San Diego, CA, March 28, 2007 -- Researchers at UC San Diego have discovered that iron-containing nanoparticles being tested for use in several biomedical applications can be toxic to nerve cells and interfere with the formation of their signal-transmitting extensions.
|These PC12 cells were exposed to no (left), low (center), or high (right) concentrations of iron oxide nanoparticles in the presence of nerve growth factor, which normally stimulates these neuronal cells to form thread-like extensions called neurites. Higher-res version.|
“Iron is an essential nutrient for mammals and most life forms and iron oxide nanoparticles were generally assumed to be safe,” said Sungho Jin, a professor of materials science at UCSD and senior author of a paper published in the June issue of Biomaterials. “However, there are recent reports that this type of nanoparticle can be toxic in some cell types, and our discovery of their nano-toxicity in yet another type of cell suggests that these particles may not be as safe as we had once thought.”
|Iron oxide nanoparticles|
Jin and the other co-authors of the paper, Thomas R Pisanic, II, Jennifer D. Blackwell, Veronica Shubayev, and Rita Finoñes began their laboratory experiments by coating iron oxide nanoparticles with DMSA (dimercaptosuccinic acid), a metal binding agent that polymerizes on the particles’ surface. This coating keeps the particles from clumping together in an aqueous solution, and facilitates their engulfment by the PC12 cells via an inward pouching of the cell membrane called endocytosis. What happened next was a surprise.
|Recent UC San Diego Ph.D. graduate Thomas R Pisanic, II (left) and materials science professor Sungho Jin. Higher-res version.|
In their experiments, PC12 cells that had not been exposed to magnetic nanoparticles generated three neurites in response to nerve growth factor. However, exposure to a low concentration of iron oxide nanoparticles resulted in the production of fewer than three neurites per cell in response to growth factor addition. A 10-fold increase in the concentration of nanoparticles led to the production of two neurites per cell, and a 10-fold increase of that concentration resulted in only one neurite per cell. Additionally, neurites produced in response to the growth factor in the presence of iron oxide nanoparticles were less well formed and also showed abnormal morphology and neurobiological characteristics.
The researchers also studied long protein polymers inside the PC12 cells that make up the cytoskeletal structure. They found that iron oxide nanoparticles resulted in fewer and less organized microtubules and microfilaments, protein polymers involved in cell motility and cell shape.
“It’s worth noting that neither iron oxide nanoparticles alone, nor the coating material alone are overtly toxic, but combining the two to create water-soluble nanoparticles has a completely different effect,” said Pisanic, who carried out the studies as a part of a Ph.D. thesis project at UCSD.
Iron oxide nanoparticles are considered promising because they are maneuverable by remote magnetic fields, and can be coated with various marker molecules to make them stick selectively to tumors and other targets within the body. The particles can also be made to carry anti-cancer drugs or radioactive materials directly to a tumor. Magnetic nanoparticles designed to attach to cancerous tissue can also be made to heat up by using a remote, alternating magnetic field, thereby selectively killing cancer cells in a process called magnetic hyperthermia.
Many researchers throughout the world are also studying the use of iron-containing nanoparticles in gene therapy, magnetic resonance imaging (MRI), and other medically important applications. While studies have focused primarily on the many potential uses of nanoparticles, the UCSD researchers said more attention should be paid to their safety. “Our experience leads us to conclude that any analysis of the biocompatibility of nanoparticles should include not just a toxicological study of the component parts,” said Pisanic, “but also an examination of the total structure as a whole.”