Mesothelial differentiation, mesothelioma and tumor markers in serous cavities

Abstract

Mesothelial cells covering the serous membranes are of mesodermal origin although they acquire several characteristics of epithelial cells. The cytological and histological diagnosis of malignant tumors in serous cavities is sometimes a major challenge: the main diagnostic alternatives are reactive benign mesothelium, mesothelioma and adenocarcinoma. The aims of this thesis were to improve the differential diagnosis of these conditions and to study mechanisms related to mesothelial differentiation.

None of the antibodies recommended for distinguishing between mesothelioma and adenocarcinoma are entirely specific. Using logistic regression analysis the tissue immunoreactivities of 10 antibodies were ranked, 2 rejected and a panel of the best 8 retained. The optimized panel should preferably be applied in two steps, starting with the 4 diagnostically most valuable parameters.

The additional diagnostic value of two biochemical assays in ascitic fluid was assessed in cases having inconclusive effusion cytology. An increased concentration of CEA indicated a carcinoma. A higher cholesterol level indicated carcinomatosis, but was not entirely specific for this condition. These two chemical assays measure different effects of tumors (CEA production versus cell disintegration), and when they were combined, increases in both parameters were specific for carcinomatosis and improved the sensitivity of "single" effusion cytology.

Mesothelial cells obtained from benign effusions are not terminally differentiated and grow uniformly with a fibroblastic or epithelioid morphology in vitro. To investigate how this differentiation correlated with Wilms' tumor susceptibility gene I (WTI) and proteoglycans (PGs), their expressions were studied by semiquantitative RT-PCR. mRNAs for WTI and cell surface PGs syndecans-1, -2, -4 and glypican-1 were more abundant in epithelioid cells, while fibroblastic cells expressed more of the matrix PGs biglycan and versican.

To assess the involvement of other molecules in mesothelial differentiation, suppression subtractive hybridization was used. This showed a pattern of genes coordinately regulated during this process. The genes overexpressed in fibroblastic cells were either matrix-associated or related to proliferation, while those upregulated in the epithelioid phenotype were related to a more differentiated cell type. Immunohistochemistry with available antibodies confirmed that the differences were also present in vivo at sites of mild mesothelial activation and regeneration.

Since PGs and WTI were related to phenotypic differentiation in benign mesothelial cells, their expression was also studied in adenocarcinomas and mesotheliomas. Adenocarcinoma cells produced more mRNA for syndecan-1, but mesothelially derived cells expressed WTI, biglycan and larger amounts of syndecan-2. Syndecan-4 was highly expressed in all malignant cell lines, as compared to benign mesothelial ones. Versican expression was associated with a high rate of proliferation. Pilot experiments suggest that the reciprocal appearance of syndecan-1 and syndecan-2 may be a useful diagnostic marker with dominance of syndecan-2 reactivity in mesothelial tissues.

In summary, these data show the role of PGs in mesothelial differentiation, and suggest that they can be used as markers to distinguish malignant mesothelioma from other conditions. The findings also indicate that epithelioid mesothelial cells may be derived from subserosal fibroblastic cells, which would then correspond to a less differentiated mesothelial phenotype.