O6-methylguanine repair in liver cells in vivo: Comparison between G1- and S-phase of the cell cycle

Summary To compare the formation and persistance of alkylated DNA bases in the G₁- and S-phase compartments in liver in vivo, regnerating rat liver was exposed to [¹⁴C]dimethylnitrosamine (0.57 mg/kg, IP injection) or N-[methyl ¹⁴C]-N-nitrosourea (3.3 mg/kg, intraportal injection) during the G₁ phase of the cell cyle (12 h after partial hepatectomy), or at 24 h after partial hepatectomy with 30% hepatocytes in DNA synthesis, or at 43 h after partial hepatectomy, 4 h after an hydroxyurea block from 14 to 39 h after operation with 80% hepatocytes in DNA synthesis. At 120 min after dimethylnitrosamine and 90 s, 5, 10, or 60 min after the intraportal pulse of N-methyl-N-nitrosourea the molar fractions of 7-methylguanine (7megua), O⁶-methylguanine (O⁶megua), and 3-methyladenine (3mead) and of metabolically labeled guanine were determined from DNA hydrolysates by Sephadex-G10 radiochromatography. After dimethylnitrosamine only minor differences were observed for 7megua formation in the three groups; the 3mead/7megua ratio remained constant irrespective of the number of cells in S phase. In contrast, the O⁶megua/7megua ratio revealed a loss of O⁶megua, the extent of which appeared proportional to the fraction of DNA-synthesizing cells in the liver. The rapid loss of O⁶megua in S-phase cells was confirmed after intraportal administration of N-methyl-N-nitrosourea. During the first 10 min after the methylnitrosourea pulse the O⁶megua/7megua ratio was constant in G₁ cells and dropped from 90 s to 10 min by about 15% in liver containing 30% S-phase cells and by about 40% with 80% cells in DNA synthesis. DNA-synthesizing hepatocytes are apparently endowed with a higher O⁶megua DNA transferase activity than nonproliferating liver cells. The rapid, though exhaustible elimination of O⁶megua during S-phase might result in partial protection of DNA-synthesizing cells from base-mispairing and/or from hypomethylation at G-C sites.Dedicated to Professor Hermann Druckrey on the occasion of his 80th birthdaySupported by grants from the Deutsche Forschungsgemeinschaft and the Dr. Mildred Scheel-Stiftung