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© 1991 Oxford University Press

research-article

Cellular levels of O6-methylguanine-DNA methyltransferase in mammary epithelial cells and liver from virgin, pregnant and pituitary grafted mice

Tapu A. Dutta-Choudhury, Benjamin H. Bak and Raphael C. Guzman

Department of Molecular and Cell Biology and the Cancer Research Laboratory, University of California Berkeley, CA 94720, USA

O6 methyltransferase (O6 is a DNA repair protein that reverses alkylation damage at the O6 position of guanine. In the process, O6-MT undergoes suicide inactivation. To determine if this enzyme might be regulated by pregnancy-associated hormones we measured changes in the level of O6-MT in isolated mouse mammary epithelial cell homogenates during different reproductive states. These were pregnancy, ectopic pituitary transplantation, proestrus/estrus and diestrus. O6 levels were found to be similar in mice in proestrus/estrus (0.95 fmolµg DNA) as compared to diestrus (0.94 fmol/µg DNA) and also mixed populations of virgin mice (1.09 fmol/µg DNA). A mean for all virgin mice (0.97 fmol/µg DNA) was i as a comparative Index. 0 decreased 2-fold during pregnancy in mammary epithelial cells to a mean value of 0.45 fmol/µg DNA (P < 0.05). A smaller decrease (0.65 fmol/µg DNA; P <0.01) in mammary epithelial cells was found at 3 weeks following pituitary isograft. The repair capacity of mammary epithelial cells to liver was compared by measurements made in liver homogenates from the same mice and are ~ 3-fold higher in liver from virgin mice (3.2 fmol/µ DNA) than mammary gland. Liver levels of O6-MT increased in pregnant (5.3 fmol/µ DNA) and pituitary transplanted (3.9 fmol/µ DNA) mice, and were 5- and 4-fold higher than the concentration in virgin mammary epithelial cells respectively.


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