Fiber Dimensions and Mesothelioma: A Reappraisal of the Stanton Hypothesis

Abstract

Fiber dimensions are postulated to be critical factors in the toxicity, fibrogenicity, and carcinogenicity of asbestos fibers. Recent in vitro experiments have provided evidence that the chemical composition of mineral fibers, especially surface iron content, is important in catalyzing the formation of highly reactive hydroxyl radicals that may cause acute toxicity, lipid peroxidation, and DNA damage. We have reexamined the roles of fiber length in the acute toxicity of crocidolite asbestos fibers in vitro and in vivo and in the induction of mesotheliomas in mice. Native UICC crocidolite asbestos fibers were separated into long and short fiber preparations by differential centrifugation. Both long and short fiber preparations stimulated the production of reactive oxygen species by elicited mouse peritoneal macrophages. Whether compared on the basis of equal mass, fiber number, or surface area, both long and short fiber preparations were toxic to macrophages. In vitro toxicity was prevented by the iron chelator, deferoxamine, or by exogenous superoxide dismutase or catalase. A single intraperitoneal injection of long crocidolite asbestos fibers caused deposition of fibers on the mesothelial surface at sites of lymphatic stomata, while short fibers were cleared to regional lymph nodes. Only the long fiber preparation caused an intense inflammatory reaction, local production of superoxide anions, and mesothelial cell injury. Similar to in vitro toxicity, mesothelial cell injury in vivo was ameliorated by deferoxamine or PEG-conjugated superoxide dismutase or catalase. If lymphatic clearance was prevented by daily repeated injections, short crocidolite asbestos fibers (but not titanium dioxide particles) accumulated at the mesothelial surface and stimulated an inflammatory reaction with local production of superoxide anions and injury to adjacent mesothelial cells. We tested whether repeated injections of short crocidolite asbestos fibers would prevent lymphatic clearance and produce mesotheliomas. Mice were injected weekly with equal numbers of native, long, or short crocidolite asbestos fiber preparations. After 22–60 weekly injections, 37.5% of mice injected with native crocidolite asbestos fibers developed malignant mesotheliomas. In contrast, 50.0% of mice injected with short fibers and 23.5% of mice injected with long fibers developed tumors. In summary, both long and short crocidolite asbestos fibers are toxic in vitro via an oxidant-dependent mechanism. In vivo, short fibers are also toxic and carcinogenic if lymphatic clearance is prevented.