Regulation of O6-methylguanine-DNA methyltransferase by methionine in human tumour cells

Methionine (MET)-dependent cell lines require MET to proliferate, and homocysteine (HCY) does not act as a substitute for this requirement. From six O6-methylguanine-DNA methyltransferase (MGMT)-efficient (mer+) cell lines tested, two medulloblastomas (Daoy and D-341) and a lung non-small-cell adenocarcinoma with metastatic potential (H-1623) were most sensitive to MET deprivation, while two glioblastomas (U-138, D-263) and a small-cell lung carcinoma H-1944 were moderately to weakly dependent. Regardless of the degree of MET dependence, all of these lines down-regulated their MGMT activity within 48-72 h of transfer from MET+HCY- to MET-HCY+ media, long before the eradication of the culture. Reduction of MGMT activity was due to a decline of both MGMT mRNA and protein levels. However, the reduction was not related to the methylation status of the MGMT promoter at the SmaI site or the HpaII sites in the body of the gene; such sites have been shown to be associated in MGMT regulation and in defining the mer phenotype. MET-dependent, mer+ tumour cells cultured in MET-HCY+ were more sensitive to BCNU (IC50 = 5-10 microM) than those cultured in MET+HCY-(IC50 = 45-90 microM), while MET-independent or mer- cell lines were unaffected. This indicates that reduction of MGMT, imposed by the absence of MET, renders mer+ tumour cells more susceptible to alkylating agents. The relatively selective suppression of MGMT activity in mer+ MET-dependent 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.

Maternal factors influencing the occurrence of low birthweight in northern Vietnam

The relationship between the birthweight of newborn infants and their mothers' health characteristics as well as socio-economic status were studied in 1474 consecutive deliveries from eight areas in the delta region of north Vietnam. The results showed that the proportion of low birthweight (LBW) infants varied from 7.9% to 12.5%. Body mass index (BMI) of the mothers, good nutrition during pregnancy and maternity leave before delivery were negatively associated with the occurrence of LBW. The association between chronic energy deficiency (low BMI) and LBW was seen only among the farming women. Measures should be taken to reduce the workload and to supply sufficient food for pregnant women, and special priority should be given to rural areas.

Mechanism of depletion of O6-methylguanine-DNA methyltransferase activity in rat tissues by O6-Benzyl-2'-deoxyguanosine. Role of metabolism

O6-Benzyl-2'-deoxyguanosine (O6-BzldGuo) was found to be a potent inhibitor of O6-methylguanine-DNA methyltransferase (MGMT) activity in vivo, despite its demonstrated lower activity compared to O6-benzylguanine (O6-BzlGua) for inactivation of MGMT in cell cultures or cell free extracts. This difference is probably due to metabolism and possibly to enhanced systemic availability of O6-BzldGuo compared to O6-BzlGua in animals. At doses above 50 mg/kg, O6-BzldGuo completely suppresses the MGMT activity in liver, lung and brain for 12 to 18 hrs following treatment. However, it fails to suppress the activity in jejunum and possibly in other tissues with low blood perfusion rates. O6-BzldGuo is metabolized mainly in the liver, and possibly in kidney and lung, to yield the more potent MGMT inhibitors O6-BzlGua and O6-Benzyl-7,8-dihydro-8-oxoguanine (O6-Bzl(8-OX)Gua), that reach maximum levels following the decline of the parent compound both in tissues and the circulation. Although O6-BzldGuo may cause the initial drop of MGMT activity, continued prolonged suppression is likely due to the persistence of its MGMT-inhibiting metabolites. Severe suppression of MGMT activity and the presence of O6-BzldGuo and its metabolites in the brain indicates that this inhibitor crosses the blood-brain barrier. Prolonged suppression of MGMT in brain following a complete disappearance of the parent compound from the tissue also demonstrates the importance of metabolites in MGMT suppression. Such metabolites, especially O6-Bzl(8-OX)Gua, appear to accumulate in brain, possibly due to their hydrophobic nature. Accumulation of the parent compound in the pancreatic ducts may explain the unusual high concentrations of this compound in pancreas as compared to the spleen. Similarly, high concentrations of O6-BzldGuo in kidneys may be related to accumulation of the compound in proximal tubules and its poor reabsorption into the circulation. Overall, O6-BzldGuo shows tissue specificity which is modulated by dose. Such specificity seems to be related to accumulation of the parent compound in certain tissues, but more importantly, to site-specific metabolism and the persistence of MGMT inhibitory metabolites in these tissues. The combination of enhanced systemic availability and site specific metabolic activation and/or inactivation of O6-BzldGuo may be related to the enhanced antitumor therapeutic index against human tumor xenografts for this compound which equals or even exceeds that of O6-BzlGua.