REGULATION OF O-6-METHYLGUANINE-DNA METHYLTRANSFERASE BY METHIONINE IN HUMAN TUMOR-CELLS

Methionine (MET)-dependent cell lines require MET to proliferate, and homocysteine (HCY) does not act as a substitute for this requirement. From six O-6-methylguanine-DNA methyltransferase (MGMT)-efficient (mer (+)) cell lines tested, two medulloblastomas (Daoy and D-341) and a lu ng non-small-cell adenocarcinoma with metastatic potential (H-1623) we re most sensitive to MET deprivation, while two glioblastomas (U-138, D-263) and a small-cell lung carcinoma H-1944 were moderately to weakl y dependent. Regardless of the degree of MET dependence, all of these lines down-regulated their MGMT activity within 48-72 h of transfer fr om MET(+)HCY(-) to MET(-)HCY(+) media, long before the eradication of the culture. Reduction of MGMT activity was due to a decline of both M GMT mRNA and protein levels. However, the reduction was not related to the methylation status of the MGMT promoter at the SmaI site or the H paII sites in the body of the gene; such sites have been shown to be a ssociated in MGMT regulation and in defining the mer phenotype. MET-de pendent, mer(+) tumour cells cultured in MET(-)HCY(+) were more sensit ive to BCNU (IC50 = 5-10 mu M) than those cultured in MET(+)HCY(-) (IC 50 = 45-90 mu M), while MET-independent or mer cell lines were unaffec ted. This indicates that reduction of MGMT imposed by the absence of M ET, renders mer(+) tumour cells more susceptible to alkylating agents. The relatively selective suppression of MGMT activity in mer(+) MET-d ependent tumour cells, in combination with the inability of such cells to proliferate in the absence of MET, may lead to the development of more effective treatment strategies for mer MET-dependent tumours.