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

Abstract

To compare the formation and persistence of alkylated DNA bases in the G1- and S-phase compartments in liver in vivo, regenerating rat liver was exposed to [14C]dimethylnitrosamine (0.57 mg/kg, IP injection) or N-[methyl14C]-N-nitrosourea (3.3 mg/kg, intraportal injection) during the G1 phase of the cell cycle (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.

O4-ethyldeoxythymidine, but not O6-ethyldeoxyguanosine, accumulates in hepatocyte DNA of rats exposed continuously to diethylnitrosamine.

“…Rat liver eliminates O6-alkylguanine from DNA more rapidly than any other rodent tissue. This removal is enhanced by chronic exposure to nitrosamines (16)(17)(18)(19) and during the proliferative response after partial hepatectomy (20,21). More detailed investigations utilizing separated hepatocytes and nonparenchymal cells (NPC) indicated that rapid and enhanced removal of 06-alkylguanine were properties of hepatocytes only (22).…”

A Critical Appraisal of the Evolution ofN-Nitrosoureas as Anticancer Drugs

“…Rat liver cells in the DNA-synthesizing S phase appeared to have a higher level of MGMT activity than cells in other phases as judged by the lower O 6 -methylguanine/ N 7 -methylguanine ratio in the DNA-synthesizing cells. After treatment with N -methyl- N -nitrosourea ( 25 ) and dimethylnitrosamine the alkylated guanine base ratio was constant in G 1 cells, decreased by 15% in 30% S phase cells, and decreased by 40% in 80% S phase cells.…”

“…A series of unblocked carbohydrate-amino acid conjugates, their tetraacetates, and methylglycosides (887)(888)(889)(890)(891)(892)(893)(894)(895)(896)(897)(898)(899)(900)(901)(902) were synthesized 465,615-617 and screened 465,616,617 against the ip-implanted L1210 leukemia (Table 47). Representative synthetic sequences are illustrated in Scheme 88a-c.…”

“…After treatment with Nmethyl-N-nitrosourea (25) and dimethylnitrosamine the alkylated guanine base ratio was constant in G 1 cells, decreased by 15% in 30% S phase cells, and decreased by 40% in 80% S phase cells. It was concluded 899 that the elimination of O 6 -methylguanine during the S phase could protect the DNA synthesis from base-mispairings and/or from hypomethylation at G-C sites.…”

Βιοχημικες Μελετες Της Επιδρασης Του Μεθυλιωτικου Καρκινογονου Μεθυλοσουλφονικος Μεθυλεστερας Στον Μεταβολισμο Του Rna Σε Μυες C57bl. Συγκριση Και Συσχετισμοσμε Την Επιδραση Της Διμεθυλονιτρωδοαμινης

“…Η μεθυλίωση της κυτοσίνης εμφανίζεται πάντα σε διπλέτες CpG και δεν κατανέμεται τυχαία, αλλά σε περιοχές εμπλουτισμένες σε CpG που σχετίζονται με συγκεκριμένα γονίδια (Jones, 1986 Boehm & Drahovsky, 1981), (β. , (Wilson & Jones, 1983 (Hicks, 1983), (Becker, 1981). (Rabes et al 1979), (Rabes, 1983). (Gaugas, 1980).…”