Insight into the Mechanisms of Combined Toxicity of Single-Walled Carbon Nanotubes and Nickel Ions in Macrophages: Role of P2X7 Receptor

Coexistence of nanomaterials and environmental pollutants requires in-depth understanding of combined toxicity and underlying mechanism. In this work, we found that coexposure to the mixture of noncytotoxic level of single-walled carbon nanotubes (SWCNTs) (10 μg/mL) and Ni 2+ (20 μM) induced significant cytotoxicity in macrophages. However, almost equal amount of intracellular Ni 2+ was detected after Ni 2+ /SWCNT coexposure or Ni 2+ single exposure, indicating no enhanced cellular uptake of Ni 2+ occurred. SDS-PAGE analysis revealed 50% more SWCNTs retained in Ni 2+ /SWCNT exposed cells than that with SWCNT exposure alone, regardless of the exposure sequence (coexposure, Ni 2+ pre- or post-treatment), suggesting inhibited SWCNT exocytosis by Ni 2+ . The increased cellular dose of SWCNTs could quantitatively account for the elevated toxicity of Ni 2+ /SWCNT mixture to cells. It was then found that agonist (ATP) and antagonist (o-ATP) of P2X 7 R could regulate intracellular SWCNT amount and the cytotoxicity accordingly. In addition, inhibition of P2X 7 R by P2X 7 -targeting siRNA diminished the inhibitory effect of Ni 2+ . It was therefore concluded that Ni 2+ impeded SWCNT exocytosis by inhibiting P2X 7 R, leading to higher intracellular retention of SWCNTs and elevated cytotoxicity. Our work identified exocytosis inhibition as an important mechanism for SWCNT/Ni 2+ oxicity, and revealed the crucial role of P2X 7 R in mediating such inhibitory effect.