Potential toxicity of superparamagnetic iron oxide nanoparticles (SPION)

Abstract

Superparamagnetic iron oxide nanoparticles (SPION) are being widely used for various biomedical applications, for example, magnetic resonance imaging, targeted delivery of drugs or genes, and in hyperthermia. Although, the potential benefits of SPION are considerable, there is a distinct need to identify any potential cellular damage associated with these nanoparticles. Besides focussing on cytotoxicity, the most commonly used determinant of toxicity as a result of exposure to SPION, this review also mentions the importance of studying the subtle cellular alterations in the form of DNA damage and oxidative stress. We review current studies and discuss how SPION, with or without different surface coating, may cause cellular perturbations including modulation of actin cytoskeleton, alteration in gene expression profiles, disturbance in iron homeostasis and altered cellular responses such as activation of signalling pathways and impairment of cell cycle regulation. The importance of protein-SPION interaction and various safety considerations relating to SPION exposure are also addressed.

Keywords: DNA damage; SPION; cellular stress; cytotoxicity.

Fig. 1

Cellular toxicity induced by SPION. Exposure to SPION could potentially lead to toxic side effects such as membrane leakage of lactate dehydrogenase, impaired mitochondrial function, inflammation, formation of apoptotic bodies, chromosome condensation, generation of reactive oxygen species (ROS) and DNA damage.

Fig. 2

Methods of SPION synthesis. Various methods can be employed for the synthesis of SPION with the desired physico-chemical characteristics. These can be also coated with biocompatible molecules either in situ or via post-synthesis methods wherein the uncoated SPION are surface-coated subsequent to their synthesis.

Fig. 3

Schematic representation of different intracellular uptake pathways of SPION. Possible mechanisms of uptake include passive diffusion, receptor-mediated endocytosis, clathrin-mediated endocytosis, caveolin-mediated internalisation, and other calthrin and caveolin-independent endocytosis (105, 106). Upon internalisation, the SPION may presumably be degraded into iron ions in the lysosomes. This ‘free iron’ can potentially cross the nuclear or mitochondrial membrane and in the latter case the free iron in the form of ferrous ions (Fe²⁺) can react with hydrogen peroxide and oxygen produced by the mitochondria to produce highly reactive hydroxyl radicals and ferric ions (Fe³⁺) via the Fenton reaction. Hydoxyl radicals (^·OH) generated could indirectly damage DNA, proteins and lipids (8-OH-dG=8 hydroxydeoxyguanosine, MDA=malondialdehyde, HNE=4-hydroxy-2-nonenal).

Fig. 4

Preliminary data to demonstrate the effect of dextran-coated SPION on the expression of genes involved in iron homeostasis using real-time RT-PCR. (A) TfR1 and (B) hepcidin. The students’ paired t-test was used to determine if down-regulation proved to be significant change in expression (with error bars representing standard deviation (*P<0.05); as compared to the control where water was used in place of the nanoparticles.

Fig. 5

Schematic representation of SPION-induced toxicity at cellular level. SPION may cause direct DNA damage or result in the generation of oxidative radicals that in turn have the potential to cause DNA damage (indirect), have an impact on actin cytoskeleton by modulating the Akt signalling pathway, and also alter the expression of various genes such as those involved in cell cycle regulation, iron homeostasis and pancreatic functioning.