Calretinin is essential for mesothelioma cell growth/survival in vitro: a potential new target for malignant mesothelioma therapy?

Malignant mesothelioma (MM) are highly aggressive asbestos-related neoplasms, which show strong chemotherapy resistance, and there is no effective cure for MM so far. Calretinin (CR) is widely used as a diagnostic marker for epithelioid and mixed (biphasic) mesothelioma; however, little is known about CR's putative functions in tumorigenesis. CR protects against asbestos-induced acute cytotoxicity mediated by the AKT/PI3K pathway, and furthermore, SV40 early region genes are able to upregulate CR in mesothelial cells. However, the precise role of CR in mesothelioma is still unknown. Downregulation of CR via lentiviral-mediated short-hairpin RNA significantly decreased the viability and proliferation of mesothelioma cells in vitro. The effect was strong in epithelioid-dominated cell lines (ZL55 and MSTO-211H). A weaker and delayed effect was observed in mesothelioma cells with prevalent sarcomatoid morphology (SPC111, SPC212 and ZL34). The specificity of the effect was confirmed by stable enhanced green fluorescent protein-CR expression in mesothelioma cell lines and subsequent downregulation. Depletion of CR led these cancer cell lines to enter apoptosis within 72 hr postinfection via strong activation of the intrinsic caspase 9-dependent pathway. Downregulation of CR in immortalized mesothelial cells LP9/TERT-1 strongly blocked proliferation and caused a G1 block without decreasing viability or activating apoptosis pathways. These results demonstrate that downregulation of CR had a strong effect on the viability of MM cells and that CR is essential for cells derived from MM. The authors anticipate these findings to reveal CR as a highly interesting new putative therapeutic target for mesothelioma treatment of especially the epithelioid, as well as of the mixed and sarcomatoid type.