PbCrO₄ mediates cellular responses via reactive oxygen species

Exposure to certain particulate hexavalent chromium [Cr(VI)] compounds, such as lead chromate (PbCrO₄), has been associated with lung cancer and respiratory tract toxicity. Previous studies indicate that the solubility of Cr(VI)-compounds is an important factor in Cr(VI)-induced carcinogenesis. The present study investigates reactive oxygen species (ROS) generation by PbCrO₄ particles and cellular responses using RAW 264.7 cells. A mixture containing PbCrO₄ and RAW 264.7 cells generated hydroxyl radical (•OH), using cellularly generated H₂O₂ as a precursor, as measured by electron spin resonance (ESR) spin trapping in combination with H₂O₂ and •OH scavengers, catalase and sodium formate. The effect of ascorbic acid on •OH radicals was also measured using ESR. Confocal microscopy showed that particles could become either bound to the cell surface or engulfed over a 120 min time period. H₂O₂ generation and O₂ consumption were also increased after treatment of the cells with PbCrO₄. Both NF-κB and AP-1 were activated after exposure to PbCrO₄ particles as measured by the NF-κB or AP-1 luciferase reporter plasmid assay. Our investigation thus demonstrated that the RAW 264.7 cells phagocytized the PbCrO₄ particles leading to accumulation of the particles within vacuoles in the cytoplasm. These particles could induce chronic production of ROS and activation of NF-κB and AP-1. Such induction of transcription pathways may be involved in the inflammatory and carcinogenic responses induced by Cr(VI)-containing particles.