Human mesothelial cells are unusually susceptible to simian virus 40-mediated transformation and asbestos cocarcinogenicity

Abstract

Mesothelioma, a malignancy associated with asbestos, has been recently linked to simian virus 40 (SV40). We found that infection of human mesothelial cells by SV40 is very different from the semipermissive infection thought to be characteristic of human cells. Mesothelial cells are uniformly infected but not lysed by SV40, a mechanism related to p53, and undergo cell transformation at an extremely high rate. Exposure of mesothelial cells to asbestos complemented SV40 mutants in transformation. Our data provide a mechanistic explanation for the ability of SV40 to transform mesothelial cells preferentially and indicate that asbestos and SV40 may be cocarcinogens. M alignant mesothelioma (MM) is a tumor of the serosal lining the pleural, pericardial, and peritoneal cavities that causes about 2,500 deaths per year in the United States (1). MM arises from the malignant transformation of mesothelial cells, which are undifferentiated cells representing the adult remnants of the surface coelomic mesoderm (1). Although MM has been associated with past exposure to asbestos fibers, the mechanisms through which asbestos causes mesothelial cell transformation are unclear. The capacity of asbestos to induce autophosphorylation of the epidermal growth factor receptor, which leads to activation protein-1 activity in human mesothelial cells (HM; ref.2); the production of reactive oxygen species by cells exposed to asbestos (3); and the local and systemic immunosuppressive effects of asbestos (4) may all contribute to carcinogenesis (1). Other factors act alone or synergistically with asbestos in causing MM, because only 5-10% of individuals exposed to high levels of asbestos develop MM, and 10-20% of MM occurs in individuals with no known exposure (1).Recently, simian virus 40 (SV40) has been associated with human mesothelioma and brain and bone tumors (reviewed in refs. 1 and 5-7). SV40 (5-8) is a DNA tumor virus encoding two transforming proteins (the large tumor antigen, or Tag; and the small tumor antigen, or tag), and three capsid proteins (VP1-3). Tag is the replicase of SV40. Expression of Tag in the absence of cell lysis leads to cellular transformation through several mechanisms, including Tag-mediated inhibition of cellular p53 and Rb family proteins, induction of insulin-like growth factor-I and its receptor, and the direct mutagenic effect of Tag. SV40 tag enhances Tag functions by inhibiting protein phosphatase 2A, contributing to malignancy (1, 9). SV40 infects cells from different species, and the cell type determines the outcome of SV40 infection (5-8). Permissive monkey cells support SV40 replication, which results in cell lysis. In nonpermissive rodent cells SV40 DNA cannot be replicated, and cells are not lysed and can be transformed. Human cells are termed semipermissive because only a fraction of cells express SV40 Tag after infection, these infected cells are lysed, and cell transformation is a very rare event.SV40 is highly oncogenic in rodents (5-7). We found that intracardial injection of SV40 induced MM...

Different simian virus 40 genomic regions and sequences homologous with SV40 large T antigen in DNA of human brain and bone tumors and of leukocytes from blood donors

“…Indeed, by using a semiquantitative PCR assay we determined 10 Ϫ2 to 10 Ϫ4 SV40 genome equivalents per cell in DNA samples from human lymphoproliferative disorders and from PBMCs of blood donors. 18 SV40 reactivation, by transfection of SV40 positive human DNA into permissive monkey cells, was reported in only a single case, 23 in agreement with the observation that in human cells that are semipermissive for SV40, this virus replicates poorly, 24 reaches low viral titers, 18,24,25 and generates at high-rate heterogeneous defective genomes, 26 which may interfere with the production of a complete, infectious viral progeny.…”

91 Crocidolite asbestos and SV40 are co-carcinogens in human mesothelial cells and in causing mesothelioma in hamsters

“…Wild-type SV40 can cause HM transformation in tissue culture and MM in hamsters independently of asbestos; therefore, it would have been difficult to study the mechanisms underlying the cocarcinogenesis of crocidolite and SV40 (14,18). However, neither crocidolite alone nor SV40 viruses that do not express the small t antigen (tag), such as SV40 dl883, can cause malignant transformation of HM (14). Moreover, SV40 dl883 does not cause MM when injected into hamsters (6).…”

“…Western blotting was as described in ref 14. The intensities of the bands were determined by using the rectangular box method from TIFF files imported into Scion Image 1.62c densitometry program of the public domain software NIH image (available at http:͞͞rsb.info.nih.gov͞nih-image͞) using three separate Western blots͞experiments.…”

“…We previously reported that SV40 and crocidolite asbestos were cocarcinogens in causing malignant transformation of HM in tissue culture (14). A recent study demonstrated that SV40 activates the AKT pathway in primary human mesothelial cells (HM), inducing cell survival and thus making HM less sensitive to asbestos cytotoxicity (15).…”

SV40 reporter viruses

“…This may result in transformation of the cells toward a more tumorigenic phenotype. Primary human mesothelial cells offer a unique system for studying SV40 virus infection as they become infected persistently with SV40 (Bocchetta et al, 2000). During persistent infection, contrary to other SV40 infections, the viral genome is maintained as an episome in the cells, and the cells become transformed by the early gene expression of large-T (LT) and small-t (ST) antigens.…”