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This section
provides an abbreviated review of various experimental research studies.
The reader is encouraged to consult the cited materials for complete
information.
4.1
Biomarkers
A biomarker
can indicate
- the
occurrence of exposure,
- the
effects of exposure,
- the
presence of early or frank disease, or
- the
susceptibility to disease or early effects of exposure [Committee
on Biological Markers of the National Research Council 1987; Schulte
1995].
Useful
biomarkers require
- a definitive,
validated link with the exposure or the risk of disease and
- evidence
of a dose-response relationship between the marker and the exposure
[Schulte 1995].
The relationship
between respirable silica dust exposure and silicosis is well established.
However, the complex chain of cellular responses that leads to fibrosis
and silicosis has not been fully discovered. The usefulness of biomarkers
as a screening tool for silicosis risk will be realized when biomarkers
in the chain of complex cellular responses are validated for their relationship
to disease. In addition, the studies of blood, serum, sputum, bronchoalveolar
lavage samples, and gene patterns of silica-exposed workers or silicotics
(Table 20) are inconclusive for the following
reasons:
- The
numbers of subjects are small, and few studies of similar markers
exist for comparison.
- The
studies lack control for factors other than silica exposure that could
change immunoglobulin concentrations.
- The
studies lack information about control groups, diagnostic criteria
for silicosis, and baseline levels of markers.
- Study
results are inconsistent.
Further
research on biomarkers in silica-exposed workers is needed to do the
following:
- Quantify
the exact amount of soluble products in bronchoalveolar lavage in
individual patients to provide more information about the mechanisms
of fibrogenesis [Sweeney and Brain 1996]
- Determine
whether silicosis or silica-related lung cancers are associated with
a specific gene or gene pattern
- Determine
whether a relationship exists between changes in immunoglobulin concentrations
and silica exposure
- Determine
whether a dose-response relationship exists between changes in certain
cellular components (lymphocytes and Clara cell protein) and silica
exposure
Detailed
reviews of the immunologic response to silica and other mineral dusts
are available elsewhere (i.e., Heppleston [1994]; Haslam [1994]; Weill
et al. [1994]; Davis [1991,1996]; Kane [1996]; Driscoll [1996]; Sweeney
and Brain [1996]; Hook and Viviano [1996]; Gu and Ong [1996]; Iyer and
Holian [1996]; Weissman et al. [1996]; Mossman and Churg [1998]).
4.2
Cytotoxicity
Respirable
crystalline silica is known to cause
silicosis; however, the molecular mechanism responsible for the cellular
injury that precedes the lung disease is unknown. Extensive in vitro
and in vivo research has been conducted to evaluate the effects of crystalline
silica on mammalian cells. Several mechanisms have been proposed to
explain the cause of the cellular damage [Lapp and Castranova 1993]:
- Direct
cytotoxicity of crystalline silica
- Stimulation
of the alveolar macrophages by silica and subsequent release of cytotoxic
enzymes or oxidants
- Stimulation
of the alveolar macrophages to release inflammatory factors (e.g.,
interleukin-8, leukotriene B4, platelet-activating factor,
tumor necrosis factor, platelet-derived growth factor) that recruit
polymorphonuclear leukocytes, which in turn may release cytotoxins
- Stimulation
of the alveolar macrophages to release factors that initiate fibroblast
production and collagen synthesis (e.g., interleukin-1, tumor necrosis
factor, platelet-derived growth factor, fibronectin, and alveolar
macrophage-derived growth factor)
4.3
Genotoxicity and Related Effects
Some studies
have demonstrated the ability of quartz
to induce micronuclei in mammalian cells in culture [Hesterberg et al.
1986; Nagalakshmi et al. 1995; Oshimura et al. 1984] (Table
21). However, other in vitro studies did not observe chromosomal
aberration [Nagalakshmi et al. 1995; Oshimura et al. 1984], hprt
(hypoxanthine guanine phosphoribosyl transferase) gene mutation [Driscoll
et al. 1997], or aneuploid or tetraploid cells [Price-Jones et al. 1980;
Oshimura et al. 1984; Hesterberg et al. 1986]. An in vivo treatment
of rats with quartz induced mutation in rat alveolar epithelial cells
(Table 21) [Driscoll 1995; 1997].
Pairon
et al. [1990] tested tridymite (i.e., Tridymite 118) and quartz (i.e.,
Min-U-Sil 5) particles for genotoxic effects. Tridymite induced a significant
number of sister chromatid exchanges (SCEs) in co-cultures of human
lymphocytes and monocytes (P<0.05 compared with control cells)
at doses of 5 and 50 µg/cm2
(87.9% of the tridymite particles had a diameter <1
µm).
