TiO2 Nanoparticles Impair Rat Bone Growth by Suppressing HDAC9-Mediated p53 Deacetylation in Chondrocytes

Abnormal bone growth and short stature in children can result from environmental factors, including exposure to pollutants. Titanium dioxide nanoparticles (TiO2 NPs), widely used in various products, are increasingly recognized as potential environmental toxins. Excessive intake of TiO2 NPs has been linked to abnormal development of cartilage growth plates, critical structures for longitudinal bone growth. Current understanding of the precise molecular mechanisms by which TiO2 NPs exert their toxic effects on chondrocytes — the cells responsible for cartilage formation — remains incomplete, hindering effective prevention and treatment strategies.

Researchers investigated the toxicity of TiO2 NPs on chondrocytes using both in vitro and in vivo models. In vitro, chondrocytes were exposed to TiO2 NPs, and their proliferation, differentiation, oxidative stress levels, and ferroptosis activation were assessed. Gene expression (qPCR) and protein levels (Western blot) of key regulatory molecules like HDAC9, TDP-43, p53, BCL6, and p21 were analyzed. For in vivo validation, young rats were exposed to TiO2 NPs, and the effects on bone growth were observed. A combination therapy involving HDAC9 overexpression and the p53 inhibitor Pifithrin-α was tested in vivo to rescue TiO2 NP-mediated detrimental effects.

Exposure to TiO2 NPs significantly inhibited chondrocyte proliferation and differentiation. This was accompanied by elevated levels of oxidative stress and activation of ferroptosis in the exposed chondrocytes. A key finding was that HDAC9 expression was downregulated in TiO2 NP-exposed chondrocytes, and its overexpression reversed these detrimental effects. Mechanistically, TiO2 NPs inhibited TDP-43, which in turn impaired the nucleocytoplasmic translocation and mRNA stability of HDAC9. Overexpression of TDP-43 protected growth plate chondrocytes from TiO2 NP exposure, an effect blocked by HDAC9 knockdown. TiO2 NPs were found to inhibit p53 deacetylation by suppressing the nucleocytoplasmic translocation of HDAC9. Furthermore, HDAC9 upregulated BCL6 and strengthened the interaction between BCL6 and Miz-1 to suppress p21 transcription in chondrocytes. The study concluded that TiO2 NPs suppress p53 deacetylation via inhibiting HDAC9 nucleocytoplasmic translocation, thereby impairing chondrocyte proliferation and differentiation through IGF1/mTOR signaling. > The combination of HDAC9 overexpression and Pifithrin-α efficiently abolished TiO2 NP-mediated detrimental effects in vivo, highlighting a potential therapeutic avenue.