Studies on mesothelial differentiation : Prognostic and therapeutic approaches to malignant mesothelioma

Abstract: Malignant mesothelioma (MM), a tumor with a poor prognosis, causes the death of most patients within a year of diagnosis since various treatments seem to have little effect on its outcome. The biological behavior of MM varies greatly, and this heterogeneity is related to its morphology, which may be epithelioid, sarcomatoid (fibroblastoid) or biphasic, the sarcomatoid phenotype being associated with a shorter survival. The mechanisms by which the MM cells differentiate into epithelioid or fibroblastoid phenotypes are largely unknown. We used a MM cell line that, depending on the serum supplementation, differentiates into sublines with two phenotypes, the epithelioid STAV-AB and fibroblastoid STAV-FCS. A similar variability in growth phenotype can also be seen in benign mesothelial cells that have exfoliated into effusions. In short-term cultures, such cells may grow with a fibroblastoid or epithelioid morphology. The growth patterns obtained remain stable during several passages and the benign and malignant cells can be used as in vitro models when studying mechanisms of mesothelial differentiation. The aims of the present study were to determine the candidate genes involved in the differentiation of benign and malignant mesothelium. This was done by using Suppression Subtractive Hybridization (SSH) and Microarray techniques, and the findings were confirmed in vitro and in vivo. Such screening of gene expressions may be of value in finding diagnostic markers and targets for novel treatments of MM. Both the benign and malignant epithelioid cells expressed more genes related to specialized functions associated with metabolism, cellular defense, apoptosis and differentiation, while those related to growth factors and their receptors were more abundantly expressed in the sarcomatoid phenotype. These findings accord with the view that the fibroblastoid cell type represents a less differentiated stage than the epithelioid ones. Our data indicate that the different tumor phenotypes use different driving mechanisms, and the heterogeneity of the tumor may therefore be one explanation for the poor response to chemotherapy. A better response might be expected if targets were selected according to the individual phenotypes. The targeting of two differentiation-associated functions induced apoptosis in vitro. The thioredoxin/thioredoxin reductase (Trx/TR) system was differentially expressed, although extremely large amounts were present in both cell sublines. This may make the cells particularly sensitive to selenite, and this salt reduced the cell viability of MM cells, especially in those with fibroblast-like phenotype. A combination with doxorubicin was synergistic, and markedly affected on both cell phenotypes. A similar induction of apoptosis occurred in the epithelioid cells, after giving proteasome inhibitor (PSI). Here the dose- and time- dependent sensitivity were mainly confined to cells with an epithelioid morphology. Selenite and proteasome inhibitors are potentially new contributions for the treatment of MM.