Titanium dioxide nanoparticles induce mitochondria-associated apoptosis in HepG2 cells

Abstract

Widespread applications of nanosized materials over the past decade have prompted investigations of desirable properties and potential hazards to humans and the environment. Titanium dioxide (TiO₂) nanoparticles are one of the most widely used nanoparticles. To investigate the effect of biological functions induced by TiO₂ nanoparticles (10 nm: TiO₂ NPs) on human liver cell lines, normal liver cell line L02 and hepatoma cell line HepG2 were co-cultured with exogenous TiO₂ NPs. Cell growth and proliferation, cell cycle, and the apoptosis rate were analyzed. The effects of TiO₂ NPs on the expression levels of apoptosis-associated protein caspase-3 and the membrane channel protein αENaC and caspase-3/7 activity were determined. Moreover, the influence of TiO₂ NPs on the expression levels of the mitochondria-related proteins SIRT3, VDAC1, and ACSS1, the mitochondrial membrane potential and the ADP/ATP ratio were also examined. Our results revealed that TiO₂ NPs inhibited the growth and proliferation of HepG2 cells, suppressed the S phase of cell cycling, and induced apoptosis of HepG2 cells. Following an increase in concentration, the inhibitory effect induced by TiO₂ NPs on proliferation and cell cycle was more evident, and the apoptosis rate increased in a significant concentration-dependent manner, whereas there was no significant effect on the growth, proliferation, apoptosis, and cell cycle of L02 cells. In addition, the results of western blot showed that in HepG2 cells, TiO₂ NPs upregulated the expressions of the apoptosis-related protein caspase-3 and the membrane channel protein αENaC in a concentration-dependent manner. However, in L02 cells, there was no significant difference in the expression levels of caspase-3 or αENaC. Furthermore, TiO₂ NPs induced depolarization of the mitochondrial membrane, upregulated the expression levels of the mitochondria-related proteins SIRT3 and VDAC1, and downregulated the expression level of the key respiratory chain protein ACSS1 in HepG2 cells. However, in L02 cells, the expressions of SIRT3, VDAC1, and ACSS1 exhibited no clear change. The apoptosis of HepG2 cells induced by TiO₂ NPs may be achieved by regulating intracellular osmotic pressure; moreover, upregulating the expression of the channel protein αENaC or the mitochondrial porin VDAC1 and depolarizing the mitochondrial membrane of HepG2 cells resulted in the loss of Cyt-c and ATP and further activated caspase-3. To further confirm the above results, a nude mouse xenograft model was employed. After a certain period of treatment with TiO₂ NPs, the nude mice were sacrificed, tumors were removed, and the expression of related proteins was detected. Immunohistochemistry and western blot results showed that the expressions of the proteins VDAC1 and SIRT3 were clearly upregulated in tissues treated to TiO₂ NPs, whereas the expression of ACSS1 was downregulated. The results were consistent with the above in vitro results. All the above results confirmed that TiO₂ NPs can act as a safe antitumor agent.