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Circadian variation in O6-methylguanine-DNA methyltransferase activity in mouse liver
N Martineau-Pivoteau1, C Cussac-Buchdahl, P Chollet
1Laboratoire 'Rythmes biologiques et Chronothérapeutique' (Université Paris XI), ICIG, Hôpital Paul Brousse, Villejuif, France.
Abstract:
Bifunctional chloroethylating cytostatic agents produce lethal DNA lesions, as a result of the formation of O6-alkylguanines. These lesions can be repaired by O6-methylguanine-DNA methyltransferase (MGMT). This ubiquitous nuclear and cytosolic enzyme removes the alkyl group by accepting it to the cysteine residue of its active site, thus preventing the formation of DNA interstrand cross-links. The role of the circadian organization in cellular protection against such DNA insults was examined in male B6D2F1 mice, synchronized with an alternation of 12 h of light and 12 h of darkness (LD12:12). MGMT activity was determined in liver of mice obtained at eight different circadian times, located 3 h apart. MGMT activity varied 5-fold along the 24 h time-scale, from 7 +/- 1 pmol/g of tissue at 7 h after light onset (HALO), during the rest span, up to 32 +/- 9 pmol/g at 19 HALO (second mid to late activity span). This large amplitude circadian rhythm in MGMT activity may be an important determinant of the susceptibility rhythms to alkylating agents. The greatest DNA repair activity occurred at night when mice were active, eat and drink, and thus are at a higher risk of being exposed to chemical insults.
Insights
The study found that O6-methylguanine-DNA methyltransferase (MGMT) activity in mice fluctuates significantly over a 24-hour period. This circadian rhythm in DNA repair capacity may influence how animals respond to DNA-damaging agents.
Area of Science:
- Chronobiology
- Molecular Biology
- Toxicology
Background:
- Bifunctional chloroethylating agents cause lethal DNA damage via O6-alkylguanines.
- O6-methylguanine-DNA methyltransferase (MGMT) repairs these lesions by removing alkyl groups, preventing DNA interstrand cross-links.
Purpose of the Study:
- To investigate the influence of circadian rhythms on cellular protection against DNA damage.
- To determine if MGMT activity exhibits daily variations in mice.
Main Methods:
- Male B6D2F1 mice were synchronized to a 12h light:12h dark cycle.
- Liver MGMT activity was measured at eight different time points across a 24-hour period.
Main Results:
- MGMT activity showed a 5-fold variation over 24 hours.
- Peak activity (32 +/- 9 pmol/g) occurred at 19:00 (7 hours after light onset), during the active phase.
- Lowest activity (7 +/- 1 pmol/g) was observed at 07:00 (7 hours after light onset), during the rest phase.
Conclusions:
- A significant circadian rhythm exists in mouse liver MGMT activity.
- This rhythm likely impacts susceptibility to alkylating agents, with higher repair capacity during the active (night) phase.