However, the number of SCEs in purified human lymphocytes that were
treated with the same doses of tridymite particles did not differ significantly
from control cells [Pairon et al. 1990]. Results of the same experiments
with quartz did not yield a clear conclusion about the ability of quartz
to induce a significant number of SCEs (Table
21) [Pairon et al. 1990].
In vitro
cellular transformation systems model the in vivo process of carcinogenesis
[Gao et al. 1997; Gu and Ong 1996]. The ability of quartz to induce
dose-dependent morphological transformation of cells in vitro has been
demonstrated in experiments with Syrian hamster embryo cells [Hesterberg
and Barrett 1984] and mouse embryo BALB/c-3T3 cells [Saffiotti and Ahmed
1995]. Gu and Ong [1996] also reported a significant increase in the
frequency of transformed foci of mouse embryo BALB/c-3T3 cells after
treatment with Min-U-Sil-5 quartz. These studies indicate that crystalline
silica can morphologically transform mammalian cells. However, further
studies are needed to determine whether the transforming activity of
silica is related to its carcinogenic potential.
Researchers
at the National Cancer Institute have examined the ability of quartz,
cristobalite, and tridymite particles to cause deoxyribonucleic acid
(DNA) damage (i.e., strand breakage) [Saffiotti et al. 1993; Shi et
al. 1994; Daniel et al. 1993; Daniel 1993, 1995]. Although the results
of those studies demonstrated the ability of crystalline silica to cause
damage to isolated DNA in acellular systems, reviewers at IARC [1997]
recently stated that the relevance of these assays to assess quartz
related genetic effects in vivo was questionable because
- the
nonphysiological experimental conditions did not apply to intracellular
silica exposure and
- very
high doses of silica were used in the DNA breakage assays [IARC 1997].
Several
studies conducted since the IARC review found that crystalline silica
induced DNA damage (i.e., DNA migration). Zhong et al. [1997] found
that by using the alkaline single cell gel/comet (SCG) assay, crystalline
silica (Min-U-Sil 5) induced DNA damage in cultured Chinese hamster
lung fibroblasts (V79 cells) and human embryonic lung fibroblasts (Hel
299 cells) [Zhong et al. 1997]. Amorphous silica (Spherisorb), but not
carbon black, was also found to induce DNA damage in these mammalian
cells. However, the DNA-damaging activity of amorphous silica was not
as high as the damaging activity of crystalline silica [Zhong et al.
1997]. Liu et al. [1996, 1998] challenged Chinese hamster lung fibroblasts
with dusts pretreated with a phospholipid surfactant to simulate the
condition of particles immediately after deposition on the pulmonary
alveolar surface. Results of the experiments showed that untreated Min-U-Sil
5, Min-U-Sil 10, and noncrystalline silica induced micronucleus formation
in a dose-dependent manner, but surfactant pretreatment suppressed that
activity [Liu et al. 1996]. A subsequent experiment found that surfactant
pretreatment suppressed quartz induced DNA damage in lavaged rat pulmonary
macrophages, but DNA damaging activity was restored with time as the
phospholipid surfactant was removed by intracellular digestion [Liu
et al. 1998].
Shi et
al. [1998] recently reviewed published literature on
- the
generation of reactive oxygen species (ROS) directly from silica and
from silica-stimulated cells,
- the
role of ROS in silica-induced DNA damage and silica-induced cell proliferation,
and
- other
silica-mediated reactions.
A proposed
mechanism for silica-induced generation of ROS species and carcinogenesis
is described by Shi et al. [1998]. Experimental research is continuing
to determine whether crystalline silica
particles have a direct genotoxic effect that could cause lung tumor
formation in humans.
4.4
Carcinogenicity
Experimental
evidence of the carcinogenicity of quartz
particles is based on the results of long-term inhalation and intratracheal
instillation studies of rats, which are summarized in Tables
22 and 23 [Saffiotti et al. 1996].
Several issues are apparent from the results of the rat studies [Holland
1995]:
- The
appearance of tumors (usually adeno-carcinomas or epidermoid carcinomas)
is a late phenomenon.
- Lung
fibrosis is usually present in the rats with tumors.
- No
adequate dose-response data exist because multiple-dose experiments
have not been conducted in the rat except for the inhalation study
by Spiethoff et al. [1992].
- Comparability
of the intratracheal instillation and inhalation studies is difficult
because of notable differences in methods and materials.
Although
new long-term carcinogenesis studies in animals may provide information
about dose-response relationships and inhibition of quartz toxicity
or reactivity in vivo, in vitro studies are needed to develop effective
cellular and molecular models of carcinogenesis [Holland 1995; Saffiotti
et al. 1996].
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