species (ROS), that can damage cells by inducing oxidative stress. Certain physical and chemical properties—including size, surface area, and surface charge—interact to determine how likely nanomaterials are to cause oxidative stress in biological systems.
But these same properties can be dramatically altered in solution, explains Greg Lowry, a professor of environmental engineering at Carnegie Mellon University and a coauthor of the paper. Nanoparticles may cluster and form larger aggregates, for example, changing their effective size and surface area. “When you put these metal nanoparticles in water, they don’t behave [ideally],” says Lowry, “and you have to understand that response.”
After characterizing solutions of commercially available TiO2 nanoparticles—Degussa’s Aeroxide P25, used as a thermal stabilizer and in catalysis applications—Veronesi and her colleagues exposed cultured microglia to the particles at concentrations ranging from 2.5 to 120 parts per million. Microglia respond to stimuli by engulfing them, in a process known as phagocytosis, and releasing chemicals in an “oxidative burst” designed to eliminate the offending stimuli. By monitoring a chemical signature of ROS formation over the course of 2 hours, the researchers found that TiO2 nanoparticles provoked a rapid and prolonged release of ROS by the microglia.
Although the microglia generate ROS as a defensive mechanism, a prolonged release can actually be harmful to the brain. “[Microglia] themselves are almost invincible to the danger of oxidative stress,” explains Veronesi. “When they release ROS to the [brain] environment, however, they can damage surrounding cells.” A similar mechanism has been implicated as the cause of neuronal damage in certain neurodegenerative diseases, including Parkinson’s and Alzheimer’s.
The authors’ next step is to determine whether neurons are harmed by the ROS triggered by nanoscale TiO2. According to Veronesi, a pilot study has shown that some neurons exposed to TiO2 initiate cellular processes that can ultimately progress to cell death, although she stresses that the findings are still preliminary: “It’s a very lengthy process and... this [study] is just the ABCs. But it’s solid data, and it can be pursued with confidence.”
“I think they set the stage for future work,” agrees Lisa Opanashuk, assistant professor of environmental medicine at the University of Rochester. “The most important aspect [of this research] is the [extensive] particle characterization.” Opanashuk notes that “ROS analysis is a good step to look at potential reactivity at the cellular level.” But she adds that researchers should also look for signs that nanoparticles are activating antioxidant defense systems or leading to inflammation.
“I think taking this altogether, it’s a very nice story and it emphasizes [that] we need to be careful,” says Wolfgang Kreyling of the GSF Institute for Inhalation Biology (Germany). Kreyling, who has extensively studied the translocation of nanoparticles in the body, notes that certain nanomaterials have been found to cross the blood–brain barrier and persist in the brain.
Although Kreyling has not determined whether TiO2 nanoparticles reach the brain, he has already seen that they can spread from the lungs to other organs. He cautions that the concentrations used in Veronesi’s research may be higher than actual exposures, but researchers cannot know for sure. Future translocation studies will be critical, Kreyling says, “because if you don’t have particles in the brain, then you don’t have to look for interactions with microglia. Even if it’s only a fraction, then you have to look at the interaction.”
