Phenotypic stability and metastatic behaviour of serially xenografted rat mesotheliomas

Abstract

Summary Mesotheliomas induced in rats by intrapleural injection of the fibrous zeolite, erionite, were serially transplanted in nude mice for up to ten generations. The cell phenotypes (epithelial or sarcomatous) were well maintained during passaging, as determined morphologically and by the expression of the cytokeratin markers demonstrated in normal mesothelial cells. Some of the tumours occasionally produced metastasis in nude mice. In contrast, a cloned epithelial cell mesothelioma and sarcomatous cell mesothelioma, the original cells of which were isolated in tissue culture, both produced regular multiple metastases when passaged in nude mice. These metastases were frequently found on the visceral pleura, rather than in the lung parenchyma, in nude mice. The high metastatic rate of the xenograph mesotheliomas derived by in vitro isolation of cells from mesotheliomas is atypical of the usual behaviour of xenografts of mesotheliomas.The fibrous zeolite, erionite, has been shown to be particularly carcinogenic by inhalation both in man (Baris et al., 1979) and animals (Wagner, 1983) and by intrapleural inoculation into rats (Hill et al., 1990).Previously mesotheliomas induced by crocidolite asbestos have been serially transplanted (Wagner et al., 1982) in syngeneic rats with success. However, during passaging it was found that the tumour phenotype changed with passage number, often alternating between epithelial and sarcomatous cell type within a relatively short number of passages. One explanation of this behaviour would be the existence of a stem cell population within the tumour which has the capacity to differentiate into epithelial or sarcomatous cells, possibly due to the various local stimuli (Wagner et al., 1982; Johnson et al., 1984). There is certainly no doubt that the erionite-induced mesotheliomas can have the appearance of producing histological elements, such as bone, which are more fully differentiated forms of tissue than the more simple epithelial or sarcomatous forms (Johnson et al., 1984).One way of resolving whether pluripotential stem cells really do exist in mesotheliomas is to clone the cells derived from them and to grow them in culture before injecting them into a suitable host for tumour production. If cloned mesothelioma cells will produce the same pattern of mixed cell tumours with differentiated tissue elements, such as bone, when passaged in animals, then there can be little doubt as to the existence of a pluripotential stem cell responsible for this. Previous attempts to examine the morphological pattern of the mesotheliomas induced by asbestos using in vitro as well as in vivo techniques have shown that the in vitro cell cultures did not correspond well with the morphology of the original tumours (Gormley et al., 1980). Only one cell line of the uncloned cell lines established produced a tumour resembling a typical mesothelioma in vivo, although tumours were produced from the cell lines established. The selection of malignant elements using soft agar cloning methods (Brown et ...