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O6-methylguanine-DNA methyltransferase activity in human liver tumors
G Isowa1, K Ishizaki, T Sadamoto
1Radiation Biology Center, Faculty of Medicine, Kyoto University, Japan.
Abstract:
Previous studies have shown that in about one-fifth of human tumor cell strains, the activity of O6-methylguanine-DNA methyltransferase (MGMT), which can repair O6-alkylguanine in DNA produced by alkylating agents, is deficient. These strains are termed Mer- cells. To see if there is any human tumor lacking MGMT activity, we measured the MGMT activity in extracts from liver tumors of 21 patients, and compared it to the activity in normal peritumoral tissues derived from the same patients. The MGMT activity was assayed by measuring the 3H radioactivity transferred from the substrate DNA containing [methyl-3H]-labeled O6-methylguanine to an acid-insoluble protein fraction. There was considerable variation in MGMT activity among individual extracts; the interindividual variation was approximately 6-fold in normal liver tissue and much larger in liver tumors. Although in many cases similar high levels of MGMT activity were found both in liver tumors and in the normal counterpart, six tumors had greater than 3-fold less activity compared with the normal liver tissue from the same patient. Liver tumors from two patients did not have any detectable level of MGMT activity by the present method used, in spite of the fact that the corresponding normal liver samples demonstrated significant activities. We also measured in the same tissue extracts the activities of two common enzymes, glutamic pyruvic transaminase (GPT) and lactic dehydrogenase (LDH). The activities of GPT and LDH in the liver tumor samples that showed undetectable levels of MGMT activity were similar to those in the surrounding normal liver tissues. These results may suggest the existence of human Mer- tumors, deficient or very low MGMT activity.
Insights
Some human liver tumors show deficient O6-methylguanine-DNA methyltransferase (MGMT) activity, an enzyme crucial for DNA repair. This suggests the potential existence of Mer- tumors lacking MGMT, impacting cancer treatment strategies.
Area of Science:
- Oncology
- Molecular Biology
- Biochemistry
Background:
- O6-methylguanine-DNA methyltransferase (MGMT) repairs DNA damage from alkylating agents.
- Deficient MGMT activity (Mer- cells) is observed in some human tumor cell strains.
- The presence of MGMT-deficient human tumors is not fully established.
Purpose of the Study:
- To investigate MGMT activity in human liver tumors and compare it with adjacent normal tissue.
- To determine if human liver tumors exhibit deficient or undetectable MGMT activity.
Main Methods:
- Assayed MGMT activity in liver tumor and normal tissue extracts using a radiolabeled DNA substrate.
- Measured MGMT activity by quantifying the transfer of [methyl-3H]-labeled O6-methylguanine to proteins.
- Assessed activities of control enzymes glutamic pyruvic transaminase (GPT) and lactic dehydrogenase (LDH).
Main Results:
- Significant interindividual variation in MGMT activity was observed in both normal and tumor tissues.
- Six of 21 liver tumors showed >3-fold lower MGMT activity than their normal counterparts.
- Two liver tumors exhibited undetectable MGMT activity, while normal tissues had significant levels.
Conclusions:
- The study suggests the existence of human liver tumors with deficient or very low MGMT activity (Mer- tumors).
- The observed MGMT deficiency in tumors was not associated with altered GPT or LDH levels.
- Findings have implications for understanding tumor resistance to alkylating agents and developing targeted therapies.