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Although asbestos is known to be a complete carcinogen for the production of mesothelioma, its mechanisms of action are still in question (1, 2). The asbestos fiber appears to have two major sources of genotoxicity: generation of reactive oxygen species (either from its own surface by reactions involving catalytic iron or from its phagocytosis by professional phagocytes) (3) or mechanical effects such as interference with mitotic spindle formation and the segregation of chromosomes (6, 7). Neither mechanism alone can account for all the known facts about asbestos-induced toxicity. For example, although reactive oxygen species undoubtedly are involved in toxicity, oxidant generation alone cannot easily explain why long fibers are more carcinogenic than short fibers. A mechanical explanation alone cannot explain why other long, thin fibers such as fiberglass are not as genotoxic as asbestos. The questions about asbestos pathogenesis are relevant today (8). Asbestos serves as the paradigm for a fibrous carcinogen. Even if all the fibers could be removed from our schools, we would be using other fibers, natural or man-made, in our lives. Understanding the mechanisms of asbestos-induced toxicity will enable us to avoid repeating the toxic cycle with another fiber.
In the current issue of the AJRCMB, Levresse and colleagues from the laboratory of Dr. Marie-Claude Jaurand report on the effects of two types of asbestos fiber (crocidolite, an amphibole-type fiber and chrysotile, a serpentine-type fiber) on the rat pleural mesothelial cell (9). This group as well as others have previously documented asbestos-induced DNA and chromosomal damage in the mesothelial cell (7, 10). Now, they have turned their attention to the effects of asbestos-induced DNA damage on the cell cycle and checkpoints leading either to cell cycle arrest or apoptosis. These life-or-death decisions may be at the heart of malignant transformation (11). It is the cells that evade the protective mechanisms of arrest or apoptosis that can survive with a damaged, unstable genome and thereby enter the multistep process leading to malignancy.
In response to DNA damage, the cell can either arrest (allowing time for repair) or initiate an active form of cell death termed apoptosis. The means by which one pathway or the other is chosen remain unclear, particularly because both pathways appear to be dependent on p53 (12). The choice of a cell to arrest or to commit apoptotic suicide is currently one of the most intriguing questions regarding the fate of cells and the role of p53. Nonetheless, either path, to cell cycle arrest or to apoptosis, serves to maintain the stability and fidelity of the genome.
Cell cycle arrest in response to DNA damage allows the cell time to repair damage before progression (13). The cell may arrest either at G1 before initiation of synthesis (G1/S), at G2 before mitosis (G2/M), or within S phase. The particular stage of arrest depends on the timing of the damage, but is also sensitive to the type of DNA damage. Arrests at G1/S are particularly sensitive to single-strand gaps or breaks, perhaps because progression through synthesis without repair could convert them to double-strand breaks. Arrests at G2/M are sensitive to even one double-strand break, perhaps because progression through mitosis would lead to loss of chromosomal fragments (14). Many of these responses are dependent on p53, especially the G1/S checkpoint. After detection of DNA damage, p53 protein increases rapidly by posttranscriptional means and initiates transcription of several genes, including that encoding p21/WAF/Cip1 (an inhibitor of cyclin-dependent kinases) and GADD45, to arrest the cell in G1/S (12).
Apoptosis may remove cells with DNA damage that is too extensive to repair and thereby delete potentially harmful cells from the organism (15). Again, p53 plays an important role, although its mode of action is less clear. p53 acts transcriptionally, perhaps by upregulating expression of Bax, a proapoptotic protein that dimerizes with and is inhibited by Bcl-2, but also nontranscriptionally, perhaps by protein-protein interactions, to initiate apoptosis.
