Neutron-activated
crocidolite, containing 55Fe and 59Fe, was used to determine whether
iron was mobilized from
crocidolite phagocytized by cultured human lung
carcinoma cells (A549 cells). Cells were treated with neutron-activated
crocidolite in medium at pH 6.8 or 7.4 for 24 h. The mobilization of
iron into two subcellular fractions, 10,000g supernatant (total
iron) or < 10,000 MW [low-molecular-weight (LMW)] was monitored using scintillation counting.
Iron was mobilized from
crocidolite at a rate similar to that observed in vitro when
citrate was incubated with
crocidolite for 24 h at pH 7.4, but the amount mobilized was greater when cells were cultured at pH 6.8 than at 7.4.
Iron mobilization was not due to the medium nor did it appear to be due to differences in the amount of
crocidolite phagocytized. At the highest concentration of
crocidolite used for treatment at pH 7.4 (4.5 micrograms/cm2), a total of 3600 pmol
iron/10(6) cells was mobilized of which 54 pmol/10(6) cells was in a LMW fraction. After estimation of the volume of the cells, this was calculated to be equivalent to an intracellular concentration of 1.4 mM
iron of which 22 microM was in the LMW fraction. Cell survival decreased linearly as the
iron mobilized into the LMW fraction increased, independent of the pH of the culture medium being used. These results suggest that
iron mobilization from
crocidolite into a LMW fraction may represent "
iron overload" in cells which have phagocytized the fibers and may be responsible for
crocidolite-dependent cytotoxicity and possibly other
crocidolite-dependent
biological effects.