Researchers at the Georgia Institute of Technology used high-throughput screening techniques to study the effects of titanium dioxide nanoparticles on the expression of 84 genes related to cellular oxidative stress. Their work found that six genes, four of them from a single gene family, were affected by a 24-hour exposure to the nanoparticles.
The effect was seen in two different kinds of cells exposed to the nanoparticles: human HeLa cancer cells commonly used in research, and a line of monkey kidney cells. Polystyrene nanoparticles similar in size and surface electrical charge to the titanium dioxide nanoparticles did not produce a similar effect on gene expression.
‘This is important because every standard measure of cell health shows that cells are not affected by these titanium dioxide nanoparticles,’ said Christine Payne, an associate professor in Georgia Tech’s School of Chemistry and Biochemistry. ‘Our results show that there is a more subtle change in oxidative stress that could be damaging to cells or lead to long-term changes. This suggests that other nanoparticles should be screened for similar low-level effects.’
The study process
Titanium dioxide nanoparticles help make powdered donuts white, protect skin from the sun’s rays and reflect light in painted surfaces. In concentrations commonly used, they are considered non-toxic, though several other studies have raised concern about potential effects on gene expression that may not directly impact the short-term health of cells.
To determine whether the nanoparticles could affect genes involved in managing oxidative stress in cells, Payne and colleague Melissa Kemp – an associate professor in the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory University – designed a study to broadly evaluate the nanoparticle’s impact on the two cell lines.
They separately incubated HeLa cells and monkey kidney cells with titanium oxide at levels 100 times less than the minimum concentration known to initiate effects on cell health. After incubating the cells for 24 hours with the TiO2, the cells were lysed and their contents analyzed using both PCR and Western Blot techniques to study the expression of 84 genes associated with the cells’ ability to address oxidative processes.
Payne and Kemp were surprised to find changes in the expression of six genes, including four from the peroxiredoxin family of enzymes that helps cells degrade hydrogen peroxide, a by-product of cellular oxidation processes. Too much hydrogen peroxide can create oxidative stress which can damage DNA and other molecules.
The effect measured was significant – changes of about 50 percent in enzyme expression compared to cells that had not been incubated with nanoparticles. The tests were conducted in triplicate and produced similar results each time.
More studies needed
The research suggests a re-evaluation may be necessary for other nanoparticles that could create subtle effects even though they’ve been deemed safe.
‘Earlier work had suggested that nanoparticles can lead to oxidative stress, but nobody had really looked at this level and at so many different proteins at the same time,’ Payne said. ‘Our research looked at such low concentrations that it does raise questions about what else might be affected. We looked specifically at oxidative stress, but there may be other genes that are affected, too.’
Those subtle differences may matter when they’re added to other factors.
‘Oxidative stress is implicated in all kinds of inflammatory and immune responses,’ Kemp noted. ‘While the titanium dioxide alone may just be modulating the expression levels of this family of proteins, if that is happening at the same time you have other types of oxidative stress for different reasons, then you may have a cumulative effect.’
The research TiO2 Nanoparticles Alter the Expression of Peroxiredoxin Antioxidant Genes was reported online May 6 in the Journal of Physical Chemistry C.
• Common Nanoparticle has Subtle Effects on Oxidative Stress Genes, by John Toon, May 10, 2016, Georgia Tech News Center