For chrysotile asbestos, and to a lesser extent for crocidolite asbestos, Levresse and colleagues show evidence of
a cell cycle arrest in G1/S and G2/M as well as an increase
in p53 protein, all strongly suggestive of a cellular response
to DNA damage (9). Similar findings with 
irradiation
strengthen the connection with DNA damage. Interestingly, these authors find little evidence of apoptosis (0.05%
of cells exposed to crocidolite [10 µg/cm2] for 24 h), a finding more striking in earlier studies (between 17 and 25%
of cells exposed to crocidolite [5-10 µg/cm2] for 24 h) (16,
17). The different paths taken by asbestos-exposed mesothelial cells in these studies are noteworthy and may illustrate the different choices of cells following DNA damage.
The choice of life or death may be guided by several factors including the particular cell type involved (e.g., fibroblast versus thymocyte), intrinsic factors such as the degree of DNA damage (18), or extrinsic factors such as the input from growth factor signals (19). Thus, the lower incidence of apoptosis in this study compared to that of earlier studies may reflect their different experimental conditions. For example, the crocidolite asbestos used in the study by Levresse and colleagues (9) is significantly shorter (mean length, 2.1 µm) than in those studies in which a higher degree of apoptosis was found (mean length, 19 µm) (16, 17, 23). If the length of asbestos is indeed important for its carcinogenic potential, then fiber length may be an independent factor in the damage sustained by the DNA and/or chromosomes and in the ultimate life-or-death path of the mesothelial cell. Other differences in the studies include the cellular growth phase (log phase versus confluent) and the presence of serum (present versus absent), all factors that may explain the different outcome of cells in response to similar concentrations of asbestos. For example, in the presence of serum, proteins such as vitronectin adsorb onto the crocidolite asbestos fiber, permitting the coated fiber to interact with cellular integrins and alter cellular responses (24). Of note, we have recently found that incubating crocidolite asbestos with serum suppresses asbestos-induced apoptosis without decreasing asbestos-induced DNA damage (25). Clearly, environmental factors as well as intrinsic factors participate in directing the damaged cell toward life or death.
Nonetheless, the key question, whether a damaged cell undergoes predominantly growth arrest or apoptosis, is how genetic abnormalities persist and become embedded in the genome of a population of cells. In a cell cycle arrest model, DNA damage is normally repaired before progression. DNA repair, however, may occasionally introduce error. Checkpoints may fail to prevent progression of a cell with a damaged DNA template either because the checkpoint itself fails or because it adapts to a continuing signal of DNA damage (26). In an apoptosis model, the damaged cell is normally deleted from the organism. Apoptosis may be suppressed, however, by genetic influences (p53 mutations, overexpression of Bcl-2) or perhaps environmental influences (signals from growth factors or integrins) thereby allowing survival of cells with abnormal genomes.
Presumably, bypassing the growth arrest or apoptotic responses to asbestos-induced DNA damage would allow the multistep accumulation of genetic abnormalities necessary for the evolution of mesothelioma. Perhaps the most common mechanism by which cells bypass growth arrest or apoptotic checks is by mutations in p53, the proximal effector of each of these pathways. The importance of p53 as the central surveillance mechanism is illustrated by the frequency of p53 mutations in cancers (27). Unlike most tumors, however, mesothelioma appears to express wild-type p53 (28, 29). Thus, the evasion of growth arrest and/or apoptosis by this tumor can be expected to arise from other pathways, either those downstream of p53 (30, 31), those that may inactivate p53 (32), or those that could potentially override p53, such as growth factor signals or oncogenic pathways that mimic growth factor signals (19- 22, 25).
From earlier work, we know that asbestos induces DNA damage in mesothelial cells, although the exact mechanisms remain in doubt. Now, with the new information provided by Levresse and colleagues (9), we have learned that asbestos can direct mesothelial cells toward either cell cycle arrest or apoptosis. An important question for the future will be how mesothelial cells evade these two protective mechanisms so that asbestos-induced genetic abnormalities persist and accumulate.
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Address correspondence to: V. Courtney Broaddus, M.D., Associate Professor of Medicine, Lung Biology Center, Box 0854, University of California at San Francisco, San Francisco, CA 94143-0854.
(Received in original form July 11, 1997 and in revised form July 22, 1997).
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