Simian virus 40, asbestos and malignant mesothelioma

Abstract

Malignant mesothelioma (MM) is a deadly cancer arising from the mesothelium, and is associated with asbestos exposure, simian virus 40 (SV40) infection and genetic predisposition. The term asbestos identifies six commercially used mineral fibers. Crocidolite is ranked the most oncogenic mineral, but chrysotile accounts for >90% of asbestos used worldwide and its capacity to induce malignant mesothelioma is still debated. SV40 is a monkey DNA tumor virus and has been detected prevalently in a limited panel of human tumors: mesothelioma, bone and brain tumors, and lymphoma. These are the same tumor types that are specifically induced by SV40 when injected into hamsters, a finding that has raised concerns about the possible pathogenic role of SV40 in humans. In this dissertation, we studied the possible tissue tropism of SV40 transformation, investigated the transforming potential of chrysotile, and explored the correlation of SV40 and chrysotile fiber in inducing human mesothelial cell (HM) transformation. The apparent different cell tropism could be related to the virus (i.e., possibly to the number of 72 bp elements) and to differential expression of cellular genes, known to play a critical role in SV40-mediated transformation of human cells, such as p53, Notch-1 and c-Met. To test for possible differences in tissue tropism, we used two different SV40 isolates: archetypal SV40 (1ESV40), which contains one 72 bp element, and nonarchetypal SV40 (wtSV40), which contains two 72 bp elements, to infect primary HM and primary human astrocytes (Ast). Both viruses transformed astrocytes; but only wtSV40 transformed HM. These two viruses induced the intracellular signaling of c-Met and Notch-1 differently in HM and Ast cells. We also found that the viral production is positively correlated with p53 copy numbers in Li-Fraumeni fibroblasts and in human bone and brain tumor cell lines. Our results provide a biological rationale to the observation that 1ESV40 is prevalently detected in brain tumors and wtSV40 in mesothelioma. We also compared the effects of crocidolite and chrysotile on survival, morphology, transcriptional activity and transformation of human mesothelial cells. Chrysotile and crocidolite exposures result in similar effects on human mesothelial cells. Morphological and molecular alterations suggestive of epithelial-mesenchymal transition, like E--cadherin down-regulation and β--catenin phosphorylation followed by nuclear translocation, were more evident in chrysotile-exposed cells. Microarray data suggest that a key regulator of the transcriptional alterations induced by fiber exposure is High-Mobility Group Box-1 protein (HMGB1). Both in vitro and in vivo, HMGB1 release induced by short-term exposure to high doses was sustained for crocidolite and transient for chrysotile. However, the prolonged exposure of mice to lower doses of chrysotile or crocidolite resulted in the increase of HMGB1 serum levels over time. These data support the hypothesis that the bio-persistence of the two fibers differentially affects the transformation of mesothelial cells. Finally, based on the evidence that crocidolite and SV40 have a co-stimulatory effect in inducing MM, we tested the hypothesis that chrysotile and SV40 are also cocarcinogens. The formation of flat foci suggested that chrysotile and SV40 are potential cocarcinogens.