Overexpression of γ-glutamylcysteine synthetase in human malignant mesothelioma

Mesothelioma is a fatal tumor resistant to all treatment modalities for reasons that are still unresolved. Glutathione (GSH)-associated pathways are induced by oxidants and cytotoxic drugs, and they are also involved in the progression and resistance of some tumor cells in vitro. The rate-limiting enzyme in GSH biosynthesis is γ-glutamylcysteine synthetase (γGCS). However, the expression of this enzyme has not been systematically investigated in malignant tumors, and there are no studies of γGCS in biopsy specimens of malignant mesothelioma. We investigated the immunohistochemical distribution and expression of both subunits of γGCS in healthy pleural mesothelium, pleural mesothelioma tumor biopsy samples (34 cases), and mesothelioma cells in culture (7 cell lines). Nonmalignant mesothelium showed no immunoreactivity for either subunit in any of the cases. The heavy (catalytic) subunit of γGCS was highly immunostained in 29 and weakly positive in 5 cases. High-moderate and weak immunoreactivity of the light (regulatory) subunit of γGCS was found in 15 and 7 tumors, respectively, whereas 12 cases showed no reactivity. There was no correlation with either catalytic or regulatory subunit expression and patient survival. There was, however, a significant correlation between the heavy chain and multidrug resistance protein (MRP) 2 (P =.048), whereas no correlation was observed between the light chain and MRP1 or MRP2. Treatment of cultured mesothelioma cells with buthionine sulfoximine (BSO), to inhibit γGCS, significantly potentiated cisplatin-induced cytotoxicity mainly by nonapoptotic mechanism when assessed by counting the living cells, TUNEL (terminal deoxytransferase-mediated dUTP nick-end labeling) assay, and caspase-3 cleavage. In conclusion, γGCS is highly positive in most cases of malignant mesothelioma and may play an important role in the primary drug resistance of this tumor in vivo. HUM PATHOL 33:748-755. Copyright 2002, Elsevier Science (USA). All rights reserved.