Photocatalytic pre-activation of TiO₂ nanoparticles modulates cell death in A549 lung cancer cells

Titanium dioxide (TiO₂) nanoparticles (NPs) have attracted attention as photoactivatable materials because UV illumination can induce photocatalytic reactions associated with reactive oxygen species (ROS) generation. In this study, we investigated the effects of TiO₂ NPs on human non-small cell lung cancer A549 cells, with or without UV-C activation. TiO₂ NPs were characterized by dynamic light scattering (DLS) and zeta potential measurements. The mean intensity-based hydrodynamic diameter was 87.7 nm, and the zeta potential was + 31.0 mV. A549 cells were exposed to increasing concentrations of TiO₂ NPs for 72 h. Parallel cultures were exposed to TiO₂ NPs pre-irradiated with UV-C (254 nm, 30 min) to evaluate photodynamic enhancement. Cell viability, assessed using the MTT assay, showed concentration-related cytotoxicity in both groups; the half-maximal inhibitory concentrations (IC₅₀) were 388.2 µg/mL for non-irradiated NPs and 327.5 µg/mL for UV-activated NPs, although the UV IC₅₀ was slightly lower, the confidence intervals overlapped extensively, and no statistically significant difference was observed between UV − and UV + TiO₂ at any single concentration. Flow cytometry (Annexin V/PI staining) showed that non-irradiated TiO₂ induced predominantly apoptotic death with low necrosis, whereas UV-irradiated TiO₂ increased late apoptosis and PI-positive cell population. Gene expression profiling showed upregulation of caspase-8 and BAX, accompanied by upregulation of BCL-2 and downregulation of caspase-9 at 48 h. These molecular changes are consistent with a putative Type II-like apoptotic response, but they should be interpreted as transcriptional evidence rather than definitive proof of pathway activation. Although UV-C pre-irradiation did not significantly enhance overall cytotoxicity under the tested conditions, the flow-cytometry results showed differences in cell-death distributions between the treatment groups. However, because different TiO₂ concentrations were used for the two groups and intracellular ROS levels were not directly measured, these differences should be interpreted cautiously. These findings suggest that TiO₂ nanoparticles affect cell viability and are associated with changes in apoptosis-related gene expression and cell-death-associated populations in A549 lung carcinoma cells. However, the contribution of ROS and the independent effect of UV-C pre-irradiation remain to be established.