IARC MONOGRAPHS ON THE EVALUATION OF CARCINOGENIC ...

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IARC MONOGRAPHS VOLUME 82
to identify the location, size and number of these cells in the airways. Neuroepithelial
bodies were significantly increased in number and pulmonary neuroendocrine cells were
significantly enlarged in naphthalene-treated lungs compared with controls (Peake et al.,
2000).
Immature Clara cells of neonatal mice are more susceptible to the toxicity of
naphthalene than are mature Clara cells of adult mice. Vacuolation and exfoliation
associated with cytotoxicity of Clara cells were dose-dependent when 7-day-old, 14-dayold
or adult Swiss Webster mice were given a single intraperitoneal injection of 0, 25, 50
or 100 mg/kg bw naphthalene in corn oil and killed 24 h later. The range of doses at
which Clara cell injury occurred varied with age, with the youngest animals being the
most susceptible. The 7- and 14-day-old mice were more sensitive to the toxicity of
naphthalene despite the fact that, at these ages, the airways have lower ability to activate
naphthalene to its reactive intermediates compared with adult mice (Fanucchi et al.,
1997).
To define the repair pattern of Clara cells after massive injury, male Swiss Webster
mice (2–3 months of age) were given an intraperitoneal injection of naphthalene
(200 mg/kg bw) in corn oil and the lungs were evaluated at various times up to 14 days
after treatment. Clara cells of terminal bronchioles were vacuolated and swollen on day 1
after the naphthalene injection, exfoliated on day 2 and resembled those of the controls
on day 14. Cell proliferation was increased within the epithelium and interstitium at
day 1, reached a maximum at day 2 and was close to the control level at all other time
points. Markers of Clara cell differentiation were barely detectable in the terminal bronchiolar
epithelium at days 1 and 2, clearly detectable at day 4 and returned to control
levels between days 5 and 14. The results showed that repair of the bronchiolar epithelium
after naphthalene treatment involved distinct phases of cell proliferation and
differentiation, including proliferation of cells other than Clara cells, and interaction of
multiple cell types including non-target cells (Van Winkle et al., 1995).
Swiss-Webster mice (8–10 weeks of age) were given an intraperitoneal injection of
0 or 200 mg/kg bw naphthalene and the temporal pattern of intracellular changes was
evaluated up to 6 h following treatment. Whole-lung preparations from these mice were
stained with cell-permeant and -impermeant nuclear binding fluorochromes and examined
by means of high-resolution light, electron and confocal fluorescence microscopy.
These methods allowed the assessment of Clara cell necrosis and cell permeability on the
same samples. After acute exposure to naphthalene in vivo, early stages of injury to
bronchiolar Clara cells included swelling of the smooth endoplasmic reticulum and bleb
formation, followed by increases in cell membrane permeability (Van Winkle et al.,
1999).
In a study in which mice were treated similarly, intracellular glutathione content was
measured and compared with the degree of cytotoxicity up to 3 h after treatment. Loss of
intracellular glutathione is an early event that precedes initial signs of cellular damage.
Once glutathione concentration dropped below 25% of the control, injury was irreversible
(Plopper et al., 2001).