*3.7. Effects of PSP-SeNPs on H2O2-Induced PC-12 Cells Toxicity*

Although the free radical scavenging assays proved the excellent antioxidant activity of PSP-SeNPs, the antioxidant assays based on chemical reactions may not necessarily reflect the behavior of antioxidants in biological systems [16]. Thus, the effect of different selenium species on oxidative stress-induced damage to PC-12 cells was further investigated by MTT assay. As depicted in Figure 7A, the cell viability was higher than 90% when incubated with SeNPs and PSP-SeNPs at the concentration of 1–20 μg/mL. However, the cell viability dramatically decreased to 67% after treatment with 20 μg/mL Na2SeO3, suggesting that both SeNPs and PSP-SeNPs showed lower cytotoxicity than Na2SeO3.

**Figure 7.** Effects of sodium selenite (Na2SeO3), SeNPs, and PSP-SeNPs on the viability of PC-12 cells (**A**). Values marked with \*: *p* < 0.05 and \*\*\*: *p* < 0.001 indicated significant differences when compared to the control group. The protective effect against H2O2 (0.5 μM)-induced PC-12 cells toxicity by MTT assay (**B**). Values marked with \*: *p* < 0.05 and \*\*: *p* < 0.01 indicated significant differences when compared to the H2O2 treated group.

The overproduction of reactive oxygen species (ROS) is considered to be the main cause of oxidative damage [44]. Herein, exogenous H2O2 was used as an inducer of cell damage in our model. PC-12 cells incubated with 500 μM H2O2 showed a remarkable decrease of cell viability reaching 56% (Figure 7B). However, the viability of PC-12 cells decreased to 55%, 50%, and 43% when pretreated with Na2SeO3 at concentrations of 1, 10, and 20 μg/mL, respectively. Interestingly, compared with the H2O2-induced oxidative stress model group, cells pretreated with SeNPs or PSP-SeNPs alleviated the H2O2-induced toxicity on PC-12 cells in a concentration-dependent manner, as reflected by the increase in cell viability. The viability of PC-12 cells pretreated with 20 μg/mL SeNPs or PSP-SeNPs significantly increased to 79% and 98%, respectively. In addition, the protective effect of PSP-SeNPs on H2O2-induced oxidative damage on PC-12 cells was better than that of SeNPs. The results confirmed that PSP-SeNPs had excellent antioxidant activity in cells, which may be associated with the free radical scavenging ability.
