Experimental Evidence for Role of Asbestos
Animal
experiments have confirmed the oncogenicity of asbestos. A
singleexplanation for the pathogenesis of mesothelioma. Normal human
mesothelial cells can phagocytose asbestos fibers and are 10 times more
sensitive than normal human bronchial epithelial cells to asbestos
cytotoxicity in vitro.153 Mesothelial cells are 100 times more sensitive
than fibroblasts.
Following in vitro exposure to asbestos,
mesothelial cells display chromosomal aberrations indicative of clonal
origin.153 Occurrence of DNA strand breaks has been found after exposure
of cells to asbestos in vitro.131,159 Such effects could further lead
to activation of oncogenes and/or loss of suppressor genes.23 Indeed,
karyotypic analyses of human mesotheliomas have revealed frequent
abnormalities, particularly involving chromosomes 1, 2, 3, 6, 7, 9, 11,
17, and 22.23,109,183,200 One of the most common nonrandom changes is
deletion of the short arm of chromosome 3 between the region of p14 to
21.200
This finding is of interest, especially since deletions
and loss of heterozygosity of the short arm of chromosome 3 have been
reported also in lung cancer, particularly the small cell type in the
region of p14 to 23,302 suggesting evidence for a suppressor gene
important in respiratory carcinogenesis.
A significant
correlation exists between chromosomal aberrations and pulmonary
asbestos fiber burden in patients with mesothelioma.268 An inverse
correlation between survival and the number of copies of chromosome 7
short arms has been reported.268 These cytogenetic changes may also be
important in explaining the likely constitutional susceptibility to
mesothelioma (see below).
Exposure of normal human mesothelial
cells to asbestos fibers in vitro has as yet been unsuccessful in
producing mesothelioma.153,183 Malignant transformation was achieved in
one experiment by first transfecting cells with a plasmid containing the
simian virus SV40, resulting in immortalization, followed by
transfection with the EJ-ras gene, resulting in tumorigenicity.208
Exposure to asbestos failed to produce tumorigenicity, however. It may
be extremely difficult to realize in vitro all the different conditions
and interactions which may operate in vivo.
The existence of
transforming genes has been detected in human mesothelioma, but their
exact nature remains to be identified.23,149 They do not seem to be
related to the ras gene family, which was found activated in 50% of
asbestos-induced Syrian hamster tumor cell lines,23 or to the myc, myb,
neu, or fos oncogenes.101 Loss of heterozygosity for the p53 gene
located on the short arm of chromosome 17 has recently been observed in
three of four mesothelioma cell lines.72
In another study of 20
cell lines from 17 patients with malignant mesothelioma, p53
abnormalities were found in three lines only.180 Wilms' tumor suppressor
gene (WT-1) transcripts were found to be expressed in normal human
mesothelial cells and in 7 of 7 human mesothelioma cell lines.297
Recently, changes in another suppressor gene, p16, were described, with
homozygous deletions in 85% of mesothelioma cell lines and 22% of
primary tumor specimens.65
Asbestos fibers can also transfect
cells by binding to exogenous nucleic acids, such as plasmid DNA, which
then becomes associated with chromosomal DNA, thereby altering gene
expression.14 Knowledge of the role of growth factors in the genesis and
proliferation of mesothelioma is rapidly expanding. The role of
plateletderived growth factors (PDGF) has been emphasized.108
Mesothelioma cells express messenger RNAs (mRNAs) for both PDGF-A and -B
chains at higher levels than normal human mesothelial cells, whereas
the reverse is true for transforming growth factor-‰ (TGF-‰), suggesting
that PDGF may be an autocrine growth factor for mesothelioma.
108
The corresponding genes for PDGF-A and PDGF-B (which is almost
identical to the c-sis gene) are located on chromosomes 7p21 to p22 and
22q13.1, respectively, and although visible abnormalities of these
chromosomes are not constant in mesothelioma, alterations at a molecular
level cannot be excluded.108,278 Human mesothelioma cell lines,
compared with normal human mesothelial cells, have shown strongly
increased expression of the c-sis oncogene (PDGF-B) and to a lesser
degree of the gene for PDGF-A.278
Normal mesothelial cell lines
seem to express PDGF-‡ receptor genes, whereas mesothelioma cell lines
express predominantly PDGF-‰ receptor genes.279 These findings could
conceivably provide also a role for the thrombocytosis commonly observed
in mesothelioma patients, in view of its negative prognostic
influence.62,63,225 No increased expression of epidermal intrapleural or
intraperitoneal injection of various asbestos fibers (chrysotile or
amphibole) produce mesotheliomas in rats, hamsters, and mice, often
after a relatively long delay of 7 months or more.261
Intratracheal
instillation or inhalation is less often successful.25,290 Physical
characteristics, rather than chemical properties, are incriminated,
since many durable fibers of similar size and shape but of different
nature (glass, aluminum oxide, talc, attapulgite) can also produce
mesothelioma in animals.167,251 The most oncogenic fibers are the long,
thin ones, with a length > 8 microns and a diameter < 0.25 micron,
the so-called Stanton hypothesis, whereas shorter fibers may be
inactivated by phagocytosis.117,251 These long, thin fibers may
penetrate deep in the lung parenchyma,117,221,251 eventually reaching
the subpleural and pleural structures and penetrating into cells without
killing them, thereby implementing a complex oncogenic process.
The
effect of gravity on inhaled fibers may explain the predominance of
pleural mesothelioma in the lower thorax and on the right side.138 The
pathogenesis of peritoneal mesothelioma is more obscure. Although the
disease has not been produced in animals by feeding experiments,
ingestion of asbestos fibers is likely to occur through the action of
the tracheobronchial mucociliary apparatus, and these fibers may
penetrate the gastrointestinal mucosa.138 Alternatively, retrograde
spread to the peritoneal cavity from the pleura may take place.85 In
fact, autopsy studies have revealed that asbestos fibers are found in
many organs besides the lungs, including the spleen, thyroid, pancreas,
heart, adrenals, kidneys, liver, prostate, and even brain.17
The
possibility that asbestos exposure increases the risk of other cancers
besides mesothelioma and lung cancer has been reviewed.85 The evidence
is strong for laryngeal cancer (relative risk 1.4), suggestive but not
conclusive for esophageal cancer, possible for renal cancer, and
inconclusive for gastrointestinal, pancreatic, and ovarian cancers
(where misdiagnosis of mesothelioma is difficult to exclude). There
appears also to be no overall association with lymphomas, except
possibly with large cell lymphomas of the oral cavity and
gastrointestinal tract (see below).
