Changes in secondary structure of DNA of rat embryos following treatment with diethylnitrosamine and methylazoxymethanol acetate in vivo.

Diethylnitrosamine (DEN) and methylazoxymethanol acetate (MAM) are not transplacental carcinogenic but embryotoxic to Wistar rats when administered by i.p. injection on day 12 of gestation. MAM, a weak teratogen to rats during this period, induced a dose dependent increase in the number of resorptions to 15% and 40% of the litters following doses of 15 and 25 mg/kg bw, respectively. Rats similarly treated with 70, 150, and 180 mg DEN/kg bw resulted in increases in total DNA mass of day 13 embryos by 31%, 45% and 52%, respectively, compared to the saline treated controls. Twenty percent reduction in total DNA amount was detected following 25 mg MAM/kg bw. Benzoylated DEAE-cellulose (BD-cellulose) chromatography fractionates DNA on the basis of secondary structure by stepwise elution of double-stranded DNA with 1.0M NaCl solution (SE-DNA) followed by elution of DNA containing single-stranded regions with caffeine solution (CE-DNA). Day 13 embryonic DNA was monitored by in vivo labelling with [methyl-3H]-thymidine (3H-TdR) on days 6 and 7 of gestation. Significant increases in percentages of caffeine-eluted DNA (%CE-DNA) compared to control values were detected 24 h after treatment of day 12 embryos with 70, 150, and 180 mg DEN/kg bw. Such increases were not observed after MAM. Incorporation of [methyl-14C]-thymidine (14C-TdR) into embryonic DNA demonstrated the effects of treatment with these compounds on DNA synthesis in vivo. When compared to saline controls, DEN induced significant increases in 14C-TdR incorporation into embryo DNA, 1 h prior to analysis, but the increases were not proportional to the doses administered. Similar analysis of MAM treated samples showed no significant changes to %CE-DNA values. The relative %CE-DNA is expressed as the ratio of the percentage of caffeine-eluted 14C-labelled DNA to %CE-DNA (i.e., %CE-14C-DNA:%CE-3H-DNA). In the majority of control embryos the 14C-specific activity of CE-DNA was higher than the 14C-specific activity of SE-DNA. No significant change to relative %CE-DNA values of embryos to those of the controls was observed 24 h after treatment of day 12 gestation rats with single doses of DEN and MAM. The results of this study support the hypothesis that initiation mechanisms of teratogenesis and transplacental carcinogenesis are different. The pertinence of %CE-DNA and relative %CE-DNA values to teratogenesis and transplacental carcinogenesis is also discussed.