Abstract
Drug resistance is a major clinical problem in the chemotherapy of tumor diseases and the identification of factors that make tumor cells resistant to drug treatment is therefore of crucial importance. We have investigated a possible involvement of O6-methylguanine-DNA methyltransferase (MGMT), glutathione transferase (GST) and glutathione (GSH) in the resistance to 1,3- bis(2-chloroethyl)-1-nitrosourea (BCNU) in two human lung cancer cell lines. The non-small cell lung cancer U1810 expresses MGMT while it was undetectable in the small cell lung cancer U1690. Both cell lines possess similar GST activities but they show different GST patterns. GST M3-3, which is detectable in U1810 cells only, is the only one among the tested human GSTs that is capable of inactivating BCNU.
U1810 cells were found to be more resistant to BCNU than U1690 cells suggesting that both MGMT and GST M3-3 may contribute to the BCNU resistance in U1810 cells. The contribution of MGMT, GST M3-3 and GSH to BCNU resistance was investigated by inhibition of the activities of MGMT and GST M3-3 and by depletion of cells of GSH. These experiments showed that MGMT and GSH are the predominant resistance factors to BCNU in U1810 cells.
The analyses of the MGMT expression and BCNU sensitivity during cell cycle progression in U1810 cells revealed that while the MGMT activity decreased by about 50% in the mid S phase, no significant cell cycle phase-specific alteration in BCNU sensitivity was observed. The measurement of protein and mRNA levels of U1810 cells indicated that the expression of the MGMT gene is regulated at the level of transcription.
The content and activity of MGMT in human melanoma metastases were analysed as well. We observed a considerable variation between different melanoma metastases in MGMT content and activity. Heterogeneity occurred frequently between tumors in the same patient and to a lesser extent within individual tumors. Our results underline the importance of analysing MGMT expression at single cell level to obtain correct estimates and accentuate the need for analysis of multiple tumors in each individual before MGMT can be used as a predictive marker for drug resistance.
