Lead Chromate-Induced Chromosome Damage Requires Extracellular Dissolution to Liberate Chromium Ions but Does Not Require Particle Internalization or Intracellular Dissolution

Abstract

Hexavalent chromium [Cr(VI)] is a well-established human lung carcinogen. Water solubility has proven to be a key factor in the carcinogenicity of Cr(VI), with the water insoluble or "particulate" compounds the more potent carcinogens. Pathology studies indicate that chromates target cells at bronchial bifurcation sites in human lungs. However, it is uncertain what roles particle internalization and dissolution play in the genotoxicity of these compounds to human lung cells. We investigated these mechanisms in a human lung cell line after exposure to particulate lead chromate. We found that lead chromate was clastogenic in a concentration-dependent manner with 0.1, 0.5, and 1 microg/cm(2), while 5 and 10 microg/cm(2) caused complete cell cycle arrest. We also found concentration-dependent increases in intracellular and extracellular chromium ion levels. We investigated particle internalization by using transmission electron microscopy and found an apparent relative increase with concentration but no apparent particle internalization at the lowest concentration (0.1 microg/cm(2)) even after 24 h. Furthermore, we found no lysosomal association with the vacuoles containing particles, further suggesting that intracellular dissolution did not occur. Cotreating the cells with lead chromate and vitamin C eliminated both the uptake of ionic chromium and the clastogenic activity of lead chromate but had no effect on particle internalization. These data indicate that in human bronchial cells lead chromate clastogenesis is mediated by the extracellular dissolution of the particles and not their internalization. These findings have important implications for our understanding of the physicochemical mechanism of particulate chromates as they contradict previous indirect data from human bronchial epithelial cells, which suggest that particles dissolve inside those cells. Thus, these new data suggest that there may be different mechanisms of genotoxicity for epithelial cells and fibroblasts exposed to chromate particles.

Particulate Depleted Uranium Is Cytotoxic and Clastogenic to Human Lung Cells

“…This mechanism has previously been elucidated for nickel to explain the differences between its soluble and particulate compounds (26). The WTHBF-6 cells used in our study are known to internalize metal particles by phagocytosis (27), and thus, this mechanism certainly is biologically possible in this model system. An alternative mechanism, observed for particulate chromate compounds, is that the DU particles provide chronic exposure to soluble DU with continuous occurrence of extracellular dissolution (27).…”

“…The WTHBF-6 cells used in our study are known to internalize metal particles by phagocytosis (27), and thus, this mechanism certainly is biologically possible in this model system. An alternative mechanism, observed for particulate chromate compounds, is that the DU particles provide chronic exposure to soluble DU with continuous occurrence of extracellular dissolution (27). We feel this is unlikely because chronic exposure to soluble DU was unable to cause an increase in clastogenicity.…”

Hexavalent chromium causes the oxidation of thioredoxin in human bronchial epithelial cells

“…It is possible that the size or surface structure of the ZnCrO 4 particles may make them more amenable to cellular components that facilitate dissolution or entry. With another insoluble chromate, PbCrO 4 , uptake into hamster embryo cells or human fibroblasts is facilitated by cell-enhanced extracellular dissolution of the particles (Elias et al, 1991;Xie et al, 2004). Such dissolution is not complete (Wise et al, 2002).…”

Reductive activation of hexavalent chromium by human lung epithelial cells: Generation of Cr(V) and Cr(V)-thiol species

“…With Syrian hamster embryo cells, PbCrO 4 uptake occurs gradually over 7 days and PbCrO 4 solubilization is facilitated by cells [81]. Studies with human bronchial fibroblasts similarly indicate that the cells facilitate the extracellular dissolution of PbCrO 4 [82] but that dissolution was not complete [83]. With CHO cells, intracellular Cr started to accummulate 3 hours after PbCrO 4 exposure and was maximal at 24 h [84].…”